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Matherly LH, Schneider M, Gangjee A, Hou Z. Biology and therapeutic applications of the proton-coupled folate transporter. Expert Opin Drug Metab Toxicol 2022; 18:695-706. [PMID: 36239195 PMCID: PMC9637735 DOI: 10.1080/17425255.2022.2136071] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2022] [Accepted: 10/11/2022] [Indexed: 01/19/2023]
Abstract
INTRODUCTION The proton-coupled folate transporter (PCFT; SLC46A1) was discovered in 2006 as the principal mechanism by which folates are absorbed in the intestine and the causal basis for hereditary folate malabsorption (HFM). In 2011, it was found that PCFT is highly expressed in many tumors. This stimulated interest in using PCFT for cytotoxic drug targeting, taking advantage of the substantial levels of PCFT transport and acidic pH conditions commonly associated with tumors. AREAS COVERED We summarize the literature from 2006 to 2022 that explores the role of PCFT in the intestinal absorption of dietary folates and its role in HFM and as a transporter of folates and antifolates such as pemetrexed (Alimta) in relation to cancer. We provide the rationale for the discovery of a new generation of targeted pyrrolo[2,3-d]pyrimidine antifolates with selective PCFT transport and inhibitory activity toward de novo purine biosynthesis in solid tumors. We summarize the benefits of this approach to cancer therapy and exciting new developments in the structural biology of PCFT and its potential to foster refinement of active structures of PCFT-targeted anti-cancer drugs. EXPERT OPINION We summarize the promising future and potential challenges of implementing PCFT-targeted therapeutics for HFM and a variety of cancers.
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Affiliation(s)
- Larry H. Matherly
- Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan 48201, United States
- Department of Oncology, Wayne State University School of Medicine, Detroit, Michigan 48201, United States
- Department of Pharmacology, Wayne State University School of Medicine, Detroit, Michigan 48201, United States
| | - Mathew Schneider
- Department of Oncology, Wayne State University School of Medicine, Detroit, Michigan 48201, United States
| | - Aleem Gangjee
- Division of Medicinal Chemistry, Graduate School of Pharmaceutical Sciences, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
| | - Zhanjun Hou
- Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan 48201, United States
- Department of Oncology, Wayne State University School of Medicine, Detroit, Michigan 48201, United States
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2
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Newstead S. Structural basis for recognition and transport of folic acid in mammalian cells. Curr Opin Struct Biol 2022; 74:102353. [PMID: 35303537 PMCID: PMC7612623 DOI: 10.1016/j.sbi.2022.102353] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2021] [Revised: 01/29/2022] [Accepted: 02/08/2022] [Indexed: 12/19/2022]
Abstract
Structural studies on mammalian vitamin transport lag behind other metabolites. Folates, also known as B9 vitamins, are essential cofactors in one-carbon transfer reactions in biology. Three different systems control folate uptake in the human body; folate receptors function to capture and internalise extracellular folates via endocytosis, whereas two major facilitator superfamily transporters, the reduced folate carrier (RFC; SLC19A1) and proton-coupled folate transporter (PCFT; SLC46A1) control the transport of folates across cellular membranes. Targeting specific folate transporters is being pursued as a route to developing new antifolates with improved pharmacology. Recent structures of the proton-coupled folate transporter, PCFT, revealed key insights into antifolate recognition and the mechanism of proton-coupled transport. Combined with previously determined structures of folate receptors and new predictions for the structure of the RFC, we are now able to develop a structure-based understanding of folate and antifolate recognition to accelerate efforts in antifolate drug development.
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Affiliation(s)
- Simon Newstead
- Department of Biochemistry, University of Oxford, Oxford, OX1 3QU, UK.
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3
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Hou Z, Gangjee A, Matherly LH. The evolving biology of the proton‐coupled folate transporter: New insights into regulation, structure, and mechanism. FASEB J 2022; 36:e22164. [PMID: 35061292 PMCID: PMC8978580 DOI: 10.1096/fj.202101704r] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2021] [Revised: 12/15/2021] [Accepted: 01/03/2022] [Indexed: 01/19/2023]
Abstract
The human proton‐coupled folate transporter (PCFT; SLC46A1) or hPCFT was identified in 2006 as the principal folate transporter involved in the intestinal absorption of dietary folates. A rare autosomal recessive hereditary folate malabsorption syndrome is attributable to human SLC46A1 variants. The recognition that hPCFT was highly expressed in many tumors stimulated substantial interest in its potential for cytotoxic drug targeting, taking advantage of its high‐level transport activity under acidic pH conditions that characterize many tumors and its modest expression in most normal tissues. To better understand the basis for variations in hPCFT levels between tissues including human tumors, studies have examined the transcriptional regulation of hPCFT including the roles of CpG hypermethylation and critical transcription factors and cis elements. Additional focus involved identifying key structural and functional determinants of hPCFT transport that, combined with homology models based on structural homologies to the bacterial transporters GlpT and LacY, have enabled new structural and mechanistic insights. Recently, cryo‐electron microscopy structures of chicken PCFT in a substrate‐free state and in complex with the antifolate pemetrexed were reported, providing further structural insights into determinants of (anti)folate recognition and the mechanism of pH‐regulated (anti)folate transport by PCFT. Like many major facilitator proteins, hPCFT exists as a homo‐oligomer, and evidence suggests that homo‐oligomerization of hPCFT monomeric proteins may be important for its intracellular trafficking and/or transport function. Better understanding of the structure, function and regulation of hPCFT should facilitate the rational development of new therapeutic strategies for conditions associated with folate deficiency, as well as cancer.
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Affiliation(s)
- Zhanjun Hou
- Molecular Therapeutics Program Barbara Ann Karmanos Cancer Institute Detroit Michigan USA
- Department of Oncology Wayne State University School of Medicine Detroit Michigan USA
| | - Aleem Gangjee
- Division of Medicinal Chemistry Graduate School of Pharmaceutical Sciences Duquesne University Pittsburgh Pennsylvania USA
| | - Larry H. Matherly
- Molecular Therapeutics Program Barbara Ann Karmanos Cancer Institute Detroit Michigan USA
- Department of Oncology Wayne State University School of Medicine Detroit Michigan USA
- Department of Pharmacology Wayne State University School of Medicine Detroit Michigan USA
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4
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Folate Transport and One-Carbon Metabolism in Targeted Therapies of Epithelial Ovarian Cancer. Cancers (Basel) 2021; 14:cancers14010191. [PMID: 35008360 PMCID: PMC8750473 DOI: 10.3390/cancers14010191] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2021] [Revised: 12/23/2021] [Accepted: 12/27/2021] [Indexed: 12/20/2022] Open
Abstract
New therapies are urgently needed for epithelial ovarian cancer (EOC), the most lethal gynecologic malignancy. To identify new approaches for targeting EOC, metabolic vulnerabilities must be discovered and strategies for the selective delivery of therapeutic agents must be established. Folate receptor (FR) α and the proton-coupled folate transporter (PCFT) are expressed in the majority of EOCs. FRβ is expressed on tumor-associated macrophages, a major infiltrating immune population in EOC. One-carbon (C1) metabolism is partitioned between the cytosol and mitochondria and is important for the synthesis of nucleotides, amino acids, glutathione, and other critical metabolites. Novel inhibitors are being developed with the potential for therapeutic targeting of tumors via FRs and the PCFT, as well as for inhibiting C1 metabolism. In this review, we summarize these exciting new developments in targeted therapies for both tumors and the tumor microenvironment in EOC.
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Parker JL, Deme JC, Kuteyi G, Wu Z, Huo J, Goldman ID, Owens RJ, Biggin PC, Lea SM, Newstead S. Structural basis of antifolate recognition and transport by PCFT. Nature 2021; 595:130-134. [PMID: 34040256 PMCID: PMC9990147 DOI: 10.1038/s41586-021-03579-z] [Citation(s) in RCA: 29] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2020] [Accepted: 04/23/2021] [Indexed: 12/20/2022]
Abstract
Folates (also known as vitamin B9) have a critical role in cellular metabolism as the starting point in the synthesis of nucleic acids, amino acids and the universal methylating agent S-adenylsmethionine1,2. Folate deficiency is associated with a number of developmental, immune and neurological disorders3-5. Mammals cannot synthesize folates de novo; several systems have therefore evolved to take up folates from the diet and distribute them within the body3,6. The proton-coupled folate transporter (PCFT) (also known as SLC46A1) mediates folate uptake across the intestinal brush border membrane and the choroid plexus4,7, and is an important route for the delivery of antifolate drugs in cancer chemotherapy8-10. How PCFT recognizes folates or antifolate agents is currently unclear. Here we present cryo-electron microscopy structures of PCFT in a substrate-free state and in complex with a new-generation antifolate drug (pemetrexed). Our results provide a structural basis for understanding antifolate recognition and provide insights into the pH-regulated mechanism of folate transport mediated by PCFT.
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Affiliation(s)
- Joanne L Parker
- Department of Biochemistry, University of Oxford, Oxford, UK.
| | - Justin C Deme
- Dunn School of Pathology, University of Oxford, Oxford, UK
- Central Oxford Structural Molecular Imaging Centre, University of Oxford, Oxford, UK
- Center for Structural Biology, Center for Cancer Research, National Cancer Institute, Frederick, MD, USA
| | - Gabriel Kuteyi
- Department of Biochemistry, University of Oxford, Oxford, UK
| | - Zhiyi Wu
- Department of Biochemistry, University of Oxford, Oxford, UK
| | - Jiandong Huo
- Structural Biology, The Rosalind Franklin Institute, Didcot, UK
- Division of Structural Biology, The Wellcome Centre for Human Genetics, University of Oxford, Oxford, UK
- Protein Production UK, The Research Complex at Harwell, Didcot, UK
| | - I David Goldman
- Departments of Molecular Pharmacology and Medicine, Albert Einstein College of Medicine, New York, NY, USA
| | - Raymond J Owens
- Structural Biology, The Rosalind Franklin Institute, Didcot, UK
- Division of Structural Biology, The Wellcome Centre for Human Genetics, University of Oxford, Oxford, UK
- Protein Production UK, The Research Complex at Harwell, Didcot, UK
| | - Philip C Biggin
- Department of Biochemistry, University of Oxford, Oxford, UK
| | - Susan M Lea
- Dunn School of Pathology, University of Oxford, Oxford, UK.
- Central Oxford Structural Molecular Imaging Centre, University of Oxford, Oxford, UK.
- Center for Structural Biology, Center for Cancer Research, National Cancer Institute, Frederick, MD, USA.
| | - Simon Newstead
- Department of Biochemistry, University of Oxford, Oxford, UK.
- The Kavli Institute for Nanoscience Discovery, University of Oxford, Oxford, UK.
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Zhan HQ, Najmi M, Lin K, Aluri S, Fiser A, Goldman ID, Zhao R. A proton-coupled folate transporter mutation causing hereditary folate malabsorption locks the protein in an inward-open conformation. J Biol Chem 2020; 295:15650-15661. [PMID: 32893190 DOI: 10.1074/jbc.ra120.014757] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2020] [Revised: 08/26/2020] [Indexed: 11/06/2022] Open
Abstract
The proton-coupled folate transporter (PCFT, SLC46A1) is required for folate intestinal absorption and transport across the choroid plexus. Recent work has identified a F392V mutation causing hereditary folate malabsorption. However, the residue properties responsible for this loss of function remains unknown. Using site-directed mutagenesis, we observed complete loss of function with charged (Lys, Asp, and Glu) and polar (Thr, Ser, and Gln) Phe-392 substitutions and minimal function with some neutral substitutions; however, F392M retained full function. Using the substituted-cysteine accessibility method (with N-biotinyl aminoethyl methanethiosulfonate labeling), Phe-392 mutations causing loss of function, although preserving membrane expression and trafficking, also resulted in loss of accessibility of the substituted cysteine in P314C-PCFT located within the aqueous translocation pathway. F392V function and accessibility of the P314C cysteine were restored by insertion of a G305L (suppressor) mutation. A S196L mutation localized in proximity to Gly-305 by homology modeling was inactive. However, when inserted into the inactive F392V scaffold, function was restored (mutually compensatory mutations), as was accessibility of the P314C cysteine residue. Reduced function, documented with F392H PCFT, was due to a 15-fold decrease in methotrexate influx V max, accompanied by a decreased influx Kt (4.5-fold) and Ki (3-fold). The data indicate that Phe-392 is required for rapid oscillation of the carrier among its conformational states and suggest that this is achieved by dampening affinity of the protein for its folate substrates. F392V and other inactivating Phe-392 PCFT mutations lock the protein in its inward-open conformation. Reach (length) and hydrophobicity of Phe-392 appear to be features required for full activity.
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Affiliation(s)
- He-Qin Zhan
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA; Department of Pathology, School of Basic Medical Sciences, Anhui Medical University, Hefei, China
| | - Mitra Najmi
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA
| | - Kai Lin
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA; Air Force Medical Center, People's Liberation Army, Beijing, China
| | - Srinivas Aluri
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA
| | - Andras Fiser
- Department of Systems and Computational Biology, Albert Einstein College of Medicine, Bronx, New York, USA; Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York, USA
| | - I David Goldman
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York, USA.
| | - Rongbao Zhao
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York, USA
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7
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Sarkisjan D, Julsing JR, El Hassouni B, Honeywell RJ, Kathmann I, Matherly LH, Lee YB, Kim DJ, Peters GJ. RX-3117 (Fluorocyclopentenyl-Cytosine)-Mediated Down-Regulation of DNA Methyltransferase 1 Leads to Protein Expression of Tumor-Suppressor Genes and Increased Functionality of the Proton-Coupled Folate Carrier. Int J Mol Sci 2020; 21:ijms21082717. [PMID: 32295203 PMCID: PMC7215832 DOI: 10.3390/ijms21082717] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2020] [Revised: 03/31/2020] [Accepted: 04/10/2020] [Indexed: 12/31/2022] Open
Abstract
(1) Background: RX-3117 (fluorocyclopentenyl-cytosine) is a cytidine analog that inhibits DNA methyltransferase 1 (DNMT1). We investigated the mechanism and potential of RX-3117 as a demethylating agent in several in vitro models. (2) Methods: we used western blotting to measure expression of several proteins known to be down-regulated by DNA methylation: O6-methylguanine-DNA methyltransferase (MGMT) and the tumor-suppressor genes, p16 and E-cadherin. Transport of methotrexate (MTX) mediated by the proton-coupled folate transporter (PCFT) was used as a functional assay. (3) Results: RX-3117 treatment decreased total DNA-cytosine-methylation in A549 non-small cell lung cancer (NSCLC) cells, and induced protein expression of MGMT, p16 and E-cadherin in A549 and SW1573 NSCLC cells. Leukemic CCRF-CEM cells and the MTX-resistant variant (CEM/MTX, with a deficient reduced folate carrier) have a very low expression of PCFT due to promoter hypermethylation. In CEM/MTX cells, pre-treatment with RX-3117 increased PCFT-mediated MTX uptake 8-fold, and in CEM cells 4-fold. With the reference hypomethylating agent 5-aza-2′-deoxycytidine similar values were obtained. RX-3117 also increased PCFT gene expression and PCFT protein. (4) Conclusion: RX-3117 down-regulates DNMT1, leading to hypomethylation of DNA. From the increased protein expression of tumor-suppressor genes and functional activation of PCFT, we concluded that RX-3117 might have induced hypomethylation of the promotor.
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Affiliation(s)
- Dzjemma Sarkisjan
- Laboratory Medical Oncology, Amsterdam UMC, location VU University Medical Center, 1081 HV Amsterdam, The Netherlands; (D.S.); (J.R.J.); (B.E.H.); (R.J.H.); (I.K.)
| | - Joris R. Julsing
- Laboratory Medical Oncology, Amsterdam UMC, location VU University Medical Center, 1081 HV Amsterdam, The Netherlands; (D.S.); (J.R.J.); (B.E.H.); (R.J.H.); (I.K.)
| | - Btissame El Hassouni
- Laboratory Medical Oncology, Amsterdam UMC, location VU University Medical Center, 1081 HV Amsterdam, The Netherlands; (D.S.); (J.R.J.); (B.E.H.); (R.J.H.); (I.K.)
| | - Richard J. Honeywell
- Laboratory Medical Oncology, Amsterdam UMC, location VU University Medical Center, 1081 HV Amsterdam, The Netherlands; (D.S.); (J.R.J.); (B.E.H.); (R.J.H.); (I.K.)
| | - Ietje Kathmann
- Laboratory Medical Oncology, Amsterdam UMC, location VU University Medical Center, 1081 HV Amsterdam, The Netherlands; (D.S.); (J.R.J.); (B.E.H.); (R.J.H.); (I.K.)
| | - Larry H. Matherly
- Department of Oncology, Wayne State University School of Medicine, Detroit, and Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, MI 48201-1976, USA;
| | - Young B. Lee
- Rexahn Pharmaceuticals, Inc., Rockville, MD 20850, USA; (Y.B.L.); (D.J.K.)
| | - Deog J. Kim
- Rexahn Pharmaceuticals, Inc., Rockville, MD 20850, USA; (Y.B.L.); (D.J.K.)
| | - Godefridus J. Peters
- Laboratory Medical Oncology, Amsterdam UMC, location VU University Medical Center, 1081 HV Amsterdam, The Netherlands; (D.S.); (J.R.J.); (B.E.H.); (R.J.H.); (I.K.)
- Department of Biochemistry, Medical University of Gdansk, 80-210 Gdansk, Poland
- Correspondence: ; Tel.: +31-20-4442633
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Jekic B, Maksimovic N, Damnjanovic T. Methotrexate pharmacogenetics in the treatment of rheumatoid arthritis. Pharmacogenomics 2019; 20:1235-1245. [DOI: 10.2217/pgs-2019-0121] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
For many decades, methotrexate (MXT) has remained the drug of choice in the treatment of rheumatoid arthritis (RA). Unfortunately, a considerable number of patients do not achieve an appropriate therapeutic response. Pharmacogenetics studies do not give usable results regarding differences in MTX response among RA patients. The mechanism of MTX action in RA is not completely understood. We present and discuss data regarding the molecular basis of folate and adenosine pathways, the most obvious MTX targets, to explain possible causes of therapy failure. The molecular basis of the disease could also have an impact on therapy outcomes and in this review we explore this. Finally, we make a short review of available pharmacogenetics study results.
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Affiliation(s)
- Biljana Jekic
- Institute of Human Genetics, Faculty of Medicine, University of Belgrade,11000 Belgrade, Serbia
| | - Nela Maksimovic
- Institute of Human Genetics, Faculty of Medicine, University of Belgrade,11000 Belgrade, Serbia
| | - Tatjana Damnjanovic
- Institute of Human Genetics, Faculty of Medicine, University of Belgrade,11000 Belgrade, Serbia
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9
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Min DJ, Vural S, Krushkal J. Association of transcriptional levels of folate-mediated one-carbon metabolism-related genes in cancer cell lines with drug treatment response. Cancer Genet 2019; 237:19-38. [DOI: 10.1016/j.cancergen.2019.05.005] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/2018] [Revised: 05/09/2019] [Accepted: 05/29/2019] [Indexed: 02/08/2023]
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Regulation of differential proton-coupled folate transporter gene expression in human tumors: transactivation by KLF15 with NRF-1 and the role of Sp1. Biochem J 2019; 476:1247-1266. [PMID: 30914440 DOI: 10.1042/bcj20180394] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2018] [Revised: 03/07/2019] [Accepted: 03/26/2019] [Indexed: 12/18/2022]
Abstract
Tumors can be therapeutically targeted with novel antifolates (e.g. AGF94) that are selectively transported by the human proton-coupled folate transporter (hPCFT). Studies were performed to determine the transcription regulation of hPCFT in tumors and identify possible mechanisms that contribute to the highly disparate levels of hPCFT in HepG2 versus HT1080 tumor cells. Transfection of hPCFT-null HT1080 cells with hPCFT restored transport and sensitivity to AGF94 Progressive deletions of the hPCFT promoter construct (-2005 to +96) and reporter gene assays in HepG2 and HT1080 cells confirmed differences in hPCFT transactivation and localized a minimal promoter to between positions -50 and +96. The minimal promoter included KLF15, GC-Box and NRF-1 cis-binding elements whose functional importance was confirmed by promoter deletions and mutations of core consensus sequences and reporter gene assays. In HepG2 cells, NRF-1, KLF15 and Sp1 transcripts were increased over HT1080 cells by ∼5.1-, ∼44-, and ∼2.4-fold, respectively. In Drosophila SL2 cells, transfection with KLF15 and NRF-1 synergistically activated the hPCFT promoter; Sp1 was modestly activating or inhibitory. Chromatin immunoprecipitation and electrophoretic mobility shift assay (EMSA) and supershifts confirmed differential binding of KLF15, Sp1, and NRF-1 to the hPCFT promoter in HepG2 and HT1080 cells that paralleled hPCFT levels. Treatment of HT1080 nuclear extracts (NE) with protein kinase A increased Sp1 binding to its consensus sequence by EMSA, suggesting a role for Sp1 phosphorylation in regulating hPCFT transcription. A better understanding of determinants of hPCFT transcriptional control may identify new therapeutic strategies for cancer by modulating hPCFT levels in combination with hPCFT-targeted antifolates.
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11
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Aluri S, Zhao R, Lin K, Shin DS, Fiser A, Goldman ID. Substitutions that lock and unlock the proton-coupled folate transporter (PCFT-SLC46A1) in an inward-open conformation. J Biol Chem 2019; 294:7245-7258. [PMID: 30858177 DOI: 10.1074/jbc.ra118.005533] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2018] [Revised: 03/06/2019] [Indexed: 11/06/2022] Open
Abstract
The proton-coupled folate transporter (PCFT) mediates intestinal absorption of folates and their transport from blood to cerebrospinal fluid across the choroid plexus. Substitutions at Asp-109 in the first intracellular loop between the first and second transmembrane domains (TMDs) abolish PCFT function, but protein expression and trafficking to the cell membrane are retained. Here, we used site-directed mutagenesis, the substituted-cysteine accessibility method, functional analyses, and homology modeling to determine whether the D109A substitution locks PCFT in one of its conformational states. Cys-substituted residues lining the PCFT aqueous translocation pathway and accessible in WT PCFT to the membrane-impermeable cysteine-biotinylation reagent, MTSEA-biotin, lost accessibility when introduced into the D109A scaffold. Substitutions at Gly-305 located exofacially within the eighth TMD, particularly with bulky residues, when introduced into the D109A scaffold largely restored function and MTSEA-biotin accessibility to Cys-substituted residues within the pathway. Likewise, Ser-196 substitution in the fifth TMD, predicted by homology modeling to be in proximity to Gly-305, also partially restored function found in solute transporters, is critical to oscillation of the carrier among its conformational states. Substitutions at Asp-109 and Gly-112 lock PCFT in an inward-open conformation, resulting in the loss of function. However, the integrity of the locked protein is preserved, indicated by the restoration of function after insertion of a second "unlocking" mutation. and accessibility. Similarly, the inactivating G112K substitution within the first intracellular loop was partially reactivated by introducing the G305L substitution. These data indicate that the first intracellular loop, with a sequence identical to "motif A" (GXXXDXXGR(R/K)).
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Affiliation(s)
| | | | - Kai Lin
- From the Departments of Pharmacology.,the Air Force Medical Center, PLA, Beijing 100142, China
| | | | - Andras Fiser
- Systems and Computational Biology, and.,Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461 and
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12
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Petri GL, Cascioferro S, Parrino B, Peters GJ, Diana P, Giovannetti E. Proton-coupled folate transporter as a biomarker of outcome to treatment for pleural mesothelioma. Pharmacogenomics 2018; 19:811-814. [PMID: 29916298 DOI: 10.2217/pgs-2018-0071] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Affiliation(s)
- Giovanna Li Petri
- Department of Medical Oncology, VU University Medical Center, Cancer Center Amsterdam, De Boelelaan 1117, 1081HV Amsterdam, The Netherlands
- Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Sezione di Chimica e Tecnologie Farmaceutiche, Università degli Studi di Palermo, Via Archirafi 32, 90123 Palermo, Italy
| | - Stella Cascioferro
- Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Sezione di Chimica e Tecnologie Farmaceutiche, Università degli Studi di Palermo, Via Archirafi 32, 90123 Palermo, Italy
| | - Barbara Parrino
- Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Sezione di Chimica e Tecnologie Farmaceutiche, Università degli Studi di Palermo, Via Archirafi 32, 90123 Palermo, Italy
| | - Godefridus J Peters
- Department of Medical Oncology, VU University Medical Center, Cancer Center Amsterdam, De Boelelaan 1117, 1081HV Amsterdam, The Netherlands
| | - Patrizia Diana
- Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Sezione di Chimica e Tecnologie Farmaceutiche, Università degli Studi di Palermo, Via Archirafi 32, 90123 Palermo, Italy
| | - Elisa Giovannetti
- Department of Medical Oncology, VU University Medical Center, Cancer Center Amsterdam, De Boelelaan 1117, 1081HV Amsterdam, The Netherlands
- Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche, Sezione di Chimica e Tecnologie Farmaceutiche, Università degli Studi di Palermo, Via Archirafi 32, 90123 Palermo, Italy
- Cancer Pharmacology Lab, AIRC Start-Up Unit, University of Pisa, via Paradisa, 56100, Pisa, Italy
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13
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Zhao R, Najmi M, Aluri S, Spray DC, Goldman ID. Concentrative Transport of Antifolates Mediated by the Proton-Coupled Folate Transporter (SLC46A1); Augmentation by a HEPES Buffer. Mol Pharmacol 2018; 93:208-215. [PMID: 29326243 DOI: 10.1124/mol.117.110445] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2017] [Accepted: 01/08/2018] [Indexed: 11/22/2022] Open
Abstract
The proton-coupled folate transporter (PCFT) is ubiquitously expressed in solid tumors to which it delivers antifolates, particularly pemetrexed, into cancer cells. Studies of PCFT-mediated transport, to date, have focused exclusively on the influx of folates and antifolates. This article addresses the impact of PCFT on concentrative transport, critical to the formation of the active polyglutamate congeners, and at pH levels relevant to the tumor microenvironment. An HeLa-derived cell line was employed, in which folate-specific transport was mediated exclusively by PCFT. At pH 7.0, there was a substantial chemical gradient for methotrexate that decreased as the extracellular pH was increased. A chemical gradient was still detected at pH 7.4 in the usual HEPES-based transport buffer in contrast to what was observed in a bicarbonate/CO2-buffered medium. This antifolate gradient correlated with an alkaline intracellular pH in the former (pH 7.85), but not the latter (pH 7.39), buffer and was abolished by the protonophore carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone. The gradient in HEPES buffer at pH 7.4 was the result of the activity of Na+/H+ exchanger(s); it was eliminated by inhibitors of Na+/H+ exchanger (s) or Na+/K+ ATPase. An antifolate chemical gradient was also detected in bicarbonate buffer at pH 6.9 versus 7.4, also suppressed by carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone. When the membrane potential is considered, PCFT generates substantial transmembrane electrochemical-potential gradients at extracellular pH levels relevant to the tumor microenvironment. The augmentation of intracellular pH, when cells are in a HEPES buffer, should be taken into consideration in studies that encompass all proton-coupled transporter families.
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Affiliation(s)
- Rongbao Zhao
- Departments of Molecular Pharmacology (R.Z., M.N., S.A., I.D.G.), Medicine (R.Z., I.D.G.), and Dominick P. Purpura Department of Neuroscience (D.C.S.), Albert Einstein College of Medicine, Bronx, New York
| | - Mitra Najmi
- Departments of Molecular Pharmacology (R.Z., M.N., S.A., I.D.G.), Medicine (R.Z., I.D.G.), and Dominick P. Purpura Department of Neuroscience (D.C.S.), Albert Einstein College of Medicine, Bronx, New York
| | - Srinivas Aluri
- Departments of Molecular Pharmacology (R.Z., M.N., S.A., I.D.G.), Medicine (R.Z., I.D.G.), and Dominick P. Purpura Department of Neuroscience (D.C.S.), Albert Einstein College of Medicine, Bronx, New York
| | - David C Spray
- Departments of Molecular Pharmacology (R.Z., M.N., S.A., I.D.G.), Medicine (R.Z., I.D.G.), and Dominick P. Purpura Department of Neuroscience (D.C.S.), Albert Einstein College of Medicine, Bronx, New York
| | - I David Goldman
- Departments of Molecular Pharmacology (R.Z., M.N., S.A., I.D.G.), Medicine (R.Z., I.D.G.), and Dominick P. Purpura Department of Neuroscience (D.C.S.), Albert Einstein College of Medicine, Bronx, New York
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14
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Hereditary folate malabsorption due to a mutation in the external gate of the proton-coupled folate transporter SLC46A1. Blood Adv 2018; 2:61-68. [PMID: 29344585 DOI: 10.1182/bloodadvances.2017012690] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2017] [Accepted: 11/28/2017] [Indexed: 01/08/2023] Open
Abstract
Hereditary folate malabsorption (HFM) is an autosomal recessive disorder characterized by impaired intestinal folate absorption and impaired folate transport across the choroid plexus due to loss of function of the proton-coupled folate transporter (PCFT-SLC46A1). We report a novel mutation, causing HFM, affecting a residue located in the 11th transmembrane helix within the external gate. The mutant N411K-PCFT was stable, trafficked to the cell membrane, and had sufficient residual activity to characterize the transport defect and the structural requirements at this site for gate function. The influx Vmax of the N411K mutant was markedly decreased, as was the affinity for most, but not all, folate/antifolate substrates. The greatest loss of activity was for 5-methyltetrahydrofolate. Substitutions with positive charged residues resulted in a loss of activity (arginine > lysine > histidine). Function was retained for the negative charged aspartate, but not the larger glutamate substitutions, whereas the bulky hydrophobic (leucine), or polar (glutamine) substitutions, were tolerated. Homology models of PCFT, in the inward and outward open conformations, based upon the mammalian Glut5 fructose transporter structures, localize Asn411 protruding into the aqueous pathway. This is most prominent when the carrier is in the inward open conformation when the external gate is closed. Mutations at this site likely result in highly specific steric and electrostatic interactions between the Asn411-substituted, and other, residues in the gate region that impede carrier function. The substrate specificity of the N411K mutant may be due to alterations of substrate flows through the external gate, downstream allosteric alterations in the folate-binding pocket, or both.
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15
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Matherly LH, Hou Z, Gangjee A. The promise and challenges of exploiting the proton-coupled folate transporter for selective therapeutic targeting of cancer. Cancer Chemother Pharmacol 2018; 81:1-15. [PMID: 29127457 PMCID: PMC5756103 DOI: 10.1007/s00280-017-3473-8] [Citation(s) in RCA: 33] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2017] [Accepted: 10/20/2017] [Indexed: 12/17/2022]
Abstract
This review considers the "promise" of exploiting the proton-coupled folate transporter (PCFT) for selective therapeutic targeting of cancer. PCFT was discovered in 2006 and was identified as the principal folate transporter involved in the intestinal absorption of dietary folates. The recognition that PCFT was highly expressed in many tumors stimulated substantial interest in using PCFT for cytotoxic drug targeting, taking advantage of its high level transport activity under the acidic pH conditions that characterize many tumors. For pemetrexed, among the best PCFT substrates, transport by PCFT establishes its importance as a clinically important transporter in malignant pleural mesothelioma and non-small cell lung cancer. In recent years, the notion of PCFT-targeting has been extended to a new generation of tumor-targeted 6-substituted pyrrolo[2,3-d]pyrimidine compounds that are structurally and functionally distinct from pemetrexed, and that exhibit near exclusive transport by PCFT and potent inhibition of de novo purine nucleotide biosynthesis. Based on compelling preclinical evidence in a wide range of human tumor models, it is now time to advance the most optimized PCFT-targeted agents with the best balance of PCFT transport specificity and potent antitumor efficacy to the clinic to validate this novel paradigm of highly selective tumor targeting.
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Affiliation(s)
- Larry H Matherly
- Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, 421 East Canfield Street, Detroit, MI, 48201, USA.
- Department of Oncology, Wayne State University School of Medicine, Detroit, MI, 48201, USA.
- Department of Pharmacology, Wayne State University School of Medicine, Detroit, MI, 48201, USA.
| | - Zhanjun Hou
- Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, 421 East Canfield Street, Detroit, MI, 48201, USA
- Department of Oncology, Wayne State University School of Medicine, Detroit, MI, 48201, USA
| | - Aleem Gangjee
- Division of Medicinal Chemistry, Graduate School of Pharmaceutical Sciences, Duquesne University, 600 Forbes Avenue, Pittsburgh, PA, 15282, USA
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16
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Giovannetti E, Zucali PA, Assaraf YG, Funel N, Gemelli M, Stark M, Thunnissen E, Hou Z, Muller IB, Struys EA, Perrino M, Jansen G, Matherly LH, Peters GJ. Role of proton-coupled folate transporter in pemetrexed resistance of mesothelioma: clinical evidence and new pharmacological tools. Ann Oncol 2017; 28:2725-2732. [PMID: 28945836 PMCID: PMC5808668 DOI: 10.1093/annonc/mdx499] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
BACKGROUND Thymidylate synthase (TS) has a predictive role in pemetrexed treatment of mesothelioma; however, additional chemoresistance mechanisms are poorly understood. Here, we explored the role of the reduced-folate carrier (RFC/SLC19A1) and proton-coupled folate transporter (PCFT/SLC46A1) in antifolate resistance in mesothelioma. PATIENTS AND METHODS PCFT, RFC and TS RNA and PCFT protein levels were determined by quantitative RT-PCR of frozen tissues and immunohistochemistry of tissue-microarrays, respectively, in two cohorts of pemetrexed-treated patients. Data were analyzed by t-test, Fisher's/log-rank test and Cox proportional models. The contribution of PCFT expression and PCFT-promoter methylation to pemetrexed activity were evaluated in mesothelioma cells and spheroids, through 5-aza-2'-deoxycytidine-mediated demethylation and siRNA-knockdown. RESULTS Pemetrexed-treated patients with low PCFT had significantly lower rates of disease control, and shorter overall survival (OS), in both the test (N = 73, 11.3 versus 20.1 months, P = 0.01) and validation (N = 51, 12.6 versus 30.3 months, P = 0.02) cohorts. Multivariate analysis confirmed PCFT-independent prognostic role. Low-PCFT protein levels were also associated with shorter OS. Patients with both low-PCFT and high-TS levels had the worst prognosis (OS, 5.5 months), whereas associations were neither found for RFC nor in pemetrexed-untreated patients. PCFT silencing reduced pemetrexed sensitivity, whereas 5-aza-2'-deoxycytidine overcame resistance. CONCLUSIONS These findings identify for the first time PCFT as a novel mesothelioma prognostic biomarker, prompting prospective trials for its validation. Moreover, preclinical data suggest that targeting PCFT-promoter methylation might eradicate pemetrexed-resistant cells characterized by low-PCFT expression.
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Affiliation(s)
- E Giovannetti
- Department of Medical Oncology, VU University Medical Center, Cancer Center Amsterdam, Amsterdam, The Netherlands; Cancer Pharmacology Lab, AIRC Start-Up Unit, Department of Translational Research and The New Technologies in Medicine and Surgery, University of Pisa, Pisa
| | - P A Zucali
- Department of Oncology, University of Milan, Humanitas Clinical and Research Hospital, Rozzano (Milan), Italy
| | - Y G Assaraf
- Department of Biology, Fred Wyszkowski Cancer Research Laboratory, Technion-Institute of Technology, Haifa, Israel
| | - N Funel
- Cancer Pharmacology Lab, AIRC Start-Up Unit, Department of Translational Research and The New Technologies in Medicine and Surgery, University of Pisa, Pisa
| | - M Gemelli
- Department of Oncology, University of Milan, Humanitas Clinical and Research Hospital, Rozzano (Milan), Italy
| | - M Stark
- Department of Biology, Fred Wyszkowski Cancer Research Laboratory, Technion-Institute of Technology, Haifa, Israel
| | - E Thunnissen
- Department of Pathology, VU University Medical Center, Amsterdam, The Netherlands
| | - Z Hou
- Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, USA
| | - I B Muller
- Department of Clinical Chemistry, VU University Medical Center, Amsterdam
| | - E A Struys
- Department of Clinical Chemistry, VU University Medical Center, Amsterdam
| | - M Perrino
- Department of Oncology, University of Milan, Humanitas Clinical and Research Hospital, Rozzano (Milan), Italy
| | - G Jansen
- Amsterdam Rheumatology and Immunology Center - Location VUmc, Amsterdam, The Netherlands
| | - L H Matherly
- Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, USA
| | - G J Peters
- Department of Medical Oncology, VU University Medical Center, Cancer Center Amsterdam, Amsterdam, The Netherlands.
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17
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Aluri S, Zhao R, Fiser A, Goldman ID. Residues in the eighth transmembrane domain of the proton-coupled folate transporter (SLC46A1) play an important role in defining the aqueous translocation pathway and in folate substrate binding. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1859:2193-2202. [PMID: 28802835 DOI: 10.1016/j.bbamem.2017.08.006] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/16/2017] [Revised: 07/28/2017] [Accepted: 08/08/2017] [Indexed: 01/12/2023]
Abstract
The proton-coupled folate transporter (PCFT-SLC46A1) is required for intestinal folate absorption and folate transport across the choroid plexus. This report addresses the structure/function of the 8th transmembrane helix. Based upon biotinylation of cysteine-substituted residues by MTSEA-biotin, 14 contiguous exofacial residues to Leu316 were accessible to the extracellular compartment of the 23 residues in this helix (Leu303-Leu325). Pemetrexed blocked biotinylation of six Cys-substituted residues deep within the helix implicating an important role for this region in folate binding. Accessibility decreased at 4°C vs RT. The influx Kt, Ki and Vmax were markedly increased for the P314C mutant, similar to what was observed for Y315A and Y315P mutants. However, the Kt, alone, was increased for the P314Y mutant. To correlate these observations with PCFT structural changes during the transport cycle, homology models were built for PCFT based upon the recently reported structures of bovine and rodent GLUT5 fructose transporters in the inward-open and outward- open conformations, respectively. The models predict substantial structural alterations in the exofacial region of the eighth transmembrane helix as it cycles between its conformational states that can account for the extended and contiguous aqueous accessibility of this region of the helix. Further, a helix break in one of the two conformations can account for the critical roles Pro314 and Tyr315, located in this region, play in PCFT function. The data indicates that the 8th transmembrane helix of PCFT plays an important role in defining the aqueous channel and the folate binding pocket.
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Affiliation(s)
- Srinivas Aluri
- Department of Pharmacology, Albert Einstein College of Medicine, Bronx, NY 10461, United States; Department of Medicine, Albert Einstein College of Medicine, Bronx, NY 10461, United States
| | - Rongbao Zhao
- Department of Pharmacology, Albert Einstein College of Medicine, Bronx, NY 10461, United States; Department of Medicine, Albert Einstein College of Medicine, Bronx, NY 10461, United States
| | - Andras Fiser
- Department of Systems and Computational Biology, Albert Einstein College of Medicine, Bronx, NY 10461, United States; Department of Biochemistry, Albert Einstein College of Medicine, Bronx, NY 10461, United States
| | - I David Goldman
- Department of Pharmacology, Albert Einstein College of Medicine, Bronx, NY 10461, United States; Department of Medicine, Albert Einstein College of Medicine, Bronx, NY 10461, United States.
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18
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Zhao R, Najmi M, Aluri S, Goldman ID. Impact of posttranslational modifications of engineered cysteines on the substituted cysteine accessibility method: evidence for glutathionylation. Am J Physiol Cell Physiol 2017; 312:C517-C526. [PMID: 28122733 DOI: 10.1152/ajpcell.00350.2016] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2016] [Revised: 01/18/2017] [Accepted: 01/19/2017] [Indexed: 01/10/2023]
Abstract
The substituted cysteine accessibility method (SCAM) is widely used to study the structure and function of channels, receptors and transporters. In its usual application, a cysteine residue is introduced into a protein which lacks native cysteines following which the accessibility of the residue to the aqueous compartment is assessed. Implicit, and generally assumed, is that if the cysteine-substituted residue is not available to react with sulfhydryl reagents it is not exposed to the extracellular compartment or within the aqueous translocation pathway. We demonstrate here, in a Hela-derived cell line, that some cysteine-substituted residues of the proton-coupled folate transporter (PCFT, SLC46A1) that are inaccessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate are glutathionylated by biotinylated glutathione ethyl ester in the absence of an oxidizing agent. Intramolecular disulfide formation involving cysteine-substituted residues was also identified in some instances. These posttranslational modifications limit the accessibility of the cysteine residues to sulfhydryl-reactive reagents and can have a profound impact on the interpretation of SCAM but may not alter function. When a posttranslationally modified residue is used as a reference extracellular control, the high level of exposure required for detection on Western blot results in erroneous detection of otherwise inaccessible intracellular cysteine-substituted residues. The data indicate that in the application of SCAM, when a cysteine-substituted residue does not appear to be accessible to sulfhydryl-reactive reagents, the possibility of a posttranslational modification should be excluded. The data explain the discrepancies in the assessment, and confirm the localization, of the first intracellular loop of PCFT.
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Affiliation(s)
- Rongbao Zhao
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; and.,Department of Medicine, Albert Einstein College of Medicine, Bronx, New York
| | - Mitra Najmi
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; and
| | - Srinivas Aluri
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; and
| | - I David Goldman
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; and .,Department of Medicine, Albert Einstein College of Medicine, Bronx, New York
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19
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Zhao R, Aluri S, Goldman ID. The proton-coupled folate transporter (PCFT-SLC46A1) and the syndrome of systemic and cerebral folate deficiency of infancy: Hereditary folate malabsorption. Mol Aspects Med 2016; 53:57-72. [PMID: 27664775 DOI: 10.1016/j.mam.2016.09.002] [Citation(s) in RCA: 44] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2016] [Accepted: 09/18/2016] [Indexed: 02/07/2023]
Abstract
The proton-coupled folate transporter (PCFT-SLC46A1) is the mechanism by which folates are absorbed across the brush-border membrane of the small intestine. The transporter is also expressed in the choroid plexus and is required for transport of folates into the cerebrospinal fluid. Loss of PCFT function, as occurs in the autosomal recessive disorder "hereditary folate malabsorption" (HFM), results in a syndrome characterized by severe systemic and cerebral folate deficiency. Folate-receptor alpha (FRα) is expressed in the choroid plexus, and loss of function of this protein, as also occurs in an autosomal recessive disorder, results solely in "cerebral folate deficiency" (CFD), the designation for this disorder. This paper reviews the current understanding of the functional and structural properties and regulation of PCFT, an electrogenic proton symporter, and contrasts PCFT properties with those of the reduced folate carrier (RFC), an organic anion antiporter, that is the major route of folate transport to systemic tissues. The clinical characteristics of HFM and its treatment, based upon the thirty-seven known cases with the clinical syndrome, of which thirty have been verified by genotype, are presented. The ways in which PCFT and FRα might interact at the level of the choroid plexus such that each is required for folate transport from blood to cerebrospinal fluid are considered along with the different clinical presentations of HFM and CFD.
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Affiliation(s)
- Rongbao Zhao
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York, USA
| | - Srinivas Aluri
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York, USA
| | - I David Goldman
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York, USA.
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20
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Zhao R, Najmi M, Fiser A, Goldman ID. Identification of an Extracellular Gate for the Proton-coupled Folate Transporter (PCFT-SLC46A1) by Cysteine Cross-linking. J Biol Chem 2016; 291:8162-72. [PMID: 26884338 DOI: 10.1074/jbc.m115.693929] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2015] [Indexed: 01/04/2023] Open
Abstract
The proton-coupled folate transporter (PCFT, SLC46A1) is required for intestinal folate absorption and folate homeostasis in humans. A homology model of PCFT, based upon theEscherichia coliglycerol 3-phosphate transporter structure, predicted that PCFT transmembrane domains (TMDs) 1, 2, 7, and 11 form an extracellular gate in the inward-open conformation. To assess this model, five residues (Gln(45)-TMD1, Asn(90)-TMD2, Leu(290)-TMD7, Ser(407)-TMD11 and Asn(411)-TMD11) in the predicted gate were substituted with Cys to generate single and nine double mutants. Transport function of the mutants was assayed in transient transfectants by measurement of [(3)H]substrate influx as was accessibility of the Cys residues to biotinylation. Pairs of Cys residues were assessed for spontaneous formation of a disulfide bond, induction of a disulfide bond by oxidization with dichloro(1,10-phenanthroline)copper (II) (CuPh), or the formation of a Cd(2+)complex. The data were consistent with the formation of a spontaneous disulfide bond between the N90C/S407C pair and a CuPh- and Cd(2+)-induced disulfide bond and complex, respectively, for the Q45C/L290C and L290C/N411C pairs. The decrease in activity induced by cross-linkage of the Cys residue pairs was due to a decrease in the influxVmaxconsistent with restriction in the mobility of the transporter. The presence of folate substrate decreased the CuPh-induced inhibition of transport. Hence, the data support the glycerol 3-phosphate transporter-based homology model of PCFT and the presence of an extracellular gate formed by TMDs 1, 2, 7, and 11.
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Affiliation(s)
- Rongbao Zhao
- From the Departments of Molecular Pharmacology, Medicine
| | - Mitra Najmi
- From the Departments of Molecular Pharmacology
| | - Andras Fiser
- Biochemistry, and Systems and Computational Biology, Albert Einstein College of Medicine, Bronx, New York 10461
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21
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Zhao R, Visentin M, Goldman ID. Determinants of the activities of antifolates delivered into cells by folate-receptor-mediated endocytosis. Cancer Chemother Pharmacol 2015; 75:1163-73. [PMID: 25847479 PMCID: PMC4442060 DOI: 10.1007/s00280-015-2733-8] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2014] [Accepted: 03/23/2015] [Indexed: 12/14/2022]
Abstract
PURPOSE Elements in the endocytic process that are determinants of the activities of antifolates delivered by folate-receptor alpha (FRα) were explored. METHODS Antifolate growth inhibition was assessed with a 1- or 5-day exposure in reduced folate carrier-null HeLa cell lines that express a high level of FRα in the presence or absence of the proton-coupled folate transporter (PCFT). pH-dependent rates of dissociation from FRα were also determined. RESULTS With a 1-day drug exposure which is relevant to the pulse clinical administration of these drugs, FRα expression enhanced raltitrexed activity and modestly enhanced ZD9331 activity, but did not significantly augment the activity of pemetrexed or lomotrexol. With a 5-day drug exposure, FRα-mediated growth inhibition was increased for raltitrexed and ZD9331 and emerged for lomotrexol. While the FRα-augmented activity of lomotrexol and raltitrexed did not require PCFT, augmentation of ZD9331 activity required the co-expression of PCFT with both 1- and 5-day exposures. In contrast, there was no augmentation of pemetrexed activity by FRα under any condition. The activities of these agents correlated with their rate of dissociation from the receptor at acidic pH: raltitrexed > ZD9331 > lomotrexol > pemetrexed consistent with insufficient pemetrexed release from FRα for export from the endosomes. CONCLUSIONS FRα is unlikely to contribute to the pharmacological activity of antifolates, such as pemetrexed, that bind tightly to, and dissociate slowly from, the receptor particularly when the exposure time is brief. While PCFT was required for FRα-mediated ZD9931 activity, the activities of the other antifolates was independent of PCFT.
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Affiliation(s)
- Rongbao Zhao
- Departments of Medicine, Albert Einstein College of Medicine, Chanin 628, 1300 Morris Park Ave, Bronx, NY, 10461, USA,
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22
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Structural determinants of human proton-coupled folate transporter oligomerization: role of GXXXG motifs and identification of oligomeric interfaces at transmembrane domains 3 and 6. Biochem J 2015; 469:33-44. [PMID: 25877470 DOI: 10.1042/bj20150169] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2015] [Accepted: 04/16/2015] [Indexed: 11/17/2022]
Abstract
The human proton-coupled folate transporter (hPCFT) is expressed in solid tumours and is active at pHs characterizing the tumour microenvironment. Recent attention focused on exploiting hPCFT for targeting solid tumours with novel cytotoxic anti-folates. hPCFT has 12 transmembrane domains (TMDs) and forms homo-oligomers with functional significance. The hPCFT primary sequence includes GXXXG motifs in TMD2 (G(93)XXXG(97)) and TMD4 (G(155)XXXG(159)). To investigate roles of these motifs in hPCFT function, stability and surface expression, we mutated glycine to leucine to generate single or multiple substitution mutants. Only the G93L and G159L mutants preserved substantial [(3)H]methotrexate (Mtx) transport when expressed in hPCFT-null (R1-11) HeLa cells. Transport activity of the glycine-to-leucine mutants correlated with surface hPCFT by surface biotinylation and confocal microscopy with ECFP*-tagged hPCFTs, suggesting a role for GXXXG in hPCFT stability and intracellular trafficking. When co-expressed in R1-11 cells, haemagglutinin-tagged glycine-to-leucine mutants and His10-tagged wild-type (WT) hPCFT co-associated on nickel affinity columns, suggesting that the GXXXG motifs are not directly involved in hPCFT oligomerization. This was substantiated by in situ FRET experiments with co-expressed ECFP*- and YFP-tagged hPCFT. Molecular modelling of dimeric hPCFT structures showed juxtaposed TMDs 2, 3, 4 and 6 as potential structural interfaces between monomers. hPCFT cysteine insertion mutants in TMD3 (Q136C and L137C) and TMD6 (W213C, L214C, L224C, A227C, F228C, F230C and G231C) were expressed in R1-11 cells and cross-linked with 1,6-hexanediyl bismethanethiosulfonate, confirming TMD juxtapositions. Altogether, our results imply that TMDs 3 and 6 provide critical interfaces for formation of hPCFT oligomers, which might be facilitated by the GXXXG motifs in TMD2 and TMD4.
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23
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DNA methylation and expression of the folate transporter genes in colorectal cancer. Tumour Biol 2015; 36:5581-90. [PMID: 25697897 DOI: 10.1007/s13277-015-3228-2] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2014] [Accepted: 02/05/2015] [Indexed: 12/20/2022] Open
Abstract
Folate has a central role in the cell metabolism. This study aims to explore the DNA methylation pattern of the folate transporter genes FOLR1, PCFT, and RFC1 as well as the corresponding protein expressions in colorectal cancer (CRC) tissue and adjacent non-cancerous mucosa (ANCM). Our results showed statistically significant differences in the DNA-methylated fraction of all three genes at several gene regions; we identified three differentially methylated CpG sites in the FOLR1 gene, five CpG sites in the PCFT gene, and six CpG sites in the RFC1 gene. There was a pronounced expression of the FRα and RFC proteins in both the CRC and ANCM tissues, though the expression was attenuated in cancer compared to the paired ANCM tissues. The PCFT protein was undetectable or expressed at a very low level in both tissue types. Higher methylated fractions of the CpG sites 3-5 in the RFC1 gene were associated with a lower protein expression, suggestive of epigenetic regulation by DNA methylation of the RFC1 gene in the colorectal cancer. Our results did not show any association between the RFC and FRα protein expression and tumor stage, TNM classification, or tumor location. In conclusion, this is the first study to simultaneously evaluate both DNA methylation and protein expression of all three folate transporter genes, FOLR1, PCFT, and RFC1, in colorectal cancer. The results encourage further investigation into the possible prognostic implications of folate transporter expression and DNA methylation.
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Visentin M, Unal ES, Najmi M, Fiser A, Zhao R, Goldman ID. Identification of Tyr residues that enhance folate substrate binding and constrain oscillation of the proton-coupled folate transporter (PCFT-SLC46A1). Am J Physiol Cell Physiol 2015; 308:C631-41. [PMID: 25608532 DOI: 10.1152/ajpcell.00238.2014] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2014] [Accepted: 01/20/2015] [Indexed: 12/15/2022]
Abstract
The proton-coupled folate transporter (PCFT) mediates intestinal folate absorption and transport of folates across the choroid plexus. This study focuses on the role of Tyr residues in PCFT function. The substituted Cys-accessibility method identified four Tyr residues (Y291, Y362, Y315, and Y414) that are accessible to the extracellular compartment; three of these (Y291, Y362, and Y315) are located within or near the folate binding pocket. When the Tyr residues were replaced with Cys or Ala, these mutants showed similar (up to 6-fold) increases in influx Vmax and Kt/Ki for [(3)H]methotrexate and [(3)H]pemetrexed. When the Tyr residues were replaced with Phe, these changes were moderated or absent. When Y315A PCFT was used as representative of the mutants and [(3)H]pemetrexed as the transport substrate, this substitution did not increase the efflux rate constant. Furthermore, neither influx nor efflux mediated by Y315A PCFT was transstimulated by the presence of substrate in the opposite compartment; however, substantial bidirectional transstimulation of transport was mediated by wild-type PCFT. This resulted in a threefold greater efflux rate constant for cells that express wild-type PCFT than for cells that express Y315 PCFT under exchange conditions. These data suggest that these Tyr residues, possibly through their rigid side chains, secure the carrier in a high-affinity state for its folate substrates. However, this may be achieved at the expense of constraining the carrier's mobility, thereby decreasing the rate at which the protein oscillates between its conformational states. The Vmax generated by these Tyr mutants may be so rapid that further augmentation during transstimulation may not be possible.
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Affiliation(s)
- Michele Visentin
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York
| | - Ersin Selcuk Unal
- Department of Medicine, Albert Einstein College of Medicine, Bronx, New York
| | - Mitra Najmi
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York
| | - Andras Fiser
- Department of Systems and Computational Biology, Albert Einstein College of Medicine, Bronx, New York; and Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York
| | - Rongbao Zhao
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York
| | - I David Goldman
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York;
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Abstract
The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.
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Affiliation(s)
- Michele Visentin
- Departments of Molecular Pharmacology and Medicine, Albert Einstein College of Medicine, Bronx, New York 10461; , , ,
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Wilson MR, Hou Z, Matherly LH. Substituted cysteine accessibility reveals a novel transmembrane 2-3 reentrant loop and functional role for transmembrane domain 2 in the human proton-coupled folate transporter. J Biol Chem 2014; 289:25287-95. [PMID: 25053408 DOI: 10.1074/jbc.m114.578252] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
The proton-coupled folate transporter (PCFT) is a folate-proton symporter highly expressed in solid tumors that can selectively target cytotoxic antifolates to tumors under acidic microenvironment conditions. Predicted topology models for PCFT suggest that the loop domain between transmembrane domains (TMDs) 2 and 3 resides in the cytosol. Mutations involving Asp-109 or Arg-113 in the TMD2-3 loop result in loss of activity. By structural homology to other solute carriers, TMD2 may form part of the PCFT substrate binding domain. In this study we mutated the seven cysteine (Cys) residues of human PCFT to serine, creating Cys-less PCFT. Thirty-three single-Cys mutants spanning TMD2 and the TMD2-3 loop in a Cys-less PCFT background were transfected into PCFT-null HeLa cells. All 33 mutants were detected by Western blotting, and 28 were active for [(3)H]methotrexate uptake at pH 5.5. For the active residues, we performed pulldown assays with membrane-impermeable 2-aminoethyl methanethiosulfonate-biotin and streptavidin beads to determine their aqueous-accessibilities. Multiple residues in TMD2 and the TMD2-3 loop domain reacted with 2-aminoethyl methanethiosulfonate-biotin, establishing aqueous accessibilities. Pemetrexed pretreatment inhibited biotinylation of TMD2 mutants G93C and F94C, and biotinylation of these residues inhibited methotrexate transport activity. Our results suggest that the TMD 2-3 loop domain is aqueous-accessible and forms a novel reentrant loop structure. Residues in TMD2 form an aqueous transmembrane pathway for folate substrates, and Gly-93 and Phe-94 may contribute to a substrate binding domain. Characterization of PCFT structure is essential to understanding the transport mechanism including the critical determinants of substrate binding.
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Affiliation(s)
| | - Zhanjun Hou
- From the Department of Oncology and the Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan 48201
| | - Larry H Matherly
- From the Department of Oncology and the Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan 48201 Department of Pharmacology, Wayne State University School of Medicine and
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Zhao R, Goldman ID. The proton-coupled folate transporter: physiological and pharmacological roles. Curr Opin Pharmacol 2014; 13:875-80. [PMID: 24383099 DOI: 10.1016/j.coph.2013.09.011] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Recent studies have identified the proton-coupled folate transporter (PCFT) as the mechanism by which folates are absorbed across the apical brush-border membrane of the small intestine and across the basolateral membrane of the choroid plexus into the cerebrospinal fluid. Both processes are defective when there are loss-of-function mutations in this gene as occurs in the autosomal recessive disorder hereditary folate malabsorption. Because this transporter functions optimally at low pH, antifolates are being developed that are highly specific for PCFT in order to achieve selective delivery to malignant cells within the acidic environment of solid tumors. PCFT has a spectrum of affinities for folates and antifolates that narrows and increases at low pH. Residues have been identified that play a role in folate and proton binding, proton coupling, and oscillation of the carrier between its conformational states.
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Matherly LH, Wilson MR, Hou Z. The major facilitative folate transporters solute carrier 19A1 and solute carrier 46A1: biology and role in antifolate chemotherapy of cancer. Drug Metab Dispos 2014; 42:632-49. [PMID: 24396145 PMCID: PMC3965896 DOI: 10.1124/dmd.113.055723] [Citation(s) in RCA: 64] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2013] [Accepted: 01/06/2014] [Indexed: 01/19/2023] Open
Abstract
This review summarizes the biology of the major facilitative membrane transporters, the reduced folate carrier (RFC) (Solute Carrier 19A1) and the proton-coupled folate transporter (PCFT) (Solute Carrier 46A1). Folates are essential vitamins, and folate deficiency contributes to a variety of health disorders. RFC is ubiquitously expressed and is the major folate transporter in mammalian cells and tissues. PCFT mediates the intestinal absorption of dietary folates and appears to be important for transport of folates into the central nervous system. Clinically relevant antifolates for cancer, such as methotrexate and pralatrexate, are transported by RFC, and loss of RFC transport is an important mechanism of methotrexate resistance in cancer cell lines and in patients. PCFT is expressed in human tumors, and is active at pH conditions associated with the tumor microenvironment. Pemetrexed is an excellent substrate for both RFC and PCFT. Novel tumor-targeted antifolates related to pemetrexed with selective membrane transport by PCFT over RFC are being developed. In recent years, there have been major advances in understanding the structural and functional properties and the regulation of RFC and PCFT. The molecular bases for methotrexate resistance associated with loss of RFC transport and for hereditary folate malabsorption, attributable to mutant PCFT, were determined. Future studies should continue to translate molecular insights from basic studies of RFC and PCFT biology into new therapeutic strategies for cancer and other diseases.
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Affiliation(s)
- Larry H Matherly
- Department of Oncology (L.H.M., M.R.W., Z.H.) and Department of Pharmacology (L.H.M.), Wayne State University School of Medicine, Detroit, Michigan; and Molecular Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Detroit, Michigan (L.H.M., Z.H.)
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Inoue K, Yuasa H. Molecular basis for pharmacokinetics and pharmacodynamics of methotrexate in rheumatoid arthritis therapy. Drug Metab Pharmacokinet 2013; 29:12-9. [PMID: 24284432 DOI: 10.2133/dmpk.dmpk-13-rv-119] [Citation(s) in RCA: 104] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Methotrexate (MTX) is a derivative of folic acid (folate) and commonly used as an anchor drug for the treatment of rheumatoid arthritis (RA). The pharmacokinetics (PK) and pharmacodynamics (PD) of MTX entirely depends on the function of specific transporters that belong to the two major superfamilies, solute carrier transporters and ATP-binding cassette transporters. Several transporters have been identified as being able to mediate the transport of MTX, and suggested to be involved in the disposition in the body and in the regulation of intracellular metabolism in target cells, together with several enzymes involved in folate metabolism. Thus, drug-drug interactions through the transporters and their genetic polymorphisms may alter the PK and PD of MTX, resulting in an interpatient variability of efficacy. This review summarizes the PK and PD of MTX, particularly in relation to RA therapy and focuses on the roles of transporters involved in PK and PD with the aim of facilitating an understanding of the molecular basis of the mechanism of MTX action to achieve its effective use in RA therapy.
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Affiliation(s)
- Katsuhisa Inoue
- Department of Biopharmaceutics, School of Pharmacy, Tokyo University of Pharmacy and Life Sciences
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Zhao R, Diop-Bove N, Goldman ID. Enhanced receptor-mediated endocytosis and cytotoxicity of a folic acid-desacetylvinblastine monohydrazide conjugate in a pemetrexed-resistant cell line lacking folate-specific facilitative carriers but with increased folate receptor expression. Mol Pharmacol 2013; 85:310-21. [PMID: 24249723 DOI: 10.1124/mol.113.089110] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptors (FR) are folate-specific transporters. Antifolates currently in the clinic, such as pemetrexed, methotrexate, and pralatrexate, are transported into tumor cells primarily via RFC. Folic acid conjugated to cytotoxics, a new class of antineoplastics, are transported into cells via FR-mediated endocytosis. To better define the role of PCFT in antifolate resistance, a methotrexate-resistant cell line, M160-8, was selected from a HeLa subline in which the RFC gene was deleted and PCFT was highly overexpressed. These cells were cross-resistant to pemetrexed. PCFT function and the PCFT mRNA level in M160-8 cells were barely detectable, and FR-α function and mRNA level were increased as compared with the parent cells. While pemetrexed rapidly associated with FR and was internalized within endosomes in M160-8 cells, consistent with FR-mediated transport, subsequent pemetrexed and (6S)-5-formyltetrahydrofolate export into the cytosol was markedly impaired. In contrast, M160-8 cells were collaterally sensitive to EC0905, a folic acid-desacetylvinblastine monohydrazide conjugate also transported by FR-mediated endocytosis. However, in this case a sulfhydryl bond is cleaved to release the lipophilic cytotoxic moiety into the endosome, which passively diffuses out of the endosome into the cytosol. Hence, resistance to pemetrexed in M160-8 cells was due to entrapment of the drug within the endosome due to the absence of PCFT under conditions in which the FR cycling function was intact.
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Affiliation(s)
- Rongbao Zhao
- Departments of Medicine (R.Z., I.D.G.) and Molecular Pharmacology (R.Z., N.D.-P., I.D.G.), Albert Einstein College of Medicine, Bronx, New York
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Zhao R, Goldman ID. Folate and thiamine transporters mediated by facilitative carriers (SLC19A1-3 and SLC46A1) and folate receptors. Mol Aspects Med 2013; 34:373-85. [PMID: 23506878 DOI: 10.1016/j.mam.2012.07.006] [Citation(s) in RCA: 120] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2012] [Accepted: 07/03/2012] [Indexed: 01/19/2023]
Abstract
The reduced folate carrier (RFC, SLC19A1), thiamine transporter-1 (ThTr1, SLC19A2) and thiamine transporter-2 (ThTr2, SLC19A3) evolved from the same family of solute carriers. SLC19A1 transports folates but not thiamine. SLC19A2 and SLC19A3 transport thiamine but not folates. SLC19A1 and SLC19A2 deliver their substrates to systemic tissues; SLC19A3 mediates intestinal thiamine absorption. The proton-coupled folate transporter (PCFT, SLC46A1) is the mechanism by which folates are absorbed across the apical-brush-border membrane of the proximal small intestine. Two folate receptors (FOLR1 and FOLR2) mediate folate transport across epithelia by an endocytic process. Folate transporters are routes of delivery of drugs for the treatment of cancer and inflammatory diseases. There are autosomal recessive disorders associated with mutations in genes encoded for SLC46A1 (hereditary folate malabsorption), FOLR1 (cerebral folate deficiency), SLC19A2 (thiamine-responsive megaloblastic anemia), and SLC19A3 (biotin-responsive basal ganglia disease).
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Affiliation(s)
- Rongbao Zhao
- Departments of Medicine and Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461, USA
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Visentin M, Unal ES, Zhao R, Goldman ID. The membrane transport and polyglutamation of pralatrexate: a new-generation dihydrofolate reductase inhibitor. Cancer Chemother Pharmacol 2013; 72:597-606. [PMID: 23881211 DOI: 10.1007/s00280-013-2231-9] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2013] [Accepted: 07/06/2013] [Indexed: 01/30/2023]
Abstract
PURPOSE To characterize, directly and for the first time, the membrane transport and metabolism of pralatrexate, a new-generation dihydrofolate reductase inhibitor approved for the treatment for peripheral T-cell lymphoma. EXPERIMENTAL DESIGN [(3)H]pralatrexate transport was studied in unique HeLa cell lines that express either the reduced folate carrier (RFC) or the proton-coupled folate transporter (PCFT). Metabolism to active polyglutamate derivatives was assessed by liquid chromatography. These properties were compared to those of methotrexate (MTX). RESULTS The pralatrexate influx K t, mediated by RFC, the major route of folate/antifolate transport at systemic pH, was 0.52 μΜ, 1/10th the MTX influx K i. The electrochemical potential of pralatrexate within HeLa cells far exceeded the extracellular level and was greater than for MTX. In contrast, MTX transport mediated by PCFT, the mechanism of folate/antifolate absorption in the small intestine, exceeded that for pralatrexate. After a 6 h exposure of HeLa cells to 0.5 μM pralatrexate, 80 % of intracellular drug was its active polyglutamate forms, predominantly the tetraglutamate, and was suppressed when cells were loaded with natural folates. There was negligible formation of MTX polyglutamates. The difference in pralatrexate and MTX growth inhibition was far greater after transient exposures (375-fold) than continuous exposure (25-fold) to the drugs. CONCLUSIONS Pralatrexate's enhanced activity relative to MTX is due to its much more rapid rate of transport and polyglutamation, the former less important when the carrier is saturated. The low affinity of pralatrexate for PCFT predicts a lower level of enterohepatic circulation and increased fecal excretion of the drug relative to MTX.
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Affiliation(s)
- Michele Visentin
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY, USA
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Zhao R, Visentin M, Suadicani SO, Goldman ID. Inhibition of the proton-coupled folate transporter (PCFT-SLC46A1) by bicarbonate and other anions. Mol Pharmacol 2013; 84:95-103. [PMID: 23609145 DOI: 10.1124/mol.113.085605] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The proton-coupled folate transporter (PCFT) plays a key role in intestinal folate absorption, and loss-of-function mutations in the gene encoding this transporter are the molecular basis for hereditary folate malabsorption. Using a stable transfectant with high expression of PCFT, physiologic levels of bicarbonate produced potent and rapidly reversible inhibition of PCFT-mediated transport at neutral pH. Bisulfite and nitrite also inhibited PCFT function at neutral pH, whereas sulfate, nitrate, and phosphate had no impact at all. At weakly acidic pH (6.5), bisulfite and nitrite exhibited much stronger inhibition of PCFT-mediated transport, whereas sulfate and nitrate remained noninhibitory. Inhibition by bisulfite and nitrite at pH 6.5 was associated with a marked decrease in the influx Vmax and collapse of the transmembrane proton gradient attributed to the diffusion of the protonated forms into these cells. Monocarboxylates such as pyruvate and acetate also collapsed the pH gradient and were also inhibitory, whereas citrate and glycine neither altered the proton gradient nor inhibited PCFT-mediated transport. These observations add another dimension to the unfavorable pH environment for PCFT function in systemic tissues: the presence of high concentrations of bicarbonate.
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Affiliation(s)
- Rongbao Zhao
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Chanin 628, 1300 Morris Park Avenue, Bronx, NY 10461, USA.
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Shin DS, Zhao R, Fiser A, Goldman ID. Role of the fourth transmembrane domain in proton-coupled folate transporter function as assessed by the substituted cysteine accessibility method. Am J Physiol Cell Physiol 2013; 304:C1159-67. [PMID: 23552283 DOI: 10.1152/ajpcell.00353.2012] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The proton-coupled folate transporter (PCFT, SLC46A1) mediates folate transport across the apical brush-border membrane of the proximal small intestine and the basolateral membrane of choroid plexus ependymal cells. Two loss-of-function mutations in PCFT, which are the basis for hereditary folate malabsorption, have been identified within the fourth transmembrane domain (TMD4) in subjects with this disorder. We have employed the substituted Cys accessibility method (SCAM) to study the accessibilities of all residues in TMD4 and their roles in folate substrate binding to the carrier. When residues 146-167 were replaced by Cys, all except R148C were expressed at the cell surface. Modification of five of these substituted Cys residues (positions 147, 152, 157, 158, and 161) by methanethiosulfonate (MTS) reagents led to reduction of PCFT function. All five residues could be labeled with N-biotinylaminoethyl-MTS, and this could be blocked by the high-affinity PCFT substrate pemetrexed. Pemetrexed also protected PCFT mutant function from inhibitory modification of the substituted Cys at positions 157, 158, and 161 by a MTS. The findings indicate that these five residues in TMD4 are accessible to the aqueous translocation pathway, play a role in folate substrate binding, and are likely located within or near the folate binding pocket. A homology model of PCFT places three of these residues, Phe¹⁵⁷, Gly¹⁵⁸, and Leu¹⁶¹, within a breakpoint in the midportion of TMD4, a region that likely participates in alterations in the PCFT conformational state during carrier cycling.
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Affiliation(s)
- Daniel Sanghoon Shin
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
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Transcriptional regulation of PCFT by KLF4, HNF4α, CDX2 and C/EBPα: implication in its site-specific expression in the small intestine. Biochem Biophys Res Commun 2013; 431:158-63. [PMID: 23313509 DOI: 10.1016/j.bbrc.2013.01.004] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/25/2012] [Accepted: 01/03/2013] [Indexed: 01/10/2023]
Abstract
Proton-coupled folate transporter (PCFT), which is responsible for the intestinal uptake of folates and analogs, is expressed only in the proximal region in the small intestine. The present study was to examine its transcriptional regulation, which may be involved in such a unique expression profile and potentially in its alteration, using dual-luciferase reporter assays in human embryonic kidney (HEK) 293 cells. The luciferase activity derived from the reporter construct containing the 5'-flanking sequence of -1695/+96 of the human PCFT gene was enhanced most extensively by the introduction of Krüppel-like factor 4 (KLF4). The KLF4-induced luciferase activity was further enhanced by hepatocyte nuclear factor 4α (HNF4α) synergistically. To the contrary, caudal-type homeobox transcription factor 2 (CDX2) and CCAAT/enhancer-binding protein α (C/EBPα) extensively suppressed the luciferase activity induced by KLF4 alone and also that induced by KLF4 and HNF4α. Western blot analysis using the rat small intestine indicated uniform expression of KLF4 along the intestinal tract, proximal-oriented expression of HNF4α, distal-oriented expression of CDX2 and C/EBPα. These results suggest that the activity of PCFT promoter is basically induced by KLF4 and the gradiented expression profile of PCFT may be at least in part accounted for by those of HNF4α, CDX2 and C/EBPα.
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Desmoulin SK, Hou Z, Gangjee A, Matherly LH. The human proton-coupled folate transporter: Biology and therapeutic applications to cancer. Cancer Biol Ther 2012; 13:1355-73. [PMID: 22954694 PMCID: PMC3542225 DOI: 10.4161/cbt.22020] [Citation(s) in RCA: 116] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
This review summarizes the biology of the proton-coupled folate transporter (PCFT). PCFT was identified in 2006 as the primary transporter for intestinal absorption of dietary folates, as mutations in PCFT are causal in hereditary folate malabsorption (HFM) syndrome. Since 2006, there have been major advances in understanding the mechanistic roles of critical amino acids and/or domains in the PCFT protein, many of which were identified as mutated in HFM patients, and in characterizing transcriptional control of the human PCFT gene. With the recognition that PCFT is abundantly expressed in human tumors and is active at pHs characterizing the tumor microenvironment, attention turned to exploiting PCFT for delivering novel cytotoxic antifolates for solid tumors. The finding that pemetrexed is an excellent PCFT substrate explains its demonstrated clinical efficacy for mesothelioma and non-small cell lung cancer, and prompted development of more PCFT-selective tumor-targeted 6-substituted pyrrolo[2,3-d]pyrimidine antifolates that derive their cytotoxic effects by targeting de novo purine nucleotide biosynthesis.
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Affiliation(s)
- Sita Kugel Desmoulin
- Cancer Biology Graduate Program in Cancer Biology, Department of Oncology, Wayne State University School of Medicine; Detroit, MI USA
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Gonen N, Assaraf YG. Antifolates in cancer therapy: Structure, activity and mechanisms of drug resistance. Drug Resist Updat 2012; 15:183-210. [DOI: 10.1016/j.drup.2012.07.002] [Citation(s) in RCA: 269] [Impact Index Per Article: 22.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2012] [Revised: 06/25/2012] [Accepted: 07/11/2012] [Indexed: 01/19/2023]
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Shin DS, Zhao R, Fiser A, Goldman DI. Functional roles of the A335 and G338 residues of the proton-coupled folate transporter (PCFT-SLC46A1) mutated in hereditary folate malabsorption. Am J Physiol Cell Physiol 2012; 303:C834-42. [PMID: 22843796 DOI: 10.1152/ajpcell.00171.2012] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
The proton-coupled folate transporter (PCFT-SLC46A1) mediates intestinal folate absorption and folate transport across the choroid plexus, processes defective in hereditary folate malabsorption (HFM). This paper characterizes the functional defect, and the roles of two mutated PCFT residues, associated with HFM (G338R and A335D). The A335D-PCFT and other mutations at this residue result in an unstable protein; when expression of a mutant protein was preserved, function was always retained. The G338R and other charged mutants resulted in an unstable protein; substitutions with small neutral and polar amino acids preserved protein but with impaired function. Pemetrexed and methotrexate (MTX) influx kinetics mediated by the G338C mutant PCFT revealed marked (15- to 20-fold) decreases in K(t) and V(max) compared with wild-type PCFT. In contrast, there was only a small (∼2-fold) decrease in the MTX influx K(i) and an increase (∼3-fold) in the pemetrexed influx K(i) for the G338C-PCFT mutant. Neither a decrease in pH to 4.5, nor an increase to 7.4, restored function of the G338C mutant relative to wild-type PCFT excluding a role for this residue in proton binding or proton coupling. Homology modeling localized the A335 and G338 residues embedded in the 9th transmembrane, consistent with the inaccessibility of the A335C and G338C proteins to MTS reagents. Hence, the loss of intrinsic G338C-PCFT function was due solely to impaired oscillation of the carrier between its conformational states. The data illustrate how alterations in carrier cycling can impact influx K(t) without comparable alterations in substrate binding to the carrier.
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Affiliation(s)
- Daniel Sanghoon Shin
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461, USA
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Zhao R, Shin DS, Fiser A, Goldman ID. Identification of a functionally critical GXXG motif and its relationship to the folate binding site of the proton-coupled folate transporter (PCFT-SLC46A1). Am J Physiol Cell Physiol 2012; 303:C673-81. [PMID: 22785121 DOI: 10.1152/ajpcell.00123.2012] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The proton-coupled folate transporter (PCFT) mediates intestinal folate absorption, and loss-of-function mutations in this gene result in the autosomal recessive disorder hereditary folate malabsorption. The current study, focused on a structure-functional analysis of this transporter, identified Gly-189 and Gly-192 (a GxxG motif) located in the fifth transmembrane domain as residues that could not be replaced with alanine without a loss of function. In contrast, function was preserved when Gly-56 and Gly-59 (the other conservative GXXG motif in human PCFT) were replaced with alanine. Similarly, Gly-93 and Gly-97, which constitute the only conserved GXXXG dimerization motif in human PCFT, tolerated alanine substitution. To explore the role of this region in folate binding, the residues around Gly-189 and Gly-192 were analyzed by the substituted cysteine accessibility method. Both I188C and M193C mutants were functional and were inhibited by membrane-impermeable sulfhydryl-reactive reagents; this could be prevented with PCFT substrate, but the protection was sustained at 0°C only for the I188C mutant, consistent with localization of Ile-188 in the PCFT folate binding pocket. The functional role of residues around Gly-189 and Gly-192 is consistent with a molecular structural model in which these two residues along with Ieu-188 are accessible to the PCFT aqueous translocation pathway.
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Affiliation(s)
- Rongbao Zhao
- Depts. of Medicine and Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
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Visentin M, Zhao R, Goldman ID. Augmentation of reduced folate carrier-mediated folate/antifolate transport through an antiport mechanism with 5-aminoimidazole-4-carboxamide riboside monophosphate. Mol Pharmacol 2012; 82:209-16. [PMID: 22554803 DOI: 10.1124/mol.112.078642] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
5-Aminoimidazole-4-carboxamide riboside (AICAR), an agent with diverse pharmacological properties, augments transport of folates and antifolates. This report further characterizes this phenomenon and defines the mechanism by which it occurs. Exposure of HeLa cells to AICAR resulted in augmentation of methotrexate, 5-formyltetrahydrofolate, and 5-methyltetrahydrofolate initial rates and net uptake in cells that express the reduced folate carrier (RFC). This did not occur in cells that express only the proton-coupled folate transporter and accumulated folates by this mechanism. Transport stimulation correlated with the accumulation of 5-aminoimidazole-4-carboxamide ribotide monophosphate (ZMP), the monophosphate derivative of AICAR, within cells as established by liquid chromatography. When ZMP formation was blocked with 5-iodotubercidin, an inhibitor of adenosine kinase, folate transport stimulation by AICAR was absent. When cells first accumulated ZMP and were then exposed to 5-iodotubercidin or AICAR-free buffer, the ZMP level markedly decreased and folate transport stimulation was abolished. Extracellular ZMP inhibited RFC-mediated folate influx, and the presence of intracellular ZMP correlated with inhibition of folate efflux. The data indicate that intracellular ZMP trans-stimulates folate influx and inhibits folate efflux, which, together, produce a marked augmentation in the net cellular folate level. This interaction among ZMP, folates, and RFC, a folate/organic phosphate antiporter, is consistent with a classic exchange reaction. The transmembrane gradient for one transport substrate (ZMP) drives the uphill transport of another (folate) via a carrier used by both substrates, a phenomenon intrinsic to the energetics of RFC-mediated folate transport.
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Affiliation(s)
- Michele Visentin
- Albert Einstein Cancer Center, 1300 Morris Park Ave., Bronx, NY 10461, USA
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Shin DS, Zhao R, Yap EH, Fiser A, Goldman ID. A P425R mutation of the proton-coupled folate transporter causing hereditary folate malabsorption produces a highly selective alteration in folate binding. Am J Physiol Cell Physiol 2012; 302:C1405-12. [PMID: 22345511 DOI: 10.1152/ajpcell.00435.2011] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Proton-coupled folate transporter (PCFT) mediates folate intestinal absorption and transport across the choroid plexus, processes defective in subjects with hereditary folate malabsorption (HFM). PCFT is also widely expressed in human solid tumors where it contributes to the transport of pemetrexed and other antifolates. This study defines the basis for the functional changes due to a P425R mutation detected in a subject with HFM. Among various substitutions, only positively charged mutants (P425R and P425K) lost function but in a highly selective manner. Transport of reduced folates mediated by P425R-PCFT was virtually abolished; the methotrexate influx K(t) was increased fivefold (from 2 to 10 μM). In contrast, the pemetrexed influx K(t) mediated by P425R-PCFT was decreased 30% compared with wild-type (WT)-PCFT. Methotrexate inhibition of pemetrexed influx was competitive with a K(i) for WT-PCFT comparable to its influx K(t). However, the methotrexate influx K(i) for P425R-PCFT was ∼15-fold higher than the WT-PCFT influx K(t) and threefold higher than the methotrexate influx K(t) for the P425R-PCFT mutant. The confirmed secondary structure and homology modeling place the P425 residue at the junction of the 6th external loop and 12th transmembrane domain, remote from the aqueous translocation pathway, a prediction confirmed by the failure to label P425C-PCFT with N-biotinylaminoethyl methanethiosulfonate-biotin and the absence of inhibition of P425C-PCFT function by water-soluble sulfhydryl reagents. Hence, despite its location, the P425R-PCFT mutation produces a conformational change that fully preserves pemetrexed binding but markedly impairs binding of methotrexate and other folates to the carrier.
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Affiliation(s)
- Daniel Sanghoon Shin
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461, USA
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Zhao R, Diop-Bove N, Visentin M, Goldman ID. Mechanisms of membrane transport of folates into cells and across epithelia. Annu Rev Nutr 2011; 31:177-201. [PMID: 21568705 DOI: 10.1146/annurev-nutr-072610-145133] [Citation(s) in RCA: 241] [Impact Index Per Article: 18.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.
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Affiliation(s)
- Rongbao Zhao
- Departments of Medicine and Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
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Visentin M, Chang MH, Romero MF, Zhao R, Goldman ID. Substrate- and pH-specific antifolate transport mediated by organic anion-transporting polypeptide 2B1 (OATP2B1-SLCO2B1). Mol Pharmacol 2011; 81:134-42. [PMID: 22021325 DOI: 10.1124/mol.111.074823] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Human organic anion-transporting polypeptide (OATP) 2B1 (OATP-B; SLCO2B1) is expressed in the apical membrane of the small intestine and the hepatocyte basolateral membrane and transports structurally diverse organic anions with a wide spectrum of pH sensitivities. This article describes highly pH-dependent OATP2B1-mediated antifolate transport and compares this property with that of sulfobromophthalein (BSP), a preferred OATP2B1 substrate. At pH 5.5 and low substrate concentrations (~2.5 μM), only [(3)H]pemetrexed influx [in contrast to methotrexate (MTX), folic acid, and reduced folates] could be detected in OATP2B1-transfected HeLa R1-11 cells that lack endogenous folate-specific transporters. Influx was optimal at pH 4.5 to 5.5, falling precipitously with an increase in pH >6.0; BSP influx was independent of pH. Influx of both substrates at low pH was markedly inhibited by the proton ionophore 4-(trifluoromethoxy)phenylhydrazone; BSP influx was also suppressed at pH 7.4. At 300 μM MTX, influx was one-third that of pemetrexed; influx of folic acid, (6S)5-methyltetrahydrofolate, or (6S)5-formyltetrahydrofolate was not detected. There were similar findings in OATP2B1-expressing Xenopus laevis oocytes. The pemetrexed influx K(m) was ~300 μM; the raltitrexed influx K(i) was ~70 μM at pH 5.5. Stable expression of OAPT2B1 in HeLa R1-11 cells resulted in substantial raltitrexed, but modest pemetrexed, growth inhibition consistent with their affinities for this carrier. Hence, OATP2B1 represents a low-affinity transport route for antifolates (relative affinities: raltitrexed > pemetrexed > MTX) at low pH. In contrast, the high affinity of this transporter for BSP relative to antifolates seems to be intrinsic to its binding site and independent of the proton concentration.
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Affiliation(s)
- Michele Visentin
- Experimental and Clinical Pharmacology Unit, Centro di Riferimento Oncologico, National Cancer Institute, Aviano, Italy
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Galvani E, Peters GJ, Giovannetti E. Thymidylate synthase inhibitors for non-small cell lung cancer. Expert Opin Investig Drugs 2011; 20:1343-56. [PMID: 21905922 DOI: 10.1517/13543784.2011.617742] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
INTRODUCTION The folate-dependent enzyme thymidylate synthase (TS) plays a pivotal role in DNA replication/repair and cancer cell proliferation, and represents a valid target for the treatment of several tumor types, including NSCLC. NSCLC is the leading cause of cancer-related mortality, and several TS inhibitors have gone into preclinical and clinical testing, with pemetrexed emerging for its approval and widespread use as first-/second-line and maintenance therapy for this disease. AREAS COVERED This review summarizes the therapeutic options in NSCLC, as well as the background and rationale for targeting TS. The authors also review recent pharmacogenetic studies and data from clinical trials evaluating novel TS inhibitors, hoping that the reader will gain a comprehensive overview of the field of TS inhibition, specifically relating to drugs used or being developed for lung cancer patients. EXPERT OPINION TS is a validated target in NSCLC. However, benefits from conventional chemotherapy in NSCLC have plateaued, and more cost-effective results should be obtained with individualized treatment. Accordingly, the clinical success for TS inhibitors may depend on our ability to correctly administer these agents following biomarker-driven patient selection, including TS genotype and expression, and using the right combination therapy.
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Affiliation(s)
- Elena Galvani
- VU University Medical Center, Department of Medical Oncology, Amsterdam, The Netherlands
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Zhao R, Shin DS, Diop-Bove N, Ovits CG, Goldman ID. Random mutagenesis of the proton-coupled folate transporter (SLC46A1), clustering of mutations, and the bases for associated losses of function. J Biol Chem 2011; 286:24150-8. [PMID: 21602279 PMCID: PMC3129196 DOI: 10.1074/jbc.m111.236539] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2011] [Revised: 04/25/2011] [Indexed: 01/19/2023] Open
Abstract
Loss-of-function mutations in the proton-coupled folate transporter (PCFT, SLC46A1) result in the autosomal recessive disorder, hereditary folate malabsorption (HFM). Identification and characterization of HFM mutations provide a wealth of information on the structure-function relationship of this transporter. In the current study, PCR-based random mutagenesis was employed to generate unbiased loss-of-function mutations of PCFT, simulating the spectrum of alterations that might occur in the human disorder. A total of 26 mutations were generated and 4 were identical to HFM mutations. Eleven were base deletion or insertion mutations that led to a frameshift and, along with similar HFM mutations, are predominantly localized to two narrow regions of the pcft gene at the 5'-end. Base substitution mutations identified in the current study and HFM patients were largely distributed across the pcft gene. Elimination of the ATG initiation codon by a one-base substitution (G > A) did not result in a complete lack of translation at the same codon consistent with rare non-ATG translation initiation. Among six missense mutants evaluated, three mutant PCFTs were not detected at the plasma membrane, one mutation resulted in decreased binding to folate substrate, and one had a reduced rate of conformational change associated with substrate translocation. The remaining PCFT mutant had defects in both processes. These results broaden understanding of the regions of the pcft gene prone to base insertion and deletion and inform further approaches to the analysis of the structure-function of PCFT.
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Affiliation(s)
- Rongbao Zhao
- Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
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46
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Shin DS, Mahadeo K, Min SH, Diop-Bove N, Clayton P, Zhao R, Goldman ID. Identification of novel mutations in the proton-coupled folate transporter (PCFT-SLC46A1) associated with hereditary folate malabsorption. Mol Genet Metab 2011; 103:33-7. [PMID: 21333572 PMCID: PMC3081934 DOI: 10.1016/j.ymgme.2011.01.008] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/07/2011] [Revised: 01/19/2011] [Accepted: 01/20/2011] [Indexed: 01/19/2023]
Abstract
Hereditary folate malabsorption (HFM) is an autosomal recessive disorder, recently shown to be due to loss-of-function mutations of the proton-coupled folate transporter (PCFT-SLC46A1), resulting in systemic and central nervous system folate deficiency. Data is emerging on the spectrum of PCFT mutations associated with this disorder. In this report, novel mutations are described in three subjects with HFM: A335D/N68Kfs (c.1004C>A/c.204-205delCC), compound heterozygous mutations, and two homozygous PCFT mutations, G338R (c.1012G>C) and E9Gfs (c.17-18insC). Functional assessment of A335D and G338R PCFT mutants transfected into folate transporter-deficient HeLa R1-11 cells indicated a complete loss of transport activity. There were neurological deficiencies in two of the families reported; in particular, late-onset seizures. The importance of early diagnosis and treatment to achieve physiological cerebrospinal fluid folate levels is emphasized.
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Affiliation(s)
- Daniel Sanghoon Shin
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461
- Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461
| | - Kris Mahadeo
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461
- Department of Pediatrics, Albert Einstein College of Medicine, Bronx, New York 10461
| | - Sang Hee Min
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461
- Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461
| | - Ndeye Diop-Bove
- Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461
| | - Peter Clayton
- Clinical and Molecular Genetics Unit, UCL Institute of Child Health, Great Ormond Street Hospital for Children NHS Trust, London WC1N 1EH, UK
| | - Rongbao Zhao
- Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461
| | - I. David Goldman
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461
- Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461
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Zhao R, Shin DS, Goldman ID. Vulnerability of the cysteine-less proton-coupled folate transporter (PCFT-SLC46A1) to mutational stress associated with the substituted cysteine accessibility method. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1808:1140-5. [PMID: 21256110 DOI: 10.1016/j.bbamem.2011.01.008] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/06/2010] [Revised: 01/07/2011] [Accepted: 01/13/2011] [Indexed: 01/19/2023]
Abstract
The proton-coupled transporter (PCFT) mediates intestinal folate absorption and folate transport from blood across the choroid plexus. The membrane topology of PCFT has been defined using the substituted cysteine accessibility method; an intramolecular disulfide bond between the Cys 66 and 298 residues, in the first and fourth extracellular loops, respectively, is present but not essential for function. The current report describes Lys 422 mutations (K422C, K422E) that have no effect on transport activity when introduced into wild-type PCFT but result in a marked loss of activity when introduced into a Cys-less PCFT which is otherwise near-fully functional. The loss of activity of both mutant PCFTs was shown to be due to impaired protein stability and expression. Additional studies were conducted with the K422C mutation in Cys-less PCFT. The impact of re-introduction of one, two, three or five, Cys residues was assessed. While there were some differences in the impact of the different Cys residues re-introduced, restoration was attributed more to a cumulative effect rather than the specific role of individual Cys residues. Preservation of the Cys66-Cys298 intramolecular disulfide bond was not required for stability of the K422C protein. These observations are relevant to studies with Cys-less transporters utilized for the characterization of proteins with the substituted cysteine accessibility method and indicate that functional defects detected in a Cys-less protein, when the tertiary structure of the molecule is stressed, are not necessarily relevant to the wild-type protein.
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Affiliation(s)
- Rongbao Zhao
- Department of Medicine and Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, NY, USA
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Hagner N, Joerger M. Cancer chemotherapy: targeting folic acid synthesis. Cancer Manag Res 2010; 2:293-301. [PMID: 21301589 PMCID: PMC3033035 DOI: 10.2147/cmr.s10043] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2010] [Indexed: 12/14/2022] Open
Abstract
Antifolates are structural analogs of folates, essential one-carbon donors in the synthesis of DNA in mammalian cells. Antifolates are inhibitors of key enzymes in folate metabolism, namely dihydrofolate reductase, β-glycinamide ribonucleotide transformylase, 5'-amino-4'-imidazolecarboxamide ribonucleotide transformylase, and thymidylate synthetase. Methotrexate is one of the earliest anticancer drugs and is extensively used in lymphoma, acute lymphoblastic leukemia, and osteosarcoma, among others. Pemetrexed has been approved in combination with cisplatin as first-line treatment for advanced non-squamous-cell lung cancer, as a single agent for relapsed non-small-cell lung cancer after platinum-containing chemotherapy, and in combination with cisplatin for the treatment of pleural mesothelioma. Raltitrexed is approved in many countries (except in the United States) for advanced colorectal cancer, but its utilization is mainly limited to patients intolerant to 5-fluorouracil. Pralatrexate has recently been approved in the United States for relapsed or refractory peripheral T-cell lymphoma. This article gives an overview of the cellular mechanism, pharmacology, and clinical use of classical and newer antifolates and discusses some of the main resistance mechanisms to antifolate drugs.
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Affiliation(s)
- Nicole Hagner
- Department of Medical Oncology, Cantonal Hospital, St Gallen, Switzerland
| | - Markus Joerger
- Department of Medical Oncology, Cantonal Hospital, St Gallen, Switzerland
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Mahadeo K, Diop-Bove N, Shin D, Unal ES, Teo J, Zhao R, Chang MH, Fulterer A, Romero MF, Goldman ID. Properties of the Arg376 residue of the proton-coupled folate transporter (PCFT-SLC46A1) and a glutamine mutant causing hereditary folate malabsorption. Am J Physiol Cell Physiol 2010; 299:C1153-61. [PMID: 20686069 PMCID: PMC2980313 DOI: 10.1152/ajpcell.00113.2010] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2010] [Accepted: 08/03/2010] [Indexed: 01/19/2023]
Abstract
The proton-coupled folate transporter (PCFT-SLC46A1) is required for intestinal folate absorption and is mutated in the autosomal recessive disorder, hereditary folate malabsorption (HFM). This report characterizes properties and requirements of the R376 residue in PCFT function, including a R376Q mutant associated with HFM. Gln, Cys, and Ala substitutions resulted in markedly impaired transport of 5-formyltetrahydrofolate (5-FTHF) and 5-methyltetrahydrofolate (5-MTHF) due to an increase in K(m) and decrease in V(max) in HeLa R1-11 transfectants lacking endogenous folate transport function. In contrast, although the influx K(m) for pemetrexed was increased, transport was fully preserved at saturating concentrations and enhanced for the like-charged R376K- and R376H-PCFT. Pemetrexed and 5-FTHF influx mediated by R376Q-PCFT was markedly decreased at pH 5.5 compared with wild-type PCFT. However, while pemetrexed transport was substantially preserved at low pH (4.5-5.0), 5-FTHF transport remained very low. Electrophysiological studies in Xenopus oocytes demonstrated that 1) the R376Q mutant, like wild-type PCFT, transports protons in the absence of folate substrate, and in this respect has channel-like properties; and 2) the influx K(m) mediated by R376Q-PCFT is increased for 5-MTHF, 5-FTHF, and pemetrexed. The data suggest that mutation of the R376 residue to Gln impairs proton binding which, in turn, modulates the folate-binding pocket and depresses the rate of conformational alteration of the carrier, a change that appears to be, in part, substrate dependent.
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Affiliation(s)
- Kris Mahadeo
- Department of Pediatrics, Albert Einstein College of Medicine, Bronx, New York 10461, USA
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50
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Functional roles of aspartate residues of the proton-coupled folate transporter (PCFT-SLC46A1); a D156Y mutation causing hereditary folate malabsorption. Blood 2010; 116:5162-9. [PMID: 20805364 DOI: 10.1182/blood-2010-06-291237] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
The proton-coupled folate transporter (PCFT; SLC46A1) mediates folate transport into enterocytes in the proximal small intestine; pcft loss-of-function mutations are the basis for hereditary folate malabsorption. The current study explored the roles of Asp residues in PCFT function. A novel, homozygous, loss-of-function mutation, D156Y, was identified in a child of Pakistani origin with hereditary folate malabsorption. Of the 6 other conserved Asp residues, only one, D109, is shown to be required for function. D156Y, along with a variety of other substitutions at this site (Trp, Phe, Val, Asn, or Lys), lacked function due to instability of the PCFT protein. Substantial function was preserved with Glu, Gly, and, to a lesser extent, with Ser, Thr, and Ala substitutions. This correlated with PCFT bio-tinylated at the cell surface. In contrast, all D109 mutants, including D109E, lacked function irrespective of pH (4.5, 5.5, and 7.4) or substrate concentration (0.5-100 μM), despite surface expression comparable to wild-type PCFT. Hence, D156 plays a critical role in PCFT protein stability, and D109, located in the first intracellular loop between the second and third transmembrane domains, is absolutely required for PCFT function.
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