1
|
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: 0] [Impact Index Per Article: 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.
Collapse
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
| |
Collapse
|
2
|
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.
Collapse
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
| |
Collapse
|
3
|
Do HQ, Bassil CM, Andersen EI, Jansen M. Impact of nanodisc lipid composition on cell-free expression of proton-coupled folate transporter. PLoS One 2021; 16:e0253184. [PMID: 34793461 PMCID: PMC8601550 DOI: 10.1371/journal.pone.0253184] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2021] [Accepted: 10/31/2021] [Indexed: 01/19/2023] Open
Abstract
The Proton-Coupled Folate Transporter (PCFT) is a transmembrane transport protein that controls the absorption of dietary folates in the small intestine. PCFT also mediates uptake of chemotherapeutically used antifolates into tumor cells. PCFT has been identified within lipid rafts observed in phospholipid bilayers of plasma membranes, a micro environment that is altered in tumor cells. The present study aimed at investigating the impact of different lipids within Lipid-protein nanodiscs (LPNs), discoidal lipid structures stabilized by membrane scaffold proteins, to yield soluble PCFT expression in an E. coli lysate-based cell-free transcription/translation system. In the absence of detergents or lipids, we observed PCFT quantitatively as precipitate in this system. We then explored the ability of LPNs to support solubilized PCFT expression when present during in-vitro translation. LPNs consisted of either dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), or dimyristoyl phosphatidylglycerol (DMPG). While POPC did not lead to soluble PCFT expression, both DMPG and DMPC supported PCFT translation directly into LPNs, the latter in a concentration dependent manner. The results obtained through this study provide insights into the lipid preferences of PCFT. Membrane-embedded or solubilized PCFT will enable further studies with diverse biophysical approaches to enhance the understanding of the structure and molecular mechanism of folate transport through PCFT.
Collapse
Affiliation(s)
- Hoa Quynh Do
- Department of Cell Physiology and Molecular Biophysics and Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, United States of America
| | - Carla M. Bassil
- Department of Cell Physiology and Molecular Biophysics and Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, United States of America
- The Clark Scholar Program, Texas Tech University, Lubbock, TX, United States of America
| | - Elizabeth I. Andersen
- Department of Cell Physiology and Molecular Biophysics and Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, United States of America
| | - Michaela Jansen
- Department of Cell Physiology and Molecular Biophysics and Center for Membrane Protein Research, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas, United States of America
| |
Collapse
|
4
|
Narawa T, Narita Y, Hosokawa S, Itoh T. Functional role of serine 318 of the proton-coupled folate transporter in methotrexate transport. Drug Metab Pharmacokinet 2021; 41:100421. [PMID: 34619546 DOI: 10.1016/j.dmpk.2021.100421] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2021] [Revised: 08/08/2021] [Accepted: 09/16/2021] [Indexed: 10/20/2022]
Abstract
This study revealed the importance of serine 318 (S318) residue for proton-coupled folate transporter (PCFT, SLC46A1) functioning. Substitution of S318 with arginine or lysine impaired transport of methotrexate (MTX), but substitution with alanine (has a simple side chain structure), or cysteine (structurally similar to serine), had no significant effect on MTX transport. The initial uptake rate of MTX by S318A and S318C mutant at pH 5.0, followed by Michaelis-Menten kinetics with a Km value of approximately 2.3 μM (for S318A) and 2.9 μM (for S318C), was similar to that of the wild-type. The normalized Vmax value of the S318A mutant, calculated by dividing the Vmax value by the Western blot protein band's relative intensity, was approximately 2-fold greater than that of the wild-type. The normalized Vmax value of the S318C mutant was approximately 0.8-fold smaller than the wild-type. Results obtained showed that the substitution of S318 with basic amino acid residues results in the loss of transport activity, even though PCFT mutants are expressed at the cell membrane. Alternatively, the substitution of S318 with neutral amino acids did not significantly affect the transport function of PCFT.
Collapse
Affiliation(s)
- Tomoya Narawa
- Laboratory of Pharmaceutics, School of Pharmacy, Kitasato University, 5-9-1 Shirokane, Tokyo, 108-8641, Japan.
| | - Yuuki Narita
- Laboratory of Pharmaceutics, School of Pharmacy, Kitasato University, 5-9-1 Shirokane, Tokyo, 108-8641, Japan
| | - Sayuri Hosokawa
- Laboratory of Pharmaceutics, School of Pharmacy, Kitasato University, 5-9-1 Shirokane, Tokyo, 108-8641, Japan
| | - Tomoo Itoh
- Laboratory of Pharmaceutics, School of Pharmacy, Kitasato University, 5-9-1 Shirokane, Tokyo, 108-8641, Japan
| |
Collapse
|
5
|
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.
Collapse
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
| |
Collapse
|
6
|
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)).
Collapse
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
| | | |
Collapse
|
7
|
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.
Collapse
|
8
|
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.
Collapse
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
| |
Collapse
|
9
|
Aluri S, Zhao R, Fiser A, Goldman ID. Substituted-cysteine accessibility and cross-linking identify an exofacial cleft in the 7th and 8th helices of the proton-coupled folate transporter (SLC46A1). Am J Physiol Cell Physiol 2017; 314:C289-C296. [PMID: 29167151 DOI: 10.1152/ajpcell.00215.2017] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
The proton-coupled folate transporter (PCFT-SLC46A1) is required for folate transport across the apical membrane of the small intestine and across the choroid plexus. This study focuses on the structure/function of the 7th transmembrane domain (TMD), and its relationship to the 8th TMD as assessed by the substituted cysteine accessibility method (SCAM) and dicysteine cross-linking. Nine exofacial residues (I278C; H281C-L288C) of 23 residues in the 7th TMD were accessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate (MTSEA-biotin). Pemetrexed, a high-affinity substrate for PCFT, decreased or abolished biotinylation of seven of these residues consistent with their location in or near the folate binding pocket. Homology models of PCFT based on Glut5 fructose transporter structures in both inward- and outward- open conformations were constructed and predicted that two pairs of residues (T289-I304C and Q285-Q311C) from the 7th and 8th TMDs should be in sufficiently close proximity to form a disulfide bond when substituted with cysteines. The single Cys-substituted mutants were accessible to MTSEA-biotin and functional with and without pretreatment with dithiotreitol. However, the double mutants were either not accessible at all, or accessibility was markedly reduced and function markedly impaired. This occurred spontaneously without inclusion of an oxidizing agent. Dithiotreitol restored accessibility and function consistent with disulfide bond disruption. The data establish the proximity of exofacial regions of the 7th and 8th TMDs and their role in defining the aqueous translocation pathway and suggest that these helices may be a component of an exofacial cleft through which substrates enter the protein binding pocket in its outward-open conformation.
Collapse
Affiliation(s)
- Srinivas Aluri
- Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York.,Department of Medicine, 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
| | - Andras Fiser
- Department of Systems and Computational Biology, Albert Einstein College of Medicine, Bronx, New York.,Department of Biochemistry, 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
| |
Collapse
|