A novel class of genetic variants of the L1210 cell up-regulated for folate analogue transport inward. Isolation, characterization, and degree of metabolic instability of the system

Reduced folate carrier 1 is present in retinal microvessels and crucial for the inner blood retinal barrier integrity

BACKGROUND

Reduced folate carrier 1 (RFC1; SLC19a1) is the main responsible transporter for the B9 family of vitamins named folates, which are essential for normal tissue growth and development. While folate deficiency resulted in retinal vasculopathy, the expression and the role of RFC1 in blood-retinal barrier (BRB) are not well known.

METHODS

We used whole mount retinas and trypsin digested microvessel samples of adult mice. To knockdown RFC1, we delivered RFC1-targeted short interfering RNA (RFC1-siRNA) intravitreally; while, to upregulate RFC1 we delivered lentiviral vector overexpressing RFC1. Retinal ischemia was induced 1-h by applying FeCl₃ to central retinal artery. We used RT-qPCR and Western blotting to determine RFC1. Endothelium (CD31), pericytes (PDGFR-beta, CD13, NG2), tight-junctions (Occludin, Claudin-5 and ZO-1), main basal membrane protein (Collagen-4), endogenous IgG and RFC1 were determined immunohistochemically.

RESULTS

Our analyses on whole mount retinas and trypsin digested microvessel samples of adult mice revealed the presence of RFC1 in the inner BRB and colocalization with endothelial cells and pericytes. Knocking down RFC1 expression via siRNA delivery resulted in the disintegration of tight junction proteins and collagen-4 in twenty-four hours, which was accompanied by significant endogenous IgG extravasation. This indicated the impairment of BRB integrity after an abrupt RFC1 decrease. Furthermore, lentiviral vector-mediated RFC1 overexpression resulted in increased tight junction proteins and collagen-4, confirming the structural role of RFC1 in the inner BRB. Acute retinal ischemia decreased collagen-4 and occludin levels and led to an increase in RFC1. Besides, the pre-ischemic overexpression of RFC1 partially rescued collagen-4 and occludin levels which would be decreased after ischemia.

CONCLUSION

In conclusion, our study clarifies the presence of RFC1 protein in the inner BRB, which has recently been defined as hypoxia-immune-related gene in other tissues and offers a novel perspective of retinal RFC1. Hence, other than being a folate carrier, RFC1 is an acute regulator of the inner BRB in healthy and ischemic retinas.

Post-transcriptional regulation of the human reduced folate carrier as a novel adaptive mechanism in response to folate excess or deficiency

The RFC (reduced folate carrier) is the principal mechanism by which folates and clinically used antifolates are delivered to mammalian cells. hRFC (human RFC) is subject to complex transcriptional controls and exists as homo-oligomer. To explore the post-transcriptional regulation of hRFC by exogenous folates, hRFC-null HeLa cells were stably transfected with hRFC under control of a constitutive promoter. hRFC transcripts and the total membrane protein increased with increasing LCV [(6R,S)5-formyl tetrahydrofolate (leucovorin)] with a maximum at 20 nM LCV, attributable to reduced turnover of hRFC transcripts. hRFC homo-oligomerization was unaffected by increasing LCV. Cell surface hRFC paralleled [³H]methotrexate transport and increased from 0.5 to 2 nM LCV, and then decreased (~2-fold) with increasing LCV up to 20 nM. hRFC was localized to the cell surface at low LCV concentrations (0.5–1.5 nM). However, at higher LCV concentrations, significant intracellular hRFC was localized to the ER (endoplasmic reticulum), such that at 20 nM LCV, intracellular hRFC was predominated. Our results demonstrate a novel post-transcriptional regulation of hRFC involving: (i) increased hRFC transcripts and proteins, accompanying increased extracellular folates, attributable to differences in hRFC transcript stabilities; and (ii) increased retention of hRFC in the ER under conditions of folate excess, because of impaired intracellular trafficking and plasma membrane targeting.

A novel regulation of the physiologically/pharmacologically important human reduced folate carrier was demonstrated in response to increasing extracellular folates, involving: (i) increased transcripts and total protein, reflecting increased transcript stabilities; and (ii) increased endoplasmic reticulum trapping, due to impaired intracellular trafficking.

The major facilitative folate transporters solute carrier 19A1 and solute carrier 46A1: biology and role in antifolate chemotherapy of cancer

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.

Structure and function of the reduced folate carrier a paradigm of a major facilitator superfamily mammalian nutrient transporter

Folates are essential for life and folate deficiency contributes to a host of health problems including cardiovascular disease, fetal abnormalities, neurological disorders, and cancer. Antifolates, represented by methotrexate, continue to occupy a unique niche among the modern day pharmacopoeia for cancer along with other pathological conditions. This article focuses on the biology of the membrane transport system termed the "reduced folate carrier" or RFC with a particular emphasis on RFC structure and function. The ubiquitously expressed RFC is the major transporter for folates in mammalian cells and tissues. Loss of RFC expression or function portends potentially profound physiological or developmental consequences. For chemotherapeutic antifolates used for cancer, loss of RFC expression or synthesis of mutant RFC protein with impaired function results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and low levels of substrate for polyglutamate synthesis. The functional properties for RFC were first documented nearly 40 years ago in murine leukemia cells. Since 1994, when RFC was first cloned, tremendous advances in the molecular biology of RFC and biochemical approaches for studying the structure of polytopic membrane proteins have led to an increasingly detailed picture of the molecular structure of the carrier, including its membrane topology, its N-glycosylation, identification of functionally and structurally important domains and amino acids, and helix packing associations. Although no crystal structure for RFC is yet available, biochemical and molecular studies, combined with homology modeling, based on homologous bacterial major facilitator superfamily transporters such as LacY, now permit the development of experimentally testable hypotheses designed to establish RFC structure and mechanism.

Molecular and cellular biology of the human reduced folate carrier

The natural folates are water-soluble members of the B class of vitamins that are essential for cell proliferation and tissue regeneration. Since mammalian cells cannot synthesize folates de novo, tightly regulated and sophisticated cellular uptake processes have evolved to sustain sufficient levels of intracellular tetrahydrofolate cofactors to support the biosynthesis of purines, pyrimidines, serine, and methione. Membrane transport is also a critical determinant of the antitumor activity of antifolate therapeutics (methotrexate, Tomudex) used in cancer chemotherapy, and impaired uptake of antifolates is a frequent mode of drug resistance. The reduced folate carrier is the major transport system for folates and classical antifolates in mammalian cells and tissues. This review summarizes the remarkable advances in the cellular and molecular biology of the human reduced folate carrier over the past decade, relating to its molecular structure and transport function, mechanisms of transcriptional and posttranscriptional regulation, and its critical role in antifolate response and resistance. Many key in vitro findings have now begun to be extended to studies of reduced folate carrier levels and function in patient specimens, paving the way for translating basic laboratory studies in cultured cells to improvements in human health and treatment of disease. The results of research into the human reduced folate carrier should clarify the roles of changes in expression and function of this system that accompany nutritional folate deficiency and human disease, and may lead to improved therapeutic strategies for enhancing drug response and circumventing resistance in cancer patients undergoing chemotherapy with antifolates.

Characterization of a human alternatively spliced truncated reduced folate carrier increasing folate accumulation in parental leukemia cells

Human CEM-7A cells established by gradual deprivation of leucovorin from the growth medium, display 100-fold overexpression of methotrexate transport activity. We found that this was associated with 10-fold reduced folate carrier gene amplification and 50-fold overexpression of both the principal 3 kb reduced folate carrier transcript and, surprisingly, a novel truncated 2 kb reduced folate carrier mRNA poorly expressed in parental CEM cells. The molecular basis for the generation of this truncated reduced folate carrier transcript and its potential functional role in folate accumulation were studied. Reduced folate carrier genomic and cDNA sequencing revealed that the truncated transcript had an internal deletion of 987 nucleotides which was a result of an alternative splicing utilizing a cryptic acceptor splice site within exon 6. This deletion consisted of the 3'-most 480 nucleotides of the reduced folate carrier ORF and the following 507 nucleotides of the 3'-UTR. These resulted in a truncated reduced folate carrier protein, which lacks the C-terminal 160 amino acids, but instead contains 58 new C-terminal amino acids obtained from reading through the 3'-UTR. Consequently, a truncated reduced folate carrier protein is generated that lacks the 12th transmembrane domain and contains a new and much shorter C-terminus predicted to reside at the extracellular face. Western analysis with plasma-membrane fraction from CEM-7A cells revealed marked overexpression of both a broadly migrating approximately 65-90 kDa native reduced folate carrier and a approximately 40-45 kDa truncated reduced folate carrier, the core molecular masses of which were confirmed by in vitro translation. However, unlike the native reduced folate carrier, the truncated reduced folate carrier protein failed to bind the affinity labels NHS-[3H]MTX and NHS-[3H]folic acid. Stable transfection of the truncated reduced folate carrier cDNA into mouse L1210 leukemia cells: increased folate accumulation, decreased their leucovorin and folic acid growth requirements, and increased their sensitivity to methotrexate. This constitutes the first documentation of an expressed alternatively spliced truncated reduced folate carrier that, when coexpressed along with the native carrier, augments folate accumulation and consequently decreases the cellular folate growth requirement. The possible mechanisms by which the truncated reduced folate carrier may increase folate accumulation and/or metabolism in cells coexpressing the truncated and native reduced folate carrier are discussed.

Inhibitory effects of prostaglandin A1 on membrane transport of folates mediated by both the reduced folate carrier and ATP-driven exporters

Studies are reported that describe the multifaceted inhibitory effects of prostaglandin A1 (PGA1) on processes that govern the transport of folates across the plasma membrane of Chinese hamster ovary (CHO) cells: the reduced folate carrier, RFC1, and ATP-dependent exporters. PGA1 was a noncompetitive inhibitor of MTX influx mediated by RFC1 with a Ki of approximately 21 microM. The onset of inhibition was virtually instantaneous, not reversible, and appeared to require the incorporation of PGA1 into the lipid membrane; surface adsorption alone was insufficient for inhibition of RFC1 transport activity. In contrast, the effect of PGA1 on folic acid transport was small (approximately 20% inhibition of total influx), consistent with the observation that the major portion of folic acid transport in CHO cells is mediated by a low pH mechanism distinct from RFC1. PGA1 was also a potent inhibitor of the ATP-driven efflux of both MTX and folic acid. At a concentration of 7 microM PGA1, the efflux rate constants for these folates were depressed by approximately 70 and approximately 50%, respectively. The net effects of PGA1 on the bidirectional folate fluxes translated into marked alterations in net transport. The addition of 7 microM PGA1 to cells at steady state with 1 microM MTX produced a rapid onset of net uptake and the achievement of an approximately 3-fold increase in the steady-state free MTX level as compared with untreated CHO cells. The addition of 7 microM PGA1 to cells at steady state with 1 microM folic acid produced an approximately 5-fold increase in the free folate level. These studies establish PGA1 as a potent inhibitor of both the reduced folate carrier and ATP-driven folate exporter(s). The noncompetitive nature of the inhibition of RFC1 is unique among anionic compounds, which are usually competitive inhibitors of the carrier.

Chromosomal localization of the murine RFC-1 gene encoding a folate transporter and its amplification in an antifolate resistant variant overproducing the transporter

A variant of the L1210 cell (L1210/R83) selected in the presence of the lipophilic antifolate, metoprine, and a concentration of the natural diastereoisomer of 5-formyltetrahydrofolate, lL5CHO-folateH4, suboptimum for growth exhibited a 35-fold increase compared to parental L1210 cells in one-carbon, reduced folate transport. This was evidenced by the increase in Vmax for [3H]MTX (methotrexate) influx and a commensurate increase in the amount of the 46 kilodalton (kDa) transport protein and reduced folate carrier (RFC-1) mRNA. The variant is resistant to lipophilic antifolates, but shows collateral sensitivity to classical folate analogues. Karyotype analysis of L1210/R83 cells revealed the presence of several new chromosome abnormalities. One of these was a large, submetacentric marker chromosome comprising a normal #10 and a longer, abnormally banded arm of uncertain origin which exhibited an interstitial, palely staining, HSR-like segment. The results of Southern and Northern blotting showed that the RFC-1 gene copy number and RNA transcript level were markedly increased (30-35 fold) in L1210/R83 cells. Fluorescence in situ hybridization (FISH) analysis revealed that the HSR-like segment in these cells was the site of amplified RFC-1 genes. Independent revertant subclones, obtained following growth in the absence of selection pressure, showed four- to 12-fold decreases in [3H]MTX influx Vmax and in amount of NHS (N-hydroxysuccinimide)-[3H]MTX affinity labeled one-carbon, reduced folate transporter compared to L1210/R83 cells. RFC-1 gene copy number also decreased, and the mean length of the HSR in these revertants declined 1.6- to 5-fold. Based upon genomic nucleotide sequencing, the RFC-1 gene in the normal mouse genome was localized to chromosome 10 in close association with the alpha 1 (Col18a1) collagen gene at 10B3(locus 41cM). The close association of these genes was confirmed by other data showing that the alpha 1 collagen gene was co-amplified in L1210/R83 cells. These results document the amplification at the site of a putative HSR in an L1210 cell variant of the RFC-1 gene regulating expression of the one-carbon, reduced folate transporter.

Methotrexate transport and resistance

Methotrexate (MTX), the antifolate drug widely used as both an anticancer chemotherapeutic drug and as an immunosuppressive agent, mimics natural folates to inhibit critical cellular biosynthetic pathways. One of the most important determinants of cellular sensitivity to MTX is the degree to which this drug is internalized by cancer cells, and one of the major pathways of folate uptake results from the activity of the reduced folate carrier (RFC). Decreased RFC activity has been associated with several models of transport-mediated MTX resistance. Recently, the rodent and human genes which encode this protein have been isolated (RFC1), and defects in the expression of RFC1 genes have been identified in transport-deficient, MTX-resistant cell lines. Therefore, these studies have demonstrated the importance of RFC1 expression in transport-mediated antifolate drug resistance. In addition, however, studies of both MTX uptake in cancer cells and of folate transport in physiologic systems indicate that there are other proteins with uptake characteristics similar to RFC, and which maybe encoded by genes other than RFC1.

Concentrating capacity of the human reduced folate carrier (hRFC1) in human ZR-75 breast cancer cell lines

Human RFC1 (hRFC1) transfected in transport-deficient methotrexate MTXR(R)ZR-75-1 human breast carcinoma cells (MTX(R)ZR-75/RFC) were used to investigate the impact of hRFC1 overexpression on influx and concentrative transport of methotrexate (MTX). Eight-fold overexpression of hRFC1, as determined by northern analysis, resulted in a 4-fold increase in MTX influx accompanied by a 2.4-fold increase in the steady-state level of free drug as compared with wild-type ZR-75-1 cells when the extracellular MTX level was 0.5 microM. When extracellular MTX was increased to 10 microM, the increase in influx equaled the increase in the transmembrane chemical gradient for MTX in the transfectant relative to wild-type cells. By 50 min, approximately 16-20 and 25% of the intracellular 3H represented MTX polyglutamates by HPLC analysis at [MTX]e = 0.5 and 10 microM in wild-type and transfected cells, respectively. Overexpression of hRFC1 enhanced sensitivity to MTX in MTX(R)ZR-75-1 cells by more than 250-fold. The data indicate that overexpression of hRFC1 in human cells results in comparable increases in influx and transmembrane gradients. This is different from what was reported when mouse RFC1 was transfected into murine leukemia cells, resulting in large, more symmetrical increases in the MTX bidirectional transport kinetics with a much smaller change in steady-state levels. The changes in the human cells transfected with hRFC1 however, were similar to what has been observed by other investigators when RFC1 expression is increased by low folate selective pressure.

Gene amplification and increased expression of the reduced folate carrier in transport elevated K562 cells

The molecular bases for the 6-fold elevated methotrexate transport capacity of K562.4CF cells (Matherly et al., Cancer Res. 51: 3420-3426, 1991) were studied with reduced folate carrier (RFC) cDNA, genomic, and antibody probes. Southern analysis showed that RFC gene copies were increased (approximately 4- to 5-fold) in K562.4CF over wild-type K562 cells. Fluorescence in situ hybridization using a genomic RFC probe confirmed the localization of the RFC gene to the q-arm of chromosome 21. In K562.4CF cells, the frequent loss of a normal copy of chromosome 21 (61% of metaphases) was accompanied by RFC gene amplification and translocations of amplified RFC gene fragments to several (2 to 6) different chromosomal loci not seen in wild-type cells. Particularly intense RFC signals were mapped to homogeneously staining regions in chromosomes 2 and 15. Increased RFC gene copies were accompanied by a similar increase in the major 3.1 kb RFC transcript by northern blotting and an approximately 7-fold elevated level of the broadly migrating (80-95 kDa) RFC protein on a western blot probed with an RFC C-terminal peptide antibody. These results demonstrate that selection of cells with a growth-limiting concentration of reduced folates (0.4 nM of leucovorin) is sufficient to promote chromosomal aberrations, including gene amplification and translocations that result in increased RFC expression and folate transport.

Impact of overexpression of the reduced folate carrier (RFC1), an anion exchanger, on concentrative transport in murine L1210 leukemia cells

Transport of reduced folates in murine leukemia cells is mediated by the bidirectional reduced folate carrier (RFC1) and independent unidirectional exit pumps. RFC1 has been proposed to be intrinsically equilibrating, generating transmembrane gradients by exchange with inorganic and organic anions. This paper defines the role of high level carrier expression, through transfection with RFC1 cDNA, on concentrative transport of the folate analog, methotrexate (MTX) in murine L1210 leukemia cells. RFC1 was expressed in the MTXrA line, which lacks a functional endogenous carrier to obtain the MTXrA-R16 clonal derivative. Influx was increased approximately 9-fold in MTXrA-R16 cells without a change in Km. The efflux rate constant was increased by a factor of 5.1 relative to L1210 cells, and this resulted in only a 2.1-fold increase in the steady-state level of free intracellular MTX, [MTX]i, when [MTX]e was 1 microM. The concentrative advantage for RFC1 (the ratio of [MTX]i in MTXrA-R16 to L1210 cells) increased from 1.8 at 0.1 microM MTX to 3.8 at an [MTX]e level of 30 microM. Augmented transport in MTXrA-R16 cells was accompanied by a 2-fold increase in accumulation of MTX polyglutamate derivatives and a approximately 50% decrease in the EC50 for 5-formyltetrahydrofolate and folic acid and the MTX IC50 relative to L1210 cells. These alterations paralleled changes in [MTX]i and not the much larger change in influx at low [MTX]e levels, consistent with the critical role that free intracellular folates and drug play in meeting cellular needs for folates and as a determinant of antifolate activity, respectively. The data indicate that RFC1 produces a large and near symmetrical increase in the bidirectional fluxes of MTX resulting in only a small increase in the transmembrane chemical gradient at low extracellular folate levels. Hence, increased expression of RFC1, alone, may not be an efficient adaptive response to folate deprivation, and other factors may come into play to account for the marked increases in concentrative folate transport which occur when cells are subjected to low folate-selective pressure.

Loss of folic acid exporter function with markedly augmented folate accumulation in lipophilic antifolate-resistant mammalian cells

We previously described a 1,000-fold pyrimethamine-resistant Chinese hamster ovary cell line (PyrR100) which retains parental dihydrofolate reductase activity and methotrexate (MTX) sensitivity. This study characterizes the basis for the 14-fold decrease in folic acid and leucovorin concentrations required for clonogenic growth of PyrR100 cells relative to parental AA8 cells. Under conditions in which folic acid reduction was blocked by trimetrexate, PyrR100 cells displayed relative to parental AA8 cells a: 1) 17- and 5-fold increase in the net transport of folic acid and MTX, respectively; 2) 23- and 5-fold decrease in the efflux rate constant for folic acid and MTX, respectively; and 3) 2-fold increase in folic acid influx with no significant change in MTX influx. The markedly increased net folic acid transport in PyrR100 cells could not be explained by cellular folic acid binding, mitochondrial sequestration, polyglutamylation, nor by a decreased membrane potential. The effect of energy deprivation on folic acid and MTX transport in both cell lines was quite different. Glucose and pyruvate deprivation nearly abolished the increase in net folic acid transport in PyrR100 cells. In contrast, energy deprivation increased net MTX transport in AA8 cells, whereas no change was seen with PyrR100 cells. Furthermore, while folic acid influx in PyrR100 and AA8 cells was markedly reduced with energy deprivation, MTX influx was not affected. Provision of glucose and pyruvate to energy-deprived cells resulted in a rapid onset of MTX efflux from parental AA8 cells but not from PyrR100 cells. Taken together these results indicate that the markedly enhanced net transport of folic acid and MTX in PyrR100 cells is largely due to the complete loss of exit pump activity. Furthermore, the energy source that sustains the augmented levels of folic acid appears linked to the influx process and is distinct from the energy source that sustains MTX gradients under these conditions. We conclude that the loss of folic acid efflux is an efficient means of augmenting cellular uptake of folate cofactors and subsequent survival on picomolar folate concentrations. This constitutes the first demonstration of the loss of folic acid exporter activity in mammalian cells as a response to lipophilic antifolate selective pressure.

Human K562 transfectants expressing high levels of reduced folate carrier but exhibiting low transport activity

A human reduced folate carrier (hRFC) cDNA was transfected into transport-deficient K562 cells to circumvent complications that may result from carrier expression in a heterologous mammalian species. Relative to wild-type cells, hRFC transcript levels were increased 11- and 19-fold, respectively, in the K43-6 and K43-1 transfectants. Although photoaffinity labeling of hRFC protein revealed similar increases of 15- and 19-fold, respectively, only a 2-fold enhancement in methotrexate (Mtx) transport was observed. This suggests that only a small portion of the cDNA-encoded hRFC protein is actively engaged in membrane transport. Kinetic analysis of [3H]Mtx transport indicated that K43-6 cells exhibited a similar affinity (Kt) but an increased Vmax (1.7-fold) when compared with K562 cells. The restored transport was similar to that of wild-type cells in its capacity to be trans-stimulated by intracellular folates and in its sensitivity to competitive transport inhibitors (1843U89, bromosulfophthalein, folic acid, leucovorin, and ZD1694) and to irreversible inhibition by N-hydroxysuccinimide-methotrexate. Further, deglycosylated photoaffinity-labeled hRFC protein in both K562 and K43-6 cells migrated at approximately 65-70 kDa on SDS-gels, consistent with the molecular mass from the predicted amino acid sequence. These data further establish that the expression of hRFC, alone, is sufficient to confer transport properties typical of the "classical" hRFC. However, the discrepancy between the stoichiometry of carrier expression and transport activity implies that membrane translocation of bound substrate may be regulated by additional undefined mechanisms.

Ligand-directed immunoaffinity purification and properties of the one-carbon, reduced folate transporter. Interspecies immuno-cross-reactivity and expression of the native transporter in murine and human tumor cells and their transport-altered variants

Almost complete purification (> 95%) of the 46-kDa murine, one-carbon, reduced folate transporter (RFT) at a recovery of 20% was obtained by ligand-directed immunoaffinity fractionation from transporter overproducing L1210/R83 cells. These cells were labeled with the N-hydroxysuccinimide ester of [3H]aminopterin (AMT), the isolated plasma membrane alkaline washed to remove nonintegral membrane proteins, detergent-solubilized, and RFT-separated on an anti-AMT antibody-protein G-Sepharose column followed by preparative SDS-polyacrylamide gel electrophoresis. Anti-RFT antibody, subsequently derived, differentially blotted (L1210/R83 >> L1210/0) a 46-kDa protein during SDS-polyacrylamide gel electrophoresis of plasma membrane from L1210/R83 and L1210 cells and in L1210/R83 cells after trichloroacetic acid precipitation. In contrast to that reported for human tumor cells, glycosidase treatment of RFT revealed no common N- or O-linked core oligosaccharides associated with this protein. The same 46-kDa protein at different relative levels was revealed in a Western blot of plasma membrane from other murine tumors. Blotting of plasma membrane from methotrexate resistant, transport defective L1210 cell variants exhibited wild-type levels of a less electrophoretically mobile RFT or greater levels of the same 46-kDa RFT which could not be affinity labeled with N-hydroxysuccinimide-[3H]AMT. The same antibody differentially blotted a 83-kDa plasma membrane protein from human HL-60 and CCRF-CEM cells with different levels of reduced folate transport and affinity labeling of RFT, verifying the conserved nature of this protein consistent with earlier functional studies.

Different antifolate-resistant L1210 cell variants with either increased or decreased folylpolyglutamate synthetase gene expression at the level of mRNA transcription

L1210 cell variants selected in the presence of the lipophilic dihydrofolate reductase inhibitor, metoprine, expressed increased levels of one-carbon, reduced folate transport inward (Sirotnak, F. M., Moccio, D. M., and Yang, C.-H. (1984) J. Biol. Chem. 259, 13139-13144). Growth of one of these variants (L1210/R69), with metoprine in the presence of decreasing concentrations of 1,L5-CHO-folateH4 (natural diastereoisomer of 5-formyl-tetrahydrofolate), resulted in the selection of other variants (L1210/R82, R83, and R84) with further reduction in one-carbon, reduced folate transport and in two cases (L1210/R83 and R84) with 3-8-fold increased folylpolyglutamate synthetase (FPGS) activity and folate compound polyglutamate formation in situ. Metoprine resistance was further increased, and the requirement for exogenous folate during growth was decreased as well in these variants. The increase in FPGS activity observed in L1210/R83 and R84 was characterized by 3- and 8-fold increases in value for Vmax with no change in Km and the same increase in a 60-61-kDa protein as shown by immunoblotting. Northern blotting revealed the same increases in these two variants in the level of a 2.3-kilobase FPGS mRNA when compared with control, while Southern blotting of genomic DNA did not reveal any increase in FPGS gene-copy number or restriction polymorphisms. Also, no difference in stability of FPGS mRNA was found between parental and variant cells. In contrast, nuclear run-on assays revealed differences among these cell types in the rate of FPGS mRNA transcription that correlated with increased FPGS activity, protein, and mRNA level in the variants. Similar studies with a transport-defective, methotrexate-resistant L1210 cell variant (L1210/R25) documented a 2-3-fold decrease in FPGS activity, protein, and mRNA levels that was accounted for by a decrease in FPGS mRNA transcription. These results provide the first examples of constitutively altered transcriptional regulation of FPGS activity associated with acquired resistance to antifolates.

The effect of folate on the methotrexate/indomethacin interaction in a murine cancer cell line

1. The effect of folate on the interaction between methotrexate (a folate analogue) and indomethacin has been examined in murine NC carcinoma cells. 2. Conditioning of NC cells to a physiological (20 nM) folate concentration after culture in a high folate concentration increased the response to methotrexate. The sensitivity of these conditioned cells to methotrexate related inversely to the folate concentration. 3. At 20 nM and 2 microM folate, indomethacin 1 micrograms ml-1 potentiated the cytotoxicity of methotrexate 4 and 8 ng ml-1 (both P < 0.03). 4. When NC cells were incubated with [3H]-methotrexate at 20 nM and 2 microM folate, there was a trend for increased tritium accumulation with indomethacin 0.36 micrograms ml-1 (1 microM; P < 0.01). 5. We conclude that the folate concentration can affect the sensitivity of NC cells to methotrexate, although the degree of potentiation of cytotoxicity by indomethacin remains similar.

Congruence of SQM1 protein expression with methotrexate sensitivity and transport

Expression of SQM1 protein, a membrane protein, was shown to be reduced in human squamous carcinoma of the head and neck (SqCHN) cell lines made less sensitive to methotrexate (MTX). High correlation of SQM1 protein expression with MTX transport (r = .94) and MTX sensitivity (r = .94) was found in MTX-resistant sublines developed from a clonally drived SqCHN cell line, SCC 15S1. Unlike SQM1 protein, there was no significant difference in amount or molecular weight of reduced-folate binding protein found in the membrane of a MTX-resistant subline, SCC15R1-3 and SCC15S1. Furthermore, MTX surface membrane binding was not significantly altered in SCC15R1-3. Compared to SCC15S1 parent cell line, SCC15R1-3 subline with similar DHFR enzyme level and MTX polyglutamylation showed a marked reduction in MTX uptake due to a decrease in Vmax without a significant change in Kt. These findings suggest the existence of membrane molecules like SQM1 protein that may indirectly affect MTX transport.

Molecular events in the membrane transport of methotrexate in human CCRF-CEM leukemia cell lines

A variant line (CEM-7A) "overproducing" the reduced folate/MTX carrier system was isolated from human CCRF-CEM leukemia cells grown under selective conditions in medium containing 0.25 nM 5-formyl-THF as the sole folate source. This line exhibits a 95-fold increased Vmax for [3H]-MTX influx as compared to parental cells. The values for [3H]-MTX influx Km, efflux t1/2 and structural specificity for other (anti)folate compounds were unchanged. The amount of carrier protein, estimated by NHS-[3H]-MTX affinity labeling, was approximately 30-fold higher in CEM-7A cells than in parental cells. Influx of [3H]-MTX in CEM-7A cells was found to be down-regulated 6-7-fold after preincubation of cells with adenosine, 5-formyl-THF or 5-methyl-THF, but could be prevented exclusively by inhibitors of dihydrofolate reductase. The underlying mechanism(s) of these effects have not as yet been elucidated. A radioiodinated photoaffinity analog of MTX was used to prove the molecular events in carrier-mediated MTX uptake in parental CCRF-CEM cells, CEM-7A cells, and a line exhibiting a MTX-transport defect (CEM-MTX). Specific labeling of an 80-85 kDa membrane protein was observed in parental cells, but not in CEM/MTX cells. Uptake of photoprobe and levels of the 80-85 kDa membrane protein were significantly increased in CEM-7A cells. Due to extensive glycosylation the MW of the carrier protein in human cells seems to be substantially higher than that of its counterpart in murine L1210 leukemia cells (46-48 kDa). Pulse-labeling experiments at 37 degrees C demonstrated that in CEM-7A cells photoprobe uptake proceeds via a specific pathway. The 80-85 kDa membrane protein is involved in the initial binding and translocation of photoprobe, after which a 38 kDa cytosolic protein is responsible for further intracellular distribution. At this time, the combination of photoaffinity labeling techniques and the availability of variant cell lines overexpressing the reduced folate/MTX carrier protein has provided new insights into the MTX transport process in human leukemia cell lines. In the near future this approach should also allow a further elucidation of the regulatory aspects of carrier function.

Mechanisms of acquired resistance to methotrexate in a human squamous carcinoma cell line of the head and neck, exposed to different treatment schedules

Mechanisms of acquired resistance to methotrexate (MTX) were evaluated in HNSCC-11B cells which were made resistant to methotrexate either by continuous (11B-MTX-C) or by pulse exposure (11B-MTX-P) to the drug. 11B-MTX-C cells were 91-fold resistant to methotrexate and 30-fold or 49-fold crossresistant to trimetrexate and 10-EdAM, respectively. Dihydrofolate reductase (DHFR) activity was increased 63-fold in 11B-MTX-C cells together with a decrease in [3H]-methotrexate transport and folylpolyglutamate synthase (FPGS) activity (2.5-fold and 3.8-fold, respectively). Against two novel antifolates targetting enzymes other than DHFR, minor crossresistance was observed for ICI-198, 583, but full sensitivity was retained for DDATHF. 11B-MTX-P cells were 46-fold resistant to methotrexate and 47-fold crossresistant to ICI-198,583 in short-term drug exposure, but showed only minor changes in methotrexate sensitivity following prolonged drug exposure. The resistant phenotype in 11B-MTX-P cells were characterised by a 5.6-fold decrease in FPGS activity. These results suggest that different mechanisms of methotrexate resistance in HNSCC cells in vitro can be obtained dependent on the schedule of exposure to the drug.

Studies on the mechanism of antitumor action of 2-desamino-2-methyl-5,8-dideazaisofolic acid

The new folate analogue, 2-desamino-2-methyl-5,8-dideazaisofolic acid, 2c, was synthesized and evaluated using a variety of biochemical and antitumor assays. For purposes of comparison, its 2-desamino, 2b, and 2-amino, 2a, counterparts, as well as N10-propargly-5,8-dideazafolic acid, 1a, and the corresponding 2-desamino, 1b, and 2-desamino-2-methyl, 1c, modifications were included in these studies. Compound 2c was found to be a potent inhibitor of the growth of L1210 and MCF-7 cells in culture, being only 2-fold and 5-fold less effective than 1c, respectively. However, although analogue 2c was 189-fold less inhibitory toward L1210 thymidylate synthase (TS) than 1c, its cytotoxicity was reversed completely by thymidine alone which suggests that the compound behaves as a TS inhibitor in cells. Enzymatically synthesized polyglutamates of 2c were substantially more inhibitory toward human TS than the parent compound. Compound 2c was the most efficient substrate for mammalian folyl-polyglutamate synthetase of the compounds studied having a Vmax/Km nearly 12-fold larger than 1c. Both 1c and 2c were effective inhibitors of the uptake of [3H]methotrexate into MOLT-4 cells, implying that each is efficiently transported into tumor cells. These results suggest that a weak inhibitor of TS in vitro can be a potent cytotoxic agent if it can readily gain entry into target cells and be converted to polyglutamated metabolites.

The transport of pteridines in CCRF-CEM human lymphoblastic cells

The transport routes used by CCRF-CEM human lymphoblastoid cells for the influx and efflux of unconjugated pteridines were analyzed using [3H]6-hydroxymethylpterin as a model compound. Influx proceeds by a mechanism that exhibits a Km of 66.7 microM and a Vmax of 0.077 nmol/min per mg cellular protein. The process is somewhat sensitive to metabolic inhibitors, particularly uncouplers of oxidative phosphorylation, and is significantly affected by the presence of other pteridines in the extracellular medium. The results suggest that pterins with either no 6-substituent (pterin) or those with methyl, hydroxyl, or formyl groups in this position, which exhibit Ki values between 25 and 77 microM, may share the same pathway for uptake. 6-Carboxypterin exhibits low affinity for the system (Ki greater than 500 microM), as do 7-substituted and 6,7-di-substituted derivatives and compounds with larger groups at the 6-position, such as neopterin and biopterin (Ki = 250-300 microM). Efflux of [3H]6-hydroxymethylpterin occurs rapidly and can proceed by at least two routes. The first, comprising approximately 50% of total efflux, is inhibited by extracellular pterins and exhibits similar properties to the uptake system in both its pattern of sensitivity to metabolic inhibitors and its specificity for pteridine structure. The route by which the remaining efflux occurs is relatively insensitive to metabolic inhibition. Adenine significantly inhibits 6-hydroxymethylpterin influx and efflux (Ki = 10.6 microM for uptake) but does not appear to share the same transport system. Similarly, methotrexate and folic acid exhibit little affinity for the unconjugated pteridine transport routes.

Mutant Chinese hamster ovary cells with defective methotrexate uptake are distinguishable by reversion analysis

Chinese hamster ovary cells that are about 50x more resistant to the cytotoxic action of methotrexate than wild-type cells were deficient in the ability to take up methotrexate. In the absence of any exogeneous folates, these cells require 100-250x the level of folinic acid as wild-type cells to support growth at a similar level. Two classes of mutants were distinguishable by their revertability for growth on folinic acid. Revertants derived from one class were similar to wild-type cells in both their ability to grow in medium containing low levels of folinic acid and in their sensitivity to methotrexate. In contrast, partial revertants from a second class were able to grow in medium containing low or no folinic acid, but retained their methotrexate resistance. Furthermore, mutants of the first class were unable to take up folic acid while the second class of mutants accumulated folic acid to levels similar to that of wild-type cells. Somatic cell hybrids formed between these two classes of mutants were noncomplementing. These observations suggested that some, but not all, components may be shared between the transport systems mediating methotrexate and folic acid uptake.

Mediated uptake of folate by a high-affinity binding protein in sublines of L1210 cells adapted to nanomolar concentrations of folate

An L1210 cell line (JT-1), which can grow in medium supplemented with 1 nM folate, has been isolated. These cells exhibit a slower growth rate than folate-replete parental cells and have a lower ability to transport folate or methotrexate via the reduced folate transport system. Measurements at nanomolar concentrations of folate revealed that the adapted cells have acquired a high-affinity folate-binding protein. Binding to this component at 37 degrees C was rapid and reached a maximum value after 30 min which corresponded in amount to 0.23 +/- 0.3 pmol/mg protein, and excess unlabeled folate added 30 min subsequent to the [3H]folate led to a rapid release of the bound substrate. Radioactivity bound to or released from the cells after 30 min at 37 degrees C remained as unmetabolized folic acid. Binding was also rapid at 0 degrees C but uptake at the plateau was only one-half the value obtained at 37 degrees C. Half-maximal saturation of the binding component (KD) occurred at a folate concentration of 0.065 nM at pH 7.4, while the affinity for folate decreased 30-fold when the pH was reduced to 6.2 (KD = 2.0 nM). 5-Methyltetrahydrofolate was also bound by this component (Ki = 13 nM at pH 7.4) but with a much lower affinity than for folate, while progressively weaker interactions were observed with 5-formyltetrahydrofolate (Ki = 45 nM) and methotrexate (Ki = 325 nM). When the same adaptation procedure was performed with limiting amounts of 5-formyltetrahydrofolate, two additional cell lines, JT-2 and JT-3, were isolated which expressed elevated levels of the folate-binding protein. The binding activity of the latter cells was 0.46 and 1.4 pmol/mg protein, respectively. When the level of binding protein was compared in cells grown at different concentrations of folate, an increase in medium folate from 1 to 500 nM caused a sevenfold reduction in binding activity in the JT-3 cell line, while these same growth conditions had no effect on binding by the other cells. These results indicate that L1210 cells adapted to low concentrations of folate or 5-formyltetrahydrofolate contain elevated levels of a high-affinity binding protein and that this protein is able to mediate the intracellular accumulation of folate compounds. L1210 cells thus appear to have two potential uptake routes for folate compounds, the previously characterized anion-exchange system and a second route mediated by a high-affinity binding protein.(ABSTRACT TRUNCATED AT 400 WORDS)