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

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.

The role of impaired transport in (pre)clinical resistance to methotrexate: insights on new antifolates

Impaired transport appears to be a common mechanism of resistance of neoplastic cells to the antifolate methotrexate. The extensive knowledge of the molecular, biochemical and functional properties of the membrane transport systems for folates, in particular the reduced folate carrier (RFC) and membrane folate receptors (MFR), has made an important contribution to the rational design of novel antifolates that are either more efficiently internalized via these transporters or can simply bypass these transport routes. The current status of exploiting the RFC and MFR for transport of novel antifolates active in preclinical model systems and a clinical setting will be reviewed, with a special reference to childhood acute lymphoblastic leukemia (ALL) and acute non-lymphoblastic leukemia (ANLL).

Human reduced folate carrier: translation of basic biology to cancer etiology and therapy

This review attempts to provide a comprehensive overview of the biology of the physiologically and pharmacologically important transport system termed the "reduced folate carrier" (RFC). The ubiquitously expressed RFC has unequivocally established itself as the major transport system in mammalian cells and tissues for a group of compounds including folate cofactors and classical antifolate therapeutics. Loss of RFC expression or function may have potentially profound pathophysiologic consequences including cancer. For chemotherapeutic antifolates used for cancer such as methotrexate or pemetrexed, synthesis of mutant RFCs or loss of RFC transcripts and proteins results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and insufficient substrate for polyglutamate synthesis. Since RFC was first cloned in 1994, tremendous advances have been made in understanding the complex transcriptional and posttranscriptional regulation of RFC, in identifying structurally and functionally important domains and amino acids in the RFC molecule as a prelude to establishing the mechanism of transport, and in characterizing the molecular defects in RFC associated with loss of transport in antifolate resistant cell line models. Many of the insights gained from laboratory models of RFC portend opportunities for modulating carrier expression in drug resistant tumors, and for designing a new generation of agents with improved transport by RFC or substantially enhanced transport by other folate transporters over RFC. Many of the advances in the basic biology of RFC in cell line models are now being directly applied to human cancers in the clinical setting, most notably pediatric acute lymphoblastic leukemia and osteogenic sarcoma.

The Reduced Folate Carrier Gene Is a Novel Selectable Marker for Recombinant Protein Overexpression

Folate cofactors are one-carbon donors essential for the biosynthesis of purines and thymidylate. Mammalian cells are devoid of folate biosynthesis and are therefore folate auxotrophs that take up folate vitamins primarily via the reduced folate carrier (RFC). In this study, we showed that the human RFC (hRFC) gene can serve as a novel selectable marker for the overproduction of recombinant proteins. Toward this end, a hemagglutinin (HA) epitope tagged hRFC (hRFC-HA) was introduced into a bicistronic vector (pIRES2-EGFP), upstream of an enhanced green fluorescent protein (EGFP) reporter gene. Chinese hamster ovary cells deficient in RFC activity were isolated and transfected with this construct, followed by gradual deprivation of leucovorin, the sole folate source in the growth medium. Only cells with hRFC-HA overexpression were able to take up leucovorin and thereby survive these selective conditions. Western blot and immunofluorescence analyses confirmed that the hRFC-HA was overexpressed at extremely high levels, properly glycosylated and sorted out to the plasma membrane. This resulted in a approximately 450-fold increase in [3H]methotrexate influx and approximately 100-fold increased sensitivity to methotrexate, relative to untransfected RFC-deficient cells. Flow cytometric analysis consistently revealed that EGFP was overexpressed approximately 100-fold above the autofluorescence level. Overproduction of hRFC-HA and EGFP was stably maintained for at least 2 months in a constant concentration of leucovorin. These results establish a novel RFC-based metabolic selection system for the efficient overexpression of recombinant proteins. Furthermore, the possible implications to subcellular transporter localization and restoration of MTX sensitivity in drug-resistant tumors by RFC-based gene therapy are discussed.

Membrane transport of folates

The chapter reviews the current understanding of the transport mechanisms for folates in mammalian cells--their molecular identities and organization, tissue expression, regulation, structures, and their kinetic and thermodynamic properties. This encompasses a variety of diverse processes. Best characterized is the reduced folate carrier, a member of the SLC19 family of facilitative carriers. But other facilitative organic anion carriers (SLC21), largely expressed in epithelial tissues, transport folates as well. In addition to these bi-directional carrier systems are the membrane-localized folate receptors alpha and beta, that mediate folate uptake unidirectionally into cells via an endocytotic process. There are also several transporters, typified by the family of multidrug resistance-associated proteins, that unidirectionally export folates from cells. There are transport activities for folates, that function optimally at low pH, related in part to the reduced folate carrier, with at least one activity that is independent of this carrier. The reduced folate carrier-associated low-pH route mediates intestinal folate transport. This review considers how these different transport processes contribute to the generation of transmembrane folate gradients and to vectorial flows of folates across epithelia. The role of folate transporters in mouse development, as assessed by homologous deletion of folate receptors and the reduced folate carrier, is described. Much of the focus is on antifolate cancer chemotherapeutic agents that are often model surrogates for natural folates in transport studies. In particular, antifolate transport mediated by the reduced folate carrier is a major determinant of the activity of, and resistance to, these agents. Finally, many of the key in vitro findings on the properties of antifolate transporters are now beginning to be extended to patient specimens, thus setting the stage for understanding response to these drugs in the clinical setting at the molecular level.

Resistance to multiple novel antifolates is mediated via defective drug transport resulting from clustered mutations in the reduced folate carrier gene in human leukaemia cell lines

We have studied the molecular basis of resistance of multiple human leukaemia CCRF-CEM sublines to the novel antifolates ZD9331, GW1843, AG2034, PT523 and edatrexate, which use the reduced folate carrier (RFC) as their main cellular uptake route and that target different folate-dependent enzymes. Antifolate-resistant sublines established by stepwise and single-step selections displayed up to 2135-fold resistance to the selection drug, and up to 2323-fold cross-resistance to various hydrophilic antifolates. In contrast, these sublines were up to 17- and 20-fold hypersensitive to the lipophilic antifolates AG377 and trimetrexate, respectively. The total reduced folate pool of these antifolate-resistant sublines shrunk by 87-96%, resulting in up to 42-fold increased folic acid growth requirement. These sublines lost 92-97% of parental [(3)H]methotrexate influx rates. Genomic PCR single-strand conformational polymorphism analysis and sequencing revealed that most of these drug-resistant sublines harboured RFC mutations that surprisingly clustered in two confined regions in exons 2 and 3. The majority of these mutations resulted in frame-shift and/or premature translation termination and lack of RFC protein expression. The remaining mutations involved single amino acid substitutions predominantly residing in the first transmembrane domain (TMD1). Some RFC-inactivating mutations emerged during the early stages of antifolate selection and were stably retained during further drug selection. Furthermore, some sublines displayed a markedly decreased or abolished RFC mRNA and/or protein expression. This constitutes the first demonstration of clustering of multiple human RFC mutations in TMD1, thereby suggesting that it plays a functional role in folate/antifolate binding and/or translocation. This is the first molecular characterization of human RFC-associated modalities of resistance to various novel antifolates in multiple leukaemia sublines.

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.

Clustering of mutations in the first transmembrane domain of the human reduced folate carrier in GW1843U89-resistant leukemia cells with impaired antifolate transport and augmented folate uptake

We have studied the molecular basis for the resistance of human CEM leukemia cells to GW1843, a thymidylate synthase inhibitor. GW1843-resistant cells displayed a approximately 100-fold resistance to GW1843 and methotrexate but were collaterally sensitive to the lipophilic antifolates trimetrexate and AG337, which enter cells by diffusion. These cells exhibited a 12-fold decreased methotrexate influx but surprisingly had a 2-fold decreased folic acid growth requirement. This was associated with a 4-fold increased influx of folic acid, a 3.5-fold increased steady-state level of folic acid, and a 2.3-fold expansion of the cellular folate pool. Characterization of the transport kinetic properties revealed that GW1843-resistant cells had the following alterations: (a) 11-fold decreased transport K(m) for folic acid; (b) 6-fold increased transport K(m) for GW1843; and (c) a slightly increased transport V(max) for folic acid. Sequence analysis showed that GW1843-resistant cells contained the mutations Val-29 --> Leu, Glu-45 --> Lys, and Ser-46 --> Ile in the first transmembrane domain of the reduced folate carrier. Transfection of the mutant-reduced folate carrier cDNA into methotrexate transport null cells conferred resistance to GW1843. This is the first demonstration of multiple mutations in a confined region of the human reduced folate carrier in an antifolate-resistant mutant. We conclude that certain amino acid residues in the first transmembrane domain play a key role in (anti)folate binding and in the conferring of drug resistance.

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.

A structurally altered human reduced folate carrier with increased folic acid transport mediates a novel mechanism of antifolate resistance

CEM/MTX is a subline of human CCRF-CEM leukemia cells which displays >200-fold resistance to methotrexate (MTX) due to defective transport via the reduced folate carrier (RFC). CEM/MTX-low folate (LF) cells, derived by a gradual deprivation of folic acid from 2.3 microM to 2 nM (LF) in the cell culture medium of CEM/MTX cells, resulted in a >20-fold overexpression of a structurally altered RFC featuring; 1) a wild type Km value for MTX transport but a 31-fold and 9-fold lower Km values for folic acid and leucovorin, respectively, relative to wild type RFC; 2) a 10-fold RFC1 gene amplification along with a >20-fold increased expression of the main 3.1-kilobase RFC1 mRNA; 3) a marked stimulation of MTX transport by anions (i.e. chloride); and 4) a G --> A mutation at nucleotide 227 of the RFC cDNA in both CEM/MTX-LF and CEM/MTX, resulting in a lysine for glutamate substitution at amino acid residue 45 predicted to reside within the first transmembrane domain of the human RFC. Upon transfer of CEM/MTX-LF cells to folate-replete medium (2.3 microM folic acid), the more efficient folic acid uptake in CEM/MTX-LF cells resulted in a 7- and 24-fold elevated total folate pool compared with CEM and CEM/MTX cells, respectively (500 versus 69 and 21 pmol/mg of protein, respectively). This markedly elevated intracellular folate pool conferred a novel mechanism of resistance to polyglutamatable (e.g. ZD1694, DDATHF, and AG2034) and lipophilic antifolates (e.g. trimetrexate and pyrimethamine) by abolishing their polyglutamylation and circumventing target enzyme inhibition.

Neural tube and craniofacial defects with special emphasis on folate pathway genes

Neural tube and orofacial defects are common congenital malformations in humans. While etiologically heterogeneous, they are for the most part multifactorial in their pathogenesis, having both genetic and environmental components in their development. In recent years, there has been a great deal of epidemiologic evidence demonstrating that women who received multivitamins containing folic acid periconceptionally had significantly reduced occurrence and recurrence risks for producing infants with such malformations. This risk reduction is not observed in all populations, further suggestive of a genetic regulation of this phenomenon. Unfortunately, the mechanisms underlying the beneficial effects of folic acid are not well-understood. In this article, we review the relevant epidemiologic data on both neural tube defects and orofacial malformations, the fundamental embryological processes involved in closing the neural tube, and the development of the craniofacies, and propose a working hypothesis for susceptibility to these malformations. This hypothesis is based on the interworkings of cellular folate transport, focusing on the key elements involved in potocytosis. We propose that infants with mutations in the folate receptor alpha gene might be at increased risk for congenital anomalies due to a reduced binding affinity for 5-methyltetrahydrofolate, the physiologic form of folic acid. Various experimental approaches to test the working hypothesis are considered.

Selective labelling of cell-surface polyamine-binding proteins on leukaemic and solid-tumour cell types using a new polyamine photoprobe

Polyamine transport is an active process which contributes to the regulation and maintenance of intracellular polyamine pools. Although the biochemical properties of polyamine transport in mammalian cells have been extensively studied, attempts to isolate and characterize the actual protein(s) have met with limited success. As one approach, photoaffinity labelling of cell surface proteins using a polyamine-conjugated photoprobe may lead to the identification of polyamine-binding proteins (pbps) associated with the transport apparatus and/or other regulatory responses. In a previous study [Felschow, MacDiarmid, Bardos, Wu, Woster and Porter (1995) J. Biol. Chem. 270, 28705-28711], we demonstrated that the photoprobes N4-ASA-spermidine and N1-ASA-norspermine [where the ASA (azidosalicylamidoethyl) group represents the photoreactive moiety] competed effectively with polyamines for transport and selectively labelled two major pbps at 118 and 50 kDa on the surface of murine and human leukaemia cells. In the present study, a new and more potent polyamine-conjugated photoprobe, N1-ASA-spermine, has been synthesized and used to develop a method based on detergent lysis for identifying putative cell-surface pbps on solid-tumour cell types. Transport kinetic assays showed that the new photoprobe competed with spermidine uptake with an apparent Ki of 1 microM, a value 20-50-fold lower than those of earlier probes. In L1210 cells, the new probe identified pbp50 and pbp118 thus reaffirming their identity as pbps. Two new bands were also detected. In A549 human lung adenocarcinoma cells, N1-ASA-spermine identified pbps at 39, 62, 73 and 130 kDa, the latter believed to be a size variant of pbp118. The presence of pbp130/118 in two very different cell types suggests the generality of the protein among mammalian cell types as well as its importance for further study. The high affinity of the photoprobe for the polyamine-transport system strongly suggests that at least some of the identified pbps may be associated with that function.

Regulation of carrier-mediated transport of folates and antifolates in methotrexate-sensitive and-resistant leukemia cells

Prolonged cell culture of human leukemia cells at folate concentrations in the (sub)physiological range (1-5 nM) rather than at 'standard' supraphysiological concentrations of 2-10 microM folic acid elicited a number of regulatory aspects of the reduced folate carrier (RFC), the membrane transport protein for natural reduced folate cofactors and folate-based chemotherapeutic drugs such as methotrexate (MTX). One subline of human CCRF-CEM leukemia cells grown under folate-restricted conditions (CEM-7A) exhibited a 95-fold increased Vmax for uptake of [3H]-MTX. The increased uptake of MTX in CEM-7A cells is based on at least two factors: (a) a constitutive 10-fold overexpression of the RFC1 gene and RFC1 message; and (b) a 7-9-fold up-regulation of RFC transport activity under low intracellular reduced folate concentrations. This second component appeared to be regulatable by changes in the cellular folate, purine and methylation status as judged from a 7-9 fold down-regulation of RFC transport activity after short term (1-2 hr) incubation of CEM-7A cells with reduced folate cofactors (25 nM LV), purines (100 microM adenosine) or S-adenosylmethionine (100 microM), respectively. Gradual folate restriction in the cell culture medium of CEM/MTX cells, a subline of CCRF-CEM resistant to MTX due to defective transport via the RFC, revealed the up-regulated expression of an altered RFC protein that is characterized by a 35-fold decreased Km for folic acid and a 10-fold decreased Km for the reduced folate cofactor LV compared to the RFC expressed in CCRF-CEM and CEM-7A cells. As a result of the markedly increased efficiency of folic acid uptake in CEM/MTX cells, intracellular folate pools were 7-fold higher than in CCRF-CEM cells when both cell lines were incubated in the presence of 2 microM folic acid. The high intracellular folate pools in CEM/MTX cells appeared to impair the polyglutamylation of antifolates and confer resistance to ZD1694, an antifolate drug that depends on polyglutamylation for its biological activity. Collectively, these studies provide a better insight into the basic regulation of RFC-mediated membrane transport of clinically active antifolates. In addition, these studies may also provide an opportunity to exploit the transport system as a target for biochemical modulation by which it may contribute to an improved efficacy of folate-based chemotherapy in a clinical setting.

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.

Methotrexate pharmacology and resistance in childhood acute lymphoblastic leukemia

Impressive gains have been made in the therapy of childhood acute lymphoblastic leukemia (ALL) in recent years such that remissions today are commonly achieved in up to 95% of patients and long term disease-free survival rates approach 70%. Methotrexate is a key component in ALL consolidation and maintenance therapies and is administered intrathecally in the prophylaxis and treatment of central nervous system leukemia. Critical determinants of methotrexate sensitivity and resistance (dihydrofolate reductase levels, methotrexate membrane transport, methotrexate polyglutamylation) previously described in cultured cells have recently been identified in lymphoblasts from children with ALL. Heterogenous expressions of increased dihydrofolate reductase or impaired methotrexate transport can be detected in both diagnostic and relapsed ALL specimens by flow cytometry with fluorescent methotrexate analogues. Lymphoblasts from children with ALL synthesize long chain polyglutamates and correlations have been established between the accumulation of methotrexate polyglutamates in ALL blasts and characteristic patient prognostic features. Variations in methotrexate polyglutamate accumulation may reflect changes in polyglutamate synthetic or degradative enzymes, or may be secondary to changes in methotrexate influx or dihydrofolate reductase levels. Other critical elements in treatment response to methotrexate include the dose and route of methotrexate administration, its catabolism to 7-hydroxymethotrexate, and the rate of methotrexate plasma clearance. A unique relationship exists between chromosome 21 and ALL leukemogenesis, and response to treatment including methotrexate. A better understanding of the molecular bases of methotrexate response and the development of methotrexate resistance in childhood ALL should facilitate further improvements in the effectiveness of methotrexate-based chemotherapy for this disease.

Endocytosis of GPI-linked membrane folate receptor-alpha

GPI-linked membrane folate receptors (MFRs) have been implicated in the receptor-mediated uptake of reduced folate cofactors and folate-based chemotherapeutic drugs. We have studied the biosynthetic transport to and internalization of MFR isoform alpha in KB-cells. MFR-alpha was synthesized as a 32-kD protein and converted in a maturely glycosylated 36-38-kD protein 1 h after synthesis. 32-kD MFR-alpha was completely soluble in Triton X-100 at 0 degree C. In contrast, only 33% of the 36-38-kD species could be solubilized at these conditions whereas complete solubilization was obtained in Triton X-100 at 37 degrees C or in the presence of saponin at 0 degree C. Similar solubilization characteristics were found when MFR-alpha at the plasma membrane was labeled with a crosslinkable 125I-labeled photoaffinity-analog of folic acid as a ligand. Triton X-100-insoluble membrane domains containing MFR-alpha could be separated from soluble MFR-alpha on sucrose flotation gradients. Only Triton X-100 soluble MFR-alpha was internalized from the plasma membrane. The reduced-folate-carrier, an integral membrane protein capable of translocating (anti-)folates across membranes, was completely excluded from the Triton X-100-resistant membrane domains. Internalized MFR-alpha recycled slowly to the cell surface during which it remained soluble in Triton X-100 at 0 degree C. Using immunoelectron microscopy, we found MFR-alpha along the entire endocytic pathway: in clathrin-coated buds and vesicles, and in small and large endosomal vacuoles. In conclusion, our data indicate that a large fraction, if not all, of internalizing MFR-alpha bypasses caveolae.

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.

Isolation of human cDNAs that restore methotrexate sensitivity and reduced folate carrier activity in methotrexate transport-defective Chinese hamster ovary cells

This report describes the isolation, nucleotide sequencing, and functional expression of human cDNAs that restore reduced folate carrier activity in transport-defective cells. Based on homology to a partial murine cDNA probe, two functional cDNAs were isolated from a lambda gt11 library prepared from methotrexate transport upregulated K562 cells (K562.4CF). A 2.8-kilobase (kb) clone, KS43, contained a 1776-base pair open reading frame. The 2.5-kb clone, KS32, contained an internal deletion (626 base pairs) resulting a shortened open reading frame and 3'-untranslated region. KS43 and KS32 encoded proteins with multiple hydrophobic domains, one consensus N-glycosylation site, and predicted molecular masses of 65 and 58 kDa, respectively. The deduced amino acid sequence of KS43 is 79% and 80% homologous to the mouse and hamster sequences, respectively (Dixon, K. H., Lanpher, B. C., Chiu, J., Kelley, K., and Cowan, K. H. (1994) J. Biol. Chem. 269, 17-20; Williams, F. M. R., Murray R. C., Underhill, T. M., and Flintoff, W. F. (1994) J. Biol. Chem. 269, 5810-5816). Northern blots identified one primary transcript at 3.1 kb in parental K562, K562.4CF, and transport-impaired K500E cells; transcript levels varied by 7-fold. The expression of both KS43 and KS32 in methotrexate transport-defective Chinese hamster ovary cells restored methotrexate sensitivity and transport. Certain transport characteristics of the transfected cells resembled both the wild type human (K562) and hamster "classical" reduced folate carriers, suggesting the expression of a hybrid system. For instance, based on Ki values, up to a 4-fold increased affinity for 1843U89 over wild type hamster cells (typical of human cells), and a 19-fold increased affinity for methotrexate over K562 cells (typical of hamster cells) was observed. Further, a photoaffinity probe with high specificity for the reduced folate carrier labeled 94-kDa proteins in K562 cells and the transfectant containing the full-length KS43, and a 85-kDa protein in the transfectant containing the 3'-truncated KS32. No specifically labeled proteins were detected in wild type or mock-transfected hamster cells. Collectively, our results suggest that the KS43/KS32 cDNAs encode the human reduced folate carrier; however, additional modulatory/regulatory factors may be required to manifest the full spectrum of transport substrate activities typical of this system.

Activation and cytotoxicity of 2-alpha-aminoacyl prodrugs of methotrexate

In an effort to improve the selectivity of the anticancer drug methotrexate (MTX), a series of potential prodrugs in which the 2-amino group was acylated with various alpha-amino acids (as well as L-pyroglutamic acid) was synthesized. Such derivatives are anticipated to be hydrolysed to MTX by appropriate aminopeptidases localized (over-expressed naturally or targeted as anti-tumor antibody conjugates) in the vicinity of the tumor. The L-leucyl, L-valyl, L-isoleucyl, D-alanyl and L-pyroglutamyl derivatives were assessed as to their suitability as prodrugs. Except for the L-pyroglutamyl compound, all derivatives decomposed slowly when incubated in phosphate buffer, pH 7.3; the formation of MTX was minimal. No major differences were observed when serum was included in the incubation medium, except for the L-leucyl compound, which was hydrolysed to MTX. The L-leucyl, L-valyl and L-isoleucyl derivatives were hydrolysed readily to MTX by aminopeptidase M (EC 3.4.11.2), while the L-pyroglutamyl and D-alanyl compounds were activated by pyroglutamate aminopeptidase (EC 3.4.19.3) (from Bacillus amyloliquefaciens) and D-aminopeptidase (from Ochrobactrum anthropi), respectively. When tested for inhibition of the target enzyme dihydrofolate reductase (DHFR; EC 1.5.1.3), 2-L-valyl-MTX showed inhibition two orders of magnitude poorer than that given by MTX, in agreement with the expectation that acylation of the 2-amino group reduces binding to DHFR. After treatment of this derivative with aminopeptidase M, the extent of inhibition correlated with the amount of MTX formed. MTX derivatives alone or in combination with the complementary peptidase were tested for cytotoxicity on murine L1210 cells in culture. The above-listed derivatives were considerably less cytotoxic than MTX, except for the L-leucyl derivative which showed considerable cytotoxicity. When the appropriate exogenous peptidase was included, the cytotoxicity of the activated prodrugs approached that of MTX. These results indicate that 2-L-leucyl-MTX is unsuitable as a prodrug since it is activated prematurely by serum enzymes. Although the L-valyl and L-isoleucyl derivatives do not hydrolyse to MTX in serum and are readily activated, they are not ideal prodrugs since they decompose under physiological conditions; the properties of the decomposition product will have a bearing on the ultimate suitability of these compounds. 2-L-Pyroglutamyl-MTX is the best candidate prodrug, showing stability and ready activation by the appropriate aminopeptidase.

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.

Role of N-glycosylation in the structure and function of the methotrexate membrane transporter from CCRF-CEM human lymphoblastic leukemia cells

The carrier protein for methotrexate and tetrahydrofolate cofactors (GP-MTX) in CCRF-CEM human lymphoblastic leukemia cells in a 117 kDa glycoprotein containing both N- and O-linked oligosaccharides (Matherly et al., J Biol Chem 267: 23253-23260, 1992). Tunicamycin, an inhibitor of N-glycosylation, was used to investigate the roles of asparagine-linked oligosaccharides in the structure, intracellular routing, and transport function of GP-MTX. Tunicamycin was growth inhibitory toward CCRF-CEM cells (IC50-0.80 micrograms/mL) and caused a potent suppression of [3H]mannose incorporation into nascent glycoproteins. From 1-3 micrograms/mL, inhibition of [3H]mannose incorporation was 66-87%, exceeding that for [35S]methionine incorporation by 2 to 4-fold. Tunicamycin (1 and 2 micrograms/mL) exposures decreased the median molecular masses of GP-MTX on immunoblots (to 82 and 67 kDa, respectively) and were accompanied by reduced maximal rates of methotrexate uptake (31 and 37%, respectively, of control levels). Conversely, the Ki values for methotrexate binding to the transporter were unaffected by tunicamycin treatments. The effects of tunicamycin on methotrexate influx closely correlated with lower levels of immunoreactive GP-MTX in plasma membranes and specific [3H]methotrexate binding to intact cells, suggesting that the transport effect was due to decreased numbers of carrier proteins at the membrane surface. The reduced molecular mass values for GP-MTX, which accompanied tunicamycin exposures, were further decreased (to 55 and 50 kDa at 1 and 2 micrograms/mL, respectively) by digestions with N-glycanase. Hence, despite the large loss of N-glycan from GP-MTX in tunicamycin-treated cells, residual core oligosaccharides remained. The sizes of hypoglycosylated GP-MTX following both treatments were similar to that of the functionally homologous methotrexate membrane carrier previously identified in L1210 murine leukemia cells.