Contrasting effects of oncogene expression on two carrier-mediated systems internalizing folate compounds in Fisher rat 3T3 cells

Humanizing mouse folate metabolism: conversion of the dual-promoter mouse folylpolyglutamate synthetase gene to the human single-promoter structure

The mouse is extensively used to model human folate metabolism and therapeutic outcomes with antifolates. However, the folylpoly-γ-glutamate synthetase (fpgs) gene, whose product determines folate/antifolate intracellular retention and antifolate antitumor activity, displays a pronounced species difference. The human gene uses only a single promoter, whereas the mouse uses two: P2, akin to the human promoter, at low levels in most tissues; and P1, an upstream promoter used extensively in liver and kidney. We deleted the mouse P1 promoter through homologous recombination to study the dual-promoter mouse system and to create a mouse with a humanized fpgs gene structure. Despite the loss of the predominant fpgs mRNA species in liver and kidney (representing 95 and 75% of fpgs transcripts in these tissues, respectively), P1-knockout mice developed and reproduced normally. The survival of these mice was explained by increased P2 transcription due to relief of transcriptional interference, by a 3-fold more efficient translation of P2-derived than P1-derived transcripts, and by 2-fold higher stability of P2-derived FPGS. In combination, all 3 effects reinstated FPGS function, even in liver. By eliminating mouse P1, we created a mouse model that mimicked the human housekeeping pattern of fpgs gene expression.

Biology of the major facilitative folate transporters SLC19A1 and SLC46A1

This chapter focuses on the biology of the major facilitative membrane folate transporters, the reduced folate carrier (RFC), and the proton-coupled folate transporter (PCFT). Folates are essential vitamins, and folate deficiency contributes to a variety of heath disorders. RFC is ubiquitously expressed and is the major folate transporter in mammalian cells and tissues. PCFT mediates intestinal absorption of dietary folates. Clinically relevant antifolates such as methotrexate (MTX) are transported by RFC, and the loss of RFC transport is an important mechanism of MTX resistance. PCFT is abundantly expressed in human tumors and is active under pH conditions associated with the tumor microenvironment. Pemetrexed (PMX) is an excellent substrate for PCFT as well as for RFC. Novel tumor-targeted antifolates related to PMX with selective membrane transport by PCFT over RFC are being developed. The molecular picture of RFC and PCFT continues to evolve relating to membrane topology, N-glycosylation, energetics, and identification of structurally and functionally important domains and amino acids. The molecular bases for MTX resistance associated with loss of RFC function, and for the rare autosomal recessive condition, hereditary folate malabsorption (HFM), attributable to mutant PCFT, have been established. From structural homologies to the bacterial transporters GlpT and LacY, homology models were developed for RFC and PCFT, enabling new mechanistic insights and experimentally testable hypotheses. RFC and PCFT exist as homo-oligomers, and evidence suggests that homo-oligomerization of RFC and PCFT monomeric proteins may be important for intracellular trafficking and/or transport function. Better understanding of the structure and function of RFC and PCFT should facilitate the rational development of new therapeutic strategies for cancer as well as for HFM.

Novel therapies for peripheral T-cell lymphomas

Peripheral T-cell lymphomas (PTCLs) are a diverse family of lymphoid neoplasms with poor prognosis. They represent approximately 6-10% of non-Hodgkin lymphomas with significant geographic variation. The median age at diagnosis varies with histology, however the majority of patients with PTCL are in their fifth or sixth decade of life. Until recently clinical development of new agents for PTCL was slow due to difficulties in making the correct diagnosis, lack of uniform classification and combination of rarity and biologic diversity of the group. In the last 5 years, significant advances were made to overcome these obstacles, leading to the approval of three new agents for relapsed and refractory PTCL by the Food and Drug Administration, based on well conducted prospective studies. Pralatrexate, a unique antifol, was the first agent granted approval, followed by romidepsin, a histone deacetylase inhibitor, and brentuximab vedotin, an immunoconjugate. Owing to the unique nature of these agents, durable responses were seen in patients with highly refractory disease, and some of these responses are long lasting after discontinuation of therapy. Accumulating data indicate that these novel agents have little cumulative toxicity and can be administered continuously to patients who are not candidates for consolidative stem-cell transplantation (SCT), with little impact on quality of life. They might also provide a new salvage option for patients eligible for SCT with no impact on autologous stem-cell collection or subsequent engraftment. New studies are underway to evaluate efficacy and safety of new agents in combination regimens for both newly diagnosed and relapsed/refractory PTCL. Several other investigational drugs showed promise in recent trials. This review focuses on novel therapies for T-cell lymphomas, their place in current treatment paradigms and future directions.

Hypermethylation of the human proton-coupled folate transporter (SLC46A1) minimal transcriptional regulatory region in an antifolate-resistant HeLa cell line

This laboratory recently identified a novel proton-coupled folate transporter (PCFT) that mediates intestinal folate absorption and transport of folates into the central nervous system. The present study focuses on the definition of the minimum transcriptional regulatory region of this gene in HeLa cells and the mechanism(s) underlying the loss of PCFT expression in the methotrexate-resistant HeLa R1-11 cell line. The PCFT transcriptional regulatory controls were localized between -42 and +96 bases from the transcriptional start site using a luciferase-reporter gene system. The promoter is a G + C rich region of 139 nucleotides contained in a CpG island. HeLa R1-11 cells have no mutations in the PCFT open reading frame and its promoter; the transcription/translation machinery is intact because transient transfections in HeLa R1-11 and wild-type HeLa cells produced similar luciferase activities. Hypermethylation at CpG sites within the minimal transcriptional regulatory region was shown in HeLa R1-11 cells as compared with the parental PCFT-competent HeLa cells, using bisulfite conversion and sequence analysis. Treatment with 5-aza-2'-deoxycytidine resulted in a substantial restoration of transport and PCFT mRNA expression and small but significant decreases in methylation in the promoter region. In vitro methylation of the transfected reporter plasmid inhibited luciferase gene expression. Cytogenetics/fluorescence in situ hybridization indicated a loss of half the PCFT gene copies in HeLa R1-11 as compared with PCFT-competent HeLa cells. Taken together, promoter silencing through methylation and gene copy loss accounted for the loss of PCFT activity in antifolate-resistant HeLa R1-11 cells.

Phase II-I-II study of two different doses and schedules of pralatrexate, a high-affinity substrate for the reduced folate carrier, in patients with relapsed or refractory lymphoma reveals marked activity in T-cell malignancies

PURPOSE

To determine the maximum-tolerated dose (MTD) and efficacy of pralatrexate in patients with lymphoma.

PATIENTS AND METHODS

Pralatrexate, initially given at a dose of 135 mg/m(2) on an every-other-week basis, was associated with stomatitis. A redesigned, weekly phase I/II study established an MTD of 30 mg/m(2) weekly for six weeks every 7 weeks. Patients were required to have relapsed/refractory disease, an absolute neutrophil greater than 1,000/microL, and a platelet count greater than 50,000/microL for the first dose of any cycle.

RESULTS

The every-other-week, phase II experience was associated with an increased risk of stomatitis and hematologic toxicity. On a weekly schedule, the MTD was 30 mg/m(2) weekly for 6 weeks every 7 weeks. This schedule modification resulted in a 50% reduction in the major hematologic toxicities and abrogation of the grades 3 to 4 stomatitis. Stomatitis was associated with elevated homocysteine and methylmalonic acid, which were reduced by folate and vitamin B12 supplementation. Of 48 assessable patients, the overall response rate was 31% (26% by intention to treat), including 17% who experienced complete remission (CR). When analyzed by lineage, the overall response rates were 10% and 54% in patients with B- and T-cell lymphomas, respectively. All eight patients who experienced CR had T-cell lymphoma, and four of the six patients with a partial remission were positron emission tomography negative. The duration of responses ranged from 3 to 26 months.

CONCLUSION

Pralatrexate has significant single-agent activity in patients with relapsed/refractory T-cell lymphoma.

Membrane transporters and folate homeostasis: intestinal absorption and transport into systemic compartments and tissues

Members of the family of B9 vitamins are commonly known as folates. They are derived entirely from dietary sources and are key one-carbon donors required for de novo nucleotide and methionine synthesis. These highly hydrophilic molecules use several genetically distinct and functionally diverse transport systems to enter cells: the reduced folate carrier, the proton-coupled folate transporter and the folate receptors. Each plays a unique role in mediating folate transport across epithelia and into systemic tissues. The mechanism of intestinal folate absorption was recently uncovered, revealing the genetic basis for the autosomal recessive disorder hereditary folate malabsorption, which results from loss-of-function mutations in the proton-coupled folate transporter gene. It is therefore now possible to piece together how these folate transporters contribute, both individually and collectively, to folate homeostasis in humans. This review focuses on the physiological roles of the major folate transporters, with a brief consideration of their impact on the pharmacological activities of antifolates.

The molecular identity and characterization of a Proton-coupled Folate Transporter--PCFT; biological ramifications and impact on the activity of pemetrexed

Membrane transport of folates is essential for the survival of all mammalian cells and transport of antifolates is an important determinant of antifolate activity. While a major focus of attention has been on transport mediated by the reduced folate carrier and folate receptors, a very prominent carrier-mediated folate transport activity has been recognized for decades with a low-pH optimum and substrate specificity distinct from that of the reduced folate carrier which operates most efficiently at neutral pH. This low-pH transporter represents the mechanism by which folates are absorbed in the small intestine and it is also widely expressed in other human tissues and solid tumors. Recently, this laboratory discovered the molecular identity of this transporter which is genetically unrelated to the reduced folate carrier. This transporter is proton-coupled, electrogenic, and manifests a substrate specificity that is similar to that of the low-pH transport activity previously described in mammalian cells. The key role this transporter plays in intestinal folate absorption has been confirmed by the demonstration of a mutation in this gene in the rare autosomal recessive disorder, hereditary folate malabsorption. This article reviews (1) the characteristics and prevalence of the low-pH folate transport activity, (2) its relationship to, and properties of, the recently identified Proton-Coupled Folate Transporter (PCFT), (3) the physiological and pharmacological roles of this transporter, particularly with respect to pemetrexed, and (4) the historical controversy, now resolved, on the mechanism of intestinal folate absorption.

The Schedule-Dependent Effects of the Novel Antifolate Pralatrexate and Gemcitabine Are Superior to Methotrexate and Cytarabine in Models of Human Non-Hodgkin's Lymphoma

PURPOSE

Methotrexate is known to synergize with cytarabine [1-beta-D-arabinofuranosylcytosine (ara-C)] in a schedule-dependent manner. The purpose of this article is to compare and contrast the activity of pralatrexate (10-propargyl-10-deazaminopterin)/gemcitabine to the standard combination of methotrexate/ara-C and to determine if schedule dependency of this combination is important in lymphoma.

EXPERIMENT DESIGN

Cytotoxicity assays using the standard trypan blue exclusion assay were used to explore the in vitro activity of pralatrexate and gemcitabine against a panel of lymphoma cell lines. Both severe combined imunodeficient beige and irradiated nonobese diabetic/severe combined imunodeficient mouse xenograft models were used to compare and contrast the in vivo activity of these combinations as a function of schedule. In addition, apoptosis assays were conducted.

RESULTS

Compared with methotrexate-containing combinations, pralatrexate plus gemcitabine combinations displayed improved therapeutic activity with some schedule dependency. The combination of pralatrexate and gemcitabine was superior to any methotrexate and ara-C combination in inducing apoptosis and in activating caspase-3. In vivo, the best therapeutic effects were obtained with the sequence of pralatrexate --> gemcitabine. Complete remissions were only appreciated in animals receiving pralatrexate followed by gemcitabine.

CONCLUSIONS

These data show that the combination of pralatrexate followed by gemcitabine was superior to methotrexate/ara-C in vitro and in vivo, and was far more potent in inducing apoptosis in a large B-cell lymphoma. These data provide strong rationale for further study of this combination in lymphomas where methotrexate and ara-C are used.

Comparison of folic acid uptake characteristics by human placental choriocarcinoma cells at acidic and physiological pH

The aim of this work was to characterize the placental uptake of folic acid from the maternal circulation. Using 2 human trophoblast cell lines (BeWo and JAR), we verified that uptake of3H-folic acid was pH-dependent, increasing significantly with decreasing extracellular pH. In BeWo cells, uptake of3H-folic acid at pH 5.5 was (i) Na+-independent; (ii) inhibited by folic acid, 5-methyltetrahydrofolate (5-MTHF), and methotrexate (MTX); (iii) inhibited by the anion transport inhibitors 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS) and 4-acetamido-4′-isothiocyano-2,2′-disulfonic acid stilbene (SITS); (iv) inhibited by the proton ionophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP); (v) not inhibited by blockers of receptor-mediated endocytosis (cytochalasin D and monensin); (vi) trans-inhibited by MTX and folic acid; and (vii) not affected by an anti-reduced folate transporter-1 (RFC) antibody. At pH 7.5, uptake of3H-folic acid was (i) Na+-independent; (ii) inhibited by folic acid and MTX, but not by 5-MTHF; (iii) inhibited by SITS, but not by DIDS; (iv) not affected by FCCP; (v) inhibited by monensin (but not by cytochalasin D); (vi) trans-inhibited by folic acid (but not by MTX); and (vii) inhibited by an anti-RFC antibody. In conclusion, in BeWo cells, both RFC and receptor-mediated endocytosis seem to be involved in3H-folic acid uptake at pH 7.5, whereas at pH 5.5, RFC and (or) a low pH-operating transporter distinct from RFC are involved.

Antifolate Resistance in a HeLa Cell Line Associated With Impaired Transport Independent of the Reduced Folate Carrier

Prior studies from this laboratory documented the prevalence of methotrexate (MTX) transport activity with a low pH optimum in human solid tumor cell lines. In HeLa cells, this low pH activity has high affinity for pemetrexed [PMX (Alimta)] and is reduced folate carrier (RFC)-independent because it is not diminished in a RFC-null subline (R5). R5 cells also have residual transport activity, with high specificity for PMX, at neutral pH. In the current study, a R5 subline, R1, was selected under MTX selective pressure at a modest reduction in pH. There was markedly decreased MTX and PMX transport at both pH 5.5 and pH 7.4. When MTX was removed, there was a slow return of transport activity, and when MTX was added back, there was loss of transport at both pH values within 8 weeks. In R1 cells, there was a marked decrease in accumulation of PMX, MTX, and folic acid along with a decrease in growth inhibition by these and other antifolates that require a facilitative process to gain entry into cells. These data demonstrate that (i) RFC-independent transport in HeLa cells at low and neutral pH contributes to antifolate activity (in particular, to PMX activity) and can be diminished by antifolate selective pressure and (ii) the loss of these activities results in marked resistance to PMX, an agent for which there is little or no loss of activity when transport mediated by RFC is abolished. These observations suggest that transport activity in RFC-null HeLa R5 cells at neutral and low pH may reflect the same carrier-mediated process.

A prominent low-pH methotrexate transport activity in human solid tumors: contribution to the preservation of methotrexate pharmacologic activity in HeLa cells lacking the reduced folate carrier

Whereas the major folate transporter, the reduced folate carrier (RFC), has a physiological pH optimum, transport activities for folates and antifolates have been detected with low pH optima. Because the interstitial pH in solid tumors is generally acidic, the mechanisms by which antifolates are transported at low pH could be an important determinant of drug activity under these conditions. The current study quantitated the low pH methotrexate (MTX) transport activity in human solid tumor cell lines from the National Cancer Institute tumor panel and other sources. MTX influx at pH 5.5 was equal to, or greater than, influx at pH 7.4 in 29 of 32 cell lines. To assess the role of RFC in transport at low pH in one of these cell lines, a HeLa clonal line (R5) was selected for MTX resistance due to a genomic deletion of the carrier gene. MTX influx was depressed by 70% in R5 versus wild-type HeLa cells at pH 7.4. At pH 6.5, influx in these two lines was similar; as the pH was decreased to 5.5 influx increased in both cell lines. Similarly, whereas net MTX uptake over 1 h was markedly decreased in R5 cells at pH 7.4, net uptake in HeLa and R5 cells was comparable at pH 6.5. Also, as compared with MCF7 breast cancer cells, MTX uptake was markedly decreased at pH 7.4, but only minimally at pH 6.5, in the MDA-MB-231 human breast cancer cell line that lacks RFC expression. When grown with folic acid (2 micro M) at pH 7.4, the IC(50) for MTX was 14-fold higher in R5 as compared with wild-type HeLa cells; the difference was only 4-fold when cells when grown at pH 6.9; the IC(50)s were identical at this pH when the medium folate was 25 nM 5-formyltetrahydrofolate. These data demonstrate that transport activity at low pH is prevalent in human solid tumors, is RFC-independent in R5 cells and MDA-MB-231 breast cancer cells, and can preserve MTX activity in the absence of RFC at an acidic pH relevant to solid tumors in vivo.

Characterization of a Folate Transporter in HeLa Cells with a Low pH Optimum and High Affinity for Pemetrexed Distinct from the Reduced Folate Carrier

Studies were undertaken to characterize a low pH transport activity in a reduced folate carrier (RFC)-null HeLa-derived cell line (R5). This transport activity has a 20-fold higher affinity for pemetrexed (PMX; Kt, approximately 45 nmol/L) than methotrexate (MTX; Kt, approximately 1 micromol/L) with comparable Vmax values. The Ki values for folic acid, ZD9331, and ZD1694 were approximately 400-600 nmol/L, and the Ki values for PT523, PT632, and trimetrexate were >50 micromol/L. The transporter is stereospecific and has a 7-fold higher affinity for the 6S isomer than the 6R isomer of 5-formyltetrahydrofolate but a 4-fold higher affinity for the 6R isomer than the 6S isomer of dideazatetrahydrofolic acid. Properties of RFC-independent transport were compared with transport mediated by RFC at low pH using HepG2 cells, with minimal constitutive low pH transport activity, transfected to high levels of RFC. MTX influx Kt was comparable at pH 7.4 and pH 5.5 (1.7 versus 3.8 micromol/L), but Vmax was decreased 4.5-fold. There was no difference in the Kt for PMX (approximately 1.2 micromol/L) or the Ki for folic acid (approximately 130 micromol/L) or PT523 ( approximately 0.2 micromol/L) at pH 7.4 and pH 5.5. MTX influx in R5 and HepG2 transfectants at pH 5.5 was trans-stimulated in cells loaded with 5-formyltetrahydrofolate, inhibited by Cl- (HepG2-B > R5), Na+ independent, and uninhibited by energy depletion. Hence, RFC-independent low pH transport activity in HeLa R5 cells is consistent with a carrier-mediated process with high affinity for PMX. Potential alterations in protonation of RFC or the folate molecule as a function of pH do not result in changes in affinity constants for antifolates. Whereas both activities at low pH have similarities, they can be distinguished by folic acid and PT523, agents for which they have very different structural specificities.

Preservation of folate transport activity with a low-pH optimum in rat IEC-6 intestinal epithelial cell lines that lack reduced folate carrier function

Intestinal folate transport has been well characterized, and rat small intestinal epithelial (IEC-6) cells have been used as a model system for the study of this process on the cellular level. The major intestinal folate transport activity has a low-pH optimum, and the current paradigm is that this process is mediated by the reduced folate carrier (RFC), despite the fact that this carrier has a neutral pH optimum in leukemia cells. The current study addressed the question of whether constitutive low-pH folate transport activity in IEC-6 cells is mediated by RFC. Two independent IEC-6 sublines, IEC-6/A4 and IEC-6/PT1, were generated by chemical mutagenesis followed by selective pressure with antifolates. In IEC-6/A4 cells, a premature stop resulted in truncation of RFC at Gln(420). A green fluorescent protein (GFP) fusion with the truncated protein was not stable. In IEC-6/PT1 cells, Ser(135) was deleted, and this alteration resulted in the failure of localization of the GFP fusion protein in the plasma membrane. In both cell lines, methotrexate (MTX) influx at neutral pH was markedly decreased compared with wild-type IEC-6 cells, but MTX influx at pH 5.5 was not depressed. Transient transfection of the GFP-mutated RFC constructs into RFC-null HeLa cells confirmed their lack of transport function. These results indicate that in IEC-6 cells, folate transport at neutral pH is mediated predominantly by RFC; however, the folate transport activity at pH 5.5 is RFC independent. Hence, constitutive folate transport activity with a low-pH optimum in this intestinal cell model is mediated by a process entirely distinct from that of RFC.

Resistance to antifolates

The antifolates were the first class of antimetabolites to enter the clinics more than 50 years ago. Over the following decades, a full understanding of their mechanisms of action and chemotherapeutic potential evolved along with the mechanisms by which cells develop resistance to these drugs. These principals served as a basis for the subsequent exploration and understanding of the mechanisms of resistance to a variety of diverse antineoplastics with different cellular targets. This section describes the bases for intrinsic and acquired antifolate resistance within the context of the current understanding of the mechanisms of actions and cytotoxic determinants of these agents. This encompasses impaired drug transport into cells, augmented drug export, impaired activation of antifolates through polyglutamylation, augmented hydrolysis of antifolate polyglutamates, increased expression and mutation of target enzymes, and the augmentation of cellular tetrahydrofolate-cofactor pools in cells. This chapter also describes how these insights are being utilized to develop gene therapy approaches to protect normal bone marrow progenitor cells as a strategy to improve the efficacy of bone marrow transplantation. Finally, clinical studies are reviewed that correlate the cellular pharmacology of methotrexate with the clinical outcome in children with neoplastic diseases treated with this antifolate.

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.

Expression of the reduced folate carrier SLC19A1 in IEC-6 cells results in two distinct transport activities

Intestinal absorption of folates has been characterized as a facilitative process with a low pH optimum. Studies with intestinal epithelial cells have suggested that this activity is mediated by the reduced folate carrier (RFC1). In this paper, we report on folate transport characteristics in an immortalized rat IEC-6 cell line that was found to exhibit the predominant influx activity for methotrexate (MTX) at pH 5.5 with a low level of activity at pH 7.4. Transfection of this cell line with an RFC1 construct resulted in clones exhibiting increased MTX uptake at both the pHs and high folic acid uptake only at the low pH. For the two clones with the highest level of transport activity, relative MTX influx at the two pHs was reversed. Moreover, the low pH MTX influx activity ([MTX](e) = 0.5 microM) was markedly inhibited by 20 microM folic acid while influx at neutral pH was not. Furthermore, in the presence and absence of glucose at low pH, MTX and folic acid influx activity was inhibited by azide, while MTX influx at pH 7.4 was stimulated by azide in the absence of glucose but was unchanged in the presence of glucose and azide. This was contrasted with the results of transfection of the same RFC1 construct into an L1210 murine leukemia cell line bearing a nonfunctional endogenous carrier. In this case, the activity expressed was only at pH 7.4. These data indicate that RFC1 can exhibit two distinct types of folate transport activities in intestinal cells that must depend on tissue-specific modulators.