Effects of metabolic deprivation on methotrexate transport in L1210 leukemia cells: further evidence for separate influx and efflux systems with different energetic requirements

The Concept of Folic Acid in Health and Disease

Folates have a pterine core structure and high metabolic activity due to their ability to accept electrons and react with O-, S-, N-, C-bounds. Folates play a role as cofactors in essential one-carbon pathways donating methyl-groups to choline phospholipids, creatine, epinephrine, DNA. Compounds similar to folates are ubiquitous and have been found in different animals, plants, and microorganisms. Folates enter the body from the diet and are also synthesized by intestinal bacteria with consequent adsorption from the colon. Three types of folate and antifolate cellular transporters have been found, differing in tissue localization, substrate affinity, type of transferring, and optimal pH for function. Laboratory criteria of folate deficiency are accepted by WHO. Severe folate deficiencies, manifesting in early life, are seen in hereditary folate malabsorption and cerebral folate deficiency. Acquired folate deficiency is quite common and is associated with poor diet and malabsorption, alcohol consumption, obesity, and kidney failure. Given the observational data that folates have a protective effect against neural tube defects, ischemic events, and cancer, food folic acid fortification was introduced in many countries. However, high physiological folate concentrations and folate overload may increase the risk of impaired brain development in embryogenesis and possess a growth advantage for precancerous altered cells.

Folate and thiamine transporters mediated by facilitative carriers (SLC19A1-3 and SLC46A1) and folate receptors

The reduced folate carrier (RFC, SLC19A1), thiamine transporter-1 (ThTr1, SLC19A2) and thiamine transporter-2 (ThTr2, SLC19A3) evolved from the same family of solute carriers. SLC19A1 transports folates but not thiamine. SLC19A2 and SLC19A3 transport thiamine but not folates. SLC19A1 and SLC19A2 deliver their substrates to systemic tissues; SLC19A3 mediates intestinal thiamine absorption. The proton-coupled folate transporter (PCFT, SLC46A1) is the mechanism by which folates are absorbed across the apical-brush-border membrane of the proximal small intestine. Two folate receptors (FOLR1 and FOLR2) mediate folate transport across epithelia by an endocytic process. Folate transporters are routes of delivery of drugs for the treatment of cancer and inflammatory diseases. There are autosomal recessive disorders associated with mutations in genes encoded for SLC46A1 (hereditary folate malabsorption), FOLR1 (cerebral folate deficiency), SLC19A2 (thiamine-responsive megaloblastic anemia), and SLC19A3 (biotin-responsive basal ganglia disease).

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.

Molecular basis of antifolate resistance

Folates play a key role in one-carbon metabolism essential for the biosynthesis of purines, thymidylate and hence DNA replication. The antifolate methotrexate has been rationally-designed nearly 60 years ago to potently block the folate-dependent enzyme dihydrofolate reductase (DHFR) thereby achieving temporary remissions in childhood acute leukemia. Recently, the novel antifolates raltitrexed and pemetrexed that target thymidylate synthase (TS) and glycineamide ribonucleotide transformylase (GARTF) were introduced for the treatment of colorectal cancer and malignant pleural mesothelioma. (Anti)folates are divalent anions which predominantly use the reduced folate carrier (RFC) for their cellular uptake. (Anti)folates are retained intracellularly via polyglutamylation catalyzed by folylpoly-gamma-glutamate synthetase (FPGS). As the intracellular concentration of antifolates is critical for their pharmacologic activity, polyglutamylation is a key determinant of antifolate cytotoxicity. However, anticancer drug resistance phenomena pose major obstacles towards curative cancer chemotherapy. Pre-clinical and clinical studies have identified a plethora of mechanisms of antifolate-resistance; these are frequently associated with qualitative and/or quantitative alterations in influx and/or efflux transporters of (anti)folates as well as in folate-dependent enzymes. These include inactivating mutations and/or down-regulation of the RFC and various alterations in the target enzymes DHFR, TS and FPGS. Furthermore, it has been recently shown that members of the ATP-binding cassette (ABC) superfamily including multidrug resistance proteins (MRP/ABCC) and breast cancer resistance protein (BCRP/ABCG2) are low affinity, high capacity ATP-driven (anti)folate efflux transporters. This transport activity is in addition to their established facility to extrude multiple cytotoxic agents. Hence, by actively extruding antifolates, overexpressed MRPs and/or BCRP confer antifolate resistance. Moreover, down-regulation of MRPs and/or BCRP results in decreased folate efflux thereby leading to expansion of the intracellular folate pool and antifolate resistance. This chapter reviews and discusses the panoply of molecular modalities of antifolate-resistance in pre-clinical tumor cell systems in vitro and in vivo as well as in cancer patients. Currently emerging novel strategies for the overcoming of antifolate-resistance are presented. Finally, experimental evidence is provided that the identification and characterization of the molecular mechanisms of antifolate-resistance may prove instrumental in the future development of rationally-based novel antifolates and strategies that could conceivably overcome drug-resistance phenomena.

The role of multidrug resistance efflux transporters in antifolate resistance and folate homeostasis

Members of the ATP-binding cassette (ABC) transporters including P-glycoprotein (Pgp/ABCB1), multidrug resistance proteins (MRPs/ABCC) as well as breast cancer resistance protein (BCRP/ABCG2) function as ATP-dependent drug efflux transporters, which form a unique defense network against multiple chemotherapeutic drugs and cellular toxins. Among antitumor agents is the important group of folic acid antimetabolites known as antifolates. Antifolates such as methotrexate (MTX), pemetrexed and raltitrexed exert their cytotoxic activity via potent inhibition of folate-dependent enzymes essential for purine and pyrimidine nucleotide biosynthesis and thereby block DNA replication. Overexpression of MRPs and BCRP confers resistance upon malignant cells to various hydrophilic and lipophilic antifolates. Apart from their central role in mediating resistance to antifolates and other anticancer drugs, MRPs and BCRP have been recently shown to transport naturally occurring reduced folates. This was inferred from various complementary systems as follows: (a) Cell-free systems including ATP-dependent uptake of radiolabeled folate/MTX into purified inside-out membrane vesicles from stable transfectants and/or cells overexpressing these transporters, (b) Decreased accumulation of radiolabeled folate/MTX in cultured tumor cells overexpressing these transporters, as well as (c) In vivo rodent models such as Eisi hyperbillirubinemic rats (EHBR) that hereditarily lack MRP2 in their canalicular membrane and thereby display a bile that is highly deficient in various reduced folate cofactors and MTX, when compared with wild type Sprague-Dawley (SD) rats. In all cases, these folate/antifolate transporters functioned as high capacity, low affinity ATP-driven exporters. While the mechanism of cellular retention of (anti)folates is mediated via (anti)folylpolyglutamylation, certain efflux transporters including MRP5 (ABCC5) and BCRP were shown to transport both mono-, di- as well as triglutamate derivatives of MTX and folic acid. Furthermore, overexpression of MRPs and BCRP has been shown to result in decreased cellular folate pools, whereas loss of ABC transporter expression brought about a significant expansion in the intracellular reduced folate pool. The latter finding has important implications to antifolate-based chemotherapy as an augmented cellular folate pool results in a significant level of resistance to certain antifolates. Hence, the aims of the present review are: (a) To summarize and discuss the cumulative evidence supporting a functional role for various multidrug resistance efflux transporters of the ABC superfamily which mediate resistance to hydrophilic and lipophilic antifolates, (b) To describe and evaluate the recent data suggesting a role for these efflux transporters in regulation of cellular folate homeostasis under folate replete and deplete conditions. Furthermore, novel developments and future perspectives regarding the identification of novel antifolate target proteins and mechanisms of action, as well as rationally designed emerging drug combinations containing antifolates along with receptor tyrosine kinase inhibitors are being discussed.

A phase I clinical pharmacologic study of pralatrexate in combination with probenecid in adults with advanced solid tumors

PURPOSE

The antifolate pralatrexate (10-propargyl-10-deazaaminopterin, PDX) demonstrates greater in vitro and in vivo antitumor efficacy than methotrexate. Preclinical models indicated that the efficacy of pralatrexate may be enhanced by coadministration with probenecid. The aim of this phase I study was to determine the maximum-tolerated dose of pralatrexate when combined with probenecid given every 2 weeks in humans.

METHODS

The starting dose was pralatrexate 40 mg/m(2) intravenously and probenecid 70 mg/m(2) intravenously administered every 14 days, where one cycle of treatment was every 28 days. The pralatrexate dose was initially fixed while probenecid dose escalation was explored. The pralatrexate area under the curve (AUC), terminal-half life (t1/2), and maximum plasma concentration (Cmax) were determined in cycle 1.

RESULTS

Seventeen patients with advanced solid tumors were treated with a median of two prior chemotherapy regimens. Stomatitis was dose-limiting with pralatrexate 40 mg/m(2) and probenecid 233 mg/m(2). Mean pralatrexate AUC and half life (t1/2) increased with increasing doses of probenecid. No objective responses were seen.

CONCLUSION

For patients with advanced solid tumors, the maximum-tolerated dose of this drug combination was pralatrexate 40 mg/m(2) and probenecid 140 mg/m(2). Vitamin B(12) and folate supplementation may allow for further dose escalation of pralatrexate and probenecid. This is a suitable question for a future study.

Lack of an effect of breast cancer resistance protein (BCRP/ABCG2) overexpression on methotrexate polyglutamate export and folate accumulation in a human breast cancer cell line

Accumulation of methotrexate (MTX) and its polyglutamates (PGs) has been recognized as an important factor in MTX efficacy. We have previously described a multidrug-resistant human breast cancer cell line, MCF7/MX, that exhibits reduced accumulation of total MTX as well as MTX-PGs, and that is resistant to continuous MTX exposure [Volk EL, Rohde K, Rhee M, McGuire JJ, Doyle LA, Ross DD, et al. Methotrexate cross-resistance in a mitoxantrone selected multidrug-resistant MCF7 breast cancer cell line is due to enhanced energy-dependent drug efflux. Cancer Res 2000;60:3514-21]. These cells express high levels of the breast cancer resistance protein (BCRP/ABCG2) that has been shown to actively transport MTX and short-chain MTX-PGs in vitro. However, the effect of BCRP on MTX-PG accumulation in intact cells was unclear. Here, we show that MTX transport by BCRP is required for the observed lower levels of MTX-PGs in the resistant cells. When BCRP was inhibited with fumitremorgin C, or in cells expressing a mutated form of BCRP that is unable to transport MTX, MTX-PG accumulation was similar or even higher than that in the parental cells that do not express BCRP. Concomitantly, there was increased inhibition of thymidylate synthase. It had previously been suggested that BCRP-mediated efflux of MTX-PGs contributed to the reduced MTX-PG accumulation. However, we found no evidence of BCRP-mediated efflux of MTX-PGs from intact cells, suggesting that direct efflux of MTX-PGs does not play a major role in MTX resistance. Together, these data show that BCRP overexpression can cause a reduction in total MTX accumulation as well as a reduction in the proportion of long-chain MTX-PGs. In contrast, BCRP overexpression did not affect natural folate accumulation or the relative distribution of folylpolyglutamates in the resistant, as compared to the parental, cells. Thus, it appears that BCRP overexpression affects the metabolism of the antifolate MTX, but not that of natural folates, although indirect effects cannot be excluded.

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.

The role of multidrug resistance proteins MRP1, MRP2 and MRP3 in cellular folate homeostasis

Previously, we reported that the multidrug resistance proteins MRP1, MRP2 and MRP3 confer resistance to therapeutic antifolates by mediating their cellular extrusion. We now determined whether MRPs also play a role in controlling cellular homeostasis of natural folates. In MRP1, MRP2 and MRP3-transfected 2008 human ovarian carcinoma cells total cellular folate content was 32-38% lower than in 2008 cells (105+/-14pmolfolate/mgprotein) when grown in medium containing 2.3 microM folic acid (FA). Under these conditions cellular growth rates were not compromised. However, when cells were challenged under folate-depleted conditions with a short exposure (4 hr) to FA or leucovorin, MRP1 and MRP3 overexpressing cells were impaired in their growth. In contrast to wild-type cells, MRP1 transfected cells retained only 60% of the maximum growth when exposed to 500 nM leucovorin or 500 microM FA. For 2008/MRP1 and 2008/MRP3 cells FA growth stimulation capacity was dramatically decreased when, during a 4 hr exposure, metabolism into rapidly polyglutamatable and retainable dihydrofolate was blocked by the dihydrofolate reductase inhibitor trimetrexate. To retain growth under such conditions MRP1 overexpressing cells required much higher concentrations of FA (EC(50) > 500 microM) compared to 2008 cells (EC(50): 12 microM). These results suggest that down- and up-regulation of MRP1 (and MRP3) expression can influence cellular folate homeostasis, in particular when cellular retention by polyglutamylation of folates is attenuated.

Loss of multidrug resistance protein 1 expression and folate efflux activity results in a highly concentrative folate transport in human leukemia cells

We studied the molecular basis of the up to 46-fold increased accumulation of folates and methotrexate (MTX) in human leukemia CEM-7A cells established by gradual deprivation of leucovorin (LCV). CEM-7A cells consequently exhibited 10- and 68-fold decreased LCV and folic acid growth requirements and 23-25-fold hypersensitivity to MTX and edatrexate. Although CEM-7A cells displayed a 74-86-fold increase in the reduced folate carrier (RFC)-mediated influx of LCV and MTX, RFC overexpression per se cannot induce a prominently increased folate/MTX accumulation because RFC functions as a nonconcentrative anion exchanger. We therefore explored the possibility that folate efflux activity mediated by members of the multidrug resistance protein (MRP) family was impaired in CEM-7A cells. Parental CEM cells expressed substantial levels of MRP1, MRP4, poor MRP5 levels, whereas MRP2, MRP3 and breast cancer resistance protein were undetectable. In contrast, CEM-7A cells lost 95% of MRP1 levels while retaining parental expression of MRP4 and MRP5. Consequently, CEM-7A cells displayed a 5-fold decrease in the [(3)H]folic acid efflux rate constant, which was identical to that obtained with parental CEM cells, when their folic acid efflux was blocked (78%) with probenecid. Furthermore, when compared with parental CEM, CEM-7A cells accumulated 2-fold more calcein fluorescence. Treatment of parental cells with the MRP1 efflux inhibitors MK571 and probenecid resulted in a 60-100% increase in calcein fluorescence. In contrast, these inhibitors failed to alter the calcein fluorescence in CEM-7A cells, which markedly lost MRP1 expression. Replenishment of LCV in the growth medium of CEM-7A cells resulted in resumption of normal MRP1 expression. These results establish for the first time that MRP1 is the primary folate efflux route in CEM leukemia cells and that the loss of folate efflux activity is an efficient means of markedly augmenting cellular folate pools. These findings suggest a functional role for MRP1 in the maintenance of cellular folate homeostasis.

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.

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.

Structural preferences among folate compounds and their analogues for ATPase-mediated efflux by inside-out plasma membrane vesicles derived from L1210 cells

Our prior studies with inside-out plasma membrane vesicles from L1210 cells (Schlemmer SR and Sirotnak FM, J Biol Chem 267: 14746-14752, 1992) identified an outwardly directed, translocating ATPase as mediating the majority of methotrexate (MTX) efflux in these cells. In the current studies, we examined by competitive inhibition with [3H]MTX as permeant some of the structural features that determine preferences among folate compounds and their analogues as permeants for this ATPase. The results show that folate compounds are preferred over simple quinazolines (5,8-dideaza-pteridines), and IL5-CH3-folateH4, and probably other 5-substituted folates are preferred over folic acid. In the latter regard, the observed equivalence in preference to IL5-CH3-folateH4 of the 4-oxa-pyridopyrimidine, lometrexol (DDATHF), probably relates to its close similarity to folateH4. The results also suggest that the 4-position in the case of folate analogues, but not in the case of the quinazoline analogues, is an important determinant with 4-amino preferred over 4-oxa. They also suggest that the N10 position on the bridge region in both series of compounds, and probably for the pyridopyrimidine lometrexol, is not an important determinant. In contrast to results seen with the simple quinazolines, the 2-CH3 desamino quinazoline ZEN D1694, modified as well by a 2-benzyl to thienyl replacement on the side chain, was highly preferred. The same relative differences seen among some of these analogues as inhibitors of [3H]MTX efflux in inside-out vesicles were documented for their effectiveness as permeants for ATP-dependent efflux in intact L1210 cells.

Inhibition of tumoral cell respiration and growth by nordihydroguaiaretic acid

The effects of nordihydroguaiaretic acid (NDGA), best known as an inhibitor of lipoxygenase activities, on the culture growth, oxygen consumption, ATP level, viability, and redox state of some electron carriers of intact TA3 and 786A ascites tumor cells have been studied. NDGA inhibited the respiration rate of these two tumor cell lines by preventing electron flow through the respiratory chain. Consequently, ATP levels, cell viability and culture growth rates were decreased. NDGA did not noticeably inhibit electron flow through both cytochrome oxidase and ubiquinone-cytochrome b-c1 complex. Also, the presence of NDGA changed to redox state of NAD(P)+ to a more reduced level, and the redox states of ubiquinone, cytochrome b and cytochromes c + c1 changed to a more oxidized level. These observations suggest that the electron transport in the tumor mitochondria was inhibited by NDGA at the NADH-dehydrogenase-ubiquinone level (energy-conserving site 1). As a consequence, mitochondrial ATP synthesis would be interrupted. This event could be related to the cytotoxic effect of NDGA.

Separation and inhibitor specificity of a second unidirectional efflux route for methotrexate in L1210 cells

L1210 cells mediate the unidirectional and energy-dependent efflux of methotrexate. Efflux occurs primarily via a system which has a high sensitivity to prostaglandin A1, vincristine, reserpine, verapamil, and bromosulfophthalein, but evidence has also been obtained for a second efflux component with a lower response to these inhibitors. Pretreatment of L1210 cells with low concentrations of vincristine reduces methotrexate efflux by three fold and uncovers a second efflux component with an inhibitor specificity which is distinctly different from the primary efflux route. Vincristine treatment increased by 8-20-fold the concentration required for half-maximal efflux inhibition by prostaglandin A1, reserpine, bromosulfophthalein, and verapamil but had no effect on inhibition by probenecid, quinidine, or carbonylcyanide m-chlorophenylhydrazone. A selective block in the primary efflux system and retention of the second component was also achieved in cells exposed to low concentrations of prostaglandin A1 or bromosulfophthalein. These results support prior conclusions that L1210 cells contain both a primary and secondary unidirectional efflux route for methotrexate. The second system has been difficult to detect and quantitate since it comprises only 25% of total unidirectional efflux and shows a relatively low response to various efflux inhibitors.

Characterization by flow cytometry of fluorescein-methotrexate transport in Chinese hamster ovary cells

We have studied by flow cytometry the transport of fluorescein-methotrexate in Chinese hamster ovary cells. Fluorescein-methotrexate appears to enter cells via a mechanism different from the carrier-mediated system for methotrexate. This conclusion is supported by the following observations: 1) Fluorescein-methotrexate is transported equally well into normal and mutant cells defective in the inward methotrexate uptake. 2) Folic acid and its reduced states, which competitively inhibit methotrexate uptake, do not alter fluorescein-methotrexate transport. 3) Fluorescein-methotrexate accumulation exhibits a low temperature coefficient (Q10 = 1.6) compared with the influx of methotrexate (Q10 = 6-8). 4) Initial rates of fluorescein-methotrexate uptake are concentration dependent but are not saturable. 5) Fluorescein-methotrexate uptake is very slow and reaches steady state after 8 h, whereas at an equimolar concentration methotrexate reaches saturation after 20 min. 6) Initial influx rates of fluorescein-methotrexate are not affected by the presence of methotrexate. 7) Sulfhydryl-reactive mercurials, which block methotrexate transport, do not reduce fluorescein-methotrexate influx, but rather stimulate it. Thus, based on the nonsaturability of fluorescein-methotrexate inward transport, its low temperature coefficient, and lack of inhibition with structural analogs, we conclude that fluorescein-methotrexate is accumulated in hamster cells by a passive diffusion process.

Mediation of cellular anion detoxification in leukemic cells by unidirectional efflux pumps

Methotrexate influx in L1210 cells is mediated almost entirely by a single system, whereas efflux occurs via three routes, the influx carrier functioning in reverse and two additional systems which are operationally unidirectional. The two unidirectional routes show considerable similarities in energy-dependence and inhibitor sensitivities but can be separated by their differential inhibition by bromosulfophthalein (BSP), probenecid, and vincristine. The predominant route is inhibited by BSP and vincristine, whereas the other route is sensitive to probenecid. A search for additional anion substrates for the unidirectional efflux systems for methotrexate led to the finding that cholate exits L1210 cells via the same two unidirectional efflux systems as methotrexate. Cholate efflux is carrier-mediated, energy-dependent, and unidirectional, and it can be inhibited by several compounds which inhibit the efflux of methotrexate. Moreover, the same concentration of each compound which produced a half-maximal inhibition of methotrexate efflux also inhibited the efflux of cholate by 50%. Cholate efflux occurred predominantly (85%) via the BSP-sensitive route. The observation that methotrexate and cholate share the same efflux systems in L1210 cells suggest that perhaps other large anions are also accommodated by these systems since methotrexate and cholate differ in net negative charge and have few common structural features. The hypothesis advanced from these studies has been that the function of the unidirectional efflux systems is to extrude various organic anion catabolites which might otherwise become toxic if allowed to accumulate within cells. Possible intracellular anions in this latter category include sulfonated or carboxylated steroids, bilirubin, and products of vitamin D and prostaglandin catabolism. Unidirectional efflux pumps have been identified for other anionic compounds including cyclic AMP and oxidized glutathione, and in both cases similarities were apparent with the efflux of methotrexate and cholate. The most compelling comparison was with prostaglandin A1 which inhibited the efflux of cyclic AMP, methotrexate, and cholate half-maximally at the same low concentration of 0.1 microM. Energy-dependent unidirectional efflux pumps for large neutral or cationic drugs have also been identified in cells with acquired multidrug resistance. The latter efflux activity is thought to be mediated by a membrane glycoprotein (p170) which is also sensitive to several of the various inhibitors (reserpine, verapamil, and quinidine) which reduce the efflux of methotrexate and cholate.

Methotrexate: studies on the cellular metabolism. I. Effect on mitochondrial oxygen uptake and oxidative phosphorylation

Effect of methotrexate (MTX) on mitochondrial oxygen uptake, oxidative phosphorylation and on the activity of several enzymes linked to respiratory chain was studied. MTX was able to inhibit state III respiration activated by ADP and to decrease the respiratory coefficient with the substrates alpha-ketoglutarate and glutamate; these effects became pronounced when mitochondria were pre-incubated with MTX for 10 min. No effect was observed on ATPase activity of undamaged or broken mitochondria; the same was true for NADH-oxidase, NADH-dehydrogenase, NADH-cytochrome c reductase, succinate oxidase, and cytochrome c oxidase activity. The effect on the steady-state of cytochrome b, as well as, the inhibitory effect on state III of respiration with NAD+-linked substrates, offers a reasonable possibility to suggesting that the inhibition site of MTX could be in a place anterior to cytochrome b region, and not linked to respiratory chain.

Fluorometric high-performance liquid chromatographic analysis of 10-deazaaminopterin, 10-ethyl-10-deazaaminopterin, and known metabolites

The antifolate compounds 10-deazaaminopterin (10-dAM) and 10-ethyl-10-deazaaminopterin (10-EdAM) are therapeutically superior to methotrexate in transplanted murine tumor systems and in human tumor xenografts growing in immunodeficient "nude" mice. The increased therapeutic index of these analogs correlates with their selective uptake, retention, and polyglutamation within neoplastic cells. We have developed a fluorescence high-performance liquid chromatographic assay applicable to 10-dAM, 10-EdAM, their polyglutamate anabolites, and their 7-hydroxy (7-OH) and deglutamate catabolites. The assay is based upon the high native fluorescence of pteridine-containing compounds which contain carbon in the 10 position. The assay employs a reverse-phase C-18 column and an ascending acetonitrile gradient in 50 mM phosphate, pH 7.0. The compounds are extracted from plasma and urine with 95 +/- 7% and 98 +/- 2% recoveries, respectively, using C-18 Sep-Paks. The linear range of the assay is, for 10-dAM, 2-100 nM, and for 10-EdAM, 1-100 nM. Polyglutamated metabolites of [3H]10-EdAM isolated from L1210 cells have been separated by HPLC with identification of five derivatives (Glu 1-5) confirmed by enzymatic peak shift using serum conjugase and by quantitative correlation of fluorescence intensity, radioactivity, and titration inhibition of dihydrofolate reductase. The assay has been used successfully in pharmacokinetic analyses of plasma and urine samples from patients receiving 10-dAM and 10-EdAM. In patients who had received 10-EdAM, 7-OH-10-EdAM, and the deglutamate catabolite were also detected. This HPLC fluorescence assay is superior to the dihydrofolate reductase inhibition and binding assays with regard to specificity and precision; moreover, it can provide a means for simultaneous assay of the physiologically important anabolites and catabolites of these new antifolates.

Characterization of the multiple transport routes for methotrexate in L1210 cells using phthalate as a model anion substrate

o-Phthalate is actively transported into L1210 cells and the primary route for cell entry is the same transport system which mediates the influx of methotrexate and other folate compounds. The identity of the influx route has been established by the following observations: (A) Phthalate influx is competitively inhibited by methotrexate and the inhibition constant (Ki) is comparable to the Kt for half-maximal influx of methotrexate: (B) Various anions inhibit the influx of phthalate and methotrexate with comparable Ki values; (C) The influx of phthalate and methotrexate both fluctuate in parallel with changes in the anionic composition of the external medium; and (D) A specific covalent inhibitor of the methotrexate transport system (NHS-methotrexate) also blocks the transport of phthalate. In contrast, the efflux of phthalate does not occur via the methotrexate influx carrier, but rather by two separate processes which can be distinguished by their sensitivities to bromosulfophthalein. Efflux via the bromosulfophthalein-sensitive route constitutes 75% of total efflux and is enhanced by glucose and inhibited by oligomycin. The inability of phthalate to exit via the methotrexate influx carrier is due to competing intracellular anions which prevent phthalate from interacting with the methotrexate binding site at the inner membrane surface.

Inhibitory effects of probenecid on the individual transport routes which mediate the influx and efflux of methotrexate in L1210 cells

L1210 cells contain a single transport system which mediates the influx of methotrexate and at least three routes for drug efflux [G. B. Henderson and E. M. Zevely, J. biol. Chem. 259, 1526 (1984)]; each of these processes is sensitive to probenecid. The influx carrier was inhibited reversibly and completely by probenecid with a Ki of 0.25 mM, while efflux via the same system was relatively unaffected by this compound (50% inhibition above 2.0 mM). The two remaining efflux routes (which do not contribute to methotrexate influx) showed a much higher sensitivity to probenecid. Efflux via these components was reduced half-maximally at probenecid concentrations of 0.08 and 0.22 mM, respectively, and a complete block was achieved with excess amounts (2.0 mM) of the inhibitor. Intracellular levels of ATP, glucose metabolism, and the membrane potential were also reduced by probenecid, indicating that the mechanism for inhibiting methotrexate efflux may involve the ability of probenecid to act as a metabolic inhibitor. Probenecid may have a broad capacity for inhibiting anion transport processes since it also reduced sulfate influx and efflux via the general anion carrier system.

Effect of depletion of cellular glutathione on methotrexate influx, efflux and retention in hepatocytes

In isolated hepatocytes the influence of cellular glutathione (GSH) on initial influx, net uptake and efflux of methotrexate (MTX) was determined. Endogenous glutathione in rat liver cells was depleted by either fasting of rats or by in vivo administration of phorone prior to cell preparation. The initial rate of influx of MTX was found to be higher in hepatocytes of fasted and phorone-treated rats than in those of untreated, fed control rats. The Km values for the methotrexate influx in GSH-deficient hepatocytes were up to 3 times lower than in normal cells, whereas Vmax remained unchanged. These results disclose an increased efficiency of the MTX transport system in cells with diminished cellular GSH levels. On the other hand, titration of external membrane SH groups by 203Hg p-CMBS revealed up to three times higher amounts of free SH groups on cells from starved and phorone-treated rats than on hepatocytes of fed rats. Increased efficiency of the MTX transport system in GSH-deficient cells may, therefore, be interpreted as increased capacity of the MTX transport carrier for which free membrane SH groups are known to be essential. Despite activation of initial transport of MTX here, later net accumulation of MTX became smaller than in cells with normal GSH levels. Efflux of MTX from liver cells was not influenced by fasting or phorone treatment of rats, however, the "nonexchangeable" pool of MTX was found to be decreased, which indicates inhibition of formation of MTX polyglutamates here. This inhibition was most likely responsible for the decreased amounts of MTX finally accumulated in GSH-deficient hepatocytes.

Interaction between anions and the reduced folate/methotrexate transport system in L1210 cell plasma membrane vesicles: directional symmetry and anion specificity for differential mobility of loaded and unloaded carrier

The effect of various anions on the mediated influx and efflux of [3H]methotrexate by L1210 cell plasma membrane vesicles in a HEPES buffer system was studied. Our results show that flux is stimulated to the same extent in either direction when SO4, Pi, or folate compounds (1,L5-CHO-folate-H4, methotrexate), but not Cl- was present in the opposite compartment. This implies the property of directional symmetry, a condition in which differential mobility of loaded and unloaded carriers occurs in both directions. We also observed a similarity in the specificity of the interaction between various anions and carrier in each orientation of the membrane, in the order, Cl- much less than Pi approximately equal to SO2-4 much less than methotrexate less than 1,L5-CHO-folate-H4. Also, the absolute differential in mobility of loaded and unloaded carrier (assumed from the extent of transstimulation obtained) varied substantially among the anions examined. No stimulation was obtained with Cl-, and stimulation was twofold with Pi, SO2-4 and methotrexate and fourfold with 1,L5-CHO-folate-H4. Transstimulation of flux from either external or internal compartment only occurred when a positive gradient of total anions was maintained in the opposite compartment. Also, no stimulation occurred when the same equivalence of two different anions are present in opposing compartments. The concentration of anions required to transstimulate [3H]methotrexate influx was increased four- to 10-fold when vesicles were equilibrated in 145 mM NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)