Reduced folate carrier

Quantitative molecular-property relationship Segregated-flow model Reduced folate carrier protein... 

Fig. 14.1 Cellular pathway of methotrexate. ABCBl, ABCCl-4, ABC transporters ADA, adenosine deaminase ADP, adenosine diphosphate AICAR, aminoimidazole carboxamide ribonucleotide AMP, adenosine monophosphate ATIC, AICAR transformylase ATP, adenosine triphosphate SjlO-CH -THF, 5,10-methylene tetrahydrofolate 5-CHj-THF, 5-methyl tetrahydro-folate DHFR, dihydrofolate reductase dTMP, deoxythymidine monophosphate dUMP, deoxy-uridine monophosphate FAICAR, 10-formyl AICAR FH, dihydrofolate FPGS, folylpolyglutamyl synthase GGH, y-glutamyl hydrolase IMP, inosine monophosphate MTHFR, methylene tetrahydrofolate reductase MTR, methyl tetrahydrofolate reductase MTX-PG, methotrexate polyglutamate RFCl, reduced folate carrier 1 TYMS, thymidylate synthase. Italicized genes have been targets of pharmacogenetic analyses in studies published so far. (Reproduced from ref. 73 by permission of John Wiley and Sons Inc.)...

Rothem, L., Aronheim, A., and Assaraf, Y.G. (2003) Alterations in the expression of transcription factors and the reduced folate carrier as a novel mechanism of antifolate resistance in human leukemia cells. Journal of Biological Chemistry. 278, 8935-8941. 

Whetstine, J.R., Witt, T.L., and Matherly, L.H. (2002) The human reduced folate carrier gene is regulated by the AP2 and spl transcription factor families and a functional 61-base pair polymorphism. Journal of Biological Chemistry. 277, 43873-43880. 

Dervieux, T., Eurst, D., Lein, D. O., et al. (2004) Polyglutamation of methotrexate with common polymorphisms in reduced folate carrier, aminoimidazole carboxamide ribonucleotide transformylase, and thymidylate synthase are associated with methotrexate effects in rheumatoid arthritis. Arthritis and Rheumatism. 50, 2766-2774. 

Thymidylate Synthase Gene Reduced Folate Carrier (RFC) Gene Discussion References... 

The reduced folate carrier is the major transporter of methotrexate into cells. The RFC gene is comprised of 40,719 base pairs, has 12 exons, and is located on 21q22.3. The primary polymorphisms studied to date is the RFCl G80A variant, which is associated with a decrease in MTX transport into cells. Rady et al. noted similar rates of variation in several ethnic groups variant homozygosity rates were 28.7% in individuals of Ashkenazi-Jewish decent, 20.8% in African-American, 29% in Caucasians, and 26% in Hispanics (23). 

Rady PL, Szucs S, Matalon RK et al. Genetic polymorphism (G80A) of reduced folate carrier gene in ethnic populations. Mol Genet Metab 2001 73 285-286. 

Laverdiere C, Chiasson S, Costea 1 et al. Polymorphism G80A in the reduced folate carrier gene and its relationship to methotrexate plasma levels and outcome of childhood acute lymphoblastic leukemia. B/oorf 2002 100 3832-3834. 

Nozaki Y, Kusuhara H, Endou H, et al. Quantitative evaluation of the drug-drug interactions between methotrexate and nonsteroidal anti-inflammatory drugs in the renal uptake process based on the contribution of organic anion transporters and reduced folate carrier. J Pharmacol Exp Ther 2004 309 226-234. 

Corona, G., Giannini, F., Fabris, M., Toffoli, G., and Boiocchi, M. (1998). Role of folate receptor and reduced folate carrier in the transport of 5-methyltetrahydrofolic acid in human ovarian carcinoma cells. Int. ]. Cancer 75(1), 125-133. 

Rothem L, Ifergan I, Kaufman Y, Priest DG, Jansen G, Assaraf YG. 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. Biochem J 2002 367 741-50. 

MTX is actively transported into the cell by solute carrier 19A1 (SLC 19A1), also called reduced folate carrier 1 (RFC1) (Fig. 1). MTX is pumped out of the cell by members of the ATP binding cassette (ABC) family of transporters, also known as multidrug resistant transporters (MDRs), and multidrug resistance-associated... 

Rothem L, Stark M, Kaufman Y, Mayo L, Assaraf YG (2004) Reduced folate carrier gene silencing in multiple antifolate-resistant tumor cell lines is due to a simultaneous loss of function of multiple transcription factors but not promoter methylation. J Biol Chem 279 374-384... 

Dervieux T, Kremer J, Lein DO, Capps R, Barham R, Meyer G, Smith K et al (2004) Contribution of common polymorphisms in reduced folate carrier and gamma-glutamylhydrolase to methotrexate polyglutamate levels in patients with rheumatoid arthritis. Pharmacogenetics 14 733-739... 

Matherly, L.H., Hou, Z. and Deng, Y. (2007) Human reduced folate carrier translation of basic biology to cancer etiology and therapy. Cancer Metastasis Reviews, 26 (1), 111-128. 

The folate receptor facilitates the cellular uptake of folate and 5-methyltetrahydrofolate via receptor-mediated endocytosis at caveolae (caveolae are plasma membrane invaginations distinct from the classical clathrin-coated pits) (6). It has been hypothesized that the folate receptor is functionally coupled to an anion transporter to mediate cytosolic folate delivery by a process defined as potocytosis (6). More recent studies suggest that folate receptor endocytosis also occurs at clathrin-coated pits (7). Studies by Low and coworkers at Purdue University have shown that folate conjugates are also taken up by the folate receptor (8-10), but not by the reduced folate carrier. Figure 2 illustrates an endocytic pathway of the type envisioned for folate conjugates. Unfortunately, the subcellular transport pathway of the folate conjugates has been only partially characterized and may well be affected by the properties of the molecule attached to folate. 

The classic antifolate methotrexate MTX continues to be an important component of the chemotherapeutic armamentarium for a variety of cancers including pediatric ALL, osteogenic sarcoma, lymphoma, and breast cancer. Raltitrexed, another anti-folate, is used throughout much of the world outside of the United States for advanced colorectal cancer. Finally, Peme-trexed was approved in 2004 for the treatment of pleural mesothelioma and shortly thereafter as a second line treatment for nonsmall cell lung cancer. These drugs mimic the natural folate molecule and in so doing utilize the membrane-bound reduced folate carrier (RFC) protein system to gain entry into the cell. 

As with other classical antifolates, the ability of 5,8-dideazaisoAMT (IV.273) and its 5-substituted derivatives to cross the cell membrane is an important determinant of growth-inhibitory potency. Susten et al [50] addressed this issue by measuring the kinetics of transport of [ H]MTX in the presence of the quinazolines. The IC50 of IV.273 as an inhibitor of 1.0 /xM MTX influx was 1.6 /xM, whereas the IC50 values of (IV.274), (IV.275), and MTX (unlabelled) were 2.0, 1.4, and 1.0 uM. Thus substitution at position 5 caused a slight decrease in binding of the quinazolines for the MTX/reduced folate carrier protein. 

New folate antagonists have been identified that are better substrates for the reduced folate carrier and appear to have significant advantages in clinical chemotherapy (see pemetrexed below). In efforts to bypass the obligatory membrane transport system and to facilitate penetration of the blood—brain barrier, lipid-soluble folate antagonists also have been synthesized. Trime-trexate (NEVTREXIN) has modest antitumor activity, primarily in combination with leucovorin rescue. However, it is beneficial in the treatment Pneumocystis jiroveci pneumonia, where leucovorin provides differential rescue of the host but not the parasite. 

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