title: Folate transport and the modulation of antifolate sensitivity in a methotrexate-resistant human breast cancer cell line.
abstract: The mechanism of acquired methotrexate-resistance in an estrogen-receptor positive human breast cancer cell line (MTX(R)ZR-75-1) was studied. MTX(R) ZR-75-1 cells are 250-fold resistant to methotrexate when grown in the presence of 1 microM folinic acid and 2,400-fold resistant in the presence of 1 microM folic acid. This drug resistant cell line also showed collateral sensitivity (10-fold) to trimetrexate (TMQ), when grown in the presence of folinic acid. Using fluoresceinated methotrexate (F-MTX), FACS analysis indicated that there is no intracellular accumulation of methotrexate into MTX(R) ZR-75-1 cells, as determined by competition of F-MTX and methotrexate binding to dihydrofolate reductase. These characteristics strongly indicate that the mechanism of resistance involved down regulation of the reduced-folate transporter. To investigate this further, the transport kinetics of parental and MTX(R) ZR-75-1 cells were examined. Although the V(max) for methotrexate transport in wild-type (WT) ZR-75-1 breast cancer cells was 1-2 orders of magnitude lower than that in the well characterized leukemia cell lines, such as L1210 and CCRF-CEM cells, kinetic analysis indicated that transport of methotrexate into WT ZR-75-1 cells involved a mechanism that was similar if not identical to the reduced folate transporter. In contrast, no specific uptake of methotrexate was detected in MTX(R) ZR-75-1cells. Furthermore, neither cell line expressed detectable levels of folate binding protein, a binding protein with high affinity for folic acid as well as for reduced folates and antifolates. These results indicate that the level of expression of the reduced-folate carrier may be an important factor in determining the sensitivity of breast cancer cells as well as leukemia cells to antifolate compounds.
authors.affiliation: Building 10, Room 12N226, Medicine Branch and Clinical Pharmacology Branch, National Cancer Institute, Bethesda, Maryland 20892, USA.
authors.forename: K H
authors.initials: KH
authors.lastname: Dixon
authors.forename: J B
authors.initials: JB
authors.lastname: Trepel
authors.forename: S C
authors.initials: SC
authors.lastname: Eng
authors.forename: K H
authors.initials: KH
authors.lastname: Cowan
mesh_terms: D000818:Animals; D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D001692:Biological Transport; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D019008:Drug Resistance, Neoplasm; D005434:Flow Cytometry; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000378:metabolism*; D005493:Folic Acid Antagonists / Q000633:toxicity*; D006801:Humans; D002955:Leucovorin / Q000494:pharmacology; D007938:Leukemia / Q000188:drug therapy / Q000378:metabolism; D008727:Methotrexate / Q000031:analogs & derivatives / Q000494:pharmacology*; D051379:Mice; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011960:Receptors, Estrogen / Q000378:metabolism; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism; D016597:Trimetrexate / Q000494:pharmacology; D014407:Tumor Cells, Cultured; D014803:Vitamin B Complex / Q000494:pharmacology
chemical_list: D000964:Antimetabolites, Antineoplastic; D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D005493:Folic Acid Antagonists; D011956:Receptors, Cell Surface; D011960:Receptors, Estrogen; C036539:fluoresceinated methotrexate; D014803:Vitamin B Complex; D005492:Folic Acid; D013762:Tetrahydrofolate Dehydrogenase; D002955:Leucovorin; D016597:Trimetrexate; D008727:Methotrexate