title: Cellular folates prevent polyglutamation of 5, 10-dideazatetrahydrofolate. A novel mechanism of resistance to folate antimetabolites.
abstract: Mouse L1210 cell variants were selected for resistance to 5, 10-dideazatetrahydrofolate, a potent inhibitor of the first folate-dependent enzyme in de novo purine synthesis, glycinamide ribonucleotide formyltransferase. The drug-resistant phenotype selected was conditional to the folate compound used to support growth: grown on folic acid cells were 400-fold resistant, whereas they were 2.5-fold more sensitive to 5,10-dideazatetrahydrofolate than wild-type L1210 cells when grown on folinic acid. In folic acid-containing media, polyglutamation of 5, 10-dideazatetrahydrofolate was markedly reduced, yet folylpolyglutamate synthetase activity was not different from that in parental L1210 cells. Resistance was due to two changes in membrane transport: a minor increase in the Km for 5, 10-dideazatetrahydrofolate influx, and a major increase in folic acid transport. Enhanced folic acid transport resulted in an expanded cellular content of folates which blocked polyglutamation of 5,10-dideazatetrahydrofolate. We propose that polyglutamation of 5,10-dideazatetrahydrofolate is limited by feedback inhibition by cellular folates on folylpolyglutamate synthetase, an effect which reflects a mechanism in place to control the level of cellular folates. Although the primary alteration causative of resistance is different from those reported previously, all 5, 10-dideazatetrahydrofolate resistance phenotypes result in decreased drug polyglutamation, reflecting the centrality of this reaction to the action of 5,10-dideazatetrahydrofolate.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Southern California, Los Angeles, California 90033, USA.
authors.forename: A
authors.initials: A
authors.lastname: Tse
authors.forename: R G
authors.initials: RG
authors.lastname: Moran
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000235:genetics; D002455:Cell Division / Q000187:drug effects / Q000235:genetics; D004351:Drug Resistance / Q000235:genetics*; D005246:Feedback / Q000502:physiology; D005492:Folic Acid / Q000378:metabolism*; D019877:Hydroxymethyl and Formyl Transferases / Q000037:antagonists & inhibitors; D007700:Kinetics; D007939:Leukemia L1210 / Q000235:genetics; D051379:Mice; D010453:Peptide Synthases / Q000378:metabolism; D010641:Phenotype; D051026:Phosphoribosylglycinamide Formyltransferase; D011624:Pteroylpolyglutamic Acids / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D013763:Tetrahydrofolates / Q000378:metabolism*; D014407:Tumor Cells, Cultured
chemical_list: D011624:Pteroylpolyglutamic Acids; D012333:RNA, Messenger; D013763:Tetrahydrofolates; C045894:lometrexol; D005492:Folic Acid; D019877:Hydroxymethyl and Formyl Transferases; D051026:Phosphoribosylglycinamide Formyltransferase; D010453:Peptide Synthases; C023103:folylpolyglutamate synthetase