CELLULAR FOLATES PREVENT POLYGLUTAMATION OF 5,10-DIDEAZATETRAHYDROFOLATE - A NOVEL MECHANISM OF RESISTANCE TO FOLATE ANTIMETABOLITES

Mouse L1210 cell variants were selected for resistance to 5,10-dideaza tetrahydrofolate, a potent inhibitor of the first folate-dependent enz yme in de novo purine synthesis, glycinamide ribonucleotide formyltran sferase. The drug-resistant phenotype selected was conditional to the folate compound used to support growth: grown on folic acid cells mere 400-fold resistant, whereas they were 2.5-fold more sensitive to 5,10 -dideazatetrahydrofolate than wild-type L1210 cells when grown on foli nic acid. In folic acid-containing media, polyglutamation of 5,10-dide azatetrahydrofolate was markedly reduced, yet folylpolyglutamate synth etase activity was not different from that in parental L1210 cells. Re sistance was due to two changes in membrane transport: a minor increas e in the K-m for 5,10-dideazatetrahydrofolate influx, and a major incr ease in folic acid transport. Enhanced folic acid transport resulted i n an expanded cellular content of folates which blocked polyglutamatio n of 5,10-dideazatetrahydrofolate.We propose that polyglutamation of 5 ,10-dideazatetrahydrofolate is limited by feedback inhibition by cellu lar folates on folylpolyglutamate synthetase, an effect which reflects a mechanism in place to control the level of cellular folates. Althou gh the primary alteration causative of resistance is different from th ose reported previously, all 5,10-dideazatetrahydrofolate resistance p henotypes result in decreased drug polyglutamation, reflecting the cen trality of this reaction to the action of 5,10-dideazatetrahydrofolate .