Amplification of Dihydrofolate Reductase Genes in Methotrexate-resistant Cultured Mouse Cells

Our laboratory has been investigating the mechanism whereby cultured cells acquire resistance to the four amino analogs of folic acid, a phenomenon often associated both in cultured cells and in human neoplasms (Bertino et al. 1963) with elevated levels of dihydrofolate reductase activity. Resistant cell variants containing up to a 300-fold increase in dihydrofolate reductase activity can be obtained by growth of cells in progressively increasing concentrations of methotrexate (Hakala et al. 1961; Fisher 1961; Friedkin et al. 1962). Previous reports from this laboratory, using a resistant cell line (AT-3000) and several clones (R-1 to R-4) obtained from the AT-3000 line, all of which have been derived from a mouse sarcoma S-180 cell line, have demonstrated that the increase in dihydrofolate reductase activity results solely from an increased rate of enzyme synthesis (Alt et al. 1976), and that there is a proportionality between the rate of enzyme synthesis and the...