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

Abstract

A number of methotrexate (MTX)-resistant murine and hamster cell lines with elevated levels of dihydrofolate reductase (DHFR) have been obtained by a stepwise selection process resulting from progressive increases in the concentration of MTX in the culture medium (Freidkin et al. 1962; Hakala et al. 1961; Littlefield 1969; Albrecht et al. 1972). We have reported that the elevated levels of DHFR in MTX-resistant murine sarcoma 180 cells result from increased synthesis of DHFR (Alt et al. 1976). Hanggi and Littlefield (1976) have also made a similar observation with a MTX-resistant Hne of baby hamster kidney (BHK) cells. In our studies, DHFR appears to be the only soluble cell protein whose rate of synthesis is increased markedly, and it can constitute as much as 20% of the soluble protein in highly resistant cells. More recently we have found that the high DHFR content is associated with a corresponding increase in specific...