Increased dihydrofolate reductase activity in methotrexate-resistant human promyelocytic-leukaemia (HL-60) cells. Lack of correlation between increased activity and overproduction

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Abstract
Methotrexate (MTX)-resistant human promyelocytic-leukemia cells (HL-60) derived from MTX-sensitive cells have a 20-fold increase in dihydrofolate reductase (DHFR) activity as compared with the sensitive cells. This increase is not associated with a concomitant increase in DHFR protein as determined by sodium dodecyl sulfate polyacrylamide-gel electrophoresis and by immunological methods using mouse anti-DHFR antibody. The rate of DHFR synthesis is similar in both cell lines. Both the sensitive and resistant cells have similar amounts of RNA hybridizing to a DHFR complementary-DNA probe, correlating well with the lack of increase in DHFR protein. DHFR-gene dosages were similar in both types of cells. The 20-fold increase in DHFR activity present in these MTX-resistant cells is not due to the overproduction of DHFR but due to the expression of a more active form of the enzyme.

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