Mechanism of the induction of the differentiation of HL-60 leukemia cells by antifolates.

The classic inhibitor of dihydrofolate reductase (DHFR), methotrexate (MTX), has been shown to be an effective inducer of the differentiation of HL-60 promyelocytic leukemia cells (Bodner A.J. et al.; J. Natl. Cancer Inst. 67:1025-1030; 1981). We have obtained evidence that induction of the differentiation of these cells by MTX, as well as by other folic acid antagonists, is the result of the effects of these agents on purine and thymine nucleotide biosynthesis. Thymidine (10 microM) completely blocked both the cytotoxicity and induction of differentiation produced by the specific inhibitor of thymidylate synthase (TS), N10-propargyl-5,8-dideazafolic acid (CB-3717). Thymidine also blocked the acute cytotoxicity caused by MTX and trimetrexate (TMQ); the induction of differentiation and the loss of proliferative capacity, however, were only partially prevented by thymidine. Hypoxanthine (100 microM), which completely restored antifolate-depleted purine nucleotide levels, had no effect on either the cytotoxicity or the induction of maturation produced by these agents. The growth inhibitory effects and the induction of differentiation caused by dideazatetrahydrofolic acid (DDATHF), which acts on de novo purine nucleotide biosynthesis rather than on DHFR or TS, was completely prevented by hypoxanthine. Hypoxanthine also completely prevented the inhibition of cellular replication and induction of differentiation by MTX and TMQ when combined with thymidine. The findings suggest that the depletion of intracellular thymine nucleotide levels by the antifolates, MTX, TMQ, and CB-3717 is the primary event involved in the maturation of HL-60 leukemia cells produced by these agents and that maturation occurs concomitantly with a high level of cytotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)