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)