Inhibition of c- myc expression in human promyelocytic leukemia and colon adenocarcinoma cells by 6-thioguanine

Abstract

A rapid decrease in expression of the oncogene c- myc has been associated with the induction of differentiation of HL-60 human leukemia cells. In this manner, the treatment of a hypoxanthine phosphoribosyltransferase (HPRT)-deficient HL-60 variant (HL-60/var) with 6-thioguanine (TG) was accompanied by lower c- myc mRNA levels. This occurred in the absence of 6-thioguanosine 5'-monophosphate (TGMP) synthesis and without alterations in cellular nucleotide pool sizes. Paradoxically, inhibition of c- myc expression in the wild type HL-60 (HL-60/wt) cell, which is only weakly induced to differentiate by TG, was 5-fold more sensitive to the thiopurine ( ic 50 = 35 μ M) . Furthermore, inosine, which blocks the formation of TGMP and enhances the extent of differentiation of HL-60/wt cells, decreased the sensitivity of c- myc expression in the HL-60/wt to TG. These actions of TG and inosine on c- myc were also observed in the human colon carcinoma cell line COLO 320, further dissociating some of the effects of TG on c- myc expression from granylocytic differentiation. The hematopoietic granulocyte-macrophage colony stimulating factor (GM-CSF) elevated c- myc expression and antagonized the actions of TG on c- myc in the HL-60 cells. GM-CSF more readily antagonized the inhibitory action of TG in the HL-60/var cell line when compared to the HL-60/wt cells, restoring c- myc levels to that of the untreated controls. Hence, TG inhibited c- myc expression by two distinct mechanisms in cells which express high levels of the oncogene: a TGMP-dependent, differentiation-independent process with an ic 50 of 35 μM, and a TGMP-independent action with an ic 50 of 175 μM that was associated with induction of differentiation and was reversed more readily by GM-CSF.