Characterization of the metabolic forms of 6‐thioguanine responsible for cytotoxicity and induction of differentiation of HL‐60 acute promyelocytic leukemia cells

HL‐60 human acute promyelocytic leukemia cells that lack hypoxanthineguanine phosphoribosyltransferase (HGPRT) activity have been developed by mutagenization and selection. These cells exhibited markedly decreased sensitivity to the cytotoxic action of 6‐thioguanine (TG) and, in contrast to parental HL‐60 cells, had the capacity to undergo terminal granulocytic differentiation after treatment with this purine antimetabolite. Analysis of extracellular and intracellular metabolites of TG revealed negligible metabolism of TG in these HGPRT⁻ HL‐60 cells. These findings are consistent with the concept that inhibition of cellular replication requires generation of analog nucleotide and suggest that TG itself is capable of initiation of differentiation. 6‐Thioguanosine (TGuo) had limited activity, while β‐2′‐deoxythioguanosine (dTGuo) was inactive, as an inducer of maturation of HGPRT⁻ HL‐60 cells. These cells converted relatively large amounts of the nucleosides to the free base TG; the simultaneous exposure of cells to 8‐aminoguanosine (AGuo), an inhibitor of purine nucleoside phosphorylase activity, decreased the degradation of TGuo and dTGuo to TG and promoted the intracellular accumulation of TG nucleotides, presumably through the action of nucleoside kinase activities. In a double mutant deficient in both HGPRT and deoxycytidine kinase (DCK) activities, dTGuo was devoid of cytotoxicity and was an effective inducer of maturation. The potency of dTGuo as an inducer in this system was not significantly affected by the presence of AGuo. These results suggested that dTGuo itself was also an active initiator of maturation. Thus, induction of differentiation appeared to be due to the free base, TG, as well as its deoxynucleoside form, dTGuo, whereas the formation of TG nucleotides appeared to antagonize maturation and produce cytotoxicity.