EVIDENCE FOR A RELATIONSHIP BETWEEN INTRACELLULAR GTB LEVELS AND THE INDUCTION OF HL-60 LEUKEMIA-CELL DIFFERENTIATION BY 5,10-DIDEAZATETRAHYDROFOLIC ACID (DDATHF)

5,10-Dideazatelrahydrofolic acid (DDATHF) is an inhibitor of glycinami de ribonucleotide transformylase, the first of two tetrahydrofolate re quiring enzymes in the de novo purine nucleotide biosynthetic pathway, and is a potent inducer of the maturation of HL-60 promyelocytic leuk emia cells. The inhibition of cellular growth by DDATHF was effectivel y prevented by adenosine or deoxyadenosine, whereas guanosine or deoxy guanosine only partially prevented the growth inhibition produced by t his folate antimetabolite, implying that the depletion of both P;TP an d GTP, which occurs with this agent, was responsible for its growth in hibitory effects. In contrast, the induction of differentiation by DDA THF was completely abolished by the presence of guanosine or deoxyguan osine, suggesting that the depletion of intracellular guanine nucleoti des by DDATHF represents the event that is essential to the induction of differentiation by this folate analog. This possibility was support ed by the observation that the concentration of dGTP was not decreased in cells treated with DDATHF under the conditions employed. Both guan ine nucleosides selectively restored intracellular GTP pools depleted by the treatment with DDATHF to their normal level, whereas only adeni ne nucleosides completely restored the levels of both ATP and GTP to t heir normal intracellular concentrations. The relationship between gua nine nucleotide pools and the induction of HL-60 differentiation by DD ATHF was further supported by the finding that maturation and the depl etion of intracellular GTP by DDATHF were not reversed by guanine nucl eosides in HL-60 cells deficient in hypoxanthine-guanine phosphoribosy ltransferase activity. The findings provide support for the hypothesis that the terminal differentiation of these leukemic cells by DDATHF i s the result of the depletion of intracellular GTP pools.