A DECREASE IN THE INTRACELLULAR GUANOSINE 5'-TRIPHOSPHATE CONCENTRATION IS NECESSARY FOR GRANULOCYTIC DIFFERENTIATION OF HL-60 CELLS, BUT GROWTH CESSATION AND DIFFERENTIATION ARE NOT ASSOCIATED WITH A CHANGE IN THE ACTIVATION STATE OF RAS, THE TRANSFORMING PRINCIPLE OF HL-60 CELLS

We found that when human promyelocytic leukemic cells (HL-60 cells) we re induced to differentiate along the granulocytic lineage by two dive rse mechanisms (starvation for an essential amino acid or treatment wi th DMSO), there was a marked decrease in the intracellular guanosine 5 '-triphosphate (GTP) concentration with no change in the guanosine 5'- diphosphate (GDP) concentration, Differentiation was prevented by guan ine or guanosine in a dose-dependent manner, We showed that: (a) guani ne had to be converted to a nucleotide because it did not prevent diff erentiation of hypoxanthine-guanine phosphoribosyltransferase-deficien t HL-60 cells; (b) the effect of guanine correlated with a return of t he cytosolic GTP:GDP ratio to normal; and (c) other purine bases were not effective, We hypothesized that the decreased GTP:GDP ratio in dif ferentiating HL-60 cells might decrease the relative amount of GTP bou nd to Ras, a key regulatory GTP-binding protein important to cell grow th and differentiation, Consistent with data showing that HL-60 cells harbor an activating N-Ras mutation, we found that the percentage of R as molecules in the GTP-bound state was high in proliferating HL-60 ce lls (27 +/- 3%) compared with other cultured mammalian cells (< 1%); h owever, we found no change in the activation state of Ras when cells c eased to proliferate and differentiated in response to DMSO, amino aci d deprivation, or inhibitors of guanylate synthesis, We conclude that: (a) a decrease in the intracellular GTP concentration is necessary fo r HL-60 cells to undergo granulocytic differentiation; and (b) althoug h a high degree of Ras activation contributes to the malignant phenoty pe of the cell, there is no change in the activation state of Ras duri ng granulocytic differentiation.