REGULATION OF THE SALVAGE PATHWAY OF DEOXYNUCLEOTIDES SYNTHESIS IN APOPTOSIS INDUCED BY GROWTH-FACTOR DEPRIVATION

Here we describe changes in dNTP metabolism that precede DNA fragmenta tion in a model of apoptosis driven by deprivation of the cytokine int erleukin 3 (IL-3). In haemopoietic BAF3 cells, IL-3 withdrawal leads t o a rapid decrease in the size of dATP, dTTP and dGTP pools without af fecting dCTP levels. This imbalance in dNTP pools precedes DNA fragmen tation and is accompanied by down-regulation of enzymes controlling th e de novo and salvage pathways of dNTP synthesis, ribonucleotide reduc tase and thymidine kinase (TK) respectively. Readdition of IL-3 result s in a rapid, protein synthesis-independent restoration of normal dNTP pools, enhanced TK activity and increased precursor incorporation thr ough the salvage pathway. Up-regulation of TK activity after IL-3 read dition is prevented by the protein kinase C (PKC) inhibitor staurospor in, but not by tyrosine kinase inhibitors. Furthermore activation of P KC by phorbol esters mimics the stimulatory effect of IL-3 on TK activ ity, suggesting that PKC might be involved in regulating this effect. These results indicate that regulation by IL-3 of the salvage pathway of dNTP synthesis plays a role in the maintenance of cellular dNTP poo l balance and suggests that alterations in dNTP metabolism after IL-3 deprivation could be a relevant event in the commitment of haemopoieti c cells to apoptosis.