Effect of deoxycytidine on 2-chloro-deoxyadenosine-mediated growth inhibition of normal human erythroid and myeloid progenitor cells.

The cytotoxic effect of chlorodeoxyadenosine (CdA) on lymphocytes and monocytes requires phosphorylation by the enzyme deoxycytidine kinase and can be antagonized by coadministration of deoxycytidine (dCyt), a competitive substrate of deoxycytidine kinase. It has also been shown for lymphocytes that coadministration of 3-aminobenzamide (3-ABA), an inhibitor of the enzyme poly-(ADP ribose) synthetase, is activated by CdA-mediated DNA strand breaks, consumes intracellular nicotinamide-dinucleotide (NAD) and can antagonize the lethal effect of CdA. Recent in vitro studies have shown that not only growth of lymphocytes and monocytes, but also colony formation by erythroid and myeloid progenitors derived from normal human bone marrow, is inhibited by CdA in a dose-dependent manner. In this study we examined the effect of various doses of dCyt (10(-6) to 10(-3) M) on CdA-mediated growth inhibition of erythroid and myeloid progenitor cells in vitro. Our results show that colony formation by human bone marrow-derived progenitor cells--CFU-E (colony-forming unit erythroid), BFU-E (burst-forming unit erythroid) and CFU-GM (colony-forming unit granulocyte/macrophage)-in semisolid medium is protected by a high, but clinically achievable and non-toxic, concentration of dCyt (> 10(-4) M) against the inhibitory effects of coadministered high concentrations of CdA. The protective effect of dCyt was markedly different on the various subclasses of progenitor cells, however. Thus, with coadministration of 10(-4) M dCyt, the CFU-E colony formation could be restored to almost 100% despite the presence of high concentrations of CdA (160 nM) compared to control cultures, whereas the colony formation of BFU-E and CFU-GM was restored to only 50%. At a concentration of 10(-3) M dCyt, colony formation of BFU-E and CFU-GM was raised to 80% of control cultures even in the presence of high concentrations of CdA (160 nM). Further experiments in which 3-ABA was coadministered to CdA-treated cultures showed that in all concentrations tested (0.3 to 5 mM) 3-ABA was not able to prevent CdA-mediated cytotoxicity on bone marrow progenitors. Based on these studies, we suggest that the CdA toxicity on CFU-E is mainly mediated by phosphorylation by deoxycytidine kinase, whereas additional mechanisms may be operative in BFU-E and CFU-GM. Considerable biochemical differences seem to exist between hematopoietic stem cells on the one hand and lymphocytes and monocytes from peripheral blood on the other.