Potentiation of 1-β-D-arabinofuranosylcytosine cytotoxicity to HL-60 cells by 1,25-dihydroxyvitamin D₃ correlates with reduced rate of maturation of DNA replication intermediates

S-phase-active cytotoxic drugs selectively damage leukemic cells, but the mechanisms of this action are not clear. We have investigated the previously reported potentiation of toxicity of 1-β-D-arabinofuranosylcytosine (ara-C) to HL-60 cells by the differentiation-inducing steroid, 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], and compared the results with the effects of other drugs which inhibit DNA synthesis. Determination of the intracellular content of the active metabolite of ara-C, ara-CTP, excluded more prolonged retention of the drug as the basis for potentiation of cytotoxicity. Alkaline elution of replicating DNA showed that 1,25(OH)₂D₃ added with or immediately after ara-C or hydroxyurea reduced the rate maturation of the replicating DNA and resulted in an increased proportion of smaller DNA fragments. However, pretreatment of the cells with 1,25(OH)₂D₃ inhibited this effect of the drugs on replication of DNA. No direct effect of 1,25(OH)₂D₃ on replicating DNA could be detected. The results suggest that the early events which initiate cell differentiation may protect an intact DNA replicative machinery from S-phase-active drugs but reduce the rate of DNA maturation once DNA integrity has been compromised by inhibitors of DNA synthesis.