Induction of cell differentiation potentiates apoptosis triggered by prior exposure to DNA-damaging drugs. 
At the end of their life span, differentiated cells die by apoptosis.  Subsets of cells also die, in some cell systems, shortly after exposure to differentiating agents.  This suggests that early during differentiation the cells may undergo "priming," during which synthesis and/or activation and accumulation of effectors of apoptosis occurs.  The objective of the present study was to test the hypothesis that the signal for apoptosis provided by DNA-damaging drugs given prior to induction of differentiation will be more effective in triggering apoptosis than when given following induction of differentiation.  Human promyelocytic HL-60 cells were treated with the topoisomerase I inhibitor camptothecin, the alkylating agent nitrogen mustard, or 5'-azacytidine, an antimetabolite affecting predominantly RNA metabolism.  Following drug removal, the cells were postincubated with n-butyrate, which induces differentiation of HL-60 cells along the monocytic pathway, or with all-trans-retinoic acid, which triggers myelocytic differentiation.  Multiparameter flow cytometry using two different methods of analysis of apoptosis-associated DNA breakage in situ, as well as evaluation of cell morphology and DNA gel electrophoresis, were used to ascertain the mode of cell death.  Increases of 100-200% in the percentage of apoptotic cells were seen when cells were first treated with camptothecin or nitrogen mustard, followed by n-butyrate or retinoic acid, compared to the combined percentage of apoptotic cells when these agents were used individually.(ABSTRACT TRUNCATED AT 250 WORDS)