ALTERED SUSCEPTIBILITY OF DIFFERENTIATING HL-60 CELLS TO APOPTOSIS INDUCED BY ANTITUMOR DRUGS

It has been reported that human promyelocytic leukemic HL-60 cells whi ch undergo differentiation fail to respond by apoptosis when treated w ith antitumor drugs, predominantly DNA topoisomerase inhibitors. Becau se S phase cells are selectively sensitive to these drugs, and during differentiation there is a reduction in the proportion of cells in S p hase, the reported decrease in the number of apoptotic cells could sim ply be a reflection of the paucity of sensitive cells in these culture s. Using cytometric methods which allow apoptosis to be related to cel l cycle position, we have compared the apoptotic response of HL-60 cel ls growing exponentially and induced to myeloid differentiation by dim ethyl sulfoxide (DMSO). The cells were treated with: (i) the DNA topoi somerase I inhibitor camptothecin (CAM), which selectively triggers ap optosis or S phase cells; (ii) the nucleoside antimetabolite 5-azacyti dine (AZC) and hyperthermia, both of which preferentially affects G(1) cells; and (iii) gamma radiation, which causes apoptosis predominantl y of G(2) + M cells, The cells exposed to 1.4% DMSO for 24 or 48 h wer e significantly more resistant to response by apoptosis, regardless of the nature of the agent and regardless of their position in the cell cycle. Thus, induction of differentiation lowers the cell's ability to respond to a variety of damaging agents by apoptosis and this effect is not correlated with cell cycle position. In addition, the differenc e in response was unrelated to expression of the apoptosis-modulating protein bcl-2, which appeared unchanged following 48 h exposure to DMS O. On the other hand, when the cells were pretreated with low concentr ations of CAM or AZC, washed free of drug, and then treated with DMSO, the proportion of cells undergoing apoptosis was markedly increased, relative to drug-treated cells returned to DMSO-free medium. The prese nt data may indicate that while the drug-induced damage screening mech anisms, which are linked to triggering apoptosis, may be more proficie nt in proliferating cells, the effecters of apoptosis are more express ed in cells undergoing differentiation. The data also suggest that the efficiency of chemotherapeutic agents or radiation may be reduced if a differentiating agent is used in combination therapy and is administ ered first. An enhancement of apoptosis, however, may be expected if t he differentiating drug is administered in the reverse sequence.