DIFFERENTIAL INDUCTION OF ETOPOSIDE-MEDIATED APOPTOSIS IN HUMAN LEUKEMIA HL-60 AND K562 CELLS

Etoposide (VP-16) is one of several DNA-damaging agents that induce subcellular structural changes associated with apoptosis. VP-16 exerts its DNA-damaging and cytotoxic effects subsequent to interference with DNA topoisomerase II activity. VP-16 also stimulates c-jun and c-fos mRNA expression in some cell lines, including human leukemia K562 and HL-60 cells. To compare the temporal relationship between drug-induced c-jun expression and apoptosis, we examined cell morphology, cell viability, DNA integrity, and c-jun induction during VP-16 treatment of K562 and HL-60 cells. VP-16 (10 mu M)-induced internucleosomal DNA damage and nuclear fragmentation were readily apparent within 6 hr in HL-60 cells but were absent in K562 cells treated for up to 24 hr. Some internucleosomal DNA damage was observed in K562 cells but only after treatment with 100 mu M VP-16 for 24 hr. In contrast, VP-16-induced DNA single-strand breaks, VP-16-induced topoisomerase II/DNA covalent complex formation, and VP-16-mediated growth inhibition were similar in K562 and HL-60 cells. Also, the time course of VP-16-induced c-jun mRNA expression was comparable for both K562 and HL-60 cell lines. Western blot analysis of whole-cell lysates showed that Bcl-2 protein levels were 13-fold greater in HL-60 cells than in K562 cells. Thus, the resistance of VP-16-treated K562 cells to apoptosis was not attributable to protection by Bcl-2. Furthermore, the relatively high levels of Bcl-2 in HL-60 cells were not sufficient to protect these cells against apoptosis. Together, our results indicate that the temporal coupling of VP-16-induced DNA damage, c-jun expression, and apoptosis is cell type specific and suggest that different signaling pathways for apoptosis are operating in these two human leukemia cell lines.