EVIDENCE FOR A G(2) CHECKPOINT IN P53-INDEPENDENT APOPTOSIS INDUCTIONBY X-IRRADIATION

The p53 tumor suppressor gene is thought to be required for the induct ion of programmed cell death (apoptosis) initiated by DNA damage. We s how here, however, that the human promyelocytic leukemia cell line HL- 60, which is known to be deficient in p53 because of large deletions i n the p53 gene, can be induced to undergo apoptosis following X-irradi ation. We demonstrate that the decision to undergo apoptosis in this c ell line appears to be made at a G(2) checkpoint. In addition, we char acterize an HL-60 variant, HCW-2, which is radioresistant. HCW-2 cells display DNA damage induction and repair capabilities identical to tho se of the parental HL-60 cell line. Thus, the difference between the t wo cell lines appears to be that X-irradiation induces apoptosis in HL -60, but not in HCW-2, cells. Paradoxically, HCW-2 cells display high levels of expression of bax, which enhances apoptosis, and no longer e xpress bcl-2, which blocks apoptosis. HCW-2 cells' resistance to apopt osis may be due to the acquisition of expression of bcl-x(L), a bcl-2- related inhibitor of apoptosis. In summary, apoptosis can be induced i n X-irradiated HL-60 cells by a p53-independent mechanism at a G(2) ch eckpoint, despite the presence of endogenous bcl-2. The resistance sho wn by HCW-2 cells suggests that bcl-x(L) can block this process.