title: Evidence for a G2 checkpoint in p53-independent apoptosis induction by X-irradiation.
abstract: The p53 tumor suppressor gene is thought to be required for the induction of programmed cell death (apoptosis) initiated by DNA damage. We show here, however, that the human promyelocytic leukemia cell line HL-60, which is known to be deficient in p53 because of large deletions in the p53 gene, can be induced to undergo apoptosis following X-irradiation. We demonstrate that the decision to undergo apoptosis in this cell line appears to be made at a G2 checkpoint. In addition, we characterize an HL-60 variant, HCW-2, which is radioresistant. HCW-2 cells display DNA damage induction and repair capabilities identical to those of the parental HL-60 cell line. Thus, the difference between the two 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 express bcl-2, which blocks apoptosis. HCW-2 cells' resistance to apoptosis may be due to the acquisition of expression of bcl-XL, a bcl-2-related inhibitor of apoptosis. In summary, apoptosis can be induced in X-irradiated HL-60 cells by a p53-independent mechanism at a G2 checkpoint, despite the presence of endogenous bcl-2. The resistance shown by HCW-2 cells suggests that bcl-XL can block this process.
authors.affiliation: Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, Rhode Island 02912, USA.
authors.forename: Z
authors.initials: Z
authors.lastname: Han
authors.forename: D
authors.initials: D
authors.lastname: Chatterjee
authors.forename: D M
authors.initials: DM
authors.lastname: He
authors.forename: J
authors.initials: J
authors.lastname: Early
authors.forename: P
authors.initials: P
authors.lastname: Pantazis
authors.forename: J H
authors.initials: JH
authors.lastname: Wyche
authors.forename: E A
authors.initials: EA
authors.lastname: Hendrickson
mesh_terms: D017209:Apoptosis / Q000528:radiation effects*; D004249:DNA Damage / Q000528:radiation effects; D004260:DNA Repair / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D016195:G2 Phase; D015870:Gene Expression; D018922:HL-60 Cells / Q000528:radiation effects*; D006801:Humans; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology*; D019253:Proto-Oncogene Proteins c-bcl-2; D011519:Proto-Oncogenes; D012333:RNA, Messenger / Q000235:genetics; D016159:Tumor Suppressor Protein p53 / Q000502:physiology*; D014965:X-Rays; D051028:bcl-2-Associated X Protein
chemical_list: C494826:BAX protein, human; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D016159:Tumor Suppressor Protein p53; D051028:bcl-2-Associated X Protein