Cell death

Another Way to Die

Science Signaling  03 Jun 2008:
Vol. 1, Issue 22, pp. ec206
DOI: 10.1126/scisignal.122ec206

Although p53-deficiency is a common factor in many cancers and loss of p53 activity contributes to the inappropriate survival of cells that have DNA damage, in vitro studies suggest that there are p53-independent cell death pathways. To facilitate the discovery of such p53-independent pathways, Sidi et al. targeted kinases involved in the DNA damage response with morpholino antisense oligonucleotides in zebrafish embryos defective for p53 activity and found that when the kinase Chk1 was knocked down, cell death (detected with acridine orange) due to irradiation was restored to that of wild-type embryos exposed to irradiation. A specific inhibitor of Chk1 (Gö6976), but not inhibitors for the DNA damage checkpoint kinases ATM or Chk2, also restored irradiation-induced cell death in p53-deficient zebrafish embryos. Although the morphological and ultrastructural characteristics of apoptosis were present in the irradiated p53e7/e7;chk1MO embryos (DNA fragmentation, chromatin compaction, and cytoplasmic condensation), activated caspase-3 was not detectable with an activation-specific antibody. Knockdown of either ATM or ATR in the p53e7/e7;chk1MO embryos severely impaired cell death induced by irradiation. Depletion of Puma, Bcl-XL, or caspase-9 failed to prevent irradiation-induced cell death in the p53e7/e7;chk1MO embryos, which suggests that the mitochondrial pathway of cell death was not involved. Depletion or inhibition of components of the extrinsic cell death pathway (caspase-9 or death receptor signaling) also did not interfere with cell death in the p53e7/e7;chk1MO embryos. Instead, irradiation-induced cell death was decreased if caspase-2 was knocked down in the p53e7/e7;chk1MO embryos. This caspase-2-dependent pathway was present in human cancer cells and was detected after irradiation in cells deficient for p53 activity and in which Chk1 was inhibited with Gö6976. Inhibition of Chk1 may be an attractive target for sensitizing human cancers to irradiation because each of the human cancer cell lines tested, regardless of p53 genotype, exhibited enhanced apoptosis in response to irradiation when exposed to the Chk1 inhibitor Gö6976.

S. Sidi, T. Sanda, R. D. Kennedy, A. T. Hagen, C. A. Jette, R. Hoffmans, J. Pascual, S. Imamura, S. Kishi, J. F. Amatruda, J. P. Kanki, D. R. Green, A. A. D’Andrea, A. T. Look, Chk1 suppresses a caspase-2 apoptotic response to DNA damage that bypasses p53, Bcl-2, and caspase-3. Cell 133, 864-877 (2008). [PubMed]