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J Biol Chem. 2009 May 29;284(22):15061-70. doi: 10.1074/jbc.M808810200. Epub 2009 Mar 30.

Ionizing radiation induces ATM-independent degradation of p21Cip1 in transformed cells.

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  • 1Biomedical Sciences Graduate Program, Division of Hematology-Oncology, Department of Medicine, and Moores Cancer Center, University of California San Diego School of Medicine, La Jolla, California 92093-0820, USA.


The cyclin-dependent kinase inhibitor p21(Cip1) plays an important role in the cellular response to DNA damage. In normal cells, genotoxic stress activates the ATM-p53 pathway that up-regulates the expression of p21(Cip1) leading to cell cycle arrest. However, we have found that in several neoplastic cell lines, ionizing radiation (IR) induces ubiquitin-dependent degradation of p21(Cip1). This process is independent of the ATM pathway as it occurs in immortalized A-T fibroblasts. Knockdown of Skp2, an F-box protein capable of regulating the normal turnover of p21(Cip1), does not prevent the IR-induced degradation. Instead, this process requires the Cul4-DDB1(Cdt2) E3 ligase as knockdown of either DDB1 or Cdt2 rescues p21(Cip1) degradation after IR. Mutating the proliferating cell nuclear antigen-binding site of p21(Cip1) also prevents its IR-induced degradation suggesting that the p21(Cip1)-proliferating cell nuclear antigen interaction is critical for this event. Although ectopic expression of a nondegradable p21(Cip1) did not by itself affect the clonogenic survival of HEK293 cells after IR, the degradation of p21(Cip1) and other targets of the Cul4-DDB1(Cdt2) E3 ligase may collectively contribute to the survival of neoplastic cells after ionizing radiation.

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