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DNA Damage and Cellular Stress Responses

Promotion of Mitosis by Activated Protein Kinase B After DNA Damage Involves Polo-Like Kinase 1 and Checkpoint Protein CHFR¹

Cancer Research Institute, Comprehensive Cancer Center, University of California San Francisco, San Francisco, CA

Requests for reprints: Emma Shtivelman, Cancer Research Institute, Comprehensive Cancer Center, University of California San Francisco, 2340 Sutter Street, San Francisco, CA 94115. Phone: (415) 502-1985; Fax: (415) 502-3179. E-mail: eshtivel{at}cc.ucsf.edu

The role of the protein kinase B (PKB/Akt) in the regulation of cell survival and proliferation is well established. PKB is a key effector in the phosphatidylinositol 3-kinase pathway and plays a role in the initiation of S phase and in the G₂-M transition. I report here that activated PKB shortens the G₂ arrest induced by DNA damage and promotes early entry into mitosis. Activated PKB supports high levels of expression and activity of the polo-like kinase 1 (Plk1) after DNA damage as cells accumulate in G₂. The checkpoint protein CHFR implicated in degradation of Plk1 is involved in the regulation of Plk1 by PKB. PKB phosphorylates CHFR in vitro and in vivo. Expression of a mutant form of CHFR that cannot be phosphorylated by PKB results in reduction of levels of Plk1 and inhibition of mitotic entry under normal conditions and after DNA damage. Results of this study support a model in which PKB facilitates mitotic resolution of DNA damage-induced G₂ arrest by inhibiting the checkpoint function of CHFR. The deregulated activation of PKB that occurs frequently in tumors might inhibit CHFR activity after DNA damage and therefore promote Plk1 accumulation leading to the disruption of the DNA damage checkpoint.

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