title: Akt activation emulates Chk1 inhibition and Bcl2 overexpression and abrogates G2 cell cycle checkpoint by inhibiting BRCA1 foci.
abstract: Akt is perhaps the most frequently activated oncoprotein in human cancers. Overriding cell cycle checkpoint in combination with the inhibition of apoptosis are two principal requirements for predisposition to cancer. Here we show that the activation of Akt is sufficient to promote these two principal processes, by inhibiting Chk1 activation with concomitant inhibition of apoptosis. These activities of Akt cannot be recapitulated by the knockdown of Chk1 alone or by overexpression of Bcl2. Rather the combination of Chk1 knockdown and Bcl2 overexpression is required to recapitulate Akt activities. Akt was shown to directly phosphorylate Chk1. However, we found that Chk1 mutants in the Akt phosphorylation sites behave like wild-type Chk1 in mediating G2 arrest, suggesting that the phosphorylation of Chk1 by Akt is either dispensable for Chk1 activity or insufficient by itself to exert an effect on Chk1 activity. Here we report a new mechanism by which Akt affects G2 cell cycle arrest. We show that Akt inhibits BRCA1 function that induces G2 cell cycle arrest. Akt prevents the translocation of BRCA1 to DNA damage foci and, thereby, inhibiting the activation of Chk1 following DNA damage.
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, IL 60607, USA.
authors.forename: Ivana
authors.initials: I
authors.lastname: Tonic
authors.forename: Wan-Ni
authors.initials: WN
authors.lastname: Yu
authors.forename: Youngku
authors.initials: Y
authors.lastname: Park
authors.forename: Chia-Chen
authors.initials: CC
authors.lastname: Chen
authors.forename: Nissim
authors.initials: N
authors.lastname: Hay
mesh_terms: D000818:Animals; D019313:BRCA1 Protein / Q000378:metabolism*; D045744:Cell Line, Tumor; D000071877:Checkpoint Kinase 1; D004249:DNA Damage; D005347:Fibroblasts / Q000378:metabolism; D016195:G2 Phase; D015971:Gene Expression Regulation, Enzymologic; D006801:Humans; D009154:Mutation; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D034622:RNA Interference; D051381:Rats
chemical_list: D019313:BRCA1 Protein; C492913:BRCA1 protein, human; D019253:Proto-Oncogene Proteins c-bcl-2; D011494:Protein Kinases; C000605867:CHEK1 protein, human; D000071877:Checkpoint Kinase 1; C000605871:Chek1 protein, rat; D051057:Proto-Oncogene Proteins c-akt