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PKB/Akt mediates cell-cycle progression by phosphorylation of p27Kip1 at threonine 157 and modulation of its cellular localization

Abstract

We have shown a novel mechanism of Akt-mediated regulation of the CDK inhibitor p27kip1. Blockade of HER2/neu in tumor cells inhibits Akt kinase activity and upregulates nuclear levels of the CDK inhibitor p27Kip1. Recombinant Akt and Akt precipitated from tumor cells phosphorylated wild-type p27 in vitro. p27 contains an Akt consensus RXRXXT157D within its nuclear localization motif. Active (myristoylated) Akt phosphorylated wild-type p27 in vivo but was unable to phosphorylate a T157A-p27 mutant. Wild-type p27 localized in the cytosol and nucleus, whereas T157A-p27 localized exclusively in the nucleus and was resistant to nuclear exclusion by Akt. T157A-p27 was more effective than wild-type p27 in inhibiting cyclin E/CDK2 activity and cell proliferation; these effects were not rescued by active Akt. Expression of Ser473 phospho Akt in primary human breast cancers statistically correlated with expression of p27 in tumor cytosol. These data indicate that Akt may contribute to tumor-cell proliferation by phosphorylation and cytosolic retention of p27, thus relieving CDK2 from p27-induced inhibition.

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Acknowledgements

This work was supported by NIH grant R01 CA80195, Vanderbilt-Ingram Comprehensive Cancer Center support grant CA68485 and a research grant from Genentech.

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Correspondence to Carlos L. Arteaga.

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The authors declare no competing financial interests.

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Figure 1: Cellular Akt phosphorylates p27 in vitro.
Figure 2: Akt associates with and phosphorylates p27 on Thr157.
Figure 3: Akt phosphorylates p27 at Thr157 in vivo.
Figure 4: Phosphorylation of Thr157 in p27 is required for Akt-induced cytosolic localization.
Figure 5: Anti-proliferative effect of T157A-p27 is not rescued by active Akt.
Figure 6: Expression of P-S473 Akt in invasive breast carcinomas correlates with cytosolic p27.