Regulation of Akt/Protein Kinase B Signaling by a Novel Protein Phosphatase in Breast Cancer Cells

摘要

The major purpose of the proposed research was to determine if a novel phosphatase, entitled PHLPP2, negatively regulates the protoncogene Akt, by dephosphorylating this kinase at a key residue, Ser 473, where phosphorylation is required formaximal activity. Furthermore, we sought to determine if this phosphatase played a role in regulating downstream physiological effects of Akt signaling including: cell proliferation, growth, and apoptosis. Finally, since this phosphatase resides in a location of frequent loss of heterozygosity in breast cancer, we sought to determine if this phosphatase played a role in breast tumorigenesis. Major findings, are consistent with originally proposed research, I was able to successfully clone the full length PHLPP2 protein and determine phosphatase activity in vitro and in vivo. Full length PHLPP2 is a functional phosphatase that preferentially dephosphorylates Akt at Ser 473, thereby decreasing kinase activity, promoting apoptosis, and inhibiting cellular proliferation. Consistent with these results, siRNA mediated knockdown of endogenous PHLPP2 increases phosphorylation of Akt at Ser 473, and promotes cellular proliferation and survival. Finally, we have distinguished functional roles for PHLPP1 and PHLPP2 by examining downstream signaling of the Akt kinase. PHLPP2 preferentially regulates the cell cycle inhibitor p27, while PHLPP1 regulates GSK-3alpha, and HDM2. Both phosphatases regulate the phosphorylation status of GSK-beta, TSC-2 and FoxO1. We have elucidated the molecular mechanism where PHLPP phosphatases regulate unique downstream substrates of Akt: by interacting with and dephosphorylating specific isoforms of Akt. PHLPP2 regulates Akt1 and Akt3 while PHLPP1 regulates Akt2 and Akt3.