Protein kinase B (also known as Akt) signaling controls cell survival, and its hyperactivation has been associated with certain cancers. Kinase activation occurs when the lipid phosphatidylinositol 3,4,5-trisphosphate (PIP3) associates with the pleckstrin homology (PH) domain of Akt, thus recruiting the enzyme to the plasma membrane when cells are stimulated with growth factors. Once phosphorylated on both its activation loop and at a serine residue (Ser473) in a C-terminal hydrophobic motif, Akt is released into the cytosol and nucleus, where it acts upon specific target substrates. Although it is known that the lipid phosphatase PTEN prevents initiation of Akt activation by reducing available PIP3, the mechanism that terminates already-activated Akt has not been clear. Gao et al. have identified a protein phosphatase that dephosphorylates the hydrophobic motif of Akt and inactivates the enzyme. The phosphatase, PH domain leucine-rich repeat protein phosphatase (PHLPP), was discovered in a database search of the human genome for phosphatases linked to a PH domain that might dephosphorylate the hydrophobic motif of Akt. Overexpression of PHLPP in cultured cells resulted in dephosphorylation of Ser473 in endogenous Akt. Akt and PHLPP were also isolated in an immune complex from lysates of cells overexpressing both proteins. The recombinant phosphatase domain dephosphorylated the hydrophobic motif of Akt in vitro. In cells derived from small-cell lung cancers with elevated Akt phosphorylation and low apoptosis, overexpression of PHLPP decreased Akt phosphorylation on Ser473, reduced Akt activity, and increased the number of apoptotic cells. Knockdown of PHLPP by RNA interference increased Akt phosphorylation at Ser473 and reduced apoptosis. Injection into mice of a human glioblastoma cell line overexpressing PHLPP reduced tumor size by nearly 70% compared with control animals, which suggests that PHLPP acts as a tumor suppressor.
T. Gao, F. Furnari, A. C. Newton, PHLPP: A phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth. Mol. Cell 18, 13-24 (2005). [Online Journal]