Many cell-surface receptors induce production of second messengers like PIP3, phosphatidylinositol 3,4,5-trisphosphate, that convey signals to the cytoplasm from the cell surface. PIP3 signals activates the kinase PDK1, 3-phosphoinositide-dependent protein kinase-1, which in turn activates the kinase AKT, also known as protein kinase B. Proteins phosphorylated by activated AKT promote cell survival. Phosphorylation of Ikappa-B kinase leads to activation of the transcription factor NF-kB to oppose apoptosis. Bad is a protein in the Bcl-2 gene family that opposes Bcl-2 to induce apoptosis. Phosphorylation of Bad by AKT blocks anti-apoptotic activity to promote cell survival. Similarly, phosphorylation of the protease caspase 9 or forkhead transcription factors by AKT block the induction of apoptosis by these factors. AKT promotes cell survival and opposes apoptosis by a variety of routes.
Contributor: Glenn Croston, PhD.
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