Akt/protein kinase B isoforms are differentially regulated by epidermal growth factor stimulation

Overexpression of epidermal growth factor receptor (EGFR) in certain cancer s is well established. There is growing evidence that epidermal growth fact or (EGF) activates Akt/protein kinase B (PKB) in a phosphoinositide 3-OH ki nase (PI3K)-dependent manner, but it is not yet clear which Akt isoforms ar e involved in this signal transduction pathway. We investigated the functio nal regulation of three Akt isoforms, Akt1/PKB alpha, Akt2/PKB beta, and Ak t3/PKB gamma, in esophageal cancer cells where EGFR is frequently overexpre ssed. Upon EGF simulation, phosphorylation of Akt1 at the Ser-473 residue w as remarkably induced. This result was corroborated by in vitro Akt kinase assays using glycogen synthase kinase 3 beta as the substrate. PI3K inhibit ors, wortmannin or LY294002, significantly blocked the Akt kinase activity induced by EGF. Akt2 activity was evaluated by electrophoretic mobility shi ft assays. Robust activation of Akt2 by EGF was observed in some cell lines in a PI3K-dependent manner. EGF-induced Akt3 activation was demonstrated b y Ser-472 phosphorylation of Akt3 but in a restrictive fashion. In aggregat e, EGF-mediated activation of Akt isoforms is overlapping and distinctive. The mechanism by which EGFR recruits the PI3K/Akt pathway was in part diffe rentially regulated at the level of Ras but independent of heterodimerizati on of EGFR with either ErbB2 or ErbB3 based upon functional dissection of p athways in esophageal cancer cell lines.