Freud-1/Aki1, a novel PDK1-interacting protein, functions as a scaffold to activate the PDK1/Akt pathway in epidermal growth factor signaling

The phosphoinositide 3-kinase (PI3K)/3-phosphoinositide-dependent protein kinase 1 (PDK1)/Akt pathway regulates various cellular functions, especially cell survival and cell cycle progression. In contrast to other survival pathways, there have been few reports of scaffold proteins that regulate signaling cascade specificity in this pathway. Here we identify a 5' repressor element under dual-repression binding protein 1 (Freud-1)/Akt kinase-interacting protein 1 (Aki1) as a novel scaffold for the PDK1/Akt pathway. Freud-1/Aki1 (also known as CC2D1A) expression induced formation of a PDK1/Akt complex and regulated Akt activation in a concentration-dependent biphasic manner. Freud-1/Aki1 also associated with epidermal growth factor (EGF) receptor in response to EGF stimulation and was required for Akt activation induced by EGF, but not by insulin-like growth factor 1. Freud-1/Aki1 gene silencing decreased Akt kinase activity, resulting in induction of apoptosis and increased sensitivity toward chemotherapeutic agents. Our results suggest that Freud-1/Aki1 is a novel receptor-selective scaffold protein for the PDK1/Akt pathway and present a new activation mechanism of Akt.