THE KIT RECEPTOR PROMOTES CELL-SURVIVAL VIA ACTIVATION OF PI-3-KINASEAND SUBSEQUENT AKT-MEDIATED PHOSPHORYLATION OF BAD ON SER136

The c-kit-encoded receptor protein tyrosine kinase for stem cell facto r (Kit/SCF-R) is essential for the development of cells within the hem atopoietic, melanogenic and gametogenic lineages [1]. SCF stimulation induces activation of phosphatidylinositol (PI) 3-kinase, which is req uired for SCF-induced mitogenesis and cell survival [2-4], and for act ivation of the serine/threonine protein kinase Akt [5-7]. Using Kit/SC F-R mutants, we found that, in response to SCF, Akt became activated a nd mediated phosphorylation of Bad, a pro-apoptotic molecule, in a PI- 3-kinase-dependent manner. Phosphorylation of Bad was restricted to Se r112 and Ser136 in vivo, but only the Akt phosphorylation site Ser136 was essential for SCF-promoted cell survival. Furthermore, Bad and Akt interacted and colocalized in intact cells. A Kit/SCF-R gain oi-funct ion mutant that has increased mitogenic and PI 3-kinase activation pot ential, due to the absence of the two protein kinase C negative feedba ck phosphorylation sites [8,9], enhanced both Akt activation and Bad p hosphorylation and also resulted in increased cell survival. Such a me chanism may account for how deregulated PI 8 kinase activity and natur ally occurring gain-of-function point mutants of Kit/SCF-R lead to cel lular transformation and fatal malignancies in man [10-12]. (C) Curren t Biology Ltd.