The Akt proto-oncogene links Ras to Pak and cell survival signals

The Res oncogene regulates cellular proliferation, differentiation, transformation, and survival through multiple downstream signals. Res signals through its effector phosphoinositide 3 (PI3) kinase to the Pak protein kinase (p65(pak)), but the steps from Res to Pak remain to be elucidated. PI3 kinase can stimulate the small G protein, Rac, a direct activator of Pak, as well as the Akt proto-oncogene, a serine-threonine protein kinase, We found that activated Akt stimulated Pak, whereas a dominant negative Akt inhibited Res activation of Bah in transfection assays. Akt stimulation of Pak was not inhibited by dominant negative mutants of either Rac or Cdc42 suggesting that Akt activated Bah through a GTPase-independent mechanism. We also developed a novel cell-free system to study Res activation of Pak, In this system Res activated Pak only in the presence of a crude cell extract but failed to activate Pak when Akt was immunodepleted from the extract. Akt protects cells from apoptosis through phosphorylation of downstream targets such as the Bcl-2 family member, Bad, We found that activated Bah decreased apoptosis and increased phosphorylation of Bad, whereas dominant negative Pak increased apoptosis and decreased phosphorylation of Bad. These studies define a new oncogene mediated cell survival signal.