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

The Res oncogene regulates cellular proliferation, differentiation, transfo rmation, and survival through multiple downstream signals. Res signals thro ugh 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 kina se can stimulate the small G protein, Rac, a direct activator of Pak, as we ll as the Akt proto-oncogene, a serine-threonine protein kinase, We found t hat activated Akt stimulated Pak, whereas a dominant negative Akt inhibited Res activation of Bah in transfection assays. Akt stimulation of Pak was n ot inhibited by dominant negative mutants of either Rac or Cdc42 suggesting that Akt activated Bah through a GTPase-independent mechanism. We also dev eloped a novel cell-free system to study Res activation of Pak, In this sys tem Res activated Pak only in the presence of a crude cell extract but fail ed to activate Pak when Akt was immunodepleted from the extract. Akt protec ts cells from apoptosis through phosphorylation of downstream targets such as the Bcl-2 family member, Bad, We found that activated Bah decreased apop tosis and increased phosphorylation of Bad, whereas dominant negative Pak i ncreased apoptosis and decreased phosphorylation of Bad. These studies defi ne a new oncogene mediated cell survival signal.