Protein kinase Cζ is a negative regulator of protein kinase B activity

Protein kinase B (PKB), also known as Akt or RAC-PK, is a serine/threonine kinase that can be activated by growth factors via phosphatidylinositol 3-kinase. In this article we show that PKC zeta but not PKC alpha and PKC delta can co-immunoprecipitate PKB from CHO cell lysates. Association of PKB with PKC zeta was also found in COS-1 cells transiently expressing PKB and PKC zeta, and moreover we found that this association is mediated by the AH domain of PKB. Stimulation of COS-1 cells with platelet-derived growth factor (PDGF) resulted in a decrease in the PKB-PKC zeta interaction. The use of kinase-inactive mutants of both kinases revealed that dissociation of the complex depends upon PKB activity. Analysis of the activities of the interacting kinases showed that PDGF-induced activation of PKC zeta was not affected by co expression of PKB. However, both PDGF- and p110-CAAX-induced activation of PKB were significantly abolished in cells co-expressing PKC zeta. In contrast, co-expression of a kinase-dead PKC zeta mutant showed an increased induction of PKB activity upon PDGF treatment. Downstream signaling of PKB, such as the inhibition of glycogen synthase kinase-3, was also reduced by coexpression of PKC zeta. A clear inhibitory effect of PKC zeta was found on the constitutively active double PKB mutant (T308D/S473D). In summary, our results demonstrate that PKB interacts with PKC zeta in vivo and that PKC zeta acts as a negative regulator of PKB.