The phosphatidylinositide 3 '-kinase/Akt survival pathway is a target for the anticancer and radiosensitizing agent PKC412, an inhibitor of protein kinase C

Activation of the phosphatidylinositol 3'-kinase (PI3K)/Akt survival pathwa y protects against apoptotic stress stimuli. Therefore, compounds that down -regulate this pathway are of clinical interest for single and combined ant icancer treatment modalities. Here we demonstrate that the cytotoxic effect of the protein kinase C (PKC)-inhibitor N-benzoylated staurosporine (PKC41 2) is mediated via the PI3K/Akt pathway. Dose-dependent down-regulation of the proliferative activity, activation of the apoptotic machinery, and cell killing by PKC412 (0-1 mum) in Rat1a-fibroblasts and H-ras-oneogene-transf ormed fibroblasts correlated with a decrease of Akt phosphorylation and a r educed phosphorylation of the endogenous Akt-substrate GSK3-alpha. Expressi on of the dominant-active myristoylated form of Akt abrogated this cytotoxi c effect of PKC412. Experiments with Apaf-1-deficient cells revealed that P KC412-induced cytotoxicity depends on an intact apoptosome but that the dec rease of Akt phosphorylation is not attributable to apoptosis execution. Co mparative experiments indicate that PKC412 and the parent-compound staurosp orine down-regulate this survival pathway upstream or at the level of Akt b ut by a different mechanism than the P13K-inhibitor LY294002. Furthermore, inhibition of this pathway by PKC412 is relevant for sensitization to ioniz ing radiation. These results demonstrate the specific role of this signalin g pathway for the PKC412-mediated down-regulation of an apoptotic threshold and its cytotoxicity.