Differential activation of the PI 3-kinase effectors AKT/PKB and p70 S6 kinase by compound 48/80 is mediated by PKCα

The secretagogue compound 48/80 (c48/80) is a well known activator of calcium mediated processes and PKCs, and is a potent inducer of mast cell degranulation. As the latter process is a phosphoinositide 3-kinase (PI 3-kinase) mediated event, we wished to address whether or not c48/80 was an activator of PI 3-kinases. The data presented here reveal that c48/80 is an effective activator of PI 3-kinases as judged by the increased phosphorylation of PKB and p70^S6K in fibroblasts in a PI 3-kinase dependent fashion. Compound 48/80 effectively translocates PKB to the plasma membrane and induces phosphorylation at serine 473 (S473), detected by fluorescence imaging of fixed cells. At higher concentrations the secretagogue is inhibitory towards PKB phosphorylation on S473. Conversely, p70^S6K phosphorylation on T389 is unaffected at high doses. We provide evidence that the differential effect on the two PI 3-kinase effectors is due to activation of PKCα by c48/80, itself a PI 3-kinase dependent process. We conclude that compound 48/80 is an effective activator of PI 3-kinase dependent pathways, leading to the activation of effectors including PKB/Akt, p70^S6K and PKCα. The latter is only activated by higher doses of c48/80 resulting in an inhibition of the c48/80 induced PKB phosphorylation, thus explaining the observed biphasic activation profile for PKB in response to this secretagogue.