Inhibition of insulin-induced activation of Akt by a kinase-deficient mutant of the epsilon isozyme of protein kinase C

Akt, also known as protein kinase B, is a protein-serine/threonine kinase t hat is activated by growth factors in a phosphoinositide (PI) 3-kinase-depe ndent manner. Although Akt mediates a variety of biological activities, the mechanisms by which its activity is regulated remain unclear. The potentia l role of the a isozyme of protein kinase C (PKC) in the activation of Akt induced by insulin has now been examined. Expression of a kinase-deficient mutant of PKC epsilon (epsilon KD), but not that of wild-type PKC epsilon o r of kinase-deficient mutants of PKC alpha or PKC lambda, with the use of a denovirus-mediated gene transfer inhibited the phosphorylation and activati on of Akt induced by insulin in Chinese hamster ovary cells or L6 myotubes, Whereas the epsilon KD mutant did not affect insulin stimulation of PI 3-k inase activity, the phosphorylation and activation of Akt induced by a cons titutively active mutant of PI 3-kinase were inhibited by epsilon KD, sugge sting that epsilon KD affects insulin signaling downstream of PI 3-kinase. PDK1 (3'-phosphoinositide-dependent kinase 1) is thought to participate in Akt activation. Overexpression of PDK1 with the use of an adenovirus vector induced the phosphorylation and activation of Akt; epsilon KD inhibited, w hereas wild-type PKC epsilon had no effect on, these actions of PDK1. These results suggest that epsilon KD inhibits the insulin-induced phosphorylati on and activation of Akt by interfering with the ability of PDK1 to phospho rylate Akt.