α-Tocopherol specifically inactivates cellular protein kinase C α by changing its phosphorylation state

Abstract

The mechanism of protein kinase C (PKC) regulation by α-tocopherol has been investigated in smooth-muscle cells. Treatment of rat aortic A7r5 smooth-muscle cells with α-tocopherol resulted in a time- and dose-dependent inhibition of PKC. The inhibition was not related to a direct interaction of α-tocopherol with the enzyme nor with a diminution of its expression. Western analysis demonstrated the presence of PKCα, β, δ, ε, ζ and µ isoforms in these cells. Autophosphorylation and kinase activities of the different isoforms have shown that only PKCα was inhibited by α-tocopherol. The inhibitory effects were not mimicked by β-tocopherol, an analogue of α-tocopherol with similar antioxidant properties. The inhibition of PKCα by α-tocopherol has been found to be associated with its dephosphorylation. Moreover the finding of an activation of protein phosphatase type 2A in vitro by α-tocopherol suggests that this enzyme might be responsible for the observed dephosphorylation and subsequent deactivation of PKCα. It is therefore proposed that PKCα inhibition by α-tocopherol is linked to the activation of a protein phosphatase, which in turn dephosphorylates PKCα and inhibits its activity.