ALPHA-TOCOPHEROL SPECIFICALLY INACTIVATES CELLULAR PROTEIN-KINASE-C-ALPHA BY CHANGING ITS PHOSPHORYLATION STATE

The mechanism of protein kinase C (PKC) regulation by alpha-tocopherol has been investigated in smooth-muscle cells. Treatment of rat aortic A7r5 smooth-muscle cells with alpha-tocopherol resulted in a time- an d dose-dependent inhibition of PKC. The inhibition was not related to a direct interaction of alpha-tocopherol with the enzyme nor with a di minution of its expression. Western analysis demonstrated the presence of PKC alpha, beta, delta, epsilon, zeta and mu isoforms in these cel ls. Autophosphorylation and kinase activities of the different isoform s have shown that only PKC alpha: was inhibited by alpha-tocopherol. T he inhibitory effects were not mimicked by beta-tocopherol, an analogu e of alpha-tocopherol with similar antioxidant properties. The inhibit ion of PKC alpha by alpha-tocopherol has been found to be associated w ith its dephosphorylation. Moreover the finding of an activation of pr otein phosphatase type 2A in vitro by alpha-tocopherol suggests that t his enzyme might be responsible for the observed dephosphorylation and subsequent deactivation of PKC alpha. It is therefore proposed that P KC alpha inhibition by alpha-tocopherol is linked to the activation of a protein phosphatase, which in turn dephosphorylates PKC alpha and i nhibits its activity.