THE EFFECT OF ALPHA-TOCOPHEROL ON THE SYNTHESIS, PHOSPHORYLATION AND ACTIVITY OF PROTEIN-KINASE-C IN SMOOTH-MUSCLE CELLS AFTER PHORBOL 12-MYRISTATE 13-ACETATE DOWN-REGULATION

Previous work had established that, in smooth muscle cells, alpha-toco pherol negatively regulates protein kinase C by preventing its activat ion [Tasinato, A., Boscoboinik, D., Bartoli, G. M., Maroni, P. & Azzi, A. (1995) Proc. Natl Acad. Sci. USA 92, 12190-12194]. In this study, the mechanism by which this event takes place has been analyzed. The r egulation by cc-tocopherol of protein kinase C expression, activity an d phosphorylation has been followed during the synthesis of protein ki nase C after its downregulation by phorbol 12-myristate 13-acetate. Th e data show that protein kinase C isoenzyme alpha is synthesised signi ficantly more (30% 72 h after down-regulation) in the presence of alph a-tocopherol. However, its activity is significantly less (45% diminut ion) and its phosphorylation state is also decreased (60% diminution). The effect of alpha-tocopherol appears not to be shared by the analog ue beta-tocopherol, provided with similar radical-scavenging propertie s. The data are interpreted in terms of a diminution of protein kinase C phosphorylation, specifically caused by alpha-tocopherol, resulting in a decreased enzyme specific activity.