Okadaic acid interferes with phorbol-ester-mediated down-regulation of protein kinase C-alpha, C-delta and C-epsilon

A prolonged cell exposure of all examined cell types to tumour-promoting phorbol esters leads to a substantial inactivation and degradation of protein kinase C (PKC), a phenomenon known as downregulation. With a combination of one-and two-dimensional immunoblot analyses we have previously shown the existence in PC12 cells of distinct PKC-alpha forms that differentially respond to cell treatment with phorbol ester [Gatti, A, & Robinson, P. J, (1996) J, Biol. Chem. 271, 31718-31722]. Using the same experimental model. in the present study we investigated a possible relationship between PKC-alpha phosphorylation and its down-regulation, The exposure of PC12 cells to okadaic acid, a potent inhibitor of biologically relevant protein phosphatases was found to partially protect PKC-alpha against phorbol-ester-mediated down-regulation. Further, a similar protective effect of okadaic acid was observed for PKC-delta and PKC-epsilon, which are also expressed in PC12 cells, These results indicate that the tumour-promoting activity of okadaic acid itself may be due to a sustained phosphorylation of PKC.