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 down-regulation. 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, 31 718-31722]. 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.
Gatti
A
A
Department of Experimental Oncology, European Institute of Oncology, Milan, Italy. andrea@pop.ucr.edu
Robinson
P J
PJ
D000818:Animals; D015536:Down-Regulation / Q000187:drug effects; D015151:Immunoblotting / Q000379:methods; D007527:Isoenzymes / Q000037:antagonists & inhibitors; D019319:Okadaic Acid / Q000494:pharmacology*; D016716:PC12 Cells; D010766:Phosphorylation; D011493:Protein Kinase C / Q000037:antagonists & inhibitors*; D051571:Protein Kinase C-alpha; D051745:Protein Kinase C-delta; D051744:Protein Kinase C-epsilon; D051381:Rats; D013347:Subcellular Fractions / Q000201:enzymology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*
D007527:Isoenzymes; D019319:Okadaic Acid; C497260:Prkcd protein, rat; C497257:Prkce protein, rat; D011493:Protein Kinase C; D051571:Protein Kinase C-alpha; D051745:Protein Kinase C-delta; D051744:Protein Kinase C-epsilon; D013755:Tetradecanoylphorbol Acetate