THE LONG-TERM COMBINED STIMULATORY EFFECTS OF ETHANOL AND PHORBOL ESTER ON PHOSPHATIDYLETHANOLAMINE HYDROLYSIS ARE MEDIATED BY A PHOSPHOLIPASE-C AND PREVENTED BY OVEREXPRESSED ALPHA-PROTEIN KINASE-C IN FIBROBLASTS

The protein kinase C (PKC) activator 12-0-tetradecanoylphorbol 13-acetate (TPA) has been shown to potentiate the stimulatory effect of ethanol on the hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts. Following an initial 20-min period, the main product of PtdEtn degradation in cells treated with TPA plus ethanol was ethanolamine phosphate. Here, we have examined the regulatory role of PKC and the possible catalytic role of phospholipase C in the formation of ethanolamine phosphate. TPA, bryostatin, and bombesin, direct or indirect activators of PKC, had similar potentiating effects on ethanol-induced formation of [C-14]ethanolamine phosphate from [C-14]PtdEtn in [C-14]ethanolamine-prelabelled NIH 3T3 fibroblasts. At lower concentrations of ethanol (40 - 80 mM), significant stimulation of ethanolamine phosphate formation required longer treatments (2 h or longer). The combined effects of TPA (100 nM) and ethanol (50 - 200 mM) on ethanolamine phosphate formation were not inhibited by the PKC inhibitors staurosporine or 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7). In contrast, these inhibitors significantly inhibited TPA-induced formation of ethanolamine, catalyzed by a phospholipase-D-type enzyme. In membranes isolated from TPA + ethanol-treated cells, enhanced formation of ethanolamine phosphate was maintained for at least 20 min. Down-regulation of PKC by prolonged (24-h) treatment of NIH 3T3 fibroblasts by 300 nM TPA enhanced, while overexpression of alpha-PKC in Balb/c fibroblasts diminished, the stimulatory effect of ethanol on the formation of ethanolamine phosphate. Finally, addition of the protein phosphatase inhibitor okadaic acid (2 muM) to fibroblasts inhibited TPA + ethanol-induced formation of ethanolamine phosphate. These results suggest that alpha-PKC-mediated protein phosphorylation may negatively regulate PtdEtn hydrolysis and that the potentiating effect of TPA may result, at least partly, from increased degradation of this PKC isoform.