Expression of Protein Kinase C-β Promotes the Stimulatory Effect of Phorbol Ester on Phosphatidylethanolamine Synthesis

Stimulation of phosphatidylethanolamine (PtdEtn) synthesis by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) has reportedly been found only in hepatocytes expressing the α-, βII-, ϵ-, and ζ-PKC isozymes. In contrast, stimulation of phosphatidylcholine synthesis by PKC activators, known to be mediated by PKC-α, is widespread in mammalian cells. In this work, various cell lines exhibiting characteristic differences in their PKC systems were used to determine the role of specific PKC isozymes in the mediation of PMA effect on PtdEtn synthesis. In NIH 3T3 fibroblasts, which express high levels of PKC-α but none of the β (βI or βII) isoforms, PMA did not stimulate PtEtn synthesis. In contrast, in Rat-6 fibroblasts overexpressing PKC-βI, 10–100 nmPMA considerably (1.7- to 2.6-fold) enhanced PtdEtn synthesis. In wild-type or multidrug resistant MCF-7 human breast carcinoma cells, which express PKC-α and PKC-βII (to varying extents) but not PKC-βI, PMA had only small or no effects on PtdEtn synthesis. In contrast, in MCF-7 cells overexpressing PKC-α, and as a consequence also expressing the βI- and βII-PKC isoforms, PMA effectively stimulated the synthesis of PtdEtn. Finally, in HL60 human leukemia cells, which contains PKC-βII as the major PKC isoform, PMA again stimulated PtdEtn synthesis. The results establish that while stimulation of PtdEtn synthesis by PMA occurs only in selected cell lines, this phenomenon is not restricted to hepatocytes. Furthermore, the data indicate that expression of either PKC-βI or PKC-βII, but not PKC-α, correlates with the effect of PMA on PtdEtn synthesis. Overall, these observations strongly suggest that regulation of PtdEtn and PtdCho synthesis by PMA involves separate PKC isozymes, i.e., PKC-β and PKC-α, respectively.