Evidence that protein kinase Cε mediates phorbol ester inhibition of calphostin C- and tumor necrosis factor-α-induced apoptosis in U937 histiocytic lymphoma cells

Abstract

Protein kinase C (PKC) activators, such as the tumor-promoting phorbol esters, have been reported to protect several cell lines from apoptosis induced by a variety of agents. Recent evidence suggests that PKCε is involved in protection of cardiac myocytes from hypoxia-induced cell death (Gray, M. O., Karliner, J. S., and Mochly-Rosen, D. (1997) J. Biol. Chem. 272, 30945-30951). We investigated the protective effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on U937 histiocytic lymphoma cells induced to undergo apoptosis by tumor necrosis factor-α (TNF-α) or by the specific PKC inhibitor calphostin C. U937 cells were transiently permeabilized with a peptide (εV1-2) derived from the V1 region of PKCε that has been reported to specifically block translocation of PKCε. The εV1-2 peptide blocked the inhibitory effect of TPA on both TNF-α- and calphostin C-induced apoptosis. A scrambled version of εV1-2 and a peptide reported to inhibit PKCβ translocation (βC2-4) had no effect on the ability of TPA to inhibit apoptosis. These results suggest that PKCε is required for the protective effect of TPA in TNF-α- and calphostin C-induced apoptosis. Furthermore, calphostin C reduced membrane-associated PKCε activity and immunoreactivity, suggesting that PKCε may play an important role in leukemic cell survival.