THE PROTEIN-KINASE-C ACTIVATORS PHORBOL ESTERS AND PHOSPHATIDYLSERINEINHIBIT NEUTRAL SPHINGOMYELINASE ACTIVATION, CERAMIDE GENERATION, ANDAPOPTOSIS TRIGGERED BY DAUNORUBICIN

To address the role of protein kinase C CPI(C) in the regulation of ce ramide production, vie evaluated the impact of the PKC activators 12-O -tetradecanoylphorbol-13-acetate and phosphatidylserine on the apoptot ic signaling pathway; triggered by the chemotherapeutic drug daunorubi cin, Treatment of U937 and HL-60 cells with 0.5-1 mu M daunorubicin in duced a greater than 30% activation of neutral sphingomyelinase activi ty within 4-10 min with concomitant sphingomyelin hydrolysis and ceram ide generation. Activation of PKC by 12-O-tetradecanoylphorbol-13-acet ate and phosphatidylserine inhibited daunorubicin-induced neutral sphi ngomyelinase activation, sphingomyelin hydrolysis, ceramide generation , and apoptosis. The apoptotic response could be restored bg the addit ion of 25 mu M cell-permeant CG-ceramide, in conclusion, PKC emerges a s a potentially critical negative regulator of the anthracycline-activ ated sphingomyelin-ceramide apoptotic pathway.