Protein kinase C inhibits singlet oxygen-induced apoptosis by decreasing caspase-8 activation

Although activation of protein kinase C (PKC) inhibits apoptosis induced by a variety of stimuli including singlet oxygen, the step at which PKC activ ation interferes with apoptotic signaling is not well defined. We have show n previously that caspase-8 and p38 mediate singlet oxygen-induced apoptosi s in HL-60 cells. In this study, we investigated the influence of PKC on re gulation of the caspase-8 and p38 pathways initiated by singlet oxygen. Sin glet oxygen induced Fas clustering and subsequent recruitment of FADD and c aspase-8. Treatment of cells with the phorbol ester 12-O-tetradecanoylphorb ol-13-acetate (TPA), a PKC activator, did not affect the binding of caspase -8 to the aggregated Fas. Surprisingly, under the same conditions PKC activ ation was still able to prevent singlet oxygen-induced activation of caspas e-8 and block its downstream signaling events including cleavage of Bid and caspase-3, decrease in mitochondrial transmembrane potential and release o f cytochrome c from mitochondria. Inhibition of PKC by GF109203 or H7 count eracted the TPA-mediated effects on the cleavage of caspases -3 and -8. How ever, neither activation nor inhibition of PKC affected p38 phosphorylation . These data indicate that PKC inhibits singlet oxygen-induced apoptosis by blocking activation of caspase-8.