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 activation interferes with apoptotic signaling is not well defined. We have shown previously that caspase-8 and p38 mediate singlet oxygen-induced apoptosis in HL-60 cells. In this study, we investigated the influence of PKC on regulation of the caspase-8 and p38 pathways initiated by singlet oxygen. Singlet oxygen induced Fas clustering and subsequent recruitment of FADD and caspase-8. Treatment of cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a PKC activator, did not affect the binding of caspase-8 to the aggregated Fas. Surprisingly, under the same conditions PKC activation was still able to prevent singlet oxygen-induced activation of caspase-8 and block its downstream signaling events including cleavage of Bid and caspase-3, decrease in mitochondrial transmembrane potential and release of cytochrome c from mitochondria. Inhibition of PKC by GF109203 or H7 counteracted the TPA-mediated effects on the cleavage of caspases -3 and -8. However, 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.