NF-kappa B inducers upregulate cFLIP, a cycloheximide-sensitive inhibitor of death receptor signaling

The caspase 8 homologue FLICE-inhibitory protein (cFLIP) is a potent negati ve regulator of death receptor-induced apoptosis. We found that cFLIP can b e upregulated in some cell lines under critical involvement of the NF-kappa B pathway, but MF-kappaB activation was clearly not sufficient for cFLIP in duction in all cell lines. Treatment of SV80 cells with the proteasome inhi bitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG-132) or geldanamycin, a drug interfering with tumor necrosis factor (TNF)-induced NF-kappaB activa tion, inhibited TNF-induced upregulation of cFLIP. Overexpression of a nond egradable I kappaB alpha mutant (I kappaB alpha -SR) or lack of I kappaB ki nase gamma expression completely prevented phorbol myristate acetate-induce d upregulation of cFLIP mRNA in Jurkat cells. These data point to an import ant role for NF-kappaB in the regulation of the cFLIP gene. SV80 cells norm ally show resistance to TNF-related apoptosis-inducing ligand (TRAIL) and T NF, as apoptosis can be induced only in the presence of low concentrations of cycloheximide (CHX). However, overexpression of I kappaB alpha -SR rende red SV80 cells sensitive to TRAIL-induced apoptosis in the absence of CHX, and cFLIP expression was able to reverse the proapoptotic effect of NF-kapp aB inhibition. Western blot analysis further revealed that cFLIP, but not T RAF1, A20, and cIAP2, expression levels rapidly decrease upon CHX treatment . In conclusion, these data suggest a key role for cFLIP in the antiapoptot ic response of NF-kappaB activation.