Caspase inhibition reveals functional cooperation between p55- and p75-TNF receptors in cell necrosis.

TNF-induced caspase activation is critically involved in both apoptosis and protection from cell necrosis. We have investigated the roles of the p55- and p75-TNF receptors (TNFR1 and TNFR2) in the induction of mouse L-M cell death in the presence of a caspase inhibitor (zVAD-fmk) and a transcription inhibitor (actinomycin D), i.e. under conditions in which protective pathways requiring caspase activation and protein synthesis were blocked. Cytometric analysis after TNF treatment showed that apoptosis was inhibited, while necrosis was highly activated. In contrast, apoptosis was observed in cells treated with TNF and actinomycin D alone. Stimulation of TNFR1 was sufficient to induce either cell necrosis or apoptosis, even when we blocked endogenous TNF with an anti-murine TNF antibody. Experiments based on the use of receptor-agonist and antagonist antibodies also showed that TNFR2 contributes to cell necrosis and apoptosis. Simultaneous stimulation of TNFR2 and TNFR1 with specific agonists indicated that TNFR2 functionally cooperates with TNFR1 to potentiate the response indirectly, by inducing endogenous TNF cytotoxicity. Caspase inhibitors enhanced the cytotoxic effect of endogenous TNF, suggesting that TNFR2 modulation can regulate the global necrotic response to TNF. TNFR2 modulation could play an important role in determining the response to TNF in pathophysiological conditions characterized by caspase down-regulation and local TNF production.