Editors' ChoiceCell Biology

Activation and Inactivation of Caspase-8 by Ubiquitylation

Sci. Signal.  15 Jan 2013:
Vol. 6, Issue 258, pp. ec18
DOI: 10.1126/scisignal.2003963

Death receptors (DRs) are members of the family of tumor necrosis factor receptors (TNFRs) that contain a cytoplasmic death domain and stimulate the extrinsic apoptosis pathway. Binding of DRs to their extracellular ligands results in the recruitment of adaptor proteins and the initiator caspase, caspase-8, into a membrane-proximal death-inducing signaling complex (DISC), which leads to activation of the executioner caspases and apoptosis. Ubiquitylation of caspase-8 by the E3 ubiquitin ligase Cullin3 enhances its clustering and activation. Clustering and autocatalytic processing of caspase-8 releases the catalytically active products p43 and p18. Noting that mouse embryonic fibroblasts (MEFs) lacking the E3 ligase TRAF2 (TNFR-associated factor 2) exhibit enhanced apoptosis in response to TNF compared with that of wild-type MEFs, Gonzalvez et al. investigated whether TRAF2 regulated caspase-8 activity. Inhibition of the proteasome increased the sensitivity of various cultured cell lines to apoptosis induced by ligation of DRs. In the presence of a proteasomal inhibitor, lysine 48 (K48)–linked polyubiquitylated caspase-8 accumulated in cells in response to DR ligands; the p18 region of caspase-8 was ubiquitylated. DR ligation resulted in the recruitment of TRAF2 to the DISC, where it associated with caspase-8 through an interaction with the p18 region of the protein; however, the kinetics of TRAF2 recruitment were delayed compared with that of Cullin3. Knockdown of TRAF2 inhibited ligand-induced, K48-linked polyubiquitylation of the p18 region of caspase-8 and enhanced the stability of p43 and p18. In a mouse model of liver toxicity stimulated by an antibody that activates caspase-8, mice treated with TRAF2-specific small-interfering RNA (siRNA) showed accelerated lethality compared with that of antibody-treated mice exposed to a nonspecific siRNA. Together, these data suggest that through targeting caspase-8 for degradation, TRAF2 determines the threshold for activation of the extrinsic apoptosis pathway.

F. Gonzalvez, D. Lawrence, B. Yang, S. Yee, R. Pitti, S. Marsters, V. C. Pham, J.-P. Stephan, J. Lill, A. Ashkenazi, TRAF2 sets a threshold for extrinsic apoptosis by tagging caspase-8 with a ubiquitin shutoff timer. Mol. Cell 48, 888–899 (2012). [PubMed]