TRAF1 is a substrate of caspases activated during tumor necrosis factor receptor-alpha-induced apoptosis

TRAF family proteins are signal-transducing adapter proteins that interact with the cytosolic domains of tumor necrosis factor (TNF) family receptors, Here we show that TRAF1 (but not TRAF2-6) is cleaved by certain caspases i n vitro and during TNF-alpha- and Fas-induced apoptosis in vivo. (LEVD163)- L-160 was identified as the caspase cleavage site within TRAF1, generating two distinct fragments. Significant enhancement of TNF receptor-1 (CD120a)- and, to a lesser extent, Fas (CD95)-mediated apoptosis was observed when o verexpressing the C-terminal TRAF1 fragment in HEK293T and HT1080 cells. Th e same fragment was capable of potently suppressing TNF receptor-1- and TRA F2-mediated nuclear factor-kappaB activation in reporter gene assays, provi ding a potential mechanism for the enhancement of TNF-mediated apoptosis. C ell death induced by other death receptor-independent stimuli such as cispl atin, staurosporine, and UV irradiation did not result in cleavage of TRAF1 , and overexpression of the C-terminal TRAF1 fragment did not enhance cell death in these cases. TRAF1 cleavage was markedly reduced in cells that con tain little procaspase-8 protein, suggesting that this apical protease in t he TNF/Fas death receptor pathway is largely responsible. These data identi fy TRAF1 as a specific target of caspases activated during TNF- and Fas-ind uced apoptosis and illustrate differences in the repertoire of protease sub strates cleaved during activation of different apoptotic pathways.