Caspase-mediated degradation of T-cell receptor zeta-chain

We recently reported an association between loss in T-cell receptor (TcR) z eta-chain expression and tumor-induced apoptosis of T lymphocytes. In this study, the possibility that zeta-chain serves as a direct substrate for act ivated caspases was investigated. Here, we report that two DXXD motifs, whi ch are putative recognition sequences for caspase-3-related proteases and a re present in the amino acid sequence of the zeta-chain, are cleaved in apo ptotic Jurkat T lymphocytes. Cleavage of zeta-chain in Jurkat cells ligated by agonistic anti-Bas antibody was inhibited in the presence of peptide in hibitors of caspases, including the pan-caspase inhibitor N-benzyloxycarbon yl-Val-Ala-Asp-fluoromethyl ketone and N-benzyloxycarbonyl-Asp-Glu-Val-Asp- fluoromethyl ketone, an inhibitor of caspase-3-like activity, Fas-induced c leavage of zeta-chain was also inhibited in Jurkat cells overexpressing the intracellular inhibitors of caspase activity, Bcl-2 or cytokine response-m odifier A. In vitro translated zeta-chain was cleaved in a similar fashion by recombinant caspase-3 or caspase-7 in a dose-dependent manner. In the pr esence of N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethyl ketone, no cleav age of in vitro translated zeta-chain was observed. These results suggest t hat the loss of TcR zeta-chain, previously associated with tumor-induced im mune dysfunction and more recently associated with tumor-induced apoptosis of T lymphocytes, is mediated by a direct degradation of the zeta-chain by activated caspases, This is the first report of involvement of caspases in degradation of the zeta protein.