| Ligation of T cell receptors on stimulated T cells leads to activation-induced cell death resulting in the downregulation of immune responses, a process essential for T-cell homeostasis. In this study, using transformed T-cell lines as cellular models of T cell receptor-mediated apoptosis, we have demonstrated that the proapoptotic protein Siva-1 is required for T cell receptor-induced apoptosis. Knockdown of Siva-1 rendered T cells specifically resistant to anti-T cell receptor but not Fas-induced apoptosis. Further, we observed that in Siva-1 knockout Jurkat cells, T cell receptor-mediated activation of the canonical and non-canonical limbs of the NF-kappaB pathway are significantly enhanced as reflected by elevated nuclear levels of p65 and RelB, respectively. In addition, loss of endogenous Siva-1 also resulted in the enhanced expression of NF-kappaB-responsive anti-apoptosic genes such as Bcl-xL and c-FLIP. Interestingly, the c-FLIPshort was detected only in TCR-ligated Siva-1 knockdown Jurkat cells. These results demonstrate a significant role for endogenous Siva-1, through its inhibitory effect on NF-kappaB activity, in TCR-mediated AICD with implications in peripheral tolerance, T-cell homeostasis and cancer. The mechanism by which Siva-1 promotes apoptosis is by inhibiting activity of the crucial prosurvival transcription factor NF-kappaB. NF-kappaB plays an important role in cell survival and inflammation. Siva-1 inhibits the transcriptional activity of AP-1 in addition to NF-kappaB and specifically interacts with TRAF2, the common mediator of both pathways. The N-terminal region and not the amphipathic helical or the C-terminal cysteine rich region in Siva-1 is required for binding to TRAF2. Ubiquitination plays a quintessential role in the NF-kappaB signaling pathway and interestingly, Siva-1 promotes the polyubiquitination of TRAF2 at lysine residue position 48. T cells devoid of Siva-1 demonstrate polyubiquitination that is significantly lowered at position 48 but elevated at position 63 of TRAF2, which essentially contributes to the observed increase in the NF-kappaB activity. |
