TAK1 Is Required for Survival of Mouse Fibroblasts Treated with TRAIL, and Does So by NF-κB Dependent Induction of cFLIPL

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is known as a “death ligand”—a member of the TNF superfamily that binds to receptors bearing death domains. As well as causing apoptosis of certain types of tumor cells, TRAIL can activate both NF-κB and JNK signalling pathways. To determine the role of TGF-β-Activated Kinase-1 (TAK1) in TRAIL signalling, we analyzed the effects of adding TRAIL to mouse embryonic fibroblasts (MEFs) derived from TAK1 conditional knockout mice. TAK1−/− MEFs were significantly more sensitive to killing by TRAIL than wild-type MEFs, and failed to activate NF-κB or JNK. Overexpression of IKK2-EE, a constitutive activator of NF-κB, protected TAK1−/− MEFs against TRAIL killing, suggesting that TAK1 activation of NF-κB is critical for the viability of cells treated with TRAIL. Consistent with this model, TRAIL failed to induce the survival genes cIAP2 and cFlipL in the absence of TAK1, whereas activation of NF-κB by IKK2-EE restored the levels of both proteins. Moreover, ectopic expression of cFlipL, but not cIAP2, in TAK1−/− MEFs strongly inhibited TRAIL-induced cell death. These results indicate that cells that survive TRAIL treatment may do so by activation of a TAK1–NF-κB pathway that drives expression of cFlipL, and suggest that TAK1 may be a good target for overcoming TRAIL resistance

Regulation of activation-induced Fas (CD95/Apo-1) ligand expression in T cells by the cyclin B1/Cdc2 complex

Fas (CD95/Apo-1) ligand-mediated apoptosis has been recognized as an important mechanism of cell-mediated cytotoxicity and maintenance of immune homeostasis. Chronically activated T cells undergo activation-induced cell death (AICD), which depends on simultaneous Fas and Fas ligand expression. Previous reports have suggested that AICD might be linked to cell cycle progression of T cells and therefore to the expression of cell cycle-related molecules. In particular, cyclin B1 has been implicated in the induction of AICD in T cells. In this study, we have investigated the role of cyclin B1 in AICD and the expression of effector molecules involved in this form of cell death. Our results show that inhibition of cyclin B1 blocks AICD in T cells through specific inhibition of Fas ligand expression but not Fas-induced apoptosis. This effect of cyclin B1 appears to be mediated through the cyclin B1/cyclin-dependent kinase 1 (Cdk1/Cdc2) complex because overexpression of cyclin B1 enhances FasL promoter activity, whereas a dominant-negative version of Cdk1 blocks Fas ligand promoter induction. We provide further evidence that cyclin B1/Cdk1 regulates FasL transcription through the regulation of NFκB activation because dominant-negative Cdk1 inhibits activation-induced NFκB reporter and Rel A-induced FasL promoter activity. In conclusion, our data support a link between cell cycle progression, activation-induced Fas ligand expression, and apoptosis in T cells