Fas ligand induces cell-autonomous NF-kappaB activation and interleukin-8 production by a mechanism distinct from that of tumor necrosis factor-alpha

Fas ligand (FasL) has been well characterized as a death factor. However, recent studies revealed that FasL possesses inflammatory activity. Here we found that FasL induces production of the inflammatory chemokine IL-8 without inducing apoptosis in HEK293 cells. Reporter gene assays involving wild-type and mutated IL-8 promoters and NF-kappaB- and AP-1 reporter constructs indicated that an FasL-induced NF-kappaB and AP-1 activity are required for maximal promoter activity. FasL induced NF-kappaB activation with slower kinetics than did TNF-alpha, yet this response was cell autonomous and not mediated by secondary paracrine factors. The death domain of Fas, FADD, and caspase-8 were required for NF-kappaB activation by FasL. A dominant-negative mutant of IKKgamma inhibited the FasL-induced NF-kappaB activation. However, TRADD and RIP, which are essential for the TNF-alpha-induced NF-kappaB activation, were not involved in the FasL-induced NF-kappaB activation. Moreover, CLARP/FLIP inhibited the FasL- but not the TNF-alpha-induced NF-kappaB activation. These results show that FasL induces NF-kappaB activation and IL-8 production by a novel mechanism, distinct from that of TNF-alpha. In addition, we found that mouse FADD had a dominant-negative effect on the FasL-induced NF-kappaB activation in HEK293 cells, which may indicate a species difference between human and mouse in the FasL-induced NF-kappaB activation.     

Lymphotoxin beta receptor induces interleukin 8 gene expression via NF-kappaB and AP-1 activation

The human lymphotoxin beta receptor (LTbetaR), a member of the tumor necrosis factor (TNF) receptor superfamily, is essential for not only the development and organization of secondary lymphoid tissues, but also for chemokine release. Even though LTbetaR was shown to recruit TNF-receptor-associated factor (TRAF) 2, 3, and 5, and to induce cell apoptosis or NF-kappaB activation, however, the downstream signaling leading to chemokine expression is not illustrated yet. In this study, we find that overexpression of LTbetaR in HEK293 cells increases IL-8 promoter activity and leads to IL-8 release. LTbetaR-induced IL-8 gene expression requires NF-kappaB (-80 to -71) and AP-1 (-126 to -12) binding sites located in IL-8 promoter, and NF-kappaB is more crucial than AP-1 for IL-8 gene expression. Reporter assay with dominant-negative mutants of TRAFs reveals that TRAF2, 3, and 5, as well as the downstream signal molecules NIK, IKKalpha, and IKKbeta, are involved in IL-8 gene expression. LTbetaR-mediated IL-8 response was inhibited by the dominant-negative mutants of ASK1, MKK4, MKK7, and JNK, but not by those of MEKK1, TAK1, MEK, ERK, and p38 MAPK. This suggests that IL-8 induction by LTbetaR is via TRAFs-elicited signaling pathways, including NIK/IKK-dependent NF-kappaB activation and ASK/MKK/JNK-dependent AP-1 activation.