Background information: Death receptors (DRs) induce intracellular signalling upon engagement of their cognate ligands, leading to apoptosis, cell survival or pro-inflammatory responses. In mammals, DR signalling is mediated by the recruitment of several DD (death domain)-containing molecules, such as FADD (Fas-associated DD) and RIP1 (receptor-interacting protein 1).
Results: To elucidate the molecular mechanisms of intracellular DR signalling in Xenopus, we have isolated cDNAs encoding xFADD (Xenopus FADD), and xRIP1 and its short isoform xRIP1beta, which is produced by alternative splicing of the xRIP1 gene. These DD-containing proteins interacted with Xenopus DR members xDR-M1 and xDR-M2 through their DDs in co-transfected HEK-293T cells. Overexpression of xFADD activated not only xCaspase 8, but also AP-1 (activator protein 1), which reflects activation of JNK (c-Jun N-terminal kinase) and NF-kappaB (nuclear factor kappaB). A comparative analysis of xRIP1, a kinase-dead mutant of xRIP1 and xRIP1beta indicated that the kinase activity of xRIP1 was required for the activation of AP-1 and NF-kappaB. Interestingly, xFADD and xRIP1 interacted with each other via their DDs, and the expression of a mutant xRIP1 containing only the DD (xRIP1-DD) repressed the xFADD-induced activation of NF-kappaB and AP-1. xFADD and xRIP1 synergistically induced the activation of AP-1 and NF-kappaB, both of which were partially mediated by TRAF2 (tumour-necrosis-factor-receptor-associated factor 2) and TAK1 (transforming-growth-factor-beta-activated kinase 1). We also found that the activation pathways of NF-kappaB induced by xDR-M2 were inhibited by xRIP1-DD.
Conclusions: Xenopus FADD, RIP1 and its splice variant RIP1beta have been characterized. Interaction of xFADD and xRIP1 induced synergistic activation of JNK and NF-kappaB. In addition, the NF-kappaB activation induced by xDR-M2 was partially mediated by xRIP1.