Eiger and its receptor, Wengen, comprise a TNF-like system in Drosophila.

In mammals, members of the tumor necrosis factor (TNF) family play an important role in the regulation of cellular proliferation, differentiation and programmed cell death. We describe isolation and characterization of an orthologous ligand/receptor axis in Drosophila. The ligand, designated Eiger, is a type II membrane glycosylated protein, which can be cleaved at residue 145 and released from the cell surface as a soluble factor, thereby representing the first potential cytokine to be described in Drosophila. Eiger exists in two alternatively spliced isoforms, Eiger long (Eiger-L) and Eiger short (Eiger-s), both of which are expressed throughout development and in the adult. We also describe the isolation and characterization of a novel Drosophila member of the TNF receptor family, designated Wengen, which is a type I membrane protein that can physically interact with the recently described TRAF2 homolog dTRAF2. Both Eiger and Wengen are expressed in distinctive patterns during embryogenesis and Eiger is responsive to genotoxic stress. Forced expression of Eiger-L, Eiger-s or Wengen, caused apoptotic cell death which could be rescued by caspase inhibitors or the JNK phosphatase Puckered. In addition, Eiger-induced cell killing was attenuated by RNAi-mediated suppression of Wengen. Our results illustrate that Eiger and Wengen represent proximal components of an evolutionarily conserved TNF-like signaling pathway in Drosophila.

Induction of apoptosis by X-linked ectodermal dysplasia receptor via a caspase 8-dependent mechanism.

X-linked ectodermal dysplasia receptor (XEDAR) is a recently isolated member of the tumor necrosis factor receptor family that is highly expressed during embryonic development and binds to ectodysplasin-A2 (EDA-A2). In this report, we demonstrate that although XEDAR lacks a death domain, it nevertheless induces apoptosis in an EDA-A2-dependent fashion. The apoptosis-inducing ability of XEDAR is dependent on the activation of caspase 8 and can be blocked by its genetic and pharmacological inhibitors. Although XEDAR-induced apoptosis can be blocked by dominant-negative Fas-associated death domain (FADD) protein and FADD small interfering RNA, XEDAR does not directly bind to FADD, tumor necrosis factor receptor-associated death domain (TRADD) protein, or RIP1. Instead, XEDAR signaling leads to the formation of a secondary complex containing FADD, caspase 8, and caspase 10, which results in caspase activation. Thus, XEDAR belongs to a novel class of death receptors that lack a discernible death domain but are capable of activating apoptosis in a caspase 8- and FADD-dependent fashion. XEDAR may represent an early stage in the evolution of death receptors prior to the emergence of the death domain and may play a role in the induction of apoptosis during embryonic development and adult life.

Role of TRAF3 and -6 in the activation of the NF-kappa B and JNK pathways by X-linked ectodermal dysplasia receptor.

X-linked ectodermal dysplasia receptor (XEDAR) is a recently isolated member of the tumor necrosis factor receptor family that has been shown to be highly expressed in ectodermal derivatives during embryonic development and binds to ectodysplasin-A2 (EDA-A2). By using a subclone of 293F cells with stable expression of XEDAR, we report that XEDAR activates the NF-kappaB and JNK pathways in an EDA-A2-dependent fashion. Treatment with EDA-A2 leads to the recruitment of TRAF3 and -6 to the aggregated XEDAR complex, suggesting a central role of these adaptors in the proximal aspect of XEDAR signaling. Whereas TRAF3 and -6, IKK1/IKKalpha, IKK2/IKKbeta, and NEMO/IKKgamma are involved in XEDAR-induced NF-kappaB activation, XEDAR-induced JNK activation seems to be mediated via a pathway dependent on TRAF3, TRAF6, and ASK1. Deletion and point mutagenesis studies delineate two distinct regions in the cytoplasmic domain of XEDAR, which are involved in binding to TRAF3 and -6, respectively, and play a major role in the activation of the NF-kappaB and JNK pathways. Taken together, our results establish a major role of TRAF3 and -6 in XEDAR signaling and in the process of ectodermal differentiation.

The human herpes virus 8-encoded viral FLICE inhibitory protein physically associates with and persistently activates the Ikappa B kinase complex.

The human herpesvirus 8 (HHV8, also called Kaposi's sarcoma-associated herpesvirus) has been linked to Kaposi's sarcoma and primary effusion lymphoma (PEL) in immunocompromised individuals. We demonstrate that PEL cell lines have a constitutively active NF-kappaB pathway, which is associated with persistent phosphorylation of IkappaBalpha. To elucidate the mechanism of NF-kappaB activation in PEL cell lines, we have investigated the role of viral FLICE inhibitory protein (vFLIP) in this process. We report that stable expression of HHV8 vFLIP in a variety of cell lines is associated with persistent NF-kappaB activation caused by constitutive phosphorylation of IkappaBalpha. HHV8 vFLIP gets recruited to a approximately 700-kDa IkappaB kinase (IKK) complex and physically associates with IKKalpha, IKKbeta, NEMO/IKKgamma, and RIP. HHV8 vFLIP is incapable of activating NF-kappaB in cells deficient in NEMO/IKKgamma, thereby suggesting an essential role of an intact IKK complex in this process. Our results suggest that HHV8 vFLIP might contribute to the persistent NF-kappaB activation observed in PEL cells by associating with and stimulating the activity of the cellular IKK complex.