Death receptor 4 (DR4) is a recently described receptor for the cytotoxic ligand TRAIL that reportedly uses a FADD-independent pathway to induce apoptosis and does not activate the NF-kappaB pathway. We have isolated a new member of the tumor necrosis factor receptor (TNFR) family, designated DR5, which bears a high degree of sequence homology to DR4. However, contrary to the previous reports, both DR4- and DR5-induced apoptosis can be blocked by dominant-negative FADD, and both receptors can activate NF-kappaB using a TRADD-dependent pathway. Finally, both receptors can interact with FADD, TRADD, and RIP. Thus, both DR5 and DR4 use FADD, TRADD, and RIP in their signal transduction pathways, and FADD is the common mediator of apoptosis by all known death domain-containing receptors.

Death E ector Domains (DEDs) have been known to mediate the recruitment of Caspase 8 and its homologs to the aggregated death-inducing signaling complex (DISC), consisting of the death domain (DD)-containing receptors and various signaling proteins. In addition, several viruses were recently shown to encode proteins with DEDs (also called FLICE inhibitory proteins or vFLIPs) which have the ability of blocking cell death induced by DD-containing receptors. We provide evidence that vFLIPs can also modulate the NF-kB pathway and physically interact with several signaling proteins, such as the TRAFs, RIP, NIK and the IKKs. Modulation of the NF-kB pathway may play a role in the natural history of infection by these viruses.

Caspase 8 is the most proximal caspase in the caspase cascade and has been known for its role in the mediation of cell death by various death receptors belonging to the TNFR family. We have discovered that Caspase 8 can activate the NF-kB pathway independent of its activity as a pro-apoptotic protease. This property is localized to its N-terminal prodomain, which contains two homologous death e ector domains (DEDs). Caspase 10 and MRIT, two DEDs-containing homologs of Caspase 8, can similarly activate the NF-kB pathway. Dominantnegative mutants of the Caspase 8 prodomain can block NF-kB induced by Caspase 8, FADD and several death receptors belonging to the TNFR family. Caspase 8 can interact with multiple proteins known to be involved in the activation of the NF-kB pathway, including the serine-threonine kinases RIP, NIK, IKK1 and IKK2. Thus, DEDs-containing caspases and caspase homolog(s) may have functions beyond their known role in the mediation of cell death. Oncogene (2000) 19, 4451 ± 4460.

The ectodermal dysplasia receptor (EDAR) is a recently isolated member of the tumor necrosis factor receptor family that has been shown to play a key role in the process of ectodermal differentiation. We present evidence that EDAR is capable of activating the nuclear factor-B, JNK, and caspase-independent cell death pathways and that these activities are impaired in mutants lacking its death domain or those associated with anhidrotic ectodermal dysplasia and the downless phenotype. Although EDAR possesses a death domain, it did not interact with the death domain-containing adaptor proteins TRADD and FADD. EDAR successfully interacted with various TRAF family members; however, a dominant-negative mutant of TRAF2 was incapable of blocking EDAR-induced nuclear factor-B or JNK activation. Collectively, the above results suggest that EDAR utilizes a novel signal transduction pathway. Finally, ectodysplasin A can physically interact with the extracellular domain of EDAR and thus represents its biological ligand.Anhidrotic (or hypohidrotic) ectodermal dysplasia is a disorder of ectodermal differentiation characterized by a triad of signs consisting of sparse hair, abnormal or missing teeth, and an inability to sweat (1). A similar phenotype is seen in mice with mutations involving the downless locus, suggesting the existence of a common underlying genetic defect. Recently, mutations in EDAR, 1 a novel receptor of the TNFR family, were found in several families with autosomal dominant and recessive forms of anhidrotic ectodermal dysplasia and in downless mice (2, 3). Although the above studies established the role of EDAR in ectodermal differentiation, the signaling pathways activated by this receptor remain to be elucidated.Death domain-containing receptors of the TNFR family are believed to activate two main signaling cascades: a kinase cascade leading to NF-B and JNK activation and a caspase cascade leading to cell death (4). As the cytoplasmic domain of EDAR was reported to contain a region with partial homology to the death domain, we decided to test its abilities to activate the above signaling cascades (2, 3). In this report, we present evidence that, like the classical death domain-containing receptors, EDAR is capable of activating the NF-B, JNK, and cell death pathways. However, EDAR does not interact with the death domain-containing adaptor proteins TRADD or FADD and does not activate the caspase cascade. Our results suggest the existence of a novel signaling pathway utilized by EDAR. MATERIALS AND METHODS Cell Lines and Reagents-293Tand MCF7 cells were obtained from Dr. David Han (University of Washington, Seattle, WA). 293 EBNA cells were obtained from Invitrogen. Rabbit polyclonal antibodies against FLAG, Myc, and hemagglutinin tags were obtained from Santa Cruz Biotechnology. FLAG beads and control beads were obtained from Sigma. The pull-down kinase assay kit for JNK was obtained from New England Biolabs Inc., and the constructs for the Pathdetect luciferase reporter assay were purchased from Stratagene....

We have isolated a novel member of the TNFR family, designated TAJ, that is highly expressed during embryonic development. TAJ possesses a unique cytoplasmic domain with no sequence homology to the previously characterized members of the TNFR family. TAJ interacts with the TRAF family members and activates the JNK pathway when overexpressed in mammalian cells. Although it lacks a death domain, TAJ is capable of inducing apoptosis by a caspase-independent mechanism. Based on its unique expression profile and signaling properties, TAJ may play an essential role in embryonic development.