The intracellular signaling pathway by which tumor necrosis factor (TNF) induces its pleiotropic actions is well characterized and includes unique components as well as modules shared with other signaling pathways. In addition to the currently known key effectors, further molecules may however modulate the biological response to TNF. In our attempt to characterize novel regulators of the TNF signaling cascade, we have identified transmembrane protein 9B (TMEM9B, c11orf15) as an important component of TNF signaling and a module shared with the interleukin 1 (IL-1) and Toll-like receptor (TLR) pathways. TMEM9B is a glycosylated protein localized in membranes of the lysosome and partially in early endosomes. The expression of TMEM9B is required for the production of proinflammatory cytokines induced by TNF, IL-1, and TLR ligands but not for apoptotic cell death triggered by TNF or Fas ligand. TMEM9B is essential in TNF activation of both the NF-B and MAPK pathways. It acts downstream of RIP1 and upstream of the MAPK and IB kinases at the level of the TAK1 complex. These findings indicate that TMEM9B is a key component of inflammatory signaling pathways and suggest that endosomal or lysosomal compartments regulate these pathways.
Tumor necrosis factor (TNF)2 is a pleiotropic mediator of a wide range of cellular responses to infection, such as cytokine and chemokine production, cell migration, cell death, and cell differentiation and maturation (1). TNF plays a pivotal role in several autoimmune disorders such as rheumatoid arthritis; this is underscored by the clinical success of neutralizing TNF with antibodies or soluble receptors (2). A better understanding of intracellular TNF signaling is therefore of high clinical relevance.The two TNF receptors, TNFR1 (p55, TNFRSF1A) and TNFR2 (p75, TNFRSF1B), show high homology in their extracellular domains but less in their intracellular domains. Although soluble TNF binds TNFR1 with higher affinity than TNFR2 and therefore acts primarily via TNFR1, membranebound TNF activates equally TNFR1 and TNFR2 (3). In most tissues TNF signaling is mediated by TNFR1, whereas TNFR2 is restricted to fewer specific tissues, mostly of an immunological nature (3). Upon ligand binding, TNFR1 trimerizes and recruits TNF receptor-associated death domain protein (TRADD), receptor-interacting protein 1 (RIP1), and TNF receptor-associated factor 2 (TRAF2). This first complex acts as a platform at the plasma membrane to activate the NF-B and MAPK signaling cascades, promoting cell survival and the expression of inflammatory cytokines. In a second step, TNFR1 is internalized into endocytic vesicles together with TRADD and RIP1 and recruits the proapoptotic molecules Fas-associated death domain (FADD) and caspase-8. This complex will initiate the apoptotic cell death program if concurrent anti-apoptotic NF-B activation is absent (4).The TGF-activated kinase 1 (TAK1) complex has a central function in many inflammatory pathways such as the TNFR, interleukin 1 receptor (IL-1R), and several Toll-...