The inflammatory cytokine tumor necrosis factor (TNF) alpha exerts its biological activity through the membrane bound (tmTNF-alpha) or soluble (sTNF-alpha) forms. Both sTNF-alpha and tmTNF-alpha ligands interact with either TNFR1 (p55, CD120a) or TNFR2 (p75, CD120b) on a variety of immune and nonimmune cell types.
Transmembrane TNF-alpha functions as a bipolar molecule that can transmit signals both as a ligand and as a receptor in a cell-to-cell contact fashion (Eissner G et al. 2004; Zhang H et al. 2008; Juhász K et al. 2013). As a ligand tmTNF-alpha binds to receptors TNFR1 or -R2 on TNF-responsive cells to initiate signaling pathways that lead among other things to cell death or NFkB-mediated expression of inflammatory genes. Binding of tmTNF-alpha by TNFRs can simultaneously induce a reverse signals transmiting back to the tmTNF-bearing cell. The molecular basis of TNF reverse signaling remains largely unexplored.
BAG4, also known as silencer of death domain (SODD), belongs to the BAG family of anti-apoptotic proteins. Mammalian BAG4 was found to associate with TNFR1 preventing receptor signaling in the absence of ligand (Jiang Y et al. 1999; Miki K and Eddy EM 2002). Furthermore, crystallographic data and biochemical analysis showed that TNFR1 forms inactive homodimers or homotrimers in the absence of TNF by the N-terminal domain, the pre ligand assembly domain (PLAD) (Chan FK et al. 2000; Wang YL et al. 2011). Upon TNF-alpha binding BAG4 is quickly released from TNFR1 and three receptor molecules form a complex with the TNF trimer.
The TNF-alpha:TNFR1 receptor complex then transmits the signal leading to cell death or survival. However, it remains unclear whether BAG4 binds to death domain of monomeric TNFR1 to prevent receptor oligomerization or recognizes receptor trimers to facilitate ATP-dependent TNFR1 trimer disassembly (Jiang Y et al. 1999; Miki K and Eddy EM 2002). Additionally, BAG4 is known to interact with HSP70, death receptor 3, and the anti-apoptotic protein Bcl-2 (Antoku et al. 2001; Brockmann et al. 2004; Jiang et al. 1999).
BAG4-overexpressing HeLa cells showed reduced cellular sensitivity to treatment with extracellular TNFalpha and CD95 ligand (Eichholtz-Wirth H et al. 2003). In addition, increased expression level of BAG4 in tumor cells leads to resistance of TNFalpha-induced cell death and is associated with pancreatic cancer, some types of melanoma, acute lymphoblastic leukemia etc.(Ozawa et al. 2000; Tao H et ql. 2007; Reuland SN et al. 2013). The physiological relevance of BAG4 for TNFR1 signaling, however, is difficult to judge because BAG4 knockout mice have no or only a mild effect on pro-inflammatory TNF signaling and give no evidence for an inhibitory role of BAG4 in TNFR1-induced cell death (Takada H et al. 2003; Endres R et al. 2003).