TARGETING TNF-RECEPTOR ASSOCIATED FACTORS (TRAFs) FOR THERAPEUTIC INTERVENTION

TNF-Receptors Associated Factors (TRAFs) are the molecules that upon engagement of the TNF-receptor (TNFR) by a TNF-family ligand come first in contact with the activated TNFR, initially acting as docking molecules for kinases and other effector proteins that are recruited to the activated receptor. TRAFs later regulate the subcellular relocalization of the receptor-ligand complex and finally they modulate the extent of the response by controlling the degradation of key proteins in the pathway. In this chapter, we review the involvement of different TRAF family members in the etiology of a variety of pathologies and address the question of whether the use of TNFR-mimic-peptides or small molecule modulators targeting TRAFs might be suitable for therapeutic intervention, discussing the advantages and disadvantages of this strategy. TNF-Receptor Associated Factors (TRAFs) A total of seven TRAF-family members participate in the regulation of as many as 20 TNFRs. TRAF3 and TRAF6 are also involved in the regulation of different members of the Toll-like Receptor (TLR) and interleukin-1 receptor (IL-1R) family. Furthermore, TNFR-family members generally utilize more than one TRAF family member for signaling, often activating similar pathways and even the same downstream effectors. Therefore, the levels of expression of the different TRAF-family members and downstream effectors will likely play an important role in the outcome of the response. The consensus amino-acid motif supporting binding of TRAF1, TRAF2, TRAF3 and TRAF5 to TNFR-family proteins is (P/S/A/T)x(Q/E)E 1, , implying that TRAF1, 2, 3 and 5 potentially interact with the same TNFR family members and that they might compete among themselves for the binding. In contrast, the consensus sequence for TRAF6 is PxExx(Ar/Ac) (where the last amino-acid residue is aromatic or acidic) . The binding motif for TRAF4 is yet to be identified. TRAF7 lacks a TRAF domain and does not directly interact with TNFRs . The crystal structures of TRAFs bound to different TNFR family members have confirmed that the peptide core motif provides the specificity of the binding. However, the actual composition of the core motif as well as other amino-acids adjacent to this core can affect the interaction, by establishing molecular interactions with residues in the TRAF-domain, by decreasing the binding affinity by steric impediments or electrostatic repulsions, or by intramolecular interactions that affect the conformation of the TRAFbinding peptidyl motifs. These results provide a molecular explanation for the differences in binding specificity and affinity of the members of the TRAF family for the different TNFR family members 3, . These results also imply that it would be conceivable to design peptides that could act as agonist or antagonist of the function of different TNFR family members, either by modulating the binding of particular TRAF proteins to those receptors; or by activating TRAF-signaling pathways independently of the activation of the TNFR. In this regard, it has been shown that 11-residue linear peptides bearing the intracellular CD40/TRAF binding motif were sufficient to induce NF-κB activation in WEHI-231 lymphoma cells , thus indicating that small peptides can mimic TNFR signaling. Also, Ye et al. 3 using RANK peptides mimicking its TRAF6 docking site could block osteoclast differentiation in vitro in both primary cells and cell lines, without affecting cell viability. These results support the suitability of using peptides mimicking TRAF-binding motifs to modulate TRAF-family signaling and associated biological