The TRAF protein family comprises seven E3 ligase members, including TRAF6. Distinct receptor signaling pathways have different members, such as the IL-1 receptor family, T-cell receptor (TCR), IL-17 receptor, TNF receptor. TRAF-dependent signaling pathways contribute to the control of diverse cellular processes, including survival, proliferation, differentiation, and cytokine production.  TRAF6 interacts with the heterodimeric E2 enzyme complex Ubc13–Uev1a to attach Lys⁶³-linked ubiquitin chains to its substrate proteins. These Lys⁶³-linked ubiquitin chains are necessary to activate innate and adaptive immune responses mainly through the NF-κB axis. Of note, the TRAF6 ligase activity is strictly determined by its protein-protein interaction with a specific set of E2 enzymes, including Ubc13. Thus, targeting TRAF6 E3 activity requires the identification of protein-protein interaction inhibitors, which disrupt the TRAF6–Ubc13 interaction.  C25-140 is a first-in-class, fairly selective TRAF6-Ubc13 inhibitor. It directly binds to TRAF6 and blocks the interaction of TRAF6 with Ubc13.

C25-140 is a first-in-class, fairly selective TRAF6-Ubc13 inhibitor, which combats autoimmunity.

C25-140 affects cIAP1, an E3 ligase generating Lys⁶³-linked polyubiquitin chains similar to TRAF6. Notably, C25-140 does not influence several other E3 ligases (MDM2, TRIM63, ITCH, E6AP, and RNF4) building other types of poly-ubiquitin chains. Therefore, It shows C25-140 acts on the E3 ligase side. Furthermore, C25-140 efficiently inhibits IL-1β– and TNFα–mediated receptor signaling in the context of cytokine activation. C25-140 also affects TCR-mediated immune response in a human-derived cell line. Besides, C25-140 is effective in cytokine signaling of primary human and murine cells.

C25-140 has appropriate properties in mouse studies for in vivo efficacy testing. C25-140 is able to ameliorate symptoms of autoimmune psoriasis. In addition, C25-140 ameliorates symptoms of rheumatoid arthritis (RA) in a preclinical mouse model.

In summary, the first-in-class TRAF6-Ubc13 inhibitor C25-140 expands the toolbox for studying the impact of the ubiquitin system on immune signaling and underscores the importance of TRAF6 E3 ligase activity in psoriasis and rheumatoid arthritis.

Reference:

Brenke JK, et al. J Biol Chem. 2018 Aug 24;293(34):13191-13203.