E3 Ubiquitin Ligase Tripartite Motif 38 Negatively Regulates TLR-Mediated Immune Responses by Proteasomal Degradation of TNF Receptor-Associated Factor 6 in Macrophages

Activation of TLR signaling in the innate immune cells is critical for the elimination of invading microorganisms. However, uncontrolled activation may lead to autoimmune and inflammatory diseases. In this article, we report the identification of tripartite motif (TRIM) 38 as a negative feedback regulator in TLR signaling by targeting TNFR-associated factor 6 (TRAF6). TRIM38 was induced by TLR stimulation in an NF-kappa B-dependent manner in macrophages. Knockdown of TRIM38 expression by small interfering RNA resulted in augmented activation of NF-kappa B and MAPKs, and enhanced expression of proinflammatory cytokines, whereas overexpression of TRIM38 has an opposite effect. As an E3 ligase, TRIM38 bound to TRAF6 and promoted K48-linked polyubiquitination, which led to the proteasomal degradation of TRAF6. Consistently, knockdown of TRIM38 expression resulted in higher protein level of TRAF6 in primary macrophages. Our findings defined a novel function for TRIM38 to prevent excessive TLR-induced inflammatory responses through proteasomal degradation of TRAF6. The Journal of Immunology, 2012, 188: 2567-2574.