Cytoplasmic Linker Protein CLIP170 Negatively Regulates TLR4 Signaling by Targeting the TLR Adaptor Protein TIRAP

Abstract

Cytoplasmic linker protein 170 (CLIP170) is a CAP-Gly domain-containing protein that is associated with the plus end of growing microtubules and implicated in various cellular processes, including the regulation of microtubule dynamics, cell migration, and intracellular transport. Our studies revealed a previously unrecognized property and role of CLIP170. We identified CLIP170 as one of the interacting partners of a effector protein TcpB that negatively regulates TLR2 and TLR4 signaling. In this study, we demonstrate that CLIP170 interacts with the TLR2 and TLR4 adaptor protein TIRAP. Furthermore, our studies revealed that CLIP170 induces ubiquitination and subsequent degradation of TIRAP to negatively regulate TLR4-mediated proinflammatory responses. Overexpression of CLIP170 in mouse macrophages suppressed the LPS-induced expression of IL-6 and TNF-α whereas silencing of endogenous CLIP170 potentiated the levels of proinflammatory cytokines. In vivo silencing of CLIP170 in C57BL/6 mice by CLIP170-specific small interfering RNA enhanced LPS-induced IL-6 and TNF-α expression. Furthermore, we found that LPS modulates the expression of CLIP170 in mouse macrophages. Overall, our experimental data suggest that CLIP170 serves as an intrinsic negative regulator of TLR4 signaling that targets TIRAP.

The neurosteroid pregnenolone promotes degradation of key proteins in the innate immune signaling to suppress inflammation

“…We previously reported that CLIP170 promotes enhanced ubiquitination and degradation of TIRAP, which attenuated TLR2/4-mediated signaling (19). Because pregnenolone induces activation of CLIP170, we sought to analyze whether pregnenolone enhances the TIRAP degradation and TLR suppression properties of CLIP170.…”

“…Pregnenolone-induced degradation of TIRAP and TLR2 resulted in attenuation of LPS or Pam3CysSerLys4 (Pam3CSK4) mediated secretion of TNF␣ in macrophages and microglial cells. We previously reported that CLIP170 induces ubiquitination and degradation of TIRAP (19). Pregnenolone induced activation of CLIP170, which enhanced its ubiquitin ligase-like property toward TIRAP and suppression of LPS-induced TLR4 signaling.…”

Paradoxical Roles of the MAL/Tirap Adaptor in Pathologies

“…Similarly, Brucella produces a TIR domain-containing protein (TcpB/Btp1) to selectively target MAL and inhibit NF-κB activation, which is essential for intracellular Brucella survival and replication ( Figure 1C) (71). Moreover, TcpB/Btp1 was also described to target CLIP170 enhancing MAL proteasome-mediated degradation (61). Furthermore, a number of viral proteins were described to interfere with innate immune signaling, highlighting the implication of the TLR pathway in antiviral immunity (72).…”

“…Tyrosine phosphorylation of MAL via Btk is necessary for the SOCS-1-mediated degradation. Moreover, MAL ubiquitination and degradation was also shown to be mediated by Cytoplasmic Linker Protein 170 (CLIP170) that is implicated in regulation of microtubule dynamics, cell migration and intracellular transport (7,61). A study also demonstrated that MAL phosphorylation at Thr28 within its PBM reduces PI interactions and cell membrane targeting, leading to its ubiquitination and degradation (62).…”

The metabolic regulator Lamtor5 suppresses inflammatory signaling via regulating mTOR-mediated TLR4 degradation

“… 8 – 11 Thus, the intracellular trafficking and subcellular compartmentalization of TLR4 constitute a key mechanism that dictates the direction and amplitude of inflammatory signaling. 12 , 13 In particular, cellular degradation systems, such as lysosome- and proteasome-mediated catabolic processes, play a pivotal role in controlling TLR4 destiny and immune homeostasis.…”