Lysosome-associated small Rab GTPase Rab7b negatively regulates TLR4 signaling in macrophages by promoting lysosomal degradation of TLR4

Abstract

Toll-like receptor 4 (TLR4) initiates both myeloid differentiation factor 88 (MyD88)-dependent and Toll/interleukin (IL)-1R domain-containing adapter, inducing interferon (IFN)-␤-dependent signaling, leading to production of proinflammatory mediators and type I interferon (IFN) to eliminate pathogens. However, uncontrolled TLR4 activation may contribute to pathogenesis of autoimmune and inflammatory diseases. TLR4 is transported from the plasma membrane to the endosome for ubiqutination and to the lysosome for degradation, and downregulation of TLR4 expression or promotion of TLR4 degradation are important ways for negative regulation of TLR4 signaling. We previously identified a lysosome-associated small guanosine triphosphatase (GTPase) Rab7b that may be involved in lysosomal trafficking and degradation of proteins. Here we demonstrate that Rab7b can negatively regulate lipopolysaccharide (LPS)-induced production of tumor necrosis factor (TNF)-␣, IL-6, nitric oxide, and IFN-␤, and potentiate LPS-induced activation of mitogen-activated protein kinase, nuclear factor B, and IFN regulatory factor 3 signaling pathways in macrophages by promoting the degradation of TLR4. Rab7b is localized in LAMP-1-positive subcellular compartments and colocalized with TLR4 after LPS treatment and can decrease the protein level of TLR4. Our findings suggest that Rab7b is a negative regulator of TLR4 signaling, potentially by promoting the translocation of TLR4 into lysosomes for degradation. IntroductionToll-like receptors (TLRs) play a critical role in innate immunity by recognizing structurally conserved bacterial and viral components termed pathogen-associated molecular patterns. [1][2][3] TLRs activate signaling through the Toll/IL-1R (TIR) domain found in the cytoplasmic tails of these proteins, which in turn triggers the binding of the adaptor protein myeloid differentiation factor 88 (MyD88) to the TIR domain, allowing for interaction and phosphorylation of IL-1R-associated kinases and subsequent activation of tumor necrosis factor receptor (TNF)-associated factor 6 (TRAF-6), leading to the activation of nuclear factor B (NF-B) and mitogen-activated protein kinase (MAPK) pathways and the induction of proinflammatory cytokines. [1][2][3] However, recent studies have identified molecules involved in the MyD88-independent signaling for TLR3 and TLR4, including the TIR domain-containing adaptor protein (TIRAP/MAL), as a second adaptor instead of MyD88 for activation of 4,5 and the TIR domain-containing adapter inducing interferon (IFN)-␤ (TRIF) as a critical MyD88-independent adaptor used by TLR3 and TLR4 to regulate type I IFN production. 6,7 The signaling receptor for lipopolysaccharide (LPS) is TLR4/MD-2, which receives LPS from CD14. LPS delivered by CD14 to TLR4/MD-2 initiates a signaling cascade through the TIR domain containing adaptors MyD88, TIRAP/ MAL, TRIF, and TRAM (TRIF-related adaptor molecule), which eventually leads to activation of MAPK (including extracellular signal-regulated kinase [ERK], c-Jun N-terminal k...

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