MicroRNA in TLR signaling and endotoxin tolerance

Abstract

Toll-like receptors (TLRs) in innate immune cells are the prime cellular sensors for microbial components. TLR activation leads to the production of proinflammatory mediators and thus TLR signaling must be properly regulated by various mechanisms to maintain homeostasis. TLR4-ligand lipopolysaccharide (LPS)-induced tolerance or cross-tolerance is one such mechanism, and it plays an important role in innate immunity. Tolerance is established and sustained by the activity of the microRNA miR-146a, which is known to target key elements of the myeloid differentiation factor 88 (MyD88) signaling pathway, including IL-1 receptor-associated kinase (IRAK1), IRAK2 and tumor-necrosis factor (TNF) receptor-associated factor 6 (TRAF6). In this review, we comprehensively examine the TLR signaling involved in innate immunity, with special focus on LPS-induced tolerance. The function of TLR ligand-induced microRNAs, including miR-146a, miR-155 and miR-132, in regulating inflammatory mediators, and their impact on the immune system and human diseases, are discussed. Modulation of these microRNAs may affect TLR pathway activation and help to develop therapeutics against inflammatory diseases.

Figure 1

A schematic summary of the response to LPS stimulation in THP-1 cells. (a) Unprimed cells challenged with high-dose LPS at 1000 ng/ml produce a prominent TNF-α response. There are respective increases in both IRAK1 and TRAF6, which peaked at 2–4 h. The increase in miR-146a expression starts at 2 h and continues to increase in the presence of LPS. (b) Tolerized cells are generated by priming with a low dose LPS at 10 ng/ml leading to a rapid and transient TNF-α, IRAK1 and TRAF6 responses. TNF-α production decreases as miR-146a expression starts to increase. Tolerized cells do not respond to high-dose LPS challenge unlike the untolerized control, which is responsive to LPS at this stage. The sustained level of miR-146a at 18 h apparently blocks the otherwise robust TNF-α response. IRAK, IL-1 receptor-associated kinase; LPS, lipopolysaccharide; TNF, tumor necrosis factor; TRAF6, TNF receptor-associated factor 6.

Figure 2

A model of the role of miR-146a in LPS–TLR4-mediated signal transduction contributing to endotoxin tolerance and cross-tolerance. LPS binds to the LPS-binding protein, which in turn is coupled to CD14 on the cell surface of monocytes. Subsequently, LPS-CD14 interacts with TLR4 and forms a complex with another accessory protein MD-2. The TLR4 signaling cascade is initiated after binding with the adaptor protein MyD88. The activation leads to the helical assembly of the so-called myddosome complex, involving six MyD88, four IRAK4 and four IRAK2/1 molecules, which in turn recruits TRAF6. This chain of events triggers the activation and translocation of NF-κB and results in the transcription of cytokines, such as TNF-α and miR-146a. As shown, miR-146a downregulates expression of IRAK1/2 and TRAF6^{, ,} and the high level of expressed miR-146a blocks subsequent LPS and other TLR ligand challenges. See text for other miRNAs that are also induced by LPS. IRAK, IL-1 receptor-associated kinase; LPS, lipopolysaccharide; MD2, myeloid differentiation protein-2; microRNA, miRNA; MyD88, myeloid differentiation factor 88; NF-κB, nuclear factor-kappa B; TLR, Toll-like receptor; TNF, tumor necrosis factor; TRAF6, TNF receptor-associated factor 6.