3.1.3. TLR4

TLR4 is the most thoroughly studied PRR in the setting of CLD. Hepatic and serum TLR4 is significantly upregulated in NASH patients, with elevated levels of circulating LPS in peripheral [100, 116] and portal vein blood [155]. High serum levels of TLR4 have also been proposed as a predictive non-invasive marker for liver fibrosis development in NASH patients [156]. Although hepatic expression of TLR4 has not been studied in patients with ALD, peripheral blood mononuclear cells (PBMCs) from patients with ALD showed sensitized responses towards LPS treatment [117].

The role of TLR4 in murine models of liver inflammation has been well studied. TLR4-dependent ROS production and TLR4-dependent interferon regulatory factor (IRF)3 activation in the liver are required to drive hepatic inflammation in mice with alcoholic hepatitis [120]. A similar study showed that hepatic inflammatory cytokines were significantly downregulated in hepatocyte-selective-TLR4-deficient mice fed with a liquid diet containing 5% ethanol [119]. The importance of TLR4 in NASH development was further emphasized in a murine NASH model using high-fat, highcholesterol (HFHC)-diet fed ApoE KO mice, showing a TLR4-mediated ROS production and triggering pro-inflammatory cytokine expression in KC [122]. Linking TLR4 to NAFLD pathogenesis, fatty acids such as palmitate can also trigger ROS production in a TLR4-dependent manner, inducing IL-1β and TNF-α production from liver macrophages [121].

TLR4-mediated fibrosis has been interrogated in a variety of mouse models. In BDL mice, the TLR4–MyD88–NF-κB pathway in HSCs has been shown to upregulate pro-inflammatory cytokine production, α-SMA, TIMP1, and TGF-β expression, and ECM deposition [105,123]. In addition, TLR4-mediated downregulation of Bambi (a TGFβ pseudoreceptor) was shown to sensitize quiescent HSCs for subsequent activation [105,123]. Using the transforming growth factor beta-activated kinase 1 (TAK1) KO murine model of fibrosis [124], TLR4 and MyD88 double KO mice also demonstrated reduced α-SMA, TIMP1, and TGFβ expression and collagen deposition, supporting the involvement of TLR4–MyD88–NF-κB signaling in hepatic fibrogenesis [124].
