Toll-like receptor-4 signaling and Kupffer cells play pivotal roles in the pathogenesis of non-alcoholic steatohepatitis

Abstract

BACKGROUND/AIMS-Studies in animal models and humans suggest a link between endotoxemia and non-alcoholic steatohepatitis. Since Kupffer cells are responsible for clearing endotoxin and are activated via endotoxin interaction with Toll-like receptor 4 (TLR-4), we examined the relationship between hepatic TLR-4 expression and Kupffer cell content during the genesis of steatohepatitis.METHODS-Male C57BL/6, C3H/HouJ and TLR-4 mutant C3H/HeJ mice were fed control or methionine/choline-deficient diet (MCDD). In one group of C57BL/6 mice, Kupffer cells were depleted by weekly intravenous injections of clodronate liposomes. After 3 weeks, serum ALT activity and portal endotoxin levels were measured. Real-time PCR was used to examine mRNA expression of TLR-4, TLR-2, CD14, MD-2, TGFβ, TNFα CD36 PPAR-α, liver fatty acid binding protein (L-FABP) and collagen α1.RESULTS-We observed histological evidence typical of steatohepatitis, portal endotoxemia and enhanced TLR-4 expression in wild type mice fed MCDD. In contrast, injury and lipid accumulation markers were significantly lower in TLR-4 mutant mice. Destruction of Kupffer cells with clodronate liposomes blunted histological evidence of steatohepatitis and prevented increases in TLR-4 expression.CONCLUSIONS-These findings demonstrate the importance of TLR-4 signaling and underscore a direct link between TLR-4 and Kupffer cells in the pathogenesis of steatohepatitis.

Cellular and molecular mechanisms in liver fibrogenesis

“…However, similar to what reported for ASH, also in experimental NASH and in human NASH patients a clear increase in LPS levels has been detected, which is likely to be due to intestinal bypass [122]. Although the role of fatty acids in activating TLRs is still controversial, although described, interesting studies on transgenic mice have shown that the absence of either TLR4 or TLR9 results in a significant reduction of NASH related parameters like histological score, hepatocyte death (apoptosis and necrosis) and markers of fibrogenesis [123][124][125]. These studies pointed out that TLR9 seems to be relevant for significant IL-1β production from Kupffer cells in experimental NASH not only in relation to steatosis but also in relation to HSC activation and hepatocyte death.…”

“…The question is whether IL-1β, which is known to increase during experimental liver fibrosis, may be considered as a profibrogenic cytokine. The few data available at the moment seems to suggest that this may be in some way true, since liver fibrosis is significantly lower in IL-1R deficient mice [124]. In wild type mice experimental chronic liver injury resulted in up-regulation of IL-1, MMP-9 and MMP-13 levels that were elevated before the onset of HSC activation and liver fibrogenesis, whereas IL-1 receptor-deficient mice exhibited less liver damage and reduced fibrogenesis.…”

Toll-like receptor 4 is involved in the development of fructose-induced hepatic steatosis in mice

“…15,16). Recently, it was shown by Rivera et al 17 that TLR-4 mutant mice fed a methioninecholine-deficient diet are protected against nonalcoholic steatohepatitis. However, whether TLR-4 is also involved in mediating the pivotal effects of fructose on the liver remains to be determined.…”

Toll-like receptor 4: A starting point for proinflammatory signals in fatty liver disease

“…6 Unfortunately, the study by Spruss et al does not provide additional clues to the mechanisms by which fructose intake, endoxemia, and the resulting activation of TLR-4 signaling might promote NAFLD. On the other hand, the experimental results in this work allow the exclusion of the involvement of some important TLR-4 -dependent proinflammatory inducing transcriptional factors (i.e., IRF3 and IF37), suggesting that fructose feeding may lead to NAFLD through an insulin-independent de novo lipogenesis and/or an endotoxin-dependent activation of Kupffer cells.…”

“…3,4 To this purpose, 125 subjects (40 nondiabetic patients with biopsyproven NAFLD and 85 healthy controls) underwent an oral fat tolerance test, 5 with measurement of postprandial plasma lipid responses, and a standard oral glucose tolerance test (OGTT), whose results were elaborated by Minimal Model analysis to assess whole-body, hepatic, and muscle insulin sensitivity and indexes of pancreatic ␤-cell function (namely, CP-genic index [CGI] and Adaptation Index [AI]), as previously described. [5][6][7] Finally, circulating markers of inflammation (Creactive protein), endothelial dysfunction (E-selectin and intercellular adhesion molecule-1 [ICAM-1]) and oxidative stress (nitrotyrosine and oxidized low-density lipoproteins) were measured.…”