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

Abstract

A link between dietary fructose intake, gut-derived endotoxemia, and nonalcoholic fatty liver disease (NAFLD) has been suggested by the results of human and animal studies. To further investigate the role of gut-derived endotoxin in the onset of fructose-induced NAFLD, Toll-like receptor (TLR-) 4-mutant (C3H/HeJ) mice and wildtype (C3H/HouJ) mice were either fed plain water or water enriched with 30% fructose for 8 weeks. Hepatic steatosis, plasma alanine aminotransferase (ALT), and markers of insulin resistance as well as portal endotoxin levels were determined. Hepatic levels of myeloid differentiation factor 88 (MyD88), interferon regulatory factor (IRF) 3 and 7, and tumor necrosis factor alpha (TNF␣) as well as markers of lipid peroxidation were assessed. Chronic intake of 30% fructose solution caused a significant increase in hepatic steatosis and plasma ALT levels in wildtype animals in comparison to water controls. In fructose-fed TLR-4 mutant mice, hepatic triglyceride accumulation was significantly reduced by Ϸ40% in comparison to fructose-fed wildtype mice and plasma ALT levels were at the level of water-fed controls. No difference in portal endotoxin concentration between fructose-fed wildtype and TLR-4-mutant animals was detected. In contrast, hepatic lipid peroxidation, MyD88, and TNF␣ levels were significantly decreased in fructose-fed TLR-4-mutant mice in comparison to fructose-fed wildtype mice, whereas IRF3 and IRF7 expression remained unchanged. Markers of insulin resistance (e.g., plasma TNF␣, retinol binding protein 4, and hepatic phospho-AKT) were only altered in fructose-fed wildtype animals. Conclusion: Taken together, these data further support the hypothesis that in mice the onset of fructose-induced NAFLD is associated with intestinal bacterial overgrowth and increased intestinal permeability, subsequently leading to an endotoxin-dependent activation of hepatic Kupffer cells.

Metformin protects against the development of fructose-induced steatosis in mice: role of the intestinal barrier function

“…In line with previous studies of our own group and those of others, 3,4,7,9,16 chronic intake of fructose resulted in a marked accumulation of triglycerides in the liver (B3-fold in comparison with controls, Po0.05; Figure 1). In contrast, in fructose-fed mice concomitantly treated with metformin triglyceride levels were only increased by B50% in comparison with the respective controls (Po0.05).…”

“…[5][6][7][8] We recently found that the development of fructose-induced steatosis was associated with an increased translocation of bacterial endotoxin and activation of toll-like receptor (TLR)-4-dependent signalling cascades in the liver. 7,9 Interestingly, similar alterations were not found in mice chronically exposed to glucose; however, chronic intake of glucose resulted in a markedly higher weight gain of mice in comparison with fructose. 7 Metformin (dimethylbiguanide) is a drug used in the treatment of type 2 diabetes for 450 years.…”

“…4,20 Indeed, we showed that chronic intake of fructose, but not glucose, may lead to a decrease in protein levels of the tight junction protein occludin and an increased translocation of bacterial endotoxins subsequently leading to a TLR-4-dependent activation of hepatic Kupffer cells and an increased release of the proinflammatory cytokine TNFa. 7,9 A sterilization of the gut with non-resorbable antibiotics or the loss of the endotoxin receptor TLR-4 was found to be associated with a marked protection on animals from the onset of Effect of metformin on fructose-induced steatosis A Spruss et al fructose-induced NAFLD. 7,9 The oral antidiabetic drug metformin has been shown to have a beneficial effect not only on the development but also on the progression of NAFLD in humans and animals (eg, ob/ob mice) as well as on the development of alcohol-induced liver damage.…”

“…7,9 A sterilization of the gut with non-resorbable antibiotics or the loss of the endotoxin receptor TLR-4 was found to be associated with a marked protection on animals from the onset of Effect of metformin on fructose-induced steatosis A Spruss et al fructose-induced NAFLD. 7,9 The oral antidiabetic drug metformin has been shown to have a beneficial effect not only on the development but also on the progression of NAFLD in humans and animals (eg, ob/ob mice) as well as on the development of alcohol-induced liver damage. 13,15,24 In these studies, it was shown that the protective effects of metformin on the liver were mainly attributed to an improvement of insulin signalling the liver.…”

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.…”