We read with great interest the article entitled “Toll-like receptor 4 is involved in the development of fructose-induced hepatic steatosis in mice”, published in a recent issue of HEPATOLOGY.1 In this study, Spruss et al. verified the hypothesis that Toll-like receptor 4 (TLR-4) may play a central role in the onset of fructose-induced nonalcoholic fatty liver disease (NAFLD). To this aim, the authors used wild-type (C3H/HouJ) mice and TLR-4 mutant (C3H/HeJ) mice, both fed plain water or 30% fructose-enriched solution for 8 weeks.
As already described by other studies,2, 3 chronic intake of 30% fructose solution leads to hepatic steatosis and some features of metabolic syndrome in wild-type animals, including the increase of body and liver weight, hepatic triglyceride levels, and plasma levels of alanine aminotransferase (ALT). Interestingly, TLR-4 mutants fed water presented only a weak decrease of liver weight and hepatic triglycerides with respect to wild-type animals fed water, and the enrichment with fructose exclusively caused the restoration of the significantly increased levels of these two parameters. These results clearly suggest that the presence of TLR-4 is essential to explain liver damage, body weight gain, and ALT impairment due to the fructose intake.
Furthermore, the authors found that plasma endotoxin levels were significantly increased both in wild-type and mutant mice fed chronically with a 30% fructose solution, in comparison to water-fed controls.
The role of fructose in NAFLD development was not entirely unknown to researchers. In particular, a recent work4 demonstrates that patients with NAFLD have a significantly greater consumption of fructose than controls, and an increased hepatic expression of fructokinase messenger RNA. Although the role of TLR-4 in carbohydrate-dependent NAFLD has been only recently suggested by Thuy and colleagues,5 they have pinpointed one of the potential mechanisms through which fructose could participate in NAFLD development and progression in humans: a carbohydrate-rich diet may produce ethanol when intestinal stasis favors bacterial overgrowth in the upper parts of the gastrointestinal tract. The increased portal endotoxemia could initiate TLR signaling and induce necroinflammation, which characterizes steatohepatitis, the most advanced form of NAFLD. Accordingly, this study also highlighted significant correlations between hepatic expression of TLR-4, plasminogen activator inhibitor 1, and endotoxin, even though they are still unable to explain the molecular signaling pathways.
Interestingly, another recent study investigated the potential importance of Kupffer cells and TLR-4 in the pathogenic mechanisms underlying nonalcoholic steatohepatitis induced by a methionine-deficient and choline-deficient diet.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. In this last hypothesis, an interaction network which involves TLR-4, Myd88, c-Jun N-terminal kinase, and nuclear factor κB might induce tumor necrosis factor-alpha production and release, oxidative stress, and insulin resistance.7
We believe that although Spruss et al. present a well-conducted study, the precise role of TLR-4–dependent pathways in NAFLD requires further experimentation. In fact, it is possible that new additional signaling proteins of innate immunity, as yet uncovered, may be involved in the necroinflammatory process and in the progression to steatohepatitis and fibrosis.