Background Non-alcoholic fatty liver disease is an obesity-related chronic liver disorder ranging from simple steatosis to non-alcoholic steatohepatitis (NASH), which may progress to liver fibrosis and cirrhosis.

Objective Tto investigate the role of Toll-like receptor (TLR) 4 in mediating the transition from steatosis to inflammation.

Methods ApoE^−/−/TLR4^mut mice and ApoE^−/−/TLR4 wild-type mice (ApoE^−/−/TLR4-WT) were generated by cross-breeding an ApoE-deficient (ApoE^−/−) strain with TLR4-mutant (TLR4^mut) mice, which were fed with high-fat, high-cholesterol (HFHC) diet to induce obesity.

Results ApoE^−/−/TLR4-WT mice fed with an HFHC diet for 12 weeks developed typical pathological features of NASH, which is associated with obesity and the metabolic syndrome. By contrast, ApoE^−/−/TLR4^mut mice lacking functional TLR4 were resistant to HFHC diet-induced liver inflammation and injury and were less susceptible to the diet-induced production of reactive oxygen species (ROS) and proinflammatory cytokines. In ApoE^−/−/TLR4-WT mice, X-box binding protein-1 (XBP-1), a transcription factor involved in the unfolded protein responses, was activated in the liver by an HFHC diet, whereas XBP-1 activation was abrogated in ApoE^−/−/TLR4^mut mice. In primary rat Kupffer cells, endotoxin induced XBP-1 activation through ROS production, whereas siRNA-mediated knockdown of XBP-1 expression resulted in a marked attenuation in endotoxin-evoked NF-κB activation and cytokine production. Furthermore, adenovirus-mediated expression of dominant negative XBP-1 led to a significant attenuation in HFHC diet-induced liver inflammation and injury in mice.

Conclusions These findings support the key role of TLR4 in Kupffer cells in mediating the progression of simple steatosis to NASH, by inducing ROS-dependent activation of XBP-1.