Background: Nonalcoholic steatohepatitis (NASH) is predicted to become the primary cause of end-stage liver disease necessitating transplantation. However, recurrence of NASH and fibrosis post-transplantation is common. The etiology of NAFLD progression, and why some patients develop recurrent NASH vs. uncomplicated NAFLD, are unknown. High fat diets have been implicated in the development of metabolic syndrome and NASH. However, current murine models of obesity do not induce representative hepatic sequelae. We hypothesized that mice exposed to a diet poor in unsaturated fats would develop a hepatic steatosis with more severe inflammation.

Methods: Adult male C57Bl/6 mice were fed either control diet (CD), traditional lard fat diet (LD) high in unsaturated fat, or a milkfat diet (MD) high in saturated fat, for 16 weeks.

Results: As compared to LD, MD promoted more severe steatosis as marked by total liver weight and Oil red O staining. MD feeding also promoted a NASH like phenotype characterized by inflammatory cell infiltrate, hepatocellular ballooning, fibrosis, oxidative stress, and elevated serum ALT. Hepatic expression of inflammatory cytokines (CCL2 and TNF-Α), as well as TGF-Β, were elevated in MD livers above levels seen in LD or CD. These mice also expressed elevated sphingosine kinase 1 (SK1). We identified SK1 expression to be increased in human NASH livers. Thus, we proposed SK1, which generates the pro-inflammatory signaling lipid sphingosine-1-phosphate, as a mediator of NAFLD progression. SK1 null mice on MD developed less severe hepatic steatosis and were largely protected from histopathologic changes occurring in the wild type MD livers, including fibrosis. SK1 KO animals had fewer total neutrophils and F4/80 positive cells (monocytes/macrophages) within the liver. Serum ALT levels and hepatic expression of CCL2 and TGF-Β were also decreased in the SK1 knockout MD fed animals. Because SK1 is activated and transcriptionally unregulated by TLR4, which is activated within the steatotic liver, we investigated the role of TLR4. MD fed TLR4 KO animals were protected from NAFLD progression as indicated by ALT and liver weights.

Conclusions: In summary, we demonstrate that dietary fat composition deferentially regulates high fat diet-induced hepatic steatosis and inflammation, and present a novel model of fatty liver disease. Finally, we provide evidence that SK1 and TLR4 activity play roles in the progression of NAFLD to NASH.

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