Nonalcoholic fatty liver disease (NAFLD) is the most common cause of human liver dysfunction, and an estimated 30% of the general population of the USA suffer from excessive fat accumulation in the liver. NAFLD is broadly divided into two subgroups: (1) non-progressive simple steatosis, and (2) nonalcoholic steatohepatitis (NASH) with worsening degeneration and fibrosis. In cases of progressive chronic liver disease, NASH may progress from steatohepatitis to liver cirrhosis, and may eventually lead to hepatocellular carcinoma. A lack of knowledge about the mechanisms that lead to NASH progression prevent advances in drug development.
The authors establish a NASH model using the ricefish medaka (Oryzias latipes), which were fed a high-fat diet (HFD) for 12 weeks (HFD-medaka). HFDmedaka exhibited hyperlipidemia, hyperglycemia and hepatic steatosis, with progressive hepatocyte degeneration that led to liver dysfunction. These findings are consistent with NASH progression in humans.
Expected genetic changes were also detected in the NASH model, including increased expression of lipogenic genes and decreased expression of lipolytic genes after HFD feeding. The concentrations of n-3 polyunsaturated fatty acids (PUFAs) and n-6 PUFAs declined, and the n-3:n-6 ratio was reduced in HFD-fed animals, which reflects the biochemistry in humans. Treating HFD-fed medaka with a drug used to treat human NASH [n-3 PUFA eicosapentaenoic acid (EPA)] mitigated disease, restoring normal liver fatty acid composition and normal expression levels of lipogenic and lipolytic genes. Moreover, medaka that were fed a diet deficient in n-3 PUFAs developed features of NASH. This NASH model demonstrates that the proportion of n-3 PUFAs that are present in the liver plays an important role in the progress of NASH pathology.
Implications and future directions
The medaka genome was recently sequenced, making it amenable to forward genetic screens with N-ethyl-N-nitrosourea (ENU), and morpholino knockdown. These techniques promote the analysis of specific gene mutations in these fish. Thus, with their high reproductive rate, rapid maturation and low maintenance costs, medaka compare favorably with rodents as experimental animals. This NASH model is a useful tool to study the unknown mechanisms underlying human liver disease, and should eventually be useful for therapeutic screen tests.