Epidemiological studies have revealed that obesity and diabetes mellitus are independent risk factors for the development of non-alcoholic steatohepatitis (NASH) and hepatocellular carcinoma. However, the debate continues on whether insulin resistance as such is directly associated with NASH and liver tumourigenesis. Here, we investigated the incidence of NASH and liver tumourigenesis in Irs1 ^−/− mice subjected to a long-term high-fat (HF) diet. Our hypothesis was that hepatic steatosis, rather than insulin resistance may be related to the pathophysiology of these conditions.

Mice (8 weeks old, C57Bl/6J) were given free access to standard chow (SC) or an HF diet. The development of NASH and liver tumourigenesis was evaluated after mice had been on the above-mentioned diets for 60 weeks. Similarly, Irs1 ^−/− mice were also subjected to an HF diet for 60 weeks.

Long-term HF diet loading, which causes obesity and insulin resistance, was sufficient to induce NASH and liver tumourigenesis in the C57Bl/6J mice. Obesity and insulin resistance were reduced by switching mice from the HF diet to SC, which also protected these mice against the development of NASH and liver tumourigenesis. However, compared with wild-type mice fed the HF diet, Irs1 ^−/− mice fed the HF diet were dramatically protected against NASH and liver tumourigenesis despite the presence of severe insulin resistance and marked postprandial hyperglycaemia.

IRS-1 inhibition might protect against HF diet-induced NASH and liver tumourigenesis, despite the presence of insulin resistance.