Urea cycle dysregulation in non-alcoholic fatty liver disease

Abstract

Background & aims: In non-alcoholic steatohepatitis (NASH), the function of urea cycle enzymes (UCEs) may be affected, resulting in hyperammonemia and the risk of disease progression. We aimed to determine whether the expression and function of UCEs are altered in an animal model of NASH and in patients with non-alcoholic fatty liver disease (NAFLD), and whether this process is reversible.

Methods: Rats were first fed a high-fat, high-cholesterol diet for 10 months to induce NASH, before being switched onto a normal chow diet to recover. In humans, we obtained liver biopsies from 20 patients with steatosis and 15 with NASH. Primary rat hepatocytes were isolated and cultured with free fatty acids. We measured the gene and protein expression of ornithine transcarbamylase (OTC) and carbamoylphosphate synthetase (CPS1), as well as OTC activity, and ammonia concentrations. Moreover, we assessed the promoter methylation status of OTC and CPS1 in rats, humans and steatotic hepatocytes.

Results: In NASH animals, gene and protein expression of OTC and CPS1, and the activity of OTC, were reversibly reduced. Hypermethylation of Otc promoter genes was also observed. Additionally, in patients with NAFLD, OTC enzyme concentration and activity were reduced and ammonia concentrations were increased, which was further exacerbated in those with NASH. Furthermore, OTC and CPS1 promoter regions were hypermethylated. In primary hepatocytes, induction of steatosis was associated with Otc promoter hypermethylation, a reduction in the gene expression of Otc and Cps1, and an increase in ammonia concentration in the supernatant.

Conclusion: NASH is associated with a reduction in the gene and protein expression, and activity, of UCEs. This results in hyperammonemia, possibly through hypermethylation of UCE genes and impairment of urea synthesis. Our investigations are the first to describe a link between NASH, the function of UCEs, and hyperammonemia, providing a novel therapeutic target.

Lay summary: In patients with fatty liver disease, the enzymes that convert nitrogen waste into urea may be affected, leading to the accumulation of ammonia, which is toxic. This accumulation of ammonia can lead to scar tissue development, increasing the risk of disease progression. In this study, we show that fat accumulation in the liver produces a reversible reduction in the function of the enzymes that are involved in detoxification of ammonia. These data provide potential new targets for the treatment of fatty liver disease.

Keywords: Ammonia; Epigenetic modifications; Non-alcoholic fatty liver disease; Non-alcoholic steatohepatitis; Urea synthesis.