Glutaminolysis-ammonia-urea Cycle Axis, Non-alcoholic Fatty Liver Disease Progression and Development of Novel Therapies

Abstract

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing worldwide, reflecting the current epidemics of obesity, insulin resistance, type 2 diabetes mellitus, and metabolic syndrome. NAFLD is characterized by the accumulation of fat in the liver, and is known to be a cause of cirrhosis. Although many pathways have been proposed, the cause of NAFLD-linked fibrosis progression is still unclear, which posed challenges for the development of new therapies to prevent NASH-related cirrhosis and hepatocellular carcinoma. Cirrhosis is associated with activation of hepatic stellate cells (HSC) and accumulation of excess extracellular matrix proteins, and inhibiting the activation of HSCs would be expected to slow the progression of NAFLD-cirrhosis. Multiple molecular signals and pathways such as oxidative stress and glutaminolysis have been reported to promote HSC activation. Both mechanisms are plausible antifibrotic targets in NASH, as the activation of HSCs the proliferation of myofibroblasts depend on those processes. This review summarizes the role of the glutaminolysis-ammonia-urea cycle axis in the context of NAFLD progression, and shows how the axis could be a novel therapeutic target.

Keywords: Ammonia; Cirrhosis; Fibrosis; Glutaminolysis; Non-alcoholic fatty liver disease; Urea.

Fig. 1. Spectrum of non-alcoholic fatty liver disease (NAFLD).

Fig. 2. Glutamine metabolism pathway and its association with NASH-fibrogenesis.

Upregulated pathways are shown in red. CPS, carbamoyl phosphate dehydrogenase; FXR, farnesoid X receptor; GLS, glutaminase; GLUD, glutamate dehydrogenase; GS, glutamine synthetase; NASH, non-alcoholic steatohepatitis; OCT, ornithine transcarbamoylase; SDH, succinate dehydrogenase.

Fig. 3. Drug discovery process.

Fig. 4. Targeting glutaminolysis: examples of selected glutaminase inhibitors.