Hepatic Accumulation of Hypoxanthine: A Link Between Hyperuricemia and Nonalcoholic Fatty Liver Disease

Abstract

An elevated level of plasma uric acid has been widely recognized as a risk factor for non-alcoholic fatty liver disease (NAFLD), where oxidative stress and inflammation play an important role in the pathophysiology of the disease. Although the complete molecular mechanisms involved remain unknown, while under physiological conditions uric acid presents antioxidant properties, hyperuricemia has been linked to oxidative stress, chronic low-grade inflammation, and insulin resistance, basic signs of NAFLD. Employing in vivo experimentation, we aim to investigate whether a high-fat diet rich in cholesterol (HFD), modifies the metabolism of purines in close relationship to molecular events associated with the development of NAFLD. In vitro experiments employing HepG2 cells are also carried out to study the phenomenon of oxidative stress. Adult male rabbits were fed for 8 weeks an HFD to induce NAFLD. At the beginning of the experiment and every 15 d until the completion of the study, plasma levels of lipids, lipoproteins, and uric acid were measured. Liver tissue was isolated, and histology performed followed by the biochemical determination of hypoxanthine, protein expression of xanthine oxidoreductase (XOR) by western blot analysis, and xanthine oxidase (XO) activity using an enzymatic kinetic assay. Furthermore, we employed in vitro experimentation studying HepG2 cells to measure the effect of hypoxanthine and H2O2 upon the production of radical oxygen species (ROS), XO activity, and cell viability. Hepatic tissue from rabbits fed the HFD diet showed signs of NAFLD associated with an increased ROS concentration and an altered purine metabolism characterized by the increase in hypoxanthine, together with an apparent equilibrium displacement of XOR towards the xanthine dehydrogenase (XDH) isoform of the enzyme. This protein shift visualized by a western blot analysis, associated with an increase in plasma uric acid and hepatocyte hypoxanthine could be understood as a compensatory series of events secondary to the establishment of oxidative stress associated with the chronic establishment of fatty liver disease.