The role of quantitative changes in the epxression of insulin receptor substrate-1 and nuclear ubiquitin in abnormal glycometabolism in the livers of KKay mice and the relative therapeutic mechanisms of Astragalus polysaccharide

Abstract

Ubiquitin and the ubiquitination pathway are important regulators of insulin signaling. The insulin receptor substrate‑1 (IRS-1), an ubiquitin-interacting adaptor protein, serves as the key docking protein in insulin signaling. The effects of this dynamic interaction and the changes in ubiquitin expression on hepatic insulin signaling, as well as the relative therapeutic effects of Astragalus polysaccharide (APS) have not yet been elucidated. In this study, we aimed to investigate the abnormal changes which occur in the levels of IRS-1 and ubiquitin in the livers of mice (mice with insulin resistance and diabetes), and to elucidate the possible mechanisms responsible for these changes. A control group (CG), an insulin resistance group (IG) and a diabetes group (DG) were respectively composed of 12-week-old C57BL/6J mice fed a normal diet, C57BL/6J mice fed a high‑fat diet and KKay mice fed a high‑fat diet, and treatment groups were composed of corresponding groups treated with APS (CG + A, IG + A, DG + A). All the mice were age-matched and grouped at random. After eight weeks, the mouse models were successfully established and the related physiological or biochemical indexes were detected using corresponding methods. Ubiquitin expression in the liver was detected by immunohistochemisty, and western blot analysis was used to detect the expression of IRS-1 and ubiquitin. The results revealed that the expression of IRS-1 in the DG was significantly lower compared to that in the CG and IG; however, the nuclear expression of ubiquitin and the ubiquitination levels of IRS-1, including body weight and blood glucose and triglyceride levels in the DG were significantly higher compared to those in the CG or IG (P<0.05). There was a significant improvement in the ubiquitination levels in DG + A, including the blood glucose and triglyceride levels compared with the DG (P<0.05). From the stage of insulin resistance to the stage of diabetes, the reduced expression of IRS-1 and its enhanced ubiquitination levels combined with the overexpression of nuclear ubiquitin contributed to the abnormal glycometabolism and the disruption of insulin signaling. APS showed beneficial effects, such as lowering body weight, as well as blood glucose and triglyceride levels, and these effects correlated with the downregulation of the ubiquitination levels of IRS-1 and the nuclear expression of ubiquitin.