The Role And Mechanism Of Uric Acid In Nonalcoholic Fatty Liver Disease

The first part Xanthine oxidase regulates hepatic fat accumulation through the NLRP3 inflammasome-dependent mechanismAims:Serum uric acid is significantly elevated in patients with nonalcoholic fatty liver disease (NAFLD). However, the mechanisms behind this association are unclear so far. We will take uric acid metabolism key enzyme-xanthine oxidase (XO) as the breakthrough point and investigate the effects and underlying mechanisms of uric acid on development of NAFLD.Methods and Materials:For evaluating the effect of uric acid on development of hepatic steatosis, eight-to ten-week-old male mice were randomly divided into four groups of eight each, feeding standard chow, high fat diet (HFD), hyperuricemia-inducing diet, or HFD combined with hyperuricemia-inducing diet for 8 weeks, respectively. To establish a cellular model of hepatic steatosis, HepG2 cells were exposed to a mixture of free fatty acids (FFA) for 48 hours. For evaluating the effect of uric acid on development of hepatic steatosis, HepG2 cells were exposed to different concentrations of uric acid for 48 hours. We used different biochemistry methods to analyze the mRNA and protein expression of XO, the activity of XO. Furthermore, Small interfering RNA was used to inhibit the expression of XO and NLRP3, Plasmid transfection was used to up-regulated the expression of XO, Allopurinol was used to inhibit the activity of XO. In addition, we used triglyceride detection and Oil Red O to analyze the role of XO and uric acid in hepatic fat accumulation, and explore the role of XO and uric acid in NLRP3 inflammasome activation.Results:Uric acid directly induces hepatocyte fat accumulation both in vivo and in vitro. The expression and activity of XO are significantly increased in cellular and mouse models of NAFLD. Knocking down XO expression significantly inhibits uric acid production and attenuatesfree fatty acids (FFA)-induced fat accumulation in HepG2 cells. Overexpression of XO aggravated free fatty acids (FFA)-induced fat accumulation in HepG2 cells. Inhibition of XO activity also significantly ameliorates high fat diet-induced hepatic steatosis in mice. Further experiments indicated that XO regulates the activation of NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome, which may be essential for the regulatory effect of XO on fat accumulation in hepatocytes. Conclusions:Our results suggest that XO-uric acid-NLRP3 inflammasome axis contributes to hepatic fat accumulation. XO may be a potential therapeutic target for NAFLD.The second part Uric acid regulates hepatic insulin resistance through the NLRP3 inflammasome-dependent mechanismAim:Accumulating clinical evidence suggests that hyperuricemia is strongly associated with abnormal glucose metabolism and insulin resistance. However, the mechanisms behind this association are unclear so far. This study aimed to investigate the effects and underlying mechanisms of uric acid on development of hepatic insulin resistance.Methods and Materials:For evaluating the effect of uric acid on hepatic insulin resistance, eight-to ten-week-old male mice were randomly divided into four groups of eight each, feeding standard chow diet, high fat diet (HFD), hyperuricemia-inducing diet, or HFD combined with hyperuricemia-inducing diet for 8 weeks, respectively. For further evaluating the effect of uric acid on hepatic insulin resistance, allopurinol was used to inhibit uric acid levels. For investigating the effect of uric acid insulin signaling, HepG2 cells were exposed to different concentrations of uric acid for 48 hours. Furthermore, Small interfering RNA was used to inhibit the expression of NLRP3. In addition, we used different biochemistry methods to analyze the role of uric acid and NLRP3 in hepatic insulin resistance.Results:We found that mice fed with HFD induce insulin resistance, Uric acid directly induces insulin resistance and insulin signaling impairment both in vivo and in vitro, and Inhibition uric levels with allopurinol significantly ameliorates HFD induced insulin resistance. Knocking down NLRP3 expression also rescued uric acid-induced insulin signaling impairment in vitro.Conclusions:Uric acid regulates insulin resistance through the NLRP3 inflammasome dependent mechanism. Uric acid may be a new therapeutic target for insulin resistance.