Aldose Reductase Regulates IRS-1 And LKB1-AMPK? Signaling To Promote Insulin Resistance,Obesity And NAFLD

The polyol pathway(PP)is the third most important pathway of glucose metabolism besides the glycolysis-tricarboxylic acid cycle and the pentose phosphate pathway.Aldose reductase(AR)is the first and rate-limited enzyme of PP,it catalyzes glucose reduction to sorbitol,and the latter generates fructose with the aid of sorbitol dehydrogenase(SDH).AR/PP plays an important role in cell metabolism,inflammation,immune response and other biological processes,closely related to diabetes and diabetic complication.In addition,the endogenous fructose produced by overactivation of AR/PP is closely associated with lipogenesis in the liver and the development of nonalcoholic fatty liver disease(NAFLD).Partly due to the fact that fructose metabolism is affected by neither the cellular energy level(AMP/ATP)nor the feedback inhibition of metabolic intermediates,exogenous or dietary fructose was shown to promote do novo synthesis of fat and suppression of fatty acid oxidation,thereby causing liver fat accumulation.However,whether and how overproduction of endogenous fructose due to overactivation of AR/PP might affect glucose and lipid homeostasis in vivo remains to be further elucidated.Using cell models and animal models,we investigated the effects of the alteration in AR/PP activities on IRS-1 and LKB1-AMPK? signaling,as well as the effects on insulin resistance,obesity and NAFLD.In mouse hepatocyte AML12 cells we showed that overexpression of AR significantly decreased the protein expression or phosphorylation of IRS-1,LKB1,AMPK? and ACC,while lentivirus-mediated Ar stable knockdown increased the protein expression or phosphorylation of IRS-1,LKB1,AMPKa and ACC,suggesting that alterations in AR/PP activities profoundly affected IRS-1 and LKBl-AMPKa signaling and disturbed overall animal metabolism.We investigated the effects of AR on inflammatory cytokines IL-6 and TNFa in AML12 cells and macrophage RAW264.7 cells.We found that AR inhibition significantly reduced the mRNA expression of IL-6 and TNFa.Analysis of epididymal fat inflammation cytokines and serum alanine aminotransferase(ALT)level also showed that lack of Ar reduced the release of inflammation cytokines and improved liver inflammation,which suggests that AR/PP inhibition could reduce the immune activity of macrophage and ameliorate inflammation.In Agouti yellow obese mice(Ay/a),we found that loss of Ar significantly up-regulated the protein expression or phosphorylation of IRS-1,LKB1,AMPK? and ACC.As a consequence,insulin resistance,glucose intolerance,bodyweight gain,liver lipid droplets accumulation and epididymal fat aggregation,were all improved.These results indicate the important roles AR/PP plays in various aspects of animal metabolism.We created lines of transgenic mice overexpressing AR liver-specifically(Tg).We fed 8 weeks transgenic mice(Tg)and its control(WT)with 10%(M/V)glucose for 14 weeks to induce the activation of AR/PP.Following the glucose feeding,the protein expression or phosphorylation of liver IRS-1,LKB1,AMPKa and ACC in Tg mice were lower than that in the WT mice.Moreover,the serum levels of fructose,ALT and proinflammatory cytokines,liver fructose and lipid droplets,and bodyweight and epididymal fat inflammation in Tg mice were significantly increased,accompanied by significant insulin resistance.These results suggest that endogenous fructose produced by overactivation of AR/PP leads to significant disturbance in animal metabolism.In conclusion,overactivation of AR/PP tended to suppress hepatic IRS-1,LKB1,AMPKa and ACC protein expression or phosphorylation and increase adipose tissue inflammation,promote insulin resistance,obesity and NAFLD.Conversely,loss of Ar tended to activate hepatic IRS-1,LKB1,AMPKa signaling and reduce adipose tissue inflammation,improve insulin resistance,obesity and NAFLD.Together these results suggest that aberrant regulation of AR/PP is tightly associated with metabolic disorders such as insulin resistance,obesity and NAFLD.Inhibition of AR potentially might be beneficial for combating insulin resistance,obesity and NAFLD.