RXRα Regulates Myocardial Remodeling In STZ-induced Diabetic Rats And Cardiac Fibroblasts Collagen Synthesis In High Glucose By LKB1/P70S6K Signaling Pathway

Aims To observe the effects of RXRαagonist Bexarotene on myocardial remodeling and left ventricular function in STZ-induced diabetic rats by animal experiments,and observe the effects of RXRαagonist 9-cis-RA on LKB1/P70S6K signaling pathway and myocardial collagen synthesis in rat cardiac fibroblasts under high glucose conditions,Furthermore,to explore the possible regulatory mechanism of RXRαon diabetic cardiomyopathy and myocardial fibrosis.Methods 1.Animal experiments Diabetic animal model was made by intraperitoneal injection of streptozotocin（60mg/kg）（intraperitoneal injection of citrate buffer 0.5ml/100g,citrate buffer contains STZ 12 mg/ml）.32 SD rats were divided into four groups,8 in each group:normal control group（Ctr）,intraperitoneal injection of citrate buffer（0.5ml/100g）;diabetic group（DM）;DM+Bex10（Bex 10 mg/kg/d）group;DM+Bex20（Bex 20mg/kg/d）group;Continuous Bex treatment for 12 weeks,fasting blood glucose and body mass were measured every 4 weeks.Fasting blood glucose level was determined by oxidase method;Echocardiography was applied to assess cardiac structure and function;Heart and left ventricular mass,and left ventricular mass index（heart or left ventricular mass/tibia length）were calculated;Sirus red staining were used to determine collagen desiposition in rat myocardium;Acetic acid homogeneous method was used for lysising myocardium collegen;Enzyme Linked Immunosorbent Assay（ELISA）was performed for determining the level of Collagen I and III in myocardium tissue homogenate;Western-blotting was used for determination the levels of RXRα、LKB1/P-LKB1 and P70S6K/P-P70S6K in myocardium.2.Cell experiments Myocardial fibroblasts were cultured by tissue dry method and identified by immunoflurescence.ELISA was performed to assay the levels of collagenⅠ、Ⅲin cardiac fibroblasts supernatant.RXRα、LKB1/P-LKB1 and P70S6K/P-P70S6K were determined with Western-blotting.The effects of LKB1/P70S6K pathway mediated by RXRαon collagen synthesis in cardiac fibroblasts were observed by transfection of specific RXRαSiRNA.Results 1.Animal experiments 1)Fasting blood glucose levels were significantly higher in DM group than that in control,Fasting blood glucose levels were significantly lower in DM+Bex20 group than that in DM group.2)Echocardiography results:IVSd、LVPWd、IVSs、LVEDd、LVEDs、EDV、ESV、EF were significantly decreased in DM group.IVSd、LVPWd、LVEDd、LVEDs、EDV、ESV、EF were significantly increased in DM+Bex20 group compared with DM group.3)Diabetic rats body mass、left ventricular mass,and left ventricular mass/tibial length ratios were decreased;Bexarotene could inhibit the the decline in body mass and ventricular mass index of diabetic rats.4)Myocardial collagen（ColⅠColⅢ）increased in diabetic rats,and Bexarotene could inhibit the increase of myocardial collagen in diabetic rats.5)Western-blotting results:LKB1/P-LKB1 was significantly decreased in DM group and P-P70S6K was increased;However,Bexarotene could inhibit the decrease of P-LKB1 and increase of P-P70S6K in diabetic rats.2.Cell experiments P-LKB1 in cardiac fibroblasts was decreased and P-P70S6K increased in high glucose（glucose 25mM）.9-cis-RA could inhibit the decrease of P-LKB1 and increase of P-P70S6K in cardiac fibroblasts under high glucose conditions.High glucose（25mM）could promote the synthesis of ColⅠand ColⅢin cardiac fibroblasts,while 9-cis-RA(10^-7M)could inhibit colⅠand colⅢsynthesis.RXR-specific siRNA decreased the expression of RXR?gene and attenuated the effect of9-cis-RA on anti-myocardial fibroblast collagen synthesis in high glucose,meanwhile,attenuated the effect of 9-cis-RA on the phosphorylation of LKB1 and the inhibition of P-P70S6K in high glucose.Conclusions 1)RXRαagonist Bexarotene could inhibit myocardial fibrosis,reverse left ventricular remodeling and improve cardiac function in STZ-induced diabetic rats.2)RXRαgonist Bexarotene could improve the dysregulation of LKB1/P70S6K signal pathway in STZ-induced diabetic rats.3)RXRαagonist9-cis-RA increased the activity of LKB1/P70S6K signaling pathway and inhibited the synthesis of ColⅠand ColⅢinduced by high glucose.4)RXRαmay inhibit diabetic myocardial fibrosis and collagen synthesis in cardiac fibroblasts by regulating the LKB1/P70S6K signaling pathway.