Background Myocardial fibrosis plays a critical role in the process of diabetic cardiac remolding.MicroRNAs (miRNAs) are endogenous,small non-coding RNAs that negatively regulate gene expression in diverse biological and pathological processes.However,the roles of miRNAs in myocardial fibrosis have not been well elucidated.In the present study,miRNAs profiles in the fibrotic myocardium of db/db mice and miRNAs expression in TGF-β1-stimulated mouse cardiac myofibroblasts was examined.Methods Heart function of 18-week-old db/db mice and db/m control mice was detected by echocardiography.miRNA expression profile in diabetic myocardium was detected by miRNA microarray.Quantitative real-time PCR was used to determine the expression of fibrosis-related genes and miRNA precursors of interest.Western blot was used to detect the levels of fibrosis-related proteins,activated Smad3 and total Smad3.Results The result of echocardiography showed that left ventricular systolic and diastolic function was impaired in 18-week-old db/db mice without significant change of ejection fraction (EF) and fractional shortening (FS).Fibrosis-related genes expression was upregulated and the amount of phosphorylated Smad3 was increased significantly in the diabetic myocardium.miRNAs dysregulation was shown in diabetic myocardium,sixty-eight miRNAs,including miR-208b,miR-29b,miR-26b and miR-30e,were increased over two-fold,meanwhile,sixty-two miRNAs were decreased more than two-fold in the myocardium of db/db mice compared to db/m controls.In parallel with a significant upregulation of Col1a1,Col3a1 and CTGF miRNA expression,miR-208b,miR-29b,miR-26b and miR-30e precursors were also shown to be upregulated in TGF-β1-induced C57bl/6 mouse cardiac myofibroblasts.Conclusions microRNAs were dysregulated in diabetic myocardium,with the activation of TGF-β/smad3 pathway,contributing to diabetic myocardial fibrosis. Background Myocardial fibrosis plays a critical role in the process of diabetic cardiac remolding. MicroRNAs (miRNAs) are endogenous, small non-coding RNAs that negatively regulate gene expression in diverse biological and pathological processes. Despite, the roles of miRNAs in myocardial fibrosis have not was well elucidated. In the present study, miRNA profiles in the fibrotic myocardium of db / db mice and miRNAs expression in TGF-β1-stimulated mouse cardiac myofibroblasts was examined. Methods Heart function of 18-week-old db / db mice and db / m control mice was detected by echocardiography. miRNA expression profile in diabetic myocardium was detected by miRNA microarray. Quantitative real-time PCR was used to determine the expression of fibrosis-related genes and miRNA precursors of interest. Western blot was used to detect the levels of fibrosis-related proteins, activated Smad3 and total Smad3.Results The result of echocardiography showed that left ventricular systolic and diastolic function was impaired in 18-week-old db / db mice without significant change of ejection fraction (EF) and fractional shortening (FS). Fibrosis-related genes expression was upregulated and the amount of phosphorylated Smad3 was increased significantly in the diabetic myocardium. miRNAs dysregulation was shown in the diabetic myocardium, sixty-eight miRNAs including miR-208b, miR-29b, miR-26b and miR-30e, were increased more than two-fold, whilewhile sixty-two miRNAs were decreased more than two- fold in the myocardium of db / db mice compared to db / m controls. In parallel with a significant upregulation of Col 1a1, Col3a1 and CTGF miRNA expression, miR-208b, miR-29b, miR- 26b and miR- 30e precursors were also shown to be upregulated in TGF-β1-induced C57bl / 6 mouse cardiac myofibroblasts. Confcnotic microRNAs were dysregulated in diabetic myocardium, with the activation of TGF-β / smad3 pathway, contributing to diabetic myocardial fibrosis.