Backgroud: Myocardial fibrosis results in myocardial remodelling and dysfunction. Celastrol, a traditional oriental medicine, has been suggested to have cardioprotective effects. However, its underlying mechanism is unknown. This study investigated the ability of celastrol to prevent cardiac fibrosis and dysfunction and explored the underlying mechanisms. Methods: Animal and cell models of cardiac fibrosis were used in this study. Myocardial fibrosis was induced by transverse aortic constriction (TAC) in mice. Cardiac hypertrophy and fibrosis were evaluated based on histological and biochemical measurements. Cardiac function was evaluated by echocardiography. The levels of transforming growth factor beta 1 (TGF-beta 1), extracellular signal regulated kinases 1/2 (ERK1/2) signalling were measured using Western blotting, while the expression of miR-21was analyzed by real-time qRT-PCR in vitro and in vivo. In vitro studies, cultured cardiac fibroblasts (CFs) were treated with TGF-81 and transfected with microRNA-21(miR21). Results: Celastrol treatment reduced the increased collagen deposition and down regulated a-smooth muscle actin (alpha-SMA), atrial natriuretic peptide (ANP), brain natriuretic peptides (BNP), beta-myosin heavy chain (p-MHC), miR-21 and p-FRK/ERK. Cardiac dysfunction was significantly attenuated by celastrol treatment in the TAC mice model. Celastrol treatment reduced myocardial fibroblast viability and collagen content and down-regulated alpha-SMA in cultured CFs in vitro. Celastrol also inhibited the miR-21/ERK signalling pathway. Celastrol attenuated miR-21 up-regulation by TGF-beta 1 and decreased elevated p-ERK/ERK levels in CFs transfected with miR-21. Conclusion: MiR-21/ERK signalling could be a potential therapeutic pathway for the prevention of myocardial fibrosis. Celastrol ameliorates myocardial fibrosis and cardiac dysfunction, these probably related to miR-21/ERK signaling pathways in vitro and in vivo. (C) 2016 The Author(s) Published by S. Karger AG, Basel