Therapeutic management of diabetic myocardial fibrosis remains an unsolved clinical problem. Pin1, a peptidyl prolyl isomerase, impacts diverse cellular processes and plays a pivotal role in regulating cardiac pathophysiology. Here we investigate the potential mechanism of action of Pinl and its role in diabetes-induced myocardial fibrosis and dysfunction in mice. Cardiac Pinl, transforming growth factor beta 1 (TGF-(beta 1), alpha-smooth muscle actin (alpha-SMA) and extracellular matrix deposits (collagen I and III) are found to be increased in diabetic mice, which are effectively prevented by Pinl inhibition by juglone. Pinl inhibition alleviates cardiac fibrosis and dysfunction. In vitro, high glucose increases Pinl expression with an accompanying increase in phospho-Akt (Ser 473), p-Smad2, p-Smad3, TGF-beta 1, and alpha-SMA in cardiac fibroblasts (CFs). These increases are effectively prevented by the inhibition of Pin1 by juglone. Furthermore, Pin1 inhibition inhibits HG-induced CF proliferation and migration. Our results indicate that Pinl inhibition attenuates cardiac extracellular matrix deposition by regulating the phosphorylation of Akt, TGF-beta 1 ISmads, MMP activities, and alpha-SMA expression in diabetic mice. (C) 2016 Elsevier Inc. All rights reserved.