TGF-beta 1-induced epithelial to mesenchymal transition (EMT) process of tubular epithelial cells plays a leading role in the occurrence and progression of renal fibrosis as seen in diabetic nephropathy (DN). High mobility group AT-hook 2 (HMGA2) is considered to be involved in TGF-beta 1-mediated EMT via multifactorial mechanisms. Specific microRNAs (miRNAs) are closely associated with EMT, and here we focused on let-7d miRNA as a regulator of HMGA2. This study aims to investigate the effects of HMGA2 on EMT process induced by TGF-beta 1 using small interfering RNA (siRNA) technique in vitro, and further explore the potential role of let-7d miRNA during renal fibrosis in DN. We demonstrated that siRNA targeting HMGA2 was sufficient to inhibit TGF-beta 1-induced EMT and fibrogenesis in rat kidney tubular epithelial cells (NRK52E). Furthermore, let-7d expression was significantly reduced by TGF-beta 1 stimulation, we focused on let-7d and found that overexpression of let-7d down-regulated the expression of HMGA2 and in turn suppressed TGF-beta 1-induced EMT and renal fibrogenesis. Inhibition of let-7d increased HMGA2 expression and enhanced the profibrogenic effects of TGF-beta 1 on NRK-52E cells. Consistent with the above observations in vitro, let-7d expression was also decreased in the kidneys of unilateral ureter obstruction model, accompanied by the correspondingly increased expression of HMGA2 and fibrotic genes in this model. Collectively, HMGA2 and let-7d miRNA significantly impact on the progression of TGF-beta 1-induced EMT and fibrogenesis both in vitro and in vivo, and they may represent novel targets for the prevention strategies of renal fibrosis in the context of DN. (C) 2016 Elsevier Inc. All rights reserved.