Kidney fibrosis is the final common pathway of all progressive chronic kidney diseases, of which diabetic nephropathy is the leading cause. Endothelial-to-mesenchymal transition (EndMT) has emerged as one of the most important origins of matrix-producing fibroblasts. Dipeptidyl peptidase-4 (DPP-4) inhibitors have been introduced into the market as antidiabetes drugs. Here, we found that the DPP-4 inhibitor linagliptin ameliorated kidney fibrosis in diabetic mice without altering the blood glucose levels associated with the inhibition of EndMT and the restoration of microRNA 29s. Streptozotocin-induced diabetic CD-1 mice exhibited kidney fibrosis and strong immunoreactivity for DPP-4 by 24 weeks after the onset of diabetes. At 20 weeks after the onset of diabetes, mice were treated with linagliptin for 4 weeks. Linagliptin-treated diabetic mice exhibited a suppression of DPP-4 activity/protein expression and an amelioration of kidney fibrosis associated with the inhibition of EndMT. The therapeutic effects of linagliptin on diabetic kidneys were associated with the suppression of profibrotic programs, as assessed by mRNA microarray analysis. We found that the induction of DPP-4 observed in diabetic kidneys may be associated with suppressed levels of microRNA 29s in diabetic mice; linagliptin restored microRNA 29s and suppressed DPP-4 protein levels. Using cultured endothelial cells, we found that linagliptin inhibited TGF-β2–induced EndMT, and such anti-EndMT effects of linagliptin were mediated through microRNA 29 induction. These results indicate the possible novel pleiotropic action of linagliptin to restore normal kidney function in diabetic patients with renal impairment.

The regulation of aberrant glucose metabolism in diabetes associated-kidney fibrosis is not well known. In this study we found the suppression of SIRT3 protein level in diabetic kidney, displays responsibility in fibrogenic programming associated with aberrant glycolysis and such abnormal glycolysis is the therapeutic target in diabetes associated-kidney fibrosis. When analyzing different strains of streptozotocin-induced diabetic mice model (fibrotic model: CD-1, less fibrotic model: C57Bl6), we found SIRT3 suppression was associated with kidney fibrosis in fibrotic CD-1; further SIRT3 suppression by systemic administration of SIRT3 siRNA in the diabetic mice, showed profound fibrogenic phenotype in the kidney. Such suppression in SIRT3 was associated with the induction of transforming growth factor-β (TGF-β)/smad signaling, higher level of HIF1α accumulation and PKM2 dimer formation; these alterations subsequently led to abnormal glycolysis and linked abnormal mesenchymal transformations in vivo and in vitro. Inhibition of such aberrant glycolysis suppressed fibrogenic programming and restored SIRT3 level as well. Such aberrant glycolysis was confirmed in the KK/Ta-Ins2Akita mouse, the mouse model of progressive diabetic kidney disease. These data demonstrate that SIRT3 deficiency promotes abnormal glycolysis which is responsible for the fibrogenic pathway in diabetic kidney. Restoration of SIRT3 could be an alternative strategy in combating diabetes associated-kidney fibrosis via inhibition of aberrant glycolysis.

Integrin β1 and dipeptidyl peptidase (DPP)-4 play roles in endothelial cell biology. Vascular endothelial growth factor (VEGF)-A inhibits endothelial-to-mesenchymal transition (EndMT) through VEGF-R2, but through VEGF-R1 promotes EndMT by reducing the bioavailability of VEGF-A. Here we tested whether DPP-4-integrin β1 interactions have a role in EndMT in the renal fibrosis of diabetic nephropathy. In streptozotocin-induced fibrotic kidneys in diabetic CD-1 mice, levels of endothelial DPP-4, integrin β1, and phospho-integrin β1 were all higher and associated with plasma cystatin C elevation. The DPP-4 inhibitor linagliptin ameliorated kidney fibrosis, reduced plasma cystatin C levels, and suppressed endothelial levels of DPP-4, integrin β1, and phospho-integrin β1. In cultured endothelial cells, DPP-4 and integrin β1 physically interacted. Suppression of DPP-4 by siRNA was associated with suppression of integrin β1 and vice versa. Knockdown of either integrin β1 or DPP-4 resulted in the silencing of TGF-β2-induced TGF-β receptor heterodimer formation, smad3 phosphorylation, and EndMT. DPP-4 negatively regulated endothelial viability signaling by VEGF-R2 suppression and VEGF-R1 induction in endothelial cells. Thus, DPP-4 and integrin β1 interactions regulate key endothelial cell signal transduction in both physiological and pathological conditions including EndMT. Hence, inhibiting DPP-4 may be a therapeutic target for treating kidney fibrosis in diabetes.

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MicroRNAs (miRNAs) are a family of small, noncoding RNAs that regulate gene expression in diverse biological and pathological processes, including cell proliferation, differentiation, apoptosis, and carcinogenesis. As a result, miRNAs emerged as major area of biomedical research with relevance to kidney fibrosis. Fibrosis is characterized by the excess deposition of extracellular matrix (ECM) components, which is the end result of an imbalance of metabolism of the ECM molecule. Recent evidence suggests that miRNAs participate in the fibrotic process in a number of organs including the heart, kidney, liver, and lung. Epithelial mesenchymal transition (EMT) and endothelial mesenchymal transition (EndMT) programs play vital roles in the development of fibrosis in the kidney. A growing number of the extracellular and intracellular molecules that control EMT and EndMT have been identified and could be exploited in developing therapeutics for fibrosis. This review highlights recent advances on the role of miRNAs in the kidney diseases; diabetic nephropathy especially focused on EMT and EndMT program responsible for the development of kidney fibrosis. These miRNAs can be utilized as a potential novel drug target for the studying of underlying mechanism and treatment of kidney fibrosis.