Abstract: BACKGROUND: The incidence of renal interstitial fibrosis as a complication of type 2 diabetes is increasing. However, the mechanism by which exercises intervene with renal interstitial fibrosis as a complication of type 2 diabetes remains to be revealed.
OBJECTIVE: To explore the effects of different exercises on renal interstitial fibrosis in type 2 diabetic mice and the regulatory mechanism of Klotho-mediated transforming growth factor-β1/Smad3 pathway in this process. 
METHODS: Forty-four 4-week-old C57BL/6 male mice were randomly divided into a normal control group and a type 2 diabetic model group after 1 week of adaptive feeding. The mouse model of type 2 diabetes mellitus was prepared by high-fat diet and one injection of streptozotocin, and the mouse models were randomly divided into a model control group, a high-intensity intermittent training group, and a downhill running group, followed by 8 weeks of exercise intervention. After the end, ELISA was used to detect the biochemical indicators of renal function; hematoxylin-eosin staining was used to detect renal tissue structure; Masson staining was used to detect the degree of renal interstitial fibrosis; RT-PCR was used to detect the mRNA expression of relevant factors; western blot assay was used to detect the protein expression of relevant factors; and immunohistochemical staining was used to detect the expression of type I collagen protein in the kidney. 
RESULTS AND CONCLUSION: Compared with the normal control group, the levels of serum creatinine, urea nitrogen and 24-hour urine protein were significantly increased in the model control group; the renal pelvis was expanded, the renal parenchyma narrowed, and the number of renal tubules decreased; the percentage of collagen area increased; the expression of Klotho mRNA and protein in the kidney was down-regulated and the expressions of transformed growth factor β1, type I collagen, type III collagen at mRNA and protein levels were up-regulated, and the expressions of Smad3 mRNA and p-Smad3 protein were significantly up-regulated; the positive area of type I collagen protein expression in the kidney was significantly increased. Compared with the model control group, the mRNA expression of Klotho, type I collagen, and type III collagen in the high-intensity interval training group showed remarkable changes, and the expression of transforming growth factor β1 and type III collagen was up-regulated. Compared with the high-intensity intermittent training group, the levels of serum creatinine, urea nitrogen and 24-hour urine protein were significantly down-regulated in the downhill running group; renal structural lesions involving the renal pelvis, renal tubules, and glomeruli were remarkably improved; the percentage of collagen area was significantly reduced; Klotho mRNA and protein expression was up-regulated, the expressions of transforming growth factor β1, type I collagen and type III collagen at mRNA and protein levels were down-regulated, the expressions of Smad3 mRNA and p-Smad3 protein were significantly down-regulated; the positive area of type I collagen protein expression was significantly decreased. To conclude, type 2 diabetic mice develop fibrosis in the renal interstitium; downhill running improves renal interstitial fibrosis in type 2 diabetic mice by up-regulating Klotho and inhibiting the transforming growth factor-β1/Smad3 pathway; however, high-intensity intermittent exercises have no remarkable effects.

Key words: exercise, Klotho, transforming growth factor-β1, type 2 diabetic mouse, renal interstitial fibrosis