In this study, we proved for the first time that ursolic acid can prevent renal interstitial fibrosis after unilateral ureteral obstruction, and its mechanism of action may be achieved by activating the Nrf2/HO-1 signaling pathway. We found in this experiment that after unilateral ureteral obstruction in rats, renal function is significantly impaired, renal tissue pathological damage is aggravated, renal interstitial fibrosis is significantly increased, and the apoptosis level of renal tubular epithelial cells is increased; When the rats were infused with ursolic acid preparations, the above-mentioned injury was obviously alleviated; it was proved that ursolic acid preparations could alleviate the level of renal interstitial fibrosis in rats.
Renal interstitial fibrosis is the common result of the development of a variety of kidney diseases to the end stage, in which excessive deposition of extracellular matrix (ECM) is a typical pathological change of renal interstitial fibrosis . Among the many cytokines that regulate renal interstitial fibrosis, transforming growth factor β1 is currently the most studied cytokine with the most clear effect . It has been confirmed that the up-regulation of TGFβ1 is closely related to the excessive deposition of ECM . Myofibroblasts are the main cells producing ECM. Some scholars have proved that the activation of renal interstitial fibroblasts to become α-SMA-positive myofibroblasts is the main factor of renal interstitial fibrosis injury, Among them, TGFβ1 is its primary stimulus and activates fibroblasts through a variety of signaling pathways. For example: oxidative stress pathway  and Smad3 signaling pathway  and so on. In our research, it was found that after unilateral ureteral obstruction in rats, the expression levels of TGFβ1 mRNA and protein in the kidney tissue were significantly higher than those in the sham operation group, and the degree of renal interstitial fibrosis was significantly increased; When given ursolic acid preparations, the expression levels of TGFβ1 mRNA and protein decreased, and the degree of renal interstitial fibrosis decreased. Once again proved the close relationship between TGFβ1 and the degree of renal interstitial fibrosis, which is often used as an important indicator of the prognosis of kidney disease.
Current research suggests that during oxidative stress damage, the production of reactive oxygen species (ROS) is considered to be an important factor leading to renal interstitial fibrosis . Under normal circumstances, the production and removal of ROS in the body are in a dynamic balance. When the body is stimulated by exogenous or endogenous factors, a large amount of ROS is produced. Excessive ROS promotes the expression of inflammatory factors, Initiate inflammation, promote the expression of fibrosis-related factors, For example: TGFβ, connective tissue growth factor, platelet-derived growth factor (PDGF), etc.accelerate the process of tissue fibrosis . Therefore, weakening the body's oxidative stress response and reducing the production of ROS are considered to be an important treatment for alleviating renal interstitial fibrosis .
Ursolic acid is a natural triterpene carboxylic acid compound with anti-inflammatory, antibacterial and anti-tumor effects . Some scholars have proven that ursolic acid can resist tissue oxidative stress and have a protective effect on tissue damage . The Nrf2/HO-1 signaling pathway is an important anti-oxidative stress pathway in the body. Under normal circumstances, Nrf2 binds to Kelch-like epichlorohydrin-related protein-1 (Keap1) and is widely distributed in the cytoplasm in an inactive form . When the body is stimulated by oxidative stress, the conformation of Keap1 changes, dissociating Nrf2 from its inhibitory protein Keap-1, and the dissociated Nrf2 migrates to the nucleus to form a heterodimer with Maf protein, which activates the expression of downstream antioxidant genes. For example: heme oxidase-1 (HO-1), SOD, catalase, etc., thereby increasing the ability of cells to resist oxidative stress [18–20]. Among them, HO-1 is the most important, an important inducible enzyme in the body. On the one hand, by reducing the production of heme, on the other hand, by increasing the production of antioxidants such as CO and bilirubin, they can jointly play the role of anti-oxidative stress and anti-inflammatory . In this study, we found that when rats were given oral ursolic acid preparations, the expression levels of Keap1 mRNA and protein in the kidney tissue decreased, and the expression levels of Nrf2 and HO-1 mRNA and protein increased, The content of antioxidant substance SOD increased, and the levels of oxidative stress damage markers MDA and 8-oxo-dG decreased, which proves that the effect of ursolic acid against renal interstitial fibrosis may be achieved by activating the Nrf2/HO-1 signaling pathway. In order to further verify the feasibility of this pathway, we found that after intraperitoneal injection of ZnPP, a specific blocker of the Nrf2/HO-1 signaling pathway in rats, compared with rats in the ursolic acid treatment group, the protein expression level of Keap1 mRNA in the kidney tissue increased, the expression levels of Nrf2 and HO-1 mRNA and protein decreased, the content of SOD decreased, and the levels of MDA and 8-oxo-dG increased, proving that Nrf2/HO-1 signal pathway is blocked. Compared with rats treated with ursolic acid, this group of rats has worsened renal function damage, increased renal pathological damage, increased renal interstitial fibrosis, increased apoptosis, and increased TGFβ1mRNA and protein expression levels. It was proved that after blocking the Nrf2/HO-1 signaling pathway, the protective effect of ursolic acid on renal interstitial fibrosis was reversed.
In summary, ursolic acid, as an antioxidant substance, can reduce the body's oxidative stress level, increase antioxidant substances, and reduce the expression of fibrotic factor TGFβ1 by activating the Nrf2/HO-1 signaling pathway. Thereby, it has a protective effect on the renal interstitium after unilateral ureteral obstruction in rats. Provide new ideas for clinical prevention and treatment of renal interstitial fibrosis.