Renal fibrosis is the key pathological change of diabetic nephropathy (DKD), and significantly increases the mortality of patients with advanced DKD. Different signaling pathways are involved in renal fibrosis, including the TGF-β, MAPK, PI3K/Akt, JAK/STAT, Wnt/β-catenin, and Notch pathways[3, 4]. These pathways all play important roles in the accumulation of ECM, the expression of collagen and fibronectin, and the secretion of other related proteins. In addition, more and more new therapies are being investigated in clinical trials, and many efforts have been made to delay or even attempt to reverse the progression of renal fibrosis[22-24].
Evidence accumulated in recent years indicates that inflammation plays an important role in the occurrence and aggravation of DN kidney injury[7, 8]. Increasing numbers of studies have shown that increases in inflammatory marker levels are related to the anti-DN effects of some renoprotective molecules. However, the mechanisms behind these phenomena are not fully understood. Therefore, further study on the mechanism of renal immunity and inflammation and search for drugs to inhibit immune inflammatory response may find new targets for DN anti-inflammatory therapy. Natural anti-inflammatory products are a safe alternative to traditional methods for regulating inflammatory diseases[25-28].
Li et al. showed that Ori exerts protective effect in diabetes-induced renal injury through the TLR4/NF-κB signaling pathways. Lin et al. demonstrated that Ori reduced proteinuria and attenuated renal damage in a spontaneous SLE mouse model by regulating the inflammatory responses. These results suggested that Ori attenuates proteinuria and protects the kidney from injury. Moreover, Ori also acts on a variety of cells, including immune cells, hepatocytes and vascular endothelial cells, to exert its protective effect. Bohanon et al. reported that Ori inhibited hepatic stellate cell proliferation and fibrogenesis by suppressing endogenous and TGF-β1-induced ECM proteins. Current study demonstrated that oridonin inhibits collagen deposition and inflammation to attenuate CCl4-induced liver fibrosis in mice through inhibition of the NLRP3 inflammasome. However, to date, whether Ori suppresses NLRP3 pathway and thus exerts beneficial effects on diabetes-induced renal fibrosis has not been explored. To confirm these effects, we investigated the inhibitory effects of Ori on the inflammatory response and fibrosis in a diabetic rat model. In our study, compared with rats in the control groups, rats in the DM group displayed increased plasma glucose levels, as well as an increased kidney weight-to-body weight ratio, which is indicative of renal injury. Significantly increased BUN, Scr, and UA concentrations were also noted in rats in the DM group. However, treatment with Ori effectively reversed these changes, as it lowered BUN, Scr, UA concentrations, and the kidney weight-to-body weight ratio. Previous studies have shown that Ori hardly affected plasma glucose levels. The discrepancy may be due to difference of the duration of the experiment. Previous experiment lasted for 12 weeks and we conducted for 6 weeks.
The typical pathological changes of DN are mesangial cell proliferation, dilatation of renal tubules with accumulation of extracellular matrix and thickening of glomerular capillary wall, accompanied by nodular sclerosis, and eventually progressed to complete diabetic nephropathy. In this study, Ori treatment significantly ameliorated these diabetes-induced histopathological alterations. These data indicated that Ori improves renal function, ameliorates diabetes-induced renal injury and delays progressive nephrotoxicity in rats with DM.
The significance of the Trx-TXNIP signaling system is increasingly recognized. Recent studies have shown a complex thiol-dependent interaction between TXNIP and the inflammation-related pathway of progressive diabetic nephropathy, the interaction of NLRP3 and TXNIP may be a significant signal of the formation of NOX4-derived NLRP3 inflammation in hyperhomocysteinemia-induced glomerular damage. To verify whether the TXNIP-NLRP3 axis plays an important role in the therapeutic effects of Ori, we investigated the expression of NLRP3, TXNIP, and IL-1β. Our findings confirm the previous view that hyperglycemia-induced mitochondrial dysfunction plays an important role in the occurrence of DN, and also confirm to some extent that Ori can regulate TXNIP-NLRP3 axis to affect ROS levels to ameliorate mitochondrial damage.
In DN, the NLRP3 inflammasome is an intracellular platform that converts pro-IL-1β into active forms (IL-1β p17) responding to danger signals and triggers inflammatory programmed cell death. In recent studies, MCC950 (inhibitor of NLRP3) reduced liver inflammation and fibrosis by suppression collagen I, α-SMA and hepatic connective tissue growth factor expression in a mouse model of non-alcoholic steatohepatitis. TGF-β1 is a major cytokine secreted by mesangial cells that mediates the development of DN. TGF-β1 is a key cytokine mediating collagen deposition in kidney, including promoting the production of ECM, inhibiting the degradation of ECM and participating in renal fibrosis[34, 35]. ECM of DN patients is produced by mesangial cells and mainly consists of fibronectin, type IV collagen and a small amount of type I collagen. α-SMA was weakly expressed in normal mesangial cells, but was significantly increased under high glucose stimulation. IL-1β has been demonstrated to stimulate production of TGF-β1, fibronectin, collagen I and mesangial proliferation. Our experimental results showed that Ori has a good inhibitory effect on these indicators promoting renal fibrosis.
In conclusion, our present study suggests that oridonin inhibits collagen deposition and inflammation, thereby alleviating diabetic induced renal injury and fibrosis in rats. However, the experiments conducted in this study covered only a narrow and superficial scope of pharmacological identification. Therefore, the renal protective effect of Ori remains to be further studied. These results should also be compared with current first-line drugs for renal fibrosis.