Osteoarthritis is a common joint disease and seriously affects the quality of life in the elderly. At present, some therapeutic options, including nonsteroidal anti-inflammatory drugs (NSAIDs), glucocorticoids, and hyaluronic acid are available for the treatment of OA (Richards et al., 2016) ; however, they are unable to inhibit the progression of OA and may cause cardiovascular, renal, and gastrointestinal side effects (Katz, 2013). Recently, many studies have focused on the application of natural plant extracts in the treatment of OA because of their potential anti-inflammatory properties and reduced risk of adverse events (Dragos et al., 2017). Dio has been demonstrated to inhibit the release of inflammatory cytokines in mouse and rat models of various diseases, such as dimethylnitrosamine-induced acute liver injury (Zhang et al., 2016), intestinal ischemia/reperfusion injury (Zheng et al., 2019), and lipopolysaccharide-induced acute lung injury (Yao et al., 2017a). In addition, Dio also showed a protective effect in chronic inflammatory diseases including gouty arthritis(Han et al., 2021), silicosis(Du et al., 2019), and atherosclerosis(Yang et al., 2019b). In this study, the protective effects of Dio were demonstrated in vitro using primary chondrocytes, and further confirmed in a rat osteoarthritis model.
IL-1β is a major pro-inflammatory cytokine that participates in the pathological process of OA by inducing inflammation. There is increasing evidence that the production of iNOS and COX-2 is upregulated by IL-1β during OA progression (Kapoor et al., 2011; Lei et al., 2012). Moreover, iNOS and COX-2 promote the release of NO and PGE2, which are well-known inflammatory mediators (Dumond et al., 2004). In this study, our results indicated that the production of iNOS and COX-2 was significantly increased after incubation with IL-1β. However, Dio reduced the IL-1β-upregulated expression levels of iNOS and COX-2. This was consistent with the findings of a previous study which indicated that Dio could markedly inhibit the production of iNOS and COX-2 in lipopolysaccharide-induced mouse mammary epithelial cells (Ran et al., 2020). Furthermore, Dio has been reported to inhibit the expression of iNOS and COX-2 in IL-1β-induced human osteoarthritis chondrocytes(Wang et al., 2020a). Therefore, Dio may exert its anti-inflammatory effects by repressing the synthesis of iNOS and COX-2.
Under normal circumstances, the anabolism and catabolism of cartilage are dynamic. Dysfunction in cartilage metabolism is key to the development of OA. Collagen II and aggrecan are the two main constituents of the extracellular matrix in cartilage, while MMPs and ADAMTS5 are the two catabolic enzymes responsible for the degradation of collagen II and Aggrecan, respectively (Stanton et al., 2005; Sondergaard et al., 2006). Excessive release of IL-1β in joints can disrupt this balance. In the study, our findings indicated that IL-1β dramatically increased the expression of the catabolic enzymes and downregulated the synthesis of collagen II and aggrecan in chondrocytes. However, the application of Dio repressed the overexpression of MMPs (MMP1, MMP3, and MMP13) and ADAMTS5 induced by IL-1β and improved the production of collagen II and aggrecan, which contribute to the maintenance of chondrocyte matrix homeostasis. Thus, the results revealed that Dio could alleviate OA progression through an anti-catabolic mechanism.
It has been previously demonstrated that MAPK and NF-κB signaling pathways are involved in the progression of OA (Saklatvala, 2007; Rigoglou and Papavassiliou, 2013). The decisive role of MAPK and NF-κB signaling pathways in OA onset and progression provides evidence that intervention of these signaling pathways might have beneficial therapeutic effects. Drugs that have been clinically applied in the treatment of OA, including NSAIDs and glucocorticoids, are pharmacologically active compounds that can block the NF-κB signaling pathway (Niederberger and Geisslinger, 2008). Therefore, in this research, we focused on examining these pathways. IL-1β has been shown to activate all three MAPK signaling molecules (P38, ERK, JNK) and NF-κB signaling pathways, and induces the production of catabolic factors to further promote the destruction of the extracellular matrix (Jenei-Lanzl et al., 2019). Our results corroborated the activity of IL-1β described previously and demonstrated that Dio could partly inhibit IL-1β-induced activation of MAPK and NF-κB signaling pathways. This was consistent with the anti-inflammatory mechanism of Dio described in previous studies(Zheng et al., 2019; Ran et al., 2020; Wang et al., 2020b; Han et al., 2021). Therefore, the protective effects of Dio on OA may be related to the inhibition of the MAPK and NF-κB pathways (Fig. 8).
To further demonstrate the protective effect of Dio against OA in vivo, we established an OA rat model. The in vivo results indicated that Dio could alleviate joint degeneration in the OA rat model. The results of H&E and Safranin-O-Fast green staining showed that characteristics of OA, such as cartilage surface erosion, chondrocyte diminution, and cartilage matrix loss, were significantly reversed by Dio. Similarly, Lu et al.(Lu et al., 2018) demonstrated that DIO has a protective effect on sodium iodoacetate induced cartilage destruction in rats. Moreover, immunohistochemical staining analysis further confirmed the anti-inflammatory and anti-catabolic effects of Dio, which are consistent with the trends observed in the in vitro studies. In addition, the OARSI score in the rat OA model was significantly reduced after Dio treatment. Thus, taken together, our results strongly suggest that Dio can ameliorate the progression of OA.
However, this research is not without limitation. Only the concentration of Dio at 80mg/kg is examined in this study. The most effective dose of Dio in vivo application is unclear and needs further evaluation. Despite this, the dioscin-containing DA-9801 completed its Phase II clinical trial in the United States for the treatment of diabetic neuropathy (Kang et al., 2017), indicating that Dio is potentially safe for therapeutic use.