Existing studies have shown that SMRR not only can prevent OA cartilage degeneration through antioxidative stress but can also reduce OA cartilage damage and thus be used to treat OA[20, 28]. The effective active SMRR component-target gene relationship network showed that the treatment of OA by SMRR is achieved through multiple components, multiple pathways, and multiple targets. Furthermore, the results showing the target genes involved with OA indicated that there are multiple targets for OA treatment. Therefore, it is particularly important to clarify the target genes and pathways of the active components of SMRR in OA through network pharmacology. Although some studies on the mechanism of action of SMRR in the treatment of KOA have been carried out, most studies are limited to a focus on the pharmacological effects of drug monomers, and there is still much area remaining for exploration in this field. In this study, the network pharmacology strategy using various related databases was adopted to screen 57 active components of SMRR and 58 potential targets for preventing and treating KOA. The chemical components of SMRR were divided into fat-soluble diterpene quinones and water-soluble phenolic acids. These compounds include flavonoids, phenolic acids, diterpenoids, sugars, proteins, alkaloids and volatile oils. Among these compounds, the active compounds were luteolin, tanshinone IIA cryptotanshinone, dihydrotanshinlactone, Isotanshinone II, dan-shexinkum d, etc., play important roles in the prevention and treatment of KOA.
Luteolin is a natural flavonoid compound that exists in a variety of plants and has a various pharmacological activities, such as anti-inflammatory, anti-allergic, and uric acid activity. Studies have shown that luteolin can reduce inflammation, protect chondrocytes, and delay cartilage degradation, which may be achieved by inhibiting the proliferation of OA chondrocytes and downregulating the expression of JNK and p38 MAPK in OA chondrocytes[29]. Similarly, luteolin can reduce IL-1β-induced inflammatory factor production and inhibit IL-1β-induced NF-κB phosphorylation, as shown by reducing cartilage destruction in OA rats[30]. In addition, experimental studies in rats showed that luteolin could improve the effect of superoxide dismutase (SOD) and catalase induced by selenite, enhance the activity of glutathione peroxidase and glutathione reductase, and then, enhance the activity of the antioxidant enzyme system in organisms[31]. It has also been reported that luteolin can regulate the expression, secretion and activity of MMP-3 by directly acting on articular chondrocytes [32].
Two fat-soluble terpenoids of tanshinone IIA from SMRR were identified. Tanshinone IIA protects ATDC5 cells from lipopolysaccharide-induced inflammatory injury by downregulating microRNA-203a and inhibiting the JAK/STAT and JNK pathways[32]. Tanshinone IIA can also inhibit the proliferation of chondrocytes, and the degree of inhibition is positively correlated with the concentration of tanshinone IIA. As the concentration of tanshinone IIA increases, the ratio of chondrocytes in G0/G1 phases increases, while the ratio in S phase decreases[34]. Cryptotanshinone is another important component of SMRR. It can inhibit the activation of the NF-κB and MAPK signaling pathways induced by LPS in macrophages, prevent IL-1β-induced human osteoarthritis chondrocyte inflammation and attenuate the progression of mouse osteoarthritis[9, 35].
According to the topological analysis of PPI network, there are 10 core targets: including MYC, STAT3, CASP3, JUN, CCND1, PTGS2, EGFR, MAPK1, AKT1, and VEGFA. These targets may be potential targets of SMRR for the treatment of OA.
Cytokines, including proinflammatory factors, chemokines and other signal molecules, can activate signal transduction and regulate various biological functions by binding with cell surface receptors [36]. Proinflammatory factors are widely present in joints. For example, IL-1β, IL-6, TNF-α and TGF-β can promote the development of inflammation, indirectly or directly affecting sensory neurons, sensitizing nociceptors and pain [37, 38]. Chemokines and their receptors are closely related to peripheral hyperalgesia. In particular, peripheral sensory neurons can be activated directly by chemokines [39].
There is a consensus suggesting that the characteristics of KOA, the degeneration of articular cartilage and the changes in synovium and subchondral bone lead to joint pain and joint dysfunction. Aging, metabolic dysfunction and mechanical injury can lead to further degeneration of articular cartilage. Interestingly, it seems that the difference in age and OA stage drives EGFR to play a dual role in articular cartilage [40]. On the one hand, EGFR stimulates the proliferation of chondrocytes through anabolism and then plays a role in maintaining cartilage. In the early stages of OA, EGFR promotes the production of joint lubricating molecules. On the other hand, EGFR can inhibit the expression of Sox9 and reduce the synthesis of cartilage matrix protein [41].
MAPK1 is a serine/threonine protein kinase that is mainly involved in the regulation of cell proliferation, differentiation, growth and apoptosis. The activation of the MAPK signaling pathway can affect the activity of a variety of transcription factors, thereby regulating the expression of cytokine genes such as TNF, IL-1, and IL-6 [42, 43]. The KEGG enrichment analysis showed that MAPK1 directly or indirectly participates in the TNF signaling pathway, MAPK signaling pathway, PI3K-Akt signaling pathway, and many other signaling pathways, forming complex interaction relationships within the "SMRR active component-signaling pathway-KOA" network.
With respect to the pathological progression of OA, VEGF is increased in articular cartilage, synovial membranes and subchondral bones. Moreover, VEGF in articular fluid is related to the severity and pain degree of OA in KOA patients; therefore VEGF may be an OA biomarker[44, 45]. A genome-wide association study and meta-analysis also showed that VEGF is significantly related to OA[46].
STAT3 is a DNA-binding molecule that can regulate the levels of many cytokines. A report showed that the activation of STAT3 in chondrocytes of patients with OA was significantly higher than that in the normal controls [47]. Studies have shown that injection of STAT3 inhibitors can reduce the expression of inflammatory mediators and chemokines in cartilage areas. Moreover, in MIa-induced OA rats, pain level and TRPV1 production in the dorsal root ganglion were decreased with the injection of a STAT3 inhibitor[48]. An analysis of 16 OA dysfunction modules and the aggregation of 3239 module genes showed that STAT3 may be the core transcription factor of OA and may promote the development of OA through NF-κB signaling [49]. These reports suggest that STAT3 inhibitors may have therapeutic potential for inflammatory diseases.
Prostaglandin endoperoxide synthase (PTGS), also known as COX, is a key enzyme in prostaglandin biosynthesis. PTGS has two isoenzymes, PTGS1 (COX-1) and PTGS2 (COX-2). PTGS2 expression is induced by cytokines and growth factors and upregulated during inflammation [50]. PTGS2 is activated to produce PGE2, which acts on a variety of cell signaling pathways, such as cell proliferation, apoptosis and immunosuppression [51]. PTGS2 plays an important role in the destruction of bone tissue, and COX-2-dependent PGE2 synthesis is considered to be an important mediator of tissue destruction in inflammatory bone diseases[52, 53].
The PI3K/Akt signaling pathway can maintain cell survival in a variety of ways. Akt not only can inhibit the activation of pro-apoptotic molecules but can also act as the substrate of Caspase-3[54]. CASP3 is an important link in the process of apoptosis. Studies have shown that the pathological progression of OA is related to apoptosis and the endoplasmic reticulum stress response, and CASP3 activation is related to cartilage degeneration and chondrocyte apoptosis[55]. Akt can activate cell survival signals and maintain cell survival[56, 57]. The increase in PI3K activity can also upregulate the activity of the Akt signaling pathway and promote the activity of Akt. [58]。
Aging is one of the risk factors in the pathological progression of OA. Advanced glycation end products (AGEs) are a series of highly active end products formed in the context of nonenzymatic glycosylation reactions between the amino groups of proteins, fatty acids or nucleic acids and the aldehyde group of reducing sugars. In the aging process, especially in the elderly, AGEs accumulate in the articular cartilage[59, 60]. AGE may also increase the levels of PGE2 and NO through induction of the MAPK pathway to enhance the inflammatory response of OA chondrocytes [61, 62]. Other studies have shown that the activation of RAGE stimulates inflammation-related signaling pathways [63]. In OA patients, RAGE activation can stimulate chondrocytes and synovial cells, leading to an increase in matrix metalloproteinase 1 (MMP-1) [64].
IL-17 is a pro-inflammatory cytokine, particularly IL-17A. IL-17A can act through NF-κB, MAPK and other signaling pathways [65]. In the pathogenesis of KOA, IL-17 may be involved with a variety of cytokines and pathways, such as IL-1, IL-8, IL-6, and macrophage inflammatory proteins[66]. Studies have found that IL-17 can activate NF-κB and promote cell apoptosis through this pathway [67, 68]. IL-17 can also promote bone resorption and collagen degradation; enhance the expression of chondrocytes MMP-9, MMP-13 and other metal matrix proteases; strengthen cartilage plate destruction and cartilage tissue decomposition; and induce joint destruction [69, 70]. Studies have shown that IL-17 can enhance the formation of blood vessels in arthritic cartilage while activating and increasing vascular endothelial growth factor, aggravating inflammation, and causing bone fibrosis and hyperplasia [71]. Moreover, clinical studies have shown that the expression level of IL-17 in OA synovium and synovial fluid is increased, and the increase in IL-17 level in OA synovial fluid is related to the severity of OA [72–74].
As an important regulator of hypoxia, HIF-1α is expressed in articular chondrocytes and plays an important role in regulating the metabolism of articular chondrocytes, maintaining the survival of chondrocytes and expressing specific phenotypes[75]. VEGF is an important target gene of HIF-1α. In the process of cartilage development, VEGF can promote the formation of new blood vessels, maintain the survival of chondrocytes and accelerate bone turnover [76, 77]. Studies have shown that the expression of HIF-1α increases with the degeneration of articular cartilage [78]. In addition, the activated HIF-1 pathway may also interfere with the combination of osteoblasts and osteoclasts, thereby inhibiting the production of osteoclasts [79].
The relationship between TNF-α and inflammation has been established. TNF-α is a multifunctional inflammatory cytokine that can stimulate the production of PGE2 in synovial cells and can stimulate human chondrocytes to secrete plasmin, thereby aggravating arthritis damage and leading to the destruction of bone and cartilage [80, 81]. In addition, TNF-α is an important mediator of cartilage matrix degradation and can play an important role in the process of synovitis and cartilage tissue degradation. TNF-α participates in inflammation by binding to the receptors TNFR1 and TNFR2 [82]. It combines with TNFRI to induce COX-Ⅱ and stimulate synovial cells to produce PGE and chondrocytes to produce peroxidation, leading to the destruction of bone and cartilage [82]. Furthermore, TNF-α activates the NF-κB signaling pathway by binding to receptors TNFR1 and TNFR2, leading to IκBα phosphorylation. The subsequent nuclear translocation of P65 leads to the activation of the NF-κB signaling pathway, leading to further enhanced proinflammatory function[84].
In conclusion, this study used network pharmacology to preliminarily predict that SMRR may regulate MYC, STAT3, CASP3, JUN, CCND1, PTGS2, EGFR, MAPK1, AKT1, VEGFA and other related targets through active components such as luteolin, tanshinone IIA, and cryptotanshinone. Through the IL-17 signaling pathway, HIF-1 signaling pathway, and TNF signaling pathway, as well as the AGE-RAGE signaling pathway in the context of diabetic complications signaling pathway, that can promote the proliferation of chondrocytes, improve microcirculation, eliminate free radicals, and inhibit inflammatory factors. However, this study still has limitations. Due to the limitations of the development of computer science, systems biology, network science and mathematics, to confirm the results of this research and determine the future application scenarios and scientific value of SMRR, further experimental research is still needed. However, this study predicted some of the potential targets, biological function modules and signaling pathways affected by SMRR in the treatment of KOA, which provides a theoretical basis and direction for further research.