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  • The Wnt catenin signaling pathway


    The Wnt/β-catenin signaling pathway is a downstream target of SOX9 and is involved in a variety of cellular responses, including cell migration, proliferation, and differentiation [14]. Several studies have identified the Wnt/β-catenin signaling pathway as essential for cartilage function, and it is also involved in chondrocyte differentiation and cartilage development [15], [16]. Intervertebral disc degeneration is closely related to the Wnt/β-catenin signaling pathway [17], [18]. Endothelin-1 (ET-1) is a potent vasoconstrictor and vasopressor agent composed of three structurally related 21 amino FK 866 hydrochloride peptides, and is abundantly and widely expressed in both vascular and non-vascular tissues [19]. Endothelin-1 participates in the formation of ECM in the cartilage [20] and is also associated with various other biological processes, including angiogenesis, fibrosis, and inflammation [21], [22], [23]. Furthermore, ET-1 signaling is key to the destruction of the bone-cartilage unit in osteoarthritis [24]. The results of our previous study suggest that ET-1 can induce matrix metalloprotease (MMP)-1 and MMP-13 production, reduce the levels of tissue inhibitors involved in the production of metalloproteases-1, and promote nitric oxide release and nitric oxide synthase activity in CECs, which are associated with CEP degeneration [25]; however, the precise role of ET-1 in regulating the synthesis of ECM by CECs is not completely understood.
    Materials and methods
    Discussion Degeneration of the CEP of IVDs is a complex pathologic process related to a reduction in nutritional supply, the release of inflammatory mediators, increased activity of various degradative enzymes, changes in different types of collagen and proteoglycans, and a reduction in water content [28]. In the present study, we investigated the interaction between inflammatory mediators and the CEP by establishing an animal model of IVD degeneration that could simulate the natural IVD degeneration process, allowing us to explore the pathogenesis of IVD and providing a theoretical basis for clinical intervention. Based on a previous study [26], we successfully established an IVD degeneration model using vertebral end plate injury. Cartilage end plate degeneration invariably involves inflammatory processes. Our animal model allowed us to determine the relationship between CEP degeneration and ET-1 expression, thus providing a breakthrough point for the late repair of CEP degeneration, which could also be useful for etiologic studies of IVD degeneration. BQ-123 is a specific antagonist of the ET-1 receptor, which blocks the ET-1-ETAR pathway [29]. BQ-123 also has anti-inflammatory effects; a previous study reported that BQ-123 can inhibit the TNF-alpha and IL-6 inflammatory response and can also exhibit antioxidant effects [30]. Therefore, besides exerting an antagonistic effect on the endothelin-A receptor, BQ-123 may also function in the repair of CEP degeneration via its anti-inflammatory effects. Our results indicate that BQ-123 may prevent CEP degeneration through its effects on ET-1 signaling and identify a potential novel molecular mechanism that could be exploited to develop new clinical treatments. Our data demonstrate that ET-1 inhibits collagen II, aggrecan, and SOX9 synthesis in CECs. Moreover, antagonism of the ET-1 receptor by BQ-123 attenuated the negative effects of ET-1 in this context. In the CEP of the IVD, CECs depend on an optimal chemical, mechanical, and biological environment within the ECM. The end plate cartilage plays a major role in maintaining disc function, as the nucleus and annulus fibrosus of the disc are vulnerable to degeneration [31]. CECs are involved in the maintenance and turnover of the cartilage-specific matrix proteins, collagen II and aggrecan, which endow the end plate cartilage with its load-bearing properties, and act as a nutrient exchange channel. SOX9, a member of the SOX family (SRY-type high-mobility group box), is a master regulator of chondrocyte differentiation [32]. A reduction in SOX9 expression inhibits the expression of cartilage markers in the chondrogenic differentiation pathway, including collagen II and aggrecan [33], [34]. This hypothesis is supported by our current data. First, reduced SOX9 expression correlated with the downregulation of collagen II and aggrecan in response to ET-1 treatment. Second, the reduced expression of collagen II and aggrecan by ET-1 was significantly ameliorated by the ET-1 receptor antagonist, BQ-123. As the expression of SOX9 is crucial to chondrogenesis [35], [36], the inhibitory effect of ET-1 on the end plate chondrocyte ECM is likely mediated via SOX9.