Gelatinases belongs to the matrix metalloproteinase family. MMP-2 and MMP-9 were discovered in the early 1970s [27] and early 1980s [28], respectively, and named gelatinase A and gelatinase B in 1991 to distinguish the molecular mass of the two (gelatinase A, 72kDa; gelatinase B, 92kDa) [29]. Based on the naming rules established by the International MMP Conference held in Florida in 1989, gelatinases were renamed in the form of MMP abbreviation with number, namely MMP-2 (gelatinase A) and MMP-9 (gelatinase B) [30]. As an important regulator of cell activity, gelatinases not only affected reproduction, angiogenesis and tissue remodeling, bone development, wound healing, neuronal network learning, memory and other broad sides of life, but also participated in periodontitis, allergy, osteochondral condition, cardiovascular disease, diabetes and other pathological processes [31–34]. Studies had reported that MMP-2-deficient mice developed normally and were fertile without any serious anatomical abnormalities [35]. Human MMP-2 deficiency which resulted from inactivating autosomal recessive mutations in MMP-2 gene, manifested as severe osteolysis and arthritis [13, 36]. MMP-2-deficient mice also showed similar symptoms, but relatively mild [37]. Winchester syndrome was another condition related to changes in MMP-2 activity. Due to decreased activation of MMP-2, Winchester syndrome was characterized by osteolysis-related symptoms [38]. MMP-9 deficiency was non-lethal in mice, but abnormal development of growth plates in the long bones and delayed endochondral ossification were observed in MMP-9-null mice. This abnormality seemed to be caused by abnormal vascular invasion. MMP-9 was considered to be a key regulator of growth plate angiogenesis and hypertrophic chondrocyte apoptosis [14]. Consistent with this, later studies in chickens also found tibial chondrodysplasias related to the decreased expression of the MMP-9 gene. During the development of mice, it has been reported that there was high expression of MMP-2 in osteoclasts [39]. Indeed, MMP-2 and MMP-9 played a significant role in maintaining homeostasis in joints. In adult tissues, the basic expression of MMP-2 has been observed in normal articular cartilage, suggesting that MMP-2 might be involved in the physiological ECM turnover of articular cartilage, and the expression of MMP-9 was relatively low and limited to some chondrocytes in the very superficial layer. However, gelatinase expression was elevated in many pathologies such as tumor metastasis and osteoarthritis. And in this work, FGF-8 promoted gelatinase expression and activity by cultured articular chondrocytes. We suggested that this increased expression of gelatinases might have negative effects on cartilage, such as promoting cartilage matrix degradation and activating various pro-inflammatory factors. As has been reported previously, the cartilage section of OA patients showed that the expression and distribution of MMP-2 and MMP-9 were increased compared with the normal control group [40–42]. However recent studies showed that the increase of MMP-2 in joint synovial fluid might have a protective effect. MMP-2 deficient mice had been found with increased arthritis grades [43, 44], and injection of fibroblasts that secreted MMP-2 into the joints could reduce the response. So, it requires further studies that the effect of FGF-8-induced gelatinase expression on cartilage.
The functions of FGF-8 on developing tissues has been widely reported. FGF-8 was expressed during gastrulation[45]. Researches on embryogenesis in mice and chicks found that FGF-8 played a very important role in the formation of central nervous system, craniofacial organs, heart system, limbs and urogenital system [46–49]. FGF-8-deficient mouse could not develop through gastrulation [50]. In the early limb ectoderm, the inactivation of FGF-8 resulted in a noteworthy reduction in the size of limb buds [51]. The skull was formed through intraperiosteal osteogenesis. Schmidt and colleagues found that moderately increased FGF-8 expression led to craniosynostosis, while higher FGF-8 level shifted the fate of mesenchymal cells from ossification to abnormal cartilage formation [50]. FGF-8 might also be involved in ectopic bone and cartilage formation in breast cancer cells that produced large amounts of FGF-8 [52]. But its functions on adult tissues were less studied. Uchii and his colleague provided new findings on the role of FGF-8 in joint inflammation. They found that FGF-8 could promote the destruction of articular cartilage by inducting catabolic factors such as MMP in joints [5]. Consistently, Liu and his colleague found that FGF-8 and FGF receptor-3 (FGFR-3) were all increased in the cartilage of children with Kashin-Beck disease, one type of OA [53]. The role of FGF-8 in cultured chondrocytes was studied in this work. Through western blot, zymography and immunofluorescence, we found that recombinant FGF-8 could increase gelatinase expression of culture chondrocytes. Given what we found, we hypothesized that FGF-8 might participate in the degradation of cartilage and exacerbation of osteoarthritis by enhancing expression of gelatinases. But other MMPs, like MMP-1, MMP-3 and MMP-13, were not measured.
In addition, FGF-8-induced expression of gelatinases was regulated through activation of NF-κB p65 signaling with acetylated p65 accumulating in the nucleus of the cell (Fig. 5). And we further found that NF-κB inhibitor could inhibit up-regulation of gelatinase induced by FGF-8. Activation of NF-κB signaling by FGFs has been reported, like FGF19 in hepatocytes [54], FGF1 In LX-2 cell [55], FGF2 in vascular smooth muscle cells [56]. It has also been reported that FGF-8 mediated NF-κB signaling in the nervous system [57]. This paper demonstrated for the first time that FGF-8 activated the NFkB signaling pathway in mouse knee chondrocytes. However the details by which FGF-8 activated p65 remained to be explored.
Our work provided a further insight into the regulation of FGF-8 in the expression of MMP-2 and MMP-9 in cultured chondrocytes. It indicated the potential role of FGF-8 in participating in OA. Most of studies focused on the pivotal role of FGF-8 in embryogenesis and morphogenesis, while it was less reported about its role in adult tissues, especially in joints. Uchii and his colleagues reported FGF-8 was expressed low in normal knees but higher in OA models, which indicated that FGF-8 might act as a catabolic mediator of cartilage[5]. Further studies are required for the role of FGF-8 in articular chondrocytes.
In conclusion, FGF-8 enhanced the expression and activity of MMP-2 and MMP-9 of cultured chondrocytes of mouse through upregulating NF-κB p65 signaling.