TGFβ stimulates fibroblast-like synovial cells to produce ED-A fibronectin in osteoarthritis

Introduction: The pathophysiology of osteoarthritis (OA) involves wear and tear, and a state of low-grade inflammation. Wear and tear leads to tissue degradation and tissue repair responses, including tissue growth factor beta (TGFβ)-induced myofibroblast production of extracellular matrix (ECM). Fibronectins are an essential part of the ECM, and injection of fibronectin fragments into rabbit joints is a previously established animal model of OA. Alternatively-spliced fibronectin contains the ED-A domain (ED-A FN) and has been shown to activate Toll-like receptor 4. In this study, we tested the hypothesis that FN fragments containing the ED-A domain could be one mechanism transducing mechanical events into inflammatory signals in OA. Methods: Samples of synovial membrane and cartilage were obtained from patients with knee OA undergoing joint replacement surgery. Immunostaining for ED-A FN and the myofibroblast marker alpha smooth muscle actin (αSMA) was performed on synovial membranes and fibroblast-like synovial cells (FLS). FLS were stimulated with TGFβ, TNFα, lipopolysaccharide, IL-6, OA synovial fluid, or chondrocyte lysate, and analyzed for ED-A FN. Synovial cells isolated by enzymatic digestion and human monocyte-derived macrophages (MDM) were incubated with recombinant ED-A FN, plasmin, cellular FN, or cellular FN digested with plasmin; and culture supernatants were analyzed for MCP-1 and TNFα.Results: We hypothesized that ED-A FN is produced by OA FLS in response to factors found in the OA synovial joint. Indeed, the production of ED-A FN by OA FLS was increased by TGFβ, OA synovial fluid, and lysed chondrocytes in all experiments (n=3). ED-A FN co-localized with the myofibroblast marker αSMA in both the OA FLS (n=3) and in the OA synovial membranes (n=8). We further hypothesized that ED-A FN expression is associated with cellular density and expression of inflammatory molecules in OA. ED-A FN staining was associated with both number of lining layer cells (rho=0.85 and p=0.011) and sublining cells (rho=0.88 and p=0.007) in the OA synovium (n=8), and co-localized with both MCP-1 and TNFα (n=5). Recombinant ED-A FN increased the production of both MCP-1 and TNFα by MDM (n=3) and OA FLS (n=3). Finally, we demonstrated that the FN fragments containing the ED-A domain generated the same production of both MCP-1 and TNFα as recombinant ED-A FN.Conclusion: The disease process in OA shares features with the chronic wound healing response including myofibroblast differentiation and production of mediators that promote myofibroblast production of ED-A FN. We show that recombinant and plasmin-derived ED-A fragments stimulate FLS and MDM to produce pro-inflammatory mediators. Our findings support utilizing ED-A FN for drug delivery or therapeutic targeting of the formation of ED-A FN or the enzymatic fragmentation of FN to reduce pro-inflammatory responses in OA.