Osteoarthritic synovial fibroblasts drive inflammatory responses and impair chondrocyte function in response to a fibronectin matrikine

To investigate the effects of a matrikine, fibronectin fragment (FN7-10), on human osteoarthritic (OA) synovial fibroblasts and implications for inflammation and cartilage degradation. Joint tissue was obtained from 50 OA patients undergoing knee arthroplasty. Isolated synovial fibroblasts were treated with 1 µM of FN7-10 or PBS as control. Cytokine protein arrays were used to identify differentially secreted inflammatory mediators in conditioned media (CM). Bulk RNA-seq was used to evaluate transcriptional changes. Monocytic THP-1 cells in transwell assays were used to evaluate chemotactic activity of CM. Macrophage differentiation of THP-1 cells was assessed via qPCR and flow cytometry. Chondrocyte gene expression was evaluated by qPCR and glycosaminoglycan (GAG) production by Alcian blue assays. FN7-10 stimulation of synovial fibroblasts induced an inflammatory secretome, including production of chemokines. RNA-seq analysis confirmed FN7-10 induced upregulation of chemokines, including CCL5, CCL7, CCL8, CCL20, CXCL5 and CXCL10, which displayed mean transcriptional log-2-fold changes of 3.89 (CI, 2.81 - 4.97). CM from FN7-10-stimulated synovial fibroblasts increased THP-1 chemotaxis by 7.8-fold (CI, 3.26 - 12.35) which was reduced by CCR2 or CXCR2 inhibitors and promoted monocyte to macrophage differentiation characterized by an increase in pro-inflammatory gene expression and CD14. Chondrocytes treated with FN7-10 synovial fibroblast CM showed elevated IL6 and MMP1 and decreased ACAN and COL2A1 expression, along with reduced GAG levels. The matrikine FN7-10 promotes an inflammatory synovial fibroblast phenotype. Proinflammatory mediators, including chemokines, released from activated synovial fibroblasts recruit monocytes that may exacerbate joint inflammation and can also promote increased catabolic activity in chondrocytes.