Stimulation of Fibrotic Processes by the Infrapatellar Fat Pad in Cultured Synoviocytes From Patients With Osteoarthritis: A Possible Role for Prostaglandin F2

Objective. Stiffening of the joint is a feature of knee osteoarthritis (OA) that can be caused by fibrosis of the synovium. The infrapatellar fat pad (IPFP) present in the knee joint produces immune-modulatory and angiogenic factors. The goal of the present study was to investigate whether the IPFP can influence fibrotic processes in synovial fibroblasts, and to determine the role of transforming growth factor beta (TGF beta) and prostaglandin F-2 alpha (PGF(2 alpha)) in these processes. Methods. Batches of fat-conditioned medium (FCM) were made by culturing pieces of IPFP obtained from the knees of 13 patients with OA. Human OA fibroblast-like synoviocytes (FLS) (from passage 3) were cultured in FCM with or without inhibitors of TGF beta/activin receptor-like kinase 5 or PGF(2 alpha) for 4 days. The FLS were analyzed for production of collagen and expression of the gene for procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2 (PLOD2; encoding lysyl hydroxylase 2b, an enzyme involved in collagen cross-linking) as well as the genes encoding alpha-smooth muscle actin and type I collagen alpha 1 chain. In parallel, proliferation and migration of the synoviocytes were analyzed. Results. Collagen production and PLOD2 gene expression by the FLS were increased 1.8- fold (P < 0.05) and 6.0-fold (P < 0.01), respectively, in the presence of FCM, relative to control cultures without FCM. Moreover, the migration and proliferation of synoviocytes were stimulated by FCM. Collagen production was positively associated with PGF(2 alpha) levels in the FCM (R = 0.89, P < 0.05), and inhibition of PGF(2 alpha) levels reduced the extent of FCM-induced collagen production and PLOD2 expression. Inhibition of TGF beta signaling had no effect on the profibrotic changes. Conclusion. These results indicate that the IPFP can contribute to the development of synovial fibrosis in the knee joint by increasing collagen production, PLOD2 expression, cell proliferation, and cell migration. In addition, whereas the findings showed that TGF beta is not involved, the more recently discovered profibrotic factor PGF(2 alpha) appears to be partially involved in the regulation of profibrotic changes.