cAMP attenuates TGF-β's profibrotic responses in osteoarthritic synoviocytes: involvement of hyaluronan and PRG4.

Osteoarthritis (OA) is characterized by synovitis and synovial fibrosis. Synoviocytes are fibroblast-like resident cells of the synovium that are activated by transforming growth factor (TGF)-β to proliferate, migrate, and produce extracellular matrix. Synoviocytes secrete hyaluronan (HA) and proteoglycan-4 (PRG4). HA reduces synovial fibrosis in vivo, and the Prg4-/- mouse exhibits synovial hyperplasia. We investigated the antifibrotic effects of increased intracellular cAMP in TGF-β-stimulated human OA synoviocytes. TGF-β1 stimulated collagen I (COL1A1), α-smooth muscle actin (α-SMA), tissue inhibitor of metalloproteinase (TIMP)-1, and procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2 (PLOD2) expression, and procollagen I, α-SMA, HA, and PRG4 production, migration, and proliferation of OA synoviocytes were measured. Treatment of OA synoviocytes with forskolin (10 μM) increased intracellular cAMP levels and reduced TGF-β1-stimulated COL1A1, α-SMA, and TIMP-1 expression, with no change in PLOD2 expression. Forskolin also reduced TGF-β1-stimulated procollagen I and α-SMA content as well as synoviocyte migration and proliferation. Forskolin (10 μM) increased HA secretion and PRG4 expression and production. A cell-permeant cAMP analog reduced COL1A1 and α-SMA expression and enhanced HA and PRG4 secretion by OA synoviocytes. HA and PRG4 reduced α-SMA expression and content, and PRG4 reduced COL1A1 expression and procollagen I content in OA synoviocytes. Prg4-/- synovium exhibited increased α-SMA, COL1A1, and TIMP-1 expression compared with Prg4+/+ synovium. Prg4-/- synoviocytes demonstrated strong α-SMA and collagen type I staining, whereas these were undetected in Prg4+/+ synoviocytes and were reduced with PRG4 treatment. We conclude that increasing intracellular cAMP levels in synoviocytes mitigates synovial fibrosis through enhanced production of HA and PRG4, possibly representing a novel approach for treatment of OA synovial fibrosis.