Fibronectin fragments induce the expression of stromelysin-1 mRNA and protein in bovine chondrocytes in monolayer culture

Addition of proteolytically generated fibronectin fragments (Fn-f) to cultured cartilage tissue causes greatly enhanced release of metalloproteinases (MMPs), such as pro-stromelysin-1 (proSln-1), and suppression of proteoglycan (PG) synthesis, through release of catabolic cytokines, while native fibronectin is ineffective. We have investigated whether enhanced release of proSln-1 was due to up-regulation of pro-Sln-1 mRNA. We report that the addition of a 29-kDa (amino-terminal heparin-binding Fn-f) or a 140-kDa (central cell-binding Fn-f) to bovine chondrocytes in monolayer culture causes a dose dependent increase in the expression of pro-Sln-1 mRNA and the greatly enhanced release of pro-Sln-1 protein into the culture media. Up to 700 nM pro-Sln-1 was found in the conditioned media and metabolic labeling showed that it constituted a major portion of newly synthesized protein. A potential activator of pro-Sln-1, urokinase (u-PA), was released at elevated levels in the presence of the Fn-f while other activators, tissue plasminogen activator (t-PA) and plasmin activities were not detected. Addition of these activators to conditioned media did not allow conversion of pro-Sln-1 to active Sln-1. However, aminophenyl mercuric acid activated pro-Sln-1 to a 48-kDa Sln-1 form capable of degrading PG when added to cartilage suspensions. Gelatinase A mRNA was also enhanced, suggesting that the Fn-f may induce MMPs in general. However, the major regulator of Sln-1 activity, tissue inhibitor of MMPs form 1 (TIMP-1), was not induced at the gene level. Thus, a major effect of Fn-f on chondrocytes is to up-regulate pro-Sln-1 expression at the gene level, resulting in pro-Sln-1 as a major protein product.