Human synovial fibroblasts produce urokinase-type plasminogen activator

The plasminogen activator produced by cultured human synovial fibroblasts was investigated both biochemically and immunologically. Stimulated either by all-trans-retinoic acid or by monocyte-conditioned medium, these fibroblasts elaborated a plasminogen activator with electrophoretic mobility similar to that of urokinase (Mr = 52 kilodaltons), and which also had immunologic cross-reactivity with urokinase. The plasminogen activator found in rheumatoid synovial fluids has been shown to be of the urokinase type. The findings reported here are consistent with the notion that synovial fibroblasts are a source of this proteinase.