Extracellular Collagen Promotes Interleukin-1β–Induced Urokinase-Type Plasminogen Activator Production by Human Corneal Fibroblasts

PURPOSE. Keratocytes maintain homeostasis of the corneal stroma through synthesis, secretion, and degradation of collagen fibrils of the extracellular matrix. Given that these cells are essentially embedded in a collagen matrix, keratocyte-collagen interactions may play a key role in regulation of the expression or activation of enzymes responsible for matrix degradation including urokinase-type plasminogen activator (uPA), plasmin, and matrix metalloproteinases (MMPs). We examined the effect of extracellular collagen on the production of uPA by corneal fibroblasts (activated keratocytes) stimulated with the proinflammatory cytokine interleukin-1 beta (IL-1 beta).METHODS. Human corneal fibroblasts were cultured either on plastic or in a three-dimensional gel of type I collagen. Plasminogen activators were detected by fibrin zymography, whereas the IL-1 receptor (IL-1R) and MMPs were detected by immunoblot analysis. Collagen degradation by corneal fibroblasts was assessed by measurement of hydroxyproline in acid hydrolysates of culture supernatants.RESULTS. Collagen and IL-1 beta synergistically increased the synthesis and secretion of uPA in corneal fibroblasts. Collagen also upregulated IL-1R expression in the cells in a concentration-dependent manner. The conversion of extracellular plasminogen to plasmin, as well as the plasminogen-dependent activation of MMP-1 and MMP-3 and degradation of collagen apparent in three-dimensional cultures of corneal fibroblasts exposed to IL-1 beta, were all abolished by a selective uPA inhibitor.CONCLUSIONS. Collagen and IL-1 beta cooperate to upregulate uPA production by corneal fibroblasts. Furthermore, IL-1 beta-induced collagen degradation by these cells appears to be strictly dependent on uPA expression and mediated by a uPA-plasmin-MMP pathway.