Regulatory Mechanism of Collagen Degradation by Keratocytes and Corneal Inflammation: The Role of Urokinase-Type Plasminogen Activator.

Keratocytes, corneal resident cells in the corneal stroma, exist between collagen lamellae and maintain the corneal stromal structure. When the corneal stroma is damaged, keratocytes are transformed to myofibroblasts to aid corneal wound healing by phagocytizing debris. Keratocytes and extracellular collagen influence each other because keratocytes cultured in a 3D collagen gel undergo morphological changes and keratocytes produce metalloproteases that degrade extracellular collagen. IL-1 and plasminogen are critical mediators for collagen degradation. The plasminogen system contributes to tissue repair by activating matrix metalloproteinases (MMPs), releasing growth factors from the extracellular matrix and extracellular matrix degradation. Urokinase-type plasminogen activator (uPA) is thought to be involved in corneal disorders and regulates corneal wound healing. uPA is a serine protease synthesized by various cells such as corneal epithelial cells, corneal fibroblasts, vascular endothelial cells, smooth muscle cells, monocytes, macrophages, and malignant tumor cells of different origins. Here, we review the role of uPA in corneal stromal wound healing. uPA is expressed in leukocytes and corneal fibroblasts in the corneas of patients with corneal ulcerations suggesting it is a key regulator of corneal stromal wound healing. uPA is directly involved in plasmin-mediated collagen degradation induced by IL-1. Moreover, uPA is critically involved in promoting leukocyte infiltration in corneal inflammation by activating MMP-9. This activation is presumably directly and indirectly mediated by the plasminogen/plasmin cascade. Moreover, uPA mediates the release of inflammatory cytokines from corneal fibroblasts to promote leukocyte infiltration.