Regulation of Plasminogen Activation

The main components of plasminogen activation include plasminogen, tissue plasminogen activator (tPA), urokinase plasminogen activator (uPA), urokinase plasminogen activator receptor (uPAR), and plasminogen activator inhibitors-1 and -2 (PAI-1, PAI-2 ). uPA and its receptor uPAR are known to be important in cell migration/adhesion. In these studies, we show that uPAR was anchored in focal contacts in a uPA-dependent manner, whereas in its unoccupied form uPAR appeared diffusely on the cell surface. Moreover, αvβ3-integrin was found in close proximity to uPAR in the focal contacts. We also studied the effect of transforming growth factor-β (TGF-β) and interferon γ (IFN-γ) retinal pigment epithelial cells (RPE) and observed that TGF-β increased uPAR expression. tPA has been considered to be mainly a fibrinolytic enzyme. We observed that in human uterine cervical epithelial cells immortalized with HPV-16 (human papillomavirus type 16) DNA, the tPA activity levels were very low or even undetectable due to an inhibitory effect of hydrocortisone in the growth medium. Withdrawal of hydrocortisone increased tPA mRNA expression, which seemed to be a later event than uPA mRNA expression. tPA activity levels increased dramatically at 72 hours when hydrocortisone was eliminated from the growth medium. Elimination of hydrocortisone also increased tPA and uPA activity levels in two other nonmalignant human epithelial cells. Based on results from in vitro observations, the presence of tPA was examined in normal, precancerous, and malignant epithelial tissue lesions by immunohistochemistry. In addition, in situ hybridization was used to demostrate tPA mRNA expression. Immunohistochemistry revealed that tPA was expressed at high levels in normal, condylomatous, and dysplastic squamous epithelium. Expression of tPA mRNA was restricted to the proliferating basal and parabasal cell layers, whereas tPA protein synthesis became evident in the next cell layers. tPA immunostaining in cancer cells was mostly negative or weak in adenocarcinoma and strong in epidermoid carcinoma. We detected tPA mRNA in adenocarcinoma but not in epidermoid carcinoma. In epidermoid carcinoma, the surrounding stromal cells were positive.