Transient and cell‐specific expression of tissue‐type plasminogen activator and plasminogen‐activator‐inhibitor type 1 results in controlled and directed proteolysis during gonadotropin‐induced ovulation

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Abstract
Proteolytic activity generated by the plasminogen-activator system (PA system) is associated with many biological processes. However, it is not known how the proteolytic activity is regulated in vivo in order to obtain directed proteolysis while, at the same time, protecting unrestrained tissue destruction. Using gonadotropin-induced ovulation as a model, we have studied how two components of the PA system, tissue-type plasminogen activator (tPA) and plasminogen-activator-inhibitor type 1 (PAI-1), are regulated temporally and spatially by gonadotropins, leading to the initiation and termination of a well-directed proteolytic process. In-situ hybridization and in-situ zymography were used to analyze the expression of tPA and PAI-1 mRNA and PA-activity in specific ovarian cell types. Both tPA and PAI-1 were found to be regulated and to have a distinct expression pattern in different ovarian compartments. tPA was expressed in both granulosa and thecal-interstitial cells; the highest levels of tPA mRNA were found in the granulosa cells of preovulatory follicles, just prior to ovulation. Consistent with a role for luteinizing hormone/chorionic gonadotropin (LH/CG) in triggering ovulation, the cells and follicles that actively expressed tPA also contained high levels of LH-receptor mRNA while cumulus cells that contain undetectable amounts of tPA mRNA were devoid of LH-receptor expression. The highest levels of PAI-1 mRNA were found about 6 h before ovulation and mainly in the thecal-interstitial cells and ovarian stroma tissue which encapsulate the follicle. Preovulatory follicles, protruding onto the surface of the ovary with less surrounding stroma tissue, expressed less PAI-1 compared to small non-ovulatory follicles embedded in inner part of the ovary. In-situ zymography also revealed that the PA activity was colocalized to the surface of the ovary just prior to ovulation. Our studies suggest that a proteolytic activity provided by tPA and modulated by PAI-1 is responsible for a controlled and directed proteolysis leading to rupture of selected follicles during ovulation.