Urokinase redistribution from the secreted to the cell-bound fraction in granulosa cells of rat preovulatory follicles

Plasminogen activators (PAs) have been shown to be synthesized in ovarian f ollicles of several mammalian species, where they contribute to the ovulati on process. The type of PA secreted by granulosa cells is species-specific. In fact, whereas in the rat, gonadotropins stimulate tissue-type PA (tPA) production, the same hormonal stimulation induces urokinase PA (uPA) secret ion in mouse cells. To investigate in more detail the hormonal regulation o f this system, we used the rat ovary as a model in which we analyzed the pr oduction of PAs by theca-interstitial (TI) and granulosa cells obtained fro m preovulatory follicles after gonadotropin stimulation. In untreated rats, uPA was the predominant enzyme in both TI and granulosa cells. After hormo nal stimulation, an increase in uPA and tPA activity was observed in both c ell types. Surprisingly, only tPA mRNA increased in a time-dependent manner in both cell types, while uPA mRNA increased only in TI cells and actually decreased in granulosa cells. These divergent results between uPA enzyme a ctivity and mRNA levels in granulosa cells were explained by studying the l ocalization of the enzyme. Analysis of granulosa cell lysates showed that a fter hormonal stimulation, 60-70% of the uPA behaved as a cell-associated p rotein, suggesting that uPA, already present in the follicle, accumulates o n the granulosa cell surface through binding to specific uPA receptors. The redistribution of uPA in granulosa cells and the differing regulation of t he two PAs by gonadotropins in the rat ovary suggest that the two enzymes m ight have different functions during the ovulation process. Moreover, the a bility of antibodies anti-tPA and anti-uPA to significantly inhibit ovulati on only when coinjected with hCG confirmed that the PA contribution to ovul ation occurs at the initial steps.