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

Abstract

Plasminogen activators (PAs) have been shown to be synthesized in ovarian follicles of several mammalian species, where they contribute to the ovulation 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) secretion in mouse cells. To investigate in more detail the hormonal regulation of this system, we used the rat ovary as a model in which we analyzed the production of PAs by theca-interstitial (TI) and granulosa cells obtained from preovulatory follicles after gonadotropin stimulation. In untreated rats, uPA was the predominant enzyme in both TI and granulosa cells. After hormonal stimulation, an increase in uPA and tPA activity was observed in both cell 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 activity and mRNA levels in granulosa cells were explained by studying the localization of the enzyme. Analysis of granulosa cell lysates showed that after hormonal stimulation, 60-70% of the uPA behaved as a cell-associated protein, suggesting that uPA, already present in the follicle, accumulates on the granulosa cell surface through binding to specific uPA receptors. The redistribution of uPA in granulosa cells and the differing regulation of the two PAs by gonadotropins in the rat ovary suggest that the two enzymes might have different functions during the ovulation process. Moreover, the ability of antibodies anti-tPA and anti-uPA to significantly inhibit ovulation only when coinjected with hCG confirmed that the PA contribution to ovulation occurs at the initial steps.