The steroid hormone progesterone (P) plays a pivotal role during ovulation. Mice lacking P receptor (Pgr) gene fail to ovulate due to a defect in follicular rupture. The P receptor (PGR)-regulated pathways that modulate ovulation, however, remain poorly understood. To identify these pathways, we performed gene expression profiling using ovaries from mice subjected to gonadotropin-induced superovulation in the presence and in the absence of CDB-2914, a synthetic PGR antagonist. Prominent among the genes that were down-regulated in response to CDB-2914 was endothelin (ET)-2, a potent vasoactive molecule. ET-2 mRNA was transiently induced in mural granulosa cells of the preovulatory follicles immediately preceding ovulation. This induction was absent in the ovaries of PGR null mice, indicating a critical role of this receptor in ET-2 expression. To investigate the functional role of ET-2 during ovulation, we employed selective antagonists of endothelin receptors, ETR-A and ETR-B. Mice treated with an ETR-B antagonist exhibited a dramatic (>85%) decline in the number of released oocytes. Strong expression of ETR-B was observed in the mural and cumulus granulosa cells of the preovulatory follicles as well as in the capillaries lining the inner border of the theca interna. We also identified cGMP-dependent protein kinase II, a previously reported PGR-regulated gene, as a downstream target of ET-2 during ovulation. Collectively, our studies uncovered a unique pathway in which ET-2, produced by PGR in mural granulosa cells, acts in a paracrine or autocrine manner on multiple cell types within the preovulatory follicle to control the final events leading to its rupture.

The progesterone receptor (PR) plays a critical role during ovulation. Mice lacking the PR gene are anovulatory due to a failure in the rupture of the preovulatory follicles. The pathways that operate downstream of PR to control ovulation are poorly understood. Using gene expression profiling, we identified peroxisome proliferator-activated receptor ␥ (PPAR␥) as a target of regulation by PR in the granulosa cells of the preovulatory follicles during the ovulatory process. To investigate the function of PPAR␥ during ovulation, we created a conditional knockout mouse in which this gene was deleted via Cre-Lox-mediated excision in granulosa cells. When these mutant mice were subjected to gonadotropin-induced superovulation, the preovulatory follicles failed to rupture and the number of eggs released from the mutant ovaries declined drastically. Gene expression analysis identified endothelin-2, interleukin-6, and cyclic GMP-dependent protein kinase II as novel targets of regulation by PPAR␥ in the ovary. Our studies also suggested that cycloxygenase 2-derived metabolites of long-chain fatty acids function as endogenous activating ligands of PPAR␥ in the preovulatory follicles. Collectively, these studies revealed that PPAR␥ is a key mediator of the biological actions of PR in the granulosa cells and activation of its downstream pathways critically controls ovulation.Ovulation is a key female reproductive event which involves the release of a fertilizable oocyte from a mature ovarian follicle (28). In mammals, it is a highly coordinated process regulated by endocrine/paracrine factors produced by the hypothalamic-pituitary-ovarian axis. During each reproductive cycle in the rodent, follicle-stimulating hormone, synthesized by the pituitary gonadotrophs, promotes the growth and development of a selected pool of preantral follicles to the preovulatory stage. The action of follicle-stimulating hormone is followed by a timely surge of another pituitary gonadotropin, luteinizing hormone (LH), which triggers ovulation. The preovulatory surge of LH sets in motion a complex cascade of gene expression events in the follicular cells that culminates in the rupture of the preovulatory follicle (27). Genetic analyses of the mouse established that the genes encoding progesterone (P) receptor (PR) (26), cyclooxygenase 2 (COX-2) (7, 21), and CCAAT/enhancer binding protein ␤ (36), which are induced in the granulosa cells following LH action, play obligatory roles during ovulation.The steroid hormone P, acting via its cellular receptor, plays an essential role during ovulation (22,23). Previous studies have shown that the PR is rapidly induced in the mural granulosa cells of the preovulatory follicles following the LH surge (26, 29). Development of a mouse model lacking PR by Lydon et al. revealed an indispensable role for this hormone receptor during ovulation (23). In PR-null mice, follicular development to the antral stage proceeds unaffected. These mutant mice, however, fail to ovulate due to a block in follicular rupture, even wh...

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