Ovulation has been described as a pathophysiological event in which the controlled dissolution and rupture followed by repair of otherwise healthy tissue occurs. Follicular rupture occurs as a result of coordination of several different cell types and functions. The specific roles of each cellular compartment involved in successful ovulation is still under investigation. The long-range goals of this research are to assess and understand the role of the theca in the events of ovulation. The experiments described in this proposal make use of a novel model of blockade of ovulation to specifically investigate the role of the theca. Immature hypophysectomized rats treated with a single dose of TCDD (2,3,7,8-tetrachlordibenzo-p-dioxin) followed by standard regimens of gonadotropins (PMSG and hCG) fail to ovulate. Ovaries examined from rats treated with TCDD have large unruptured follicles containing ova. Preliminary studies described here together with previously published data indicate TCDD acts directly at the level of the theca. This model system provides a unique tool to specifically investigate the role of the theca in ovulation. The process of ovulation is initiated by luteinizing hormone (LH) released from the pituitary. LH, acting at the level of the ovary increases intracellular cAMP and thus initiates a cascade of intracellular events. One set of genes regulated by LH and requisite for the process of ovulation are the plasminogen activator (PA) and plasminogen activator inhibitor (PAI) genes. In response to LH, PAI is decreased and PA increases. Preliminary studies indicate that when TCDD is present, ovulation does not occur. The hypothesis to be investigated is that TCDD blocks ovulation by inhibiting the plasminogen activator system in the ovary either directly at the level of the gene, and/or indirectly by altering the action of LH. TCDD may function directly at the level of the PA/PAI genes, and/or indirectly via a TCDD induced inhibition of LH-receptor mediated events. Preliminary studies indicate LH receptor binding following TCDD treatment is not altered, thus TCDD might act at points after LH receptor binding.
The first Aim outlined in this proposal will address ovarian PA and PAI gene expression and activity following in vivo treatment with TCDD. For these studies immature hypophysectomized rats will be treated with TCDD followed by PMSG and hCG. Ovaries will be collected at various time points after hCG administration, preovulatory follicles will be dissected from the ovaries and separated into theca and granulosa. Thus, PA/PAI expression and activity will be assessed separately in the thecal and granulosal compartments. TCDD might block ovulation by altering the ability of the ovarian cells to respond to LH. Preliminary studies show LH receptor binding to be unaffected by TCDD (TCDD vs. control), however, in vitro data indicate TCDD inhibits thecal LH-stimulated cAMP accumulation.
The second Aim of these studies will assess the effect of TCDD on ovarian LH receptor signaling events. Using a similar in vivo model system, immature hypophysectomized rats will be treated with TCDD followed by PMSG and hCG. Four hours after hCG administration preovulatory follicles will be dissected and separated into theca and granulosa. hCG stimulated cAMP, adenylyl cyclase, phosphodiesterase, and protein kinase C activities will be determined and compared between control and TCDD treatment. This novel model for blockade of ovulation by treatment with TCDD provides a unique tool to investigate the role of the theca in ovulation. The studies outlined in this proposal begin to assess the mechanism whereby TCDD blocks ovulation.
| Son, Deok-Soo; Roby, Katherine F; Rozman, Karl K et al. (2002) Estradiol enhances and estriol inhibits the expression of CYP1A1 induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in a mouse ovarian cancer cell line. Toxicology 176:229-43 |
| Petroff, B K; Roby, K F; Gao, X et al. (2001) A review of mechanisms controlling ovulation with implications for the anovulatory effects of polychlorinated dibenzo-p-dioxins in rodents. Toxicology 158:91-107 |
| Son, D S; Ushinohama, K; Gao, X et al. (1999) 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) blocks ovulation by a direct action on the ovary without alteration of ovarian steroidogenesis: lack of a direct effect on ovarian granulosa and thecal-interstitial cell steroidogenesis in vitro. Reprod Toxicol 13:521-30 |
