Presence and localization of endothelin receptor in the rat ovary and its regulation by pituitary gonadotropins

Current status of the role of endothelins in regulating ovarian follicular function: A review

Endothelins (EDN) are a group of vasoactive 21 amino acid peptides reported to play roles in steroidogenesis, folliculogenesis, and ovulation. EDN1, EDN2 and EDN3 have all been shown to affect granulosa cell (GC) function in a variety of mammalians species. Herewithin, the role of EDN in regulating steroidogenesis and ovarian follicular development is reviewed, focusing on the localization and function of EDN and their receptors in ovarian follicular function emphasizing species differences. For example, in single ovulating species such as humans and cattle, in the presence of trophic hormones such as FSH and IGF1, EDN1 and EDN2 significantly inhibited GC estradiol production in 2 of 4 studies, while no effect was observed for GC progesterone production in 2 of 4 studies. In contrast, EDN1 exhibited inhibitory effects on progesterone production by GC in 3 of 3 studies in pigs and 3 of 4 studies in rats. Also, EDN1 inhibited GC estradiol production in 4 of 5 studies in rats. Altogether, these results indicate that EDN are produced by ovarian follicles and are involved in the regulation of steroidogenesis of GC of several mammalian species including humans, cattle, pigs and rats, but that these effects may vary with species and culture condition.

Paracrine regulation of the resumption of oocyte meiosis by endothelin-1

Mammalian oocytes remain dormant in the diplotene stage of prophase I until the resumption of meiosis characterized by germinal vesicle breakdown (GVBD) following the preovulatory gonadotropin stimulation. Based on genome-wide analysis of peri-ovulatory DNA microarray to identify paracrine hormone-receptor pairs, we found increases in ovarian transcripts for endothelin-1 and endothelin receptor type A (EDNRA) in response to the preovulatory luteinizing hormone (LH)/human chorionic gonadotropin (hCG) stimulation. Immunohistochemical analyses demonstrated localization of EDNRA in granulosa and cumulus cells. In cultured preovulatory follicles, treatment with endothelin-1 promoted oocyte GVBD. The stimulatory effect of endothelin-1 was blocked by cotreatment with antagonists for the type A, but not related type B, receptor. The stimulatory effect of hCG on GVBD was partially blocked by the same antagonist. The endothelin-1 promotion of GVBD was found to be mediated by the MAPK/ERK pathway but not by the inhibitory G protein. Studies using cumulus-oocyte complexes and denuded oocytes demonstrated that the endothelin-1 actions are mediated by cumulus cells. Furthermore, intrabursal administration with endothelin-1 induced oocyte GVBD in preovulatory follicles. Our findings demonstrate a paracrine role of endothelin-1 in the induction of the resumption of meiosis and provide further understanding on the molecular mechanisms underlying the nuclear maturation of oocytes induced by the preovulatory LH surge.

Apoptosis in the ovary: molecular mechanisms

Cell death was first described in rabbit ovaries (Graaffian follicles), the phenomenon being called 'chromatolysis' rather than apoptosis. In humans, the ovarian endowment of primordial follicles is established during fetal life. Apoptotic cell death depletes this endowment by at least two-thirds before birth, executed with the help of several players and pathways conserved from worms to humans. To date, apoptosis has been reported to be involved in oogenesis, folliculogenesis, oocyte loss/selection and atresia. Several pro-survival and pro-apoptotic molecules are involved in ovarian apoptosis with the delicate balance between them being the determinant for the final destiny of the follicular cells. This review critically analyses the current knowledge about the biological roles of these molecules and their relevance to the dynamics of follicle development. It also presents the existing literature and assesses the gaps in our knowledge.

Correlation between 21 amino acid endothelin, intrafollicular steroids and follicle size in stimulated cycles

Several in-vitro studies have shown that endothelins (ET) may inhibit synthesis of progesterone and prevent luteinization of granulosa cells. In the present study, a specific radioimmunoassay was used to evaluate the correlation between concentrations of active (21 residue) ET and ovarian steroids in 47 samples of human follicular fluid (FF) following gonadotrophin stimulation for in-vitro fertilization (IVF) protocols. An isoform non-selective antibody was used in the radioimmunoassay, which recognized the C-terminal structure of the 21 residue ET, and therefore did not crossreact with their weakly active precursors - big ET. In pooled samples of follicular fluid (FF), the concentration of 21 amino acid ET correlated negatively with diameter of the follicles (r = -0.31, P < 0.05) and progesterone concentrations in FF (r = -0.56, P < 0. 001). A positive relationship (non-significant) was found between ET and testosterone concentrations. No correlation between ET and oestradiol was observed. The within-patient correlation coefficients were also evaluated in women from whom three or more samples of FF were obtained. ET were markedly inversely correlated with follicle size in all cases, and with progesterone in five of seven women. Five of seven patients also showed significant positive correlation of ET with testosterone. The results demonstrate clinical evidence that active ET play an important role in regulation of follicle development, especially in the inhibition of premature luteinization of granulosa cells.

Local release of steroid hormones, prostaglandin E2, and endothelin-1 from bovine mature follicles In vitro: effects of luteinizing hormone, endothelin-1, and cytokines

Local regulation of ovulation involves the interaction of LH and intrafollicular factors including steroids, prostaglandins, and peptides derived from endothelial cells, leukocytes, fibroblasts, and steroidogenic cells. To estimate the intrafollicular role of endothelin-1 (ET-1) and its possible interaction with LH, tumor necrosis factor alpha (TNFalpha), and interleukin-1beta (IL-1beta), a microdialysis system was implanted into the theca layer of preovulatory bovine follicles that were maintained in organ culture chambers. The effects of LH, ET-1, TNFalpha, and IL-1beta on the local release of steroids, prostaglandin E2 (PGE2), and ET-1 from the cells surrounding the implanted capillary membrane were investigated. Each preovulatory follicle (selected based on the concentrations of steroids and PGE2) was dissected from surrounding stromal tissue and implanted with 4 capillary dialysis membranes (control, LH, cytokines or ET-1, and LH+cytokine or LH+ET-1) into the theca layer. They were then incubated in organ culture chambers and perfused with Ringer's solution for 14 h after pre-perfusion for 2 h. The stimulation with LH (5 microg/ml) between 4 and 6 h increased the release of progesterone (P4), androstenedione (A), estradiol-17beta (E2), PGE2 (p < 0.001), and ET-1 (p < 0.05). The infusion of ET-1 (250 ng/ml) between 8 and 10 h inhibited P4 and A release but stimulated E2 release (p < 0.05). The infusion of TNFalpha (100 ng/ml) between 8 and 10 h after LH exposure suppressed the release of A and E2 (p < 0.05). IL-1beta (10 ng/ml) between 8 and 10 h stimulated E2 release but inhibited A release (p < 0.05). Moreover, ET-1 and cytokines clearly stimulated PGE2 release (p < 0.05). ET-1 and TNFalpha induced further release of PGE2 stimulated by LH (p < 0. 05). Also, TNFalpha and IL-1beta induced further release of ET-1 stimulated by LH (p < 0.05). These results show that ET-1 is released from the theca layer of mature bovine follicles in vitro and that it affects follicular steroids and PGE2 secretion. The overall results suggest that interactions among ET-1, PGE2, and cytokines may have key roles in a local intermediatory/amplifying system of the LH-triggered ovulatory cascade in the bovine preovulatory follicle.