Localization of endothelin receptor messenger ribonucleic acid in the rat ovary and fallopian tube by in situ hybridization

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.

Altered endothelin expression in granulosa-lutein cells of women with polycystic ovary syndrome

AIMS

To examine the levels of endothelin system components in granulosa lutein cells (GLCs) of women with PCOS and compare them to normally ovulating women undergoing In Vitro Fertilization (IVF). Polycystic ovary syndrome (PCOS) is one of the most common endocrine-metabolic disorders in women of reproductive age. Endothelins are locally produced by endothelial and granulosa cells of the preovulatory follicle. Abnormal expression or production of endothelins may be a contributing factor in PCOS pathogenesis.

MAIN METHODS

Follicular aspirates containing GLCs were obtained from PCOS and normally ovulating patients undergoing oocyte retrieval during the IVF cycle. RNA was extracted and endothelin system components were quantified using real-time PCR. GLCs were cultured in basal media for 7 days, and then challenged with various luteinizing agents (luteinizing hormone, hCG, or forskolin) for 24 h.

KEY FINDINGS

In GLCs from women with PCOS, Endothelin-1 mRNA expression was elevated (2.2-fold) as compared with normally ovulating women, whereas endothelin-2 mRNA was reduced (1.8-fold). ET receptors and endothelin-converting enzyme showed the same expression levels in the two groups. In vitro modeling showed that although the steroidogenic response was preserved in GLC, endothelin expression levels were not exhibited in vitro in their original pattern.

SIGNIFICANCE

Dysregulation of ovarian endothelin expression may induce a pathologic ovulation pattern characteristic of PCOS.

Endothelins enhance prostaglandin (PGE(2) and PGF(2alpha)) biosynthesis and release by human luteal cells: evidence of a new paracrine/autocrine regulation of luteal function

We have previously shown that endothelin-1 (ET-1) is normally found in human luteal cells, where it is able to significantly inhibit both basal and hCG-induced progesterone production. To further expand our comprehension of the possible roles of endothelins (ETs) in luteal physiology, in this study we used primary cultures of luteal cells exposed to graded doses of ET-1 and ET-3; PGF(2alpha) and PGE(2) were assayed in the culture medium to investigate whether ETs also influence cyclooxygenase activity in these cells. We found that both ETs are able to significantly stimulate PGF(2alpha) and PGE(2) release in a dose- and time-dependent manner. ET-1 was always more effective than ET-3. Experiments with two endothelin receptor antagonists (the BQ485 and BQ788 compounds, which block the ET-A and ET-B receptors, respectively) showed that the two endothelins induce PG production through different receptors and signaling pathways. In conclusion, here we demonstrate the ability of ETs to influence PG synthesis and release from human luteal cells. As PGs are deeply involved in corpus luteum activity, and ETs were also able to influence progesterone production, the present new data suggest an interesting interplay among progesterone, PGs, and ETs in the control of corpus luteum physiology.

Gene expression of endothelin-1 in the porcine ovary: follicular development

We have investigated which follicular compartment and stage of follicular development are associated with endothelin-1 (ET-1) gene expression in the porcine ovary. The localization of mature ET-1 peptide and of its mRNA was determined by immunohistochemistry and by in situ hybridization. Stage of follicular development associated with ET-1 expression was investigated in terms of follicular class and occurrence of atresia. The latter was investigated by determining the occurrence of DNA fragmentation in apoptotic cells on adjacent sections to those used for ET-1 gene expression. Fifteen ovaries from 10 prepubertal pigs stimulated with gonadotropin were collected; a total of 1050 follicles were examined. Specific ET-1 immunoreactivity was restricted to the ovarian vasculature and to the granulosa cell compartment of antral follicles. The pattern of ET-1 mRNA expression was similar to that found for ET-1 immunoreactivity. Primordial, primary, and most secondary follicles did not express ET-1. The theca cell layer did not express ET-1 regardless of follicle developmental stage. ET-1 expression occurred with a significantly greater probability (P < 0.001 by the likelihood ratio test) in the granulosa cell compartment of antral follicles than in any other follicle class. Furthermore, in antral follicles, ET-1 expression occurred with a greater likelihood in large antral follicles than in small antral follicles (P < 0.001 by the likelihood ratio test). In small antral follicles, only 16.8% expressed ET-1; in contrast, 66.7% of large antral follicles exhibited ET-1 expression. It is interesting that in follicles in which ovulation had already occurred, intense ET-1 expression was found only in the prominent developing vasculature, the other cells present in the luteinized follicle did not display any ET-1 expression. The pattern of ET-1 gene expression observed in this study would be in agreement with our previous suggestion of a plausible physiological role for ET-1 in preventing premature progesterone production by granulosa cells of an antral follicle. The occurrence of atresia and expression of ET-1 in the same follicle was rare. Small and large antral follicles constituted 5.1% and 5.6%, respectively, of the examined follicles in this category. The majority of atretic follicles did not express ET-1 and, conversely, follicles that expressed ET-1 were not atretic. To the best of our knowledge, this is the first report in which large, nonatretic follicles are clearly identified as the population of follicles expressing ET-1. The results of this study delineate the follicular developmental stage and the compartment of when and where ET-1 may be physiologically meaningful.

Mechanisms underlying endothelin's inhibition of FSH-stimulated progesterone production by ovarian granulosa cells

In previous studies in porcine granulosa cell cultures, endothelin-1 (ET-1) was shown to inhibit FSH-stimulated cAMP and progesterone accumulation, and to increase inositol phosphate formation and cytosolic calcium ion concentration. The latter results suggest an action of ET-1 via the activation of phospholipase C. Here we have investigated the following experimental questions. (1) Does ET-1 activate PKC in ovarian cells? (2) Does the cellular mechanism(s) whereby ET-1 interferes with the steroidogenic action of FSH in granulosa cells involve an impairment of cAMP generation or action? And (3) how does the site(s) of the inhibitory effect(s) of ET-1 and TPA on FSH-stimulated progesterone accumulation in cultured granulosa cells compare? In the present investigation, ET-1 (1 microM) induced rapid cytosol-to-membrane translocation of [3H]phorbol 12,13-dibutyrate binding sites, indicating protein kinase C (PKC) activation. At 24 or 48 h, ET-1 inhibited FSH-, but not forskolin (1 microM)-induced, cAMP accumulation. Cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc) messenger RNA (mRNA) accumulation was stimulated by FSH, 8-bromo-cAMP (8Br-cAMP, 0.5 mM) and forskolin. ET-1 significantly inhibited this effect of FSH, but not the effects of 8Br-cAMP and forskolin. Progesterone production decreased commensurately with this inhibitory action of ET-1 on the FSH-stimulated accumulation P450scc mRNA. The PKC activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), suppressed steroidogenesis stimulated by forskolin and 8Br-cAMP as well as FSH. In conclusion, ET-1 inhibited FSH-stimulated cAMP accumulation, P450scc expression, and progesterone production in porcine granulosa cell cultures. The data are compatible with pre-adenylate cyclase site of action. Although ET-1 activated PKC, TPA, unlike ET-1, seems to inhibit steroidogenesis by interfering with cAMP action.

Characterization and regulation of type A endothelin receptor gene expression in bovine luteal cell types

Our previous studies demonstrated that endothelin-1 (ET-1), a 21-amino acid vasoconstrictor peptide, has a paracrine regulatory role in bovine corpus luteum (CL). The peptide is produced within the gland where it inhibits progesterone production by acting via the selective type A endothelin (ETA) receptors. The present study was designed to characterize ETA receptor gene expression in different ovarian cell types and its hormonal regulation. ETA receptor messenger RNA (mRNA) levels were high in follicular cells as well as in CL during luteal regression. At this latter stage, high ETA receptor expression concurred with low prostaglandin F2alpha receptor mRNA. The ETA receptor gene was expressed by all three major cell populations of the bovine CL; i.e. small and large luteal cells, as well as in luteal endothelial cells. Among these various cell populations, the highest ETA receptor mRNA levels were found in endothelial cells. cAMP elevating agents, forskolin and LH, suppressed ETA receptor mRNA expression in luteinized theca cells (LTC). This inhibition was dose dependent and was evident already after 24 h of incubation. In luteinized granulosa cells (LGC), 10 and 100 ng/ml of insulin-like growth factor I and insulin (only at a concentration of 2000 ng/ml) markedly decreased ETA receptor mRNA levels. In both LGC and LTC there was an inverse relationship between ETA receptor gene expression and progesterone production; insulin (in LGC) and forskolin (in LTC) enhanced progesterone production while inhibiting ETA receptor mRNA levels. Our findings may therefore suggest that, during early stages of luteinization when peak levels of both LH and insulin-like growth factor I exist, the expression of ETA receptors in the gland are suppressed. This study demonstrates physiologically relevant regulatory mechanisms controlling ETA receptor gene expression and further supports the inhibitory role of ET-1 in CL function.

Endothelin-1: expression and role in human corpus luteum

PROBLEM

Several recent data suggest an involvement of endothelin (ET)-1, a powerful vasoconstrictor peptide, in reproductive function. This study was designed to investigate the presence and role of ET-1 in human corpus luteum.

METHOD OF STUDY

Purified luteal cells were incubated for different times with ET-1 or ET-3 alone or associated with human chorionic gonadotropin. In another set of experiments cells were treated with ET-1 and BQ485, an ET-A receptor antagonist, or with phorbol 12-myristate-13 acetate (PMA), an activator of protein kinase C.

RESULTS

ET-1 reduced both basal and human chorionic gonadotropin-induced progesterone production at all examined times, similarly PMA inhibited basal progesterone synthesis. BQ485 prevented the inhibitory effect of ET-1, while no effect was observed with ET-3. Finally, ET-1 mRNA was detected in the luteal cells.

CONCLUSION

ET-1 is expressed by human luteal cells and reduces basal and human chorionic gonadotropin-induced progesterone synthesis through the ET-A receptors and the protein kinase C pathway. Conversely, ET-3 does not affect luteal steroidogenesis.

Effects of follicle-stimulating hormone on endothelin receptors in cultured rat ovarian granulosa cells

Two subtypes of the endothelin (ET) receptor (ETA and ETB) were studied in cultured ovarian granulosa cells. Immature 21-day-old female Wistar-Imamichi rats were implanted with diethylstilbestrol (DES) pellets for 5 days and granulosa cells were collected by repeated puncturing. Viable cells (2.5 or 5 x 10(5)) were cultured with 50--400 ng/ml of ovine NIH follicle-stimulating hormone (FSH) in the presence or absence of [125I-Tyr13]ET-1 (50 pM) in 1 ml McCoy's 5a medium for 72 h. FSH gradually increased the [125I-Tyr13]ET-1 binding to granulosa cells, whereas FSH-untreated granulosa cells had no significant changes. The dose of 200 ng/ml of FSH was most effective for [125I-Tyr13]ET-1 binding for 48-h culture, thereafter revealing a plateau. After 48 h of culture with 200 ng/ml of FSH, granulosa cells were further incubated with [125I-Tyr13]ET-1 (10 pM-1 nM) and/or [125I]IRL1620, the selective ETB receptor agonist (10 pM-1 nM) for 2 h for equilibrium study, and then the dissociation constant and the maximal binding capacity between receptors and ligands were determined by saturation curve and Scatchard plot analysis. ETA + ETB, ETB, and ETA (sites/cells) showed a 4.4-, 2.6-, and 7.5-fold increase, respectively. As for steroidogenesis, ET-1 (100 nM) or ET-3 (100 nM) suppressed FSH-induced progesterone and 17 beta-estradiol production. These results indicate that FSH upregulates both ETA and ETB receptors in DES-treated immature rat granulosa cells, with no significant differences between ET-1 and ET-3, and that ET-1 or ET-3 suppresses FSH-induced steroidogenesis. ETs may affect the granulosa cell function through the ETA and ETB receptors, and the increase in amount of ET binding does not reflect ET effects on granulosa cell function. The ET receptor plays an important role in the development of the ovary.

Does endothelin-1 affect human spermatozoa function?

PROBLEM

It has been reported that massive amounts of immunoreactive endothelins (ETs) exist in human seminal plasma. However, the physiological role of ETs in seminal plasma remains to be determined. We speculated that ETs might affect sperm function.

METHOD

The present study was designed to investigate the effect of endothelium-1 (ET-1) on: (a) sperm motion parameters, (b) hyperactivated motility, (c) sperm-zona pellucida-binding capacity, (d) sperm-oocyte penetration capacity, (e) acrosome reaction and its prerequisite, an increase in intracellular Ca2+ concentration ([Ca2+]i), and to examine (f) the presence of binding sites for ET-1 in human sperm. Forty-six semen ejaculates from 14 fertile men were assessed under capacitating conditions after separation of the motile sperm fraction by wash and swim-up.

RESULTS

ET-1 (1 micron) exhibited significant stimulatory effects on sperm velocity at 30 min (P = 0.01), amplitude of lateral head displacement (ALH)max (P = 0.05), and ALHmean at 60 min (P = 0.04) in some samples (n = 10). However, these effects were not observed in experiments using a larger number of samples (n = 39). ET-1 had no effect on hyperactivated motility of sperm at 30 min to 24 h. Neither ET-3 nor IRL 1620, a selective ETB receptor agonist, affected sperm motion parameters or hyperactivated motility. ET-1 did not affect sperm-zona-binding capacity, sperm-oocyte penetration capacity, acrosome reaction, or [Ca2+]i of sperm. Specific binding sites for ET-1 were not detected on the cell surface of human sperm.

CONCLUSIONS

Although ET-1 is present in massive amounts in human seminal plasma and may have the capacity to alter the quality of motile sperm in some samples, a physiological role of ET-1 in the modulation of the function of mature, ejaculated sperm still remains unknown.