Mechanism of luteinizing hormone regulation of prostaglandin synthesis in rat granulosa cells

Potential role of polyunsaturated fatty acids, with particular regard to the signaling pathways of arachidonic acid and its derivatives in the process of maturation of the oocytes: Contemporary review

Oocyte meiotic maturation is one of the significant physiological requirements for ovulation and fertility. It is believed that Cyclic Adenosine Monophosphate, protein kinase A and protein kinase C pathways along with eicosanoids, particularly prostaglandin E_2, and steroids are the key factors regulating mammalian oocyte maturation. The aim of the current study was to highlight the molecular events triggered by arachidonic acid during oocyte meiotic arrest and resumption at the time of gonadotrophin surge. It should be noted that arachidonic acid release is tightly regulated by Follicle-stimulating and Luteinizing hormones during oocyte development.

Expression of prostaglandin synthesizing enzymes (cyclooxygenase 1 and cyclooxygenase 2) in the ovary of the ostrich (Struthio camelus)

Cyclooxygenase is the rate limiting enzyme in the production of prostaglandins. In birds two isoforms are present: cyclooxygenase 1 (COX-1) and cyclooxygenase 2 (COX-2). Despite evidence implicating that cyclooxygenases and PGs are critical factors in female reproduction in birds, little is known about COX expression in the avian ovary. In birds, cyclooxygenases have been studied in very few species only. In this study we report on the expression of COX-1 and COX-2 in the ovary of the ostrich (Struthio camelus) using immunohistochemistry and non-radioactive in situ hybridization techniques. Our results demonstrate that COX-1 is strongly expressed in the cytoplasm of oocytes of previtellogenic follicles, whereas COX-2 shows the strongest immunostaining in the granulosa cells of previtellogenic follicles. The signals of both isoenzymes fade significantly with increasing diameter and finally nearly vanish in the vitellogenic follicles with a size >1.8 cm. This expression pattern in the ostrich (S. camelus) is, therefore, completely different from the localization of COX-1 and COX-2 in the hen (Gallus gallus), a finding which also suggests different functions of the cyclooxygenases in the ostrich species. Non-radioactive in situ hybridization confirmed that COX-1 is synthesized in the ooplasm and COX-2 in the granulosa layers of early previtellogenic follicles. According to the results of this study it appears unlikely that COX-1 or COX-2 play a major role in ovulation and oviposition in the ostrich.

Cyclooxygenase-2 and its role in ovulation: a 2004 account

The pre-ovulatory surge of gonadotrophins triggers a marked and obligatory increase in follicular prostaglandin synthesis prior to ovulation, and the cyclooxygenase (COX) enzyme is a key rate-limiting step in the biosynthesis of prostaglandins. In the early 1990s, the pre-ovulatory rise in follicular prostaglandin synthesis was shown to result from the selective induction of a novel COX isoform, now referred to as COX-2. Differences in the time-course of COX-2 induction in species with a short versus a long ovulatory process suggest that the enzyme could be a molecular determinant that sets the alarm of the mammalian ovulatory clock. Some of the fine molecular mechanisms involved in the transcriptional activation of the COX-2 gene in granulosa cells have also been elucidated. The binding of trans-activating upstream stimulatory factors (USF) to a consensus E-box cis-element in the proximal region of the promoter was shown to play a predominant role in COX-2 transcription. Studies showed that COX-2 expression could also serve as a valuable marker for follicular commitment to ovulation during hyperstimulatory cycles. This paper presents a comprehensive review of the events that led to the characterization of COX-2 in pre-ovulatory follicles, updates current concepts on the control of COX-2 expression in pre-ovulatory follicles, and addresses the consequences of COX-2 inhibition to women fertility and potential implications of COX-2 expression in ovarian cancer.

Prostaglandins in the ovary and fallopian tube

More than 20 years following the recognition of a possible role for eicosanoids in ovarian function a physiological role for prostaglandins and/or leukotrienes in human ovulation, corpus luteum function and tubal motility remains to be demonstrated. With respect to ovarian function, the well-characterized preovulatory rise in eicosanoid production in animal species and humans, in conjunction with the large body of experimental evidence employing inhibitors of prostaglandin synthesis and replacement of individual prostaglandins, has provided strong evidence for a role in follicular rupture independent of other LH-mediated ovulatory events. The possible mechanism of prostaglandin-induced follicle rupture may involve stimulation of proteolytic activity via substances such as plasmin and PA; however, this is controversial. A role for prostaglandins in ovarian luteal function is well established in laboratory animals and large ruminant species, where PGF2 alpha derived from the uterus has been demonstrated to be the luteolytic factor. In humans, luteal function may be influenced by local intraovarian eicosanoid production, which has been suggested to involve the paracrine interaction of local ovarian hormones such as oxytocin, noradrenaline, insulin and IGFs, to name but a few. Several lines of evidence have also implicated prostaglandins as an aetiological factor in ovarian pathological states such as seen in the OHSS. However, the bulk of clinical experimental evidence to date has failed to support this contention. Prostaglandin production has likewise been well characterized in the fallopian tube in both humans and animal species. Whereas a role for prostaglandins in tubal transport has been demonstrated with animal species such as the rabbit, several studies have failed to define a similar function in humans. More recently, direct injections of prostaglandin analogues into the fallopian tube and the corpus luteum have been shown to be efficacious as a treatment for ectopic pregnancy. Whether the primary mechanism of action involves effects on tubal musculature or corpus luteum function, or is simply a local vascular effect, remains to be demonstrated. Therefore, although the physiological role for eicosanoids in ovarian and tubal function remains unclear, particularly in the human, an increasing body of recent evidence has suggested an important paracrine function for this class of cellular mediators whose interaction with other more recently characterized local ovarian factors has only begun to be recognized.

Effects of mammalian gonadotropin-releasing hormone on plasma level of prostaglandin F2 alpha in the water frog, Rana esculenta

The present investigation was performed to evaluate the effects of mammalian gonadotropin-releasing hormone (mGnRH) on prostaglandin F2 alpha (PGF2 alpha) plasma level in adult male and female water frog, Rana esculenta, during three different periods of the reproductive cycle: recovery period (October), breeding period (May), and postreproductive period (June). Intact, hypophysectomized (HYP), gonadectomized (GON), and hypophysectomized-and-gonadectomized (HYP/GON) animals were injected with 0.6 micrograms of mGnRH and sacrificed 1 hr and 5 hr after peptide administration. Some of each of the groups were sacrificed without having received mGnRH. PGF2 alpha plasma levels were assessed by radioimmunoassay. Hypophysectomy induced a significant increase of PGF2 alpha levels in October and June males. mGnRH induced a significant increase of PGF2 alpha plasma levels only in HYP and HYP/GON frogs. The tissue target of this GnRH action is, at present, unknown, although interrenals could be putative responsive tissues. At present, it is also difficult to assign any physiological role to observed phenomena unless to suppose that the pituitary inhibition is not constant throughout the year. It cannot be excluded that the prostaglandin induction depends on a local paracrine action of GnRH, which could be performed outside any pituitary control.

Regulation of prostaglandin biosynthesis by luteinizing hormone and bradykinin in rat preovulatory follicles in vitro

Luteinizing hormone (LH) stimulates prostaglandin biosynthesis and steroidogenesis in preovulatory (PO) follicles prior to ovulation. Since the ovulatory process shares many similarities with an inflammatory reaction, mediators of the inflammatory response, such as bradykinin (BK) have been suggested to modulate the effects of LH. In the present study the effect of BK (5 microM) on: 1) prostaglandin biosynthesis (PGE2, PGF2 alpha and 6-keto-PGF1 alpha), 2) the levels of two enzymes in the cyclo-oxygenase pathway, prostaglandin endoperoxide synthase (PGS) and prostacyclin synthase (PCS), and 3) cyclic adenosine 3'5'-monophosphate (cAMP) and progesterone response of PO follicles incubated in vitro were examined. LH (0.1 microgram/ml) stimulated the accumulation of cAMP and progesterone in the medium, while BK had no effect on these parameters. BK exerted a slight stimulatory effect on PGE2, and PGF2 alpha, (p less than or equal to 0.01) but not on 6-keto-PGF1 alpha synthesis, but no changes in PGS or PCS levels could be detected. The effect of LH on prostaglandin biosynthesis was much more pronounced, with an increase of PGE2, PGF2 alpha and 6-keto-PGF1 alpha. LH also induced PGS. The combination of LH and BK did not alter these responses compared to that of LH alone. This study demonstrates that BK stimulates prostaglandin biosynthesis in PO follicles. In contrast to LH, this effect of BK does not seem to involve the adenylate cyclase system, since BK did not stimulate cAMP production. BK did not affect the levels of PGS or PCS, and the stimulatory effect of BK is suggested to involve an increase in the availability of substrate for the cyclo-oxygenase pathway.

Relationship between the production capacity of ovarian 13,14-dihydro-prostaglandin F2-alpha and the process of ovulation in immature female rats pretreated with gonadotropin

The purpose of this work was to investigate the effects of gonadotropin on the production capacity of ovarian 13,14-dihydro-prostaglandin F2-alpha (13,14H2-PGF2 alpha) and whether or not this capacity had any relation to the process of ovulation in rat. To induce the first ovulation, immature rats were injected subcutaneously with PMSG (5 IU/rat) at 8:00 at 26 days of age and some of these rats were followed by an intraperitoneal injection of hCG (10 IU/rat) at 57 hrs after PMSG treatment. The 13,14H2-PGF2 alpha production capacity was unchanged as compared with vehicle control until 57 hrs after PMSG treatment. However, the capacity showed a striking increase at 60 hrs after PMSG treatment. A maximal increase of about 7 fold was observed at 9 hrs after hCG injection just before ovulation. The production capacity of the Graafian follicle (GF) and the part (WO-GF) of the whole ovary (WO) from which the GF is removed at 2:00 on day 29 and the capacity of early corpus luteum at 8:00 on day 29 was greater than that of GF and WO-GF at 0:00 on day 29. These results suggest that the 13,14H2-PGF2 alpha production capacity in rat ovary is regulated by gonadotropin and is closely associated with the process of ovulation.

Human chorionic gonadotropin stimulation of immunoreactive prostaglandin synthase in the rat ovary

Follicular prostaglandins (PG) increase markedly in the hours after the preovulatory gonadotropin rise in the rat. The present investigation was performed to determine if the increased prostaglandins result from elevation of the amount of the principal enzyme in the conversion of arachidonic acid to prostaglandins, i.e. PG synthase. PG synthase was purified from sheep seminal vesicles and rat ovaries for the preparation of monoclonal antibodies. A monoclonal antibody was utilized in a competitive, microtiter plate-based enzyme immunoassay to quantitate PG synthase protein. Follicular development was stimulated in 26-day-old rats by injection of 20 IU of PMSG, and 51 h later 20 IU of hCG was injected. Ovaries were removed from rats before and 8 h after the hCG injection for quantitation of PG synthase by enzyme immunoassay. PG synthase immunologic activity was increased three-fold by hCG stimulation. These findings support the hypothesis that the preovulatory gonadotropin rise causes increased PG synthase protein in the rat ovary.

Studies on the prostaglandin synthesizing capacity of several blood vessels from male and female rats

Prostaglandin (PG) I2 (measured as 6-keto-PGF1 alpha) was the major PG synthesized by homogenates of the aorta from male rats, followed by lesser quantities of PGF2 alpha and PGE2. The amounts of 6-keto-PGF1 alpha synthesized by homogenates of the vena cava, mesenteric artery and femoral artery were much less than synthesized by the aorta, and were similar to the amounts of PGF2 alpha and PGE2 synthesized. The amounts of 6-keto-PGF1 alpha synthesized by homogenates of the aorta were 30% lower in female rats than in male rats. The amounts of 6-keto-PGF1 alpha and PGF2 alpha synthesized by homogenates of the aorta from female rats were similar, but the amounts of PGE2 synthesized were lower. The amounts of 6-keto-PGF1 alpha synthesized by homogenates of the aorta and femoral artery of female rats were similar, but the amounts of 6-keto-PGF1 alpha synthesized by homogenates of the mesenteric artery and vena cava were lower and were similar to the amounts of PGF2 alpha and PGE2 synthesized. The amounts of 6-keto-PGF1 alpha synthesized by the vena cava were significantly lower (P less than 0.05) and the amounts of 6-keto-PGF1 alpha and PGF2 alpha synthesized by the femoral artery and aorta, respectively, were significantly higher (P less than 0.05) in female rats than in male rats. The total amount of 6-keto-PGF1 alpha, PGF2 alpha and PGE2 synthesized by the aortae of male and female rats did not differ, indicating that the higher output of 6-keto-PGF1 alpha from the aorta of male rats compared to female rats is not due to a higher concentration of PGH2 synthetase in the aortic tissue of male rats. However, the shift in the relative amounts of 6-keto-PGF1 alpha and PGF2 alpha synthesized by the aorta of female rats in favour of PGF2 alpha may be responsible for the lower output of 6-keto-PGF1 alpha from the aorta of female rats. Oestradiol and progesterone had no effect on the amounts of 6-keto-PGF1 alpha, PGF2 alpha and PGE2 synthesized by homogenates of the aorta and vena cava from intact and ovariectomized, female rats. However, ovariectomy increased the amounts of 6-keto-PGF1 alpha and PGE2 synthesized by the aorta and vena cava, respectively, an effect not reversed by oestradiol and progesterone treatment.

Mechanisms of action of gonadotropin releasing hormone on rat granulosa cells

To elucidate the mechanisms of stimulatory actions of GnRH on rat granulosa cells (GC), we have compared the actions of a GnRH agonist with those of a tumor-promoting phorbol ester, 12-0-tetradecanoylphorbol 13-acetate (TPA) and Ca+2 ionophore, A23187. GC were obtained from immature (28-29 days old) rats 48 h after injection of 20 IU PMSG. Following prelabeling with 3[H]arachidonic acid (AA), the cells were incubated with the test substances for 10 min and AA release determined. A GnRH agonist, [D-Ala6, des-Gly-NH2(10)] GnRH ethylamide (GnRHa; 10 ng/ml) increased AA release 175% compared to the control value. AA release in the presence of GnRHa was larger than that due to 1 microM A23187 or 40 nM TPA alone. A23187 or TPA increased GnRHa-stimulated AA release further. GC were incubated with the test substances for longer time periods, i.e., up to 5 h. GnRHa caused a 4-fold increase in prostaglandin (PG) synthase activity at 5 h. GnRHa increased PGE accumulation to the same extent as TPA, but only increased PG synthase activity about half as much. In combination with TPA, GnRHa had no influence on TPA-stimulated PG synthase activity, but increased PGE accumulation to levels comparable to those with A23187 plus TPA. GnRHa caused a 2.5 fold increase in progesterone (P) accumulation, which was the same as TPA. P accumulation in the presence of GnRHa was affected by neither A23187 nor TPA. These data indicate that the combination of TPA and A23187 can substitute for GnRH action on PGE and P accumulation in rat GC.

Release of prostaglandin E2 and beta-endorphin-like immunoreactivity from rat adenohypophysis in vitro: variations after adrenalectomy or lesions of the paraventricular nuclei

Previous studies in vitro have shown that prostaglandin (PG) E2 is formed in rat adenohypophysis upon stimulation by arginine-vasopressin (AVP) and synthetic ovine corticotropin-releasing factor (CRF-(1-41]. The aim of the present study was to examine whether long-term changes in the hypothalamic stimulation of the pituitary corticotrophs in vivo may influence PG synthesis in subsequent in vitro incubations of rat anterior pituitary quarters. The release of PGE2 from adenohypophyses obtained from adrenalectomized rats was increased to about 300% of controls both under basal conditions and after stimulation by AVP; by contrast, the release of PG D2 was changed neither by adrenalectomy nor by AVP. Simultaneously, basal release of beta-endorphin-like immunoreactivity (beta-EI) was increased after adrenalectomy to about 300% of controls, parallel to the increase in the tissue content, whereas AVP-induced beta-EI release was unchanged. Addition of PG E2 inhibited, whereas blockade of PG formation by indomethacin enhanced AVP-induced beta-EI release both in controls and after adrenalectomy. When anterior pituitary glands were taken from rats with lesions of the paraventricular nuclei, release of PG E2 was decreased as compared to controls both under basal conditions and after stimulation by AVP or CRF-(1-41). Simultaneously, basal and evoked release of beta-EI was unchanged. We conclude that the formation of PG E2 in the adenohypophysis varies according to long-term changes in the hypothalamic stimulation of adrenocorticotropin and beta-endorphin release supporting the view that PG E2 synthesis is related to, and may be involved in mechanisms controlling peptide hormone release from the corticotrophs.

A radioimmunoassay for prostaglandin endoperoxide synthetase

A radioimmunoassay for prostaglandin endoperoxide synthetase has been developed. The antiserum was produced in rabbits by repeated immunization with the purified sheep seminal vesicular enzyme. The enzyme was labeled by chloramine T mediated radioiodination and purified by Sephadex G-25 gel filtration followed by hydroxyapatite column chromatography. Separation of bound from free enzyme was achieved by double antibody method. The radioimmunoassay was capable of detecting 1 ng per assay tube. The antiserum was found to crossreact with the enzyme from various animal tissues. Subcellular localization of the enzyme in sheep seminal vesicular gland indicated that distribution of immunoreactive enzyme in subcellular fractions correlated well with that of enzyme activity with highest enrichment in the microsomal fraction. Application of radioimmunoassay to the measurement of immunoreactive enzyme in kidneys of normotensive and genetically hypertensive rats revealed that the level was significantly increased in hypertensive rats at 18 weeks but not at 10 weeks of age. The level of immunoreactive enzyme also correlated well with that of enzyme activity. The availability of a radioimmunoassay will provide a valuable tool for probing the structure and function of the enzyme in various physiological and pathophysiological states.

Prostaglandin levels in preovulatory follicles from rabbit ovaries perfused in vitro

Prostaglandin (PG) levels in follicular fluid from preovulatory follicles of rabbit ovaries perfused in vitro were measured in order to compare PG changes in this model system with those that occur in vivo and in isolated, LH-treated follicles in vitro. One ovary from each rabbit was perfused without further treatment (control). The other ovary was exposed to LH (0.1 or 1 microgram/ml) beginning 1 hour (h) after initiation of perfusion. Samples of perfusion medium were taken at frequent intervals for measurement of PGE, PGF, progesterone and estradiol 17 beta. The perfusions were terminated when the first ovulation occurred or appeared imminent as judged by changes in the size and shape of the follicles. Follicular fluid was then rapidly aspirated from all large follicles on both ovaries for PGE and PGF measurement. Ovulations occurred only in the LH-treated ovaries. Progesterone and estradiol levels were significantly elevated in the perfusion medium within 1 h of LH treatment in comparison to controls. PG levels in perfusion medium from the control and LH-treated ovaries were not different throughout perfusion and increased in both groups. In contrast, PG levels measured in follicular fluid from LH-treated ovaries were 4- to 5-fold greater than in fluid from control ovaries. It is concluded that ovulation induced by LH in this experimental model is accompanied by an increase in follicular PG levels similar to that seen in other in vivo and in vitro models. This difference in follicular PG levels between the LH-treated and control ovaries is, however, not reflected in the perfusion medium.

Luteinizing hormone stimulates the production of prostacyclin by isolated ovarian cell in vitro

Granulosa cells isolated from mature Graafian follicles of swine produced significant quantities of immunoreactive 6-keto-PGF1 alpha under chemically defined conditions in vitro. Luteinizing hormone elicited a dose-dependent stimulation of 6-keto-PGF1 alpha accumulation, but follicle stimulating hormone, prolactin, L-epinephrine, estradiol-17B, or PGE2 were devoid of effect. The time-dependent in vitro production of 6-keto-PGF1 alpha by ovarian cells was susceptible to inhibition by indomethacin, U-51506, cycloheximide, and actinomycin D. These observations implicate granulosa cells in the specific and hormonally regulated production of prostacyclin.

Elevation of apparent membrane viscosity in ovarian granulosa cells by follicle-stimulating hormone

Continued exposure of cultured granulosa cells to follicle-stimulating hormone (FSH) induced: (i) a rise in apparent membrane microviscosity, as reflected by an increase in fluorescence polarization of the lipid-soluble probe, 1,6-diphenyl-1,3,5,-hexatriene; and (ii) a progressive decline in the cyclic AMP response to renewed challenge with the same hormone. Both changes were reduced or prevented by pretreatment of the cells with oleic or linoleic acid, agents which reduce membrane viscosity, but not by elaidic or palmitic acid which increase the rigidity of membrane lipids. Other agents that inhibited FSH-induced changes in membrane fluidity (gonadotropin-releasing hormone, actinomycin D and cycloheximide) also prevented desensitization to FSH. Cyclic AMP and cyclic GMP derivatives did not mimic the effects of FSH on apparent membrane viscosity or desensitization. Changes in membrane fluidity are unlikely to be the sole cause of desensitization since (i) pretreatment of the cells with fatty acids that increase lipid viscosity did not induce desensitization to FSH, and (ii) desensitization of granulosa cells to lutropin and prostaglandin E2 by exposure to the homologous hormone was not attended by increased membrane viscosity. The experiments described provide the first example of a hormonally induced increase in the target cell apparent membrane viscosity.