The role of protein synthesis in the stimulation by LH of prostaglandin accumulation in rat preovulatory follicles in vitro

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.

Leukocyte networks and ovulation

Ovulation, the process whereby the oocyte is expelled from the interior of the follicle, is the final process of folliculogenesis. During the last decade, data have accumulated to suggest that tissue-bound leukocytes are major effector cells in several physiological processes within the reproductive tract. Some specific subclasses of leukocytes seem to be critically involved in the process of ovulation. The main components of this ovulatory process are degradation of the extracellular matrix (ECM) at the follicular apex and changes in the follicular vasculature. The leukocytes participate actively in these events by secretion of proteases and vasoactive substances. This review covers our current understanding of the mechanisms by which the leukocytes are attracted to the preovulatory follicle after the LH-surge and the roles that the activated leukocytes play in the follicle during the ovulatory period.

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.

Regulation of prostaglandin biosynthesis by human ovarian follicular fluid: a mechanism for ovulation?

We have studied the effect of human ovarian follicular fluid on PG production by bovine seminal vesicles in vitro and found that hFF1 contains a factor of high molecular weight (Mr greater than 30,000) which inhibits PG synthase in a dose-dependent manner. Exposure of this substance to protease activity produced a factor of lower molecular weight (Mr less than 1000) which stimulated PG synthase activity. If this is true of ovarian follicles in vivo, it is possible that increased follicular protease activity stimulates PG synthesis at the time of ovulation.

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.

Luteal phase variations in endogenous concentrations of prostaglandins PGE and PGF and in the capacity for their in vitro formation in the human corpus luteum

One evidence for a luteolytic role for prostaglandin F2 alpha in the human is the increase in luteal PGF at times corresponding to luteolysis as reported earlier by us and other groups. There have been other contradictory reports on this point. In the present experiments we have measured the concentrations of PGE and PGF in 16 more human corpora lutea and have determined the capacity of those tissues to form PGE and PGF in vitro. PGF concentrations were highest in the mid luteal phase but were accompanied by high PGE concentrations. On the other hand, in the late luteal phase PGF concentrations, lower than in mid luteal but generally higher than in early luteal phase, were significantly higher than PGE concentrations. This pattern in PGE and PGF concentrations was also evident in the capacity of these tissues to form these compounds in vitro. In view of the known capacity of PGE2 to counteract the luteolytic effect of PGF2 alpha, these variations in the relative concentrations of PGE and PGF during the luteal phase may be of significance in the process of luteolysis in the human.

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.

Acute effects of gonadotrophins and cyclic AMP on protein synthesis and progesterone production by isolated rat granulosa cells

Amino acid uptake, protein synthesis and progesterone production were studied in rat granulosa cells, isolated from follicles of different stages of development. The amino acid uptake in granulosa cells from prepubertal rats was rapid with a distribution ratio above 1 within 10 min. No significant effects of gonadotrophins were observed on this parameter. The acute influence of exogenous LH and FSH or dibutyryl cyclic AMP (dbcAMP) on incorporation of 3H-leucine or 3H-phenylalanine was investigated as a measure of the rate of protein synthesis. In most experiments progesterone production was determined concomitantly. Both FSH (10 micrograms/ml or 100 micrograms/100 g b.wt.) and dbcAMP (a mM) stimulated (1.6--1.8 fold) the incorporation of leucine into granulosa cell proteins from prepubertal rats while LH was without effect. Progesterone production in these granulosa cells was very low and neither the gonadotrophins nor dbcAMP were stimulatory. Similarly designed experiments were performed on granulosa cells isolated from preovulatory rat follicles. It was then found that both FSH (10 micrograms/ml or 100 micrograms/199 g bst.) and dbcAMP (1 and 5 mM) as well as LH (10 micrograms/ml or 100 micrograms/100 g b.wt.) significantly (1.2--2 fold) stimulated protein synthesis. Furthermore, basal progesterone production was higher and was markedly stimulated (3--6 fold) by all three substances tested. The observations that the protein synthesis in immature granulosa cells is increased by exposure to FSH and dbcAMP while progesterone production in not, whereas both these parameters are stimulated with FSH, LH and dbcAMP in the preovulatory granulosa cells suggest that there might be certain differences in the nature of the proteins synthesized by immature and preovulatory granulosa cells.

Peptide-induced prostaglandin biosynthesis in the renal-vein-constricted kidney

The ipsilateral kidney was removed from a rabbit 48h after unilateral partial renal-vein-constriction and was perfused with Krebs-Henseleit media at 37 degrees C. Hourly administration of a fixed dose of bradykinin to the renal-vein-constricted kidney demonstrated a marked time-dependent increase in the release of bioassayable prostaglandin E(2) and thromboxane A(2) into the venous effluent as compared with the response of the contralateral control kidney. The renal-vein-constricted kidney produced up to 60 times more prostaglandin E(2) in response to bradykinin after 6h of perfusion as compared with the contralateral kidney; thromboxane A(2) was not demonstratable in the contralateral kidney. Inhibition of protein synthesis de novo in the perfused renal-vein-constricted kidney with cycloheximide lessened the hormone-stimulated increase in prostaglandin E(2) by 94% and in thromboxane A(2) by 90% at 6h of perfusion. Covalent acetylation of the renal cyclo-oxygenase by prior oral administration of aspirin to the rabbit inhibited initial bradykinin-stimulated prostaglandin E(2) biosynthesis 71% at 1h of perfusion. However, there was total recovery from aspirin in the renal-vein-constricted kidney by 2h of perfusion after bradykinin stimulation. Total cyclo-oxygenase activity as measured by [(14)C]arachidonate metabolism to labelled prostaglandins by renal cortical and renal medullary microsomal fractions prepared from 6h-perfused kidneys demonstrated that renal-vein-constricted kidney-cortical cyclo-oxygenase activity was significantly greater than the contralateral-kidney-cortical conversion, whereas medullary arachidonate metabolism was comparable in both the renal-vein-constricted kidney and contralateral kidney. These data suggest that perfusion of a renal-vein-constricted kidney initiates a time-dependent induction of synthesis of prostaglandin-producing enzymes, which appear to be primarily localized in the renal cortex. The presence of the synthetic capacity to generate very potent vasodilator and vasoconstrictor prostaglandins in the renal cortex suggests that these substances could mediate or modulate changes in renal vascular resistance in pathological states.

Biosynthesis of prostaglandins from arachidonic acid by the rat ovary

Biosynthesis of prostaglandins (PGs) from 14C-arachidonic acid was studied using homogenates of the ovaries from immature rats. In ascending order of metabolizing potency were, the ovaries from untreated rats, from rats treated with pregnant mare's serum gonadotropin (PMS), and from PMS-human chorionic gonadotropin (hCG) treated rats. Among the radioactive metabolites extracted, PGE2 and 6-keto PGF1 were purified and identified by silicic acid column-, thin layer-, reversed phase partition chromatographies, and radiogaschromatography. Production of PGE2 and 6-keto PGF1 was observed in homogenates of the ovaries of intact and PMS-hCG treated rats at conversion rates of 0.72; 0.43% and 7.62; 2.31%, but not by PMS treated rat ovaries. Treatment with PMS-hCG activated metabolism of arachidonic acid into radioactive metabolites including PGE2 and 6-keto PGF1 to a large extent. Accordingly, it is concluded that luteinizing hormone and hCG play a significant role in the biosynthesis of PGs by the rat ovarian follicle.

Effects of inhibitors of steroid biosynthesis on prostaglandin formation in rabbit ovarian follicles

Rabbit ovarian follicles were incubated without stimulation, with LH and with LH + an inhibitor or steroid biosynthesis. Formation of prostaglandins PGE and PGF and of progesterone and estradiol was measured in these incubates. It was found that aminoglutethimide phosphate (AGP) inhibited the LH stimulated biosynthesis of both prostaglandins and steroids. However U 30870 and Metyrapone, while completely inhibiting the LH stimulated biosynthesis of progesterone and estradiol respectively, had no effect on the formation of prostaglandins. Further, the inhibition of prostaglandin formation by AGP could not be reversed by exogenou steroids. It, therefore, appears that the effect of AGP on prostaglandin biosynthesis may not be related to its effect on steroid biosynthesis. However, the response of rabbit follicles to AGP is contrary to that reported for rat follicles and indicates different control mechanisms for prostaglandin formation in the follicles of the two species.

Effect of oestradiol-17 beta and gonadotrophins on prostaglandin production by pre-ovulatory pig follicles superfused in vitro

Pre-ovulatory pig follicles were superfused in vitro with oestradiol, LH or FSH in the presence and absence of indomethacin. Prostaglandin E2 ANd F2 alpha and oestradiol were measured in the same samples of superfusate. LH and FSH caused an increase in levels of both prostaglandins (approximately 2-fold) but the most marked effect was of oestradiol in PGE2 where a 4-fold increase in levels above controls was observed. In all cases indomethacin inhibited the effects. Both gonadotrophins caused an increase in oestradiol production although the response to LH was much slower than to FSH. The results show a possible relationship between FSH, oestradiol and PG synthesis in the pre-ovulatory pig follicle.

Stimulation by cyclic nucleotides of prostaglandin E production in isolated graafian follicles

Rat Graafian follicles isolated intact responded to 8-Br-cyclic AMP and 8-Br-cyclic GMP with increased prostaglandin E (PGE) production during a 6 h incubation. By contrast, 8-Br-cyclic IMP, 8-Br-5' AMP and 8-Br-5' GMP were inactive in this respect. The effect of 8-Br-cyclic AMP and 8-Br-cyclic GMP was noted only after a lag period of about 4 h. Choleragen, LH, and the phosphodiesterase inhibitor (3-isobutyl-l-methyl-xanthine; IBMX) also stimulated PGE production. Actinomycin D and cycloheximide given simultaneously with 8-Br-cyclic AMP or LH prevented the stimulatory effect of these agents. Concomitant addition of arachidonic acid did not overcome the effect of these inhibitors. Administration of hCG in vivo or incubation with LH in vitro did not elevate endogenous ovarian free arachidonate, while PGE production was enhanced. Dexamethasone prevented this stimulatory effect of hCG. Collectively, the results suggest that stimulation of ovarian PGE production by cyclic nucleotides and LH is dependent on de novo synthesis of one or more components of the PG synthetase system rather than on substrate availability. Cyclic nucleotides may mediate the stimulatory effect of gonadotropins on PGE production.