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Gonadotropin-Induced Expression of Messenger Ribonucleic Acid for Cyclooxygenase-2 and Production of Prostaglandins E and F₂ in Bovine Preovulatory Follicles Are Regulated by the Progesterone Receptor

Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca New York 14853

Address all correspondence and requests for reprints to: Dr. J. E. Fortune, Department of Biomedical Sciences, Cornell University, Ithaca, New York 14853. E-mail: JF11{at}cornell.edu.

	Abstract

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Follicular production of prostaglandins (PGs) is essential for ovulation, but the factors mediating gonadotropin-induced secretion of PGE and PGF₂ remain largely unknown. We tested the hypothesis that gonadotropin-induced changes in progesterone and its receptor (PR) mediate the increase in periovulatory PGs. Heifers were treated with PGF₂ and GnRH to induce luteolysis and the LH/FSH surge (ovulation occurs 30 h after GnRH). Because there are two increases in intrafollicular progesterone/PR mRNA during the bovine periovulatory period, we first examined the temporal pattern of PG production by follicles collected at 0, 3.5, 6, 12, 18, and 24 h after GnRH. Although PGs did not increase in the follicular fluid until 24 h after GnRH, acute secretion of PGs by follicle wall (theca + granulosa cells) was initiated by 18 h and had increased manyfold by 24 h after GnRH. In vitro, FSH and LH induced dramatic transient increases in PG production by follicle wall and granulosa, but not theca, cells isolated from preovulatory follicles (0 h after GnRH). PG accumulation peaked on d 2 of culture, mimicking the secretion pattern after a gonadotropin surge in vivo. In cultures of follicle wall and granulosa cells, the PR antagonist mifepristone (MIFE, 1 µM) inhibited LH-induced PG secretion and the progestin medroxyprogesterone acetate (1 or 10 µM), but not the glucocorticoid dexamethasone (1 or 10 µM), overcame the effect of MIFE on PGs. Semiquantitative RT-PCR revealed that MIFE inhibited LH-induced expression of cyclooxygenase-2 mRNA in granulosa cells in vitro. Again, treatment with medroxyprogesterone acetate overcame the effect of MIFE. Together these results provide strong evidence that periovulatory increases in cyclooxygenase-2 mRNA, PGE, and PGF₂ are mediated by gonadotropin-induced increases in progesterone/PR, indicating that in some species there is an important functional relationship between these pathways in the ovulatory cascade.

	Introduction

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OVULATION MARKS THE culmination of follicular growth and development and involves a complex cascade of events stimulated by an estradiol-induced surge of gonadotropins. Progesterone and prostaglandins (PGs) are two key players in the ovulatory cascade. Inhibition of progesterone or PGs in vivo blocks ovulation in a variety of species (1, 2, 3, 4, 5, 6, 7, 8). In mice, genetic deletion of the enzyme responsible for the conversion of arachidonic acid to PGH₂, cyclooxygenase (COX)-2 or deletion of the progesterone receptor (PR) inhibits or dramatically reduces ovulation (9, 10, 11).

The relationship between progesterone and PGs during the periovulatory period is unclear. In PR-deficient mice, human chorionic gonadotropin (hCG)-induced expression of mRNA for COX-2 appears normal (12), whereas in rats progesterone was reported to decrease LH-induced expression of mRNA for COX-2 and PGE secretion by preovulatory follicles in vitro (13). In contrast, experiments with sheep provided evidence that progesterone regulates PGE₂-9-ketoreductase, the enzyme responsible for the conversion of PGE₂ to PGF₂ (2, 14). Reports of the primary PG required for ovulation are also inconsistent. Ovulation rate is reduced in mice null mutant for the PGE receptor subtype EP₂ (15, 16) but not in mice null mutant for FP, the PGF₂ receptor (17), whereas experimental evidence points to PGF₂ as the PG required for ovulation in ewes (2).

It is not clear whether the differences and contradictions summarized above are due to differences in experimental conditions and/or methods or whether they reflect true species differences in key mediators of the ovulatory cascade. Cattle are an excellent model for studying the periovulatory period (i.e. the interval between the gonadotropin surge and ovulation) because luteolysis, a follicular phase, and ovulation can be induced by sequential treatment of animals that have a growing dominant follicle with PGF₂ to induce luteal regression and GnRH (or hCG) to induce (or mimic) the LH/FSH surge (18, 19). This allows the retrieval of follicles at defined stages of the long (about 28 h) interval between the LH/FSH surge and ovulation and the large size of the preovulatory follicle permits the application of multiple treatments in vitro to cells isolated from one follicle. In the studies described herein, we used cattle as a model to investigate the mechanisms that regulate the periovulatory rise in PGs, particularly the potential role of progesterone and its receptor.

Sirois and colleagues (19, 20) have shown that there is a dramatic increase in COX-2 and its mRNA in bovine granulosa cells late in the periovulatory period, coincident with increases in PGE and PGF₂ in follicular fluid at 24 h after hCG or estrus, but the earlier stages of the periovulatory interval have not been examined as carefully. Studies in our laboratory showed that at 3.5–6 h after GnRH, shortly after the preovulatory LH/FSH surge, concentrations of progesterone in the follicular fluid and expression of mRNA for the PR in follicular tissue increase transiently, followed by a second increase late in the periovulatory period at 24 h after GnRH (21, 22, 23). Because the increases in follicular progesterone and expression of mRNA for the PR precede the dramatic increase in follicular PGs after the ovulatory surge of gonadotropins, we hypothesized that gonadotropin-induced secretion of PGs by bovine follicular cells is mediated through progesterone and the PR. The specific objectives were: 1) to examine the temporal pattern of secretion of PGF₂ and PGE during the periovulatory period in vivo and determine whether it could be replicated in vitro, 2) to examine the relative contributions of theca vs. granulosa cells and of LH vs. FSH to PG production by preovulatory follicles, and 3) to examine the potential role of progesterone/PR in mediating the effects of gonadotropins on PG production by bovine ovulatory follicles.

	Materials and Methods

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Animals
Holstein heifers were housed at the research farm and monitored for estrous behavior. Heifers that had exhibited normal and regular estrous cycles were assigned to experimental groups at estrus (d 0). A protocol developed and validated previously (18) was used to induce the development of preovulatory (before the LH/FSH surge) and periovulatory (between the LH/FSH surge and ovulation) follicles. On d 6 (afternoon) or d 7 (morning), heifers were injected im with 25 mg PGF₂ (Lutalyse; Pharmacia & Upjohn, Kalamazoo, MI) to induce regression of the corpus luteum and thus initiate a follicular phase. Because the first follicular wave of the estrous cycle begins at a more predictable time than succeeding waves, this experimental model ensures that the development of preovulatory follicles is consistent among heifers. Follicular and luteal development were monitored in vivo by daily transrectal ultrasonography and jugular venous blood was collected by daily venipuncture for the determination of concentrations of progesterone to verify regression of the corpus luteum. At 36 h after PGF₂, the ovary bearing the preovulatory follicle was removed by colpotomy (0 h group), or heifers were injected im with 100 µg of an analog of GnRH (Cystorelin; Sanofi Animal Health Inc., Overland Park, KS) to induce the ovulatory surge of gonadotropins and 3.5, 6, 12, 18, or 24 h later; the ovary bearing the periovulatory follicle was removed. In this model, the surge of LH peaks at 2 h after GnRH and follicles ovulate approximately 30 h after GnRH (18). All procedures with animals were approved by the Cornell University Animal Care and Use Committee.

Isolation and culture of follicle wall, granulosa cells, and theca interna
Ovaries were transported to the laboratory (10 min) in Eagle’s MEM supplemented with penicillin-streptomycin (50 U-50 µg/ml; Life Technologies, Inc., Grand Island, NY). The ovulatory follicle was dissected from the ovary, follicular fluid aspirated, and the follicle wall peeled away from the surrounding stroma. Depending on the experiment, follicle wall (theca interna plus attached granulosa cells), theca interna, and/or isolated granulosa cells were cultured. Within an experiment, all cell types were obtained from the same follicles. Granulosa cells were separated from the theca interna by scraping the theca with a fine glass needle, as described previously (24). The follicle wall and theca interna were cut into small pieces that were then distributed at random (three pieces/well) to 24-well culture plates (Costar, Cambridge, MA) and cultured in 0.5 ml of culture medium [modified Eagle’s MEM (25)]. Granulosa cells were collected by centrifugation, counted with a hemacytometer, and distributed to 24-well (200,000 cells/well; experiments II and IIIa) or six-well (10⁶ cells/well; experiment IV) Primaria culture plates (Falcon, Becton Dickinson, Lincoln Park, NJ) and cultured in 0.5 or 2 ml of culture medium, respectively.

Treatments were applied to duplicate cultures from each follicle in experiments I-III and to single cultures from each follicle in (experiment IV. Treatments included LH (NIH LH-S26, 100 ng/ml), FSH (NIH FSH-S17, 100 ng/ml), the PR antagonist mifepristone (1 µM), the synthetic progestogen medroxyprogesterone acetate (MPA, 1 or 10 µM), and the synthetic glucocorticoid dexamethasone (DEX, 1 or 10 µM). Mifepristone (RU-486) is a reversible PR antagonist (26); the dose used (1 µM) is the lowest dose that was maximally effective in pilot experiments. MIFE, DEX, and MPA were obtained from Sigma-Aldrich Corp. (St. Louis, MO). Cultures were maintained at 37 C in humidified modular incubation chambers (Billups-Rothenberg, Del Mar, CA) gassed with 5% CO₂-95% air for 72 h (experiment 1), 96 h (experiments II and III), or 36 h (experiment IV). Medium was collected and replaced at 24-h intervals except in experiment I (further described in Results), IIIb, and IV (12-h intervals) and stored frozen for later measurement of PGs. Each experiment was replicated with three (experiment I), four (experiments. II, IIIb, and IV), or five (experiment IIIa) separate follicles on separate occasions.

RIAs for PGE and PGF₂
Concentrations of PGE and PGF₂ were determined in duplicate aliquots of culture media (0.5–25 µl) without extraction or in ether-extracted aliquots of follicular fluid. Briefly, standards ranging from 6.25 to 800 pg/100 µl and pools ranging from 12.5 to 200 pg/100 µl were prepared from PGE₂ and PGF₂ tromethamine salt (Cayman Chemical, Ann Arbor, MI) in assay buffer (0.1 M PBS, 0.1% gelatin). The antibody to PGE₂ was provided by Dr. T. G. Kennedy (University of Western Ontario, London, Ontario, Canada) and has been described previously (27). Because this antibody has 46% cross-reactivity with PGE₁, total concentrations of PGE are referred to throughout the text. The antibody to PGF₂ (Assay Designs, Inc., Ann Arbor MI) was polyclonal, produced by immunization of sheep with PGF₂ conjugated to bovine albumin, and used at a final dilution of 1:15,000. The PGF₂ antibody was tested for cross-reactivities and determined to cross-react with PGA₁ (<0.5%), PGA₂ (<0.5%), PGB₁ (<0.5%), PGB₂ (<0.5%), PGD₂ (3.1%), PGE₁ (<0.5%), PGE₂ (1.6%), PGF₁ (3.1%), 15-keto PGF₂ (<0.5%), and 13,14-dihydro-15-keto PGF₂ (<0.5%) (all from Cayman Chemical). ³H(N) PGE₂ (5, 6, 8, 11, 12, 14, 15) and ³H(N) PGF₂ (5, 6, 8, 9, 11, 12, 14, 15) were obtained from NEN Life Science Products (Boston, MA) and used at a final concentration of 0.01 µCi/reaction. The sensitivity of the assays for PGE and PGF₂ was approximately 25 pg/tube. Standards, pools, and samples were incubated overnight at 4 C with ³H(N)-PGE₂ or -PGF₂ and appropriate antibody in assay buffer. Bound and free ligand were separated by a 10-min incubation at 4 C with 1 ml dextran-coated activated charcoal (Sigma-Aldrich), followed by centrifugation for 15 min at 3000 rpm. The supernatant was decanted and radiolabeled, and PGs were counted in a liquid scintillation counter. The inter- and intraassay coefficients of variation were 13.8 and 4.5% for PGE and 10.8 and 3.9% for PGF₂.

Semiquantitative RT-PCR
Total RNA was extracted from freshly isolated granulosa cells and a placentome from a pregnant cow (positive control) using TRIzol reagent (Invitrogen Corp., Carlsbad, CA) and from cultured granulosa cells using Absolutely RNA microprep kit (Stratagene, La Jolla, CA) according to the manufacturer’s directions. Samples of total RNA were treated with amplification grade DNase-I to control for possible contamination with genomic DNA and then reverse transcribed to cDNA using Superscript II RNase H^– reverse transcriptase and random primers (Invitrogen).

Samples of first strand cDNA were amplified with Taq DNA polymerase (Invitrogen) using pairs of primers specific for COX-2 (5'-GTCTGGAACAACTGCTCATCGC-3', 5'-CACAGTGCACTACATACTTACCC-3') (28) and the18S subunit of rRNA (5'-GCTCGCTCCTCTCCTACTTG-3', 5'-GATCGGCCCGAGGTTATCTA-3') (29). For COX-2, amplification consisted of 37 cycles at 94 C for 30 sec, 59 C for 60 sec, and 72 C for 2 min, followed by a final 5 min extension at 72 C. For 18S rRNA, amplification consisted of 17 cycles at 94 C for 30 sec, 55 C for 30 sec, and 72 C for 60 sec, followed by a final 5-min extension at 72 C. The numbers of cycles of amplification for COX-2 and 18S rRNA were based on the results of amplification curves and were within the linear range of those curves. PCR products were electrophoresed in 2% agarose/Tris borate EDTA gels containing ethidium bromide and the intensity of ethidium bromide staining was digitally quantified under UV light (Kodak Image Station 440CF; Eastman Kodak Co., Rochester, NY). Relative levels of mRNA for COX-2 were calculated by dividing the intensity of the respective band by the corresponding 18S rRNA band. PCR products were purified using a gel purification kit (QIAGEN Inc., Valencia, CA) according to the manufacturer’s directions and sequenced using BIG Dye Terminator chemistry with AmpliTaq-FS DNA polymerase (Applied Biosystems Automated 3730xl DNA Analyzer, Biotechnology Resource Center, Cornell University, Ithaca, NY). The integrity and identity of the PCR product sequences were verified using the BLAST database (www.ncbi.nlm.nih.gov/blast) (30).

Statistical analyses
Data sets were first tested for homogeneity of variance by Hartley’s test (31). Heterogeneity was detected in all data sets and hence data were log (base 10) transformed before statistical analysis. However, for ease of comprehension, means ± SEM of nontransformed data are depicted in the figures. The concentration of PGs in follicular fluid and levels of mRNA for COX-2 were analyzed by ANOVA using the general linear model of SAS (SAS Institute, Inc., Cary, NC). The concentration of PGs in culture medium was analyzed by repeated measures ANOVA. If differences were detected, Duncan’s multiple range test was used to determine which means differed. To simplify presentation, the data shown in some of the figures (see Figs. 3 and 6) were summed over time before analysis.

View larger version (19K): [in this window] [in a new window]   FIG. 3. Secretion of PGF2 by pieces of follicle wall from preovulatory follicles collected at 0 h after GnRH and periovulatory follicles collected at 3.5, 6, 12, 18, and 24 h after GnRH in vivo and cultured for 72 h in medium alone. Data are the means ± SEM of duplicate cultures from each of three follicles per time point. Means were compared within each time in vitro (0–24, 24–48, or 48–72 h) across time in vivo. Within a time period in vitro, values without a common superscript differ (P < 0.05).

View larger version (27K): [in this window] [in a new window]   FIG. 6. Secretion of PGE (A) and PGF2 (B) by pieces of follicle wall from periovulatory follicles collected at 6 h after GnRH in vivo and cultured for 96 h in medium alone or with mifepristone (MIFE, 1 µM), MIFE + MPA (1 or 10 µM), or MIFE + DEX (10 µM). A and B, Accumulation of PGs during the first 72 h of culture; treatment effects from 72 to 96 h were similar to 48–72 h, but absolute values were very low (data not shown). Data are the means ± SEM of duplicate cultures from each of four periovulatory follicles. In each panel, within a time in vitro values with different superscripts differ (P < 0.05).

	Results

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View larger version (18K): [in this window] [in a new window]   FIG. 1. Concentrations of PGE and PGF2 in the follicular fluid of preovulatory follicles collected 36 h after PGF2 (0 h after GnRH) and periovulatory follicles collected at 3.5, 6, 12, 18, and 24 h after heifers were injected with GnRH to induce the LH/FSH surge. Data are means ± SEM of three follicles per time point. For each PG, values with different superscripts differ (P < 0.05).

View larger version (22K): [in this window] [in a new window]   FIG. 2. Secretion of PGF2 by pieces of follicle wall from preovulatory follicles collected at 0 h after GnRH and periovulatory follicles collected at 3.5, 6, 12, 18, and 24 h after GnRH in vivo and cultured for 8 h in medium alone. Data are the means ± SEM of duplicate cultures from each of three follicles per time point. Values with different superscripts differ (P < 0.05).

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Experiment II. Secretion of PGs by follicular cells of preovulatory follicles in vitro
The next objective was to determine the relative contributions of theca and granulosa cells to PG secretion and the relative roles of LH and FSH in stimulating PG production by follicle cells by culturing granulosa cells and pieces of follicle wall and theca interna from preovulatory follicles (i.e. not exposed to the gonadotropin surge in vivo) with luteinizing doses (100 ng/ml) of gonadotropins in vitro. In the absence of gonadotropins, secretion of PGs by follicle wall, theca interna, and isolated granulosa cells was very low (Fig. 4). The response to gonadotropins by follicle wall and granulosa cells was dramatic. Treatment with 100 ng/ml LH or FSH transiently, yet markedly and similarly, stimulated secretion of PGE and PGF₂ in vitro (P < 0.05). Peak production of PGE and PGF₂ by gonadotropin-treated pieces of follicle wall or granulosa cells was observed between 24 and 48 h in vitro. Secretion of PGE and PGF₂ by pieces of theca interna was also transiently increased by treatment with 100 ng/ml LH (P < 0.05); however, concentrations were very low relative to LH-induced production of PGs by granulosa cells or pieces of follicle wall. These results indicate that treatment of follicle cells from preovulatory follicles with gonadotropins in vitro can be used as an in vitro model for studying the mechanisms that subserve the gonadotropin-induced production of PGs during the periovulatory period, and this model was then used for experiments III and IV, as detailed below.

View larger version (35K): [in this window] [in a new window]   FIG. 4. Secretion of PGE and PGF2 by pieces of follicle wall (A and B), granulosa cells (C and D), and theca interna (E and F) from preovulatory follicles collected 36 h after PGF2 (0 h after GnRH) and cultured for 96 h with 0 or 100 ng/ml LH or FSH (follicle wall and granulosa cells) or 0 or 100 ng/ml LH (theca interna). Data are the means ± SEM of duplicate cultures from each of four preovulatory follicles. In each panel, within a time in vitro values with different superscripts differ (P < 0.05).

Gonadotropin treatment in vitro.
In experiment IIIa the effects of mifepristone on secretion of PGs by pieces of follicle wall and isolated granulosa cells were determined using tissue from preovulatory follicles collected at 36 h after PGF₂ (i.e. time 0 after GnRH). Pieces of follicle wall (theca interna plus attached granulosa cells) were used in addition to isolated granulosa cells to compare the responses of granulosa cells cultured with their adjacent thecal layer vs. isolated granulosa cells and because mRNA for the PR has been localized to both follicular cell types in cattle (22). Because experiment II did not reveal the theca interna as a major site of PG production, pieces of theca interna were not cultured in this and the following experiments. As expected based on the results of experiment II, treatment of pieces of follicle wall with a luteinizing dose of LH induced a marked but transient increase in secretion of both PGE and PGF₂ in vitro (P < 0.05; Fig. 5, A and B). The stimulatory effect of LH on secretion of PGE and PGF₂ by pieces of follicle wall was completely inhibited by treatment with the PR antagonist mifepristone. Concurrent treatment with the synthetic progestogen (MPA) completely overcame the inhibition by mifepristone of LH-induced secretion of both PGE and PGF₂ in vitro. In contrast, concurrent treatment with mifepristone and the synthetic glucocorticoid DEX had no effect, indicating the specificity of mifepristone for the PR. Treatment with MPA or DEX alone did not induce secretion of PGE and PGF₂ in vitro (P > 0.05, data not shown). A similar response to these treatments was observed in cultures of granulosa cells (Fig. 5, C and D). The luteinizing dose of LH induced a transient increase in secretion of PGE and PGF₂ in vitro (P < 0.05). Mifepristone inhibited LH-induced secretion of PGE and PGF₂ and MPA, but not DEX, overcame the inhibitory effect of mifepristone on LH-induced secretion of PGs. Similar to the results for follicle wall cultures, treatment with MPA or DEX alone did not stimulate secretion of PGs by granulosa cells in vitro (P > 0.05, data not shown).

View larger version (32K): [in this window] [in a new window]   FIG. 5. Secretion of PGE and PGF2 by pieces of follicle wall (A and B) and granulosa cells (C and D) from preovulatory follicles collected at 0 h after GnRH and cultured for 96 h in medium alone or with LH (100 ng/ml), LH + mifepristone (MIFE, 1 µM), LH + MIFE + MPA (1 or 10 µM), and LH + MIFE + DEX (1 or 10 µM). A–D, Accumulation of PGs during the first 72 h of culture; treatment effects from 72 to 96 h were similar to 48–72 h, but absolute values were very low (data not shown). Data are the means ± SEM of duplicate cultures from each of five preovulatory follicles. nd, Not detectable. In each panel, within a time in vitro values with no common superscript differ (P < 0.05).

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As expected (Fig. 2), secretion of PGE and PGF₂ by pieces of follicle wall from periovulatory follicles collected 6 h after GnRH in vivo and cultured in medium alone was low during the first 24 h of culture and increased markedly between 24 and 48 h in vitro (Fig. 6). Similar to LH- or FSH-induced secretion of PGs from follicular cells collected at 0 h after GnRH (Figs. 4 and 5), secretion of PGE and PGF₂ by pieces of follicle wall collected at 6 h after GnRH was transient in nature, with accumulation of PGs in culture medium declining during the later times in culture. The stimulatory effect of the gonadotropin-surge in vivo on secretion of PGE and PGF₂ in vitro was completely inhibited by treatment with mifepristone (P < 0.05). Concurrent treatment with 10 µM MPA, but not DEX, overcame the inhibition by mifepristone of the secretion of PGs in vitro. The lower dose of MPA (1 µM), which was equally effective in overcoming the effect of the PR antagonist on LH-induced PGs from preovulatory follicles (Fig. 5), was only partially able to overcome the inhibitory effect of mifepristone on PG secretion from periovulatory follicles between 24 and 48 h in vitro, the peak period of PG secretion in vitro (Fig. 6). Treatment with 1 µg/ml (3 µM) progesterone was also partially effective in overcoming the inhibition of PG secretion by mifepristone, restoring PGs to a level intermediate between that observed after treatment with 1 and 10 µM of the synthetic progestogen (data not shown). As expected, treatment with MPA or DEX alone did not affect secretion of PGE or PGF₂ in vitro (P > 0.05; data not shown).

Experiment IV. Effect of a PR antagonist on LH-induced expression of mRNA for COX-2
In experiment IV we began to explore the mechanisms by which progesterone/PR affects PG secretion by testing the hypothesis that progesterone/PR mediates LH-induced expression of mRNA for COX-2 in bovine granulosa cells by determining the effects of a luteinizing dose of LH, LH + mifepristone, and LH + mifepristone + MPA on levels of mRNA for COX-2 in granulosa cells from preovulatory follicles (collected 36 h after PGF₂). Expression of mRNA for COX-2 was not detectable in granulosa cells freshly isolated from preovulatory follicles (0 h control) or after 12 h in vitro (Fig. 7). Compared with control cultures without LH, treatment of granulosa cells with LH increased mRNA for COX-2 by 2.4- and 4-fold after 24 and 36 h in vitro, respectively (P < 0.05). Concurrent treatment with mifepristone completely inhibited LH-induced expression of mRNA for COX-2, and MPA overcame the inhibitory effect of mifepristone. The effects of these treatments on COX-2 mRNA were reflected in the concentrations of PGE and PGF₂ in culture media collected at 36 h in vitro (data not shown). As in the previous experiments, LH increased the secretion of PGE and PGF₂ above control levels and concurrent treatment with mifepristone inhibited this effect (P < 0.05). Treatment with MPA overcame the inhibitory effect of mifepristone on LH-induced secretion of PGs.

View larger version (37K): [in this window] [in a new window]   FIG. 7. Semiquantitative RT-PCR showing that the inhibition of LH-induced mRNA for COX-2 by mifepristone is overcome by concurrent progestogen treatment. A, Photographs of ethidium bromide staining in representative agarose gels show the pattern of expression of COX-2 mRNA and 18S rRNA in granulosa cells of preovulatory follicles collected at 0 h after GnRH and cultured for 0, 12, 24, or 36 h in medium alone or with LH (100 ng/ml), LH + mifepristone (MIFE, 1 µM), LH + MIFE + MPA (10 µM). na, Not applicable. B, Relative levels of mRNA for COX-2, calculated by correcting for the intensity of the band for the 18S subunit of rRNA for each sample. Data are the means ± SEM of cultures from each of four preovulatory follicles. nd, Not detectable. Within a time in vitro values with different superscripts differ (P < 0.05).

	Discussion

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Previous studies have provided evidence that genetic deletion of either the PR or COX-2 prevents ovulation (9, 10, 11). Interestingly, in at least three species (rats, cattle, horses), the induction of mRNA for COX-2 occurs at around 10 h before follicular rupture, rather than at a specific time after the gonadotropin surge (reviewed in Ref. 32), whereas in rhesus monkeys mRNA for COX-2 is induced much earlier relative to the time of ovulation (around 28 h before rupture), but COX-2 protein and an increase in PGs in the follicular fluid were not detected until 4–16 h before ovulation (33). Previous experiments in our lab revealed two separate increases in both progesterone in the follicular fluid and levels of mRNA for PR, with one peak occurring early in the periovulatory period at 3.5–6 h after GnRH and the second at 24 h after GnRH, much closer to ovulation (21, 22, 34). Thus, in experiment I we examined temporal patterns of PG production in vivo, between the injection of GnRH and the anticipated time of ovulation, to determine their relationship to temporal changes in progesterone/PR. The dramatic increase in both PGF₂ and PGE in the follicular fluid collected 24 h after GnRH, about 6 h before the expected time of ovulation, is consistent with previous reports on PGs in follicular fluid (19, 20). Because it is not clear to what extent concentrations of substances in the follicular fluid reflect their production by granulosa and theca cells, secretion of PGs by follicular wall tissue was examined during the first few hours in vitro, when the cells are still under the influence of the in vivo environment, and those results suggest that the periovulatory increase in PG secretion begins between 12 and 18 h after GnRH. The fact that secretion of PG by follicular tissue collected 18 h after GnRH tripled during the second 4 h of culture, compared with the first 4 h, suggests that this increase is indeed the initial stage of the periovulatory rise.

It is intriguing that there was a smaller, but significant, transient increase in acute PG secretion by follicular tissue collected 3.5 h after GnRH because previous studies revealed 7- and 5-fold increases in mRNA for PR in follicular tissue and in progesterone in the follicular fluid at 3.5 h vs. 0 h after GnRH (21, 22). This is consistent with results of Acosta et al. (35) showing a small, transient increase in PGF₂ in the ovarian venous effluent before a large increase closer to the time of ovulation. The physiological significance of the early transient increase in PG in the follicular fluid at 3.5 h after GnRH, and any potential functional relationship to the contemporaneous transient increases in progesterone and PR, remain to be determined. Studies in our laboratory have shown a complex pattern of expression of mRNA for receptors for PGE (EP₂, EP₃, and EP₄) and PGF₂ (FP) in both theca and granulosa cells, including up-regulation of mRNA for EP₂ in granulosa cells and for FP and EP₄ in theca cells early in the periovulatory period at 6 h after GnRH (36). Those results suggest a complex pattern of PG action throughout the periovulatory period.

Analysis of the patterns of secretion of PGs by pre- and periovulatory follicular tissue over 3 d of culture suggested that prior exposure to the LH/FSH surge in vivo is sufficient to reproduce the effects of the surge in vitro qualitatively but not quantitatively. Regardless of the time that the tissue was isolated, there was always a robust but transient increase in PG accumulation in the culture medium. As would be expected if secretion in vitro mirrors that in vivo, the later during the periovulatory period the follicle was removed from the animals, the earlier was the rise in PGs and the earlier the termination of the rise. However, the quantity of PGs secreted in vitro was affected by the time of isolation of the follicle, with follicular tissue obtained at 3.6 and 6 h, 12 and 18 h, and 24 h secreting low, intermediate, and large amounts of PG, respectively, over the 3-d culture. Whether these quantitative differences are due to the absence in vitro of optimal levels of substrates for PG synthesis and/or additional regulatory signals that modify overall expression of PG synthetic enzymes is yet to be determined.

Comparisons of follicular wall tissue with isolated follicular cell types suggested that the granulosa cells are the source of the PGs produced by follicular wall preparations containing both cell types. This is not surprising because the laboratory of Sirois and colleagues reported induction of COX-2 and its mRNA in granulosa, but not theca, cells late in the periovulatory period, at 24 h after an endogenous surge of LH/FSH (20), or after injection of hCG as an ovulatory stimulus (19). However, Murdoch et al. (37) reported an increase in mRNA for COX in the theca interna of periovulatory ovine follicles 8–24 h after injection of GnRH, suggesting that theca interna cells of ruminant follicles may produce PGs early in the periovulatory interval. In the current studies, secretion of PGs by theca interna in vitro was very low, compared with granulosa cells or follicle wall. Although there was a small stimulatory effect of LH on the secretion of PGs by theca interna, a similar effect was observed in additional cultures in which theca was treated with a luteinizing dose of FSH (data not shown), suggesting that the increased concentration of PGs was due to very minor contamination of the theca preparations with granulosa cells. In contrast, immunostaining for COX-2 in rhesus money periovulatory follicles was positive in theca as well as granulosa cells (33).

Ovulation can be induced in cattle by treatment with either GnRH, which induces an LH/FSH surge, or with hCG, which has only LH bioactivity (38). An increase in intrafollicular prostaglandin production has been observed in cattle after injection of either hCG (19) or GnRH (current study) and after a spontaneous LH/FSH surge (20). Thus, it was not known whether FSH, as well as LH, plays a role in the periovulatory increase in PG production. The results of the current in vitro studies showed that luteinizing doses of the two gonadotropins have equivalent ability to stimulate secretion of both PGE and PGF₂ by follicle wall and granulosa cells. Over 4 d of culture, both gonadotropins stimulated a large transient increase in PG production that peaked during the second day of culture and then declined.

Periovulatory production of PGs is an essential component of the ovulatory cascade in cattle (1, 7) as well as in other mammalian species that have been studied (3, 4, 5, 6, 8). The hypothesis that gonadotropin-induced production of PGs is mediated by progesterone acting through the PR was strongly supported by the finding that addition of exogenous progestins (MPA or progesterone) overcame the inhibitory effects of the PR antagonist mifepristone on PG production in vitro by both follicle wall preparations and granulosa cells. Whether follicular cells were isolated before the endogenous surge and exposed to LH in vitro or were exposed to an LH/FSH surge in vivo and isolated 6 h later, mifepristone completely inhibited the positive effects of gonadotropins on PG production and the synthetic progestin MPA restored PGs to levels not significantly different from controls. In the experiments with granulosa cells obtained 6 h after GnRH, progesterone was as effective as MPA at reversing the inhibitory effects of mifepristone on PG production, indicating that this effect was not specific to the synthetic progestin. The specificity of the effects of mifepristone was demonstrated by the failure of the glucocorticoid DEX to mimic the effects of progestin in overcoming the inhibition of PR effects. It is interesting that, in the experiments with follicular cells obtained before the endogenous gonadotropin surge, both doses of MPA (1 and 10 µM) completely overcame the inhibition of PGs by mifepristone, whereas when follicular cells were isolated 6 h after GnRH administration, only the higher dose of MPA restored PGs to control levels. This suggests that exposure of follicular cells to the LH/FSH surge in vivo in advance of their exposure to mifepristone and MPA in vitro modulates some aspects(s) of their environment that affects their responses in vitro. Understanding how this occurs will require further experimentation.

Whether follicular cells were obtained before or after the LH/FSH surge, neither progesterone nor the progesterone agonist MPA alone stimulated the production of PGs by follicular wall tissue or isolated granulosa cells. This may seem contradictory to the conclusion that progesterone/PR is an obligatory mediator between gonadotropins and periovulatory PG production. However, when follicular cells are obtained before the gonadotropin surge, they lack PR (22) and thus would not be capable of responding to progestin treatment. When bovine follicular cells were obtained at 3.5 or 6 h after GnRH, induction of a transient increase in mRNA for PR had already occurred (22), coincident with a transient rise in progesterone in the follicular fluid (21). In fact, the concentrations of progesterone in the follicular fluid during this initial transient increase (about 175 ng/ml or 0.6 µM) are not significantly different from the concentration observed during the second increase at 24 h after GnRH, close to the time of ovulation (21, 23). Taken together, these results suggest that by 6 h after the LH/FSH surge maximally effective concentrations of endogenous progesterone are present, and this may explain why addition of exogenous progestins produces no additional effects.

When granulosa cells were obtained before the LH surge (0 h after GnRH) levels of mRNA for COX-2 were nondetectable, but a luteinizing dose of LH induced the mRNA between 12 and 24 h of culture. This is consistent with the timing of the increase in PGs observed in cultures treated with LH and with the induction of mRNA for COX-2 in preovulatory follicles of cattle treated with hCG in vivo or exposed to an endogenous LH surge (19, 20). In the current experiments, the PR antagonist mifepristone inhibited the LH-stimulated rise in COX-2 mRNA and concurrent treatment with the synthetic progestogen MPA completely overcame the inhibition of expression of COX-2 mRNA, restoring it to the levels observed with LH alone. These results show that progesterone/PR mediate the effects of LH on PG production by increasing mRNA for this key enzyme in the pathway of prostaglandin synthesis.

In summary, the induction of the intrafollicular periovulatory increase in PGE and PGF₂ by the gonadotropin surge in vivo was replicated in bovine follicular wall and granulosa cells in vitro by luteinizing doses of LH or FSH. Experimental evidence presented herein supports an obligatory role for progesterone acting through the PR in the induction of mRNA for COX-2 and the increase in PGs. Because ovulation is of such critical importance to the survival of a species, different mammalian species may well have evolved different variations on the complex cascade of intrafollicular changes that allow ovulation to occur successfully. Indeed, hCG induced an increase in mRNA for COX-2 in the ovaries of pregnant mare serum gonadotropin-primed mice null mutant for the PR (12), suggesting that up-regulation of progesterone/PR and of COX-2/prostaglandins by gonadotropins occurs via separate intracellular pathways in that species. The current results illustrate the importance of using various animal models to study periovulatory events, especially models of practical significance, such as domestic animals and primates.

	Acknowledgments

 
We thank D. Bianchi for care of animals and surgical procedures, K. Sierzega for technical assistance, Dr. M. Roberson for critical reading of the manuscript, Dr. T. G. Kennedy for kindly providing the antibody to PGE, and the National Hormone and Pituitary Program (supported by the National Institute of Diabetes and Digestive and Kidney Diseases) for providing LH and FSH.

	Footnotes

 
This research was supported by National Institutes of Health Grant HD41592 (to J.E.F.) and a Lalor Foundation Fellowship (to P.J.B.).

Current address for P.J.B.: Department of Clinical Sciences, University of Kentucky, Lexington, Kentucky 40536.

Current address for C.M.K.: Department of Animal Science, Iowa State University, Ames, Iowa 50011.

Author disclosure summary: the authors have nothing to declare.

First Published Online July 6, 2006

Abbreviations: COX, Cyclooxygenase; DEX, dexamethasone; hCG, human chorionic gonadotropin; MPA, medroxyprogesterone acetate; PG, prostaglandin; PR, progesterone receptor.

Received December 13, 2005.

Accepted for publication June 26, 2006.

	References

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