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Endocrinology Vol. 142, No. 6 2311-2317
Copyright © 2001 by The Endocrine Society

ARTICLES

Expression of Messenger Ribonucleic Acid for the Antiapoptosis Gene P11 in the Rat Ovary: Gonadotropin Stimulation in Granulosa Cells of Preovulatory Follicles1

Sang-Young Chun, Hyun-Wook Bae, Wan-Ju Kim, Jeong-Ho Park, Sheau Yu Hsu and Aaron J. W. Hsueh

Hormone Research Center (S.-Y.C., H.-W.B., W.-J.K.), Chonnam National University, Kwangju 500–757, Republic of Korea; and Division of Reproductive Biology (S.Y.H., A.J.W.H.), Department of Gynecology and Obstetrics, Stanford University School of Medicine, Stanford, California 94305-5317

Address all correspondence and requests for reprints to: Aaron J. W. Hsueh, Division of Reproductive Biology, Department of Gynecology and Obstetrics, Stanford University School of Medicine, Stanford, California 94305-5317. E-mail: aaron.hsueh{at}stanford.edu


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
P11, a member of the S100 family of calcium-binding proteins, has been shown to interact with BAD (Bcl-xL/Bcl-2-associated death promoter) in the yeast two-hybrid protein-protein interaction assay. Because overexpression of P11 dampens the proapoptotic activity of BAD in transfected cells, we tested the possibility that the expression of this antiapoptotic protein may be regulated by gonadotropins and other survival factors in the ovary. Northern blot analysis of ovaries obtained from prepubertal rats revealed an increased expression of P11 messenger RNA (mRNA) during prepubertal development in the theca cells of preantral and early antral follicles. Treatment of immature rats with PMSG did not affect P11 expression, whereas treatment of PMSG-primed rats with an ovulatory dose of human (h)CG stimulated ovarian P11 mRNA within 6–9 h in the granulosa cells of preovulatory follicles. Treatment of cultured preovulatory follicles in vitro with LH further confirmed the time-dependent stimulation of P11 by gonadotropins. In addition, treatment of cultured preovulatory follicles with MDL-12,330A, an inhibitor of adenylate cyclase, inhibited LH-stimulated P11 mRNA, whereas treatment with forskolin, an adenylate cyclase activator, but not the protein kinase C activator, 2-O-tetradecanol-phorbal-13-acetate, mimicked the LH action, suggesting the role of adenylate cyclase activation in P11 expression. Treatment with other follicle survival factors, including the epidermal growth factor, the basic fibroblast growth factor, and interleukin-1ß, could also stimulate P11 expression in cultured preovulatory follicles. These results demonstrate the expression of P11 mRNA in theca cells of different-sized follicles and in granulosa cells of preovulatory follicles following gonadotropin stimulation, and suggest that P11 may mediate, at least partially, the survival action of gonadotropins during the ovulatory process.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
DURING FOLLICLE development in the mammalian ovary, only a few follicles reach the ovulatory stage, whereas the majority of them undergo apoptosis (1, 2, 3). Studies on the hormonal control of follicle cell death demonstrate that deprivation of pituitary-derived gonadotropins in vivo results in follicle atresia, indicating that gonadotropins are required to prevent follicle apoptosis (4). In addition to gonadotropins, intrafollicular regulators, such as epidermal growth factor (EGF) (5), insulin-like growth factor I (IGF-I) (6), basic fibroblast growth factor (bFGF) (5, 7), and interleukin-1ß (IL-1ß) (8), have been shown to suppress follicle cell apoptosis in vitro. Although the extracellular signals regulating follicle cell demise have been extensively studied, the intracellular and molecular events that control follicle atresia are largely unknown.

An abundance of evidence has been accumulating to document that the final series of intracellular events that triggers apoptosis, regardless of cell lineage, involves changes in the expression of a common subset of genes including Bcl-2 and caspase family members (9, 10). The Bcl-2 family of proteins consists of anti- and proapoptotic members, and the balance between the anti- and proapoptotic Bcl-2 family of proteins determines the fate of the cell (2, 11). In the ovary, alterations in the expression of bcl-2, bax, and bcl-x genes have been shown to be associated with follicle atresia (12, 13). Moreover, targeted overexpression of Bcl-2 in ovarian cells leads to decreased follicle cell apoptosis and tumorigenesis in transgenic mice (14), whereas targeted deletion of the bcl-2 gene in mutant mice decreases the number of oocytes and primordial follicles at birth (15). Most recently, the ovarian expression of myeloid cell leukemia-1 (Mcl-1), an antiapoptotic Bcl-2 family protein, has been shown to be stimulated by gonadotropins in the rat (16). The expression of Bcl-xL/Bcl-2-associated death promoter (BAD), a proapoptotic member of the Bcl-2 family, is also found in the granulosa cells of rat ovary, and BAD overexpression in granulosa cells leads to apoptosis (17). Collectively, these observations indicate that a group of anti- and proapoptotic Bcl-2 family of proteins is important in the regulation of ovarian follicle cell apoptosis.

Recent studies further indicate interactions between the Bcl-2 family of proteins and signal transduction molecules activated by extracellular signals. BAD possesses a unique structural feature in that it does not have the C-terminal transmembrane domain found in most Bcl-2 family proteins, thus implying that BAD in the cytoplasm may interact with intracellular proteins in addition to its dimerization with other members of the Bcl-2 family (18). Indeed, phosphorylated BAD has been shown to bind to 14–3-3, a group of proteins involved in intracellular signaling and cell cycle progression (19). Of interest, in the yeast two-hybrid screens using an ovarian fusion complementary DNA (cDNA) library, a BAD phosphorylation mutant not capable of binding 14–3-3 preferentially interacts with P11 (20), a survival gene induced by the nerve growth factor (NGF) in the PC12 cells (21). P11, also known as 42C or calpactin I light chain, belongs to the S100 family of calcium-binding proteins (21). Furthermore, overexpression of P11 suppresses apoptosis induced by BAD (20), suggesting that P11 may be one of the intracellular signal molecules interacting with the Bcl-2 family of proteins to control apoptosis. Because the P11 gene has been identified in an ovarian cDNA library (20), we examined the changes in the expression of P11 mRNA during follicle development in the immature rat ovary. The present study demonstrates that P11 mRNA was induced by gonadotropins in the granulosa cells of preovulatory follicles both in vivo and in vitro through a protein kinase A pathway.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Hormones and reagents
Ovine LH (LH-S-26; 2,300 IU/mg) was obtained from the National Hormone and Pituitary Distribution Program, NIDDK, NIH (Baltimore, MD). hCG, PMSG, forskolin, 2-O-tetradecanol-phorbol-13-acetate (TPA), and EGF were purchased from Sigma (St. Louis, MO). MDL-12,330A and chelerythrine (CE) chloride were purchased from Calbiochem (La Jolla, CA). Recombinant human IGF-I, ß-NGF, and recombinant murine Il-1ß were purchased from R&D Systems Inc. (Minneapolis, MN). The purified recombinant human activin A was kindly donated by Dr. Yuzuru Eto (Ajinomoto Inc., Kawasaki, Japan).

Animals
Immature female rats of the Sprague Dawley strain were purchased from Daehan Laboratories (Chungbuk, Korea). They were housed in groups of five with controlled temperature and photoperiod (10-h darkness, 14-h light, lights on from 0600–2000 h). The animals had ad libitum access to food and water. Rats, ranging in age from 3–21 days, were killed by cervical dislocation and their ovaries were removed for RNA analysis. Ovaries were also collected from immature (26-day-old; BW 60–65 g) rats at various times after treatment with 10 IU PMSG to induce follicle growth. Some rats received a single ip injection of 10 IU hCG to induce ovulation, and ovaries were obtained at different time intervals for Northern blot and in situ hybridization analyses. In pilot experiments, these rats responded optimally to an ovulatory dose (10 IU) of hCG 2 days after 10 IU PMSG treatment based on their ovulation rate (34.4 ± 5.5 ova per rat; n = 9).

Culture of preovulatory follicles
Preovulatory follicles (>800 µm in diameter) were isolated from ovaries collected 48–52 h after PMSG injection, and a follicle culture was performed as previously described (7). Fifteen to twenty follicles were cultured in glass vials containing 400 µl Eagle’s MEM (Life Technologies, Inc., Grand Island, NY) supplemented with penicillin, streptomycin, L-glutamine, and 0.1% BSA (wt/vol, Fraction V, Sigma) in the absence or presence of different hormones. Cultures were maintained in serum-free conditions for up to 24 h at 37 C under 5% CO2-95% O2. At the end of incubation, follicles were snap-frozen for RNA isolation.

Northern blot analysis
Total RNA from ovaries or cultured follicles was isolated using Tri Reagent solution (Sigma). Ten to twenty micrograms of total RNA were fractionated by electrophoresis on a 1% agarose gel containing formaldehyde and were transferred to nylon membranes by capillary blotting with 10x sodium citrate-sodium chloride (SSC). After UV cross-linking and prehybridization, membranes were hybridized overnight at 42 C in a solution containing 50% formamide, 5x SSC, 1 mM EDTA, 250 µg/ml denatured salmon sperm DNA, 500 µg/ml yeast transfer RNA, and a total of 2 x 106 cpm of a 32P-labeled full-length rat P11 cDNA probe (20). After hybridization, membranes were washed twice for 5 min at room temperature in 2 x SSC and 0.1 SDS, followed by washing for 1 h at 65 C in 0.5 x SSC and 0.1% SDS. Membranes were then exposed using Kodak RX film (Eastman Kodak Co., Rochester, NY) for 1–3 days at -80 C. For normalization of data, blots were stripped by boiling in 0.1x SSC and 0.5% SDS twice for 20 min before reprobing with a cDNA probe for rat glyceraldehyde-3-phosphate-dehydrogenase (GAPDH). The intensities of hybridization signals were subsequently measured using a phosphorimager (Bio-Rad Laboratories, Inc., Hercules, CA), and normalized with the GAPDH RNA levels as internal controls.

In situ hybridization analysis
Ovaries were fixed at 4 C for 6 h in 4% paraformaldehyde in PBS, followed by immersion in 0.5 M sucrose in PBS overnight. Cryostat sections (14-µm thick) were mounted on poly-L-lysine (Sigma)-coated microscope slides, fixed in 4% paraformaldehyde in PBS, and stored at -80 C until analyzed. The hybridization procedure was essentially the same as previously described (16). In brief, sections were pretreated serially with 0.2 M HCl, 2x SSC, pronase (0.125 mg/ml), 4% paraformaldehyde, and acetic anhydride in triethanolamine. Hybridization was carried out at 52-55 C overnight in a mixture containing 35S-labeled rat P11 complementary RNA (cRNA) probe (2 x 108 cpm/ml), 50% formamide, 0.3 M NaCl, 10 mM Tris-HCl, 5 mM EDTA, 1 x Denhardt’s solution, 10% dextran sulfate, 1 µg/ml carrier transfer RNA, and 10 mM dithiothreitol. Posthybridization washing was performed under stringent conditions that included ribonuclease A (25 µg/ml) treatment at 37 C for 30 min and a final stringency of 0.1 x SSC. Slides were dipped into NTB-2 emulsion (Eastman Kodak Co.), exposed at 4 C, and developed after 2 weeks. The slides were stained with hematoxylin and eosin and examined under the light microscope with bright- and dark-field illumination.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Ovarian P11 mRNA expression during prepubertal development and following gonadotropin stimulation
Developmental changes in P11 mRNA levels in the ovary were determined by Northern blot analysis using the full-length P11 cDNA probe. As shown in Fig. 1AGo, a 0.8-kb P11 transcript of similar size as found in PC12 cells (22) was detected in ovaries of 3-day-old rats and the hybridization signal markedly increased during development. The levels of ovarian P11 expression were 2.1- and 3.7-fold higher in 6- and 21-day-old rats, respectively, than in 3-day-old rats (Fig. 1BGo).



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  		Figure 1. Developmental expression of rat ovarian P11 mRNA. A, Aliquots of total RNA (20 µg) isolated from ovaries on the indicated postnatal days were assayed for P11 mRNA levels by Northern blotting using a rat P11 cDNA probe. The expression of GAPDH was used as an internal standard. B, Quantitative estimation of ovarian P11 mRNA levels during development. The 0.8-kb P11 transcript was quantified using a phosphorimager and normalized for GAPDH RNA levels in each sample. Results are expressed relative to ovarian P11 mRNA levels found at 3 days of age. Each data point represents the mean ± SEM from three independently performed experiments.

 
To characterize gonadotropin regulation of P11 mRNA expression in the ovary, the total RNA extracted from ovaries at different times after gonadotropin treatment was analyzed by Northern blotting. As shown in Fig. 2AGo, PMSG treatment of 26-day-old rats for 6 h caused a 34% increase (n = 3) in ovarian P11 mRNA expression. Treatment with hCG of PMSG-primed rats, however, markedly stimulated ovarian P11 mRNA expression within 6–9 h. Quantitative analysis of ovarian P11 signals (Fig. 2BGo) showed a 3.7-fold increase at 6 h after hCG treatment.



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  		Figure 2. Changes in P11 mRNA levels in ovaries of PMSG/hCG-treated immature rats. A, Aliquots of total RNA (20 µg) isolated from ovaries at the indicated time intervals after PMSG (upper panel) and PMSG/hCG stimulation (lower panel) were assayed for P11 mRNA levels by Northern blotting using a rat P11 cDNA probe. The expression of GAPDH was used as an internal standard. B, Quantitative estimation of ovarian P11 mRNA levels after hCG stimulation. The 0.8-kb P11 transcript was quantified using a phosphorimager and normalized for GAPDH RNA levels in each sample. Results are expressed relative to ovarian P11 mRNA levels found at 0 h after PMSG/hCG treatment. Each data point represents the mean ± SEM from three independently performed experiments.

 
To determine the cell types expressing P11 mRNA in gonadotropin-treated ovaries, antisense and sense cRNA probes for P11 were generated for in situ hybridization analysis. The ovaries of 26-day-old rats (control) contained numerous preantral follicles (PaF) and some early antral follicles (AnF; Fig. 3AGo). In these ovaries, P11 expression was confined to theca cells of preantral and early antral follicles (Fig. 3Go, B and D; arrowhead). P11 signals were also detected in atretic follicles (AtF) with thinner granulosa cells showing pyknotic bodies (Fig. 3Go, A and B). At 12 h after PMSG treatment, the ovaries contained follicles at varying stages of growth with an increased number of antral follicles (AnF; Fig, 3E). P11 signals were still confined to theca cells, but not granulosa cells, of preantral and growing antral follicles (Fig. 3Go, F and H; arrowhead). Some atretic follicles also expressed P11 mRNA (Fig. 3Go, E and F). At 6 h after hCG treatment, the ovaries contained a large number of preovulatory follicles (Fig. 3IGo). High levels of P11 mRNA were detected in the granulosa cells of preovulatory follicles (PoF), but not of growing antral follicles (Fig. 3Go, J and L). P11 signals were also detected in the theca cells of follicles of different sizes (Fig. 3JGo; arrowhead). Ovarian sections hybridized with the sense P11 riboprobe showed only background hybridization (Fig. 3Go; C, G, and K).



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  		Figure 3. Localization of P11 mRNA in PMSG/hCG-treated ovaries. Ovarian sections from rats at 26 days of age (Cont; left column) treated with PMSG for 6 h (middle column), or with PMSG/hCG for 6 h (right column) were hybridized with 35S-labeled P11 cRNA probes. Photomicrographs were taken under bright (A, E, and I) and darkfield (B–D, F–H, and J–L) illumination. Arrowheads (B, D, F, and H) and the asterisk indicate signals in the theca cells and atretic follicles, respectively. Sections hybridized with the P11 sense probe showed only background signals (C, G, and K). PaF, Preantral follicles; AnF, antral follicles; AtF, atretic follicles; PoF, preovulatory follicles; Gc, granulosa cells; Oo,oocyte. A–C, E–G, and I–K, x40; D, H and L, x100.

 
Stimulation of P11 mRNA expression following treatment of cultured preovulatory follicles with gonadotropins and intraovarian survival factors as well as a protein kinase A activator
To study the regulation of P11 mRNA expression by different survival factors and intracellular regulators, preovulatory follicles obtained from ovaries of rats primed for 2 days with PMSG were incubated in a serum-free condition. In this follicle culture model, LH, EGF, bFGF, IGF-I, and IL-1ß have been shown to suppress spontaneous follicle cell apoptosis (23). Northern blot analysis revealed that LH (200 ng/ml) treatment resulted in a transient stimulation of P11 mRNA in cultured preovulatory follicles, reaching a peak at 6 h (3.7-fold increase) and declining at 12 h after treatment (Fig. 4Go).



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  		Figure 4. Stimulation of P11 mRNA expression by LH in preovulatory follicles cultured in vitro. Preovulatory follicles, obtained from the ovaries of PMSG-primed immature rats, were cultured in serum-free conditions under 5% CO2-95% O2 at 37 C in the presence of LH. Total RNA was extracted from follicles collected at the indicated time intervals after LH (200 ng/ml) stimulation. Twenty micrograms of follicular total RNA were then analyzed for P11 mRNA levels by Northern blotting using a rat P11 cDNA probe. The expression of GAPDH was used as an internal standard. Data are representative of two independently performed experiments.

 
To determine the signaling pathway for P11 mRNA stimulation by LH, preovulatory follicles were incubated for 6 h in the presence of LH with or without 10 µM MDL, an inhibitor of adenylate cyclase, or 10 µM chelerythrine (CE), an inhibitor of protein kinase C. In addition, some follicles were treated for 6 h with either 10 µM forskolin (an activator for protein kinase A) or 200 nM phorbol ester TPA (an activator for protein kinase C). As shown in Fig. 5Go, forskolin treatment stimulated the P11 gene expression thus mimicking the action of LH, whereas treatment with TPA had no effect. Moreover, inclusion of MDL markedly decreased LH-stimulated P11 expression whereas the addition of CE did not affect the action of LH. Follicles incubated with MDL or CE alone did not show any effect.



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  		Figure 5. Effect of adenylate cyclase activation on P11 mRNA expression. Preovulatory follicles were incubated in serum-free conditions in the absence (Control; C) or presence of forskolin (FSK; 10 µM), MDL (10 µM), TPA (200 nM), and chelerythrine (CE; 10 µM) with or without LH (200 ng/ml). Ten micrograms of follicular total RNA were analyzed for P11 mRNA levels by Northern blotting using a rat P11 cDNA probe. The expression of GAPDH was used as an internal standard. Data are representative of two independently performed experiments.

 
To test the role of intraovarian survival factors, P11 mRNA expression was evaluated after incubation of preovulatory follicles in the presence of LH (200 ng/ml), EGF (200 ng/ml), bFGF (200 ng/ml), or IL-1ß (100 ng/ml). Inclusion of EGF, bFGF, or IL-1ß during a 9-h culture stimulated the P11 gene (Fig. 6Go). Maximum stimulation was detected 6 h after treatment (3.0- to 4.0-fold increases) with the same levels found in follicles treated with LH. In contrast, other intraovarian regulators including stem cell factor, NGF, activin, and IGF-I did not alter P11 mRNA expression (data not shown).



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  		Figure 6. Effect of intraovarian survival factors on P11 mRNA expression in cultured preovulatory follicles. Preovulatory follicles were incubated in serum-free conditions in the absence (Control; C) or presence of EGF (200 ng/ml), bFGF (200 ng/ml), or IL-1ß (IL-ß, 100 ng/ml) for up to 9 h. Follicles cultured in the presence of LH (200 ng/ml) for 6 h were used as a positive control. Ten micrograms of follicular total RNA were analyzed for P11 mRNA levels by Northern blotting using a rat P11 cDNA probe. The expression of GAPDH was used as an internal standard. Data are representative of two independently performed experiments.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
P11, a hormone-inducible survival factor, regulates apoptosis by interacting with hypophosphorylated BAD to dampen the proapoptotic action of BAD (20). In this study, we examined the developmental and gonadotropin regulation of P11 mRNA expression in the rat ovary. Additionally, we employed an in vitro follicle culture model to evaluate the hormonal regulation and signaling pathway of P11 mRNA expression. The present results indicate that ovarian P11 mRNA expression was increased during prepubertal development in theca cells of growing follicles and was induced after LH/hCG treatment in granulosa cells of preovulatory follicles. A recent report demonstrates that Mcl-1, an antiapoptotic Bcl-2 family protein capable of binding to the hypophosphorylated BAD, is also expressed in theca cells during prepubertal development (16). Interestingly, LH/hCG treatment induced a transient expression of P11 mRNA in the granulosa cells of preovulatory follicles by activating the protein kinase A pathway. This gonadotropin induction of P11 mRNA is consistent with the concept that P11 is a hormonally regulated and tissue-specific cell survival gene (21). Because LH/hCG rescues preovulatory follicles from death by activating the cAMP pathway (3, 23), P11 induced by LH/hCG may play a role in gonadotropin-mediated follicular survival. EGF, bFGF and IL-1ß, intraovarian hormones known to suppress follicle cell apoptosis (23), were also able to stimulate P11 mRNA expression in cultured preovulatory follicles.

P11, also known as 42C or calpactin I light chain, has been shown to play a role in cell survival. P11 has been identified as an early inducible gene following NGF stimulation in the rat PC12 cell line (21). Overexpression of P11 stimulates neurite outgrowth and promotes PC12 cell survival in the absence of NGF (22), indicating a role for P11 in apoptosis regulation. Although P11 belongs to the S-100 family of small calcium binding proteins (24), it does not have the ability to bind calcium ions due to amino acid deletions and substitutions in calcium binding motifs (25, 26). Instead, P11 is present in some cells as a monomer or as a heterotetramer bound to annexin II (27, 28). Of interest, induction of apoptosis by BAD can be attenuated through the interaction with the hormone-inducible P11 (20), implying the importance of P11 in the regulation of apoptosis by interacting with the Bcl-2-associated apoptotic machinery. The present observation that LH/hCG induces a rapid and transient expression of P11 mRNA in the granulosa cells of preovulatory follicles raises the possibility that P11 may play a role in the prevention of follicular cell apoptosis through interactions with the Bcl-2 family.

The mature ovarian follicles deprived of the preovulatory surge of gonadotropins, undergo atresia with apoptotic degeneration of their granulosa cells (29, 30). In addition, activation of the cAMP pathway is important in the gonadotropin suppression of preovulatory follicle atresia (23). Our present observation, showing the transient stimulation of P11 mRNA in the granulosa cells of preovulatory follicles by LH through a cAMP-dependent pathway, indicates that P11, like Mcl-1 (16), may be one of the intracellular signaling molecules mediating extracellular survival signals in the granulosa cells of preovulatory follicles. Because some degree of apoptosis has been found in ovarian surface epithelium during ovulation, the survival role of P11 in the ovary is likely to be cell type-specific.

As in other cell systems, a common subset of Bcl-2 family proteins including death promoters such as BAD (17), Bok (31), and BOD/Bim (32), and survival factors such as Bcl-2 (12) and Mcl-1 (16), is present in the ovary, and the fate of the granulosa cells is determined by the balance of these opposing activities. The survival of the granulosa cells of preovulatory follicles may occur through up-regulation of the survival factors and/or removal of the cell death promoters. Because BAD, a death promoter, interacts with P11 (20), our results raise the possibility that induction of P11 mRNA expression in the granulosa cells of preovulatory follicles by LH/hCG may be an important mechanism underlying the antiatretogenic action of gonadotropin in the ovary. It is possible that by inducing P11 overexpression in the granulosa cells of preovulatory follicles, LH/hCG is able to regulate the rheostat of apoptosis in favor of granulosa cell survival, and thus successful ovulation. Similar to P11 mRNA expression, inhibitor of apoptosis proteins (IAPs), a novel family of intracellular proteins that suppress apoptosis, is also expressed in the granulosa cells of preovulatory follicles following LH/hCG stimulation (33). The relative importance of P11 expression and the interactions of P11 with IAPs and/or intracellular regulators of apoptosis in the survival of the granulosa cells of preovulatory follicles remains to be determined.

Apoptosis of atretic follicles in rodents is confined to granulosa cells. Interestingly, in the present study, P11 mRNA was also expressed in theca cells in follicles of different sizes. The constitutive expression of P11 mRNA in theca cells may be responsible for the scarcity of theca cell degeneration in rats. Alternatively, P11 may be involved in other as yet undetermined physiological processes such as theca cell differentiation. Because theca cells undergo differentiation in immature follicles of neonatal rats (34), our observation showing increased levels of P11 mRNA during prepubertal development suggests the involvement of P11 in theca cell development. Unexpectedly, granulosa cells of some follicles at the advanced stage of atresia also expressed high levels of P11 mRNA. At the present time, the reason(s) for the expression of P11 mRNA in granulosa cells of these atretic follicles is not known.

In addition to apoptosis regulation, P11 could have other roles in the regulation of follicular development by interacting with unknown factors. P11 has been shown to inhibit the release of arachidonic acid by interacting with cytoplasmic phospholipase A2 in HeLa and BEAS-2B cells (35). In the ovary, prostaglandins and leukotrienes converted from arachidonic acid play an important role in ovulation (36, 37). These eicosanoids increase in rat ovarian follicles during the first several hours of ovulation induction and then decline to preovulatory levels at 6–12 h preceding ovulation (38, 39). Thus, it may be possible that P11, produced during the 6–9 h period after LH/hCG treatment, plays a role in follicle rupture by interacting with phospholipase A2, resulting in the modulation of eicosanoid activities. Studies are currently underway to identify the P11 interacting proteins in ovarian cells by using the yeast two-hybrid system. Indeed, P11 has been shown to interact with the FSH receptor as demonstrated by the yeast two-hybrid screening of the human ovarian cDNA library (40).

In summary, the present study has demonstrated that P11 mRNA is expressed in the rat ovary in a stage- and cell-specific manner during follicle development. P11 mRNA is expressed in theca cells regardless of follicle size and in granulosa cells of preovulatory follicles following gonadotropin stimulation. P11 appears to be an intracellular protein important in the decision of granulosa cell fate and may play a critical role as a cell survival factor during the ovulatory process.


   Acknowledgments
 
We thank The National Hormone and Pituitary Distribution Program (NIDDK, NIH) for the ovine LH preparation.


   Footnotes
 
1 This work was supported by Korean Research Foundation Grants KRF-99–015-DP0361 and HRC-98k1–0405, Republic of Korea (to S.Y.C.), and by NIH Grant HD-31566 (to A.J.H.). Back

Received November 8, 2000.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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