In vitro DNA synthesis by isolated preantral to preovulatory follicles from the cyclic mouse

In recent studies, we have shown that the smallest preantral follicles in the cyclic hamster increase DNA synthesis in the periovulatory period in response to surge levels of FSH. The current investigation was designed to determine whether the same phenomenon occurs in the cyclic mouse. Intact mouse follicles were isolated with watchmaker forceps (stages 4-6) or by enzymatic digestion (stages 1-4) at 0900 h and 1500 h on each day of the 5-day estrous cycle. The isolated follicles were classified into 6 stages: stages 1 and 2: follicles with 1 and 2 layers of granulosa cells; stage 3: follicles with 3 or more layers of granulosa cells and formation of theca; stages 4-6: incipient, small, and preovulatory antral follicles. The follicles at each stage were incubated for 3 h with [3H]thymidine. DNA content in stages 1-4 of follicles remained unchanged during the estrous cycle; for stages 5 and 6, DNA content was higher on the afternoon of proestrus than on other days of the cycle. Incorporation of [3H]thymidine for stages 1-3 (preantral follicles) started to increase at 1500 h of proestrus and peaked at 0900 h on estrus, whereas for stages 4-6, DNA synthesis peaked on proestrus (1500 h) and then fell by the morning of estrus. Thus, the rate of DNA replication in preantral and antral mouse follicles were different. Similarities and differences in folliculogenesis between mouse and hamster are discussed. These results suggest that DNA synthesis and the growth of all stages of follicles in the cyclic mouse may be associated with changing levels of periovulatory gonadotropins.     

A novel mechanism of FSH regulation of DNA synthesis in the granulosa cells of hamster preantral follicles: involvement of a protein kinase C-mediated MAP kinase 3/1 self-activation loop

The objective was to reveal whether a protein kinase C (PKC [all isozymes])-mediated self-sustaining MAPK3/1 (3/1 extracellular signal regulated kinase 2/1, also known as ERK2/1) activation loop was necessary for FSH- or epidermal growth factor (EGF)-induced DNA synthesis in the granulosa cells of intact preantral follicles. For this purpose, hamster preantral follicles were cultured with FSH or EGF in the presence of selective kinase inhibitors FSH or EGF phosphorylated RAF1, MAP2K1, and MAPK3/1. However, a relatively higher dose of EGF was necessary to sustain the MAPK3/1 activity, which was essential for cyclin-dependent kinase 4 (CDK4) activation and DNA synthesis. In intact preantral follicles, FSH or EGF stimulated DNA synthesis only in the granulosa cells. Sustained activation of MAPK3/1 beyond 3 h was independent of EGFR kinase activity but dependent on PKC activity, which appeared to form a self-sustaining MAPK3/1 activation loop by activating RAF1, MAP2K1, and PLA2G4 (phospholipase A2 [all cytosolic isozymes]). Inhibition of PKC activity as late as 4 h after the administration of FSH or EGF arrested DNA synthesis, which corresponded with attenuated phosphorylation of RAF1 and MAPK3/1, thus suggesting an essential role of PKC in MAPK3/1 activation. Collectively, these data present a novel self-sustaining mechanism comprised of MAPK3/1, PLA2G4, PKC, and RAF1 for CDK4 activation leading to DNA synthesis in granulosa cells. Either FSH or EGF can activate the loop to activate CDK4 and initiate DNA synthesis; however, consistent with our previous findings, FSH effect seems to be mediated by EGF, which initiates the event by stimulating EGFR kinase.     

Transforming growth factor B1 stimulated DNA synthesis in the granulosa cells of preantral follicles: negative interaction with epidermal growth factor

EGF or TGFB1 alone stimulates but together attenuate granulosa cell DNA synthesis. Intact preantral follicles from hamsters were cultured with TGFB1, EGF, or both to reveal the mechanisms of such unique regulation. Follicular CCND2 (also known as cyclin D2), CDKN1B (also known as p27(kip1)), and the involvement of appropriate signaling intermediaries and kinases were examined. TGFB1, acting via SMAD2 and SMAD3, antagonized the degradation of CCND2 protein by blocking its phosphorylation. In contrast, TGFB1 supported CDKN1B degradation by involving MAPK14 (also known as p38 Map Kinase) and PKC, resulting in CDK4 activation and DNA synthesis. EGF via MAPK3/1 maintained functional levels of CCND2 through CCND2 synthesis as well as degradation. EGF and TGFB1 together inhibited CDK4 activation and DNA synthesis. EGF attenuated TGFB1 stimulated phosphorylation of SMAD3, TGFB1-induced activation of MAPK14 and PKC, and TGFB1 suppression of CCND2 degradation. In contrast, TGFB1 suppressed EGF-induced increase in CCND2 mRNA levels. The final outcome was CCND2 degradation without replenishment and decreased activities of MAPK14 and PKC leading to suppression of CDK4 activation. The results indicate that each growth factor involves a separate mechanism to maintain an effective level of CCND2 in granulosa cells for the activation of CDK4 and induction of DNA synthesis. However, their simultaneous action is inhibitory to follicular DNA synthesis because they counteract each other's activity by interfering at specific sites. Because both EGF and TGFB1 are present in granulosa cells, this mechanism may explain how their effects are temporally modulated for granulosa cell proliferation and folliculogenesis.     

Cryopreservation of ovarian tissues temporarily suppresses the proliferation of granulosa cells in mouse preantral follicles

Cryopreservation of ovarian tissue has been reported to delay the development of preantral follicles during in vitro culture, but the mechanism causing this impairment has not been brought to light. In order to elucidate the underlying mechanism of delayed follicular development, we evaluated the effects of cryopreservation on the proliferation of granulosa cells during culture of mouse preantral follicles, as a sufficient population of granulosa cells is critical for normal follicular development. Additionally the initial cell death of granulosa cells was estimated immediately after cryopreservation. The ovarian tissues obtained from 12-day-old female mice were cryopreservation by vitrification. The granulosa cell proliferation was evaluated by measuring the PCNA expression and the expression of cell cycle regulators such as cyclin D2, CDK4, cyclin E and CDK2 in preantral follicles isolated from fresh and cryopreserved ovarian tissues that were cultured for 48 h. The viability of granulosa cells was evaluated by measuring the proportion of necrotic areas. The granulosa cell proliferation of the cryopreserved preantral follicles was decreased significantly compared to that of the fresh controls at 0 and 24 h after culture (P < 0.05), and this was increased to the control levels after 48 h of culture. The expressions of cyclin D2, Cdk 4, cyclin E and Cdk2 were also decreased in the cryopreserved ovarian tissues at 0 and 24 h after culture (P < 0.05), but they were increased to the control levels after 48 h of culture. The proportion of the necrotic area was significantly higher in cryopreserved preantral follicles compared to that of the fresh preantral follicles (P < 0.05). This suggests that cryopreservation of ovarian tissues may delay the preantral follicle development by temporary suppressing the granulosa cell proliferation through the cell cycle regulators (cyclin D2, Cdk4, cyclin E and Cdk2) and by granulosa cell death immediately after warming.     

Genetic differences in follicular DNA synthesis between two strains of hamsters

This study was designed to compare our previous results on ovarian follicular DNA synthesis by hamsters obtained from Sasco Laboratories with a different breeding colony: Harlan. Follicles from proestrous Harlan hamsters required twice as much [3H] thymidine and a minimum of 4 hr of in vitro exposure to 100 ng of ovine follicle-stimulating hormone (FSH) before a significant increase in DNA synthesis was elicited compared with 30-120 min for the Sasco breed. Peak responsiveness to FSH was observed at 8-hr incubation for the Harlan strain with significant increases in DNA per follicle at 8-12 hr. Both strains increased DNA synthesis with as little as 25 ng of ovine FSH and the response was elicited in all growing follicles, from preantral stages with one to four layers of granulosa cells, lacking theca (Stages 1-4) to mature antral follicles (Stages 8-10). A recombinant bovine FSH, devoid of luteinizing hormone activity, was not as effective as ovine FSH (which has 4% luteinizing hormone contamination) in stimulating DNA synthesis by large preantral and antral follicles. In vitro responsiveness to ovine FSH was abolished in the absence of Ca2+ in the culture medium and 0.05 mM Ca2+ was the optimal amount. For both strains of hamsters, the highest rate of DNA synthesis in response to endogenous gonadotropins was on the morning of estrus--when the second surge of FSH was in progress--and Harlan follicles in vitro also showed maximal stimulation by FSH on this day. Where the two strains differed was that the Harlan strain did not show an increase in follicular DNA synthesis on the afternoon of proestrus--when the preovulatory increase in gonadotropins commenced. When expressed as DNA per follicle, DNA approximately doubled from Stages 1 to 5 and then entered a new growth phase at Stage 6 (large preantral follicles) with a steeper increase. Collectively, these experiments show that strain characteristics can alter the latency and degree of follicular DNA replication in response to endogenous or exogenous FSH.     

Cell-type-specific localization of transforming growth factor-beta 2 and transforming growth factor-beta 1 in the hamster ovary: differential regulation by follicle-stimulating hormone and luteinizing hormone.

Cell-type-specific localization and gonadotropin regulation of transforming growth factor-beta 1 (TGF-beta 1) and transforming growth factor-beta 2 (TGF-beta 2) in the hamster ovary were evaluated immunohistochemically under three conditions: (1) during the estrous cycle (Day 1 = estrus; Day 4 = proestrus); (2) after the blockade of periovulatory gonadotropin surges by phenobarbital, and (3) after FSH and/or LH treatment of long-term hypophysectomized hamsters. Ovarian TGF-beta 1 activity was primarily localized in theca and interstitial cells. The activity increased moderately but significantly after the preovulatory LH surge and reached a peak at 0900 h, Day 2 h; oocytes showed considerable activity. TGF-beta 1 immunoreactivity subsequently fell to low levels in theca-interstitial cells through 0900 h, Day 4. Significant TGF-beta 2 immunoreactivity appeared after the surge, mainly in the granulosa cells of both preantral and antral follicles; a few interstitial cells surrounding preantral follicles showed discrete staining. TGF-beta 2 immunoreactivity in granulosa cells and in interstitial cells next to preantral follicles reached a peak at 0900 h, Day 1, and persisted up to 0900 h, Day 2; oocytes showed no staining. Phenobarbital treatment blocked the appearance of TGF-beta 1 and TGF-beta 2 immunoreactivities at 1600 h, Day 4; however, a rebound in immunoreactivities was observed with the onset of the surge after a 1-day delay. Replacement of LH to long-term hypophysectomized hamsters resulted in a marked increase in TGF-beta 1 immunoreactivity in the interstitial cells, but FSH, although it induced follicular development, did not influence ovarian TGF-beta 1 activity. Treatment with FSH, however, induced a massive increase in TGF-beta 2 immunoreactivity in the granulosa cells of newly developed antral and preantral follicles but not in the interstitial cells; LH, on the other hand, had no significant effect on TGF-beta 2 activity. Treatment with FSH and LH combined resulted in a dramatic increase in TGF-beta 2 immunoreactivity in granulosa and interstitial cells and in TGF-beta 1 in theca and interstitial cells comparable to their peak activity in intact animals. Western analyses substantiated the presence of TGF-beta 1 and TGF-beta 2 in the hamster ovary and the specificity of immunolocalization. These studies, therefore, provide critical evidence that TGF-beta 1 and TGF-beta 2 in the hamster ovary are expressed in specific cell types and that their expression is differentially regulated by LH and FSH, respectively.     

An enzymatic method for dissociation of intact follicles from the hamster ovary: histological and quantitative aspects

An enzymatic method was developed to collect intact follicles at different stages of development from cyclic hamsters to study ovarian folliculogenesis under various circumstances. Ovaries from 6 adult hamsters on each day of the cycle (Day 1 = ovulation) were collected, corpora lutea and large preantral and antral follicles were dissected, and follicles saved. Minced ovaries were then incubated with a mixture of collagenase, DNAse and pronase at 37 degrees C for 20 min to disperse intact follicles. Histological studies with 2191 isolated follicles revealed 10 different stages of follicular development (depending on the number of granulosa cell layers surrounding the oocyte and development of the antrum). Of the total follicular population, 14% showed signs of atresia, with 50% of those having 1-3 layers of granulosa cells (Stages 1-3); a second peak of 18% was observed in antral follicles (Stages 8-10). No signs of thecal cells were evident until the follicles reached Stage 6 (7-8 layers of granulosa cells), which possibly accounts for reduced atresia in this class and beyond. Ultrastructural study revealed that there were no signs of morphological damage to the basement membrane or to other subcellular organelles in the small preantral follicles. The presence of subnuclear lipid droplets in follicles with 3 layers of granulosa cells provided evidence for potential steroidogenesis by small follicles. The number of Stage 1-10 follicles was remarkably constant throughout the estrous cycle (460 +/- 34 per animal on Day 1 vs. 492 +/- 66 on Day 4). The usefulness of this method in analyzing follicular kinetics is illustrated in experiments involving hypophysectomy and the effects of unilateral ovariectomy. This procedure offers an improved method to study the factors responsible for the growth and the differentiation of small preantral follicles in the mammalian ovary.     

Quantitative analysis of in-vitro incorporation of [3H]thymidine into hamster follicles during the oestrous cycle

Follicles were isolated from hamster ovaries at 09:00 h and 15:00 h on each of the 4 days of the oestrous cycle (Day 1 = oestrus; Day 4 = pro-oestrus) by microdissection and by a mixture of enzymes and classified into 10 stages with pre-calibrated pipettes (stage 1 = preantral follicles with 1 layer of granulosa cells; stage 10 = preovulatory antral follicles). The follicles at each stage were incubated for 4 h with [3H]thymidine with incorporation expressed per microgram follicular DNA or per follicle. A significant increase in thymidine per follicle occurred at 15:00 h on Days 1 and 3 of the cycle from stage 2 (bilaminar follicle) to stage 6 (7-8 layers granulosa cells plus theca). When expressed as thymidine per follicle or microgram DNA, there was a significant increase in incorporation for stages 1-4 (4 layers granulosa cells) on Day 4 at 15:00 h compared to 09:00 h, presumably as a consequence of the preovulatory increase in gonadotrophins. Follicles in stages 5 to 8 (preantral follicles with 5 or more layers of granulosa cells to small antral follicles), from which the next set of ovulatory follicles will be selected, did not show a significant peak in incorporation per microgram DNA until Day 1 at 09:00 and 15:00 h when the second increase in FSH is in progress. DNA synthesis was similarly sustained throughout Day 1 for stage 1-4 follicles. These results suggest that periovulatory changes in FSH and LH, directly or indirectly, are not only responsible for ovulation and the recruitment of the next set of follicles destined to ovulate but also stimulate DNA replication in smaller follicles which develop over the course of several cycles before they ovulate or become atretic.     

Cyclic AMP inhibits the proliferation of thyroid carcinoma cell lines through regulation of CDK4 phosphorylation

How cyclic AMP (cAMP) could positively or negatively regulate G1 phase progression in different cell types or in cancer cells versus normal differentiated counterparts has remained an intriguing question for decades. At variance with the cAMP-dependent mitogenesis of normal thyroid epithelial cells, we show here that cAMP and cAMP-dependent protein kinase activation inhibit S-phase entry in four thyroid carcinoma cell lines that harbor a permanent activation of the Raf/ERK pathway by different oncogenes. Only in Ret/PTC1-positive TPC-1 cells did cAMP markedly inhibit the Raf/ERK cascade, leading to mTOR pathway inhibition, repression of cyclin D1 and p21 and p27 accumulation. p27 knockdown did not prevent the DNA synthesis inhibition. In the other cells, cAMP little affected these signaling cascades and levels of cyclins D or CDK inhibitors. However, cAMP differentially inhibited the pRb-kinase activity and T172-phosphorylation of CDK4 complexed to cyclin D1 or cyclin D3, whereas CDK-activating kinase activity remained unaffected. At variance with current conceptions, our studies in thyroid carcinoma cell lines and previously in normal thyrocytes identify the activating phosphorylation of CDK4 as a common target of opposite cell cycle regulations by cAMP, irrespective of its impact on classical mitogenic signaling cascades and expression of CDK4 regulatory partners.     

Spatiotemporal expression of epidermal growth factor receptor messenger RNA and protein in the hamster ovary: follicle stage-specific differential modulation by follicle-stimulating hormone,luteinizing hormone,estradiol, and progesterone

Spatiotemporal expression, endocrine regulation, and activation of epidermal growth factor receptor (EGFR) in the hamster ovary were evaluated by immunofluorescence and in situ hybridization localization. Whereas granulosa cells (GC) of primordial through large preantral (stage 6, 7-8 layers GC) follicles had low immunoreactivity, granulosa cells of antral follicles, theca, and interstitial cells had intense EGFR immunoreactivity. EGFR expression in GC of primordial and small preantral follicles increased progressively from estrous through proestrous, but a significant increase occurred in mural GC of antral follicles following the gonadotropin surge. Interstitial cells around small preantral follicles had strong immunofluorescence, and the intensity increased significantly in fully differentiated thecal cells. Distinct EGFR protein was localized in the nucleus of the oocytes and granulosa cells. FSH significantly stimulated EGFR expression in the GC, especially the mural GC, theca, and interstitial cells in hypophysectomized hamster. Estrogen stimulated EGFR expression in preantral GC as well as in interstitial cells. Progesterone and hCG effect was limited to theca and interstitial cells. EGFR expression correlated well with EGFR activation following endogenous or exogenous gonadotropin exposure. Receptor mRNA expression closely followed the protein expression, with increased mRNA expression in mural GC of antral follicles. These results suggest that low levels of EGF signal as a consequence of low levels of receptors in preantral GC may be critical for cell proliferation, but higher receptor density may evoke increased signal intensity due to activation of other intracellular signal pathways, which activate cellular processes related to granulosa, theca, and interstitial cell differentiation. The spatiotemporal cell type and follicle stage-specific expression of receptor mRNA and protein and EGFR activation is critically regulated by gonadotropins and ovarian steroids, primarily estradiol.     

Hormonal requirements for the growth and differentiation of hamster preantral follicles in long-term culture

Preantral follicles from pro-oestrous and oestrous hamsters were isolated enzymically (Stages 1-5) and by microdissection (Stage 6) and cultured for up to 168 h in the absence or presence of 100 ng ovine FSH or LH separately or combined or 1 or 10 micrograms progesterone or estradiol-17 beta in serum-free defined medium and exposed to 1 muCi [3H]thymidine for 24 h before termination. In the presence of insulin and hydrocortisone but not gonadotrophins, the morphology of follicles from pro-oestrous animals at Stages 1-4 (1-4 layers granulosa cells; no theca) were unaffected for up to 48 h whereas for Stages 5 (5-6 layers granulosa cells and developing theca) and 6 (7-8 layers granulosa cells and theca), atresia was prominent by 24 h. FSH significantly reduced the percentage of atretic follicles in Stages 1-5 throughout the culture period; but was effective only up to 96 h for Stage-6 follicles. LH was also effective, albeit to a lesser extent. FSH increased follicular labelling indexes during every 24-h labelling period and, during a pulse-chase period, follicular DNA content and granulosa cell numbers. FSH, but not LH, induced differentiation by 96 h of preantral follicles at Stage 6 into small antral stages (Stages 7-8). FSH and LH together induced almost the same effect as FSH alone. However, neither progesterone nor oestradiol had any significant long-term effects on DNA synthesis and oestradiol induced atresia beyond 24 h. Both FSH and LH induced follicular maturation in vitro as evident from increases in progesterone, androstenedione and oestradiol production. Follicles (Stages 1-4) collected from oestrous hamsters responded to FSH to a lesser extent than did those from pro-oestrous animals, possibly because of in-vivo exposure to periovulatory changes in gonadotrophins; however, an antrum formed in Stage-6 follicles by 72 h.     

Effects of progesterone or luteinizing hormone on folliculogenesis in intact or hypophysectomized immature hamsters

Since exogenous progesterone (P4) causes superovulation in hypophysectomized (hypoxed) cyclic hamsters treated with gonadotropins, the current study was performed to evaluate the roles of P4 and luteinizing hormone (LH) as a folliculotropic complex in the immature hamster. Intact or hypoxed immature hamsters were injected daily, beginning on Day 23, with 1 mg P4 and/or 20 micrograms LH for 4 days. Treatment with P4 alone or combined with LH in intact immature hamsters increased the number of antral follicles (6.7 and 4.3, respectively, vs. 1.5 per ovary in controls), but neither treatment maintained large follicles in hypoxed animals. In contrast, in hypoxed hamsters, the number of small preantral follicles was enhanced by P4 or LH (406 and 409, respectively, compared to 302 per ovary in untreated controls), but with no additive effects by combined treatment. The stimulatory effect of P4 in intact hamsters was unrelated to serum levels of follicle-stimulating hormone, estradiol, or LH. Moreover, in the hypoxed hamster, P4 or LH acts directly to increase the numbers of small preantral follicles with 2 to 5 layers of granulosa cells, whereas equally large doses of stilbestrol or estradiol cyclopentylpropionate are ineffective. In the hypoxed or intact hamster, the effects of P4 or LH may involve either recruitment of smaller follicles into larger stages or prevention of atresia. The present experimental design can not distinguish between these possibilities.     

FSH and growth factors affect the growth and endocrine function in vitro of granulosa cells of bovine preantral follicles

The hypothesis was tested that bovine preantral follicles can be stimulated to grow in vitro by FSH and by the mitogens, epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF), but not by transforming growth factor-beta (TGFbeta), which generally inhibits EGF and bFGF action. Preantral follicles, 60 to 179 mum in diameter, were isolated from fetal ovaries by treatment with collagenase and DNase and cultured for 6 d in serum-free medium, with or without FSH and growth factors. Basic FGF (50 ng/ml), and to a lesser extent FSH (100 ng/ml) and EGF (50 ng/ml), stimulated thymidine incorporation by granulosa cells in bovine preantral follicles compared to control cultures (8-, 4- and 2.5-fold the labeling index of the controls; P < 0.05). Alone TGFbeta (10 ng/ml) had no effect on (3)H-thymidine incorporation, but it completely inhibited the bFGF- but not the FSH-stimulated increase in the labeling index and mean follicular diameter of preantral follicles (P < 0.05). By the end of the culture period oocytes in most treatments had degenerated, and the few surviving oocytes were in preantral follicles cultured with FSH or bFGF. Progesterone accumulation was greater (P < 0.05) in the presence of FSH (100 ng/ml) or EGF (50 ng/ml) than with bFGF, TGFbeta or control medium. Basic FGF strongly inhibited the effect of FSH on progesterone secretion (P < 0.05). Only FSH stimulated the conversion of exogenous testosterone to estradiol and both bFGF and TGFbeta markedly inhibited FSH-stimulated estradiol accumulation. These results indicate that proliferation of granulosa cells of bovine preantral follicles can be stimulated by bFGF, FSH and EGF, whereas TGFbeta inhibits growth, and that they are steroidogenically active in culture. Basic FGF and TGFbeta antagonize FSH-stimulated steroid production by granulosa cells of cultured bovine preantral follicles.     

DeltaRaf-1:ER* bypasses the cyclic AMP block of extracellular signal-regulated kinase 1 and 2 activation but not CDK2 activation or cell cycle reentry

Elevation of cellular cyclic AMP (cAMP) levels inhibits cell cycle reentry in a variety of cell types. While cAMP can prevent the activation of Raf-1 and extracellular signal-regulated kinases 1 and 2 (ERK1/2) by growth factors, we now show that activation of ERK1/2 by DeltaRaf-1:ER is insensitive to cAMP. Despite this, DeltaRaf-1:ER-stimulated DNA synthesis is still inhibited by cAMP, indicating a cAMP-sensitive step downstream of ERK1/2. Although cyclin D1 expression has been proposed as an alternative target for cAMP, we found that cAMP could inhibit DeltaRaf-1:ER-induced cyclin D1 expression only in Rat-1 cells, not in CCl39 or NIH 3T3 cells. DeltaRaf-1:ER-stimulated activation of CDK2 was strongly inhibited by cAMP in all three cell lines, but cAMP had no effect on the induction of p21(CIP1). cAMP blocked the fetal bovine serum (FBS)-induced degradation of p27(KIP1); however, loss of p27(KIP1) in response to DeltaRaf-1:ER was less sensitive in CCl39 and Rat-1 cells and was completely independent of cAMP in NIH 3T3 cells. The most consistent effect of cAMP was to block both FBS- and DeltaRaf-1:ER-induced expression of Cdc25A and cyclin A, two important activators of CDK2. When CDK2 activity was bypassed by activation of the ER-E2F1 fusion protein, cAMP no longer inhibited expression of Cdc25A or cyclin A but still inhibited DNA synthesis. These studies reveal multiple points of cAMP sensitivity during cell cycle reentry. Inhibition of Raf-1 and ERK1/2 activation may operate early in G(1), but when this early block is bypassed by DeltaRaf-1:ER, cells still fail to enter S phase due to inhibition of CDK2 or targets downstream of E2F1.     

Changes in expression of inhibin subunits in the cyclic golden hamster (Mesocricetus auratus) and the regulation of inhibin alpha subunit production by luteinizing hormone

In the present study, changes in localization of each inhibin subunit in the ovary were investigated during the estrous cycle of the golden hamster. The effect of LH surge on changes in localization in inhibin alpha subunit in the ovary was also investigated. Inhibin alpha subunit was localized in granulosa cells of various stages of follicles throughout the estrous cycle. Inhibin alpha subunit was also present in numerous interstitial cells on days 1 and 2 (day 1 = day of ovulation), but the number of positive interstitial cells was fewer on days 3 and almost disappeared on day 4 of the estrous cycle. Newly formed luteal cells were also positive for inhibin alpha subunit on days 1 and 2. On the other hand, positive reactions for inhibin beta A and beta B subunits were only present in the granulosa cells of healthy antral follicles. However, a positive reaction for inhibin beta B subunit in peripheral mural granulosa cells disappeared on days 3 and 4 of the estrous cycle. Treatment with LHRH-AS at 1100 h on day 4 completely blocked the luteinizing hormone (LH) surge and ovulation, although relatively high concentrations of plasma follicle-stimulating hormone (FSH) were maintained throughout the experiment. There were few positive reactions for inhibin alpha subunit in theca and interstitial cells 24 hr after LHRH-AS injection. The effect of LHRH-AS treatment was blocked by a single injection of 10 IU human chorionic gonadotropin. These results suggest that the major source of dimeric inhibin in the cyclic hamster was granulosa cells of healthy antral follicles. Different distribution pattern of inhibin beta A from beta B subunits in large antral follicles on days 3 and 4 of the estrous cycle suggests different secretion patterns of inhibin A from B on these days. Furthermore, the LH surge may be an important factor to induce production of inhibin alpha subunit in interstitial cells of the cyclic hamster.     

Müllerian inhibitory substance induces growth of rat preantral ovarian follicles

Müllerian inhibitory substance (MIS), also known as anti-Müllerian hormone, is best known as the hormone that regulates the regression of the Müllerian duct in males. In females, MIS is expressed in granulosa cells of preantral and early antral follicles. The specific MIS type II receptor is present in granulosa and theca cells of these small, growing follicles. Because the role of MIS in preantral follicle development is unknown, we have evaluated the effect of MIS on the growth, differentiation, and apoptosis of intact preantral follicles in a serum-free culture system. In this system, treatment with FSH induces an increase in both follicle diameter, cell number, and follicle cell differentiation based on increased inhibin-alpha synthesis. Of interest, treatment with MIS enhances the effect of FSH both on follicle diameter and cell number. Although treatment with activin A also enhances FSH effects on follicle growth, treatment with transforming growth factor (TGF)-ss inhibits the FSH effects on follicle growth. Based on in situ staining of fragmented DNA, MIS was found to have no effect on follicle cell apoptosis, unlike its proapoptotic action on Müllerian ducts. In contrast to MIS and activin, TGF-ss was a potent proapoptotic factor for preantral follicles in culture. Analysis of inhibin-alpha expression of cultured preantral follicles further indicated that in contrast to activin, treatment with MIS did not enhance FSH-stimulated follicle differentiation. Thus, MIS is a unique factor that promotes preantral follicle growth but not preantral follicle cell differentiation and apoptosis.     

Bovine preantral follicles and activin: immunohistochemistry for activin and activin receptor and the effect of bovine activin A in vitro

Activin was originally isolated from follicular fluid as a factor stimulating FSH from the pituitary. Recent studies also suggest a local role for activin in the development of preantral and early antral follicles. In the present study, activin and activin receptor immunoreactivity are shown in oocyte and granulosa cells of bovine preantral follicles. In addition, activin immunoreactivity was observed in the theca of secondary follicles. During culture of isolated preantral follicles, activin increased follicular growth and granulosa cell proliferation in a dose-dependent manner. This increase was further stimulated by addition of FSH. In conclusion, activin and its receptor are present on bovine preantral follicles, and additional activin stimulates development of those follicles.