Repression of the inhibin alpha-subunit gene by the transcription factor CCAAT/enhancer-binding protein-beta

Roles of epidermal growth factor (EGF)-like factor in the ovulation process

Luteinizing hormone (LH) surge stimulates preovulatory follicles to induce the ovulation process, including oocyte maturation, cumulus expansion, and granulosa cell luteinization. The matured oocytes surrounded by an expanded cumulus cell layer are released from follicles to the oviduct. However, LH receptors are dominantly expressed in granulosa cells, but less in cumulus cells and are not expressed in oocytes, indicating that the secondary factors expressed and secreted from LH-stimulated granulosa cells are required for the induction of the ovulation process. Prostaglandin and progesterone are well-known factors that are produced in granulosa cells and then stimulate in both granulosa and cumulus cells. The mutant mice of prostaglandin synthase (Ptgs2KO mice) or progesterone receptor (PRKO mice) revealed that the functions were essential to accomplish the ovulation process, but not to induce the ovulation process. To identify the factors initiating the transfer of the stimuli of LH surge from granulosa cells to cumulus cells, M. Conti's lab and our group performed microarray analysis of granulosa cells and identified the epidermal growth factor (EGF)-like factor, amphiregulin (AREG), epiregulin (EREG), and β-cellulin (BTC) that act on EGF receptor (EGFR) and then induce the ERK1/2 and Ca²⁺-PLC pathways in cumulus cells. When each of the pathways was down-regulated using a pharmacological approach or gene targeting study, the induction of cumulus expansion and oocyte maturation were dramatically suppressed, indicating that both pathways are inducers of the ovulation process. However, an in vitro culture study also revealed that the EGFR-induced unphysiological activation of PKC in cumulus cells accelerated oocyte maturation with low cytostatic activity. Thus, the matured oocytes are not arrested at the metaphase II (MII) stage and then spontaneously form pronuclei. The expression of another type of EGF-like factor, neuregulin 1 (NRG1), that does not act on EGFR, but selectively binds to ErbB3 is observed in granulosa cells after the LH surge. NRG1 supports EGFR-induced ERK1/2 phosphorylation, but reduces PKC activity to physiological level in the cumulus cells, which delays the timing of meiotic maturation of oocytes to adjust the timing of ovulation. Thus, both types of EGF-like factor are rapidly induced by LH surge and then stimulate cumulus cells to control ERK1/2 and PKC pathways, which results in the release of matured oocytes with a fertilization competence.

Growth Arrest Specific-1 (GAS1) Is a C/EBP Target Gene That Functions in Ovulation and Corpus Luteum Formation in Mice

Ovulation and luteinization are initiated in preovulatory follicles by the luteinizing hormone (LH) surge; however, the signaling events that mediate LH actions in these follicles remain incompletely defined. Two key transcription factors that are targets of LH surge are C/EBPalpha and C/EBPbeta, and their depletion in granulosa cells results in complete infertility. Microarray analyses of these mutant mice revealed altered expression of a number of genes, including growth arrest specific-1 (Gas1). To investigate functions of Gas1 in ovulation- and luteinization-related processes, we crossed Cyp19a1-Cre and Gas1^flox/flox mice to conditionally delete Gas1 in granulosa and cumulus cells. While expression of Gas1 is dramatically increased in granulosa and cumulus cells around 12–16 h post-human chorionic gonadotropin (hCG) stimulation in wild-type mice, this increase is abolished in Cebpa/b double mutant and in Gas1 mutant mice. GAS1 is also dynamically expressed in stromal cells of the ovary independent of C/EBPalpha/beta. Female Gas1 mutant mice are fertile, exhibit enhanced rates of ovulation, increased fertility, and higher levels of Areg and Lhcgr mRNA in granulosa cells. The morphological appearance and vascularization of corpora lutea appeared normal in these mutant females. Interestingly, levels of mRNA for a number of genes (Cyp11a1, Star, Wnt4, Prlr, Cd52, and Sema3a) associated with luteinization are decreased in corpora lutea of Gas1 mutant mice as compared with controls at 24 h post-hCG; these differences were no longer detectable by 48 h post-hCG. The C/EBP target Gas1 is induced in granulosa cells and is associated with ovulation and luteinization.

Polymorphisms in inhibin α gene promoter associated with male infertility

Inhibins play important roles in normal gonadal function, including regulation of proliferation, differentiation, and steroidogenesis of Leydig and Sertoli cells via paracrine and autocrine processes. In adult males, circulating inhibin levels are correlated with fertility by regulating the number of Sertoli cells, total sperm count, and testicular volume. Given this important role, inhibin-α subunit (INHA) is a strong candidate gene in male fertility. However, limited data regarding the association of polymorphisms of INHA with male fertility are available. This study was based on the hypothesis that polymorphisms in the promoter of INHA are associated with male fertility. Han Chinese patients with non-normozoospermia (n=153) and normozoospermia (n=72) from Northern China were screened, and genotypes were analyzed by polymerase chain reaction-restriction fragment length polymorphism after INHA promoter was amplified. Statistical analysis results revealed a significant difference in the allele frequency of INHA promoter between males with non-normozoospermia and normozoospermia. For c.-124G>A, males carrying c.-124GG genotype and c.-124GA genotype showed an increased risk of non-normozoospermic syndrome. For c.-16C>T polymorphism, no significant difference in allele frequency was observed between the two groups. Therefore, the haplotype AC possibly displayed a considerable reduced risk of non-normozoospermic syndrome.

Inhibin alpha gene expression in human trophoblasts is regulated by interactions between TFAP2 and cAMP signaling pathways

Inhibin α (Inha) gene expression is regulated, in rat granulosa cells, via a cyclic 3',5'-adenosine monophosphate (AMP)-response element (CRE) found in a region of the promoter that is homologous to the human INHA promoter. We previously found that during in vitro cytotrophoblast differentiation, human INHA gene expression was regulated by TFAP2A via association with an AP-2 site located upstream of this CRE. The aim of this study was to evaluate if the human INHA gene was also regulated by cAMP in trophoblasts, and to investigate the possible crosstalk between TFAP2 and cAMP signaling pathways in the regulation of INHA gene expression. Treatment with cAMP or forskolin increased INHA mRNA expression by 7- and 2-fold in primary cytotrophoblasts and choriocarcinoma-derived BeWo cells, respectively. Treatment with the protein kinase A inhibitor H-89 reduced forskolin-induced luciferase activity by ∼40% in BeWo cells transfected with an INHA promoter-driven luciferase reporter vector. TFAP2 overexpression increased basal luciferase activity, whereas the dominant repressor KCREB abolished it. Surprisingly, mutation of the CRE also eliminated the TFAP2-induced transcription, although TFAP2 overexpression was still able to increase forskolin-induced luciferase activity when the AP-2 binding site, but not the CRE site, was mutated. Thus, INHA gene expression is upregulated by cAMP via CRE in human trophoblasts, and TFAP2 regulates this expression by interacting with CRE.

Inhibin at 90: from discovery to clinical application, a historical review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

Follicle growth, ovulation, and luteal formation in primates and rodents: A comparative perspective

Ovarian function has a great deal of functional overlap between species; antral follicles grow in response to FSH, ovulation involves proteolysis, and the steroidogenic pathway is largely the same. However, embedded in these similarities are important differences that reflect the evolutionary and natural history of species and may focus future research into these critical areas. This review compares ovarian function of rats and mice with primates, focusing on estradiol and follicle growth, steroidogenesis and rupture during the periovulatory interval, and the formation of a functional corpus luteum, drawing the conclusion that careful comparison of species yields more functional information about both than studying them in isolation.

DNA methylation and histone modifications are associated with repression of the inhibin α promoter in the rat corpus luteum

The transition from follicle to corpus luteum after ovulation is associated with profound morphological and functional changes and is accompanied by corresponding changes in gene expression. The gene encoding the α subunit of the dimeric reproductive hormone inhibin is maximally expressed in the granulosa cells of the preovulatory follicle, is rapidly repressed by the ovulatory LH surge, and is expressed at only very low levels in the corpus luteum. Although previous studies have identified transient repressors of inhibin α gene transcription, little is known about how this repression is maintained in the corpus luteum. This study examines the role of epigenetic changes, including DNA methylation and histone modification, in silencing of inhibin α gene expression. Bisulfite sequencing reveals that methylation of the inhibin α proximal promoter is low in preovulatory and ovulatory follicles but is elevated in the corpus luteum. Increased methylation during luteinization is observed within the cAMP response element in the promoter, and EMSA demonstrate that methylation of this site inhibits cAMP response element binding protein binding in vitro. Chromatin immunoprecipitation reveals that repressive histone marks H3K9 and H3K27 trimethylation are increased on the inhibin α promoter in primary luteal cells, whereas the activation mark H3K4 trimethylation is decreased. The changes in histone modification precede the alterations in DNA methylation, suggesting that they facilitate the recruitment of DNA methyltransferases. We show that the DNA methyltransferase DNMT3a is present in the ovary and in luteal cells when the inhibin α promoter becomes methylated and observe recruitment of DNMT3a to the inhibin promoter during luteinization.

Luteinizing hormone regulates inhibin-α subunit expression through multiple signaling pathways involving steroidogenic factor-1 and beta-catenin in the macaque corpus luteum

We employed different experimental model systems to define the role of GATA4, beta-catenin, and steroidogenic factor (SF-1) transcriptional factors in the regulation of monkey luteal inhibin secretion. Reverse transcription polymerase chain reactions and western blotting analyses show high expression of inhibin-α, GATA4, and beta-catenin in corpus luteum (CL) of the mid-luteal phase. Gonadotropin-releasing hormone receptor antagonist-induced luteolysis model suggested the significance of luteinizing hormone (LH) in regulating these transcriptional factors. Inducible cyclic AMP early repressor mRNA expression was detected in the CL and no change was observed in different stages of CL. Following amino acid sequence analysis, interaction between SF-1 and beta-catenin in mid-stage CL was verified by reciprocal co-immunoprecipitation experiments coupled to immunoblot analysis. Electrophoretic mobility shift analysis support the role of SF-1 in regulating luteal inhibin-α expression. Our results suggest a possible multiple crosstalk of Wnt, cAMP, and SF-1 in the regulation of luteal inhibin secretion.

Herpes simplex virus type 2 infection of human epithelial cells induces CXCL9 expression and CD4+ T cell migration via activation of p38-CCAAT/enhancer-binding protein-β pathway

Recruitment of CD4(+) T cells to infection areas after HSV-2 infection may be one of the mechanisms that account for increased HIV-1 sexual transmission. Lymphocytes recruited by chemokine CXCL9 are known to be important in control of HSV-2 infection in mice, although the underlying mechanism remains to be addressed. Based on our observation that CXCL9 expression is augmented in the cervical mucus of HSV-2-positive women, in this study we demonstrate that HSV-2 infection directly induces CXCL9 expression in primary cervical epithelial cells and cell lines, the principal targets of HSV-2, at both mRNA and protein levels. Further studies reveal that the induction of CXCL9 expression by HSV-2 is dependent upon a binding site for C/EBP-β within CXCL9 promoter sequence. Furthermore, CXCL9 expression is promoted at the transcriptional level through phosphorylating C/EBP-β via p38 MAPK pathway, leading to binding of C/EBP-β to the CXCL9 promoter. Chemotaxis assays indicate that upregulation of CXCL9 expression at the protein level by HSV-2 infection enhances the migration of PBLs and CD4(+) T cells, whereas neutralization of CXCL9 or inhibition of p38-C/EBP-β pathway can significantly decrease the migration. Our data together demonstrate that HSV-2 induces CXCL9 expression in human cervical epithelial cells by activation of p38-C/EBP-β pathway through promoting the binding of C/EBP-β to CXCL9 promoter, which may recruit activated CD4(+) T cells to mucosal HSV-2 infection sites and potentially increase the risk of HIV-1 sexual transmission.

Negative regulation of miR-145 by C/EBP-β through the Akt pathway in cancer cells

MicroRNAs are master gene regulators that can also be under the control of transcriptional regulation. We have previously shown that miR-145 is a tumor suppressor capable of silencing c-Myc and the tumor suppressor p53 induces miR-145 by directly binding to the miR-145 promoter, demonstrating the role of miR-145 in p53-mediated c-Myc repression. However, little is known as to why miR-145 is often downregulated in tumors. In this study, we identify CCAAT/enhancer binding protein beta (C/EBP-β) as a negative regulator for miR-145 expression by direct interaction with the putative C/EBP-β binding site in the miR-145 promoter. In the wild-type p53 background, C/EBP-β counteracts the ability of p53 to induce miR-145. Moreover, C/EBP-β is able to suppress miR-145 in the mutant p53 background, suggesting the p53 independent regulation of miR-145. Of interest, both the large isoform (LAP-2) and the small isoform (LIP) of C/EBP-β can exert suppressive function for miR-145. Finally, we further show that, like serum starvation and PI3K inhibitor LY29, the antioxidant resveratrol suppresses pAkt and phosphorylation of C/EBP-β and at the same time, it induces miR-145. Together, these results suggest a miR-145 regulatory system involving the Akt and C/EBP-β, which may contribute to the downregulation of miR-145 in cancer cells.

Role of PCSK5 expression in mouse ovarian follicle development: identification of the inhibin α- and β-subunits as candidate substrates

Inhibin and activin are essential dimeric glycoproteins belonging to the transforming growth factor-beta (TGFβ) superfamily. Inhibin is a heterodimer of α- and β-subunits, whereas activin is a homodimer of β-subunits. Production of inhibin is regulated during the reproductive cycle and requires the processing of pro-ligands to produce mature hormone. Furin is a subtilisin-like proprotein convertase (proconvertase) that activates precursor proteins by cleavage at basic sites during their transit through the secretory pathway and/or at the cell surface. We hypothesized that furin-like proconvertases are central regulators of inhibin α- and β-subunit processing within the ovary. We analyzed the expression of the proconvertases furin, PCSK5, PCSK6, and PCSK7 in the developing mouse ovary by real-time quantitative RT-PCR. The data showed that proconvertase enzymes are temporally expressed in ovarian cells. With the transition from two-layer secondary to pre-antral follicle, only PCSK5 mRNA was significantly elevated. Activin A selectively enhanced expression of PCSK5 mRNA and decreased expression of furin and PCSK6 in cultured two-layer secondary follicles. Inhibition of proconvertase enzyme activity by dec-RVKR-chloromethylketone (CMK), a highly specific and potent competitive inhibitor of subtilisin-like proconvertases, significantly impeded both inhibin α- and β-subunit maturation in murine granulosa cells. Overexpression of PC5/6 in furin-deficient cells led to increased inhibin α- and β_B-subunit maturation. Our data support the role of proconvertase PCSK5 in the processing of ovarian inhibin subunits during folliculogenesis and suggest that this enzyme may be an important regulator of inhibin and activin bioavailability.

The synthesis and secretion of inhibins

Inhibin A and B, dimeric glycoproteins comprising an α- and β((A/B))-subunit, negatively regulate follicle stimulating hormone (FSH) synthesis by the pituitary. The expression of α- and β-subunits within Sertoli cells of the testis and granulosa cells of the ovary is controlled by a range of transcription factors, including CREB, SP-1, Smads, and GATA factors. The inhibin α- and β-subunits are synthesized as precursor molecules consisting of an N-terminal propeptide and a C-terminal mature domain. Recently, we showed that hydrophobic residues within the propeptides of the α- and β-subunits interact noncovalently with their mature domains, maintaining the molecules in a conformation competent for dimerization. Dimeric precursors are cleaved by proprotein convertases and mature inhibins are secreted from the cell noncovalently associated with their propeptides. Propeptides may increase the half-life of inhibin A and B in circulation, but they are readily displaced in the presence of the high-affinity receptors, betaglycan, and ActRII.

CCAAT/enhancer-binding proteins (C/EBP)-α and -β are essential for ovulation, luteinization, and the expression of key target genes

LH activation of the epidermal growth factor receptor/RAS/ERK1/2 pathway is essential for ovulation and luteinization because granulosa cell (GC) depletion of ERK1/2 (ERK1/2(gc)(-/-) mice) renders mice infertile. As mediators of ERK1/2-dependent GC differentiation, the CCAAT/enhancer-binding proteins, (C/EBP)α and C/EBPβ, were also disrupted. Female Cebpb(gc)(-/-) mutant mice, but not Cebpa(gc)(-/-) mice, were subfertile whereas Cebpa/b(gc)(-/-) double-mutant females were sterile. Follicles failed to ovulate, ovaries were devoid of corpora lutea, luteal cell marker genes (Lhcgr, Prlr, Ptgfr, Cyp11a1, and Star) were absent, and serum progesterone levels were low. Microarray analyses identified numerous C/EBPα/β target genes in equine chorionic gonadotropin (eCG)-human (h)CG-treated mice. At 4 h post-hCG, a subset (19%) of genes altered in the Cebpa/b-depleted cells was also altered in ERK1/2-depleted cells; hence they are common effectors of ERK1/2. Additional genes down-regulated in the Cebpa/b-depleted cells at 8 and 24 h post-hCG include known (Akr1b7, Runx2, Star, Saa3) and novel (Abcb1b, Apln, Igfbp4, Prlr, Ptgfr Timp4) C/EBP targets and effectors of luteal and vascular cell development. Bhmt, a gene controlling methionine metabolism and thought to be expressed exclusively in liver and kidney, was high in wild-type luteal cells but totally absent in Cebpa/b mutant cells. Because numerous genes potentially associated with vascular development were suppressed in the mutant cells, C/EBPα/β appear to dictate the luteinization process by also controlling genes that regulate the formation of the extensive vascular network required to sustain luteal cells. Thus, C/EBPα/β mediate the terminal differentiation of GCs during the complex process of luteinization.

RUNX2 transcription factor regulates gene expression in luteinizing granulosa cells of rat ovaries

The LH surge promotes terminal differentiation of follicular cells to become luteal cells. RUNX2 has been shown to play an important role in cell differentiation, but the regulation of Runx2 expression and its function in the ovary remain to be determined. The present study examined 1) the expression profile of Runx2 and its partner CBFbeta during the periovulatory period, 2) regulatory mechanisms of Runx2 expression, and 3) its potential function in the ovary. Runx2 expression was induced in periovulatory granulosa cells of human and rodent ovaries. RUNX2 and core binding factor-beta (CBFbeta) proteins in nuclear extracts and RUNX2 binding to a consensus binding sequence increased after human chorionic gonadotropin (hCG) administration. This in vivo up-regulation of Runx2 expression was recapitulated in vitro in preovulatory granulosa cells by stimulation with hCG. The hCG-induced Runx2 expression was reduced by antiprogestin (RU486) and EGF-receptor tyrosine kinase inhibitor (AG1478), indicating the involvement of EGF-signaling and progesterone-mediated pathways. We also found that in the C/EBPbeta knockout mouse ovary, Runx2 expression was reduced, indicating C/EBPbeta-mediated expression. Next, the function of RUNX2 was investigated by suppressing Runx2 expression by small interfering RNA in vitro. Runx2 knockdown resulted in reduced levels of mRNA for Rgc32, Ptgds, Fabp6, Mmp13, and Abcb1a genes. Chromatin immunoprecipitation analysis demonstrated the binding of RUNX2 in the promoter region of these genes, suggesting that these genes are direct downstream targets of RUNX2. Collectively, the present data indicate that the LH surge-induced RUNX2 is involved in various aspects of luteal function by directly regulating the expression of diverse luteal genes.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

The biology of gonadotroph regulation

PURPOSE OF REVIEW

To discuss recent progress in our understanding of pituitary gonadotroph development and gonadotropin gene regulation, with an emphasis on differential luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion and subunit synthesis, and the implications this may have on female reproductive health.

RECENT FINDINGS

In the mature gonadotroph, there is an emerging concept that differential synthesis of gonadotropin beta-subunit genes, essential for cyclic reproductive function, is associated with modification of activation and/or stability of important regulatory proteins and transcription factors. Recent studies suggest that cellular events, which affect histone modification, play an essential role in both gonadotroph development and the ontogeny of gonadotropin subunit gene expression. Such dynamic events are under the orchestration of the hypothalamic neuropeptide gonadotropin-releasing hormone (GnRH), potentially through the ability of GnRH to activate several distinct signaling cascades within the gonadotroph.

SUMMARY

Greater insight into the cellular events that are key to gonadotroph physiology will contribute to our understanding of abnormal gonadotropin secretion in disorders such as hypothalamic amenorrhea and polycystic ovarian syndrome (PCOS), and provide a context for the design of novel therapeutic approaches.

Suppression of Notch signaling in the neonatal mouse ovary decreases primordial follicle formation

Notch signaling directs cell fate during embryogenesis by influencing cell proliferation, differentiation, and apoptosis. Notch genes are expressed in the adult mouse ovary, and roles for Notch in regulating folliculogenesis are beginning to emerge from mouse genetic models. We investigated how Notch signaling might influence the formation of primordial follicles. Follicle assembly takes place when germ cell syncytia within the ovary break down and germ cells are encapsulated by pregranulosa cells. In the mouse, this occurs during the first 4-5 d of postnatal life. The expression of Notch family genes in the neonatal mouse ovary was determined through RT-PCR measurements. Jagged1, Notch2, and Hes1 transcripts were the most abundantly expressed ligand, receptor, and target gene, respectively. Jagged1 and Hey2 mRNAs were up-regulated over the period of follicle formation. Localization studies demonstrated that JAGGED1 is expressed in germ cells prior to follicle assembly and in the oocytes of primordial follicles. Pregranulosa cells that surround germ cell nests express HES1. In addition, pregranulosa cells of primordial follicles expressed NOTCH2 and Hey2 mRNA. We used an ex vivo ovary culture system to assess the requirement for Notch signaling during early follicle development. Newborn ovaries cultured in the presence of gamma-secretase inhibitors, compounds that attenuate Notch signaling, had a marked reduction in primordial follicles compared with vehicle-treated ovaries, and there was a corresponding increase in germ cells that remained within nests. These data support a functional role for Notch signaling in regulating primordial follicle formation.

C/EBPbeta represses p53 to promote cell survival downstream of DNA damage independent of oncogenic Ras and p19(Arf)

CCAAT/enhancer-binding protein-beta (C/EBPbeta) is a mediator of cell survival and tumorigenesis. When C/EBPbeta(-/-) mice are treated with carcinogens that produce oncogenic Ras mutations in keratinocytes, they respond with abnormally elevated keratinocyte apoptosis and a block in skin tumorigenesis. Although this aberrant carcinogen-induced apoptosis results from abnormal upregulation of p53, it is not known whether upregulated p53 results from oncogenic Ras and its ability to induce p19(Arf) and/or activate DNA-damage response pathways or from direct carcinogen-induced DNA damage. We report that p19(Arf) is dramatically elevated in C/EBPbeta(-/-) epidermis and that C/EBPbeta represses a p19(Arf) promoter reporter. To determine whether p19(Arf) is responsible for the proapoptotic phenotype in C/EBPbeta(-/-) mice, C/EBPbeta(-/-);p19(Arf-/-) mice were generated. C/EBPbeta(-/-);p19(Arf-/-) mice responded to carcinogen treatment with increased p53 and apoptosis, indicating p19(Arf) is not essential. To ascertain whether oncogenic Ras activation induces aberrant p53 and apoptosis in C/EBPbeta(-/-) epidermis, we generated K14-ER:Ras;C/EBPbeta(-/-) mice. Oncogenic Ras activation induced by 4-hydroxytamoxifen did not produce increased p53 or apoptosis. Finally, when C/EBPbeta(-/-) mice were treated with differing types of DNA-damaging agents, including alkylating chemotherapeutic agents, they displayed aberrant levels of p53 and apoptosis. These results indicate that C/EBPbeta represses p53 to promote cell survival downstream of DNA damage and suggest that inhibition of C/EBPbeta may be a target for cancer cotherapy to increase the efficacy of alkylating chemotherapeutic agents.

Investigating the association between inhibin alpha gene promoter polymorphisms and premature ovarian failure

OBJECTIVE

To determine whether variants in the promoter region of the inhibin alpha gene (INHA) are associated with premature ovarian failure (POF).

DESIGN

Mutational analysis of the INHA gene promoter in women with POF.

SETTING

Academic institution.

PATIENT(S)

Patients with POF (n = 194) and controls (n = 162) from New Zealand and Slovenia.

INTERVENTION(S)

Peripheral blood samples were screened for known polymorphisms in the INHA promoter (c.-16C-->T, c.-124A-->G, and an imperfect TG repeat at approximately -300 base pairs). Genotyping was performed by restriction fragment length polymorphism, forced restriction fragment length polymorphism, and nondenaturing high-performance liquid chromatography analysis.

MAIN OUTCOME MEASURE(S)

Genotypic status of INHA promoter polymorphisms.

RESULT(S)

Significant differences in INHA promoter allele frequencies were observed between POF patient populations and controls. Significant reductions in allele frequency were observed for the -16T allele (New Zealand POF) and -124G allele (total POF) and for INHA promoter haplotypes C (New Zealand POF) and D (Slovenian POF).

CONCLUSION(S)

We conclude that INHA promoter variants are associated with the development of POF.

Activin regulates estrogen receptor gene expression in the mouse ovary

Activin, a member of the transforming growth factor-beta superfamily, is an important modulator of follicle-stimulating hormone synthesis and secretion in the pituitary and plays autocrine/paracrine roles in the regulation of ovarian follicle development. From a microarray study on mouse ovarian granulosa cells, we discovered that the estrogen receptor beta (ERbeta) is inducible by activin. We previously demonstrated that estrogen suppresses activin gene expression, suggesting a feedback relationship between these two follicle-regulating hormones. The purpose of this study was to investigate fully activin A regulation of ER expression. Real time reverse transcription-PCR assays on cultured granulosa cells showed that both ERalpha and ERbeta mRNAs were induced by activin A at 4, 12, and 24 h in a dose-responsive manner. Western blots confirmed an increase in their protein levels. Consistent with increased ERalpha and ERbeta expression, activin A stimulated estradiol-induced estrogen response element promoter activity. Activin A stimulation of ER expression was a direct effect at the level of gene transcription, as it was not abolished by cycloheximide but was abolished by actinomycin D, and in transfected granulosa cells activin A stimulated ERalpha promoter activity. To investigate the effect of activin in vivo and, thus, its biological significance, we examined ER expression in inhibin transgenic mice that have decreased activin expression and discovered that these mice had decreased ERalpha and ERbeta expression in the ovary. We also found that ER mRNA levels were decreased in Müllerian inhibiting substance promoter (MIS)-Smad2 dominant negative mice that have impaired activin signaling through Smad2, and small interfering RNAs targeting Smad2 or Smad3 suppressed ERalpha promoter activation, suggesting that Smad2 and Smad3 are involved in regulating ER levels. Therefore, this study reveals an important role for activin in inducing the expression of ERs in the mouse ovary and suggests important interplay between activin and estrogen signaling.

LRH-1/NR5A2 cooperates with GATA factors to regulate inhibin alpha-subunit promoter activity

Inhibin alpha is the common subunit of the dimeric inhibin proteins known for their role in suppressing pituitary FSH secretion. In this study, we have examined the role of GATA factors and the nuclear receptor, LRH-1/NR5A2, in the regulation of inhibin alpha-subunit promoter activity. The inhibin alpha promoter contains two GATA-binding motifs that can be activated by GATA4 or GATA6. The GATA-dependence of the promoter was demonstrated by downregulating GATA expression in MA-10 cells using siRNA technology. We next examined whether GATA factors could cooperate with LRH-1, a factor recently proposed to be an important regulator of inhibin alpha-subunit transcription. Both GATA4 and GATA6 strongly synergized with LRH-1. Consistent with the cAMP-dependence of the inhibin alpha-subunit promoter, GATA/LRH-1 synergism was markedly enhanced by PKA and the co-activator protein CBP. Thus, our results identify LRH-1 as a new transcriptional partner for GATA factors in the regulation of inhibin alpha-subunit gene expression.

Decreased survival of C/EBP beta-deficient keratinocytes is due to aberrant regulation of p53 levels and function

Recent studies have identified roles for C/EBPbeta in cellular survival and tumorigenesis, however, the mechanisms through which C/EBPbeta regulates these processes are not fully understood. Previously, we demonstrated that C/EBPbeta(-/-) mice are resistant to carcinogen-induced skin tumorigenesis and in response to topical carcinogen treatment display a 17-fold increase in keratinocyte apoptosis compared to wild-type mice. Here, we have investigated the mechanisms through which C/EBPbeta regulates apoptosis in response to carcinogenic stress. Analysis of carcinogen-treated C/EBPbeta(-/-) mouse skin revealed a striking increase in the number of p53 immunopositive keratinocytes in the epidermis of C/EBPbeta(-/-) mice compared to wild-type mice and this increase was temporally associated with a concomitant anomalous increase in apoptosis. The increased levels of p53 were functional as Mdm2, Bcl-2, C/EBPalpha and p21 were differentially regulated in the epidermis of carcinogen-treated C/EBPbeta(-/-) mice. The increase in p53 protein was not associated with an increase in p53 mRNA levels. To determine whether p53 is required for the increased apoptosis in C/EBPbeta(-/-) mice, C/EBPbeta/p53 compound knockout mice were generated. Carcinogen-treated C/EBPbeta/p53 compound knockout mice did not display increased apoptosis demonstrating p53 is required for the proapoptotic phenotype in C/EBPbeta(-/-) mice. Our results demonstrate that altered keratinocyte survival in C/EBPbeta(-/-) mice results from aberrant regulation of p53 protein and function and indicate C/EBPbeta has a role in the negative regulation of p53 protein levels in response to carcinogen-induced stress.

Transcriptional activation of the steroidogenic acute regulatory protein (StAR) gene: GATA-4 and CCAAT/enhancer-binding protein beta confer synergistic responsiveness in hormone-treated rat granulosa and HEK293 cell models

Steroidogenic acute regulatory protein (StAR) mediates translocation of cholesterol to the inner membranes of steroidogenic mitochondria, where it serves as a substrate for steroid synthesis. Transcription of StAR in the gonads and adrenal cells is upregulated by trophic hormones, involves downstream signaling pathways and a cohort of trans-factors acting as activators or suppressors of StAR transcription. This study suggests that a 21 basepair long sequence positioned at -81/-61 of the murine StAR promoter is sufficient to confer a robust hormonal activation of transcription in ovarian granulosa cells treated with FSH. We show that recombinant GATA-4 and CCAAT/enhancer-binding protein beta (C/EBPbeta) bind to the promoter at -66/-61 and -81/-70 and activate transcription of a reporter gene when co-expressed in heterologous human embryonic kidney 293 (HEK293) cells. In this cell model, C/EBPbeta and GATA-4 synergize in a sequence dependent manner and p300/CBP further maximizes their joint activities. Inhibitors of the transcriptional activators, such as liver-enriched inhibiting protein (C/EBPbeta-LIP), Friend of GATA-4 (FOG-2) protein and the viral E1A protein abolished the respective factor-dependent activities in HEK293 cells. Binding assays suggest that a dual binding of C/EBPbeta and GATA-4 to the promoter depends on the molar ratio of the factors present while demonstrating GATA-4 predominant association with the promoter DNA. This pattern may reflect on StAR expression at the time of corpus luteum formation when C/EBPbeta levels peak, as does StAR expression.

Mechanism of Repression of the Inhibin α-Subunit Gene by Inducible 3′,5′-Cyclic Adenosine Monophosphate Early Repressor

The rodent ovary is regulated throughout the reproductive cycle to maintain normal cyclicity. Ovarian follicular development is controlled by changes in gene expression in response to the gonadotropins FSH and LH. The inhibin alpha-subunit gene belongs to a group of genes that is positively regulated by FSH and negatively regulated by LH. Previous studies established an important role for inducible cAMP early repressor (ICER) in repression of alpha-inhibin. These current studies investigate the mechanisms of repression by ICER. It is not clear whether all four ICER isoforms expressed in the ovary can act as repressors of the inhibin alpha-subunit gene. EMSAs demonstrate binding of all isoforms to the inhibin alpha-subunit CRE (cAMP response element), and transfection studies demonstrate that all isoforms can repress the inhibin alpha-subunit gene. Repression by ICER is dependent on its binding to DNA as demonstrated by mutations to ICER's DNA-binding domain. These mutational studies also demonstrate that repression by ICER is not dependent on heterodimerization with CREB (CRE-binding protein). Competitive EMSAs show that ICER effectively competes with CREB for binding to the inhibin alpha CRE in vitro. Chromatin immunoprecipitation assays demonstrate a replacement of CREB dimers bound to the inhibin alpha CRE by ICER dimers in ovarian granulosa cells in response to LH signaling. Thus, there is a temporal association of transcription factors bound to the inhibin alpha-CRE controlling inhibin alpha-subunit gene expression.

Transcriptional repression of peroxisome proliferator-activated receptor beta/delta in murine keratinocytes by CCAAT/enhancer-binding proteins

The roles of peroxisome proliferator-activated receptors (PPARs) and CCAAT/enhancer-binding proteins (C/EBPs) in keratinocyte and sebocyte differentiation suggest that both families of transcription factors closely interact in the skin. Initial characterization of the mouse PPARbeta promoter revealed an AP-1 site that is crucial for the regulation of PPARbeta expression in response to inflammatory cytokines in the skin. We now present evidence for a novel regulatory mechanism of the expression of the PPARbeta gene by which two members of the C/EBP family of transcription factors inhibit its basal promoter activity in mouse keratinocytes. We first demonstrate that C/EBPalpha and C/EBPbeta, but not C/EBPdelta, inhibit the expression of PPARbeta through the recruitment of a transcriptional repressor complex containing HDAC-1 to a specific C/EBP binding site on the PPARbeta promoter. Consistent with this repression, the expression patterns of PPARbeta and C/EBPs are mutually exclusive in keratinocytes of the interfollicular epidermis and hair follicles in mouse developing skin. This work reveals the importance of the regulatory interplay between PPARbeta and C/EBP transcription factors in the control of proliferation and differentiation in this organ. Such insights are crucial for the understanding of the molecular control regulating the balance between proliferation and differentiation in many cell types including keratinocytes.