Mechanisms of insulin-like growth factor I augmentation of follicle-stimulating hormone-induced porcine steroidogenic acute regulatory protein gene promoter activity in granulosa cells

Molecular crosstalk between insulin-like growth factors and follicle-stimulating hormone in the regulation of granulosa cell function

Background

The last phase of folliculogenesis is driven by follicle-stimulating hormone (FSH) and locally produced insulin-like growth factors (IGFs), both essential for forming preovulatory follicles.

Methods

This review discusses the molecular crosstalk of the FSH and IGF signaling pathways in regulating follicular granulosa cells (GCs) during the antral-to-preovulatory phase.

Main findings

IGFs were considered co-gonadotropins since they amplify FSH actions in GCs. However, this view is not compatible with data showing that FSH requires IGFs to stimulate GCs, that FSH renders GCs sensitive to IGFs, and that FSH signaling interacts with factors downstream of AKT to stimulate GCs. New evidence suggests that FSH and IGF signaling pathways intersect at several levels to regulate gene expression and GC function.

Conclusion

FSH and locally produced IGFs form a positive feedback loop essential for preovulatory follicle formation in all species. Understanding the mechanisms by which FSH and IGFs interact to control GC function will help design new interventions to optimize follicle maturation, perfect treatment of ovulatory defects, improve in vitro fertilization, and develop new contraceptive approaches.

Effect of IGF1 and FSH on the function of granulosa cells from prehierarchal follicles in chickens†

Insulin-like growth factor 1 (IGF1) is an essential regulator of mammalian follicle development and synergizes with follicle-stimulating hormone (FSH) to amplify its effects. In avian preovulatory follicles, IGF1 increases the expression of genes involved in steroidogenesis and progesterone and inhibin A production. The role of IGF1 in prehierarchal follicles has not been well studied in chickens. The aim of this study was to investigate the role of IGF1 in granulosa cells from prehierarchal follicles and to determine whether IGF1 and FSH synergize to promote follicle development. Granulosa cells of 3-5 and 6-8 mm prehierarchal follicles were cultured with IGF1 (0, 10, 100 ng/mL) in the presence or absence of FSH (0, 10 ng/mL). Cell proliferation, expression of genes important in follicle development (FSHR, IGF1R, AMH, STAR, CYP11A1, INHA, and INHBA), and progesterone production were evaluated following treatment. IGF1 treatment alone significantly increased STAR, CYP11A1, and INHBA mRNA expression and cell proliferation in granulosa cells of 6-8 mm follicles. IGF1 and FSH synergized to increase STAR mRNA expression in 6-8 mm follicles. IGF1 and FSH co-treatment were necessary to increase INHA mRNA expression in 6-8 mm follicles. Although IGF1 significantly increased the expression of genes involved in steroidogenesis, progesterone production in granulosa cells of 6-8 mm follicles was not affected. IGF1 did not affect AMH mRNA expression, although FSH significantly decreased AMH expression in granulosa cells of 3-5 mm follicles. These results suggest that IGF1 may act with FSH to promote follicle selection at the prehierarchal follicle stage.

A brief history of the search for the protein(s) involved in the acute regulation of steroidogenesis

The synthesis of steroid hormones occurs in specific cells and tissues in the body in response to trophic hormones and other signals. In order to synthesize steroids de novo, cholesterol, the precursor of all steroid hormones, must be mobilized from cellular stores to the inner mitochondrial membrane (IMM) to be converted into the first steroid formed, pregnenolone. This delivery of cholesterol to the IMM is the rate-limiting step in this process, and has long been known to require the rapid synthesis of a new protein(s) in response to stimulation. Although several possibilities for this protein have arisen over the past few decades, most of the recent attention to fill this role has centered on the candidacies of the proteins the Translocator Protein (TSPO) and the Steroidogenic Acute Regulatory Protein (StAR). In this review, the process of regulating steroidogenesis is briefly described, the characteristics of the candidate proteins and the data supporting their candidacies summarized, and some recent findings that propose a serious challenge for the role of TSPO in this process are discussed.

Hypoxia-inducible factor 1 mediates hypoxia-enhanced synthesis of progesterone during luteinization of granulosa cells

Hypoxia has been suggested to enhance progesterone (P4) synthesis in luteinizing granulosa cells (GCs), but the mechanism is unclear. The present study was designed to test the hypothesis that the hypoxia-induced increase in P4 synthesis during luteinization in bovine GCs is mediated by hypoxia-inducible factor 1 (HIF-1). GCs obtained from small antral follicles were cultured with 2 µg/ml insulin in combination with 10 µM forskolin for 24 h as a model of luteinizing GCs. To examine the influence of HIF-1 on P4 synthesis, we determined the effect of changes in protein expression of the α-subunit of HIF-1 (HIF1A) on P4 production and on the expression levels of StAR, P450scc, and 3β-HSD. CoCl₂ (100 µM), a hypoxia-mimicking chemical, increased HIF-1α protein expression in luteinizing GCs. After the upregulation of HIF-1α, we observed an increase in P4 production and in the gene and protein expression levels of StAR in CoCl₂-treated luteinizing GCs. In contrast, CoCl₂ did not affect the expression of either P450scc or 3β-HSD. Echinomycin, a small-molecule inhibitor of HIF-1's DNA-binding activity, attenuated the effects of CoCl₂ and of low oxygen tension (10% O₂) on P4 production and StAR expression in luteinizing GCs. Overall, these findings suggest that HIF-1 is one of the factors that upregulate P4 in GCs during luteinization.

Insulin Receptor Substrate 1, the Hub Linking Follicle-stimulating Hormone to Phosphatidylinositol 3-Kinase Activation

The ubiquitous phosphatidylinositol 3-kinase (PI3K) signaling pathway regulates many cellular functions. However, the mechanism by which G protein-coupled receptors (GPCRs) signal to activate PI3K is poorly understood. We have used ovarian granulosa cells as a model to investigate this pathway, based on evidence that the GPCR agonist follicle-stimulating hormone (FSH) promotes the protein kinase A (PKA)-dependent phosphorylation of insulin receptor substrate 1 (IRS1) on tyrosine residues that activate PI3K. We report that in the absence of FSH, granulosa cells secrete a subthreshold concentration of insulin-like growth factor-1 (IGF-1) that primes the IGF-1 receptor (IGF-1R) but fails to promote tyrosine phosphorylation of IRS1. FSH via PKA acts to sensitize IRS1 to the tyrosine kinase activity of the IGF-1R by activating protein phosphatase 1 (PP1) to promote dephosphorylation of inhibitory Ser/Thr residues on IRS1, including Ser(789). Knockdown of PP1β blocks the ability of FSH to activate PI3K in the presence of endogenous IGF-1. Activation of PI3K thus requires both PKA-mediated relief of IRS1 inhibition and IGF-1R-dependent tyrosine phosphorylation of IRS1. Treatment with FSH and increasing concentrations of exogenous IGF-1 triggers synergistic IRS1 tyrosine phosphorylation at PI3K-activating residues that persists downstream through protein kinase B (AKT) and FOXO1 (forkhead box protein O1) to drive synergistic expression of genes that underlies follicle maturation. Based on the ability of GPCR agonists to synergize with IGFs to enhance gene expression in other cell types, PP1 activation to relieve IRS1 inhibition may be a more general mechanism by which GPCRs act with the IGF-1R to activate PI3K/AKT.

Growth Differentiation Factor-8 Decreases StAR Expression Through ALK5-Mediated Smad3 and ERK1/2 Signaling Pathways in Luteinized Human Granulosa Cells

Growth differentiation factor-8 (GDF-8) has been recently shown to be expressed in human granulosa cells, and the mature form of GDF-8 protein can be detected in the follicular fluid. However, the biological function and significance of this growth factor in the human ovary remains to be determined. Here, we investigated the effects of GDF-8 on steroidogenic enzyme expression and the potential mechanisms of action in luteinized human granulosa cells. We demonstrated that treatment with GDF-8 did not affect the mRNA levels of P450 side-chain cleavage enzyme and 3β-hydroxysteroid dehydrogenase, whereas it significantly down-regulated steroidogenic acute regulatory protein (StAR) expression and decreased progesterone production. The suppressive effect of GDF-8 on StAR expression was abolished by the inhibition of the TGF-β type I receptor. In addition, treatment with GDF-8 activated both Smad2/3 and ERK1/2 signaling pathways. Furthermore, knockdown of activin receptor-like kinase 5 reversed the effects of GDF-8 on Smad2/3 phosphorylation and StAR expression. The inhibition of Smad3 or ERK1/2 signaling pathways attenuated the GDF-8-induced down-regulation of StAR and production of progesterone. Interestingly, the concentrations of GDF-8 were negatively correlated with those of progesterone in human follicular fluid. These results indicate a novel autocrine function of GDF-8 to down-regulate StAR expression and decrease progesterone production in luteinized human granulosa cells, most likely through activin receptor-like kinase 5-mediated Smad3 and ERK1/2 signaling pathways. Our findings suggest that granulosa cells might play a critical role in the regulation of progesterone production to prevent premature luteinization during the final stage of folliculogenesis.

T-2 toxin regulates steroid hormone secretion of rat ovarian granulosa cells through cAMP-PKA pathway

T-2 toxin is a secondary metabolite produced by Fusarium genus and is a common contaminant in food and feedstuffs of cereal origin. In porcine granulosa cells(GC), T-2 toxin has been shown to inhibit the steroidogenesis; however, the mechanism has not been well understood. Gonadotropin-stimulated steroidogenesis is regulated by the cAMP-PKA pathway. In this study, we investigated potential mechanisms for T-2 toxin-induced reproductive toxicity focusing on the critical steps of the cAMP-PKA pathway affected by T-2 toxin. We first analyzed the effects of T-2 toxin on progesterone and estrogen production in rat granulosa cells. For this purpose the granulosa cells were cultured for 48 h in 10% fetal bovine serum-containing medium followed by 24h in serum-free medium containing FSH (10 ng/ml) and androstenedione (3 ng/ml), both are required for normal steroidogenesis. Treatment of these cells with T-2 toxin dose-dependently inhibited the growth of cells and the steroid hormone production. Cellular cyclic AMP levels were dose-dependently inhibited by T-2 toxin (0, 1, 10 and 100 nM, 24 h). Furthermore, we found that although the induction of progesterone by 8-Br-cAMP (a FSH mimetic) and 22R-HC (substrate for progesterone) could both be inhibited by T-2 toxin treatment, the T-2-imposed inhibitory effects could be reversed by increasing doses of 22R-HC, while increasing 8-Br-cAMP had no effects, suggesting that T2 toxin targeted at distinct mechanisms. cAMP-stimulated steroidogenic acute regulatory protein (StAR) is a rate limiting protein in progesterone synthesis. Exposure to T2 toxin caused significant suppression of StAR expression as determined by Western blotting and semi-quantitative RT-PCR suggesting StAR is a sensitive target for T-2 toxin. Taken together, our results strongly suggest that T2 toxin inhibits steroidogenesis by suppressing cAMP-PKA pathway and StAR is a target for T-2-toxin. The antisteroidogenesis effects were observable at low T-2 dose (1 ng/ml) suggesting T-2 toxin has an endocrine disruptive effect.

Hypoxia promotes progesterone synthesis during luteinization in bovine granulosa cells

To determine whether hypoxia has an effect on luteinization, we examined the influence of hypoxia on a model of bovine luteinizing and non-luteinizing granulosa cell culture. The granulosa cells were obtained from small antral follicles (≤ 6 mm in diameter). To induce luteinization, the cells were treated for 24 h with insulin (2 µg/ml), forskolin (10 µM) or insulin in combination with forskolin at 20% O2. After 24 h, progesterone (P4) production was higher in the treated cells, which we defined as luteinizing granulosa cells, than in non-treated cells, which we defined as non-luteinizing granulosa cells. P4 production by non-luteinizing granulosa cells was not affected by hypoxia (24 h at 10% and 5% O2), while P4 production by granulosa cells treated with insulin in combination with forskolin was significantly increased under hypoxia (24 h at 10% and 5% O2). Because hypoxia affected P4 production by the luteinizing granulosa cells but not by the non-luteinizing granulosa cells, hypoxia seems to promote P4 production during, rather than before, luteinization. In the cells treated with insulin in combination with forskolin, mRNA and protein expression of steroidogenic acute regulatory protein (StAR) and protein expression of 3β-hydroxysteroid dehydrogenase (3β-HSD) increased under 10% O2, while mRNA and protein expressions of key protein and enzymes in P4 biosynthesis did not increase under 5% O2. The overall results suggest that hypoxia plays a role in progressing and completing the luteinization by enhancing P4 production through StAR as well as 3β-HSD expressions in the early time of establishing the corpus luteum.

Insulin-like growth factor 1 physiology: lessons from mouse models

Insulin-like growth factor 1 (IGF-1) is a pleiotropic polypeptide. Its expression is tightly regulated and it plays significant roles during early development, maturation, and adulthood. This article discusses the roles of IGF-1 in determination of body size, skeletal acquisition, muscle growth, carbohydrate metabolism, and longevity, as learned from mouse models.

GATA depletion impacts insulin-like growth factor 1 mRNA and protein levels in luteinizing porcine granulosa cells

GATA4 and GATA6 are zinc-finger transcription factors that regulate specific genes involved in steroidogenesis. Using RNA interference (RNAi)-mediated reduction of GATA4 and/or GATA6 with microarray analysis, we aimed to identify novel GATA target genes in luteinizing porcine granulosa cells under vehicle- and cAMP-treated conditions. Microarray analysis identified IGF1 mRNA to be cAMP- and GATA-responsive, and real-time PCR demonstrated that the cAMP-induced increase in IGF1 mRNA was reduced under conditions of GATA6 depletion and GATA4 plus GATA6 depletion, but not GATA4 depletion. Insulin-like growth factor 1 protein levels in media were also decreased by GATA6 or GATA4 plus GATA6 reduction. IGFBP2 and IGFBP4 mRNAs were increased and IGFBP5 mRNA decreased with vehicle and cAMP treatment under GATA4 plus GATA6 RNAi conditions. GATA6 reduction alone increased basal IGFBP4 and decreased IGFBP5 with both vehicle and cAMP, and GATA4 reduction alone lowered cAMP IGFBP5 levels with cAMP. No changes in IGFBP3 mRNA were observed with GATA reduction relative to the control RNAi condition. Levels of insulin-like growth factor binding proteins 2-5 in media as assessed by Western ligand blotting were not altered by GATA reduction. Electromobility gel shift assays with two GATA-containing oligonucleotides of the IGF1 5'-regulatory region showed GATA4 and GATA6 could bind the more proximal GATA-B site. These studies indicate that although GATA4 and GATA6 can bind the porcine IGF1 5'-region, GATA6 is functionally most important for cAMP-stimulated mRNA levels. Using microarray analysis, we identified other mRNAs that were altered by GATA-reduced conditions, including ALDH1, DIO2, and EDNRB. Our findings further support GATA as a coordinator of endocrine/paracrine/autocrine signals in the ovary.

Identification of a novel distal control region upstream of the human steroidogenic acute regulatory protein (StAR) gene that participates in SF-1-dependent chromatin architecture

StAR (steroidogenic acute regulatory protein) mediates the transport of cholesterol from the outer to the inner mitochondrial membrane, the process of which is the rate-limiting step for steroidogenesis. Transcriptional regulation of the proximal promoter of the human StAR gene has been well characterized, whereas analysis of its distal control region has not. Recently, we found that SF-1 (steroidogenic factor 1) induced the differentiation of mesenchymal stem cells (MSCs) into steroidogenic cells with the concomitant strong induction of StAR expression. Here, we show, using differentiated MSCs, that StAR expression is regulated by a novel distal control region. Using electrophoretic mobility shift (EMSA) and chromatin immunoprecipitation (ChIP) assays, we identified novel SF-1 binding sites between 3,000 and 3,400 bp upstream of StAR. A luciferase reporter assay revealed that the region worked as a strong regulator to exert maximal transcription of StAR. ChIP analysis of histone H3 revealed that upon SF-1 expression, nucleosome eviction took place at the SF-1 binding sites, not only in the promoter but also in the distal SF-1 binding sites. Chromosome conformation capture analysis revealed that the region upstream of StAR formed a chromatin loop both in the differentiated MSCs and in KGN cells, a human granulosa cell tumor cell line, where SF-1 is endogenously expressed. Finally, SF-1 knockdown resulted in disrupted formation of this chromatin loop in KGN cells. These results indicate that the novel distal control region participate in StAR activation through SF-1 dependent alterations of chromatin structure, including histone eviction and chromatin loop formation.

GATA4 reduction enhances 3',5'-cyclic adenosine 5'-monophosphate-stimulated steroidogenic acute regulatory protein messenger ribonucleic acid and progesterone production in luteinized porcine granulosa cells

Previous studies with cultured granulosa cells implicated GATA4 in gonadotropin regulation of the steroidogenic acute regulatory protein (STAR) gene. Caveats to these prior studies exist. First, GATA4 levels are reduced in granulosa-luteal cells after the LH surge when GATA6 expression is relatively high. Second, STAR mRNA expression is negligible in granulosa cells until after the LH surge. Both exogenous GATA4 and GATA6 can transactivate STAR gene promoter constructs. We used an RNA interference (RNAi) approach to determine the contributions of GATA4 and GATA6 to cAMP analog regulation of the endogenous STAR gene in luteinizing granulosa cells. STAR mRNA was stimulated by cAMP under control RNAi conditions. Surprisingly, GATA4 reduction by its respective RNAi approximately doubled the cAMP induction of STAR mRNA. At 24 h cAMP treatment, this augmentation was abolished by co-down-regulation of GATA4+GATA6. GATA6 down-regulation by itself did not alter STAR mRNA levels. GATA4+GATA6 co-down-regulation elevated basal CYP11A mRNA at 24 h treatment but did not affect its induction by cAMP. Basal levels of HSD3B mRNA were reduced by GATA4 RNAi conditions leading to a greater fold induction of its mRNA by cAMP. Fold cAMP-stimulated progesterone production was enhanced by GATA4 down-regulation but not by GATA4+GATA6 co-down-regulation. These data implicate GATA6 as the facilitator in cAMP-stimulated STAR mRNA and downstream progesterone accumulation under reduced GATA4 conditions. Data also demonstrate that basal levels of GATA4/6 are not required for cAMP induction of the STAR gene. The altered ratio of GATA4 to GATA6 after ovulation may allow GATA6 to enhance STAR mRNA accumulation.

Regulation of Kruppel-like factor 4, 9, and 13 genes and the steroidogenic genes LDLR, StAR, and CYP11A in ovarian granulosa cells

Kruppel-like factors (KLFs) are important Sp1-like eukaryotic transcriptional proteins. The LDLR, StAR, and CYP11A genes exhibit GC-rich Sp1-like sites, which have the potential to bind KLFs in multiprotein complexes. We now report that KLF4, KLF9, and KLF13 transcripts are expressed in and regulate ovarian cells. KLF4 and 13, but not KLF9, mRNA expression was induced and then repressed over time (P < 0.001). Combined LH and IGF-I stimulation increased KLF4 mRNA at 2 h (P < 0.01), whereas LH decreased KLF13 mRNA at 6 h (P < 0.05), and IGF-I reduced KLF13 at 24 h (P < 0.01) compared with untreated control. KLF9 was not regulated by either hormone. Transient transfection of KLF4, KLF9, and KLF13 suppressed LDLR/luc, StAR/luc, and CYP11A/luc by 80-90% (P < 0.001). Histone-deacetylase (HDAC) inhibitors stimulated LDLR/luc five- to sixfold and StAR/luc and CYP11A/luc activity twofold (P < 0.001) and partially reversed suppression by all three KLFs (P < 0.001). Deletion of the zinc finger domain of KLF13 abrogated repression of LDLR/luc. Lentiviral overexpression of the KLF13 gene suppressed LDLR mRNA (P < 0.001) and CYP11A mRNA (P = 0.003) but increased StAR mRNA (P = 0.007). Collectively, these data suggest that KLFs may recruit inhibitory complexes containing HDAC corepressors, thereby repressing LDLR and CYP11A transcription. Conversely, KLF13 may recruit unknown coactivators or stabilize StAR mRNA, thereby explaining enhancement of in situ StAR gene expression. These data introduce new potent gonadal transregulators of genes encoding proteins that mediate sterol uptake and steroid biosynthesis.

Transcriptional regulation of adrenocortical steroidogenic gene expression

By serving as ligands for nuclear and plasma membrane receptors, steroid hormones are key regulators of a diverse array of physiological processes. These hormones are synthesized from cholesterol in tissues such as the adrenal cortex, ovaries, testes, and placenta. Because steroid hormones control the expression of numerous genes, steroidogenic cells utilize multiple mechanisms that ensure tight control of the synthesis of these molecules. This review will give an overview of the molecular mechanisms by which the expression of steroidogenic genes is regulated in the human adrenal cortex.

Polymorphism in the insulin-like growth factor 1 gene is associated with age at menarche in caucasian females

BACKGROUND

The insulin-like growth factor 1 (IGF1) gene, which plays a crucial role in hypothalamic-pituitary-ovarian hormone-controlled metabolic processes, may influence the onset of menarche. Our study aimed to test association between IGF1 polymorphisms with the variation of age at menarche (AAM) in Caucasian females.

METHODS

We recruited a sample of 1048 females from 354 Caucasian nuclear families and genotyped 19 single-nucleotide polymorphisms (SNPs) spanning the entire IGF1 gene. Pairwise linkage disequilibrium among SNPs was measured, and the haplotype blocks were inferred. Both single SNP markers and haplotypes were tested for association with AAM using the quantitative transmission disequilibrium test.

RESULTS

Significant association (P = 0.0153) between AAM and SNP3 (rs6214) in block1 was detected.

CONCLUSIONS

Our results suggested a potential effect of SNP3 in the IGF1 gene on AAM variation in Caucasian women for the first time. However, further independent studies are needed to confirm our findings.

Molecular mechanisms of insulin-like growth factor 1 promoted synthesis and retention of hyaluronic acid in porcine oocyte-cumulus complexes

The purpose of the present study was to elucidate signaling pathways by which insulin like-growth factor 1 (IGF1) promotes FSH-stimulated synthesis and retention of hyaluronic acid (HA) in pig oocyte-cumulus complexes (OCCs) cultured in serum-free medium. We found that IGF1 had no effects on FSH-stimulated production of cAMP and activation of protein kinase A in the OCCs. Immunoblotting with phospho-specific antibodies showed that FSH moderately phosphorylated v-akt murine thymoma viral oncogene homolog (AKT) and mitogen-activated kinase 3 and 1 (MAPK3/1) in cumulus cells. The exposure of OCCs to both FSH and IGF1 resulted in a significant (P < 0.05) increase in AKT and MAPK3/1 phosphorylation. An inhibitor of phosphoinositide-3-kinase (PIK3), LY 294002, significantly (P < 0.05) reduced the IGF1-enhanced phosphorylation of AKT, and inhibitors of AKT (SH6) and MAPK3/1 (U0126) significantly (P < 0.05) decreased the synthesis and retention of HA stimulated by concomitant exposure of OCCs to both FSH and IGF1. The IGF1-promoted synthesis of HA was not accompanied by an increase in the relative abundance of hyaluronan synthase 2 (HAS2) mRNA in the cumulus cells. We conclude that IGF1 promotes the FSH-stimulated synthesis and retention of HA in pig OCCs by PIK3/AKT- and MAPK3/1-dependent mechanisms.

Ovarian steroids: the good, the bad, and the signals that raise them

Ovarian steroid production and subsequent local steroid-mediated signaling are critical for normal ovarian processes, including follicle growth, oocyte maturation, and ovulation. In contrast, elevated steroidogenesis and/or increased steroid signaling in the ovary can lead to profound ovarian pathology, such as polycystic ovarian syndrome, the leading cause of infertility in reproductive age women. Through the use of several in vitro and animal models, great strides have been made toward characterizing the mechanisms regulating local steroid production and action in the ovary. Examples of this progress include insights into luteinizing hormone (LH)- and growth factor-mediated signaling, steroidogenic acute regulatory protein (StAR) activation, and both genomic and nongenomic steroid-mediated signaling in somatic and germ cells, respectively. The following review will address these advances, focusing on how this rapidly expanding knowledge base can be used to better understand female reproduction, and to further improve treatments for common diseases of infertility.

Molecular Mechanisms of Insulin-like Growth Factor-I Mediated Regulation of the Steroidogenic Acute Regulatory Protein in Mouse Leydig Cells

Growth factors are known to play diverse roles in steroidogenesis, a process regulated by the mitochondrial steroidogenic acute regulatory (StAR) protein. The mechanism of action of one such growth factor, IGF-I, was investigated in mouse Leydig tumor (mLTC-1) cells to determine its potential role in the regulation of StAR expression. mLTC-1 cells treated with IGF-I demonstrated temporal and concentration-dependent increases in StAR expression and steroid synthesis. However, IGF-I had no effect on cytochrome P450 side-chain cleavage or 3beta-hydroxysteroid dehydrogenase protein levels. IGF-I was capable of augmenting N,O'-dibutyrl-cAMP-stimulated steroidogenic responsiveness in these cells. The steroidogenic potential of IGF-I was also confirmed in primary cultures of isolated mouse Leydig cells. IGF-I increased phosphorylation of ERK1/2, an event inhibited by the MAPK/ERK inhibitors, PD98059 and U0126. Interestingly, inhibition of ERK activity enhanced IGF-I-mediated StAR protein expression, but phosphorylation of StAR was undetectable, an observation in contrast to that seen with N,O'-dibutyrl-cAMP signaling. Further studies demonstrated that these events were tightly correlated with the expression of dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 and scavenger receptor class B type 1. Whereas both protein kinase A and protein kinase C signaling were involved in the IGF-I-mediated steroidogenic response, the majority of the effects of IGF-I were found to be mediated by the protein kinase C pathway. Transcriptional activation of the StAR gene by IGF-I was influenced by several transcription factors, its up-regulation being dependent on phosphorylation of the cAMP response element-binding protein (CREB) and the activator protein 1 family member, c-Jun. Conversely, StAR gene transcription was markedly inhibited by expression of nonphosphorylatable CREB (Ser(133)Ala), dominant negative A-CREB, and dominant negative c-Jun (TAM-67) mutants. Collectively, the present studies identify molecular events in IGF-I signaling that may influence testicular growth, development, and the Leydig cell steroidogenic machinery through autocrine/paracrine regulation.

Stimulation of the novel estrogen receptor-alpha intronic TERP-1 promoter by estrogens, androgen, pituitary adenylate cyclase-activating peptide, and forskolin, and autoregulation by TERP-1 protein

The estrogen receptor-alpha (ERalpha) pituitary-specific variant, TERP-1, is regulated dramatically by physiological status. We examined hormonal regulation of the TERP-1 promoter in transient transfection assays in GH3 somatolactotrope cells. We found that 17beta-estradiol (E2), genistein, androgen, pituitary adenylate cyclase-activating peptide, and forskolin (FSK) all stimulated TERP-1 promoter activity, whereas progesterone had no effect. ERalpha bound to a palindromic estrogen response element (ERE) and two half-site EREs; mutation of any of these sites decreased basal expression and completely obliterated E2 stimulation. In contrast, mutation of an activator protein-1 site decreased basal and FSK-stimulated promoter activity, but not E2 or androgen stimulation. The pure antiestrogen ICI 182,780 suppressed E2 and genistein, but not FSK or androgen, stimulation. Similarly, mutation of the ERE palindrome or half-site EREs suppressed promoter stimulation by E2 and genistein, but not by androgen or FSK. Because TERP-1 levels regulate ERalpha function on model promoters, we tested TERP-1 modulation of its own and other physiological promoters. TERP-1 suppressed basal and E2-stimulated expression of its own promoter. TERP-1 suppression required the ERE regions of the promoter, and the dimerization domain of TERP-1. TERP-1 overexpression also suppressed E2 stimulation of the progesterone receptor and prolactin promoters. Thus, estrogens, androgen, and FSK can stimulate TERP-1 promoter activity, and increased TERP-1 expression modulates E2 stimulation of physiological promoters. These data suggest that TERP-1 regulation may play a significant role in modifying pituitary ERalpha responses.

Expression analysis of the steroidogenic acute regulatory protein (STAR) gene in developing porcine conceptuses

Steroidogenesis in porcine non-conceptus tissue is regulated by the STAR-dependent transport of cholesterol from the outer to inner mitochondrial membrane. Previous serial analysis of gene expression (SAGE) identified a STAR mRNA transcript in the porcine peri-implantation conceptus during trophectoderm elongation and increased conceptus estrogen synthesis between gestational day 11 and 12. To assess a potential role for STAR in the modulation of conceptus steroidogenesis via cholesterol transport, the conceptus STAR transcript was PCR cloned and temporal expression of mRNA and protein were examined. Northern analysis of day 12 corpora lutea and pig conceptus RNA detected multiple STAR transcripts in both tissues and identified the cloned transcript as the longest variant. The transcript had a 99% similarity to a truncated ovarian STAR transcript. The conceptus STAR transcript was temporally regulated during elongation but trace expression was present in day 6 blastocysts and day 25 conceptuses. Differential regulation of STAR mRNA was concomitant with the presence of the stimulatory transcription factor steroidogenic factor 1, and absence of the inhibitory transcription factor dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene-1, transcripts. In contrast to peak STAR mRNA expression at the filamentous stage, Western blot analyses revealed STAR protein levels were highest in tubular conceptuses. These data confirm the presence of STAR mRNA and protein during porcine conceptus elongation and suggest regulation of STAR at two levels, transcriptionally, in part, through differential regulation of transcription factors, and post-transcriptionally, evidenced by the disparity of protein to RNA in filamentous conceptuses.

Association of the mSin3A-histone deacetylase 1/2 corepressor complex with the mouse steroidogenic acute regulatory protein gene

Several factors have been identified in the transcriptional repression of the steroidogenic acute regulatory protein (StAR) gene promoter; yet, no associating corepressor complexes have been characterized for the mouse promoter in MA-10 mouse Leydig tumor cells. We now report that Sp3, CAGA element binding proteins, and a corepressor complex consisting of mSin3A, histone deacetylase (HDAC)1, and HDAC2 associates with a transcriptional repressor region within the mouse StAR promoter. 5'-Promoter deletion analysis localized the negative regulatory region between -180 and -150 bp upstream of the transcription start site, and mutations in both the CAGA and Sp binding elements were required to relieve the repression of basal StAR promoter activity. Protein-DNA binding analysis revealed Sp3 and specific CAGA element-binding protein(s) associated with the repressor region. Coimmunoprecipitation analysis identified the presence of the mSin3A, HDAC1, and HDAC2 corepressor complex in MA-10 cells. Furthermore, chromatin immunoprecipitation assays revealed Sp3, mSin3A, and HDAC1/2 association with the proximal region of the StAR promoter in situ. In addition, HDAC inhibition resulted in a dose-dependent activation of a mouse StAR reporter construct, whereas mutations within the repressor region diminished this effect by 44%. In sum, these data support a novel regulatory mechanism for transcriptional repression of the mouse StAR promoter by DNA binding of Sp3 and CAGA element-binding proteins, and association of the Sin3 corepressor complex exhibiting HDAC activity.

Epidermal growth factor-mediated inhibition of follicle-stimulating hormone-stimulated StAR gene expression in porcine granulosa cells is associated with reduced histone H3 acetylation

Steroidogenic acute regulatory protein (StAR) mediates cholesterol transport into the mitochondria and is essential for ovarian steroidogenesis. Epidermal growth factor (EGF) has been reported to inhibit FSH-stimulated differentiation in porcine granulosa cells. Previous studies have demonstrated FSH stimulates StAR mRNA accumulation and gene promoter activation in granulosa cells. Treatment of granulosa cells with FSH (5 ng/ml, 6 h) increased StAR mRNA, whereas coaddition of EGF (10 ng/ ml) significantly reduced (P < 0.05) FSH-stimulated mRNA accumulation by 62.7% +/- 13.9%. Under these same conditions, FSH-stimulated cAMP accumulation in cultures was unaltered by coincubation with EGF. RNA stability studies showed that cotreatment with FSH and EGF did not alter the StAR mRNA half-life compared with FSH alone, t(1/2) = 1.9-3.8 and 2.7-4.1 h, respectively. EGF significantly inhibited (P < 0.05) FSH-stimulated StAR heterogeneous nuclear RNA levels by 47.6% +/- 6.8 %, implicating a repressive effect on transcription. Surprisingly, EGF (1-50 ng/ml) did not affect FSH stimulation of a 1423-base pair StAR gene promoter-luciferase construct in transient transfection assays in porcine granulosa cells. To evaluate FSH and EGF effects on the endogenous StAR gene, chromatin immunoprecipitation assays were performed in combination with real-time polymerase chain reaction. FSH increased histone H3 acetylation (lysines 9, 14) within the proximal region of the StAR gene promoter and coincubation with EGF blocked this effect. Dimethylation (lysine 9) of histone H3 was not influenced by treatments. In conclusion, EGF repression of FSH-stimulated StAR transcription in porcine granulosa cells is accompanied by reductions in histone H3 acetylation associated with the StAR gene promoter.

Interactions Between Androgen and Growth Factors in Granulosa Cell Subtypes of Porcine Antral Follicles1

Androgens acting via the androgen receptor (AR) have been implicated in regulation of folliculogenesis in many animal species. These effects are possibly mediated via enhancement of FSH and/or insulin-like growth factor (IGF)-I activity in granulosa cells, which contain high levels of AR protein. We examined the in vitro effect of dihydrotestosterone (DHT) on DNA synthesis and progesterone secretion by follicular cells in response to FSH and IGF-I, alone or in combination. Cells from separate pools of 1- to 3-mm and 3- to 5-mm antral follicles were aspirated from gilt ovaries and fractioned into mural granulosa cells (MGCs) and cumulus-oocyte complexes (COCs) for subsequent cell culture. Androgen alone or with any combination of mitogen had minimal effect on proliferative and no effect on steroidogenic responses of MGCs from 3- to 5-mm antral follicles. Conversely, in MGCs from 1- to 3-mm follicles, DHT significantly enhanced IFG-I-stimulated proliferation and had variable influence on progesterone secretion. The effects of DHT on proliferative responses of COCs were also dependent on follicle size: DHT significantly augmented either IGF-I-stimulated proliferation (1- to 3-mm follicles) or FSH-stimulated proliferation (3- to 5-mm follicles). However, the steroidogenic responses of all COCs were identical, whereby DHT significantly suppressed progesterone secretion, predominantly in the presence of FSH. Addition of an AR antagonist, hydroxyflutamide, generally reversed the proliferative responses invoked by DHT but not the steroidogenic responses. We conclude that androgen-receptor-mediated activity in granulosa cells of antral follicles is dependent on follicle size, is influenced by proximity of cells to the oocyte, and possibly involves both classic and nonclassic steroid mechanisms.

Downregulation of steroidogenic acute regulatory protein (StAR) gene expression by cyclic AMP in cultured Schwann cells

Steroidogenic acute regulatory protein (StAR) plays a key role in the availability of cholesterol to the inner mitochondrial membrane, where the first step of steroidogenesis, its conversion to pregnenolone, takes place. Here, we demonstrate for the first time that the StAR gene is also expressed in the rat sciatic nerve and in cultured Schwann cells. The addition to the culture medium of the cAMP-elevating agent forskolin or of the cAMP analogue 8Br-cAMP produced a time-course extinction of StAR gene expression. An inverse relationship was demonstrated between StAR gene expression and the intracellular cAMP content. Accordingly, pharmacological inhibition of the activities of Schwann cell adenylyl cyclase or of phosphodiesterase IV resulted in modifications of StAR gene expression. Since StAR gene expression is stimulated by cAMP in classical steroidogenic cells, our work is the first demonstration of a negative regulation of StAR gene by cAMP.

Characterization of binding between SF-1 and Sp1: predominant interaction of SF-1 with the N-terminal region of Sp1

The transcription factor specificity protein 1 (Sp1) binds to GC boxes and interacts with many transcription factors to regulate gene expression. Steroidogenic factor-1 (SF-1) is an orphan nuclear receptor and plays a major role in regulation of the human steroidogenic acute regulatory (StAR) gene. We demonstrated that there is interaction between SF-1 and Sp1 on the human StAR promoter. In the present study, we examined the mechanism of the interaction between Sp1 and SF-1 on the human StAR gene promoter. Results of glutathione S-transferase (GST) pull-down assays and a mammalian two-hybrid assay showed that SF-1 interacted with Sp1 through the N-terminal domains of Sp1. Results of electrophoretic mobility shift assays using nuclear extracts showed that Sp1 is associated with SF-1-DNA complex formation. The density of SF-1-DNA complex was much greater when recombinant Sp1 was added to the incubation mixture. These results suggest that Sp1 interacts with SF-1 and that Sp1 enhances SF-1-DNA complex formation to regulate human StAR transcription.

Cholesterol transport and steroidogenesis by the corpus luteum

The synthesis of progesterone by the corpus luteum is essential for the establishment and maintenance of early pregnancy. Regulation of luteal steroidogenesis can be broken down into three major events; luteinization (i.e., conversion of an ovulatory follicle), luteal regression, and pregnancy induced luteal maintenance/rescue. While the factors that control these events and dictate the final steroid end products are widely varied among different species, the composition of the corpus luteum (luteinized thecal and granulosa cells) and the enzymes and proteins involved in the steroidogenic pathway are relatively similar among all species. The key factors involved in luteal steroidogenesis and several new exciting observations regarding regulation of luteal steroidogenic function are discussed in this review.

ACTH modulation of transcription factors responsible for steroid hydroxylase gene expression in the adrenal cortex

Steroid hormone biosynthesis in the adrenal cortex and gonads involves the coordinated transcription of the genes encoding the steroid hydroxylases, 3beta-hydroxysteroid dehydrogenase (3betaHSD), the steroidogenic acute regulatory protein (StAR), and adrenodoxin (Adx). Transcriptional regulation of steroidogenic genes is multifactorial, entailing developmental, tissue-specific, constitutive, and cAMP-dependent mechanisms. Optimal steroidogenic capacity is achieved by the actions of ACTH which exerts transcriptional pressure on all steroidogenic genes. The actions of ACTH in the adrenal cortex have been studied in great detail and is mediated by cAMP and protein kinase A (PKA) via two temporally distinct pathways. The acute response leads to mobilization of cholesterol, the initial substrate for all steroidogenic pathways, from cellular stores to the inner mitochondrial membrane where cholesterol sidechain cleavage cytochrome P450 (P45011A1) resides. The slower, chronic response of ACTH in the adrenal cortex directs transcription of the genes encoding the steroidogenic enzymes. Although steroidogenic gene transcription in response to ACTH is cAMP-dependent, the consensus cAMP response pathway (CRE/CREB) is not involved. Instead, each steroidogenic gene utilizes unique cAMP-responsive sequences (CRS) found in the promoters of each gene, which bind a diverse array of transcription factors. Moreover, once specific transcription factors are bound to the promoters of the steroidogenic genes, increased gene expression requires posttranslational modification (phosphorylation/dephosphorylation) of the transcription factors and binding of coactivator proteins. This review provides a general view (with emphasis on the human) of the important factors involved in regulating steroidogenic gene expression and ultimately steroid hormone biosynthesis.

Adrenocorticotropin regulation of steroidogenic acute regulatory protein

We studied the effect of the adrenocorticotropin hormone (ACTH) on the expression of the steroidogenic acute regulatory protein (StAR) in vivo in rat and hamster adrenals and also in transfection experiments using COS-1 cells. In vivo, ACTH increased the level of StAR mRNA within 30-60 minutes and also increased the quantity of StAR, but with a 2-3-hour delay. ACTH induced the formation of many acidic StAR species as analyzed by two-dimensional gel electrophoresis and immunoblotting. In the transfection experiments, (Bu)(2)-cAMP also induced the formation of many acidic species for the hamster StAR; in COS-1 cells, StAR is phosphorylated mainly on serine (S) residue(s). When alanine (A) was substituted for serine, S13A, S185A, and S194A mutants had decreased StAR activity compared to wildtype, thus determining the importance of these amino acid residues in StAR action. The full-length WT, N46-truncated StAR lacking its mitochondrial import sequence, and N46-S194A had similar activities, whereas N46-S185A had completely lost its activity. Our results suggest that S194, but not S185, functions in association with the mitochondrial import sequence for the initiation of StAR activation. Further studies showed that S185 is implicated in salt bridge stability, not in StAR phosphorylation, suggesting its importance for StAR folding. Thermodynamic calculations of the hamster StAR homology model based on MLN64 show that StAR can partially unfold to bind cholesterol and serve as a rapid transfer mechanism for eventual translocation into mitochondria. This is supportive of a StAR functioning either outside the mitochondria or in the mitochondrial intermembrane space.

GATA-4 and GATA-6 transcription factors: expression, immunohistochemical localization, and possible function in the porcine ovary

The expression and localization of GATA-4 and GATA-6 mRNAs and proteins were assessed in porcine ovaries at different stages of the estrous cycle. Reverse transcription polymerase chain reaction and Western blot analyses revealed that GATA-4 and GATA-6 transcripts and proteins were strongly expressed in granulosa cells isolated from antral follicles, intact antral follicles, corpora hemorrhagica (CH), and midluteal phase corpora lutea (CL). Immunoblot analyses showed two predominant proteins with molecular masses of approximately 53 and 55 kDa for GATA-4 and one 55-kDa protein for GATA-6. Immunohistochemical studies revealed GATA-4 and GATA-6 nuclear staining in granulosa cells of healthy primordial and primary antral follicles and antral follicle of various sizes. The percentage of immunopositive thecal cell nuclei increased with follicular development. In CH and CL, luteal cells displayed nuclear immunoreactivity for both transcription factors. Regressing CL showed a decrease in GATA-immunopositive cells. Immunoreactivity for GATA-4 and GATA-6 was present in most blood vessels. In electrophoretic mobility shift assays, nuclear protein extracts isolated from granulosa cells and CL exhibited both GATA-4 and GATA-6 binding to a GATA consensus oligonucleotide, with GATA-4 the predominant binding protein. GATA-4 and GATA-6 DNA binding was elevated in granulosa cell nuclear extracts from preovulatory (8-10 mm) follicles. Cotransfection of primary cultures of luteinizing granulosa cells with GATA-4 or GATA-6 expression vectors increased the activity of the porcine steroidogenic acute regulatory protein gene promoter significantly but did not significantly activate the inhibin alpha gene promoter. The detection of GATA-4 and GATA-6 mRNAs and proteins in porcine ovaries and the identification of at least one possible target gene may help to establish roles for these GATA factors in follicular development and luteal function.

Relationship Between Steroidogenic Acute Regulatory Protein Expression and Progesterone Production in Hen Granulosa Cells During Follicle Development1

The present studies were conducted to address cellular mechanisms responsible for regulating steroidogenic acute regulatory protein (StAR) expression and progesterone synthesis at maturational stages corresponding to both the time of hen follicle selection, as well as before and after the LH surge in preovulatory follicle granulosa cells. A recently published report has established that mitogen-activated protein (MAP) kinase signaling induced by transforming growth factor alpha (TGFalpha) treatment blocks FSH-induced differentiation and StAR expression in cultured hen granulosa cells, whereas inhibitors of MAP kinase signaling enhance FSH-induced differentiation. The present in vitro studies demonstrate that in addition to MAP kinase signaling, activation of protein kinase C (PKC) blocks both FSH-induced StAR expression and the initiation of progesterone production in prehierarchal follicle granulosa cells, whereas the pharmacologic inhibitor of PKC, GF109203X, potentiates FSH-induced StAR expression and, as a consequence, the initiation of progesterone synthesis. Moreover, we demonstrate in granulosa cells collected from preovulatory follicles that although an acute increase in progesterone production in response to LH treatment requires rapid transcription and translation of StAR, the magnitude of progesterone production is rate-limited by one or more factors other than StAR (e.g., the P450 cholesterol side-chain enzyme). Finally, the rapid turnover of StAR protein, such as occurs following the withdrawal of LH, provides an additional mechanism for the tight regulation of progesterone production that occurs during the hen ovulatory cycle, and explains the rapid loss of steroidogenesis in the postovulatory follicle. In summary, data reported herein support the proposal that paracrine/autocrine factors (including but not necessarily limited to TGFalpha) prevent premature expression of StAR in prehierarchal follicle granulosa cells by more than one receptor-mediated signaling pathway. Furthermore, subsequent to follicle selection into the preovulatory hierarchy, StAR transcription and translation is necessary but not sufficient for the full potentiation of the preovulatory surge of serum progesterone.

Cloning and characterization of porcine ovarian estrogen receptor beta isoforms

The cDNA for the full-length porcine estrogen receptor beta (ER beta) and an alternatively spliced transcript with a deletion of exon 5 (ER beta delta 5) was cloned from pig ovary. RNase protection assays revealed that ER beta mRNA was expressed in the preovulatory follicles and early, midluteal, and regressing corpora lutea (CL) of eCG +/- hCG-primed gilts. ER beta and ER beta delta 5 transcripts were shown by semiquantitative reverse transcription polymerase chain reaction to be expressed at a ratio of approximately 2:1 in granulosa cells, small, medium, and large antral follicles, and midluteal phase corpora lutea of unprimed animals. Immunoreactive ER beta proteins corresponding to the size of in vitro translated ER beta and ER beta delta 5 were detected by immunoblot. Full-length ER beta was detected in granulosa, small, medium, and large antral follicles, and midluteal phase CL of unprimed animals. Putative ER beta delta 5 immunoreactive bands were abundant only in granulosa cell extracts. In COS-1 cells, transfected ER beta delta 5 had no effect on basal transcription of an estrogen-responsive reporter construct but did repress wild-type ER beta transactivation when cotransfected at 10-fold excess plasmid. No repression of ER alpha transactivation was observed. In primary granulosa cell cultures, transfected ER beta delta 5 plasmid did not inhibit basal reporter activation. ER beta delta 5 was shown by immunofluorescence to localize to the nucleus in transfected COS-1 cells. In vitro translated ER beta delta 5 proteins bound estrogen response elements in DNA in electrophoretic mobility shift assays, as indicated by supershift analysis. ER beta is abundant in porcine ovary, and a naturally occurring splice variant missing exon 5 may have biological function.

Regulation of steroidogenesis and the steroidogenic acute regulatory protein by a member of the cAMP response-element binding protein family

The mitochondrial phosphoprotein, the steroidogenic acute regulatory (StAR) protein, is an essential component in the regulation of steroid biosynthesis in adrenal and gonadal cells through cAMP-dependent pathways. In many cases transcriptional induction by cAMP is mediated through the interaction of a cAMP response-element binding protein (CREB) family member with a consensus cAMP response element (CRE; 5'-TGACGTCA-3') found in the promoter of target genes. The present investigation was carried out to determine whether a CRE-binding protein (CREB) family member [CREB/CRE modulator (CREM) family] was involved in the regulation of steroidogenesis and StAR protein expression. Transient expression of wild- type CREB in MA-10 mouse Leydig tumor cells further increased the levels of (Bu)2cAMP-induced progesterone synthesis, StAR promoter activity, StAR mRNA, and StAR protein. These responses were significantly inhibited by transfection with a dominant-negative CREB (A-CREB), or with a CREB mutant that cannot be phosphorylated (CREB-M1), the latter observation indicating the importance of phosphorylation of a CREB/CREM family member in steroidogenesis and StAR expression. The CREB/CREM-responsive region in the mouse StAR gene was located between -110 and -67 bp upstream of the transcriptional start site. An oligonucleotide probe (-96/-67 bp) containing three putative half-sites for 5'-canonical CRE sequences (TGAC) demonstrated the formation of protein-DNA complexes in EMSAs with recombinant CREB protein as well as with nuclear extracts from MA-10 or Y-1 mouse adrenal tumor cells. The predominant binding factor observed with EMSA was found to be the CREM protein as demonstrated using specific antibodies and RT-PCR analyses. The CRE elements identified within the -96/-67 bp region were tested for cAMP responsiveness by generating mutations in each of the CRE half-sites either alone or in combination. Although each of the CRE sites contribute in part to the CREM response, the CRE2 appears to be the most important site as determined by EMSA and by reporter gene analyses. Binding specificity was further assessed using specific antibodies to CREB/CREM family members, cold competitors, and mutations in the target sites that resulted in either supershift and/or inhibition of these complexes. We also demonstrate that the inducible cAMP early repressor markedly diminished the endogenous effects of CREM on cAMP-induced StAR promoter activity and on StAR mRNA expression. These are the first observations to provide evidence for the functional involvement of a CREB/CREM family member in the acute regulation of trophic hormone-stimulated steroidogenesis and StAR gene expression.

RIP 140 modulates transcription of the steroidogenic acute regulatory protein gene through interactions with both SF-1 and DAX-1

Coregulators have been suggested to act as a bridging apparatus between nuclear receptors and the transcriptional machinery. The orphan receptor SF-1 plays a role in controlling the basal and cAMP-stimulated expression of the human steroidogenic acute regulatory protein gene. DAX-1 is the gene responsible for X-linked adrenal hypoplasia congenita and blocks steroid biosynthesis by impairing the expression of steroidogenic acute regulatory protein. In the present study we examined the role of coregulators in the actions of SF-1 and DAX-1 on the human steroidogenic acute regulatory protein promoter. We found that the coregulator RIP 140 interacts with SF-1 in the yeast two-hybrid system. Glutathione-S-transferase pull-down assays and coimmunoprecipitations confirmed the interaction between RIP 140 and SF-1. RIP 140 was also shown to interact with DAX-1. When an RIP 140 expression vector was introduced into Y-1 cells, basal and cAMP-stimulated human steroidogenic acute regulatory protein promoter activities decreased. The inhibitory effect of RIP 140 on human steroidogenic acute regulatory protein promoter activity was dependent upon the presence of SF-1. The cAMP response of an SF-1 response element was inhibited by both RIP 140 and DAX-1 expression vectors at low concentrations of plasmids. We conclude that RIP 140 binds to the orphan nuclear receptor SF-1 and DAX-1 and modulates their actions on the human steroidogenic acute regulatory protein promoter.

Quantitative analysis of the hormone-induced hyperacetylation of histone H3 associated with the steroidogenic acute regulatory protein gene promoter

Transcriptional regulation of steroidogenic acute regulatory protein (StAR) determines adrenal and gonadal cell steroidogenesis. Chromatin immunoprecipitation assays were combined with quantitative real-time polymerase chain reaction to assess histone acetylation associated with the StAR promoter. MA-10 cells treated with 8-bromo-cAMP had increased acetylated histone H3 associated with the proximal (but not distal) StAR promoter, nascent StAR transcripts, and progesterone production within 15 min, whereas StAR mRNA increased at 30 min. At 360 min, steroidogenesis remained elevated, but mRNA, nascent RNA, and StAR promoter-associated H3 acetylation all declined. StAR promoter-associated H4 acetylation was unchanged by 8-bromo-cAMP treatment of MA-10 cells. In vivo analysis of macaque and human granulosa cells showed that luteinization was associated with increased StAR promoter-associated H3 acetylation. We conclude that acetylation of H3 (but not H4) associated with the proximal promoter is associated with StAR gene transcription, that chromatin modification occurs in discrete regions of the promoter, that the initial steroidogenic response to 8-bromo-cAMP occurs prior to increased StAR mRNA accumulation, and that MA-10 cell StAR gene transcription and promoter-associated H3 acetylation are biphasic during a 6-h treatment period. The union of the chromatin immunoprecipitation assay with quantitative real-time polymerase chain reaction described and validated here should enhance the analysis of gene expression.

Are estrogens of import to primate/human ovarian folliculogenesis?

The notion that estrogens play a meaningful role in ovarian folliculogenesis stems from a large body of in vitro and in vivo experiments carried out in certain rodent models, (e.g., rats) wherein the stimulatory role of estrogen on granulosa cell growth and differentiation is undisputed. However, evidence derived from these polyovulatory species may not be readily generalizable to the monoovulatory subhuman primates, let alone the human. Only recently, significant observations on the ovarian role(s) of estrogen have been reported for the primate/human. It is thus the objective of this communication to review the evidence for and against a role for estrogens in primate/human ovarian follicular development with an emphasis toward the application of the concepts so developed to contemporary reproductive physiology and to the practice of reproductive medicine. The role(s) of estrogens will be examined not only by analyzing the physiological evidence to the effect that these hormones control ovarian function and follicular growth, but also by summarizing the molecular evidence for the existence and distribution of the cognate receptors.

Post-translational regulation of steroidogenic acute regulatory protein by cAMP-dependent protein kinase A

Adrenal steroid production is stimulated by adrenocorticotropin hormone activation of the cAMP-dependent protein kinase A (PKA) signaling pathway and subsequent induction of Steroidogenic Acute Regulatory (StAR) protein expression. Herein we have compared StAR mRNA and protein levels in 8-Br-cAMP-treated mouse adrenocortical Y1 and the derived PKA mutant Kin-8 cell lines to evaluate the PKA requirement in StAR expression. StAR mRNA was induced by 8-Br-cAMP-treatment of both Y1 and Kin-8 cells with maximal expression levels in Kin-8 cells approximately 50% of that observed in Y1 cells. StAR protein levels, as detected by Western analysis, were concomitantly increased in Y1 cells but were not detected in the Kin-8 cells. StAR mRNA colocalized with the active polysome fractions in both 8-Br-cAMP-treated Y1 and Kin-8 cells, indicating translation was not blocked in Kin-8 cells. Consistent with this data, a 2-fold increase in incorporation of [35S]methionine into StAR was also observed after 8-Br-cAMP treatment of both cell lines. Since StAR protein levels were not sufficient to detect by Western analysis, these data indicate that PKA functions at the post-translational level to regulate StAR expression and we propose that phosphorylation of StAR by PKA contributes to protein stability.

Sp1 and SF-1 interact and cooperate in the regulation of human steroidogenic acute regulatory protein gene expression

Steroidogenic acute regulatory (StAR) protein plays a critical role in the movement of cholesterol from the outer to the inner mitochondrial membrane. Steroidogenic factor 1 (SF-1) controls basal and cAMP-stimulated transcription of the StAR gene. The 1.3-kb StAR promoter has three SF-1 binding sites, and two consensus transcription factor Spl binding sequences near the two most distal SF-1 binding sites. Spl mediates cAMP-dependent transcription of steroidogenic P450 enzyme genes, raising the possibility of Sp1 involvement in cAMP regulation of the StAR gene. However, the mechanism of Spl-mediated, cAMP-stimulated responsiveness is not known. In this study, we elucidated the roles of Sp1 and SF-1 in the regulation of the human StAR gene promoter. We found that there was negligible promoter activity in a pGL2 StAR construct (-235 to +39) in which Spl and SF-1 binding sites were mutated in Y-1 adrenal tumor cells. An Sp1 binding site mutation (pGL2Sp1M) did not support promoter activity, suggesting that Spl cooperates with SF-1 in regulating StAR promoter function. In gel shift assays, the SF-1 binding site formed a complex with an SF-1-GST fusion protein and Spl. Coimmunoprecipitation cross-linking experiments indicated that SF-1 physically interacts with Sp1 in vitro. Finally, a mammalian two-hybrid system was employed to demonstrate that Spl and SF-1 associate in vivo. In conclusion, our data indicate that Spl and SF-1 physically interact and cooperate in the regulation of human StAR promoter activity.

Calcium ions positively modulate follicle-stimulating hormone- and exogenous cyclic 3',5'-adenosine monophosphate-driven transcription of the P450(scc) gene in porcine granulosa cells

Given the evident modulation of FSH-induced steroidogenesis by Ca2+ in granulosa cells, we here test the hypothesis that Ca2+ controls expression of the enzymatically rate-limiting cytochrome P450(scc) (CYP11A) gene. To test this postulate, we quantitated the ability of Ca2+ to regulate: 1) transcriptional activity of a transiently transfected luciferase reporter gene driven by a 2.32-kb 5'-upstream fragment of the porcine P450(scc) gene promoter region; and 2) accumulation of endogenous P450(scc) transcripts in primary monolayer cultures of porcine granulosa cells. To this end, granulosa cells were stimulated for 4 h with FSH (15 ng/ml, NIDDK-oFSH-20) or 8-Bromo-cAMP (8 Br-cAMP, 1 mM) in serum-free medium containing either 1.8 mM Ca2- or no added Ca2+ with 100 microM EGTA or 100 microM CoCl2. In the presence of extracellular Ca2+, FSH and 8 Br-cAMP stimulated expression of the transfected P450(scc) promoter-reporter fusion construct by 5.6 +/- 1.1 and 3.6 +/- 0.67-fold, respectively over Ca2+-containing unstimulated control (P < or = 0.04, n = 5-6 experiments). The foregoing two agonists augmented 4-h progesterone production by cultured granulosa cells by 1.8 +/- 0.11 and 1.6 +/- 0.16-fold, respectively (P < or = 0.001 for FSH and P < or = 0.01 for 8 Br-cAMP). FSH and 8 Br-cAMP also significantly elevated endogenous P450(scc) transcript levels as measured by homologous solution-hybridization RNase protection assay; i.e. by 3.1 +/- 0.49 and 2.9 +/- 0.45-fold, respectively (P < or = 0.001). In Ca2+-free/EGTA-supplemented medium, basal luciferase reporter-gene activity and endogenous P450(scc) messenger RNA accumulation in granulosa cells declined to 34 +/- 12% and 78 +/- 12%, respectively, of corresponding values in control (unstimulated Ca2+-containing) cultures. Extracellular Ca2+ deprivation inhibited the stimulatory effect of FSH (and 8 Br-cAMP) on P450(scc) promoter-luciferase reporter expression to 58 +/- 30% (and 58 +/- 23%), and restrained endogenous P450(scc) message accumulation to 86 +/- 15% (and 96 +/- 18%) of the value in Ca2+-containing control. Extracellular Ca2+ withdrawal suppressed FSH (and 8 Br-cAMP)-driven progesterone production over 4 h to basal levels but did not alter FSH-stimulated cAMP accumulation by granulosa cells. Ca2+-deprived cells exposed to serum-containing media regained P450(scc) responsiveness to both agonists. Antagonism of cellular uptake of Ca2+ and other divalent cations via administration of cobalt chloride (100 microM) inhibited FSH and 8 Br-cAMP's stimulation of endogenous (but not exogenous promoter-driven) P450(scc) gene expression. In contrast, granulosa-cell concentrations of messenger RNA's encoding sterol-carrier protein-2 (SCP-2) and the low density lipoprotein receptor were not altered by Ca2+ withdrawal. In summary, uptake of extracellular Ca2+ by porcine granulosa cells significantly potentiates transactivation of the endogenously expressed and exogenously transfected P450(scc) gene by FSH and 8 Br-cAMP. The agonistic impact of Ca2+ on P450(scc) promoter activity is requisite downstream of FSH-induced cAMP second-messenger signaling.

Models of luteinization

Luteinization is essential to the success of early gestation. It is the process by which elements of the ovarian follicle, usually including both theca interna and granulosa cells, are provoked by the ovulatory stimulus to develop into the corpus luteum. Although there are significant species differences in luteinization, some elements pervade, including the morphological and functional differentiation to produce and secrete progesterone. There is evidence that luteinization results in granulosa cell exit from the cell cycle. The mechanisms that appear to control luteinization include intracellular signalling pathways, cell adhesion factors, intracellular cholesterol and oxysterols, and perhaps progesterone itself as a paracrine or intracrine regulator. Cell models of luteinization, along with some of the conflicting observations on the luteinization process, are discussed in this review.

Regulation by adrenocorticotropin (ACTH), angiotensin II, transforming growth factor-beta, and insulin-like growth factor I of bovine adrenal cell steroidogenic capacity and expression of ACTH receptor, steroidogenic acute regulatory protein, cytochrome P450c17, and 3beta-hydroxysteroid dehydrogenase

The purpose of this study was to evaluate the time-course effect of a 36-h treatment with ACTH (10(-8) M), transforming growth factor-beta1 (TGFbeta1; 10(-10) M), angiotensin II (AngII; 10 (-7) M), and insulin-like growth factor I (IGF-I; 10(-8) M) on the steroidogenic capacity of bovine adrenocortical cells (BAC) and on messenger RNA (mRNA) levels of ACTH receptor, cytochrome P450c17, 3beta-hydroxysteroid dehydrogenase (3betaHSD), steroidogenic acute regulatory protein (StAR), and StAR protein. ACTH and IGF-I enhanced, in a time-dependent manner, the acute 2-h ACTH-induced cortisol production, whereas TGFbeta 1 and AngII markedly reduced it. ACTH, IGF-I, and AngII increased ACTH receptor mRNA, but the opposite was observed after TGFbeta1 treatment. ACTH and IGF-I increased P450c17 and 3betaHSD mRNAs, whereas AngII and TGFbeta1 had the opposite effects. However, the effects of the four peptides on ACTH-induced cortisol production appeared before any significant alterations of the mRNA levels occurred. The most marked and rapid effect of the four peptides was on StAR mRNA. The stimulatory effect of ACTH was seen within 1.5 h, peaked at 4-6 h, and declined thereafter, but at the end of the 36-h pretreatment, the levels of StAR mRNA and protein were higher than those in control cells. IGF-I also enhanced StAR mRNA levels within 1.5 h, and these levels remained fairly constant. The effects of AngII on StAR mRNA expression were biphasic, with a peak within 1.5-3 h, followed by a rapid decline to almost undetectable levels of both mRNA and protein. TGFbeta1 had no significant effect during the first 3 h, but thereafter StAR mRNA declined, and at the end of the experiment the StAR mRNA and protein were almost undetectable. Similar results were observed when cells were treated with ACTH plus TGFbeta1. A 2-h acute ACTH stimulation at the end of the 36-h pretreatment caused a higher increase in StAR mRNA and protein in ACTH- or IGF-I-pretreated cells than in control cells, which, in turn, had higher levels than cells pretreated with TGFbeta1, ACTH plus TGFbeta1, or AngII. These results and the fact that the stimulatory (IGF-I) or inhibitory (AngII and TGFbeta1) effects on ACTH-induced cortisol production were more pronounced than those on the ability of cells to transform pregnenolone into cortisol strongly suggest that regulation of StAR expression is one of the main factors, but not the only one, involved in the positive (IGF-I) or negative (TGFbeta1 and AngII) regulation of BAC for ACTH steroidogenic responsiveness. A high correlation between steady state mRNA level and acute ACTH-induced cortisol production favors this conclusion.

Steroidogenic factor-1 influences protein-deoxyribonucleic acid interactions within the cyclic adenosine 3,5-monophosphate-responsive regions of the murine steroidogenic acute regulatory protein gene

De novo synthesis of the steroidogenic acute regulatory protein (StAR) in response to trophic hormonal stimulation of steroidogenic cells is required for the delivery of cholesterol from the mitochondrial outer membrane to the mitochondrial inner membrane and the cytochrome P450 side-chain cleavage enzyme. StAR expression is transcriptionally regulated by cAMP-mediated mechanisms, and we have identified a 45-bp region within the mouse promoter that is important for cAMP responsiveness of the gene. This region, located between -105 and -60 of the start site of transcription, contains a SF-1-binding site, a highly conserved C/EBPbeta-AP-1-nuclear receptor half-site sequences (CAN region), and a GATA-4-binding site. The SF-1 element and CAN region are required for full basal activity, whereas the GATA-4 element may account for 20% of the cAMP response in MA-10 mouse Leydig tumor cells. A cAMP-dependent protein-DNA complex was observed with the CAN region and mutation of a nonconsensus AP-1 site within this region greatly diminished promoter strength. Complex protein-DNA interactions within the cAMP response region (-105/-60) were shown to require the SF-1 element (-95), suggesting that SF-1 is required for protein-DNA interaction at the CAN (-79) region and maximal activity of the promoter.

Steroidogenic acute regulatory (StAR) protein: what's new?

In response to trophic hormone stimulation of steroidogenic adrenal and gonadal cells, the acute biosynthesis of steroid hormones occurs in the order of minutes to tens of minutes and can be contrasted to chronic regulation, which occurs on the order of hours. The steroidogenic acute regulatory (StAR) protein is an indispensable component in the acute regulatory phase and functions by rapidly mediating the transfer of the substrate for all steroid hormones, cholesterol, from the outer to the inner mitochondrial membrane where it is cleaved to pregnenolone, the first steroid formed. This transfer of cholesterol constitutes the rate-limiting step in steroidogenesis. To underscore its importance, mutations in the StAR gene have been shown to be the only cause of the potentially fatal disease lipoid congenital adrenal hyperplasia, in which affected individuals synthesize virtually no steroids. Since the cloning of the murine cDNA in 1994, many observations have substantiated the critical role of StAR in regulated steroidogenesis. The purpose of this review will be to summarize briefly some background material on StAR and then attempt to update several recent and interesting findings on the StAR protein.

CCAAT enhancer-binding protein beta and GATA-4 binding regions within the promoter of the steroidogenic acute regulatory protein (StAR) gene are required for transcription in rat ovarian cells

Steroidogenic acute regulatory protein (StAR) is a vital accessory protein required for biosynthesis of steroid hormones from cholesterol. The present study shows that in primary granulosa cells from prepubertal rat ovary, StAR transcript and protein are acutely induced by gonadotropin (FSH). To determine the sequence elements required for hormone inducibility of the StAR promoter, truncated regions of the -1002/+6 sequence of the mouse gene were ligated to pCAT-Basic plasmid and transfected by electroporation to freshly prepared cells. FSH inducibility determined over a 6-h incubation was 10-40-fold above basal levels of chloramphenicol acetyltransferase activity. These functional studies, supported by electrophoretic mobility shift assays indicated that two sites were sufficient for transcription of the StAR promoter constructs: a non-consensus binding sequence (-81/-72) for CCAAT enhancer-binding protein beta (C/EBPbeta) and a consensus motif for GATA-4 binding (-61/-66). Western analyses showed that GATA-4 is constitutively expressed in the granulosa cells, while all isoforms of C/EBPbeta were markedly inducible by FSH. Site-directed mutations of both binding sequences practically ablated both basal and hormone-driven chloramphenicol acetyltransferase activities to less than 5% of the parental -96/+6 construct. Unlike earlier notions, elimination of potential binding sites for steroidogenic factor-1, a well known tissue-specific transcription factor, did not impair StAR transcription. Consequently, we propose that C/EBPbeta and GATA-4 represent a novel combination of transcription factors capable of conferring an acute response to hormones upon their concomitant binding to the StAR promoter.

Growth factor modulation of steroidogenic acute regulatory protein and luteinization in the pig ovary

In vivo and in vitro luteinization were investigated in the porcine ovary, with emphasis on expression of steroidogenic acute regulatory protein (StAR). StAR mRNA and protein as well as cytochrome P450 side-chain cleavage mRNA (P450scc) increased during the luteal phase in the corpus luteum (CL) and were absent in regressed CL. Cytochrome P450 aromatase mRNA (P450arom) was not detectable at any time in CL. In vitro luteinization of granulosa cells occurred over 96 h in culture, during which P450arom mRNA was present at 1 h after cell isolation but not detectable at 6 h; and P450scc and StAR mRNAs were first detectable at 6 h and 48 h, respectively. Incubation of cultures with insulin-like growth factor I (IGF-I, 10 ng/ml), dibutyryl cAMP (cAMP, 300 microM), or their combination, induced measurable StAR mRNA at 24 h (p < 0.05), increased progesterone accumulation at 48 h, and elevated both StAR and P450scc expression through 96 h. Incubation of luteinized granulosa cells with epidermal growth factor (EGF, 10 nM) changed their phenotype from epithelioid to fibroblastic, eliminated steady-state StAR expression, and interfered with cAMP induction of StAR mRNA and progesterone accumulation. EGF had little apparent effect on P450scc mRNA abundance. It is concluded that StAR expression characterizes luteinization, and early luteinization is induced by cAMP and IGF-I in vitro. Further, EGF induces a morphological and functional phenotype that appears similar to an earlier stage of granulosa cell function.

Transcriptional regulation of the StAR gene

The steroidogenic acute regulatory (StAR) protein regulates the rate-limiting step of steroidogenesis. In steroidogenic tissues, the StAR gene is regulated acutely by trophic hormone through a cAMP second messenger pathway. Thus, the gene encoding StAR must be finely regulated so that it is expressed in steroidogenic tissues at the proper time in development, and must be rapidly induced in response to cAMP stimulation. We have summarized the available information concerning the regulation of StAR mRNA levels including promoter mapping and transactivation studies. We also discuss the various transcription factors which have been implicated in the regulation of the StAR gene thus far, and propose models of how StAR transcription may be regulated.

SF-1 (steroidogenic factor-1) and C/EBP beta (CCAAT/enhancer binding protein-beta) cooperate to regulate the murine StAR (steroidogenic acute regulatory) promoter

The steroidogenic acute regulatory (StAR) protein mediates the rate-limiting step of steroidogenesis, which is the transfer of cholesterol to the inner mitochondrial membrane. In steroidogenic tissues, StAR expression is acutely regulated by trophic hormones through a cAMP second messenger pathway, leading to increased StAR mRNA levels within 30 min, reaching maximal levels after 4-6 h of stimulation. The molecular mechanisms underlying such regulation remain unknown. We have examined the StAR promoter for putative transcription factor-binding sites that may regulate transcription in a developmental and/or hormone-induced context. Through sequence analysis, deoxyribonuclease I (DNAse I) footprinting and electrophoretic mobility shift assays (EMSAs), we have identified two putative CCAAT/enhancer binding protein (C/EBP) DNA elements at -113 (C1) and -87 (C2) in the mouse StAR promoter. Characterization of these sites by EMSA indicated that C/EBP beta bound with high affinity to C1 and C2 was a low-affinity C/EBP site. Functional analysis of these sites in the murine StAR promoter showed that mutation of one or both of these binding sites decreases both basal and (Bu)2cAMP-stimulated StAR promoter activity in MA-10 Leydig tumor cells, without affecting the fold activation [(Bu)2cAMP-stimulated/basal] of the promoter. Furthermore, we have demonstrated that these two C/EBP binding sites are required for steroidogenic factor-1 (SF-1)-dependent transactivation of the StAR promoter in a nonsteroidogenic cell line. These data indicate that in addition to SF-1, C/EBP beta is involved in the transcriptional regulation of the StAR gene and may play an important role in developmental and hormone-responsive regulation of steroidogenesis.