Steroidogenic factor-1 mediates cyclic 3',5'-adenosine monophosphate regulation of the high density lipoprotein receptor

Scavenger receptor BI attenuates oxidized phospholipid-induced pulmonary inflammation

Damage associated molecular patterns (DAMPs) are molecules released from dead/dying cells following toxicant and/or environmental exposures that activate the immune response through binding of pattern recognition receptors (PRRs). Excessive production of DAMPs or failed clearance leads to chronic inflammation and delayed inflammation resolution. One category of DAMPs are oxidized phospholipids (oxPLs) produced upon exposure to high levels of oxidative stress, such as following ozone (O3) induced inflammation. OxPLs are bound by multiple classes of PRRs that include scavenger receptors (SRs) such as SR class B-1 (SR-BI) and toll-like receptors (TLRs). Interactions between oxPLs and PRRs appear to regulate inflammation; however, the role of SR-BI in oxPL-induced lung inflammation has not been defined. Therefore, we hypothesize that SR-BI is critical in protecting the lung from oxPL-induced pulmonary inflammation/injury. To test this hypothesis, C57BL/6J (WT) female mice were dosed with oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphatidylcholine (oxPAPC) by oropharyngeal aspiration which increased pulmonary SR-BI expression. Following oxPAPC exposure, SR-BI deficient (SR-BI-/-) mice exhibited increased lung pathology and inflammatory cytokine/chemokine production. Lipidomic analysis revealed that SR-BI-/- mice had an altered pulmonary lipidome prior to and following oxPAPC exposure, which correlated with increased oxidized phosphatidylcholines (PCs). Finally, we characterized TLR4-mediated activation of NF-κB following oxPAPC exposure and discovered that SR-BI-/- mice had increased TLR4 mRNA expression in lung tissue and macrophages, increased nuclear p65, and decreased cytoplasmic IκBα. Overall, we conclude that SR-BI is required for limiting oxPAPC-induced lung pathology by maintaining lipid homeostasis, reducing oxidized PCs, and attenuating TLR4-NF-κB activation, thereby preventing excessive and persistent inflammation.

SR-BI as a target of natural products and its significance in cancer

Scavenger receptor class B type I (SR-BI) protein is an integral membrane glycoprotein. SR-BI is emerging as a multifunctional protein, which regulates autophagy, efferocytosis, cell survival and inflammation. It is well known that SR-BI plays a critical role in lipoprotein metabolism by mediating cholesteryl esters selective uptake and the bi-directional flux of free cholesterol. Recently, SR-BI has also been identified as a potential marker for cancer diagnosis, prognosis, or even a treatment target. Natural products are a promising source for the discovery of new drug leads. Multiple natural products were identified to regulate SR-BI protein expression. There are still a number of challenges in modulating SR-BI expression in cancer and in using natural products for modulation of such protein expression. In this review, our purpose is to discuss the relationship between SR-BI protein and cancer, and the molecular mechanisms regulating SR-BI expression, as well as to provide an overview of natural products that regulate SR-BI expression.

Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.

Ad4BP/SF-1 regulates cholesterol synthesis to boost the production of steroids

Housekeeping metabolic pathways such as glycolysis are active in all cell types. In addition, many types of cells are equipped with cell-specific metabolic pathways. To properly perform their functions, housekeeping and cell-specific metabolic pathways must function cooperatively. However, the regulatory mechanisms that couple metabolic pathways remain largely unknown. Recently, we showed that the steroidogenic cell-specific nuclear receptor Ad4BP/SF-1, which regulates steroidogenic genes, also regulates housekeeping glycolytic genes. Here, we identify cholesterogenic genes as the targets of Ad4BP/SF-1. Further, we reveal that Ad4BP/SF-1 regulates Hummr, a candidate mediator of cholesterol transport from endoplasmic reticula to mitochondria. Given that cholesterol is the starting material for steroidogenesis and is synthesized from acetyl-CoA, which partly originates from glucose, our results suggest that multiple biological processes involved in synthesizing steroid hormones are governed by Ad4BP/SF-1. To our knowledge, this study provides the first example where housekeeping and cell-specific metabolism are coordinated at the transcriptional level.

SR-B1: A Unique Multifunctional Receptor for Cholesterol Influx and Efflux

The scavenger receptor, class B type 1 (SR-B1), is a multiligand membrane receptor protein that functions as a physiologically relevant high-density lipoprotein (HDL) receptor whose primary role is to mediate selective uptake or influx of HDL-derived cholesteryl esters into cells and tissues. SR-B1 also facilitates the efflux of cholesterol from peripheral tissues, including macrophages, back to liver. As a regulator of plasma membrane cholesterol content, SR-B1 promotes the uptake of lipid soluble vitamins as well as viral entry into host cells. These collective functions of SR-B1 ultimately affect programmed cell death, female fertility, platelet function, vasculature inflammation, and diet-induced atherosclerosis and myocardial infarction. SR-B1 has also been identified as a potential marker for cancer diagnosis and prognosis. Finally, the SR-B1-linked selective HDL-cholesteryl ester uptake pathway is now being evaluated as a gateway for the delivery of therapeutic and diagnostic agents. In this review, we focus on the regulation and functional significance of SR-B1 in mediating cholesterol movement into and out of cells.

ACTH Regulation of Adrenal SR-B1

The adrenal gland is one of the prominent sites for steroid hormone synthesis. Lipoprotein-derived cholesterol esters (CEs) delivered via SR-B1 constitute the dominant source of cholesterol for steroidogenesis, particularly in rodents. Adrenocorticotropic hormone (ACTH) stimulates steroidogenesis through downstream actions on multiple components involved in steroidogenesis. Both acute and chronic ACTH treatments can modulate SR-B1 function, including its transcription, posttranscriptional stability, phosphorylation and dimerization status, as well as the interaction with other protein partners, all of which result in changes in the ability of SR-B1 to mediate HDL-CE uptake and the supply of cholesterol for conversion to steroids. Here, we provide a review of the recent findings on the regulation of adrenal SR-B1 function by ACTH.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

Identification of transcription factors and coactivators affected by dibutylphthalate interactions in fetal rat testes

Previous analysis of in utero dibutylphthalate (DBP)-exposed fetal rat testes indicated that DBP's antiandrogenic effects were mediated, in part, by indirect inhibition of steroidogenic factor 1 (SF1), suggesting that peroxisome proliferator-activated receptor alpha (PPARα) might be involved through coactivator (CREB-binding protein [CBP]) sequestration. To test this hypothesis, we have performed chromatin immunoprecipitation (ChIP) microarray analysis to assess the DNA binding of PPARα, SF1, CBP, and RNA polymerase II in DBP-induced testicular maldevelopment target genes. Pathway analysis of expression array data in fetal rat testes examined at gestational day (GD) 15, 17, or 19 indicated that lipid metabolism genes regulated by SF1 and PPARα, respectively, were overrepresented, and the time dependency of changes to PPARα-regulated lipid metabolism genes correlated with DBP-mediated repression of SF1-regulated steroidogenesis genes. ChIP microarrays were used to investigate whether DBP-mediated repression of SF1-regulated genes was associated with changes in SF1 binding to genes involved in DBP-induced testicular maldevelopment. DBP treatment caused reductions in SF1 binding in CYP11a, StAR, and CYP17a. Follicle-stimulating hormone receptor (FSHR), regulated by SF1 but unaffected by DBP-treatment, also contained SF1-binding peaks, but DBP did not change this compared with control. GD15 and GD19 fetal testes contained PPARα protein-binding peaks in CYP11a, StAR, and CYP17a regulatory regions. In contrast to its repressive effect on SF1, DBP treatment caused increases in these peaks compared with control. PPARα-binding peaks in the FSHR promoter were not detected in GD15 samples. Hence, the repressive effect of DBP on SF1-regulated steroidogenic genes correlates with inhibition of SF1-DNA binding and increased PPARα-DNA binding. The data indicate that PPARα may act as an indirect transrepressor of SF1 on steroidogenic genes in fetal rat testes in response to DBP treatment.

Feedback inhibition of human scavenger receptor class B type I gene expression by glucocorticoid in adrenal and ovarian cells

Scavenger receptor class B type I (SR-BI) facilitates the reverse transport of excess cholesterol from peripheral tissues to the liver via high-density lipoproteins. In steroidogenic tissues, SR-BI supplies cholesterol for steroid hormone production. We show here that the transcription of the human SR-BI gene is subject to feedback inhibition by glucocorticoid in adrenal and ovarian cells. SR-BI mRNA levels were increased in adrenals from corticosterone-insufficient Crh(-/-) mice, whereas corticosterone replacement by oral administration inhibited SR-BI gene expression in these mice. SR-BI mRNA levels were increased in adrenals from wild-type mice treated with metyrapone, a drug that blocks corticosterone synthesis. Experiments in adrenocortical H295R and ovarian SKOV-3 cells using cycloheximide and siRNA-mediated gene silencing revealed that glucocorticoid-mediated inhibition of SR-BI gene transcription requires de novo protein synthesis and the glucocorticoid receptor (GR). No direct binding of GR to the SR-BI promoter could be demonstrated in vitro and in vivo, suggesting an indirect mechanism of repression of SR-BI gene transcription by GR in adrenal cells. Deletion analysis established that the region of the human SR-BI promoter between nucleotides -201 and -62 is sufficient to mediate repression by glucocorticoid. This region contains putative binding sites for transcriptional repressors that could play a role in SR-BI gene regulation in response to glucocorticoid. In summary, this is the first report showing that glucocorticoid suppress SR-BI expression suggesting that steroidogenic tissues maintain steroid hormone homeostasis by prohibiting SR-BI-mediated high-density lipoprotein cholesterol uptake when the endogenous levels of glucocorticoid are elevated.

Minireview: steroidogenic factor 1: its roles in differentiation, development, and disease

The orphan nuclear receptor steroidogenic factor 1 (SF-1, also called Ad4BP, encoded by the NR5A1 gene) is an essential regulator of endocrine development and function. Initially identified as a tissue-specific transcriptional regulator of cytochrome P450 steroid hydroxylases, studies of both global and tissue-specific knockout mice have demonstrated that SF-1 is required for the development of the adrenal glands, gonads, and ventromedial hypothalamus and for the proper functioning of pituitary gonadotropes. Many genes are transcriptionally regulated by SF-1, and many proteins, in turn, interact with SF-1 and modulate its activity. Whereas mice with heterozygous mutations that disrupt SF-1 function have only subtle abnormalities, humans with heterozygous SF-1 mutations can present with XY sex reversal (i.e. testicular failure), ovarian failure, and occasionally adrenal insufficiency; dysregulation of SF-1 has been linked to diseases such as endometriosis and adrenocortical carcinoma. The current state of knowledge of this important transcription factor will be reviewed with a particular emphasis on the pioneering work on SF-1 by the late Keith Parker.

The transcriptional factor prolactin regulatory element-binding protein mediates the gene transcription of adrenal scavenger receptor class B type I via 3',5'-cyclic adenosine 5'-monophosphate

Prolactin regulatory element-binding (PREB) protein is a transcription factor that regulates prolactin promoter activity in the rat anterior pituitary. The PREB protein is not only expressed in the anterior pituitary but also in the adrenal gland. However, the role of PREB in the adrenal gland is not clearly understood. Scavenger receptor class B type I (SR-BI) is a receptor for high-density lipoprotein that mediates the cellular uptake of high-density lipoprotein-cholesteryl ester and is a major route for cholesterol delivery to the steroidogenic pathway in the adrenal gland. In the present study, we have examined the role of PREB in regulating SR-BI. SR-BI expression was found to be regulated by cAMP, which stimulated the expression of PREB in a dose-dependent manner. Conversely, overexpression of PREB using a PREB-expressing adenovirus increased the expression of the SR-BI protein in the adrenocortical cell line Y-1. In addition, PREB induced the expression of the luciferase reporter protein that was under the control of the SR-BI promoter. EMSA showed that PREB mediates its transcriptional effect by binding to the PREB-responsive cis-element of the SR-BI promoter. Finally, we used small interfering RNA to inhibit PREB expression in the Y-1 cells and demonstrated that the knockdown of PREB expression attenuated the effects of cAMP on SR-BI expression. In summary, our data showed that in the adrenal gland, PREB regulates the transcription of the SR-BI gene via cAMP.

Activation of the SCPx promoter in mouse adrenocortical Y1 cells

Sterol carrier protein X (SCPx) is a peroxisomal protein with both lipid transfer and thiolase activity. Treatment of mouse adrenal Y1 cells with cAMP for 24h caused a significant induction of SCPx mRNA levels. Reporter gene studies demonstrated that treatment with cAMP and SF-1 was able to activate the SCPx promoter. Sequence analysis revealed the presence of three putative steroidogenic factor-1 (SF-1) binding motifs (designated SFB1, SFB2, and SFB3) and one CRE. Only SFB1 and SFB3 were able to bind recombinant SF-1 protein in electrophoretic mobility shift assays. The CRE was able to form a DNA/protein complex in the presence of Y1 nuclear extracts. Mutational analysis studies demonstrated that SFB3 is required for full activation of the SCPx promoter by cAMP treatment. Regulation of the SCPx gene by SF-1 and cAMP is similar to the regulatory mechanisms observed for other steroidogenic genes.

Activation of the hepatic LDL receptor promoter by thyroid hormone

The question of whether mature sterol regulatory element binding protein-2 (SREBP-2) mediates transcriptional activation of the hepatic low density lipoprotein (LDL) receptor by thyroid hormone was investigated. Western blotting analysis and electrophoretic mobility shift assays demonstrated that mature nuclear SREBP-2 protein could be detected in liver nuclear extracts prepared from normal animals but not in extracts prepared from rats rendered hypothyroid either by hypophysectomy (Hx) or thyroidectomy (Tx). Treatment of Hx rats with T3 restored LDL receptor mRNA levels in about 1 h and caused a 6-fold increase 2.5 h after T3 administration. However, no detectable mature SREBP-2 was seen in this time period despite a substantial reduction in serum cholesterol levels caused by the T3 treatment. Deletion of the SRE region from the LDL receptor promoter did not decrease the T3 response. Thus, the possibility that T3 may be mediating LDL receptor induction directly via a thyroid response element (TRE) was investigated. Reporter gene analysis and electrophoretic mobility shift assays demonstrated that the rat LDL receptor promoter contains two functional TREs (US-TRE and 2H-TRE). Either one of these elements could support T3 induction. However, the stronger of these elements is US-TRE at-612 which binds TRbeta1 more tightly and when mutated results in a diminished T3 response. These results indicate that the rapid induction of the hepatic LDL receptor by thyroid hormone is likely due to direct interaction with TREs rather than indirectly by a mechanism involving SREBP-2.

Cholesterol 25-hydroxylase on chromosome 10q is a susceptibility gene for sporadic Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia. It is characterized by beta-amyloid (A beta) plaques, neurofibrillary tangles and the degeneration of specifically vulnerable brain neurons. We observed high expression of the cholesterol 25-hydroxylase (CH25H) gene in specifically vulnerable brain regions of AD patients. CH25H maps to a region within 10q23 that has been previously linked to sporadic AD. Sequencing of the 5' region of CH25H revealed three common haplotypes, CH25Hchi2, CH25Hchi3 and CH25Hchi4; CSF levels of the cholesterol precursor lathosterol were higher in carriers of the CH25Hchi4 haplotype. In 1,282 patients with AD and 1,312 healthy control subjects from five independent populations, a common variation in the vicinity of CH25H was significantly associated with the risk for sporadic AD (p = 0.006). Quantitative neuropathology of brains from elderly non-demented subjects showed brain A beta deposits in carriers of CH25Hchi4 and CH25Hchi3 haplotypes, whereas no A beta deposits were present in CH25Hchi2 carriers. Together, these results are compatible with a role of CH25Hchi4 as a putative susceptibility factor for sporadic AD; they may explain part of the linkage of chromosome 10 markers with sporadic AD, and they suggest the possibility that CH25H polymorphisms are associated with different rates of brain A beta deposition.

Characterization of the rat LDL receptor 5′-flanking region

A 1.5-kb genomic DNA fragment corresponding to the 5'-flanking region of the rat LDL receptor gene was cloned and putative regulatory regions were identified. A major transcription start site was identified at -154 bp relative to the ATG translation initiation codon, within a region containing two thyroid hormone response element half-site motifs (2H-TRE). Binding of thyroid hormone receptors alpha and beta1 to this element was demonstrated. Mutations within this 2H-TRE region abolished basal transcription levels of the rat LDL receptor gene. Reporter gene studies indicated that the promoter region between -300 and -200 bp, which contains one sterol response element (SRE) and two specificity protein-1 sites (Sp1) sites, is crucial for basal transcription of the rat LDL receptor gene. The functionality of the SRE motif was confirmed using electrophoretic mobility shift assays and reporter gene studies.

Adrenocorticotropic Hormone-Mediated Signaling Cascades Coordinate a Cyclic Pattern of Steroidogenic Factor 1-Dependent Transcriptional Activation

Steroidogenic factor 1 (SF-1) is an orphan nuclear receptor that has emerged as a critical mediator of endocrine function at multiple levels of the hypothalamic-pituitary-steroidogenic axis. Within the adrenal cortex, ACTH-dependent transcriptional responses, including transcriptional activation of several key steroidogenic enzymes within the steroid biosynthetic pathway, are largely dependent upon SF-1 action. The absence of a bona fide endogenous eukaryotic ligand for SF-1 suggests that signaling pathway activation downstream of the melanocortin 2 receptor (Mc2r) modulates this transcriptional response. We have used the chromatin immunoprecipitation assay to examine the temporal formation of ACTH-dependent transcription complexes on the Mc2r gene promoter. In parallel, ACTH-dependent signaling events were examined in an attempt to correlate transcriptional events with the upstream activation of signaling pathways. Our results demonstrate that ACTH-dependent signaling cascades modulate the temporal dynamics of SF-1-dependent complex assembly on the Mc2r promoter. Strikingly, the pattern of SF-1 recruitment and the subsequent attainment of active rounds of transcription support a kinetic model of SF-1 transcriptional activation, a model originally established in the context of ligand-dependent transcription by several classical nuclear hormone receptors. An assessment of the major ACTH-dependent signaling pathways highlights pivotal roles for the MAPK as well as the cAMP-dependent protein kinase A pathway in the entrainment of SF-1-mediated transcriptional events. In addition, the current study demonstrates that specific enzymatic activities are capable of regulating distinct facets of a highly ordered transcriptional response.

Adrenocorticotropin/3',5'-cyclic AMP-mediated transcription of the scavenger akr1-b7 gene in adrenocortical cells is dependent on three functionally distinct steroidogenic factor-1-responsive elements

The akr1-b7 gene encodes a scavenger enzyme expressed in steroidogenic glands under pituitary control. In the zona fasciculata of the adrenal cortex where its expression is controlled by ACTH, AKR1-B7 detoxifies isocaproaldehyde produced during the first step of steroidogenesis. Three steroidogenic factor-1 (SF-1)-responsive elements (SFREs) are contained within the -510/+41 promoter region, which was previously demonstrated to drive gene expression in transgenic mice adrenal cortex. All these sequences bind at least SF-1 in Y1 adrenocortical cell nuclear extracts and can be activated by overexpression of this factor in HeLa cells. However, the three SFREs show distinct properties regarding akr1-b7 promoter activity in Y1 cells. Whereas the proximal -102 SFRE supports basal promoter activity, the -458 bona fide SFRE is essential for both basal promoter activity and cAMP responsiveness, although it is unresponsive to cAMP when isolated from its promoter context. This suggests that SF-1 is not a cAMP-responsive factor per se. The neighboring SFRE at -503 is a palindromic sequence that binds monomeric and heteromeric SF-1 as well as an adrenal-specific complex. Using MA-10 Leydig cells and Y1-10r9 mutant cells, we provide evidence that its activity in adrenocortical cells depends on the binding of the adrenal-specific factor, which is required for basal and cAMP-induced promoter activity. Furthermore, the -503 site has intrinsic cAMP-sensing ability in Y1 cells, which is correlated with increased adrenal-specific complex binding. Collectively, our results suggest that cAMP responsiveness of the akr1-b7 promoter is achieved through cooperation between the adrenal-specific factor bound to the -503 site and SF-1 bound to the -458 site.

Cloning and characterization of a novel zinc finger protein that modulates the transcriptional activity of nuclear receptors

The orphan nuclear receptor steroidogenic factor-1 (SF-1) plays pivotal roles in the development and function of steroidogenic organs. It transcriptionally regulates an array of factors required for biosynthesis of steroid hormones and is also necessary for the expression of genes in the pituitary and the male reproductive tract. Here we describe the identification of a novel zinc finger protein that modifies the transcriptional potential of SF-1. This factor, which we call Zip67 (zinc finger protein 67 kDa), was cloned through a two-hybrid screen of a human testis cDNA library using the C-terminal part of SF-1 as the bait. Transient transfection experiments demonstrated that Zip67 represses SF-1-dependent transcription in the context of both multimerized SF-1-binding sites and natural SF-1-inducible promoters. The interaction between Zip67 and SF-1 was dependent on an intact activation function-2 domain of SF-1, and we propose a mechanism whereby Zip67 represses transcription through competition with p160 coactivators for binding to SF-1. Zip67 was detected in SF-1 expressing tissues such as testis, adrenal, ovary and spleen in addition to other tissues. In line with the broader expression pattern, we found that Zip67 also affected transcription mediated by several other nuclear receptors. In conclusion, we have isolated a novel zinc-finger protein that influences gene activation through interaction with the functionally important activation function-2 domain of nuclear receptors.

SF-1 a key player in the development and differentiation of steroidogenic tissues

Since its discovery in the early 1990s, the orphan nuclear receptor SF-1 has been attributed a central role in the development and differentiation of steroidogenic tissues. SF-1 controls the expression of all the steroidogenic enzymes and cholesterol transporters required for steroidogenesis as well as the expression of steroidogenesis-stimulating hormones and their cognate receptors. SF-1 is also an essential regulator of genes involved in the sex determination cascade. The study of SF-1 null mice and of human mutants has been of great value to demonstrate the essential role of this factor in vivo, although the complete adrenal and gonadal agenesis in knock-out animals has impeded studies of its function as a transcriptional regulator. In particular, the role of SF-1 in the hormonal responsiveness of steroidogenic genes promoters is still a subject of debate. This extensive review takes into account recent data obtained from SF-1 haploinsufficient mice, pituitary-specific knock-outs and from transgenic mice experiments carried out with SF-1 target gene promoters. It also summarizes the pros and cons regarding the presumed role of SF-1 in cAMP signalling.

Identification of two novel CT antigens and their capacity to elicit antibody response in hepatocellular carcinoma patients

FATE and TPTE genes were originally reported to be specifically expressed in the adult testis. We searched for the databases of Unigene and serial analysis of gene expression (SAGE) implying that these two gene transcripts might also be expressed in tumours. Herein, we demonstrated that FATE and TPTE mRNA transcripts were expressed in different histological types of tumours and normal testis. Both are cancer-testis (CT) antigens and renamed as FATE/BJ-HCC-2 and TPTE/BJ-HCC-5, respectively. Comparison at nucleotide sequence, the FATE/BJ-HCC-2 cDNA, was identical to that of FATE, whereas the TPTE/BJ-HCC-5 was found to have two isoforms in both cancers and testis: one was identical in cDNA sequence to TPTE, encoding a protein of 551 amino acids, and the other variant lacked an exon of 54 bp, encoding a protein of 533 amino acids. The mRNA expression was analysed by RT-PCR and real-time PCR. FATE/BJ-HCC-2 mRNA was detected in 66% (41 out of 62) in hepatocellular carcinoma (HCC) samples and 21% (three out of 14) in colon cancer samples, whereas the TPTE/BJ-HCC-5 mRNA was detected in 39% (24 out of 62) and 36% (five out of 14) in HCC and non-small lung cancer samples, respectively. The recombinant proteins were prepared and the reactivity of allogenic sera to these two antigens was screened. The frequency of antibody response against FATE/BJ-HCC-2 and TPTE/BJ-HCC-5 proteins was 7.3% (three out of 41) and 25.0% (six out of 24), respectively, in HCC patients bearing respective gene transcripts. Therefore, FATE/BJ-HCC-2 and TPTE/BJ-HCC-5 are the novel CT antigens capable of eliciting antibody response in cancer patients.

Characterization of receptor-interacting protein RIP140 in the regulation of SF-1 responsive target genes

Receptor-interacting protein (RIP) 140 interacts with several nuclear receptors, but its function in regulation of nuclear receptor action has been debated. Here we have examined the role of RIP140 in regulation of Steroidogenic factor-1 (SF-1)-dependent transcription. SF-1 interacts with RIP140 through its activation function-2 (AF-2) domain. Several domains of RIP140 interact directly with SF-1, but the carboxyl-terminal region containing 4 of its 9 LXXLL motifs showed the strongest SF-1 interaction. Coexpression of RIP140 and SF-1 in different cell types demonstrated that RIP140 acts as a potent corepressor of transcription from the SF-1 responsive cAMP regulatory sequence 2 (CRS2) element of the CYP17 gene and a variety of SF-1 responsive promoter genes. RIP140 also counteracted the stimulatory action of p160/SRC coactivators. The inhibitory effect of RIP140 was partially reversed by Trichostatin A, suggesting a role of histone deacetylase (HDAC) activity in RIP140-mediated repression of SF-1. Quantitation of endogenous coregulator mRNA levels revealed cell type specific differences that could affect the repressor action by overexpressed RIP140.

Steroidogenic factor-1 controls the aldose reductase akr1b7 gene promoter in transgenic mice through an atypical binding site

Aldo-keto-reductase 1B7/mouse vas deferens protein (AKR1B7/MVDP) is expressed in rodent steroidogenic glands and in the mouse vas deferens. In steroidogenic organs, AKR1B7/MVDP scavenges isocaproaldehyde produced from the cholesterol side-chain cleavage reaction. Akr1b7/mvdp is responsive to ACTH in adrenals and to androgens in vas deferens. Using transgenic mice, we previously delimited the regulatory DNA sequences necessary for expression in both organs and identified by cell transfections, a cryptic steroidogenic factor-1 (SF-1) response element (SFRE) at -102 that overlaps a proximal androgen-responsive element. To address its in vivo functions in adrenals, we devised a transgenic mouse study using wild-type and mutant akr1b7 promoters driving the chloramphenol acetyltransferase reporter gene. Adrenal expression in adults was impaired in all lines mutant for -102 SFRE. This effect is linked to impaired SF-1 binding and not to impaired androgen receptor binding, because akr1b7 expression is not affected in adrenals of androgen receptor-defective Tfm mice. Triphasic developmental patterns of both AKR1B7 and wild-type transgene expression paralleled changes in SF-1 levels/binding activity; expression was maximal in late embryos, minimal in 6- to 15-d-old neonates, and thereafter progressively restored. Differences in developmental expression between wild-type and mutant transgenes revealed that requirement for the -102 SFRE appears stage specific, as its integrity is an absolute prerequisite for reinduction of gene expression after postnatal d 15. Further, mutation of this site did not affect transgene responsiveness to ACTH. These findings demonstrate a new function for SFRE in vivo, via influencing promoter sensibility to postnatal changes of SF-1 contents, in controlling promoter strength in adults without affecting adrenal targeting, hormonal control, or early gene expression.

Targeted replacement of mouse apolipoprotein A-I with human ApoA-I or the mutant ApoA-IMilano. Evidence of APOA-IM impaired hepatic secretion

Despite a pro-atherogenic profile, individuals carrying the molecular variant (R173C) of apolipoprotein (apo)A-I, named apoA-I(Milano) (apoA-I(M)), appear to be at reduced risk for cardiovascular disease. To develop an in vivo system to explore, in a controlled manner, the effects of apoA-I(M) on lipid metabolism, we have used the gene targeting technology, or "gene knock-in" (gene k-in), to replace the murine apoA-I gene with either human apoA-I or apoA-I(M) genes in embryonic stem cells. As in human carriers, mice expressing apoA-I(M) (A-I(M) k-in) are characterized by low concentrations of the human apolipoprotein and reduced high density lipoprotein cholesterol levels, compared with A-I k-in animals. The aim of the present study was to investigate the basic mechanisms of hypoalphalipoproteinemia associated with the apoA-I(M) mutation. ApoA-I and apoA-I(M) mRNA expression, as assessed by Northern blot analysis and quantitative real time reverse transcription-PCR, did not exhibit significant differences in either liver or intestine. Moreover, human apolipoprotein synthesis rates were similar in the k-in lines. When the secretion rate of the human apolipoproteins was assessed in cultured hepatocytes from the mouse lines, secretion from apoA-I(M)-expressing cells was markedly reduced (42% for A-I(M) k-in and 36% for A-I/A-I(M) k-in mice) as compared with that of A-I k-in hepatocytes. These results provide the first evidence that the hypoalphalipoproteinemia in apoA-I(M) human carriers may be partially explained by impaired apoA-I(M) secretion.

Trophoblast-like human choriocarcinoma cells serve as a suitable in vitro model for selective cholesteryl ester uptake from high density lipoproteins

As human choriocarcinoma cells display many of the biochemical and morphological characteristics reported for in utero invasive trophoblast cells we have studied cholesterol supply from high density lipoproteins (HDL) to these cells. Binding properties of 125I-labeled HDL subclass 3 (HDL3) at 4 degrees C were similar for BeWo, JAr, and Jeg3 choriocarcinoma cell lines while degradation rates at 37 degrees C were highest for BeWo. Calculating the selective cholesteryl ester (CE)-uptake as the difference between specific cell association of [3H]CE-labeled HDL3 and holoparticle association of 125I-labeled HDL3 revealed that in BeWo cells, the selective CE-uptake was slightly lower than holoparticle association. However, the pronounced capacity for specific cell association of [3H]CE-HDL3 and selective [3H]CE-uptake in excess of HDL3-holoparticle association, and cAMP-mediated enhanced cell association of [3H]CE-HDL3 in JAr and Jeg3 suggested the scavenger receptor class B, type I (SR-BI) to be responsible for this pathway. Abundant expression of SR-BI (but not SR-BII, a splice variant of SR-BI) could be observed in JAr and Jeg3 but not in BeWo cells using RT-PCR, Northern and Western blot analysis, and immunocytochemical technique. Adenovirus-mediated overexpression of SR-BI in all three choriocarcinoma cell lines resulted in an enhanced capacity for cell association of [3H]CE-HDL3 (20-fold in BeWo; fivefold in JAr and Jeg3). The fact that exogenous HDL3 remarkably increases proliferation in JAr and Jeg3 supports the notion that selective CE-uptake and subsequent intracellular generation of cholesterol is coupled to cellular growth. From our findings we propose that JAr and Jeg3 cells serve as a suitable in vitro model to study selective CE-supply to human placental cells.

Differential regulation of steroid hormone biosynthesis in R2C and MA-10 Leydig tumor cells: role of SR-B1-mediated selective cholesteryl ester transport

The rat R2C Leydig tumor cell line is constitutively steroidogenic in nature, while the mouse MA-10 Leydig tumor cell line synthesizes large amounts of steroids only in response to hormonal stimulation. Earlier studies showed abundant cAMP-independent steroid production and constitutive expression of steroidogenic acute regulatory (StAR) protein in R2C cells. The objective of the current study was to identify possible genetic alterations in the R2C cell line responsible for rendering it a constitutively steroidogenic cell line, especially those that might have altered its cholesterol homeostatic mechanisms. Measurement of the levels of cholesterol esters and free cholesterol, precursors for steroidogenesis, indicated that R2C mitochondria were fourfold enriched in free cholesterol content compared with MA-10 mitochondria. In addition to the previously demonstrated increased expression of StAR protein, we show that R2C cells possess marginally enhanced protein kinase A activity, exhibit higher capacity to take up extracellular cholesterol esters, and express much higher levels of scavenger receptor-type B class 1 (SR-B1) and hormone sensitive lipase (HSL). These observations suggest that the high level of steroid biosynthesis in R2C cells is a result of the constitutive expression of the components involved in the uptake of cholesterol esters (SR-B1), their conversion to free cholesterol (HSL), and its mobilization to the inner mitochondrial membrane (StAR).

Estrogen activates the high-density lipoprotein receptor gene via binding to estrogen response elements and interaction with sterol regulatory element binding protein-1A

The effects of E2 on the high-density lipoprotein receptor (HDL-R) scavenger receptor class B type I (SR-BI) gene were examined. Four putative estrogen response element half-site motifs (ERE(1/2)) (-2176, -1726, -1622, and -1211, designated ERE(1/2)-1, 2, 3, and 4, respectively) were identified in the HDL-R SR-BI promoter. Transfection studies and mutation analysis demonstrated that E2 significantly increased HDL-R SR-BI promoter activity and that mutating ERE(1/2)-1, 2, and 4 resulted in a loss of E2 responsiveness. Both ER alpha and ER beta formed specific complexes with ERE(1/2)-1, 2, and 4 but did not bind ERE(1/2)-3 in vitro. Interestingly, ERE(1/2)-3 was the motif shown not to be important for E2-activation of the HDL-R SR-BI promoter in the mutational analysis studies. The influence of SREBP-1a (sterol regulatory element binding protein-1a) on E2 regulation of the HDL-R SR-BI gene was also examined. SREBP-1a was able to bind directly to the ERE(1/2) motifs and enhanced ER binding when both ER subtypes were present. ER alpha and beta also bound to a sterol response element motif, but they did not enhance SREBP-1a binding. Cotransfection studies demonstrated that the presence of the three factors, ER alpha, ER beta, and SREBP-1a, enhanced the overall luciferase activity produced from the HDL-R SR-BI promoter construct in the presence of only one of the factors. Interaction of SREBP-1a with both ERs was demonstrated using a mammalian two-hybrid assay. The data confirmed that E2 through the ERs can positively regulate the HDL-R SR-BI through binding and activation of three ERE(1/2) motifs and identified SREBP-1a as a potential coactivator of the E2-ER-dependent effects on the HDL-R SR-BI gene.

The nuclear receptor coactivators p300/CBP/cointegrator-associated protein (p/CIP) and transcription intermediary factor 2 (TIF2) differentially regulate PKA-stimulated transcriptional activity of steroidogenic factor 1

Steroidogenic factor-1 (SF-1) is a member of the nuclear receptor superfamily that plays essential roles in the development of endocrine organs. Steroid receptor coactivator 1 and transcription intermediary factor 2 (TIF2) belong to the p160 coactivator family that mediates transcriptional activation by several nuclear receptors, including SF-1. Here, it is reported that another of the p160 coactivators, p/CIP, interacts with SF-1 through the activation function-2 domain. Both p300/CBP/cointegrator-associated protein (p/CIP) and TIF2 potentiated SF-1-mediated transcription from two reporter gene constructs in transfected nonsteroidogenic COS-1 cells and in adrenocortical Y1 cells. PKA was shown to stimulate SF-1 transcriptional activity, and coexpression of p/CIP together with the PKA catalytic subunit stimulated SF-1-mediated transactivation even further. In contrast, PKA catalytic subunit overexpression impaired the ability of TIF2 to potentiate SF-1-dependent transcription. Activation of PKA also inhibited the TIF2-mediated coactivation of other nuclear receptors such as PPAR alpha/-gamma and liver X receptor-alpha. The TIF2 mRNA levels were not affected by PKA, but instead we found that PKA activation led to a decrease in the levels of TIF2 protein. Moreover, the C-terminal activation domain 2 of TIF2 was required for the inhibitory effect of PKA, suggesting that this region is the target for the PKA-mediated down-regulation. Thus, in contrast to the regulation of p/CIP and steroid receptor coactivator 1, we suggest that activation of PKA leads to selective down-regulation of TIF2 and subsequently repression of TIF2 coactivator function.

Activation of cAMP-dependent protein kinase increases the protein level of steroidogenic factor-1

The orphan nuclear receptor steroidogenic factor 1 (SF-1) is an essential regulator of endocrine organogenesis, sexual differentiation, and steroidogenesis. SF-1 is a transcriptional regulator of cAMP responsive genes, but the exact mechanisms by which cAMP-dependent PKA modulates SF-1 dependent transcription leading to increased steroidogenic output have not been determined. In this report the effects of PKA activation on SF-1 in living cells have been examined by the use of full-length SF-1 cDNA fused to the cDNA encoding green fluorescent protein (GFP). The GFP-SF-1 fusion protein localized to the nucleus of both steroidogenic Y1 cells and nonsteroidogenic COS-1 cells, and the functional properties of wild-type SF-1 were conserved. When the catalytic subunit of PKA was coexpressed with GFP-SF-1, we observed that the fluorescence emission was markedly elevated. These findings were confirmed by Western blot analysis, showing that stimulation of PKA increased SF-1 protein levels. The PKA- induced expression of SF-1 protein was not accompanied by an increase in SF-1 mRNA levels. However, pulse-chase studies showed a decrease in SF-1 degradation rate in response to activation of PKA, indicating that PKA elevates the level of SF-1 by increasing the stability of SF-1 protein.

DAX-1 represses the high-density lipoprotein receptor through interaction with positive regulators sterol regulatory element-binding protein-1a and steroidogenic factor-1

The high-density lipoprotein receptor (HDL-R) mediates the selective uptake of high-density lipoprotein cholesterol in nonplacental steroidogenic tissues. We have previously demonstrated that sterol regulatory element-binding protein-1a (SREBP-1a) and steroidogenic factor-1 (SF-1) positively regulate HDL-R gene transcription. In the present study, we examined whether DAX-1 (dosage-sensitive sex adrenal hypoplasia congenital critical region on the X chromosome, gene-1) could influence the expression of the HDL-R gene. Cotransfection studies demonstrated that DAX-1 was able to repress SREBP-1a and SF-1-dependent activation of the HDL-R promoter. Mammalian two-hybrid assays demonstrated that DAX-1 could interact with SREBP-1a. In addition, electrophoretic mobility shift assays demonstrated that initial incubation of DAX-1 with SREBP-1a protein in the absence of DNA prevented subsequent binding of SREBP-1a to the HDL-R sterol regulatory elements in a dose-dependent manner, whereas, in the case of SF-1, DAX-1 formed a complex with SF-1 protein on the DNA. These data suggest that DAX-1 inhibits SREBP-1a- and SF-1-dependent activation of the HDL-R promoter through different mechanisms. This investigation confirms that DAX-1 has an important role in regulating steroidogenesis by interfering with SREBP-1a and SF-1 induction of a gene involved in the transport of cholesterol, thereby limiting the amount of substrate available for steroid hormone production.

Follicle-stimulating hormone stimulates protein kinase A-mediated histone H3 phosphorylation and acetylation leading to select gene activation in ovarian granulosa cells

We examined the phosphorylation and acetylation of histone H3 in ovarian granulosa cells stimulated to differentiate by follicle-stimulating hormone (FSH). We found that protein kinase A (PKA) mediates H3 phosphorylation on serine 10, based on inhibition exclusively by PKA inhibitors. FSH-stimulated H3 phosphorylation in granulosa cells is not downstream of mitogen-activated protein kinase/extracellular signal-regulated kinase, ribosomal S6 kinase-2, mitogen- and stress-activated protein kinase-1, p38 MAPK, phosphatidylinositol-3 kinase, or protein kinase C. Transcriptional activation-associated H3 phosphorylation on serine 10 and acetylation of lysine 14 leads to activation of serum glucocorticoid kinase, inhibin alpha, and c-fos genes. We propose that phosphorylation of histone H3 on serine 10 by PKA in coordination with acetylation of H3 on lysine 14 results in reorganization of the promoters of select FSH responsive genes into a more accessible configuration for activation. The unique role of PKA as the physiological histone H3 kinase is consistent with the central role of PKA in initiating granulosa cell differentiation.

Angiotensin II promotes selective uptake of high density lipoprotein cholesterol esters in bovine adrenal glomerulosa and human adrenocortical carcinoma cells through induction of scavenger receptor class B type I

Angiotensin II is one of the main physiological regulators of aldosterone biosynthesis in the zona glomerulosa of the adrenal cortex. The hormone stimulates intracellular cholesterol mobilization to the mitochondrion for steroid biosynthesis. Here we have examined whether angiotensin II also modulates exogenous lipoprotein cholesterol ester supply to the steroidogenic machinery and whether this control is exerted on the selective transport of high density lipoprotein-derived cholesterol ester to intracellular lipid droplets through the scavenger receptor class B type I. In bovine adrenal glomerulosa and human NCI H295R adrenocortical carcinoma cells, high density lipoprotein stimulated steroid production. Angiotensin II pretreatment for 24 h potentiated this response. Fluorescence microscopy of cellular uptake of reconstituted high density lipoprotein containing a fluorescent cholesterol ester revealed an initial, time-dependent narrow labeling of the cell membrane followed by an intense accumulation of the fluorescent cholesterol ester within lipid droplets. At all time points, labeling was more pronounced in cells that had been treated for 24 h with angiotensin II. Fluorescence incorporation into cells was prevented by a monoclonal antibody directed against apolipoprotein A-I. Upon quantitative fluorometric determination, cholesterol ester uptake in angiotensin II-treated bovine cells was increased to 175 +/- 15% of controls after 2 h and to 260 +/- 10% after 4 h of exposure to fluorescent high density lipoprotein. The amount of scavenger receptor class B type I protein detected in cells treated with angiotensin II for 24 h reached 203 +/- 12% of that measured in control cells (n = 3, P < 0.01). In contrast, low density lipoprotein receptors were only minimally affected by angiotensin II treatment. This increase in scavenger receptor class B type I protein was associated with a 3-fold induction of scavenger receptor class B type I mRNA, which could be prevented by actinomycin D but not by cycloheximide. Similar results were obtained in the human adenocarcinoma cell line H295R. These observations show that angiotensin II regulates the scavenger receptor class B type I-mediated selective transport of lipoprotein cholesterol ester across the cell membrane as a major source of precursor for mineralocorticoid biosynthesis in both human and bovine adrenal cells.

Pituitary adenylate cyclase-activating polypeptide and cyclic adenosine 3',5'-monophosphate stimulate the promoter activity of the rat gonadotropin-releasing hormone receptor gene via a bipartite response element in gonadotrope-derived cells

Specific type I receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are present in gonadotrope cells of the anterior pituitary gland. By transient transfection of mouse gonadotrope-derived alphaT3-1 cells, which are direct targets for PACAP and express gonadotropin-releasing hormone receptor (GnRH-R), a marker of the gonadotrope lineage, we provide the first evidence that PACAP stimulates rat GnRH-R gene promoter activity. The EC(50) of this stimulation is compatible with a mediation via activation of the cyclic AMP-dependent signaling pathway and, consistently, co-transfection of an expression vector expressing the protein kinase A inhibitor causes reduction in PACAP as well as cholera toxin-stimulated promoter activity. Deletion and mutational analyses indicate that PACAP activation necessitates a bipartite response element that consists of a first region (-272/-237) termed PACAP response element (PARE) I that includes a steroidogenic factor-1 (SF-1)-binding site and a second region (-136/-101) referred to as PARE II that contains an imperfect cyclic AMP response element. Gel shift experiments indicate the specific binding of the SF-1 and a potential SF-1-interacting factor to PARE I while a protein immunologically related to the cyclic AMP response element-binding protein interacts with PARE II. These findings suggest that PACAP might regulate the GnRH-R gene at the transcriptional level, providing novel insights into the regulation of pituitary-specific genes by hypothalamic hypophysiotropic signals.

Activation of the rat follicle-stimulating hormone receptor promoter by steroidogenic factor 1 is blocked by protein kinase a and requires upstream stimulatory factor binding to a proximal E box element

The receptor for the pituitary glycoprotein hormone FSH (FSHR) and the nuclear hormone receptor steroidogenic factor 1 (SF-1) play important roles in control of the hypothalamic-pituitary- gonadal axis. FSHR is essential for integrating the pituitary FSH signal to gonadal response, while SF-1 is an important transcriptional regulator of many genes that function within this axis and is essential for the development of gonads and adrenal glands. Given the critical role of SF-1 in regulation of the gonads and the coexpression of FSHR and SF-1 in Sertoli and granulosa cells, we examined the ability of SF-1 to regulate transcription of the FSHR gene. We found that SF-1 stimulated rat FSHR promoter activity in a dose-dependent and promoter-specific manner. Examination of various promoter deletion mutants indicated that SF-1 acts through the proximal promoter region and upstream promoter sequences. An E box element within the proximal promoter is essential for activation of the FSHR promoter by SF-1. This element binds the transcriptional regulators USF1 and USF2 (upstream stimulatory factors 1 and 2) but not SF-1, as shown by electrophoretic mobility shift assays. In addition, functional studies identified a requirement for the USF proteins in SF-1 activation of FSHR and mapped an important regulatory domain within exons 4 and 5 of USF2. Cotransfection studies revealed that activation of protein kinase A leads to inhibition of SF-1-stimulated transcription of FSHR, while it synergized with SF-1 to activate the equine LH beta-promoter (ebeta). Thus, stimulation of the cAMP pathway differentially regulates SF-1 activation of the FSHR and ebeta-promoters.

Effects of mutating different steroidogenic factor-1 protein regions on gene regulation

The involvement of cyclic adenosine monophosphate cAMP-dependent protein kinase A (PKA) in the regulation of the steroidogenic acute regulatory protein (StAR) and the high-density lipoprotein receptor (HDL-R) genes by steroidogenic factor-1 (SF-1) and cAMP were examined. Cotransfection studies carried out in Kin 8 cells, a Y1 cell line (mouse adrenal) with a mutation in the type I PKA regulatory subunit, demonstrated that an intact PKA is required for maximal activation and that SF-1 participates in cAMP regulation of these genes. Site-directed mutational analysis was performed to examine which SF-1 regions could be involved in SF-1 transcriptional activation of the StAR and HDL-R genes. SF-1 regions protein analyzed were amino acids Thr 60, Ser 203, Ser 431, Thr 462, and the activation function-2 domain (amino acids 449-462). Plasmids encoding each of the mutated SF-1 proteins were cotransfected with the StAR and HDL-R promoter constructs into human bladder carcinoma (HTB-9) cells in the presence or absence of dibutyryl cAMP. The results of these studies suggest that although SF-1 is required for optimal promoter response to cAMP, transcriptional activation of genes by SF-1 and cAMP are promoter dependent, perhaps resulting from gene-specific interactions of this transcription factor with other regulatory proteins.

Characterization of a steroidogenic factor-1-binding site found in promoter of sterol carrier protein-2 gene

Sterol carrier protein-2 (SCP2) is thought to mediate intracellular cholesterol transport in steroidogenic tissues. To elucidate the mechanism underlying the expression of this gene, a 300-bp fragment of the SCP2 promoter was cloned and analyzed for regulatory motifs. This promoter region contained a SF-1 binding motif, three activator protein-1 elements, an insulin response element, and a peroxisomal proliferator response element. The putative SF-1 binding region reacted with recombinant SF-1 DNA-binding domain in a mobility shift assay. The SCP2 promoter fragment was linked to a luciferase reporter gene and cotransfected in the presence or absence of SF-1 into HTB-9 cells. The results indicated that SF-1 was able to increase SCP2 promoter activity, an effect that was enhanced by cAMP. Similar results were obtained when the SCP2 promoter construct was cotransfected into Y1 cells. Cotransfection studies carried out in Kin 8 cells, a Y1 cell line with a mutation that prevents cAMP activation of PKA, revealed that a functional PKA is required for cAMP induction of SCP2 gene transcription. These results demonstrated that SF-1 confers cAMP responsiveness to the SCP2 promoter suggesting that SF-1 activation may be critical in regulation of this cholesterol transport protein.

Steroidogenic factor-1 contributes to the cyclic-adenosine monophosphate down-regulation of human SRY gene expression

In mammals, male sex determination is initiated by SRY (sex-determining region of the Y chromosome) gene expression and followed by testicular development. This study describes specific down-regulation of the human SRY gene transcription by cAMP stimulation using reverse transcription-polymerase chain reaction experiments. Using transfection experiments, conserved nuclear hormone receptor (NHR1) and Sp1 consensus binding sites were identified as essential for this cAMP transcriptional response. Steroidogenic factor-1 (SF-1), a component of the sex-determination cascade, binds specifically to the NHR1 site and activates the SRY promoter. Activation of SF-1 was abolished by cAMP pretreatment of the cells, suggesting a possible effect of cAMP on the SF-1 protein itself. Indeed, human SF-1 protein contains at least two in vitro cAMP-dependent protein kinase (PKA) phosphorylation sites, leading after phosphorylation to a modification of both DNA-binding activity and interaction with general transcription factors such as Sp1. Taken together, these data suggest that cAMP responsiveness of human SRY promoter involves both SF-1 and Sp1 sites and could act via PKA phosphorylation of the SF-1 protein itself.

Expression and regulation of transcripts encoding two members of the NR5A nuclear receptor subfamily of orphan nuclear receptors, steroidogenic factor-1 and NR5A2, in equine ovarian cells during the ovulatory process

Steroidogenic factor-1 (SF-1, NR5A1a) is a member of the NR5A nuclear receptor subfamily and has been implicated as a key transcriptional regulator of all ovarian steroidogenic genes in vitro. To establish links between the expression of SF-1 and that of the steroidogenic genes in vivo, the objectives of this study were to clone equine SF-1 and examine the regulation of its messenger RNA (mRNA) in follicular cells during human CG (hCG)-induced ovulation. The equine SF-1 primary transcript was cloned by a combination of RT-PCR techniques. Results showed that the transcript was composed of a 5'-untranslated region (UTR) of 161 bp, an open reading frame (ORF) of 1386 bp that encodes a highly-conserved 461-amino acid protein, and a 3'-UTR of 518 bp. The cloning of SF-1 also led to the unexpected and serendipitous isolation of the highly-related orphan nuclear receptor NR5A2, which was shown to include a 5'-UTR of 243 bp, an ORF of 1488 bp, and a 3'-UTR of 1358 bp. The NR5A2 ORF encodes a 495-amino acid protein that is 60% identical to SF-1, including 99%-similar DNA-binding domains. Northern blot analysis revealed that SF-1 and NR5A2 were expressed in all major steroidogenic tissues, with the exception that NR5A2 was not present in the adrenal. Interestingly, NR5A2 was found to be, by far, the major NR5A subfamily member expressed in the preovulatory follicle and the corpus luteum. Using a semiquantitative RT-PCR/Southern blotting approach, the regulation of SF-1 and NR5A2 mRNAs in vivo was studied in equine follicular cells obtained from preovulatory follicles isolated between 0 and 39 h post hCG. Results showed that the theca interna was the predominant site of SF-1 mRNA expression in the follicle, and that hCG caused a significant decrease in SF-1 levels between 12-39 h in theca interna and between 24-39 h post hCG in granulosa cells (P < 0.05). In contrast, the granulosa cell layer was the predominant, if not the sole, site of NR5A2 mRNA expression in the follicle. Importantly, NR5A2 was much more highly expressed in granulosa cells than SF-1. The administration of hCG caused a significant decrease in NR5A2 transcripts in granulosa cells at 30, 36, and 39 h post hCG (P < 0.05). Thus, this study is the first to report the concomitant regulation of SF-1 in theca interna and granulosa cells throughout the ovulation/luteinization process, and to demonstrate the novel expression and hormonal regulation of NR5A2 in ovarian cells. Based on the marked expression of NR5A2 in equine granulosa and luteal cells and on mounting evidence of a functional redundancy between SF-1 and NR5A2 in other species, it is proposed that NR5A2 may play a key role in the regulation of gonadal steroidogenic gene expression.

Transcriptional regulation of rat scavenger receptor class B type I gene

The scavenger receptor class B type I (SR-BI) mediates the selective transport of lipids from high density lipoprotein to cells and plays an important role in the reverse uptake of cholesterol to the liver and in the delivery of substrates for steroidogenesis in steroidogenic organs. We report here on the isolation and characterization of the upstream promoter region of the rat SR-BI gene. The transcription start site for rat SR-BI was mapped, and DNA sequence analysis revealed the presence of binding sites for the Sp1 family in the proximal 5'-flanking region. Analysis of deletion mutants with different 5' lengths revealed that the region between -121 and -90 base pairs from the transcription start site is essential for the efficient transcription of SR-BI. Both Sp1 and Sp3 bind to three GC boxes in the region (-141 to -1 base pairs) in a sequence-specific manner. Mutations in any of the GC boxes decreased efficient transcription from this promoter in MA-10 mouse Leydig tumor cells. The overexpression of Sp1 or Sp3 protein enhanced the rat SR-BI promoter activity. These results indicate that Sp1 family members of transcription factors are essential for transcription of the rat SR-BI gene.

LXRalpha functions as a cAMP-responsive transcriptional regulator of gene expression

LXRalpha is a member of a nuclear receptor superfamily that regulates transcription. LXRalpha forms a heterodimer with RXRalpha, another member of this family, to regulate the expression of cholesterol 7alpha-hydroxylase by means of binding to the DR4-type cis-element. Here, we describe a function for LXRalpha as a cAMP-responsive regulator of renin and c-myc gene transcriptions by the interaction with a specific cis-acting DNA element, CNRE (an overlapping cAMP response element and a negative response element). Our previous studies showed that renin gene expression is regulated by cAMP, at least partly, through the CNRE sequence in its 5'-flanking region. This sequence is also found in c-myc and several other genes. Based on our cloning results using the yeast one-hybrid system, we discovered that the mouse homologue of human LXRalpha binds to the CNRE and demonstrated that it binds as a monomer. To define the function of LXRalpha on gene expression, we transfected the renin-producing renal As4.1 cells with LXRalpha expression plasmid. Overexpression of LXRalpha in As4.1 cells confers cAMP inducibility to reporter constructs containing the renin CNRE. After stable transfection of LXRalpha, As4.1 cells show a cAMP-inducible up-regulation of renin mRNA expression. In parallel experiments, we demonstrated that LXRalpha can also bind to the homologous CNRE in the c-myc promoter. cAMP promotes transcription through c-myc/CNRE:LXRalpha interaction in LXRalpha transiently transfected cells and increases c-myc mRNA expression in stably transfected cells. Identification of LXRalpha as a cAMP-responsive nuclear modulator of renin and c-myc expression not only has cardiovascular significance but may have generalized implication in the regulation of gene transcription.

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.

Influence of the HDL receptor SR-BI on atherosclerosis

The scavenger receptor class B, type I (SR-BI) is an HDL receptor that mediates selective cholesterol uptake from HDL to cells. In rodents, SR-BI has a critical influence on plasma HDL-cholesterol concentration and structure, the delivery of cholesterol to steroidogenic tissues, female fertility, and biliary cholesterol concentration. SR-BI can also serve as a receptor for non-HDL lipoproteins and appears to play an important role in reverse cholesterol transport. Recent studies involving the manipulation of SR-BI expression in mice, either using adenovirus-mediated or transgenic hepatic overexpression or using homologous recombination for complete functional ablation, indicate that the expression of SR-BI protects against atherosclerosis. If SR-BI has a similar activity in humans, it may become an attractive target for therapeutic intervention.

Sterol regulatory element-binding protein-1a binds to cis elements in the promoter of the rat high density lipoprotein receptor SR-BI gene

The high density lipoprotein (HDL) receptor, or scavenger receptor class B type I (SR-BI), is critical for cholesterol transport and a potential target for hypercholesterolemic drugs. Thus, elucidation of the mechanism underlying regulation of the HDL receptor SR-BI gene is essential. It has been previously shown that there is a correlation between depletion in ovarian cholesteryl ester content and increased HDL receptor SR-BI expression in response to hormonal stimulation. We wanted to determine whether the levels of mature sterol response element-binding protein-1a (SREBP-1a), a key protein in the transcriptional regulation of several genes by sterols, are affected under these conditions. Thus, Western blot analysis was carried out. Consistent with the possibility that SREBP-1a may be involved in the regulation of the HDL receptor SR-BI gene, we found that mature SREBP-1a levels increased up to 11-fold in the ovary after treatment with 50 U hCG. This increase in mature SREBP-1a protein levels correlated with a 30% decrease in ovarian cholesterol levels. These changes in both SREBP-1a and cholesterol levels preceded a 2-fold induction of HDL receptor SR-BI protein levels. To determine whether SREBP-1a could directly regulate the expression of the rat HDL receptor SR-BI gene, approximately 2.2 kb of the receptor SR-BI promoter were cloned and sequenced, and deletion analysis and mobility shift assays were performed. The results of these studies demonstrate that the rat HDL receptor SR-BI promoter contains two sterol response elements (pSRE and dSRE) through which SREBP-1a can bind and activate transcription of this gene. These motifs are similar to known SRE motifs reported for sterol-sensitive genes, and the pSRE is located between two Sp1 sites, similar to the SRE-1 motif in the low density lipoprotein receptor. The cysteine protease inhibitor N-acetyl-leucyl-leucyl-norleucinal, which inhibits SREBP degradation, enhanced the effect of SREBP-1a on the regulation of the rat HDL receptor SR-BI gene. It has previously been shown that tropic hormones such as hCG can also influence gene expression by increasing cAMP levels. Consistent with this fact, we have recently shown that steroidogenic factor-1 (SF-1) mediates cAMP activation of the HDL receptor SR-BI gene. Thus, we decided to examine whether SREBP-1a could cooperate with SF-1 to enhance transcription this gene. The results confirm that indeed both SF-1 and SREBP-1a synergize to induce HDL receptor SR-BI gene expression.