Phosphorylation and intramolecular stabilization of the ligand binding domain in the nuclear receptor steroidogenic factor 1

Steroidogenic Factor-1 form and function: From phospholipids to physiology

Steroidogenic Factor-1 (SF-1, NR5A1) is a member of the nuclear receptor superfamily of ligand-regulated transcription factors, consisting of a DNA-binding domain (DBD) connected to a transcriptional regulatory ligand binding domain (LBD) via an unstructured hinge domain. SF-1 is a master regulator of development and adult function along the hypothalamic pituitary adrenal and gonadal axes, with strong pathophysiological association with endometriosis and adrenocortical carcinoma. SF-1 was shown to bind and be regulated by phospholipids, one of the most interesting aspects of SF-1 regulation is the manner in which SF-1 interacts with phospholipids: SF-1 buries the phospholipid acyl chains deep in the hydrophobic core of the SF-1 protein, while the lipid headgroups remain solvent-exposed on the exterior of the SF-1 protein surface. Here, we have reviewed several aspects of SF-1 structure, function and physiology, touching on other transcription factors that help regulate SF-1 target genes, non-canonical functions of SF-1, the DNA-binding properties of SF-1, the use of mass spectrometry to identify lipids that associate with SF-1, how protein phosphorylation regulates SF-1 and the structural biology of the phospholipid-ligand binding domain. Together this review summarizes the form and function of Steroidogenic Factor-1 in physiology and in human disease, with particular emphasis on adrenal cancer.

Steroidogenic Factor 1, a Goldilocks Transcription Factor from Adrenocortical Organogenesis to Malignancy

Steroidogenic factor-1 (SF-1, also termed Ad4BP; NR5A1 in the official nomenclature) is a nuclear receptor transcription factor that plays a crucial role in the regulation of adrenal and gonadal development, function and maintenance. In addition to its classical role in regulating the expression of P450 steroid hydroxylases and other steroidogenic genes, involvement in other key processes such as cell survival/proliferation and cytoskeleton dynamics have also been highlighted for SF-1. SF-1 has a restricted pattern of expression, being expressed along the hypothalamic-pituitary axis and in steroidogenic organs since the time of their establishment. Reduced SF-1 expression affects proper gonadal and adrenal organogenesis and function. On the other hand, SF-1 overexpression is found in adrenocortical carcinoma and represents a prognostic marker for patients' survival. This review is focused on the current knowledge about SF-1 and the crucial importance of its dosage for adrenal gland development and function, from its involvement in adrenal cortex formation to tumorigenesis. Overall, data converge towards SF-1 being a key player in the complex network of transcriptional regulation within the adrenal gland in a dosage-dependent manner.

Variants of STAR, AMH and ZFPM2/FOG2 May Contribute towards the Broad Phenotype Observed in 46,XY DSD Patients with Heterozygous Variants of NR5A1

Variants of NR5A1 are often found in individuals with 46,XY disorders of sex development (DSD) and manifest with a very broad spectrum of clinical characteristics and variable sex hormone levels. Such complex phenotypic expression can be due to the inheritance of additional genetic hits in DSD-associated genes that modify sex determination, differentiation and organ function in patients with heterozygous NR5A1 variants. Here we describe the clinical, biochemical and genetic features of a series of seven patients harboring monoallelic variants in the NR5A1 gene. We tested the transactivation activity of novel NR5A1 variants. We additionally included six of these patients in a targeted diagnostic gene panel for DSD and identified a second genetic hit in known DSD-causing genes STAR, AMH and ZFPM2/FOG2 in three individuals. Our study increases the number of NR5A1 variants related to 46,XY DSD and supports the hypothesis that a digenic mode of inheritance may contribute towards the broad spectrum of phenotypes observed in individuals with a heterozygous NR5A1 variation.

The Orphan Nuclear Receptors Steroidogenic Factor-1 and Liver Receptor Homolog-1: Structure, Regulation, and Essential Roles in Mammalian Reproduction

Nuclear receptors are intracellular proteins that act as transcription factors. Proteins with classic nuclear receptor domain structure lacking identified signaling ligands are designated orphan nuclear receptors. Two of these, steroidogenic factor-1 (NR5A1, also known as SF-1) and liver receptor homolog-1 (NR5A2, also known as LRH-1), bind to the same DNA sequences, with different and nonoverlapping effects on targets. Endogenous regulation of both is achieved predominantly by cofactor interactions. SF-1 is expressed primarily in steroidogenic tissues, LRH-1 in tissues of endodermal origin and the gonads. Both receptors modulate cholesterol homeostasis, steroidogenesis, tissue-specific cell proliferation, and stem cell pluripotency. LRH-1 is essential for development beyond gastrulation and SF-1 for genesis of the adrenal, sexual differentiation, and Leydig cell function. Ovary-specific depletion of SF-1 disrupts follicle development, while LRH-1 depletion prevents ovulation, cumulus expansion, and luteinization. Uterine depletion of LRH-1 compromises decidualization and pregnancy. In humans, SF-1 is present in endometriotic tissue, where it regulates estrogen synthesis. SF-1 is underexpressed in ovarian cancer cells and overexpressed in Leydig cell tumors. In breast cancer cells, proliferation, migration and invasion, and chemotherapy resistance are regulated by LRH-1. In conclusion, the NR5A orphan nuclear receptors are nonredundant factors that are crucial regulators of a panoply of biological processes, across multiple reproductive tissues.

Novel NR5A1 mutations found in Chinese patients with 46, XY disorders of sex development

OBJECTIVE

To analyze nuclear receptor subfamily 5 group A member 1 (NR5A1) gene mutations in a cohort of Chinese patients with 46, XY Disorders of Sex Development (DSD).

METHODS

Sixty 46, XY DSD patients were recruited at Peking Union Medical College Hospital. Targeted next-generation and Sanger sequencing were performed to investigate pathogenic gene variants and validate NR5A1 gene variants, respectively. In silico tools and in vitro function studies were used to analyze the pathogenicity of rare variants. The clinical and endocrinological characteristics of patients with NR5A1 variants were retrospectively analyzed.

RESULTS

A total of four novel and three recurrent NR5A1 variants were identified in seven 46, XY DSD patients. These variants widely spread almost all the functional domains. Functional studies showed that novel mutations including p.S32N, p.N44del and p.G91D reduced transactivation of CYP11A1, while the other missense variant p.A168E did not impact protein function. All patients with NR5A1 rare variants had normal adrenal function and showed genital defects. Results of the genitalia examination showed female external genitalia (three patients), ambiguous external genitalia (two patients), female external genitalia with clitoromegaly (one patient), and hypospadias (one patient). All seven patients had bilateral testis and five of seven patients lacked Müllerian structures.

CONCLUSIONS

Four novel mutations in the NR5A1 gene were identified in our cohort with 46, XY DSD, expanding the spectrum of NR5A1 gene mutations. All patients with NR5A1 rare variants had normal adrenal function and showed genital defects.

ERK/MAPK regulates ERRγ expression, transcriptional activity and receptor-mediated tamoxifen resistance in ER+ breast cancer

Selective estrogen receptor modulators such as tamoxifen (TAM) significantly improve breast cancer-specific survival for women with estrogen receptor-positive (ER+) disease. However, resistance to TAM remains a major clinical problem. The resistant phenotype is usually not driven by loss or mutation of the estrogen receptor; instead, changes in multiple proliferative and/or survival pathways over-ride the inhibitory effects of TAM. Estrogen-related receptor γ (ERRγ) is an orphan member of the nuclear receptor superfamily that promotes TAM resistance in ER+ breast cancer cells. This study sought to clarify the mechanism(s) by which this orphan nuclear receptor is regulated, and hence affects TAM resistance. mRNA and protein expression/phosphorylation were monitored by RT-PCR and western blotting, respectively. Site-directed mutagenesis was used to disrupt consensus extracellular signal-regulated kinase (ERK) target sites. Cell proliferation and cell-cycle progression were measured by flow cytometric methods. ERRγ transcriptional activity was assessed by dual-luciferase promoter-reporter assays. We show that ERRγ protein levels are affected by the activation state of ERK/mitogen-activated protein kinase, and mutation of consensus ERK target sites impairs ERRγ-driven transcriptional activity and TAM resistance. These findings shed new light on the functional significance of ERRγ in ER+ breast cancer, and are the first to demonstrate a role for kinase regulation of this orphan nuclear receptor.

Mutational screening of the NR5A1 in azoospermia

Nuclear receptor subfamily 5 group A member 1 (NR5A1) encodes a nuclear receptor that regulates transcription of multiple genes involved in adrenal and gonadal development, steroidogenesis and the reproductive axis. Human mutations in NR5A1were initially found in two 46, XY female patients suffering from severe gonadal dysgenesis and primary adrenal failure. However, more recent case reports have suggested that heterozygous mutations in NR5A1 may also contribute to the male infertility aetiology. We have analysed the coding sequence of NR5A1 in a cohort of 90 well-characterised idiopathic Iranian azoospermic infertile men versus 112 fertile men. Heterozygous NR5A1 mutations were found in 2 of 90 (2.2%) of cases. These two patients harboured missense mutations within the hinge region (p.P97T) and ligand-binding domain (p.E237K) of the NR5A1 protein. In silico analysis of the mutations showed that founded mutations could be detrimental. In conclusion, findings of the current and previous studies suggest that mutations in the NR5A1 gene are not common in azoospermia, and male infertility and inclusion of NR5A1 mutation screening in the diagnostic workup of male infertility may seem unnecessary.

The pERK of being a target: Kinase regulation of the orphan nuclear receptor ERRγ

Estrogen-related receptors (ERRs) are orphan members of the nuclear receptor superfamily that are important regulators of mitochondrial metabolism with emerging roles in cancer. In the absence of an endogenous ligand, ERRs are reliant upon other regulatory mechanisms that include protein/protein interactions and post-translational modification, though the cellular and clinical significance of this latter mechanism is unclear. We recently published a study in which we establish estrogen-related receptor gamma (ERRγ) as a target for extracellular signal-regulated kinase (ERK), and show that regulation of ERRγ by ERK has important consequences for the function of this receptor in cellular models of estrogen receptor-positive (ER+) breast cancer. In this Research Highlight, we discuss the implications of these findings from a molecular and clinical perspective.

The orphan nuclear receptors at their 25-year reunion

The nuclear receptor superfamily includes many receptors, identified based on their similarity to steroid hormone receptors but without a known ligand. The study of how these receptors are diversely regulated to interact with genomic regions to control a plethora of biological processes has provided critical insight into development, physiology, and the molecular pathology of disease. Here we provide a compendium of these so-called orphan receptors and focus on what has been learned about their modes of action, physiological functions, and therapeutic promise.

An Iranian family with azoospermia and premature ovarian insufficiency segregating NR5A1 mutation

In brief, we report an Iranian family with a history of both azoospermia and premature ovarian insufficiency with the same heterozygote mutation in the NR5A1 gene that can be transmitted. As far as we know, this is the first observation that a common mutation in NR5A1 can cause these above-mentioned phenotypes in a family.

GADD45G functions in male sex determination by promoting p38 signaling and Sry expression

Male sex determination in mammals is induced by Sry, a gene whose regulation is poorly understood. Here we show that mice mutant for the stress-response gene Gadd45g display complete male-to-female sex reversal. Gadd45g and Sry have a strikingly similar expression pattern in the genital ridge, and they are coexpressed in gonadal somatic cells. In Gadd45g mutants, Sry expression is delayed and reduced, and yet Sry seemed to remain poised for expression, because its promoter is demethylated on schedule and is occupied by active histone marks. Instead, p38 MAPK signaling is impaired in Gadd45g mutants. Moreover, the transcription factor GATA4, which is required for Sry expression, binds to the Sry promoter in vivo in a MAPK-dependent manner. The results suggest that a signaling cascade, involving GADD45G → p38 MAPK → GATA4 → SRY, regulates male sex determination.

Acid ceramidase (ASAH1) represses steroidogenic factor 1-dependent gene transcription in H295R human adrenocortical cells by binding to the receptor

Adrenocorticotropin (ACTH) signaling increases glucocorticoid production by promoting the interaction of transcription factors and coactivator proteins with the promoter of steroidogenic genes. The nuclear receptor steroidogenic factor 1 (SF-1) is essential for steroidogenic gene transcription. Sphingosine (SPH) is a ligand for SF-1. Moreover, suppression of expression of acid ceramidase (ASAH1), an enzyme that produces SPH, increases the transcription of multiple steroidogenic genes. Given that SF-1 is a nuclear protein, we sought to define the molecular mechanisms by which ASAH1 regulates SF-1 function. We show that ASAH1 is localized in the nuclei of H295R adrenocortical cells and that cyclic AMP (cAMP) signaling promotes nuclear sphingolipid metabolism in an ASAH1-dependent manner. ASAH1 suppresses SF-1 activity by directly interacting with the receptor. Chromatin immunoprecipitation (ChIP) assays revealed that ASAH1 is recruited to the promoter of various SF-1 target genes and that ASAH1 and SF-1 colocalize on the same promoter region of the CYP17A1 and steroidogenic acute regulatory protein (StAR) genes. Taken together, these results demonstrate that ASAH1 is a novel coregulatory protein that represses SF-1 function by directly binding to the receptor on SF-1 target gene promoters and identify a key role for nuclear lipid metabolism in regulating gene transcription.

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

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

Transcriptional control of hepatocyte differentiation

The liver is the largest glandular organ in the body and plays a central role in controlling metabolism. During hepatogenesis, complex developmental processes must generate an array of cell types that are spatially arranged to generate a hepatic architecture that is essential to support liver function. The processes that control the ultimate formation of the liver are diverse and complex and in many cases poorly defined. Much of the focus of research during the past three decades has been on understanding how hepatocytes, which are the predominant liver parenchymal cells, differentiate during embryogenesis. Through a combination of mouse molecular genetics, embryology, and molecular biochemistry, investigators have defined a myriad of transcription factors that combine to control formation and function of hepatocytes. Here, we will review the major discoveries that underlie our current understanding of transcriptional regulation of hepatocyte differentiation.

Coactivation of SF-1-mediated transcription of steroidogenic enzymes by Ubc9 and PIAS1

Steroidogenic factor-1 (SF-1) is a nuclear orphan receptor, which is essential for adrenal development and regulation of steroidogenic enzyme expression. SF-1 is posttranslationally modified by small ubiquitin-related modifier-1 (SUMO-1), thus mostly resulting in attenuation of transcription. We investigated the role of sumoylation enzymes, Ubc9 and protein inhibitors of activated STAT1 (PIAS1), in SF-1-mediated transcription of steroidogenic enzyme genes in the adrenal cortex. Coimmunoprecipitation assays showed that both Ubc9 and PIAS1 interacted with SF-1. Transient transfection assays in adrenocortical H295R cells showed Ubc9 and PIAS1 potentiated SF-1-mediated transactivation of reporter constructs containing human CYP17, CYP11A1, and CYP11B1 but not CYP11B2 promoters. Reduction of endogenous Ubc9 and PIAS1 by introducing corresponding small interfering RNA significantly reduced endogenous CYP17, CYP11A1, and CYP11B1 mRNA levels, indicating that they normally function as coactivators of SF-1. Wild type and sumoylation-inactive mutants of Ubc9 and PIAS1 can similarly enhance the SF-1-mediated transactivation of the CYP17 gene, indicating that the coactivation potency of Ubc9 and PIAS1 is independent of sumoylation activity. Chromatin immunoprecipitation assays demonstrated that SF-1, Ubc9, and PIAS1 were recruited to an endogenous CYP17 gene promoter in the context of chromatin in vivo. Immunohistochemistry and Western blotting showed that SF-1, Ubc9, and PIAS1 were expressed in the nuclei of the human adrenal cortex. In cortisol-producing adenomas, the expression pattern of SF-1 and Ubc9 were markedly increased, whereas that of PIAS1 was decreased compared with adjacent normal adrenals. These results showed the physiological roles of Ubc9 and PIAS1 as SF-1 coactivators beyond sumoylation enzymes in adrenocortical steroidogenesis and suggested their possible pathophysiological roles in human cortisol-producing adenomas.

NR5A1/SF-1 and development and function of the ovary

Primary ovarian insufficiency (POI) is defined as cessation of menstruation with associated elevation of gonadotropin levels as a result of decreased ovarian function before the age of 40. The incidence of POI is 1% in women prior to age 40, and 0.1% prior to age 30. There is evidence of a strong genetic component associated with POI. However, the gene mutations/variations influencing POI still remain uncharacterized. NR5A1, a member of the nuclear receptor superfamily, is a key transcriptional regulator of genes involved in the hypothalamic-pituitary-gonadal steroidogenic axis. Newborn mice deficient in NR5A1 lack both gonads and adrenal glands and have impaired expression of pituitary gonadotrophins. NR5A1 is also expressed in multiple cell types in the fetal, postnatal, prepubertal and mature ovary. Until 2008, 18 NR5A1 mutations were described in the human. Three of these were identified in individuals with adrenal insufficiency, two associated with 46,XY disorders of sex development (DSD) and the third a 46,XX female with conserved ovarian function. Other mutations were associated with various anomalies of testis development with no evidence of adrenal failure. We have identified further 19 mutations in NR5A1 including mutations in four familial cases having individuals with 46,XY DSD as well as POI. A further analysis of 25 sporadic cases of POI revealed two additional mutations. Functional analysis revealed that each mutant protein had altered transactivational properties on gonadal promoters. These data reveal novels insights into the role of NR5A1 in ovarian developmental and function and indicate that mutations of the NR5A1 gene may be a significant cause of human ovarian insufficiency.

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.

Molecular aspects of steroidogenic factor 1 (SF-1)

Steroidogenic factor 1 (SF-1, also called Ad4BP and NR5A1) is a nuclear receptor with critical roles in steroidogenic tissues, as well as in the brain and pituitary. In particular, SF-1 has emerged as an essential regulator of adrenal and gonadal functions and development. In the last few years, our knowledge on SF-1 has increased considerably at all levels, from the gene to the protein, and on its specific roles in different physiological processes. In this review, we discuss the current understanding on SF-1 with focus on the parameters that control the transcriptional capacity of SF-1 and the mechanisms that ensure proper stage- and tissue-specific expression of the gene encoding SF-1.

Pin1 facilitates the phosphorylation-dependent ubiquitination of SF-1 to regulate gonadotropin beta-subunit gene transcription

Pin1 is a peptidyl-prolyl cis-trans isomerase which catalyzes the isomerization of phosphorylated Ser/Thr-Pro peptide bonds. Pin1 knockout mice have marked abnormalities in their reproductive development and function. However, the molecular mechanisms underlying their reproductive defects are poorly understood. Herein, we demonstrate that Pin1 is required for both basal and GnRH-induced gonadotropin beta-subunit gene transcription, through interactions with the transcription factors SF-1, Pitx1, and Egr-1. Pin1 activates transcription of the gonadotropin beta-subunit genes synergistically with these transcription factors, either by modulating their stability or by increasing their protein-protein interactions. Notably, we provide evidence that Pin1 is required for the Ser203 phosphorylation-dependent ubiquitination of SF-1, which facilitates SF-1-Pitx1 interactions and therefore results in an enhancement of SF-1 transcriptional activity. Furthermore, we demonstrate that in gonadotrope cells, sufficient levels of activated Pin1 are maintained through transcriptional and posttranslational regulation by GnRH-induced signaling cascades. Our results suggest that Pin1 functions as a novel player in GnRH-induced signal pathways and is involved in gonadotropin beta-subunit gene transcription by modulating the activity of various specific transcription factors.

Loss of mitogen-activated protein kinase kinase kinase 4 (MAP3K4) reveals a requirement for MAPK signalling in mouse sex determination

Sex determination in mammals is controlled by the presence or absence of the Y-linked gene SRY. In the developing male (XY) gonad, sex-determining region of the Y (SRY) protein acts to up-regulate expression of the related gene, SOX9, a transcriptional regulator that in turn initiates a downstream pathway of testis development, whilst also suppressing ovary development. Despite the requirement for a number of transcription factors and secreted signalling molecules in sex determination, intracellular signalling components functioning in this process have not been defined. Here we report a role for the phylogenetically ancient mitogen-activated protein kinase (MAPK) signalling pathway in mouse sex determination. Using a forward genetic screen, we identified the recessive boygirl (byg) mutation. On the C57BL/6J background, embryos homozygous for byg exhibit consistent XY gonadal sex reversal. The byg mutation is an A to T transversion causing a premature stop codon in the gene encoding MAP3K4 (also known as MEKK4), a mitogen-activated protein kinase kinase kinase. Analysis of XY byg/byg gonads at 11.5 d post coitum reveals a growth deficit and a failure to support mesonephric cell migration, both early cellular processes normally associated with testis development. Expression analysis of mutant XY gonads at the same stage also reveals a dramatic reduction in Sox9 and, crucially, Sry at the transcript and protein levels. Moreover, we describe experiments showing the presence of activated MKK4, a direct target of MAP3K4, and activated p38 in the coelomic region of the XY gonad at 11.5 d post coitum, establishing a link between MAPK signalling in proliferating gonadal somatic cells and regulation of Sry expression. Finally, we provide evidence that haploinsufficiency for Map3k4 accounts for T-associated sex reversal (Tas). These data demonstrate that MAP3K4-dependent signalling events are required for normal expression of Sry during testis development, and create a novel entry point into the molecular and cellular mechanisms underlying sex determination in mice and disorders of sexual development in humans.

Heat shock protein 27 is involved in SUMO-2/3 modification of heat shock factor 1 and thereby modulates the transcription factor activity

Heat shock protein 27 (HSP27) accumulates in stressed cells and helps them to survive adverse conditions. We have already shown that HSP27 has a function in the ubiquitination process that is modulated by its oligomerization/phosphorylation status. Here, we show that HSP27 is also involved in protein sumoylation, a ubiquitination-related process. HSP27 increases the number of cell proteins modified by small ubiquitin-like modifier (SUMO)-2/3 but this effect shows some selectivity as it neither affects all proteins nor concerns SUMO-1. Moreover, no such alteration in SUMO-2/3 conjugation is achievable by another HSP, such as HSP70. Heat shock factor 1 (HSF1), a transcription factor responsible for HSP expression, is one of the targets of HSP27. In stressed cells, HSP27 enters the nucleus and, in the form of large oligomers, binds to HSF1 and induces its modification by SUMO-2/3 on lysine 298. HSP27-induced HSF1 modification by SUMO-2/3 takes place downstream of the transcription factor phosphorylation on S303 and S307 and does not affect its DNA-binding ability. In contrast, this modification blocks HSF1 transactivation capacity. These data show that HSP27 exerts a feedback inhibition of HSF1 transactivation and enlighten the strictly regulated interplay between HSPs and HSF1. As we also show that HSP27 binds to the SUMO-E2-conjugating enzyme, Ubc9, our study raises the possibility that HSP27 may act as a SUMO-E3 ligase specific for SUMO-2/3.

Mutations in NR5A1 associated with ovarian insufficiency

BACKGROUND

The genetic causes of nonsyndromic ovarian insufficiency are largely unknown. A nuclear receptor, NR5A1 (also called steroidogenic factor 1), is a key transcriptional regulator of genes involved in the hypothalamic-pituitary-steroidogenic axis. Mutation of NR5A1 causes 46,XY disorders of sex development, with or without adrenal failure, but growing experimental evidence from studies in mice suggests a key role for this factor in ovarian development and function as well.

METHODS

To test the hypothesis that mutations in NR5A1 cause disorders of ovarian development and function, we sequenced NR5A1 in four families with histories of both 46,XY disorders of sex development and 46,XX primary ovarian insufficiency and in 25 subjects with sporadic ovarian insufficiency. None of the affected subjects had clinical signs of adrenal insufficiency.

RESULTS

Members of each of the four families and 2 of the 25 subjects with isolated ovarian insufficiency carried mutations in the NR5A1 gene. In-frame deletions and frameshift and missense mutations were detected. Functional studies indicated that these mutations substantially impaired NR5A1 transactivational activity. Mutations were associated with a range of ovarian anomalies, including 46,XX gonadal dysgenesis and 46,XX primary ovarian insufficiency. We did not observe these mutations in more than 700 control alleles.

CONCLUSIONS

NR5A1 mutations are associated with 46,XX primary ovarian insufficiency and 46,XY disorders of sex development.

SUMO-specific protease 1 (SENP1) reverses the hormone-augmented SUMOylation of androgen receptor and modulates gene responses in prostate cancer cells

The acceptor sites for small ubiquitin-like modifier (SUMO) are conserved in the N-terminal domains of several nuclear receptors. Here, we show that androgens induce rapid and dynamic conjugation of SUMO-1 to androgen receptor (AR). Nuclear import of AR is not sufficient for SUMOylation, because constitutively nuclear apo-ARs or antagonist-bound ARs are only very weakly modified by SUMO-1 in comparison with agonist-bound ARs. Of the SUMO-specific proteases (SENP)-1, -2, -3, -5, and -6, only SENP1 and SENP2 are efficient in cleaving AR-SUMO-1 conjugates in intact cells and in vitro. Both SENP1 and -2 are nuclear and found at sites proximal to AR. Their expression promotes AR-dependent transcription, but in a promoter-selective fashion. SENP1 and -2 stimulated the activity of holo-AR on compound androgen response element-containing promoters. The effects of SENP1 and -2 on AR-dependent transcription were dependent on catalytic activity and required intact SUMO acceptor sites in AR, indicating that their coactivating effects are mainly due to their direct isopeptidase activity on holo-AR. In prostate cancer cells, ectopic expression of SENP1, but not that of SENP2, increased the transcription activity of endogenous AR. Silencing of SENP1 attenuated the expression of several AR target genes and blunted androgen-stimulated growth of LNCaP cells. Our results indicate that SENP1 reverses the ligand-induced SUMOylation of AR and helps fine tune the cellular responses to androgens in a target promoter-selective manner.

Conditional deletion of beta-catenin mediated by Amhr2cre in mice causes female infertility

Follicle-stimulating hormone (FSH) regulation of aromatase gene expression in vitro requires the transcriptional coactivator beta-catenin. To ascertain the physiological significance of beta-catenin in granulosa cells during folliculogenesis, mice homozygous for floxed alleles of beta-catenin were intercrossed with Amhr2cre mice. Conditional deletion of beta-catenin in 8-wk-old females occurred in derivatives of the Müllerian duct, granulosa cells and, surprisingly, in brain, pituitary, heart, liver, and tail. Female mice deficient for beta-catenin were infertile, despite reaching puberty and ovulating at the expected age, indications of apparently normal ovarian function. In contrast, their oviducts were grossly distended, with fewer but healthy oocytes. In addition, their uteri lacked implantation sites. Together, these two phenotypes could explain the complete loss of fertility. Nevertheless, although the ovary appeared normal, with serum estradiol concentrations in the normal range, there was marked animal-to-animal variation of mRNAs encoding beta-catenin and aromatase. Similarly, inhibin-alpha and luteinizing hormone receptor mRNAs varied considerably in whole ovaries, whereas pituitary Fshb mRNA was significantly reduced. Collectively, these features suggested cyclization recombination (CRE)-mediated recombination of beta-catenin may be unstable in proliferating granulosa cells, and therefore may mask the suspected steroidogenic requirement for beta-catenin. We tested this possibility by transducing primary cultures of granulosa cells from mice homozygous for floxed alleles of beta-catenin with a CRE-expressing adenovirus. Reduction of beta-catenin significantly compromised FSH stimulation of aromatase mRNA and subsequent production of estradiol. Collectively, these data suggest that FSH regulation of steroidogenesis requires beta-catenin, a role that remains hidden when tested through Amhr2cre-mediated recombination in vivo.

Role of dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 in protein kinase A- and protein kinase C-mediated regulation of the steroidogenic acute regulatory protein expression in mouse Leydig tumor cells: mechanism of action

Dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 (DAX-1) is an orphan nuclear receptor that has been demonstrated to be instrumental to the expression of the steroidogenic acute regulatory (StAR) protein that regulates steroid biosynthesis in steroidogenic cells. However, its mechanism of action remains obscure. The present investigation was aimed at exploring the molecular involvement of DAX-1 in protein kinase A (PKA)- and protein kinase C (PKC)-mediated regulation of StAR expression and its concomitant impact on steroid synthesis using MA-10 mouse Leydig tumor cells. We demonstrate that activation of the PKA and PKC pathways, by a cAMP analog dibutyryl (Bu)2cAMP [(Bu)2cAMP] and phorbol 12-myristate 13-acetate (PMA), respectively, markedly decreased DAX-1 expression, an event that was inversely correlated with StAR protein, StAR mRNA, and progesterone levels. Notably, the suppression of DAX-1 requires de novo transcription and translation, suggesting that the effect of DAX-1 in regulating StAR expression is dynamic. Chromatin immunoprecipitation studies revealed the association of DAX-1 with the proximal but not the distal region of the StAR promoter, and both (Bu)2cAMP and PMA decreased in vivo DAX-1-DNA interactions. EMSA and reporter gene analyses demonstrated the functional integrity of this interaction by showing that DAX-1 binds to a DNA hairpin at position -44/-20 bp of the mouse StAR promoter and that the binding of DAX-1 to this region decreases progesterone synthesis by impairing transcription of the StAR gene. In support of this, targeted silencing of endogenous DAX-1 elevated basal, (Bu)2cAMP-, and PMA-stimulated StAR expression and progesterone synthesis. Transrepression of the StAR gene by DAX-1 was tightly associated with expression of the nuclear receptors Nur77 and steroidogenic factor-1, demonstrating these factors negatively modulate the steroidogenic response. These findings provide insight into the molecular events by which DAX-1 influences the PKA and PKC signaling pathways involved in the regulation of the StAR protein and steroidogenesis in mouse Leydig tumor cells.

Steroidogenic factor-1 (SF-1, Ad4BP, NR5A1) and disorders of testis development

Steroidogenic factor-1 (SF-1) (Ad4BP, NR5A1) is a nuclear receptor that regulates many aspects of adrenal and reproductive development and function. Consequently, deletion of the gene (Nr5a1) encoding Sf-1 in XY mice results in impaired adrenal development, complete testicular dysgenesis with Mullerian structures, and female external genitalia. Initial efforts to identify NR5A1 changes in humans focused on 46,XY individuals with combined adrenogonadal failure and Mullerian structures. Although this combination of clinical features is rare, 2 such patients harboring NR5A1 mutations have been described within the past decade. More recently, however, it has emerged that heterozygous loss of function mutations in NR5A1 can be found relatively frequently in children and adults with 46,XY disorders of sex development (DSD) but with apparently normal adrenal function. The phenotypic spectrum associated with these changes ranges from complete testicular dysgenesis with Mullerian structures, through individuals with mild clitoromegaly or genital ambiguity, to severe penoscrotal hypospadias or even anorchia. Furthermore, a non-synonymous polymorphism in NR5A1 may be associated with micropenis or undescended testes within the population. Taken together, these reports suggest that variable loss of SF-1 function can be associated with a wide range of reproductive phenotypes in humans.

Identification of a novel nurr1-interacting protein

The orphan nuclear receptor Nurr1 is required for the development of ventral mesencephalic dopaminergic neurons in mice. One of the possible mechanisms that might contribute to the regulation activity of Nurr1 is through interaction with other proteins. To identify potential partners of Nurr1, we screened a yeast two-hybrid library from developing mouse embryonic mesencephalon with the Nurr1 ligand-binding domain (NLBD). We identified a novel interacting protein, termed the Nurr1-interacting protein (NuIP). We demonstrate that it specifically interacts with NLBD using the mammalian two-hybrid assay and coimmunoprecipitation studies in MN9D cells. In addition, we show that NuIP interacts with Nurr1 in lysates from substantia nigra. Coexpression of NuIP with Nurr1 results in potentiation of the transcriptional activity of Nurr1 on an nerve growth factor inducible-B response element reporter, as well as reporters driven by the endogenous tyrosine hydroxylase promoter. The mechanism underlying the regulatory action of NuIP on Nurr1 is demonstrated to be through assembly of distinct helical domains of the NLBD. Using a NuIP specific antibody, we show that expression of NuIP protein is mainly colocalized with Nurr1 in adult midbrain dopaminergic neurons. Finally, we demonstrate that suppression of NuIP expression in MN9D cells by NuIP-specific small interfering RNA leads to decreased cell division and decreased expression of a Nurr1 target gene, the dopamine transporter. These results suggest NuIP interacts with and positively regulates the activity of Nurr1 protein and modulates the phenotype of dopaminergic cells.

Androstenedione up-regulation of endometrial aromatase expression via local conversion to estrogen: potential relevance to the pathogenesis of endometriosis

CONTEXT

Up-regulation of aromatase expression in endometrial cells disseminated into the peritoneal cavity may enhance their survival via local estrogen synthesis, which may lead to endometriosis. The factors that mediate induction of aromatase in the endometrium are not well defined, but increased expression of steroidogenic factor (SF)-1 may play a role.

OBJECTIVE

The objective of the study was to determine whether androstenedione (A4), the predominant sex steroid in peritoneal fluid, regulates endometrial aromatase expression.

DESIGN

This was a cell/tissue culture study.

SETTING

The study was conducted at an academic center.

METHODS

Quantitative real-time PCR, HPLC, and chromatin immunoprecipitation were used in this study.

RESULTS

Treatment of cultured human endometrial explants and stromal cells with A4 (10 nm) significantly up-regulated expression of aromatase mRNA transcripts containing exon IIa at their 5'-ends. In endometrial stromal cells and the human endometrial surface epithelial (HES) cell line, induction of aromatase mRNA by A4 was associated with increased expression of SF-1. In HES cells, tritiated A4 was metabolized to estradiol, testosterone (T), dihydrotestosterone, and androstanediol. Both estradiol and T, but not nonaromatizable androgens, up-regulated aromatase and SF-1 mRNA in HES cells. Chromatin immunoprecipitation revealed that A4 enhanced recruitment of SF-1 to its response element (-136 bp) upstream of CYP19 exon IIa. This, together with the findings that both estrogen receptor antagonist, ICI 182,780, and aromatase inhibitor, fadrozole, suppressed A4 and T induction of aromatase and SF-1 mRNA, indicates that the inductive effects of A4 and T are mediated by their conversion to estrogens.

CONCLUSIONS

Exposure of endometrial cells to A4 may enhance CYP19 gene expression through its aromatization to estrogens.

Identification of the first synthetic steroidogenic factor 1 inverse agonists: pharmacological modulation of steroidogenic enzymes

Steroidogenic factor SF-1, a constitutively active nuclear hormone receptor, is essential to the development of adrenal and gonadal glands and acts as a shaping factor of sexual determination and differentiation. Its effects are exerted primarily through the control of the synthesis of steroid hormones. The functional cell-based assay Receptor Selection and Amplification Technology (R-SAT) was used to identify potent and selective SF-1 inverse agonists through the screening of a chemical library of drug-like small-molecule entities. Among them, 4-(heptyloxy)phenol (AC-45594), a prototype inverse agonist lead, was used to show that SF-1 constitutive activity can be pharmacologically modulated by a synthetic ligand. In a physiological system of endocrine function, the expression of several reported SF-1 target genes, including SF-1 itself, was inhibited by treatment with AC-45594 and analogs. Thus, pharmacological modulation of SF-1 is critical to its function as an endocrine master regulator and has potentially important consequences to diseases in which SF-1 activity is critical.

Phosphorylation of liver X receptor alpha selectively regulates target gene expression in macrophages

Dysregulation of liver X receptor alpha (LXRalpha) activity has been linked to cardiovascular and metabolic diseases. Here, we show that LXRalpha target gene selectivity is achieved by modulation of LXRalpha phosphorylation. Under basal conditions, LXRalpha is phosphorylated at S198; phosphorylation is enhanced by LXR ligands and reduced both by casein kinase 2 (CK2) inhibitors and by activation of its heterodimeric partner RXR with 9-cis-retinoic acid (9cRA). Expression of some (AIM and LPL), but not other (ABCA1 or SREBPc1) established LXR target genes is increased in RAW 264.7 cells expressing the LXRalpha S198A phosphorylation-deficient mutant compared to those with WT receptors. Surprisingly, a gene normally not expressed in macrophages, the chemokine CCL24, is activated specifically in cells expressing LXRalpha S198A. Furthermore, inhibition of S198 phosphorylation by 9cRA or by a CK2 inhibitor similarly promotes CCL24 expression, thereby phenocopying the S198A mutation. Thus, our findings reveal a previously unrecognized role for phosphorylation in restricting the repertoire of LXRalpha-responsive genes.

Welcoming beta-catenin to the gonadotropin-releasing hormone transcriptional network in gonadotropes

GnRH binds its G-coupled protein receptor, GnRHR, on pituitary gonadotropes and stimulates transcription of Cga, Lhb, and Fshb. These three genes encode two heterodimeric glycoprotein hormones, LH and FSH, that act as gonadotropins by regulating gametogenesis and steroidogenesis in both the testes and ovary. GnRH also regulates transcription of Gnrhr. Thus, regulated expression of Cga, Lhb, Fshb, and Gnrhr provides a genomic signature unique to functional gonadotropes. Steadily increasing evidence now indicates that GnRH regulates transcription of its four signature genes indirectly through a hierarchical transcriptional network that includes distinct subclasses of DNA-binding proteins that comprise the immediate early gene (IEG) family. These IEGs, in turn, confer hormonal responsiveness to the four signature genes. Although the IEGs confer responsiveness to GnRH, they cannot act alone. Instead, additional DNA-binding proteins, including the orphan nuclear receptor steroidogenic factor 1, act permissively to allow the four signature genes to respond to GnRH-induced changes in IEG levels. Emerging new findings now indicate that beta-catenin, a transcriptional coactivator and member of the canonical WNT signaling pathway, also plays an essential role in transducing the GnRH signal by interacting with multiple DNA-binding proteins in gonadotropes. Herein we propose that these interactions with beta-catenin define a multicomponent transcriptional network required for regulated expression of the four signature genes of the gonadotrope, Cga, Lhb, Fshb, and Gnrhr.

Multiple Signaling Pathways Coordinate CYP17 Gene Expression in the Human Adrenal Cortex

Optimal levels of steroid hormone biosynthesis are assured by the integration of several regulatory mechanisms, including substrate delivery, enzymatic activity, and gene transcription. In the human adrenal cortex, optimal glucocorticoid secretion is achieved by the actions of adrenocorticotropin (ACTH), which exerts transcriptional pressure on all genes involved in steroidogenesis. One of these genes is CYP17, which encodes P450 17alpha-hydroxylase-17,20 lyase, a key enzyme in the production of cortisol and adrenal androgens. Levels of CYP17 transcription are regulated by multiple regulatory mechanisms that act to respond to various signaling cues. These cues are coordinated in a developmental, species-, and tissue-specific manner, with an additional time/circadian-dependent level of regulation. This brief review will highlight some of the signal transduction cascades and transcription factors that have been shown to modulate CYP17 gene expression in the adrenal cortex.

Insulin-like growth factor-I, regulating aromatase expression through steroidogenic factor 1, supports estrogen-dependent tumor Leydig cell proliferation

The aim of this study was to investigate the role of estrogens in Leydig cell tumor proliferation. We used R2C rat Leydig tumor cells and testicular samples from Fischer rats with a developed Leydig tumor. Both experimental models express high levels of aromatase and estrogen receptor alpha (ERalpha). Treatment with exogenous 17beta-estradiol (E(2)) induced proliferation of R2C cells and up-regulation of cell cycle regulators cyclin D1 and cyclin E, the expression of which was blocked by addition of antiestrogens. These observations led us to hypothesize an E(2)/ERalpha-dependent mechanism for Leydig cell tumor proliferation. In determining the molecular mechanism responsible for aromatase overexpression, we found that total and phosphorylated levels of transcription factors cyclic AMP-responsive element binding protein and steroidogenic factor 1 (SF-1) were higher in tumor samples. Moreover, we found that tumor Leydig cells produce high levels of insulin-like growth factor I (IGF-I), which increased aromatase mRNA, protein, and activity as a consequence of increased total and phosphorylated SF-1 levels. Specific inhibitors of IGF-I receptor, protein kinase C, and phosphatidylinositol 3-kinase determined a reduction in SF-1 expression and in IGF-I-dependent SF-1 recruitment to the aromatase PII promoter. The same inhibitors also inhibited aromatase expression and activity and, consequently, R2C cell proliferation. We can conclude that one of the molecular mechanisms determining Leydig cell tumorigenesis is an excessive estrogen production that stimulates a short autocrine loop determining cell proliferation. In addition, cell-produced IGF-I amplifies estrogen signaling through an SF-1-dependent up-regulation of aromatase expression. The identification of this molecular mechanism will be helpful in defining new therapeutic approaches for Leydig cell tumors.

Phosphorylation of steroidogenic factor 1 is mediated by cyclin-dependent kinase 7

The nuclear receptor steroidogenic factor-1 (SF1) is critical for development and function of steroidogenic tissues. Posttranslational modifications are known to influence the transcriptional capacity of SF1, and it was previously demonstrated that serine 203 is phosphorylated. In this paper we report that serine 203 is phosphorylated by a cyclin-dependent kinase 7 (CDK7)-mediated process. As part of the CDK-activating kinase complex, CDK7 is a component of the basal transcription factor TFIIH, and phosphorylation of SF1 as well as SF1-dependent transcription was clearly reduced in cells carrying a mutation that renders the CDK-activating kinase complex unable to interact with the TFIIH core. Coimmunoprecipitation analyses revealed that SF1 and CDK7 reside in the same complex, and kinase assays demonstrated that immunoprecipitated CDK7 and purified TFIIH phosphorylate SF1 in vitro. The CDK inhibitor roscovitine blocked phosphorylation of SF1, and an inactive form of CDK7 repressed the phosphorylation level and the transactivation capacity of SF1. Structural studies have identified phosphoinositides as potential ligands for SF1. Interestingly, we found that mutations designed to block phospholipid binding dramatically decreased the level of SF1 phosphorylation. Together our results suggest a connection between ligand occupation and phosphorylation and association with the basic transcriptional machinery, indicating an intricate regulation of SF1 transactivation.

SMAD3 inhibits SF-1-dependent activation of the CYP17 promoter in H295R cells

Cytochrome P450c17, encoded by the CYP17 gene, is a component of 17alpha-hydroxylase/17,20 lyase which catalyses 17alpha-hydroxylation of pregnenolone or progesterone, required for glucocorticosteroid and androgen synthesis. It has been reported that transforming growth factor beta (TGF-beta) decreases both basal and cAMP-stimulated levels of CYP17 mRNA, but the mechanism of TGF-beta action on CYP17 expression remains unknown. We investigated an inhibitory effect of TGF-beta on CYP17 expression in H295R cells using constructs containing the CYP17 promoter region fused with the luciferase gene. In the H295R cells, TGF-beta decreased endogenous SF-1 level and inhibited activity of the 300 bp fragment of CYP17 promoter, which was stimulated by coexpression of SF-1. Overexpression of SMAD3 caused an inhibition of SF-1-stimulated CYP17 promoter activity, whereas overexpression of SMAD7 was ineffective. In conclusion, our results suggest that the inhibitory action of TGF-beta on CYP17 transcription involve at least two mechanisms: SMAD3 dependent inactivation of CYP17 promoter activity and repression of SF-1 expression.

Structural insight into the constitutive repression function of the nuclear receptor Rev-erbbeta

The Rev-erb family is an orphan nuclear receptor acting as a negative regulator of transcription. Rev-erbalpha and Rev-erbbeta are crucial components of the circadian clock and involved in various lipid homeostasis. They are unique nuclear receptors that lack the activation function 2 helix (AF2-helix) required for ligand-dependent activation by other members of nuclear receptors. Here, we report the crystal structure of Rev-erbbeta (NR1D2) in a dimeric arrangement. The putative ligand-binding pocket (LBP) of Rev-erbbeta is filled with bulky hydrophobic residues resulting in a residual cavity size that is too small to allow binding of any known ligand molecules. However, an alternative conformation of the putative LBP observed in another crystal form suggests the flexibility of this region. The kinked conformation of helix H11 allows helix H11 to bend toward helix H3 over the putative ligand binding pocket by filling and closing the cavity with its side-chains. In the absence of the AF2-helix and a cognate ligand, Rev-erbbeta appears to stabilize the hydrophobic cluster in the putative ligand binding pocket and provide a structural platform for co-repressor binding by adopting the unique geometry of helix H11, a suitable conformation for the constitutive repression activity.

Histone deacetylase inhibitors reduce steroidogenesis through SCF-mediated ubiquitination and degradation of steroidogenic factor 1 (NR5A1)

Histone deacetylase (HDAC) inhibitors such as trichostatin A and valproic acid modulate transcription of many genes by inhibiting the activities of HDACs, resulting in the remodeling of chromatin. Yet this effect is not universal for all genes. Here we show that HDAC inhibitors suppressed the expression of steroidogenic gene CYP11A1 and decreased steroid secretion by increasing the ubiquitination and degradation of SF-1, a factor important for the transcription of all steroidogenic genes. This was accompanied by increased expression of Ube2D1 and SKP1A, an E2 ubiquitin conjugase and a subunit of the E3 ubiquitin ligase in the Skp1/Cul1/F-box protein (SCF) family, respectively. Reducing SKP1A expression with small interfering RNA resulted in recovery of SF-1 levels, demonstrating that the activity of SCF E3 ubiquitin ligase is required for the SF-1 degradation induced by HDAC inhibitors. Overexpression of exogenous SF-1 restored steroidogenic activities even in the presence of HDAC inhibitors. Thus, increased SF-1 degradation is the cause of the reduction in steroidogenesis caused by HDAC inhibitors. The increased SKP1A expression and SCF-mediated protein degradation could be the mechanism underlying the mode of action of HDAC inhibitors.

Cyclic AMP-stimulated interaction between steroidogenic factor 1 and diacylglycerol kinase theta facilitates induction of CYP17

In the human adrenal cortex, adrenocorticotropin (ACTH) activates CYP17 transcription by promoting the binding of the nuclear receptor steroidogenic factor 1 (SF1) (Ad4BP, NR5A1) to the promoter. We recently found that sphingosine is an antagonist for SF1 and inhibits cyclic AMP (cAMP)-dependent CYP17 gene transcription. The aim of the current study was to identify phospholipids that bind to SF1 and to characterize the mechanism by which ACTH/cAMP regulates the biosynthesis of this molecule(s). Using tandem mass spectrometry, we show that in H295R human adrenocortical cells, SF1 is bound to phosphatidic acid (PA). Activation of the ACTH/cAMP signal transduction cascade rapidly increases nuclear diacylglycerol kinase (DGK) activity and PA production. PA stimulates SF1-dependent transcription of CYP17 reporter plasmids, promotes coactivator recruitment, and induces the mRNA expression of CYP17 and several other steroidogenic genes. Inhibition of DGK activity attenuates the binding of SF1 to the CYP17 promoter, and silencing of DGK-theta expression inhibits cAMP-dependent CYP17 transcription. LXXLL motifs in DGK-theta mediate a direct interaction of SF1 with the kinase and may facilitate binding of PA to the receptor. We conclude that ACTH/cAMP stimulates PA production in the nucleus of H295R cells and that this increase in PA concentrations facilitates CYP17 induction.

Phosphorylation of a conserved serine in the deoxyribonucleic acid binding domain of nuclear receptors alters intracellular localization

Nuclear receptors (NRs) are a superfamily of transcription factors whose genomic functions are known to be activated by lipophilic ligands, but little is known about how to deactivate them or how to turn on their nongenomic functions. One obvious mechanism is to alter the nuclear localization of the receptors. Here, we show that protein kinase C (PKC) phosphorylates a highly conserved serine (Ser) between the two zinc fingers of the DNA binding domain of orphan receptor hepatocyte nuclear factor 4alpha (HNF4alpha). This Ser (S78) is adjacent to several positively charged residues (Arg or Lys), which we show here are involved in nuclear localization of HNF4alpha and are conserved in nearly all other NRs, along with the Ser/threonine (Thr). A phosphomimetic mutant of HNF4alpha (S78D) reduced DNA binding, transactivation ability, and protein stability. It also impaired nuclear localization, an effect that was greatly enhanced in the MODY1 mutant Q268X. Treatment of the hepatocellular carcinoma cell line HepG2 with PKC activator phorbol 12-myristate 13-acetate also resulted in increased cytoplasmic localization of HNF4alpha as well as decreased endogenous HNF4alpha protein levels in a proteasome-dependent fashion. We also show that PKC phosphorylates the DNA binding domain of other NRs (retinoic acid receptor alpha, retinoid X receptor alpha, and thyroid hormone receptor beta) and that phosphomimetic mutants of the same Ser/Thr result in cytoplasmic localization of retinoid X receptor alpha and peroxisome proliferator-activated receptor alpha. Thus, phosphorylation of this conserved Ser between the two zinc fingers may be a common mechanism for regulating the function of NRs.

Steroidogenic factor-1 is a sphingolipid binding protein

Steroidogenic factor (SF1, NR5A1, Ad4BP) is an orphan nuclear receptor that is essential for steroid hormone-biosynthesis and endocrine development. Studies have found that the ability of this receptor to increase target gene expression can be regulated by post-translational modification, subnuclear localization, and protein-protein interactions. Recent crystallographic studies and our mass spectrometric analyses of the endogenous receptor have demonstrated an integral role for ligand-binding in the control of SF1 transactivation activity. Herein, we discuss our findings that sphingosine is an endogenous ligand for SF1. These studies and the structural findings of others have demonstrated that the receptor can bind both sphingolipids and phospholipids. Thus, it is likely that multiple bioactive lipids are ligands for SF1 and that these lipids will differentially act to control SF1 activity in a context-dependent manner. Finally, these findings highlight a central role for bioactive lipids as mediators of trophic hormone-stimulated steroid hormone biosynthesis.

Cyclic AMP stimulates SF-1-dependent CYP11A1 expression through homeodomain-interacting protein kinase 3-mediated Jun N-terminal kinase and c-Jun phosphorylation

Steroids are synthesized in adrenal glands and gonads under the control of pituitary peptides. These peptides bind to cell surface receptors to activate the cyclic AMP (cAMP) signaling pathway leading to an increase of steroidogenic gene expression. Exactly how cAMP activates steroidogenic gene expression is not clear, except for the knowledge that transcription factor SF-1 plays a key role. Investigating the factors participating in SF-1 action, we found that c-Jun and homeodomain-interacting protein kinase 3 (HIPK3) were required for basal and cAMP-stimulated expression of one major steroidogenic gene, CYP11A1. HIPK3 enhanced SF-1 activity, and c-Jun was required for the functional interaction of HIPK3 with SF-1. Furthermore, after cAMP stimulation, both c-Jun and Jun N-terminal kinase (JNK) were phosphorylated through HIPK3. These phosphorylations were important for SF-1 activity and CYP11A1 expression. Thus, we have defined HIPK3-mediated JNK activity and c-Jun phosphorylation as important events that increase SF-1 activity for CYP11A1 transcription in response to cAMP. This finding has linked three common factors, HIPK3, JNK, and c-Jun, to the cAMP signaling pathway leading to increased steroidogenic gene expression.

Differentiation-specific action of orphan nuclear receptor NR5A1 (SF-1): transcriptional regulation in luteinizing bovine theca cells

BACKGROUND

The orphan nuclear receptor NR5A1 (steroidogenic factor-1, SF-1) is a master regulator of tissue-specific gene expression in reproductive and steroidogenic tissues. Two activating functions, AF-1 and AF-2, have been described to function in a cooperative manner to recruit transcriptional coactivators to the promoter regions of NR5A1-controlled genes.

METHODS

The role of the NR5A1 activating functions AF-1 and AF-2 was studied in primary bovine theca cells. Bovine theca cells were infected with recombinant adenovirus vectors over-expressing wild-type NR5A1 or NR5A1 mutants, in which one of the activating functions of this orphan nuclear receptor had been impaired. Under different culture conditions, theca cell-specific transcript levels were measured by reverse transcription and real-time PCR.

RESULTS

Under culture conditions optimized for cell growth, transcriptional up-regulation of CYP11A1 (P450 side chain-cleavage enzyme) and INSL3 (Insulin-like factor 3, Relaxin-like factor (RLF)) was found to be dependent on the presence of NR5A1 carrying an intact AF-2. Under conditions inducing luteal differentiation of theca cells, CYP11A1 and STAR (Steroidogenic acute regulatory protein) were up-regulated by the action of luteinizing hormone (LH), whereas the differentiation-specific up-regulation of INSL3 was suppressed by LH in luteinizing theca cells. Inhibition of insulin- or IGF1- (insulin-like growth factor I) dependent signal transduction by the RAF1 kinase inhibitor GW5074 and the mitogen-activated protein kinase kinase inhibitor PD98059 resulted in the finding that RAF1 kinase inhibition was able to counteract the LH-dependent regulation of NR5A1-controlled genes, whereas inhibition of the mitogen-activated protein kinase (MAP kinase) pathway did not have any significant effect.

CONCLUSION

The regulation of the three NR5A1-controlled genes CYPA11, STAR, and INSL3 in luteinizing theca cells apparently is not dependent on NR5A1 activating functions AF-1 or AF-2. Activation of AF-1 here even appears to have an impairing effect on NR5A1 transcriptional activity, implying that up-regulation of NR5A1-controlled genes uses a different pathway. Our results might be explained by the possible existence of an interconnection between the RAF1 kinase and the cyclic AMP-protein kinase A pathway. Such a non-classical regulatory pathway might play an important role in the control of gene expression in reproductive and steroidogenic tissues.

Sphingosine regulates the transcription of CYP17 by binding to steroidogenic factor-1

Steroidogenic factor (SF1, Ad4BP, NR5A1) is a nuclear receptor that is essential for steroid hormone biosynthesis and endocrine development. Recent crystallographic studies have found that phospholipids are ligands for SF1. In the present study, our aim was to identify endogenous ligands for SF1 and characterize their functional significance in mediating cAMP-dependent transcription of human CYP17. Using tandem mass spectrometry, we show that in H295R adrenocortical cells, SF1 is bound to sphingosine (SPH) and lyso-sphingomyelin (lysoSM) under basal conditions and that cAMP stimulation decreases the amount of SPH and lysoSM bound to the receptor. Silencing both acid and neutral ceramidases using small interfering RNA induces CYP17 mRNA expression, suggesting that SPH acts as an inhibitory ligand. SPH antagonized the ability of cAMP and the coactivator steroid receptor coactivator-1 to increase CYP17 reporter gene activity. These studies demonstrate that SPH is a bonafide endogenous ligand for SF1 and a negative regulator of CYP17 gene expression.

The Octopus vulgaris estrogen receptor is a constitutive transcriptional activator: evolutionary and functional implications

Steroid hormones such as estrogens and androgens are important regulators of reproduction, physiology, and development in a variety of animal taxa, including vertebrates and mollusks. Steroid hormone receptors, which mediate the classic cellular responses to these hormones, were thought to be vertebrate specific, which left the molecular mechanisms of steroid action in invertebrates unresolved. Recently an estrogen receptor (ER) ortholog was isolated from the sea hare Aplysia californica, but the functional significance of the receptor was unclear because estrogens and other steroids are not known to be important in that species. Furthermore, the Aplysia ER was found to be a constitutive transcriptional activator, but it was unclear whether the estrogen independence of the ER was an Aplysia-specific novelty or a more ancient character general to the mollusks. Here we report on the isolation and functional characterization of the first ER ortholog from an invertebrate in which estrogens are produced and play an apparent role, the cephalopod Octopus vulgaris. We show that the Octopus ER is a strong constitutive transcriptional activator from canonical estrogen response elements. The receptor does not bind estradiol and is unresponsive to estrogens and other vertebrate steroid hormones. These characteristics are similar to those observed with the Aplysia ER and support the hypothesis that the evolving ER gained constitutive activity deep in the mollusk lineage. The apparent reproductive role of estrogens in Octopus and other mollusks is unlikely to be mediated by the ER and may take place through an ancient, non-ER-mediated pathway.

FSH signaling pathways in immature granulosa cells that regulate target gene expression: branching out from protein kinase A

Follicle-stimulating hormone (FSH) is necessary and sufficient to induce maturation of ovarian follicles to a mature, preovulatory phenotype in the intact animal, resulting in the generation of mature eggs and production of estrogen. FSH accomplishes these actions by inducing a complex pattern of gene expression in target granulosa cells that is regulated by input from many different signaling cascades, including those for the extracellular regulated kinases (ERKs), p38 mitogen-activated protein kinases (MAPKs), and phosphatidylinositol-3 kinase (PI3K). The upstream kinase that appears to be responsible for initiating all of the signaling that regulates gene expression in these epithelial cells is protein kinase A (PKA). PKA not only signals to directly phosphorylate transcription factors like cAMP response element binding protein and to promote chromatin remodeling by phosphorylating histone H3, this versatile kinase also enhances the activity of the p38 MAPK, ERK, and PI3K pathways. Additionally, accumulating evidence suggests that activation of a single signaling cascade downstream of PKA is not sufficient to activate target gene expression. Rather, cross-talk between and among signaling cascades is required. We will review the signaling cascades activated by FSH in granulosa cells and how these cascades contribute to the regulation of select target gene expression.

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.

Orphan nuclear receptors adopted by crystallography

Of the large nuclear hormone receptor superfamily of proteins, orphan nuclear receptors have remained a mystery owing to their lack of identified ligands and their constitutive nature. Now, structures of several ligand-binding domains of orphan receptors have provided some surprising insights that were not anticipated from molecular studies. Therefore, most orphan nuclear receptors have now been 'adopted' and their regulation has been shown to range from true ligand-independence to highly promiscuous ligand-dependence. Former orphan receptors have been found to contain ligand-binding pockets that range in volume from vast (>1600A3) to non-existent and have been shown to generate surface AF2 motifs that range from being multifunctionally active to distinctly inactive. Insights from these new structures illustrate how powerful a structural biology approach can be when integrated with molecular and cellular physiology.