The nonconserved hinge region and distinct amino-terminal domains of the ROR alpha orphan nuclear receptor isoforms are required for proper DNA bending and ROR alpha-DNA interactions

RAR-related orphan receptor alpha and the staggerer mice: a fine molecular story

The retinoic acid-related orphan receptor alpha (RORα) protein first came into the limelight due to a set of staggerer mice, discovered at the Jackson Laboratories in the United States of America by Sidman, Lane, and Dickie (1962) and genetically deciphered by Hamilton et al. in 1996. These staggerer mice exhibited cerebellar defects, an ataxic gait, a stagger along with several other developmental abnormalities, compensatory mechanisms, and, most importantly, a deletion of 160 kilobases (kb), encompassing the RORα ligand binding domain (LBD). The discovery of the staggerer mice and the subsequent discovery of a loss of the LBD within the RORα gene of these mice at the genetic level clearly indicated that RORα's LBD played a crucial role in patterning during embryogenesis. Moreover, a chance study by Roffler-Tarlov and Sidman (1978) noted reduced concentrations of glutamic acid levels in the staggerer mice, indicating a possible role for the essence of a nutritionally balanced diet. The sequential organisation of the building blocks of intact genes, requires the nucleotide bases of deoxyribonucleic acid (DNA): purines and pyrimidines, both of which are synthesized, upon a constant supply of glutamine, an amino acid fortified in a balanced diet and a byproduct of the carbohydrate and lipid metabolic pathways. A nutritionally balanced diet, along with a metabolic "enzymatic machinery" devoid of mutations/aberrations, was essential in the uninterrupted transcription of RORα during embryogenesis. In addition to the above, following translation, a ligand-responsive RORα acts as a "molecular circadian regulator" during embryogenesis and not only is expressed selectively and differentially, but also promotes differential activity depending on the anatomical and pathological site of its expression. RORα is highly expressed in the central nervous system (CNS) and the endocrine organs. Additionally, RORα and the clock genes are core components of the circadian rhythmicity, with the expression of RORα fluctuating in a night-day-night sigmoidal pattern and undoubtedly serves as an endocrine-like, albeit "molecular-circadian regulator". Melatonin, a circadian hormone, along with tri-iodothyronine and some steroid hormones are known to regulate RORα-mediated molecular activity, with each of these hormones themselves being regulated rhythmically by the hypothalamic-pituitary axis (HPA). The HPA regulates the circadian rhythm and cyclical release of hormones, in a self-regulatory feedback loop. Irregular sleep-wake patterns affect circadian rhythmicity and the ability of the immune system to withstand infections. The staggerer mice with their thinner bones, an altered skeletal musculature, an aberrant metabolic profile, the ataxic gait and an underdeveloped cerebellar cortex; exhibited compensatory mechanisms, that not only allowed the survival of the staggerer mice, but also enhanced protection from microbial invasions and resistance to high-fat-diet induced obesity. This review has been compiled in its present form, more than 14 years later after a chromatin immunoprecipitation (ChIP) cloning and sequencing methodology helped me identify signal transducer and activator of transcription 5 (STAT5) target sequences, one of which was mapped to the first intron of the RORα gene. The 599-base-long sequence containing one consensus TTCNNNGAA (TTCN3GAA) gamma-activated sequence (GAS) and five other non-consensus TTN5AA sequences had been identified from the clones isolated from the STAT5 target sites (fragments) in human phytohemagglutinin-activated CD8+ T lymphocytes, during my doctoral studies between 2006 and 2009. Most importantly, preliminary studies noted a unique RORα expression profile, during a time-course study on the ribonucleic acid (RNA), extracted from human phytohemagglutinin (PHA) activated CD8+ T lymphocytes stimulated with interleukin-2 (IL-2). This review mainly focuses on the "staggerer mice" with one of its first roles materialising during embryogenesis, a molecular-endocrine mediated circadian-like regulatory process.

Structural characterization of the DNA binding mechanism of retinoic acid-related orphan receptor gamma

Retinoic acid-related orphan receptor gamma (RORγ) plays critical roles in regulating various biological processes and has been linked to immunodeficiency disorders and cancers. DNA recognition is essential for RORγ to exert its functions. However, the underlying mechanism of the DNA binding by RORγ remains unclear. In this study, we present the crystal structure of the complex of RORγ1 DNA-binding domain (RORγ1-DBD)/direct repeat DNA element DR2 at 2.3 Å resolution. We demonstrate that RORγ1-DBD binds the DR2 motif as a homodimer, with the C-terminal extension (CTE) region of RORγ1-DBD contributing to the DNA recognition and the formation of dimeric interface. Further studies reveal that REV-ERB-DBD and RXR-DBD, also bind the DR2 site as a homodimer, while NR4A2-DBD binds DR2 as a monomer. Our research uncovers a binding mechanism of RORγ1 to the DR2 site and provides insights into the biological functions of RORγ1 and the broader RORs subfamily.

RORγT agonists as immune modulators in anticancer therapy

RORγT is a transcription factor that directs the development of Th17 lymphocytes and other IL-17-expressing cells (e.g., Tc17 and ILC3 cells). These cells are involved in the body's defense against pathogenic bacteria and fungi, but they also participate in maintaining the proinflammatory environment in some autoimmune diseases and play a role in the immune system's response to cancer. Similar to other members of the nuclear receptor superfamily, the activity of RORγT is regulated by low-molecular-weight ligands. Therefore, extensive efforts have been dedicated to identifying inverse agonists that diminish the activity of this receptor and subsequently inhibit the development of autoimmune diseases. Unfortunately, in the pursuit of an ideal inverse agonist, the development of agonists has been overlooked. It is important to remember that these types of compounds, by stimulating lymphocytes expressing RORγT (Th17 and Tc17), can enhance the immune system's response to tumors. In this review, we present recent advancements in the biology of RORγT agonists and their potential application in anticancer therapy.

Identification of dietary compounds that interact with the circadian clock machinery: Molecular docking and structural similarity analysis

The molecular clock is vital for regulating circadian rhythms in various physiological processes, and its dysregulation is associated with multiple diseases. As such, the use of small molecule modulators to regulate the molecular clock presents a promising therapeutic approach. In this study, we generated a homology model of the human circadian locomotor output cycles kaput (CLOCK) protein to evaluate its ligand binding sites. Using molecular docking, we obtained further insights into the binding mode of the control compound CLK8 and explored a selection of dietary compounds. Our investigation of dietary compounds was guided by their potential interactions with the retinoic acid-related orphan receptors RORα/γ, which are involved in circadian regulation. Through the molecular similarity and docking analyses, we identified oleanolic acid demethyl, 3-epi-lupeol, and taraxasterol as potential ROR-interacting compounds. These compounds may exert therapeutic effects through their modulation of RORα/γ activity and subsequently influence the molecular clock. Overall, our study highlights the potential of small molecule modulators in regulating the molecular clock and the importance of exploring dietary compounds as a source of such modulators. Our findings also provide insights into the binding mechanisms of CLK8 and shed light on potential compounds that can interact with RORs to regulate the molecular clock. Future investigations could focus on validating the efficacy of these compounds in modulating the molecular clock and their potential use as therapeutic agents.

Transcription factor RORα enforces stability of the Th17 cell effector program by binding to a Rorc cis-regulatory element

T helper 17 (Th17) cells regulate mucosal barrier defenses but also promote multiple autoinflammatory diseases. Although many molecular determinants of Th17 cell differentiation have been elucidated, the transcriptional programs that sustain Th17 cells in vivo remain obscure. The transcription factor RORγt is critical for Th17 cell differentiation; however, it is not clear whether the closely related RORα, which is co-expressed in Th17 cells, has a distinct role. Here, we demonstrated that although dispensable for Th17 cell differentiation, RORα was necessary for optimal Th17 responses in peripheral tissues. The absence of RORα in T cells led to reductions in both RORγt expression and effector function among Th17 cells. Cooperative binding of RORα and RORγt to a previously unidentified Rorc cis-regulatory element was essential for Th17 lineage maintenance in vivo. These data point to a non-redundant role of RORα in Th17 lineage maintenance via reinforcement of the RORγt transcriptional program.

RORγ Structural Plasticity and Druggability

Retinoic acid receptor-related orphan receptor γ (RORγ) is a transcription factor regulating the expression of the pro-inflammatory cytokine IL-17 in human T helper 17 (Th17) cells. Activating RORγ can induce multiple IL-17-mediated autoimmune diseases but may also be useful for anticancer therapy. Its deep immunological functions make RORɣ an attractive drug target. Over 100 crystal structures have been published describing atomic interactions between RORɣ and agonists and inverse agonists. In this review, we focus on the role of dynamic properties and plasticity of the RORɣ orthosteric and allosteric binding sites by examining structural information from crystal structures and simulated models. We discuss the possible influences of allosteric ligands on the orthosteric binding site. We find that high structural plasticity favors the druggability of RORɣ, especially for allosteric ligands.

Ecdysteroid-mimicking compounds act as both agonists and antagonists to the crustacean ecdysone receptor

To characterize the potential endocrine-disrupting chemicals (EDCs) in the environment that interact with the crustacean ecdysone receptor (EcR), we established a method involving in silico modeling/molecular docking and in vitro reporter gene assay. Cherry shrimp (Neocaridina davidi) EcR (NdEcR) and retinoid X receptor (NdRxR) were identified and cloned for use in this method. A theoretical 3D model of NdEcR ligand-binding domain (LBD) was built in silico based on sequence homology with the established X-ray structure of insect EcR. The interaction of the NdEcR LBD with ecdysteroids, diacylhydrazine (DAH) pesticides, and other potential EDCs was evaluated using molecular docking programs. The results revealed that the ligand-binding pocket in the NdEcR LBD was flexible and adaptive for accommodating ligands of different shapes. The agonistic and antagonistic activities of the candidate compounds were further assessed by in vitro reporter gene assay using human cell lines transiently transfected with NdEcR and NdRxR expression plasmids and a reporter plasmid containing synthesized ecdysone response element. The assay was validated by the dose-dependent responses of EcR-mediated gene transcription after treating the transfected cell lines with ecdysteroids, 20-hydroxyecdysone, and ponasterone A. Examination of the candidate compounds using the reporter gene assay revealed restricted functional specificity to ecdysteroids and DAHs. Three of the tested DAH pesticides originally targeting the insect EcR were found to be weak agonists and strong antagonists of NdEcR. These results suggest that DAHs are potential EDCs for crustaceans that disrupt their ecdysteroid signals by functioning as EcR agonists or antagonists.

Melatonin-induced demethylation of antioxidant genes increases antioxidant capacity through RORα in cumulus cells of prepubertal lambs

Physical damage and oxidative stress may occur in prepubertal cumulus cells, due to insufficient glutathione synthesis. To determine potential epigenetic mechanisms related to antioxidant effects of melatonin on ovine prepubertal cumulus cells, 30 lambs, 4-wk-old were randomly allocated into two groups: a control (C, n = 20) group and a melatonin (M, n = 10) group given a subcutaneous implant containing 18 mg melatonin. All lambs were superovulated (250 IU FSH and 250 IU eCG). Cumulus cells from germinal vesicle stage cumulus oocyte complexes (COCs) were collected by ovarian follicular aspiration and dissociated with hyaluronidase. Compared to the C group, the M group had greater superovulation, better antioxidant capacity, a higher proportion of fully expanded COCs and a lower proportion of apoptotic cumulus cells (P < 0.05). Melatonin up-regulated mRNA expression of genes for melatonin receptors MT1 and nuclear binding site RORα, antioxidants (SOD1, GPx4 and CAT) and cumulus cell expansion (PTX3, HAS2 and PTGS2), as well as Bcl2, but down-regulated expression of Bax (P < 0.05). Regarding epigenetics, there were less methylation at five CpG sites of SOD1, three CpG sites of GPx4 and two CpG sites of CAT in M versus C groups (P <  0.05), leading to lower total methylation of SOD1, GPx4 and CAT promoters region on M group (P < 0.05). In a mechanistic study, addition of MT1 or RORα antagonist increased ROS and MDA concentrations, but decreased T-AOC, GPx, CAT and T-SOD concentrations (P < 0.05), whereas there were no significant difference between the melatonin and MT2 antagonist treatment groups for T-AOC, GPx, CAT and T-SOD concentrations. Furthermore, addition of RORα agonist decreased total DNA methylation of SOD1, GPx4 and CAT, with no significant difference after MT1 agonist treatment. These studies provided new information regarding epigenetic mechanisms by which melatonin promoted ovine prepubertal cumulus cells antioxidant through RORα, both in vitro and in vivo.

A divergent role of COOH-terminal domains in Nurr1 and Nur77 transactivation

Orphan nuclear receptors such as Nurr1 and Nur77 have conserved amino acid sequences in the zinc finger DNA binding domains and similar COOH-terminal regions, but have no known ligands. These receptors can bind DNA sequences (response elements) as monomers and can also heterodimerize with the retinoid X receptor to activate transcription. We report here the identification and initial characterization of a novel COOH-terminal truncated isoform of Nurr1, Nurr1a. Internal splicing of Nurr1 generates a frameshift such that a stop codon is prematurely encoded resulting in a naturally occurring COOH-terminal truncation. Embryonic and postnatal mouse brain showed both Nurr1 and Nurr1a mRNAs expressed during development. To characterize essential COOH-terminal elements that may be deleted from Nurr1a and determine function in putative ligand binding, we created COOH-terminal deletion mutants. Nurr1, Nur77, and 3'-truncated mutants bind in gel mobility shift assays to the monomeric Nur77 response element (B1A-RE). However, in transient transfection assays, a truncation of as little as 15 Nurr1 COOH-terminal amino acids diminished transcriptional activation of B1A-thymidine kinase-chloramphenicol acetyltransferase reporter. This result was not seen for a similar Nur77 deletion mutant, Nur77-586. Unlike full-length Nurr1 and Nur77, transactivation by Nur77-586 was not augmented in response to the presence of retinoid-like receptor and 9-cis-retinoic acid. Thus, the interaction of putative ligand binding and transactivation for Nurr1 and Nur77 may function differently.

RORα2 requires LSD1 to enhance tumor progression in breast cancer

Retinoic acid-related orphan receptor α (RORα) regulates diverse physiological processes, including inflammatory responses, lipid metabolism, circadian rhythm, and cancer biology. RORα has four different isoforms which have distinct N-terminal domains but share identical DNA binding domain and ligand binding domain in human. However, lack of specific antibody against each RORα isoform makes biochemical studies on each RORα isoform remain unclear. Here, we generate RORα2-specific antibody and characterize the role of RORα2 in promoting tumor progression in breast cancer. RORα2 requires lysine specific demethylase 1 (LSD1/KDM1A) as a coactivator for transcriptional activation of RORα2 target genes, exemplified by CTNND1. Intriguingly, RORα2 and LSD1 protein levels are dramatically elevated in human breast cancer specimens compared to normal counterparts. Taken together, our studies indicate that LSD1-mediated RORα2 transcriptional activity is important to promote tumor cell migration in human breast cancer as well as breast cancer cell lines. Therefore, our data establish that suppression of LSD1-mediated RORα2 transcriptional activity may be potent therapeutic strategy to attenuate tumor cell migration in human breast cancer.

Nuclear receptors in bone physiology and diseases

During the last decade, our view on the skeleton as a mere solid physical support structure has been transformed, as bone emerged as a dynamic, constantly remodeling tissue with systemic regulatory functions including those of an endocrine organ. Reflecting this remarkable functional complexity, distinct classes of humoral and intracellular regulatory factors have been shown to control vital processes in the bone. Among these regulators, nuclear receptors (NRs) play fundamental roles in bone development, growth, and maintenance. NRs are DNA-binding transcription factors that act as intracellular transducers of the respective ligand signaling pathways through modulation of expression of specific sets of cognate target genes. Aberrant NR signaling caused by receptor or ligand deficiency may profoundly affect bone health and compromise skeletal functions. Ligand dependency of NR action underlies a major strategy of therapeutic intervention to correct aberrant NR signaling, and significant efforts have been made to design novel synthetic NR ligands with enhanced beneficial properties and reduced potential negative side effects. As an example, estrogen deficiency causes bone loss and leads to development of osteoporosis, the most prevalent skeletal disorder in postmenopausal women. Since administration of natural estrogens for the treatment of osteoporosis often associates with undesirable side effects, several synthetic estrogen receptor ligands have been developed with higher therapeutic efficacy and specificity. This review presents current progress in our understanding of the roles of various nuclear receptor-mediated signaling pathways in bone physiology and disease, and in development of advanced NR ligands for treatment of common skeletal disorders.

Retinoid-related orphan receptor alpha and the regulation of lipid homeostasis

Many nuclear hormone receptors (NRs) control lipid, glucose and energy homeostasis in an organ specific manner. Concordantly, dysfunctional NR signalling results in metabolic disease. The Retinoic acid receptor-related orphan receptor alpha (RORα), a member of the NR1F subgroup, is expressed in metabolic tissues. Previous studies identified the role of this NR in dyslipidemia, apo-lipoprotein metabolism and atherosclerosis. Recent data is underscoring the significant role of this orphan NR in the regulation of phase I/II metabolism (bile acids, xenobiotics, steroids etc.), adiposity, insulin signalling, and glucose tolerance. Moreover, oxygenated sterols, have been demonstrated to function as native ligands and inverse agonists. This review focuses on the rapidly emerging and evolving role of RORα in the control of lipid and glucose homeostasis in major mass metabolic tissues. Article from the special issue orphan receptors.

General molecular biology and architecture of nuclear receptors

Nuclear receptors (NRs) regulate and coordinate multiple processes by integrating internal and external signals, thereby maintaining homeostasis in front of nutritional, behavioral and environmental challenges. NRs exhibit strong similarities in their structure and mode of action: by selective transcriptional activation or repression of cognate target genes, which can either be controlled through a direct, DNA binding-dependent mechanism or through crosstalk with other transcriptional regulators, NRs modulate the expression of gene clusters thus achieving coordinated tissue responses. Additionally, non genomic effects of NR ligands appear mediated by ill-defined mechanisms at the plasma membrane. These effects mediate potential therapeutic effects as small lipophilic molecule targets, and many efforts have been put in elucidating their precise mechanism of action and pathophysiological roles. Currently, numerous nuclear receptor ligand analogs are used in therapy or are tested in clinical trials against various diseases such as hypertriglyceridemia, atherosclerosis, diabetes, allergies and cancer and others.

Homozygous staggerer (sg/sg) mice display improved insulin sensitivity and enhanced glucose uptake in skeletal muscle

AIMS/HYPOTHESIS

Homozygous staggerer (sg/sg) mice, which have decreased and dysfunctional Rorα (also known as Rora) expression in all tissues, display a lean and dyslipidaemic phenotype. They are also resistant to (high fat) diet-induced obesity. We explored whether retinoic acid receptor-related orphan receptor (ROR) α action in skeletal muscle was involved in the regulation of glucose metabolism.

METHODS

We used a three-armed genomic approach, including expression profiling, ingenuity analysis and quantitative PCR validation to identify the signalling pathway(s) in skeletal muscle that are perturbed in sg/sg mice. Moreover, western analysis, functional insulin and glucose tolerance tests, and ex vivo glucose uptake assays were used to phenotypically characterise the impact of aberrant v-AKT murine thymoma viral oncogene homologue (AKT) signalling.

RESULTS

Homozygous and heterozygous (sg/sg and sg/+) animals exhibited decreased fasting blood glucose levels, mildly improved glucose tolerance and increased insulin sensitivity. Illumina expression profiling and bioinformatic analysis indicated the involvement of RORα in metabolic disease and phosphatidylinositol 3-kinase-AKT signalling. Quantitative PCR and western analysis validated increased AKT2 (mRNA and protein) and phosphorylation in sg/sg mice in the basal state. This was associated with increased expression of Tbc1d1 and Glut4 (also known as Slc2a4) mRNA and protein. Finally, in agreement with the phenotype, we observed increased (absolute) levels of AKT and phosphorylated AKT (in the basal and insulin stimulated states), and of (ex vivo) glucose uptake in skeletal muscle from sg/sg mice relative to wild-type littermates.

CONCLUSIONS/INTERPRETATION

We propose that Rorα plays an important role in regulation of the AKT2 signalling cascade, which controls glucose uptake in skeletal muscle.

Identification and validation of the pathways and functions regulated by the orphan nuclear receptor, ROR alpha1, in skeletal muscle

The retinoic acid receptor-related orphan receptor (ROR) alpha has been demonstrated to regulate lipid metabolism. We were interested in the RORα1 dependent physiological functions in skeletal muscle. This major mass organ accounts for ∼40% of the total body mass and significant levels of lipid catabolism, glucose disposal and energy expenditure. We utilized the strategy of targeted muscle-specific expression of a truncated (dominant negative) RORα1ΔDE in transgenic mice to investigate RORα1 signaling in this tissue. Expression profiling and pathway analysis indicated that RORα influenced genes involved in: (i) lipid and carbohydrate metabolism, cardiovascular and metabolic disease; (ii) LXR nuclear receptor signaling and (iii) Akt and AMPK signaling. This analysis was validated by quantitative PCR analysis using TaqMan low-density arrays, coupled to statistical analysis (with Empirical Bayes and Benjamini–Hochberg). Moreover, westerns and metabolic profiling were utilized to validate the genes, proteins and pathways (lipogenic, Akt, AMPK and fatty acid oxidation) involved in the regulation of metabolism by RORα1. The identified genes and pathways were in concordance with the demonstration of hyperglycemia, glucose intolerance, attenuated insulin-stimulated phosphorylation of Akt and impaired glucose uptake in the transgenic heterozygous Tg-RORα1ΔDE animals. In conclusion, we propose that RORα1 is involved in regulating the Akt2-AMPK signaling pathways in the context of lipid homeostasis in skeletal muscle.

Activation of aromatase expression by retinoic acid receptor-related orphan receptor (ROR) alpha in breast cancer cells: identification of a novel ROR response element

Estrogen is a key regulator of the proliferation and differentiation of breast cancer cells. In addition to the estrogen supply from the ovary, estrogen is produced locally from androgen by aromatase. However, the regulation of aromatase gene expression in breast cancer has not yet been fully clarified. Retinoic acid receptor-related orphan receptor (ROR) alpha plays an important role in the differentiation of many organs by regulating the transcription of target genes. Because aromatase and RORalpha are expressed in breast cancer, the effect of RORalpha on aromatase gene expression was studied. RORalpha significantly augmented the expression of aromatase mRNA, particularly those containing exon I.4, in MCF7 cells, and aromatase activities in T47D and MCF7 cells. RORalpha also stimulated the proliferation of these cells. Transient transfection-based reporter gene assays using the promoter at exon I.4 showed that RORalpha augmented the transcription. A series of truncated mutation studies revealed that RORalpha activated the transcription through -147 to +14 bp of the promoter I.4. Furthermore, RORalpha bound to the fragment containing -119 to -107 bp of the promoter in vitro, indicating that this region may contain a novel ROR response element. Chromatin immunoprecipitation assay showed that RORalpha bound to the region containing this site of the promoter I.4 in MCF7 cells. Moreover, we examined clinical samples and found a correlation between RORalpha and aromatase expression. These results suggest that RORalpha directly activates the aromatase expression to accelerate the local production of estrogen, which results in the proliferation of breast cancer cells.

Retinoid-related orphan receptors (RORs): critical roles in development, immunity, circadian rhythm, and cellular metabolism

The last few years have witnessed a rapid increase in our knowledge of the retinoid-related orphan receptors RORα, -β, and -γ (NR1F1-3), their mechanism of action, physiological functions, and their potential role in several pathologies. The characterization of ROR-deficient mice and gene expression profiling in particular have provided great insights into the critical functions of RORs in the regulation of a variety of physiological processes. These studies revealed that RORα plays a critical role in the development of the cerebellum, that both RORα and RORβ are required for the maturation of photoreceptors in the retina, and that RORγ is essential for the development of several secondary lymphoid tissues, including lymph nodes. RORs have been further implicated in the regulation of various metabolic pathways, energy homeostasis, and thymopoiesis. Recent studies identified a critical role for RORγ in lineage specification of uncommitted CD4⁺ T helper cells into Th17 cells. In addition, RORs regulate the expression of several components of the circadian clock and may play a role in integrating the circadian clock and the rhythmic pattern of expression of downstream (metabolic) genes. Study of ROR target genes has provided insights into the mechanisms by which RORs control these processes. Moreover, several reports have presented evidence for a potential role of RORs in several pathologies, including osteoporosis, several autoimmune diseases, asthma, cancer, and obesity, and raised the possibility that RORs may serve as potential targets for chemotherapeutic intervention. This prospect was strengthened by recent evidence showing that RORs can function as ligand-dependent transcription factors.

Primers on molecular pathways: nuclear receptors in pancreatic cancer. The ligand-independent way

Nuclear receptors (NRs) are ligand-activated transcription factors that regulate gene expression. These receptors share a quite similar structure and mechanism of action. Upon ligand binding, NRs translocate from the cytoplasm to the nucleus and bind to specific DNA sequences that regulate expression of different genes. In the absence of ligand the cascade of signaling events is different and either activation or repression may occur. Interestingly, several NRs are implicated in the pathogenesis of pancreatic cancer (PanCa). Specifically, androgen and estrogen receptor-related pathways are active in patients with pancreatic tumors, thus representing a suitable target for PanCa treatment. Although anti-hormone treatment has a modest effect on PanCa, ligand-independent activation of NRs remains overall unexplored in this disease.

Regulation of T helper 17 differentiation by orphan nuclear receptors: it's not just ROR gamma t anymore

T cells that produce IL-17 (T helper 17 cells) are implicated in autoimmune pathogenesis. In this issue of Immunity, Yang et al. (2008) report that the closely related orphan nuclear receptors ROR alpha and ROR gamma t work together to regulate T helper (Th)17 cell differentiation.

Extracellular signal-regulated kinase-2 phosphorylates RORalpha4 in vitro

The retinoic acid related orphan receptor RORalpha activates transcription of genes that play an important role in cerebellar development, the protection against age-related degenerative processes, the regulation of inflammatory responses, and is one of the pivotal participants that control the circadian rhythmicity in the core-clock of mammals. We identified the extracellular signal-regulated kinase 2 (ERK-2) as RORalpha4 phosphorylating kinase in vitro. The primary sequence of RORalpha4 contains an ERK-2 recognition motif (P-L-T(128)-P) within the hinge domain, and mutation of Thr-128 to Ala prevents RORalpha4 phosphorylation by ERK. The RORalpha4-T128A mutant exhibits an increased DNA-binding affinity, an increased transcriptional activity and, in the interplay with the opponent RevErbalpha, acts as a stronger competitor at ROR response elements than RORalpha4-WT.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

RORalpha, a pivotal nuclear receptor for Purkinje neuron survival and differentiation: from development to ageing

RORalpha (Retinoid-related Orphan Receptor) is a transcription factor belonging to the superfamily of nuclear receptors. The spontaneous staggerer (sg) mutation, which consists of a deletion in the Rora gene, has been shown to cause the loss of function of the RORalpha protein. The total loss of RORalpha expression leads to cerebellar developmental defects, particularly to a dramatic decreased survival of Purkinje cells and an early block in the differentiation process. This review focuses on recent studies which position RORalpha as a pivotal factor controlling Purkinje cell survival and differentiation, from development to ageing.

Essential hypertension seems to result from melatonin-induced epigenetic modifications in area postrema

Essential hypertension is a complex multifactorial disorder with epigenetic and environmental factors contributing to its prevalence. Epigenetic system is a genetic regulatory mechanism that allows humans to maintain extraordinarily stable patterns of gene expression over many generations. Sympathetic nervous system plays a major role in the maintenance of hypertension and the rostral ventrolateral medulla is the main source of this sympathetic activation. A possible mechanism to explain the sympathetic hyperactivity in the rostral ventrolateral medulla is an action of the area postrema. Area postrema seems to be the region where a shift of the set-point to a higher operating pressure occurs resulting in hypertension. But, how can a shift occur in the area postrema. We propose that melatonin-induced epigenetic modifications in the neurons of area postrema plays a role in this shift. Area postrema is reported to contain high levels of melatonin receptors that play a role in the epigenetic modifications in certain cells. Environmental stressors cause epigenetic modifications in the neurons of area postrema via the pineal hormone melatonin and these changes lead to a shift in the set-point to a higher operating pressure. This signal is then sent via efferent projections to key medullary sympathetic nuclei in rostral ventrolateral medulla resulting in increases in sympathetic nerve activity. This model may explain the long-term alterations in sympathetic activity in essential hypertension.

Retinoid-related Orphan Receptors (RORs): Roles in Cellular Differentiation and Development

Retinoid-related orphan receptors RORalpha, -beta, and -gamma are transcription factors belonging to the steroid hormone receptor superfamily. During embryonic development RORs are expressed in a spatial and temporal manner and are critical in the regulation of cellular differentiation and the development of several tissues. RORalpha plays a key role in the development of the cerebellum particularly in the regulation of the maturation and survival of Purkinje cells. In RORalpha-deficient mice, the reduced production of sonic hedgehog by these cells appears to be the major cause of the decreased proliferation of granule cell precursors and the observed cerebellar atrophy. RORalpha has been implicated in the regulation of a number of other physiological processes, including bone formation. RORbeta expression is largely restricted to several regions of the brain, the retina, and pineal gland. Mice deficient in RORbeta develop retinal degeneration that results in blindness. RORgamma is essential for lymph node organogenesis. In the intestine RORgamma is required for the formation of several other lymphoid tissues: Peyer's patches, cryptopatches, and isolated lymphoid follicles. RORgamma plays a key role in the generation of lymphoid tissue inducer (LTi) cells that are essential for the development of these lymphoid tissues. In addition, RORgamma is a critical regulator of thymopoiesis. It controls the differentiation of immature single-positive thymocytes into double-positive thymocytes and promotes the survival of double-positive thymocytes by inducing the expression of the anti-apoptotic gene Bcl-X(L). Interestingly, all three ROR receptors appear to play a role in the control of circadian rhythms. RORalpha positively regulates the expression of Bmal1, a transcription factor that is critical in the control of the circadian clock. This review intends to provide an overview of the current status of the functions RORs have in these biological processes.

The gene encoding human retinoic acid-receptor-related orphan receptor alpha is a target for hypoxia-inducible factor 1

Retinoic acid-receptor-related orphan receptor (ROR) alpha is a nuclear receptor involved in many pathophysiological processes such as cerebellar ataxia, inflammation, atherosclerosis and angiogenesis. In the present study we first demonstrate that hypoxia increases the amount of Rora transcripts in a wide panel of cell lines derived from diverse tissues. In addition, we identified a functional promoter sequence upstream of the first exon of the human Rora gene, spanning -487 and -45 from the translation initiation site of RORalpha1. When cloned in a luciferase reporter vector, this sequence allowed the efficient transcription of the luciferase gene in several cell lines. Interestingly, the activity of the Rora promoter was enhanced by hypoxia in HepG2 human hepatoma cells, and this effect was dependent on an HRE (hypoxia response element) spanning from -229 to -225. Using electrophoretic-mobility-shift assays, we showed that HIF-1 (hypoxia-inducible factor 1), which plays a key role in the transcriptional response to hypoxia, bound to this HRE. Overexpression of HIF-1alpha increased the activity of the Rora promoter through the HRE. Overexpression of a dominant-negative form of HIF-1alpha producing transcriptionally inactive HIF-1alpha/HIF-1beta dimers abolished hypoxic activation of the Rora promoter. This indicated that HIF-1 is involved in the response of RORalpha to hypoxia. Taken together, our data reveal Rora as a new HIF-1 target gene. This illustrates, at the molecular level, the existence of cross-talk between signalling pathways mediated by HIF-1 and those mediated by nuclear receptors.

DNA sequence-dependent regulation of SF-1-mediated transcription

Rat P450c17 gene transcription is regulated by several nuclear factors, including steroidogenic factor-1 (SF-1), nerve growth factor-inducible protein B (NGF-IB, Nurr77), COUP-TF, SET, and Ku autoimmune antigen. A region of this gene, -447/-419, that mediates both basal and cAMP-stimulated transcription, contains two binding sites for orphan nuclear receptors. While SF-1 activates transcription through a single binding site, we show that both binding sites at -447/-419 are required for transcriptional activation by SF-1 and cAMP. Both SF-1 and a novel factor, Steroidogenic Factor-Inducer of Transcription-2 (StF-IT-2) bind to this region, suggesting that a DNA-dependent interaction between StF-IT-2 and SF-1 may be required for full transcriptional activity. Each of the two orphan nuclear receptor sites -429/-424 and at -444/-439 are sufficient for SF-1 binding but are insufficient for SF-1-mediated transcription. Increasing the distance between or changing the orientation of these two sites does not affect basal or SF-1-stimulated activity. Circular permutation analysis, which measures the degree of DNA bending caused by protein binding, indicates that SF-1 binding to -447/-419 induces a different degree of DNA bending than it does at another SF-1-responsive site. However, similar domains of the SF-1 protein are required for its actions at these two regions. Southwestern blots suggest that StF-IT-2 is a approximately 33 kDa protein, and gel shift assays suggest it is expressed primarily in the gonad and brain early in rodent development. These data suggest that the mechanism by which SF-1 stimulates transcription is DNA sequence dependent, and may require additional proteins, such as StF-IT-2, for activation at specific regions of DNA.

Melatonin seems to be a mediator that transfers the environmental stimuli to oocytes for inheritance of adaptive changes through epigenetic inheritance system

Possibility of inheritance of epigenetic modifications have led us to consider that adaptive geographic variations in humans may result from interactions between environmental factors and epigenetic inheritance system. In this system melatonin seems to be a mediator that transfers the environmental stimuli to germ cells (oocytes). While environmental factors produce modifications in the body, they simultaneously induce epigenetic modifications in the oocytes with the help of melatonin, and these changes are inherited to offspring. In this way, adaptive changes could be passed on to the next generation. This kind of heritable long-term changes is generally labeled biological adaptation. But, how can melatonin cause epigenetic changes in oocytes? We suggest that melatonin induces epigenetic modifications by affecting the nuclear melatonin receptors that can in turn change the superstructure of DNA. It was previously suggested that biological adaptation is limited to neural crest derivatives such as, craniofacial tissues, melanocytes, and structures related to stature, hair form and body proportions. Thus, inheritance of adaptive changes is possible only where environmental factors affect the neural crest derivatives, including the cells that produce the next generation.

RORalpha regulates the expression of genes involved in lipid homeostasis in skeletal muscle cells: caveolin-3 and CPT-1 are direct targets of ROR

The staggerer mice carry a deletion in the RORalpha gene and have a prolonged humoral response, overproduce inflammatory cytokines, and are immunodeficient. Furthermore, the staggerer mice display lowered plasma apoA-I/-II, decreased plasma high density lipoprotein cholesterol and triglycerides, and develop hypo-alpha-lipoproteinemia and atherosclerosis. However, relatively little is known about RORalpha in the context of target tissues, target genes, and lipid homeostasis. For example, RORalpha is abundantly expressed in skeletal muscle, a major mass peripheral tissue that accounts for approximately 40% of total body weight and 50% of energy expenditure. This lean tissue is a primary site of glucose disposal and fatty acid oxidation. Consequently, muscle has a significant role in insulin sensitivity, obesity, and the blood-lipid profile. In particular, the role of RORalpha in skeletal muscle metabolism has not been investigated, and the contribution of skeletal muscle to the ROR-/- phenotype has not been resolved. We utilize ectopic dominant negative RORalpha expression in skeletal muscle cells to understand the regulatory role of RORs in this major mass peripheral tissue. Exogenous dominant negative RORalpha expression in skeletal muscle cells represses the endogenous levels of RORalpha and -gamma mRNAs and ROR-dependent gene expression. Moreover, we observed attenuated expression of many genes involved in lipid homeostasis. Furthermore, we show that the muscle carnitine palmitoyltransferase-1 and caveolin-3 promoters are directly regulated by ROR and coactivated by p300 and PGC-1. This study implicates RORs in the control of lipid homeostasis in skeletal muscle. In conclusion, we speculate that ROR agonists would increase fatty acid catabolism in muscle and suggest selective activators of ROR may have therapeutic utility in the treatment of obesity and atherosclerosis.

Bidirectional role of orphan nuclear receptor RORalpha in clock gene transcriptions demonstrated by a novel reporter assay system

Circadian rhythms are generated by an extremely complicated transcription-translation feedback loop. To precisely analyze the molecular mechanisms of the circadian clock, it is critical to monitor multiple gene expressions and/or interactions with their transcription factors simultaneously. We have developed a novel reporter assay system, the tricolor reporter in vitro assay system, which consists of green- and red-emitting Phrixothrix luciferases as dual reporters and blue-emitting Renilla luciferase as internal control. We have successfully employed this system in analyzing the effects of clock gene products on the enhancer elements of Per1 and Bmal1 promoters. The results indicate that the orphan nuclear receptor RORalpha regulates bidirectionally Bmal1 (positively) and Per1 (negatively) transcriptions simultaneously.

The “CholesteROR” Protective Pathway in the Vascular System

Retinoic acid receptor-related Orphan Receptor α (RORα) is a member of the nuclear hormone receptor superfamily. RORα has long been considered as a constitutive activator of transcription in the absence of exogenous ligand; however, cholesterol has recently been identified as a natural ligand of RORα. The spontaneous staggerer (sg/sg) mutation is a deletion in the Rora gene that prevents the translation of the ligand-binding domain (LBD), leading to the loss of RORα activity. The homozygous Rora sg/sg mutant mouse, of which the most obvious phenotype is ataxia associated with cerebellar degeneration, also displays a variety of other phenotypes, including several vascular ones; in particular, dysfunction of smooth muscle cells and enhanced susceptibility to atherosclerosis. Moreover, RORα appears to participate in the regulation of plasma cholesterol levels, and has been shown to positively regulate apolipoprotein (apo)A-I and apoC-III gene expression. Yet its activity is regulated by cholesterol itself, making RORα an intracellular cholesterol target.

Gene Silencing by Nuclear Orphan Receptors

Nuclear orphan receptors represent a large and diverse subgroup in the nuclear receptor superfamily. Although putative ligands for these orphan members remain to be identified, some of these receptors possess intrinsic activating, inhibitory, or dual regulatory functions in development, differentiation, homeostasis, and reproduction. In particular, gene-silencing events elicited by chicken ovalbumin upstream promoter-transcription factors (COUP-TFs); dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1 (DAX-1); germ cell nuclear factor (GCNF); short heterodimer partner (SHP); and testicular receptors 2 and 4 (TR2 and TR4) are among the best characterized. These orphan receptors are critical in controlling basal activities or hormonal responsiveness of numerous target genes. They employ multiple and distinct mechanisms to mediate target gene repression. Complex cross-talk exists between these orphan receptors at their cognate DNA binding elements and an array of steroid?nonsteroid hormone receptors, other transcriptional activators, coactivators and corepressors, histone modification enzyme complexes, and components of basal transcriptional components. Therefore, perturbation induced by these orphan receptors at multiple levels, including DNA binding activities, receptor homo- or heterodimerization, recruitment of cofactor proteins, communication with general transcriptional machinery, and changes at histone acetylation status and chromatin structures, may contribute to silencing of target gene expression in a specific promoter or cell-type context. Moreover, the findings derived from gene-targeting studies have demonstrated the significance of these orphan receptors' function in physiologic settings. Thus, COUP-TFs, DAX-1, GCNF, SHP, and TR2 and 4 are known to be required for multiple physiologic and biologic functions, including neurogenesis and development of the heart and vascular system steroidogenesis and sex determination, gametogenesis and embryonic development, and cholesterol?lipid homeostasis.

The ecdysone regulatory cascade and ovarian development in lepidopteran insects: insights from the silkmoth paradigm

The developing ovariole of the silkmoth Bombyx mori represents an excellent model for studies on the changes that occur in gene expression during the execution of long-term developmental programs. All stages of follicle development, which differ from each other by 2-2.5 h of developmental time, are organized in a single array and can be isolated simultaneously for physiological, biochemical and gene expression studies from a single animal. Recently, significant progress has been made toward the understanding of the hormonal regulation and autonomous implementation of the developmental program that governs follicular cell differentiation during oogenesis in Bombyx. In this review, the developmental career of the ovarian follicle during pharate adult development is discussed in view of the new physiological, biochemical and gene expression data that have recently accumulated. An overview is presented of the changes in expression of structural and regulatory genes and their hormonal regulation in the developing follicle during the transitions among the broad developmental periods of previtellogenesis, vitellogenesis and choriogenesis. Ovarian development in silkmoth pharate adults is induced by 20-hydroxy-ecdysone (20E) through the activation of a regulatory cascade similar to the one observed during Drosophila development. The transition from previtellogenesis to vitellogenesis corresponds to a late response to the hormone and is characterized by the induction of the expression of a unique isoform of the nuclear receptor BmHR3 and the follicular cell-specific yolk protein ESP. The transition from early to middle and late vitellogenesis and choriogenesis, on the other hand, is regulated by positively and negatively acting intra- and extra-ovarian factors. In vitro cultures of developing ovarioles reveal the requirement for the presence of an as yet unidentified growth factor(s) in the hemolymph, while the follicle developmental arrest that is observed after treatment with the ecdysone agonist tebufenozide indicates the requirement for a decline in the level of 20E. The initiation of choriogenesis is characterized by the transcriptional activation of the gene BmGATAbeta that encodes GATA transcription factors, and the chorion genes in the follicular cells. Furthermore, modulation of the activity of BmGATAbeta at the posttranscriptional level is crucial for the stage-specific activation of chorion genes during late choriogenesis. The developing ovariole of the silkmoth is, therefore, emerging as an important model for the study of insect oogenesis and the action of the steroid hormone 20E at the molecular level.

X-ray structure of the hRORalpha LBD at 1.63 A: structural and functional data that cholesterol or a cholesterol derivative is the natural ligand of RORalpha

The retinoic acid-related orphan receptor alpha (RORalpha) is an orphan member of the subfamily 1 of nuclear hormone receptors. No X-ray structure of RORalpha has been described so far, and no ligand has been identified. We describe the first crystal structure of the ligand binding domain (LBD) of RORalpha, at 1.63 A resolution. This structure revealed a ligand present in the ligand binding pocket (LBP), which was identified by X-ray crystallography as cholest-5-en-3beta-ol (cholesterol). Moreover, RORalpha transcriptional activity could be modulated by changes in intracellular cholesterol level or mutation of residues involved in cholesterol binding. These findings suggest that RORalpha could play a key role in the regulation of cholesterol homeostasis and thus represents an important drug target in cholesterol-related diseases.

Identification of Reverb(alpha) as a novel ROR(alpha) target gene

The nuclear receptor superfamily comprises a large number of ligand-activated transcription factors that are involved in numerous biological processes such as cell proliferation, differentiation, and homeostasis. ROR(alpha) (NR1F1) and Reverb(alpha) (NR1D1) are two members of this family whose biological functions are largely unknown. In addition, no ligand has been yet identified for these two receptors; therefore, they are referred as orphan receptors. Here, we show that ROR(alpha) and Reverb(alpha) are expressed with a similar tissue distribution and are both induced during the differentiation of rat L6 myoblastic cells. Ectopic expression of ROR(alpha)1 in L6 cells significantly induces Reverb(alpha) expression as demonstrated by Northern blot analysis. Using reverse transcription-PCR to analyze Reverb(alpha) gene expression from staggerer mice, we found that there was a significant reduction of Reverb(alpha) mRNA in the skeletal muscle comparing it with the wild-type mice, which suggests that ROR(alpha) is involved in the regulation of Reverb(alpha) gene expression. Transient transfection assays using the Reverb(alpha) promoter demonstrate that ROR(alpha) regulates the Reverb(alpha) gene at the transcriptional level. Furthermore, mutagenesis experiments indicate that ROR(alpha) regulates Reverb(alpha) transcription via a monomeric ROR response element located in the Reverb(alpha) gene promoter. Electrophoretic mobility shift assays show that ROR(alpha) binds strongly to this site in a specific-manner. Finally, overexpression of GRIP-1/TIF-2, but not SRC-1, potentiates ROR(alpha)-stimulated Reverb(alpha) promoter activity in transient transfection experiments. Together, our results identify Reverb(alpha) as a novel target gene for ROR(alpha).

Retinoic acid receptor-related orphan receptor (ROR) alpha4 is the predominant isoform of the nuclear receptor RORalpha in the liver and is up-regulated by hypoxia in HepG2 human hepatoma cells

The retinoic acid receptor-related orphan receptor alpha (RORalpha) is critically involved in many physiological functions in several organs. We find that the main RORalpha isoform in the mouse liver is the RORalpha4 isoform, in terms of both mRNA and protein levels, while the RORalpha1 isoform is less abundant. Because hypoxia is a major feature of liver physiology and pathology, we examined the effect of this stress on Rora gene expression and RORalpha transcriptional activity. HepG2 human hepatoma cells were cultured for 24 h under normoxia (20% O2) or hypoxia (10, 2, and 0.1% O2) and the abundance of the Rora transcripts measured by Northern blot and semi-quantitative RT-PCR. Hypoxic HepG2 cells contained more Rora mRNA than controls. This was also observed in rat hepatocytes in primary culture. Cobalt chloride and desferrioxamine also increased the amount of Rora mRNA in HepG2 cells. It is likely that these treatments increase the amount of the RORalpha4 protein in HepG2 cells as evidenced by Western blotting in the case of desferrioxamine. Transient transfection experiments indicated that hypoxia, cobalt chloride, and desferrioxamine all stimulate RORalpha transcriptional activity in HepG2 cells. Hence, we believe that RORalpha participates in the control of gene transcription in hepatic cells and modulates gene expression in response to hypoxic stress.

Expression and regulation of the nuclear receptor RORalpha in human vascular cells

Retinoic acid receptor-related orphan receptor alpha (RORalpha) is a member of the nuclear receptor superfamily. Using RT-PCR, RORalpha mRNA was identified in human aortic smooth muscle cells (hASMC), endothelial cells (EC), as well as in human mammary arteries and atherosclerotic plaques. We found a predominant expression of RORalpha1 in hASMC, and RORalpha4 in EC. RORalpha2 and RORalpha3 were not detected. In arteries, RORalpha4 was predominant compared with RORalpha1. In atherosclerotic plaques, RORalpha expression was significantly decreased. In hASMC stimulated with cytokines, RORalpha expression was increased by 2.5-fold. RORalpha mRNA was also significantly increased (approximately 2-fold) in hASMC and EC cultured under hypoxia.

The ROR nuclear orphan receptor subfamily: critical regulators of multiple biological processes

The nuclear receptor superfamily, a group of structurally related, ligand-dependent transcription factors, includes a large number of orphan receptors for which no ligand has yet been identified. These proteins function as key regulators of many physiological processes that occur during embryonic development and in the adult. The retinoid-related orphan receptors (RORs) alpha, beta, and gamma comprise one nuclear orphan receptor gene subfamily. RORs exhibit a modular structure that is characteristic for nuclear receptors; the DNA-binding domain is highly conserved and the ligand-binding domain is moderately conserved among RORs. By a combination of alternative promoter usage and exon splicing, each ROR gene generates several isoforms that differ only in their amino terminus. RORs bind as monomers to specific ROR response elements (ROREs) consisting of the consensus core motif AGGTCA preceded by a 5-bp A/T-rich sequence. RORE-dependent transcriptional activation by RORs is cell type-specific and mediated through interactions with nuclear cofactors. RORs have been shown to interact with certain corepressors as well as coactivators, suggesting that RORs are not constitutively active but that their activity is under some regulatory control. RORs likely can assume at least two different conformations: a repressive state, which allows interaction with corepressor complexes, and an active state, which promotes binding of coactivator complexes. Whether the transition between these two states is regulated by ligand binding and/or by phosphorylation remains to be determined. Ca2+/calmodulin-dependent kinase IV (CaMKIV) can dramatically enhance ROR-mediated transcriptional activation. This stimulation involves CaMKIV-mediated phosphorylation not of RORs, but likely of specific nuclear cofactors that interact with RORs. RORalpha is widely expressed. In the cerebellum, its expression is limited to the Purkinje cells. RORalpha-/- mice and the natural RORalpha-deficient staggerer mice exhibit severe cerebellar ataxia due to a defect in Purkinje cell development. In addition, these mice have thin long bones, suggesting a role for RORalpha in bone metabolism, and develop severe atherosclerosis when placed on a high-fat diet. Expression of RORbeta is very restricted. RORbeta is highly expressed in different parts of the neurophotoendocrine system, the pineal gland, the retina, and suprachiasmatic nuclei, suggesting a role in the control of circadian rhythm. This is supported by observations showing alterations in circadian behavior in RORbeta-/- mice. RORgamma, which is most highly expressed in the thymus, plays an important role in thymopoiesis. Thymocytes from RORgamma-/- mice undergo accelerated apoptosis. The induction of apoptosis is, at least in part, due to a down-regulation of the expression of the antiapoptotic gene Bcl-XL. In addition to the thynic phenotype, RORgamma-/- mice lack lymph nodes, indicating that RORgamma is essential for lymph node organogenesis. Overexpression of RORgamma has been shown to inhibit T cell receptor-mediated apoptosis in T cell hybridomas and to repress the induction of Fas-ligand and interleukin 2. These studies demonstrate that RORs play critical roles in the regulation of a variety of physiological processes. Further characterization of the mechanisms of action of RORs will not only lead to the identification of ROR target genes and provide additional insight into their normal physiological functions, but will also determine their roles in disease.

Transcriptional repression of RORalpha activity in human breast cancer cells by melatonin

Melatonin has repeatedly been shown to inhibit the proliferation of MCF-7 human breast cancer cells. Previous reports suggest that the actions of melatonin can be mediated either through G-protein coupled membrane receptors or via retinoid orphan receptors (RORalphas). In this study, we demonstrated the expression of RORalpha2, 3, and 4 transcripts in MCF-7 cells. These cells exhibited a high basal level of RORalpha transcriptional activity, which was further stimulated by serum. In the presence of serum, RORalpha transactivation and DNA-binding activity was repressed by melatonin even though melatonin had no effect on RORalpha protein levels. We found that RORalpha transcriptional activity in MCF-7 cells was regulated by modulators of the Ca2+/CaM signaling pathway. Given that melatonin has been reported to modulate the Ca2+/CaM signaling pathway in other tissues, our data indicate that melatonin may affect RORalpha transcriptional activity, expression of RORalpha regulated genes, and even breast cancer cell proliferation via modulation of the Ca2+/CaM signaling pathway.

The orphan nuclear receptor BmHR3A of Bombyx mori: hormonal control, ovarian expression and functional properties

Ovarian development in the domesticated silkmoth, Bombyx mori, is induced by the molting hormone 20-hydroxy-ecdysone (20E) shortly after larval-pupal ecdysis. Studies of the ecdysone response in Drosophila and other insects have shown that 20E exerts its effects initially by the induction of a small number of early genes, including the orphan nuclear receptors HR3, that transduce and amplify the hormone signal. Here we show that the silkmoth orphan receptor BmHR3A acts in the 20E-induced regulatory cascade in the ovary during pupal and pharate adult development in a manner different than that observed in the classical ecdysone regulatory hierarchy in Drosophila salivary glands at the end of the third instar. While other isoforms of BmHR3 are induced as early gene products in the ecdysone response, BmHR3A is induced 2 days after 20E administration in the silkmoth ovary and, thus, behaves as late product. The period of accumulation of BmHR3A in ovarian follicular cells occurs during vitellogenesis and coincides with the period of transcriptional expression of the ESP (egg-specific protein) gene, whose product constitutes a major component of the egg yolk, while it is reciprocal to the period of expression of BmGATAbeta, a gene encoding a regulator of late chorion gene expression. Bandshift experiments demonstrate that BmHR3A binds specifically to RORE (Retinoic acid-related Orphan receptor Response Element)-like sequences in the promoters of both genes, thus suggesting a direct role for BmHR3A in regulating the expression of BmGATAbeta and ESP genes during vitellogenesis. Finally, we show that BmHR3A functions as a constitutive transcriptional activator in a B. mori derived cell line. We propose that BmHR3A may function as a regulator of vitellogenesis in the silkmoth ovary.

DNA recognition by the aberrant retinoic acid receptors implicated in human acute promyelocytic leukemia

Human acute promyelocytic leukemias (APLs) are associated with chromosomal translocations that replace the NH2 terminus of wild-type retinoic acid receptor (RAR) alpha with portions of the promyelocytic leukemia protein (PML) or promyelocytic leukemia zinc-finger protein (PLZF). The wild-type RARalpha readily forms heterodimers with the retinoid X receptors (RXRs), and these RAR/RXR heterodimers appear to be the principal mediators of retinoid signaling in normal cells. In contrast, PML-RARalpha and PLZF-RARa display an enhanced ability to form homodimers, and this enhanced homodimer formation is believed to contribute to the neoplastic properties of these chimeric oncoproteins. We report here that the DNA recognition specificity of the RXRalpha/RARa heterodimer, which is presumed to be the dominant receptor species in normal cells, differs from that of the PML-RARalpha and PLZF-RARalpha homodimers, which are thought to prevail in the oncogenic cell. We suggest that differences in target gene recognition by the normal and oncogenic RARalpha proteins may contribute to the leukemogenic phenotype.

The orphan nuclear receptor ROR alpha is a negative regulator of the inflammatory response

Retinoid-related orphan receptor alpha (ROR alpha) (NR1F1) is a member of the nuclear receptor superfamily whose biological functions are largely unknown. Since staggerer mice, which carry a deletion in the ROR alpha gene, suffer from immune abnormalities, we generated an adenovirus encoding ROR alpha1 to investigate its potential role in control of the inflammatory response. We demonstrated that ROR alpha is expressed in human primary smooth-muscle cells and that ectopic expression of ROR alpha1 inhibits TNFalpha-induced IL-6, IL-8 and COX-2 expression in these cells. ROR alpha1 negatively interferes with the NF-kappaB signalling pathway by reducing p65 translocation as demonstrated by western blotting, immunostaining and electrophoretic mobility shift assays. This action of ROR alpha1 on NF-kappaB is associated with the induction of IkappaB alpha, the major inhibitory protein of the NF-kappaB signalling pathway, whose expression was found to be transcriptionally upregulated by ROR alpha1 via a ROR response element in the IkappaB alpha promoter. Taken together, these data identify ROR alpha1 as a potential target in the treatment of chronic inflammatory diseases, including atherosclerosis and rheumatoid arthritis.

Differential binding and transcriptional behaviour of two highly related orphan receptors, ROR alpha(4) and ROR beta(1)

Nuclear receptors are ligand-inducible transcription factors that can be classified into two major groups according to their DNA-binding properties. Members of the first group bind to DNA as dimers, either homo- or heterodimers; members of the second group are also able to bind as monomers. While the first group has been extensively studied biochemically, very little is known about nuclear receptors that bind and act as monomers. In this study, we compared the binding and transcriptional behaviour of ROR alpha (NR1F1) and ROR beta (NR1F2), two representatives of the subgroup of monomer-binding receptors. We show that although they are highly related in their amino acid structures, they display remarkably different binding behaviours. Furthermore, we provide evidence that ROR beta can efficiently activate transcription in vitro as a monomer.

Cloning of cDNAs encoding retinoic acid receptors RAR gamma 1, RAR gamma 2, and a new splicing variant, RAR gamma 3, from Aambystoma mexicanum and characterization of their expression during early development

To analyze retinoic acid (RA) receptor (RAR) expression during early development in the urodele embryo, we have isolated cDNAs for four members of the axolotl (Ambystoma mexicanum) RAR family, namely RAR alpha (NR1B1), aRAR gamma 1 (NR1B3a), aRAR gamma 2 (NR1B3b), and a new splicing variant of aRAR gamma 2, aRAR gamma 3 (NR1B3c), which contains an insertion of five hydrophobic amino acids in the C-terminal region of the DNA binding domain. The temporal expression pattern of the RAR gamma isoforms was established by RT-PCR using total RNA from embryos of different stages. The expression of aRAR gamma 2 coincides with neurulation and is enhanced in the extremities of the embryo's anteroposterior axis. The aRAR gamma 3 is specifically expressed during gastrulation and early neurulation, whereas aRAR gamma 1 is expressed later during organogenesis. Global aRAR gamma 2 mRNA levels, as well as their spatio-temporal expression pattern in the neurula, were not affected by treatment with RA. These results show that several RARs are expressed in the axolotl embryo during early development, and reveal the existence of a new RAR gamma variant.

Increased cell death and delayed development in the cerebellum of mice lacking the rev-erbA(alpha) orphan receptor

The rev-erbA(alpha) gene, belonging to the steroid receptor superfamily of transcription factors, is highly conserved during evolution but little is known so far about its functions in development or in adult physiology. Here, we describe genetically altered mice lacking the rev-erbA(alpha) gene. These animals do not show any obvious phenotype in either fat tissue or skeletal muscle, despite the known regulation of rev-erbA(alpha) expression during adipocyte and myotube differentiation in vitro. However, during the second week of life, the cerebellum of rev-erbA(alpha) mutants presents several unexpected abnormalities, such as alterations in the development of Purkinje cells, delay in the proliferation and migration of granule cells from the external granule cell layer and increased apoptosis of neurons in the internal granule cell layer. Interestingly, the expression pattern of rev-erbA(alpha) suggests that the abnormalities observed in the external granule cell layer could be secondary to Purkinje cell alterations. Taken together, our data underline the importance of rev-erbA(alpha)expression for the appropriate balance of transcriptional activators and repressors during postnatal cerebellar development.

Expression of the early-late gene encoding the nuclear receptor HR3 suggests its involvement in regulating the vitellogenic response to ecdysone in the adult mosquito

The insect steroid hormone, 20-hydroxyecdysone (20E), is a key factor controlling critical developmental events of embryogenesis, larval molting, metamorphosis, and, in some insects, reproduction. We are interested in understanding the molecular basis of the steroid hormone ecdysone action in insect egg development. The yellow fever mosquito, Aedes aegypti, in addition to being an important vector of human diseases, represents an outstanding model for studying molecular mechanisms underlying egg maturation due to stringently controlled, blood meal-activated reproductive events in this insect. To elucidate the genetic regulatory hierarchy controlling the reproductive ecdysone response, we have investigated ecdysone-regulated gene expression in vitellogenic mosquito ovaries and fat bodies. We have previously demonstrated the conservation of a primary ecdysone-triggered regulatory hierarchy, implicated in development of immature stages of Drosophila, represented by the ecdysone receptor/Ultraspiracle complex and an early gene E75 during the reproductive ecdysone response (Wang, S.-F., Miura, K., Miksicek, R.J., Segraves, W.A., Raikhel, A.S., 1998. DNA binding and transactivation characteristics of the mosquito ecdysone receptor - Ultraspiracle complex. J. Biol. Chem. 273, 27531-27540; Pierceall, W. E., Li, C., Biran, A., Miura, K., Raikhel, A.S., Segraves, W.A., 1999. E75 expression in mosquito ovary and fat body suggests reiterative use of ecdysone-regulated hierarchies in development and reproduction. Mol. Cell. Endocrinol. 150, 73-89). The present paper demonstrates that conservation of the factors involved in the ecdysone-responsive genetic hierarchy regulating female reproduction extends beyond the early genes. Here, we identify AHR3, a highly conserved homologue of the Drosophila HR3 early-late ecdysone-inducible gene in the mosquito. We show that AHR3 is expressed in both vitellogenic tissues of the female mosquito, the fat body and the ovary. The expression of AHR3 correlates with the ecdysteroid titer, reaching a peak at 24 h after a blood meal. Moreover, in vitro fat body culture experiments demonstrate that the kinetics and dose response of AHR3 to 20-hydroxyecdysone (20E), an active ecdysteroid in the mosquito, is similar to those of the late vitellogenic genes rather than the early E75 gene. However, as shown for other early and early-late genes, the 20E activation of AHR3 is not inhibited by the presence of cycloheximide, a protein synthesis inhibitor. Taken together, these findings strongly suggest AHR3 involvement in regulating the vitellogenic response to ecdysone in the adult mosquito.

Activation of orphan receptor-mediated transcription by Ca(2+)/calmodulin-dependent protein kinase IV

Retinoid-related receptor alpha (RORalpha) is an orphan nuclear receptor that constitutively activates transcription from its cognate response element. We show that RORalpha is Ca(2+ )responsive, and a Ca(2+)/calmodulin-independent form of Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) potentiates RORalpha-dependent transcription 20- to 30-fold. Other orphan receptors including RORalpha2, RORgamma and COUP-TFI are also potentiated by CaMKIV. Transcriptional activation by CaMKIV is orphan receptor selective and does not occur with either the thyroid hormone or estrogen receptor. CaMKIV does not phosphorylate RORalpha or its ligand-binding domain (LBD) in vitro, although the LBD is essential for transactivation. Therefore, the RORalpha LBD was used in the mammalian two-hybrid assay to identify a single class of small peptide molecules containing LXXLL motifs that interacted with greater affinity in the presence of CaMKIV. This class of peptides antagonized activation of orphan receptor-mediated transcription by CaMKIV. These studies demonstrate a pivotal role for CaMKIV in the regulation of orphan receptor-mediated transcription.

Coactivators for the orphan nuclear receptor RORalpha

A mutation in the nuclear orphan receptor RORalpha results in a severe impairment of cerebellar development by unknown mechanisms. We have shown previously that RORalpha contains a strong constitutive activation domain in its C terminus. We therefore searched for mammalian RORalpha coactivators using the minimal activation domain as bait in a two-hybrid screen. Several known and putative coactivators were isolated, including glucocorticoid receptor-interacting protein-1 (GRIP-1) and peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP/TRAP220/DRIP205). These interactions were confirmed in vitro and require the intact activation domain of RORalpha although different requirements for interaction with GRIP-1 and PBP were detected. Even in the absence of exogenous ligand, RORalpha interacts with a complex or complexes of endogenous proteins, similar to those that bind to ligand-occupied thyroid hormone and vitamin D receptors. Both PBP and GRIP-1 were shown to be present in these complexes. Thus we have identified several potential RORalpha coactivators that, in contrast to the interactions with hormone receptors, interact with RORalpha in yeast, in bacterial extracts, and in mammalian cells in vivo and in vitro in the absence of exogenous ligand. GRIP-1 functioned as a coactivator for the RORalpha both in yeast and in mammalian cells. Thus, GRIP-1 is the first proven coactivator for RORalpha.