Genomic organization of the retinoic acid receptor gamma gene

Targeting Androgen, Thyroid Hormone, and Vitamin A and D Receptors to Treat Prostate Cancer

The nuclear hormone family of receptors regulates gene expression. The androgen receptor (AR), upon ligand binding and homodimerization, shuttles from the cytosol into the nucleus to activate gene expression. Thyroid hormone receptors (TRs), retinoic acid receptors (RARs), and the vitamin D receptor (VDR) are present in the nucleus bound to chromatin as a heterodimer with the retinoid X receptors (RXRs) and repress gene expression. Ligand binding leads to transcription activation. The hormonal ligands for these receptors play crucial roles to ensure the proper conduct of very many tissues and exert effects on prostate cancer (PCa) cells. Androgens support PCa proliferation and androgen deprivation alone or with chemotherapy is the standard therapy for PCa. RARγ activation and 3,5,3'-triiodo-L-thyronine (T3) stimulation of TRβ support the growth of PCa cells. Ligand stimulation of VDR drives growth arrest, differentiation, and apoptosis of PCa cells. Often these receptors are explored as separate avenues to find treatments for PCa and other cancers. However, there is accumulating evidence to support receptor interactions and crosstalk of regulatory events whereby a better understanding might lead to new combinatorial treatments.

A decade of USFDA-approved small molecules as anti-inflammatory agents: Recent trends and Commentaries on the "industrial" perspective

Inflammation is the human body's defence process against various pathogens, toxic substances, irradiation, and physically injured cells that have been damaged. Inflammation is characterized by swelling, pain, redness, heat, as well as diminished tissue function. Multiple important inflammatory markers determine the prognosis of inflammatory processes, which include likes of pro-inflammatory cytokines which are controlled by nuclear factor kappa-B (NF-kB), mitogen-activated protein kinase (MAPK), Janus kinase signal transducer and activator of transcription (JAK-STAT) pathway, all of which are activated in response to the stimulation of specific receptors. Besides these, the cyclooxygenase (COX) enzyme family also plays a significant role in inflammation. The current review is kept forth to compile a summary of small molecules-based drugs approved by the USFDA during the study period of 2013-2023. A thorough discussion has also been made to focus on biologics, macromolecules, and small chemical entities approved during this study period and their greener synthetic routes with a brief discussion on the chemical spacing parameters of anti-inflammatory drugs. The compilation is expected to assist the medicinal chemist and the scientist actively engaged in drug discovery and development of anti-inflammatory agents from newer perspectives during the current years.

Therapeutic use of selective synthetic ligands for retinoic acid receptors: a patent review

INTRODUCTION

Differentiation therapy using all-trans retinoic acid (ATRA) revolutionised the treatment of acute promyelocytic leukaemia to such an extent that it is now one of the most curable types of leukaemia, with ATRA and anthracycline-based chemotherapy providing cure rates above 80%. Isotretinoin is used to treat chronic acne. Here, we examine the information described in recent patents and the extent to which new findings are influencing extending retinoid-based differentiation therapy to other cancers, as well as the development of new therapies for other disorders.

AREAS COVERED

A search has been performed on the literature and worldwide patents filed during 2014 to the present time, focusing on synthetic agonists and antagonists of retinoic acid receptors and novel compositions for the delivery of these agents.

EXPERT OPINION

New potential therapeutic applications have been described, including lung, breast and head and neck cancers, T cell lymphoma and neurodegenerative, metabolic, ophthalmic, muscle, and inflammatory disorders. Recent patents have described the means to maximise retinoid activity. Two decades of efforts to extend retinoid-based therapies have been disappointing and new synthetic retinoids, target diseases and modes of delivery may well resolve this long standing issue.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Macrophages engulfing apoptotic thymocytes produce retinoids to promote selection, differentiation, removal and replacement of double positive thymocytes

The thymus provides the microenvironment in which thymocytes develop into mature T-cells, and interactions with thymic stromal cells are thought to provide the necessary signals for thymocyte maturation. Recognition of self-MHC by T-cells is a basic requirement for mature T-cell functions, and those thymocytes that do not recognize or respond too strongly to the peptide-loaded self-MHC molecules found in the thymus undergo apoptosis. As a result, 95% of the thymocytes produced will die and be subsequently cleared by macrophages. This review describes a complex crosstalk between developing thymocytes and engulfing macrophages which is mediated by retinoids produced by engulfing macrophages. The interaction results in the harmonization of the rate of cell death of dying double positive cells with their clearance and replacement, and in promotion of the differentiation of the selected cells in the thymus.

MIR-34c regulates mouse embryonic stem cells differentiation into male germ-like cells through RARg

Embryonic stem cells (ESCs) have the capacity to differentiate into nearly all sorts of cell types, including germ cells, which were regarded as one type of highly specialized cells in mammals, taking the responsibility of transferring genetic materials to the next generation. Studies on induction differentiation of murine embryonic stem cells (mESCs) into male germ cells, but with a low efficiency, basic reason is that the regulation mechanism of germ cell development in mammals is still unclear. miRNA might play an important role in spermatogenesis in mammals. In this study, several miRNAs, which might be related to spermatogenesis, were initially selected and detected in the mouse tissues by semi-polymerase chain reaction (PCR) and quantitative real time (qRT)-PCR to find a testis-specific miRNA. To study its effect on mESCs differentiation into male germ cells, miR-34c mimics were synthesized and pri-miR-34c-GFP plasmid was constructed, transfected into mESCs and combined with retinoic acid induction. The effects of miR-34c were analysed by morphology, alkaline phosphatase staining, qRT-PCR_and immunofluorescent staining. The results showed that miR-34c promoted mESCs differentiation into male germ-like cells, to some extent. Then miR-34c targeted genes were predicted by bioinformatics; Retinoic acid receptor gamma (RARg) was selected, and two dual-luciferase reporter vectors contained the normal and mutated 3'untranslated region of RARg were constructed, respectively. By miRNA mimics and vector co-transfection experiment, the predicted target gene-RARg was confirmed. In conclusion, we found a mammalian male germ cell specific miRNA--miR-34c, and it might be pivotal in mESCs differentiation into male germ cells through its target--RARg.

International Union of Pharmacology. LX. Retinoic acid receptors

Retinoid is a term for compounds that bind to and activate retinoic acid receptors (RARalpha, RARbeta, and RARgamma), members of the nuclear hormone receptor superfamily. The most important endogenous retinoid is all-trans-retinoic acid. Retinoids regulate a wide variety of essential biological processes, such as vertebrate embryonic morphogenesis and organogenesis, cell growth arrest, differentiation and apoptosis, and homeostasis, as well as their disorders. This review summarizes the considerable amount of knowledge generated on these receptors.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Characterization of the mouse peroxisome proliferator-activated receptor delta gene

Peroxisome proliferator-activated receptors (alpha, delta and gamma) are ligand-activated transcription factors that are involved in multiple cellular responses. The PPARdelta subtype is the least understood of all PPAR subtypes. PPARdelta is activated by unsaturated fatty acids, PGI2, and by synthetic ligands. PPARdelta-regulated genes have not been identified and the factors that control PPARdelta expression are not known. The gene that encodes the mouse PPARdelta gene is contained in >30 kb DNA sequence and organized in eight exons, six of which encode the PPARdelta receptor. A PPARdelta-luciferase reporter containing 694 bp 5' upstream regulatory and 127 bp untranslated was introduced to primary brain cultures to begin a characterization of the DNA sequences that mediate transcriptional regulation of PPARdelta. PPARdelta-luciferase expression was 10 times higher in oligodendrocyte-containing mature cultures than in immature cultures, indicating that PPARdelta may play a role during oligodendrocyte migration, proliferation, and/or maturation.

4HPR triggers apoptosis but not differentiation in retinoid sensitive and resistant human embryonal carcinoma cells through an RARgamma independent pathway

Retinoids signal biological effects through retinoic acid receptors (RAR) and retinoid X receptors (RXR) and their co-regulators. We previously reported that all-trans retinoic acid (RA) triggers terminal differentiation in the human embryonal carcinoma cell line NTERA-2 clone D1 (NT2/D1), through an RARgamma dependent pathway. RARgamma repression in NT2/D1-R1 cells accounts for RA resistance in this line. This report finds RARgamma repression is due to selective repression of RARgamma but not RARbeta transcription in NT2/D1-R1 cells. The repression is neither due to mutations in RARgamma nor its promoter containing the RA response element. Prior work was confirmed and extended by demonstrating that an RARgamma selective agonist preferentially signals differentiation of NT2/D1 cells, while RARalpha/beta, RARbeta, RXR agonists and an RAR pan-antagonist do not even when NT2/D1 cells are treated with these retinoids at 10 microM dosages. None of these examined retinoids induced differentiation of the RA resistant NT2/D1-R1 cells. In contrast, N-(4-hydroxyphenyl)retinamide (4HPR), a reported transcriptional activator of RARgamma was shown to potently induce growth inhibition and apoptosis in both NT2/D1 and NT2/D1-R1 cells. 4HPR-induced apoptosis was unaffected by co-treatment of both cell lines with equimolar RAR antagonist. Semi-quantitative reverse transcription-polymerase chain reaction (RT - PCR) assays of total RNA from 4HPR-treated NT2/D1 and NT2/D1-R1 cells did not reveal RARgamma induction. Since 4HPR signals in RA-resistant NT2/D1-R1 cells having an RARgamma transcriptional block, these results indicate that 4HPR triggers apoptosis but not differentiation through an RARgamma independent pathway. Taken together, these findings implicate a therapeutic role for 4HPR mediated apoptosis in germ cell tumors even when a maturation block is present.

Redox-dependent regulation of nuclear import of the glucocorticoid receptor

A number of transcription factors including the glucocorticoid receptor (GR) are regulated in a redox-dependent fashion. We have previously reported that the functional activity of the GR is suppressed under oxidative conditions and restored in the presence of reducing reagents. In the present study, we have used a chimeric human GR fused to the Aequorea green fluorescent protein and demonstrated that both ligand-dependent and -independent nuclear translocation of the GR is impaired under oxidative conditions in living cells. Substitution of Cys-481 for Ser within NL1 of the human GR resulted in reduction of sensitivity to oxidative treatment, strongly indicating that Cys-481 is one of the target amino acids for redox regulation of the receptor. Taken together, we may conclude that redox-dependent regulation of nuclear translocation of the GR constitutes an important mechanism for modulation of glucocorticoid-dependent signal transduction.

Quanititation of gene expression by means of HPLC analysis of RT-PCR products

A method for the quantitative analysis of specific mRNA species by reverse transcription-polymerase chain reaction (RT-PCR) and subsequent detection of products by means of ion-pair reversed-phase high-performance liquid chromatography (IP-RP-HPLC) on alkylated micropellicular polystyrene-divinylbenzene particles has been developed. For RT-PCR, we used the EZrTth RNA PCR kit (Perkin Elmer). This method allows reverse transcription and amplification of specific target mRNA in a single reaction tube. RT-PCR products were analyzed qualitatively and quantitatively by means of IP-RP-HPLC and UV detection at 254 nm. The enzymatic amplification combined with chromatographic separation and UV detection permitted high precision (and intra assay CV < 10%), with good practicability (two pipetting steps only). A total RNA preparation of a tissue that highly expressed the sequence of interest and that was stored in multiple aliquots, was diluted to give a standard curve. This external standard curve was used to define when samples have to be diluted, i.e., when the signal is in the plateau phase of amplification. The validity of the method is demonstrated with the example of human retinoic acid receptor mRNA.

Genomic structure and chromosomal mapping of the nuclear orphan receptor ROR gamma (RORC) gene

The nuclear orphan receptor subfamily ROR/RZR is part of the steroid and thyroid hormone/retinoid receptor superfamily and consists of three different genes, alpha, beta, and gamma. In this study, we determined the genomic structure of mouse ROR gamma and the chromosomal localization of both mouse ROR gamma and human ROR gamma (HGMW-approved symbol RORC). The genomic structure of the mouse ROR gamma gene was derived from the analysis of P1 vector clones containing large genomic fragments encoding ROR gamma. These results revealed that the mROR gamma gene has a complex structure consisting of 11 exons separated by 10 introns spanning more than 21 kb of genomic DNA. The DNA-binding domain is contained in two exons, 3 and 4, each encoding one zinc-finger. The splice site between exon 3 and exon 4 is identical to that found in RAR and TR3 receptors. ROR gamma is expressed as two mRNAs, 2.3 and 3.0 kb in size, that are derived by the use of alternative polyadenylation signals. We show by fluorescence in situ hybridization that the mouse ROR gamma gene is located on chromosome 3, in a region that corresponds to band 3F2.1-2.2. The human ROR gamma was mapped to chromosome region 1q21. The results demonstrate that the ROR gamma genes are located in chromosomal regions that are syntenic between mouse and human.

Alternative splicing of the estrogen receptor primary transcript normally occurs in estrogen receptor positive tissues and cell lines

Several laboratories have described estrogen receptor mRNA variants created by skipping internal exons. Some of the putative proteins encoded for by these variants have been functionally characterized by transfection analyses. The variant lacking exon 5 would lead, if translated, to a truncated receptor which shows dominant positive transactivation activity in the absence of hormone. It has been postulated that the variant could account for anti-estrogen resistant tumor growth and for expression of the progesterone receptor in estrogen negative tumors. In order to understand the possible role this and other variants may have in the tumorigenesis of mammary tissue we have carried out a thorough analysis of variants expressed in a tumor cell line (MCF-7), in a tumor sample and in a sample of normal breast tissue derived from mammary reduction surgery. We performed rt-PCR analyses followed by hybridization with exon specific oligonucleotide probes. By these means we have detected nine different variants co-expressed in MCF-7 cells and at least the major variants were equally expressed in normal and neoplastic breast tissue. The same is true for the variant lacking exon 5 which, however, resulted to be a variant of low expression in the three samples analyzed. Variant formation appeared to be restricted to the estrogen receptor messenger since several other members of the superfamily of nuclear receptors did not show variant formation. We also have analyzed the effect of the most abundantly expressed variant, the exon 4 lacking variant, on normal estrogen receptor function, on the growth and on the response to estradiol and to tamoxifen of MCF-7 cells. Although over-expressed at high levels this variant has, if any, only marginal effects on the expression of endogenous estrogen regulated genes and on growth and response to the hormone and its antagonist. Although the lack of function of this variant cannot be extrapolated to other variants, their involvement in tumor formation appears rather unlikely since they are also expressed in normal tissue and the single variant is expressed in addition to many others, some of which might have opposing effects. Variant formation is, however, specific for the estrogen receptor and apparently regulated with tissue specificity as our expression analysis in normal mouse tissues shows. Therefore the variants probably have a physiological significance yet to be discovered.

The mouse bone morphogenetic protein-4 gene. Analysis of promoter utilization in fetal rat calvarial osteoblasts and regulation by COUP-TFI orphan receptor

Bone morphogenetic protein-4 (BMP-4) is one of a member of related polypeptides that are important in bone formation and other developmental processes. We isolated the BMP-4 gene from a mouse genomic library and characterized the exon-intron structure and one of the candidate promoters. Two alternative 5'-noncoding exons, 1A and 1B, were identified by reverse transcription polymerase chain reaction assays. Quantitative competitive polymerase chain reaction using Exon 1A, Exon 1B, and Exon 3 primers indicate the 1A-containing transcript is the primary BMP-4 mRNA expressed in bone cell cultures. Primer extension analysis supports that 1A is the major promoter utilized in bone cell cultures as well as in 9.5-day mouse embryos. 1A promoter activity indicate selective DNA regions functional in bone cells. We found potential regulatory response regions in the 1A 5'-flanking region of the BMP-4 gene for the chicken ovalbumin upstream-transcription factor I (COUP-TFI). Specific binding to the COUP-TFI response regions in the BMP-4 1A promoter was demonstrated. By co-transfection of a COUP-TFI expression plasmid with the BMP-4 1A promoter in fetal rat calvarial osteoblasts, we demonstrated that COUP-TFI inhibits the BMP-4 promoter activity. This suggests that COUP-TFI could act as a silencer for BMP-4 transcription in vivo.

Characterization of the mouse HNF-4 gene and its expression during mouse embryogenesis

Hepatocyte nuclear factor 4 (HNF-4) is a member of the nuclear receptor gene superfamily with unknown ligand. It has been assumed to play an important role in the regulation of gene expression in the liver. Here, we report the cloning and characterization of the mouse HNF-4 gene, as well as its expression during embryogenesis. The HNF-4 protein is encoded by ten exons. The gene structure is unique in the steroid receptor superfamily in that the second zinc finger is encoded by two exons. HNF-4 mRNA is expressed in a limited number of mouse adult tissues: liver, kidney, intestine, stomach and skin. HNF-4 could play an important role in the formation and function of visceral yolk sac and in the development of the liver and kidney since its mRNA, as determined by in situ hybridization, appears upon primary differentiation of these organs. As a first step in the study of the regulatory elements of the HNF-4 gene, we mapped the transcription start site and carried out DNase I hypersensitive site (HS) analysis over a region of approximately 22kb upstream of the gene. The complexity of the HSs suggests that multiple elements might contribute to the transcriptional regulation of the HNF-4 gene.

The genomic organisation, sequence and functional analysis of the 5' flanking region of the chicken estrogen receptor gene

The cDNA of many members of the nuclear receptor superfamily has been cloned. Recently more effort has been expended on the analysis of these genes at the genomic level and on the factors controlling their expression. The genomic organization of the chicken estrogen receptor gene is presented and compared to the other members of the superfamily of hormone receptor genes with emphasis on the relationship to the functional domains. The results show that the gene is divided into eight exons and that the position of the intron/exon boundaries are as in the human gene but different to the trout estrogen receptor gene. Primer extension and cDNA clone isolation was used to determine the transcription start site and 3.0 kb of 5' flanking sequence was generated. There is striking sequence homology to the human estrogen receptor promoter and there is a well positioned "typical" TATA sequence, with potential candidate CAAT box sequences close to the start site of transcription. In transient transfection assays, subfragments of this region drove CAT expression in chicken embryo fibroblasts, and the level was increased further with the addition of forskolin, but not phorbol myristate acetate. Including sequences more distal to the cap site in promoter constructs, completely abolished the promoter activity and forskolin inducibility, indicating the presence of strong silencing activity.

The mouse Rxrb gene encoding RXR beta: genomic organization and two mRNA isoforms generated by alternative splicing of transcripts initiated from CpG island promoters

Two major isoforms of retinoid X receptor beta (RXR beta; H-2RIIBP), encoded by the Rxrb gene, have been identified in the mouse. Northern analysis of Rxrb mRNA showed two close bands of 2.8 and 2.6 kb in many tissues and cell lines. They are designated as mRxr beta 1 and mRxr beta 2, respectively. Some rapidly growing cell lines and spleen tissue had about twofold more Rxr beta 1 mRNA than Rxr beta 2 whereas most adult tissues had similar amounts of both beta 1 and beta 2. Amino acid (aa) sequences deduced from cDNAs show an extra N-terminal domain of 72 aa for RXR beta 1 that is well conserved between mouse and human, but not found in RXR beta 2. These isoforms are generated from separate exons transcribed from different CpG island promoters and spliced into the common acceptor site in the transactivation domain by an alternative splicing. The Rxrb gene contains an intron in the midst of the first zinc-finger coding region. This is different from the retinoic acid receptor (RAR) and other nuclear receptor superfamily genes that contain an intron between the first and the second zinc-finger coding regions. These results, together with their unique ability to form heterodimers with other members of the superfamily, suggest a distinct phylogenic position for the Rxr genes.

Correlation of the ability of retinoids to inhibit promoter-induced anchorage-independent growth of JB6 mouse epidermal cells with their activation of retinoic acid receptor gamma

Retinoids inhibit the biological effects induced in mouse epidermal cells by the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Specific nuclear retinoic acid receptors (RARs) have been identified in the epidermis, but the specific receptor that mediates the inhibitory response by retinoids is not established. Retinoic acid and six conformationally restricted retinoids were evaluated in an in vitro bioassay using the JB6 mouse epidermal cell line. These activities were then compared with the ability of these retinoids to activate the RARs in transient transfection assays for transcriptional activation to identify the retinoid receptor involved in inhibiting TPA-induced anchorage-independent growth. The retinoids inhibited TPA-induced colony formation of JB6 cells in semisolid medium at concentrations that were not toxic based on colony formation of attached cells. These concentrations ranged from less than 10(-9)-10(-6) M. 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethylanthracen-2-yl)benzoic acid (TTAB) was the most potent retinoid, with an EC50 of 0.8 nM. Both RAR alpha and RAR gamma were expressed in JB6 cells. Expression of RAR beta was not detected in these cells using a polymerase chain reaction assay, consistent with its extremely low level in mouse skin. Inhibition of the TPA response by these retinoids in JB6 cells correlated only with their transcriptional activation of RAR alpha, but not with that of RAR alpha. These results suggest that RAR gamma is most probably the receptor that mediates the chemopreventive effects of retinoids in mouse epidermis.

Genomic organization of the human retinoic acid receptor beta 2

Recently three isoforms of the mouse retinoic acid receptor (mRAR beta 1, mRAR beta 2, mRAR beta 3) have been described, generated from the same gene (Zelent et al., 1991). The isoforms differ in their 5'-untranslated (5'-UTR) and A region, but have identical B to F regions. The N-terminal variability of mRAR beta 1/beta 3 is encoded in the first two exons (E1 and E2), while exon E3 includes N-terminal sequences of the mRAR beta 2 isoform. We have determined the structure of the human RAR beta 2 gene, using a genomic library from K562 cells. The open reading frame is split into eight exons: E3 contains sequences for the N-terminal A region and E4 to E10 encode the common part of the receptor, including the DNA-binding domain and ligand-binding domain. Corresponding to other nuclear receptors, both 'zinc-fingers' of the DNA-binding domain are encoded separately in two exons and the ligand-binding domain is assembled from five exons.

COUP orphan receptors are negative regulators of retinoic acid response pathways

The vitamin hormone retinoic acid (RA) regulates many complex biological programs. The hormonal signals are mediated at the level of transcription by multiple nuclear receptors. These receptors belong to the steroid/thyroid hormone receptor superfamily that also includes a large number of orphan receptors whose biological roles have not yet been determined. Although much has been learned in recent years about RA receptor (RAR) functions, little is known about how specific RA response programs are restricted to certain tissues and cell types during development and in the adult. It has been recently shown that RAR activities are regulated by retinoid X receptors (RXR) through heterodimer formation. In an effort to isolate and further characterize nuclear receptors that modulate RAR and/or RXR activities, we have screened cDNA libraries by using a RXR alpha cDNA probe. Two clones, COUP alpha and COUP beta, identical and closely related to the orphan receptor COUP-TF, were obtained. We show that COUP proteins dramatically inhibit retinoid receptor activities on certain response elements that are activated by RAR/RXR heterodimers or RXR homodimers. COUP alpha and -beta bind strongly to these response elements, including a palindromic thyroid hormone response element and a direct repeat RA response element as well as an RXR-specific response element. In addition, we found that the previously identified COUP-TF binding site in the ovalbumin gene functions in vitro as an RA response element that is repressed in the presence of COUP. Our data suggest that the COUP receptors are a novel class of RAR and RXR regulators that can restrict RA signaling to certain elements. The COUP orphan receptors may thus play an important role in cell- or tissue-specific repression of subsets of RA-sensitive programs during development and in the adult.

COUP orphan receptors are negative regulators of retinoic acid response pathways

The vitamin hormone retinoic acid (RA) regulates many complex biological programs. The hormonal signals are mediated at the level of transcription by multiple nuclear receptors. These receptors belong to the steroid/thyroid hormone receptor superfamily that also includes a large number of orphan receptors whose biological roles have not yet been determined. Although much has been learned in recent years about RA receptor (RAR) functions, little is known about how specific RA response programs are restricted to certain tissues and cell types during development and in the adult. It has been recently shown that RAR activities are regulated by retinoid X receptors (RXR) through heterodimer formation. In an effort to isolate and further characterize nuclear receptors that modulate RAR and/or RXR activities, we have screened cDNA libraries by using a RXR alpha cDNA probe. Two clones, COUP alpha and COUP beta, identical and closely related to the orphan receptor COUP-TF, were obtained. We show that COUP proteins dramatically inhibit retinoid receptor activities on certain response elements that are activated by RAR/RXR heterodimers or RXR homodimers. COUP alpha and -beta bind strongly to these response elements, including a palindromic thyroid hormone response element and a direct repeat RA response element as well as an RXR-specific response element. In addition, we found that the previously identified COUP-TF binding site in the ovalbumin gene functions in vitro as an RA response element that is repressed in the presence of COUP. Our data suggest that the COUP receptors are a novel class of RAR and RXR regulators that can restrict RA signaling to certain elements. The COUP orphan receptors may thus play an important role in cell- or tissue-specific repression of subsets of RA-sensitive programs during development and in the adult.

RAR gamma 2 expression is regulated through a retinoic acid response element embedded in Sp1 sites

At the level of transcription, all signals of the vitamin A derivative retinoic acid (RA) are mediated by the RA receptors (RARs) as well as the retinoid X receptors (RXRs). The control of expression of the various receptor subtypes and their specific isoforms appears to be strictly regulated and can be assumed to play a pivotal role during development and in the adult tissue. It has previously been shown that the RAR beta 2 isoform can regulate its own synthesis through an RA response element (RARE) in its promoter. Recent evidence suggests that the expression of other RAR isoforms, including that of RAR gamma 2, are also regulated by RA. We present evidence that expression of the RAR gamma 2 isoform can be regulated through the RARE in its own promoter region. Similar to the beta 2 RARE, the gamma 2 RARE consists of a 6-bp direct repeat with a 5-nucleotide spacer, but it has different functional features, including receptor specificity, basal-level activity, and affinity for RAR. In agreement with recent observations, this response element is bound most effectively by RAR/RXR heterodimers. Single-base-pair mutations had different effects on the activity of this RARE. The gamma 2 RARE is surrounded by several binding sites for the transcription factor Sp1. Cotransfected Sp1 enhanced strongly the activity of gamma 2 promoter reporter constructs in Drosophila cells. Our data suggest an important role for RAR-containing heterodimers and Sp1 in the regulation of RAR gamma 2 expression.

At least three promoters direct expression of the mouse glucocorticoid receptor gene

We have characterized the gene for the mouse glucocorticoid receptor. The gene spans approximately 110 kilobases, and glucocorticoid receptor transcripts are assembled from nine exons. Expression of the gene is controlled by at least three promoters, resulting in glucocorticoid receptor transcripts with different 5' nontranslated exons. One promoter is cell-specific, found to be active only in T lymphocytes. The other two promoters are active to various degrees in all cell lines and tissues so far analyzed and are located in a CpG island. The promoter activities are accompanied by DNase I hypersensitivity sites in chromatin. In contrast to a conservation of exon-intron structure, differences in promoter organization suggest a divergence between the evolution of regulatory and coding regions among members of the steroid receptor super-family.

RAR gamma 2 expression is regulated through a retinoic acid response element embedded in Sp1 sites

At the level of transcription, all signals of the vitamin A derivative retinoic acid (RA) are mediated by the RA receptors (RARs) as well as the retinoid X receptors (RXRs). The control of expression of the various receptor subtypes and their specific isoforms appears to be strictly regulated and can be assumed to play a pivotal role during development and in the adult tissue. It has previously been shown that the RAR beta 2 isoform can regulate its own synthesis through an RA response element (RARE) in its promoter. Recent evidence suggests that the expression of other RAR isoforms, including that of RAR gamma 2, are also regulated by RA. We present evidence that expression of the RAR gamma 2 isoform can be regulated through the RARE in its own promoter region. Similar to the beta 2 RARE, the gamma 2 RARE consists of a 6-bp direct repeat with a 5-nucleotide spacer, but it has different functional features, including receptor specificity, basal-level activity, and affinity for RAR. In agreement with recent observations, this response element is bound most effectively by RAR/RXR heterodimers. Single-base-pair mutations had different effects on the activity of this RARE. The gamma 2 RARE is surrounded by several binding sites for the transcription factor Sp1. Cotransfected Sp1 enhanced strongly the activity of gamma 2 promoter reporter constructs in Drosophila cells. Our data suggest an important role for RAR-containing heterodimers and Sp1 in the regulation of RAR gamma 2 expression.

Evolution of the nuclear receptor gene superfamily

Nuclear receptor genes represent a large family of genes encoding receptors for various hydrophobic ligands such as steroids, vitamin D, retinoic acid and thyroid hormones. This family also contains genes encoding putative receptors for unknown ligands. Nuclear receptor gene products are composed of several domains important for transcriptional activation, DNA binding (C domain), hormone binding and dimerization (E domain). It is not known whether these genes have evolved through gene duplication from a common ancestor or if their different domains came from different independent sources. To test these possibilities we have constructed and compared the phylogenetic trees derived from two different domains of 30 nuclear receptor genes. The tree built from the DNA binding C domain clearly shows a common progeny of all nuclear receptors, which can be grouped into three subfamilies: (i) thyroid hormone and retinoic acid receptors, (ii) orphan receptors and (iii) steroid hormone receptors. The tree constructed from the central part of the E domain which is implicated in transcriptional regulation and dimerization shows the same distribution in three subfamilies but two groups of receptors are in a different position from that in the C domain tree: (i) the Drosophila knirps family genes have acquired very different E domains during evolution, and (ii) the vitamin D and ecdysone receptors, as well as the FTZ-F1 and the NGF1B genes, seem to have DNA binding and hormone binding domains belonging to different classes. These data suggest a complex evolutionary history for nuclear receptor genes in which gene duplication events and swapping between domains of different origins took place.

Retinoid X receptor is an auxiliary protein for thyroid hormone and retinoic acid receptors

Thyroid hormones and retinoic acid function through nuclear receptors that belong to the steroid/thyroid-hormone receptor superfamily. Thyroid hormone receptors (TRs) and retinoic acid receptors (RARs) require auxiliary nuclear proteins for efficient DNA binding. Here we report that retinoid X receptors RXR alpha is one of these nuclear proteins. RXR alpha interacts both with TRs and with RARs, forming heterodimers in solution that strongly interact with a variety of T3/retinoic acid response elements. Transfection experiments show that RXR alpha can greatly enhance the transcriptional activity of TR and RAR at low retinoic acid concentrations that do not significantly activate RXR alpha itself. Thus, RXR alpha enhances the transcriptional activity of other receptors and its own ligand sensitivity by heterodimer formation. Our studies reveal a new subclass of receptors and a regulatory pathway controlling nuclear receptor activities by heterodimer formation.

6'-substituted naphthalene-2-carboxylic acid analogs, a new class of retinoic acid receptor subtype-specific ligands

The biological response to retinoic acid (RA) and synthetic derivatives (retinoids) is mediated by three nuclear retinoic acid receptors, RAR alpha, beta and gamma. To explore the potential of retinoids as receptor subtype selective activators, we employed a transcriptional activation assay. Hybrid receptors that recognize an estrogen response element were used to avoid measuring activities of endogenous retinoic acid receptors. In response to retinoic acid, the three hybrid receptors ER-RAR alpha, ER-RAR beta and ER-RAR gamma exhibited the same induction profile as the corresponding wild type receptors RAR alpha, RAR beta, and RAR gamma. Three different retinoids, analogs of 6'-substituted naphthalene-2-carboxylic acid, elicited strong transcriptional activation of gamma receptor while no activation of alpha receptor was observed. Conversely, two retinobenzoic acid analogs showed a limited alpha selectivity. We conclude that retinoids with unique profiles of retinoic acid receptor subtype selectivity can be defined and tested for their impact on cellular differentiation and for therapeutical applications.

Retinoic acid affects the expression of nuclear retinoic acid receptors in tissues of retinol-deficient rats

The multitude of biological effects of the vitamin A metabolite, retinoic acid, are mediated by nuclear retinoic acid receptors (RARs), which are members of the steroid/thyroid hormone receptor superfamily. RAR-alpha, -beta, and -gamma are encoded by three genes from which multiple isoforms can be generated. Recent studies suggest that the expression of at least some RAR isoforms can be regulated by retinoic acid in certain cell lines. Here we examined regulation of RAR expression in the adult animal. RARs were analyzed by Northern blots from lung, liver, and testes of retinol-deficient rats. Retinol deficiency caused a 65-70% decrease in the mRNA levels of lung and liver RAR-beta, whereas no change was observed in RAR-alpha and -gamma mRNA levels in these organs. In the testes of retinol-deficient animals, two transcripts, RAR-alpha 1 (3.7 kb) and RAR-alpha 2 (2.8 kb), were detected as compared with one RAR-alpha 1 (3.7 kb) transcript in retinol-sufficient testes. When retinol-deficient rats were orally administered 1 dose of retinoic acid (100 micrograms per rat), lung RAR-beta mRNA levels started to increase after 1 hr and reached a 16-fold higher level after 4 hr; after 4 hr these retinoic acid-fed rats also showed a 7-fold increase in liver RAR-beta mRNA levels as compared with levels in the retinol-deficient rats. In contrast, liver, lung, and testes RAR-alpha transcripts remained either unchanged or showed only a slight increase in response to retinoic acid. RAR-gamma was constitutively expressed in lung, and its mRNA levels were induced 2-fold by retinoic acid. These results show tissue diversity in the rapid induction of RAR-beta and RAR-gamma by retinoic acid in the adult animal and suggest distinct roles for the various receptor isoforms in the control of the retinoid response.

Antagonism between retinoic acid receptors

In the developing mouse, retinoic acid receptors (RARs) beta and gamma 1 are expressed in characteristic spatiotemporal patterns which are correlated with different developmental fates of the respective tissues. Understanding the cues that regulate the expression of the various RARs may therefore provide insights into the process of tissue diversification. Transcription of RAR beta is rapidly upregulated through a retinoic acid-responsive element (here referred to as the beta RARE) in its promoter. Like RAR alpha and RAR beta, RAR gamma 1 has been implicated in the activation of the beta RARE. Therefore, it is puzzling that RAR beta and RAR gamma 1 appear to be expressed in reciprocal patterns. In the present report, we show that RAR gamma 1, one of the two predominant RAR gamma isoforms, can inhibit the activity of RAR gamma 2, RAR beta, and endogenous RAR on the beta RARE. In contrast, the three RAR gamma isoforms tested and RAR beta activated a palindromic thyroid hormone response element with similar levels of efficiency. The differential activity of RAR gamma 1 compared with that of RAR beta appears to reside in both the N-terminal and the C-terminal halves of RAR gamma 1. RAR gamma 1-mediated inhibition of other RARs may involve competition for the response element as well as direct interaction with other receptors and might be part of a regulatory system contributing to the characteristic tissue distribution of the various RARs.

Antagonism between retinoic acid receptors

In the developing mouse, retinoic acid receptors (RARs) beta and gamma 1 are expressed in characteristic spatiotemporal patterns which are correlated with different developmental fates of the respective tissues. Understanding the cues that regulate the expression of the various RARs may therefore provide insights into the process of tissue diversification. Transcription of RAR beta is rapidly upregulated through a retinoic acid-responsive element (here referred to as the beta RARE) in its promoter. Like RAR alpha and RAR beta, RAR gamma 1 has been implicated in the activation of the beta RARE. Therefore, it is puzzling that RAR beta and RAR gamma 1 appear to be expressed in reciprocal patterns. In the present report, we show that RAR gamma 1, one of the two predominant RAR gamma isoforms, can inhibit the activity of RAR gamma 2, RAR beta, and endogenous RAR on the beta RARE. In contrast, the three RAR gamma isoforms tested and RAR beta activated a palindromic thyroid hormone response element with similar levels of efficiency. The differential activity of RAR gamma 1 compared with that of RAR beta appears to reside in both the N-terminal and the C-terminal halves of RAR gamma 1. RAR gamma 1-mediated inhibition of other RARs may involve competition for the response element as well as direct interaction with other receptors and might be part of a regulatory system contributing to the characteristic tissue distribution of the various RARs.