Segmentation gene product Fushi tarazu is an LXXLL motif-dependent coactivator for orphan receptor FTZ-F1

Orphan receptors for whom cognate ligands have not yet been identified form a large subclass within the nuclear receptor superfamily. To address one aspect of how they might regulate transcription, we analyzed the mode of interaction between the Drosophila orphan receptor FTZ-F1 (NR5A3) and a segmentation gene product Fushi tarazu (FTZ). Strong interaction between these two factors was detected by use of the mammalian one- and two-hybrid interaction assays without addition of ligand. This interaction required the AF-2 core and putative ligand-binding domain of FTZ-F1 and the LXXLL motif of FTZ. The requirement of these elements was further confirmed by examination of their target gene expression in Drosophila embryos and observation of a cuticle phenotype in transgenic fly lines that express mutated factors. In Drosophila cultured cells, FTZ is required for FTZ-F1 activation of a FTZ-F1 reporter gene. These results reveal a resemblance in the mode of interaction between FTZ-F1 and FTZ and that of nuclear receptor-coactivator and indicate that direct interaction is required for regulation of gene expression by FTZ-F1. Thus, we propose that FTZ may represent a category of LXXLL motif-dependent coactivators for nuclear receptors.

Active repression of RAR signaling is required for head formation

The retinoic acid receptors (RARs) recruit coactivator and corepressor proteins to activate or repress the transcription of target genes depending on the presence of retinoic acid (RA). Despite a detailed molecular understanding of how corepressor complexes function, there is no in vivo evidence to support a necessary function for RAR-mediated repression. Signaling through RARs is required for patterning along the anteroposterior (A-P) axis, particularly in the hindbrain and posterior, although the absence of RA is required for correct anterior patterning. Because RARs and corepressors are present in regions in which RA is absent, we hypothesized that repression mediated through unliganded RARs might be important for anterior patterning. To test this hypothesis, specific reagents were used that either reduce or augment RAR-mediated repression. Derepression of RAR signaling by expressing a dominant-negative corepressor resulted in embryos that exhibited phenotypes similar to those treated by RA. Anterior structures such as forebrain and cement gland were greatly reduced, as was the expression of molecular markers. Enhancement of target gene repression using an RAR inverse agonist resulted in up-regulation of anterior neural markers and expansion of anterior structures. Morpholino antisense oligonucleotide-mediated RARalpha loss-of-function phenocopied the effects of RA treatment and dominant-negative corepressor expression. Microinjection of wild-type or dominant-negative RARalpha rescued the morpholino phenotype, confirming that RAR is functioning anteriorly as a transcriptional repressor. Lastly, increasing RAR-mediated repression potentiated head-inducing activity of the growth factor inhibitor cerberus, whereas releasing RAR-mediated repression blocked cerberus from inducing ectopic heads. We conclude that RAR-mediated repression of target genes is critical for head formation. This requirement establishes an important biological role for active repression of target genes by nuclear hormone receptors and illustrates a novel function for RARs during vertebrate development.

Aldehyde dehydrogenase 6, a cytosolic retinaldehyde dehydrogenase prominently expressed in sensory neuroepithelia during development

We have isolated the chick and mouse homologs of human aldehyde dehydrogenase 6 (ALDH6) that encode a third cytosolic retinaldehyde-specific aldehyde dehydrogenase. In both chick and mouse embryos, strong expression is observed in the sensory neuroepithelia of the head. In situ hybridization analysis in chick shows compartmentalized expression primarily in the ventral retina, olfactory epithelium, and otic vesicle; additional sites of expression include the isthmus, Rathke's pouch, posterior spinal cord interneurons, and developing limbs. Recombinant chick ALDH6 has a K(0.5) = 0.26 microm, V(max) = 48.4 nmol/min/mg and exhibits strong positive cooperativity (H = 1.9) toward all-trans-retinaldehyde; mouse ALDH6 has similar kinetic parameters. Expression constructs can confer 1000-fold increased sensitivity to retinoic acid receptor-dependent signaling from retinol in transient transfections experiments. The localization of ALDH6 to the developing sensory neuroepithelia of the eye, nose, and ear and discreet sites within the CNS suggests a role for RA signaling during primary neurogenesis at these sites.

Cell-type-specific regulation of the retinoic acid receptor mediated by the orphan nuclear receptor TLX

Malformations in the eye can be caused by either an excess or deficiency of retinoids. An early target gene of the retinoid metabolite, retinoic acid (RA), is that encoding one of its own receptors, the retinoic acid receptor beta (RARbeta). To better understand the mechanisms underlying this autologous regulation, we characterized the chick RARbeta2 promoter. The region surrounding the transcription start site of the avian RARbeta2 promoter is over 90% conserved with the corresponding region in mammals and confers strong RA-dependent transactivation in primary cultured embryonic retina cells. This response is selective for RAR but not retinoid X receptor-specific agonists, demonstrating a principal role for RAR(s) in retina cells. Retina cells exhibit a far higher sensitivity to RA than do fibroblasts or osteoblasts, a property we found likely due to expression of the orphan nuclear receptor TLX. Ectopic expression of TLX in fibroblasts resulted in increased sensitivity to RA induction, an effect that is conserved between chick and mammals. We have identified a cis element, the silencing element relieved by TLX (SET), within the RARbeta2 promoter region which confers TLX- and RA-dependent transactivation. These results indicate an important role for TLX in autologous regulation of the RARbeta gene in the eye.

Feedback control of cyclooxygenase-2 expression through PPARgamma

Cyclooxygenase-2 (COX-2), a rate-limiting enzyme for prostaglandins (PG), plays a key role in inflammation, tumorigenesis, development, and circulatory homeostasis. The PGD(2) metabolite 15-deoxy-Delta(12, 14) PGJ(2) (15d-PGJ(2)) was identified as a potent natural ligand for the peroxisome proliferator-activated receptor-gamma (PPARgamma). PPARgamma expressed in macrophages has been postulated as a negative regulator of inflammation and a positive regulator of differentiation into foam cell associated with atherogenesis. Here, we show that 15d-PGJ(2) suppresses the lipopolysaccharide (LPS)-induced expression of COX-2 in the macrophage-like differentiated U937 cells but not in vascular endothelial cells. PPARgamma mRNA abundantly expressed in the U937 cells, not in the endothelial cells, is down-regulated by LPS. In contrast, LPS up-regulates mRNA for the glucocorticoid receptor which ligand anti-inflammatory steroid dexamethasone (DEX) strongly suppresses the LPS-induced expression of COX-2, although both 15d-PGJ(2) and DEX suppressed COX-2 promoter activity by interfering with the NF-kappaB signaling pathway. Transfection of a PPARgamma expression vector into the endothelial cells acquires this suppressive regulation of COX-2 gene by 15d-PGJ(2) but not by DEX. A selective COX-2 inhibitor, NS-398, inhibits production of PGD(2) in the U937 cells. Taking these findings together, we propose that expression of COX-2 is regulated by a negative feedback loop mediated through PPARgamma, which makes possible a dynamic production of PG, especially in macrophages, and may be attributed to various expression patterns and physiological functions of COX-2.

Alteration of a single amino acid in peroxisome proliferator-activated receptor-alpha (PPAR alpha) generates a PPAR delta phenotype

Three pharmacologically important nuclear receptors, the peroxisome proliferator-activated receptors (PPARs alpha, gamma, and delta), mediate key transcriptional responses involved in lipid homeostasis. The PPAR alpha and gamma subtypes are well conserved from Xenopus to man, but the beta/delta subtypes display substantial species variations in both structure and ligand activation profiles. Characterization of the avian cognates revealed a close relationship between chick (c) alpha and gamma subtypes to their mammalian counterparts, whereas the third chicken subtype was intermediate to Xenopus (x) beta and mammalian delta, establishing that beta and delta are orthologs. Like xPPAR beta, cPPAR beta responded efficiently to hypolipidemic compounds that fail to activate the human counterpart. This provided the opportunity to address the pharmacological problem as to how drug selectivity is achieved and the more global evolutionary question as to the minimal changes needed to generate a new class of receptor. X-ray crystallography and chimeric analyses combined with site-directed mutagenesis of avian and mammalian cognates revealed that a Met to Val change at residue 417 was sufficient to switch the human and chick phenotype. These results establish that the genetic drive to evolve a novel and functionally selectable receptor can be modulated by a single amino acid change and suggest how nuclear receptors can accommodate natural variation in species physiology.

A new thiazolidinedione, NC-2100, which is a weak PPAR-gamma activator, exhibits potent antidiabetic effects and induces uncoupling protein 1 in white adipose tissue of KKAy obese mice

Thiazolidinediones (TZDs) reduce insulin resistance in type 2 diabetes by increasing peripheral uptake of glucose, and they bind to and activate the transcriptional factor peroxisome proliferator-activated receptor-gamma (PPAR-gamma). Studies have suggested that TZD-induced activation of PPAR-gamma correlates with antidiabetic action, but the mechanism by which the activated PPAR-gamma is involved in reducing insulin resistance is not known. To examine whether activation of PPAR-gamma directly correlates with antidiabetic activities, we compared the effects of 4 TZDs (troglitazone, pioglitazone, BRL-49653, and a new derivative, NC-2100) on the activation of PPAR-gamma in a reporter assay, transcription of the target genes, adipogenesis, plasma glucose and triglyceride levels, and body weight using obese KKAy mice. There were 10- to 30-fold higher concentrations of NC-2100 required for maximal activation of PPAR-gamma in a reporter assay system, and only high concentrations of NC-2100 weakly induced transcription of the PPAR-gamma but not PPAR-alpha target genes in a whole mouse and adipogenesis of cultured 3T3L1 cells, which indicates that NC-2100 is a weak PPAR-gamma activator. However, low concentrations of NC-2100 efficiently lowered plasma glucose levels in KKAy obese mice. These results strongly suggest that TZD-induced activation of PPAR-gamma does not directly correlate with antidiabetic (glucose-lowering) action. Furthermore, NC-2100 caused the smallest body weight increase of the 4 TZDs, which may be partly explained by the finding that NC-2100 efficiently induces uncoupling protein (UCP)-2 mRNA and significantly induces UCP1 mRNA in white adipose tissue (WAT). NC-2100 induced UCP1 efficiently in mesenteric WAT and less efficiently in subcutaneous WAT, although pioglitazone and troglitazone also slightly induced UCP1 only in mesenteric WAT. These characteristics of NC-2100 should be beneficial for humans with limited amounts of brown adipose tissue.

The orphan nuclear receptor Tlx regulates Pax2 and is essential for vision

Although the development of the vertebrate eye is well described, the number of transcription factors known to be key to this process is still limited. The localized expression of the orphan nuclear receptor Tlx in the optic cup and discrete parts of the central nervous system suggested the possible role of Tlx in the formation or function of these structures. Analyses of Tlx targeted mice revealed that, in addition to the central nervous system cortical defects, lack of Tlx function results in progressive retinal and optic nerve degeneration with associated blindness. An extensive screen of Tlx-positive and Tlx-negative P19 neural precursors identified Pax2 as a candidate target gene. This identification is significant, because Pax2 is known to be involved in retinal development in both the human and the mouse eye. We find that Pax2 is a direct target and that the Tlx binding site in its promoter is conserved between mouse and human. These studies show that Tlx is a key component of retinal development and vision and an upstream regulator of the Pax2 signaling cascade.

Identification of a response element for vitamin D3 and retinoic acid in the promoter region of the human fructose-1,6-bisphosphatase gene

Fructose-1,6-bisphosphatase (FBPase) is a key gluconeogenic enzyme. The data herein show that both the enzyme activity and mRNA level of the human FBPase gene are enhanced by 9-cis retinoic acid (9cRA) and all-trans retinoic acid (atRA) as well as by 1,25-dihydroxyvitamin D3 (VD3) in human promyelocytic HL60 cells and normal monocytes in peripheral blood, which were used as an alternative source to liver for the DNA diagnosis of FBPase deficiency. To understand the molecular mechanism of this enhancing action, the 2.4 kb 5'-regulatory region of the human FBPase gene was isolated and sequenced. Using luciferase reporter gene assays, a 0.5 kb FBPase basal promoter fragment was found to confer induction by VD3, 9cRA, and atRA that was mediated by the vitamin D3 receptor (VDR), retinoid X receptor (RXR), and retinoic acid receptor (RAR). Within this region, a direct repeat sequence, 5'-TAACCTttcTGAACT-3' (-340 to -326), which functions as a common response element for VD3, 9cRA, and atRA, was identified. The results of electrophoretic mobility shift assays indicated that VDR-RXR and RAR-RXR heterodimers bind this response element. Collectively, these observations indicate that VD3 and RA are important modulators of the expression of the human FBPase gene in monocytic cells.

Three-dimensional modeling of and ligand docking to vitamin D receptor ligand binding domain

The ligand binding domain of the human vitamin D receptor (VDR) was modeled based on the crystal structure of the retinoic acid receptor. The ligand binding pocket of our VDR model is spacious at the helix 11 site and confined at the beta-turn site. The ligand 1alpha, 25-dihydroxyvitamin D(3) was assumed to be anchored in the ligand binding pocket with its side chain heading to helix 11 (site 2) and the A-ring toward the beta-turn (site 1). Three residues forming hydrogen bonds with the functionally important 1alpha- and 25-hydroxyl groups of 1alpha,25-dihydroxyvitamin D(3) were identified and confirmed by mutational analysis: the 1alpha-hydroxyl group is forming pincer-type hydrogen bonds with S237 and R274 and the 25-hydroxyl group is interacting with H397. Docking potential for various ligands to the VDR model was examined, and the results are in good agreement with our previous three-dimensional structure-function theory.

Efficient targeting of gene expression in chick embryos by microelectroporation

During vertebrate embryonic development, a key to unraveling specific functions of gene products is the capability to manipulate expression of the gene of interest at the desired time and place. For this, we developed a 'microelectroporation' technique by which DNA can be locally introduced into a targeted site of avian embryos, restricting spatial expression of the protein products during development. This technique involved injection of DNA solution in ovo around the target tissue and pinpoint application of an electric field by tungsten electrodes, allowing efficient and reproducible targeted gene transfer, for example, into an optic vesicle, somites, cranial mesoderm and limb mesenchyme. Because of the locality of gene introduction and its expression, survival rates of the embryos were high: approximately 90% of the embryos injected in optic vesicles were alive for at least 1 day after microelectroporation. The instantaneous gene transfer into embryonic cells allowed rapid expression of protein products such as green fluorescence protein within 2.5 h with fluorescence maintained for 3 days of incubation. This improved technique provides a convenient and efficient way to express transgenes in a spatially and temporally restricted manner in chicken embryos.

Identification of a photoreceptor cell-specific nuclear receptor

Nuclear receptors comprise a large and expanding family of transcription factors involved in diverse aspects of animal physiology and development, the functions of which can be modulated in a spatial and temporal manner by access to small lipophilic ligands and/or the specificity of their own localized expression. Here we report the identification of a human nuclear receptor that reveals a unique proximal box (CNGCSG) in the DNA-binding domain. The conservation of this feature in its nematode counterpart suggests the requirement for this type of P box in the genetic cascades mediated by nuclear receptors in a wide variety of animal species. The expression of this receptor, PNR (photoreceptor-specific nuclear receptor), appears strongly restricted in the retina, exclusively in photoreceptor cells. In human cell lines, PNR expression was observed in Y79 retinoblastoma along with other photoreceptor marker genes such as CRX. Among vertebrate receptors, PNR shares structural kinship with an orphan receptor TLX, and despite distinct differences in the DNA binding domain, PNR is able to recognize a subset of TLX target sequences in vitro. Analyses of the human PNR gene revealed its chromosomal position as 15q24, a site that has recently been reported as a susceptible region for retinal degeneration. These data support a role for PNR in the regulation of signalling pathways intrinsic to the photoreceptor cell function.

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.

Three-dimensional structure-function relationship of vitamin D: side chain location and various activities

The various biological activities of side-chain mobility restricted analogs, four diastereomers at C(20) and C(22) of 22-methyl-1alpha,25-dihydroxyvitamin D3, were evaluated. The relationship between structure and the various activities of the analogs was discussed in terms of the active space region concept that we previously suggested.

Glucocorticoid-mediated suppression of the promoter activity of the cyclooxygenase-2 gene is modulated by expression of its receptor in vascular endothelial cells

Cyclooxygenase-2 (COX-2), an inducible isozyme of cyclooxygenase, is expressed selectively in response to various inflammatory stimuli such as lipopolysaccharide (LPS) and its expression is suppressed by the glucocorticoid dexamethasone (DEX) in numerous types of cells. However, LPS-enhanced production of prostacyclin in bovine arterial endothelial cells (BAEC) was not significantly decreased by treatment with DEX but was suppressed by selective COX-2 inhibitors. This is consistent with the finding that DEX was not effective at preventing the expression of LPS-induced COX-2 mRNA. Transient transfection analysis showed that DEX did not suppress the LPS-induced promoter activity of the 5'-flanking region of the human COX-2 gene (nucleotides -327 to +59). Since RNA blot analysis indicated low-level expression of glucocorticoid receptor (GR) mRNA in BAEC, a GR-expression vector was transfected to evaluate the role of the GR in the COX-2 promoter activity. It was found that DEX mediated the suppression of the LPS-induced COX-2 promoter activity in a dose-dependent manner. These results suggest that the DEX-mediated suppression of LPS-induced promoter activity of the COX-2 gene is modulated by expression of the GR, which will be possible to account for a unique expression pattern of the COX-2 gene in BAEC.

[Endocrine signaling system mediated by nuclear receptors]

Nuclear receptors act as ligand-dependent DNA-binding transcription factors that transduce specific hormonal signals to modulate pattern of gene expression. Their unique modular structure defines the superfamily of intracellular signal mediators which include receptors for steroids, thyroid hormones, and several lipophilic vitamins as well as a large number of orphan receptors. Almost 50 distinct genes have been identified in the human genome, and their products can be classified into 5 different subfamilies based upon structural similarity, the mode of DNA binding, and evolutionary conservation. Recent advances in understanding the nuclear receptor system have uncovered complex yet well-organized regulatory networks of these signal transducers at the level of receptor dimerization, target gene recognition, and cofactor interaction.

Induction of differentiation in acute promyelocytic leukemia cells by 9-cis retinoic acid alpha-tocopherol ester (9-cis tretinoin tocoferil)

Acute promyelocytic leukemia (APL) has a specific genetic rearrangement between the retinoic acid receptor (RAR)-alpha gene and the pml nuclear protein gene. All-trans retinoic acid (ATRA) induces granulocytic differentiation of APL-derived cells and is used to treat APL patients. However, ATRA interacts with normal cells with RAR throughout the entire body, and when used at high doses or over a long duration, it induces several adverse effects. The development of drugs that selectively act on APL cells may contribute to increasing the therapeutic efficacy of APL treatment as well as elucidating the mechanisms of response to ATRA. In this study, 9-cis retinoic acid alpha-tocopherol ester (9CTT) inhibited the proliferation of APL-derived NB4 and HT93 cells and induced differentiation markers, such as granulocytic maturation, nitroblue tetrazolium reduction, and CD11b expression, in these cells. The effects of 9CTT on non-APL cells, including HL-60 and U937 cells, were much weaker than those on APL cells, and tretinoin tocoferil (TT), which is an alpha-tocopherol ester of ATRA, did not induce the differentiation of APL cells as effectively as 9CTT. The differentiation-inducing effects of 9CTT were inhibited by RAR antagonists. 9CTT and TT similarly induced the transactivating activity of RARs, but were not effective on RXRs. 9CTT downregulated the expression of PML/RAR-alpha protein more effectively than TT, which suggests that it may be involved in the selectivity of 9CTT against APL cells. Interestingly, 9CTT enhanced the differentiation of APL cells induced by ATRA, 9-cis retinoic acid, and synthetic retinobenzoic acids. Combined with 1alpha,25-dihydroxyvitamin D3 (VD3), 9CTT also more than additively induced the differentiation of APL cells. Thus, 9CTT, alone or in combination with other retinoids or VD3, may be useful for the treatment of APL.

BXR, an embryonic orphan nuclear receptor activated by a novel class of endogenous benzoate metabolites

Nuclear receptors are ligand-modulated transcription factors that respond to steroids, retinoids, and thyroid hormones to control development and body physiology. Orphan nuclear receptors, which lack identified ligands, provide a unique, and largely untapped, resource to discover new principles of physiologic homeostasis. We describe the isolation and characterization of the vertebrate orphan receptor, BXR, which heterodimerizes with RXR and binds high-affinity DNA sites composed of a variant thyroid hormone response element. A bioactivity-guided screen of embryonic extracts revealed that BXR is activatable by low-molecular-weight molecules with spectral patterns distinct from known nuclear receptor ligands. Mass spectrometry and 1H NMR analysis identified alkyl esters of amino and hydroxy benzoic acids as potent, stereoselective activators. In vitro cofactor association studies, along with competable binding of radiolabeled compounds, establish these molecules as bona fide ligands. Benzoates comprise a new molecular class of nuclear receptor ligand and their activity suggests that BXR may control a previously unsuspected vertebrate signaling pathway.

Troglitazone increases the number of small adipocytes without the change of white adipose tissue mass in obese Zucker rats

Troglitazone (CS-045) is one of the thiazolidinediones that activate the peroxisome proliferator-activated receptor gamma (PPARgamma), which is expressed primarily in adipose tissues. To elucidate the mechanism by which troglitazone relieves insulin resistance in vivo, we studied its effects on the white adipose tissues of an obese animal model (obese Zucker rat). Administration of troglitazone for 15 d normalized mild hyperglycemia and marked hyperinsulinemia in these rats. Plasma triglyceride level was decreased by troglitazone in both obese and lean rats. Troglitazone did not change the total weight of white adipose tissues but increased the number of small adipocytes (< 2,500 micron2) approximately fourfold in both retroperitoneal and subcutaneous adipose tissues of obese rats. It also decreased the number of large adipocytes (> 5,000 micron2) by approximately 50%. In fact, the percentage of apoptotic nuclei was approximately 2.5-fold higher in the troglitazone-treated retroperitoneal white adipose tissue than control. Concomitantly, troglitazone normalized the expression levels of TNF-alpha which were elevated by 2- and 1.4-fold in the retroperitoneal and mesenteric white adipose tissues of the obese rats, respectively. Troglitazone also caused a dramatic decrease in the expression levels of leptin, which were increased by 4-10-fold in the white adipose tissues of obese rats. These results suggest that the primary action of troglitazone may be to increase the number of small adipocytes in white adipose tissues, presumably via PPARgamma. The increased number of small adipocytes and the decreased number of large adipocytes in white adipose tissues of troglitazone-treated obese rats appear to be an important mechanism by which increased expression levels of TNF-alpha and higher levels of plasma lipids are normalized, leading to alleviation of insulin resistance.

A bacterial cloning vector using a mutated Aequorea green fluorescent protein as an indicator

The bacterial cloning vector, pGreenscript A, derived from the mutated Aequorea green fluorescent protein (GFP-S65A) gene, when expressed in E. coli produced colonies that showed yellow color under daylight and strong green fluorescence under long-wave ultraviolet light. The vector was used to select for inserted foreign genes based on the loss of the yellow color/green fluorescence of E. coli cells caused by the insertional inactivation of GFP production.

Transcriptional regulation by competition between ELP isoforms and nuclear receptors

ELP is a transcription factor belonging to the nuclear receptor superfamily. The consensus binding sequence for ELP contains a half site of the nuclear receptor recognition element. We demonstrated previously that ELP1, the repressor type isoform of ELP, competes for binding with the retinoic acid receptor and represses retinoic acid-induced transactivation. In this study, competitive repression by ELP1 was investigated for several other nuclear receptors. As in the case of the retinoic acid receptor, binding of vitamin D receptor, thyroid hormone receptor, and estrogen receptor could be competed by ELP1, resulting in repression of their ligand-dependent transactivation. Interestingly, the activator-type ELP isoforms were capable of repressing retinoic acid-induced transactivation through binding to the retinoic acid receptor binding element. These data suggest that competition for target DNA binding is a general mechanism of transcriptional repression by ELP isoforms.

Thioredoxin: a redox-regulating cellular cofactor for glucocorticoid hormone action. Cross talk between endocrine control of stress response and cellular antioxidant defense system

Adaptation to stress evokes a variety of biological responses, including activation of the hypothalamic-pituitary-adrenal (HPA) axis and synthesis of a panel of stress-response proteins at cellular levels: for example, expression of thioredoxin (TRX) is significantly induced under oxidative conditions. Glucocorticoids, as a peripheral effector of the HPA axis, exert their actions via interaction with a ligand-inducible transcription factor glucocorticoid receptor (GR). However, how these stress responses coordinately regulate cellular metabolism is still unknown. In this study, we demonstrated that either antisense TRX expression or cellular treatment with H2O2 negatively modulates GR function and decreases glucocorticoid-inducible gene expression. Impaired cellular response to glucocorticoids is rescued by overexpression of TRX, most possibly through the functional replenishment of the GR. Moreover, not only the ligand binding domain but the DNA binding domain of the GR is also suggested to be a direct target of TRX. Together, we here present evidence showing that cellular glucocorticoid responsiveness is coordinately modulated by redox state and TRX level and propose that cross talk between neuroendocrine control of stress responses and cellular antioxidant systems may be essential for mammalian adaptation processes.

Two distinct dimerization interfaces differentially modulate target gene specificity of nuclear hormone receptors

Several nuclear receptors including the all-trans retinoic acid receptor RAR, form heterodimers with the 9-cis retinoic acid receptor, RXR. RXR-RAR heterodimers show an impressive flexibility in DNA binding and can recognize palindromic, inverted palindromes and direct repeats of the core half-site sequence AGGTCA. Dimerization interfaces in the DNA-binding domains of RXR, RAR, and thyroid hormone receptor (TR) that promote selective binding to strictly spaced direct repeats have previously been identified. However, an additional dimerization domain is present within the ligand-binding domains (LBDs) of these receptors. Here we localize a transferable 40-amino acid region within the LBDs of RXR, RAR, TR, and chicken ovalbumin upstream promoter transcription factor that is critical for determining identity in the heterodimeric interaction and for high-affinity DNA binding. This region overlaps almost perfectly with a helical segment in the RXR LBD crystal structure that was recently demonstrated to be part of the dimer interface. Our data suggest a sequential pathway for nuclear receptor dimerization whereby the LBD dimerization interface initiates the formation of solution heterodimers that, in turn, acquire the capacity to bind to a number of differently organized repeats. Formation of a second dimer interface within the DNA-binding domain (DBD) restricts receptors to direct repeat targets. Accordingly, the combination of an obligatory (LBD) and an optional (DBD) dimerization domain imparts a dynamic DNA-binding potential to the heterodimerizing receptors that both increases the diversity of the hormonal response as well as providing a restricted set of target sequences in direct repeat elements that ensures physiological specificity.

SMRT isoforms mediate repression and anti-repression of nuclear receptor heterodimers

Transcriptional repression represents an important component in the regulation of cell differentiation and oncogenesis mediated by nuclear hormone receptors. Hormones act to relieve repression, thus allowing receptors to function as transcriptional activators. The transcriptional corepressor SMRT was identified as a silencing mediator for retinoid and thyroid hormone receptors. SMRT is highly related to another corepressor, N-CoR, suggesting the existence of a new family of receptor-interacting proteins. We demonstrate that SMRT is a ubiquitous nuclear protein that interacts with unliganded receptor heterodimers in mammalian cells. Furthermore, expression of the receptor-interacting domain of SMRT acts as an antirepressor, suggesting the potential importance of splicing variants as modulators of thyroid hormone and retinoic acid signaling.

Localization, trafficking, and temperature-dependence of the Aequorea green fluorescent protein in cultured vertebrate cells

The localization, trafficking, and fluorescence of Aequorea green fluorescent protein (GFP) in cultured vertebrate cells transiently transfected with GFP cDNA were studied. Fluorescence of GFP in UV light was found to be strongest when cells were incubated at 30 degrees C but was barely visible at an incubation temperature of 37 degrees C. COS-1 cells, primary chicken embryonic retina cells, and carp epithelial cells were fluorescently labeled under these conditions. GFP was distributed uniformly throughout the cytoplasm and nucleus independent of cell type examined. When GFP was fused to PML protooncogene product, fluorescence was detected in a unique nuclear organelle pattern indistinguishable from that of PML protein, showing the potential use of GFP as a fluorescent tag. To analyze both function and intracellular trafficking of proteins fused to GFP, a GFP-human glucocorticoid receptor fusion construct was prepared. The GFP-human glucocorticoid receptor efficiently transactivated the mouse mammary tumor virus promoter in response to dexamethasone at 30 degrees C but not at 37 degrees C, indicating that temperature is important, even for function of the GFP fusion protein. The dexamethasone-induced translocation of GFP-human glucocorticoid receptor from cytoplasm to nucleus was complete within 15 min; the translocation could be monitored in a single living cell in real time.

Activin and its receptors during gastrulation and the later phases of mesoderm development in the chick embryo

We have cloned chick homologues of the type-II activin receptor, which we have designated cActR-IIA and -IIB. Binding assays show that the two receptors are indistinguishable in their ability to bind activin-A, with comparable kds. Injection of mRNAs encoding these receptors into Xenopus embryos causes axial duplications. Expression of both receptors can first be detected in the primitive streak by in situ hybridization. This suggests that these genes may be activated in response to mesoderm induction. In agreement with this, we find that treatment of preprimitive streak chick embryos with activin-A leads to rapid induction of the expression of cActR-IIB. At later stages, cActR-IIA transcripts become localized mainly in the notochord and myotome and cActR-IIB in the dorsal neural tube, proximal-anterior part of the limb bud, sensory placodes, and specific regions of the fore- and midbrain. To test the response of early chick embryonic tissues to activin, we designed a new in vitro assay for differentiation. We find that explants of area opaca epiblast or posterior primitive streak from various stages can respond to activin treatment by differentiating into a variety of mesodermal cell types in a dose-dependent manner. These results suggest that the importance of activin-related signaling pathways is not confined to pregastrulation stages and that these receptors may be involved in mediating the effects of inducing signals during later stages of development of the mesoderm, limbs, and nervous system.

Unique response pathways are established by allosteric interactions among nuclear hormone receptors

Heterodimerization is a common paradigm among eukaryotic transcription factors. The 9-cis retinoic acid receptor (RXR) serves as a common heterodimerization partner for several nuclear receptors, including the thyroid hormone receptor (T3R) and retinoic acid receptor (RAR). This raises the question as to whether these complexes possess dual hormonal responsiveness. We devised a strategy to examine the transcriptional properties of each receptor individually or when tethered to a heterodimeric partner. We find that the intrinsic binding properties of RXR are masked in T3R-RXR and RAR-RXR heterodimers. In contrast, RXR is active as a non-DNA-binding cofactor with the NGFI-B/Nurr1 orphan receptors. Heterodimerization of RXR with constitutively active NGFI-B/Nurr1 creates a novel hormone-dependent complex. These findings suggest that allosteric interactions among heterodimers create complexes with unique properties. We suggest that allostery is a critical feature underlying the generation of diversity in hormone response networks.

LXR, a nuclear receptor that defines a distinct retinoid response pathway

We have identified a new retinoid response pathway through which 9-cis retinoic acid (9cRA) activates transcription in the presence of LXR alpha, a member of the nuclear receptor superfamily. LXR alpha shows a specific pattern of expression in visceral organs, thereby restricting the response to certain tissues. Retinoid trans-activation occurs selectively on a distinct response element termed an LXRE. Significantly, neither RXR homodimers nor RXR/RAR heterodimers are able to substitute for LXR alpha in mediating this retinoid response. We provide evidence that the retinoid response on the LXRE is the result of a unique interaction between LXR alpha and endogenous RXR, which, unlike in the RXR/RAR heterodimer, makes RXR competent to respond to retinoids. Thus, the interaction with LXR alpha shifts RXR from its role described previously as a silent, DNA-binding partner to an active ligand-binding subunit in mediating retinoid responses through target genes defined by LXREs.

Repression of retinoic acid-induced transactivation by embryonal LTR binding protein

The mechanism of repression of transcription by ELP, the embryonal long terminal repeat binding protein, was investigated. ELP represses the Moloney murine leukemia virus long terminal repeat by binding to a site which overlaps with a sequence element for retinoic acid receptor binding. This suggests possible competition of ELP with retinoic acid receptor for the same sequence elements. Oligonucleotides corresponding to ELP and/or retinoic acid receptor binding elements were placed upstream of the SV40 promoter and their effect on gene expression was analyzed by CAT assay. Elements which have affinity to both ELP and retinoic acid receptor were activated by retinoic acid receptor and these activations were repressed by ELP. An ELP binding element without affinity to retinoic acid receptor was insensitive to both activation by retinoic acid receptor and repression by ELP. Furthermore, cellular ELP binding elements and the Moloney leukemia virus long terminal repeat were activated by retinoic acid. These data suggest that one of the mechanism of transcriptional repression by ELP is competition for binding sites with transactivators such as retinoic acid receptors.

A novel pathway for retinoic acid-induced differentiation of F9 cells that is distinct from receptor-mediated trans-activation

Retinoic acid (RA) has striking effects on vertebrate development and induces differentiation of several lines of cells including embryonal carcinoma F9 cells. It is generally accepted that the actions of RA are mediated by nuclear receptors for RA. However, we now provide evidence that F9 cells can differentiate in response to RA without trans-activation by nuclear receptors. Irreversible differentiation of F9 cells was induced by 18 h of exposure to RA with subsequent incubation in the absence of RA. This induction of differentiation was not blocked after inhibition of protein synthesis and mRNA synthesis during the 18-h treatment with RA, but the endogenous RA receptors failed to activate transcription from their target genes that contain the receptor-binding sequences. During the commitment to RA-induced differentiation, at least five sets of four phosphorylated proteins underwent changes in the absence of protein synthesis de novo. These results suggest that there is a novel pathway for the action of RA that is independent of nuclear receptor-mediated trans-activation.

Relationship between Drosophila gap gene tailless and a vertebrate nuclear receptor Tlx

We report here the identification of a unique vertebrate nuclear receptor, Tlx, which is expressed exclusively in the neuroepithelium of the embryonic brain. Sequence comparison reveals striking similarity to the product of the Drosophila terminal/gap gene tailless (tll), which is expressed in the embryonic brain and is required for brain development in flies. In vitro DNA-binding assays demonstrated that Tlx and Tll proteins share a target gene specificity that is unique among the nuclear receptor superfamily. Ectopic expression of Tlx in fly embryos caused a repression of segmentation comparable to that elicited by Tll. The similarities in structure, expression pattern, target gene specificity and phenotypes in transgenic flies suggest conservation of genetic programs upstream and downstream of this Tlx/Tll class of nuclear receptors during embryogenesis.

Differential expression and activation of a family of murine peroxisome proliferator-activated receptors

To gain insight into the function of peroxisome proliferator-activated receptor (PPAR) isoforms in mammals, we have cloned and characterized two PPAR alpha-related cDNAs (designated PPAR gamma and -delta, respectively) from mouse. The three PPAR isoforms display widely divergent patterns of expression during embryogenesis and in the adult. Surprisingly, PPAR gamma and -delta are not activated by pirinixic acid (Wy 14,643), a potent peroxisome proliferator and activator of PPAR alpha. However, PPAR gamma and -delta are activated by the structurally distinct peroxisome proliferator LY-171883 and linoleic acid, respectively, indicating that each of the isoforms can act as a regulated activator of transcription. These data suggest that tissue-specific responsiveness to peroxisome proliferators, including certain fatty acids, is in part a consequence of differential expression of multiple, pharmacologically distinct PPAR isoforms.

Determinants for selective RAR and TR recognition of direct repeat HREs

Recently, we have shown that receptors for vitamin D3 (VDR), thyroid hormone (TR), and retinoic acid (RAR) activate preferentially through direct repeats (DRs) spaced by 3, 4, and 5 nucleotides, respectively. In addition, the RAR can activate weakly through DRs spaced by 2 nucleotides. A common feature of RAR, TR, and VDR is their ability to heterodimerize with the retinoid X receptor (RXR) through their ligand-binding domains (LBDs) to form high-affinity DNA-binding complexes that are specific for appropriately spaced repeats. In this paper we demonstrate that selective binding of RAR-RXR and TR-RXR heterodimers to their cognate DRs is a consequence of a novel cooperative dimer interaction within the DNA-binding domains (DBDs). Accordingly, a region in the first zinc finger of the TR and RAR DBDs interacts with the second zinc finger in the RXR DBD to promote selective DNA-binding to DRs spaced by 4 and 5 nucleotides, respectively. The resulting polarity established by this interaction places RXR in the 5' position of the direct repeats. These data provide a mechanism for selective receptor recognition of a restricted set of target sequences in DR DNA and explains the structural basis for physiological specificity.

Functional inhibition of retinoic acid response by dominant negative retinoic acid receptor mutants

The diverse effects of retinoids on the development, growth, and homeostasis of vertebrate organisms are mediated in part by three distinct isoforms of retinoic acid receptors (RARs). These proteins, which are structurally and functionally closely related to thyroid hormone receptors and the oncogene product v-ErbA, regulate patterns of gene expression in target tissues. One approach to study the distinct effects of retinoic acid in cells is to subvert this activity of endogenous receptors by expression of dominant negative receptor derivatives. We demonstrate here that RAR alpha, RAR beta, and RAR gamma can be converted into potent negative transcriptional regulators that block wild-type RAR function. Furthermore, these mutant RARs, but not the wild-type receptors, actively repress the basal transcription level of target promoters. When expressed in transgenic mice, the most potent of these inhibitory receptor mutants is apparently able to disturb developmental processes by inducing a cleft palate in transgenic offspring.

Modulation of glucocorticoid receptor function by protein kinase A

Protein kinase A (PKA) has been shown to modulate the pattern of gene expression via transcription factors such as cAMP response element binding protein. However, in F9 embryonal carcinoma cells which lack endogenous functional cAMP response element binding protein, we have found that PKA is still able to control gene transcription through the glucocorticoid receptor (GR) by up-regulating its hormone-dependent trans-activation. Dose-response analysis indicates that PKA does not alter the ligand binding affinity of GR. PKA seems to act through the DNA binding domain of GR, since GR mutants which lack either the amino-terminal or the ligand binding domain are still able to be up-regulated by PKA. In support of this proposal, we demonstrate that PKA can enhance the DNA binding activity of GR. Our results suggest a novel mechanism by which PKA modulates the steroid sensitivity of a target cell by enhancing the DNA binding activity of GR for its cognate hormone response elements.

Convergence of 9-cis retinoic acid and peroxisome proliferator signalling pathways through heterodimer formation of their receptors

Peroxisomes are cytoplasmic organelles which are important in mammals in modulation of lipid homeostasis, including the metabolism of long-chain fatty acids and conversion of cholesterol to bile salts (reviewed in refs 1 and 2). Amphipathic carboxylates such as clofibric acid have been used in man as hypolipidaemic agents and in rodents they stimulate the proliferation of peroxisomes. These agents, termed peroxisome proliferators, and all-trans retinoic acid activate genes involved in peroxisomal-mediated beta-oxidation of fatty acids. Here we show that the receptor activated by peroxisome proliferators and the retinoid X receptor-alpha (ref. 6) form a heterodimer that activates acyl-CoA oxidase gene expression in response to either clofibric acid or the retinoid X receptor-alpha ligand, 9-cis retinoic acid, an all-trans retinoic acid metabolite; simultaneous exposure to both activators results in a synergistic induction of gene expression. These data demonstrate the coupling of the peroxisome proliferator and retinoid signalling pathways and provide evidence for a physiological role for 9-cis retinoic acid in modulating lipid metabolism.

Retinoic acid receptors initiate induction of the cytomegalovirus enhancer in embryonal cells

Reactivation of latent virus is believed to result from a signal transduction event that induces immediate-early (IE) gene transcription. Evidence is presented that the major IE promoter (MIEP) of human cytomegalovirus (hCMV) is activated by physiological levels of retinoic acid (RA) in human embryonal carcinoma cells. Mutagenesis experiments localized in the MIEP enhancer, a retinoic acid-responsive element composed of a direct repeat separated by five nucleotides. Protein-DNA binding experiments revealed that this element functions as a specific target site for the direct interaction of nuclear receptor proteins for RA. These findings implicate the biologically active derivative of vitamin A (RA) as a potential modulator of hCMV pathogenesis in infants and immunocompromised adults.

Retinoid X receptor-COUP-TF interactions modulate retinoic acid signaling

We have recently described the properties of direct repeats (DRs) of the half-site AGGTCA as hormone response elements (HREs). According to our results, spacing the half sites by 3, 4, or 5 nucleotides determines specificity of response for vitamin D3, thyroid hormone, and retinoic acid receptors, respectively. This so-called 3-4-5 rule led to the prediction that remaining spacing options of 0, 1, and 2 might serve as targets for other nuclear receptors. A concurrent prediction is that receptors recognizing common sites might display more complex or combinatorial interactions. In exploring these predictions, we discovered that both the retinoid X receptor (RXR) and COUP-TF bind preferentially to a DR-1 motif. In vivo, RXR and COUP-TF display antagonistic action such that RXR-mediated activation is fully repressed by COUP-TF. In vitro studies reveal that COUP-TF and RXR form heterodimers on DR-1. Thus, these results support a general proposal in which the half-site spacing preferences may be used as a means to decipher potentially complex and interactive regulatory circuits.

Retinoid X receptor interacts with nuclear receptors in retinoic acid, thyroid hormone and vitamin D3 signalling

Cellular responsiveness to retinoic acid and its metabolites is conferred through two structurally and pharmacologically distinct families of receptors: the retinoic acid receptors (RAR) and the retinoid X receptors (RXR). Here we report that the transcriptional activity of RAR and RXR can be reciprocally modulated by direct interactions between the two proteins. RAR and RXR have a high degree of cooperativity in binding to target DNA, consistent with previous reports indicating that the binding of either RAR or RXR to their cognate response elements is enhanced by factors present in nuclear extracts. RXR also interacts directly with and enhances the binding of nuclear receptors conferring responsiveness to vitamin D3 and thyroid hormone T3; the DNA-binding activities of these receptors are also stimulated by the presence of nuclear extracts. Together these data indicate that RXR has a central role in multiple hormonal signalling pathways.

Transcriptional regulation of the c-jun gene by retinoic acid and E1A during differentiation of F9 cells

Differentiation of mouse F9 embryonal carcinoma (EC) cells can be induced by exposure to retinoic acid (RA) or by expression of adenovirus E1A. The transcription of the c-jun gene is stimulated by either RA or E1A. We report here that both RA and E1A strongly induce the expression of chloramphenicol acetyltransferase (CAT) from c-jun promoter/CAT reporter construct (c-jun/CAT), which is stably integrated into F9 cells, in a manner that is independent of both copy number and integration locus. The induction of c-jun/CAT expression is observed in undifferentiated F9 cells, but not in differentiated F9 cells, adenovirus-infected F9 cells or HeLa cells. Deletion analysis of the promoter region of the c-jun gene indicates that the sequence elements required for the RA- and E1A-mediated induction are identical and they have been defined as a region of 145 bp between -190 and -46 of the 5' flanking region of c-jun. This RA and E1A response element (RERE) contains five variants of the motif CGCGGTGACGNT. The upstream two motifs are adjacent and extend in opposite directions, creating an imperfect palindrome. The downstream four motifs are located at 35 or 36 bp intervals in the same orientation. Substitution and insertion analysis indicates that these motifs and their regular intervals are important for the activity of the RERE.

Chromosomal translocation t(15;17) in human acute promyelocytic leukemia fuses RAR alpha with a novel putative transcription factor, PML

A unique mRNA produced in leukemic cells from a t(15;17) acute promyelocytic leukemia (APL) patient encodes a fusion protein between the retinoic acid receptor alpha (RAR alpha) and a myeloid gene product called PML. PML contains a cysteine-rich region present in a new family of apparent DNA-binding proteins that includes a regulator of the interleukin-2 receptor gene (Rpt-1) and the recombination-activating gene product (RAG-1). Accordingly, PML may represent a novel transcription factor or recombinase. The aberrant PML-RAR fusion product, while typically retinoic acid responsive, displays both cell type- and promoter-specific differences from the wild-type RAR alpha. Because patients with APL can be induced into remission with high dose RA therapy, we propose that the nonliganded PML-RAR protein is a new class of dominant negative oncogene product. Treatment with RA would not only relieve this inhibition, but the activated PML-RAR protein may actually promote myelocyte differentiation.

A direct repeat in the cellular retinol-binding protein type II gene confers differential regulation by RXR and RAR

The vitamin A derivative retinoic acid exerts its effects on transcription through two distinct classes of nuclear receptors, the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). We provide evidence that expression of the gene for cellular retinol-binding protein type II (CRBPII), a key protein in the intestinal absorption of vitamin A, is dramatically up-regulated by retinoic acid in the presence of RXR but not RAR. This regulation is conferred through a specific cis element in the CRBPII promoter that contains five nearly perfect tandem repeats of the sequence AGGTCA spaced by a single nucleotide. The discovery of this new RX response element provides a means for distinguishing between the two retinoid receptor systems and suggests that an RXR-mediated pathway exists for modulating vitamin A metabolism.

Direct repeats as selective response elements for the thyroid hormone, retinoic acid, and vitamin D3 receptors

We report here the identification of thyroid hormone response elements (TREs) that consist of a direct repeat, not a palindrome, of the half-sites. Unlike palindromic TREs, direct repeat TREs do not confer a retinoic acid response. The tandem TRE can be converted into a retinoic acid response element by increasing the spacing between the half-sites by 1 nucleotide, and the resulting retinoic acid response element is no longer a TRE. Decreasing the half-site spacing by 1 nucleotide converts the TRE to a vitamin D3 response element, while eliminating response to T3. These results correlate well with DNA-binding affinities of the thyroid hormone, retinoic acid, and vitamin D3 receptors. This study points to the general importance of tandem repeat hormone response elements and suggests a simple physiologic code exists in which half-site spacing plays a critical role in achieving selective hormonal response.

A functional retinoic acid receptor encoded by the gene on human chromosome 12

A cDNA clone derived from a human hepatocellular carcinoma has been isolated on the basis of homology to the alpha human retinoic acid receptor (RAR alpha) gene. Expression of this cDNA produces a high affinity nuclear binding protein for retinoic acid. The product of this clone when expressed in transfected cells is able to activate transcription of a reporter plasmid through specific DNA sequences in response to the addition of retinoic acid to the medium. Dose-dependent profiles upon trans-activation of the reporter indicate that apparent sensitivity to retinoic acid of this protein is approximately 10-fold higher than that of human RAR alpha and is comparable to that of the second human RAR, RAR beta. This gene has been mapped to human chromosome 12, which is distinct from those coding for either alpha or beta RAR, and thus encodes a third human RAR.