Allosteric regulation of the discriminative responsiveness of retinoic acid receptor to natural and synthetic ligands by retinoid X receptor and DNA

The action of retinoic acid on spermatogonia in the testis

For mammalian spermatogenesis to proceed normally, it is essential that the population of testicular progenitor cells, A undifferentiated spermatogonia (Aundiff), undergoes differentiation during the A to A1 transition that occurs at the onset of spermatogenesis. The commitment of the Aundiff population to differentiation and leaving a quiescent, stem-like state gives rise to all the spermatozoa produced across the lifespan of an individual, and ultimately determines male fertility. The action of all-trans retinoic acid (atRA) on the Aundiff population is the determining factor that induces this change. Sertoli cells, omnipresent, nurse cells within the mammalian testis are responsible for synthesizing the atRA that prompts this change in the neonatal testicular environment. The mechanism of atRA synthesis and signaling has been robustly explored and, in this review, we have summarized what is currently known about the action of testicular atRA at the onset of spermatogenesis. We have combined this with evidence gained from prominent genetic studies that have further elucidated the function of genes critical to atRA synthesis. We have additionally described the effects of the first pulse of atRA delivered to the germ cells of the testis, which has been investigated using WIN 18,446 treatment which prevents atRA synthesis and induces spermatogenic synchrony. This method provides unparalleled resolution into cell and stage specific testicular changes, and combined with transgenic animal models, has allowed researchers to elucidate much regarding the onset of spermatogenesis.

Structural basis for DNA recognition and allosteric control of the retinoic acid receptors RAR-RXR

Retinoic acid receptors (RARs) as a functional heterodimer with retinoid X receptors (RXRs), bind a diverse series of RA-response elements (RAREs) in regulated genes. Among them, the non-canonical DR0 elements are bound by RXR–RAR with comparable affinities to DR5 elements but DR0 elements do not act transcriptionally as independent RAREs. In this work, we present structural insights for the recognition of DR5 and DR0 elements by RXR–RAR heterodimer using x-ray crystallography, small angle x-ray scattering, and hydrogen/deuterium exchange coupled to mass spectrometry. We solved the crystal structures of RXR–RAR DNA-binding domain in complex with the Rarb2 DR5 and RXR–RXR DNA-binding domain in complex with Hoxb13 DR0. While cooperative binding was observed on DR5, the two molecules bound non-cooperatively on DR0 on opposite sides of the DNA. In addition, our data unveil the structural organization and dynamics of the multi-domain RXR–RAR DNA complexes providing evidence for DNA-dependent allosteric communication between domains. Differential binding modes between DR0 and DR5 were observed leading to differences in conformation and structural dynamics of the multi-domain RXR–RAR DNA complexes. These results reveal that the topological organization of the RAR binding element confer regulatory information by modulating the overall topology and structural dynamics of the RXR–RAR heterodimers.

Characterization of the differential coregulator binding signatures of the Retinoic Acid Receptor subtypes upon (ant)agonist action

Retinoic Acid Receptor alpha (RARα/NR1B1), Retinoic Acid Receptor beta (RARβ/NR1B2) and Retinoic Acid Receptor gamma (RARγ/NR1B3) are transcription factors regulating gene expression in response to retinoids. Within the RAR genomic pathways, binding of RARs to coregulators is a key intermediate regulatory phase. However, ligand-dependent interactions between the wide variety of coregulators that may be present in a cell and the different RAR subtypes are largely unknown. The aim of this study is to characterize the coregulator binding profiles of RARs in the presence of the pan-agonist all-trans-Retinoic Acid (AtRA); the subtype-selective agonists Am80 (RARα), CD2314 (RARβ) and BMS961 (RARγ); and the antagonist Ro415253. To this end, we used a microarray assay for coregulator-nuclear receptor interactions to assess RAR binding to 154 motifs belonging to >60 coregulators. The results revealed a high number of ligand-dependent RAR-coregulator interactions among all RAR variants, including many binding events not yet described in literature. Next, this work confirmed a greater ligand-independent activity of RARβ compared to the other RAR subtypes based on both higher basal and lower ligand-driven coregulator binding. Further, several coregulator motifs showed selective binding to a specific RAR subtype. Next, this work showed that subtype-selective agonists can be successfully discriminated by using coregulator binding assays. Finally this study demonstrated the possible applications of a coregulator binding assay as a tool to discriminate between agonistic/antagonistic actions of ligands. The RAR-coregulator interactions found will be of use to direct further studies to better understand the mechanisms driving the eventual actions of retinoids.

Structural basis of natural promoter recognition by the retinoid X nuclear receptor

Retinoid X receptors (RXRs) act as homodimers or heterodimerisation partners of class II nuclear receptors. RXR homo- and heterodimers bind direct repeats of the half-site (A/G)G(G/T)TCA separated by 1 nucleotide (DR1). We present a structural characterization of RXR-DNA binding domain (DBD) homodimers on several natural DR1s and an idealized symmetric DR1. Homodimers displayed asymmetric binding, with critical high-affinity interactions accounting for the 3' positioning of RXR in heterodimers on DR1s. Differing half-site and spacer DNA sequence induce changes in RXR-DBD homodimer conformation notably in the dimerization interface such that natural DR1s are bound with higher affinity than an idealized symmetric DR1. Subtle changes in the consensus DR1 DNA sequence therefore specify binding affinity through altered RXR-DBD-DNA contacts and changes in DBD conformation suggesting a general model whereby preferential half-site recognition determines polarity of heterodimer binding to response elements.

General molecular biology and architecture of nuclear receptors

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

Proteasomal degradation of retinoid X receptor alpha reprograms transcriptional activity of PPARgamma in obese mice and humans

Obese patients have chronic, low-grade inflammation that predisposes to type 2 diabetes and results, in part, from dysregulated visceral white adipose tissue (WAT) functions. The specific signaling pathways underlying WAT dysregulation, however, remain unclear. Here we report that the PPARgamma signaling pathway operates differently in the visceral WAT of lean and obese mice. PPARgamma in visceral, but not subcutaneous, WAT from obese mice displayed increased sensitivity to activation by its agonist rosiglitazone. This increased sensitivity correlated with increased expression of the gene encoding the ubiquitin hydrolase/ligase ubiquitin carboxyterminal esterase L1 (UCH-L1) and with increased degradation of the PPARgamma heterodimerization partner retinoid X receptor alpha (RXRalpha), but not RXRbeta, in visceral WAT from obese humans and mice. Interestingly, increased UCH-L1 expression and RXRalpha proteasomal degradation was induced in vitro by conditions mimicking hypoxia, a condition that occurs in obese visceral WAT. Finally, PPARgamma-RXRbeta heterodimers, but not PPARgamma-RXRalpha complexes, were able to efficiently dismiss the transcriptional corepressor silencing mediator for retinoid and thyroid hormone receptors (SMRT) upon agonist binding. Increasing the RXRalpha/RXRbeta ratio resulted in increased PPARgamma responsiveness following agonist stimulation. Thus, the selective proteasomal degradation of RXRalpha initiated by UCH-L1 upregulation modulates the relative affinity of PPARgamma heterodimers for SMRT and their responsiveness to PPARgamma agonists, ultimately activating the PPARgamma-controlled gene network in visceral WAT of obese animals and humans.

Direct interdomain interactions can mediate allosterism in the thyroid receptor

The thyroid (TR) and retinoid X receptors (RXR) belong to the nuclear receptor (NR) superfamily of ligand-mediated transcription factors. At the molecular level, TR activity is specifically modulated by interactions with the ligand 3,3',5 triiodo-l-thyronine (T3), RXR, DNA, and co-activators such as SRC1, occurring in concert or sequentially. Although binding sites for DNA and coregulators such as SRC1 are distinct and at distal regions of these receptors, cell-based and EMSA studies have suggested that these molecules can regulate binding of each other to the receptor. We present evidence of direct, DNA-dependent, communication between the DNA and ligand binding domains (DBD and LBD) that can allosterically regulate interactions with SRC1 and DNA, respectively, using isothermal titration calorimetry (ITC) and cell-based assays. Additionally, we note that interdomain communication is affected by RXR in RXR:TR. We also noticed a DNA-dependent cross-talk between RXR and TR within RXR:TR. Finally, we suggest that differences in transactivation on different TRE may be the consequence of different affinities between TRE and RXR:TR.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

S 26948: a new specific peroxisome proliferator activated receptor gamma modulator with potent antidiabetes and antiatherogenic effects

OBJECTIVE

Rosiglitazone displays powerful antidiabetes benefits but is associated with increased body weight and adipogenesis. Keeping in mind the concept of selective peroxisome proliferator-activated receptor (PPAR)gamma modulator, the aim of this study was to characterize the properties of a new PPARgamma ligand, S 26948, with special attention in body-weight gain.

RESEARCH DESIGN AND METHODS

We used transient transfection and binding assays to characterized the binding characteristics of S 26948 and GST pull-down experiments to investigate its pattern of coactivator recruitment compared with rosiglitazone. We also assessed its adipogenic capacity in vitro using the 3T3-F442A cell line and its in vivo effects in ob/ob mice (for antidiabetes and antiobesity properties), as well as the homozygous human apolipoprotein E2 knocking mice (E2-KI) (for antiatherogenic capacity).

RESULTS

S 26948 displayed pharmacological features of a high selective ligand for PPARgamma with low potency in promoting adipocyte differentiation. It also displayed a different coactivator recruitment profile compared with rosiglitazone, being unable to recruit DRIP205 or PPARgamma coactivator-1 alpha. In vivo experiments showed that S 26948 was as efficient in ameliorating glucose and lipid homeostasis as rosiglitazone, but it did not increase body and white adipose tissue weights and improved lipid oxidation in liver. In addition, S 26948 represented one of the few molecules of the PPARgamma ligand class able to decrease atherosclerotic lesions.

CONCLUSIONS

These findings establish S 26948 as a selective PPARgamma ligand with distinctive coactivator recruitment and gene expression profile, reduced adipogenic effect, and improved biological responses in vivo.

The core component of the mammalian SWI/SNF complex SMARCD3/BAF60c is a coactivator for the nuclear retinoic acid receptor

Retinoic acid receptors (RARs) activate transcription by recruiting coactivator complexes such as histone acetyltransferases (HAT) and the mediator complex, to increase chromatin accessibility by general transcription factors and to promote transcription initiation. Indirect evidences have suggested a role for the ATP-dependent chromatin remodeling complex SWI/SNF in RAR-mediated transcription. Here we demonstrate that two highly related subunits of the core SWI/SNF complex, BAF60c1 and BAF60c2, interact physically with retinoid receptors and are coactivators for RARs. This coactivating property is dependent on SRC1 expression, showing that HATs and SWI/SNF cooperate in this retinoid-controlled transcriptional process.

Down-Regulation of the Tumor Suppressor Gene Retinoic Acid Receptor β2 through the Phosphoinositide 3-Kinase/Akt Signaling Pathway

The retinoic acid receptor β2 (RARβ2) is a potent, retinoid-inducible tumor suppressor gene, which is a critical molecular relay for retinoid actions in cells. Its down-regulation, or loss of expression, leads to resistance of cancer cells to retinoid treatment. Up to now, no primary mechanism underlying the repression of the RARβ2 gene expression, hence affecting cellular retinoid sensitivity, has been identified. Here, we demonstrate that the phosphoinositide 3-kinase/Akt signaling pathway affects cellular retinoid sensitivity, by regulating corepressor recruitment to the RARβ2 promoter. Through direct phosphorylation of the corepressor silencing mediator for retinoic and thyroid hormone receptors (SMRT), Akt stabilized RAR/SMRT interaction, leading to an increased tethering of SMRT to the RARβ2 promoter, decreased histone acetylation, down-regulation of the RARβ2 expression, and impaired cellular differentiation in response to retinoid. The phosphoinositide 3-kinase/Akt signaling pathway, an important modulator of cellular survival, has thus a direct impact on cellular retinoid sensitivity, and its deregulation may be the triggering event in retinoid resistance of cancer cells.

Distinct Roles of the Steroid Receptor Coactivator 1 and of MED1 in Retinoid-induced Transcription and Cellular Differentiation

Retinoic acid receptors (RARs) are the molecular relays of retinoid action on transcription, cellular differentiation and apoptosis. Transcriptional activation of retinoid-regulated promoters requires the dismissal of corepressors and the recruitment of coactivators to promoter-bound RAR. RARs recruit in vitro a plethora of coactivators whose actual contribution to retinoid-induced transcription is poorly characterized in vivo. Embryonal carcinoma P19 cells, which are highly sensitive to retinoids, were depleted from archetypical coactivators by RNAi. SRC1-deficient P19 cells showed severely compromised retinoid-induced responses, in agreement with the supposed role of SRC1 as a RAR coactivator. Unexpectedly, Med1/TRAP220/DRIP205-depleted cells exhibited an exacerbated response to retinoids, both in terms transcriptional responses and of cellular differentiation. Med1 depletion affected TFIIH and cdk9 detection at the prototypical retinoid-regulated RARbeta2 promoter, and favored a higher RNA polymerase II detection in transcribed regions of the RARbeta2 gene. Furthermore, the nature of the ligand impacted strongly on the ability of RARs to interact with a given coactivator and to activate transcription in intact cells. Thus RAR accomplishes transcriptional activation as a function of the ligand structure, by recruiting regulatory complexes which control distinct molecular events at retinoid-regulated promoters.

The geminivirus nuclear shuttle protein NSP inhibits the activity of AtNSI, a vascular-expressed Arabidopsis acetyltransferase regulated with the sink-to-source transition

DNA viruses can suppress or enhance the activity of cellular acetyltransferases to regulate virus gene expression and to affect cell cycle progression in support of virus replication. A role for protein acetylation in regulating the nuclear export of the bipartite geminivirus (Begomovirus) DNA genome was recently suggested by the findings that the viral movement protein NSP, a nuclear shuttle protein, interacts with the Arabidopsis (Arabidopsis thaliana) nuclear acetyltransferase AtNSI (nuclear shuttle protein interactor), and that this interaction and NSI expression are necessary for cabbage leaf curl virus infection and pathogenicity. To further investigate the consequences of NSI-NSP interactions, and the potential role of NSI in Arabidopsis growth and development, we used a reverse yeast two-hybrid selection and deletion analysis to identify NSI mutants that failed to interact with NSP, and promoter fusions to a uidA reporter gene to analyze the pattern of NSI expression during plant development. We found that NSI self assembles into highly active enzyme complexes and that high concentrations of NSP, in the absence of viral DNA, can inhibit NSI activity in vitro. Based on our detailed analysis of three NSI missense mutants, we identified an 88-amino acid putative domain, which spans NSI residues 107 to 194, as being required for both NSI oligomerization and its interaction with NSP. Finally, we found that NSI is predominantly transcribed in vascular cells, and that its expression is developmentally regulated in a manner that resembles the sink-to-source transition. Our data indicate that NSP can inhibit NSI activity by interfering with its assembly into highly active complexes, and suggest a mechanism by which NSP can both recruit NSI to regulate nuclear export of the viral genome and down-regulate NSI activity on cellular targets, perhaps to affect cellular differentiation and favor virus replication.

The proliferating cell nuclear antigen regulates retinoic acid receptor transcriptional activity through direct protein-protein interaction

Retinoic acid receptors (RARs) interact, in a ligand-dependent fashion, with many coregulators that participate in a wide spectrum of biological responses, ranging from embryonic development to cellular growth control. The transactivating function of these ligand-inducible transcription factors reside mainly, but not exclusively, in their ligand-binding domain (AF2), which recruits or dismiss coregulators in a ligand-dependent fashion. However, little is known about AF2-independent function(s) of RARs. We have isolated the proliferating cell nuclear antigen (PCNA) as a repressor of RAR transcriptional activity, able to interact with an AF2-crippled RAR. The N-terminus of PCNA interacts directly with the DNA-binding domain of RAR, and PCNA is recruited to a retinoid-regulated promoter in intact cells. This interaction affects the transcriptional response to retinoic acid in a promoter-specific manner, conferring an unanticipated role to PCNA in transcriptional regulation. Our findings also suggest a role for RAR as a factor coordinating DNA transcription and repair.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Regulation of human apoA-I by gemfibrozil and fenofibrate through selective peroxisome proliferator-activated receptor alpha modulation

OBJECTIVE

The objective of this trial was to study the effects of fenofibrate (FF) and gemfibrozil (GF), the most commonly used fibrates, on high-density lipoprotein (HDL) and apolipoprotein (apo) A-I.

METHODS AND RESULTS

In a head-to-head double-blind clinical trial, both FF and GF decreased triglycerides and increased HDL cholesterol levels to a similar extent, whereas plasma apoA-I only increased after FF but not GF. Results in human (h) apoA-Itransgenic (hA-ITg) peroxisome proliferator-activated receptor (PPAR) alpha-/- mice demonstrated that PPARalpha mediates the effects of FF and GF on HDL in vivo. Although plasma and hepatic mRNA levels of hapoA-I increased more pronouncedly after FF than GF in hA-ITgPPARalpha+/+ mice, both fibrates induced acylCoAoxidase mRNA similarly. FF and GF transactivated PPARalpha with similar activity and affinity on a DR-1 PPAR response element, but maximal activation on the hapoA-I DR-2 PPAR response element was significantly lower for GF than for FF. Moreover, GF induced recruitment of the coactivator DRIP205 on the DR-2 site less efficiently than did FF.

CONCLUSIONS

Both GF and FF exert their effects on HDL through PPARalpha. Whereas FF behaves as a full agonist, GF appears to act as a partial agonist due to a differential recruitment of coactivators to the promoter. These observations provide an explanation for the differences in the activity of these fibrates on apoA-I.

Interaction of the movement protein NSP and the Arabidopsis acetyltransferase AtNSI is necessary for Cabbage leaf curl geminivirus infection and pathogenicity

DNA viruses can modulate the activity of cellular acetyltransferases to regulate virus gene expression and to affect cell cycle progression in order to support virus replication. A role for protein acetylation in regulating the nuclear export of the bipartite geminivirus DNA genome was recently suggested by the findings that the viral movement protein NSP, which shuttles the viral genome between the nucleus and the cytoplasm, interacts with a novel Arabidopsis acetyltransferase, AtNSI, and the increased expression of AtNSI enhances susceptibility to Cabbage leaf curl virus infection. To further investigate the interaction of NSP and AtNSI and to establish the importance of this interaction in virus infections, we used a reverse yeast two-hybrid selection and deletion analysis to identify NSP mutants that were impaired in their ability to bind AtNSI. These mutants identified a 38-amino-acid region of NSP, to which no function had so far been assigned, as being necessary for NSP-AtNSI interaction. Three NSP missense mutants were analyzed in detail and were found to be comparable to wild-type NSP in their levels of accumulation, nucleocytoplasmic shuttling, DNA binding, and cooperative interaction with the viral cell-to-cell movement protein MP. Despite this, Cabbage leaf curl virus that expressed each mutated NSP was defective in its ability to infect Arabidopsis, exhibiting lower levels of infectivity than the wild-type virus, and delayed systemic spread of the virus and attenuated disease symptoms. Our data demonstrate the importance of the interaction of NSP with AtNSI for virus infection and pathogenicity.

Interaction of the movement protein NSP and the Arabidopsis acetyltransferase AtNSI is necessary for Cabbage leaf curl geminivirus infection and pathogenicity

DNA viruses can modulate the activity of cellular acetyltransferases to regulate virus gene expression and to affect cell cycle progression in order to support virus replication. A role for protein acetylation in regulating the nuclear export of the bipartite geminivirus DNA genome was recently suggested by the findings that the viral movement protein NSP, which shuttles the viral genome between the nucleus and the cytoplasm, interacts with a novel Arabidopsis acetyltransferase, AtNSI, and the increased expression of AtNSI enhances susceptibility to Cabbage leaf curl virus infection. To further investigate the interaction of NSP and AtNSI and to establish the importance of this interaction in virus infections, we used a reverse yeast two-hybrid selection and deletion analysis to identify NSP mutants that were impaired in their ability to bind AtNSI. These mutants identified a 38-amino-acid region of NSP, to which no function had so far been assigned, as being necessary for NSP-AtNSI interaction. Three NSP missense mutants were analyzed in detail and were found to be comparable to wild-type NSP in their levels of accumulation, nucleocytoplasmic shuttling, DNA binding, and cooperative interaction with the viral cell-to-cell movement protein MP. Despite this, Cabbage leaf curl virus that expressed each mutated NSP was defective in its ability to infect Arabidopsis, exhibiting lower levels of infectivity than the wild-type virus, and delayed systemic spread of the virus and attenuated disease symptoms. Our data demonstrate the importance of the interaction of NSP with AtNSI for virus infection and pathogenicity.

A single amino acid change in the first zinc finger of the DNA binding domain of the glucocorticoid receptor regulates differential promoter selectivity

Mammalian species are well known to differ in their sensitivity to glucocorticoids, but the molecular basis for this difference remains largely uncharacterized. To address this issue, the transcriptional activity of the mouse and human glucocorticoid receptor (GR) was analyzed on two model glucocorticoid-responsive promoters. Mouse GR (mGR) displayed unique promoter discrimination in response to a range of glucocorticoids, with enhanced activity on a simple glucocorticoid response element (GRE)-based promoter and diminished activity on the complex mouse mammary tumor virus promoter compared with human GR (hGR). Promoter discrimination between mGR and hGR was mapped to a single amino acid change at residue 437 (glycine to valine) of mGR and to sequence differences within individual GREs of the different promoters. Mouse GR displayed higher activation on GREs with a guanine rather than a thymine at the -6 position. Binding studies indicated mGR (mGR437V) displayed a weaker affinity for GREs containing a thymine at the -6 position than a mGR mutant containing a glycine at residue 437 (mGR437G). Despite distinct transcriptional activities, both receptors had similar affinities for response elements that contain a guanine at the -6 position. Our findings support a model by which the presence of a valine residue at position 437 of mGR induces a conformational change that leads to alterations in affinity and/or transcriptional activation in a promoter-dependent context.

Differential regulation of protein expression, growth and apoptosis by natural and synthetic retinoids

All-trans retinoic acid (ATRA) can down regulate the anti-apoptotic protein Bcl-2 and the cell cycle proteins cyclin D1 and cdk2 in estrogen receptor-positive breast cancer cells. We show here that retinoids can also reduce expression of the inhibitor of apoptosis protein, survivin. Here we have compared the regulation of these proteins in MCF-7 and ZR-75 breast cancer cells by natural and synthetic retinoids selective for the RA receptors (RARs) alpha, beta, and gamma then correlated these with growth inhibition, induction of apoptosis and chemosensitization to Taxol. In both cell lines ATRA and 9-cis RA induced the most profound decreases in cyclin D1 and cdk2 expression and also mediated the largest growth inhibition. The RARalpha agonist, Ro 40-6055 also strongly downregulated these proteins although did not produce an equivalent decrease in S-phase cells. Only ATRA induced RARbeta expression. ATRA, 9-cis RA and 4-HPR initiated the highest level of apoptosis as determined by mitochondrial Bax translocation, while only ATRA and 9-cis RA strongly reduced Bcl-2 and survivin protein expression. Enumeration of dead cells over 96 h correlated well with downregulation of both survivin and Bcl-2. Simultaneous retinoid-mediated reduction of both these proteins also predicted optimal Taxol sensitization. 4-HPR was much weaker than the natural retinoids with respect to Taxol sensitization, consistent with the proposed requirement for reduced Bcl-2 in this synergy. Neither the extent of cell cycle protein regulation nor AP-1 inhibition fully predicted the antiproliferative effect of the synthetic retinoids suggesting that growth inhibition requires regulation of a spectrum of RAR-regulated gene products in addition even to pivotal cell cycle proteins.

PLZF is a negative regulator of retinoic acid receptor transcriptional activity

BACKGROUND: Retinoic acid receptors (RARs) are ligand-regulated transcription factors controlling cellular proliferation and differentiation. Receptor-interacting proteins such as corepressors and coactivators play a crucial role in specifying the overall transcriptional activity of the receptor in response to ligand treatment. Little is known however on how receptor activity is controlled by intermediary factors which interact with RARs in a ligand-independent manner. RESULTS: We have identified the promyelocytic leukemia zinc finger protein (PLZF), a transcriptional corepressor, to be a RAR-interacting protein using the yeast two-hybrid assay. We confirmed this interaction by GST-pull down assays and show that the PLZF N-terminal zinc finger domain is necessary and sufficient for PLZF to bind RAR. The RAR ligand binding domain displayed the highest affinity for PLZF, but corepressor and coactivator binding interfaces did not contribute to PLZF recruitment. The interaction was ligand-independent and correlated to a decreased transcriptional activity of the RXR-RAR heterodimer upon overexpression of PLZF. A similar transcriptional interference could be observed with the estrogen receptor alpha and the glucocorticoid receptor. We further show that PLZF is likely to act by preventing RXR-RAR heterodimerization, both in-vitro and in intact cells. CONCLUSION: Thus RAR and PLZF interact physically and functionally. Intriguingly, these two transcription factors play a determining role in hematopoiesis and regionalization of the hindbrain and may, upon chromosomal translocation, form fusion proteins. Our observations therefore define a novel mechanism by which RARs activity may be controlled.

A complex deoxyribonucleic acid response element in the rat Ca(2+)/calmodulin-dependent protein kinase IV gene 5'-flanking region mediates thyroid hormone induction and chicken ovalbumin upstream promoter transcription factor 1 repression

Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is regulated by T(3) in a time- and concentration-dependent manner in the developing rat brain and plays an important role in neuronal-specific gene regulation. T(3) treatment, but not retinoic acid (RA), stimulated endogenous CaMKIV mRNA 5-fold in mouse embryonic stem (ES) cells differentiated into neurons. We localized a region -750 to -700 in the CaMKIV gene 5'-flanking region that conferred T(3) responsiveness and bound thyroid hormone receptor (TR), retinoic acid receptor (RAR), and chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1). T(3) and RA treatment stimulated the CaMKIV hormone response element. Cotransfection of a COUP-TF1 expression vector repressed the T(3) response and augmented the RA response. Mutational analysis identified three half-sites arranged in a direct repeat (AB) and overlapping inverted repeat (BC), required for functional induction and receptor binding. TR and RAR bound predominantly to the BC portion of the element and COUP-TF1 to the AB region, with a close correlation of binding and functional studies. COUP-TF1 binding did not influence TR/retinoid X receptor binding but modestly augmented RAR/retinoid X receptor binding. A single element confers T(3) and COUP-TF1 regulation of CaMKIV expression.

Requirements for heterodimerization between the orphan nuclear receptor Nurr1 and retinoid X receptors

The nuclear receptor nurr1 is a transcription factor involved in the development and maintenance of neurons synthesizing the neurotransmitter dopamine. Although the lack of nurr1 expression has dramatic consequences for these cells either in terms of differentiation or survival, the mechanisms by which nurr1 controls gene transcription still remain unclear. In the intent to understand better the modalities of action of this nuclear receptor, we have undertaken a systematic analysis of the transcriptional effects and DNA binding properties of nurr1 as a monomer or when forming dimers with the different isotypes of the retinoic X receptor (RXR). Here, we show that nurr1 acts as a gene activator independently of RXR and through an AF2-independent mechanism. In addition, heterodimerization with RXR is isotype-specific, involves multiple domains in the C-terminal region of nurr1, and requires RXR binding to DNA. RXR(alpha)-nurr1 and RXRgamma-nurr1 heterodimers bind direct repeat response elements and display no specific requirements with respect to half-site spacing. However, the retinoid responsiveness of DNA-bound heterodimers requires the reiteration of at least three nurr1 binding sites, thereby limiting retinoid-induced nurr1 transcriptional activity to specific direct response elements.

Retinoic acid receptors inhibit AP1 activation by regulating extracellular signal-regulated kinase and CBP recruitment to an AP1-responsive promoter

Retinoids exhibit antineoplastic activities that may be linked to retinoid receptor-mediated transrepression of activating protein 1 (AP1), a heterodimeric transcription factor composed of fos- and jun-related proteins. Here we show that transcriptional activation of an AP1-regulated gene through the mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) pathway (MAPK(ERK)) is characterized, in intact cells, by a switch from a fra2-junD dimer to a junD-fosB dimer loading on its promoter and by simultaneous recruitment of ERKs, CREB-binding protein (CBP), and RNA polymerase II. All-trans-retinoic acid (atRA) receptor (RAR) was tethered constitutively to the AP1 promoter. AP1 transrepression by retinoic acid was concomitant to glycogen synthase kinase 3 activation, negative regulation of junD hyperphosphorylation, and to decreased RNA polymerase II recruitment. Under these conditions, fra1 loading to the AP1 response element was strongly increased. Importantly, CBP and ERKs were excluded from the promoter in the presence of atRA. AP1 transrepression by retinoids was RAR and ligand dependent, but none of the functions required for RAR-mediated transactivation was necessary for AP1 transrepression. These results indicate that transrepressive effects of retinoids are mediated through a mechanism unrelated to transcriptional activation, involving the RAR-dependent control of transcription factors and cofactor assembly on AP1-regulated promoters.

Selective alteration of gene expression in response to natural and synthetic retinoids

BACKGROUND

Retinoids are very potent inducers of cellular differentiation and apoptosis, and are efficient anti-tumoral agents. Synthetic retinoids are designed to restrict their toxicity and side effects, mostly by increasing their selectivity toward each isotype of retinoic acids receptors (RARalpha,beta, gamma and RXRalpha, beta, gamma). We however previously showed that retinoids displayed very different abilities to activate retinoid-inducible reporter genes, and that these differential properties were correlated to the ability of a given ligand to promote SRC-1 recruitment by DNA-bound RXR:RAR heterodimers. This suggested that gene-selective modulation could be achieved by structurally distinct retinoids.

RESULTS

Using the differential display mRNA technique, we identified several genes on the basis of their differential induction by natural or synthetic retinoids in human cervix adenocarcinoma cells. Furthermore, this differential ability to regulate promoter activities was also observed in murine P19 cells for the RARbeta2 and CRABPII gene, showing conclusively that retinoid structure has a dramatic impact on the regulation of endogenous genes.

CONCLUSIONS

Our findings therefore show that some degree of selective induction or repression of gene expression may be achieved when using appropriately designed ligands for retinoic acid receptors, extending the concept of selective modulators from estrogen and peroxisome proliferator activated receptors to the class of retinoid receptors.

Phosphorylation of histone H3 is functionally linked to retinoic acid receptor beta promoter activation

Ligand-dependent transcriptional activation of retinoic acid receptors (RARs) is a multistep process culminating in the formation of a multimeric co-activator complex on regulated promoters. Several co-activator complexes harbor an acetyl transferase activity, which is required for retinoid-induced transcription of reporter genes. Using murine P19 embryonal carcinoma cells, we examined the relationship between histone post-translational modifications and activation of the endogenous RARbeta2 promoter, which is under the control of a canonical retinoic acid response element and rapidly induced upon retinoid treatment. While histones H3 and H4 were constitutively acetylated at this promoter, retinoid agonists induced a rapid phosphorylation at Ser10 of histone H3. A retinoid antagonist, whose activity was independent of co-repressor binding to RAR, could oppose this agonist-induced H3 phosphorylation. Since such post-translational modifications were not observed at several other promoters, we conclude that histone H3 phosphorylation may be a molecular signature of the activated, retinoid-controlled mRARbeta2 gene promoter.

In vivo and in vitro influence of etretinate on erythrocyte membrane fluidity

The molecular mechanisms underlying the action of synthetic retinoids have been studied intensively, but they are not fully understood yet. It is well known that retinoids exert their effects on gene expression via the retinoic acid receptor. Some observations suggest that the main aromatic retinoid etretinate (Tigason) exerts its therapeutic effect in psoriasis also through an action on the cell membrane. In this paper, we present the results of previously unreleased experiments (when Tigason was still in use) concerning the in vivo and in vitro influence of etretinate on erythrocyte membrane fluidity in psoriatic patients. Erythrocytes from healthy subjects and topically treated psoriatics were chosen as control groups. Membrane fluidity was measured by the electron paramagnetic resonance (EPR) spin-labelling technique. Erythrocytes from psoriatic patients had lower membrane fluidity, a lower antioxidant activity and a greater susceptibility to peroxidation than those from healthy subjects. After treatment with etretinate, a significant increase in erythrocyte membrane fluidity and in antioxidant activity as well as a decrease in lipid peroxidation were observed in erythrocytes from patients. Local therapy of psoriatic lesions had no influence on the improvement in membrane fluidity and antioxidant activity of erythrocytes. Incubation of erythrocytes from healthy controls and topically treated psoriatics with etretinate in vitro confirmed its fluidizing effect on erythrocyte membranes. These data may indicate that two mechanisms lead to an increase in erythrocyte membrane fluidity in psoriatic patients treated with Tigason: the first one, indirect, by improvement of the antioxidant defence system and cell protection against lipid peroxidation, and the second one, by a direct fluidizing effect of etretinate on the erythrocyte membrane.

In vitro effect of Triac on resistance to thyroid hormone receptor mutants: potential basis for therapy

Resistance to thyroid hormone (RTH) is a syndrome caused by a mutation in the carboxyl-terminal domain of the thyroid hormone receptor beta (TRbeta) gene. 3,5,3'-triiodothyroacetic acid (Triac) has been used on an empirical basis to treat RTH but its efficacy is still controversial. In previous studies, we demonstrated that Triac has TR isoform- and TRE-specific effects. In this report, we used five natural RTH mutations of the ligand-binding domain in both TRbeta1 and TRbeta2 isoforms for the evaluation of the effect of T3 and Triac on regulation of transcription and binding affinity. We show that Triac has superior activity on negatively and positively regulated promoters and higher binding affinity than T3 for a majority of TRbeta1 and TRbeta2 mutants. However, the difference of transcriptional activity and binding affinity between both ligands is less for RTH mutants than for wild type receptors. These results suggest that Triac could be a potential treatment for RTH patients.

Control of retinoic acid receptor heterodimerization by ligand-induced structural transitions. A novel mechanism of action for retinoid antagonists

Heterodimerization of retinoic acid receptors (RARs) with 9-cis-retinoic receptors (RXRs) is a prerequisite for binding of RXR.RAR dimers to DNA and for retinoic acid-induced gene regulation. Whether retinoids control RXR/RAR solution interaction remains a debated question, and we have used in vitro and in vivo protein interaction assays to investigate the role of ligand in modulating RXR/RAR interaction in the absence of DNA. Two-hybrid assay in mammalian cells demonstrated that only RAR agonists were able to increase significantly RAR interaction with RXR, whereas RAR antagonists inhibited RXR binding to RAR. Quantitative glutathione S-transferase pull-down assays established that there was a strict correlation between agonist binding affinity for the RAR monomer and the affinity of RXR for liganded RAR, but RAR antagonists were inactive in inducing RXR recruitment to RAR in vitro. Alteration of coactivator- or corepressor-binding interfaces of RXR or RAR did not alter ligand-enhanced dimerization. In contrast, preventing the formation of a stable holoreceptor structure upon agonist binding strongly altered RXR.RAR dimerization. Finally, we observed that RAR interaction with RXR silenced RXR ligand-dependent activation function. We propose that ligand-controlled dimerization of RAR with RXR is an important step in the RXR.RAR activation process. This interaction is dependent upon adequate remodeling of the AF-2 structure and amenable to pharmacological inhibition by structurally modified retinoids.

Mutations in the glucocorticoid receptor DNA-binding domain mimic an allosteric effect of DNA

Two previously isolated mutations in the glucocorticoid receptor DNA-binding domain (DBD), S459A and P493R, have been postulated to mimic DNA-induced conformational changes in the glucocorticoid receptor DBD, thereby constitutively triggering an allosteric mechanism in which binding of specific DNA normally induces the exposure of otherwise silent glucocorticoid receptor transcriptional activation surfaces. Here we report the three-dimensional structure of the free S459A and P493R mutant DBDs as determined by NMR spectroscopy. The free S459A and P493R structures both display the conformational changes in the DBD dimerization interface that are characteristic of the DNA-bound wild-type DBD, confirming that these mutations mimic an allosteric effect of DNA. A transition between two packing arrangements of the DBD hydrophobic core provides a mechanism for long-range transmission of conformational changes, induced either by the mutations or by DNA binding, to protein-protein contact surfaces.

Properties of the glucocorticoid modulatory element binding proteins GMEB-1 and -2: potential new modifiers of glucocorticoid receptor transactivation and members of the family of KDWK proteins

An important component of glucocorticoid steroid induction of tyrosine aminotransferase (TAT) gene expression is the glucocorticoid modulatory element (GME), which is located at -3.6 kb of the rat TAT gene. The GME both mediates a greater sensitivity to hormone, due to a left shift in the dose-response curve of agonists, and increases the partial agonist activity of antiglucocorticoids. These properties of the GME are intimately related to the binding of a heteromeric complex of two proteins (GMEB-1 and -2). We previously cloned the rat GMEB-2 as a 67-kDa protein. We now report the cloning of the other member of the GME binding complex, the 88-kDa human GMEB-1, and various properties of both proteins. GMEB-1 and -2 each possess an intrinsic transactivation activity in mammalian one-hybrid assays, consistent with our proposed model in which they modify glucocorticoid receptor (GR)-regulated gene induction. This hypothesis is supported by interactions between GR and both GMEB-1 and -2 in mammalian two-hybrid and in pull-down assays. Furthermore, overexpression of GMEB-1 and -2, either alone or in combination, results in a reversible right shift in the dose-response curve, and decreased agonist activity of antisteroids, as expected from the squelching of other limiting factors. Additional mechanistic details that are compatible with the model of GME action are suggested by the interactions in a two-hybrid assay of both GMEBs with CREB-binding protein (CBP) and the absence of histone acetyl transferase (HAT) activity in both proteins. GMEB-1 and -2 share a sequence of 90 amino acids that is 80% identical. This region also displays homology to several other proteins containing a core sequence of KDWK. Thus, the GMEBs may be members of a new family of factors with interesting transcriptional properties.

Recruitment of nuclear receptor corepressor and coactivator to the retinoic acid receptor by retinoid ligands. Influence of DNA-heterodimer interactions

Ligand activation of retinoic acid receptors (RARs) involves coordinated changes in their interaction with coregulatory molecules. Binding of the agonist all-trans-retinoic acid to the RAR results in increased interaction with coactivator molecules as well as a decreased interaction with corepressor molecules. Thus, an all-trans-retinoic acid antagonist might function either by preventing agonist induction of such events or, additionally, by actively increasing repression via corepressor recruitment. We demonstrate that the repression of the transcriptional activity of a constitutively active RARgamma-VP-16 chimeric receptor by the inverse agonist AGN193109 requires a functional Co-R box and that binding of this ligand to RARgamma leads to an increased interaction with the corepressor N-CoR both in glutathione S-transferase pull-down and yeast two-hybrid analyses. Detection of nuclear receptor corepressor (N-CoR) association with RARgamma was greatly facilitated by inclusion of a RARE oligonucleotide in coimmunoprecipitation analyses, a result of an increase in association of the ternary complex consisting of RAR, RXR, and DNA. Similarly, this DNA-dependent increase in heterodimer formation likewise resulted in an increase in agonist-mediated recruitment efficiency of the coactivator SRC-1. Under conditions which favor ternary complex formation, a RAR neutral antagonist is distinguished from an inverse agonist with respect to corepressor recruitment as is a RAR partial agonist distinguished from an agonist with respect to coactivator recruitment. These results indicate that it is possible to design RAR ligands with distinct recruitment capabilities for coregulators, both coactivators as well as corepressors. In addition, using this recruitment assay, we show that SRC-1 and the related coactivator molecule ACTR associate with the ternary complex via utilization of different helical motifs within their conserved receptor interaction domains.

Serine 157, a retinoic acid receptor alpha residue phosphorylated by protein kinase C in vitro, is involved in RXR.RARalpha heterodimerization and transcriptional activity

Retinoic acid (RA) regulation of cellular proliferation and differentiation is mediated, at least in part, through two related nuclear receptors, RAR and RXR. RA-induced modulation of gene expression leads generally to cellular differentiation, whereas stimulation of the protein kinase C (PKC) signaling pathway is associated with cellular proliferation. Pursuant to our discovery that prolonged activation of PKCs induced a strong decrease in RA responsiveness of a retinoid-inducible reporter gene, we have further investigated the connections between these two signaling pathways. We demonstrate that PKC isoforms alpha and gamma are able to phosphorylate human RARalpha (hRARalpha) in vitro on a single serine residue located in the extended DNA binding domain (T box). The introduction of a negative charge at this position (serine 157) strongly decreased hRARalpha transcriptional activity, whereas a similar mutation at other PKC consensus phosphorylation sites had no effect. The effect on transcriptional activation was correlated with a decrease in the capacity of hRARalpha to heterodimerize with hRXRalpha. Thus hRARalpha is a direct target for PKCalpha and gamma, which may control retinoid receptor transcriptional activities during cellular proliferation and differentiation.

Effect of ligand and DNA binding on the interaction between human transcription intermediary factor 1alpha and estrogen receptors

Hormonal regulation of gene activity is mediated by nuclear receptors acting as ligand-activated transcription factors. To achieve efficient regulation of gene expression, these receptors must interact with different type of molecules: 1) the steroid hormone, 2) the DNA response element, and 3) various proteins acting as transcriptional cofactors. In the present study, we have investigated how ligand and DNA binding influence the in vitro interaction between estrogen receptors (ERs) and the transcription intermediary factor hTIF1alpha (human transcriptional intermediary factor 1alpha). We first optimized conditions for the coactivator-dependent receptor ligand assay to lower ED50, and we then analyzed the ability of various natural and synthetic estrogens to allow the binding of the two types of proteins. Results were compared with the respective affinities of these ligands for the receptor. We then developed a protein-protein-DNA assay allowing the quantification of cofactor-ER-estrogen response element (ERE) complex formation in the presence of ligand and used measurements of fluorescence anisotropy to define the equilibrium binding parameters of the interaction. We demonstrated that the leucine-charged domain of hTIF1alpha is sufficient to interact with ERE-bound ERalpha in a ligand-dependent manner and showed that binding of ERalpha onto DNA does not significantly affect its hormone-dependent association with TIF1alpha. Finally, we show that, mainly in the absence of hormone, hTIF1alpha interacts better with ERbeta than with ERalpha independently of the presence of ERE.