Auto-silencing by the retinoid X receptor

Cytoplasmic Localization of RXRα Determines Outcome in Breast Cancer

Simple Summary

Considering the immense development of today’s therapeutic approaches in oncology towards customized therapy, this study aimed to assess the prognostic value of nuclear versus cytoplasmic retinoid X receptor α (RXRα) expression in breast cancer. Our results demonstrate that RXRα expression may have different roles in tumorigenesis according to its subcellular localization. This study strengthens the need for further research on the behavior of RXRα, depending on its intracellular localization.

Abstract

The aim of this retrospective study was to assess the prognostic value of cytoplasmic versus nuclear RXRα expression in breast cancer (BC) tissue samples and to correlate the results with clinicopathological parameters. In 319 BC patients, the expression of RXRα was evaluated via immunohistochemistry. Prognosis-determining aspects were calculated through uni- and multivariate analyses. Correlation analysis revealed a trend association with nuclear RXRα expression regarding an improved overall survival (OS) (p = 0.078), whereas cytoplasmic RXRα expression was significantly correlated with a poor outcomes in terms of both OS (p = 0.038) and disease-free survival (DFS) (p = 0.037). Strengthening these results, cytoplasmic RXRα was found to be an independent marker for DFS (p = 0.023), when adjusted to clinicopathological parameters, whereas nuclear RXRα expression was positively associated with lower TNM-staging, i.e., pT (p = 0.01), pN (p = 0.029) and pM (p = 0.001). Additionally, cytoplasmic RXRα expression was positively associated with a higher histopathological tumor grading (p = 0.02). Cytoplasmic RXRα was also found to be a negative prognosticator for Her-2neu-negative and triple-negative patients. Altogether, these findings support the hypothesis that the subcellular localization of RXRα plays an important role in carcinogenesis and the prognosis of BC. The expression of cytoplasmic RXRα is correlated with a more aggressive course of the disease, whereas nuclear RXRα expression appears to be a protective factor. These data may help to identify high-risk BC subgroups in order to find possible specific options in targeted tumor therapy.

RXRA DT448/9PP generates a dominant active variant capable of inducing maturation in acute myeloid leukemia cells

RARA and RXRA contribute to myeloid maturation in both mice and humans, and deletion of Rxra and Rxrb augments leukemic growth in mice. While defining the domains of RXRA that are required for anti-leukemic effects in murine KMT2A-MLLT3 leukemia cells, we unexpectedly identified RXRA DT448/9PP as a constitutively active variant capable of inducing maturation and loss of their proliferative phenotype. RXRA DT448/9PP was associated with ligand-independent activity in reporter assays, with enhanced co-activator interactions, reduced engraftment in vivo, and activation of myeloid maturation transcriptional signatures that overlapped with those of cells treated with the potent RXRA agonist bexarotene, suggestive of constitutive activity that leads to leukemic maturation. Phenotypes of RXRA DT448/9PP appear to differ from those of two other RXRA mutations with forms of constitutive activity (F318A and S427F), in that DT448/9PP activity was resistant to mutations at critical ligand-interacting amino acids (R316A/L326A) and was resistant to pharmacological antagonists, suggesting it may be ligand-independent. These data provide further evidence that activated retinoid X receptors can regulate myeloid maturation and provide a novel constitutively active variant that may be germane for broader studies of retinoid X receptors in other settings.

Rosiglitazone binds to RXRα to induce RXRα tetramerization and NB4 cell differentiation

Rosiglitazone is a ligand of peroxisome proliferation-activated receptor gamma (PPARγ). However, it exerts biological activities and therapeutic effects through both PPARγ-dependent and independent mechanisms. In this study, we defined that rosiglitazone was also a ligand of retinoid X receptor alpha (RXRα) and displayed RXRα-dependent activities. We found that rosiglitazone directly bound to the ligand binding domain (LBD) of RXRα and induced RXRα/LBD tetramerization. Rosiglitazone inhibited the agonist-induced transcriptional activity of RXRα homodimers and heterodimers likely through inhibiting RXRα homo- and hetero-dimerization. In acute promyelocytic leukemia (APL) NB4 cells, rosiglitazone inhibited cell proliferation and induced cell differentiation, resulting from inhibiting RXRα/PML-RARα complex formation and down-regulating PML-RARα. Together, our study identified RXRα as a novel target of rosiglitazone and RXRα mediating the anti-APL activity of rosiglitazone.

Berberine binds RXRα to suppress β-catenin signaling in colon cancer cells

Berberine, an isoquinoline alkaloid, is a traditional oriental medicine used to treat diarrhea and gastroenteritis. Recently, we reported that it could inhibit the growth of intestinal polyp in animals and in patients with the familial adenomatous polyposis by downregulating β-catenin signaling. However, the intracellular target mediating the effects of berberine remains elusive. Here, we provide evidence that berberine inhibits β-catenin function via directly binding to a unique region comprising residues Gln275, Arg316 and Arg371 in nuclear receptor retinoid X receptor alpha (RXRα), where berberine concomitantly binding to and synergistically activating RXRα with 9-cis-retinoic acid (9-cis-RA), a natural ligand binding to the classical ligand-binding pocket of RXRα. Berberine binding promotes RXRα interaction with nuclear β-catenin, leading to c-Cbl mediated degradation of β-catenin, and consequently inhibits the proliferation of colon cancer cells. Furthermore, berberine suppresses the growth of human colon carcinoma xenograft in nude mice in an RXRα-dependent manner. Together, our study not only identifies RXRα as a direct protein target for berberine but also dissects their binding mode and validates that berberine indeed suppresses β-catenin signaling and cell growth in colon cancer via binding RXRα, which provide new strategies for the design of new RXRα-based antitumor agents and drug combinations.

Modulation of nongenomic activation of PI3K signalling by tetramerization of N-terminally-cleaved RXRα

Retinoid X receptor-alpha (RXRα) binds to DNA either as homodimers or heterodimers, but it also forms homotetramers whose function is poorly defined. We previously discovered that an N-terminally-cleaved form of RXRα (tRXRα), produced in tumour cells, activates phosphoinositide 3-kinase (PI3K) signalling by binding to the p85α subunit of PI3K and that K-80003, an anti-cancer agent, inhibits this process. Here, we report through crystallographic and biochemical studies that K-80003 binds to and stabilizes tRXRα tetramers via a ‘three-pronged’ combination of canonical and non-canonical mechanisms. K-80003 binding has no effect on tetramerization of RXRα, owing to the head–tail interaction that is absent in tRXRα. We also identify an LxxLL motif in p85α, which binds to the coactivator-binding groove on tRXRα and dissociates from tRXRα upon tRXRα tetramerization. These results identify conformational selection as the mechanism for inhibiting the nongenomic action of tRXRα and provide molecular insights into the development of RXRα cancer therapeutics.

The transcription factor retinoid X receptor-alpha (RXRα) can also form homotetramers. Here the authors show that the anti-cancer agent K-80003 selectively inhibits the nongenomic action of N-terminally-cleaved RXRα in tumour cells by stabilizing its tetramerization but not that of full-length RXRα.

Trialkyltin Rexinoid-X Receptor Agonists Selectively Potentiate Thyroid Hormone Induced Programs of Xenopus laevis Metamorphosis

The trialkyltins tributyltin (TBT) and triphenyltin (TPT) can function as rexinoid-X receptor (RXR) agonists. We recently showed that RXR agonists can alter thyroid hormone (TH) signaling in a mammalian pituitary TH-responsive reporter cell line, GH3.TRE-Luc. The prevalence of TBT and TPT in the environment prompted us to test whether they could also affect TH signaling. Both trialkyltins induced the integrated luciferase reporter alone and potentiated TH activation at low doses. Trimethyltin, which is not an RXR agonist, did not. We turned to a simple, robust, and specific in vivo model system of TH action: metamorphosis of Xenopus laevis, the African clawed frog. Using a precocious metamorphosis assay, we found that 1nM TBT and TPT, but not trimethyltin, greatly potentiated the effect of TH treatment on resorption phenotypes of the tail, which is lost at metamorphosis, and in the head, which undergoes extensive remodeling including gill loss. Consistent with these responses, TH-induced caspase-3 activation in the tail was enhanced by cotreatment with TBT. Induction of a transgenic reporter gene and endogenous collagenase 3 (mmp13) and fibroblast-activating protein-α (fap) genes were not induced by TBT alone, but TH induction was significantly potentiated by TBT. However, induction of other TH receptor target genes such as TRβ and deiodinase 3 by TH were not affected by TBT cotreatment. These data indicate that trialkyltins that can function as RXR agonists can selectively potentiate gene expression and resultant morphological programs directed by TH signaling in vivo.

β-Apo-13-carotenone regulates retinoid X receptor transcriptional activity through tetramerization of the receptor

Retinoid X receptor (RXRα) is activated by 9-cis-retinoic acid (9cRA) and regulates transcription as a homodimer or as a heterodimer with other nuclear receptors. We have previously demonstrated that β-apo-13-carotenone, an eccentric cleavage product of β-carotene, antagonizes the activation of RXRα by 9cRA in mammalian cells overexpressing this receptor. However, the molecular mechanism of β-apo-13-carotenone's modulation on the transcriptional activity of RXRα is not understood and is the subject of this report. We performed transactivation assays using full-length RXRα and reporter gene constructs (RXRE-Luc) transfected into COS-7 cells, and luciferase activity was examined. β-Apo-13-carotenone was compared with the RXRα antagonist UVI3003. The results showed that both β-apo-13-carotenone and UVI3003 shifted the dose-dependent RXRα activation by 9cRA. In contrast, the results of assays using a hybrid Gal4-DBD:RXRαLBD receptor reporter cell assay that detects 9cRA-induced coactivator binding to the ligand binding domain demonstrated that UVI3003 significantly inhibited 9cRA-induced coactivator binding to RXRαLBD, but β-apo-13-carotenone did not. However, both β-apo-13-carotenone and UVI3003 inhibited 9-cRA induction of caspase 9 gene expression in the mammary carcinoma cell line MCF-7. To resolve this apparent contradiction, we investigated the effect of β-apo-13-carotenone on the oligomeric state of purified recombinant RXRαLBD. β-Apo-13-carotenone induces tetramerization of the RXRαLBD, although UVI3003 had no effect on the oligomeric state. These observations suggest that β-apo-13-carotenone regulates RXRα transcriptional activity by inducing the formation of the "transcriptionally silent" RXRα tetramer.

Sulindac-derived RXRα modulators inhibit cancer cell growth by binding to a novel site

Retinoid X receptor-alpha (RXRα), an intriguing and unique drug target, can serve as an intracellular target mediating the anticancer effects of certain nonsteroidal anti-inflammatory drugs (NSAIDs), including sulindac. We report the synthesis and characterization of two sulindac analogs, K-8008 and K-8012, which exert improved anticancer activities over sulindac in a RXRα-dependent manner. The analogs inhibit the interaction of the N-terminally truncated RXRα (tRXRα) with the p85α subunit of PI3K, leading to suppression of AKT activation and induction of apoptosis. Crystal structures of the RXRα ligand-binding domain (LBD) with K-8008 or K-8012 reveal that both compounds bind to tetrameric RXRα LBD at a site different from the classical ligand-binding pocket. Thus, these results identify K-8008 and K-8012 as tRXRα modulators and define a binding mechanism for regulating the nongenomic action of tRXRα.

Targeting nuclear receptors with marine natural products

Nuclear receptors (NRs) are important pharmaceutical targets because they are key regulators of many metabolic and inflammatory diseases, including diabetes, dyslipidemia, cirrhosis, and fibrosis. As ligands play a pivotal role in modulating nuclear receptor activity, the discovery of novel ligands for nuclear receptors represents an interesting and promising therapeutic approach. The search for novel NR agonists and antagonists with enhanced selectivities prompted the exploration of the extraordinary chemical diversity associated with natural products. Recent studies involving nuclear receptors have disclosed a number of natural products as nuclear receptor ligands, serving to re-emphasize the translational possibilities of natural products in drug discovery. In this review, the natural ligands of nuclear receptors will be described with an emphasis on their mechanisms of action and their therapeutic potentials, as well as on strategies to determine potential marine natural products as nuclear receptor modulators.

RXRs: collegial partners

Retinoid X Receptors (RXR) were initially identified as nuclear receptors binding with stereo-selectivity the vitamin A derivative 9-cis retinoic acid, although the relevance of this molecule as endogenous activator of RXRs is still elusive. Importantly, within the nuclear receptor superfamily, RXRs occupy a peculiar place, as they are obligatory partners for a number of other nuclear receptors, thus integrating the corresponding signaling pathways. In this chapter, we describe the structural features allowing RXR to form homo- and heterodimers, and the functional consequences of this unique ability. Furthermore, we discuss the importance of studying RXR activity at a genome-wide level in order to comprehensively address the biological implications of their action that is fundamental to understand to what extent RXRs could be exploited as new therapeutic targets.

Retinoid metabolism and its effects on the vasculature

Retinoids, the metabolically-active structural derivatives of vitamin A, are critical signaling molecules in many fundamental biological processes including cell survival, proliferation and differentiation. Emerging evidence, both clinical and molecular, implicates retinoids in atherosclerosis and other vasculoproliferative disorders such as restenosis. Although the data from clinical trials examining effect of vitamin A and vitamin precursors on cardiac events have been contradictory, this data does suggest that retinoids do influence fundamental processes relevant to atherosclerosis. Preclinical animal model and cellular studies support these concepts. Retinoids exhibit complex effects on proliferation, growth, differentiation and migration of vascular smooth muscle cells (VSMC), including responses to injury and atherosclerosis. Retinoids also appear to exert important inhibitory effects on thrombosis and inflammatory responses relevant to atherogenesis. Recent studies suggest retinoids may also be involved in vascular calcification and endothelial function, for example, by modulating nitric oxide pathways. In addition, established retinoid effects on lipid metabolism and adipogenesis may indirectly influence inflammation and atherosclerosis. Collectively, these observations underscore the scope and complexity of retinoid effects relevant to vascular disease. Additional studies are needed to elucidate how context and metabolite-specific retinoid effects affect atherosclerosis. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

DNA binding alters coactivator interaction surfaces of the intact VDR-RXR complex

The vitamin D receptor (VDR) functions as an obligate heterodimer with the retinoid X receptor (RXR). These nuclear receptors (NRs) are multidomain proteins and it is unclear how various domains interact with one another within the NR heterodimer. Here we show that binding of intact heterodimer to DNA alters the receptor dynamics in regions remote from the DNA binding domains (DBDs), including the coactivator binding surfaces of both coreceptors, and the sequence of the DNA response element can specify the dynamics. Furthermore, agonist binding to the heterodimer results in changes in the stability of the VDR DBD, indicating that ligand itself may play a role in DNA recognition. These data suggest a mechanism by which NRs can display promoter-specific activity and impart differential effects on various target genes, which provides mechanistic insight for the function of selective NR modulators.

Danthron functions as a retinoic X receptor antagonist by stabilizing tetramers of the receptor

Retinoic X receptor (RXR) is a promising target for drug discovery against cancer and metabolic syndromes. Here, we identified a specific RXRα antagonist, danthron, from the traditional Chinese medicine rhubarb. Danthron repressed all tested RXRα-involved response element transcription, including the RXRE, PPRE, FXRE, and LXRE. Results from native PAGE and isothermal titration calorimetry (ITC)-based assays indicated that danthron bound to the tetrameric RXRα-LBD in a specific stoichimetric ratio, and such a binding could influence the corepressor SMRT affinity to the receptor. Additionally, a unique tetrameric structure of the apo-RXRα ligand-binding domain (LBD) was determined, which exhibited a larger tetramer interface and different ligand-binding pocket size compared with the one previously reported. Together with the biochemical and biophysical results, the determined crystal structure of danthron-soaked RXRα-LBD suggested a new mechanism for danthron antagonism to tetrameric RXRα. Moreover, the in vivo efficient improvement of insulin sensitivity by danthron was observed in diet-induced obese (DIO) mice. Thus, our findings were expected to supply new insights into the structural basis of RXRα antagonist for its further potential therapeutic application.

Between death and survival: retinoic acid in regulation of apoptosis

The vitamin A metabolite all-trans-retinoic acid (RA) regulates multiple biological processes by virtue of its ability to regulate gene expression. It thus plays critical roles in embryonic development and is involved in regulating growth, remodeling, and metabolic responses in adult tissues. RA can also suppress carcinoma cell growth and is currently used in treatment of some cancers. Growth inhibition by RA may be exerted by induction of differentiation, cell cycle arrest, or apoptosis, or by a combination of these activities. Paradoxically, in the context of some cells, RA not only fails to inhibit growth but, instead, enhances proliferation and survival. This review focuses on the involvement of RA in regulating apoptotic responses. It includes brief overviews of transcriptional signaling by RA and of apoptotic pathways, and then addresses available information on the mechanisms by which RA induces apoptosis or, conversely, inhibits cell death and enhances survival.

Inhibition of mammary carcinoma cell growth by RXR is mediated by the receptor's oligomeric switch

Ligands that activate the nuclear receptor retinoid X receptor (RXR) display potent anticarcinogenic activities, but the mechanisms by which these compounds inhibit carcinoma cell growth are poorly understood. While RXR can regulate gene expression due to its intrinsic ligand-activated transcription function, this receptor can also regulate transcription by functioning as a ligand-controlled DNA architectural factor. It was thus reported that apo-RXR self-associates into tetramers and that each dimer within these tetramers can separately bind to an RXR response element. Hence, DNA binding by RXR tetramers may bring distant genomic regions into close physical proximity. As ligand binding induces the dissociation of RXR tetramers into dimers, it can alter gene expression by modulating the DNA architecture. Here, we show that inhibition of mammary carcinoma cell growth by RXR ligands stems from the ability of these compounds to regulate the oligomeric state of RXR and is independent of the direct intrinsic transcriptional activity of the receptor. The data suggest that compounds that trigger dissociation of RXR tetramers may comprise a novel class of anticarcinogenic agents.

Interleukin-22 forms dimers that are recognized by two interleukin-22R1 receptor chains

Interleukin-22 (IL-22) is a class 2 cytokine whose primary structure is similar to that of interleukin 10 (IL-10) and interferon-gamma (IFN-gamma). IL-22 induction during acute phase immune response indicates its involvement in mechanisms of inflammation. Structurally different from IL-10 and a number of other members of IL-10 family, which form intertwined inseparable V-shaped dimers of two identical polypeptide chains, a single polypeptide chain of IL-22 folds on itself in a relatively globular structure. Here we present evidence, based on native gel electrophoresis, glutaraldehyde cross-linking, dynamic light scattering, and small angle x-ray scattering experiments, that human IL-22 forms dimers and tetramers in solution under protein concentrations assessable by these experiments. Unexpectedly, low-resolution molecular shape of IL-22 dimers is strikingly similar to that of IL-10 and other intertwined cytokine dimeric forms. Furthermore, we determine an ab initio molecular shape of the IL-22/IL-22R1 complex which reveals the V-shaped IL-22 dimer interacting with two cognate IL-22R1 molecules. Based on this collective evidence, we argue that dimerization might be a common mechanism of all class 2 cytokines for the molecular recognition with their respective membrane receptor. We also speculate that the IL-22 tetramer formation could represent a way to store the cytokine in nonactive form at high concentrations that could be readily converted into functionally active monomers and dimers upon interaction with the cognate cellular receptors.

Ligand specificity of nuclear hormone receptors: sifting through promiscuity

The superfamily of nuclear hormone receptors includes transcription factors that play key roles in regulating multiple biological functions during embryonic development and in adult tissues, as well as in many disease states. The quintessential characteristic of nuclear receptors, and the basis for the name of the family, is that their transcriptional activities can be regulated by small molecules, usually comprised of hydrophobic compounds. However, the endogenous ligands for approximately half of the members of the nuclear receptor family are unknown, and these receptors are thus designated as "orphan receptors". One class of orphan receptors encompasses receptors that display a broad ligand selectivity; i.e., they can promiscuously bind to and may be activated by multiple ligands. This characteristic complicates the identification of physiologically meaningful ligands that activate these receptors in vivo. Here, we discuss a few examples of promiscuous receptors and outline strategies that may be employed in shedding light on the nature of bona fide ligands for such receptors.

Low-resolution structures of thyroid hormone receptor dimers and tetramers in solution

High-resolution X-ray structures of thyroid hormone (TH) receptor (TR) DNA and ligand binding domains (DBD and LBD) have yielded significant insights into TR action. Nevertheless, the TR DBD and LBD act in concert to mediate TH effects upon gene expression, and TRs form multiple oligomers; however, structures of full-length TRs or DBD-LBD constructs that would clarify these influences are not available. Here, we report low-resolution X-ray structures of the TRbeta DBD-LBD construct in solution which define the shape of dimers and tetramers and likely positions of the DBDs and LBDs. The holo TRbeta DBD-LBD construct forms a homodimer with LBD-DBD pairs in close contact and DBDs protruding from the base in the same direction. The DBDs are connected to the LBDs by crossed extended D domains. The apo hTRbeta DBD-LBD construct forms tetramers that resemble bulged cylinders with pairs of LBD dimers in a head-to-head arrangement with DBD pairs packed tightly against the LBD core. Overall, there are similarities with our previous low-resolution structures of retinoid X receptors, but TRs exhibit two unique features. First, TR DBDs are closely juxtaposed in the dimer and tetramer forms. Second, TR DBDs are closely packed against LBDs in the tetramer, but not the dimer. These findings suggest that TRs may be able to engage in hitherto unknown interdomain interactions and that the D domain must rearrange in different oligomeric forms. Finally, the data corroborate our suggestion that apo TRs form tetramers in solution which dissociate into dimers upon hormone binding.

RXR: From Partnership to Leadership in Metabolic Regulations

Vitamin A signaling occurs through nuclear receptors recognizing diverse forms of retinoic acid (RA). The retinoic acid receptors (RARs) bind all-trans RA and its 9-cis isomer (9-cis RA). They convey most of the activity of RA, particularly during embryogenesis. The second subset of receptors, the rexinoid receptors (RXRs), binds 9-cis RA only. However, RXRs are obligatory DNA-binding partners for a number of nuclear receptors, broadening the spectrum of their biological activity to the corresponding nuclear receptor-signaling pathways. The present chapter more particularly focuses on RXR-containing transcriptional complexes for which RXR is not only a structural component necessary for DNA binding but also acts as a ligand-activated partner. After positioning RXR among the nuclear receptor superfamily in the first part, we will give an overview of three major signaling pathways involved in metabolism, which are sensitive to RXR activation: LXR:RXR, FXR:RXR, and PPAR:RXR. The third and last part is focused on RXR signaling and its potential role in metabolic regulation. Indeed, while the nature of the endogenous ligand for RXR is still in question, as we will discuss herein, a better understanding of RXR activities is necessary to envisage the potential therapeutic applications of synthetic RXR ligands.

Nuclear import of the retinoid X receptor, the vitamin D receptor, and their mutual heterodimer

The nuclear receptor retinoid X receptor (RXR) can regulate transcription through homotetramers, homodimers, and heterodimers with other nuclear receptors such as the vitamin D receptor (VDR). The mechanisms that underlie the nuclear import of RXR, VDR, and RXR-VDR heterodimers were investigated. We show that RXR and VDR translocate into the nucleus by distinct pathways. RXR strongly bound to importinbeta and was predominantly nuclear in the absence of ligand. Importin binding and nuclear localization of RXR were modestly enhanced by its ligand, 9-cis-retinoic acid. On the other hand, VDR selectively associated with importinalpha. Importin association and correspondingly nuclear import of VDR were markedly augmented by 1,25(OH)2D3. RXR-VDR dimerization inhibited the ability of RXR to bind importinbeta and to mobilize into the nucleus using its own nuclear localization signal. In contrast, VDR recruited RXR-VDR heterodimers to importinalpha and mediated nuclear import of the heterodimers in response to 1,25(OH)2D3. Hence nuclear import of RXR-VDR heterodimers is mediated preferentially by VDR and is controlled by the VDR ligand. The observations reveal a novel mechanism by which an RXR heterodimerization partner dominates the activity of the heterodimers.

Environmental xenobiotics and nuclear receptors--interactions, effects and in vitro assessment

A group of intracellular nuclear receptors is a protein superfamily including arylhydrocarbon AhR, estrogen ER, androgen AR, thyroid TR and retinoid receptors RAR/RXR as well as molecules with unknown function known as orphan receptors. These proteins play an important role in a wide range of physiological as well as toxicological processes acting as transcription factors (ligand-dependent signalling macromolecules modulating expression of various genes in a positive or negative manner). A large number of environmental pollutants and other xenobiotics negatively affect signaling pathways, in which nuclear receptors are involved, and these modulations were related to important in vivo toxic effects such as immunosuppression, carcinogenesis, reproduction or developmental toxicity, and embryotoxicity. Presented review summarizes current knowledge on major nuclear receptors (AhR, ER, AR, RAR/RXR, TR) and their relationship to known in vivo toxic effects. Special attention is focused on priority organic environmental contaminants and experimental approaches for determination and studies of specific toxicity mechanisms.

Retinoid X receptors: X-ploring their (patho)physiological functions

Retinoid X receptor (RXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. A class of nuclear receptors requires RXR as heterodimerization partner for their function. This places RXR in the crossroad of multiple distinct biological pathways. This and the fact that the debate on the endogenous ligand requirement for RXR is not yet settled make RXR still an enigmatic transcription factor. Here, we review some of the biology of RXR. We place RXR into the evolution of nuclear receptors, review structural details and ligands of the receptor. Then processes regulated by RXR are discussed focusing on the developmental roles deduced from studies on knockout animals and metabolic roles in diseases such as diabetes and atherosclerosis deduced from pharmacological studies. Finally, aspects of RXR's involvement in myeloid differentiation and apoptosis are summarized along with issues on RXR's suitability as a therapeutic target.

DNA-looping by RXR Tetramers Permits Transcriptional Regulation “at a Distance”

RXR, a member of the superfamily of nuclear hormone receptors, regulates gene transcription in response to 9-cis-retinoic acid. We previously showed that, among nuclear receptors, RXR is unique in that it self-associates into homotetramers, and that these tetramers dissociate rapidly upon ligation. Here, we report that binding of RXR tetramers to DNA containing two RXR response elements results in a dramatic DNA-looping. RXR can thus juxtapose distant DNA sequences, enabling transcriptional regulation by far-upstream factors. We show that RXR functions as a DNA architectural factor and that, while this activity is regulated by 9-cis-retinoic acid, it is distinct from and independent of the receptor's intrinsic transcriptional activity. The data establish RXR as the first identified architectural factor whose activity is regulated by a small ligand, and demonstrate a novel mechanism of transcriptional regulation by retinoids.

Retinoid X receptor alpha represses GATA-4-mediated transcription via a retinoid-dependent interaction with the cardiac-enriched repressor FOG-2

Dietary vitamin A and its derivatives, retinoids, regulate cardiac growth and development. To delineate mechanisms involved in retinoid-mediated control of cardiac gene expression, the regulatory effects of the retinoid X receptor alpha (RXR alpha) on atrial naturietic factor (ANF) gene transcription was investigated. The transcriptional activity of an ANF promoter-reporter in rat neonatal ventricular myocytes was repressed by RXR alpha in the presence of 9-cis-RA and by the constitutively active mutant RXR alpha F318A indicating that liganded RXR confers the regulatory effect. The RXR alpha-mediated repression mapped to the proximal 147 bp of the rat ANF promoter, a region lacking a consensus retinoid response element but containing several known cardiogenic cis elements including a well characterized GATA response element. Glutathione S-transferase "pull-down" assays revealed that RXR alpha interacts directly with GATA-4, in a ligand-independent manner, via the DNA binding domain of RXR alpha and the second zinc finger of GATA-4. Liganded RXR alpha repressed the activity of a heterologous promoter-reporter construct containing GATA-response element recognition sites in cardiac myocytes but not in several other cell types, suggesting that additional cardiac-enriched factors participate in the repression complex. Co-transfection of liganded RXR alpha and the known cardiac-enriched GATA-4 repressor, FOG-2, resulted in additive repression of GATA-4 activity in ventricular myocytes. In addition, RXR alpha was found to bind FOG-2, in a 9-cis-RA-dependent manner. These data reveal a novel mechanism by which retinoids regulate cardiogenic gene expression through direct interaction with GATA-4 and its co-repressor, FOG-2.

Retinoid X receptors and retinoid response in neuroblastoma cells

Retinoic acid (RA) modulates differentiation and apoptosis of neural cells via RA receptors (RARs) and retinoid X receptors (RXRs). Neuroblastoma cells are potentially useful models for elucidating the molecular mechanisms of RA in neural cells, and responses to different isomers of RA have been interpreted in terms of differential homo- and heterodimerization of RXRs. The aim of this study was to identify the RXR types expressed in neuroblast and substrate-adherent neuroblastoma cells, and to study the participation of these RXRs in RAR heterodimers. RXRbeta was the predominant RXR type in N-type SH SY 5Y cells and S-type SH EP cells. Gel shift and supershift assays demonstrated that RARbeta and RARgamma predominantly heterodimerize with RXRbeta. In SH SY 5Y cells, RARgamma/RXRbeta was the predominant heterodimer binding to the DR5 RARE in the absence of 9-cis RA (9C), whereas the balance shifted in favor of RARbeta/RXRbeta in the presence of ligand. There was a marked difference between the N- and S-type neuroblastoma cells in retinoid receptor-DNA interactions, and this may underlie the differential effects of retinoids in these neuroblastoma cell types. There was no evidence to indicate that 9C functions via RXR homodimers in either SH SY 5Y or SH EP neuroblastoma cells. The results of this study suggest that interactions between retinoid receptors and other nuclear proteins may be critical determinants of retinoid responses in neural cells.

Interactions of RXR with coactivators are differentially mediated by helix 11 of the receptor's ligand binding domain

RXR is a nuclear hormone receptor that is activated by the vitamin A metabolite 9-cis-retinoic acid. Previously, it was shown that, in the absence of a cognate ligand, RXR self-associates into tetramers, thereby silencing its own transcriptional activity. It was also shown that the tetramerization region of RXR critically contains two of three consecutive phenylalanine residues found in helix 11 (H11) of the receptor's ligand binding domain. Mutation of these residues abolishes the ability of RXR to form tetramers but also results in a receptor that is defective in its ligand-induced transcriptional activity. These observations suggest that the region may be involved in the association of RXR with transcriptional coactivators. Here, it is demonstrated that mutation of the H11 phenylalanine residues diminishes the ability of RXR to associate with the p160 coactivators TIF2 and p/CIP, but has little effect on ligand-dependent interactions of the receptor with the unrelated coactivator TIF1. It is further shown that a peptide comprised of the H11 sequence effectively competes with RXR for binding of TIF2 but not of TIF1. Finally, transactivation assays demonstrate that the defective transcriptional activity of the H11 mutant can be rescued by ectopic expression of TIF1 but not of TIF2. Taken together, the results indicate that H11 is directly involved in stabilizing the interactions of RXR with p160 coactivators, but is not required for association with TIF1. This region is thus a novel coactivator interaction surface which selectively mediates the association of RXR with transcriptional coactivators.

Regulated assembly of transcription factors and control of transcription initiation

Proteins that function in regulation of transcription initiation are typically homo or hetero-oligomeric. Results of recent biophysical studies of transcription regulators indicate that the assembly of these proteins is often subject to regulation. This regulation of assembly dictates the frequency of transcription initiation via its influence on the affinity of a transcription regulator for DNA and its affect on target site selection. Factors that modulate transcription factor assembly include binding of small molecules, post-translational modification, DNA binding and interactions with other proteins. Here, the results of recent structural and/or thermodynamic studies of a number of transcription regulators that are subject to regulated assembly are reviewed. The accumulated data indicate that this phenomenon is ubiquitous and that mechanisms utilized in eukaryotes and prokaryotes share common features.

Existence of retinoic acid-receptor-independent retinoid X-receptor-dependent pathway in myeloid cell function

We previously reported that ER-27191 (4-[4,5,7,8,9,10-hexahydro-7,7,10,10-tetramethyl-1-(3-pyridylmethyl)anthra[1,2-b]pyrrol-3-yl]benzoic acid) is a potent antagonist of retinoic acid receptor (RAR), and ER-35795 ((2E,4E,6E)-7-[1-(1-methylethyl)-8-chloro-1,2,3,4-tetrahydroquinolin-6-yl]-6-fluoro-3-methyl-2,4,6-nonatrienoic acid) is a novel retinoid X receptor (RXR)-specific agonist. By using these compounds, we investigated whether distinct RAR-dependent and RXR-dependent pathways operate to mediate the diverse activities of retinoids, particularly, the effects of the RXR pathway on cellular function. ER-27191 completely antagonized HL60 cell differentiation induced by all-trans-retinoic acid (atRA). However, the differentiation induced by the ER-35795 was not antagonized at all by the RAR antagonist, but was inhibited by an RXR homodimer antagonist (LGD100754, (2E,4E,6Z)-7-(3-n-propoxy-5,6,7,8-tetrahydro-5,5,8,8-tetramethylnaphthalen-2-yl)-3-methylocta-2,4,6-trienoic acid). Its agonistic action on RXR/RAR heterodimer, on the other hand, was neutralized by the RAR antagonist. During HL60 cell differentiation, atRA induced RARbeta mRNA, while the RXR had no effect. Interestingly, a functional RXR-pathway was also seen in lipopolysaccharide-induced inhibition of mouse splenocyte proliferation. These results strongly suggest the existence of a pharmacological RXR-dependent pathway that is activated by a ligand that can bind to RXR.

Structural basis for autorepression of retinoid X receptor by tetramer formation and the AF-2 helix

The 9-cis-retinoic acid receptors (RXRalpha, RXRbeta, and RXRgamma) are nuclear receptors that play key roles in multiple hormone-signaling pathways. Biochemical data indicate that, in the absence of ligand, RXR can exist as an inactive tetramer and that its dissociation, induced by ligand, is important for receptor activation. In this article we report the inactivated tetramer structures of the RXRalpha ligand-binding domain (LBD), either in the absence of or in the presence of a nonactivating ligand. These structures reveal that the RXR LBD tetramer forms a compact, disc-shaped complex, consisting of two symmetric dimers that are packed along helices 3 and 11. In each monomer, the AF-2 helix protrudes away from the core domain and spans into the coactivator binding site in the adjacent monomer of the symmetric dimer. In this configuration, the AF-2 helix physically excludes the binding of coactivators and suggests an autorepression mechanism that is mediated by the AF-2 helix within the tetramer. The RXR-tetramer interface is assembled from amino acids that are conserved across several closely related receptors, including the HNF4s and COUP transcription factors, and may therefore provide a model for understanding structure and regulation of this subfamily of nuclear receptors.

On the role of the carboxyl-terminal helix of RXR in the interactions of the receptor with ligand

The retinoid X receptor (RXR), a ligand-inducible transcription factor that is activated by 9-cis-retinoic acid, is a member of the superfamily of nuclear hormone receptors. The ligand-induced transcriptional activity of nuclear receptors is coordinated by their C-terminal region termed the ligand-binding domain. Structural analyses of several nuclear receptors showed that the most dramatic ligand-induced conformational change in these proteins involves a positional shift in the receptors' C-terminal helix, termed helix 12. Consequently, in the liganded state, helix 12 is folded over the entrance to the ligand-binding pocket where it serves as a lid, and it has been proposed that this region functions to stabilize ligand binding by at least some nuclear receptors. Here, to examine the possible role of helix 12 in contributing to the association of RXR with its ligand, the equilibrium and kinetic parameters of the interactions of 9-cis-retinoic acid with RXR and with a deletion mutant lacking helix 12 were measured. Deletion of the region did not significantly alter the ligand-binding affinity of RXR at equilibrium. However, both the rate of dissociation and the rate of association of the RXR-9-cis-retinoic acid complex were significantly slower in the absence of helix 12. Taken together, these observations suggest that helix 12 of RXR facilitates both the entry and the exit of the ligand from the binding pocket without affecting the equilibrium ligand-binding affinity. The results thus point at a previously unsuspected function for this region.