Identification and characterization of a novel corepressor interaction region in RVR and Rev-erbA alpha

Diabetes and Exposure to Environmental Lead (Pb)

Although the increased incidence of type 2 diabetes since the 1950s is thought to be primarily due to coincident alterations in lifestyle factors, another potential contributing factor in industrialized countries is exposure of the population to environmental pollutants and industrial chemicals. Exposure levels of many environmental toxicants have risen in the same time-frame as the disease incidence. Of particular interest in this regard is the metal lead. Although overall lead exposure levels have diminished in recent decades, there is an under-recognized but persistent occurrence of lead exposure in poor underserved urban populations. Although the neural developmental pathologies induced by lead exposures have been well documented, very little is known about the effect of lead exposure on the incidence of chronic metabolic diseases such as type 2 diabetes. Although our understanding of the metabolic health effects of lead exposure is incomplete, there are studies in model systems and a small amount of epidemiological data that together suggest a deleterious effect of environmental lead exposure on metabolic health. This article reviews the human, animal and in vitro studies that have examined the effects of lead exposure on the development of diabetes and related metabolic conditions.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

SMRTε, a corepressor variant, interacts with a restricted subset of nuclear receptors, including the retinoic acid receptors α and β

The SMRT and NCoR corepressors bind to, and mediate transcriptional repression by, many nuclear receptors. Both SMRT and NCoR are expressed by alternative mRNA splicing, generating a series of structurally and functionally distinct corepressor "variants". We report that a splice variant of SMRT, SMRTε, recognizes a restricted subset of nuclear receptors. Unlike the other corepressor variants characterized, SMRTε possesses only a single receptor interaction domain (RID) and exhibits an unusual specificity for a subset of nuclear receptors that includes the retinoic acid receptors (RARs). The ability of the single RID in SMRTε to efficiently interact with RARs appears to be enhanced by a recently recognized β-strand/β-strand interaction between corepressor and receptor. We suggest that alternative mRNA splicing of corepressors can restrict their function to specific nuclear receptor partnerships, and we propose that this may serve to customize the transcriptional repression properties of different cell types for different biological purposes.

Alternative mRNA splicing of corepressors generates variants that play opposing roles in adipocyte differentiation

The SMRT and NCoR corepressors partner with, and help mediate repression by, a wide variety of nuclear receptors and non-receptor transcription factors. Both SMRT and NCoR are expressed by alternative mRNA splicing, resulting in the production of a series of interrelated corepressor variants that differ in their tissue distribution and in their biochemical properties. We report here that different corepressor splice variants can exert opposing transcriptional and biological effects during adipocyte differentiation. Most notably, the NCoRω splice variant inhibits, whereas the NCoRδ splice variant promotes, adipogenesis. Furthermore, the ratio of NCoRω to NCoRδ decreases during adipogenic differentiation. We propose that this alteration in corepressor splicing helps convert the cellular transcriptional program from one that maintains the pre-adipocyte in an undifferentiated state to a new transcriptional context that promotes differentiation and helps establish the proper physiology of the mature adipocyte.

The structural basis of gas-responsive transcription by the human nuclear hormone receptor REV-ERBbeta

Heme is a ligand for the human nuclear receptors (NR) REV-ERBalpha and REV-ERBbeta, which are transcriptional repressors that play important roles in circadian rhythm, lipid and glucose metabolism, and diseases such as diabetes, atherosclerosis, inflammation, and cancer. Here we show that transcription repression mediated by heme-bound REV-ERBs is reversed by the addition of nitric oxide (NO), and that the heme and NO effects are mediated by the C-terminal ligand-binding domain (LBD). A 1.9 A crystal structure of the REV-ERBbeta LBD, in complex with the oxidized Fe(III) form of heme, shows that heme binds in a prototypical NR ligand-binding pocket, where the heme iron is coordinately bound by histidine 568 and cysteine 384. Under reducing conditions, spectroscopic studies of the heme-REV-ERBbeta complex reveal that the Fe(II) form of the LBD transitions between penta-coordinated and hexa-coordinated structural states, neither of which possess the Cys384 bond observed in the oxidized state. In addition, the Fe(II) LBD is also able to bind either NO or CO, revealing a total of at least six structural states of the protein. The binding of known co-repressors is shown to be highly dependent upon these various liganded states. REV-ERBs are thus highly dynamic receptors that are responsive not only to heme, but also to redox and gas. Taken together, these findings suggest new mechanisms for the systemic coordination of molecular clocks and metabolism. They also raise the possibility for gas-based therapies for the many disorders associated with REV-ERB biological functions.

Identification of heme as the ligand for the orphan nuclear receptors REV-ERBalpha and REV-ERBbeta

The nuclear receptors REV-ERBalpha (encoded by NR1D1) and REV-ERBbeta (NR1D2) have remained orphans owing to the lack of identified physiological ligands. Here we show that heme is a physiological ligand of both receptors. Heme associates with the ligand-binding domains of the REV-ERB receptors with a 1:1 stoichiometry and enhances the thermal stability of the proteins. Results from experiments of heme depletion in mammalian cells indicate that heme binding to REV-ERB causes the recruitment of the co-repressor NCoR, leading to repression of target genes including BMAL1 (official symbol ARNTL), an essential component of the circadian oscillator. Heme extends the known types of ligands used by the human nuclear receptor family beyond the endocrine hormones and dietary lipids described so far. Our results further indicate that heme regulation of REV-ERBs may link the control of metabolism and the mammalian clock.

Expression of AFP and Rev-Erb A/Rev-Erb B and N-CoR in fetal rat liver, liver injury and liver regeneration

Background

Alpha-fetoprotein (AFP) expression can resume in the adult liver under pathophysiological conditions. Orphan nuclear receptors were supposed to regulate AFP gene expression, in vitro. We were interested to study the expression of AFP and orphan nuclear receptors, in vivo.

Results

The expression of AFP gene and orphan nuclear receptors in the liver was examined in different rat models: (a) fetal liver (b) liver regeneration [partial hepatectomy (PH) with and without 2-acetyl-aminofluren treatment (2-AAF)], (c) acute liver damage [treatment with CCl₄] and (d) acute phase reaction [treatment with turpentine oil]. After PH of 2-AAF treated rats, clusters of AFP positive cells occurred in the periportal region. In the Northern blot analysis, a positive hybridization signal for the full-length AFP-RNA was observed only in liver samples from 2-AAF treated rats after PH. In real-time PCR analysis, the full-length AFP-RNA was highly up regulated in the fetal liver (maximum at day 14: 21,500 fold); after PH of 2-AAF treated rats, the full-length AFP-RNA was also up regulated up to 400 fold (day 7 after PH). The orphan nuclear receptors were down regulated at nearly each time points in all models, also at time point of up regulation of the AFP gene.

Conclusion

Expression of "fetal" AFP could be demonstrated during liver development and during proliferation of the so-called oval cells. Changes of expression of orphan nuclear receptors, however, did not correlate with AFP expression. Other regulatory pathways were possibly involved in controlling AFP expression, in vivo.

Alternative mRNA splicing of SMRT creates functional diversity by generating corepressor isoforms with different affinities for different nuclear receptors

Many eukaryotic transcription factors are bimodal in their regulatory properties and can both repress and activate expression of their target genes. These divergent transcriptional properties are conferred through recruitment of auxiliary proteins, denoted coactivators and corepressors. Repression plays a particularly critical role in the functions of the nuclear receptors, a large family of ligand-regulated transcription factors involved in metazoan development, differentiation, reproduction, and homeostasis. The SMRT corepressor interacts directly with nuclear receptors and serves, in turn, as a platform for the assembly of a larger corepressor complex. We report here that SMRT is expressed in cells by alternative mRNA splicing to yield two distinct variants or isoforms. We designate these isoforms SMRTalpha and SMRTtau and demonstrate that these isoforms have significantly different affinities for different nuclear receptors. These isoforms are evolutionarily conserved and are expressed in a tissue-specific manner. Our results suggest that differential mRNA splicing serves to customize corepressor function in different cells, allowing the transcriptional properties of nuclear receptors to be adapted to different contexts.

Determinants of CoRNR-dependent repression complex assembly on nuclear hormone receptors

Ligand-dependent exchange of coactivators and corepressors is the fundamental regulator of nuclear hormone receptor (NHR) function. The interaction surfaces of coactivators and corepressors are similar but distinct enough to allow the ligand to function as a switch. Multiple NHRs share features that allow corepressor binding, and each of two distinct corepressors (N-CoR and SMRT) contains two similar CoRNR motifs that interact with NHRs. Here we report that the specificity of corepressor-NHR interaction is determined by the individual NHR interacting with specific CoRNR boxes within a preferred corepressor. First, receptors have distinct preferences for CoRNR1 versus CoRNR2. For example, the retinoic acid receptor binds CoRNR1, while RXR interacts almost exclusively with CoRNR2. Second, the NHR preference for N-CoR or SMRT is due to differences in CoRNR1 but not CoRNR2. Moreover, within a single corepressor, affinity for different NHRs is determined by distinct regions flanking CoRNR1. The highly specific determinants of NHR-corepressor interaction and preference suggest that repression is regulated by the permissibility of selected receptor-CoRNR-corepressor combinations. Interestingly, different NHR surfaces contribute to binding of CoRNR1 and CoRNR2, suggesting a model to explain corepressor binding to NHR heterodimers.

The corepressor N-CoR and its variants RIP13a and RIP13Delta1 directly interact with the basal transcription factors TFIIB, TAFII32 and TAFII70

Repression of transcription by the classical nuclear receptors (e.g. TR, RAR), the orphan nuclear receptors (e.g. Rev-erbAalpha/beta), Mxi-1 and Mad bHLH-zip proteins and the oncoproteins PLZF and LAZ3/BCL6 is mediated by the corepressors N-CoR and SMRT. The interaction of the corepressors with the components involved in chromatin remodelling, such as the recruiting proteins Sin3A/B and the histone deacteylases HDAc-1 and RPD3, has been analysed in detail. The N-CoR/Sin3/HDAc complexes have a key role in the regulation of cellular proliferation and differentiation. However, the interaction of these corepressors with the basal transcriptional machinery has remained obscure. In this study we demonstrated that the N-terminalrepression domains and the receptor interactiondomains (RID) of N-CoR and its splice variants, RIP13a and RIP13Delta1, directly interact with TAFII32 in vivo and in vitro . We show that interaction domain II within the N-CoR and RIP13a RID is required for the interaction with TAFII32. We also observed that N-CoR directly interacts with each of the basal factors, TFIIB and TAFII70, and can simultaneously interact with all three basal factors in a non-competitive manner. Furthermore, we provide evidence that suggests the RVR/Rev-erbbeta-corepressor complex also interacts with the general transcriptional machinery, and that the physicalassociation of TFIIB with N-CoR also occurs in the presence of Sin3B and HDAc-1. Interestingly, we observed that N-CoR expression ablated the functional interaction between TFIIB and TAFII32 that is critical to the initiation of transcription. In conclusion, this study demonstrates that the N-terminal repressor region and the C-terminal RIDs are part of the corepressor contact interface that mediates the interaction with the general transcription factors, and demonstrates that TAFs can also directly interact with corepressors to mediate signals from repressors to the basal machinery. We also suggest that N-CoR interacts with the central components of the transcriptional initiation process (TFIIB, TAFs) and locks them into a non-functional complex or conformation that is not conducive to transcription.