Receptor interacting protein 140 as a thyroid hormone-dependent, negative co-regulator for the induction of cellular retinoic acid binding protein I gene

Cellular retinoic acid binding protein 1 protects mice from high-fat diet-induced obesity by decreasing adipocyte hypertrophy

Objectives

Obesity, an emerging global health issue, involves numerous factors; understanding its underlying mechanisms for prevention and therapeutics is urgently needed. Cellular retinoic acid binding protein 1 (Crabp1) knockout (CKO) mice exhibit an obese phenotype under normal diet feedings, which prompted us to propose that Crabp1 could play a role in modulating adipose tissue development/homeostasis. Studies were designed to elucidate the underlying mechanism of Crabp1’s action in reducing obesity.

Subjects/methods

In animal studies, 6 weeks old male wild type and CKO mice were fed with normal diet (ND) or high fat diet (HFD) for 10 weeks. Body weight and food intake were regularly monitored. Glucose tolerance test and biological parameters of plasma (glucose and insulin levels) were measured after 10 weeks of ND vs. HFD feedings. Visceral adipose tissues were collected for histological and molecular analyses to determine affected signaling pathways. In cell culture studies, the 3T3L1 adipocyte differentiation model was used to examine and validate relevant signaling pathways.

Results

CKO mice, compared to WT mice, gained more body weight, exhibited more elevated fasting plasma glucose levels, and developed more severe impaired glucose tolerance under both ND and HFD. Histological examination revealed readily increased adipocyte hypertrophy and adipose tissue inflammation under HFD feedings. In 3T3L1 adipocytes, Crabp1 silencing enhanced extracellular signal-regulated kinase 1/2 (ERK1/2) activation, accompanied by elevated markers and signaling pathways of lipid accumulation and adipocyte hypertrophy.

Conclusions

This study identifies Crabp1’s physiological role against the development of obesity. The protective function of CRABP1 is likely attributed to its classically proposed (canonical) activity as a trap for RA, which will reduce RA availability, thereby dampening RA-stimulated ERK1/2 activation and adipocyte hypertrophy. The results suggest Crabp1 as a potentially new therapeutic target in managing obesity and metabolic diseases.

Thyroid Hormone Induces PGC-1α during Dendritic Outgrowth in Mouse Cerebellar Purkinje Cells

Thyroid hormone 3,3′,5-Triiodo-L-thyronine (T3) is essential for proper brain development. Perinatal loss of T3 causes severe growth defects in neurons and glia, including strong inhibition of dendrite formation in Purkinje cells in the cerebellar cortex. Here we show that T3 promotes dendritic outgrowth of Purkinje cells through induction of peroxisome proliferator-activated receptor gamma (PPARγ) co-activator 1α (PGC-1α), a master regulator of mitochondrial biogenesis. PGC-1α expression in Purkinje cells is upregulated during dendritic outgrowth in normal mice, while it is significantly retarded in hypothyroid mice or in cultures depleted of T3. In cultured Purkinje cells, PGC-1α knockdown or molecular perturbation of PGC-1α signaling inhibits enhanced dendritic outgrowth and mitochondrial generation and activation caused by T3 treatment. In contrast, PGC-1α overexpression promotes dendrite extension even in the absence of T3. PGC-1α knockdown also downregulates dendrite formation in Purkinje cells in vivo. Our findings suggest that the growth-promoting activity of T3 is partly mediated by PGC-1α signaling in developing Purkinje cells.

Coordinated repressive chromatin-remodeling of Oct4 and Nanog genes in RA-induced differentiation of embryonic stem cells involves RIP140

Maintaining pluripotency and indefinite self-renewal of embryonic stem cells requires a tight control of the expression of several key stemness factors, particularly Nanog and Oct4 transcription factors. The mammalian SWItch/Sucrose NonFermentable (SWI/SNF) complex contains Brg1 or Brm as its core subunit, along with Brg1-associated factors. Our previous studies have addressed chromatin-remodeling of the Oct4 gene locus in retinoic acid (RA)-treated embryonal carcinoma cell line P19, which involves receptor-interacting protein 140 (RIP140) for heterochromatinization on the proximal promoter region of this gene locus. However, the mechanism of RIP140 action in RA-triggered repressive chromatin-remodeling is unclear. The current study examines RA repression of the Nanog gene and compares the results with RA repression of the Oct4 gene on the chromatin level. The results show a loose nucleosome array on the Nanog gene promoter in undifferentiated embryonic stem cells. On RA treatment, the Nanog gene locus remodels specifically in the CR1 region of its proximal promoter, with the insertion of a nucleosome and compaction of this region. Further, RA induces coordinated chromatin-remodeling of both Nanog and Oct4 gene loci, which requires RA receptor-α, RIP140 and Brm. Finally, in these RA-triggered repressive chromatin-remodeling processes, lysine acetylation of RIP140 is critical for its recruiting Brm.

Class IIa HDACs: from important roles in differentiation to possible implications in tumourigenesis

Histone deacetylases (HDACs) are important regulators of gene expression. Specific structural features and distinct regulative mechanisms rationalize the separation of the 18 different human HDACs into four classes. The class II comprises a heterogeneous group of nuclear and cytosolic HDACs involved in the regulation of several cellular functions, not just limited to transcriptional repression. In particular, HDAC4, 5, 7 and 9 belong to the subclass IIa and share many transcriptional partners, including members of the MEF2 family. Genetic studies in mice have disclosed the fundamental contribution of class IIa HDACs to specific developmental/differentiation pathways. In this review, we discuss about the recent literature, which hints a role of class IIa HDACs in the development, growth and aggressiveness of cancer cells.

Chromatin remodeling and epigenetic regulation of the CrabpI gene in adipocyte differentiation

Retinoic acid (RA) acts by binding to nuclear RA receptors (RARs) to regulate a broad spectrum of downstream target genes in most cell types examined. In cytoplasm, RA binds specifically to cellular retinoic acid binding proteins I (CRABPI), and II. Although the function of CRABPI in animals remains the subject of debate, it is believed that CRABPI binding facilitates RA metabolism, thereby modulating the concentration of RA and the type of RA metabolites in cells. The basal promoter of the CrabpI gene is a housekeeping promoter that can be regulated by thyroid hormones (T3), DNA methylation, sphinganine, and ethanol acting on its upstream regulatory region. T3 regulation of CrabpI is mediated by the binding of thyroid hormone receptor (TR) to a TR response element (TRE) approximately 1 kb upstream of the basal promoter. Specifically, in the adipocyte differentiation process, T3 regulation is bimodal and closely associated with the cellular differentiation status: T3 activates CrabpI in predifferentiated cells (e.g., mesenchymal precursors or fibroblasts), but suppresses this gene once cells are committed to adipocyte differentiation. These disparate effects are functions of T3-triggered differential recruitment of coregulatory complexes in conjunction with chromatin looping/folding that alters the configuration of this genomic locus along adipocyte differentiation. Subsequent sliding, disassembly and reassembly of nucleosomes occur, resulting in specific changes in the conformation of the basal promoter chromatin at different stages of differentiation. This chapter summarizes studies illustrating the epigenetic regulation of CrabpI expression during adipocyte differentiation. Understanding the pathways regulating CrabpI in this specific context might help to illuminate the physiological role of CRABPI in vivo. This article is part of a special issue entitled: Retinoid and Lipid Metabolism.

Ligand-dependent corepressor acts as a novel corepressor of thyroid hormone receptor and represses hepatic lipogenesis in mice

BACKGROUND & AIMS

Transcriptional co-regulators assist nuclear receptors to control the transcription and maintain the metabolic homeostasis. Ligand-dependent corepressor (LCOR) was reported to function as a transcriptional corepressor in vitro. We found LCOR expression decreased in fatty livers of leptin-deficient (ob/ob) mice, diet-induced obese mice, as well as patients, suggesting LCOR may play a role in lipid homeostasis. We sought to investigate the physiological role of LCOR in vivo and elucidate the underlining molecular mechanisms.

METHODS

The effect of LCOR on hepatic lipid accumulation and thyroid hormone receptor (TR) mediated expression of lipogenic genes was studied in vitro and in vivo.

RESULTS

Ectopic expression of LCOR via intravenous infection with LCOR adenovirus decreased the hepatic triglyceride level in wild type, ob/ob, and diet-induced obese mice. Interestingly, overexpression of LCOR repressed the thyroid hormone induced expression of lipogenic genes and non-lipogenic genes, and ameliorated hepatic steatosis in obese mice, suggesting that LCOR might regulate lipogenesis as a novel TR corepressor. Furthermore, our study revealed that LCOR could interact with TRβ1 in the presence of the ligand, which resulted in competitive binding and reduced recruitment of steroid receptor coactivator-1/3 (SRC-1/3) to the promoter region of TR target genes.

CONCLUSIONS

Our data suggest that LCOR is likely to suppress TRβ1-mediated hepatic lipogenesis by decreasing binding and recruitment of SRCs to TRβ1. Our study reveals the physiological function of hepatic LCOR in lipid metabolism and the mechanism by which LCOR regulates lipogenesis. Hepatic LCOR may be a potential target for treating hepatic steatosis.

Gene repressive activity of RIP140 through direct interaction with CDK8

Receptor interacting protein 140 (RIP140) is a coregulator for numerous nuclear receptors and transcription factors and primarily exerts gene-repressive activities on various target genes. We previously identified a spectrum of posttranslational modifications on RIP140 that augment its property and biological activity. In T(3)-triggered biphasic regulation of cellular retinoic acid binding protein 1 (Crabp1) gene along the course of fibroblast-adipocyte differentiation, we found TRAP220(MED1) critical for T(3)-activated chromatin remodeling whereas RIP140 essential for T(3)-repressive chromatin remodeling of this gene promoter. In this current study, we aim to examine whether and how RIP140 replaces TRAP220(MED1) on the CrabpI promoter in differentiating adipocyte cultures. We find increasing recruitment of RIP140 to this promoter, with corresponding reduction in TRAP220(MED1) recruitment during the T(3)-repressive phase. We also uncover direct interaction of RIP140 with cyclin-dependent kinase (CDK)8 through the amino terminus of RIP140, which is stimulated by lysine acetylation on RIP140. We further validate the biological activity of lysine acetylation-mimetic RIP140, which elicits a stronger repressive effect and more efficiently recruits CDK8 and confirm CDK8's function in recruiting repressive components, such as G9a, to the RIP140 complex on this promoter. This underlies the T(3)-triggered repression of CrabpI gene. This study illustrates a new gene-repressive mechanism of RIP140 that can affect the transcription machinery by directly interacting with CDK8.

Coordination of mitochondrial biogenesis by thyroid hormone

Thyroid hormone (TH) has profound influence on metabolism that is closely linked to its effect on mitochondrial biogenesis and function. After a single injection of TH into mammals, physiological alterations (e.g. changes in oxygen consumption rates) are detectable after a lag period of ∼48h. This characteristic lag period is somewhat surprising since non-genomic responses are already detectable within minutes, and first genomic responses within some hours after administration of TH. This review provides a model to explain the characteristic lag period: TH regulates a first series of TH target genes via classical activation of gene expression by binding to thyroid hormone response elements. Some directly regulated target genes serve as intermediate factors and subsequently regulate a second series of indirect TH target genes. Intermediate factors are transcription factors (such as NRF-1, NRF-2 and PPARγ) and transcriptional coactivators (such as PGC-1α and PGC-1β). In concert with several post-translational modifications, these intermediate factors orchestrate the physiological response to thyroid hormone in vivo.

Molecular aspects of thyroid hormone actions

Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T(3) via transcriptional regulation. Two TR genes, alpha and beta, encode four T(3)-binding receptor isoforms (alpha1, beta1, beta2, and beta3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T(3), transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T(3) target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T(3)-dependent manner. In the absence of T(3), corepressors act to repress the basal transcriptional activity, whereas in the presence of T(3), coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRbeta gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T(3). Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism. Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin alphavbeta3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na(+)/H(+) exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T(4) is an agonist at the plasma membrane without conversion to T(3). Tetraiodothyroacetic acid is a T(4) analog that inhibits the actions of T(4) and T(3) at the integrin, including angiogenesis and tumor cell proliferation. T(3) can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alphavbeta3. Downstream consequences of phosphatidylinositol 3-kinase activation by T(3) include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase. Thyroid hormone, chiefly T(3) and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T(4) and rT(3), but not T(3), is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth. A truncated TRalpha1 isoform that resides in the extranuclear compartment mediates the action of thyroid hormone on the cytoskeleton.

RIP140 in thyroid hormone-repression and chromatin remodeling of Crabp1 gene during adipocyte differentiation

Cellular retinoic acid binding protein 1 (Crabp1) gene is biphasically (proliferation versus differentiation) regulated by thyroid hormone (T3) in 3T3-L1 cells. This study examines T3-repression of Crabp1 gene during adipocyte differentiation. T3 repression of Crabp1 requires receptor interacting protein 140 (RIP140). During differentiation, the juxtaposed chromatin configuration of Crabp1 promoter with its upstream region is maintained, but the 6-nucleosomes spanning thyroid hormone response element to transcription initiation site slide bi-directionally, with the third nucleosome remaining at the same position throughout differentiation. On the basal promoter, RIP140 replaces coactivators GRIP1 and PCAF and forms a repressive complex with CtBP1, HDAC3 and G9a. Initially active chromatin marks on this promoter, histone modifications H3-Ac and H3K4-me3, are weakened whereas repressive chromatin marks, H3K9-me3 and H3K27-me3 modification and recruitment of G9a, HP1α, HP1γ and H1, are intensified. This is the first study to examine chromatin remodeling, during the phase of hormone repression, of a bi-directionally regulated hormone target gene, and provides evidence for a functional role of RIP140 in chromatin remodeling to repress hormone target gene expression.

SUMOylation modulates the transcription repressor function of RIP140

RIP140/NRIP1 (receptor-interacting protein 140) functions as a corepressor of nuclear receptors. It plays an important role in the transcriptional control of energy metabolism and female fertility. RIP140 contains four distinct repression domains (RD1-RD4), and the repressive activity of RIP140 involves complex mechanisms. The function of both RD1 and RD2 is linked to recruitment of histone deacetylases and C-terminal binding protein, respectively, but the mechanism of repression for RD3 and RD4 has remained elusive. Because covalent modification by small ubiquitin-like modifiers (SUMO-1, -2, and -3; SUMOylation) is often associated with transcriptional repression, we studied whether SUMOylation is involved in the repressive activity of RIP140. We show that two conserved lysines, Lys(756) and Lys(1154), located in RD3 and RD4, respectively, are subject to reversible SUMOylation, with SUMO-1 being more efficiently conjugated than SUMO-2. Interestingly, mutations of the RIP140 SUMOylation sites compromised the transcription repressor function of RIP140 and blunted its capacity to repress estrogen receptor alpha-dependent transcription. Conjugation of SUMO-1 also influenced the subnuclear distribution pattern of RIP140. In sum, our demonstration that the function of RIP140 repression domains 3 and 4 can be modulated by reversible SUMO modification thus adds a novel level to the regulation of RIP140 activity, which may have ramifications in the control of gene networks exerted by RIP140.

Orphan nuclear receptor TR2, a mediator of preadipocyte proliferation, is differentially regulated by RA through exchange of coactivator PCAF with corepressor RIP140 on a platform molecule GRIP1

Orphan nuclear receptor TR2 is a preadipocyte proliferator. Knockdown of TR2 in 3T3-L1 preadipocytes reduced their proliferation efficiency, whereas specific elevation of TR2 in these cells facilitated their proliferation. All-trans retinoic acid (RA) stimulates cellular proliferation in 3T3-L1 preadipocytes by activating TR2 through an IR0-type RA response element, which further activates c-Myc expression. In post-differentiated adipocytes, RA becomes a repressive signal for TR2 and rapidly down-regulates its expression. The biphasic effect of RA on TR2 expression in 3T3-L1 is mediated by differential RA-dependent coregulator recruitment to the receptor/Glucocorticoid Receptor-Interacting Protein 1 (GRIP1) complex that binds IR0 on the TR2 promoter. RA induces the recruitment of histone acetyl transferase-containing/GRIP1/p300/CBP-associated factor (PCAF) complex to the TR2 promoter in undifferentiated cells, whereas it triggers recruitment of histone deacetylase-containing/GRIP1/receptor-interacting protein 140 (RIP140) complex in differentiated cells. GRIP1 directly interacts with RIP140 through its carboxyl terminal AD2 domain. GRIP1 interacts with PCAF and RIP140 directly and differentially, functioning as a platform molecule to mediate differential RA-induced coregulator recruitment to TR2 promoter target. This results in a biphasic effect of RA on the expression of TR2 in undifferentiated and differentiated cells, which is required for RA-stimulated preadipocyte proliferation.

The nuclear receptor co-repressor RIP140 controls the expression of metabolic gene networks

NRs (nuclear receptors) regulate the expression of specific gene networks in target cells by recruiting cofactor complexes involved in chromatin remodelling and in the assembly of transcription complexes. The importance of activating gene expression, in metabolic tissues, is well established, but the contribution of transcriptional inhibition is less well defined. In this review, we highlight a crucial role for RIP140 (receptor-interacting protein 140), a transcriptional co-repressor for NR, in the regulation of metabolic gene expression. Many genes involved in lipid and carbohydrate metabolism are repressed by RIP140 in adipose and muscle. The repressive function of RIP140 results from its ability to bridge NRs to repressive enzyme complexes that modify DNA and histones. In the absence of RIP140, expression from many metabolic genes is increased so that mice exhibit a lean phenotype and resistance to high-fat-diet-induced obesity and display increased glucose tolerance and insulin sensitivity. We propose that a functional interplay between transcriptional activators and the co-repressor RIP140 is an essential process in metabolic regulation.

Metabolic regulation by the nuclear receptor corepressor RIP140

Whereas the importance of activating gene expression in metabolic pathways to control energy homeostasis is well established, the contribution of transcriptional inhibition is less well defined. In this review we highlight a crucial role of RIP140, a transcriptional corepressor for nuclear receptors, in the regulation of energy expenditure. Mice devoid of the RIP140 gene are lean, exhibit resistance to high-fat-diet-induced obesity, and have increased glucose tolerance and insulin sensitivity. Consistent with these observations, RIP140 suppresses the expression of gene clusters that are involved in lipid and carbohydrate metabolism, including fatty acid oxidation, oxidative phosphorylation and mitochondrial uncoupling. Therefore, the functional interplay between transcriptional activators and the corepressor RIP140 is an essential process in metabolic regulation.

Coregulator Interactions with the Thyroid Hormone Receptor

The thyroid hormone receptor (TR) directly regulates the transcription of thyroid hormone-responsive genes in response to changing levels of thyroid hormone. Mechanistically TR utilizes a complex set of binding interactions, with hormone, response elements, and coregulatory proteins, to provide specific local control of patterns of transcriptional response that are partially responsible for inducing the tissue-selective responses to the circulating hormone. One of the apparently dominant phenomena in the regulation of thyroid hormone responses is the protein interactions between TR and its coregulators. This review summarizes the current state of knowledge with respect to the identity of these coregulators, their interaction with TR, and the consequences of those interactions.