A cold-inducible coactivator of nuclear receptors linked to adaptive thermogenesis

Combinatorial control of gene expression by nuclear receptors and coregulators

The nuclear receptor (NR) superfamily of transcription factors regulates gene expression in response to endocrine signaling, and recruitment of coregulators affords these receptors considerable functional flexibility. We will place historical aspects of NR research in context with current opinions on their mechanism of signal transduction, and we will speculate upon future trends in the field.

A dominant-negative mutant of androgen receptor coregulator ARA54 inhibits androgen receptor-mediated prostate cancer growth

The ligand-bound androgen receptor (AR) regulates target genes via a mechanism involving coregulators such as androgen receptor-associated 54 (ARA54). We investigated whether the interruption of the AR coregulator function could lead to down-regulation of AR activity. Using in vitro mutagenesis and a yeast two-hybrid screening assay, we have isolated a mutant ARA54 (mt-ARA54) carrying a point mutation at amino acid 472 changing a glutamic acid to lysine, which acts as a dominant-negative inhibitor of AR transactivation. In transient transfection assays of prostate cancer cell lines, the mt-ARA54 suppressed endogenous mutated AR-mediated and exogenous wild-type AR-mediated transactivation in LNCaP and PC-3 cells, respectively. In DU145 cells, the mt-ARA54 suppressed exogenous ARA54 but not other coregulators, such as ARA55-enhanced or SRC-1-enhanced AR transactivation. In the LNCaP cells stably transfected with the plasmids encoding the mt-ARA54 under the doxycycline inducible system, the overexpression of the mt-ARA54 inhibited cell growth and endogenous expression of prostate-specific antigen. Mammalian two-hybrid assays further demonstrated that the mt-ARA54 can disrupt the interaction between wild-type ARA54 molecules, suggesting that ARA54 dimerization or oligomerization may play an essential role in the enhancement of AR transactivation. Together, our results demonstrate that a dominant-negative AR coregulator can suppress AR transactivation and cell proliferation in prostate cancer cells. Further studies may provide a new therapeutic approach for blocking AR-mediated prostate cancer growth.

PGC-1 functions as a transcriptional coactivator for the retinoid X receptors

Ligand-dependent gene transcription mediated by the nuclear receptors involves the recruitment of transcriptional coactivators to the ligand-binding domain (LBD), which leads to interaction with the basal transcription machinery, and ultimately with RNA polymerase II. Although most of these coactivators are ubiquitously expressed, a tissue-selective coactivator, PGC-1, has recently been characterized. Because PGC-1 and the retinoid X receptors (RXRs) possess an overlapping tissue distribution, we investigated whether PGC-1 is a coactivator for the retinoid X receptors. In a transient transfection assay, PGC-1 augments ligand-stimulated RXR transcription. Furthermore, PGC-1 efficiently enhances the RXR element-driven reporter gene transcription by all three RXR isoforms. An immunoprecipitation assay reveals that PGC-1 and RXRalpha interact in vivo. In addition, a glutathione S-transferase pull-down assay showed that this interaction requires the presence of the LXXLL motif of PGC-1. We demonstrate further, in a mammalian two-hybrid assay, that this physical interaction also requires the presence of the AF-2 region of RXR to interact with the LXXLL motif of PGC-1, which is consistent with our protein-protein interaction results. A time-resolved fluorescence assay shows that a peptide within the NR box of PGC-1 is efficiently recruited by a ligand-bound RXRalpha in vitro. Finally, PGC-1 and TIF2 synergistically enhance ligand-activated RXRalpha transcriptional activity. Taken together, these results indicate that PGC-1 is a bona fide coactivator for RXRalpha.

Cold elicits the simultaneous induction of fatty acid synthesis and beta-oxidation in murine brown adipose tissue: prediction from differential gene expression and confirmation in vivo

A survey of genes differentially expressed in the brown adipose tissue (BAT) of mice exposed to a range of environmental temperatures was carried out to identify novel genes and pathways associated with the transition of this tissue toward an amplified thermogenic state. The current report focuses on an analysis of the expression patterns of 50 metabolic genes in BAT under control conditions (22 degrees C), cold exposure (4 degrees C, 1 to 48 h), warm acclimation (33 degrees C, 3 wk), or food restriction/meal feeding (animals fed the same amount as warm mice). In general, expression of genes encoding proteins involving glucose uptake and catabolism was significantly elevated in the BAT of cold-exposed mice. The levels of mRNAs encoding proteins critical to de novo lipogenesis were also increased. Gene expression for enzymes associated with procurement and combustion of long chain fatty acids (LCFAs) was increased in the cold. Thus, a model was proposed in which coordinated activation of glucose uptake, fatty acid synthesis, and fatty acid combustion occurs as part of the adaptive thermogenic processes in BAT. Confirmation emerged from in vivo assessments of cold-induced changes in BAT 2-deoxyglucose uptake (increased 2.7-fold), BAT lipogenesis (2.8-fold higher), and incorporation of LCFA carboxyl-carbon into BAT water-soluble metabolites (elevated approximately twofold). It is proposed that temperature-sensitive regulation of distinct intracellular malonyl-CoA pool sizes plays an important role in driving this unique metabolic profile via maintenance of the lipogenic pool but diminution of the carnitine palmitoyltransferase 1 inhibitory pool under cold conditions.

Peroxisome proliferator-activated receptor gamma coactivator 1beta (PGC-1beta ), a novel PGC-1-related transcription coactivator associated with host cell factor

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) plays a critical role in regulating multiple aspects of energy metabolism, including adaptive thermogenesis, mitochondrial biogenesis, and fatty acid beta-oxidation. Recently, this coactivator of nuclear receptors/transcription factors has been shown to control hepatic gluconeogenesis, an important component of the pathogenesis of both type-1 and type-2 diabetes. We described here the cloning of a novel bona fide homologue of PGC-1, PGC-1beta (PGC-1 was renamed as PGC-1alpha), first identified through searches of new data base entries. Despite the fact that PGC-1alpha and -1beta share similar tissue distributions with highest levels of expression in brown fat and heart, their mRNAs are differentially regulated in the brown adipose tissue upon cold exposure and during brown fat cell differentiation. Like PGC-1alpha, PGC-1beta mRNA levels are increased significantly in the liver during fasting, suggesting a possible role for this factor in the regulation of hepatic gluconeogenesis and/or fatty acid oxidation. Consistent with this, PGC-1beta was shown to physically interact and potently coactivate hepatic nuclear factor 4 and peroxisome proliferator-activated receptor alpha, nuclear receptors that are essential for hepatic adaptation to fasting. Finally, using sequence comparisons between PGC-1alpha and -1beta, we have identified a conserved amino acid motif that serves as a docking site for host cell factor, a cellular protein implicated in cell cycle regulation and viral infection. HCF is shown to bind to both PGC-1alpha and -1beta and augment their transcriptional activity.

Small heterodimer partner, an orphan nuclear receptor, augments peroxisome proliferator-activated receptor gamma transactivation

Small heterodimer partner (SHP, NR0B2) is an atypical orphan nuclear receptor that inhibits transcriptional activation by several other nuclear receptors. We recently reported that mutations in the SHP gene are associated with insulin resistance. In the present study, we demonstrated that the SHP gene is expressed in adipose tissues. A reporter gene assay showed that a gene product of SHP increased the transcriptional activation of peroxisome proliferator-activated receptor (PPAR) gamma. SHP-mediated activation of PPARgamma was observed both in the presence and absence of the ligand of PPARgamma. Immunoprecipitation and glutathione S-transferase pull-down assay showed that SHP directly bound to PPARgamma and competed with nuclear receptor corepressor for binding to PPARgamma. Serial deletion studies indicated that the C terminus of SHP is important for PPARgamma activation. Mutant SHP proteins, which are found in naturally occurring mutation, showed less enhancing activity for PPARgamma than wild-type SHP. Our results suggest that SHP may act as an endogenous enhancer of PPARgamma.

Molecular cloning and characterization of CAPER, a novel coactivator of activating protein-1 and estrogen receptors

Transcriptional coactivators either bridge transcription factors and the components of the basal transcription apparatus and/or remodel the chromatin structures. We isolated a novel nuclear protein based on its interaction with the recently described general coactivator activating signal cointegrator-2 (ASC-2). This protein CAPER (for coactivator of activating protein-1 (AP-1) and estrogen receptors (ERs)) selectively bound, among the many transcription factors we tested, the AP-1 component c-Jun and the estradiol-bound ligand binding domains of ERalpha and ERbeta. Interestingly, CAPER exhibited a cryptic autonomous transactivation function that becomes activated only in the presence of estradiol-bound ER. In cotransfections, CAPER stimulated transactivation by ERalpha, ERbeta, and AP-1. Thus, CAPER may represent a more selective transcriptional coactivator molecule that plays a pivotal role for the function of AP-1 and ERs in vivo in conjunction with the general coactivator ASC-2.

Photoperiodic regulation of gene expression in brown and white adipose tissue of Siberian hamsters (Phodopus sungorus)

Siberian hamsters exhibit seasonal fluctuations in white adipose tissue (WAT) mass, with peaks in long "summerlike" days (LDs) and nadirs in short "winterlike" days (SDs). These responses can be mimicked in the laboratory after transfer from LDs to SDs. The purpose of the present study was to test whether changes in WAT and brown adipose tissue (BAT) gene expression that are mediated by the sympathetic nervous system in other obesity models are also associated with seasonal adiposity changes in Siberian hamsters. SDs decreased WAT mass and leptin mRNA, increased WAT beta(3)-adrenoceptor mRNA, and induced retroperitoneal WAT uncoupling protein-1 mRNA (the latter measured by RT-PCR, others measured by ribonuclease protection assay) while increasing BAT uncoupling protein-1 and peroxisome proliferator-activated receptor-gamma coactivator-1 mRNAs. These effects were not due to SD-induced gonadal regression and largely occurred before the usual SD-induced decreases in food intake. Thus the SD-induced decreased adiposity of Siberian hamsters may be due to a coordinated suite of WAT and BAT gene transcription changes ultimately increasing lipid mobilization and utilization.

Ser-884 adjacent to the LXXLL motif of coactivator TRBP defines selectivity for ERs and TRs

Ligand-dependent interaction of nuclear receptors and coactivators is a critical step in nuclear receptor-mediated transcriptional regulation. TR-binding protein (TRBP) interacts with nuclear receptors through a single LXXLL motif. Evidence suggested that the sequences flanking the LXXLL motif in a number of coactivators determine receptor selectivity. We performed mutagenesis studies at residues adjacent to the TRBP LXXLL motif and identified S884 of TRBP at the -3 position of the LXXLL motif as a key residue for receptor selectivity. Analysis of in vitro and in vivo receptor interactions with TRBP suggested that S884 allowed selective interactions for ERbeta, TR, and RXR vs. ERalpha. Transient transfection studies further confirmed that the LXXLL-binding affinity correlates with TRBP transcriptional activity. Consistent with the structural modeling, an E380G substitution within ERalpha altered the binding to TRBP mutants, demonstrating the direct contact between TRBP S884 and ERalpha E380, which is a residue that distinguishes receptor subclasses. Furthermore, S884 can be phosphorylated by MAPK in vitro, an event that significantly altered the binding of TRBP to ER and suggests a potential mechanism for regulatory interaction. As the differential recruitment of TRBP to ERalpha and ERbeta may rely on S884, our finding provides insight into estrogen signaling and may lead to the development of therapeutic receptor-selective peptide antagonists.

Mammalian transcription factors in yeast: strangers in a familiar land

Many transcription factors in human cells have functional orthologues in yeast, and a common experimental theme has been to define the function of the yeast protein and then test whether the mammalian version behaves similarly. Although, at first glance, this approach does not seem feasible for factors that do not have yeast counterparts, mammalian transcriptional activators or repressors can be expressed directly in yeast. Often, the mammalian factor retains function in yeast, and this allows investigators to exploit the experimental tractability of yeast to ask a diverse set of questions.

Identification and characterization of a tissue-specific coactivator, GT198, that interacts with the DNA-binding domains of nuclear receptors

Gene activation mediated by nuclear receptors is regulated in a tissue-specific manner and requires interactions between nuclear receptors and their cofactors. Here, we identified and characterized a tissue-specific coactivator, GT198, that interacts with the DNA-binding domains of nuclear receptors. GT198 was originally described as a genomic transcript that mapped to the human breast cancer susceptibility locus 17q12-q21 with unknown function. We show that GT198 exhibits a tissue-specific expression pattern in which its mRNA is elevated in testis, spleen, thymus, pituitary cells, and several cancer cell lines. GT198 is a 217-amino-acid nuclear protein that contains a leucine zipper required for its dimerization. In vitro binding and yeast two-hybrid assays indicated that GT198 interacted with nuclear receptors through their DNA-binding domains. GT198 potently stimulated transcription mediated by estrogen receptor alpha and beta, thyroid hormone receptor beta1, androgen receptor, glucocorticoid receptor, and progesterone receptor. However, the action of GT198 was distinguishable from that of the ligand-binding domain-interacting nuclear receptor coactivators, such as TRBP, CBP, and SRC-1, with respect to basal activation and hormone sensitivity. Furthermore, protein kinase A, protein kinase C, and mitogen-activated protein kinase can phosphorylate GT198 in vitro, and cotransfection of these kinases regulated the transcriptional activity of GT198. These data suggest that GT198 is a tissue-specific, kinase-regulated nuclear receptor coactivator that interacts with the DNA-binding domains of nuclear receptors.

Enhancement of the aquaporin adipose gene expression by a peroxisome proliferator-activated receptor gamma

The current study demonstrates that aquaporin adipose (AQPap), an adipose-specific glycerol channel (Kishida, K., Kuriyama, H., Funahashi, T., Shimomura, I., Kihara, S., Ouchi, N., Nishida, M., Nishizawa, H., Matsuda, M., Takahashi, M., Hotta, K., Nakamura, T., Yamashita, S., Tochino, Y., and Matsuzawa, Y. (2000) J. Biol. Chem. 275, 20896-20902), is a target gene of peroxisome proliferator-activated receptor (PPAR) gamma. The AQPap mRNA amounts increased following the induction of PPARgamma in the differentiation of 3T3-L1 adipocytes. The AQPap mRNA in the adipose tissue increased when mice were treated with pioglitazone (PGZ), a synthetic PPARgamma ligand, and decreased in PPARgamma(+/-) heterozygous knockout mice. In 3T3-L1 adipocytes, PGZ augmented the AQPap mRNA expression and its promoter activity. Serial deletion of the promoter revealed the putative peroxisome proliferator-activated receptor response element (PPRE) at -93/-77. In 3T3-L1 preadipocytes, the expression of PPARgamma by transfection and PGZ activated the luciferase activity of the promoter containing the PPRE, whereas the PPRE-deleted mutant was not affected. The gel mobility shift assay showed the direct binding of PPARgamma-retinoid X receptor alpha complex to the PPRE. DeltaPPARgamma, which we generated as the dominant negative PPARgamma lacking the activation function-2 domain, suppressed the promoter activity in 3T3-L1 cells, dose-dependently. We conclude that AQPap is a novel adipose-specific target gene of PPARgamma through the binding of PPARgamma-retinoid X receptor complex to the PPRE region in its promoter.

Peroxisomal beta-oxidation and peroxisome proliferator-activated receptor alpha: an adaptive metabolic system

beta-Oxidation occurs in both mitochondria and peroxisomes. Mitochondria catalyze the beta-oxidation of the bulk of short-, medium-, and long-chain fatty acids derived from diet, and this pathway constitutes the major process by which fatty acids are oxidized to generate energy. Peroxisomes are involved in the beta-oxidation chain shortening of long-chain and very-long-chain fatty acyl-coenzyme (CoAs), long-chain dicarboxylyl-CoAs, the CoA esters of eicosanoids, 2-methyl-branched fatty acyl-CoAs, and the CoA esters of the bile acid intermediates di- and trihydroxycoprostanoic acids, and in the process they generate H2O2. Long-chain and very-long-chain fatty acids (VLCFAs) are also metabolized by the cytochrome P450 CYP4A omega-oxidation system to dicarboxylic acids that serve as substrates for peroxisomal beta-oxidation. The peroxisomal beta-oxidation system consists of (a) a classical peroxisome proliferator-inducible pathway capable of catalyzing straight-chain acyl-CoAs by fatty acyl-CoA oxidase, L-bifunctional protein, and thiolase, and (b) a second noninducible pathway catalyzing the oxidation of 2-methyl-branched fatty acyl-CoAs by branched-chain acyl-CoA oxidase (pristanoyl-CoA oxidase/trihydroxycoprostanoyl-CoA oxidase), D-bifunctional protein, and sterol carrier protein (SCP)x. The genes encoding the classical beta-oxidation pathway in liver are transcriptionally regulated by peroxisome proliferator-activated receptor alpha (PPAR alpha). Evidence derived from mice deficient in PPAR alpha, peroxisomal fatty acyl-CoA oxidase, and some of the other enzymes of the two peroxisomal beta-oxidation pathways points to the critical importance of PPAR alpha and of the classical peroxisomal fatty acyl-CoA oxidase in energy metabolism, and in the development of hepatic steatosis, steatohepatitis, and liver cancer.

BFIT, a unique acyl-CoA thioesterase induced in thermogenic brown adipose tissue: cloning, organization of the human gene and assessment of a potential link to obesity

We hypothesized that certain proteins encoded by temperature-responsive genes in brown adipose tissue (BAT) contribute to the remarkable metabolic shifts observed in this tissue, thus prompting a differential mRNA expression analysis to identify candidates involved in this process in mouse BAT. An mRNA species corresponding to a novel partial-length gene was found to be induced 2-3-fold above the control following cold exposure (4 degrees C), and repressed approximately 70% by warm acclimation (33 degrees C, 3 weeks) compared with controls (22 degrees C). The gene displayed robust BAT expression (i.e. approximately 7-100-fold higher than other tissues in controls). The full-length murine gene encodes a 594 amino acid ( approximately 67 kDa) open reading frame with significant homology to the human hypothetical acyl-CoA thioesterase KIAA0707. Based on cold-inducibility of the gene and the presence of two acyl-CoA thioesterase domains, we termed the protein brown-fat-inducible thioesterase (BFIT). Subsequent analyses and cloning efforts revealed the presence of a novel splice variant in humans (termed hBFIT2), encoding the orthologue to the murine BAT gene. BFIT was mapped to syntenic regions of chromosomes 1 (human) and 4 (mouse) associated with body fatness and diet-induced obesity, potentially linking a deficit of BFIT activity with exacerbation of these traits. Consistent with this notion, BFIT mRNA was significantly higher ( approximately 1.6-2-fold) in the BAT of obesity-resistant compared with obesity-prone mice fed a high-fat diet, and was 2.5-fold higher in controls compared with ob/ob mice. Its strong, cold-inducible BAT expression in mice suggests that BFIT supports the transition of this tissue towards increased metabolic activity, probably through alteration of intracellular fatty acyl-CoA concentration.

Two thyroid hormone-mediated gene expression patterns in vivo identified by cDNA expression arrays in rat

Thyroid hormone (T3) is essential for normal development, differentiation and metabolic balance. Only a limited number of T3-target genes have been identified so far and their complex regulation pattern is poorly understood. We performed cDNA expression array hybridisation to identify T3-regulated genes and to investigate their expression pattern after various time points in vivo. Radioactively labelled cDNA was prepared from hepatic RNA of hypothyroid and hyperthyroid rats 6, 24 and 48 h after the administration of T3. Labelled cDNA probes were hybridised to rat Atlas Arrays. Twenty-three of 588 genes were shown to be differentially regulated, 18 of which were previously not known to be regulated by T3. The expression of 19 genes was verified by independent northern blot hybridisation. Two different expression time courses of T3 expression were observed. In a first expression profile ('early' expression) the transcription level of the target genes rises within 6 h, drops by 24 h and increases again within 48 h after the administration of T3. In a second expression profile ('late' expression) the mRNA level rose in the first 6 h and rose further by 48 h, indicating an additional regulation mechanism. Nuclear respiratory factor (NRF)-1 and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1), but not NRF-2, were up-regulated within 6 h after T3 administration, suggesting NRF-1 and/or PGC-1 as key regulators for mediating the 'late' expression pattern.

Tamoxifen agonism and estrogen antagonism of c-fos gene promoter activity through non-consensus-responsive elements in MC3T3-E1 osteoblasts

We find that the activity of a 0.4-kb human c-fos gene promoter (-404/+41), which lacks consensus estrogen-responsive elements (EREs), is regulated by estrogen receptor (ER) ligands in MC3T3-E1 osteoblastic cells through ERs in a manner distinct from ERE-mediated regulation. When ERalpha is coexpressed, both estrogens and antiestrogens upregulate promoter activity. When ERbeta is coexpressed, however, three tested antiestrogens affect c-fos promoter activity, with tamoxifen exerting the greatest effect, while estrogens have no such effect. The tamoxifen agonism through ERbeta is antagonized by 17beta-estradiol, while the 17beta-estradiol agonism through ERalpha is canceled by excess-level coexpression of ERbeta. Deletion analysis revealed that the sequence -206/-110 plays a crucial role in the ERbeta-mediated tamoxifen agonism. Interestingly, there is no ERbeta-mediated tamoxifen agonism when nonosteoblastic cells are tested. Taken together, these results suggest that the transcription of the c-fos gene is regulated by ER ligands possibly through non-ERE elements in ligand structure-, cell type-, and ER subtype-dependent manners.

Domain structure of the NRIF3 family of coregulators suggests potential dual roles in transcriptional regulation

The identification of a novel coregulator for nuclear hormone receptors, designated NRIF3, was recently reported (D. Li et al., Mol. Cell. Biol. 19:7191-7202, 1999). Unlike most known coactivators, NRIF3 exhibits a distinct receptor specificity in interacting with and potentiating the activity of only TRs and RXRs but not other examined nuclear receptors. However, the molecular basis underlying such specificity is unclear. In this report, we extended our study of NRIF3-receptor interactions. Our results suggest a bivalent interaction model, where a single NRIF3 molecule utilizes both the C-terminal LXXIL (receptor-interacting domain 1 [RID1]) and the N-terminal LXXLL (RID2) modules to cooperatively interact with TR or RXR (presumably a receptor dimer), with the spacing between RID1 and RID2 playing an important role in influencing the affinity of the interactions. During the course of these studies, we also uncovered an NRIF3-NRIF3 interaction domain. Deletion and mutagenesis analyses mapped the dimerization domain to a region in the middle of NRIF3 (residues 84 to 112), which is predicted to form a coiled-coil structure and contains a putative leucine zipper-like motif. By using Gal4 fusion constructs, we identified an autonomous transactivation domain (AD1) at the C terminus of NRIF3. Somewhat surprisingly, full-length NRIF3 fused to the DNA-binding domain of Gal4 was found to repress transcription of a Gal4 reporter. Further analyses mapped a novel repression domain (RepD1) to a small region at the N-terminal portion of NRIF3 (residues 20 to 50). The NRIF3 gene encodes at least two additional isoforms due to alternative splicing. These two isoforms contain the same RepD1 region as NRIF3. Consistent with this, Gal4 fusions of these two isoforms were also found to repress transcription. Cotransfection of NRIF3 or its two isoforms did not relieve the transrepression function mediated by their corresponding Gal4 fusion proteins, suggesting that the repression involves a mechanism(s) other than the recruitment of a titratable corepressor. Interestingly, a single amino acid residue change of a potential phosphorylation site in RepD1 (Ser(28) to Ala) abolishes its transrepression function, suggesting that the coregulatory property of NRIF3 (or its isoforms) might be subjected to regulation by cellular signaling. Taken together, our results identify NRIF3 as an interesting coregulator that possesses both transactivation and transrepression domains and/or functions. Collectively, the NRIF3 family of coregulators (which includes NRIF3 and its other isoforms) may play dual roles in mediating both positive and negative regulatory effects on gene expression.

Aspirin-triggered lipoxin A4 and lipoxin A4 up-regulate transcriptional corepressor NAB1 in human neutrophils

Aspirin-triggered 15-epi-lipoxin A4 (ATL) is an endogenous lipid mediator that mimics the actions of native lipoxin A4, a putative "stop signal" involved in regulating resolution of inflammation. A metabolically more stable analog of ATL, 15-epi-16-(para-fluoro)-phenoxy-lipoxin A4 analog (ATLa), inhibits neutrophil recruitment in vitro and in vivo and displays potent anti-inflammatory actions. ATLa binds with high affinity to the lipoxin A4 receptor, a G protein-coupled receptor on the surface of leukocytes. In this study, we used freshly isolated human neutrophils to examine ATLa's potential for initiating rapid nuclear responses. Using differential display reverse transcription polymerase chain reaction, we identified a subset of genes that was selectively up-regulated upon short exposure of polymorphonuclear leukocytes to ATLa but not to the chemoattractant leukotriene B4 or vehicle alone. We further investigated ATLa regulation of one of the genes, NAB1, a transcriptional corepressor identified previously as a glucocorticoid-responsive gene in hamster smooth muscle cells. Treatment of human neutrophils with pertussis toxin blocked ATLa up-regulation of NAB1. In addition, ATLa stimulated NAB1 gene expression in murine lung vascular smooth muscle in vivo. These findings provide evidence for rapid transcriptional induction of a cassette of genes via an ATLa-stimulated G protein-coupled receptor pathway that is potentially protective and overlaps with the anti-inflammatory glucocorticoid regulatory circuit.

Foxa3 (hepatocyte nuclear factor 3gamma ) is required for the regulation of hepatic GLUT2 expression and the maintenance of glucose homeostasis during a prolonged fast

The winged helix transcription factors, hepatocyte nuclear factors 3alpha, -beta, and -gamma (HNF-3, encoded by the Foxa1, -a2, and -a3 genes, respectively), are expressed early in embryonic endoderm and play important roles in the regulation of gene expression in liver and pancreas. Foxa1 has been shown to be required for glucagon secretion in the pancreas, whereas Foxa2 is critical for the regulation of insulin secretion in pancreatic beta-cells. Here we address the role of Foxa3 in the maintenance of glucose homeostasis. Mice homozygous for a null mutation in Foxa3 appear normal under fed conditions. However, when fasted, Foxa3(-/-) mice have a significantly lower blood glucose compared with control mice. The fasting hypoglycemia in Foxa3(-/-) mice could not be attributed to defects in pancreatic hormone secretion, ketone production, or hepatic glycogen breakdown. Surprisingly, mRNA levels for several gluconeogenic enzymes were up-regulated appropriately in fasted Foxa3(-/-) mice, despite the fact that the corresponding genes had been shown to be activated by FOXA proteins in vitro. However, the mRNA for the plasma membrane glucose transporter GLUT2 was decreased by 64% in the fasted and 93% in the fed state, suggesting that efflux of newly synthesized glucose is limiting in Foxa3(-/-) hepatocytes. Thus, Foxa3 is the dominating transcriptional regulator of GLUT2 expression in hepatocytes in vivo. In addition, we investigated the hepatic transcription factor network in Foxa3(-/-) mice and found that the normal activation of HNF-4alpha, HNF-1alpha, and PGC-1 induced by fasting is attenuated in mice lacking Foxa3.

BFIT, a unique acyl-CoA thioesterase induced in thermogenic brown adipose tissue: cloning, organization of the human gene and assessment of a potential link to obesity

We hypothesized that certain proteins encoded by temperature-responsive genes in brown adipose tissue (BAT) contribute to the remarkable metabolic shifts observed in this tissue, thus prompting a differential mRNA expression analysis to identify candidates involved in this process in mouse BAT. An mRNA species corresponding to a novel partial-length gene was found to be induced 2-3-fold above the control following cold exposure (4 degrees C), and repressed approximately 70% by warm acclimation (33 degrees C, 3 weeks) compared with controls (22 degrees C). The gene displayed robust BAT expression (i.e. approximately 7-100-fold higher than other tissues in controls). The full-length murine gene encodes a 594 amino acid ( approximately 67 kDa) open reading frame with significant homology to the human hypothetical acyl-CoA thioesterase KIAA0707. Based on cold-inducibility of the gene and the presence of two acyl-CoA thioesterase domains, we termed the protein brown-fat-inducible thioesterase (BFIT). Subsequent analyses and cloning efforts revealed the presence of a novel splice variant in humans (termed hBFIT2), encoding the orthologue to the murine BAT gene. BFIT was mapped to syntenic regions of chromosomes 1 (human) and 4 (mouse) associated with body fatness and diet-induced obesity, potentially linking a deficit of BFIT activity with exacerbation of these traits. Consistent with this notion, BFIT mRNA was significantly higher ( approximately 1.6-2-fold) in the BAT of obesity-resistant compared with obesity-prone mice fed a high-fat diet, and was 2.5-fold higher in controls compared with ob/ob mice. Its strong, cold-inducible BAT expression in mice suggests that BFIT supports the transition of this tissue towards increased metabolic activity, probably through alteration of intracellular fatty acyl-CoA concentration.

Cytokine stimulation of energy expenditure through p38 MAP kinase activation of PPARgamma coactivator-1

Cachexia is a chronic state of negative energy balance and muscle wasting that is a severe complication of cancer and chronic infection. While cytokines such as IL-1alpha, IL-1beta, and TNFalpha can mediate cachectic states, how these molecules affect energy expenditure is unknown. We show here that many cytokines activate the transcriptional PPAR gamma coactivator-1 (PGC-1) through phosphorylation by p38 kinase, resulting in stabilization and activation of PGC-1 protein. Cytokine or lipopolysaccharide (LPS)-induced activation of PGC-1 in cultured muscle cells or muscle in vivo causes increased respiration and expression of genes linked to mitochondrial uncoupling and energy expenditure. These data illustrate a direct thermogenic action of cytokines and p38 MAP kinase through the transcriptional coactivator PGC-1.

Differential gene expression in white and brown preadipocytes

White (WAT) and brown (BAT) adipose tissue are tissues of energy storage and energy dissipation, respectively. Experimental evidence suggests that brown and white preadipocytes are differentially determined, but so far not much is known about the genetic control of this determination process. The aim of this study was to identify differentially expressed genes involved in brown and white preadipocyte development. Using representational difference analysis (cDNA RDA) and DNA microarray screening, we identified four genes with higher expression in white preadipocytes (three different complement factors and delta-6 fatty acid desaturase) and seven genes with higher expression levels in brown preadipocytes, of which three are structural genes implicated in cell adhesion and cytoskeleton organization (fibronectin, alpha-actinin-4, metargidin) and four that might function in gene transcription and protein synthesis (vigilin, necdin, snRNP polypeptide A, and a homolog to human hepatocellular carcinoma-associated protein). The expression profile of these genes was analyzed during preadipocyte differentiation, upon beta-adrenergic stimulation, and in WAT and BAT tissue in vivo compared with references genes such as peroxisome proliferator-activated receptor-gamma (PPARgamma), uncoupling protein 1 (UCP1), cytochrome c oxidase.

Mechanisms of estrogen action

Our appreciation of the physiological functions of estrogens and the mechanisms through which estrogens bring about these functions has changed during the past decade. Just as transgenic mice were produced in which estrogen receptors had been inactivated and we thought that we were about to understand the role of estrogen receptors in physiology and pathology, it was found that there was not one but two distinct and functional estrogen receptors, now called ER alpha and ER beta. Transgenic mice in which each of the receptors or both the receptors are inactive have revealed a much broader role for estrogens in the body than was previously thought. This decade also saw the description of a male patient who had no functional ER alpha and whose continued bone growth clearly revealed an important function of estrogen in men. The importance of estrogen in both males and females was also demonstrated in the laboratory in transgenic mice in which the aromatase gene was inactivated. Finally, crystal structures of the estrogen receptors with agonists and antagonists have revealed much about how ligand binding influences receptor conformation and how this conformation influences interaction of the receptor with coactivators or corepressors and hence determines cellular response to ligands.

PGC-1, a versatile coactivator

PGC-1 was originally identified as a transcriptional coactivator of the nuclear receptor PPARgamma. The expression pattern and induction by exposure to cold have implicated PGC-1 in the regulation of energy metabolism and adaptive thermogenesis. Remarkably, PGC-1 overexpression can induce mitochondrial biogenesis and functions. Recent studies show that PGC-1 regulates the activity of several nuclear receptors and other transcription factors, and thus acts in a broader context than previously anticipated. Furthermore, PGC-1 displays the striking ability to interact with components of the splicing machinery. PGC-1 could therefore allow coordinated regulation of transcription and splicing in response to signals relaying metabolic needs. These novel findings are discussed in the context of the proposed physiological functions of PGC-1.

Cloning of BUG demonstrates the existence of a brown preadipocyte distinct from a white one

BACKGROUND

Several indirect arguments agree with the existence of a brown preadipocyte distinct from a white one. Nevertheless, to date, no molecular marker has been available to directly in vivo demonstrate this hypothesis.

OBJECTIVE

The aim of this study was to find a gene expressed in brown preadipocyte but not in white and to use it as a molecular marker to analyse brown preadipocyte recruitment in different physiological and physiopathological situations.

METHOD

Differential display was performed on stromal-vascular and adipocyte fractions of white and brown adipose tissues in rat.

RESULTS

We identified a new gene, BUG, preferentially expressed in the stromal-vascular fraction of brown fat vs other adipose tissues fractions in adult rat. This RNA is also highly expressed in heart and, to a lesser extent, in other tissues such as kidney and brain. The BUG transcript is detected by in situ hybridization in putative preadipocytes within brown adipose tissue. Its level is transiently and specifically up-regulated during early stages of brown preadipocyte differentiation in a primary culture system, before the acquisition of late brown adipocyte phenotype. During development, BUG can be detected before the emergence of UCP-1 expression. In adult rats, BUG expression is inversely associated to brown adipose tissue (BAT) activation during cold exposure as well as in obese animals.

CONCLUSIONS

The pattern of BUG expression agrees with an early divergence between brown and white adipocyte lineages. It also reveals the existence of a pool of committed brown preadipocytes within BAT that are recruited during cold exposure. BUG expression is increased in obese animals, suggesting that an early defect in brown preadipocyte differentiation could account for impaired BAT function in genetically obese rats.

Adipose tissue reduction in mice lacking the translational inhibitor 4E-BP1

All nuclear-encoded mRNAs contain a 5' cap structure (m7GpppN, where N is any nucleotide), which is recognized by the eukaryotic translation initiation factor 4E (eIF4E) subunit of the eIF4F complex. The eIF4E-binding proteins constitute a family of three polypeptides that reversibly repress cap-dependent translation by binding to eIF4E, thus preventing the formation of the eIF4F complex. We investigated the biological function of 4E-BP1 by disrupting its gene (Eif4ebp1) in the mouse. Eif4ebp1-/- mice manifest markedly smaller white fat pads than wild-type animals, and knockout males display an increase in metabolic rate. The males' white adipose tissue contains cells that exhibit the distinctive multilocular appearance of brown adipocytes, and expresses the uncoupling protein 1 (UCP1), a specific marker of brown fat. Consistent with these observations, translation of the peroxisome proliferator-activated receptor-gamma co-activator 1 (PGC1), a transcriptional co-activator implicated in mitochondrial biogenesis and adaptive thermogenesis, is increased in white adipose tissue of Eif4ebp1-/- mice. These findings demonstrate that 4E-BP1 is a novel regulator of adipogenesis and metabolism in mammals.

Control of hepatic gluconeogenesis through the transcriptional coactivator PGC-1

Blood glucose levels are maintained by the balance between glucose uptake by peripheral tissues and glucose secretion by the liver. Gluconeogenesis is strongly stimulated during fasting and is aberrantly activated in diabetes mellitus. Here we show that the transcriptional coactivator PGC-1 is strongly induced in liver in fasting mice and in three mouse models of insulin action deficiency: streptozotocin-induced diabetes, ob/ob genotype and liver insulin-receptor knockout. PGC-1 is induced synergistically in primary liver cultures by cyclic AMP and glucocorticoids. Adenoviral-mediated expression of PGC-1 in hepatocytes in culture or in vivo strongly activates an entire programme of key gluconeogenic enzymes, including phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, leading to increased glucose output. Full transcriptional activation of the PEPCK promoter requires coactivation of the glucocorticoid receptor and the liver-enriched transcription factor HNF-4alpha (hepatic nuclear factor-4alpha) by PGC-1. These results implicate PGC-1 as a key modulator of hepatic gluconeogenesis and as a central target of the insulin-cAMP axis in liver.

CREB regulates hepatic gluconeogenesis through the coactivator PGC-1

When mammals fast, glucose homeostasis is achieved by triggering expression of gluconeogenic genes in response to glucagon and glucocorticoids. The pathways act synergistically to induce gluconeogenesis (glucose synthesis), although the underlying mechanism has not been determined. Here we show that mice carrying a targeted disruption of the cyclic AMP (cAMP) response element binding (CREB) protein gene, or overexpressing a dominant-negative CREB inhibitor, exhibit fasting hypoglycaemia [corrected] and reduced expression of gluconeogenic enzymes. CREB was found to induce expression of the gluconeogenic programme through the nuclear receptor coactivator PGC-1, which is shown here to be a direct target for CREB regulation in vivo. Overexpression of PGC-1 in CREB-deficient mice restored glucose homeostasis and rescued expression of gluconeogenic genes. In transient assays, PGC-1 potentiated glucocorticoid induction of the gene for phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting enzyme in gluconeogenesis. PGC-1 promotes cooperativity between cyclic AMP and glucocorticoid signalling pathways during hepatic gluconeogenesis. Fasting hyperglycaemia is strongly correlated with type II diabetes, so our results suggest that the activation of PGC-1 by CREB in liver contributes importantly to the pathogenesis of this disease.

Identification and characterization of RRM-containing coactivator activator (CoAA) as TRBP-interacting protein, and its splice variant as a coactivator modulator (CoAM)

We previously cloned and characterized thyroid hormone receptor-binding protein (TRBP) as an LXXLL-containing general coactivator that associates with coactivator complexes through its C terminus. To identify protein cofactors for TRBP action, a Sos-Ras yeast two-hybrid cDNA library was screened using TRBP C terminus as bait. A novel coactivator was isolated, coactivator activator (CoAA), that specifically associates with TRBP. Human CoAA is composed of 669 amino acids with a TRBP-interacting domain and two highly conserved RNA recognition motifs (RRM) commonly found in ribonucleoproteins. A splice variant lacking the entire TRBP-interacting domain was also isolated as a coactivator modulator (CoAM), a 156-amino acid protein containing only the RRM region. Human CoAA and CoAM mRNAs are encoded by a single gene located on chromosome 11q13; alternative splicing in exon 2 of CoAA yields CoAM. CoAA interacts with both TRBP and p300 in vitro. In addition, CoAA potently coactivates transcription mediated by multiple hormone-response elements and acts synergistically with TRBP and CREB-binding protein (CBP). Furthermore, CoAA is associated with the DNA-dependent protein kinase-poly(ADP-ribose) polymerase complex. Strikingly, CoAM, which lacks a TRBP-interacting domain, strongly represses both TRBP and CBP action suggesting that CoAM may modulate endogenous CoAA function. These data suggest that CoAA may serve as a mediator of coactivators such as TRBP in gene activation.

FOXC2 is a winged helix gene that counteracts obesity, hypertriglyceridemia, and diet-induced insulin resistance

Obesity, hyperlipidemia, and insulin resistance are common forerunners of type 2 diabetes mellitus. We have identified the human winged helix/forkhead transcription factor gene FOXC2 as a key regulator of adipocyte metabolism. Increased FOXC2 expression, in adipocytes, has a pleiotropic effect on gene expression, which leads to a lean and insulin sensitive phenotype. FOXC2 affects adipocyte metabolism by increasing the sensitivity of the beta-adrenergic-cAMP-protein kinase A (PKA) signaling pathway through alteration of adipocyte PKA holoenzyme composition. Increased FOXC2 levels, induced by high fat diet, seem to counteract most of the symptoms associated with obesity, including hypertriglyceridemia and diet-induced insulin resistance--a likely consequence hereof would be protection against type 2 diabetes.

Neurohumoral regulation of body weight gain

The regulation of body weight is a complex process which relies on a balance between supply of nutrients and demand on these nutrients in the form of energy expenditure. Various central and peripheral mechanisms play a crucial role in maintaining this balance. While various neuropeptides in the central nervous system (CNS), particularly in the hypothalamus, maintain the necessary harmony between hyperphagia and anorexia, peripheral signals arising from the gastrointestinal tract (cholecystokinin-8 [CCK-8], amylin), pancreas (insulin) and adipose tissue (leptin) provide the necessary stimuli or a feedback inhibition for the synthesis and secretion of these hypothalamic neuropeptides. Various metabolites of the carbohydrate and fat metabolism are also involved in regulating the neuronal activity in the hypothalamus which ultimately leads to a release of key neuropeptides. In addition to the central mechanisms, peripheral mechanisms that regulate energy expenditure, particularly in the brown adipose tissue and skeletal muscle, are critical in maintaining the overall balance. Insight into these mechanisms sets the stage for developing novel strategies in the treatment of emerging childhood diseases such as obesity, anorexia nervosa, and bulimia. Further, delineation of these processes in the fetus and newborn sets the stage for investigating their role in molding the adult phenotype due to intrauterine adaptations.

Prospects for prevention and treatment of cancer with selective PPARgamma modulators (SPARMs)

Peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor and transcription factor that regulates the expression of many genes relevant to carcinogenesis, is now an important target for development of new drugs for the prevention and treatment of cancer. Deficient expression of PPARgamma can be a significant risk factor for carcinogenesis, although in some cases overexpression enhances carcinogenesis. Ligands for PPARgamma suppress breast carcinogenesis in experimental models and induce differentiation of human liposarcoma cells. By analogy to the selective estrogen receptor modulator (SERM) concept, it is suggested that selective PPARgamma modulators (SPARMs), designed to have desired effects on specific genes and target tissues without undesirable effects on others, will be clinically important in the future for chemoprevention and chemotherapy of cancer.

Sex-dependent dietary obesity, induction of UCPs, and leptin expression in rat adipose tissues

OBJECTIVE

The aim of this study was to determine the sex-dependent differences in the response of key parameters involved in thermogenesis and control of body weight in brown adipose tissue (BAT) and white adipose tissue (WAT) in postcafeteria-fed rats, a model of dietary obesity.

RESEARCH METHODS AND PROCEDURES

BAT and WAT were obtained from male and female control and postcafeteria-fed Wistar rats. Postcafeteria-fed rats were initially fed with cafeteria diet from day 10 of life until day 110 (cafeteria period) and with standard chow diet from then until day 180 of life (postcafeteria period). Body mass and energy intake were evaluated. Biometric parameters were analyzed in interscapular BAT (IBAT). Levels of uncoupling protein 1 (UCP1), alpha(2)-adrenergic receptor (AR), and beta(3)-AR proteins and UCP1, UCP2, UCP3, beta(3)-AR, and leptin mRNAs, in IBAT or WAT, were studied by Western blot and Northern blot analyses, respectively.

RESULTS

Rats attained 59% (females) and 39% (males) increase in body weight at the end of the cafeteria period. During the postcafeteria period, the rats showed a loss of body weight, which was higher in females. Postcafeteria-fed female rats also presented higher activation of thermogenic parameters in IBAT, including UCP1, UCP2, and UCP3 mRNAs. Female control rats showed lower levels of both alpha 2 and beta(3)-ARs in BAT compared with male rats, but these levels in postcafeteria-fed female and male rats were the same, because males tended to down-regulate them. Levels of leptin mRNA in response to the postcafeteria state depended on gender and the specific WAT depot studied.

DISCUSSION

It is suggested that in postcafeteria-fed female rats, BAT thermogenic capacity becomes more efficiently activated than in males. Female rats also showed a bigger weight loss. The parallel regulation of the levels of UCP2 and UCP3 mRNAs, with respect to UCP1 mRNA, with higher activation in female postcafeteria-fed rats, suggests a possible role of both UCP2 and UCP3 in the regulation of energy expenditure and in the control of body weight. The distinct responses to overweight of alpha 2 and beta(3)-ARs--which were sex dependent--and leptin mRNA--which depended on both sex and WAT depot--also support the different response of thermogenesis-related parameters between overweight males and females.

Cloning and characterization of PIMT, a protein with a methyltransferase domain, which interacts with and enhances nuclear receptor coactivator PRIP function

The nuclear receptor coactivators participate in the transcriptional activation of specific genes by nuclear receptors. In this study, we report the isolation of a nuclear receptor coactivator-interacting protein from a human liver cDNA library by using the coactivator peroxisome proliferator-activated receptor-interacting protein (PRIP) (ASC2/AIB3/RAP250/NRC/TRBP) as bait in a yeast two-hybrid screen. Human PRIP-interacting protein cDNA has an ORF of 2,556 nucleotides, encodes a protein with 852 amino acids, and contains a 9-aa VVDAFCGVG methyltransferase motif I and an invariant GXXGXXI segment found in K-homology motifs of many RNA-binding proteins. The gene encoding this protein, designated PRIP-interacting protein with methyltransferase domain (PIMT), is localized on chromosome 8q11 and spans more than 40 kb. PIMT mRNA is ubiquitously expressed, with a high level of expression in heart, skeletal muscle, kidney, liver, and placenta. Using the immunofluorescence localization method, we found that PIMT and PRIP proteins appear colocalized in the nucleus. PIMT strongly interacts with PRIP under in vitro and in vivo conditions, and the PIMT-binding site on PRIP is in the region encompassing amino acids 773-927. PIMT binds S-adenosyl-l-methionine, the methyl donor for methyltransfer reaction, and it also binds RNA, suggesting that it is a putative RNA methyltransferase. PIMT enhances the transcriptional activity of peroxisome proliferator-activated receptor gamma and retinoid-X-receptor alpha, which is further stimulated by coexpression of PRIP, implying that PIMT is a component of nuclear receptor signal transduction apparatus acting through PRIP. Definitive identification of the specific substrate of PIMT and the role of this RNA-binding protein in transcriptional regulation remain to be determined.

DNA binding-independent transcriptional activation by the androgen receptor through triggering of coactivators

Androgens have critical roles in the development and maintenance of the male reproductive system and are important for progression of prostate cancer. The effects of androgens are mediated by the androgen receptor (AR), which is a ligand-modulated transcription factor that belongs to the nuclear receptor superfamily. In the presence of androgens, AR binds to androgen response elements in the vicinity of androgen receptor target genes and activates transcription. In addition, liganded AR can interfere with the activity of other transcription factors, such as activator protein-1 and nuclear factor kappaB, for which DNA binding by AR is not necessary. In this study, we describe a novel ligand-dependent transactivation function for AR that is independent of its DNA binding ability. AR dramatically increased the intrinsic transcriptional activity of the nuclear receptor coactivators glucocorticoid receptor-interacting protein-1 (GRIP1), cAMP response element-binding protein-binding protein, and p300 that are tethered to DNA. This "triggering" phenomenon required both similar and distinctly different regions of AR compared with those needed for ligand-dependent transactivation from androgen-responsive elements. Furthermore, the domains of GRIP1 required for triggering by AR are different from those required when GRIP1 serves as a coactivator for AR at androgen-responsive promoters. These data suggest that triggering may constitute an important part of the mechanism by which AR regulates transcription.

Regulation of the transcriptional coactivator PGC-1 via MAPK-sensitive interaction with a repressor

Mechanisms and signals that regulate transcriptional coactivators are still largely unknown. Here we provide genetic evidence for a repressor that interacts with and regulates the nuclear receptor coactivator PGC-1. Association with the repressor requires a PGC-1 protein interface that is similar to the one used by nuclear receptors. Removal of the repressor enhances PGC-1 coactivation of steroid hormone responses. We also provide evidence that interaction of the repressor with PGC-1 is regulated by mitogen-activated protein kinase (MAPK) signaling. Activation of the MAPK p38 enhances the activity of wild-type PGC-1 but not of a PGC-1 variant that no longer interacts with the repressor. Finally, p38 activation enhances steroid hormone response in a PGC-1-dependent manner. Our data suggest a model where the repressor and nuclear receptors compete for recruiting PGC-1 to an inactive and active state, respectively. Extracellular signals such as nuclear receptor ligands or activators of the MAPK p38 can shift the equilibrium between the two states.

Changes of adiposity in response to vitamin A status correlate with changes of PPAR gamma 2 expression

OBJECTIVE

To gain insight into the in vivo modulation of the expression of the adipogenic transcription factors PPAR gamma 2, C/EBP alpha, and ADD1/SREBP1c by retinoids and its relationship with whole-body adiposity.

RESEARCH METHODS AND PROCEDURES

Three-week-old mice were fed with standard chow or a vitamin A-deficient diet for 10 weeks. During the 4 days immediately before they were killed, the animals were treated either with all-trans retinoic acid (tRA; 100 mg/kg per day, subcutaneously) or vehicle. The specific levels of the mRNAs for the three transcription factors were analyzed in epididymal white adipose tissue (eWAT) and inguinal white adipose tissue and in brown adipose tissue (BAT). Other parameters determined were leptin and UCP2 levels in white adipose tissue depots, total cholesterol and triglyceride serum levels, energy intake, body weight, and adiposity.

RESULTS

Vitamin A-deficient diet feeding led to a marked increase of adiposity and to a small increase of body weight. Hypertrophy of white adipose tissue depots correlated with enhanced PPAR gamma 2 expression. Hypertrophy of BAT, in contrast, correlated with a decrease of PPAR gamma 2 expression that may contribute to the known reduced thermogenic potential of BAT under conditions of vitamin A restriction. Treatment with tRA triggered a reduction of adiposity and body weight that correlated with a down-regulation of PPAR gamma 2 expression in all adipose tissues. The effects of tRA were more pronounced in eWAT, where C/EBP alpha and ADD1/SREBP1c levels were also reduced. The response to tRA was impaired in the eWAT and BAT of animals fed the vitamin A-deficient diet.

DISCUSSION

The results emphasize the importance of retinoids as physiological regulators of adipose tissue development and function in intact animals.

Human immunodeficiency virus type 1 Tat binding protein-1 is a transcriptional coactivator specific for TR

The DNA-binding domain of nuclear hormone receptors functions as an interaction interface for other transcription factors. Using the DNA-binding domain of TRbeta1 as bait in the yeast two-hybrid system, we cloned the Tat binding protein-1 that was originally isolated as a protein binding to the human immunodeficiency virus type 1 Tat transactivator. Tat binding protein-1 has subsequently been identified as a member of the ATPase family and a component of the 26S proteasome. Tat binding protein-1 interacted with the DNA-binding domain but not with the ligand binding domain of TR in vivo and in vitro. TR bound to the amino-terminal portion of Tat binding protein-1 that contains a leucine zipper-like structure. In mammalian cells, Tat binding protein-1 potentiated the ligand-dependent transactivation by TRbeta1 and TRalpha1 via thyroid hormone response elements. Both the intact DNA-binding domain and activation function-2 of the TR were required for the transcriptional enhancement in the presence of Tat binding protein-1. Tat binding protein-1 did not augment the transactivation function of the RAR, RXR, PPARgamma, or ER. The intrinsic activation domain in Tat binding protein-1 resided within the carboxyl-terminal conserved ATPase domain, and a mutation of a putative ATP binding motif but not a helicase motif in the carboxyl-terminal conserved ATPase domain abolished the activation function. Tat binding protein-1 synergistically activated the TR-mediated transcription with the steroid receptor coactivator 1, p120, and cAMP response element-binding protein, although Tat binding protein-1 did not directly interact with these coactivators in vitro. In contrast, the N-terminal portion of Tat binding protein-1 directly interacted in vitro and in vivo with the TR-interacting protein 1 possessing an ATPase activity that interacts with the activation function-2 of liganded TR. Collectively, Tat binding protein-1 might function as a novel DNA-binding domain-binding transcriptional coactivator specific for the TR probably in cooperation with other activation function-2-interacting cofactors such as TR-interacting protein 1.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Nuclear hormone receptors and gene expression

The nuclear hormone receptor superfamily includes receptors for thyroid and steroid hormones, retinoids and vitamin D, as well as different "orphan" receptors of unknown ligand. Ligands for some of these receptors have been recently identified, showing that products of lipid metabolism such as fatty acids, prostaglandins, or cholesterol derivatives can regulate gene expression by binding to nuclear receptors. Nuclear receptors act as ligand-inducible transcription factors by directly interacting as monomers, homodimers, or heterodimers with the retinoid X receptor with DNA response elements of target genes, as well as by "cross-talking" to other signaling pathways. The effects of nuclear receptors on transcription are mediated through recruitment of coregulators. A subset of receptors binds corepressor factors and actively represses target gene expression in the absence of ligand. Corepressors are found within multicomponent complexes that contain histone deacetylase activity. Deacetylation leads to chromatin compactation and transcriptional repression. Upon ligand binding, the receptors undergo a conformational change that allows the recruitment of multiple coactivator complexes. Some of these proteins are chromatin remodeling factors or possess histone acetylase activity, whereas others may interact directly with the basic transcriptional machinery. Recruitment of coactivator complexes to the target promoter causes chromatin decompactation and transcriptional activation. The characterization of corepressor and coactivator complexes, in concert with the identification of the specific interaction motifs in the receptors, has demonstrated the existence of a general molecular mechanism by which different receptors elicit their transcriptional responses in target genes.

Peroxisome proliferator-activated receptors: from transcriptional control to clinical practice

Peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors that control energy homeostasis through genomic actions. Over the past few years significant advances have been made in unravelling the pathways that are modulated by PPARs. Gene targeting experiments in mice and genetic studies in humans have demonstrated a physiological role for these receptors in adipocyte function, glucose homeostasis, and lipid and lipoprotein metabolism. Recent data indicate that PPARs enhance the reverse cholesterol transport pathway by regulating genes that control macrophage cholesterol efflux, cholesterol transport in plasma and bile acid synthesis. Clinical and experimental evidence suggest that PPAR activation decreases the incidence of cardiovascular disease not only by correcting metabolic disorders, but also through direct actions at the level of the vascular wall. Thus, dysregulation of PPAR activity modulates the onset and evolution of metabolic disorders such as dyslipidaemia, obesity and insulin resistance, predisposing to atherosclerosis.

Pgc-1-related coactivator, a novel, serum-inducible coactivator of nuclear respiratory factor 1-dependent transcription in mammalian cells

The thermogenic peroxisome proliferator-activated receptor gamma (PPAR-gamma) coactivator 1 (PGC-1) has previously been shown to activate mitochondrial biogenesis in part through a direct interaction with nuclear respiratory factor 1 (NRF-1). In order to identify related coactivators that act through NRF-1, we searched the databases for sequences with similarities to PGC-1. Here, we describe the first characterization of a 177-kDa transcriptional coactivator, designated PGC-1-related coactivator (PRC). PRC is ubiquitously expressed in murine and human tissues and cell lines; but unlike PGC-1, PRC was not dramatically up-regulated during thermogenesis in brown fat. However, its expression was down-regulated in quiescent BALB/3T3 cells and was rapidly induced by reintroduction of serum, conditions where PGC-1 was not detected. PRC activated NRF-1-dependent promoters in a manner similar to that observed for PGC-1. Moreover, NRF-1 was immunoprecipitated from cell extracts by antibodies directed against PRC, and both proteins were colocalized to the nucleoplasm by confocal laser scanning microscopy. PRC interacts in vitro with the NRF-1 DNA binding domain through two distinct recognition motifs that are separated by an unstructured proline-rich region. PRC also contains a potent transcriptional activation domain in its amino terminus adjacent to an LXXLL motif. The spatial arrangement of these functional domains coincides with those found in PGC-1, supporting the conclusion that PRC and PGC-1 are structurally and functionally related. We conclude that PRC is a functional relative of PGC-1 that operates through NRF-1 and possibly other activators in response to proliferative signals.

Samui, a novel cold-inducible gene, encoding a protein with a BAG domain similar to silencer of death domains (SODD/BAG-4), isolated from Bombyx diapause eggs

Cellular responses to cold-acclimation have not yet been studied in depth. To explore this field, we focussed on insect diapause development. Although embryonic diapause of Bombyx mori is sustained at 25 degrees C, chilling at 5 degrees C for 2 months causes diapause termination, a transition that is marked when the sorbitol dehydrogenase gene (SDH) is activated. To clarify the relationship between this activation and incubation at 5 degrees C, we isolated a novel cold-inducible gene, Samui. Expression of Samui mRNA and protein was activated after incubation at 5 degrees C for 5-6 days, lasted for another 30 days and then weakened. Exposure to 25 degrees C suppressed both mRNA and protein expression. In nondiapause eggs incubated at 5 degrees C, Samui was also up-regulated, although the expression was weaker. Samui contained nuclear localization-signals, a ssDNA-binding motif and a BAG domain similar to that of SODD/BAG-4. Because Samui could bind to HSP70, it is a member of BAG protein family. It is proposed that Samui serves to transmit the '5 degrees C signal' for SDH expression in diapause eggs, while also protecting against cold-injures in nondiapause eggs, through binding to respective partners. This is the first report that a member of BAG protein family is up-regulated by cold.