Synthesis and accumulation of a receptor regulatory protein associated with lipid droplet accumulation in 3T3-L1 cells

Synthesis and accumulation of the recently identified prostaglandin F2alpha receptor regulatory protein (FPRP) was found to correlate closely with lipid droplet accumulation by 3T3-L1 preadipose cells. FPRP, a transmembrane glycoprotein, has been shown to regulate the binding of ligand to certain seven-transmembrane receptors. Anti-FPRP immunohistochemistry, Western blotting, and metabolic labeling/immunoprecipitation experiments demonstrated that FPRP was not detectable in undifferentiated 3T3-L1 cells. Interestingly, low levels of FPRP mRNA were detected in the undifferentiated 3T3-L1 cells. After induction of adipose differentiation, FPRP mRNA increased approximately 3 fold whereas FPRP synthesis increased approximately 50 fold. Differentiation induction with either dexamethasone/insulin/isobutylmethylxanthine or the thiazolidinedione derivative ADD 4743 were both effective at inducing FPRP accumulation and accumulation of lipid droplets. By co-immunohistochemical and lipid staining, greater than 99% of the cells accumulating lipid droplets possessed FPRP. FPRP mRNA and protein are also found in rat adipose tissue. Treatment of 3T3-L1 cells with an FPRP anti-sense oligonucleotide during differentiation decreased FPRP accumulation and resulted in a decrease in lipid droplets without altering the level of induction of a late marker of adipocyte differentiation, glycerol-3-phosphate dehydrogenase activity. Transient expression of an FPRP cDNA in undifferentiated 3T3-L1 cells was insufficient to induce lipid droplet accumulation.

The histone acetylase PCAF is a nuclear receptor coactivator

Whereas the histone acetylase PCAF has been suggested to be part of a coactivator complex mediating transcriptional activation by the nuclear hormone receptors, the physical and functional interactions between nuclear receptors and PCAF have remained unclear. Our efforts to clarify these relationships have revealed two novel properties of nuclear receptors. First, we demonstrate that the RXR/RAR heterodimer directly recruits PCAF from mammalian cell extracts in a ligand-dependent manner and that increased expression of PCAF leads to enhanced retinoid-responsive transcription. Second, we demonstrate that, in vitro, PCAF directly associates with the DNA-binding domain of nuclear receptors, independently of p300/CBP binding, therefore defining a novel cofactor interaction surface. Furthermore, our results show that dissociation of corepressors enables ligand-dependent PCAF binding to the receptors. This observation illuminates how a ligand-dependent receptor function can be propagated to regions outside the ligand-binding domain itself. On the basis of these observations, we suggest that PCAF may play a more central role in nuclear receptor function than previously anticipated.

BXR, an embryonic orphan nuclear receptor activated by a novel class of endogenous benzoate metabolites

Nuclear receptors are ligand-modulated transcription factors that respond to steroids, retinoids, and thyroid hormones to control development and body physiology. Orphan nuclear receptors, which lack identified ligands, provide a unique, and largely untapped, resource to discover new principles of physiologic homeostasis. We describe the isolation and characterization of the vertebrate orphan receptor, BXR, which heterodimerizes with RXR and binds high-affinity DNA sites composed of a variant thyroid hormone response element. A bioactivity-guided screen of embryonic extracts revealed that BXR is activatable by low-molecular-weight molecules with spectral patterns distinct from known nuclear receptor ligands. Mass spectrometry and 1H NMR analysis identified alkyl esters of amino and hydroxy benzoic acids as potent, stereoselective activators. In vitro cofactor association studies, along with competable binding of radiolabeled compounds, establish these molecules as bona fide ligands. Benzoates comprise a new molecular class of nuclear receptor ligand and their activity suggests that BXR may control a previously unsuspected vertebrate signaling pathway.

Localization of nascent RNA and CREB binding protein with the PML-containing nuclear body

The cellular role of the PML-containing nuclear bodies also known as ND10 or PODs remains elusive despite links to oncogenesis and viral replication. Although a potential role in transcription has been considered, direct evidence has been lacking. By developing a novel in vivo nucleic acid labeling approach, we demonstrate the existence of nascent RNA polymerase II transcripts within this nuclear body. In addition, PML and the transactivation cofactor, CREB binding protein (CBP), colocalize within the nucleus. Furthermore, we show that CBP in contrast to PML is distributed throughout the internal core of the structure. Collectively, these findings support a role for this nuclear body in transcriptional regulation.

PPARgamma promotes monocyte/macrophage differentiation and uptake of oxidized LDL

The formation of foam cells from macrophages in the arterial wall is characterized by dramatic changes in lipid metabolism, including increased expression of scavenger receptors and the uptake of oxidized low-density lipoprotein (oxLDL). We demonstrate here that the nuclear receptor PPARgamma is induced in human monocytes following exposure to oxLDL and is expressed at high levels in the foam cells of atherosclerotic lesions. Ligand activation of the PPARgamma:RXRalpha heterodimer in myelomonocytic cell lines induces changes characteristic of monocytic differentiation and promotes uptake of oxLDL through transcriptional induction of the scavenger receptor CD36. These results reveal a novel signaling pathway controlling differentiation and lipid metabolism in monocytic cells, and suggest that endogenous PPARgamma ligands may be important regulators of gene expression during atherogenesis.

Oxidized LDL regulates macrophage gene expression through ligand activation of PPARgamma

Macrophage uptake of oxidized low-density lipoprotein (oxLDL) is thought to play a central role in foam cell formation and the pathogenesis of atherosclerosis. We demonstrate here that oxLDL activates PPARgamma-dependent transcription through a novel signaling pathway involving scavenger receptor-mediated particle uptake. Moreover, we identify two of the major oxidized lipid components of oxLDL, 9-HODE and 13-HODE, as endogenous activators and ligands of PPARgamma. Our data suggest that the biologic effects of oxLDL are coordinated by two sets of receptors, one on the cell surface, which binds and internalizes the particle, and one in the nucleus, which is transcriptionally activated by its component lipids. These results suggest that PPARgamma may be a key regulator of foam cell gene expression.

Role of the histone deacetylase complex in acute promyelocytic leukaemia

Non-liganded retinoic acid receptors (RARs) repress transcription of target genes by recruiting the histone deacetylase complex through a class of silencing mediators termed SMRT or N-CoR. Mutant forms of RARalpha, created by chromosomal translocations with either the PML (for promyelocytic leukaemia) or the PLZF (for promyelocytic leukaemia zinc finger) locus, are oncogenic and result in human acute promyelocytic leukaemia (APL). PML-RARalpha APL patients achieve complete remission following treatments with pharmacological doses of retinoic acids (RA); in contrast, PLZF-RARalpha patients respond very poorly, if at all. Here we report that the association of these two chimaeric receptors with the histone deacetylase (HDAC) complex helps to determine both the development of APL and the ability of patients to respond to retinoids. Consistent with these observations, inhibitors of histone deacetylase dramatically potentiate retinoid-induced differentiation of RA-sensitive, and restore retinoid responses of RA-resistant, APL cell lines. Our findings suggest that oncogenic RARs mediate leukaemogenesis through aberrant chromatin acetylation, and that pharmacological manipulation of nuclear receptor co-factors may be a useful approach in the treatment of human disease.

Terminal differentiation of human breast cancer through PPAR gamma

We have previously demonstrated that PPAR gamma stimulates the terminal differentiation of adipocyte precursors when activated by synthetic ligands, such as the antidiabetic thiazolidinedione (TZD) drugs. We show here that PPAR gamma is expressed at significant levels in human primary and metastatic breast adenocarcinomas. Ligand activation of this receptor in cultured breast cancer cells causes extensive lipid accumulation, changes in breast epithelial gene expression associated with a more differentiated, less malignant state, and a reduction in growth rate and clonogenic capacity of the cells. Inhibition of MAP kinase, shown previously to be a powerful negative regulator of PPAR gamma, improves the TZD ligand sensitivity of nonresponsive cells. These data suggest that the PPAR gamma transcriptional pathway can induce terminal differentiation of malignant breast epithelial cells and thus may provide a novel, nontoxic therapy for human breast cancer.

Energy deprivation and a deficiency in downstream metabolic target genes during the onset of embryonic heart failure in RXRalpha−/− embryos

RXRalpha null mutant mice display ocular and cardiac malformations, liver developmental delay, and die from cardiac failure around embryonic day (E) 14.5 pc. To dissect the molecular basis of the RXRalpha-associated cardiomyopathy, we performed subtractive hybridization and systematically characterized putative downstream target genes that were selectively lacking in the mutant embryos, both at early (E10.5) and late (E13.5) stages of mouse embryonic development. Approximately 50% of the subtracted clones (61/115) encoded proteins involved in intermediary metabolism and electron transport, suggesting an energy deficiency in the RXRalpha−/− embryos. In particular, clone G1, which encodes subunit 14.5b of the NADH-ubiquinone dehydrogenase complex, displayed a dose-dependent expression in the wild-type, heterozygous and RXRalpha mutant mice. This gene was also downregulated in a retinoid-deficient rat embryo model. ATP content and medium Acyl-CoA dehydrogenase mRNA were lower in RXRalpha mutant hearts compared to wild-type mice. Ultrastructural studies showed that the density of mitochondria per myocyte was higher in the RXRalpha mutant compared to wild-type littermates. We propose a model whereby defects in intermediary metabolism may be a causative factor of the RXRalpha−/− phenotype and resembles an embryonic form of dilated cardiomyopathy.

Vimentin: the conundrum of the intermediate filament gene family

Intermediate filaments are a major component of the "cytoskeleton" of "higher" eukaryotes. These filaments are composed of a number of different, although structurally related, proteins. Different intermediate filament protein genes are expressed in different tissues. Spontaneous and experimentally produced mutations in the intermediate filament genes indicate that these filaments function to enhance the mechanical stability of epidermal and muscle cells. As a result, the use of transgenic mice with "knockout" or dominant negative mutations in IF genes has become an important approach for investigating the significance of IFs in other cell types. However, a knockout mutation of vimentin (-/-), the intermediate filament protein characteristically expressed in cells of mesenchymal origin, results in very subtle phenotypes that are not obviously related to cell fragility. Although experiments with cultured cells have described a variety of discrete changes in cell properties that are associated with vimentin expression or organization, there is no evidence yet that any of these properties are affected in the vimentin-/- mouse.

The germ cell nuclear factor mGCNF is expressed in the developing nervous system

Using a reinoic acid receptor hybridization probe, we have isolated a mouse embryonic cDNA that encodes the germ cell nuclear factor (mGCNF). The in vitro translated protein binds specifically to the direct repeat of the sequence-AGGTCA-which is characteristic of a subclass of nuclear receptors, albeit with a spacing of zero that is unique among the receptors. Northern analysis shows embryonic expression after mouse gastrulation and during early organogenesis, as well as expression in the adult testis. The message level decreases during embryonic development. Whereas there are two transcripts in the testis, only the larger one is found in embryos. In situ analysis of embryos at days 6.5-10.5 of gestation shows that, in the early postimplantation period, high transcript levels are found in ectodermal cells and in the primitive streak. During further development the expression becomes more restricted. Mainly cells of the developing nervous system are expressing the receptor. Our findings imply that GCNF is involved in two apparently disparate developmental events.

Ligation independent cloning irrespective of restriction site compatibility

Here we report the use of exonuclease to expose complementary DNA between an insert and vector such that annealing becomes independent of restriction site compatibility. We demonstrate that unusual and, in some cases, previously impossible cloning strategies can be readily and efficiently achieved as long as the flanking sequences of the linear vectors are highly related. Furthermore, we show that the bacterial repair system resolves the residual mismatches, overhangs or gaps in a predictable fashion to generate excisable inserts. This approach facilitates cloning regardless of restriction site compatibility and overcomes an important limitation in current cloning techniques.

Inducible gene expression in mammalian cells and transgenic mice

Advances in biomedicine have accentuated the need to develop methods to deliberately modulate gene activity. In addition to improved versions of the system based on components of the tetracycline resistance operon, several strategies have recently emerged to control gene function at the transcriptional level. Particularly promising are approaches based on non-mammalian steroid hormones, and on small molecules that bind immunophilins.

Corepressor SMRT binds the BTB/POZ repressing domain of the LAZ3/BCL6 oncoprotein

The LAZ3/BCL6 (lymphoma-associated zinc finger 3/B cell lymphomas 6) gene frequently is altered in non-Hodgkin lymphomas. It encodes a sequence-specific DNA binding transcriptional repressor that contains a conserved N-terminal domain, termed BTB/POZ (bric-à-brac tramtrack broad complex/pox viruses and zinc fingers). Using a yeast two-hybrid screen, we show here that the LAZ3/BCL6 BTB/POZ domain interacts with the SMRT (silencing mediator of retinoid and thyroid receptor) protein. SMRT originally was identified as a corepressor of unliganded retinoic acid and thyroid receptors and forms a repressive complex with a mammalian homolog of the yeast transcriptional repressor SIN3 and the HDAC-1 histone deacetylase. Protein binding assays demonstrate that the LAZ3/BCL6 BTB/POZ domain directly interacts with SMRT in vitro. Furthermore, DNA-bound LAZ3/BCL6 recruits SMRT in vivo, and both overexpressed proteins completely colocalize in nuclear dots. Finally, overexpression of SMRT enhances the LAZ3/BCL6-mediated repression. These results define SMRT as a corepressor of LAZ3/BCL6 and suggest that LAZ3/BCL6 and nuclear hormone receptors repress transcription through shared mechanisms involving SMRT recruitment and histone deacetylation.

The orphan nuclear receptor LXRalpha is positively and negatively regulated by distinct products of mevalonate metabolism

LXRalpha is an orphan member of the nuclear hormone receptor superfamily that displays constitutive transcriptional activity. We reasoned that this activity may result from the production of an endogenous activator that is a component of intermediary metabolism. The use of metabolic inhibitors revealed that mevalonic acid biosynthesis is required for LXRalpha activity. Mevalonic acid is a common metabolite used by virtually all eukaryotic cells. It serves as a precursor to a large number of important molecules including farnesyl pyrophosphate, geranylgeranyl pyrophosphate, cholesterol, and oxysterols. Inhibition of LXRalpha could be reversed by addition of mevalonic acid and certain oxysterols but not by other products of mevalonic acid metabolism. Surprisingly, the constitutive activity of LXRalpha was inhibited by geranylgeraniol, a metabolite of mevalonic acid. These findings suggest that LXRalpha may represent a central component of a signaling pathway that is both positively and negatively regulated by multiple products of mevalonate metabolism.

Tetracycline regulated expression of vimentin in fibroblasts derived from vimentin null mice

Fibroblast cell lines were derived from vim-/- mice that express a mouse vimentin transgene in a tetracycline regulatable manner. Vimentin null mouse primary embryo fibroblasts were transformed with SV-40 early genes and vim- cell lines were isolated. A vim- cell line was then serially transfected with an expression plasmid encoding the tetracycline regulatable transactivator (tTA) and a mouse vimentin cDNA expression plasmid under the regulation of Escherichia coli tet operator and minimal CMV promoter sequences. Two stable cell lines were obtained that contained little or no vimentin in the presence of low concentrations of tetracycline but rapidly expressed abundant vimentin filaments after removal of tetracycline. The vimentin content of one cell line was similar to that of control vim+/+ fibroblasts. The level of transgene expression could be regulated by the concentration of tetracycline in a dose dependent fashion. Induction of vimentin expression in these cells did not observably affect cell growth, the distribution of microfilaments or microtubules, or the shape of the nucleus. Enucleation studies indicated that while disassembly of microfilaments significantly increased the sensitivity of the cells to enucleation, the presence or absence of vimentin had no detectable effect on the degree of enucleation with increasing sedimentation force. Monolayer wounding experiments demonstrated that vimentin expression did not alter the mobility of polarized cells at the edge of the wound. Experiments to more directly test the effect of vimentin expression on the capacity of these fibroblasts to survive mechanical trauma indicated that vimentin expression had no obvious effect on the survival of suspension cells subjected to nitrogen cavitation or the fraction of cells that survived the mechanical scraping of monolayer culture. These studies indicate that vimentin expression in a single population of cells does not have an obvious effect on cytoplasmic organization and provides a useful system to study the effects of IFs on the capacity of individual cells to resist mechanical injury.

Nuclear receptor coactivator ACTR is a novel histone acetyltransferase and forms a multimeric activation complex with P/CAF and CBP/p300

We report here the identification of a novel cofactor, ACTR, that directly binds nuclear receptors and stimulates their transcriptional activities in a hormone-dependent fashion. ACTR also recruits two other nuclear factors, CBP and P/CAF, and thus plays a central role in creating a multisubunit coactivator complex. In addition, and unexpectedly, we show that purified ACTR is a potent histone acetyltransferase and appears to define a distinct evolutionary branch to this recently described family. Thus, hormonal activation by nuclear receptors involves the mutual recruitment of at least three classes of histone acetyltransferases that may act cooperatively as an enzymatic unit to reverse the effects of histone deacetylase shown to be part of the nuclear receptor corepressor complex.

NMR spectroscopic studies of the DNA-binding domain of the monomer-binding nuclear orphan receptor, human estrogen related receptor-2. The carboxyl-terminal extension to the zinc-finger region is unstructured in the free form of the protein

Unlike steroid and retinoid receptors, which associate with DNA as dimers, human estrogen related receptor-2 (hERR2) belongs to a growing subclass of nuclear hormone receptors that bind DNA with high affinity as monomers. A carboxyl-terminal extension (CTE) to the zinc-finger domain has been implicated to be responsible for determining the stoichiometry of binding by a nuclear receptor to its response element. To better understand the mechanism by which DNA specificity is achieved, the solution structure of the DNA-binding domain of hERR2 (residues 96-194) consisting of the two putative zinc fingers and the requisite 26-amino acid CTE was analyzed by multidimensional heteronuclear magnetic resonance spectroscopy. The highly conserved zinc-finger region (residues 103-168) has a fold similar to those reported for steroid and retinoid receptors, with two helices that originate from the carboxyl-terminal ends of the two zinc fingers and that pack together orthogonally, forming a hydrophobic core. The CTE element of hERR2 is unstructured and highly flexible, exhibiting nearly random coil chemical shifts, extreme sensitivity of the backbone amide protons to solvent presaturation, and reduced heteronuclear (1H-15N) nuclear Overhauser effect values. This is in contrast to the dimer-binding retinoid X and thyroid hormone receptors, where, in each case, a helix has been observed within the CTE. The implications of this property of the hERR2 CTE are discussed.

A role for ultraspiracle, the Drosophila RXR, in morphogenetic furrow movement and photoreceptor cluster formation

Many of the same genes needed for proper eye and limb development in vertebrates, such as hairy, hedgehog, patched and cyclic AMP-dependent protein kinase A, are responsible for patterning Drosophila imaginal discs, the tissues that will give rise to the adult cuticle structures. This is well demonstrated in the control of morphogenetic furrow movement and differentiation in the eye imaginal disc. We report that ultraspiracle, the gene encoding the Drosophila cognate of the Retinoid X Receptor, is required for normal morphogenetic furrow movement and ommatidial cluster formation. Examination of the expression of genes involved in regulating the furrow suggests that ultraspiracle defines a novel regulatory pathway in eye differentiation.

Compartment-selective sensitivity of cardiovascular morphogenesis to combinations of retinoic acid receptor gene mutations

Several aspects of normal cardiovascular development require signaling by the vitamin A metabolite retinoic acid. We have previously established germ-line mutations in mice in the genes that encode the RAR alpha 1, RAR beta, and RXR alpha retinoic acid receptors as a means of studying the function of these receptors in vivo. Although mutation of RXR alpha results in fetal ventricular defects, the RAR alpha 1 and RAR beta mutations are apparently nonphenotypic in the heart and elsewhere. In this study, we have established and analyzed combinations of these receptor gene mutations. Malformations of the ventricular chamber (chamber hypoplasia and muscular ventricular septal defects), conotruncus (double-outlet right ventricle, transposition, and membranous ventricular septal defects), aortic sac (persistent truncus arteriosus and aorticopulmonary window), and aortic arch-derived arteries were recovered in various combinations of the RAR alpha 1, RAR beta, and RXR alpha gene mutations. Depending on the combination of receptor mutations, selective defects were obtained in specific cardiovascular compartments, suggestive of differential expression or function of each receptor within domains of the developing heart.

Nuclear receptor repression mediated by a complex containing SMRT, mSin3A, and histone deacetylase

The transcriptional corepressors SMRT and N-CoR function as silencing mediators for retinoid and thyroid hormone receptors. Here we show that SMRT and N-CoR directly interact with mSin3A, a corepressor for the Mad-Max heterodimer and a homolog of the yeast global-transcriptional repressor Sin3p. In addition, we demonstrate that the recently characterized histone deacetylase 1 (HDAC1) interacts with Sin3A and SMRT to form a multisubunit repressor complex. Consistent with this model, we find that HDAC inhibitors synergize with retinoic acid to stimulate hormone-responsive genes and differentiation of myeloid leukemia (HL-60) cells. This work establishes a convergence of repression pathways for bHLH-Zip proteins and nuclear receptors and suggests this type of regulation may be more widely conserved than previously suspected.

Familial chronic fatigue

Images

Hypolipidemic drugs, polyunsaturated fatty acids, and eicosanoids are ligands for peroxisome proliferator-activated receptors alpha and delta

Fatty acids (FAs) and their derivatives are essential cellular metabolites whose concentrations must be closely regulated. This implies that regulatory circuits exist which can sense changes in FA levels. Indeed, the peroxisome proliferator-activated receptor alpha (PPARalpha) regulates lipid homeostasis and is transcriptionally activated by a variety of lipid-like compounds. It remains unclear as to how these structurally diverse compounds can activate a single receptor. We have developed a novel conformation-based assay that screens activators for their ability to bind to PPARalpha/delta and induce DNA binding. We show here that specific FAs, eicosanoids, and hypolipidemic drugs are ligands for PPARalpha or PPARdelta. Because altered FA levels are associated with obesity, atherosclerosis, hypertension, and diabetes, PPARs may serve as molecular sensors that are central to the development and treatment of these metabolic disorders.

Retinoid receptors in development and disease

Nuclear receptors comprise a large family of ligand-dependent transcription factors that display considerable specificity in and selectivity in regulating the genetic programs they ultimately influence. The response to retinoic acid (RA) is mediated by two families of transcription factors which include the retinoic acids receptors (RARs) and the retinoid X receptors (RXRs). In human acute promyelocytic leukemia (APL), RAR alpha becomes an activated oncogene as a consequence of its fusion to the PML locus. Because patients with APL can be induced into remission with high dose RA therapy, we propose that PML-RAR is a new class of dominant negative oncogene that disrupts a structure that includes at least five other proteins. This mega-complex, referred to as a "POD", is disrupted in leukemic cells expressing the oncoprotein and is reassembled following high dose RA therapy in both cell culture an in patients.