A nuclear hormone receptor-associated protein that inhibits transactivation by the thyroid hormone and retinoic acid receptors.

Nuclear hormone receptors are transcription factors that require multiple protein-protein interactions to regulate the expression of their target genes. Using the yeast two-hybrid system, we identified a protein, thyroid hormone receptor uncoupling protein (TRUP), that specifically interacts with a region of the human thyroid hormone receptor (TR) consisting of the hinge region and the N-terminal portion of the ligand binding domain in a hormone-independent manner. Interestingly, TRUP inhibits transactivation by TR and the retinoic acid receptor but has no effect on the estrogen receptor or the retinoid X receptor in mammalian cells. We also demonstrate that TRUP exerts its action on TR and retinoic acid receptor by interfering with their abilities to interact with their DNA. TRUP represents a type of regulatory protein that modulates the transcriptional activity of a subclass of the nuclear hormone receptor superfamily by preventing interaction with their genomic response elements.

Immunohistochemical expression of oestrogen and progesterone receptors during experimental acute and chronic murine Schistosomiasis mansoni

INTRODUCTION: The responsibility of Schistosoma mansoni in female infertility is still controversial. This study was conducted to evaluate the effect of acute and chronic schistosomiasis mansoni infection on the endometrium using immunohistochemical analysis of uterine hormone receptor expression. METHODS: Twenty-four nonpregnant swiss albino mice were divided into three groups: control, noninfected; acute; and chronic Schistosoma mansoni infection. Histological sections of uterine specimens were examined by light microscope with an image analyzing system to detect structural histological, estrogen receptor (ER) and progesterone receptor (PR) expression in the endometrium. RESULTS: No secretory phase was detected in the endometrium in acute and chronic Schistosoma infection. Hormone receptor expression (ER and PR) showed statistically significant differences among the groups (p< 0.05), with significant low ER hormone expression in chronic infection, compared to control proliferative, control secretory and acute infection cases, and statistically significant high PR expression in both acute and chronic infection cases compared to the control secretory cases (p< 0.05). CONCLUSIONS: Schistosomiasis mansoni seems to have an important impact on the hormone expression of affected women. Further studies to explore the mechanism of such changes are recommended.

Role of thyroid hormones and their receptors in peripheral nerve regeneration.

After peripheral nerve injury in adult mammals, reestablishment of functional connections depends on several parameters including neurotrophic factors, the extracellular matrix, and hormones. However, little is known about the contribution of hormones to peripheral nerve regeneration. Thyroid hormones, which are required for the development and maturation of the central nervous system, are also important for the development of peripheral nerves. The action of triiodothyronine (T3) on responsive cells is mediated through nuclear thyroid hormone receptors (TRs) which modulate the expression of specific genes in target cells. Thus, to study the effect of T3, it is first necessary to know whether the target tissues possess TRs. The fact that sciatic nerve cells possess functional TRs suggests that these cells can respond to T3 and, as a consequence, that thyroid hormone may be involved in peripheral nerve regeneration. The silicone nerve guide model provides an excellent system to study the action of local administration of T3. Evidence from such studies demonstrate that animals treated locally with T3 at the level of transection have more complete regeneration of sciatic nerve and better functional recovery. Among the possible regulatory mechanisms by which T3 enhances peripheral nerve regeneration is rapid action on both axotomized neurons and Schwann cells which, in turn, produce a lasting and stimulatory effect on peripheral nerve regeneration. It is probable that T3 up- or down-regulates gene expression of one or more growth factors, extracellular matrix, or cell adhesion molecules, all of which stimulate peripheral nerve regeneration. This could explain the greater effect of T3 on nerve regeneration compared with the effect of any one growth factor or adhesion molecule.

Feedback on hypothalamic TRH transcription is dependent on thyroid hormone receptor N terminus.

The beta thyroid hormone receptor (TRbeta), but not TRalpha1, plays a specific role in mediating T(3)-dependent repression of hypothalamic TRH transcription. To investigate the structural basis of isoform specificity, we compared the transcriptional regulation and DNA binding obtained with chimeric and N-terminally deleted TRs. Using in vivo transfection assays to follow hypothalamic TRH transcription in the mouse brain, we found that TRbeta1 and chimeras with the TRbeta1 N terminus did not affect either transcriptional activation or repression from the rat TRH promoter, whereas N-terminally deleted TRbeta1 impaired T(3)-dependent repression. TRalpha1 or chimeras with the TRalpha1 N terminus reduced T(3)-independent transcriptional activation and blocked T(3)-dependent repression of transcription. Full deletion of the TRalpha1 N terminus restored ligand-independent activation of transcription. No TR isoform specificity was seen after transcription from a positive thyroid hormone response element. Gel mobility assays showed that all TRs tested bound specifically to the main negative thyroid hormone response element in the TRH promoter (site 4). Addition of neither steroid receptor coactivator 1 nor nuclear extracts from the hypothalamic paraventricular nuclei revealed any TR isoform specificity in binding to site 4. Thus N-terminal sequences specify TR T(3)-dependent repression of TRH transcription but not DNA recognition, emphasizing as yet unknown neuron-specific contributions to protein-promoter interactions in vivo.

Peroxisome proliferator-activated receptors: insight into multiple cellular functions.

Peroxisome proliferator-activated receptors, PPARs, (NR1C) are nuclear hormone receptors implicated in energy homeostasis. Upon activation, these ligand-inducible transcription factors stimulate gene expression by binding to the promoter of target genes. The different structural domains of PPARs are presented in terms of activation mechanisms, namely ligand binding, phosphorylation, and cofactor interaction. The specificity of ligands, such as fatty acids, eicosanoids, fibrates and thiazolidinediones (TZD), is described for each of the three PPAR isotypes, alpha (NR1C1), beta (NR1C2) and gamma (NR1C3), so as the differential tissue distribution of these isotypes. Finally, general and specific functions of the PPAR isotypes are discussed, namely their implication in the control of inflammatory responses, cell proliferation and differentiation, the roles of PPARalpha in fatty acid catabolism and of PPARgamma in adipogenesis.

Expression cloning of the pancreatic beta cell receptor for the gluco-incretin hormone glucagon-like peptide 1.

Glucagon-like peptide 1 (GLP-1) is a hormone derived from the preproglucagon molecule and is secreted by intestinal L cells. It is the most potent stimulator of glucose-induced insulin secretion and also suppresses in vivo acid secretion by gastric glands. A cDNA for the GLP-1 receptor was isolated by transient expression of a rat pancreatic islet cDNA library into COS cells; this was followed by binding of radiolabeled GLP-1 and screening by photographic emulsion autoradiography. The receptor transfected into COS cells binds GLP-1 with high affinity and is coupled to activation of adenylate cyclase. The receptor binds specifically GLP-1 and does not bind peptides of related structure and similar function, such as glucagon, gastric inhibitory peptide, vasoactive intestinal peptide, or secretin. The receptor is 463 amino acids long and contains seven transmembrane domains. Sequence homology is found only with the receptors for secretin, calcitonin, and parathyroid hormone, which form a newly characterized family of G-coupled receptors.

Peroxisome proliferator-activated receptors mediate host cell proinflammatory responses to Pseudomonas aeruginosa autoinducer.

The pathogenic bacterium Pseudomonas aeruginosa utilizes the 3-oxododecanoyl homoserine lactone (3OC(12)-HSL) autoinducer as a signaling molecule to coordinate the expression of virulence genes through quorum sensing. 3OC(12)-HSL also affects responses in host cells, including the upregulation of genes encoding inflammatory cytokines. This proinflammatory response may exacerbate underlying disease during P. aeruginosa infections. The specific mechanism(s) through which 3OC(12)-HSL influences host responses is unclear, and no mammalian receptors for 3OC(12)-HSL have been identified to date. Here, we report that 3OC(12)-HSL increases mRNA levels for a common panel of proinflammatory genes in murine fibroblasts and human lung epithelial cells. To identify putative 3OC(12)-HSL receptors, we examined the expression patterns of a panel of nuclear hormone receptors in these two cell lines and determined that both peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) and PPARgamma were expressed. 3OC(12)-HSL functioned as an agonist of PPARbeta/delta transcriptional activity and an antagonist of PPARgamma transcriptional activity and inhibited the DNA binding ability of PPARgamma. The proinflammatory effect of 3OC(12)-HSL in lung epithelial cells was blocked by the PPARgamma agonist rosiglitazone, suggesting that 3OC(12)-HSL and rosiglitazone are mutually antagonistic negative and positive regulators of PPARgamma activity, respectively. These data identify PPARbeta/delta and PPARgamma as putative mammalian 3OC(12)-HSL receptors and suggest that PPARgamma agonists may be employed as anti-inflammatory therapeutics for P. aeruginosa infections.

Differential distribution of thyroid hormone receptor isoform in rat dorsal root ganglia and sciatic nerve in vivo and in vitro.

Using autoradiographic techniques carried out under precise conditions we previously demonstrated that both sensory neurons and peripheral glial cells in dorsal root ganglia (DRG) or sciatic nerve, possess specific [125I]-labeled T3 binding sites. Thyroid hormone receptors (TR) include several isoforms (TR alpha(1), TR alpha(2), TR beta(1), TR beta(2...)) The present study demonstrates that while sensory neurons and peripheral glial cells both possess functional TR, they express a differential expression of TR isoforms. Using a panel of antisera to specific for the TR alpha-common (alpha(1) and alpha(2)), TR alpha-1 or TR beta-1 isoforms, we detected TRs isoform localization at the cellular level during DRG and sciatic nerve development and regeneration. Immunohistochemical analysis revealed that during embryonic life, sensory neurons express TR alpha-common and TR beta-1 rather than TR alpha-1. The number of TR alpha-common and TR beta-1 positive neurons as well as the intensity of labeling increased during the first two postnatal weeks and remained more or less stable in adult life. TR alpha-1 immunoreactivity, which was undetectable in embryonic sensory neurons, became discreetly visible in neurons after birth. In developing DRG and sciatic nerves, Schwann cells exhibited TR alpha-common and TR alpha-1 rather than TR beta-1 immunolabeling. The appearance of TR alpha-common and alpha-1 isoform immunoreactivity in the sciatic nerve was restricted to a short period ranging from E17 up to two postnatal weeks. By comparing TR alpha-common and TR alpha-1 immunostaining we can deduce that Schwann cells primarily express TR alpha-1. Afterwards, in adult rat sciatic nerve TR alpha isoforms was no more detected. However transection of sciatic nerve caused a reexpression of TR alpha isoforms in degenerating nerve. The prevalence of TR alpha in Schwann cells in vivo was correlated with in vitro results. The differential expression of TR alpha and beta by sensory neurons and Schwann cells indicates that the feedback regulation of circulating thyroid hormone could occur by binding to either the alpha or beta TR isoforms. Moreover, the presence of multiple receptor isoforms in developing sensory neurons suggests that thyroid hormone uses multiple signaling pathways to regulate DRG and sciatic nerve development.

The N-terminal DNA-binding 'zinc finger' of the oestrogen and glucocorticoid receptors determines target gene specificity.

Steroid hormone receptors activate specific gene transcription by binding as hormone-receptor complexes to short DNA enhancer-like elements termed hormone response elements (HREs). We have shown previously that a highly conserved 66 amino acid region of the oestrogen (ER) and glucocorticoid (GR) receptors, which corresponds to part of the receptor DNA binding domain (region C) is responsible for determining the specificity of target gene activation. This region contains two sub-regions (CI and CII) analogous to the 'zinc-fingers' of the transcription factor TFIIIA. We show here that CI and CII appear to be separate domains both involved in DNA binding. Furthermore, using chimaeric ERs in which either the first (N-terminal) (CI) or second (CII) 'zinc finger' region has been exchanged with that of the GR, indicates that it is the first 'zinc finger' which largely determines target gene specificity. We suggest that receptor recognition of the HRE is analogous to that of the helix-turn-helix DNA binding motif in that the receptor binds to DNA as a dimer with the first 'zinc finger' lying in the major groove recognizing one half of the palindromic HRE, and that protein-DNA interaction is stabilized through non-specific DNA binding and dimer interactions contributed by the second 'zinc finger'.

Peroxisome proliferator-activated receptors A link between endocrinology and nutrition?

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily like the steroid, thyroid, or retinoid hormone receptors, which are ligand-activated transcription factors regulating gene expression. PPARs mediate the induction of the enzymes of the peroxisomal and microsomal fatty-acid oxidation pathways by hypolipidemic drugs such as clofibrate and are probably also involved in the gene expression of other lipid-metabolism-associated proteins that are controlled by fibrate hypolipidemic drugs. That PPARs play an important role in the regulation of lipid metabolism is reinforced by the discovery of their activation by physiologic concentrations of fatty acids. This observation raises the question of whether fatty acids are ligands of PPARs, which would imply that nutritional fatty acids can act like hormones.

Positive regulation of the peroxisomal beta-oxidation pathway by fatty acids through activation of peroxisome proliferator-activated receptors (PPAR).

Peroxisome proliferators regulate the transcription of genes by activating ligand-dependent transcription factors, which, due to their structure and function, can be assigned to the superfamily of nuclear hormone receptors. Three such peroxisome proliferator-activated receptors (PPAR alpha, beta, and gamma) have been cloned in Xenopus laevis. Their mRNAs are expressed differentially; xPPAR alpha and beta but not xPPAR gamma are expressed in oocytes and embryos. In the adult, expression of xPPAR alpha and beta appears to be ubiquitous, and xPPAR gamma is mainly observed in adipose tissue and kidney. Immunocytochemical analysis revealed that PPARs are nuclear proteins, and that their cytoplasmic-nuclear translocation is independent of exogenous activators. A target gene of PPARs is the gene encoding acyl-CoA oxidase (ACO), which catalyzes the rate-limiting step in the peroxisomal beta-oxidation of fatty acids. A peroxisome proliferator response element (PPRE), to which PPARs bind, has been identified within the promoter of the ACO gene. Besides the known xenobiotic activators of PPARs, such as hypolipidemic drugs, natural activators have been identified. Polyunsaturated fatty acids at physiological concentrations are efficient activators of PPARs, and 5,8,11,14-eicosatetraynoic acid (ETYA), which is the alkyne homolog of arachidonic acid, is the most potent activator of xPPAR alpha described to date. Taken together, our data suggest that PPARs have an important role in lipid metabolism.

Peroxisome-proliferator-activated receptors and cancers: complex stories.

Peroxisome-proliferator-activated receptors (PPARs) are nuclear hormone receptors that mediate the effects of fatty acids and their derivatives at the transcriptional level. Through these pathways, PPARs can regulate cell proliferation, differentiation and survival, so controlling carcinogenesis in various tissues. But what are the links between each PPAR isotype and carcinogenesis and what is the relevance of these findings to human pathology and therapy?

Peroxisome proliferator-activated receptors and lipid metabolism.

PPARs are nuclear hormone receptors which, like the retinoid, thyroid hormone, vitamin D, and steroid hormone receptors, are ligand-activated transcription factors mediating the hormonal control of gene expression. Two lines of evidence indicate that PPARs have an important function in fatty acid metabolism. First, PPARs are activated by hypolipidemic drugs and physiological concentrations of fatty acids, and second, PPARs control the peroxisomal beta-oxidation pathway of fatty acids through transcriptional induction of the gene encoding the acyl-CoA oxidase (ACO), which is the rate-limiting enzyme of the pathway. Furthermore, the PPAR signaling pathway appears to converge with the 9-cis retinoic acid receptor (RXR) signaling pathway in the regulation of the ACO gene because heterodimerization between PPAR and RXR is essential for in vitro binding to the PPRE and because the strongest stimulation of this gene is observed when both receptors are exposed simultaneously to their activators. Thus, it appears that PPARs are involved in the 9-cis retinoic acid signaling pathway and that they play a pivotal role in the hormonal control of lipid metabolism.

Fatty acids, eicosanoids, and hypolipidemic agents identified as ligands of peroxisome proliferator-activated receptors by coactivator-dependent receptor ligand assay.

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors controlling the expression of genes involved in lipid homeostasis. PPARs activate gene transcription in response to a variety of compounds including hypolipidemic drugs as well as natural fatty acids. From the plethora of PPAR activators, Scatchard analysis of receptor-ligand interactions has thus far identified only four ligands. These are the chemotactic agent leukotriene B4 and the hypolipidemic drug Wy 14,643 for the alpha-subtype and a prostaglandin J2 metabolite and synthetic antidiabetic thiazolidinediones for the gamma-subtype. Based on the hypothesis that ligand binding to PPAR would induce interactions of the receptor with transcriptional coactivators, we have developed a novel ligand sensor assay, termed coactivator-dependent receptor ligand assay (CARLA). With CARLA we have screened several natural and synthetic candidate ligands and have identified naturally occurring fatty acids and metabolites as well as hypolipidemic drugs as bona fide ligands of the three PPAR subtypes from Xenopus laevis. Our results suggest that PPARs, by their ability to interact with a number of structurally diverse compounds, have acquired unique ligand-binding properties among the superfamily of nuclear receptors that are compatible with their biological activity.