Vitamin D interferes with transactivation of the growth hormone gene by thyroid hormone and retinoic acid

Impaired Vitamin D Signaling in T Cells From a Family With Hereditary Vitamin D Resistant Rickets

The active form of vitamin D, 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), mediates its immunomodulatory effects by binding to the vitamin D receptor (VDR). Here, we describe a new point mutation in the DNA-binding domain of the VDR and its consequences for 1,25(OH)₂D₃ signaling in T cells from heterozygous and homozygous carriers of the mutation. The mutation did not affect the overall structure or the ability of the VDR to bind 1,25(OH)₂D₃ and the retinoid X receptor. However, the subcellular localization of the VDR was strongly affected and the transcriptional activity was abolished by the mutation. In heterozygous carriers of the mutation, 1,25(OH)₂D₃-induced gene regulation was reduced by ~ 50% indicating that the expression level of wild-type VDR determines 1,25(OH)₂D₃ responsiveness in T cells. We show that vitamin D-mediated suppression of vitamin A-induced gene regulation depends on an intact ability of the VDR to bind DNA. Furthermore, we demonstrate that vitamin A inhibits 1,25(OH)₂D₃-induced translocation of the VDR to the nucleus and 1,25(OH)₂D₃-induced up-regulation of CYP24A1. Taken together, this study unravels novel aspects of vitamin D signaling and function of the VDR in human T cells.

Vitamin D deficiency serves as a precursor to stunted growth and central adiposity in zebrafish

Emerging evidence demonstrates the importance of sufficient vitamin D (1α, 25-dihydroxyvitamin D3) levels during early life stage development with deficiencies associated with long-term effects into adulthood. While vitamin D has traditionally been associated with mineral ion homeostasis, accumulating evidence suggests non-calcemic roles for vitamin D including metabolic homeostasis. In this study, we examined the hypothesis that vitamin D deficiency (VDD) during early life stage development precedes metabolic disruption. Three dietary cohorts of zebrafish were placed on engineered diets including a standard laboratory control diet, a vitamin D null diet, and a vitamin D enriched diet. Zebrafish grown on a vitamin D null diet between 2-12 months post fertilization (mpf) exhibited diminished somatic growth and enhanced central adiposity associated with accumulation and enlargement of visceral and subcutaneous adipose depots indicative of both adipocyte hypertrophy and hyperplasia. VDD zebrafish exhibited elevated hepatic triglycerides, attenuated plasma free fatty acids and attenuated lipoprotein lipase activity consistent with hallmarks of dyslipidemia. VDD induced dysregulation of gene networks associated with growth hormone and insulin signaling, including induction of suppressor of cytokine signaling. These findings indicate that early developmental VDD impacts metabolic health by disrupting the balance between somatic growth and adipose accumulation.

Are plasma 25-hydroxyvitamin D and retinol levels and one-carbon metabolism related to metabolic syndrome in patients with a severe mental disorder?

There is a scarcity of studies assessing the influence of biomarkers in metabolic syndrome in psychiatric patients. Our aim was to correlate serum or plasma levels of 25-hydroxyvitamin D (25-OH-VD), retinol, vitamin B12 (VB12), folate and homocysteine (Hcy), with the metabolic status, in a sample of 289 outpatients with Schizophrenia or Bipolar Disorder. Logistic regression and multiple linear regressions were performed to assess the ability of biomarkers to predict the presence of MetS, the number of risk factors for MetS, and insulin resistance indexes (HOMA and QUICKI). Regarding the association between biomarkers and the QUICKI index, the model explained 6.8% of the variance, with folate and 25-OH-VD levels contributing significantly to the model. The model predicting the number of MetS risk factors was significant and explained 21.7% of the variance, being 25-OH-VD and retinol the statistically significant factors. As for the impact of biomarkers on MetS, the model was statistically significant, being 25-OH-VD and retinol levels the significant factors.  We report for the first time an association between MetS and both low 25-OH-VD and high retinol concentrations. Inflammation-related biomarkers may help identify patients with a high risk of MetS who might benefit from healthy lifestyle counselling and early intervention.

Hotspots for Vitamin-Steroid-Thyroid Hormone Response Elements Within Switch Regions of Immunoglobulin Heavy Chain Loci Predict a Direct Influence of Vitamins and Hormones on B Cell Class Switch Recombination

Vitamin A deficiencies are common throughout the world and have a significant negative influence on immune protection against viral infections. Mouse models demonstrate that the production of IgA, a first line of defense against viruses at mucosal sites, is inhibited in the context of vitamin A deficiency. In vitro, the addition of vitamin A to activated B cells can enhance IgA expression, but downregulate IgE. Previous reports have demonstrated that vitamin A modifies cytokine patterns, and in so doing may influence antibody isotype expression by an indirect mechanism. However, we have now discovered hundreds of potential response elements among Sμ, Sɛ, and Sα switch sites within immunoglobulin heavy chain loci. These hotspots appear in both mouse and human loci and include targets for vitamin receptors and related proteins (e.g., estrogen receptors) in the nuclear receptor superfamily. Full response elements with direct repeats are relatively infrequent or absent in Sγ regions although half-sites are present. Based on these results, we pose a hypothesis that nuclear receptors have a direct effect on the immunoglobulin heavy chain class switch recombination event. We propose that vitamin A may alter S site accessibility to activation-induced deaminase and nonhomologous end-joining machinery, thereby influencing the isotype switch, antibody production, and protection against viral infections at mucosal sites.

24, 25-dihydroxycholecalciferol but not 25-hydroxycholecalciferol suppresses apolipoprotein A-I gene expression

AIMS

Ligands for the vitamin D receptor (VDR) regulate apolipoprotein A-I (apo A-I) gene expression in a tissue-specific manner. The vitamin D metabolite 24, 25-dihydroxycholecalciferol (24, 25-(OH)(2)D(3)) has been shown to possess unique biological effects. To determine if 24, 25-(OH)(2)D(3) modulates apo A-I gene expression, HepG2 hepatocytes and Caco-2 intestinal cells were treated with 24, 25-(OH)(2)D(3) or its precursor 25-OHD(3).

MAIN METHODS

Apo A-I protein levels and mRNA levels were measured by Western and Northern blotting, respectively. Changes in apo A-I promoter activity were measured using the chlorampenicol acetytransferase assay.

KEY FINDINGS

Treatment with 24, 25-(OH)(2)D(3), but not 25-OHD(3), inhibited apo A-I secretion in HepG2 and Caco-2 cells and apo A-I mRNA levels and apo A-I promoter activity in HepG2 cells. To determine if 24, 25-(OH)(2)D(3) represses apo A-I gene expression through site A, the nuclear receptor binding element that is essential for VDRs effects on apo A-I gene expression, HepG2 cells were transfected with plasmids containing or lacking site A. While the site A-containing plasmid was suppressed by 24, 25-(OH)(2)D(3), the plasmid lacking site A was not. Likewise, treatment with 24, 25-(OH)(2)D(3) suppressed reporter gene expression in cells transfected with a plasmid containing site A in front of a heterologous promoter. Finally, antisense-mediated VDR depletion failed to reverse the silencing effects of 24, 25-(OH)(2)D(3) on apo A-I expression.

SIGNIFICANCE

These results suggest that the vitamin D metabolite 24, 25-(OH)(2)D(3) is an endogenous regulator of apo A-I synthesis through a VDR-independent signaling mechanism.

Transactivation of a growth hormone (GH) promoter-luciferase construct in canine mammary cells

The gene encoding growth hormone (GH) is expressed not only in the pituitary but also in a variety of non-pituitary tissues. In the female dog, progestins are known to stimulate GH expression in the mammary gland. In order to investigate the regulation of the GH gene expression in the mammary gland, we transfected the canine mammary tumor cell line CMT-U229 with 3 different canine GH promoter-luciferase constructs. The constructs, varying in length between 252 bp and 673 bp, were transfected followed by an incubation for 4 h, 24 h and 48 h with cAMP, all-trans-retinoic acid (RA), 3,3',5-triiodothyronine (T3), 1,25-dihydroxy-vitamin D (VitD), progesterone and EGF. Promoter activity was stimulated by cAMP, T3 and RA whereas VitD clearly inhibited gene expression. However, despite the presence of nuclear and membrane receptors for progesterone, no direct effects of progesterone on promoter activity could be demonstrated. It is concluded that progesterone alone has no direct stimulatory effect on GH transcription. This finding is discussed in relation to the slow onset of progesterone-stimulated GH release in vivo and the absence of Pit-1 in canine mammary tissue.

The menace of endocrine disruptors on thyroid hormone physiology and their impact on intrauterine development

The delivery of the appropriate thyroid hormones quantity to target tissues in euthyroidism is the result of unopposed synthesis, transport, metabolism, and excretion of these hormones. Thyroid hormones homeostasis depends on the maintenance of the circulating 'free' thyroid hormone reserves and on the development of a dynamic balance between the 'free' hormones reserves and those of the 'bound' hormones with the transport proteins. Disturbance of this hormone system, which is in constant interaction with other hormone systems, leads to an adaptational counter-response targeting to re-establish a new homeostatic equilibrium. An excessive disturbance is likely to result, however, in hypo- or hyper- thyroid clinical states. Endocrine disruptors are chemical substances forming part of 'natural' contaminating agents found in most ecosystems. There is abundant evidence that several key components of the thyroid hormones homeostasis are susceptible to the action of endocrine disruptors. These chemicals include some chlorinated organic compounds, polycyclic aromatic hydrocarbons, herbicides, and pharmaceutical agents. Intrauterine exposure to endocrine disruptors that either mimic or antagonize thyroid hormones can produce permanent developmental disorders in the structure and functioning of the brain, leading to behavioral changes. Steroid receptors are important determinants of the consequences of endocrine disruptors. Their interaction with thyroid hormones complicates the effect of endocrine disruptors. The aim of this review is to present the effect of endocrine disruptors on thyroid hormones physiology and their potential impact on intrauterine development.

Prepubertal testis development relies on retinoic acid but not rexinoid receptors in Sertoli cells

Sertoli cells (SC) are instrumental to stem spermatogonia differentiation, a process that critically depends on retinoic acid (RA). We show here that selective ablation of RA receptor alpha (RARalpha) gene in mouse SC, singly (Rara(Ser-/-) mutation) or in combination with RARbeta and RARgamma genes (Rara/b/g(Ser-/-) mutation), abolishes cyclical gene expression in these cells. It additionally induces testis degeneration and delays spermatogonial expression of Stra8, two hallmarks of RA deficiency. As identical defects are generated upon inactivation of RARalpha in the whole organism, our data demonstrate that all the functions exerted by RARalpha in male reproduction are Sertoli cell-autonomous. They further indicate that RARalpha is a master regulator of the cyclical activity of SC and controls paracrine pathways required for spermatogonia differentiation and germ cell survival. Most importantly, we show that the ablation of all RXR (alpha, beta and gamma isotypes) in SC does not recapitulate the phenotype generated upon ablation of all three RARs, thereby providing the first evidence that RARs exert functions in vivo independently of RXRs.

Vitamin D and its analog EB1089 induce p27 accumulation and diminish association of p27 with Skp2 independent of PTEN in pituitary corticotroph cells

Disruption of the gene for the cyclin dependent kinase inhibitor (CDKI) p27/kip1 results in pituitary corticotroph hyperplasia while diminished expression of this protein has been described in aggressive human pituitary tumors. We have previously shown that 1,25-vitamin D3 (VD) hypophosphorylates p27 and interferes with the degradation of this CDKI in thyroid carcinoma cells. In this study we investigated whether VD/EB1089 can induce p27 accumulation and cause growth arrest of pituitary corticotroph cells. VD and EB1089 exhibited a significant reduction in AtT20 corticotroph but not PRL235 lactotroph cell growth. These changes were accompanied by selective accumulation of p27 in AtT20 but not in PRL235 cells. As p27 levels are highly dependent on protein degradation, we examined the effect of VD/EB1089 on p27 association with factors that target this CDKI to the proteasome. VD/EB1089 significantly restricted the association of p27 with Skp2 as well as with cyclin dependent kinase 2 (CDK2). As the tumor suppressor and phosphatase PTEN has been implicated in p27 regulation, we tested whether the effects of VD/EB1089 on p27 accumulation in corticotrophs could be mediated through this pathway. VD/EB1089 did not appreciably alter PTEN expression. Moreover, transfection of PTEN did not influence the effect of VD on p27 accumulation in corticotrophs. We conclude that VD/EB1089 can selectively arrest pituitary corticotroph growth and induce p27 accumulation.This effect is mediated at least partially through diminished p27 association with Skp2 and with CDK2. In contrast to other cell systems, PTEN does not participate in the regulation of corticotroph p27 and is not involved in mediating the effect of VD on p27 in these cells. Our findings highlight p27 and VD analogs as targets for manipulation and drug development respectively in the treatment of inoperable corticotroph adenomas.

Cooperative action of 1α,25-dihydroxyvitamin D3 and retinoic acid in NB4 acute promyelocytic leukemia cell differentiation is transcriptionally controlled

All-trans-retinoic acid (RA) and 1alpha,25-dihydroxyvitamin D3 (1,25D3) are involved in the control of hematopoiesis and have been suggested to play a role in cellular differentiation and are as such potent inducers of differentiation of myeloid leukemia cells. In this study, we show that, in promyelocytic NB4 cells, addition of 1,25D3 enhances terminal granulocytic RA-dependent differentiation concomitant with the enhanced activation of the RA transcriptional activity through an RARbeta promoter. By EMSA and ChIP assays, we further demonstrate that, while both VDR and RAR are bound to the RARbeta promoter in NB4 cells, addition of 1,25D3 increases VDR binding to this promoter, while that of RA induces the release of VDR and increases the binding of RAR. Thus, contrary to normal myeloid cells, 1,25D3 does not act as a transrepressor of RA transcriptional activity in leukemic cells, suggesting that transcriptional regulation of RA-target genes may be modified in malignant cells. In promyelocytic leukemic cells, the combination of 1,25D3 and RA results in both enhanced transactivation and differentiation.

Peroxisome proliferator activated receptor-gamma (PPARgamma) represses thyroid hormone signaling in growth plate chondrocytes

Peroxisome proliferator activated receptors (PPARs) are DNA-binding nuclear hormone receptors that are upregulated in response to high fat diets. PPARs are structurally related to the type II nuclear receptors, including the thyroid hormone receptors (TRs). To investigate if PPARs modulate TR-mediated terminal differentiation of growth plate chondrocytes, primary cultures of epiphyseal chondrocytes transiently transfected with TRalpha and PPARgamma expression vectors were treated with the PPAR ligands ciglitazone or troglitazone. Forced overexpression of PPARgamma decreased TRalpha1-mediated transcriptional activity and suppressed T3-induced increases in alkaline phosphatase activity and type X collagen expression. Similar effects were observed when the cells were treated with the PPARgamma activator ciglitazone or troglitazone. Overexpression of retinoid X receptor-alpha (RXRalpha) partially restored not only the inhibition of transcriptional activation by PPARgamma but also T3-induced hypertrophic differentiation. These data demonstrate that activation of PPARgamma signaling by either addition of PPARgamma ligands or overexpression of PPARgamma in growth plate chondrocytes inhibits TR-mediated gene transcription and inhibits the biological effects of thyroid hormone on terminal differentiation. The molecular mechanism involved in this inhibition appears to be competition between PPARgamma and TRalpha for limiting amounts of the heterodimeric partner RXR.

The repressor DREAM acts as a transcriptional activator on Vitamin D and retinoic acid response elements

DREAM (downstream regulatory element antagonist modulator) is a transcriptional repressor, which binds DREs (downstream response elements) in a Ca2+-regulated manner. The DREs consist of core GTCA motifs, very similar to binding motifs for non-steroid nuclear receptors. In this work, we find that DREAM stimulates basal and ligand-dependent activation of promoters containing vitamin D and retinoic acid response elements (VDREs and RAREs), consisting of direct repeats of the sequence AGT/GTCA spaced by 3 or 5 nt, respectively. Stimulation occurs when the element is located upstream, but not downstream, the transcription initiation site. Activation requires both Ca2+ binding to the EF-hands and the leucine-charged domains (LCDs), analogous to those responsible for the interaction of the nuclear receptors with coregulators. Further more, DREAM can bind both 'in vitro' and in chromatin immunoprecipitation assays to these elements. Importantly, 'in vivo' binding is only observed in vitamin D- or RA-treated cells. These results show that DREAM can function as an activator of transcription on certain promoters and demonstrate a novel role for DREAM acting as a potential modulator of genes containing binding sites for nuclear receptors.

A permissive retinoid X receptor/thyroid hormone receptor heterodimer allows stimulation of prolactin gene transcription by thyroid hormone and 9-cis-retinoic acid

Heterodimers of the retinoid X receptor (RXR) with the thyroid hormone receptor (TR) are considered to be nonpermissive. It is believed that within these complexes RXR acts as a "silent partner." We demonstrate here that a permissive heterodimer mediates stimulation of prolactin expression by the thyroid hormone T3 and by 9-cis retinoic acid (9-cis-RA). A response element located in the prolactin distal enhancer mediates transactivation by both ligands in pituitary cells, and RXR recruits coactivators when bound to this element as a heterodimer with TR. Furthermore, transcription by the RXR agonist can be obtained in CV-1 cells only after overexpression of coactivators, and overexpression of corepressors inhibits the response in pituitary cells. Thus, cell type-specific differences in coregulator recruitment can determine the cellular response to both ligands. Coactivator recruitment by 9-cis-RA requires the ligand-dependent transactivation domains (AF-2) of both heterodimeric partners. Interestingly, the presence of the RXR ligand can overcome the deleterious effect of the AF-2 mutation E401Q on association with coactivators and transactivation. These results demonstrate an unexpected role for RXR in TR signaling and show that in particular cellular environments this receptor can act as a "nonsilent" partner of TR, allowing stimulation by RXR agonists.

Localization of a negative vitamin D response sequence in the human growth hormone gene

1,25-Dihydroxyvitamin D(3) [1,25(OH)(2) D(3)] exerts its biological effects by binding to the vitamin D receptor (VDR), which binds in turn to the vitamin D response elements located in the target gene's promoter. We have previously demonstrated that VDR binds in vitro with high affinity to the 5'-flanking sequence of the human growth hormone (hGH) gene. In this study, we analyzed the response to 1,25(OH)(2) D(3) of hGH-promoter constructs introduced by transfection into the MCF-7 human adenocarcinoma cell line. We found that the transcriptional activity of some of these constructs was markedly reduced by 1,25(OH)(2) D(3). Deletion analyses revealed that a 34-bp sequence located between positions -62 and -29 upstream of the transcription start site is sufficient for this repressive response. This conclusion was also confirmed by gel mobility shift assays. Our results indicate that vitamin D inhibits hGH gene transcription, directly or by interference with other transcription factors.

T3-activation of the rat growth hormone gene is inhibited by a zinc finger/homeodomain protein

Since the transcription factor Zfhep is expressed in somatotropes and binds the rat growth hormone (rGH) gene T3-response element (TRE), we investigated whether Zfhep regulates the response of this gene to T3. In cotransfection experiments, Zfhep did not regulate the native rGH promoter in the absence of T3. However, Zfhep repressed T3-mediated activation significantly in either GH(3) or JEG-3 cells. Up to 70% repression was mediated through the rGH TRE in a heterologous promoter (thymidine kinase), but was not observed with the idealized DR4 or chicken lysozyme F2 TREs. Zfhep apparently does not repress T3-mediated activation simply by competition for binding to DNA since the C-terminal DNA-binding domain of Zfhep (which is sufficient for DNA-binding) is not sufficient for repression and since cotransfection of excess thyroid hormone receptor (TR) did not prevent repression by Zfhep. These data indicate that the rGH TRE is a composite element that can integrate Zfhep and T3 regulation.

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.

Nerve growth factor activates the RARbeta2 promoter by a Ras-dependent mechanism

Nerve growth factor (NGF) and retinoic acid (RA) exert important actions on PC12 cells. We have previously shown that incubation with NGF induces retinoic acid receptor beta (RARbeta) binding to a hormone response element in PC12 cells. In this study we show that NGF increases RARbeta protein levels by enhancing basal RARbeta2 promoter activity, and potentiates stimulation by RA in transient transfection assays. The effect of RA is mediated by a RA response element (RARE) located at -37/-53 and mutation of this element abolishes activation by the retinoid, as well as cooperation with NGF. However, the action of NGF is independent of the RARE and is mediated by sequences overlapping the TATA box and the INR comprising nucleotides -59 to +14. NGF produces a strong decrease in some of the complexes that bind to the INR. These results suggest that the RARbeta2 gene could be in a basal repressed state and NGF could increase RARbeta2 transcription by inducing the release of some inhibitory factors from the INR. Functional Ras is required for RARbeta2 promoter activation by NGF because expression of oncogenic Ras increases promoter activity and a dominant inhibitory Ras mutant blocks the effect of NGF. Oncogenic Raf also mimics the effect of NGF on the promoter. Other ligands of tyrosine kinase receptors that stimulate Ras also cause RARbeta2 promoter activation and act cooperatively with RA. These results indicate the existence of cross-coupling of the Ras-Raf signal transduction pathway with retinoid receptor pathways which could increase sensitivity to RA and be important for PC12 cell function.

Nutrition-hormone receptor-gene interactions: implications for development and disease

Nutrition profoundly alters the phenotypic expression of a given genotype, particularly during fetal and postnatal development. Many hormones act as nutritional signals and their receptors play a key role in mediating the effects of nutrition on numerous genes involved in differentiation, growth and metabolism. Polypeptide hormones act on membrane-bound receptors to trigger gene transcription via complex intracellular signalling pathways. By contrast, nuclear receptors for lipid-soluble molecules such as glucocorticoids (GC) and thyroid hormones (TH) directly regulate transcription via DNA binding and chromatin remodelling. Nuclear hormone receptors are members of a large superfamily of transcriptional regulators with the ability to activate or repress many genes involved in development and disease. Nutrition influences not only hormone synthesis and metabolism but also hormone receptors, and regulation is mediated either by specific nutrients or by energy status. Recent studies on the role of early environment on development have implicated GC and their receptors in the programming of adult disease. Intrauterine growth restriction and postnatal undernutrition also induce striking differences in TH-receptor isoforms in functionally-distinct muscles, with critical implications for gene transcription of myosin isoforms. glucose transporters, uncoupling proteins and cation pumps. Such findings highlight a mechanism by which nutritional status can influence normal development, and modify nutrient utilization. thermogenesis. peripheral sensitivity to insulin and optimal cardiac function. Diet and stage of development will also influence the transcriptional activity of drugs acting as ligands for nuclear receptors. Potential interactions between nuclear receptors, including those for retinoic acid and vitamin D, should not be overlooked in intervention programmes using I or vitamin A supplementation of young and adult human populations

Cloning and characterization of two novel thyroid hormone receptor beta isoforms

Thyroid hormone (T(3)) activates nuclear receptor transcription factors, encoded by the TRalpha (NR1A1) and TRbeta (NR1A2) genes, to regulate target gene expression. Several TR isoforms exist, and studies of null mice have identified some unique functions for individual TR variants, although considerable redundancy occurs, raising questions about the specificity of T(3) action. Thus, it is not known how diverse T(3) actions are regulated in target tissues that express multiple receptor variants. I have identified two novel TRbeta isoforms that are expressed widely and result from alternative mRNA splicing. TRbeta3 is a 44.6-kDa protein that contains an unique 23-amino-acid N terminus and acts as a functional receptor. TRDeltabeta3 is a 32.8-kDa protein that lacks a DNA binding domain but retains ligand binding activity and is a potent dominant-negative antagonist. The relative concentrations of beta3 and Deltabeta3 mRNAs vary between tissues and with changes in thyroid status, indicating that alternative splicing is tissue specific and T(3) regulated. These data provide novel insights into the mechanisms of T(3) action and define a new level of specificity that may regulate thyroid status in tissue.

Synergistic activation of the prolactin promoter by vitamin D receptor and GHF-1: role of the coactivators, CREB-binding protein and steroid hormone receptor coactivator-1 (SRC-1)

PRL gene expression is dependent on the presence of the pituitary-specific transcription factor GHF-1/Pit-1, which is transcribed in a highly restricted manner in cells of the anterior pituitary. In pituitary GH3 cells, vitamin D increases the levels of PRL transcripts and stimulates the PRL promoter. We have analyzed the role of GHF-1 and of the vitamin D receptor (VDR) to confer vitamin D responsiveness to the PRL promoter. For this purpose we have used nonpituitary HeLa cells, which do not express GHF-1. We found that VDR activates the PRL promoter both in a ligand-dependent and -independent manner through a sequence located between positions -45/-27 in the proximal 5'-flanking region. This sequence also confers VDR and vitamin D responsiveness to a heterologous promoter. In the context of the PRL gene, VDR requires the presence of GHF-1 to activate the promoter. Truncation of the last 12 C-terminal amino acids of VDR, which contain the ligand-dependent activation function (AF2), abolishes regulation by vitamin D, suggesting that binding of coactivators to this region mediates ligand-dependent stimulation of the PRL promoter by the receptor. Indeed, expression of the coactivators, steroid hormone receptor coactivator-1 (SRC-1) and CREB-binding protein (CBP), significantly enhances the stimulatory effect of vitamin D mediated by the wild-type VDR but not by the AF2 mutant receptor. Furthermore, CBP also increases the activation of the PRL promoter by GHF-1 and the ligand-independent activation by both wild-type and mutant VDR.

Vitamin D represses retinoic acid-dependent transactivation of the retinoic acid receptor-beta2 promoter: the AF-2 domain of the vitamin D receptor is required for transrepression

Retinoic acid (RA)-dependent activation of the RA receptor beta2 (RARbeta2) gene in embryonal carcinoma cells is mediated by binding of retinoid receptor heterodimers (RAR/RXR) to a RA response element (RARE) located closely to the TATA box. We have analyzed the effect of vitamin D on the response of the RARbeta2 promoter to RA in pituitary GH4C1 cells that coexpress receptors for retinoids and vitamin D. Incubation with vitamin D markedly reduced the response to RA caused by transcriptional interference of the vitamin D receptor (VDR) on the RARE. This DNA element binds VDR/RXR heterodimers with high affinity, and these inactive heterodimers can displace active RAR/RXR from the RARE. Overexpression of RXR in GH4C1 cells, as well as incubation with BMS649 (a RXR-specific ligand), increased the inhibitory effect of vitamin D, suggesting that the VDR/RXR heterodimer is the repressive species and that titration of RXR is not responsible for this inhibition. Although DNA binding could be required for full potency of the inhibitory activity of VDR, it is not absolutely required because a truncated receptor (VDR delta1-111), lacking the DNA binding domain, also displays repressor activity. Furthermore, the ability to mediate transrepression by vitamin D was strongly decreased when a mutant VDR in which the last 12 C-terminal aminoacids have been deleted (VDR deltaAF-2) was used. Because this region contains the domain responsible for ligand-dependent recruitment of coactivators, titration of common coactivators for VDR and RAR could be involved in the inhibitory effect of vitamin D. In agreement with this hypothesis, overexpression of E1A, which can act as a RARbeta2 promoter-specific coactivator, significantly reversed repression by vitamin D.

The vitamin D receptor binds in a transcriptionally inactive form and without a defined polarity on a retinoic acid response element

Heterodimers of the vitamin D receptor (VDR) with the retinoid X receptor (RXR) bind in a transcriptionally unproductive manner to the retinoic acid response element present in the retinoic acid receptor-beta2 promoter. This element is composed of a direct repeat (DR) of the sequence PuGTTCA spaced by five nucleotides. However, the same sequence separated by three nucleotides (DR3) acts as a strong vitamin D response element. Here we show that the polarity of binding of the heterodimers to the DR3 was 5'-RXR-VDR-3', whereas on the DR5, both heterodimeric partners bind indistinctly to the 5' or 3' hemi-sites. These results suggest that the response elements can allosterically regulate the conformation of the receptors to determine positive or negative regulation of gene expression. Despite the altered polarity, the DR5-bound heterodimer was able to recruit the nuclear receptor coactivator ACTR in a vitamin D-dependent fashion. Furthermore, binding of the corepressor SMRT (silencing mediator of retinoid and thyroid hormone receptors) to the RXR/VDR heterodimer on a DR5 was not observed. Binding of RXR/VDR heterodimers to DRs with different transcriptional outcomes may generate selectivity and provide a greater complexity and flexibility to the vitamin D responses.

Lysine 246 of the vitamin D receptor is crucial for ligand-dependent interaction with coactivators and transcriptional activity

Mutant K246A in the predicted helix 3 of the ligand-binding domain, as well as mutants L417S and E420Q in helix 12, which contains the core ligand-dependent transcriptional activation domain (AF-2), were generated to examine AF-2 activity of the vitamin D receptor (VDR). These mutations abolished vitamin D-dependent transactivation. In addition, VDR mediates a ligand-dependent repression of the response of the retinoic acid receptor beta2 promoter to retinoic acid, and the helix 3 and helix 12 mutants were unable to mediate transrepression. Furthermore, the VDR mutants, but not the native receptor, enhanced phorbol ester induction of the activator protein-1-containing collagenase promoter. The helix 3 and helix 12 mutations strikingly reduced the ability of VDR to interact with the coactivators steroid receptor coactivator-1, ACTR, and the CREB-binding protein. As a consequence, overexpression of steroid receptor coactivator-1 increased vitamin D-dependent transactivation by VDR but not by the K246A mutant. These results indicate that the lysine 246 participates, together with residues in helix 12, in the recruitment of coactivators and that AF-2 activity is involved both in ligand-dependent transactivation and in transrepression by VDR.

Lipid soluble vitamins in gene regulation

Vitamin A (retinol) and vitamin D are lipid soluble vitamins that are precursors of the nuclear hormones all-trans retinoic acid (RA) and 1alpha,25-dihydroxyvitamin D3 (VD) that bind with high affinity to their cognate nuclear receptors, referred to as retinoic acid receptor (RAR) and vitamin D receptor (VDR). Both types of nuclear receptors are structurally related and belong to the same subclass of the nuclear receptor superfamily, a large family of ligand-inducible transcription factors. Both RAR and VDR form heterodimers preferentially with the nuclear receptor for 9-cis RA, referred to as the retinoid X receptor (RXR), but functional RAR-VDR heterodimers have also been observed. Moreover, both types of nuclear receptors interact in a ligand-dependent fashion with members of the same class of co-activator, co-repressor and co-integrator proteins. These similar molecular mechanisms of action provide several possibilities for an interaction of RARs with VDR that are all based on allosteric protein-protein interactions. These interactions can result in either an additive or a transrepressive functional interference between RA and VD. The two remaining lipid soluble vitamins, vitamins E and K, are not known to interact with nuclear receptors, but their structure does not exclude this possibility. Moreover, for vitamin E modulatory effects on transcription factors, such as AP-1, have been described. This review will discuss briefly gene regulation by the four lipid soluble vitamins.

Role of GHF-1 in the regulation of the rat growth hormone gene promoter by thyroid hormone and retinoic acid receptors

In non-pituitary HeLa cells the unliganded thyroid hormone or retinoic acid receptors cause a strong activation of the rat growth hormone promoter that is repressed by their ligands. In contrast, after expression of the pituitary-specific transcription factor GHF-1, thyroid hormone and retinoic acid produce a stimulation similar to that found in pituitary cells. Therefore, GHF-1 changes a ligand-dependent inhibition into a ligand-dependent activation. The essential role of GHF-1 on the rat growth hormone promoter was also demonstrated with AF-2-defective T3 receptor mutants that show a normal activation of this promoter in the presence of GHF-1. Furthermore, a truncated T3 receptor, which lacks the N-terminus and the DNA binding domain, was able to stimulate this promoter in the presence of GHF-1 and exogenous RXR receptors, suggesting the importance of protein to protein interactions in this regulation. This study shows that the final transcriptional effect depends not only on the type of regulatory promoter response elements but also on the presence of other transcriptional activators, in the case of the growth hormone promoter, the tissue-specific transcription factor GHF-1, which plays a coactivator-like role in this promoter.

Activation of the prolactin gene by peroxisome proliferator-activated receptor-alpha appears to be DNA binding-independent

Although the effects of the peroxisome proliferator-activated receptors (PPARs) have been studied primarily in adipocytes and liver, the wide distribution of these receptors suggests that they might also play a role in other cell types. We present evidence that PPAR activators stimulate the expression of the prolactin gene in pituitary GH4C1 cells. Transfection assays in non-pituitary HeLa cells showed that stimulation of the prolactin promoter by PPARalpha requires the presence of the transcription factor GHF-1 (or Pit-1). Proximal promoter sequences confer responsiveness to PPARalpha, and activation by this receptor is lost concomitantly with the response to GHF-1. Surprisingly, expression of the retinoid X receptor (RXR) abolishes stimulation by PPARalpha. Furthermore, the promoter region that confers PPARalpha responsiveness does not contain a PPAR response element. This suggests that the transcriptional effect of PPARalpha might be mediated by protein-protein interactions rather than by binding of PPAR/RXR to the promoter. A direct interaction between PPARalpha and GHF-1 was confirmed by in vitro binding studies. Expression of the coactivators SRC-1 and CREB-binding protein, which bind to PPAR, also enhanced the responsiveness of the prolactin promoter to PPARalpha. Furthermore, CREB-binding protein also significantly increased activation by GHF-1, and both proteins associated in vitro. Thus, PPARalpha, a receptor that normally acts as a ligand-dependent transcription factor by binding to specific DNA sequences in one context, can also stimulate the prolactin promoter by association with GHF-1 and coactivator proteins.

A direct protein-protein interaction is involved in the cooperation between thyroid hormone and retinoic acid receptors and the transcription factor GHF-1

The nuclear receptors for thyroid hormone (TRs) and retinoic acid (RARs and RXRs) cooperate with the pituitary-specific transcription factor GHF-1 to activate the rat growth hormone (GH) gene. The GH promoter contains a hormone response element (HRE), which binds TR/RXR and RAR/RXR heterodimers, located close to two binding sites for GHF-1. GHF-1 inhibits binding of TR/RXR and RAR/RXR heterodimers to an isolated HRE. Similarly, the receptors inhibit binding of GHF-1 to its cognate site. These results suggest the existence of direct protein to protein interactions between the receptors and the pituitary transcription factor. This was confirmed by in vitro binding studies with GST fusion proteins, which demonstrated a strong association of GHF-1 with RXR and a weaker interaction with RAR and TR. GHF-1 and the receptor heterodimers form a ternary complex with a fragment of the rat GH promoter, which contains binding sites for both, and GHF-1 increases receptor binding to the promoter when present in limiting conditions. These results suggest that the synergistic activation of the rat GH gene involves protein-DNA interactions as well as a physical association between the nuclear receptors and the pituitary-specific transcription factor GHF-1.

High-affinity binding sites to the vitamin D receptor DNA binding domain in the human growth hormone promoter

The regulation of the human growth hormone (hGH) gene by 1,25(OH)2D3 is a mechanism which is poorly understood. The objective of this study was to investigate whether the hGH gene has DNA recognition elements for the DNA binding domain of the vitamin D receptor. Using gel retardation assays and footprinting techniques, two high-affinity binding sites, denominated F1 and F2, were identified in the 5'-flanking sequence of hGH. The distal site, F1, located at -59 bp is made up of an imperfect direct repeat separated by 3 bp and showed a high degree of similarity with other known vitamin D response elements (VDREs). The proximal site, F2, located at -36 bp showed a single 7-bp sequence, which is different from other known VDREs. The location of both sites (F1 near the GHF-1/Pit-1 response element, F2 contacting the TATA box) suggests that the vitamin D receptor by itself or through interference with other transcriptional factors may modulate hGH expression.

A zinc finger homeodomain transcription factor binds specific thyroid hormone response elements

Thyroid hormone receptors can act through response elements (TREs) having a wide variation of sequence. We screened for transcription factors that bind the rat (r) glycoprotein hormone alpha-subunit TRE (alpha-sub), and isolated a cDNA, termed zinc finger homeodomain enhancer-binding protein (Zfhep), which encodes two separate zinc finger domains (ZD1 and ZD2), and a region similar to homeodomains. DNA-binding assays show that ZD1 or ZD2 can bind the alpha-subunit, rat growth hormone, or thyrotropin beta (TSHbeta) gene TREs, but do not bind DR4 or palindromic (pal) TREs. Methylation interference footprinting demonstrates that Zfhep binds the alpha-sub overlapping the TR-binding site. Similarly, the ZD1 protein footprints over TR-binding halfsites of the rat growth hormone (rGH) and TSHbeta TREs. Hence, Zfhep binding is dependent on sequences within and outside the AGGTCA TR-binding halfsite. Interactions of non-receptor transcription factors (such as Zfhep) with certain TREs are important to modify gene-specific regulation by thyroid hormones.

The transcription factor GHF-1, but not the splice variant GHF-2, cooperates with thyroid hormone and retinoic acid receptors to stimulate rat growth hormone gene expression

The rat growth hormone (GH) promoter was significantly activated in non-pituitary cells by the expression of unliganded trioodothyronine (T3) and retinoic acid (RA) receptors. Furthermore, a strong ligand-dependent activation was found in the presence of the pituitary-specific transcription factor GHF-1. When compared with GHF-1, the splice variant GHF-2 showed a decreased ability to bind the cognate site in the GH promoter. As a consequence, expression of GHF-2 had little stimulatory effect on the GH promoter and did not show cooperation with T3 or RA receptors even in the presence of ligands. Furthermore, over-expression of GHF-2 inhibited the response to T3 and RA in pituitary cells. These results show that alternative splicing of the GHF-1 gene gives rise to two isoforms that differ in their transactivating properties and in their ability to synergize with the nuclear thyroid hormone and retinoic acid receptors on GH gene expression.

1 alpha,25-dihydroxyvitamin D3 receptor as a mediator of transrepression of retinoid signaling

The receptors for retinoic acid (RA) and for 1 alpha,25-dihydroxyvitamin D3 (VD), RAR, RXR, and VDR are ligand-inducible members of the nuclear receptor superfamily. These receptors mediate their regulatory effects by binding as dimeric complexes to response elements located in regulatory regions of hormone target genes. Sequence scanning of the tumor necrosis factor-alpha type 1 receptor (TNF alpha RI) gene identified a 3' enhancer region composed of two directly repeated hexameric core motifs spaced by 2 nucleotides (DR2). On this novel DR2-type sequence, but not on a DR5-type RA response element, VD was shown to act through its receptor, the vitamin D receptor (VDR), as a repressor of retinoid signalling. The repression appears to be mediated by competitive protein-protein interactions between VDR, RAR, RXR, and possibly their cofactors. This VDR-mediated transrepression of retinoid signaling suggests a novel mechanism for the complex regulatory interaction between retinoids and VD.

A mutant thyroid hormone receptor beta 1 identified in a patient with resistance to thyroid hormone inhibits the activities of not only the wild-type TRs, but also other nuclear receptors

Although mutations of human thyroid hormone receptor beta (hTR beta) have been associated with resistance to thyroid hormone (RTH), the molecular basis by which the mutant TRs cause the various clinical symptoms is unknown. We show here that a mutant TR beta [corrected] identified in a patient with RTH inhibited the transcriptional activities of, not only the wild-type TR beta, but also other nuclear receptors including retinoid X receptor alpha (RXR alpha), vitamin D3 receptor (VDR) and retinoic acid receptor (RAR alpha). We provide evidence that these inhibitions by the mutant TR beta [corrected] occur by different mechanisms. Namely, the mutant TR beta interferes with VDR and RAR alpha by competition for binding to the corresponding response elements, but the pathway through RXR alpha is mainly inhibited by squelching of RXR alpha in solution. These findings suggest that in patients with RTH, not only the T3 responsive genes but also other responsive genes are inhibited by the mutant TRs, which might explain the variety of clinical symptoms in RTH.

Molecular cloning and functional characterization of the human cytosolic malic enzyme promoter: thyroid hormone responsiveness

We report the structural and functional features of the 5'-flanking region of the human cytosolic malic enzyme (ME) gene. A 2.2-kb subclone, comprising 1.5 kb upstream of the translation initiation codon, the first exon, and 0.7 kb of flanking intronic region, was sequenced and mapped to chromosome 6. The proximal promoter region is rich in G + C, lacks TATA or CCAAT boxes, and shows multiple transcription start sites, the major one 106 nucleotides upstream the ATG codon. Sequences -59/-13 and -137/-103 conferred maximal promoter activity. Deletional analysis revealed the presence of two regions positively regulated by 3,5,3'-triiodo-L-thyronine (T3). The proximal region confers the strongest T3 inducibility to the human ME as well as to a heterologous promoter. Thyroid hormone receptor beta (TRbeta) binds to an inverted palindromic T3 response element (TRE) at position -105/-87 in a manner that is prevented by T3. Nuclear extracts or in vitro-translated retinoid acid receptor alpha (RXR alpha) shifted the TRbeta retarded band to slower-mobility complexes, which are unaffected by T3. In the absence of T3, overexpression of TRbeta repressed the ME promoter activity, most probably, through binding of TRbeta homodimers to the TRE. Thus, T3 seems to control ME transcription by inducing the dissociation of TRbeta homodimers and the functional activation of liganded heterodimers.

Histone acetylation influences thyroid hormone and retinoic acid-mediated gene expression

Thyroid hormone (T3) and retinoic acid (RA) receptors regulate transcription of the rat growth hormone (GH) gene through binding to a common hormone response element (HRE) in the promoter. We have investigated the effect of histone acetylation on hormone-dependent expression of the rat GH gene. We examined the effect of butyrate, which induces histone hyperacetylation, and trichostatin A (TSA), a highly specific inhibitor of histone deacetylases. GH-mRNA levels were significantly increased in pituitary GH4C1 cells incubated with T3 and RA, and this response was further stimulated in the presence of 1 mM butyrate. The effect of butyrate was mimicked by TSA. Butyrate and TSA also enhanced the activity of recombinant constructs containing the GH promoter directing chloramphenicol acetyl transferase (CAT) reporter gene expression. CAT activity increased by 4- to 8-fold after incubation with 1 nM T3 and 1 microM RA, and this response was stimulated 2- to 4-fold further in the presence of 0.25 mM butyrate. This concentration of butyrate did not influence basal expression of CAT. TSA produced a dose-dependent increase of CAT activity in the absence of ligands, and between 5 and 200 nM potentiated the effect of T3 and RA. These compounds also increased the hormonal response of constructs in which the HRE was linked to heterologous [mouse mammary tumor virus (MMTV) and thymidine kinase (TK)] promoters. With butyrate >1 mM, basal activity of the GH promoter increased by more than 10-fold and the effect of T3 and RA was no longer observed. Overexpression of T3 receptors was able to counteract the stimulation of basal CAT levels caused by butyrate. Thus, in the absence of ligand, the T3 receptor acts as a constitutive repressor of gene expression. Upon binding of the hormone, the T3 receptor is converted into an activator. Our findings suggest that histone acetylation, which alters chromatin structure, may play an important role in hormone-mediated transcriptional regulation.