A c-erb-A binding site in rat growth hormone gene mediates trans-activation by thyroid hormone

Visual Perception of Density and Density-Dependent Growth in Medaka (Oryzias latipes): A Suitable Model for Studying Density Effects in Fish

High stocking densities have negative effects on fish. However, the mechanism mediating density perception and growth inhibition is still unknown. This study was conducted to confirm the occurrence of growth inhibition and evaluate changes in growth-related factors in fish reared under high-stocking-density conditions and to determine the role of vision in density perception of medaka. In the graduated-stocking experiment, growth inhibition was clearly observed in fish reared at higher densities, although environmental factors, such as water quality, dissolved oxygen, and feeding conditions, were the same in each experimental group. Differences in growth were observed between the 6-fish and 8-fish groups, indicating that medaka have a superior sense that allows them to accurately perceive the number of individuals in their surroundings. In the pseudo-high stocking experiment, the inner 2-L tank in both groups contained six fish; however, the outer 3-L tank in the pseudo group contained several fish, while that of the control group contained only water. Growth inhibition was observed among the fish in the inner tank of the pseudo group despite having similar spatial density with the control group. These findings suggest that vision is important for density perception. The gene expression of growth-related and metabolic-regulatory hormones decreased in the high-density group. Furthermore, neuropeptide Y expression increased, while pro-opiomelanocortin expression decreased in the high-density group. This study is the first to report that fish can visually perceive density and the resulting growth inhibition, and concluded that medaka is a suitable model for studying density effects and perception in fish.

A Historical Reflection on Scientific Advances in Understanding Thyroid Hormone Action

Background: Thyroid hormone (TH) has actions in every tissue of the body and is essential for normal development, as well as having important actions in the adult. The earliest markers of TH action that were identified and monitored clinically, even before TH could be measured in serum, included oxygen consumption, basal metabolic rate, serum cholesterol, and deep tendon reflex time. Cellular, rodent, amphibian, zebrafish, and human models have been used to study TH action. Summary: Early studies of the mechanism of TH action focused on saturable-specific triiodothyronine (T3) nuclear binding and direct actions of T3 that altered protein expression. Additional effects of TH were recognized on mitochondria, stimulation of ion transport, especially the sodium potassium ATPase, augmentation of adrenergic signaling, role as a neurotransmitter, and direct plasma membrane effects. The cloning of the thyroid hormone receptor (THR) genes in 1986 and report of the THR crystal structure in 1995 produced rapid progress in understanding the mechanism of TH nuclear action, as well as the development of modified THR ligands. These findings revealed nuances of TH signaling, including the role of nuclear receptor coactivators and corepressors, repression of positively stimulated genes by the unliganded receptor, THR isoform-specific actions of TRα (THRA) and TRβ (THRB), and THR binding DNA as a heterodimer with retinoid-x-receptor (RXR) for genes positively regulated by TH. The identification of genetic disorders of TH transport and signaling, especially Resistance to Thyroid Hormone (RTH) and monocarboxylate transporter 8 (Mct8) defects, has been highly informative with respect to the mechanism of TH action. Conclusions: The impact of THR isoform, post-translational modifications, receptor cofactors, DNA response element, and selective TH tissue uptake, on TH action, have clinical implications for diagnosing and treating thyroid disease. Additionally, these findings have led to the development of novel TH and TH analogue therapies for metabolic, neurological, and cardiovascular diseases.

Retinoic acid receptors at 35 years

For almost a century, vitamin A has been known as a nutrient critical for normal development, differentiation, and homeostasis; accordingly, there has been much interest in understanding its mechanism of action. This review is about the discovery of specific receptors for the vitamin A derivative, retinoic acid (RA), which launched extensive molecular, genetic, and structural investigations into these new members of the nuclear receptor superfamily of transcriptional regulators. These included two families of receptors, the RAR isotypes (α, β, and γ) along with three RXR isotypes (α, β, and γ), which bind as RXR/RAR heterodimers to cis-acting response elements of RA target genes to generate a high degree of complexity. Such studies have provided deep molecular insight into how the widespread pleiotropic effects of RA can be generated.

Thyroid hormone and thyroid hormone nuclear receptors: History and present state of art

The present review traces the road leading to discovery of L-thyroxine, thyroid hormone (3,5,3´-triiodo-L-thyronine, T₃) and its cognate nuclear receptors. Thyroid hormone is a pleio-tropic regulator of growth, differentiation, and tissue homeostasis in higher organisms. The major site of the thyroid hormone action is predominantly a cell nucleus. T₃ specific binding sites in the cell nuclei have opened a new era in the field of the thyroid hormone receptors (TRs) discovery. T₃ actions are mediated by high affinity nuclear TRs, TRalpha and TRbeta, which function as T₃-activated transcription factors playing an essential role as transcription-modulating proteins affecting the transcriptional responses in target genes. Discovery and characterization of nuclear retinoid X receptors (RXRs), which form with TRs a heterodimer RXR/TR, positioned RXRs at the epicenter of molecular endocrinology. Transcriptional control via nuclear RXR/TR heterodimer represents a direct action of thyroid hormone. T₃ plays a crucial role in the development of brain, it exerts significant effects on the cardiovascular system, skeletal muscle contractile function, bone development and growth, both female and male reproductive systems, and skin. It plays an important role in maintaining the hepatic, kidney and intestine homeostasis and in pancreas, it stimulates the beta-cell proliferation and survival. The TRs cross-talk with other signaling pathways intensifies the T₃ action at cellular level. The role of thyroid hormone in human cancers, acting via its cognate nuclear receptors, has not been fully elucidated yet. This review is aimed to describe the history of T₃ receptors, starting from discovery of T3 binding sites in the cell nuclei to revelation of T₃ receptors as T₃-inducible transcription factors in relation to T₃ action at cellular level. It also focuses on milestones of investigation, comprising RXR/TR dimerization, cross-talk between T₃ receptors, and other regulatory pathways within the cell and mainly on genomic action of T₃. This review also focuses on novel directions of investigation on relationships between T₃ receptors and cancer. Based on the update of available literature and the author's experimental experience, it is devoted to clinicians and medical students.

A coregulator shift, rather than the canonical switch, underlies thyroid hormone action in the liver

In this study, Shabtai et al. investigated the mechanism of thyroid hormone (TH)-dependent gene repression, generated a mouse line in which endogenous thyroid hormone receptor TRβ1 was epitope-tagged to allow precise chromatin immunoprecipitation at the low physiological levels of thyroid hormone receptors (TR), and defined high-confidence binding sites where TR functioned at enhancers regulated in the same direction as the nearest gene in a TRβ-dependent manner. Their results demonstrate that, in contrast to the canonical “all or none” coregulator switch model, TH regulates gene expression by orchestrating a shift in the relative binding of corepressors and coactivators.

Thyroid hormones (THs) are powerful regulators of metabolism with major effects on body weight, cholesterol, and liver fat that have been exploited pharmacologically for many years. Activation of gene expression by TH action is canonically ascribed to a hormone-dependent “switch” from corepressor to activator binding to thyroid hormone receptors (TRs), while the mechanism of TH-dependent repression is controversial. To address this, we generated a mouse line in which endogenous TRβ1 was epitope-tagged to allow precise chromatin immunoprecipitation at the low physiological levels of TR and defined high-confidence binding sites where TRs functioned at enhancers regulated in the same direction as the nearest gene in a TRβ-dependent manner. Remarkably, although positive and negative regulation by THs have been ascribed to different mechanisms, TR binding was highly enriched at canonical DR4 motifs irrespective of the transcriptional direction of the enhancer. The canonical NCoR1/HDAC3 corepressor complex was reduced but not completely dismissed by TH and, surprisingly, similar effects were seen at enhancers associated with negatively as well as positively regulated genes. Conversely, coactivator CBP was found at all TH-regulated enhancers, with transcriptional activity correlating with the ratio of CBP to NCoR rather than their presence or absence. These results demonstrate that, in contrast to the canonical “all or none” coregulator switch model, THs regulate gene expression by orchestrating a shift in the relative binding of corepressors and coactivators.

A Comparative Update on the Neuroendocrine Regulation of Growth Hormone in Vertebrates

Growth hormone (GH), mainly produced from the pituitary somatotrophs is a key endocrine regulator of somatic growth. GH, a pleiotropic hormone, is also involved in regulating vital processes, including nutrition, reproduction, physical activity, neuroprotection, immunity, and osmotic pressure in vertebrates. The dysregulation of the pituitary GH and hepatic insulin-like growth factors (IGFs) affects many cellular processes associated with growth promotion, including protein synthesis, cell proliferation and metabolism, leading to growth disorders. The metabolic and growth effects of GH have interesting applications in different fields, including the livestock industry and aquaculture. The latest discoveries on new regulators of pituitary GH synthesis and secretion deserve our attention. These novel regulators include the stimulators adropin, klotho, and the fibroblast growth factors, as well as the inhibitors, nucleobindin-encoded peptides (nesfatin-1 and nesfatin-1-like peptide) and irisin. This review aims for a comparative analysis of our current understanding of the endocrine regulation of GH from the pituitary of vertebrates. In addition, we will consider useful pharmacological molecules (i.e. stimulators and inhibitors of the GH signaling pathways) that are important in studying GH and somatotroph biology. The main goal of this review is to provide an overview and update on GH regulators in 2020. While an extensive review of each of the GH regulators and an in-depth analysis of specifics are beyond its scope, we have compiled information on the main endogenous and pharmacological regulators to facilitate an easy access. Overall, this review aims to serve as a resource on GH endocrinology for a beginner to intermediate level knowledge seeker on this topic.

A Transgenic Mouse Model for Detection of Tissue-Specific Thyroid Hormone Action

Thyroid hormone (TH) is present in the systemic circulation and thus should affect all cells similarly in the body. However, tissues have a complex machinery that allows tissue-specific optimization of local TH action that calls for the assessment of TH action in a tissue-specific manner. Here, we report the creation of a TH action indicator (THAI) mouse model to study tissue-specific TH action. The model uses a firefly luciferase reporter readout in the context of an intact transcriptional apparatus and all elements of TH metabolism and transport and signaling. The THAI mouse allows the assessment of the changes of TH signaling in tissue samples or in live animals using bioluminescence, both in hypothyroidism and hyperthyroidism. Beyond pharmacologically manipulated TH levels, the THAI mouse is sufficiently sensitive to detect deiodinase-mediated changes of TH action in the interscapular brown adipose tissue (BAT) that preserves thermal homeostasis during cold stress. The model revealed that in contrast to the cold-induced changes of TH action in the BAT, the TH action in this tissue, at room temperature, is independent of noradrenergic signaling. Our data demonstrate that the THAI mouse can also be used to test TH receptor isoform-specific TH action. Thus, THAI mouse constitutes a unique model to study tissue-specific TH action within a physiological/pathophysiological context and test the performance of thyromimetics. In conclusion, THAI mouse provides an in vivo model to assess a high degree of tissue specificity of TH signaling, allowing alteration of tissue function in health and disease, independently of changes in circulating levels of TH.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

Quantification of Thyromimetic Sobetirome Concentration in Biological Tissue Samples

Thyroid hormone is a principal regulator of essential processes in vertebrate physiology and homeostasis. Synthetic derivatives of thyroid hormone, known as thyromimetics, display desirable therapeutic properties. Thoroughly understanding how thyromimetics distribute throughout the body is crucial for their development and this requires appropriate bioanalytical techniques to quantify drug levels in different tissues. Here, we describe a detailed protocol for the quantification of the thyromimetic sobetirome using liquid chromatography tandem-mass spectrometry (LC-MS/MS).

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Role of co-regulators in metabolic and transcriptional actions of thyroid hormone

Thyroid hormone (TH) controls a wide range of physiological processes through TH receptor (TR) isoforms. Classically, TRs are proposed to function as tri-iodothyronine (T3)-dependent transcription factors: on positively regulated target genes, unliganded TRs mediate transcriptional repression through recruitment of co-repressor complexes, while T3 binding leads to dismissal of co-repressors and recruitment of co-activators to activate transcription. Co-repressors and co-activators were proposed to play opposite roles in the regulation of negative T3 target genes and hypothalamic-pituitary-thyroid axis, but exact mechanisms of the negative regulation by TH have remained elusive. Important insights into the roles of co-repressors and co-activators in different physiological processes have been obtained using animal models with disrupted co-regulator function. At the same time, recent studies interrogating genome-wide TR binding have generated compelling new data regarding effects of T3, local chromatin structure, and specific response element configuration on TR recruitment and function leading to the proposal of new models of transcriptional regulation by TRs. This review discusses data obtained in various mouse models with manipulated function of nuclear receptor co-repressor (NCoR or NCOR1) and silencing mediator of retinoic acid receptor and thyroid hormone receptor (SMRT or NCOR2), and family of steroid receptor co-activators (SRCs also known as NCOAs) in the context of TH action, as well as insights into the function of co-regulators that may emerge from the genome-wide TR recruitment analysis.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Novel mechanism of positive versus negative regulation by thyroid hormone receptor β1 (TRβ1) identified by genome-wide profiling of binding sites in mouse liver

Triiodothyronine (T3) regulates key metabolic processes in the liver through the thyroid hormone receptor, TRβ1. However, the number of known target genes directly regulated by TRβ1 is limited, and the mechanisms by which positive and especially negative transcriptional regulation occur are not well understood. To characterize the TRβ1 cistrome in vivo, we expressed a biotinylated TRβ1 in hypo- and hyperthyroid mouse livers, used ChIP-seq to identify genomic TRβ1 targets, and correlated these data with gene expression changes. As with other nuclear receptors, the majority of TRβ1 binding sites were not in proximal promoters but in the gene body of known genes. Remarkably, T3 can dictate changes in TRβ1 binding, with strong correlation to T3-induced gene expression changes, suggesting that differential TRβ1 binding regulates transcriptional outcome. Additionally, DR-4 and DR-0 motifs were significantly enriched at binding sites where T3 induced an increase or decrease in TRβ1 binding, respectively, leading to either positive or negative regulation by T3. Taken together, the results of this study provide new insights into the mechanisms of transcriptional regulation by TRβ1 in vivo.

The thyroid cancer PAX8-PPARG fusion protein activates Wnt/TCF-responsive cells that have a transformed phenotype

A chromosomal translocation results in the production of a paired box 8-peroxisome proliferator-activated receptor gamma (PAX8-PPARG) fusion protein (PPFP) in ∼35% of follicular thyroid carcinomas. To examine the role of PPFP in thyroid oncogenesis, the fusion protein was stably expressed in the non-transformed rat thyroid cell line PCCL3. PPFP conferred on PCCL3 cells the ability to invade through Matrigel and to form colonies in anchorage-independent conditions. PPFP also increased the fraction of cells with Wnt/TCF-responsive green fluorescent protein reporter gene expression. This Wnt/TCF-activated population was enriched for colony-forming and invading cells. These actions of PPFP required a functional PPARG DNA binding domain (DBD) within PPFP and were further stimulated by PPARG agonists. These data indicate that PPFP, through its PPARG DBD, induces Wnt/TCF pathway activation in a subpopulation of cells, and these cells have properties of cellular transformation including increased invasiveness and anchorage-independent growth.

Identification of thyroid hormone receptor binding sites in developing mouse cerebellum

Background

Thyroid hormones play an essential role in early vertebrate development as well as other key processes. One of its modes of action is to bind to the thyroid hormone receptor (TR) which, in turn, binds to thyroid response elements (TREs) in promoter regions of target genes. The sequence motif for TREs remains largely undefined as does the precise chromosomal location of the TR binding sites. A chromatin immunoprecipitation on microarray (ChIP-chip) experiment was conducted using mouse cerebellum post natal day (PND) 4 and PND15 for the thyroid hormone receptor (TR) beta 1 to map its binding sites on over 5000 gene promoter regions. We have performed a detailed computational analysis of these data.

Results

By analysing a recent spike-in study, the optimal normalization and peak identification approaches were determined for our dataset. Application of these techniques led to the identification of 211 ChIP-chip peaks enriched for TR binding in cerebellum samples. ChIP-PCR validation of 25 peaks led to the identification of 16 true positive TREs. Following a detailed literature review to identify all known mouse TREs, a position weight matrix (PWM) was created representing the classic TRE sequence motif. Various classes of promoter regions were investigated for the presence of this PWM, including permuted sequences, randomly selected promoter sequences, and genes known to be regulated by TH. We found that while the occurrence of the TRE motif is strongly correlated with gene regulation by TH for some genes, other TH-regulated genes do not exhibit an increased density of TRE half-site motifs. Furthermore, we demonstrate that an increase in the rate of occurrence of the half-site motifs does not always indicate the specific location of the TRE within the promoter region. To account for the fact that TR often operates as a dimer, we introduce a novel dual-threshold PWM scanning approach for identifying TREs with a true positive rate of 0.73 and a false positive rate of 0.2. Application of this approach to ChIP-chip peak regions revealed the presence of 85 putative TREs suitable for further in vitro validation.

Conclusions

This study further elucidates TRβ gene regulation in mouse cerebellum, with 211 promoter regions identified to bind to TR. While we have identified 85 putative TREs within these regions, future work will study other mechanisms of action that may mediate the remaining observed TR-binding activity.

Effects of polybrominated diphenyl ethers (PBDEs) and their derivatives on protein disulfide isomerase activity and growth hormone release of GH3 cells

Polybrominated diphenyl ethers (PBDEs) have been used in a variety of consumer products such as flame retardants and recently have been known to be widespread environmental pollutants, which probably affect biological functions of mammalian cells. However, the risk posed by PBDE metabolites has not been clarified. Our previous study suggested that bisphenol A (BPA), an endocrine-disrupting chemical, binds to protein disulfide isomerase (PDI) and inhibits its activity. PDI is an isomerase enzyme in the endoplasmic reticulum and facilitates the formation or cleavage of disulfide bonds. PDI consists of a, b, b', and a' domains and the c region, with the a and a' domains having isomerase active sites. In the present study, we tested the effects of 10 kinds of PBDE compounds and their metabolites on PDI. OH-PBDEs specifically inhibited the isomerase activity of PDI, with 4'-OH-PBDE more effective than 2' (or 2)-OH-PBDEs. 4'-OH-PBDE inhibited the isomerase activity of the b'a'c fragment but not that of ab and a'c, suggesting that the b' domain of PDI is essential for the inhibition by 4'-OH-PBDE. We also investigated the effects of these chemicals on the production of growth hormone (GH) in GH3 cells. In GH3 cells, levels of mRNA and protein of GH stimulated by T(3) were reduced by 4'-OH-PBDE and 4'-MeO-PBDE. The reduction in GH expression caused by these compounds was not changed by the overexpression or knockdown of PDI in GH3 cells, while these manipulations of PDI levels significantly suppressed the expression of GH. These results suggest that the biological effects of PBDEs differed depending on their brominated and hydroxylated positions.

The IGF pathway regulates ERα through a S6K1-dependent mechanism in breast cancer cells

The IGF pathway stimulates malignant behavior of breast cancer cells. Herein we identify the mammalian target of rapamycin (mTOR)/S6 kinase 1 (S6K1) axis as a critical component of IGF and estrogen receptor (ER)α cross talk. The insulin receptor substrate (IRS) adaptor molecules function downstream of IGF-I receptor and dictate a specific biological response, in which IRS-1 drives proliferation and IRS-2 is linked to motility. Although rapamycin-induced mTOR inhibition has been shown to block IGF-induced IRS degradation, we reveal differential effects on motility (up-regulation) and proliferation (down-regulation). Because a positive correlation between IRS-1 and ERα expression is thought to play a central role in the IGF growth response, we investigated the potential role of ERα as a downstream mTOR target. Small molecule inhibition and targeted knockdown of S6K1 blocked the IGF-induced ERα(S167) phosphorylation and did not influence ligand-dependent ERα(S118) phosphorylation. Inhibition of S6K1 kinase activity consequently ablated IGF-stimulated S6K1/ERα association, estrogen response element promoter binding and ERα target gene transcription. Moreover, site-specific ERα(S167) mutation reduced ERα target gene transcription and blocked IGF-induced colony formation. These findings support a novel link between the IGF pathway and ERα, in which the translation factor S6K1 affects transcription of ERα-regulated genes.

Recognition by the thyroid hormone receptor of canonical DNA response elements

To shed more light on the molecular requirements for recognition of thyroid response elements (TREs) by thyroid receptors (TRs), we compared the specific aspects of DNA TRE recognition by different TR constructs. Using fluorescence anisotropy, we performed a detailed and hierarchical study of TR-TRE binding. This was done by comparing the binding affinities of three different TR constructs for four different TRE DNA elements, including palindromic sequences and direct repeats (F2, PAL, DR-1, and DR-4) as well as their interactions with nonspecific DNA sequences. The effect of MgCl(2) on suppressing of nonselective DNA binding to TR was also investigated. Furthermore, we determined the dissociation constants of the hTRbeta DBD (DNA binding domain) and hTRbeta DBD-LBD (DNA binding and ligand binding domains) for specific TREs. We found that a minimum DNA recognition peptide derived from DBD (H1TR) is sufficient for recognition and interaction with TREs, whereas scrambled DNA sequences were unrecognized. Additionally, we determined that the TR DBD binds to F2, PAL, and DR-4 with high affinity and similar K(d) values. The TR DBD-LBD recognizes all the tested TREs but binds preferentially to F2, with even higher affinity. Finally, our results demonstrate the important role played by LBDs in modulating TR-DNA binding.

BMP7 and SHH regulate Pax2 in mouse retinal astrocytes by relieving TLX repression

Pax2 is essential for development of the neural tube, urogenital system, optic vesicle, optic cup and optic tract. In the eye, Pax2 deficiency is associated with coloboma, a loss of astrocytes in the optic nerve and retina, and abnormal axonal pathfinding of the ganglion cell axons at the optic chiasm. Thus, appropriate expression of Pax2 is essential for astrocyte determination and differentiation. Although BMP7 and SHH have been shown to regulate Pax2 expression, the molecular mechanism by which this regulation occurs is not well understood. In this study, we determined that BMP7 and SHH activate Pax2 expression in mouse retinal astrocyte precursors in vitro. SHH appeared to play a dual role in Pax2 regulation; 1) SHH may regulate BMP7 expression, and 2) the SHH pathway cooperates with the BMP pathway to regulate Pax2 expression. BMP and SHH pathway members can interact separately or together with TLX, a repressor protein in the tailless transcription factor family. Here we show that the interaction of both pathways with TLX relieves the repression of Pax2 expression in mouse retinal astrocytes. Together these data reveal a new mechanism for the cooperative actions of signaling pathways in astrocyte determination and differentiation and suggest interactions of regulatory pathways that are applicable to other developmental programs.

BMP signaling dynamics in embryonic orofacial tissue

The bone morphogenetic protein (BMP) family represents a class of signaling molecules, that plays key roles in morphogenesis, cell proliferation, survival and differentiation during normal development. Members of this family are essential for the development of the mammalian orofacial region where they regulate cell proliferation, extracellular matrix synthesis, and cellular differentiation. Perturbation of any of these processes results in orofacial clefting. Embryonic orofacial tissue expresses BMP mRNAs, their cognate proteins, and BMP-specific receptors in unique temporo-spatial patterns, suggesting functional roles in orofacial development. However, specific genes that function as downstream mediators of BMP action during orofacial ontogenesis have not been well defined. In the current study, elements of the Smad component of the BMP intracellular signaling system were identified and characterized in embryonic orofacial tissue and functional activation of the Smad pathway by BMP2 and BMP4 was demonstrated. BMP2 and BMP4-initiated Smad signaling in cells derived from embryonic orofacial tissue was found to result in: (1) phosphorylation of Smads 1 and 5; (2) nuclear translocation of Smads 1, 4, and 5; (3) binding of Smads 1, 4, and 5 to a consensus Smad binding element (SBE)-containing oligonucleotide; (4) transactivation of transfected reporter constructs, containing BMP-inducible Smad response elements; and (5) increased expression at transcriptional as well as translational levels of Id3 (endogenous gene containing BMP receptor-specific Smad response elements). Collectively, these data document the existence of a functional Smad-mediated BMP signaling system in cells of the developing murine orofacial region.

Mammalian genome projects reveal new growth hormone (GH) sequences. Characterization of the GH-encoding genes of armadillo (Dasypus novemcinctus), hedgehog (Erinaceus europaeus), bat (Myotis lucifugus), hyrax (Procavia capensis), shrew (Sorex araneus), ground squirrel (Spermophilus tridecemlineatus), elephant (Loxodonta africana), cat (Felis catus) and opossum (Monodelphis domestica)

Mammalian growth hormone (GH) sequences have been shown previously to display episodic evolution: the sequence is generally strongly conserved but on at least two occasions during mammalian evolution (on lineages leading to higher primates and ruminants) bursts of rapid evolution occurred. However, the number of mammalian orders studied previously has been relatively limited, and the availability of sequence data via mammalian genome projects provides the potential for extending the range of GH gene sequences examined. Complete or nearly complete GH gene sequences for six mammalian species for which no data were previously available have been extracted from the genome databases-Dasypus novemcinctus (nine-banded armadillo), Erinaceus europaeus (western European hedgehog), Myotis lucifugus (little brown bat), Procavia capensis (cape rock hyrax), Sorex araneus (European shrew), Spermophilus tridecemlineatus (13-lined ground squirrel). In addition incomplete data for several other species have been extended. Examination of the data in detail and comparison with previously available sequences has allowed assessment of the reliability of deduced sequences. Several of the new sequences differ substantially from the consensus sequence previously determined for eutherian GHs, indicating greater variability than previously recognised, and confirming the episodic pattern of evolution. The episodic pattern is not seen for signal sequences, 5' upstream sequence or synonymous substitutions-it is specific to the mature protein sequence, suggesting that it relates to the hormonal function. The substitutions accumulated during the course of GH evolution have occurred mainly on the side of the hormone facing away from the receptor, in a non-random fashion, and it is suggested that this may reflect interaction of the receptor-bound hormone with other proteins or small ligands.

3,5-Diiodothyronine in vivo maintains euthyroidal expression of type 2 iodothyronine deiodinase, growth hormone, and thyroid hormone receptor beta1 in the killifish

Until recently, 3,5-diiodothyronine (3,5-T(2)) has been considered an inactive by-product of triiodothyronine (T(3)) deiodination. However, studies from several laboratories have shown that 3,5-T(2) has specific, nongenomic effects on mitochondrial oxidative capacity and respiration rate that are distinct from those due to T(3). Nevertheless, little is known about the putative genomic effects of 3,5-T(2). We have previously shown that hyperthyroidism induced by supraphysiological doses of 3,5-T(2) inhibits hepatic iodothyronine deiodinase type 2 (D2) activity and lowers mRNA levels in the killifish in the same manner as T(3) and T(4), suggesting a pretranslational effect of 3,5-T(2) (Garcia-G C, Jeziorski MC, Valverde-R C, Orozco A. Gen Comp Endocrinol 135: 201-209, 2004). The question remains as to whether 3,5-T(2) would have effects under conditions similar to those that are physiological for T(3). To this end, intact killifish were rendered hypothyroid by administering methimazole. Groups of hypothyroid animals simultaneously received 30 nM of either T(3), reverse T(3), or 3,5-T(2). Under these conditions, we expected that, if it were bioactive, 3,5-T(2) would mimic T(3) and thus reverse the compensatory upregulation of D2 and tyroid receptor beta1 and downregulation of growth hormone that characterize hypothyroidism. Our results demonstrate that 3,5-T(2) is indeed bioactive, reversing both hepatic D2 and growth hormone responses during a hypothyroidal state. Furthermore, we observed that 3,5-T(2) and T(3) recruit two distinct populations of transcription factors to typical palindromic and DR4 thyroid hormone response elements. Taken together, these results add further evidence to support the notion that 3,5-T(2) is a bioactive iodothyronine.

Antithyroid drugs inhibit thyroid hormone receptor-mediated transcription

CONTEXT

Methimazole (MMI) and propylthiouracil (PTU) are widely used as antithyroid drugs (ATDs) for the treatment of Graves' disease. Both MMI and PTU reduce thyroid hormone levels by several mechanisms, including inhibition of thyroid hormone synthesis and secretion. In addition, PTU decreases 5'-deiodination of T(4) in peripheral tissues. ATDs may also interfere with T(3) binding to nuclear thyroid hormone receptors (TRs). However, the effect of ATDs on the transcriptional activities of T(3) mediated by TRs has not been studied.

OBJECTIVE

The present study was undertaken to determine whether ATDs have an effect on the gene transcription regulated by T(3) and TRs in vitro.

METHODS

Transient gene expression experiments and GH secretion assays were performed. To elucidate possible mechanisms of the antagonistic action of ATDs, the interaction between TR and nuclear cofactors was examined.

RESULTS

In the transient gene expression experiments, both MMI and PTU significantly suppressed transcriptional activities mediated by the TR and T(3) in a dose-dependent manner. In mammalian two-hybrid assays, both drugs recruited one of the nuclear corepressors, nuclear receptor corepressor, to the TR in the absence of T(3). In addition, PTU dissociated nuclear coactivators, such as steroid receptor coactivator-1 and glucocorticoid receptor interacting protein-1, from the TR in the presence of T(3). Finally, MMI decreased the GH release that was stimulated by T(3).

CONCLUSIONS

ATDs inhibit T(3) action by recruitment of transcriptional corepressors and/or dissociation of coactivators. This is the first report to show that ATDs can modulate T(3) action at the transcriptional level.

Polymorphism of the growth hormone gene of red deer (Cervus elaphus)

In mammals, pituitary growth hormone (GH) is usually encoded by a single gene, but in some caprine ruminants there are two GH genes, and higher primates have a cluster of at least 5 GH-like genes. We have previously shown that in several artiodactyls (chevrotain, giraffe, and hippopotamus) there are two GH gene sequences, differing by 5-21 nucleotides (nt), but whether these arise from two distinct gene loci is unclear. We report here that in the red deer (Cervus elaphus) also there are two main GH gene sequences (designated A and B) differing at about 23 nt. Investigation of DNA from a number of individual animals demonstrated that this variation was due to allelic polymorphism, with individuals carrying either the A-type or the B-type sequence, or both. A- and B-type sequences showed some variation between individuals. The overall difference between the A and B sequences is substantial-greater than that between the GH gene sequences of three distinct bovine species, Bos taurus (ox), Bos indicus (zebu) and Bos grunniens (yak). The biological significance of the presence of two markedly differing GH gene sequences in red deer is not clear, but it is notable that several of the differences between the A and B sequences occur in the 5' upstream region, which may be associated with differences in gene expression.

Regulation of 5'-promoter activity of the rat growth hormone and growth hormone-releasing hormone receptor genes in the MtT/S and MtT/E cells

The MtT/E and MtT/S cells have been established from a mammotrophic pituitary tumor, and postulated to be progenitor and premature growth hormone (GH) cells, respectively. The difference in the regulation of GH and GH-releasing hormone (GHRH) receptor gene transcription in relation to the developmental stage of GH cells were examined in these two cell lines. In MtT/S cells, triiodothyronine (T3), all-trans retinoic acid (RA) and 9-cis retinoic acid (9cRA) stimulated GH promoter activity but dexamethasone (DEX) did not. On the other hand, DEX stimulated GHRH-receptor promoter alone. T3, RA and 9cRA showed little effect alone but each of them augmented the effect of DEX when used together with DEX. In MtT/E cells, DEX, RA and 9cRA showed similar effect as observed in MtT/S cells on both GH and GHRH-receptor promoter activity. However, T3 neither stimulated GH promoter activity nor augmented the DEX-induced GHRH-receptor gene transcription in MtT/E cells. RT-PCR analyses revealed that both cell types expressed TRalpha1, TRbeta1 and TRalpha2, but expression of TRbeta2, a pituitary specific isoform of TR, was only detected in MtT/S cells. However, the deficiency of TRbeta2 for its own sake does not appear to be a reason why T3 action was not observed in MtT/E cells, because co-transfection of expression plasmids for TRbeta2 and RXRalpha failed in conferring on the cells an ability to respond to T3 by increased GH or GHRH-receptor promoter activity. These results suggest that the acquisition of mechanisms responsible for the regulation of GH or GHRH-receptor transcription by T3 may be involved in the process of functional development of GH cells.

Histone acetyltransferase activity of p300 is required for transcriptional repression by the promyelocytic leukemia zinc finger protein

Histone acetyltransferase (HAT) activities of proteins such as p300, CBP, and P/CAF play important roles in activation of gene expression. We now show that the HAT activity of p300 can also be required for down-regulation of transcription by a DNA binding repressor protein. Promyelocytic leukemia zinc finger (PLZF), originally identified as a fusion with retinoic acid receptor alpha in rare cases of all-trans-retinoic acid-resistant acute promyelocytic leukemia, is a transcriptional repressor that recruits histone deacetylase-containing corepressor complexes to specific DNA binding sites. PLZF associates with p300 in vivo, and its ability to repress transcription is specifically dependent on HAT activity of p300 and acetylation of lysines in its C-terminal C2-H2 zinc finger motif. An acetylation site mutant of PLZF does not repress transcription and is functionally deficient in a colony suppression assay despite retaining its abilities to interact with corepressor/histone deacetylase complexes. This is due to the fact that acetylation of PLZF activates its ability to bind specific DNA sequences both in vitro and in vivo. Taken together, our results indicate that a histone deacetylase-dependent transcriptional repressor can be positively regulated through acetylation and point to an unexpected role of a coactivator protein in transcriptional repression.

Programmed cell death during amphibian metamorphosis

The death of different types of cells occurs in regressing or remodeling organs to transform from a tadpole to a frog in both temporally and spatially regulated manners during amphibian metamorphosis. This morphological change is drastic and visible with the naked eye. This review summarizes our current understanding of the basic mechanism of the cell death during the metamorphosis. It focuses in particular on the tail resorption and the remodeling of intestine and skin where programmed cell death is executed by thyroid hormone-signaling through the cell-autonomous response (suicide) and the degradation of the extracellular matrix (murder).

The thyroid hormone receptor beta-specific agonist GC-1 selectively affects the bone development of hypothyroid rats

We investigated the effects of GC-1, a TRbeta-selective thyromimetic, on bone development of hypothyroid rats. Whereas T3 reverted the IGF-I deficiency and the skeletal defects caused by hypothyroidism, GC-1 had no effect on serum IGF-I or on IGF-I protein expression in the epiphyseal growth plate of the femur, but induced selective effects on bone development. Our findings indicate that T3 exerts some essential effects on bone development that are mediated by TRbeta1.

INTRODUCTION

We investigated the role of the thyroid hormone receptor beta1 (TRbeta1) on skeletal development of rats using the TRbeta-selective agonist GC-1.

MATERIALS AND METHODS

Twenty-one-day-old female rats (n = 6/group) were rendered hypothyroid (Hypo) and treated for 5 weeks with 0.3 ug/100 g BW/day of T3 (1xT3), 5xT3, or equimolar doses of GC-1 (1xGC-1 and 5xGC-1). Serum triiodothyronine (T3), thyroxine (T4), thyroid-stimulating hormone (TSH), and insulin-like growth factor (IGF)-I concentrations were determined by radioimmunoassay (RIA). BMD and longitudinal bone growth were determined by DXA. Trabecular bone histomorphometry and epiphyseal growth plate (EGP) morphometry were performed in the distal femur. Expressions of IGF-I protein and of collagen II and X mRNA were evaluated by immunohistochemistry and in situ hybridization, respectively. To determine hormonal effects on ossification, skeletal preparations of hypothyroid-, 5xGC-1-, and 5xT3-treated neonatal rats were compared.

RESULTS

Hypothyroidism impaired longitudinal body growth and BMD gain, delayed ossification, reduced the number of hypertrophic chondrocytes (HCs; 72% versus Euthyroid [Eut] rats; p < 0.001), and resulted in disorganized columns of EGP chondrocytes. Serum IGF-I was 67% reduced versus Eut rats (p < 0.001), and the expression of IGF-I protein and collagen II and X mRNA were undetectable in the EGP of Hypo rats. T3 completely or partially normalized all these parameters. In contrast, GC-1 did not influence serum concentrations or EGP expression of IGF-I, failed to reverse the disorganization of proliferating chondrocyte columns, and barely affected longitudinal growth. Nevertheless, GC-1 induced ossification, HC differentiation, and collagen II and X mRNA expression and increased EGP thickness to Eut values. GC-1-treated rats had higher BMD gain in the total tibia, total femur, and in the femoral diaphysis than Hypo animals (p < 0.05). These changes were associated with increased trabecular volume (48%, p < 0.01), mineralization apposition rate (2.3-fold, p < 0.05), mineralizing surface (4.3-fold, p < 0.01), and bone formation rate (10-fold, p < 0.01).

CONCLUSIONS

Treatment of hypothyroid rats with the TRbeta-specific agonist GC-1 partially reverts the skeletal development and maturation defects resultant of hypothyroidism. This finding suggests that TRbeta1 has an important role in bone development.

Generation of Thyrotropin-Releasing Hormone Receptor 1-Deficient Mice as an Animal Model of Central Hypothyroidism

To provide an animal model of central hypothyroidism, mice deficient in the TRH-receptor 1 (TRH-R1) gene were generated by homologous recombination. The pituitaries of TRH-R1−/− mice are devoid of any TRH-binding capacity, demonstrating that TRH-R1 is the only receptor localized on TRH target cells of the pituitary. With the exception of some retardation in growth rate, TRH-R1−/− mice appear normal, but compared with control animals they exhibit a considerable decrease in serum T3, T4, and prolactin (PRL) levels but not in serum TSH levels. In situ hybridization histochemistry and real-time RT-PCR analysis revealed that in adult TRH-R1−/− animals TSHβ-mRNA expression is not impaired whereas PRL mRNA and GH mRNA levels are considerably reduced compared with control mice. The numbers of thyrotropes, somatotropes, and lactotropes, however, are not affected by the deletion of the TRH-R1 gene. The mutant mice are fertile, and the dams nourish their pups well, indicating that TRH is not a decisive factor for suckling-induced PRL release. In situ hybridization and quantitative RT-PCR analysis, furthermore, revealed that, as in control animals, pituitary PRL-mRNA expression in TRH-R1−/− is considerably increased during lactation, albeit strongly reduced as compared with lactating control animals.

Intracellular dynamics of Smad-mediated TGFbeta signaling

The transforming growth factor-beta (TGFbeta) family represents a class of signaling molecules that plays a central role in morphogenesis, growth, and cell differentiation during normal embryonic development. Members of this growth factor family are particularly vital to development of the mammalian secondary palate where they regulate palate mesenchymal cell proliferation and extracellular matrix synthesis. Such regulation is particularly critical since perturbation of either cellular process results in a cleft of the palate. While the cellular and phenotypic effects of TGFbeta on embryonic craniofacial tissue have been extensively catalogued, the specific genes that function as downstream mediators of TGFbeta action in the embryo during palatal ontogenesis are poorly defined. Embryonic palatal tissue in vivo and murine embryonic palate mesenchymal (MEPM) cells in vitro secrete and respond to TGFbeta. In the current study, elements of the Smad component of the TGFbeta intracellular signaling system were identified and characterized in cells of the embryonic palate and functional activation of the Smad pathway by TGFbeta1, TGFbeta2, and TGFbeta3 was demonstrated. TGFbeta-initiated Smad signaling in cells of the embryonic palate was found to result in: (1) phosphorylation of Smad 2; (2) nuclear translocation of the Smads 2, 3, and 4 protein complex; (3) binding of Smads 3 and 4 to a consensus Smad binding element (SBE) oligonucleotide; (4) transactivation of transfected reporter constructs, containing TGFbeta-inducible Smad response elements; and (4) increased expression of gelatinases A and B (endogenous genes containing Smad response elements) whose expression is critical to matrix remodeling during palatal ontogenesis. Collectively, these data point to the presence of a functional Smad-mediated TGFbeta signaling system in cells of the developing murine palate.

Binding of the thyroid hormone receptor to a negative element in the basal growth hormone promoter is associated with histone acetylation

Nuclear thyroid hormone receptors (TRs) act as ligand-dependent activators, but paradoxically unliganded TRs can increase transcription of promoters containing negative response elements (nTRE), and hormone binding represses this activation. The rat growth hormone (GH) promoter contains a positive TRE and a nTRE. Ligand-dependent negative regulation mediated by the nTRE could play an important physiological role in restricting GH gene expression in non-pituitary cells that express TRs. With chromatin immunoprecipitation assays, we show here that the nTRE is responsible for binding of TR to the promoter in non-pituitary HeLa cells and that this element also governs transactivation by the unoccupied receptor and repression by triiodothyronine. Occupancy of the promoter by TR is concomitant with appearance of acetylated histone H3, and triiodothyronine causes release of the receptor as well as disappearance of the acetylated histone from the promoter. Although the nTRE overlaps the TATA box, the receptor does not exclude binding of TATA-binding protein, but could rather facilitate formation of the preinitiation complex. Furthermore, the proximal GH promoter is synergistically stimulated by unliganded TR and TATA-binding protein, whereas the ligand represses this cooperation. Constitutive receptor activity and synergism with TATA-binding protein require binding of corepressors. Furthermore, inhibitors of histone deacetylases enhance promoter activation by the unliganded receptor and reduce triiodothyronine-dependent repression, whereas expression of HDAC1 reverses promoter stimulation. This suggests that partitioning of histone acetylases and deacetylases between the receptors and basal transcription factors could be involved in regulation of the basal GH promoter by TRs.

Differential modulation of androgen receptor action by deoxyribonucleic acid response elements

In addition to the steroid response elements (SREs), which are recognized by several steroid receptors, a second class of DNA elements exhibiting selectivity for the androgen receptor (AR) and named androgen response elements (AREs) has been identified. Here we provide evidence for the differential role of these element classes in modulating AR function. AR complexes attached to response elements representative of each class were purified. Limited protease digests of ARE- or SRE-bound AR complexes led to the generation of different patterns, in line with differential accessibilities. In transactivation assays, mutations in the AR dimerization interface of the DNA-binding domain had various effects, depending on the response elements tested. The R598D mutant displayed much enhanced activity on SREs, whereas far less effect was seen on the selective AREs. The A596T mutant had reduced activity on AREs but not on SREs. Ectopic expression of the coactivators transcriptional intermediary factor 2 (TIF2) and ARA55 stimulated AR activity to different extents, depending on the response element. When using cysteine-rich secretory protein 1 (CRISP-1) SRE as reference, the most significant difference was observed with Pem ARE-2. A differential response of each element class was furthermore observed in the presence of two enzymes involved in the sumoylation pathway. Ubiquitin-conjugating enzyme 9 (Ubc9) overexpression enhanced AR action conveyed by SREs, whereas little effect was seen on Pem ARE-1 and repression on Pem ARE-2. Protein inhibitor of activated STAT (PIAS)xalpha overexpression had little influence on SRE-mediated AR activity but was repressive when using AREs. Altogether, these results demonstrate that DNA response elements play an important modulatory role in transmitting AR action and may be determinative for specificity of gene expression in cell or tissue types.

A chemical switch regulates fibrate specificity for peroxisome proliferator-activated receptor alpha (PPARalpha ) versus liver X receptor

Fenofibrate is clinically successful in treating hypertriglyceridemia and mixed hyperlipidemia presumably through peroxisome proliferator-activated receptor alpha (PPARalpha)-dependent induction of genes that control fatty acid beta-oxidation. Lipid homeostasis and cholesterol metabolism also are regulated by the nuclear oxysterol receptors, liver X receptors alpha and beta (LXRalpha and LXRbeta). Here we show that fenofibrate ester, but not fenofibric acid, functions as an LXR antagonist by directly binding to LXRs. Likewise, ester forms, but not carboxylic acid forms, of other members of the fibrate class of molecules antagonize the LXRs. The fibrate esters display greater affinity for LXRs than the corresponding fibric acids have for PPARalpha. Thus, these two nuclear receptors display a degree of conservation in their recognition of ligands; yet, the acid/ester moiety acts as a chemical switch that determines PPARalpha versus LXR specificity. Consistent with its LXR antagonistic activity, fenofibrate potently represses LXR agonist-induced transcription of hepatic lipogenic genes. Surprisingly, fenofibrate does not repress LXR-induced transcription of various ATP-binding cassette transporters either in liver or in macrophages, suggesting that fenofibrate manifests variable biocharacter in the context of differing gene promoters. These findings provide not only an unexpected mechanism by which fenofibrate inhibits lipogenesis but also the basis for examination of the pharmacology of an LXR ligand in humans.

Differential modulation of liver and pituitary triiodothyronine and 9-cis retinoid acid receptors by insulin-like growth factor I in rats

Triiodothyronine (T(3)) exerts most of its effects through nuclear thyroid hormone receptors (TRs) that bind mainly as heterodimers with retinoid-X receptors (RXRs) to thyroid hormone response elements in target genes. It is well known that T(3) activates the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis in rats. In turn, IGF-I inhibits the T(3)-induced GH production in cell cultures. The impact of IGF-I on T(3) action has only been partially explored. We have presented evidence that IGF-I feeds back to limit specific metabolic actions of T(3) in rat liver through a downregulation of nuclear TR number and its mRNA expression. We have also found that IGF-I injected to rats inhibited pituitary GH production. In this study we aimed at exploring whether the IGF-I-induced feedback loop on T(3)-action in the liver also operates in the pituitary gland. The mechanism of the liver TR mRNA reduction induced by IGF-I was also studied. We evaluated the effect of recombinant human (rh) IGF-I administration (240 microg/100 g of body weight subcutaneously every 12 hours for 48 hours) to adult male Wistar rats on TR and RXR proteins (Western blot) from pituitary, liver, brain, and thyroid and TR mRNA (Northern blot) from pituitary and liver. The transcriptional rate of liver TR gene (run-on assay) was also determined. In pituitary, TR protein and TR mRNA isoforms were reduced by rhIGF-I. No changes in TR proteins in brain and thyroid were observed. Nuclear run-on assay revealed that IGF-I reduced the TR gene transcriptional rate in liver. A significant increase in RXR proteins in liver and pituitary without changes in thyroid and brain was induced by IGF-I. In conclusion, these results indicate that in pituitary, IGF-I downregulates TR expression, similarly as previously found in liver. A reduced transcriptional rate of TR gene is implicated in the IGF-I effect on the liver. The increase in RXR protein levels may be also involved in the expression of T(3) specific actions in liver and pituitary.

The role of C/EBP in nutrient and hormonal regulation of gene expression

C/EBPs are a family of transcription factors that play important roles in energy metabolism. Although initially thought to be constitutive regulators of transcription, an increasing amount of evidence indicates that their transactivating capacity within the cell can be modulated by nutrients and hormones. There are several mechanisms whereby this occurs. First, hormones/nutrients are known to directly alter the expression of C/EBPs. Second, hormones/nutrients may cause an alteration in the phosphorylation state of C/EBPs, which can affect their DNA-binding activity or transactivating capacity. Third, C/EBPs can function as accessory factors on gene promoters within a hormone response unit, interacting with other transcription factors to enhance the degree of responsiveness to specific hormones. Given their role in regulating genes involved in a wide variety of metabolic events, advancing our understanding of the molecular mechanism of action of C/EBPs will undoubtedly further our appreciation for the role these transcription factors play in both health and disease.

Ligand-dependent degradation of retinoid X receptors does not require transcriptional activity or coactivator interactions

Cells utilize ubiquitin-mediated proteolysis to regulate the activity of numerous proteins involved in signal transduction, cell cycle control, and transcriptional regulation. For a number of transcription factors, there appears to be a direct correlation between transcriptional activity and protein instability, suggesting that cells use targeted destruction as one method to down-regulate or attenuate gene expression. In this report we demonstrate that retinoid X receptors (RXRs) which function as versatile mediators of nuclear hormone-dependent gene expression are marked for destruction upon binding agonist ligands. Interestingly, when RXR serves as a heterodimeric partner for retinoic acid (RAR) or thyroid hormone (TR) receptors, binding of agonists by RAR or TR leads to degradation of both the transcriptionally active RAR or TR subunits as well as the transcriptionally inactive RXR subunit. Furthermore, using a series of mutants in the ligand-dependent activation domain (activation function 2), we demonstrate that agonist-stimulated degradation of RXR does not require corepressor release, coactivator binding, or transcriptional activity. Taken together, the data suggest a model for targeted destruction of transcription factors based on structural or conformational signals as opposed to functional coupling with gene transcription.

The role of hinge domain in heterodimerization and specific DNA recognition by nuclear receptors

Four structural domains are characteristic of the members of the nuclear receptor superfamily. The hinge (D) domain which is located between the DNA binding (C) domain and the ligand binding (EF) domain, is less conserved among the nuclear receptors. In this study, we investigated the effects of the D domain on receptor function with regard to ligand binding, protein-protein interaction and DNA recognition. We found that EF domain of TR lacked T3 binding activity and additional D domain was required for its ligand binding. Using pull down assays and two-hybrid assays, we also demonstrated that the EF domain of TR did not dimerize with TR or RXR in solution, while the DEF domain was able to homo-and heterodimerize with RXR. In contrast, the RXR EF domain alone was able to heterodimerize with TR. The D domain of TR is required but that of RXR is not necessary for the interaction. We further demonstrated that the D domain was required for receptor specific DNA recognition. The ABC domain of vitamin D receptor (VDR) and TR(DEF) chimeric receptor could not bind to VDR response element (VDRE). Addition of own D domain of VDR to the ABC domain enables the chimeric receptor to bind VDRE and transactivate. The D domain of TR cannot substitute for that of VDR in context of specific DNA recognition. These data suggest that the D domain is important to maintain the integrity of the functional structure of the nuclear receptors.

Expression of peptides and other neurochemical markers in hypothalamus and olfactory bulb of mice devoid of all known thyroid hormone receptors

We have investigated with histochemical techniques the expression of peptides and other neurochemical markers in the hypothalamus and olfactory bulb of male mice, in which the genes encoding the alpha and beta thyroid hormone receptors (TRalpha1, TRbeta1 and TRbeta2) have been deleted. Thyrotropin-releasing hormone messenger RNA levels were increased in the hypothalamic paraventricular nucleus and in the medullary raphe nuclei of mutant mice lacking the thyroid hormone receptors alpha1 and beta (alpha1(-/-)beta(-/-)), as compared to wild-type mice. In contrast, galanin messenger RNA levels were lower in the hypothalamic paraventricular nucleus of mutant animals, as was galanin-like immunoreactivity in the internal layer of the median eminence. Substance P messenger RNA levels were unchanged in the medullary raphe nuclei. Thyrotropin-releasing hormone receptor messenger RNA levels were increased in motoneurons, unchanged in the subiculum, and lower in the amygdala of mutant animals. Galanin messenger RNA levels were unchanged in the hypothalamic dorsomedial and arcuate nuclei of the thyroid hormone receptor alpha1(-/-)beta(-/-) mice, as was the immunocytochemistry for oxytocin and for vasopressin in the hypothalamic paraventricular nucleus. A reduction in tyrosine hydroxylase messenger RNA levels was found in the arcuate nucleus of mutant mice. In the olfactory bulb, immunohistochemistry for calbindin and for tyrosine hydroxylase revealed a reduction in the intensity of labeling of nerve processes in the glomerular layer of thyroid hormone receptor alpha1(-/-)beta(-/-) mice. The tyrosine hydroxylase messenger RNA levels were also slightly reduced. In contrast, the levels of galanin and neuropeptide Y messenger RNA in this region were unchanged in thyroid hormone receptor alpha1(-/-)beta(-/-) mice as compared to wild-type mice. Together these studies reveal many regional and neurochemically selective alterations in neuronal phenotype of mice devoid of all known thyroid hormone receptors.

Cell type-specific roles of histone deacetylase in TR ligand-independent transcriptional repression

Recent evidence indicates that corepressor protein with histone deacetylase (HDAC) activity mediates thyroid hormone receptor (TR) transcriptional repression. In order to examine the physiological relevance of HDAC in ligand-independent TR-mediated repression, we studied the effect of trichostatin A (TSA), a specific HDAC inhibitor, in transient transfection studies with natural reporters, and assessed the expression of TR-regulated endogenous genes. Luciferase-coupled DR4-, F2-, PAL- or GH-TREs and TRbeta1 expression vectors were cotransfected in CV-1 and GH(3) cells. We did not observe any effect of TSA on TR-induced basal repression in CV-1 cells. Instead, TSA was able to induce an increase in transcription without T(3) on all TREs tested in GH(3) cells. This increase was >7-fold on F2-, >4-fold on DR4-, and 3-fold on GH-TREs. The cotransfection of a TRbeta1 mutant that exhibits decreased affinity with N-CoR (AHT) reduced the TSA effect in GH(3) cells, demonstrating a primary role for TR/N-CoR/Sin3/HDAC complex. Next, we examined the effects of TSA on endogenous GH mRNA production in GH(3) cells by Northern blot analysis. We observed an increase of 50-70% of GH mRNA in cells treated with TSA in hypothyroid medium, and an increase of GH mRNA in T(3)-treated cells after TSA treatment. Our results show that TSA can increase the expression of endogenous genes that are susceptible to TR regulation. These results support an active role of HDAC in transcriptional repression by ligand-independent TR.

Allosteric effects of Pit-1 DNA sites on long-term repression in cell type specification

Reciprocal gene activation and restriction during cell type differentiation from a common lineage is a hallmark of mammalian organogenesis. A key question, then, is whether a critical transcriptional activator of cell type-specific gene targets can also restrict expression of the same genes in other cell types. Here, we show that whereas the pituitary-specific POU domain factor Pit-1 activates growth hormone gene expression in one cell type, the somatotrope, it restricts its expression from a second cell type, the lactotrope. This distinction depends on a two-base pair spacing in accommodation of the bipartite POU domains on a conserved growth hormone promoter site. The allosteric effect on Pit-1, in combination with other DNA binding factors, results in the recruitment of a corepressor complex, including nuclear receptor corepressor N-CoR, which, unexpectedly, is required for active long-term repression of the growth hormone gene in lactotropes.

The mechanism of action of thyroid hormones

Thyroid hormone is essential for normal development, differentiation, and metabolic balance. Thyroid hormone action is mediated by multiple thyroid hormone receptor isoforms derived from two distinct genes. The thyroid hormone receptors belong to a nuclear receptor superfamily that also includes receptors for other small lipophilic hormones. Thyroid hormone receptors function by binding to specific thyroid hormone-responsive sequences in promoters of target genes and by regulating transcription. Thyroid hormone receptors often form heterodimers with retinoid X receptors. Heterodimerization is regulated through distinct mechanisms that together determine the specificity and flexibility of the sequence recognition. Amino-terminal regions appear to modulate thyroid hormone receptor function in an isoform-dependent manner. Unliganded thyroid hormone receptor represses transcription through recruitment of a corepressor complex, which also includes Sin3A and histone deacetylase. Ligand binding alters the conformation of the thyroid hormone receptor in such a way as to release the corepressor complex and recruit a coactivator complex that includes multiple histone acetyltransferases, including a steroid receptor family coactivator, p300/CREB-binding protein-associated factor (PCAF), and CREB binding protein (CBP). The existence of histone-modifying activities in the transcriptional regulatory complexes indicates an important role of chromatin structure. Stoichiometric, structural, and sequence-specific rules for coregulator interaction are beginning to be understood, as are aspects of the tissue specificity of hormone action. Moreover, knockout studies suggest that the products of two thyroid hormone receptor genes mediate distinct functions in vivo. The increased understanding of the structure and function of thyroid hormone receptors and their interacting proteins has markedly clarified the molecular mechanisms of thyroid hormone action.

A choice between transcriptional enhancement and repression by the v-erbA oncoprotein governed by one nucleotide in a thyroid hormone responsive half site

The v-erbA oncoprotein (P75gag-v-erbA) can repress thyroid hormone receptor induced transcriptional activation of target genes. A central question is how hormone responsive elements in a target gene determine the transcriptional regulation mediated by P75gag-v-erbA. We addressed this with receptors chimeric between P75gag-v-erbA and thyroid hormone receptor (TR) by testing their regulatory activities on thyroid hormone response elements (TREs) differing in the sequence of the consensus core recognition motif AGGTCA. We report here that enhances, TR dependent transcriptional activation is conferred by P75gag-v-erbA when the thymidine in the half site recognition motif is exchanged for an adenosine. The enhancement was independent of the DNA binding region of P75gag-v-erbA, whereas increased expression of corepressor abolished the enhancing effect. The data indicate that the enhancement results from an impaired DNA binding by the oncoprotein combined with an effective scavenging of corepressors. Our data thus suggest the P75gag-v-erbA indirectly can contribute to enhancement of thyroid hormone induced gene expression.

Effects of thyroid hormone receptor gene disruption on myosin isoform expression in mouse skeletal muscles

Skeletal muscle is known to be a target for the active metabolite of thyroid hormone, i.e., 3,5,3'-triiodothyronine (T(3)). T(3) acts by repressing or activating genes coding for different myosin heavy chain (MHC) isoforms via T(3) receptors (TRs). The diverse function of T(3) is presumed to be mediated by TR-alpha(1) and TR-beta, but the function of specific TRs in regulating MHC isoform expression has remained undefined. In this study, TR-deficient mice were used to expand our knowledge of the mechanisms by which T(3) regulates the expression of specific MHC isoforms via distinct TRs. In fast-twitch extensor digitorum longus (EDL) muscle, TR-alpha(1)-, TR-beta-, or TR-alpha(1)beta-deficient mice showed a small but statistically significant decrease (P < 0.05) of type IIB MHC content and an increased number of type I fibers. In the slow-twitch soleus, the beta/slow MHC (type I) isoform was significantly (P < 0. 001) upregulated in the TR-deficient mice, but this effect was highly dependent on the type of receptor deleted. The lack of TR-beta had no significant effect on the expression of MHC isoforms. An increase (P < 0.05) of type I MHC was observed in the TR-alpha(1)-deficient muscle. A dramatic overexpression (P < 0.001) of the slow type I MHC and a corresponding downregulation of the fast type IIA MHC (P < 0.001) was observed in TR-alpha(1)beta-deficient mice. The muscle- and fiber-specific differences in MHC isoform expression in the TR-alpha(1)beta-deficient mice resembled the MHC isoform transitions reported in hypothyroid animals, i.e., a mild MHC transition in the EDL, a dramatic but not complete upregulation of the beta/slow MHC isoform in the soleus, and a variable response to TR deficiency in different soleus muscle fibers. Thus the consequences on muscle are similar in the absence of thyroid hormone or absence of thyroid hormone receptors, indicating that TR-alpha(1) and TR-beta together mediate the known actions of T(3). However, it remains unknown how thyroid hormone exerts muscle- and muscle fiber-specific effects in its action. Finally, although developmental MHC transitions were not studied specifically in this study, the absence of embryonic and fetal MHC isoforms in the TR-deficient mice indicates that ultimately the transition to the adult MHC isoforms is not solely mediated by TRs.

Ca(2+)-dependent gene expression mediated by MEF2 transcription factors

Ca(2+) induction of a subset of cellular and viral immediate-early activation genes in lymphocytes has been previously mapped to response elements recognized by the MEF2 family of transcription factors. Here, we demonstrate that Ca(2+) activation of MEF2 response elements in T lymphocytes is mediated in synergy by two Ca(2+)/calmodulin-dependent enzymes, the phosphatase calcineurin, and the kinase type IV/Gr (CaMKIV/Gr), which promote transcription by the MEF2 family members MEF2A and MEF2D. Calcineurin up-regulates the activity of both factors by an NFAT-dependent mechanism, while CaMKIV/Gr selectively and independently activates MEF2D. These results identify MEF2 proteins as effectors of a pathway of gene induction in T lymphocytes which integrates diverse Ca(2+) activation signals and may be broadly operative in several tissues.