The T3R alpha gene encoding a thyroid hormone receptor is essential for post-natal development and thyroid hormone production

Thyroid hormone's action on progenitor/stem cell biology: new challenge for a classic hormone?

BACKGROUND

Thyroid hormones are involved in developmental and homeostatic processes in several tissues. Their action results in different outcomes depending on the developmental stage, tissue and/or cellular context. Interestingly, their pleiotropic roles are conserved across vertebrates. It is largely documented that thyroid hormones act via nuclear receptors, the TRs, which are transcription factors and whose activity can be modulated by the local availability of the hormone T3. In the "classical view", the T3-induced physiological response depends on the expression of specific TR isoforms and the iodothyronine deiodinase selenoenzymes that control the local level of T3, thus TR activity.

SCOPE OF THE REVIEW

Recent data have clearly established that the functionality of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and propose a new and intriguing role for thyroid hormones in two selected examples.

MAJOR CONCLUSIONS

In the intestinal epithelium and the retina, TRα1 and TRβ2 are expressed at the level of the precursors where they induce cell proliferation and differentiation, respectively. Moreover, these different functions result from the integration of the hormone signal with other intrinsic pathways, which play a fundamental role in progenitor/stem cell physiology.

GENERAL SIGNIFICANCE

Taken together, the interaction of TRs with other signaling pathways, specifically in stem/progenitor cells, is a new concept that may have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Thyroid hormone receptors: the challenge of elucidating isotype-specific functions and cell-specific response

BACKGROUND

Thyroid hormone receptors TRα1, TRβ1 and TRβ2 are broadly expressed and exert a pleiotropic influence on many developmental and homeostatic processes. Extensive genetic studies in mice precisely defined their respective function.

SCOPE OF REVIEW

The purpose of the review is to discuss two puzzling issues:

MAJOR CONCLUSIONS

Mouse genetics support a balanced contribution of expression pattern and receptor intrinsic properties in defining the receptor respective functions. The molecular mechanisms sustaining cell specific response remain hypothetical and based on studies performed with other nuclear receptors.

GENERAL SIGNIFICANCE

The isoform-specificity and cell-specificity questions have many implications for clinical research, drug development, and endocrine disruptor studies. This article is part of a Special Issue entitled Thyroid hormone signalling.

Triiodothyronine stimulates glucose transport in bone cells

Thyroid hormones increase energy expenditure and bone turnover in vivo. To study whether 3,3',5-triiodo-l-thyronine (T₃) stimulates the uptake of glucose in osteoblastic cells, PyMS (a cell line derived from rat bone) cells were kept in serum-free culture medium and treated with T₃. We measured [1-¹⁴C]-2-deoxy-D: -glucose (2DG) uptake and looked for expression of the high-affinity glucose transporters GLUT1 and GLUT3 by northern and western analysis. T₃ did not influence the cell number but slightly (1.3-fold) increased the protein content of the cell cultures. 2DG uptake was low in serum-deprived cell cultures and was increased by T₃ (up to 2.5-fold at 1 nmol l⁻¹ after 4 days) in a dose- and time-dependent manner. Triiodothyronine at 1 nmol l⁻¹ increased GLUT1 and GLUT3 abundance in membranes. Therefore, increased glucose uptake induced by T₃ in osteoblasts may be mediated by the known high-affinity glucose transporters GLUT1 and GLUT3.

Fundamentally distinct roles of thyroid hormone receptor isoforms in a thyrotroph cell line are due to differential DNA binding

Thyroid hormones have a profound influence on human development and disease. The hypothalamic-pituitary-thyroid axis involves finely tuned feedback mechanisms to maintain thyroid hormone (TH) levels. Despite the important role of TH-negative feedback in regulating this axis, the mechanism by which this occurs is not clearly defined. Previous in vivo studies suggest separate roles for the two thyroid hormone receptor isoforms, THRA and THRB, in this axis. We performed studies using a unique pituitary thyrotroph cell line (TαT1.1) to determine the relative roles of THRA and THRB in the regulation of Tshb. Using chromatin immunoprecipitation assays, we found that THRB, not THRA, bound to the Tshb promoter. By selectively depleting THRB, THRA, or both THRA and THRB in TαT1.1 cells, we found that simultaneous knockdown of both THRB and THRA abolished T(3)-mediated down-regulation of Tshb at concentrations as high as 100 nm T(3). In contrast, THRA knockdown alone had no effect on T(3)-negative regulation, whereas THRB knockdown alone abolished T(3)-mediated down-regulation of Tshb mRNA levels at 10 nm but not 100 nm T(3) concentrations. Interestingly, chromatin immunoprecipitation assays showed that THRA becomes enriched on the Tshb promoter after knockdown of THRB. Thus, a likely mechanism for the differential effects of THR isoforms on Tshb may be based on their differential DNA-binding affinity to the promoter.

Imprinted gene dosage is critical for the transition to independent life

Neonatal survival in mammals is crucially dependent upon maintenance of body temperature. Neonatal body temperature is largely maintained by thermogenesis in brown adipose tissue (BAT). BAT develops perinatally in mice requiring integration of adipogenic and thermoregulatory gene pathways. We describe a regulatory mutation in the imprinted gene cluster on mouse chromosome 12 resulting in early postnatal lethality. Maternal inheritance of this mutation impairs the ability of young mice to maintain body temperature. While mechanisms of perinatal BAT development are well understood, our work highlights a second phase of BAT recruitment necessary to support small animals newly independent of the nest. We show that the imprinted delta-like homolog 1/preadipocyte factor (Dlk1/Pref1) and iodothyronine deiodinase type 3 (Dio3) functions converge on the development of brown fat at the transition to independent life. This shows that appropriate dosage control at imprinted loci can act as a critical determinant in postnatal survival during phases of physiological adaptation.

A mutation in the thyroid hormone receptor alpha gene

Thyroid hormones exert their effects through alpha (TRα1) and beta (TRβ1 and TRβ2) receptors. Here we describe a child with classic features of hypothyroidism (growth retardation, developmental retardation, skeletal dysplasia, and severe constipation) but only borderline-abnormal thyroid hormone levels. Using whole-exome sequencing, we identified a de novo heterozygous nonsense mutation in a gene encoding thyroid hormone receptor alpha (THRA) and generating a mutant protein that inhibits wild-type receptor action in a dominant negative manner. Our observations are consistent with defective human TRα-mediated thyroid hormone resistance and substantiate the concept of hormone action through distinct receptor subtypes in different target tissues.

Thyroid hormone receptor repression is linked to type I pneumocyte-associated respiratory distress syndrome

Although the lung is a defining feature of air-breathing animals, the pathway controlling the formation of type I pneumocytes, the cells that mediate gas exchange, is poorly understood. In contrast, the glucocorticoid receptor and its cognate ligand have long been known to promote type II pneumocyte maturation; prenatal administration of glucocorticoids is commonly used to attenuate the severity of infant respiratory distress syndrome (RDS). Here we show that knock-in mutations of the nuclear co-repressor SMRT (silencing mediator of retinoid and thyroid hormone receptors) in C57BL/6 mice (SMRTmRID) produces a previously unidentified respiratory distress syndrome caused by prematurity of the type I pneumocyte. Though unresponsive to glucocorticoids, treatment with anti-thyroid hormone drugs (propylthiouracil or methimazole) completely rescues SMRT-induced RDS, suggesting an unrecognized and essential role for the thyroid hormone receptor (TR) in lung development. We show that TR and SMRT control type I pneumocyte differentiation through Klf2, which, in turn, seems to directly activate the type I pneumocyte gene program. Conversely, mice without lung Klf2 lack mature type I pneumocytes and die shortly after birth, closely recapitulating the SMRTmRID phenotype. These results identify TR as a second nuclear receptor involved in lung development, specifically type I pneumocyte differentiation, and suggest a possible new type of therapeutic option in the treatment of RDS that is unresponsive to glucocorticoids.

Thyroid hormone receptor β mediates thyroid hormone effects on bone remodeling and bone mass

Excess thyroid hormone (TH) in adults causes osteoporosis and increases fracture risk. However, the mechanisms by which TH affects bone turnover are not elucidated. In particular, the roles of thyroid hormone receptor (TR) isotypes in the mediation of TH effects on osteoblast-mediated bone formation and osteoclast-mediated bone resorption are not established. In this study we have induced experimental hypothyroidism or hyperthyroidism in adult wild-type, TRα- or TRβ-deficient mice and analyzed the effects of TH status on the structure and remodeling parameters of trabecular bone. In wild-type mice, excess TH decreased bone volume and mineralization. High TH concentrations were associated with a high bone-resorption activity, assessed by increased osteoclast surfaces and elevated concentrations of serum bone-resorption markers. Serum markers of bone formation also were higher in TH-treated mice. TRα deficiency did not prevent TH action on bone volume, bone mineralization, bone formation, or bone resorption. In contrast, TRβ deficiency blocked all the early effects of excess TH observed in wild-type mice. However, prolonged exposure to low or high TH concentrations of TRβ-deficient mice induced mild modifications of bone structure and remodeling parameters. Together our data suggest that TRβ receptors mediate the acute effects produced by transient changes of TH concentrations on bone remodeling, whereas TRα receptors mediate long-term effects of chronic alterations of TH metabolism. These data shed new light on the respective roles of TRs in the control of bone metabolism by TH.

The thyroid hormone receptors as modulators of skin proliferation and inflammation

We have analyzed the role of the thyroid hormone receptors (TRs) in epidermal homeostasis. Reduced keratinocyte proliferation is found in interfollicular epidermis of mice lacking the thyroid hormone binding isoforms TRα1 and TRβ (KO mice). Similar results were obtained in hypothyroid animals, showing the important role of the liganded TRs in epidermal proliferation. In addition, KO and hypothyroid animals display decreased hyperplasia in response to 12-O-tetradecanolyphorbol-13-acetate. Both receptor isoforms play overlapping functional roles in the skin because mice lacking individually TRα1 or TRβ also present a proliferative defect but not as marked as that found in double KO mice. Defective proliferation in KO mice is associated with reduction of cyclin D1 expression and up-regulation of the cyclin-dependent kinase inhibitors p19 and p27. Paradoxically, ERK and AKT activity and expression of downstream targets, such as AP-1 components, are increased in KO mice. Increased p65/NF-κB and STAT3 phosphorylation and, as a consequence, augmented expression of chemokines and proinflammatory cytokines is also found in these animals. These results show that thyroid hormones and their receptors are important mediators of skin proliferation and demonstrate that TRs act as endogenous inhibitors of skin inflammation, most likely due to interference with AP-1, NF-κB, and STAT3 activation.

Thyromimetics: a journey from bench to bed-side

Selective thyromimetics are synthetic analogs of thyroid hormones with tissue-specific thyroid hormone actions. Tissue selectivity is partly mediated by selectivity for the thyroid hormone receptor-β isoform, but is also enhanced by tissue-selective uptake. Several preclinical animal models and recent human clinical trials have provided sound evidence that thyromimetics can serve as pharmacological tools to improve serum lipids without affecting heart rate. Thyromimetics consistently and efficiently lowered low-density lipoprotein cholesterol and lipoprotein (a) plasma levels without positive chronotropic effects. Most importantly, thyromimetics had a synergistic action when used in addition to 3-hydroxy-3-methylglutaryl CoA reductase inhibitors. Animal data have further suggested that thyromimetics might be useful in the treatment of obesity, hepatic steatosis and atherosclerosis. However, only long-term phase III clinical trials will tell if the observed lipid lowering effects of thyromimetics will improve cardiovascular outcome in humans, too. At the moment, the treatment of dyslipidemia seems to be the major indication for the therapeutic use of thyromimetics, which are now rapidly moving from bench to bed-side.

Growth hormone treatment in children with idiopathic short stature: correlation of growth response with peripheral thyroid hormone action

OBJECTIVE

Idiopathic short stature (ISS) describes short children with normal GH secretion. Although GH treatment increases their heights, growth response to the therapy differs among patients. Thyroid hormones (TH) are essential for longitudinal growth acting mainly through TH receptors (TR) α and β. We have previously reported that GH treatment reduced peripheral TH action in Turner Syndrome by TR down-regulation. The aims of the study were to assess the effect of GH treatment to ISS on peripheral TH action and the correlation between thyroid status and growth response to the therapy. SUBJECTS, DESIGN AND MEASUREMENTS: Eighteen normal (control) and twenty-five ISS children were enrolled and evaluated before and after 12 months of life time (control) or 12 months of GH therapy (ISS). Fasting blood was used for serum biochemical evaluations, peripheral blood mononuclear cells for TR mRNA determination by QRT-PCR and growth parameters by standard methods.

RESULTS

GH treatment modified neither TR mRNA levels nor serum markers of TH action in ISS evaluated as a whole group. However, the individual change in TRβ mRNA levels correlated to the change in sex hormone-binding globulin (SHBG) levels after GH therapy. The growth response to GH correlated positively with the change in TRα mRNA level and negatively with that in TRβ mRNA, TSH and SHBG levels. The change in each TR mRNA isoform after GH treatment correlated negatively with its own basal level.

CONCLUSIONS

GH therapy induced individual changes in TR expression in ISS that correlated with their growth response. The basal TR mRNA level could predetermine the change in TR expression and therefore the sensitivity to GH treatment.

Maternal thyroid hormones are transcriptionally active during embryo-foetal development: results from a novel transgenic mouse model

Even though several studies highlighted the role of maternal thyroid hormones (THs) during embryo-foetal development, direct evidence of their interaction with embryonic thyroid receptors (TRs) is still lacking. We generated a transgenic mouse model ubiquitously expressing a reporter gene tracing TH action during development. We engineered a construct (TRE2×) containing two TH-responsive elements controlling the expression of the LacZ reporter gene, which encodes β-galactosidase (β-gal). The specificity of the TRE2× activation by TH was evaluated in NIH3T3 cells by cotransfecting TRE2× along with TRs, retinoic or oestrogen receptors in the presence of their specific ligands. TRE2× transgene was microinjected into the zygotes, implanted in pseudopregnant BDF1 (a first-generation (F1) hybrid from a cross of C57BL/6 female and a DBA/2 male) mice and transgenic mouse models were developed. β-gal expression was assayed in tissue sections of transgenic mouse embryos at different stages of development. In vitro, TRE2× transactivation was observed only following physiological T3 stimulation, mediated exclusively by TRs. In vivo, β-gal staining, absent until embryonic day 9.5-10.5 (E9.5-E10.5), was observed as early as E11.5-E12.5 in different primordia (i.e. central nervous system, sense organs, intestine, etc.) of the TRE2× transgenic embryos, while the foetal thyroid function (FTF) was still inactive. Immunohistochemistry for TRs essentially colocalized with β-gal staining. No β-gal staining was detected in embryos of hypothyroid transgenic mice. Importantly, treatment with T3 in hypothyroid TRE2× transgenic mice rescued β-gal expression. Our results provide in vivo direct evidence that during embryonic life and before the onset of FTF, maternal THs are transcriptionally active through the action of embryonic TRs. This model may have clinical relevance and may be employed to design end-point assays for new molecules affecting THs action.

The thyroid hormone, triiodothyronine, enhances fluoxetine-induced neurogenesis in rats: possible role in antidepressant-augmenting properties

The thyroid hormone triiodothyronine (T3) may accelerate and augment the action of antidepressants. Antidepressants up-regulate neurogenesis in adult rodent hippocampus. We studied the effect of T3 and T3+fluoxetine in enhancement of hippocampal neurogenesis beyond that induced by fluoxetine alone and the correlation with antidepressant behaviour in the novelty suppressed feeding test (NSFT). Rats were administered fluoxetine (5 mg/kg.d), T3 (50 mug/kg.d), fluoxetine (5 mg/kg.d)+T3 (50 mug/kg.d) or saline, for 21 d. Neurogenesis was studied by doublecortin (DCX) immunohistochemistry in the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ). In the NSFT, latency to feeding in animals deprived of food was measured. Fluoxetine and fluoxetine+T3 increased the number of doublecortin-positive (DCX+) cells in the SGZ compared to saline (p=0.00005, p=0.008, respectively). There was a trend towards an increased number of DCX+ cells by T3 compared to saline (p=0.06). Combined treatment with fluoxetine+T3 further increased the number of DCX+ cells compared to T3 or fluoxetine alone (p=0.001, p=0.014, respectively). There was no effect of any of the treatments on number of DCX+ cells in the SVZ. In the NSFT, all treatments (T3, fluoxetine+T3 and fluoxetine) reduced latency to feeding compared to saline (p=0.0004, p=0.00001, p=0.00009, respectively). Fluoxetine+T3 further reduced latency to feeding compared to T3 alone (p=0.05). The results suggest that enhancement of antidepressant action by T3 may be related to its effect of increasing hippocampal neurogenesis and that the antidepressant effect of these treatments is specific to the hippocampus and does not represent a general effect on cell proliferation.

Structural modeling of high-affinity thyroid receptor-ligand complexes

Understanding the molecular basis of the binding modes of natural and synthetic ligands to nuclear receptors is fundamental to our comprehension of the activation mechanism of this important class of hormone regulated transcription factors and to the development of new ligands. Thyroid hormone receptors (TR) are particularly important targets for pharmaceuticals development because TRs are associated with the regulation of metabolic rates, body weight, and circulating levels of cholesterol and triglycerides in humans. While several high-affinity ligands are known, structural information is only partially available. In this work we obtain structural models of several TR-ligand complexes with unknown structure by docking high affinity ligands to the receptors' ligand binding domain with subsequent relaxation by molecular dynamics simulations. The binding modes of these ligands are discussed providing novel insights into the development of TR ligands. The experimental binding free energies are reasonably well-reproduced from the proposed models using a simple linear interaction energy free-energy calculation scheme.

Molecular aspects of thyroid hormone actions

Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T(3) via transcriptional regulation. Two TR genes, alpha and beta, encode four T(3)-binding receptor isoforms (alpha1, beta1, beta2, and beta3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T(3), transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T(3) target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T(3)-dependent manner. In the absence of T(3), corepressors act to repress the basal transcriptional activity, whereas in the presence of T(3), coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRbeta gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T(3). Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism. Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin alphavbeta3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na(+)/H(+) exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T(4) is an agonist at the plasma membrane without conversion to T(3). Tetraiodothyroacetic acid is a T(4) analog that inhibits the actions of T(4) and T(3) at the integrin, including angiogenesis and tumor cell proliferation. T(3) can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alphavbeta3. Downstream consequences of phosphatidylinositol 3-kinase activation by T(3) include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase. Thyroid hormone, chiefly T(3) and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T(4) and rT(3), but not T(3), is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth. A truncated TRalpha1 isoform that resides in the extranuclear compartment mediates the action of thyroid hormone on the cytoskeleton.

Ontogenetic profile of the expression of thyroid hormone receptors in rat and human corpora cavernosa of the penis

Introduction

In the last few years, various studies have underlined a correlation between thyroid function and male sexual function, hypothesizing a direct action of thyroid hormones on the penis.

Aim

To study the spatiotemporal distribution of mRNA for the thyroid hormone nuclear receptors (TR) α1, α2 and β in the penis and smooth muscle cells (SMCs) of the corpora cavernosa of rats and humans during development.

Methods

We used several molecular biology techniques to study the TR expression in whole tissues or primary cultures from human and rodent penile tissues of different ages.

Main Outcome Measure

We measured our data by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) amplification, Northern blot and immunohistochemistry.

Results

We found that TRα1 and TRα2 are both expressed in the penis and in SMCs during ontogenesis without development-dependent changes. However, in the rodent model, TRβ shows an increase from 3 to 6 days post natum (dpn) to 20 dpn, remaining high in adulthood. The same expression profile was observed in humans. While the expression of TRβ is strictly regulated by development, TRα1 is the principal isoform present in corpora cavernosa, suggesting its importance in SMC function. These results have been confirmed by immunohistochemistry localization in SMCs and endothelial cells of the corpora cavernosa.

Conclusions

The presence of TRs in the penis provides the biological basis for the direct action of thyroid hormones on this organ. Given this evidence, physicians would be advised to investigate sexual function in men with thyroid disorders. Carosa E, Di Sante S, Rossi S, Castri A, D'Adamo F, Gravina GL, Ronchi P, Kostrouch Z, Dolci S, Lenzi A, and Jannini EA. Ontogenetic profile of the expression of thyroid hormone receptors in rat and human corpora cavernosa of the penis. J Sex Med 2010;7:1381–1390.

Regulation of bone metabolism by nuclear receptors

Bone tissue protects and supports soft organs and maintains calcium homeostasis. Steroid sex hormones and fat-soluble vitamins play a pivotal role in regulation of bone homeostasis, turnover and remodeling. These molecules act as ligands of nuclear receptors, through which they control gene expression in bone cells, namely bone-forming osteoblasts, bone-resorptive osteoclasts and osteocytes. Significant advances in our understanding of nuclear receptor physiology have been achieved due to development of novel genetic manipulation approaches and generation of experimental animal models in which nuclear receptor genes were mutated in specific cell types. In this review, we summarized some aspects of recent progress in studies on molecular mechanisms of cell-specific action of nuclear hormone receptors in bone tissue.

Thyroid hormone and cerebellar development

Thyroid hormone (TH) plays a key role in mammalian brain development. The developing brain is sensitive to both TH deficiency and excess. Brain development in the absence of TH results in motor skill deficiencies and reduced intellectual development. These functional abnormalities can be attributed to maldevelopment of specific cell types and regions of the brain including the cerebellum. TH functions at the molecular level by regulating gene transcription. Therefore, understanding how TH regulates cerebellar development requires identification of TH-regulated gene targets and the cells expressing these genes. Additionally, the process of TH-dependent regulation of gene expression is tightly controlled by mechanisms including regulation of TH transport, TH metabolism, toxicologic inhibition of TH signaling, and control of the nuclear TH response apparatus. This review will describe the functional, cellular, and molecular effects of TH deficit in the developing cerebellum and emphasize the most recent findings regarding TH action in this important brain region.

The role of the visceral mesoderm in the development of the gastrointestinal tract

The gastrointestinal (GI) tract forms from the endoderm (which gives rise to the epithelium) and the mesoderm (which develops into the smooth muscle layer, the mesenchyme, and numerous other cell types). Much of what is known of GI development has been learned from studies of the endoderm and its derivatives, because of the importance of epithelial biology in understanding and treating human diseases. Although the necessity of epithelial-mesenchymal cross talk for GI development is uncontested, the role of the mesoderm remains comparatively less well understood. The transformation of the visceral mesoderm during development is remarkable; it differentiates from a very thin layer of cells into a complex tissue comprising smooth muscle cells, myofibroblasts, neurons, immune cells, endothelial cells, lymphatics, and extracellular matrix molecules, all contributing to the form and function of the digestive system. Understanding the molecular processes that govern the development of these cell types and elucidating their respective contribution to GI patterning could offer insight into the mechanisms that regulate cell fate decisions in the intestine, which has the unique property of rapid cell renewal for the maintenance of epithelial integrity. In reviewing evidence from both mammalian and nonmammalian models, we reveal the important role of the visceral mesoderm in the ontogeny of the GI tract.

The thyroid hormone receptor (TR) beta-selective agonist GC-1 inhibits proliferation but induces differentiation and TR beta mRNA expression in mouse and rat osteoblast-like cells

Previous studies showed anabolic effects of GC-1, a triiodothyronine (T3) analogue that is selective for both binding and activation functions of thyroid hormone receptor (TR) beta1 over TRalpha1, on bone tissue in vivo. The aim of this study was to investigate the responsiveness of rat (ROS17/2.8) and mouse (MC3T3-E1) osteoblast-like cells to GC-1. As expected, T3 inhibited cellular proliferation and stimulated mRNA expression of osteocalcin or alkaline phosphatase in both cell lineages. Whereas equimolar doses of T3 and GC-1 equally affected these parameters in ROS17/2.8 cells, the effects of GC-1 were more modest compared to those of T3 in MC3T3-E1 cells. Interestingly, we showed that there is higher expression of TRalpha1 than TRbeta1 mRNA in rat (approximately 20-90%) and mouse (approximately 90-98%) cell lineages and that this difference is even higher in mouse cells, which highlights the importance of TRalpha1 to bone physiology and may partially explain the modest effects of GC-1 in comparison with T3 in MC3T3-E1 cells. Nevertheless, we showed that TRbeta1 mRNA expression increases (approximately 2.8- to 4.3-fold) as osteoblastic cells undergo maturation, suggesting a key role of TRbeta1 in mediating T3 effects in the bone forming cells, especially in mature osteoblasts. It is noteworthy that T3 and GC-1 induced TRbeta1 mRNA expression to a similar extent in both cell lineages (approximately 2- to 4-fold), indicating that both ligands may modulate the responsiveness of osteoblasts to T3. Taken together, these data show that TRbeta selective T3 analogues have the potential to directly induce the differentiation and activity of osteoblasts.

Cardiac hypertrophy and thyroid hormone signaling

Thyroid hormone exerts a large number of influences on the cardiovascular system. Increased thyroid hormone action increases the force and speed of systolic contraction and the speed of diastolic relaxation and these are largely beneficial effects. Furthermore, thyroid hormone has marked electrophysiological effects increasing heart rate and the propensity for atrial fibrillation and these effects are largely mal-adaptive. In addition, thyroid hormone markedly increases cardiac angiogenesis and decreases vascular tone. These multiple thyroid hormone effects are largely mediated by the action of nuclear based thyroid hormone receptors (TR) the thyroid hormone receptor alpha and beta. TRα is the predominant isoform in the heart. Rapid nongenomic thyroid hormone effects also occur, which can be clearly demonstrated in ex-vivo experiments. Some of the most marked thyroid hormone effects in cardiac myocytes involve influences on calcium flux, with thyroid hormone promoting expression of the gene encoding the calcium pump of the sarcoplasmic reticulum (SERCa2). In contrast, in hypothyroid animals phospholamban levels, which inhibit the SERCa2 pump, are increased. In addition, marked effects are exerted on the calcium channel of the sarcoplasmic reticulum the ryanodine channel. Related to myofibrillar proteins, myosin heavy chain alpha is increased by T3 and MHC beta is decreased. Complex and interesting interactions occur between cardiac hypertrophy induced by excess thyroid hormone action and cardiac hypertrophy occurring with heart failure. The thyroid hormone mediated cardiac hypertrophy in its initial phases presents a physiological hypertrophy with increases in SERCa2 levels and decreased expression of MHC beta. In contrast, pressure overload induced heart failure leads to a “pathological” cardiac hypertrophy which is largely mediated by activation of the calcineurin system and the MAPkinases signaling system. Recent evidence indicates that heart failure can lead to a downregulation of the thyroid hormone signaling system in the heart. In the failing heart, decreases of thyroid hormone receptor levels occur. In addition, serum levels of T4 and T3 are decreased with heart failure in the frame of the non-thyroidal illness syndrome. The decrease in T3 serves as an indicator for a bad prognosis in the heart failure patient being linked to increased mortality. In animal models, it can be shown that in pressure overload-induced cardiac hypertrophy a decrease of thyroid hormone receptor levels occurs. Cardiac function can be improved by increasing expression of thyroid hormone receptors mediated by adeno-associated virus based gene transfer. The failing heart may develop a “hypothyroid” status contributing to diminished cardiac contractile function.

Erythroid defects in TRα−/− mice

Thyroid hormone and its cognate receptor (TR) have been implicated in the production of red blood cells. Here, we show mice deficient for TRα have compromised fetal and adult erythropoiesis. Erythroid progenitor numbers were significantly reduced in TRα−/− fetal livers, and transit through the final stages of maturation was impeded. In addition, immortalized TRα−/− erythroblasts displayed increased apoptosis and reduced capacity for proliferation and differentiation. Adult TRα−/− mice had lower hematocrit levels, elevated glucocorticoid levels, and an altered stress erythropoiesis response to hemolytic anemia. Most TRα−/− animals contained markedly altered progenitor numbers in their spleens. Strikingly, 20% of TRα−/− mice failed to elicit a stress erythropoiesis response and recovered very poorly from hemolytic anemia. We conclude that an underlying erythroid defect exists in TRα−/− mice, demon-strating the importance of TRα to the erythroid compartment.

Coactivator recruitment is enhanced by thyroid hormone receptor trimers

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors. TRs are generally thought to bind to their DNA target sites as homodimers or as TR/retinoid X receptor (RXR) heterodimers. However, we have shown that certain TR isoforms, such as TRbeta0, can bind as trimers to a subset of naturally occurring DNA elements. We report here that this trimeric mode of DNA recognition by TRbeta0 also results in an enhanced recruitment of coactivators in vitro and increased transcriptional activation in cells compared to TRbeta0 dimers. At least part of this enhanced coactivator recruitment reflects a selectively enhanced avidity of the TRbeta0 trimer for a specific LXXLL interaction motif within the p160 coactivators. TRbeta0 trimers also recruit certain coactivators at lower concentrations of T3 hormone and exhibit distinct coactivator stoichiometries than do TRbeta0 dimers. We conclude that trimer formation confers isoform-specific DNA recognition and transcriptional regulatory properties that are not observed for TR dimers.

Mechanisms of action of the congenital diaphragmatic hernia-inducing teratogen nitrofen

Congenital diaphragmatic hernia (CDH) is a developmental anomaly that results in significant mortality and morbidity. The underlying etiology is poorly understood. Insights will arise from an understanding of the mechanisms by which the teratogen nitrofen induces CDH in rodent models. In this study, we use in vitro cell assays in conjunction with whole animal rodent studies to test hypotheses regarding nitrofen's mechanism of action. The first component examined the interaction of nitrofen with various aspects of the retinoid signaling pathway including uptake proteins, binding proteins, receptors, conversion, and degradation enzymes. The second component examined the interactions of nitrofen and vitamins A, C, and E to test the hypothesis that nitrofen was functioning as an antioxidant to interfere with retinoid signaling. Third, we performed a series of experiments examining the interaction of nitrofen and thyroid signaling. Collectively, the data suggest that the primary aspect of retinoid signaling affected by nitrofen is via inhibition of the rate-limiting enzymes controlling retinoic acid synthesis. Retinoid signaling perturbations do not appear to involve oxidative effects of nitrofen. Any substantial roles of nitrofen-induced perturbations of thyroid hormone signaling or receptor function are not supported.

De-phosphorylation of TRalpha-1 by p44/42 MAPK inhibition enhances T(3)-mediated GLUT5 gene expression in the intestinal cell line Caco-2 cells

Thyroid hormone and p44/42 MAPK inactivation are important in intestinal differentiation. We demonstrated not only that treatment with p44/42 MAPK inhibitor U0126 in intestinal cell line Caco-2 cells reduced the phosphorylation of serine and threonine residues of TRalpha-1, but also that T(3) and U0126 synergistically induced GLUT5 gene expression. EMSA demonstrated that the binding activity of TRalpha-1-RXR heterodimer on GLUT5-TRE in nuclear proteins of Caco-2 cells was synergistically enhanced by co-incubation in vitro with T(3) and CIAP, which strongly de-phosphorylates proteins. ChIP and transfection assays revealed that co-treatment of T(3) and U0126 induces TRalpha-1-RXR binding to GLUT5-TRE on the human GLUT5 enhancer region, and recruitment of the transcriptional complex in cells. These results suggest that inactivation of p44/42 MAPK enhances T(3)-induced GLUT5 gene expression in Caco-2 cells through increasing TRalpha-1 transactivity and binding activity to the GLUT5-TRE, probably due to de-phosphorylation of TRalpha-1.

Triiodothyronine (T3) and fructose coordinately enhance expression of the GLUT5 gene in the small intestine of rats during weaning period

Jejunal GLUT5 is elevated with triiodothyronine (T(3)) during weaning of rats. A perfusion of fructose into the small intestine of T(3)-injected rats at 21 d induced expression of the GLUT5 gene, but one into that of vehicle-injected rats did not. These results suggest that T(3) and fructose coordinately enhance jejunal expression of the GLUT5 gene in rats during weaning period.

The rat thyroid hormone receptor (TR) Deltabeta3 displays cell-, TR isoform-, and thyroid hormone response element-specific actions

The THRB gene encodes the well-described thyroid hormone (T3) receptor (TR) isoforms TRbeta1 and TRbeta2 and two additional variants, TRbeta3 and TRDeltabeta3, of unknown physiological significance. TRbeta1, TRbeta2, and TRbeta3 are bona fide T3 receptors that bind DNA and T3 and regulate expression of T3-responsive target genes. TRDeltabeta3 retains T3 binding activity but lacks a DNA binding domain and does not activate target gene transcription. TRDeltabeta3 can be translated from a specific TRDeltabeta3 mRNA or is coexpressed with TRbeta3 from a single transcript that contains an internal TRDeltabeta3 translation start site. In these studies, we provide evidence that the TRbeta3/Deltabeta3 locus is present in rat but not in other vertebrates, including humans. We compared the activity of TRbeta3 with other TR isoforms and investigated mechanisms of action of TRDeltabeta3 at specific thyroid hormone response elements (TREs) in two cell types. TRbeta3 was the most potent isoform, but TR potency was TRE dependent. TRDeltabeta3 acted as a cell-specific and TRE-dependent modulator of TRbeta3 when coexpressed at low concentrations. At higher concentrations, TRDeltabeta3 was a TRE-selective and cell-specific antagonist of TRalpha1, -beta1, and -beta3. Both TRbeta3 and TRDeltabeta3 were expressed in the nucleus in the absence and presence of hormone, and their actions were determined by cell type and TRE structure, whereas TRDeltabeta3 actions were also dependent on the TR isoform with which it interacted. Analysis of these complex responses implicates a range of nuclear corepressors and coactivators as cell-, TR isoform-, and TRE-specific modulators of T3 action.

Essential role of GATA2 in the negative regulation of thyrotropin beta gene by thyroid hormone and its receptors

Previously we reported that the negative regulation of the TSHbeta gene by T(3) and its receptor [thyroid hormone receptor (TR)] is observed in CV1 cells when GATA2 and Pit1 are introduced. Using this system, we further studied the mechanism of TSHbeta inhibition. The negative regulatory element (NRE), which had been reported to mediate T(3)-bound TR (T(3)-TR)-dependent inhibition, is dispensable, because deletion or mutation of NRE did not impair suppression. The reporter construct, TSHbeta-D4-chloramphenicol acetyltransferase, which possesses only the binding sites for Pit1 and GATA2, was activated by GATA2 alone, and this transactivation was specifically inhibited by T(3)-TR. The Zn finger region of GATA2 interacts with the DNA-binding domain of TR in a T(3)-independent manner. The suppression by T(3)-TR was impaired by overexpression of a dominant-negative type TR-associated protein (TRAP) 220, an N- and C-terminal deletion construct, indicating the participation of TRAP220. Chromatin immunoprecipitation assays with a thyrotroph cell line, TalphaT1, revealed that T(3) treatment recruited histone deacetylase 3, reduced the acetylation of histone H4, and caused the dissociation of TRAP220 within 15-30 min. The reduction of histone H4 acetylation was transient, whereas the dissociation of TRAP220 persisted for a longer period. In the negative regulation of the TSHbeta gene by T(3)-TR we report that 1) GATA2 is the major transcriptional activator of the TSHbeta gene, 2) the putative NRE previously reported is not required, 3) TR-DNA-binding domain directly interacts with the Zn finger region of GATA2, and 4) histone deacetylation and TRAP220 dissociation are important.

The critical period for thyroid hormone responsiveness through thyroid hormone receptor isoform alpha in the postnatal small intestine

During second and third weeks after birth in rats, serum thyroid hormone level is elevated. In this study, we investigated the jejunal expression of thyroid hormone receptor (TR) alpha in developing rats. The TRalpha-1 mRNA level and TRalpha-1/TRalpha-2 mRNA ratio increased two-fold from 5 to 13 days after birth. This high level of TRalpha-1 mRNA was maintained until 20 days and then decreased to the basal level by the end of weaning period at 27 days; however, the level of TRalpha-2 mRNA remained unchanged throughout the developmental period. The increase in the TRalpha-1/TRalpha-2 mRNA ratio from 5 to 13 days was accompanied by an initial rise in the levels of mRNA for hexose transporters in the jejunum. Administration of T(3) during the suckling period (8-13 days) caused a 50% increase in the TRalpha-1/TRalpha-2 mRNA ratio, while administration of T(3) on days 12-17 and days 16-21, but not on days 22-27, caused a two to four-fold increase in the levels of mRNA for hexose transporters. These results suggest that a transient variation in the TRalpha-1/TRalpha-2 expression ratio is closely related to the critical period of thyroid hormone responsiveness for hexose transporters expression in the developing rat jejunum.

Thyroid hormone and the growth plate

Thyroid hormone was first identified as a potent regulator of skeletal maturation at the growth plate more than forty years ago. Since that time, many in vitro and in vivo studies have confirmed that thyroid hormone regulates the critical transition between cell proliferation and terminal differentiation in the growth plate, specifically the maturation of growth plate chondrocytes into hypertrophic cells. However these studies have neither identified the molecular mechanisms involved in the regulation of skeletal maturation by thyroid hormone, nor demonstrated how the systemic actions of thyroid hormone interface with the local regulatory milieu of the growth plate. This article will review our current understanding of the role of thyroid hormone in regulating the process of endochondral ossification at the growth plate, as well as what is currently known about the molecular mechanisms involved in this regulation.

Behavioral inhibition and impaired spatial learning and memory in hypothyroid mice lacking thyroid hormone receptor alpha

Thyroid hormone insufficiency leads to impaired neurogenesis, behavioral alterations and cognitive deficits. Thyroid hormone receptors, expressed in brain regions involved in these behaviors, mediate the effects of thyroid hormone deficiency or excess. To determine the contribution of thyroid hormone receptor alpha (TRalpha) in these behaviors, we examined the behavior of euthyroid as well as hypo- and hyperthyroid mice lacking all isoforms of the TRalpha (TRalpha(o/o)). The hypothyroxinemic TRalpha(o/o) mice demonstrated behavioral inhibition, manifested in decreased activity and increased anxiety/fear in the open field test (OFT) and increased immobility in the forced swim test (FST) compared to C57BL/6J mice. TRalpha(o/o) mice also showed learning and recall impairments in the Morris water maze (MWM), which were exaggerated by hypothyroidism in TRalpha(o/o) mice. These impairments were concurrent with increased thigmotaxis, suggesting an increased anxiety-like state of the TRalpha(o/o) mice in the MWM. Expression of genes, known to be involved in processes modulating learning and memory, such as glucocorticoid receptor (GR), growth-associated protein 43 (GAP-43) and neurogranin (RC3), were significantly decreased in the hippocampus of TRalpha(o/o) mice. GR expression was also decreased in the frontal cortex and amygdala of TRalpha(o/o) mice, indicating that expression of GR is regulated, probably developmentally, by one or more isoforms of TRalpha in the mouse brain. Taken together these data demonstrate behavioral alterations in the TRalpha(o/o) mice, indicating the functional role of TRalpha, and a delicate interaction between TRalpha and TRbeta-regulated genes in these behaviors. Thyroid hormone-regulated genes potentially responsible for the learning deficit found in TRalpha(o/o) mice include GR, RC3 and GAP-43.

Isolation of the alligator (Alligator mississippiensis) thyroid hormone receptor alpha and beta transcripts and their responsiveness to thyroid stimulating hormone

Thyroid hormones (THs) play key regulatory roles in growth, development and metabolism in vertebrates. Modulation of the cellular hormonal response is largely through the activity of two nuclear TH receptors, TRalpha and TRbeta, which act as transcription factors and alter gene expression programs. Little information is available regarding their structure and regulation in reptiles. We have cloned the expressed sequences encoding these two receptors in the American alligator, Alligator mississippiensis. The encoded putative proteins share a high degree of amino acid sequence conservation with other vertebrates, however, both alligator TRs contain putative N-terminal truncations. This phenomenon is shared with the chicken for TRbeta, but not for TRalpha, making this the first demonstration of this type of TRalpha isoform. We measured the steady-state levels of TR transcripts in heart, lung, liver, thyroid, cliterophallus/phallus, and gonad of juvenile alligators 24 and 48 h after injection with thyroid stimulating hormone (TSH). TRalpha transcript levels were increased in the heart, decreased in the lung and cliterophallus/phallus, and unaffected in the liver, thyroid, and gonad. TRbeta transcript levels were increased in the heart, lung, and gonad whereas estrogen receptor alpha transcript levels were elevated by TSH treatment only in the gonad. Modulation of these transcripts in the gonad is consistent with TH playing an important role in this tissue's function since seasonal TH fluctuations coincide with reproductive events. These data demonstrate that alligator tissues are differentially responsive to TSH by regulation of TR expression and provide an important comparative framework among vertebrates.

Evaluation of the effect of acetochlor on thyroid hormone receptor gene expression in the brain and behavior of Rana catesbeiana tadpoles

The thyroid hormones (THs) including 3,5,3'-triiodothyronine (T3), are important regulators of growth and development of the brain in vertebrates. Previous studies showed that acetochlor, a widely used herbicide, accelerates T3 -induced frog tadpole metamorphosis and elevates the T3 -dependent accumulation of the mRNAs encoding the TH receptors, TRalpha and beta, in the tail. Here we show that acetochlor affects the expression of these TR isoforms in the brain of Rana catesbeiana tadpoles. Premetamorphic tadpoles exposed to 10 nM acetochlor with and without 100 nM T(3) for 4 days showed substantial increases in TRalpha and TRbeta transcript levels and significant decreases in the TRalpha/TRbeta ratios in their brains. This change in TR ratios is recapitulated with 10 nM acetochlor in R. catesbeiana tadpole brains during prometamorphosis, a period in which THs are endogenously produced. Tail fin biopsies revealed an elevation in TRalpha and beta mRNA levels compared to control animals when exposed to 1 and 10 nM acetochlor for 6 days. When subsequently reared in clean water for 59 days, no alterations in metamorphic hallmarks (forelimb emergence, mouth development, tail regression) were noted compared to the controls. Since alterations in TR ratios/levels may impact brain development, we tested the escape behavior in premetamorphic tadpoles exposed to 10 nM acetochlor for 4 days. We did not detect any statistically significant differences that would indicate that acetochlor affects escape behavior. However, since the gene expression data suggest that brain function may be affected, additional studies examining different behaviors upon acetochlor exposure at environmentally-relevant concentrations are warranted.

Pedomorphosis revisited: thyroid hormone receptors are functional in Necturus maculosus

Heterochrony, a difference in developmental timing, is a central concept in modern evolutionary biology. An example is pedomorphosis, retention of juvenile characteristics in sexually mature adults, a phenomenon largely represented in salamanders. The mudpuppy (Necturus maculosus) is an obligate pedomorphic amphibian, never undergoing metamorphosis. Thyroid hormone induces tissue transformation in metamorphosing species and this action is mediated by nuclear thyroid hormone (TH) receptors (TRs). The absence of metamorphosis in Necturus has been attributed to a resistance to TH action as treatment with exogenous TH fails to induce transformation. The failure to metamorphose could be due to the lack of TR expression in target tissues, or to a loss of TR function. Toward understanding the molecular basis for the failure of Necturus tissues to respond to TH, and the ultimate cause for the expression of the obligate pedomorphic life history, we characterized the structure, function, and expression of TR genes in Necturus. Strikingly, we found that Necturus TRalpha and TRbeta genes encode fully functional TR proteins. These TRs bind both DNA and TH and can transactivate target genes in response to TH. Both TRalpha and TRbeta are expressed in various tissues. TH treatment in vivo induced expression in the gill of some but not all genes known to be activated by TH in anuran larvae, caused whole organism metabolic effects, but induced no external morphological changes in adults or larvae. Thus, Necturus possesses fully functional TRs and its tissues are not generally resistant to the actions of TH. Rather, the absence of metamorphosis may be due to the loss of TH-dependent control of key genes required for tissue transformation.

Congenital hypothyroidism: From paracelsus to molecular diagnosis

Endemic cretinism was noted in alpine Europe as early as the 13th century. However, it was only in 1848 that a commission, sponsored by the King of Sardinia, first formally demonstrated its link to goiter. An important landmark was the publication of a report in 1871 describing several cases of nongoitrous hypothyroidism that were clearly distinguished from the endemic form of the disease, for which the author suggested the designation of "sporadic cretinism." Classification of the hypothyroid status was for a long time solely based on clinical observation. In the second half of the 20th century, the use of radionuclides (iodine radioisotope and technetium pertechnetate) allowed a more precise diagnosis and taxonomy into thyroid dysgenesis and dyshormonogenesis. This brief review summarizes the progress that has been achieved during the last 40 years in diagnosing the multiple variants of congenital hypothyroidism (CH). It becomes evident that while accurate diagnosis for CH is readily available, its exact etiology requires a precise molecular investigation as different genes are implicated in the differentiation, migration and growth of the thyroid gland.

Dominant Role of Thyrotropin-releasing Hormone in the Hypothalamic-Pituitary-Thyroid Axis

Hypothalamic thyrotropin-releasing hormone (TRH) stimulates thyroid-stimulating hormone (TSH) secretion from the anterior pituitary. TSH then initiates thyroid hormone (TH) synthesis and release from the thyroid gland. Although opposing TRH and TH inputs regulate the hypothalamic-pituitary-thyroid axis, TH negative feedback is thought to be the primary regulator. This hypothesis, however, has yet to be proven in vivo. To elucidate the relative importance of TRH and TH in regulating the hypothalamic-pituitary-thyroid axis, we have generated mice that lack either TRH, the beta isoforms of TH receptors (TRbeta KO), or both (double KO). TRbeta knock-out (KO) mice have significantly higher TH and TSH levels compared with wild-type mice, in contrast to double KO mice, which have reduced TH and TSH levels. Unexpectedly, hypothyroid double KO mice also failed to mount a significant rise in serum TSH levels, and pituitary TSH immunostaining was markedly reduced compared with all other hypothyroid mouse genotypes. This impaired TSH response, however, was not due to a reduced number of pituitary thyrotrophs because thyrotroph cell number, as assessed by counting TSH immunopositive cells, was restored after chronic TRH treatment. Thus, TRH is absolutely required for both TSH and TH synthesis but is not necessary for thyrotroph cell development.

Retinoic acid rescues lung hypoplasia in nitrofen-induced hypoplastic foetal rat lung explants

There is increasing evidence to suggest that the retinoid pathway is involved in the pathogenesis of congenital diaphragmatic hernia (CDH). We hypothesised that retinoids are involved in the pathogenesis of associated pulmonary hypoplasia in CDH and therefore designed this study to investigate the effects of retinoid acid on nitrofen-induced hypoplastic lungs. Pregnant rats were exposed to either olive oil or 100 mg nitrofen on day 9.5 of gestation. Foetal lungs were harvested on embryonic day 13.5 and were cultured for 96 h with or without exogenous retinoic acid (RA) (1 muM) added daily to the culture medium. Lungs were divided into four study groups: control (n=31); control + RA (n=19); nitrofen (n=19); and nitrofen + RA (n=12). Lung growth was assessed in each group by measuring branching morphogenesis, total DNA content and the proportion of proliferating cells stained by immunohistochemistry. One-way ANOVA test was used for statistical analysis. Retinoic acid significantly increased the growth of nitrofen-induced hypoplastic lungs, whilst growth of control lungs did not change. The number of lung buds and lung area of nitrofen-exposed hypoplastic lungs after 96 h of culture significantly increased after the addition of RA compared to the non-treated hypoplastic lungs (25.75+/-6.47 vs 15.11+/-3.29 and 0.98+/-0.18 mm(2) vs 0.65+/-0.13 mm(2), respectively; P<0.0001). Lung perimeter was also higher when RA was added to hypoplastic lungs compared to the non-treated ones, although it did not reach significance (12.51+/-2.53 mm vs 11.19+/-2.56 mm; P=0.17). Conversely, the addition of RA to control lungs did not affect the number of lung buds, lung area or lung perimeter after 96 h in culture compared to the non-treated ones (31.28+/-4.66 vs 31.81+/-6.67; 1.29+/-0.18(2) vs 1.29+/-0.23 mm(2) and 18.47+/-3.47 mm vs 17.89+/-2.94 mm, respectively; P=NS). Retinoic acid also increased the total DNA content and the proportion of proliferating cells in hypoplastic lungs compared to the non-treated ones (2.59+/-0.58 mug vs 1.96+/-0.31 mug and 57.89+/-9.46% vs 36.76+/-8.15%, respectively; P<0.001). The addition of RA did not affect either total DNA content or the proportion of proliferating cells in control lungs compared to the non-treated ones (4.04+/-0.64 mug vs 3.79+/-0.85 mug and 58.67+/-11.23% vs 56.03+/-10.36%, respectively; P=NS). This study demonstrates for the first time that RA rescues lung hypoplasia in nitrofen-induced hypoplastic lungs. These results suggest that retinoid pathway may be involved in the pathogenesis of associated pulmonary hypoplasia in CDH.

Congenital diaphragmatic hernia: a retinoid-signaling pathway disruption during lung development?

Congenital diaphragmatic hernia (CDH) usually occurs sporadically. The prognosis remains poor, with a 50% perinatal mortality rate. Most deaths result from hypoxemia due to lung hypoplasia and abnormal development of pulmonary vasculature that results in persistent pulmonary hypertension. Our current understanding of the pathogenesis of CDH is based on an assumption linking herniation of abdominal viscera into the thorax with compression of the developing lung. Pulmonary hypoplasia, however, can also result from reduced distension of the developing lung secondary to impaired fetal breathing movements. Moreover, a nitrofen-induced CDH model shows that lung hypoplasia precedes the diaphragmatic defect, leading to a "dual-hit hypothesis." Recent data reveal the role of a retinoid-signaling pathway disruption in the pathogenesis of CDH. We describe the clinical and epidemiological aspects of human CDH, the metabolic and molecular aspects of the retinoid-signaling pathway, and the implications of retinoids in the development of the diaphragm and the lung. Finally, we highlight the existing links between CDH and disruption of the retinoid-signaling pathway, which may suggest an eventual use of retinoids in the treatment of CDH.

Thyroid hormone action in the heart

The heart is a major target organ for thyroid hormone action, and marked changes occur in cardiac function in patients with hypo- or hyperthyroidism. T(3)-induced changes in cardiac function can result from direct or indirect T(3) effects. Direct effects result from T(3) action in the heart itself and are mediated by nuclear or extranuclear mechanisms. Extranuclear T(3) effects, which occur independent of nuclear T(3) receptor binding and increases in protein synthesis, influence primarily the transport of amino acids, sugars, and calcium across the cell membrane. Nuclear T(3) effects are mediated by the binding of T(3) to specific nuclear receptor proteins, which results in increased transcription of T(3)-responsive cardiac genes. The T(3) receptor is a member of the ligand-activated transcription factor family and is encoded by cellular erythroblastosis A (c-erb A) genes. T(3) also leads to an increase in the speed of diastolic relaxation, which is caused by the more efficient pumping of the calcium ATPase of the sarcoplasmic reticulum. This T(3) effect results from T(3)-induced increases in the level of the mRNA coding for the sarcoplasmic reticulum calcium ATPase protein, leading to an increased number of calcium ATPase pump units in the sarcoplasmic reticulum.