Retinoic X receptor subtypes exert differential effects on the regulation of Trh transcription

Modulating Thyroid Hormone Levels in Adult Mice: Impact on Behavior and Compensatory Brain Changes

Thyroid hormone (TH) perturbation is a common medical problem. Because of substantial public health impact, prior researchers have studied hyper- and hypothyroidism in animal models. Although most prior research focused on in utero and/or developmental effects, changes in circulating TH levels are commonly seen in elderly individuals: approximately 20% of persons older than 80 years have clinically impactful hypothyroidism and up to 5% have clinical hyperthyroidism, with women being more often affected than men. TH disease model methodology in mice have varied but usually focus on a single sex, and the impact(s) of TH perturbation on the adult brain are not well understood. We administered thyroxine to middle-aged (13 to 14 months) male and female mice to model hyperthyroidism and TH-lowering drugs propylthiouracil (PTU) and methimazole, to induce hypothyroidism. These pharmacological agents are used commonly in adult humans. Circulating TH-level changes were observed when thyroxine was dosed at 20 µg/mL in drinking water for two weeks. By contrast, PTU and methimazole did not elicit a consistent reproducible effect until two months of treatment. No substantial changes in TH levels were detected in brain tissues of treated animals; however, pronounced changes in gene expression, specifically for TH-processing transcripts, were observed following the treatment with thyroxine. Our study indicated a robust compensatory mechanism by which the brain tissue/cells minimize the TH fluctuation in CNS by altering gene expression. Neurobehavioral changes were related to the TH perturbation and suggested potential associations between cognitive status and hyper- and hypothyroidism.

G ATA2 mediates the negative regulation of the prepro-thyrotropin-releasing hormone gene by liganded T3 receptor β2 in the rat hypothalamic paraventricular nucleus

Thyroid hormone (T3) inhibits thyrotropin-releasing hormone (TRH) synthesis in the hypothalamic paraventricular nucleus (PVN). Although the T3 receptor (TR) β2 is known to mediate the negative regulation of the prepro-TRH gene, its molecular mechanism remains unknown. Our previous studies on the T3-dependent negative regulation of the thyrotropin β subunit (TSHβ) gene suggest that there is a tethering mechanism, whereby liganded TRβ2 interferes with the function of the transcription factor, GATA2, a critical activator of the TSHβ gene. Interestingly, the transcription factors Sim1 and Arnt2, the determinants of PVN differentiation in the hypothalamus, are reported to induce expression of TRβ2 and GATA2 in cultured neuronal cells. Here, we confirmed the expression of the GATA2 protein in the TRH neuron of the rat PVN using immunohistochemistry with an anti-GATA2 antibody. According to an experimental study from transgenic mice, a region of the rat prepro-TRH promoter from nt. -547 to nt. +84 was able to mediate its expression in the PVN. We constructed a chloramphenicol acetyltransferase (CAT) reporter gene containing this promoter sequence (rTRH(547)-CAT) and showed that GATA2 activated the promoter in monkey kidney-derived CV1 cells. Deletion and mutation analyses identified a functional GATA-responsive element (GATA-RE) between nt. -357 and nt. -352. When TRβ2 was co-expressed, T3 reduced GATA2-dependent promoter activity to approximately 30%. Unexpectedly, T3-dependent negative regulation was maintained after mutation of the reported negative T3-responsive element, site 4. T3 also inhibited the GATA2-dependent transcription enhanced by cAMP agonist, 8-bromo-cAMP. A rat thyroid medullary carcinoma cell line, CA77, is known to express the preproTRH mRNA. Using a chromatin immunoprecipitation assay with this cell line where GATA2 expression plasmid was transfected, we observed the recognition of the GATA-RE by GATA2. We also confirmed GATA2 binding using gel shift assay with the probe for the GATA-RE. In CA77 cells, the activity of rTRH(547)-CAT was potentiated by overexpression of GATA2, and it was inhibited in a T3-dependent manner. These results suggest that GATA2 transactivates the rat prepro-TRH gene and that liganded TRβ2 interferes with this activation via a tethering mechanism as in the case of the TSHβ gene.

Nuclear RXRα and RXRβ receptors exert distinct and opposite effects on RA-mediated neuroblastoma differentiation

Retinoic acid (RA) promotes differentiation in multiple neurogenic cell types by promoting gene reprogramming through retinoid receptors and also by inducing cytosolic signaling events. The nuclear RXR receptors are one of the main mediators of RA cellular effects, classically by joining the direct receptors of RA, the nuclear RAR receptors, in RAR/RXR dimers which act as transcription factors. Distinct RXR genes lead to RXRα, RXRβ and RXRγ subtypes, but their specific roles in neuronal differentiation remain unclear. We firstly investigated both RXRs and RARs expression profiles during RA-mediated neuronal differentiation of human neuroblastoma cell line SH-SY5Y, and found varying levels of retinoid receptors transcript and protein contents along the process. In order to understand the roles of the expression of distinct RXR subtypes to RA signal transduction, we performed siRNA-mediated silencing of RXRα and RXRβ during the first stages of SH-SY5Y differentiation. Our results showed that RXRα is required for RA-induced neuronal differentiation of SH-SY5Y cells, since its silencing compromised cell cycle arrest and prevented the upregulation of neuronal markers and the adoption of neuronal morphology. Besides, silencing of RXRα affected the phosphorylation of ERK1/2. By contrast, silencing of RXRβ improved neurite extension and led to increased expression of tau and synaptophysin, suggesting that RXRβ may negatively regulate neuronal parameters related to neurite outgrowth and function. Our results indicate distinct functions for RXR subtypes during RA-dependent neuronal differentiation and reveal new perspectives for studying such receptors as clinical targets in therapies aiming at restoring neuronal function.

Heme oxygenase-1 protects airway epithelium against apoptosis by targeting the proinflammatory NLRP3-RXR axis in asthma

Asthma is thought to be caused by malfunction of type 2 T helper cell (Th2)-mediated immunity, causing excessive inflammation, mucus overproduction, and apoptosis of airway epithelial cells. Heme oxygenase-1 (HO-1) functions in heme catabolism and is both cytoprotective and anti-inflammatory. We hypothesized that this dual function may be related to asthma's etiology. Using primary airway epithelial cells (pAECs) and an asthma mouse model, we demonstrate that severe lung inflammation is associated with rapid pAEC apoptosis. Surprisingly, NOD-like receptor protein 3 (NLRP3) inhibition, retinoid X receptor (RXR) deficiency, and HO-1 induction were associated with abrogated apoptosis. MCC950, a selective small-molecule inhibitor of canonical and noncanonical NLRP3 activation, reduced RXR expression, leading to decreased pAEC apoptosis that was reversed by the RXR agonist adapalene. Of note, HO-1 induction in a mouse model of ovalbumin-induced eosinophilic asthma suppressed Th2 responses and reduced apoptosis of pulmonary pAECs. In vitro, HO-1 induction desensitized cultured pAECs to ovalbumin-induced apoptosis, confirming the in vivo observations. Critically, the HO-1 products carbon monoxide and bilirubin suppressed the NLRP3-RXR axis in pAECs. Furthermore, HO-1 impaired production of NLRP3-RXR-induced cytokines (interleukin [IL]-25, IL-33, thymic stromal lymphopoietin, and granulocyte-macrophage colony-stimulating factor) in pAECs and lungs. Finally, we demonstrate that HO-1 binds to the NACHT domain of NLRP3 and the RXRα and RXRβ subunits and that this binding is not reversed by Sn-protoporphyrin. Our findings indicate that HO-1 and its products are essential for pAEC survival to maintain airway epithelium homeostasis during NLRP3-RXR-mediated apoptosis and inflammation.

Integrating Thyroid Hormone Signaling in Hypothalamic Control of Metabolism: Crosstalk Between Nuclear Receptors

The obesity epidemic is well recognized as a significant global health issue. A better understanding of the energy homeostasis mechanisms could help to identify promising anti-obesity therapeutic strategies. It is well established that the hypothalamus plays a pivotal role governing energy balance. The hypothalamus consists of tightly interconnected and specialized neurons that permit the sensing and integration of several peripheral inputs, including metabolic and hormonal signals for an appropriate physiological response. Current evidence shows that thyroid hormones (THs) constitute one of the key endocrine factors governing the regulation and the integration of metabolic homeostasis at the hypothalamic level. THs modulate numerous genes involved in the central control of metabolism, as TRH (Thyrotropin-Releasing Hormone) and MC4R (Melanocortin 4 Receptor). THs act through their interaction with thyroid hormone receptors (TRs). Interestingly, TH signaling, especially regarding metabolic regulations, involves TRs crosstalk with other metabolically linked nuclear receptors (NRs) including PPAR (Peroxisome proliferator-activated receptor) and LXR (Liver X receptor). In this review, we will summarize current knowledge on the important role of THs integration of metabolic pathways in the central regulation of metabolism. Particularly, we will shed light on the crosstalk between TRs and other NRs in controlling energy homeostasis. This could be an important track for the development of attractive therapeutic compounds.

Advances in TRH signaling

The activity of the hypothalamus-pituitary-thyroid axis (HPT) is coordinated by hypophysiotropic thyrotropin releasing hormone (TRH) neurons present in the paraventricular nucleus of the hypothalamus. Hypophysiotropic TRH neurons act as energy sensors. TRH controls the synthesis and release of thyrotropin, which activates the synthesis and secretion of thyroid hormones; in target tissues, transporters and deiodinases control their local availability. Thyroid hormones regulate many functions, including energy homeostasis. This review discusses recent evidence that covers several aspects of TRH role in HPT axis regulation. Knowledge about the mechanisms of TRH signaling has steadily increased. New transcription factors engaged in TRH gene expression have been identified, and advances made on how they interact with signaling pathways and define the dynamics of TRH neurons response to acute and/or long-term influences. Albeit yet incomplete, the relationship of TRH neurons activity with positive energy balance has emerged. The importance of tanycytes as a central relay for the feedback control of the axis, as well as for HPT responses to alterations in energy balance, and other stimuli has been reinforced. Finally, some studies have started to shed light on the interference of prenatal and postnatal stress and nutrition on HPT axis programing, which have confirmed the axis susceptibility to early insults.

Regulation of TRH neurons and energy homeostasis-related signals under stress

Energy homeostasis relies on a concerted response of the nervous and endocrine systems to signals evoked by intake, storage, and expenditure of fuels. Glucocorticoids (GCs) and thyroid hormones are involved in meeting immediate energy demands, thus placing the hypothalamo-pituitary-thyroid (HPT) and hypothalamo-pituitary-adrenal axes at a central interface. This review describes the mode of regulation of hypophysiotropic TRHergic neurons and the evidence supporting the concept that they act as metabolic integrators. Emphasis has been be placed on i) the effects of GCs on the modulation of transcription of Trh in vivo and in vitro, ii) the physiological and molecular mechanisms by which acute or chronic situations of stress and energy demands affect the activity of TRHergic neurons and the HPT axis, and iii) the less explored role of non-hypophysiotropic hypothalamic TRH neurons. The partial evidence gathered so far is indicative of a contrasting involvement of distinct TRH cell types, manifested through variability in cellular phenotype and physiology, including rapid responses to energy demands for thermogenesis or physical activity and nutritional status that may be modified according to stress history.

Liver X receptor regulation of thyrotropin-releasing hormone transcription in mouse hypothalamus is dependent on thyroid status

Reversing the escalating rate of obesity requires increased knowledge of the molecular mechanisms controlling energy balance. Liver X receptors (LXRs) and thyroid hormone receptors (TRs) are key physiological regulators of energetic metabolism. Analysing interactions between these receptors in the periphery has led to a better understanding of the mechanisms involved in metabolic diseases. However, no data is available on such interactions in the brain. We tested the hypothesis that hypothalamic LXR/TR interactions could co-regulate signalling pathways involved in the central regulation of metabolism. Using in vivo gene transfer we show that LXR activation by its synthetic agonist GW3965 represses the transcriptional activity of two key metabolic genes, Thyrotropin-releasing hormone (Trh) and Melanocortin receptor type 4 (Mc4r) in the hypothalamus of euthyroid mice. Interestingly, this repression did not occur in hypothyroid mice but was restored in the case of Trh by thyroid hormone (TH) treatment, highlighting the role of the triiodothyronine (T₃) and TRs in this dialogue. Using shLXR to knock-down LXRs in vivo in euthyroid newborn mice, not only abrogated Trh repression but actually increased Trh transcription, revealing a potential inhibitory effect of LXR on the Hypothalamic-Pituitary-Thyroid axis. In vivo chromatin immunoprecipitation (ChIP) revealed LXR to be present on the Trh promoter region in the presence of T₃ and that Retinoid X Receptor (RXR), a heterodimerization partner for both TR and LXR, was never recruited simultaneously with LXR. Interactions between the TR and LXR pathways were confirmed by qPCR experiments. T₃ treatment of newborn mice induced hypothalamic expression of certain key LXR target genes implicated in metabolism and inflammation. Taken together the results indicate that the crosstalk between LXR and TR signalling in the hypothalamus centres on metabolic and inflammatory pathways.