[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Mammalian thyroid hormone receptors (TRs) have multiple isoforms, including the bona fide receptors that bind T3 (TRα1, TRβ1 and TRβ2) and a non-hormone-binding variant, TRα2. Intriguingly, TRα2 is strongly expressed in the brain, where its mRNA levels exceed those of functional TRs. Ablation of TRα2 in mice results in over-expression of TRα1, and a complex phenotype with low levels of free T3 and T4, without elevated TSH levels, suggesting an alteration in the negative feedback at the hypothalamic-pituitary level. As the hypothesis of a potential TRH response defect has never been tested, we explored the functional role of TRα2 in negative feedback on transcription of hypothalamic thyrotropin, Trh. The in vivo transcriptional effects of TRα2 on hypothalamic Trh were analysed using an in vivo reporter gene approach. Effects on Trh-luc expression were examined to that of two, T3 positively regulated genes used as controls. Applying in vivo gene transfer showed that TRα2 over-expression in the mouse hypothαlamus abrogates T3-dependent repression of Trh and T3 activation of positively regulated promoters, blocking their physiological regulation. Surprisingly, loss of function studies carried out by introducing a shTRα2 construct in the hypothalamus also blocked physiological T3 dependent regulation. Thus, modulating hypothalamic TRα2 expression by either gain or loss of function abrogated T3 dependent regulation of Trh transcription, producing constant transcriptional levels insensitive to feedback. This loss of physiological regulation was reflected at the level of the endogenous Trh gene, were gain or loss of function held mRNA levels constant. These results reveal the as yet undescribed dominant negative role of TRα2 over TRα1 effect on hypothalamic Trh transcription.
[Show abstract][Hide abstract] ABSTRACT: How Retinoid X receptors (RXR) and thyroid hormone receptors (TR) interact on negative TREs and whether RXR subtype specificity is determinant in such regulations is unknown. In a set of functional studies, we analyzed RXR subtype effects in T3-dependent repression of hypothalamic thyrotropin-releasing hormone (Trh). Two-hybrid screening of a hypothalamic paraventricular nucleus cDNA bank revealed specific, T3-dependent interaction of TRs with RXRβ. In vivo chromatin immuno-precipitation showed recruitment of RXRs to the TRE-site 4 region of the Trh promoter in the absence of T3. In vivo overexpression of RXRα in the mouse hypothalamus heightened T3-independent Trh transcription, whereas RXRβ overexpression abrogated this activity. Loss of function of RXRα and β by shRNAs induced inverse regulations. Thus, RXRα and RXRβ display specific roles in modulating T3-dependent regulation of Trh. These results provide insight into the actions of these different TR heterodimerization partners within the context of a negatively regulated gene.
[Show abstract][Hide abstract] ABSTRACT: Objectives:
Although a relationship between obesity and metabolic consequences with thyroid function has been reported, the underlying pathogenesis is not completely known. In the current study, we evaluated the thyroid function in obese and/or diabetic patients compared to healthy normal weight peers, exploring the possible association between components of metabolic syndrome and thyroid function parameters.
We recruited 108 subjects (56 male and 52 female). In all subjects, thyroid stimulating hormone (TSH), free thyroxine (FT4), fasting plasma levels of insulin and glucose, homeostasis model assessment for insulin resistance, and obesity parameters were assessed.
We found that circulating levels of TSH and FT4 were significantly increased in overweight and obese subjects. However, the data do not reveal any change of these hormones in diabetics. Multivariate linear regression analysis showed that TSH was directly associated with both obesity and insulin resistance parameters (p < 0.05). FT4 was negatively associated only with obesity parameters (p < 0.05).
Our data strongly support that the changes of thyroid hormones may be influenced by adiposity and its metabolic consequences, such as insulin resistance. This relationship can be explained by a cross talk between adipose tissue release and thyroid function. Nevertheless, metformin treatment seems to affect thyroid function in diabetic patients by maintaining plasma thyrotropin levels to subnormal levels.
No preview · Article · Jun 2012 · Endocrine Research
[Show abstract][Hide abstract] ABSTRACT: Cellular resistance to insulin caused by reduced glucose transport and metabolism is a primary defect leading to the development of metabolic disease. While the etiology of insulin resistance is multifactorial, reduced insulin action is associated with impaired activity of the glucose transporter GLUT4 in insulin-sensitive tissues. Yet, the role of adipose tissue GLUT4 deregulation in the pathogenesis of insulin resistance, obesity, and diabetes is still unclear. In this study, we assessed the relative GLUT4 level in human subcutaneous adipose tissue from obese, diabetic, and diabetic obese versus control subjects, using a real-time PCR method. GLUT4 mRNA levels were considerably decreased among type 2 diabetic patients compared with those of the controls (P < 0.01), whereas no such difference was found between obese and normal-weight controls. Multiple linear regressions analysis in both diabetic non-obese and diabetic obese groups showed a negative correlation between GLUT4 mRNA expression and both markers of obesity or insulin resistance (P < 0.01). However, in obese group, GLUT4 was inversely associated only with HOMA-IR (P < 0.01). Our findings showed that adipose GLUT4 gene expression changes were more related to insulin resistance and type 2 diabetes rather than to obesity.
No preview · Article · May 2011 · Acta Diabetologica
[Show abstract][Hide abstract] ABSTRACT: The hypothalamus integrates metabolic and endocrine signals. As such it represents a potential target for a wide spectrum of endocrine disrupting chemicals (EDCs). We investigated hypothalamic effects of two environmentally abundant xenobiotics, the flame-retardant tetrabromo bisphenol A (TBBPA) and the anti-fouling agent tributyltin (TBT). These EDCs affect endocrine signalling through different nuclear receptors including the thyroid hormone receptor (TR) or its partner, the retinoid X receptor (RXR). Promoter sequences of two hypothalamic genes implicated in metabolic control and regulated by thyroid hormone, thyrotropin-releasing hormone (Trh) and type 4 melanocortin receptor (Mc4r), were studied in vivo using reporter assays. Chronic exposure of gestating dams or acute exposure of their newborns to TBBPA abrogated activation of both Trh and Mc4r transcription. Exposure of lactating dams to TBT amplified activation of Trh without affecting Mc4r transcription. Thus, perinatal exposure to EDCs affecting nuclear receptor signalling modulates hypothalamic set-points controlling metabolic responses.
No preview · Article · Jul 2010 · Molecular and Cellular Endocrinology
[Show abstract][Hide abstract] ABSTRACT: The type 4 melanocortin receptor MC4R, a key relay in leptin signaling, links central energy control to peripheral reserve status. MC4R activation in different brain areas reduces food intake and increases energy expenditure. Mice lacking Mc4r are obese. Mc4r is expressed by hypothalamic paraventricular Thyrotropin-releasing hormone (TRH) neurons and increases energy usage through activation of Trh and production of the thyroid hormone tri-iodothyronine (T(3)). These facts led us to test the hypothesis that energy homeostasis should require negative feedback by T(3) on Mc4r expression. Quantitative PCR and in situ hybridization showed hyperthyroidism reduces Mc4r mRNA levels in the paraventricular nucleus. Comparative in silico analysis of Mc4r regulatory regions revealed two evolutionarily conserved potential negative thyroid hormone-response elements (nTREs). In vivo ChIP assays on mouse hypothalamus demonstrated association of thyroid hormone receptors (TRs) with a region spanning one nTRE. Further, in vivo gene reporter assays revealed dose-dependent T(3) repression of transcription from the Mc4r promoter in mouse hypothalamus, in parallel with T(3)-dependent Trh repression. Mutagenesis of the nTREs in the Mc4r promoter demonstrated direct regulation by T(3), consolidating the ChIP results. In vivo shRNA knockdown, TR over-expression approaches and use of mutant mice lacking specific TRs showed that both TRalpha and TRbeta contribute to Mc4r regulation. T(3) repression of Mc4r transcription ensures that the energy-saving effects of T(3) feedback on Trh are not overridden by MC4R activation of Trh. Thus parallel repression by T(3) on hypothalamic Mc4r and Trh contributes to energy homeostasis.
Full-text · Article · Feb 2010 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Thyroid hormone receptor (TR) and peroxisome proliferator-activated receptor gamma (PPARgamma) co-regulate numerous peripheral metabolic responses. To examine potential crosstalk between PPARgamma and TRbeta in the hypothalamus, thyrotropin-releasing hormone (Trh) regulation in the newborn mouse hypothalamus was followed. QPCR showed PPARgamma to be expressed in the hypothalamus at this developmental stage. Intracerebral injection of PPARgamma agonists modified transcription from a TRH-luc construct introduced into the hypothalamus and increased serum thyroxine levels. Furthermore, shRNA-based in vivo PPARgamma knockdown amplified T(3)-independent transcription and PPARgamma overexpression dose-dependently abrogated T(3)-dependent Trh repression. Overexpression of retinoid X receptor-alpha (RXRalpha), the common heterodimeric partner of PPARgamma and TRbeta, rescued PPARgamma abrogation of T(3)-dependent repression. Thus, competition for RXR could represent one mechanism underlying this hypothalamic crosstalk between PPARgamma and TRbeta. These demonstrations of PPARgamma effects on hypothalamic Trh transcription in vivo consolidate the role of the TRH neuron as a central integrator of energy homeostasis.
Full-text · Article · Nov 2009 · Molecular and Cellular Endocrinology