The PKC and ERK/MAPK Pathways Regulate Glucocorticoid Action on TRH Transcription

The implication of ADRA2A and AVPRIB gene variants in the aetiology of stress-related bipolar disorder

OBJECTIVE

Bipolar disorder is a complex and severe mental illness characterised by manic and depressive episodes that can be triggered and exacerbated by psychosocial, environmental, and biological stressors. Genetic variations are a risk factor for bipolar disorder. However, the identification of the exact gene variants and genotypes remains complex. This study, therefore, aims to identify the potential association between genotypes of analysed single nucleotide polymorphisms and the presence of a stressor in bipolar disorder patients.

METHOD

We analysed 114 single nucleotide polymorphisms (SNPs) from bipolar and stress-related candidate genes in 550 patients with bipolar disorders (60.36 % females and 39.64 % male). We compared SNPs of patients reporting the presence (40.73 %) or absence of stressors (59.27 %) before the first episode using the Persons Chi-square test and Bayes Factor t-test. The genotyping of 114 SNPs was done using TaqMan assays. Statistical analysis was done using Statistica 13.3 software (StatSoft Poland, Krakow, Poland), R programming, and G*Power statistics.

RESULT

We found significant differences in genotype distribution (p < 0.05) in 6 polymorphisms (AVPRIB/rs28536160, FKBP4/rs2968909, ADRA2A/rs3750625, 5HTR2A/rs6311, 5HTR2A/rs6313, and GLCCI1/rs37972) when comparing BD patient with and without stressor with a small effect of d = 0.2. Of these, two gene variants (ADRA2A/rs3750625/AC and AVPRIB/rs28536160/CT) with minor alleles formed an association with the presence of a stressor prior to the disease onset and favoured the alternative hypothesis using Bayes Factor Analysis t-test for hypothesis testing.

CONCLUSION

This study presents a novel association of ADRA2A/rs3750625/AC and AVPR1B/rs28536160/CT gene variants in stress-related bipolar disorder with the AC genotype of ADRA2A/rs3750625 constituting a risk genotype and CT of AVPR1B/rs28536160 constituting a protective genotype. However, further functional analysis is required to fully understand their clinical and biological significance and interaction.

Cytokine Storm-Induced Thyroid Dysfunction in COVID-19: Insights into Pathogenesis and Therapeutic Approaches

Angiotensin-converting enzyme 2 receptors (ACE2R) are requisite to enter the host cells for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). ACE2R is constitutive and functions as a type I transmembrane metallo-carboxypeptidase in the renin-angiotensin system (RAS). On thyroid follicular cells, ACE2R allows SARS-CoV-2 to invade the thyroid gland, impose cytopathic effects and produce endocrine abnormalities, including stiff back, neck pain, muscle ache, lethargy, and enlarged, inflamed thyroid gland in COVID-19 patients. Further damage is perpetuated by the sudden bursts of pro-inflammatory cytokines, which is suggestive of a life-threatening syndrome known as a "cytokine storm". IL-1β, IL-6, IFN-γ, and TNF-α are identified as the key orchestrators of the cytokine storm. These inflammatory mediators upregulate transcriptional turnover of nuclear factor-kappa B (NF-κB), Janus kinase/signal transducer and activator of transcription (JAK/STAT), and mitogen-activated protein kinase (MAPK), paving the pathway for cytokine storm-induced thyroid dysfunctions including euthyroid sick syndrome, autoimmune thyroid diseases, and thyrotoxicosis in COVID-19 patients. Targeted therapies with corticosteroids (dexamethasone), JAK inhibitor (baricitinib), nucleotide analogue (remdesivir) and N-acetyl-cysteine have demonstrated effectiveness in terms of attenuating the severity and frequency of cytokine storm-induced thyroid dysfunctions, morbidity and mortality in severe COVID-19 patients. Here, we review the pathogenesis of cytokine storms and the mechanisms and pathways that establish the connection between thyroid disorder and COVID-19. Moreover, cross-talk interactions of signalling pathways and therapeutic strategies to address COVID-19-associated thyroid diseases are also discussed herein.

In vitro modeling of the neurobiological effects of glucocorticoids: A review

Hypothalamic-pituitary adrenal (HPA)axis dysregulation has long been implicated in stress-related disorders such as major depression and post-traumatic stress disorder. Glucocorticoids (GCs) are released from the adrenal glands as a result of HPA-axis activation. The release of GCs is implicated with several neurobiological changes that are associated with negative consequences of chronic stress and the onset and course of psychiatric disorders. Investigating the underlying neurobiological effects of GCs may help to better understand the pathophysiology of stress-related psychiatric disorders. GCs impact a plethora of neuronal processes at the genetic, epigenetic, cellular, and molecular levels. Given the scarcity and difficulty in accessing human brain samples, 2D and 3D in vitro neuronal cultures are becoming increasingly useful in studying GC effects. In this review, we provide an overview of in vitro studies investigating the effects of GCs on key neuronal processes such as proliferation and survival of progenitor cells, neurogenesis, synaptic plasticity, neuronal activity, inflammation, genetic vulnerability, and epigenetic alterations. Finally, we discuss the challenges in the field and offer suggestions for improving the use of in vitro models to investigate GC effects.

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.

Hyperactivity of Hypothalamic-Pituitary-Adrenal Axis Due to Dysfunction of the Hypothalamic Glucocorticoid Receptor in Sigma-1 Receptor Knockout Mice

Sigma-1 receptor knockout (σ₁R-KO) mice exhibit a depressive-like phenotype. Because σ₁R is highly expressed in the neuronal cells of hypothalamic paraventricular nuclei (PVN), this study investigated the influence of σ₁R deficiency on the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Here, we show that the levels of basal serum corticosterone (CORT), adrenocorticotropic hormone (ACTH) and corticotrophin releasing factor (CRF) as well as the level of CRF mRNA in PVN did not significantly differ between adult male σ₁R-KO mice and wild-type (WT) mice. Acute mild restraint stress (AMRS) induced a higher and more sustainable increase in activity of HPA axis and CRF expression in σ₁R-KO mice. Percentage of dexamethasone (Dex)-induced reduction in level of CORT was markedly attenuated in σ₁R^-/- mice. The levels of glucocorticoid receptor (GR) and protein kinase C (PKC) phosphorylation were reduced in the PVN of σ₁R-KO mice and σ₁R antagonist NE100-treated WT mice. The exposure to AMRS in σ₁R-KO mice induced a stronger phosphorylation of cAMP-response element binding protein (CREB) in PVN than that in WT mice. Intracerebroventricular (i.c.v.) injection of PKC activator PMA for 3 days in σ₁R-KO mice not only recovered the GR phosphorylation and the percentage of Dex-reduced CORT but also corrected the AMRS-induced hyperactivity of HPA axis and enhancement of CRF mRNA and CREB phosphorylation. Furthermore, the injection (i.c.v.) of PMA in σ₁R-KO mice corrected the prolongation of immobility time in forced swim test (FST) and tail suspension test (TST). These results indicate that σ₁R deficiency causes down-regulation of GR by reducing PKC phosphorylation, which attenuates GR-mediated feedback inhibition of HPA axis and facilitates the stress response of HPA axis leading to the production of depressive-like behaviors.

A screen for modulators reveals that orexin-A rapidly stimulates thyrotropin releasing hormone expression and release in hypothalamic cell culture

In the paraventricular nucleus of the mammalian hypothalamus, hypophysiotropic thyrotropin releasing hormone (TRH) neurons integrate metabolic information and control the activity of the thyroid axis. Additional populations of TRH neurons reside in various hypothalamic areas, with poorly defined connections and functions, albeit there is evidence that some may be related to energy balance. To establish extracellular modulators of TRH hypothalamic neurons activity, we performed a screen of neurotransmitters effects in hypothalamic cultures. Cell culture conditions were chosen to facilitate the full differentiation of the TRH neurons; these conditions had permitted the characterization of the effects of known modulators of hypophysiotropic TRH neurons. The major end-point of the screen was Trh mRNA levels, since they are generally rapidly (0.5-3h) modified by synaptic inputs onto TRH neurons; in some experiments, TRH cell content or release was also analyzed. Various modulators, including histamine, serotonin, β-endorphin, met-enkephalin, and melanin concentrating hormone, had no effect. Glutamate, as well as ionotropic agonists (kainate and N-Methyl-d-aspartic acid), increased Trh mRNA levels. Baclofen, a GABA_B receptor agonist, and dopamine enhanced Trh mRNA levels. An endocannabinoid receptor 1 inverse agonist promoted TRH release. Somatostatin increased Trh mRNA levels and TRH cell content. Orexin-A rapidly increased Trh mRNA levels, TRH cell content and release, while orexin-B decreased Trh mRNA levels. These data reveal unaccounted regulators, which exert potent effects on hypothalamic TRH neurons in vitro.

Hypothalamo‑hypophysial system in rats with autotransplantation of the adrenal cortex

Patients with bilateral pheochromocytoma often require an adrenalectomy. Autotransplantation of the adrenal cortex is an alternative therapy that could potentially be performed instead of receiving glucocorticoid replacement following adrenalectomy. Adrenal cortex autotransplantation aims to avoid the side effects of long‑term steroid treatment and adrenal insufficiency. Although the function of the hypothalamo‑hypophysial system is critical for patients who have undergone adrenal cortex autotransplantation, the details of that system, with the exception of adrenocorticotropic hormone in the subjects with adrenal autotransplantation, have been overlooked for a long time. To clarify the precise effect of adrenal autotransplantation on the pituitary gland and hypothalamus, the current study examined the gene expression of hormones produced from the hypothalamus and pituitary gland. Bilateral adrenalectomy and adrenal autotransplantation were performed in 8 to 9‑week‑old male rats. The hypothalamus and pituitary tissues were collected at 4 weeks after surgery. Transcriptional regulation of hypothalamic and pituitary hormones was subsequently examined by reverse transcription‑quantitative polymerase chain reaction. Proopiomelanocortin, glycoprotein hormone α polypeptide, and thyroid stimulating hormone β were significantly elevated in the pituitary gland of autotransplanted rats when compared with sham‑operated rats. In addition, there were significant differences in the levels of corticotropin releasing hormone receptor 1 (Crhr1), Crhr2, nuclear receptor subfamily 3 group C member 1 and thyrotropin releasing hormone receptor between the sham‑operated rats and autotransplanted rats in the pituitary gland. In the hypothalamus, corticotropin releasing hormone and urocortin 2 mRNA was significantly upregulated in autotransplanted rats compared with sham‑operated rats. The authors identified significant alterations in the function of not only the hypothalamus‑pituitary‑adrenal axis, but also the adenohypophysis thyrotropes in autotransplanted rats. In the future, it will be important to examine other tissues affected by glucocorticoids following adrenal cortex autotransplantation.

PKCβII-mediated cross-talk of TRPV1/CB2 modulates the glucocorticoid-induced osteoclast overactivity

In this study, we investigated the role of the endovanilloid/endocannabinoid system in the glucocorticoid-induced osteoclast overactivity. Receptorial and enzymatic component of the endovanilloid/endocannabinoid system are expressed in bone cells, and dysregulated when bone mass is reduced. Moreover, blockade or desensitization of vanilloid receptor 1 (TRPV1) and/or stimulation of cannabinoid receptor 2 (CB2) are beneficial for reducing number and activity of the bone cells modulating resorption, the osteoclasts. We have treated in vitro healthy woman derived osteoclasts with methylprednisolone in presence or not of CB2 or TRPV1 agonists/antagonists, analysing the effect on osteoclast function and morphology through a multidisciplinary approach. Moreover, a treatment with a protein kinase C inhibitor to evaluate osteoclast activity and endovanilloid/endocannabinoid component expression levels was performed in osteoclasts derived from healthy subjects in presence of not of methylprednisolone. Our results show, for the first time, that the endovanilloid/endocannabinoid system is dysregulated by the treatment with methylprednisolone, that the osteoclast activity is increased and that pharmacological compounds stimulating CB2 or inhibiting TRPV1 might reduce, possible inhibiting protein kinase C beta II, the methylprednisolone-induced osteoclast over-activation, suggesting their therapeutic use for protecting from the glucocorticoid-induced bone mass loss.

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.

11β-Hydroxysteroid Dehydrogenase Type 1(11β-HSD1) mediates insulin resistance through JNK activation in adipocytes

Glucocorticoids are used to treat a number of human diseases but often lead to insulin resistance and metabolic syndrome. 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) is a key enzyme that catalyzes the intracellular conversion of cortisone to physiologically active cortisol. Despite the known role of 11β-HSD1 and active glucocorticoid in causing insulin resistance, the molecular mechanisms by which insulin resistance is induced remain elusive. The aim of this study is to identify these mechanisms in high fat diet (HFD) experimental models. Mice on a HFD were treated with 11β-HSD1 inhibitor as well as a JNK inhibitor. We then treated 3T3-L1-derived adipocytes with prednisone, a synthetic glucocorticoid, and cells with 11β-HSD1 overexpression to study insulin resistance. Our results show that 11β-HSD1 and JNK inhibition mitigated insulin resistance in HFD mice. Prednisone stimulation or overexpression of 11β-HSD1 also caused JNK activation in cultured adipocytes. Inhibition of 11β-HSD1 blocked the activation of JNK in adipose tissue of HFD mice as well as in cultured adipocytes. Furthermore, prednisone significantly impaired the insulin signaling pathway, and these effects were reversed by 11β-HSD1 and JNK inhibition. Our study demonstrates that glucocorticoid-induced insulin resistance was dependent on 11β-HSD1, resulting in the critical activation of JNK signaling in adipocytes.

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.

CREB/TRH pathway in the central nervous system regulates energy expenditure in response to deprivation of an essential amino acid

BACKGROUND

In the central nervous system (CNS), thyrotropin-releasing hormone (TRH) has an important role in regulating energy balance. We previously showed that dietary deprivation of leucine in mice increases energy expenditure through CNS-dependent regulation. However, the involvement of central TRH in this regulation has not been reported.

METHODS

Male C57J/B6 mice were maintained on a control or leucine-deficient diet for 7 days. Leucine-deprived mice were either third intracerebroventricular (i.c.v.) injected with a TRH antibody followed by intraperitoneal (i.p.) injection of triiodothyronine (T3) or i.c.v. administrated with an adenovirus of shCREB (cAMP-response element binding protein) followed by i.c.v. injection of TRH. Food intake and body weight were monitored daily. Oxygen consumption, physical activity and rectal temperature were assessed after the treatment. After being killed, the hypothalamus and the brown adipose tissue were collected and the expression of related genes and proteins related was analyzed. In other experiments, control or leucine-deficient medium incubated primary cultured neurons were either infected with adenovirus-mediated short hairpin RNA targeting extracellular signal-regulated kinases 1 and 2 (Ad-shERK1/2) or transfected with plasmid-overexpressing protein phosphatase 1 regulatory subunit 3C (PPP1R3C).

RESULTS

I.c.v. administration of anti-TRH antibodies significantly reduced leucine deprivation-stimulated energy expenditure. Furthermore, the effects of i.c.v. TRH antibodies were reversed by i.p. injection of T3 during leucine deprivation. Moreover, i.c.v. injection of Ad-shCREB (adenovirus-mediated short hairpin RNA targeting CREB) significantly suppressed leucine deprivation-stimulated energy expenditure via modulation of TRH expression. Lastly, TRH expression was regulated by CREB, which was phosphorylated by ERK1/2 and dephosphorylated by PPP1R3C-containing protein Ser/Thr phosphatase type 1 (PP1) under leucine deprivation in vitro.

CONCLUSIONS

Our data indicate a novel role for TRH in regulating energy expenditure via T3 during leucine deprivation. Furthermore, our findings reveal that TRH expression is activated by CREB, which is phosphorylated by ERK1/2 and dephosphorylated by PPP1R3C-containing PP1. Collectively, our studies provide novel insights into the regulation of energy homeostasis by the CNS in response to an essential amino-acid deprivation.

Modulation of μ-opioid receptor signaling by RGS19 in SH-SY5Y cells

Regulator of G-protein signaling protein 19 (RGS19), also known as Gα-interacting protein (GAIP), acts as a GTPase accelerating protein for Gαz as well as Gαi/o subunits. Interactions with GAIP-interacting protein N-terminus and GAIP-interacting protein C-terminus (GIPC) link RGS19 to a variety of intracellular proteins. Here we show that RGS19 is abundantly expressed in human neuroblastoma SH-SY5Y cells that also express µ- and δ- opioid receptors (MORs and DORs, respectively) and nociceptin receptors (NOPRs). Lentiviral delivery of short hairpin RNA specifically targeted to RGS19 reduced RGS19 protein levels by 69%, with a similar reduction in GIPC. In RGS19-depleted cells, there was an increase in the ability of MOR (morphine) but not of DOR [(4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC80)] or NOPR (nociceptin) agonists to inhibit forskolin-stimulated adenylyl cyclase and increase mitogen-activated protein kinase (MAPK) activity. Overnight treatment with either MOR [D-Ala, N-Me-Phe, Gly-ol(5)-enkephalin (DAMGO) or morphine] or DOR (D-Pen(5)-enkephalin or SNC80) agonists increased RGS19 and GIPC protein levels in a time- and concentration-dependent manner. The MOR-induced increase in RGS19 protein was prevented by pretreatment with pertussis toxin or the opioid antagonist naloxone. Protein kinase C (PKC) activation alone increased the level of RGS19 and inhibitors of PKC 5,6,7,13-tetrahydro-13-methyl-5-oxo-12H-indolo[2,3-a]pyrrolo[3,4-c]carbazole-12-propanenitrile and mitogen-activated protein kinase kinase 1 2-(2-amino-3-methoxyphenyl)-4H-chromen-4-one, but not protein kinase A (H89), completely blocked DAMGO-induced RGS19 protein accumulation. The findings show that RGS19 and GIPC are jointly regulated, that RGS19 is a GTPase accelerating protein for MOR with selectivity over DOR and NOPR, and that chronic MOR or DOR agonist treatment increases RGS19 levels by a PKC and the MAPK pathway-dependent mechanism.

Creb and Sp/Krüppel response elements cooperate to control rat TRH gene transcription in response to cAMP

Expression of hypophysiotropic TRH, that controls thyroid axis activity, is increased by cold exposure; this effect is mimicked in rat hypothalamic cells incubated with norepinephrine or cAMP analogs. TRH proximal promoter contains three putative CRE: Site-4 or CRE-1 that overlaps an element recognized by thyroid hormone receptors, CRE-2 with adjacent sequences GC box or CACCC recognized by Sp/Krüppel factors (extended CRE-2), and AP-1 sites flanking a GRE(1/2). To evaluate the role of each element in the cAMP response, these sites were mutated or deleted in rat TRH promoter linked to luciferase gene (TRH-luc) and co-transfected with β-gal expression vector in various cell lines; C6 cells gave the highest response to forskolin. Basal activity was most affected by mutations or deletion of CRE-2 site, or CACCC (50-75% of wild type-WT). Forskolin-induced 3× stimulation in WT which decreased 25% with CRE-1 or AP-1 deletions, but 50% when CRE-2 or its 5' adjacent GC box was altered. SH-SY5Y cells co-transfected with CREB-expression vector increased dB-cAMP response in the wild type but not in the CRE-2 mutated plasmid; cotransfecting CREB-A (a dominant negative expression vector) strongly diminished basal or cAMP response. Primary cultures of hypothalamic cells transfected with plasmids containing deletions of CRE-1, CRE-2, or extended CRE-2 failed to respond to forskolin when CRE-2 was modified. These results corroborate the CRE-2 site as the main cAMP-response element of rat TRH promoter, not exclusive of transcription factors of hypothalamic cells, and stress the relevance of adjacent Sp-1 sites, important mediators of some metabolic hormones.

The obesogen hypothesis: a shift of focus from the periphery to the hypothalamus

The obesogen concept proposes that environmental contaminants may be contributing to the epidemic of obesity and its related pathology, metabolic disorder. The first references to such a notion appeared at the beginning of the current decade, with the hypothesis that the correlation between increasing incidence of obesity and enhanced industrial chemical production was not simply coincidental, but potentially causally related. The next event was the introduction of the term "obesogen" as representing an environmental pollutant that adversely affects various aspects of adipose tissue functions. More recently, the concept was extended to include substances that may modify metabolic balance at the central, hypothalamic level. The actions of two prime candidate obesogens, tributyltin (TBT) and tetrabromobisphenol A (TBBPA), acting at the central level are the main focus of this review. Having discussed the evidence for contaminant accumulation in the environment and in human tissues and the potential mechanisms of action, data are provided showing that these two widespread pollutants modify hypothalamic gene regulations. Our studies are based on maternal exposure and measurement of effects in the progeny, mainly based on in vivo gene reporter assays. Such models are obviously pertinent to testing current hypotheses that propose that early exposure might exert effects on later development and physiological functions. The potential molecular mechanisms involved are discussed, as are the broader physiological consequences of these hypothalamic dysregulations.

Molecular evolution of the thyrotrophin-releasing hormone precursor in vertebrates: insights from comparative genomics

Human preprothyrotrophin-releasing hormone (ppTRH) includes six copies of the TRH sequence, the rat and mouse precursors have five, and those of non-mammalian vertebrates have up to eight. In the present study, the evolutionary basis of this variation was investigated using ppTRH gene sequences extracted from available vertebrate genomic databases. A structure based on eight TRH repeats appears to be the norm for non-mammalian vertebrates, but in all mammals except monotremes this number is reduced to a maximum of six. In some species, one (or more) of the TRH repeats has been mutated, probably rendering it functionless and, in a few species, one or two copies of the TRH sequence have been deleted completely. Sequences of regions between the TRH sequences are poorly conserved, despite reports that several active peptides are produced from these regions. The 5' untranslated region of ppTRH is also very variable but, in eutherians, the promoter region immediately upstream of the gene is quite strongly conserved. In particular, those sequences identified as being involved in transcriptional regulation are well conserved in most eutherians, although they are largely absent from other vertebrates. In most species, gene order around the ppTRH locus is conserved, although exceptions include man and chimpanzee, as well as rat and mouse. The comparative genomics approach thus provides a wider view than previously available of the range of ppTRH genes in vertebrates, and of the species specificity displayed by this molecule.

A rapid interference between glucocorticoids and cAMP-activated signalling in hypothalamic neurones prevents binding of phosphorylated cAMP response element binding protein and glucocorticoid receptor at the CRE-Like and composite GRE sites of thyrotrophin-releasing hormone gene promoter

Glucocorticoids or cAMP increase, within minutes, thyrotrophin-releasing hormone (TRH) transcription in hypothalamic primary cultures, although this effect is prevented if cells are simultaneously incubated with both drugs. Rat TRH promoter contains a CRE site at -101/-94 bp and a composite GRE element (cGRE) at -218/-197 bp. Nuclear extracts of hypothalamic cells incubated with 8Br-cAMP or dexamethasone, and not their combination, bind to oligonucleotides containing the CRE or cGRE sequences. Adjacent to CRE are Sp/Krüppel response elements, and flanking the GRE half site, two AP1 binding sites. The present study aimed to identify the hypothalamic transcription factors that bind to these sites. We verified that the effects of glucocorticoid were not mimicked by corticosterone-bovine serum albumin. Footprinting and chromatin immunoprecipitation (ChIP) assays were used to examine the interaction of cAMP- and glucocorticoid-mediated regulation of TRH transcription at the CRE and cGRE regions of the TRH promoter. Nuclear extracts from hypothalamic cells incubated for 1 h with cAMP or glucocorticoids protected CRE. The GRE half site was recognised by nuclear proteins from cells stimulated with glucocorticoids and, for the adjacent AP-1 sites, by nuclear proteins from cells stimulated with cAMP or phorbol esters. Protection of CRE or cGRE was lost if cells were coincubated with dexamethasone and 8Br-cAMP. ChIP assays revealed phospho-CREB, c-Jun, Sp1, c-Fos and GR antibodies bound the TRH promoter of cells treated with cAMP or glucocorticoids; anti:RNA-polymerase II immunoprecipitated TRH promoter in a similar proportion as anti:pCREB or anti:GR. Recruitment of pCREB, SP1 or GR was lost when cells were exposed simultaneously to 8Br-cAMP and glucocorticoids. The data show that while pCREB and Sp1 bind to CRE-2, or GR to cGRE of the TRH promoter, the mutual antagonism between cAMP and glucocorticoid signalling, which prevent their binding to TRH promoter, could serve as a mechanism by which glucocorticoids rapidly suppress cAMP and noradrenaline-stimulated TRH transcription.

T3 differentially regulates TRH expression in developing hypothalamic neurons in vitro

Triiodothyronine (T3) plays an important role during development of the central nervous system. T3 effects on gene expression are determined in part by the type of thyroid hormone receptors (TRs) expressed in a given cell type. Previous studies have demonstrated that thyrotropin releasing hormone (TRH) transcription in the adult hypothalamus is subjected to negative regulation by thyroid hormones. However, the role of T3 on the development of TRH expression is unknown. In this study we used primary cultures derived from 17-day-old fetal rat hypothalamus to analyze the effects of T3 on TRH gene expression during development. T3 increased TRH mRNA expression in immature cultures, but decreased it in mature cultures. In addition, T3 up-regulated TRalpha1 and TRbeta2 mRNA expression. TRalpha1 expression coincided chronologically with that of TRH in the rat hypothalamus in vivo. Maturation of TRH expression in the hypothalamus may involve T3 acting through TRalpha1.

Elevated glucocorticoid levels are responsible for induction of tyrosine hydroxylase mRNA expression, phosphorylation, and enzyme activity in the nucleus of the solitary tract during morphine withdrawal

Chronic opiate exposure induces neurochemical adaptations in the noradrenergic system. Enhanced responsiveness of the hypothalamo-pituitary-adrenal axis after morphine withdrawal has been associated with hyperactivity of ascending noradrenergic input from the nucleus of the solitary tract (NTS-A(2)) cell group to the hypothalamic paraventricular nucleus (PVN). This study addressed the role of morphine withdrawal-induced corticosterone (CORT) release in regulation of tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine biosynthesis in adrenalectomized (ADX) rats supplemented with low CORT pellet (ADX plus CORT). Present results show that in sham-ADX rats, noradrenergic neurons in the NTS-A(2) became activated during morphine withdrawal, as indicated by increased TH mRNA expression. However, this induction of TH expression is not detected in ADX plus CORT rats that are unable to mount CORT secretory response to morphine withdrawal. Total TH protein levels were elevated in the NTS-A(2) from sham-operated rats during morphine dependence and withdrawal, whereas we did not find any alteration in ADX plus CORT animals. Furthermore, high levels of TH phosphorylated (activated) at Ser31 (but not at Ser40) were found in the A(2) area from sham-morphine withdrawn rats. Consistent with these effects, we observed an increase in the enzyme activity of TH in the PVN. However, induction of morphine withdrawal to ADX plus CORT animals did not alter the phosphorylation (activation) of TH in NTS-A(2) and decreased TH activity in the PVN. These results suggest the existence of a positive reverberating circle in which elevated glucocorticoids during morphine abstinence play a permissive role in morphine withdrawal-induced activation of noradrenergic pathway innervating the PVN.