Central overexpression of the TRH precursor gene induces hypertension in rats: antisense reversal

Non-alcoholic fatty liver disease mediates the effect of obesity on arterial hypertension

BACKGROUND

It has been consistently shown that obesity contributes directly to arterial hypertension and cardiovascular disease (CVD), independently of other risk factors. Likewise, non-alcoholic fatty liver disease (NAFLD) is acknowledged as a contributor and a risk enhancer for CVD.

OBJECTIVES

We tested the hypothesis of a causal role of NAFLD in the effect of obesity on arterial hypertension.

METHODS

Using causal mediation analysis, we quantified the magnitude of the body mass index (BMI) effect on arterial hypertension and CV-traits mediated by NAFLD. First, we analysed data from 1348 young adults in the Bogalusa Heart Study (BHS), a cohort aimed at assessing the natural history of CVD. Then, we used data from 3359 participants of the National Health and Nutrition Examination Survey (2017-2018 cycle, NHANES) to replicate the findings.

RESULTS

We found that roughly 92% of the effects of BMI on arterial hypertension in the BHS and 51% in the NHANES population are mediated by NAFLD. In addition, indirect effects of BMI on systolic (SBP) and diastolic (DBP) blood pressure, and heart rate (HR) through NAFLD explained up to 91%, 93%, and 100% of the total effect, respectively, in the BHS. In the NHANES survey, indirect effects of BMI through NAFLD on CV traits explain a significant proportion of the total effects (SBP = 60.4%, HR = 100%, and pulse pressure = 88%).

CONCLUSION

NAFLD mediates a substantial proportion of the effect of obesity on the presence of hypertension and CV-parameters independently of relevant covariates. This conclusion has implications for clinical management.

Hypothyroidism-associated immunosuppression involves induction of galectin-1-producing regulatory T cells

Hypothyroidism exerts deleterious effects on immunity, but the precise role of the hypothalamic-pituitary-thyroid (HPT) axis in immunoregulatory and tolerogenic programs is barely understood. Here, we investigated the mechanisms underlying hypothyroid-related immunosuppression by examining the regulatory role of components of the HPT axis. We first analyzed lymphocyte activity in mice overexpressing the TRH gene (Tg-Trh). T cells from Tg-Trh showed increased proliferation than wild-type (WT) euthyroid mice in response to polyclonal activation. The release of Th1 pro-inflammatory cytokines was also increased in Tg-Trh and TSH levels correlated with T-cell proliferation. To gain further mechanistic insights into hypothyroidism-related immunosuppression, we evaluated T-cell subpopulations in lymphoid tissues of hypothyroid and control mice. No differences were observed in CD3/CD19 or CD4/CD8 ratios between these strains. However, the frequency of regulatory T cells (Tregs) was significantly increased in hypothyroid mice, and not in Tg-Trh mice. Accordingly, in vitro Tregs differentiation was more pronounced in naïve T cells isolated from hypothyroid mice. Since Tregs overexpress galectin-1 (Gal-1) and mice lacking this lectin (Lgals1^-/- ) show reduced Treg function, we investigated the involvement of this immunoregulatory lectin in the control of Tregs in settings of hypothyroidism. Increased T lymphocyte reactivity and reduced frequency of Tregs were found in hypothyroid Lgals1^-/- mice when compared to hypothyroid WT animals. This effect was rescued by the addition of recombinant Gal-1. Finally, increased expression of Gal-1 was found in Tregs purified from hypothyroid WT mice compared with their euthyroid counterpart. Thus, a substantial increase in the frequency and activity of Gal-1-expressing Tregs underlies immunosuppression associated with hypothyroid conditions, with critical implications in immunopathology, metabolic disorders, and cancer.

Association between DIO2 Thr92Ala polymorphism and hypertension in patients with hypothyroidism: Korean Genome and Epidemiology Study

BACKGROUND/AIMS

Recent evidence has identified the significance of type 2 iodothyronine deiodinase (DIO2) in various diseases. However, the role of DIO2 polymorphism in metabolic parameters in patients with hypothyroidism is not fully understood.

METHODS

We assessed the polymorphism of the DIO2 gene and various clinical parameters in 118 patients who were diagnosed with hypothyroidism from the Ansan-Anseong cohort of the Korean Genome and Epidemiology Study. Furthermore, we systematically analyzed Genotype-Tissue Expression (GTEx) data.

RESULTS

A total of 118 participants with hypothyroidism were recruited; 32 (27.1%) were homozygous for the Thr allele, 86 (73.9%) were homozygous for the Ala allele or heterozygous. Patients with hypothyroidism with DIO2 polymorphism without hypertension at baseline had higher incidence of hypertension compared to patients without DIO2 polymorphism. Analysis of the GTEx database revealed that elevation of DIO2 expression is associated with enhancement of genes involved in blood vessel regulation and angiogenesis.

CONCLUSION

Commonly inherited variation in the DIO2 gene is associated with high blood pressure and prevalence of hypertension in patients with hypothyroidism. Our results suggest that genetic variation in the hypothalamic-pituitary-thyroid pathway in influencing susceptibility to hypertension.

Cardiovascular and body weight regulation changes in transgenic mice overexpressing thyrotropin-releasing hormone (TRH)

Thyrotropin-releasing hormone (TRH) plays several roles as a hormone/neuropeptide. Diencephalic TRH (dTRH) participates in the regulation of blood pressure in diverse animal models, independently of the thyroid status. The present study aimed to evaluate whether chronic overexpression of TRH in mice affects cardiovascular and metabolic variables. We developed a transgenic (TG) mouse model that overexpresses dTrh. Despite having higher food consumption and water intake, TG mice showed significantly lower body weight respect to controls. Also, TG mice presented higher blood pressure, heart rate, and locomotor activity independently of thyroid hormone levels. These results and the higher urine noradrenaline excretion observed in TG mice suggest a higher metabolic rate mediated by sympathetic overflow. Cardiovascular changes were impeded by siRNA inhibition of the diencephalic Trh overexpression. Also, the silencing of dTRH in the TG mice normalized urine noradrenaline excretion, supporting the view that the cardiovascular effects of TRH involve the sympathetic system. Overall, we show that congenital dTrh overexpression leads to an increase in blood pressure accompanied by changes in body weight and food consumption mediated by a higher sympathetic overflow. These results provide new evidence confirming the participation of TRH in cardiovascular and body weight regulation.

Thyrotropin-releasing hormone overexpression induces structural changes of the left ventricle in the normal rat heart

Thyrotropin-releasing hormone (TRH) hyperactivity has been observed in the left ventricle of spontaneously hypertensive rats. Its long-term inhibition suppresses the development of hypertrophy, specifically preventing fibrosis. The presence of diverse systemic abnormalities in spontaneously hypertensive rat hearts has raised the question of whether specific TRH overexpression might be capable of inducing structural changes in favor of the hypertrophic phenotype in normal rat hearts. We produced TRH overexpression in normal rats by injecting into their left ventricular wall a plasmid driving expression of the preproTRH gene (PCMV-TRH). TRH content and expression of preproTRH, collagen type III, brain natriuretic peptide, β-myosin heavy chain, Bax-to-Bcl-2 ratio, and caspase-3 were measured. The overexpression maneuver was a success, as we found a significant increase in both tripeptide and preproTRH mRNA levels in the PCMV-TRH group compared with the control group. Immunohistochemical staining against TRH showed markedly positive brown signals only in the PCMV-TRH group. TRH overexpression induced a significant increase in fibrosis, evident in the increase of collagen type III expression accompanied by a significant increase in extracellular matrix expansion. We found a significant increase in brain natriuretic peptide and β-myosin heavy chain expression (recognized markers of hypertrophy). Moreover, TRH overexpression induced a slight but significant increase in myocyte diameter, indicating the onset of cell hypertrophy. We confirmed the data “in vitro” using primary cardiac cell cultures (fibroblasts and myocytes). In conclusion, these results show that a specific TRH increase in the left ventricle induced structural changes in the normal heart, thus making the cardiac TRH system a promising therapeutic target.

Association between diencephalic thyroliberin and arterial blood pressure in agouti-yellow and ob/ob mice may be mediated by leptin

Leptin, a hormone secreted by the adipose tissue, stimulates anorexigenic peptides and also inhibits orexigenic peptides in hypothalamic arcuate nuclei-located neurons. It also counteracts the starvation-induced suppression of thyroid hormones by up-regulating the expression of preproTRH gene. On the other hand, in addition to its role as a modulator of the thyroid-hypothalamic-hypophysial axis, thyrotropin-releasing hormone (TRH) acts as a modulator of the cardiovascular system. In fact, we reported that overexpression of diencephalic TRH (dTRH) induces hypertension. We have recently shown that, in rats with obesity-induced hypertension, hyperleptinemia may produce an increase of dTRH together with an elevation of arterial blood pressure (ABP) through an increase of sympathetic activity and that these alterations were reversed by antisense oligonucleotide and small interfering RNA against preproTRH treatments. Here we explore the possible role of dTRH as a mediator involved in leptin-induced hypertension in 2 obesity mouse models: agouti-yellow mice, which are hyperleptinemic and hypertensive, and ob/ob mice, which lack functional circulating leptin. These 2 models share some characteristics, but ob/ob mice show lower ABP and plasma catecholamines levels. Then, for the first time, we report that there is a clear association between ABP and dTRH levels in both mouse models, as we have found that dTRH content was elevated in agouti-yellow mice and diminished in ob/ob mice compared with their controls. We also show that, after 3 days of subcutaneous leptin injections (10 microg/12 hours), ABP and dTRH increased significantly in ob/ob mice with no alterations of thyroid hormone levels. These results add evidence to the putative molecular mechanisms for the strong association between obesity and hypertension.

Knocking down the diencephalic thyrotropin-releasing hormone precursor gene normalizes obesity-induced hypertension in the rat

We recently showed that diencephalic TRH may mediate the central leptin-induced pressor effect. Here, to study the role of TRH in obesity-induced hypertension (OIH), we used a model of OIH produced by a high-fat diet (HFD, 45 days) in male Wistar rats. After 4 wk, body weight and systolic arterial blood pressure (SABP) increased in HFD animals. Plasma leptin was correlated with peritoneal adipose tissue. Then, we treated OIH animals with an antisense oligodeoxynucleotide and small interfering (si)RNA against the prepro-TRH. Antisense significantly decreased diencephalic TRH content and SABP at 24 and 48 h posttreatment. Similar effects were observed with siRNA against prepro-TRH but for up to 4 wk. Conversely, vehicle, an inverted antisense sequence and siRNA against green fluorescence protein, produced no changes. SABP decrease seems to be owing to an inhibition of the obesity-enhanced sympathetic outflow but not to an alteration in thyroid status. Using a simple OIH model we demonstrated, for the first time, that central TRH participates in the hypertension induced by body weight gain probably through its well-known action on sympathetic activity. Thus the TRH-leptin interaction may contribute to the strong association between hypertension and obesity.

Central thyrotropin-releasing hormone infusion opposes cardiovascular and metabolic suppression during caloric restriction

Inhibition of hypothalamic thyrotropin-releasing hormone (TRH) neuronal activity is a well-established adaptation to caloric restriction (CR) that suppresses pituitary secretion of thyroid-stimulating hormone, but may also participate in modulation of autonomic function. Thus, we hypothesized that decreased hypothalamic TRH activity contributes to CR-induced bradycardia and decreased metabolic rate. To test this hypothesis, male Sprague-Dawley rats were instrumented with telemetry devices for measurement of heart rate (HR) and blood pressure (BP) and a lateral intracerebroventricular (i.c.v.) guide cannula for central infusions. After recovery, rats were housed in metabolic chambers and given either ad libitum(ad-lib) or CR treatments for 7 days; half of each diet group was then given continuous i.c.v. infusions of TRH (25 nmol/h) or saline (0.25 microl/h) for 7 days via osmotic pump. This dose of TRH did not significantly alter peripheral free T(4) levels. In ad-lib rats, TRH infusion produced small reductions in food intake and small increases in HR and BP over saline-infused controls. In CR rats, TRH infusion resulted in an increase in HR and also energy expenditure over saline-infused controls. These results support the hypothesis that suppression of central TRH activity contributes to the homeostatic suppression of energy expenditure and HR observed during CR.

Thyrotropin-releasing hormone in cardiovascular pathophysiology

Thyrotropin (TSH)-releasing hormone (TRH) also known as thyroliberin was the first of a number of peptides exerting several roles as a hormone and as a neuropeptide. Its ubiquitous distribution in the hypothalamus and in the extrahypothalamic regions and its diverse pharmacological and physiological effects are all features of its dual functions. For this reason, TRH has been the subject of much research throughout the past 20 years, work that has examined the structure, function, distribution, and regulation of the tripeptide and it has been extensively reviewed elsewhere [O'Leary R., O'Connor B. Thyrotropin-releasing hormone. J Neurochem. 1995;65:953-963.; Nillni E., Sevarino K. The biology of pro-thyrotropin-releasing hormone-derived peptides. Endocrine Reviews, 1999;20:599-664.]. After a brief overview of its distribution, hypothalamic and extrahypothalamic functions, and receptors involved, this review discusses efforts devoted to support TRH role in cardiovascular regulation with a main focus on hypertension pathophysiology in experimental models and humans.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.