María S. Landa

Thyrotropin-releasing hormone receptor (TRHR) gene is associated with essential hypertension

In essential hypertension, a polygenic and multifactorial syndrome, several genes interact with the environment to produce high blood pressure. Thyrotropin-releasing hormone (TRH) plays an important role in central cardiovascular regulation. We have described that TRH overexpression induces hypertension in a normal rat, which was reversed by TRH antisense treatment. This treatment also reduces the central TRH hyperactivity in spontaneously hypertensive rats and normalizes blood pressure. Human TRH receptor (TRHR) belongs to the G protein-coupled seven-transmembrane domain receptor superfamily. Mutations of these receptors may result in constitutive activation. As it has been demonstrated that hypertensive patients have a blunted TSH response to TRH injection, suggesting a defect in the TRHR, we postulate that the TRHR gene is involved in human hypertension. We studied 2 independent populations from different geographic regions of our country: a sample of adult subjects from a referral clinic and a population-based sample of high school students. In search of molecular variants of TRHR, we disclosed that a polymorphic TG dinucleotide repeat (STR) at −68 bp and a novel single nucleotide polymorphism, a G→C conversion at −221 located in the promoter of the TRHR are associated with essential hypertension. As STRs detected in gene promoters are potential Z-DNA-forming sequences and seem to affect gene expression, we studied the potentially different transcriptional activity of these TRHR promoter variants and found that the S/−221C allele has a higher affinity than does the L/G−221 allele to nuclear protein factor(s). Our findings support the hypothesis that the TRHR gene participates in the etiopathogenesis of essential hypertension.

Clinical features of the metabolic syndrome in adolescents: Minor role of the Trp64Arg β3-Adrenergic receptor gene variant

Obesity and hypertension are increasing medical problems in adolescents. We evaluated the association between being overweight—particularly abdominal fat—and having hypertension and assessed the contribution of the Trp64Arg β3-adrenergic receptor gene variant. In a population-based study, we determined family history, anthropometric variables, and arterial blood pressure of 934 high school students, out of whom we selected 121 normotensive and 54 hypertensive students. Biochemical measurements included circulating renin and angiotensin-converting enzyme activities, leptin, glucose, insulin and lipid levels, and β3-adrenergic receptor genotypes. We used Mann-Whitney U test, χ2-test, and Spearman rank-order correlation. In the total population, hypertension prevalence increased across the entire range of body mass index (BMI) percentiles. In the sample, hypertensive students showed higher BMI, waist-to-hip ratio, triglycerides, and insulin resistance and lower HDL-cholesterol than normotensive students did. Age- and sex-adjusted systolic arterial blood pressure was correlated with BMI, waist-to-hip ratio, insulin resistance, and leptin. Leptin was correlated with BMI and homeostasis model assessment method. We found no association among hypertension, BMI, and leptin levels with β3-adrenergic receptor genotypes. Especially in girls, the waist-to-hip ratio was, however, suggestively higher in Arg64 variant carriers than in noncarriers, independent of hypertension. In fact, there was a significantly (p < 0.01) higher frequency of carriers of the Arg64 variant across the waist-to-hip ratio quartiles. In adolescents of European origin, hypertension is associated with an increased degree of obesity among other characteristics of the metabolic syndrome; the Trp64Arg variant of the β 3-adrenergic receptor gene may favor the central adiposity gain.

Cardiac Thyrotropin-Releasing Hormone Mediates Left Ventricular Hypertrophy in Spontaneously Hypertensive Rats

Local thyrotropin-releasing hormone (TRH) may be involved in cardiac pathophysiology, but its role in left ventricular hypertrophy (LVH) is still unknown. We studied whether local TRH is involved in LVH of spontaneously hypertensive rats (SHR) by investigating TRH expression and its long-term inhibition by interference RNA (TRH-iRNA) during LVH development at 2 stages (prehypertrophy and hypertrophy). SHR and their control rats (WKY) were compared. Cardiac hypertrophy was expressed as heart/total body weight (HW/BW) ratio. TRH content (radioimmuno assay), preproTRH, TRH receptor type I, brain natriuretic peptide (BNP), and collagen mRNA expressions (real-time polymerase chain reaction) were measured. For long-term inhibition of TRH, TRH-iRNA was injected into the left ventricle (LV) wall for 8 weeks. Hearts were processed for morphometric studies and immunohistochemical analysis using antibodies against &agr;-smooth muscle actin and collagen type III. LV preproTRH-mRNA abundance was similar in both strains at 7 weeks of age. At the hypertrophic stage (18 weeks old), however, there was a 15-fold increase in SHR versus WKY, consistent with a significant increase in tripeptide levels and the expression of its receptor. Specific LV-TRH inhibition at the prehypertensive stage with TRH-iRNA, which decreased >50% preproTRH expression and tripeptide levels, prevented LVH development as shown by the normal HW/BW ratio observed in TRH-iRNA–treated SHR. In addition, TRH-iRNA impeded the increase in BNP and type III collagen expressions and prevented the increase in cardiomyocyte diameter evident in mismatch iRNA-treated adult SHR. These results show for the first time that the cardiac TRH system is involved in the development of LVH in SHR.

SiRNA-mediated silencing of the diencephalic thyrotropin-releasing hormone precursor gene decreases the arterial blood pressure in the obese agouti mice.

Obesity is associated with increased cardiovascular morbidity and mortality, in part through development of hypertension. Leptin promotes weight loss by reducing food intake and increasing energy expenditure through sympathetic stimulation. It also counteracts the starvation-induced suppression of thyroid hormone by up-regulating the expression of TRH. On the other hand, it is known that the extrahypothalamic TRH system participates in cardiovascular function modulating sympathetic system activity. In order to challenge the testable hypothesis that obesity may raise arterial blood pressure (ABP) through TRH system activation, we herein analyze the participation of the TRH system in the elevation of ABP in the obese agouti yellow mice. These mice are characterized by resistance to the weight reducing effect of leptin although they show a preserved sympathetic response to leptin along with a mild hypertension compared with the control strain (121+/-2 vs 102+/-2 mmHg, p less than 0.001, n=10). We report here that hyperleptinemic agouti mice showed a 1.8-fold elevation of diencephalic TRH content compared with controls, and we demonstrate that a long lasting specific inhibition of TRH system by icv treatment with siRNA against preproTRH normalizes systolic ABP independently of the thyroid status. These results suggest that the interaction leptin-diencephalic TRH may be one of the mechanisms involved in the mild hypertension of the obese agouti mice.

Thyrotropin-Releasing Hormone Precursor Gene Knocking Down Impedes Melanocortin-Induced Hypertension in Rats

To the Editor: Recently, da Silva et al1 reported that endogenous melanocortin may cause elevation of arterial blood pressure (ABP) in spontaneously hypertensive rats. We invite authors to consider that the hypothalamic thyrotropin-releasing hormone (TRH) system may be involved, because spontaneously hypertensive rats show an hyperactivity of this system,2 and intracerebroventricular (ICV) injections of a prepro-TRH antisense oligonucleotide (AS) decreases both elevated TRH content and ABP independent of thyroid status.3 Leptin effects include increases in sympathetic activity …

Parathyroid Hormone-Related Protein Overexpression Decreases Blood Pressure in Spontaneously Hypertensive Rats

We have recently demonstrated that arterial PTHrP expression and cardiovascular responses to this protein are altered in SHR compared with normotensive animals, Wistar Kyoto (WKY) and Sprague-Dawley (SD) rats. To investigate whether the slightly, but significantly decreased, aortic PTHrP gene expression observed in SHR, compared to that of normotensive animals, may play a causative role in the maintenance of the elevated arterial blood pressure (ABP) of the SHR, we transfected a hepatic lobe with a PTHrP expression vector in a sense and antisense orientation. At 24 and 48 hours, sense pSV2neo-ECE induced a significant five-fold increase in PTHrP mRNA abundance with respect to antisense pSV2neo-ECE and vehicle. This increment in the PTHrP mRNA induced by the sense PTHrP expression vector was totally inhibited by the co-administration of the antisense PTHrP expression vector. At the same time, we observed a significant decrease of mean ABP (MABP) in SHR transfected with the sense pSV2neo-ECE to similar values as those obtained in the normotensive strain. Neither antisense PTHrP expression vector nor vehicle had any significant effect in any strain. Again, the effect of the sense PTHrP expression vector on MABP was blocked by the simultaneous treatment with the antisense PTHrP expression vector. At 48 hours, the hypotensive effect of the sense pSV2neo-ECE in SHR was reverted by the IV bolus injection of a specific competitive PTHrP receptor antagonist such as Nle8,18,Tyr34-bPTH(3–34)amide. We propose that a defect of this potent local vasodilator may contribute to the development and/or maintenance of arterial hypertension in SHR. This defect can be ameliorated by transfecting tissues with protein-exporting capabilities, such as the liver. Finally, our work adds additional data to a cumulative body of evidence suggesting that it might be possible to design an effective gene therapy to treat the common polygenic and multifactorial form of hypertension by increasing the activity of potent and physiological vasodilators.

Letter by Landa et al Regarding Article, “Protein Tyrosine Phosphatase 1B, a Major Regulator of Leptin-Mediated Control of Cardiovascular Function”

To the Editor: Recently, Belin de Chantemele et al1 reported that protein tyrosine phosphatase 1B may be a major player in the leptin-mediated control of arterial blood pressure (ABP) in mice. We invite the authors to consider that the hypothalamic thyrotropin-releasing hormone (TRH) system may be involved in this pathway. Leptin effects include increases in sympathetic activity and inhibition of the starvation-induced suppression of thyroid hormones by upregulating preproTRH gene expression. Furthermore, we showed that intracerebroventricular leptin injections induce …