Thyrotropin-releasing hormone in rat heart: effect of swelling, angiotensin II and renin gene

Novel Leptin-Cardiac TRH pathway responsible for the cardiac alterations in the Hyperleptinemic obesity

The association between hypertension and obesity-induced cardiac damage is usually accepted. However, no studies have been focused on cardiac alterations in obesity, independently of blood pressure increase. It is well known that Cardiac TRH induces Left Ventricular Hypertrophy (LVH) and fibrosis, and its inhibition prevents the development of hypertrophy. Also, it has been described that the adiponectin leptin induces TRH expression. Thus, we hypothesized that in obesity, the increase in TRH induced by hyperleptinemia is responsible for LVH, until now mostly attributed to pressure load. We studied obese Agouti mice suffering from hypertension with hyperleptinemia and found a significant LVH development with increased TRH gene expression. Consequently, we found higher fibrotic (collagens and TGF-β) and hypertrophic markers (BNP and β-MHC) expression vs lean black controls. As pressure could explain these results, we treated obese mice with diuretic (hydrochlorothiazide 20 mg/kg/day) since weaning. Diuretic treatment was successful as the diuretic group was normotensive in contrast to control obese mice. Nevertheless, both groups showed LVH development, higher cardiac precursor TRH gene and peptide expressions and elevated fibrotic and hypertrophic markers expression, pointing out that obesity-induced LVH is not due to hypertension. In addition, we performed Cardiac TRH inhibition by specific siRNA injection compared to control siRNA treatment and evaluated cardiac damage. As expected, expressions and protein increase in hypertrophic and fibrotic markers observed in the AG mouse with the native cTRH system were not seen in the AG mouse with the cTRH silencing. Indeed, the AG + TRH-siRNA group showed hypertrophic markers expression and fibrosis measurements similar to the lean BL mice. On the whole, these results point out that the novel Leptin-Cardiac TRH pathway is responsible for the cardiac alterations present in hyperleptinemic obesity, independent of blood pressure, and cTRH long-term silencing since early stages totally prevent LVH development and cardiac fibrosis.

Cardiac Thyrotropin-releasing Hormone Inhibition Improves Ventricular Function and Reduces Hypertrophy and Fibrosis After Myocardial Infarction in Rats

BACKGROUND

Cardiac thyrotropin-releasing hormone (TRH) is a tripeptide with still unknown functions. We demonstrated that the left ventricle (LV) TRH system is hyperactivated in spontaneously hypertensive rats and its inhibition prevented cardiac hypertrophy and fibrosis. Therefore, we evaluated whether in vivo cardiac TRH inhibition could improve myocardial function and attenuate ventricular remodeling in a rat model of myocardial infarction (MI).

METHODS AND RESULTS

In Wistar rats, MI was induced by a permanent left anterior descending coronary artery ligation. A coronary injection of a specific small interfering RNA against TRH was applied simultaneously. The control group received a scrambled small interfering RNA. Cardiac remodeling variables were evaluated one week later. In MI rats, TRH inhibition decreased LV end-diastolic (1.049 ± 0.102 mL vs 1.339 ± 0.102 mL, P < .05), and end-systolic volumes (0.282 ± 0.043 mL vs 0.515 ± 0.037 mL, P < .001), and increased LV ejection fraction (71.89 ± 2.80% vs 65.69 ± 2.85%, P < .05). Although both MI groups presented similar infarct size, small interfering RNA against TRH treatment attenuated the cardiac hypertrophy index and myocardial interstitial collagen deposition in the peri-infarct myocardium. These effects were accompanied by attenuation in the rise of transforming growth factor-β, collagen I, and collagen III, as well as the fetal genes (atrial natriuretic peptide, B-type natriuretic peptide, and beta myosin heavy chain) expression in the peri-infarct region. In addition, the expression of Hif1α and vascular endothelial growth factor significantly increased compared with all groups.

CONCLUSIONS

Cardiac TRH inhibition improves LV systolic function and attenuates ventricular remodeling after MI. These novel findings support the idea that TRH inhibition may serve as a new therapeutic strategy against the progression of heart failure.

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.