Downregulation of vascular angiotensin II type 1 receptor by thyroid hormone

Angiotensin II cell signaling: physiological and pathological effects in the cardiovascular system

The renin-angiotensin system is a central component of the physiological and pathological responses of cardiovascular system. Its primary effector hormone, angiotensin II (ANG II), not only mediates immediate physiological effects of vasoconstriction and blood pressure regulation, but is also implicated in inflammation, endothelial dysfunction, atherosclerosis, hypertension, and congestive heart failure. The myriad effects of ANG II depend on time (acute vs. chronic) and on the cells/tissues upon which it acts. In addition to inducing G protein- and non-G protein-related signaling pathways, ANG II, via AT(1) receptors, carries out its functions via MAP kinases (ERK 1/2, JNK, p38MAPK), receptor tyrosine kinases [PDGF, EGFR, insulin receptor], and nonreceptor tyrosine kinases [Src, JAK/STAT, focal adhesion kinase (FAK)]. AT(1)R-mediated NAD(P)H oxidase activation leads to generation of reactive oxygen species, widely implicated in vascular inflammation and fibrosis. ANG II also promotes the association of scaffolding proteins, such as paxillin, talin, and p130Cas, leading to focal adhesion and extracellular matrix formation. These signaling cascades lead to contraction, smooth muscle cell growth, hypertrophy, and cell migration, events that contribute to normal vascular function, and to disease progression. This review focuses on the structure and function of AT(1) receptors and the major signaling mechanisms by which angiotensin influences cardiovascular physiology and pathology.

Thyroid hormone inhibits vascular remodeling through suppression of cAMP response element binding protein activity

OBJECTIVE

Although accumulating evidences suggest that impaired thyroid function is a risk for ischemic heart disease, the molecular mechanism of anti-atherosclerotic effects of thyroid hormone is poorly defined. We examined whether thyroid hormone affects signaling pathway of angiotensin II (Ang II), which is critically involved in a broad aspect of cardiovascular disease process.

METHODS AND RESULTS

3,3',5-triiodo-L-thyronine (T3) did not show a significant effect on Ang II-induced activation of extracellular signal-regulated protein kinase or p38 mitogen-activated protein kinase in vascular smooth muscle cells (VSMCs), whereas T3 inhibited Ang II-induced activation of cAMP response element (CRE) binding protein (CREB), a nuclear transcription factor involved in the vascular remodeling process. Coimmunoprecipitaion assay revealed the protein-protein interaction between thyroid hormone receptor and CREB. T3 reduced an expression level of interleukin (IL)-6 mRNA, CRE-dependent promoter activity, and protein synthesis induced by Ang II. Administration of T3 (100 microg/100 g for 14 days) to rats attenuated neointimal formation after balloon injury of carotid artery with reduced CREB activation and BrdU incorporation.

CONCLUSIONS

These results suggested that T3 inhibits CREB/CRE signaling pathway and suppresses cytokine expression and VSMCs proliferation, which may account for, at least in part, an anti-atherosclerotic effect of thyroid hormone.

Regulation of iodothyronine deiodinase and roles of thyroid hormones in human coronary artery smooth muscle cells

Thyroid hormones have been reported to have significant effects on the peripheral vascular system, including relaxation of vascular smooth muscle cells and prevention of atherosclerosis. To exert its biological activity, thyroxine (T4) needs to be converted to 3,5,3'-triiodothyronine (T3) by type 1 and type 2 iodothyronine deiodinases. We have previously identified type 2 iodothyronine deiodinase (D2) expression in cultured human coronary artery smooth muscle cells (hCASMCs). In the present study, we have characterized the regulation of D2 expression in hCASMCs by stable prostacyclin analogue beraprost sodium (BPS) and platelet derived growth factor (PDGF), and the roles of thyroid hormones in the functions of hCASMCs. BPS increased D2 expression, whereas PDGF suppressed BPS stimulated D2 expression without affecting cAMP production in hCASMCs. PDGF increased DNA synthesis, while BPS, T3 or T4 suppressed PDGF stimulated DNA synthesis in hCASMCs. Inhibition of D2 activity by 3,3',5'-triiodothyronine (rT3) partially restored T4 suppression of PDGF stimulated DNA synthesis in hCASMCs. PDGF increased migration activity, whereas BPS, T3 or T4 suppressed PDGF stimulated migration activity of hCASMCs. These results suggest that D2 expression is increased by BPS and suppressed by PDGF in hCASMCs, and that intracellular thyroid hormone activation may be involved in the suppression of DNA synthesis and migration activity of hCASMCs.

Post-transcriptional downregulation of sarcolipin mRNA by triiodothyronine in the atrial myocardium

Thyroid hormone-mediated positive cardiotropic effects are differently regulated between the atria and ventricles. This regulation is, at least in part, dependent on sarcoplasmic reticulum (SR) proteins. Sarcolipin, a homologue of phospholamban, has been recently identified as an atrium-specific SR protein. The expression of sarcolipin mRNA was significantly decreased in the atria of mice with hyperthyroidism and in 3,5,3'-triiodo-l-thyronine-treated neonatal rat atrial myocytes. Promoter activity and mRNA stability analyses revealed that thyroid hormone post-transcriptionally down regulated the expression of sarcolipin mRNA. The atrium-specific effect of thyroid hormone may occur in part through the regulation of atrial sarcolipin gene expression.

[Cellular mechanisms of thyroid hormone action]

KEY POINTS

Thyroid hormones affect cardiac myocytes as well as the smooth muscle and endothelial cells of the vascular wall. Free 3,53'-L-triiodothyronine (FT3) and its specific nuclear receptor modulate the transcription of various proteins, principally those involved in the myocyte contractile apparatus (myosin heavy chains), and the regulation of intracellular calcium flux (sarcoplasmic reticulum Ca2+ATPase). Thyroid hormones also have non-genomic effects that work rapidly, complement the effects described above, and are related to alterations in the properties of many channels and membrane receptors, especially in the sinoatrial mode. Thyroid hormones also affect the smooth muscle and endothelial cells of the vascular walls and reduce systemic vascular resistance. These effects on cardiac and vascular cells globally explain the cardiac manifestations (especially the inotropic and chronotropic effects) observed during dysthyroidism, particularly in hyperthyroidism where they are often in the forefront (positive inotropic and chronotropic effects).

Thyroid hormone targets matrix Gla protein gene associated with vascular smooth muscle calcification

Thyroid hormones have marked cardiovascular effects in vivo. However, their direct effects on vascular smooth muscle cells have been unclear. Because thyroid hormones play critical roles in bone remodeling, we hypothesized that they are also associated with vascular smooth muscle calcification, one of the pathological features of vascular sclerosis. To test this hypothesis, we examined the effects of 3',3,5-triiodo-L-thyronine (T3) on the expression of calcification-associated genes in rat aortic smooth muscle cells (RAOSMCs). Quantitative RT-PCRs revealed that a physiological concentration of T3 (15 pmol/L free T3) increased mRNA level of matrix Gla protein (MGP), which acts as a potent inhibitor of vascular calcification in vivo, by 3-fold in RAOSMCs, as well as in cultured human coronary artery smooth muscle cells. In RAOSMCs transiently transfected with a luciferase reporter gene driven by the MGP promoter, T3 significantly stimulated luciferase activity. In addition, RNA interference against thyroid hormone receptor-alpha gene diminished the effect of T3 on MGP expression. Aortic smooth muscle tissues from methimazole-induced hypothyroid rats (400 mg/L drinking water; 4 weeks) also showed a 68% decrease in the MGP mRNA level, as well as a 33% increase in calcium content compared with that from the control euthyroid animals, whereas hyperthyroidism (0.2 mg T3/kg IP; 10 days) upregulated MGP mRNA by 4.5-fold and reduced calcium content by 11%. Our findings suggest that a physiological concentration of thyroid hormone directly facilitates MGP gene expression in smooth muscle cells via thyroid hormone nuclear receptors, leading to prevention of vascular calcification in vivo.