Thyroid hormones and 3,5-diiodothyropropionic acid: new keys for new locks

Partial Resistance to Thyroid Hormone-Induced Tachycardia and Cardiac Hypertrophy in Mice Lacking Thyroid Hormone Receptor β

Background: Thyroid hormones regulate cardiac functions mainly through direct actions in the heart and by binding to the thyroid hormone receptor (TR) isoforms α1 and β. While the role of the most abundantly expressed isoform, TRα1, is widely studied and well characterized, the role of TRβ in regulating heart functions is still poorly understood, primarily due to the accompanying elevation of circulating thyroid hormone in TRβ knockout mice (TRβ-KO). However, their hyperthyroidism is ameliorated at thermoneutrality, which allows studying the role of TRβ without this confounding factor. Methods: Here, we noninvasively monitored heart rate in TRβ-KO mice over several days using radiotelemetry at different housing temperatures (22°C and 30°C) and upon 3,3',5-triiodothyronine (T3) administration in comparison to wild-type animals. Results: TRβ-KO mice displayed normal average heart rate at both 22°C and 30°C with only minor changes in heart rate frequency distribution, which was confirmed by independent electrocardiogram recordings in freely-moving conscious mice. Parasympathetic nerve activity was, however, impaired in TRβ-KO mice at 22°C, and only partly rescued at 30°C. As expected, oral treatment with pharmacological doses of T3 at 30°C led to tachycardia in wild-types, accompanied by broader heart rate frequency distribution and increased heart weight. The TRβ-KO mice, in contrast, showed blunted tachycardia, as well as resistance to changes in heart rate frequency distribution and heart weight. At the molecular level, these observations were paralleled by a blunted cardiac mRNA induction of several important genes, including the pacemaker channels Hcn2 and Hcn4, as well as Kcna7. Conclusions: The phenotyping of TRβ-KO mice conducted at thermoneutrality allows novel insights on the role of TRβ in cardiac functions in the absence of the usual confounding hyperthyroidism. Even though TRβ is expressed at lower levels than TRα1 in the heart, our findings demonstrate an important role for this isoform in the cardiac response to thyroid hormones.

Thyroid disorders and cardiovascular manifestations: an update

Cardiovascular disease (CVD) remains the leading cause of death worldwide, representing a major health, social, and economic issue. Thyroid disorders are very common and affect >10% of the adult population in total. The aim of this review is to describe the physiologic role of thyroid hormones on cardiovascular system, to present cardiovascular manifestations in patients with thyroid disorders, emphasizing in molecular mechanisms and biochemical pathways, and to summarize current knowledge of treatment options. Thyroid hormone receptors are located both in myocardium and vessels, and changes in their concentrations affect cardiovascular function. Hyperthyroidism or hypothyroidism, both clinical and subclinical, without the indicated therapeutical management, may contribute to the progression of CVD. According to recent studies, even middle changes in thyroid hormones levels increase cardiovascular mortality from 20% to 80%. In more details, thyroid disorders seem to have serious effects on the cardiovascular system via plenty mechanisms, including dyslipidemia, hypertension, systolic and diastolic myocardial dysfunction, as well endothelial dysfunction. On top of clinical thyroid disorders management, current therapeutics focus on younger patients with subclinical hypothyroidism and elderly patients with subclinical hyperthyroidism.

Hypertension and Hyperthyroidism: Association and Pathogenesis

Hypertension can cause significant morbidity and reduced life expectancy. Most patients with hypertension have primary hypertension; however, 10 to 15% of patients have secondary hypertension. Endocrine disorders explain approximately 10% of hypertension in all patients, and thyroid disorders account for approximately 1% of cases with hypertension. Hyperthyroidism can cause increased cardiac output, increased systolic blood pressures, and increased levels of renin, angiotensin, and aldosterone. Treatment of hyperthyroidism can cure hypertension in some patients. Consequently, identification of patients with secondary hypertension potentially has important benefits, and understanding secondary hypertension provides a framework for investigating the pathophysiology of hypertension. Clinicians should consider the possibility of hyperthyroidism in patients with hypertension, even in those of more advanced age.

Crosstalk between integrin αvβ3 and estrogen receptor-α is involved in thyroid hormone-induced proliferation in human lung carcinoma cells

A cell surface receptor for thyroid hormone that activates extracellular regulated kinase (ERK) 1/2 has been identified on integrin αvβ3. We have examined the actions of thyroid hormone initiated at the integrin on human NCI-H522 non-small cell lung carcinoma and NCI-H510A small cell lung cancer cells. At a physiologic total hormone concentration (10(-7) M), T(4) significantly increased proliferating cell nuclear antigen (PCNA) abundance in these cell lines, as did 3, 5, 3'-triiodo-L-thyronine (T(3)) at a supraphysiologic concentration. Neutralizing antibody to integrin αvβ3 and an integrin-binding Arg-Gly-Asp (RGD) peptide blocked thyroid hormone-induced PCNA expression. Tetraiodothyroacetic acid (tetrac) lacks thyroid hormone function but inhibits binding of T(4) and T(3) to the integrin receptor; tetrac eliminated thyroid hormone-induced lung cancer cell proliferation and ERK1/2 activation. In these estrogen receptor-α (ERα)-positive lung cancer cells, thyroid hormone (T(4)>T(3)) caused phosphorylation of ERα; the specific ERα antagonist ICI 182,780 blocked T(4)-induced, but not T(3)-induced ERK1/2 activation, as well as ERα phosphorylation, proliferating-cell nuclear antigen (PCNA) expression and hormone-dependent thymidine uptake by tumor cells. Thus, in ERα-positive human lung cancer cells, the proliferative action of thyroid hormone initiated at the plasma membrane is at least in part mediated by ERα. In summary, thyroid hormone may be one of several endogenous factors capable of supporting proliferation of lung cancer cells. Activity as an inhibitor of lung cancer cell proliferation induced at the integrin receptor makes tetrac a novel anti-proliferative agent.