Molecular Mechanisms of the Relationship between Thyroid Dysfunctions and Diabetes Mellitus

Type 1 and type 2 diabetes mellitus (DM) are known to increase the incidence of thyroid gland (TG) dysfunctions. The review addresses the literature data and our experimental results on the molecular mechanisms that underlie thyroid disorders under DM. Most important of these mechanisms are the attenuation of thyrocyte adenylyl cyclase signaling system sensitivity to thyroid-stimulating hormone, the decrease in the number of thyroid hormone receptors in peripheral tissues, and the decline in activity as well as changes in the ratio of different deiodinase forms in these tissues. Decreased activity of D2 deiodinases, which convert thyroxine into the active form of triiodothyronine, is associated with the development of insulin resistance, while decreased activity of D3 deiodinases, which catalyze inactivation of triiodothyronine in pancreatic β cells, suppresses insulin secretion and leads to insulin deficiency. Thus, both the excess and the deficiency of thyroid hormones can entail diabetic pathology. Identification of thyroid disorders is of utmost importance for elaborating novel approaches to treat and prevent thyroid diseases associated with type 1 and type 2 DM.     

Roles of thyroid, adrenal and pancreatic hormones on thyroid activity of the soft-shelled turtles Lissemys punctata punctata Bonnoterre

The effects of some exogenous peripheral hormones (thyroxine, corticosterone, epinephrine, norepinephrine and insulin) on thyroid activity were investigated in juvenile female soft-shelled turtles, Lissemys punctata punctata. Each hormone was injected in three different doses (25 microg, 50 microg or 100 microg each per 100 g body weight, once daily at 9 AM) for 10 consecutive days. Thyroid activity was evaluated by gravimetry, histology (epithelial height) and thyroperoxidase assay. The findings revealed that thyroxine in low dose (25 microg) stimulated thyroid activity by increasing the relative thyroid weight, epithelial height and thyroperoxidase activity, but inhibited gland activity at a high dose (100 microg) by decreasing the values of all these parameters. The medium dose (50 microg) had no significant effect. All other hormones, in all doses, significantly decreased thyroid activity by decreasing the values of all the parameters. Thyroid responses to exogenous hormones are generally dose-dependent in turtles. The mechanisms of actions of the hormones administered are suggested.     

Thyroid hormones and the cardiomyocytes

The authors present the current knowledge on the intracellular mechanisms of thyroid hormone action in the cardiomyocytes. Many of the clinical manifestations of thyroid diseases are due to the ability of thyroid hormone to alter cardiovascular hemodynamics. Triiodothyronine affects the hemodynamic state mainly by its influence on the expression of cardiomyocyte genes. These genes encode both structural and regulatory proteins in the heart (myosin heavy chains, sarcoplasmic reticulum calcium-activated ATP-ase, phospholamban). The impaired myocardium contractile activity in hypothyreosis reminds findings in heart failure and may warrant further exploration of therapeutic approaches using thyroid hormone to improve cardiac function in heart failure.     

The immune system as a regulator of thyroid hormone activity

It has been known for decades that the neuroendocrine system can both directly and indirectly influence the developmental and functional activity of the immune system. In contrast, far less is known about the extent to which the immune system collaborates in the regulation of endocrine activity. This is particularly true for immune-endocrine interactions of the hypothalamus-pituitary-thyroid axis. Although thyroid-stimulating hormone (TSH) can be produced by many types of extra-pituitary cells--including T cells, B cells, splenic dendritic cells, bone marrow hematopoietic cells, intestinal epithelial cells, and lymphocytes--the functional significance of those TSH pathways remains elusive and historically has been largely ignored from a research perspective. There is now, however, evidence linking cells of the immune system to the regulation of thyroid hormone activity in normal physiological conditions as well as during times of immunological stress. Although the mechanisms behind this are poorly understood, they appear to reflect a process of local intrathyroidal synthesis of TSH mediated by a population of bone marrow cells that traffic to the thyroid. This hitherto undescribed cell population has the potential to microregulate thyroid hormone secretion leading to critical alterations in metabolic activity independent of pituitary TSH output, and it has expansive implications for understanding mechanisms by which the immune system may act to modulate neuroendocrine function during times of host stress. In this article, the basic underpinnings of the hematopoietic-thyroid connection are described, and a model is presented in which the immune system participates in the regulation of thyroid hormone activity during acute infection.     

Presence of adrenocorticotropin-potentiating factors in porcine thyroid glands

An extract of porcine thyroid gland in 0.1 N acetic acid exerted dose-dependent potentiation of ACTH-induced corticosterone production in isolated rat adrenal cells. The extract by itself manifested no steroidogenic activity. Upon gel-filtration of the extract, potentiating activities were demonstrated in three main peaks with molecular weights of about 10,000, 5,000 and 2,000. These findings indicate the presence of heterogeneous forms of ACTH-potentiating factors in the thyroid. Significant enhancement of ACTH-induced steroidogenesis was readily apparent with three gel-filtration fractions at a lower concentration of ACTH (4.75 pM). At this concentration, dose-dependent potentiation was observed with these three fractions. Enhanced corticosterone production responses by cells preincubated with the thyroid extract were observed and the results indicated the existence of potentiating mechanisms other than inhibition of ACTH proteolysis. The lack of T4, T3 and thyroglobulin in this activity suggests that the activity resides in other constituents of the thyroid.     

Thyroid hormones in small ruminants: effects of endogenous, environmental and nutritional factors

Appropriate thyroid gland function and thyroid hormone activity are considered crucial to sustain the productive performance in domestic animals (growth, milk or hair fibre production). Changes of blood thyroid hormone concentrations are an indirect measure of the changes in thyroid gland activity and circulating thyroid hormones can be considered as indicators of the metabolic and nutritional status of the animals. Thyroid hormones play a pivotal role in the mechanisms permitting the animals to live and breed in the surrounding environment. Variations in hormone bioactivity allow the animals to adapt their metabolic balance to different environmental conditions, changes in nutrient requirements and availability, and to homeorhetic changes during different physiological stages. This is particularly important in the free-ranging and grazing animals, such as traditionally reared small ruminants, whose main physiological functions (feed intake, reproduction, hair growth) are markedly seasonal. Many investigations dealt with the involvement of thyroid hormones in the expression of endogenous seasonal rhythms, such as reproduction and hair growth cycles in fibre-producing (wool, mohair, cashmere) sheep and goats. Important knowledge about the pattern of thyroid hormone metabolism and their role in ontogenetic development has been obtained from studies in the ovine foetus and in the newborn. Many endogenous (breed, age, gender, physiological state) and environmental factors (climate, season, with a primary role of nutrition) are able to affect thyroid activity and hormone concentrations in blood, acting at the level of hypothalamus, pituitary and/or thyroid gland, as well as on peripheral monodeiodination. Knowledge on such topics mirror physiological changes and possibly allows the monitoring and manipulation of thyroid physiology, in order to improve animal health, welfare and production.     

Evidence for a role of the cytoskeleton in the in vitro folliculogenesis of the thyroid gland of the fetal rat

Summary Thyrotropic hormone (TSH) or cAMP accelerate the formation of follicular cavities in the explanted thyroid gland of the 15-day-old rat fetus. Cytochalasin B or vinblastine and nocodazole or colchicine, which disorganize microfilamental and microtubular structures respectively, inhibit or completely block in vitro-induced folliculogenesis. Exposure of the thyroid tissue to lumicolchicine, a structural isomer of colchicine deprived of antimicrotubular activity, does not inhibit the activation of folliculogenesis induced by TSH. These results are strong evidence for the supposition that microfilaments and microtubules are involved in the TSH-stimulated mechanisms resulting in thyroid folliculogenesis. Folliculogenesis requires the integrity of both microfilaments and microtubules.     

Thyroid hormone stimulation of NADPH P450 reductase expression in liver and extrahepatic tissues. Regulation by multiple mechanisms.

The role of thyroid hormone in regulating the expression of the flavoprotein NADPH cytochrome P450 reductase was studied in adult rats. Depletion of circulating thyroid hormone by hypophysectomy, or more selectively, by treatment with the anti-thyroid drug methimazole led to a 75-85% depletion of hepatic microsomal P450 reductase activity and protein in both male and female rats. Thyroxine substantially restored P450 reductase activity at a dose that rendered the thyroid-depleted rats euthyroid. Microsomal P450 reductase activity in several extrahepatic tissues was also dependent on thyroid hormone, but to a lesser extent than in liver (30-50% decrease in kidney, adrenal, lung, and heart but not in testis from hypothyroid rats). Hepatic P450 reductase mRNA levels were also decreased in the hypothyroid state, indicating that the loss of P450 reductase activity is not a consequence of the associated decreased availability of the FMN and FAD cofactors of P450 reductase. Parallel analysis of S14 mRNA, which has been studied extensively as a model thyroid-regulated liver gene product, indicated that P450 reductase and S14 mRNA respond similarly to these changes in thyroid state. In contrast, while the expression of S14 and several other thyroid hormone-dependent hepatic mRNAs is stimulated by feeding a high carbohydrate, fat-free diet, hepatic P450 reductase expression was not increased by this lipogenic diet. Injection of hypothyroid rats with T3 at a supraphysiologic, receptor-saturating dose stimulated a major induction of hepatic P450 reductase mRNA that was detectable 4 h after the T3 injection, and peaked at approximately 650% of euthyroid levels by 12 h. However, this same treatment stimulated a biphasic increase in P450 reductase protein and activity that required 3 days to reach normal euthyroid levels. T3 treatment of euthyroid rats also stimulated a major induction of P450 reductase mRNA that was maximal (12-fold increase) by 12 h, but in this case no major increase in P450 reductase protein or activity was detectable over a 3-day period. Together, these studies establish that thyroid hormone regulates P450 reductase expression by pretranslational mechanisms. They also suggest that other regulatory mechanisms, which may involve changes in P450 reductase protein stability and/or changes in the translational efficiency of its mRNA, are likely to occur.     

Regression of thyroid hormone induced cardiac hypertrophy: Effect on cardiac beta receptors and adenyl cyclase activity

Adrenergic mechanisms may be important in the symptomatic manifestations of hyperthyroidism. The chronic administration of thyroid hormone also results in cardiac hypertrophy and increased numbers of beta-adrenergic receptors in cardiac membranes. The roles of adrenergic mechanisms in the initiation and perpetuation of this hypertrophy has been open to speculation. Rats treated chronically with L-thyroxin were sacrificed after 7 days of treatment and 1–4 days after cessation of treatment. Hearts were removed and weighed and norepinephrine measured. In other groups of identically treated rats, membranes were prepared from the left ventricle for $\text{invitro}$ measurements of beta-adrenergic receptor characteristics and adenyl cyclase activity. Regression of cardiac hypertrophy with a decrease in receptor number to control values was seen as early as 2 days after stopping thyroxine. Cardiac norepinephrine concentrations had also returned to control values at this time. Displacement of bound [H³] dihydroalprenolol by isoproterenol was not changed from control. Basal and isoproterenol stimulated adenyl cyclase activity was not changed by thyroxine administration or its cessation.The rapid reversal of the increased beta-adrenergic receptor number and cardiac hypertrophy raises the possibility that thyroid hormone may play a regulatory role in cardiac function. Although the enhancement of myocardial contractility by thyroid hormone may be mediated through cardiac hypertrophy this effect of thyroid hormone is independent of the catecholamine sensitive adenyl cyclase system.     

Autoimmunity and hypothyroidism

Primary myxedema and hypothyroid Hashimoto's disease provide a well-documented example of organ-specific autoimmunity in man. Very slight modifications or increased release of thyroglobulin or thyroid antigens in the circulation may cause the rupture of autotolerance for the normal thyroid components, at least when individuals have a genetic predisposition to autoimmune thyroiditis (possibly associated with a predisposition to other autoimmune diseases). The demonstration of an association between HLA and thyroiditis, however, requires additional studies. The basic immunological abnormality responsible for autoimmunization against thyroid components is a defect in suppressor T cells, shown in experimental animals but not firmly established in man. The result of autoimmunization will be the appearance of cytotoxic mechanisms that lead to destruction of the thyroid follicle with progressive fibrosis, antibody-dependent cell-mediated cytotoxicity apparently being of major importance. A recent report shows, in addition, that thyroid atrophy in primary hypothyroidism is associated with the production of antibodies that block the thyroid-growth-promoting activity of TSH. The recent progress made in our understanding of autoimmune thyroiditis will certainly contribute to improving our knowledge of how and when autoimmunization might develop in man.     

RET tyrosine kinase signaling in development and cancer

The variety of diseases caused by mutations in RET receptor tyrosine kinase provides a classic example of phenotypic heterogeneity. Gain-of-function mutations of RET are associated with human cancer. Gene rearrangements juxtaposing the tyrosine kinase domain to heterologous gene partners have been found in sporadic papillary carcinomas of the thyroid (PTC). These rearrangements generate chimeric RET/PTC oncogenes. In the germline, point mutations of RET are responsible for multiple endocrine neoplasia type 2 (MEN 2A and 2B) and familial medullary thyroid carcinoma (FMTC). Both MEN 2 mutations and PTC gene rearrangements potentiate the intrinsic tyrosine kinase activity of RET and, ultimately, activate the RET downstream targets. Loss-of-function mutations of RET cause Hirschsprung's disease (HSCR) or colonic aganglionosis. A deeper understanding of the molecular signaling of normal versus abnormal RET activity in cancer will enable the development of potential new treatments for patients with sporadic and inherited thyroid cancer or MEN 2 syndrome. We now review the role and mechanisms of RET signaling in development and carcinogenesis.     

Mechanistic considerations for the relevance of animal data on thyroid neoplasia to human risk assessment

There are two basic mechanisms whereby chemicals produce thyroid gland neoplasia in rodents. The first involves chemicals that exert a direct carcinogenic effect in the thyroid gland and the other involves chemicals which, through a variety of mechanisms, disrupt thyroid function and produce thyroid gland neoplasia secondary to hormone imbalance. These secondary mechanisms predominantly involve effects on thyroid hormone synthesis or peripheral hormone disposition. There are important species differences in thyroid gland physiology between rodents and humans that may account for a marked species difference in the inherent susceptibility for neoplasia to hormone imbalance. Thyroid gland neoplasia, secondary to chemically induced hormone imbalance, is mediated by thyroid-stimulating hormone (TSH) in response to altered thyroid gland function. The effect of TSH on cell proliferation and other aspects of thyroid gland function is a receptor mediated process and the plasma membrane surface of the follicular cell has receptors for TSH and other growth factors. Small organic molecules are not known to be direct TSH receptor agonists or antagonists; however, various antibodies found in autoimmune disease such as Graves' disease can directly stimulate or inhibit the TSH receptor. Certain chemicals can modulate the TSH response for autoregulation of follicular cell function and thereby increase or decrease the response of the follicular cell to TSH. It is thus important to consider mechanisms for the evaluation of potential cancer risks. There would be little if any risk for non-genotoxic chemicals that act secondary to hormone imbalance at exposure levels that do not disrupt thyroid function. Furthermore, the degree of thyroid dysfunction produced by a chemical would present a significant toxicological problem before such exposure would increase the risk for neoplasia in humans.     

Degradation and biological inactivation of thyrotropin releasing hormone (TRH): regulation of the membrane-bound TRH-degrading enzyme from rat anterior pituitary by estrogens and thyroid hormones

Thyrotropin releasing hormone (TRH, pyroGlu-His-Pro-NH2) is important in the regulation of adenohypophyseal hormone secretion and also serves important functions in extrahypothalamic brain areas, indicating that it is involved in neurotransmission and other forms of cellular communication. This hypothesis is strengthened by the observation that TRH is rapidly inactivated by a heterogeneously distributed ecto-enzyme which exhibits a high degree of substrate specificity. Moreover, in the rat, the activity of the membrane-bound TRH-degrading enzyme of the anterior pituitary is found to be stringently controlled by thyroid hormones and estrogens. In contrast, the activity of the TRH-degrading brain enzyme is neither influenced by thyroid hormones nor estrogens. These data indicate that the TRH-degrading brain enzyme serves specialized functions for the transmission of TRH signals and apparently represents the peptidergic equivalent to acetylcholine esterase, whereas the membrane-bound adenohypophyseal TRH-degrading enzyme itself fulfills a biologically important control function within feedback-regulatory mechanisms.     

Role of iodine in antioxidant defence in thyroid and breast disease

The role played in thyroid hormonogenesis by iodide oxidation to iodine (organification) is well established. Iodine deficiency may produce conditions of oxidative stress with high TSH producing a level of H_2O_2, which because of lack of iodide is not being used to form thyroid hormones. The cytotoxic actions of excess iodide in thyroid cells may depend on the formation of free radicals and can be attributed to both necrotic and apoptotic mechanisms with necrosis predominating in goiter development and apoptosis during iodide induced involution. These cytotoxic effects appear to depend on the status of antioxidative enzymes and may only be evident in conditions of selenium deficiency where the activity of selenium containing antioxidative enzymes is impaired. Less compelling evidence exists of a role for iodide as an antioxidant in the breast. However the Japanese experience may indicate a protective effect against breast cancer for an iodine rich seaweed containing diet. Similarly thyroid autoimmunity may also be associated with improved prognosis. Whether this phenomenon is breast specific and its possible relationship to iodine or selenium status awaits resolution.     

Thyroglobulin may affect telomerase activity in thyroid follicular cells

Telomerase (TA) activity is known to be present in malignant tumor cells, but not in most somatic differentiated cells. TA shows relatively high activity in thyroid cancer cells, but reports vary. This fact prompted us to elucidate whether cell component inhibitors of TA in the thyroid follicles can modulate its activity. The activity of TA extracted from Hela cells was inhibited by mixing with the supernatant fraction of human thyroid tissue extract. To examine the effect of iodine, thyroid hormones (l-T3 and l-T4) and human thyroglobulin (hTg) contained in the thyroid follicles, l-T3, l-T4 and hTg were added to the TRAP assay system in vitro, using TA from Hela cells. Iodine, l-T3 and l-T4 did not affect TA activity, but hTg inhibited the TA activity in a dose-dependent manner (IC(50) of hTg: ca 0.45 microM: inhibiting concentration of hTg was from 0.15 microM to 3.0 microM). The hTg inhibition was not evident in the RT-PCR system, suggesting no effect of hTg on Taq DNA polymerase activity. The hTg inhibition of TA activity was attenuated by dNTP but not significantly by TS primer. These data suggest that hTg contained in thyroid follicular cells of various thyroid diseases may affect the TA activity measured in biopsied thyroid specimens, and that the reduction of the TA activity by hTg may induce slow progression and growth, and low grade malignancy of thyroid cancer, particularly differentiated carcinoma.