TSH signaling and cell survival in 3T3-L1 preadipocytes

Type 2 Diabetes Mellitus Coincident with Clinical and Subclinical Thyroid Dysfunctions Results in Dysregulation of Circulating Chemerin, Resistin and Visfatin

The alterations of circulating adipocytokines have been reported in thyroid diseases or type 2 diabetes mellitus (T2DM), but such data in T2DM coincident with clinical and subclinical thyroid-dysfunctions are limited, and remain to be investigated. We studied the changes in serum chemerin, resisitin and visfatin in T2DM patients with thyroid dysfunctions, and their association with inflammatory and insulin resistance-markers. A total of 272 female and male Iranian participants were selected and divided into six groups: the euthyroid group, T2DM, T2DM coincident with clinical and sub clinical hypothyroidism (SC-HO, and C-HO), and T2DM coincident with clinical and sub clinical hyperthyroidism (SC-HR, C-HR).Demographic characteristics, serum levels of adipocytokines, thyroid hormones, inflammatory factors (IL1-β, IL-6 and CRP) and insulin resistance-markers were determined in all participants. T2DM patients with clinical thyroid dysfunctions showed higher levels of circulating resistin, visfatin, chemerin and inflammatory factors, compared with the T2DM group and T2DM coexisted with subclinical thyroid diseases. No significant differences were observed in circulating adipocytokines and inflammatory markers between T2DM coexisting with subclinical thyroid diseases and those without thyroid dysfunctions. Our results revealed that clinical thyroid dysfunction in T2DM patients was associated with elevated levels of circulating resistin, chemerin, visfatin and inflammatory factors, while no such alteration was detected in T2DM coincident with subclinical thyroid dysfunction.

A compendium of G-protein-coupled receptors and cyclic nucleotide regulation of adipose tissue metabolism and energy expenditure

With the ever-increasing burden of obesity and Type 2 diabetes, it is generally acknowledged that there remains a need for developing new therapeutics. One potential mechanism to combat obesity is to raise energy expenditure via increasing the amount of uncoupled respiration from the mitochondria-rich brown and beige adipocytes. With the recent appreciation of thermogenic adipocytes in humans, much effort is being made to elucidate the signaling pathways that regulate the browning of adipose tissue. In this review, we focus on the ligand-receptor signaling pathways that influence the cyclic nucleotides, cAMP and cGMP, in adipocytes. We chose to focus on G-protein-coupled receptor (GPCR), guanylyl cyclase and phosphodiesterase regulation of adipocytes because they are the targets of a large proportion of all currently available therapeutics. Furthermore, there is a large overlap in their signaling pathways, as signaling events that raise cAMP or cGMP generally increase adipocyte lipolysis and cause changes that are commonly referred to as browning: increasing mitochondrial biogenesis, uncoupling protein 1 (UCP1) expression and respiration.

Adipokine expression and endothelial function in subclinical hypothyroidism rats

The purpose of our study was to observe adipokine expression and endothelial function in subclinical hypothyroidism (sHT) rats and to determine whether levothyroxine (LT₄) treatment affects these changes. Sixty-five male Wistar rats were randomly divided into five groups: the control group; sHT A, B and C groups and the sHT + T₄ group. The sHT rats were induced by methimazole (MMI) and the sHT + T₄ rats were administered LT₄ treatment after 8 weeks of MMI administration. Thyroid function and lipid levels were measured using radioimmunoassays and enzymatic colorimetric methods, respectively. Serum adiponectin (APN), chemerin, TNF-α, endothelin (ET-1) and nitric oxide (NO) levels were measured using ELISA kits and a nitric-reductive assay. The expression of APN, chemerin and TNF-α in visceral adipose tissue (VAT) was measured in experimental rats using RT-PCR and Western blotting. Hematoxylin-eosin (HE) staining was used to observe changes in adipose tissue. The sHT rats had significantly higher levels of thyroid-stimulating hormone (TSH), TNF-α, chemerin, ET-1, total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) and lower levels of APN and NO than those in control and sHT + T₄ rats. Based on Pearson correlation analysis, the levels of chemerin, TNF-α, ET-1, LDL-C, TC and triglyceride (TG) were positively correlated with TSH, but APN and NO levels were negatively correlated with TSH. These findings demonstrated that high TSH levels contribute to the changes of adipokines and endothelial dysfunction in sHT, but LT₄ treatment ameliorates those changes.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Thyrostimulin Regulates Osteoblastic Bone Formation During Early Skeletal Development

The ancestral glycoprotein hormone thyrostimulin is a heterodimer of unique glycoprotein hormone subunit alpha (GPA)2 and glycoprotein hormone subunit beta (GPB)5 subunits with high affinity for the TSH receptor. Transgenic overexpression of GPB5 in mice results in cranial abnormalities, but the role of thyrostimulin in bone remains unknown. We hypothesized that thyrostimulin exerts paracrine actions in bone and determined: 1) GPA2 and GPB5 expression in osteoblasts and osteoclasts, 2) the skeletal consequences of thyrostimulin deficiency in GPB5 knockout (KO) mice, and 3) osteoblast and osteoclast responses to thyrostimulin treatment. Gpa2 and Gpb5 expression was identified in the newborn skeleton but declined rapidly thereafter. GPA2 and GPB5 mRNAs were also expressed in primary osteoblasts and osteoclasts at varying concentrations. Juvenile thyrostimulin-deficient mice had increased bone volume and mineralization as a result of increased osteoblastic bone formation. However, thyrostimulin failed to induce a canonical cAMP response or activate the noncanonical Akt, ERK, or mitogen-activated protein kinase (P38) signaling pathways in primary calvarial or bone marrow stromal cell-derived osteoblasts. Furthermore, thyrostimulin did not directly inhibit osteoblast proliferation, differentiation or mineralization in vitro. These studies identify thyrostimulin as a negative but indirect regulator of osteoblastic bone formation during skeletal development.

Foxc2 enhances proliferation and inhibits apoptosis through activating Akt/mTORC1 signaling pathway in mouse preadipocytes

Forkhead box C2 (Foxc2) protein is a transcription factor in regulation of development, metabolism, and immunology. However, the regulatory mechanisms of Foxc2 on proliferation and apoptosis of preadipocytes are unclear. In this study, we found that high-fat-diet-induced obesity elevated the expression of Foxc2 and cyclin E after 6 weeks. Additionally, Foxc2 suppressed preadipocyte differentiation, increased cell counts and augmented G1-S transition of preadipocytes, along with the elevation of cyclin E expression and the reduction levels of p27 and p53. Furthermore, Foxc2 knockdown reduced early apoptotic cells with accompanying reduction of mitochondrial membrane potential and increased fragmentation of genomic DNA. We show that Foxc2 reduces the expression of Bax, caspase-9, and caspase-3 in both serum-starved and palmitic acid-induced cell apoptotic models, which confirms the anti-apoptotic role of Foxc2. Moreover, the protein kinase B (Akt)/mammalian target of rapamycin (mTOR)C1 signaling pathway and the ERK/mTORC1 signaling pathway were activated along with preadipocyte proliferation in response to Foxc2 overexpression, whereas apoptosis marker genes were downregulated during this process. Those effects were blocked by the interference of Foxc2 or signal pathways specific inhibitors. These data collectively reveal that Foxc2 enhances proliferation of preadipocytes and inhibits apoptosis of preadipocytes by activating the Akt/mTORC1 and ERK/mTORC1 signaling pathways.

Functional thyrotropin receptor expression in the ventricle and the effects on ventricular BNP secretion

Elevated thyrotropin (TSH) and hypercholesterolemia commonly coexist in patients with subclinical hypothyroidism, which can cause and aggravate heart disease. However, it is unclear whether TSH has a direct effect on cardiac function. To determine the expression of the thyrotropin receptor (TSHR) and the effects of TSH on ventricular function, we analyzed the ventricular tissues and thyroid glands from normal rats and mice and the H9c2 cardiomyocyte cell line. The results revealed that TSHR was expressed at the transcriptional and protein levels by PCR, immunoblotting, immunohistochemistry and immunofluorescence. The mRNA levels of β-MHC and the expression of pCREB and HMGCR in the ventricle were significantly lower in Tshr (-/-) mice than in wild-type (WT) mice (p < 0.05), but serum NT-proBNP levels were similar between WT and Tshr (-/-) mice. After synchronization, H9c2 cells were stimulated with several concentrations of TSH for various time periods. TSH up-regulated β-MHC mRNA expression in H9c2 cells. Cyclic adenosine monophosphate (cAMP) production and downstream signaling, such as pCREB and HMGCR expression and NT-proBNP secretion, increased in dose- and time-dependent manners. The TSH-stimulated effects were suppressed by an adenylyl cyclase inhibitor, a protein kinase A (PKA) inhibitor and HMGCR inhibitors (all p < 0.05). The data indicate functional TSHR is expressed in ventricular myocytes and mediates TSH-induced BNP secretion and HMGCR up-regulation through the cAMP/PKA/pCREB signaling pathway. Our findings suggest a potentially novel pathophysiological role of TSH in heart failure-associated hypothyroidism.

Effects of TSH on the function of human umbilical vein endothelial cells

Recent studies have reported that subclinical hypothyroidism (SCH) is associated with atherosclerosis (AS). Thyroid hormone is maintained at normal levels in patients with SCH, whereas TSH is increased. However, the pathogenesis of AS in association with SCH is only partially understood. In addition, endothelial dysfunction plays an important role in the development of AS. The purpose of the present research was to study the direct effect of TSH on human umbilical vein endothelial cells (HUVECs). The expression of some genes associated with endothelial dysfunction after treatment with TSH was evaluated by real-time PCR and western blotting respectively. At first, we showed that the TSH receptor (TSHR) is expressed in HUVECs. We also provide evidence indicating that TSH treatment promotes tumor necrosis factor α-induced endothelial cells interactions by upregulating the expression of the adhesion molecules intercellular adhesion molecule-1. Furthermore, the expression of endothelial nitric oxide synthase (eNOS) and prostacyclin (PGI₂) was significantly attenuated following treatment with TSH in dose- and time-dependent manner. Conversely, the results indicated that TSH upregulated endothelin-1 (ET1) mRNA and protein expression in HUVECs, similar effects were observed for plasminogen activator inhibitor-1 (PAI1) after treatment with various concentrations of TSH. Taken together, these results demonstrate that elevated TSH can promote endothelial dysfunction by altering gene expression in HUVECs.

Renal nerves in blood pressure regulation

PURPOSE

This review highlights the physiological mechanisms underlying the neural regulation of the kidney, normally to maintain cardiovascular homeostasis, and in pathophysiological states of hypertension and renal disease. It is relevant because of the demonstration that bilateral renal denervation in different hypertensive groups causes a sustained reduction in blood pressure.

RECENT FINDINGS

There are patients groups in whom their hypertension is resistant to antihypertensive drugs or with renal diseases in which they are contraindicated. Recently, medical devices have been developed to manipulate the sympathetic nervous system, for example, implantation of carotid sinus nerve stimulating electrodes and ablation of the renal innervation. These approaches have been relatively successful but there remains a lack of understanding of the neural mechanisms impinging on the kidney that regulate long-term control of blood pressure.

SUMMARY

The observation that bilateral renal nerve ablation can reduce blood pressure represents an important therapeutic milestone. Nonetheless, questions arise as to the underlying mechanisms, the long-term consequences, whether there may be re-innervation over a number of years, or whether some unknown consequence to the denervation may arise. This may point to the development of novel compounds targeted to the innervation of the kidney.

Characterization of recombinant human and bovine thyroid-stimulating hormone preparations by mass spectrometry and determination of their endotoxin content

Background

The TSH stimulation test to confirm canine hypothyroidism is commonly performed using a recombinant human TSH (rhTSH), as up to date, canine TSH is not yet commercially available. Limiting factors for the use of rhTSH are its high costs and occasional difficulties in product availability. Less expensive bovine TSH preparations (bTSH) purified from bovine pituitary glands are readily commercially available. The aim of this study was to evaluate two different bTSH products as alternative to rhTSH using mass spectrometry.

Results

More than 50 proteins, including other pituitary hormones, bovine albumin, hemoglobin, and tissue proteins were identified in the bTSH preparations. In contrast, rhTSH proved to be a highly pure product. Significantly higher endotoxin levels could be detected in all bTSH products compared to the rhTSH.

Conclusions

Both bTSH products are crude mixtures and therefore not an acceptable alternative to rhTSH. Their use should be discouraged to prevent unintended side effects.

Sex differences in adipose tissue: It is not only a question of quantity and distribution

Obesity and its associated secondary complications are active areas of investigation in search of effective treatments. As a result of this intensified research numerous differences between males and females at all levels of metabolic control have come to the forefront. These differences include not only the amount and distribution of adipose tissue, but also differences in its metabolic capacity and functions between the sexes. Here, we review some of the recent advances in our understanding of these dimorphisms and emphasize the fact that these differences between males and females must be taken into consideration in hopes of obtaining successful treatments for both sexes.

Adipokines in childhood obesity

The worldwide increase in the prevalence of obesity in children and adolescents during the past decades, in addition to mounting evidence indicating that obesity is associated with an increased incidence of comorbidities and the risk of premature death, resulting in a high economical impact, has stimulated obesity-focused research. These studies have highlightened the prominent endocrine activity of adipose tissue, which is exerted through the synthesis and secretion of a wide variety of peptides and cytokines, called adipokines. In the present review, we have summarized the current knowledge and most relevant studies of adipokine dynamics and actions in children, focusing on the control of energy homeostasis, metabolic regulation (particularly, carbohydrate metabolism), and inflammation. The particularities of adipose secretion and actions in healthy children, from birth to adolescence, and the modifications induced by early-onset obesity are highlighted.

Effects of orexin A on proliferation, survival, apoptosis and differentiation of 3T3-L1 preadipocytes into mature adipocytes

Metabolic activities of orexin A (OXA) in mature adipocytes are mediated via PI3K/PKB and PPARγ. However, the effects of OXA on preadipocytes are largely unknown. We report here that OXA stimulates the proliferation and viability of 3T3-L1 preadipocytes and protects them from apoptosis via ERK1/2, but not through PKB. OXA reduces proapoptotic activity of caspase-3 via ERK1/2. Inhibition of ERK1/2 prevents the differentiation of preadipocytes into adipocytes. Unlike insulin, neither short-term nor prolonged exposure of 3T3-L1 preadipocytes to OXA induces preadipocyte differentiation to adipocytes, despite increased ERK1/2 phosphorylation. Unlike insulin, OXA fails to activate PKB, which explains its inability to induce the differentiation of preadipocytes.

Extrathyroidal expression of TSH receptor

The TSH receptor expressed on the cell surface of thyroid follicular cells plays a pivotal role in the regulation of thyroid status and growth of the thyroid gland. In recent years it has become evident that the TSH receptor is also expressed widely in a variety of extrathyroidal tissues including: anterior pituitary; hypothalamus; ovary; testis; skin; kidney; immune system; bone marrow and peripheral blood cells; white and brown adipose tissue; orbital preadipocyte fibroblasts and bone. A large body of evidence is emerging to describe the functional roles of the TSH receptor at these various sites but their physiological importance in many cases remains a subject of controversy and much interest. Current understanding of the actions of the TSH receptor in extrathyroidal tissues and their possible physiological implications is discussed.

A novel role for thyroid-stimulating hormone: up-regulation of hepatic 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase expression through the cyclic adenosine monophosphate/protein kinase A/cyclic adenosine monophosphate-responsive element binding protein pathway

Elevated thyroid-stimulating hormone (TSH) and hypercholesterolemia commonly coexist, as typically seen in hypothyroidism, but there is no known mechanism directly linking the two. Here, we demonstrated that in liver cells, TSH promoted the expression of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR), a rate-limiting enzyme in cholesterol synthesis, by acting on the TSH receptor in hepatocyte membranes and stimulating the cyclic adenosine monophosphate / protein kinase A / cyclic adenosine monophosphate-responsive element binding protein (cAMP/PKA/CREB) signaling system. In thyroidectomized rats, the production of endogenous thyroid hormone was eliminated and endogenous TSH was suppressed through pituitary suppression with constant administration of exogenous thyroid hormone, and hepatic HMGCR expression was increased by administration of exogenous TSH. These results suggested that TSH could up-regulate hepatic HMGCR expression, which indicated a potential mechanism for hypercholesterolemia involving direct action of TSH on the liver.

Acute exogenous TSH administration stimulates leptin secretion in vivo

TSH-receptor (TSHR) has been found in a variety of cell types, including preadipocytes and adipocytes. In vitro, TSH-mediated preadipocyte and adipocyte responses include proliferation, differentiation, survival, and lipolysis. Objective To measure the response of serum leptin to exogenous administration of recombinant human TSH (rhTSH) in vivo. Patients One hundred patients with differentiated thyroid cancer already treated by total thyroidectomy and (131)I remnant ablation were enrolled. Mean (+/-s.e.m.) body mass index (BMI) was 26.9+/-0.6 kg/m(2). Methods Patients received a standard dose of rhTSH for measurement of thyroglobulin in the follow-up of their disease. Blood samples were taken for the assay of TSH and leptin before the first administration of rhTSH (time 0), and 24 h (time 1), 48 h (time 2), 72 h (time 3), and 96 h (time 4) after the first administration of rhTSH. Results Significant mean serum leptin increments, with respect to basal value, were 16, 13, 18, and 11% at times 1, 2, 3, and 4 respectively. Significant positive correlations of leptin-area under the curve with respect to basal leptin levels (r=0.43; P<0.0001) and BMI (r=0.32; P<0.005) were observed. Conclusions Acute rhTSH administration in hypothyroid subjects under l-thyroxine therapy produces a rise in serum leptin. This increase is proportional to the adipose mass suggesting that a functioning TSHR is expressed on the surface of adipocytes. The role that TSHR activation in adipocytes might play in physiological and pathological conditions remains a matter of investigation.

Distinct early signaling events resulting from the expression of the PRKAG2 R302Q mutant of AMPK contribute to increased myocardial glycogen

BACKGROUND

Humans with an R302Q mutation in AMPKgamma(2) (the PRKAG2 gene) develop a glycogen storage cardiomyopathy characterized by a familial form of Wolff-Parkinson-White syndrome and cardiac hypertrophy. This phenotype is recapitulated in transgenic mice with cardiomyocyte-restricted expression of AMPKgamma(2)R302Q. Although considerable information is known regarding the consequences of harboring the gamma(2)R302Q mutation, little is known about the early signaling events that contribute to the development of this cardiomyopathy.

METHODS AND RESULTS

To distinguish the direct effects of gamma(2)R302Q expression from later compensatory alterations in signaling, we used transgenic mice expressing either the wild-type AMPKgamma(2) subunit (TGgamma(2)WT) or the mutated form (TGgamma(2)R302Q), in combination with acute expression of these proteins in neonatal rat cardiomyocytes. Although acute expression of gamma(2)R302Q induces AMPK activation and upregulation of glycogen synthase and AS160, with an associated increase in glycogen content, AMPK activity, glycogen synthase activity, and AS160 expression are reduced in hearts from TGgamma(2)R302Q mice, likely in response to the existing 37-fold increase in glycogen. Interestingly, gamma(2)WT expression has similar, yet less marked effects than gamma(2)R302Q expression in both cardiomyocytes and hearts.

CONCLUSIONS

Using acute and chronic models of gamma(2)R302Q expression, we have differentiated the direct effects of the gamma(2)R302Q mutation from eventual compensatory modifications. Our data suggest that expression of gamma(2)R302Q induces AMPK activation and the eventual increase in glycogen content, a finding that is masked in hearts from transgenic adult mice. These findings are the first to highlight temporal differences in the effects of the PRKAG2 R302Q mutation on cardiac metabolic signaling events.

Functional expression of the thyrotropin receptor in C cells: new insights into their involvement in the hypothalamic-pituitary-thyroid axis

Thyroid C cells, or parafollicular cells, are mainly known for producing calcitonin, a hormone involved in calcium homeostasis with hypocalcemic and hypophosphatemic effects. Classically, the main endocrine activity of this cell population has been believed to be restricted to its roles in serum calcium and bone metabolism. Nonetheless, in the last few years evidence has been accumulating in the literature with regard to local regulatory peptides secreted by C cells, such as somatostatin, ghrelin, thyrotropin releasing hormone or the recently described cocaine- and amphetamine-related transcript, which could modify thyroid function. As thyrotropin is the main hormone controlling the hypothalamic-pituitary-thyroid axis and, accordingly, thyroid function, we have examined the functional expression of the thyrotropin receptor in C-cell lines and in thyroid tissues. We have found that rat and human C-cell lines express the thyrotropin receptor at both mRNA and protein levels. Furthermore, incubation of C cells with thyrotropin resulted in a 10-fold inhibition of thyrotropin-receptor expression, and a concomitant decrease of the steady-state mRNA levels for calcitonin and calcitonin gene-related peptide determined by quantitative real-time PCR was found. Finally, thyrotropin receptor expression by C cells was confirmed at protein level in both normal and pathological thyroid tissues by immunohistochemistry and immunofluorescence. These results confirm that C cells, under regulation by thyrotropin, are involved in the hypothalamic-pituitary-thyroid axis and suggest a putative role in local fine-tuning of follicular cell activity.

Presence of thyrotropin receptor in hepatocytes: not a case of illegitimate transcription

The function of thyrotropin (TSH) in the thyroid gland is mediated by thyrotropin receptor (TSHR). In addition to the thyroid, TSHR expression has been described in some non-thyroidal tissues, although it is uncertain whether TSHR is present in hepatocytes. One study has reported hepatic expression of TSHR mRNA, but this was considered to be because of illegitimate transcription, and there has not been a study investigating its protein expression and function in hepatocytes. Here, we examined the expression of TSHR in human and rat liver tissues, as well as human normal hepatocyte cell line L-02. Our results demonstrated that hepatic TSHR mRNA could be detected and had the same sequence as that of thyroid-derived mRNA. TSHR protein was also expressed and mainly located in the hepatocyte cell membrane. Moreover, bovine TSH and immunoglobulin from sera of patients with Graves’ disease stimulated cAMP production in these cells. Taken together, these data show that TSHR is present and functional in hepatocytes, and this expression is not a case of illegitimate transcription. Given the pivotal role of the liver in body metabolism and many human diseases, our findings provide important implications for a potentially novel physiopathological role of TSH via acting on the TSHR in hepatocytes besides its classical role in regulating the thyroid function.

Critical role of the hypothalamic-pituitary-thyroid axis in bone

Studies in genetically modified mice have highlighted the importance of the hypothalamic-pituitary-thyroid (HPT) axis during skeletal development and the maintenance of adult bone. Recently, the conventional view that skeletal responses to abnormal thyroid status result solely from altered T3 action in bone has been complicated by studies proposing TSH as a negative regulator of bone turnover. Although skeletal consequences of thyrotoxicosis may result from thyroid hormone excess or TSH deficiency, the two alternatives are not necessarily mutually exclusive and cannot easily be differentiated because the HPT axis maintains them in a physiological reciprocal relationship. By contrast, situations in which this inverse relationship is disrupted have the potential to resolve the roles of T3 and TSH in the skeleton. We discuss these situations and the relative importance of T3 and TSH in skeletal homeostasis.

TSH stimulates adipogenesis in mouse embryonic stem cells

Although TSH is the main regulator of thyroid growth and function, TSH binding activity in fat has long been reported. Since the TSH receptor (TSHR) has been detected in both preadipocytes and adipocytes, we hypothesized that it may play a role in adipose differentiation. Here, we use an in vitro model of adipogenesis from mouse embryonic stem (ES) cells to define TSH function. Directed differentiation of ES cells into the adipose lineage can be achieved over a 3-week period. Although adipocyte differentiation is initiated early in the development of cultured ES cells, TSHR up-regulation is precisely correlated with terminal differentiation of those adipocytes. The adipocytes express TSHR on the cell surface and respond to TSH with increased intracellular cAMP production, suggesting the activation of the protein kinase A signaling pathway. To determine whether TSH impacts adipogenesis, we examined how adipocytes responded to TSH at various points during their differentiation from cultured ES cells. We found that TSH greatly increases adipogenesis when added in the presence of adipogenic factors. More importantly, our data suggest that TSH also stimulates adipogenesis in cultured ES cells even in the absence of adipogenic factors. This finding provides the first evidence of TSH being a pro-adipogenic factor that converts ES cells into adipocytes. It further highlights the potential of ES cells as a model system for use in the study of TSH's role in the regulation of physiologically relevant adipose tissue.

A lack of thyroid hormones rather than excess thyrotropin causes abnormal skeletal development in hypothyroidism

By proposing TSH as a key negative regulator of bone turnover, recent studies in TSH receptor (TSHR) null mice challenged the established view that skeletal responses to disruption of the hypothalamic-pituitary-thyroid axis result from altered thyroid hormone (T(3)) action in bone. Importantly, this hypothesis does not explain the increased risk of osteoporosis in Graves' disease patients, in which circulating TSHR-stimulating antibodies are pathognomonic. To determine the relative importance of T(3) and TSH in bone, we compared the skeletal phenotypes of two mouse models of congenital hypothyroidism in which the normal reciprocal relationship between thyroid hormones and TSH was intact or disrupted. Pax8 null (Pax8(-/-)) mice have a 1900-fold increase in TSH and a normal TSHR, whereas hyt/hyt mice have a 2300-fold elevation of TSH but a nonfunctional TSHR. We reasoned these mice must display opposing skeletal phenotypes if TSH has a major role in bone, whereas they would be similar if thyroid hormone actions predominate. Pax8(-/-) and hyt/hyt mice both displayed delayed ossification, reduced cortical bone, a trabecular bone remodeling defect, and reduced bone mineralization, thus indicating that the skeletal abnormalities of congenital hypothyroidism are independent of TSH. Treatment of primary osteoblasts and osteoclasts with TSH or a TSHR-stimulating antibody failed to induce a cAMP response. Furthermore, TSH did not affect the differentiation or function of osteoblasts or osteoclasts in vitro. These data indicate the hypothalamic-pituitary-thyroid axis regulates skeletal development via the actions of T(3).

Modulation of TSHR signaling by posttranslational modifications

Posttranslational modifications of seven transmembrane receptors (7TMRs) affect their function to a large extent. Many studies of glycosylation or phosphorylation of 7TMRs have shown that these modifications influence the cell-surface expression or signaling of the receptor. Recently, other types of posttranslational modifications of the thyrotropin-stimulating hormone receptor (TSHR) have been characterized, including sialylation and dimerization. Increased TSHR sialylation results in increased TSHR cell-surface expression. Furthermore, TSHR oligomerization and the probable modification of TSHR signaling in lipid rafts require further clarification with regard to their functional consequences. In addition to its known coupling to Galphas and Galphaq, and possibly other G proteins, the TSHR also couples to further signaling pathways, such as the mitogen-activated protein kinase (MAPK) pathway, which involves G-protein-coupled receptor kinases (GRKs) and arrestins. We discuss these emerging new findings and their implications for signaling of the TSHR.

The Asp727Glu polymorphism in the TSH receptor is associated with insulin resistance in healthy elderly men

BACKGROUND

Variations in thyroid function within the normal range are associated with differences in metabolism and body composition. For instance, TSH is positively associated with body mass index (BMI). This could be due to alterations in thyroid hormone activity, or to direct effects of TSH, as the TSH receptor (TSHR) is also expressed in adipose tissue. The TSHR-Asp727Glu polymorphism is associated with lower serum TSH levels in vivo. In this study, we analysed whether serum thyroid parameters and the TSHR-Asp727Glu polymorphism were associated with glucose metabolism and insulin resistance. In addition, we analysed the Thr92Ala polymorphism in the type 2 deiodinase (D2), which was recently associated with insulin resistance.

METHODS

Genotypes were determined in a population of 349 elderly men (age 77.7 +/- 3.5 years), for whom serum thyroid parameters and data on insulin resistance, such as fasting blood glucose, serum insulin and homeostasis model assessment (HOMA) values, were available.

RESULTS

In nondiabetic, euthyroid subjects, TSH was positively associated with leptin levels, whereas FT4 and rT3 were significantly negatively correlated with insulin and HOMA. Carriers of the TSHR-Glu727 allele had a significantly higher glucose (P = 0.01), insulin (P = 0.001), glycated haemoglobin (HbA1c) (P = 0.002), HOMA (P = 0.001) and leptin (P = 0.008). The D2-Ala(92) allele showed a trend towards higher levels of insulin (P = 0.07) and a higher HOMA (P = 0.09).

CONCLUSION

In this population of nondiabetic elderly men, serum thyroid parameters and the TSHR-Asp727Glu polymorphism were associated with relative insulin resistance. Our study suggests that genetic variation in TSHR plays a role in insulin resistance and thereby influences glucose metabolism.

Thyroid-stimulating hormone stimulates interleukin-6 release from 3T3-L1 adipocytes through a cAMP-protein kinase A pathway

OBJECTIVE

Thyroid-stimulating hormone (TSH) is a novel modulator of adipokine release from human and mouse adipocytes. The aim of our study was to identify the signal transduction pathways activated by TSH that stimulate interleukin (IL)-6 production.

RESEARCH METHODS AND PROCEDURES

Mouse 3T3-L1 preadipocyte and differentiated adipocyte cell cultures were studied. The effect of 0 to 1 microM TSH on IL-6 protein release into the medium over 0 to 24 hours was assessed. TSH signaling pathways responsible for regulating IL-6 were studied through the use of 1 muM forskolin, 100 microM 8-pCPT-2'-O-Me-cAMP, 10 microM H89, 50 microM PD98059, and 2 mug/mL actinomycin D.

RESULTS

TSH stimulated IL-6 release by 2.6-fold from 3T3-L1 adipocytes at concentrations as low as 0.01 microM but did not alter IL-6 production of corresponding preadipocytes. Forskolin (elevates intracellular cAMP) stimulated IL-6 release from 3T3-L1 adipocytes (n = 3, p < 0.005), and H89, an inhibitor of cAMP-dependent protein kinase A (PKA), reduced TSH-stimulated IL-6 release by 66% (n = 3, p < 0.01), indicating a requirement for cAMP-dependent PKA. Inhibition of the mitogen-activated protein kinase pathway with PD98059 did not affect TSH-stimulated IL-6 release. Activation of an alternate cAMP target, the exchange protein of cAMP, with 8-pCPT-2'-O-Me-cAMP, had no effect on IL-6 release. TSH raised the level of IL-6 mRNA, and blockade of transcription with actinomycin D abrogated IL-6 protein release by TSH (n = 3, p < 0.05).

DISCUSSION

TSH stimulates IL-6 release from differentiated 3T3-L1 adipocytes, but not preadipocytes, by signaling through cAMP-PKA to activate IL-6 gene transcription.

Renal denervation attenuates long-term hypertensive effects of Angiotensin ii in the rat

1. It is well accepted that some of the long-term effects of angiotensin (Ang) II are mediated via the central nervous system. Some of these actions that are mediated by the circumventricular organs and the baroreceptor reflex are thought to then alter sympathetic nervous system activity. In particular, there is some debate as to the role of renal nerves in the chronic effects of AngII. The aim of the present study was to assess the contribution of the renal nerves in a long-term model of progressive AngII-induced hypertension. 2. Male Sprague-Dawley rats were subjected to either bilateral renal denervation (RDX; n = 7) or sham surgery (SHAM; n = 8). Rats were instrumented with radiotelemetric transducers and venous catheters for the measurement of blood pressure and AngII infusion, respectively. A 4.0% NaCl diet and distilled water were provided ad libitum. The first 3 days served as the control period (7 mL/day, 0.9% NaCl, i.v.). This was followed by an infusion of AngII for 16 days (10 ng/kg per min, i.v.) and a 3 day recovery period identical to control. 3. Baseline arterial pressure between RDX and SHAM rats did not differ. Following AngII treatment, the arterial pressure of SHAM rats increased more rapidly than that of RDX rats. By Day 10 of treatment, the mean arterial pressure was significantly different between groups, having increased to 166 +/- 4 mmHg in SHAM rats and 135 +/- 11 mmHg in RDX rats. This trend continued for the remainder of AngII treatment. 4. The present results indicate that the renal nerves are necessary for the full expression of AngII-induced hypertension.

Regulation of PDGF-stimulated SHIP2 tyrosine phosphorylation and association with Shc in 3T3-L1 preadipocytes

In 3T3-L1 and human preadipocytes, insulin results in the isolated rise in phosphatidylinositol (PI)-3,4,5-P3, whereas PDGF produces PI(3,4)P2 in addition to PI(3,4,5)P3. SH2 domain-containing inositol 5-phosphatase 2 (SHIP2) converts PI(3,4,5)P3 into PI(3,4)P2. PDGF, but not insulin, stimulates SHIP2 tyrosine phosphorylation and its association with Shc in human and 3T3-L1 preadipocytes. We now demonstrate that SHIP2 tyrosine phosphorylation and association with Shc in PDGF-treated 3T3-L1 preadipocytes was reduced by bisindolylmaleimide I (BisI), an inhibitor of conventional/novel protein kinase C (PKC). However, the production of PI(3,4)P2 and PI(3,4,5)P3 by PDGF was unaffected by BisI. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) was not sufficient to induce SHIP2 tyrosine phosphorylation. Furthermore, we identified threonine 958 (T958) as a novel PDGF-responsive SHIP2 phosphorylation site. Mutation of T958 to alanine reduced PDGF-stimulated SHIP2 tyrosine phosphorylation and association with Shc, but did not alter its anti-proliferative effect on preadipocytes. This study demonstrates that SHIP2 tyrosine phosphorylation and Shc association can be regulated by serine/threonine signaling pathways, either indirectly (via PKC), or directly (via T958). Interestingly, the anti-proliferative effect of SHIP2 T958A, as well as another SHIP2 mutant (Y986F, Y987F) that also displays defective tyrosine phosphorylation and Shc association, does not depend on these molecular events.

TNFalpha mediates the skeletal effects of thyroid-stimulating hormone

We have shown recently that by acting on the thyroid-stimulating hormone (TSH) receptor (TSHR), TSH negatively regulates osteoclast differentiation. Both heterozygotic and homozygotic TSHR null mice are osteopenic with evidence of enhanced osteoclast differentiation. Here, we report that the accompanying elevation of TNFalpha, an osteoclastogenic cytokine, causes the increased osteoclast differentiation. This enhancement in TSHR-/- and TSHR+/- mice is abrogated in compound TSHR-/-/TNFalpha-/- and TSHR+/-/TNFalpha+/- mice, respectively. In parallel studies, we find that TSH directly inhibits TNFalpha production, reduces the number of TNFalpha-producing osteoclast precursors, and attenuates the induction of TNFalpha expression by IL-1, TNFalpha, and receptor activator of NF-kappaB ligand. TSH also suppresses osteoclast formation in murine macrophages and RAW-C3 cells. The suppression is more profound in cells that overexpress the TSHR than those transfected with empty vector. The overexpression of ligand-independent, constitutively active TSHR abrogates osteoclast formation even under basal conditions and in the absence of TSH. Finally, IL-1/TNFalpha and receptor activator of NF-kappaB ligand fail to stimulate AP-1 and NF-kappaB binding to DNA in cells transfected with TSHR or constitutively active TSHR. The results suggest that TNFalpha is the critical cytokine mediating the downstream antiresorptive effects of TSH on the skeleton.

The role of 'adipotropins' and the clinical importance of a potential hypothalamic–pituitary–adipose axis

Since adipocytes express specific receptors for pituitary hormones and hypothalamic releasing factors, adipose tissue has to be regarded as a fast-acting endocrine gland under the control of the brain. Expanding on this suggestion, the existence and clinical impact of a hypothalamic-pituitary-adipose axis is reviewed. The term 'adipotropins' is introduced in order to describe pituitary and hypothalamic hormones or releasing factors that directly target adipocytes by their specific receptors.

Interleukin-6 release from human abdominal adipose cells is regulated by thyroid-stimulating hormone: effect of adipocyte differentiation and anatomic depot

Adipose cells are extrathyroidal targets of thyroid-stimulating hormone (TSH). TSH stimulates interleukin-6 (IL-6) release from adipocytes. We examined TSH responsiveness as a function of stage of differentiation or adipose tissue depot in cultured adipose cells and determined the effect of TSH on extrathyroidal IL-6 production in vivo. Stromal preadipocytes, isolated from human abdominal subcutaneous or omental adipose tissue, and their differentiated counterparts were studied. IL-6 protein concentration in the medium was measured after TSH stimulation. Basal IL-6 release was greater for preadipocytes than differentiated adipocytes, whether derived from subcutaneous or omental fat depots. A depot-dependent effect (omental > subcutaneous) on basal IL-6 release was observed for preadipocytes (1.6-fold, P < 0.05); a similar trend for differentiated adipocytes was not significant (6.2-fold, P > 0.05). IL-6 responsiveness to TSH was observed upon differentiation, but only for subcutaneous adipocytes (1.9-fold over basal, P < 0.001). To determine if TSH could stimulate IL-6 release from extrathyroidal tissues in vivo, we measured serum IL-6 levels from five thyroidectomized patients who received recombinant human (rh) TSH and found that levels increased by threefold on days 3 and 4 (P < 0.05) after its administration. Our data demonstrate that stage of differentiation and fat depot origin affect basal and TSH-stimulated IL-6 release from adipose cells in culture. Furthermore, rhTSH elevates serum IL-6 response in thyroidectomized patients, indicating an extrathyroidal site of TSH action.

Activation of AMP-activated protein kinase inhibits protein synthesis associated with hypertrophy in the cardiac myocyte

A necessary mediator of cardiac myocyte enlargement is protein synthesis, which is controlled at the levels of both translation initiation and elongation. Eukaryotic elongation factor-2 (eEF2) mediates the translocation step of peptide-chain elongation and is inhibited through phosphorylation by eEF2 kinase. In addition, p70S6 kinase can regulate protein synthesis by phosphorylating eEF2 kinase or via phosphorylation of ribosomal protein S6. We have recently shown that eEF2 kinase is also controlled by phosphorylation by AMP-activated protein kinase (AMPK), a key regulator of cellular energy homeostasis. Moreover, the mammalian target of rapamycin has also been shown to be inhibited, indirectly, by AMPK, thus leading to the inhibition of p70S6 kinase. Although AMPK activation has been shown to modulate protein synthesis, it is unknown whether AMPK could also be a regulator of cardiac hypertrophic growth. Therefore, we investigated the role of AMPK activation in regulating protein synthesis during both phenylephrine- and Akt-induced cardiac hypertrophy. Metformin and 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside were used to activate AMPK in neonatal rat cardiac myocytes. Activation of AMPK significantly decreased protein synthesis induced by phenylephrine treatment or by expression of constitutively active Akt. Activation of AMPK also resulted in decreased p70S6 kinase phosphorylation and increased phosphorylation of eEF2, suggesting that inhibition of protein synthesis involves the eEF2 kinase/eEF2 axis and/or the p70S6 kinase pathway. Together, our data suggest that the inhibition of protein synthesis by pharmacological activation of AMPK may be a key regulatory mechanism by which hypertrophic growth can be controlled.

Fatty Acid Translocase/CD36 Deficiency Does Not Energetically or Functionally Compromise Hearts Before or After Ischemia

Background— Evidence from humans suggests that fatty acid translocase (FAT)/CD36 deficiency can lead to functionally and/or energetically compromised hearts, but the data are equivocal, and the subject remains controversial. In this report we assessed the contribution of FAT/CD36 to overall fatty acid oxidation rates in the intact heart and determined the effect of FAT/CD36 on energy metabolism during reperfusion of ischemic hearts.

Methods and Results— Isolated working hearts from wild-type and FAT/CD36-knockout (KO) mice were perfused with Krebs-Henseleit solution containing 0.4 or 1.2 mmol/L [U- 3 H]palmitate, 5 mmol/L [U- 14 C]glucose, 2.5 mmol/L calcium, and 100 μU/mL insulin at a preload pressure of 11.5 mm Hg and afterload pressure of 50 mm Hg. Hearts were aerobically perfused for 30 minutes or aerobically perfused for 30 minutes, followed by 18 minutes of global no-flow ischemia and 40 minutes of aerobic reperfusion. Rates of fatty acid oxidation in FAT/CD36-KO hearts were significantly lower than in wild-type hearts at both concentrations of palmitate (0.4 or 1.2 mmol/L). In addition, hearts from FAT/CD36-KO mice displayed a compensatory increase in glucose oxidation rates. On aerobic reperfusion after ischemia, cardiac work of FAT/CD36-KO hearts recovered to the same extent as wild-type hearts.

Conclusions— FAT/CD36-deficient hearts are not energetically or functionally compromised and are not more sensitive to ischemic injury because glucose oxidation can compensate for the loss of fatty acid–derived ATP.

Akt activity negatively regulates phosphorylation of AMP-activated protein kinase in the heart

In the heart, insulin stimulates a variety of kinase cascades and controls glucose utilization. Because insulin is able to activate Akt and inactivate AMP-activated protein kinase (AMPK) in the heart, we hypothesized that Akt can regulate the activity of AMPK. To address the potential existence of this novel signaling pathway, we used a number of experimental protocols to activate Akt in cardiac myocytes and monitored the activation status of AMPK. Mouse hearts perfused in the presence of insulin demonstrated accelerated glycolysis and glucose oxidation rates as compared with non-insulin-perfused hearts. In addition, insulin caused an increase in Akt phosphorylation and a decrease in AMPK phosphorylation at its major regulatory site (threonine 172 of the alpha catalytic subunit). Transgenic mice overexpressing a constitutively active mutant form of Akt1 displayed decreased phosphorylation of cardiac alpha-AMPK. Isolated neonatal cardiac myocytes infected with an adenovirus expressing constitutively active mutant forms of either Akt1 or Akt2 also suppressed AMPK phosphorylation. However, Akt-dependent depression of alpha-AMPK phosphorylation could be overcome in the presence of the AMPK activator, metformin, suggesting that an override mechanism exists that can restore AMPK activity. Taken together, this study suggests that there is cross-talk between the AMPK and Akt pathways and that Akt activation can lead to decreased AMPK activity. In addition, our data suggest that the ability of insulin to inhibit AMPK may be controlled via an Akt-mediated mechanism.

Saturated fatty acid-induced apoptosis in MDA-MB-231 breast cancer cells. A role for cardiolipin

Little is known about the biochemical basis of the action of free fatty acids (FFA) on breast cancer cell proliferation and apoptosis. Here we report that unsaturated FFAs stimulated the proliferation of human MDA-MB-231 breast cancer cells, whereas saturated FFAs inhibited it and caused apoptosis. Saturated FFA palmitate decreased the mitochondrial membrane potential and caused cytochrome c release. Palmitate-induced apoptosis was enhanced by the fat oxidation inhibitor etomoxir, whereas it was reduced by fatty-acyl CoA synthase inhibitor triacsin C. The non-metabolizable analog 2-bromopalmitate was not cytotoxic. This indicates that palmitate must be metabolized to exert its toxic effect but that its action does not involve fat oxidation. Pharmacological studies showed that the action of palmitate is not mediated via ceramides, reactive oxygen species, or changes in phosphatidylinositol 3-kinase activity. Palmitate caused early enhancement of cardiolipin turnover and decreased the levels of this mitochondrial phospholipid, which is necessary for cytochrome c retention. Cosupplementation of oleate, or increasing beta-oxidation with the AMP-activated protein kinase activator, 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside, both restored cardiolipin levels and blocked palmitate-induced apoptosis. Oleate was preferentially metabolized to triglycerides, and oleate cosupplementation channeled palmitate esterification processes to triglycerides. Overexpression of Bcl-2 family members blocked palmitate-induced apoptosis. The results provide evidence that a decrease in cardiolipin levels and altered mitochondrial function are involved in palmitate-induced breast cancer cell death. They also suggest that the antiapoptotic action of oleate on palmitate-induced cell death involves both restoration of cardiolipin levels and redirection of palmitate esterification processes to triglycerides.

Regulation of the phosphatidylinositol 3-kinase, Akt/protein kinase B, FRAP/mammalian target of rapamycin, and ribosomal S6 kinase 1 signaling pathways by thyroid-stimulating hormone (TSH) and stimulating type TSH receptor antibodies in the thyroid gland

Thyroid-stimulating hormone (TSH) regulates the growth and differentiation of thyrocytes by activating the TSH receptor (TSHR). This study investigated the roles of the phosphatidylinositol 3-kinase (PI3K), PDK1, FRAP/mammalian target of rapamycin, and ribosomal S6 kinase 1 (S6K1) signaling mechanism by which TSH and the stimulating type TSHR antibodies regulate thyrocyte proliferation and the follicle activities in vitro and in vivo. The TSHR immunoprecipitates exhibited PI3K activity, which was higher in the cells treated with either TSH or 8-bromo-cAMP. TSH and cAMP increased the tyrosine phosphorylation of TSHR and the association between TSHR and the p85alpha regulatory subunit of PI3K. TSH induced a redistribution of PDK1 from the cytoplasm to the plasma membrane in the cells in a PI3K- and protein kinase A-dependent manner. TSH induced the PDK1-dependent phosphorylation of S6K1 but did not induce Akt/protein kinase B phosphorylation. The TSH-induced S6K1 phosphorylation was inhibited by a dominant negative p85alpha regulatory subunit or by the PI3K inhibitors wortmannin and LY294002. Rapamycin inhibited the phosphorylation of S6K1 in the cells treated with either TSH or 8-bromo-cAMP. The stimulating type TSHR antibodies from patients with Graves disease also induced S6K1 activation, whereas the blocking type TSHR antibodies from patients with primary myxedema inhibited TSH- but not the insulin-induced phosphorylation of S6K1. In addition, rapamycin treatment in vivo inhibited the TSH-stimulated thyroid follicle hyperplasia and follicle activity. These findings suggest an interaction between TSHR and PI3K, which is stimulated by TSH and cAMP and might involve the downstream S6K1 but not Akt/protein kinase B. This pathway may play a role in the TSH/stimulating type TSH receptor antibody-mediated thyrocyte proliferation in vitro and in the response to TSH in vivo.