Regulation of lipolysis during the neonatal period. Importance of thyrotropin

The role of G protein-coupled receptors and their ligands in animal domestication

The domestication of plants and animals has resulted in one of the most significant cultural and socio-economical transitions in human history. Domestication of animals, including human-supervised reproduction, largely uncoupled particular animal species from their natural, evolutionary history driven by environmental and ecological factors. The primary motivations for domesticating animals were, and still are, producing food and materials (e.g. meat, eggs, honey or milk products, wool, leather products, jewelry and medication products) to support plowing in agriculture or in transportation (e.g. horse, cattle, camel and llama) and to facilitate human activities (for hunting, rescuing, therapeutic aid, guarding behavior and protecting or just as a companion). In recent years, decoded genetic information from more than 40 domesticated animal species have become available; these studies have identified genes and mutations associated with specific physiological and behavioral traits contributing to the complex genetic background of animal domestication. These breeding-altered genomes provide insights into the regulation of different physiological areas, including information on links between e.g. endocrinology and behavior, with important pathophysiological implications (e.g. for obesity and cancer), extending the interest in domestication well beyond the field. Several genes that have undergone selection during domestication and breeding encode specific G protein-coupled receptors, a class of membrane-spanning receptors involved in the regulation of a number of overarching functions such as reproduction, development, body homeostasis, metabolism, stress responses, cognition, learning and memory. Here we summarize the available literature on variations in G protein-coupled receptors and their ligands and how these have contributed to animal domestication.

Neonatal hypoglycaemia

Low blood concentrations of glucose (hypoglycaemia) soon after birth are common because of the delayed metabolic transition from maternal to endogenous neonatal sources of glucose. Because glucose is the main energy source for the brain, severe hypoglycaemia can cause neuroglycopenia (inadequate supply of glucose to the brain) and, if severe, permanent brain injury. Routine screening of infants at risk and treatment when hypoglycaemia is detected are therefore widely recommended. Robust evidence to support most aspects of management is lacking, however, including the appropriate threshold for diagnosis and optimal monitoring. Treatment is usually initially more feeding, with buccal dextrose gel, followed by intravenous dextrose. In infants at risk, developmental outcomes after mild hypoglycaemia seem to be worse than in those who do not develop hypoglycaemia, but the reasons for these observations are uncertain. Here, the current understanding of the pathophysiology of neonatal hypoglycaemia and recent evidence regarding its diagnosis, management, and outcomes are reviewed. Recommendations are made for further research priorities.

The Impact of Excessive Fructose Intake on Adipose Tissue and the Development of Childhood Obesity

Worldwide, childhood obesity cases continue to rise, and its prevalence is known to increase the risk of non-communicable diseases typically found in adults, such as cardiovascular disease and type 2 diabetes mellitus. Thus, comprehending its multiple causes to build healthier approaches and revert this scenario is urgent. Obesity development is strongly associated with high fructose intake since the excessive consumption of this highly lipogenic sugar leads to white fat accumulation and causes white adipose tissue (WAT) inflammation, oxidative stress, and dysregulated adipokine release. Unfortunately, the global consumption of fructose has increased dramatically in recent years, which is associated with the fact that fructose is not always evident to consumers, as it is commonly added as a sweetener in food and sugar-sweetened beverages (SSB). Therefore, here, we discuss the impact of excessive fructose intake on adipose tissue biology, its contribution to childhood obesity, and current strategies for reducing high fructose and/or free sugar intake. To achieve such reductions, we conclude that it is important that the population has access to reliable information about food ingredients via food labels. Consumers also need scientific education to understand potential health risks to themselves and their children.

Obesity and Thyroid Axis

Development of obesity is primarily the result of imbalance between energy intake and energy expenditure. Thyroid hormones influence energy expenditure by regulating cellular respiration and thermogenesis and by determining resting metabolic rate. Triiodothyronine influences lipid turnover in adipocytes and impacts appetite regulation through the central nervous system, mainly the hypothalamus. Thyroid-stimulating hormone may also influence thermogenesis, suppress appetite and regulate lipid storage through lipolysis and lipogenesis control. Subclinical hypothyroidism may induce changes in basal metabolic rate with subsequent increase in BMI, but obesity can also affect thyroid function via several mechanisms such as lipotoxicity and changes in adipokines and inflammatory cytokine secretion. The present study investigated the complex and mutual relationships between the thyroid axis and adiposity.

Adipose tissue development and lipid metabolism in the human fetus: The 2020 perspective focusing on maternal diabetes and obesity

During development, the human fetus accrues the highest proportion of fat of all mammals. Precursors of fat lobules can be found at week 14 of pregnancy. Thereafter, they expand, filling with triacylglycerols during pregnancy. The resultant mature lipid-filled adipocytes emerge from a developmental programme of embryonic stem cells, which is regulated differently than adult adipogenesis. Fetal triacylglycerol synthesis uses glycerol and fatty acids derived predominantly from glycolysis and lipogenesis in liver and adipocytes. The fatty acid composition of fetal adipose tissue at the end of pregnancy shows a preponderance of palmitic acid, and differs from the mother. Maternal diabetes mellitus does not influence this fatty acid profile. Glucose oxidation is the main source of energy for the fetus, but mitochondrial fatty acid oxidation also contributes. Indirect evidence suggests the presence of lipoprotein lipase in fetal adipose tissue. Its activity may be increased under hyperinsulinemic conditions as in maternal diabetes mellitus and obesity, thereby contributing to increased triacylglycerol deposition found in the newborns of such pregnancies. Fetal lipolysis is low. Changes in the expression of genes controlling metabolism in fetal adipose tissue appear to contribute actively to the increased neonatal fat mass found in diabetes and obesity. Many of these processes are under endocrine regulation, principally by insulin, and show sex-differences. Novel fatty acid derived signals such as oxylipins are present in cord blood with as yet undiscovered function. Despite many decades of research on fetal lipid deposition and metabolism, many key questions await answers.

Distinctive Features of Orbital Adipose Tissue (OAT) in Graves' Orbitopathy

Depot specific expansion of orbital-adipose-tissue (OAT) in Graves' Orbitopathy (GO) is associated with lipid metabolism signaling defects. We hypothesize that the unique adipocyte biology of OAT facilitates its expansion in GO. A comprehensive comparison of OAT and white-adipose-tissue (WAT) was performed by light/electron-microscopy, lipidomic and transcriptional analysis using ex vivo WAT, healthy OAT (OAT-H) and OAT from GO (OAT-GO). OAT-H/OAT-GO have a single lipid-vacuole and low mitochondrial number. Lower lipolytic activity and smaller adipocytes of OAT-H/OAT-GO, accompanied by similar essential linoleic fatty acid (FA) and (low) FA synthesis to WAT, revealed a hyperplastic OAT expansion through external FA-uptake via abundant SLC27A6 (FA-transporter) expression. Mitochondrial dysfunction of OAT in GO was apparent, as evidenced by the increased mRNA expression of uncoupling protein 1 (UCP1) and mitofusin-2 (MFN2) in OAT-GO compared to OAT-H. Transcriptional profiles of OAT-H revealed high expression of Iroquois homeobox-family (IRX-3&5), and low expression in HOX-family/TBX5 (essential for WAT/BAT (brown-adipose-tissue)/BRITE (BRown-in-whITE) development). We demonstrated unique features of OAT not presented in either WAT or BAT/BRITE. This study reveals that the pathologically enhanced FA-uptake driven hyperplastic expansion of OAT in GO is associated with a depot specific mechanism (the SLC27A6 FA-transporter) and mitochondrial dysfunction. We uncovered that OAT functions as a distinctive fat depot, providing novel insights into adipocyte biology and the pathological development of OAT expansion in GO.

Adipose-specific inactivation of thyroid stimulating hormone receptors in mice modifies body weight, temperature and gene expression in adipocytes

BACKGROUND

In obesity, the expression level of thyroid stimulating hormone receptor in adipose tissue is reduced and the levels of thyroid stimulating hormone (TSH) are often elevated within the normal range.

PURPOSE/AIM

To investigate the role of TSHR in brown and white adipose tissue (AT) using TSHR knockout (KO) mice and the physiological phenotypes affected by the TSHR knockout.

METHODS

AT-specific TSHR KO male mice and wild type (WT) controls were given a high-fat diet (HFD) or a control diet (CD). Body weights and food consumption were recorded for 20 weeks and body temperatures for the first 3 weeks. At termination, white and brown adipocytes were isolated. Gene expressios was investigated using real-time PCR. In a subgroup of female KO mice, glucose tolerance was investigated.

RESULTS

TSHR were partially knocked out in KO mice, which gained more weight than WT mice when fed both a CD (p = .03) and HFD (p = .003). Body temperatures were lower in KO mice on CD (p <.001) and on HFD (p <.001) than WT controls. This was in agreement with reduced gene expression of UCP1 in brown adipocytes in the KO mice. Glucose tolerance was significantly impaired in KO mice on CD mice before termination (p <.01). Expression of adipogenic and lipolytic genes were reduced in KO mice, which was exacerbated by HFD. The mRNA levels of adipokines including ADIPOQ and LEP were altered in white adipocytes of KO mice.

CONCLUSIONS

TSHR KO led to dysfunction of both white and brown AT and predisposition to excess body weight gain in mice. Our data show that TSHR in AT regulates glucose tolerance, lipid metabolism, adipokine profile, and thermogenesis.

The Role of Ketone Bodies in Improving Neurological Function and Efficiency

: Neurological coordination is essential for performing biological and mechanical activities achieved by the cooperation of biomolecules such as carbohydrates, lipids, and proteins. It plays an important role in energy production, which can be fascinatingly improved by ketone bodies. Ketone bodies are small, water-soluble lipid molecules by shifting the glycolytic phase KBs directly enters into the tricarboxylic acid cycle for ATP synthesis. It leads to the production of much more energy levels than a single molecule of glucose. Therefore, it could have a profound effect on neuro-metabolism as well as bioenergetics of ATP production. These neuro-enhancement properties are useful for epilepsy, Alzheimer's, and several neurocognitive disorders treatment. Interestingly, the cancer cells cannot use it for efficiently energy production results in decreasing cancer cells viability. This review summarized ketone bodies generation, related imperative effects on normal cells, and more importantly its application in various neurological disorders treatment by rising neuronal functions.

Five serum fatty acids are associated with subclinical hypothyroidism in a Chinese pregnant population

Subclinical hypothyroidism (SCH) is a common endocrine disorder affecting women of reproductive age. Although SCH and abnormal fatty acid composition are often associated with adverse pregnancy outcomes and metabolic syndrome later in maternal and fetal life, the longitudinal relationship between SCH and serum fatty acids during pregnancy has rarely been studied. Therefore, the aim of this study was to investigate the association between SCH and maternal serum fatty acids throughout gestation. A total of 240 women enrolled in the Complex Lipids in Mothers and Babies (CLIMB) study in Chongqing, China were included in our study. Clinical information and maternal serum samples were collected at three time points during pregnancy: 11-14th, 22-28th, and 32-34th weeks of gestation. Twenty serum fatty acids were quantified using gas chromatography-mass spectrometry (GC-MS) analysis. A majority of the 20 serum fatty acids increased as gestation progressed in women with a normal pregnancy and women experiencing SCH. Levels of arachidic acid, docosahexaenoic acid, and eicosenoic acid were significantly higher in the serum of women with SCH when compared to women with a normal pregnancy, in the second trimester. On the other hand, the levels of eicosadienoic acid and octadecanoic acid were significantly higher in SCH in the third trimester. Our findings demonstrate that serum fatty acid composition during the second and third trimesters was significantly associated with SCH in pregnant Chinese women.

Unique Pattern of N-Glycosylation, Sialylation, and Sulfonation on TSH Molecules in Serum of Children Up to 18 Months

CONTEXT

N-glycosylation and glycan composition of human TSH molecules modulate the biological properties of TSH in different physiological and clinical situations. The degree of sialylation of serum TSH was reported to be very low in normal third-trimester fetuses compared with normal adults. The circulating TSH glycoforms and their glycan compositions in young children have hitherto not been determined.

OBJECTIVE

To characterize N-glycosylation and glycan composition of circulating TSH molecules in young children.

DESIGN, PARTICIPANTS, MAIN OUTCOME MEASURES

Serum samples were obtained from euthyroid individuals: 33 children, age 2 weeks to 3 years, and 264 adults. The di-glycosylated TSH and tri-glycosylated TSH glycoforms were determined and characterized with respect to sialylation and sulfonation. The TSH N-glycosylation was also examined in pituitary extracts of 75 individuals.

RESULTS

In children up to 18 months of age, most TSH molecules were low-N-glycosylated, high-sulfonated, and low-sialylated compared with older children and adults. The degree of N-glycosylation was similar in serum and pituitary extracts up to 3 months of age and after that was higher in serum than in pituitary extracts.

CONCLUSIONS

Children up to age 18 months had low-sialylated TSH molecules, similar to those reported for third-trimester fetuses. Most TSH molecules in young children were of smaller size and less negatively charged, favoring transport into their target tissues. The low sialylation favors a high biopotency at thyroid and extrathyroidal TSH receptors. A delayed development of the liver SO3-N-acetylgalactosamine receptor function after birth is a likely explanation of the highly sulfonated TSH molecules in serum samples of infants.

Thyroid-Stimulating Hormone, Degree of Obesity, and Metabolic Risk Markers in a Cohort of Swedish Children with Obesity

BACKGROUND/AIMS

Thyroid-stimulating hormone (TSH) is affected in obesity and might influence metabolic risk. It is unclear what mechanisms cause elevated TSH in obesity. We aimed to investigate TSH status within the normal range and the association of TSH with degree of obesity and metabolic parameters in children with obesity.

METHODS

A total of 3,459 children, aged 3.0-17.9 years, were identified in the Swedish Childhood Obesity Treatment Registry, BORIS. Age, gender, TSH, free triiodothyronine (fT3), free thyroxine (fT4), body mass index standard deviation scores (BMI SDS), as well as variables of lipid and glucose metabolism were examined.

RESULTS

Children with high-normal TSH (>3.0 mU/L) (28.8%) had higher BMI SDS compared to children with low-normal TSH (<3.0 mU/L) (p < 0.001). Multivariable regression analysis adjusted for age and gender showed that TSH levels were associated with BMI SDS (β: 0.21, 95% CI: 0.14-0.28, p < 0.001). Associations of thyroid hormones with markers of lipid and glucose metabolism were observed, where TSH was associated with fasting insulin, HOMA (homeostatic model assessment of insulin resistance), total cholesterol, and triglycerides.

CONCLUSIONS

A positive association between TSH levels and BMI SDS was seen in children with obesity. Associations of TSH and free thyroid hormones with glucose metabolism indicated that TSH might be one of several factors acting to determine body weight and obesity co-morbidities, although the underlying mechanism remains unclear.

Implications of Lipids in Neonatal Body Weight and Fat Mass in Gestational Diabetic Mothers and Non-Diabetic Controls

PURPOSE OF REVIEW

Maternal lipid metabolism greatly changes during pregnancy and we review in this article how they influence fetal adiposity and growth under non-diabetic and gestational diabetic conditions.

RECENT FINDINGS

In pregnant women without diabetes (control), maternal glycemia correlates with neonatal glycemia, neonatal body weight and fat mass. In pregnant women with gestational diabetes mellitus (GDM), maternal glucose correlates with neither neonatal glycemia, neonatal birth weight nor fat mass, but maternal triacylglycerols (TAG), non-esterified fatty acids (NEFA) and glycerol do correlate with birth weight and neonatal adiposity. The proportions of maternal plasma arachidonic (AA) and docosahexaenoic (DHA) acids decrease from the first to the third trimester of pregnancy, and at term these long-chain polyunsaturated fatty acids are higher in cord blood plasma than in mothers, indicating efficient placental transfer. In control or pregnant women with GDM at term, the maternal concentration of individual fatty acids does not correlate with neonatal body weight or fat mass, but cord blood fatty acid levels correlate with birth weight and neonatal adiposity-positively in controls, but negatively in GDM. The proportion of AA and DHA in umbilical artery plasma in GDM is lower than in controls but not in umbilical vein plasma. Therefore, an increased utilization of those two fatty acids by fetal tissues, rather than impaired placental transfer, is responsible for their smaller proportion in plasma of GDM newborns. In control pregnant women, maternal glycemia controls neonatal body weight and fat mass, whereas in mothers with GDM-even with good glycemic control-maternal lipids and their greater utilization by the fetus play a critical role in neonatal body weight and fat mass. We propose that altered lipid metabolism rather than hyperglycemia constitutes a risk for macrosomia in GDM.

The Choice of Euthanasia Method Affects Metabolic Serum Biomarkers

The impact of euthanasia methods on endocrine and metabolic parameters in rodent tissues and biological fluids is highly relevant for the accuracy and reliability of the data collected. However, few studies concerning this issue are found in the literature. We compared the effects of three euthanasia methods currently used in animal experimentation (i.e. decapitation, CO₂ inhalation and pentobarbital injection) on the serum levels of corticosterone, insulin, glucose, triglycerides, cholesterol and a range of free fatty acids in rats. The corticosterone and insulin levels were not significantly affected by the euthanasia protocol used. However, euthanasia by an overdose of pentobarbital (120 mg/kg intraperitoneal injection) increased the serum levels of glucose, and decreased cholesterol, stearic and arachidonic acids levels compared with euthanasia by CO₂ inhalation and decapitation. CO₂ inhalation appears to increase the serum levels of triglycerides, while euthanasia by decapitation induced no individual discrepant biomarker level. We conclude that choice of the euthanasia methods is critical for the reliability of serum biomarkers and indicate the importance of selecting adequate euthanasia methods for metabolic analysis in rodents. Decapitation without anaesthesia may be the most adequate method of euthanasia when taking both animal welfare and data quality in consideration.

The Role of Thyrotropin Receptor Activation in Adipogenesis and Modulation of Fat Phenotype

Evidence from clinical and experimental data suggests that thyrotropin receptor (TSHR) signaling is involved in energy expenditure through its impact on white adipose tissue (WAT) and brown adipose tissue (BAT). TSHR expression increases during mesenchymal stem cell (MSC) differentiation into fat. We hypothesize that TSHR activation [TSHR*, elevated thyroid-stimulating hormone, thyroid-stimulating antibodies (TSAB), or activating mutation] influences MSC differentiation, which contributes to body composition changes seen in hypothyroidism or Graves' disease (GD). The role of TSHR activation on adipogenesis was first investigated using ex vivo samples. Neck fat (all euthyroid at surgery) was obtained from GD (n = 11, TSAB positive), toxic multinodular goiter (TMNG, TSAB negative) (n = 6), and control patients with benign euthyroid disease (n = 11, TSAB negative). The effect of TSHR activation was then analyzed using human primary abdominal subcutaneous preadipocytes (n = 16). Cells were cultured in complete medium (CM) or adipogenic medium [ADM, containing thiazolidinedione (TZD), PPARγ agonist, which is able to induce BAT formation] with or without TSHR activation (gain-of-function mutant) for 3 weeks. Adipogenesis was evaluated using oil red O (ORO), counting adipogenic foci, qPCR measurement of terminal differentiation marker (LPL). BAT [PGC-1α, uncoupling protein 1 (UCP1), and ZIC1], pre-BAT (PRDM16), BRITE- (CITED1), or WAT (LEPTIN) markers were analyzed by semiquantitative PCR or qPCR. In ex vivo analysis, there were no differences in the expression of UCP1, PGC-1α, and ZIC1. BRITE marker CITED1 levels were highest in GD followed by TMNG and control (p for trend = 0.009). This was associated with higher WAT marker LEPTIN level in GD than the other two groups (p < 0.001). In primary cell culture, TSHR activation substantially enhanced adipogenesis with 1.4 ± 0.07 (ORO), 8.6 ± 1.8 (foci), and 5.5 ± 1.6 (LPL) fold increases compared with controls. Surprisingly, TSHR activation in CM also significantly increased pre-BAT marker PRDM16; furthermore, TZD-ADM induced adipogenesis showed substantially increased BAT markers, PGC-1α and UCP1. Our study revealed that TSHR activation plays an important role in the adipogenesis process and BRITE/pre-BAT formation, which leads to WAT or BAT phenotype. It may contribute to weight loss as heat during hyperthyroidism and later transforms into WAT posttreatment of GD when patients gain excess weight.

Subclinical Hypothyroidism - What is Responsible for its Association with Cardiovascular Disease?

Subclinical hypothyroidism (SH) is a common condition, with prevalence estimates ranging from 4–20%, depending on the population demographics. Although epidemiological analysis associates it with an increased risk of cardiovascular disease, clinical practice guidelines express uncertainty about whether to monitor or to treat. As we await large-scale, well-designed randomised clinical trials regarding treatment of SH, a review of pathophysiological considerations may be informative to better understand this disorder.

Plasma amylin concentration in suckling goat neonates and its relationship with C-reactive protein, selected biochemical and hormonal indicators

Amylin is a recently discovered neuropeptide hormone that belongs to the calcitonin gene-related peptide family. It is co-secreted with insulin in response to feed intake. In goat kids, neonatal mortality and morbidity seems to be relatively higher than in other farm species. This high mortality and morbidity in goat kids may be associated with underdeveloped metabolism and immune system during the first week of life. The main objectives of this study were to determine amylin concentration and its relationship with some hormones, biochemical indicators and with a general inflammatory marker, CRP (C-reactive protein) in goat neonates. Blood samples were collected from 30 Saanen goat neonates at 20–35 days of age. Plasma amylin and other hormone concentrations were measured by ELISA, whereas serum biochemical indices were analysed by spectrophotometry. The mean values of plasma amylin concentrations were 9.07 ± 0.25 pmol/l. Plasma amylin concentrations were positively correlated with plasma non-esterified fatty acids, CRP, prolactin, cortisol, insulin; however, a negative correlation was determined between plasma amylin and serum triglyceride concentrations. The current study suggests that amylin contents are strongly associated with circulating concentrations of some hormones and with those of CRP in Saanen goat kids.

Thyroid-stimulating hormone inhibits adipose triglyceride lipase in 3T3-L1 adipocytes through the PKA pathway

Thyroid-stimulating hormone (TSH) has been shown to play an important role in the regulation of triglyceride (TG) metabolism in adipose tissue. Adipose triglyceride lipase (ATGL) is a rate-limiting enzyme controlling the hydrolysis of TG. Thus far, it is unclear whether TSH has a direct effect on the expression of ATGL. Because TSH function is mediated through the TSH receptor (TSHR), TSHR knockout mice (Tshr-/- mice) (supplemented with thyroxine) were used in this study to determine the effects of TSHR deletion on ATGL expression. These effects were verified in 3T3-L1 adipocytes and potential underlying mechanisms were explored. In the Tshr-/- mice, ATGL expression in epididymal adipose tissue was significantly increased compared with that in Tshr+/+ mice. ATGL expression was observed to increase with the differentiation process of 3T3-L1 preadipocytes. In mature 3T3-L1 adipocytes, TSH significantly suppressed ATGL expression at both the protein and mRNA levels in a dose-dependent manner. Forskolin, which is an activator of adenylate cyclase, suppressed the expression of ATGL in 3T3-L1 adipocytes. The inhibitory effects of TSH on ATGL expression were abolished by H89, which is a protein kinase A (PKA) inhibitor. These results indicate that TSH has an inhibitory effect on ATGL expression in mature adipocytes. The associated mechanism is related to PKA activation.

TSH signaling pathways that regulate MCP-1 in human differentiated adipocytes

OBJECTIVE

Adipose tissue is an extra-thyroidal thyroid-stimulating hormone (TSH) target. Increases in lipolysis and in expression and release of interleukin-6 (IL-6) occur in TSH-stimulated adipocytes, and levels of circulating free fatty acids and IL-6 rise following TSH administration to patients with previous thyroidectomy and radioablation for thyroid cancer. Our first objective was to compare how TSH stimulates protein kinase A (PKA) and inhibitor of κB (IκB) kinase (IKK)-β. Our second objective was to investigate whether TSH induces other cytokines besides IL-6.

METHODS

TSH stimulation of either CHO cells expressing human TSH receptor or human abdominal subcutaneous differentiated adipocytes.

RESULTS

Signaling studies showed TSH increased NADPH oxidase activity, and either diphenyleneiodonium (oxidase inhibitor) or N-acetyl cysteine (scavenger of reactive oxygen species) reduced IKKβ phosphorylation. Phosphorylation of protein kinase C-δ, an upstream regulator of NADPH oxidase, was increased by TSH, and rottlerin (PKCδ inhibitor) reduced TSH-stimulated IKKβ phosphorylation. TSH upregulated monocyte chemoattractant protein-1 (MCP-1) mRNA expression and the release of MCP-1 protein in human abdominal differentiated adipocytes. H89 (PKA inhibitor) and sc-514 (IKKβ inhibitor) each blocked TSH-stimulated MCP-1 mRNA expression and protein release, suggesting PKA and IKKβ participate in this pathway.

CONCLUSIONS

These data provide new information about TSH signaling in human differentiated adipocytes, and add to the evidence that TSH is a pro-inflammatory stimulus of adipocytes.

Dissecting adipose tissue lipolysis: molecular regulation and implications for metabolic disease

Lipolysis is the process by which triglycerides (TGs) are hydrolyzed to free fatty acids (FFAs) and glycerol. In adipocytes, this is achieved by sequential action of adipose TG lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase. The activity in the lipolytic pathway is tightly regulated by hormonal and nutritional factors. Under conditions of negative energy balance such as fasting and exercise, stimulation of lipolysis results in a profound increase in FFA release from adipose tissue (AT). This response is crucial in order to provide the organism with a sufficient supply of substrate for oxidative metabolism. However, failure to efficiently suppress lipolysis when FFA demands are low can have serious metabolic consequences and is believed to be a key mechanism in the development of type 2 diabetes in obesity. As the discovery of ATGL in 2004, substantial progress has been made in the delineation of the remarkable complexity of the regulatory network controlling adipocyte lipolysis. Notably, regulatory mechanisms have been identified on multiple levels of the lipolytic pathway, including gene transcription and translation, post-translational modifications, intracellular localization, protein-protein interactions, and protein stability/degradation. Here, we provide an overview of the recent advances in the field of AT lipolysis with particular focus on the molecular regulation of the two main lipases, ATGL and HSL, and the intracellular and extracellular signals affecting their activity.

Lack of relationship between cord serum angiopoietin-like protein 4 (ANGPTL4) and lipolytic activity in human neonates born by spontaneous delivery

BACKGROUND

Ligands of peroxisome-proliferator activated receptors (PPARs), such as non-esterified fatty acids (NEFAs), induce expression of angiopoietin-like protein 4 (ANGPTL4). Recently ANGPTL4 has been reported to be a mediator of intracellular adipose lipolysis induced by glucocorticoids.

OBJECTIVE

To determine the concentrations of ANGPTL4 in cord serum of neonates born by spontaneous vaginal delivery (SVD) and by pre-labor cesarean section (CS) from healthy women, and to relate them to parameters of neonatal lipolytic activity at birth.

MEASUREMENTS

In 54 neonates born by SVD and in 56 neonates born by CS, arterial cord blood was drawn to determine insulin, cortisol, triacylglycerols (TAGs), glycerol, non-esterified fatty acids (NEFAs), individual fatty acids, ANGPTL4, adiponectin, retinol binding protein 4 (RBP4) and leptin.

RESULTS

Birth weight and neonatal fat mass in SVD and CS showed no difference, but the concentrations of glycerol, adiponectin, RBP4, NEFAs and most individual fatty acids were higher in cord serum of neonates born by SVD compared to CS, indicating a higher adipose tissue breakdown in the SVD group. The concentrations of TAG and cortisol were also higher and that of insulin was lower in cord serum of SVD compared to the CS group. However, the concentration in cord serum of ANGPTL4 did not differ between the two groups and no positive correlation with either NEFA or glycerol concentrations were detected.

CONCLUSION

ANGPTL4 is known to stimulate lipolysis in adults, but does not appear to mediate the increased activity in SVD, indicating the presence of different regulatory inputs.

Neonatal liver physiology

In the neonate, the liver is relatively immature and undergoes several changes in its functional capacity during the early postnatal period. The essential liver functions can be classified into three categories: metabolism, detoxification, and bile synthesis. In general, the immature liver function has limited consequences on the healthy term neonate. However, preterm neonates are particularly susceptible to the effects of the immature liver function placing them at risk of hypoglycemia, hyperbilirubinemia, cholestasis, bleeding, and impaired drug metabolism. An appreciation of the dynamic changes in liver function during the neonatal period is essential for successful management of neonates who require medical and surgical interventions. This review will focus on the neonatal liver function as well as the changes that the liver undergoes as it matures.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.

Thyrotropin regulates IL-6 expression in CD34+ fibrocytes: clear delineation of its cAMP-independent actions

IL-6 plays diverse roles in normal and disease-associated immunity such as that associated with Graves’ disease (GD). In that syndrome, the orbit undergoes remodeling during a process known as thyroid-associated ophthalmopathy (TAO). Recently, CD34⁺ fibrocytes were found to infiltrate the orbit in TAO where they transition into CD34⁺ orbital fibroblasts. Surprisingly, fibrocytes display high levels of functional thyrotropin receptor (TSHR), the central antigen in GD. We report here that TSH and the pathogenic anti-TSHR antibodies that drive hyperthyroidism in GD induce IL-6 expression in fibrocytes and orbital fibroblasts. Unlike TSHR signaling in thyroid epithelium, that occurring in fibrocytes is completely independent of adenylate cyclase activation and cAMP generation. Instead TSH activates PDK1 and both AKT/PKB and PKC pathways. Expression and use of PKCβII switches to that of PKCµ as fibrocytes transition to TAO orbital fibroblasts. This shift is imposed by CD34⁻ orbital fibroblasts but reverts when CD34⁺ fibroblasts are isolated. The up-regulation of IL-6 by TSH results from coordinately enhanced IL-6 gene promoter activity and increased IL-6 mRNA stability. TSH-dependent IL-6 expression requires activity at both CREB (−213 to −208 nt) and NF-κB (–78 to −62 nt) binding sites. These results provide novel insights into the molecular action of TSH and signaling downstream for TSHR in non-thyroid cells. Fibrocytes neither express adenylate cyclase nor generate cAMP and thus these findings are free from any influence of cAMP-related signaling. They identify potential therapeutic targets for TAO.

Human fibrocytes coexpress thyroglobulin and thyrotropin receptor

Thyroglobulin (Tg) is the macromolecular precursor of thyroid hormones and is thought to be uniquely expressed by thyroid epithelial cells. Tg and the thyroid-stimulating hormone receptor (TSHR) are targets for autoantibody generation in the autoimmune disorder Graves disease (GD). Fully expressed GD is characterized by thyroid overactivity and orbital tissue inflammation and remodeling. This process is known as thyroid-associated ophthalmopathy (TAO). Early reports suggested that in TAO, both Tg and TSHR become overexpressed in orbital tissues. Previously, we found that CD34(+) progenitor cells, known as fibrocytes, express functional TSHR, infiltrate the orbit, and comprise a large subset of orbital fibroblasts in TAO. We now report that fibrocytes also express Tg, which resolves as a 305-kDa protein on Western blots. It can be immunoprecipitated with anti-Tg Abs. Further, (125)iodine and [(35)S]methionine are incorporated into Tg expressed by fibrocytes. De novo Tg synthesis is attenuated with a specific small interfering RNA targeting the protein. A fragment of the Tg gene promoter fused to a luciferase reporter exhibits substantial activity when transfected into fibrocytes. Unlike fibrocytes, GD orbital fibroblasts, which comprise a mixture of CD34(+) and CD34(-) cells, express much lower levels of Tg and TSHR. When sorted into pure CD34(+) and CD34(-) subsets, Tg and TSHR mRNA levels become substantially higher in CD34(+) cells. These findings indicate that human fibrocytes express multiple "thyroid-specific" proteins, the levels of which are reduced after they infiltrate tissue. Our observations establish the basis for Tg accumulation in orbital GD.

New cases of isolated congenital central hypothyroidism due to homozygous thyrotropin beta gene mutations: a pitfall to neonatal screening

BACKGROUND

Congenital central hypothyroidism (CCH) is a rare condition that is often diagnosed in late childhood in countries where neonatal screening programs rely solely on detecting thyrotropin (TSH) elevation. TSHbeta gene mutation is one of the causes of CCH. We describe two cases of c.Q49X mutation and three cases of c.C105Vfs114X mutation in exon 3 of the TSH beta-subunit gene.

SUMMARY

We found two different TSHbeta gene mutations in two families. In one family, we identified a missense mutation in exon 3 leading to a premature stop at position 49 (c.Q49X) in the two affected twins. In the other family, the three affected siblings had a 313delT nucleotide deletion leading to a frame shift responsible for premature termination at codon 114 (c.C105Vfs114X); neonatal screening showed very low TSH levels in all three patients. The presence of inappropriately low TSH levels at birth in the three affected members of the second family raises questions about the value of the TSH level for CCH screening.

CONCLUSIONS

The marked phenotypic variability in patients with the c.Q49X mutation suggests modulation by interacting genes and/or differences in the genetic background. TSHbeta gene mutations should be suspected in neonates with inappropriately low TSH levels.

Thyroid-stimulating hormone stimulates lipolysis in adipocytes in culture and raises serum free fatty acid levels in vivo

Thyroid-stimulating hormone (TSH) stimulates adipocyte lipolysis, but signal transduction pathways activated by TSH for this response have not been directly studied. Using differentiated 3T3-L1 adipocytes as well as primary human adipocytes, we characterized the lipolytic action of TSH with dose-response and time-course studies, and compared it with isoproterenol. Thyroid-stimulating hormone stimulated phosphorylation of perilipin and hormone-sensitive lipase (HSL). Inhibition of protein kinase A with H89 blocked TSH-stimulated lipolysis as well as phosphorylation of perilipin and HSL. Thyroid-stimulating hormone stimulated lipolysis in vivo, as indicated by an elevation in serum free fatty acid (FFA) levels after recombinant human TSH administration to thyroidectomized patients (42% increase, n = 19, P < .05). For patients with a body mass index less than 30 kg/m(2), the TSH-induced increase in serum FFA levels was 53% (n = 11, P < .05), whereas levels in patients with a body mass index of at least 30 kg/m(2) (n = 8) did not change after TSH treatment. In summary, TSH stimulates lipolysis and phosphorylation of perilipin and HSL in a protein kinase A-dependent manner in differentiated adipocytes in culture and raises serum FFA levels in vivo.

Increased generation of fibrocytes in thyroid-associated ophthalmopathy

CONTEXT

The pathogenic basis for Graves' disease (GD) continues to elude our understanding. Specifically why activating antibodies are generated against self-antigens remains uncertain as does the identity of the antigen(s) that provokes orbital involvement in GD, a process known as thyroid-associated ophthalmopathy (TAO).

OBJECTIVE

The aim of the study was to determine whether CD34(+) fibrocytes are generated more frequently in GD, whether they infiltrate orbital connective tissues in TAO, and whether they express the thyrotropin receptor (TSHR).

DESIGN/SETTING/PARTICIPANTS

Generation of fibrocytes from peripheral blood mononuclear cells was examined in samples from 70 patients with GD and 25 healthy control subjects. Fibrocytes were characterized by flow cytometry. Orbital tissues and fibroblast culture strains were examined for their presence.

MAIN OUTCOME MEASURES

The frequency of CD34(+) fibrocyte generation from peripheral blood cells, characterization of their phenotype, cytokine production, and their presence in affected orbital tissues were analyzed.

RESULTS

CD34(+)CXCR4(+)Col I(+) fibrocytes expressing IGF-I receptor are far more frequently generated from cultured peripheral blood mononuclear cells of donors with GD compared with healthy subjects. They express TSHR at high levels and TSH induces fibrocytes to produce IL-6 and TNF-alpha. Numerous CD34(+) fibrocytes were detected in orbital tissues in TAO but were absent in healthy orbits. Tissue-infiltrating fibrocytes express TSHR in situ and comprise a subpopulation of TAO-derived orbital fibroblasts.

CONCLUSIONS

Our findings suggest that fibrocytes may participate in the pathogenesis of TAO because they express relevant autoantigens such as IGF-I receptor and functional TSHR and differentially accumulate in orbital tissue in TAO.

Lipolysis and lipid mobilization in human adipose tissue

Triacylglycerol (TAG) stored in adipose tissue (AT) can be rapidly mobilized by the hydrolytic action of the three main lipases of the adipocyte. The non-esterified fatty acids (NEFA) released are used by other tissues during times of energy deprivation. Until recently hormone-sensitive lipase (HSL) was considered to be the key rate-limiting enzyme responsible for regulating TAG mobilization. A novel lipase named adipose triglyceride lipase/desnutrin (ATGL) has been identified as playing an important role in the control of fat cell lipolysis. Additionally perilipin and other proteins of the surface of the lipid droplets protecting or exposing the TAG core of the droplets to lipases are also potent regulators of lipolysis. Considerable progress has been made in understanding the mechanisms of activation of the various lipases. Lipolysis is under tight hormonal regulation. The best understood hormonal effects on AT lipolysis concern the opposing regulation by insulin and catecholamines. Heart-derived natriuretic peptides (i.e., stored in granules in the atrial and ventricle cardiomyocytes and exerting stimulating effects on diuresis and natriuresis) and numerous autocrine/paracrine factors originating from adipocytes and other cells of the stroma-vascular fraction may also participate in the regulation of lipolysis. Endocrine and autocrine/paracrine factors cooperate and lead to a fine regulation of lipolysis in adipocytes. Age, anatomical site, sex, genotype and species differences all play a part in the regulation of lipolysis. The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity and probably in several inborn errors of metabolism.

Neonatal glucose metabolism in offspring of mothers with varying degrees of hyperglycemia during pregnancy

The definition of neonatal hypoglycemia is controversial. Operational thresholds of blood glucose values at which intervention should be considered have been proposed. IDM and GDM infants frequently exhibit a pronounced drop of plasma glucose immediately after birth. This exaggerated physiological decline of glucose is transient and is seldom accompanied by suppressed lipolysis or clinical symptoms. It is generally attributed to hyperinsulinism elicited by maternal hyperglycemia. Alternative substrates for CNS i.e. lactate and astrocyte glycogen may explain lack of symptoms. Similarly low glucose values later on may cause clinical symptoms. Glucose production rates vary from attenuated to normal likely reflecting differences in maternal glycemic control. The HAPO study of around 25,000 non-diabetic pregnancies revealed strong associations between glucose values (75g OGTT) and increased fetal size and hyperinsulinemia at birth - findings adding strong support to the maternal hyperglycemia - fetal hypinsulinism theory. Mothers with the highest fasting glucose had infants with the highest frequency of clinical neonatal hypoglycaemia (4,6%).

Lipolysis and the integrated physiology of lipid energy metabolism

Fat cell lipolysis, the cleavage of triglycerides and release of fatty acids and glycerol, evolved to enable survival during prolonged food deprivation but is paradoxically increased in obesity, in which a surfeit of all energy metabolites is found. Essential, previously-unsuspected components have been discovered in the lipolytic machinery, at the protective interface of the lipid droplet surface and in the signaling pathways that control lipolysis. At least two adipocyte lipases are important for controlling lipolysis, hormone-sensitive lipase (HSL) and adipocyte triglyceride lipase (ATGL). Perilipin (PLIN) and possibly other proteins of the lipid droplet surface are master regulators of lipolysis, protecting or exposing the triglyceride core of the droplet to lipases. The prototypes for hormonal lipolytic control are beta adrenergic stimulation and suppression by insulin, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and PLIN. Newly-recognized mediators of lipolysis include atrial natriuretic peptide, cyclic GMP, the ketone body 3-hydroxybutyrate, AMP kinase and mitogen-activated kinases. Lipolysis must be interpreted in its physiological context since similar rates of basal or stimulated lipolysis occur under different conditions and by different mechanisms. Age, sex, anatomical site, genotype and species differences are each important variables. Manipulation of lipolysis has therapeutic potential in several inborn errors and in the metabolic syndrome that frequently complicates obesity.

Thyroid-stimulating hormone receptor in brown adipose tissue is involved in the regulation of thermogenesis

C.RF- Tshr(hyt/hyt) mice have a mutated thyroid-stimulating hormone receptor (TSHR), and, without thyroid hormone supplementation, these mice develop severe hypothyroidism. When hypothyroid Tshr(hyt/hyt) mice were exposed to cold (4 degrees C), rectal temperature rapidly dropped to 23.9 +/- 0.40 degrees C at 90 min, whereas the wild-type mice temperatures were 37.0 +/- 0.15 degrees C. When we carried out functional rat TSHR gene transfer in the brown adipose tissues by plasmid injection combined with electroporation, there was no effect on the serum levels of thyroxine, although rectal temperature of the mice transfected with pcDNA3.1/Zeo-rat TSHR 90 min after cold exposure remained at 34.6 +/- 0.34 degrees C, which was significantly higher than that of Tshr(hyt/hyt) mice. Transfection of TSHR cDNA increased mRNA and protein levels of uncoupling protein-1 (UCP-1) in brown adipose tissues, and the weight ratio of brown adipose tissue to overall body weight also increased. Exogenous thyroid hormone supplementation to Tshr(hyt/hyt) mice restored rectal temperature 90 min after exposure to cold (36.8 +/- 0.10 degrees C). These results indicate that not only thyroid hormone but also thyroid-stimulating hormone (TSH)/TSHR are involved in the expression mechanism of UCP-1 in mouse brown adipose tissue. TSH stimulates thermogenesis and functions to protect a further decrease in body temperature in the hypothyroid state.

Review on the occasion of a decade of recombinant human TSH: prospects and novel uses

The introduction of recombinant human thyroid-stimulating hormone (rhTSH) almost a decade ago represents a remarkable achievement in the history of clinical thyroidology. rhTSH now contributes substantially to the diagnostic approach to thyroid cancer, offering a reliable and safe alternative to thyroid hormone withdrawal by avoiding the morbidity of hypothyroidism. Several recent studies have also demonstrated the efficacy of radioiodine ablation of thyroid remnants after preparation with rhTSH. Moreover, the use of rhTSH in this context is associated with a lower whole body exposure to radiation compared to thyroid hormone withdrawal. Although not approved officially, rhTSH-assisted treatment of locoregional or distant metastatic disease may be the treatment of choice for patients in whom hypothyroidism may be relatively contraindicated, such as the very young and the aged, although additional efficacy studies are necessary. The compound has also been shown to be useful in the treatment of nontoxic multinodular goiter, especially when the radioiodine uptake is low. Finally, rhTSH may prove to be useful in studying the functional reserve of thyroid in the aging process, as well as the putative role of the TSH receptor in extrathyroidal tissue, such as lymphocytes, osteocytes, and adipocytes.

Insulin and carbohydrate metabolism

Fetal glucose exposure and consequent fetal insulin secretion is normally tightly regulated by glucose delivery from the mother during pregnancy. Maternal hyperglycaemia and gestational diabetes (GDM) are known to be detrimental to offspring, although defining the criteria for diagnosis of GDM is controversial. Recent data suggest that the risk of poor fetal outcome appears to be a continuous variable across the range of glucose control, and that the level of maternal blood glucose for a diagnosis of gestational diabetes needs to be reviewed. After birth, rapid adaptation is necessary for infants to be able to maintain independent glucose homeostasis. This adaptation is compromised in infants who are small for gestational age (SGA), premature, or large for gestational age (LGA). Interestingly, the infants who are born at the extremes of birth weight are also at increased risk of impaired glucose tolerance and diabetes in later life.

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.

Lipolysis and insulin sensitivity at birth in infants who are large for gestational age

OBJECTIVE

In addition to neonatal hypoglycemia, infants who are born large for gestational age are at risk for developing obesity, cardiovascular disease, and diabetes later in life. The aim of this study was to investigate glucose production, lipolysis, and insulin sensitivity in infants who were born large for gestational age to mothers without diabetes. The effect of glucagon administration on production of energy substrates was also investigated.

METHODS

Ten healthy term infants who were born large for gestational age to mothers without diabetes were studied 16 +/- 8 hours postnatally after a 3-hour fast. Rates of glucose production and lipolysis were analyzed by gas chromatography-mass spectrometry following constant rate infusion of [6,6-(2)H2]glucose and [2-(13)C]glycerol. Insulin sensitivity was assessed by the Homeostasis Assessment Model. In 8 of the infants, the effect of an intravenous injection of 0.2 mg/kg glucagon was also analyzed.

RESULTS

Plasma glucose and glycerol averaged 3.8 +/- 0.5 mmol/L and 384 +/- 183 micromol/L, respectively. The glycerol production rate, reflecting lipolysis, was 12.7 +/- 2.9 micromol/kg per min. Mean rate of glucose production was 30.2 +/- 4.6 micromol/kg per min. Homeostasis Assessment Model insulin sensitivity corresponded to 82% +/- 19%, beta-cell function to 221% +/- 73%, and insulin resistance to 1.3 +/- 0.3. After glucagon administration, rate of glucose production increased by 13.3 +/- 8.3 micromol/kg per min and blood glucose by 1.4 +/- 0.5 mmol/L. Glycerol production decreased from 12.8 +/- 3.0 to 10.7 +/- 2.9 micromol/kg per min. Mean insulin concentration increased from 10.9 +/- 3.0 to 30.9 +/- 10.3 mU/L. There was a strong inverse correlation between the decrease in lipolysis and increase in insulin after glucagon administration.

CONCLUSIONS

Infants who are born large for gestational age show increased lipolysis and a propensity for decreased insulin sensitivity already at birth. The simultaneous increase in plasma insulin correlated strongly with the noted decrease in lipolysis, indicating an antilipolytic effect of insulin in these infants.

Nocturnal secretion of growth hormone, noradrenaline, cortisol and insulin in cluster headache remission

We have previously shown decreased, nocturnal lipolysis in both phases of cluster headache (CH). Lipolysis is stimulated by noradrenaline (NA), growth hormone (GH) and cortisol, and inhibited by insulin, hormones which are directly or indirectly regulated by the hypothalamus. Our aim was to investigate the nocturnal secretion of NA, GH, cortisol and insulin in nine CH patients in remission and 10 healthy controls. Nocturnal venous blood samples were collected in hourly intervals for analysis of NA, cortisol and insulin and in 30-min intervals for GH. We found a reduced increase in GH between 24.00 h and 01.00 h (anova, P < 0.05) in CH patients. Nocturnal secretion of NA, cortisol and insulin did not differ significantly between the groups. The altered nocturnal GH pattern that was seen in CH patients in remission might in part explain the altered nocturnal lipolysis previously found and further indicate a permanent hypothalamic disturbance in CH.

Pathogenesis of graves ophthalmopathy: implications for prediction, prevention, and treatment

PURPOSE

To review current concepts regarding the pathogenesis of Graves ophthalmopathy (GO). We have presented this information in the context of potential target sites for novel disease therapies.

DESIGN

Review of recent literature.

METHODS

Synthesis of recent literature.

RESULTS

Enlargement of the extraocular muscle bodies and expansion of the orbital fatty connective tissues is apparent in patients with GO. These changes result from abnormal hyaluronic acid accumulation and edema within these tissues and expanded volume of the orbital adipose tissues. Recent studies have suggested that the increase in orbital fat volume is caused by stimulation of adipogenesis within these tissues. The orbital fibroblast appears to be the major target cell of the autoimmune process in GO. A subset of these cells is capable of producing hyaluronic acid and differentiating into mature adipocytes, given appropriate stimulation. In addition, orbital fibroblasts from patients with GO have been shown to display immunoregulatory molecules and to express both thyrotropin receptors (TSHRs) and insulin-like growth factor 1 receptors (IGF-1Rs). Increased TSHR expression in the GO orbit appears to be the result of stimulated adipocyte differentiation. The activation of IGF-1R on orbital fibroblasts by immunoglobulins from GO patients results in increased production of both hyaluronic acid and molecules that stimulate the infiltration of activated T cells into areas of inflammation.

CONCLUSIONS

Potential targets for novel therapeutic agents to be used in GO include blocking T-cell costimulation, depleting B cells, inhibiting cytokine action, targeting the IGF-1R or the TSHR, and preventing connective tissue remodeling.

beta-Receptor response to noradrenaline in cluster headache. A study of adipose tissue lipolysis

We have previously shown decreased lipolysis in both phases of cluster headache (CH), as an indication of a sympathetic dysregulation. Reduced lipolysis could be a result of diminished beta-receptor sensitivity in adipose tissue. The aim of this study was to measure the lipolytic response to noradrenaline in 10 CH patients in remission and in 10 healthy subjects, to estimate beta-receptor function. Microdialysis technique was used to measure the increase of glycerol, the end-product of lipolysis, during infusion of noradrenaline into the adipose tissue. Noradrenaline infusion resulted in a distinct elevation of glycerol. The average glycerol increase was significantly higher in CH patients (121% +/- 48) than in healthy subjects (77% +/- 41) (P < 0.05), which indicates increased beta-receptor response to noradrenaline in CH patients in remission. This may be due to up-regulated beta-receptor sensitivity, secondary to reduced sympathetic outflow and a primary autonomic disturbance in CH.

Control of fatty acid and glycerol release in adipose tissue lipolysis

Adipose tissue lipolysis is the catabolic process leading to the breakdown of triglycerides stored in fat cells and the release of fatty acids and glycerol. Recent work has revealed that lipolysis is not a simple metabolic pathway stimulated by catecholamines and inhibited by insulin. New discoveries on the regulation of lipolysis by endocrine and paracrine factors and on the proteins involved in triglyceride hydrolysis have led to a reappraisal of the complexity of the various signal transduction pathways. The steps involved in the dysregulation of lipolysis observed in obesity have partly been identified.

Adipose tissue lipolysis as a metabolic pathway to define pharmacological strategies against obesity and the metabolic syndrome

Adipose tissue lipolysis is the catabolic process leading to the breakdown of triglycerides stored in fat cells and release of fatty acids and glycerol. Recent work has revealed that lipolysis is not a simple metabolic pathway stimulated by catecholamines and inhibited by insulin. There have been new discoveries on the endocrine and paracrine regulation of lipolysis and on the molecular mechanisms of triglyceride hydrolysis. Catecholamines modulate lipolysis through lipolytic beta-adrenoceptor and antilipolytic alpha2-adrenoceptor. Recent studies have allowed a better understanding of the relative contribution of the two types of receptors and provided evidence for the in vivo involvement of alpha2-adrenoceptors in the physiological control of subcutaneous adipose tissue lipolysis. A puzzling observation is the characterization of a residual catecholamine-induced lipolysis in mice deficient in beta-adrenoceptors. A novel lipolytic system has been characterized in human fat cells. Natriuretic peptides stimulate lipolysis through a cGMP-dependent pathway. There are other lipolytic pathways active in human fat cells which importance is not fully understood. Forty years after the description of the antilipolytic effect of nicotinic acid, the receptors have been identified. Adrenomedullin which is produced by adipocytes exert an antilipolytic effect through an indirect mechanism involving nitric oxide. The molecular details of the lipolytic reaction are not fully understood. The role of the lipases has been re-evaluated with the cloning of adipose triglyceride lipase. Hormone-sensitive lipase appears as the major lipase for catecholamine and natriuretic peptide-stimulated lipolysis whereas adipose triglyceride lipase mediates the hydrolysis of triglycerides during basal lipolysis. Translocation of hormone-sensitive lipase bound to the adipocyte lipid binding protein to the lipid droplet seems to be an important step during lipolytic activation. Re-organization of the lipid droplet coating by perilipins facilitates the access of the enzyme. The role of other lipid-interacting proteins in lipolysis is still unclear. The proteins involved in the lipolytic process constitute drug targets for the treatment of obesity and the metabolic syndrome. The oldest example is nicotinic acid (niacin) used as a hypolipidaemic drug. A first approach consists in molecules stimulating lipolysis and oxidation of the released fatty acids to decrease fat stores. A second approach is a chronic inhibition of lipolysis to diminish plasma fatty acid level which is a central feature of the metabolic syndrome.

Lipolysis in smokers during tobacco withdrawal: a pilot study

OBJECTIVE

Nicotine has an influence on several metabolic events, such as lipid metabolism. Habitual smoking increases plasma levels of glycerol as well as noradrenaline, which is the main stimulating hormone of adipose tissue lipolysis. However, the long-term effect of smoking on lipolysis is unclear. We compared nocturnal lipolysis in habitual smokers during short-term tobacco withdrawal with a control group of non-smokers.

MATERIAL AND METHODS

Sixteen healthy subjects (9 heavy smokers and 7 non-smokers) were recruited in the study. The smokers were not permitted to smoke for at least 7 h before the test. The microdialysis technique was used to measure glycerol levels, the end-product of lipolysis, in subcutaneous adipose tissue. Variations in adipose tissue blood flow were measured using the ethanol technique. Glycerol, lactate and glucose concentrations as well as ethanol outflow/inflow ratio were measured between 2400 and 0600 h.

RESULTS

There were no significant differences in subcutaneous glycerol or glucose concentrations between smokers and non-smokers. Between 0300 and 0600 h, lactate levels in smokers were lower than those in non-smokers. Adipose tissue blood flow did not differ between the groups.

CONCLUSIONS

Despite potent acute and direct effects of smoking on lipolysis, we could not find any significant differences in basal lipolysis rate between smokers during short-term tobacco withdrawal and non-smokers.

Metabolism of lipids in human white adipocyte

Adipose tissue is considered as the body's largest storage organ for energy in the form of triacylglycerols, which are mobilized through lipolysis process, to provide fuel to other organs and to deliver substrates to liver for gluconeogenesis (glycerol) and lipoprotein synthesis (free fatty acids). The release of glycerol and free fatty acids from human adipose tissue is mainly dependent on hormone-sensitive lipase which is intensively regulated by hormones and agents, such as insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). A special attention is paid to the recently discovered perilipins which could regulate the activity of the lipase hormono-sensible. Most of the plasma triacylglycerols are provided by dietary lipids, secreted from the intestine in the form of chylomicron or from the liver in the form of VLDL. Released into circulation as non-esterified fatty acids by lipoprotein lipase, those are taken up by adipose tissue via specific plasma fatty acid transporters (CD36, FATP, FABPpm) and used for triacylglycerol synthesis. A small part of triacylglycerols is synthesized into adipocytes from carbohydrates (lipogenesis) but its regulation is still debated in human. Physiological factors such as dieting/fasting regulate all these metabolic pathways, which are also modified in pathological conditions e.g. obesity.

Age-dependent regulation of lipogenesis in human and rat adipocytes

The regulation of adipocyte metabolism is of importance for adipose tissue growth and therefore also for the development of obesity. This study was designed to investigate the regulation of basal and insulin-induced lipogenesis, glucose transport, and glucose transporter protein expression in human and rat adipocytes from different age groups. The study included 21 infants, 21 children, nine adults, and 80 male weaned and 20 male adult Fischer rats. The lipogenesis experiments were performed under conditions at which glucose transport is rate limiting. Basal lipogenesis was approximately three times higher in infants and children than in adults, whereas insulin-induced lipogenesis was two times higher in infants than in children and adults. In rats, basal lipogenesis, insulin-induced lipogenesis, and insulin sensitivity were two times higher in weaned than in adult animals. Moreover, basal and insulin-induced glucose transport were two times higher in weaned than in adult rats. No differences were detected in GLUT1 or GLUT4 content between any of the age groups in human or in rat adipocytes. In conclusion, basal and insulin-stimulated lipogenesis are increased in adipocytes early in life. This may promote adipose tissue growth in early age. The data indicate that age-dependent variation in basal and insulin-stimulated lipogenesis is differently regulated.

Leptin, thyrotropin, and thyroid hormones in obese/overweight women before and after two levels of energy deficit

The aim of our study was to compare serum concentration of leptin and pituitary-thyroid axis hormones in obese/overweight women before and after two levels of energy deficit with those parameters in lean women on adequate energy intake. Additionally, we attempted to elucidate if the effect of weight reduction could be related to anthropometric and hormonal parameters before treatment. Anthropometric and hormonal parameters-serum leptin, TSH, T4, fT4, T3 and leptin to fat mass (Lep/fm), T3/T4, fT4/T4, T4/TSH, fT4/TSH--were compared in two groups of women (n =18 each)--lean women (C: BMI 22.0 +/- 1.2) and overweight/obese (Ov/Ob: BMI 29.9 +/- 3.3). Ov/Ob women were subjected to weight-reducing treatment consisting of energy intake equal to 80% of calculated total energy expenditure for the first 4 wk and to 50% for subsequent 4 wk. All baseline hormone concentrations, Lep/fm, and fT4/T4 were higher in overweight/obese group. After 20% energy deficit decrease in BMI, percent body fat (fm%), leptin, T3, and TSH serum concentrations as well as in Lep/fm and T3/T4 was observed; T4/TSH increased, fT4, fT4/T4 and fT4/TSH did not change significantly. Increase in energy deficit from 20% to 50% resulted in normalization of Lep/fm, on the other hand, it provoked greater decline in thyroid hormone plasma concentration, which could hinder further mass reduction. Leptin and TSH levels were positively correlated after 50% energy deficit treatment. Changes in fm% were directly related to baseline T4/TSH, fT4/TSH, and log TSH. In conclusion, TSH serum concentration and its ratio to T4 and fT4 before weight reduction could be a good predictor of successful weight loss.

TSH receptor expression in orbital tissue and its role in the pathogenesis of Graves' ophthalmopathy

The TSH receptor (TSHr) is the autoantigen responsible for the hyperthyroidism of Graves' disease. Recent studies suggest that this receptor may also be an autoimmune target in Graves' ophthalmopathy (GO) and pretibial dermopathy (PTD). Its involvement in the pathogenesis of these conditions would help to explain the close clinical associations between hyperthyroidism, GO and PTD. TSHr has been shown to be present in normal orbital and dermal tissues and evidence supports the conviction that expression may be increased in tissues involved in GO and PTD. In the setting of Graves' disease, the expression of this antigen in connective tissues throughout the body may lead to systemic, subclinical connective tissue inflammation. Given this background, local or environmental factors such as circulating or local cytokines, gravitational dependency, anatomic constraint of the bony orbit, or trauma, may augment clinical disease involvement within the orbit and pretibial skin. Alternately, locally enhanced expression of this protein at the sites of clinical disease may not be directly involved in pathogenesis, but could be secondary to the ongoing process, and nonetheless important in disease progression.

Inducing Graves' ophthalmopathy

The majority of patients with Graves' disease (GD) have some degree of ocular involvement and this requires surgical or medical intervention in about 5% of cases. There are autoimmune and inflammatory processes operating in Graves' ophthalmopathy (GO), which together induce glycosaminoglycan production, edema and adipogenesis resulting in an increase in the volume of the orbital contents. GO is a heterogeneous disorder, i.e.: 1) whilst usually associated with hyperthyroidism it may occur in euthyroid (and even hypothyroid) patients; 2) expansion of orbital tissues may be due to 'big-fat' or 'big muscles'. The heterogeneity is further exemplified by the spectrum of protocols which have succeeded in inducing aspects of the disease both in animal models and in humans including: 1) Production of severe hypothyroidism in guinea pigs by thyroidectomy and administration of pituitary extract (TSH); 2) Induction of T cells autoreactive to the thyrotropin receptor (TSHR) in mice; 3) Depletion of regulatory T cells in humans susceptible to autoimmunity; 4) Modulation of adipose tissue metabolism in mice and men. In addition, identical induction protocols result in different pathological features depending on the environment, e.g. TSHR primed T cells produce thyroiditis and ocular pathology in BALBc mice in Brussels but thyroid stimulating antibodies accompanied by elevated thyroxine in these animals (from the same supplier) in Cardiff. Thus, experiences in the induction of GO have confirmed the polygenic, multifactorial nature of the disorder and highlight the importance of careful disease classification to promote further progress in understanding.

Regulation of lipolysis: natriuretic peptides and the development of cachexia

The development of cachexia is commonly seen in many pathological states and is associated with a markedly impaired prognosis. Loss of fat tissue appears to be of particular pathophysiological importance in this setting. Lipolysis is closely regulated in health; the major established pathways involving catecholamines (stimulation of lipolysis) and insulin (inhibition of lipolysis). The wasting process in cachexia is associated with marked metabolic dysfunction, and loss of this tight regulatory control. Natriuretic peptides are a family of related peptides with important vasodilatory, natriuretic and diuretic properties. It has recently been shown that natriuretic peptides are also potent stimuli for lipolysis in humans. In this respect, atrial and brain natriuretic peptide appear to have the greatest lipolytic effect, and are similar in potency to catecholamines. Elevated levels of circulating natriuretic peptides are found in several pathological states, and generally reflect disease severity. This article will provide a concise review of the regulation of lipolysis in humans, concentrating on the role of the natriuretic peptides. The relevance of natriuretic peptides to the development of cachexia will be discussed.