Differential recruitment and differential regulation by physiological amines of fat cell beta-1, beta-2 and beta-3 adrenergic receptors expressed in native fat cells and in transfected cell lines

Alcohol, White Adipose Tissue, and Brown Adipose Tissue: Mechanistic Links to Lipogenesis and Lipolysis

According to data from the World Health Organization, there were about 3 million deaths caused by alcohol consumption worldwide in 2016, of which about 50% were related to liver disease. Alcohol consumption interfering with the normal function of adipocytes has an important impact on the pathogenesis of alcoholic liver disease. There has been increasing recognition of the crucial role of adipose tissue in regulating systemic metabolism, far beyond that of an inert energy storage organ in recent years. The endocrine function of adipose tissue is widely recognized, and the significance of the proteins it produces and releases is still being investigated. Alcohol consumption may affect white adipose tissue (WAT) and brown adipose tissue (BAT), which interact with surrounding tissues such as the liver and intestines. This review briefly introduces the basic concept and classification of adipose tissue and summarizes the mechanism of alcohol affecting lipolysis and lipogenesis in WAT and BAT. The adipose tissue-liver axis is crucial in maintaining lipid homeostasis within the body. Therefore, this review also demonstrates the effects of alcohol consumption on the adipose tissue-liver axis to explore the role of alcohol consumption in the crosstalk between adipose tissue and the liver.

Of mice and men: Pinpointing species differences in adipose tissue biology

The prevalence of obesity and metabolic diseases continues to rise, which has led to an increased interest in studying adipose tissue to elucidate underlying disease mechanisms. The use of genetic mouse models has been critical for understanding the role of specific genes for adipose tissue function and the tissue’s impact on other organs. However, mouse adipose tissue displays key differences to human fat, which has led, in some cases, to the emergence of some confounding concepts in the adipose field. Such differences include the depot-specific characteristics of visceral and subcutaneous fat, and divergences in thermogenic fat phenotype between the species. Adipose tissue characteristics may therefore not always be directly compared between species, which is important to consider when setting up new studies or interpreting results. This mini review outlines our current knowledge about the cell biological differences between human and mouse adipocytes and fat depots, highlighting some examples where inadequate knowledge of species-specific differences can lead to confounding results, and presenting plausible anatomic explanations that may underlie the differences. The article thus provides critical insights and guidance for researchers working primarily with only human or mouse fat tissue, and may contribute to new ideas or concepts in the important and evolving field of adipose biology.

Cross-Talk between Obesity and Diabetes: Introducing Polyphenols as an Effective Phytomedicine to Combat the Dual Sword Diabesity

: Obesity-associated diabetes mellitus, a chronic metabolic affliction accounting for 90% of all diabetic patients, has been affecting humanity extremely badly and escalating the risk of developing other serious disorders. It is observed that 0.4 billion people globally have diabetes, whose major cause is obesity. Currently, innumerable synthetic drugs like alogliptin and rosiglitazone are being used to get through diabetes, but they have certain complications, restrictions with severe side effects, and toxicity issues. Recently, the frequency of plant-derived phytochemicals as advantageous substitutes against diabesity is increasing progressively due to their unparalleled benefit of producing less side effects and toxicity. Of these phytochemicals, dietary polyphenols have been accepted as potent agents against the dual sword "diabesity". These polyphenols target certain genes and molecular pathways through dual mechanisms such as adiponectin upregulation, cannabinoid receptor antagonism, free fatty acid oxidation, ghrelin antagonism, glucocorticoid inhibition, sodium-glucose cotransporter inhibition, oxidative stress and inflammation inhibition etc. which sequentially help to combat both diabetes and obesity. In this review, we have summarized the most beneficial natural polyphenols along with their complex molecular pathways during diabesity.

Environmental activation of a hypothalamic BDNF-adipocyte IL-15 axis regulates adipose-natural killer cells

Physical and social environments influence immune homeostasis within adipose tissue, yet the mechanisms remain poorly defined. We report that an enriched environment (EE) housing modulates the immune cell population in white adipose tissue of mice including an increase in the abundance of natural killer (NK) cells. EE upregulates the expression of IL-15 and its receptor IL-15Rα specifically within mature adipocytes. Mechanistically, we show that hypothalamic brain-derived neurotrophic factor (BDNF) upregulates IL-15 production in adipocytes via sympathetic β-adrenergic signaling. Overexpressing BDNF mediated by recombinant adeno-associated virus (rAAV) vector in the hypothalamus expands adipose NK cells. Conversely, inhibition of hypothalamic BDNF signaling via gene transfer of a dominant negative TrkB receptor suppresses adipose NK cells. In white adipose tissue, overexpression of IL-15 using an adipocyte-specific rAAV vector stimulates adipose NK cells and inhibits the progression of subcutaneous melanoma, whereas local IL-15 knockdown blocks the EE effect. These results suggest that bio-behavioral factors regulate adipose NK cells via a hypothalamic BDNF-sympathoneural-adipocyte IL-15 axis. Targeting this pathway may have therapeutic significance for cancer.

Cardioprotective Effects of Beta3-Adrenergic Receptor (β3-AR) Pre-, Per-, and Post-treatment in Ischemia-Reperfusion

The β3-AR (beta3-adrenergic receptor) is resistant to short-term agonist-promoted desensitization and delivers a constant intracellular signal, making this receptor a potential target in acute myocardial infarction (AMI).

AIM

To investigate whether selective modulation of β3-AR prior to or during ischemia and/or reperfusion may be cardioprotective.

METHODS

Isolated perfused rat hearts were exposed to 35-min regional ischemia (RI) and 60-min reperfusion. The β3-AR agonist (BRL37344, 1 μM) or antagonist (SR59230A, 0.1 μM) was applied: (i) before RI (PreT) or (ii) last 10 min of RI (PerT) or (iii) onset of reperfusion (PostT) or (iv) during both PerT+PostT. Nitric oxide (NO) involvement was assessed, using the NOS inhibitor, L-NAME (50 μM). Endpoints were functional recovery, infarct size (IS), cGMP levels, and Western blot analysis of eNOS, ERKp44/p42, PKB/Akt, and glycogen synthase kinase-3β (GSK-3β).

RESULTS

Selective treatment with BRL significantly reduced IS. L-NAME abolished BRL-mediated cardioprotection. BRL (PreT) and BRL (PerT) significantly increased cGMP levels (which were reduced by L-NAME) and PKB/Akt phosphorylation. BRL (PostT) produced significantly increased cGMP levels, PKB/Akt, and ERKp44/p42 phosphorylation. BRL (PerT+PostT) caused significant eNOS, PKB/Akt, ERKp44/p42, and GSK-3β phosphorylation.

CONCLUSION

β3-AR activation by BRL37344 induced significant cardioprotection regardless of the experimental protocol. However, the pattern of intracellular signaling with each BRL treatment differed to some degree and suggests the involvement of cGMP, eNOS, ERK, GSK-3β, and particularly PKB/Akt activation. The data also suggest that clinical application of β3-AR stimulation should preferably be incorporated during late ischemia or/and early reperfusion.

Perivascular Adipose Tissue: the Sixth Man of the Cardiovascular System

Perivascular adipose tissue (PVAT) refers to the local aggregate of adipose tissue surrounding the vascular tree, exhibiting phenotypes from white to brown and beige adipocytes. Although PVAT has long been regarded as simply a structural unit providing mechanical support to vasculature, it is now gaining reputation as an integral endocrine/paracrine component, in addition to the well-established modulator endothelium, in regulating vascular tone. Since the discovery of anti-contractile effect of PVAT in 1991, the use of multiple rodent models of reduced amounts of PVAT has revealed its regulatory role in vascular remodeling and cardiovascular implications, including atherosclerosis. PVAT does not only release PVAT-derived relaxing factors (PVRFs) to activate multiple subsets of endothelial and vascular smooth muscle potassium channels and anti-inflammatory signals in the vasculature, but it does also provide an interface for neuron-adipocyte interactions in the vascular wall to regulate arterial vascular tone. In this review, we outline our current understanding towards PVAT and attempt to provide hints about future studies that can sharpen the therapeutic potential of PVAT against cardiovascular diseases and their complications.

Preventive Treatment of Migraine: Effect on Weight

Weight gain is a side-effect commonly associated with drugs used for headache prophylaxis. Weight gain can adversely affect patient health, exacerbate comorbid metabolic disorders and encourage noncompliance. Few studies have been conducted specifically on the effect of headache medications on weight, and it is important for physicians to have accurate information about weight-gain side-effects when identifying appropriate pharmacological regimens. This review discusses the potential effects on weight of the more common headache medications.

FGF21 mediates alcohol-induced adipose tissue lipolysis by activation of systemic release of catecholamine in mice

Alcohol consumption leads to adipose tissue lipoatrophy and mobilization of FFAs, which contributes to hepatic fat accumulation in alcoholic liver disease. This study aimed to investigate the role of fibroblast growth factor (FGF)21, a metabolic regulator, in the regulation of chronic-binge alcohol-induced adipose tissue lipolysis. FGF21 KO mice were subjected to chronic-binge alcohol exposure, and epididymal white adipose tissue lipolysis and liver steatosis were investigated. Alcohol exposure caused adipose intracellular cAMP elevation and activation of lipolytic enzymes, leading to FFA mobilization in both WT and FGF21 KO mice. However, alcohol-induced systemic elevation of catecholamine, which is known to be a major player in adipose lipolysis by binding to the β-adrenergic receptor, was markedly inhibited in KO mice. Supplementation with recombinant human FGF21 to alcohol-exposed FGF21 KO mice resulted in an increase in fat loss in parallel with an increase of circulating norepinephrine concentration. Furthermore, alcohol consumption-induced fatty liver was blunted in the KO mice, indicating an inhibition of fatty acid reverse transport from adipose to the liver in the KO mice. Taken together, our studies demonstrate that FGF21 KO mice are protected from alcohol-induced adipose tissue excess-lipolysis through a mechanism involving systemic catecholamine release.

Neural innervation of white adipose tissue and the control of lipolysis

White adipose tissue (WAT) is innervated by the sympathetic nervous system (SNS) and its activation is necessary for lipolysis. WAT parasympathetic innervation is not supported. Fully-executed SNS-norepinephrine (NE)-mediated WAT lipolysis is dependent on β-adrenoceptor stimulation ultimately hinging on hormone sensitive lipase and perilipin A phosphorylation. WAT sympathetic drive is appropriately measured electrophysiologically and neurochemically (NE turnover) in non-human animals and this drive is fat pad-specific preventing generalizations among WAT depots and non-WAT organs. Leptin-triggered SNS-mediated lipolysis is weakly supported, whereas insulin or adenosine inhibition of SNS/NE-mediated lipolysis is strongly supported. In addition to lipolysis control, increases or decreases in WAT SNS drive/NE inhibit and stimulate white adipocyte proliferation, respectively. WAT sensory nerves are of spinal-origin and sensitive to local leptin and increases in sympathetic drive, the latter implicating lipolysis. Transsynaptic viral tract tracers revealed WAT central sympathetic and sensory circuits including SNS-sensory feedback loops that may control lipolysis.

Effect of pioglitazone on body composition and bone density in subjects with prediabetes in the ACT NOW trial

AIMS

This study examined the effects of pioglitazone on body weight and bone mineral density (BMD) prospectively in patients with impaired glucose tolerance as pioglitazone (TZD) increases body weight and body fat in diabetic patients and increases the risk of bone fractures.

METHODS

A total of 71 men and 163 women aged 49.3 (10.7) years [mean (s.d.)]; body mass index (BMI), 34.5 (5.9) kg/m(2) were recruited at five sites for measurements of body composition by dual energy X-ray absorptiometry at baseline and at conversion to diabetes or study end, if they had not converted.

RESULTS

Mean follow-up was 33.6 months in the pioglitazone group and 32.1 months in the placebo group. Body weight increased 4.63 ± 0.60 (m ± s.e.) kg in the pioglitazone group compared to 0.98 ± 0.62 kg in the PIO group (p < 0.0001). Body fat rose 4.89 ± 0.42 kg in the pioglitazone group compared to 1.41 ± 0.44 kg, (p < 0.0001) in placebo-treated subjects. The increase in fat was greater in legs and trunk than in the arms. BMD was higher in all regions in men and significantly so in most. PIO decreased BMD significantly in the pelvis in men and women, decreased BMD in the thoracic spine and ribs of women and the lumbar spine and legs of men. Bone mineral content also decreased significantly in arms, legs, trunk and in the total body.

CONCLUSIONS

Pioglitazone increased peripheral fat more than truncal fat and decreased BMD in several regions of the body.

In vitro TNF-α- and noradrenaline-stimulated lipolysis is impaired in adipocytes from growing rats fed a low-protein, high-carbohydrate diet

The aim of this study was to investigate tumor necrosis factor alpha (TNF-α)- and noradrenaline (NE)-stimulated lipolysis in retroperitoneal (RWAT) and epididymal (EAT) white adipose tissue as a means of understanding how low-protein, high-carbohydrate (LPHC) diet-fed rats maintain their lipid storage in a catabolic environment (marked by increases in serum TNF-α and corticosterone and sympathetic flux to RWAT and EAT), as previously observed. Adipocytes or tissues from the RWAT and EAT of rats fed an LPHC diet and rats fed a control (C) diet for 15 days were used in the experiments. The adipocytes from both tissues of the LPHC rats exhibited lower TNF-α- stimulated lipolysis compared to adipocytes from the C rats. The intracellular lipolytic agents IBMX, DBcAMPc and FSK increased lipolysis in both tissues from rats fed the C and LPHC diets compared to basal lipolysis; however, the effect was approximately 2.5-fold lower in adipocytes from LPHC rats. The LPHC diet induced a marked reduction in the β3 and α2-AR, adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) content in RWAT and EAT. The LPHC diet did not affect TNF-α receptor 1 content but did induce a reduction in ERK p44/42 in both tissues. The present work indicates that RWAT and EAT from LPHC rats have an impairment in the lipolysis signaling pathway activated by NE and TNF-α, and this impairment explains the reduced response to these lipolytic stimuli, which may be fundamental to the maintenance of lipid storage in LPHC rats.

Historical perspectives in fat cell biology: the fat cell as a model for the investigation of hormonal and metabolic pathways

For many years, there was little interest in the biochemistry or physiology of adipose tissue. It is now well recognized that adipocytes play an important dynamic role in metabolic regulation. They are able to sense metabolic states via their ability to perceive a large number of nervous and hormonal signals. They are also able to produce hormones, called adipokines, that affect nutrient intake, metabolism and energy expenditure. The report by Rodbell in 1964 that intact fat cells can be obtained by collagenase digestion of adipose tissue revolutionized studies on the hormonal regulation and metabolism of the fat cell. In the context of the advent of systems biology in the field of cell biology, the present seems an appropriate time to look back at the global contribution of the fat cell to cell biology knowledge. This review focuses on the very early approaches that used the fat cell as a tool to discover and understand various cellular mechanisms. Attention essentially focuses on the early investigations revealing the major contribution of mature fat cells and also fat cells originating from adipose cell lines to the discovery of major events related to hormone action (hormone receptors and transduction pathways involved in hormonal signaling) and mechanisms involved in metabolite processing (hexose uptake and uptake, storage, and efflux of fatty acids). Dormant preadipocytes exist in the stroma-vascular fraction of the adipose tissue of rodents and humans; cell culture systems have proven to be valuable models for the study of the processes involved in the formation of new fat cells. Finally, more recent insights into adipocyte secretion, a completely new role with major metabolic impact, are also briefly summarized.

The β3-adrenergic agonist (BRL35135A) acutely increases oxygen consumption and plasma intermediate metabolites in sheep

There is evidence that an atypical adrenoreceptor subtype is involved in mediating some of the physiological effects of catecholamines, particularly in some adipose tissue sites. Therefore, three experiments were conducted to determine the metabolic and energetic responses to oral administration of the purported β3-agonist BRL35135A in ruminant lambs. The post-prandial increase in O2 consumption (0.109 versus 0.139 L/min) and CO2 production (0.102 versus 0.127 L/min) at 30 min after feeding was greater (P < 0.05) in the lambs receiving 5 mg of the BRL35135A. Treatment × time interactions over the period between –50 and 220 min indicate significant increases in plasma non-esterified fatty acids (P < 0.001), glucose (P < 0.001) and lactate (P = 0.024) in lambs consuming a single oral dose of 5 mg BRL35135A. In a subsequent experiment there were similar interactions over the period between –120 and 1440 min for non-esterified fatty acids (P < 0.001), glucose (P < 0.001) and lactate (P < 0.001) in lambs consuming a lower oral dose of 1 mg BRL35135A. The effects of BRL35135A on plasma non-esterified fatty acids (P = 0.95), glucose (P = 0.84) and lactate (P = 0.68) were not modified by the β1- and β2-adrenergic antagonist alprenolol suggesting that the effects were mediated via β3-adrenergic receptor subtypes. In conclusion, these experiments indicate that BRL35135A is acutely active in sheep when given with feed, as indicated by increases in respiratory gas exchange and plasma metabolite concentrations.

Plasma non-esterified fatty acid response to a β-adrenergic challenge before or after feeding in energy underfed or overfed, dry or lactating cows

The effects of adrenergic challenge (in order to evaluate the adipose tissue lipolytic potential) and time of challenge relative to feeding on the response curve of plasma non-esterified fatty acids (NEFA) in four underfed or overfed non-lactating non-pregnant cows were studied. Basal NEFA and NEFA response to isoproterenol (ISO; 4 nmol/kg body weight) were higher when challenged before than after feeding and higher in underfed than overfed cows. Interaction between feeding level and time of challenge was significant (P < 0·001) for the rising part of NEFA response. Pooled NEFA response curves from several trials (no. = 63 challenges) were analysed in order to obtain a simplified procedure for the prediction of the NEFA response. High correlations were found between the response area or maximal NEFA response and NEFA response at 15 min (r = 0·95 and 0·98, respectively). This simplified procedure was applied on pooled data from 84 challenges in order to evaluate the effects of energy balance, physiological status, body condition score (BCS) and time of challenge relative to feeding. Energy balance had a significant effect on basal plasma NEFA and NEFA response at 15 min after ISO challenge (–7·6 and –14·2 μmol/l when the daily energy balance increased by 1 MJ, respectively). Time of ISO injection relative to feeding had a greater effect on stimulated NEFA than on basal NEFA (308 v. 239 μmol/l). There was a significant effect of BCS (41 μmol/l per unit of BCS) on the basal plasma NEFA level. The NEFA response at 15 min after ISO challenge was lower (387 μmol/l) in lactating cows than in dry cows. The NEFA response to ISO at 15 min could provide an efficient method of studying the adipose tissue lipolytic potential of cattle in vivo.

[Nebivolol: the first vasodilatory beta-blocker with a beta3-adrenergic agonist activity]

Several studies suggest that the beta(3)-adrenergic stimulation could be a new therapeutic target for the treatment of cardiovascular diseases. The vascular effects induced by beta(3)-adrenergic stimulation are able to decrease the left ventricular strain allowing to reduce after-load. In addition, the increased coronary blood flow due to vasorelaxation increases the myocardial oxygene delivery. The hypothesis about the beneficial role of beta(3)-adrenoceptors is supported by recent data about a beta-blocker of third generation, nebivolol, currently used in the treatment of heart failure and hypertension. The present review presents the beta(3)-adrenoceptors characteristics as well as its involvement in the cardiovascular effects of nebivolol.

Sensory and sympathetic nervous system control of white adipose tissue lipolysis

Circulating factors are typically invoked to explain bidirectional communication between the CNS and white adipose tissue (WAT). Thus, initiation of lipolysis has been relegated primarily to adrenal medullary secreted catecholamines and the inhibition of lipolysis primarily to pancreatic insulin, whereas signals of body fat levels to the brain have been ascribed to adipokines such as leptin. By contrast, evidence is given for bidirectional communication between brain and WAT occurring via the sympathetic nervous system (SNS) and sensory innervation of this tissue. Using retrograde transneuronal viral tract tracers, the SNS outflow from brain to WAT has been defined. Functionally, sympathetic denervation of WAT blocks lipolysis to a variety of lipolytic stimuli. Using anterograde transneuronal viral tract tracers, the sensory input from WAT to brain has been defined. Functionally, these WAT sensory nerves respond electrophysiologically to increases in WAT SNS drive suggesting a possible neural negative feedback loop to regulate lipolysis.

Role of beta3-adrenoceptors for intrahepatic resistance and portal hypertension in liver cirrhosis

Increased intrahepatic resistance and splanchnic blood flow cause portal hypertension in liver cirrhosis. Nonselective beta-adrenoceptor (beta-AR) antagonists have beneficial effects on hyperdynamic circulation and are in clinical use. In this context, the role of the beta(3)-AR is undefined. Here we investigated their expression and role in portal hypertension in patients and rats with liver cirrhosis. We analyzed cirrhotic human and rat tissues (liver, splanchnic vessels) and primary rat cells. Protein expression of beta(3)-AR was determined by western blot and messenger RNA (mRNA) levels by reverse-transcription polymerase chain reaction (RT-PCR). Activities of Rho-kinase and the nitric oxide (NO) effector protein kinase G (PKG) were assessed by way of substrate phosphorylation (moesin, vasodilator-stimulated phosphoprotein [VASP]). Cyclic 3',5' adenosine monophosphate (cAMP) accumulation was determined by an enzyme-immunoassay kit. The effects of selective beta(3)-AR agonists (CGP12177A, BRL37344) and antagonist (SR59230A) were investigated by collagen matrix contraction of hepatic stellate cells (HSCs), in situ liver perfusions, and in vivo hemodynamic parameters in bile duct ligation and carbon tetrachloride intoxication in cirrhotic rats. In cirrhosis of humans and rats, beta(3)-AR expression is markedly increased in hepatic and in splanchnic tissues. Stimulation of beta(3)-AR leads to relaxation of HSCs by way of cAMP accumulation, and by inhibition of Rho-kinase activity; any role of NO and its effector PKG was not observed. beta(3)-AR agonists decrease intrahepatic resistance and portal pressure in cirrhotic rats.

CONCLUSION

There is a marked hepatic and mesenteric up-regulation of beta(3)-ARs in human cirrhosis and in two different animal models of cirrhosis. The beta(3)-AR-agonists should be further evaluated for therapy of portal hypertension.

Adrenaline but not noradrenaline is a determinant of exercise-induced lipid mobilization in human subcutaneous adipose tissue

The relative contribution of noradrenaline (norepinephrine) and adrenaline (epinephrine) in the control of lipid mobilization in subcutaneous adipose tissue (SCAT) during exercise was evaluated in men treated with a somatostatin analogue, octreotide. Eight lean and eight obese young men matched for age and physical fitness performed 60 min exercise bouts at 50% of their maximal oxygen consumption on two occasions: (1) during i.v. infusion of octreotide, and (2) during placebo infusion. Lipolysis and local blood flow changes in SCAT were evaluated using in situ microdialysis. Infusion of octreotide suppressed plasma insulin and growth hormone levels at rest and during exercise. It blocked the exercise-induced increase in plasma adrenaline while that of noradrenaline was unchanged. Plasma natriuretic peptides (NPs) level was higher at rest and during exercise under octreotide infusion in lean men. Under placebo, no difference was found in the exercise-induced increase in glycerol between the probe perfused with Ringer solution alone and that with phentolamine (an alpha-adrenergic receptor antagonist) in lean subjects while a greater increase in glycerol was observed in the obese subjects. Under placebo, propranolol infusion in the probe containing phentolamine reduced by about 45% exercise-induced glycerol release; this effect was fully suppressed under octreotide infusion while noradrenaline was still elevated and exercise-induced lipid mobilization maintained in both lean and obese individuals. In conclusion, blockade of beta-adrenergic receptors during exercise performed during infusion of octreotide (blocking the exercise-induced rise in adrenaline but not that of noradrenaline) does not alter the exercise-induced lipolysis. This suggests that adrenaline is the main adrenergic agent contributing to exercise-induced lipolysis in SCAT. Moreover, it is the combined action of insulin suppression and NPs release which explains the lipolytic response which remains under octreotide after full local blockade of fat cell adrenergic receptors. For the moment, it is unknown if results apply specifically to SCAT and exercise only or if conclusions could be extended to all forms of lipolysis in humans.

Adrenergic beta1 receptor polymorphism (Ser49Gly) is associated with obesity in type II diabetic patients

In the process of lipolysis, adipocytes are stimulated by catecholamines through beta(1), beta(2), and beta(3) adrenergic receptors (ARs). So far, beta(2) and beta(3) AR polymorphisms have been reported related to obesity. However, the relation of beta(1)AR polymorphisms to obesity has not been evaluated. In the present study, we examined whether betaAR polymorphisms are associated with obesity-related phenotype in type II diabetic patients. Polymorphisms of beta(1)Ser49Gly, beta(1)Arg389Gly, beta(2)Arg16Gly, beta(2)Gln27Glu and beta(3)Trp64Arg were genotyped in 188 type II diabetic patients by PCR-RFLP. Among these polymorphisms, beta(1)Ser49Gly was found to be associated with obesity. Subjects with beta(1)Gly49 allele showed higher body mass index (BMI) than those with Ser49/Ser49 genotype (24.7+/-3.7 vs. 23.4+/-3.3 kg/m(2); p=0.031). Subjects with beta(1)Gly49 allele were more frequently overweight (BMI >or= 25 kg/m(2)) compared with beta(1)Ser49 homozygous group (42.1 vs. 24.4%, p=0.015). By multiple linear regression analysis, beta(1)Ser49Gly polymorphism was independently associated with higher BMI (p=0.019, beta=0.166). Our data indicate that the Gly49 allele in beta(1)AR is associated with higher BMI in type II diabetic patients. Genotyping for beta(1)Ser49Gly polymorphism in type II diabetic patients may have clinical benefit to predict obesity, thereby contributing to the prevention of insulin resistance.

Genetics of the metabolic syndrome

The concept of a metabolic syndrome (MetS), a cluster of pre-clinical metabolic alterations commonly associated with obesity, is the object of much debate. Genetic studies have the potential to contribute to some of the key questions, including the true nature of the cluster of pre-clinical features and whether it is associated with human genetic variation. This review summarizes the evidence for the presence of familial aggregation for the individual components of MetS and their heritability levels. It also provides an overview of the studies that have dealt with candidate genes for MetS. Potential leads from genome-wide linkage scans are also discussed. The assumption is made that obesity, ectopic fat deposition and abnormal adipose tissue metabolism are responsible for alterations in lipid metabolism, which in turn generates the commonly observed pre-clinical shifts in glucose tolerance, lipids and lipoprotein profile, blood pressure, inflammatory markers, endothelial function, and a prothrombotic state. Progress in the understanding of the genetic basis of MetS should occur as soon as a consensus is reached on the true nature of MetS, its components and diagnostic criteria.

Thematic review series: adipocyte biology. Sympathetic and sensory innervation of white adipose tissue

During our study of the reversal of seasonal obesity in Siberian hamsters, we found an interaction between receptors for the pineal hormone melatonin and the sympathetic nervous system (SNS) outflow from brain to white adipose tissue (WAT). This ultimately led us and others to conclude that the SNS innervation of WAT is the primary initiator of lipid mobilization in these as well as other animals, including humans. There is strong neurochemical (norepinephrine turnover), neuroanatomical (viral tract tracing), and functional (sympathetic denervation-induced blockade of lipolysis) evidence for the role of the SNS in lipid mobilization. Recent findings suggest the presence of WAT sensory innervation based on strong neuroanatomical (viral tract tracing, immunohistochemical markers of sensory nerves) and suggestive functional (capsaicin sensory denervation-induced WAT growth) evidence, the latter implying a role in conveying adiposity information to the brain. By contrast, parasympathetic nervous system innervation of WAT is characterized by largely negative neuroanatomical evidence (viral tract tracing, immunohistochemical and biochemical markers of parasympathetic nerves). Functional evidence (intraneural stimulation and in situ microdialysis) for the role of the SNS innervation in lipid mobilization in human WAT is convincing, with some controversy regarding the level of sympathetic nerve activity in human obesity.

Beta-adrenergic regulation of uncoupling protein expression in swine

This study examined the beta-adrenergic regulation of uncoupling protein (UCP) 2 and UCP3 gene expression in porcine tissues. In vitro experiments examined changes in UCP2 and UCP3 gene expression in middle (MSQ) and outer (OSQ) subcutaneous adipose tissues from crossbred neutered male pigs. Incubation of tissue slices (24 h) with 0 to 1000 nM isoproterenol increased UCP2 and UCP3 mRNA abundance in MSQ and OSQ, relative to 18S rRNA (P<0.05). For the in vivo experiment, nine randomly selected pigs (80 kg) were presented with a diet supplemented with 10.0 ppm ractopamine for 2 weeks. Another eight pigs were maintained on a control diet. Dietary ractopamine did not affect adipose UCP2 or UCP3 gene expression (P>0.05). However, UCP2 mRNA abundance was depressed in semitendinosus white (STW, P<0.05) and semitendinosus red (STR, P<0.001) by ractopamine feeding. Also, ractopamine decreased UCP3 mRNA abundance by 28% in STW (P<0.05). The in vitro data suggest that beta-adrenergic agonists directly affect adipose tissue UCP expression, although these adipose effects can be masked by the in vivo physiology. The in vivo data indicate that beta-adrenergic agonists may function in regulating UCP2 and UCP3 expression in selected muscles.

Alterations of beta3-adrenoceptors expression and their myocardial functional effects in physiological model of chronic exercise-induced cardiac hypertrophy

Physical training induces cardiovascular autonomic nervous system regulation adaptations, which could result from beta adrenergic receptor (AR) modifications. Among them, beta(3 )AR alterations have been recently reported but their functional effect remained to discuss. To explain the beta(3) AR gene expression in relation to function, we simultaneously studied the left ventricle (LV) beta(3) AR mRNA and protein levels and the myocardial functional effects of a beta(3) AR agonist following physical training. Forty rats were assigned to either a control (C; N = 20) or a trained (T; N = 20) group. The treadmill running protocol was performed for 8 weeks. Histological measurements on LV slices were quantified. The beta(3) AR mRNA abundance was studied with RT-PCR and beta(3) AR protein density with Western-Blot analysis. Myocardial functional effects of a beta(3) AR agonist, BRL37344 (10(-8) M), were studied in Langendorff-perfused hearts. Histological data confirmed the adapted patterns of the physiological cardiac hypertrophy observed in T (P < 0.01), with a significant increase in arteries density (P < 0.01) and an unchanged collagen concentration. The beta(3) AR protein density was increased in T (154 +/- 38% in T vs. 100 +/- 24% in C; P < 0.05), but no change was noted concerning the beta(3) AR mRNA level. After BRL37344 perfusion LVDP, +dP/dT and -dP/dT, in C (P < 0.01), and only +dP/dT in T (P < 0.05) were decreased. Moreover, all LV hemodynamic parameters were more altered after BRL37344 in C than in T (P < 0.01).Thus, in this physiological model of cardiac hypertrophy, an increase of beta(3) AR density without beta(3) AR mRNA alteration was observed. Classical negative myocardial lusitropic and inotropic effects induced by a specific agonist of beta(3) AR were diminished in trained rats.

Stress and cardiac beta adrenoceptors

Most modern theories about stress recognize that although stress is not a disease, it may be the trigger for the majority of diseases when allostatic overload has been generated. During stress, the glucocorticoids and catecholamines play a key role in the regulation of physiological parameters and homeostasis during stress. In the heart, positive chronotropic, inotropic, and lusitropic responses to catecholamines are mediated by various subtypes of adrenergic receptors (beta-ARs), mainly beta1- and beta2-adrenergic receptors. beta-ARs also control cardiomyocyte growth and death, thus contributing to cardiac remodelling. The structural basis of each beta-AR subtype, as well as their signalling pathways, and adaptive responses to stress are discussed. The participation of beta3- and putative beta4-ARs in the control of cardiac function is also discussed, with emphasis on low affinity beta-AR isoforms and the role they play in the response to the catecholamines under stress. The changes in beta-AR signalling under pathogenic conditions as well as under stress are reviewed.

Evidence for a role of developmental genes in the origin of obesity and body fat distribution

Obesity, especially central obesity, is a hereditable trait associated with a high risk for development of diabetes and metabolic disorders. Combined gene expression analysis of adipocyte- and preadipocyte-containing fractions from intraabdominal and subcutaneous adipose tissue of mice revealed coordinated depot-specific differences in expression of multiple genes involved in embryonic development and pattern specification. These differences were intrinsic and persisted during in vitro culture and differentiation. Similar depot-specific differences in expression of developmental genes were observed in human subcutaneous versus visceral adipose tissue. Furthermore, in humans, several genes exhibited changes in expression that correlated closely with body mass index and/or waist/hip ratio. Together, these data suggest that genetically programmed developmental differences in adipocytes and their precursors in different regions of the body play an important role in obesity, body fat distribution, and potential functional differences between internal and subcutaneous adipose tissue.

Enhanced Inhibition of L-type Ca2+ Current by β3-Adrenergic Stimulation in Failing Rat Heart

beta3-adrenergic receptors (AR) have recently been identified in mammalian hearts and shown to be up-regulated in heart failure (HF). beta3-AR stimulation reduces inotropic response associated with an inhibition of L-type Ca2+ channels in normal hearts; however, the effects of beta3-AR activation on Ca2+ channel in HF remain unknown. We compared the effects of beta(3)-AR activation on L-type Ca2+ current (ICa,L) in isolated left ventricular myocytes obtained from normal and age-matched rats with isoproterenol (ISO)-induced HF (4 months after 340 mg/kg s.c. for 2 days). ICa,L was measured using whole-cell voltage clamp and perforated-patch recording techniques. In normal myocytes, superfusion of 4-[-[2-hydroxy-(3-chlorophenyl)ethylamino]propyl]phenoxyacetate (BRL-37,344; BRL), a beta3-AR agonist, caused a dose-dependent decrease in ICa,L with maximal inhibition (21%, 1.1 +/- 0.2 versus 1.4 +/- 0.1 nA) (p < 0.01) at 10(-7) M. In HF myocytes, the same concentration of BRL produced a proportionately greater inhibition (31%) in ICa,L (1.1 +/- 0.2 versus 1.6 +/- 0.2 nA) (p < 0.05). A similar inhibition of ICa,L was also observed with ISO (10(-7) M) in the presence of a beta1- and beta2-AR antagonist, nadolol (10(-5) M). Inhibition was abolished by the beta3-AR antagonist (S)-N-[4-[2-[[3-[3-(acetamidomethyl)phenoxy]-2-hydroxypropyl]amino]ethyl]phenyl]benzenesulfonamide (L-748,337; 10(-6) M), but not by nadolol. The inhibitory effect of BRL was attenuated by a nitric-oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (10(-4) M), and was prevented by the incubation of myocytes with pertussis toxin (PTX; 2 microg/ml, 36 degrees C, 6 h). In conclusion, beta3-AR activation inhibits L-type Ca2+ channel in both normal and HF myocytes. In HF, beta3-AR stimulation-induced inhibition of Ca2+ channel is enhanced. These effects are likely coupled with PTX-sensitive G-protein and partially mediated through a NOS-dependent pathway.

Endogenous β3-adrenoreceptor activation contributes to left ventricular and cardiomyocyte dysfunction in heart failure

The objective of the present study was to test the hypothesis that endogenous β3-adrenoreceptor (AR) activation contributes to left ventricular (LV) and cardiomyocyte dysfunction in heart failure (CHF). Stimulation of the β3-AR inhibits cardiac contraction. In the failing myocardium, β3-ARs are upregulated, suggesting that stimulation of β3-ARs may contribute to depressed cardiac performance in CHF. We assessed the functional significance of endogenous β3-AR activation in 10 conscious dogs before and after pacing-induced CHF. Under normal conditions, L-748,337, a specific β3-AR antagonist, produced a mild increase in LV contractile performance assessed by the slope ( Ees) of the LV pressure-volume relation (18%, 6.2 ± 0.9 vs. 7.3 ± 1.2 mmHg/ml, P < 0.05) and the improved LV relaxation time constant (τ; 28.4 ± 1.9 vs. 26.8 ± 1.0 ms, P < 0.05). After CHF, the plasma norepinephrine concentration increased eightfold, and L-748,337 produced a larger increase in Ees (34%, 3.8 ± 0.7 vs. 5.1 ± 0.8 mmHg/ml, P < 0.05) and a greater decrease in τ (46.4 ± 4.2 vs. 41.0 ± 3.9 ms, P < 0.05). Similar responses were observed in isolated myocytes harvested from LV biopsies before and after CHF. In the normal myocyte, L-748,337 did not cause significant changes in contraction or relengthening. In contrast, in CHF myocytes, L-748,337 produced significant increases in contraction (5.8 ± 0.9 vs. 6.8 ± 0.9%, P < 0.05) and relengthening (33.5 ± 4.2 vs. 39.7 ± 4.0 μm/s, P < 0.05). The L-748,337-induced myocyte response was associated with improved intracellular Ca2+ concentration regulation. In CHF myocytes, nadolol caused a decrease in contraction and relengthening, and adding isoproterenol to nadolol caused a further depression of myocyte function. Stimulation of β3-AR by endogenous catecholamine contributes to the depression of LV contraction and relaxation in CHF.

Obesity--is it a genetic disorder?

Obesity is one of the most pressing problems in the industrialized world. Twin, adoption and family studies have shown that genetic factors play a significant role in the pathogenesis of obesity. Rare mutations in humans and model organisms have provided insights into the pathways involved in body weight regulation. Studies of candidate genes indicate that some of the genes involved in pathways regulating energy expenditure and food intake may play a role in the predisposition to obesity. Amongst these genes, sequence variations in the adrenergic receptors, uncoupling proteins, peroxisome proliferator-activated receptor, and the leptin receptor genes are of particular relevance. Results that have been replicated in at least three genome-wide scans suggest that key genes are located on chromosomes 2p, 3q, 5p, 6p, 7q, 10p, 11q, 17p and 20q. We conclude that the currently available evidence suggests four levels of genetic determination of obesity: genetic obesity, strong genetic predisposition, slight genetic predisposition, and genetically resistant. This growing body of research may help in the development of anti-obesity agents and perhaps genetic tests to predict the risk for obesity.

Beta 3-adrenoceptor in rat aorta: molecular and biochemical characterization and signalling pathway

1. We have previously demonstrated that beta(3)-adrenoceptor (beta(3)-AR) stimulation induces endothelium-dependent vasorelaxation in rat aorta through the activation of an endothelial NO synthase associated with an increase in intracellular cGMP. The aim of the present study was to localise beta(3)-AR to confirm our functional study and to complete the signalling pathway of beta(3)-AR in rat aorta. 2. By RT-PCR, we have detected beta(3)-AR transcripts both in aorta and in freshly isolated endothelial cells. The absence of markers for adipsin or hormone-sensitive lipase in endothelial cells excluded the presence of beta(3)-AR from adipocytes. The localization of beta(3)-AR in aortic endothelial cells was confirmed by immunohistochemistry using a rat beta(3)-AR antibody. 3. To identify the G protein linked to beta(3)-AR, experiments were performed in rat pre-treated with PTX (10 microg kg(-1)), a G(i/0) protein inhibitor. The blockage of G(i/0) protein by PTX was confirmed by the reduction of vasorelaxation induced by UK 14304, a selective alpha(2)-AR agonist. The cumulative concentration-response curve for SR 58611A, a beta(3)-AR agonist, was not significantly modified on aorta rings from PTX pre-treated rats. 4. At the same level of contraction, the relaxations induced by 10 microM SR 58611A were significantly reduced in 30 mM-KCl pre-constricted rings (E(max)=16.7+/-8.4%, n=5), in comparison to phenylephrine (0.3 microM) pre-constricted rings (E(max)=49.11+/-11.0%, n=5, P<0.05). In addition, iberotoxin (0.1 microM), glibenclamide (1 microM) and 4-aminopyridine (1 mM), selective potassium channels blockers of K(Ca), K(ATP), and K(v) respectively, decreased the SR 58611A-mediated relaxation. 5. We conclude that beta(3)-AR is preferentially expressed in rat aortic endothelial cells. Beta(3)-AR-mediated aortic relaxation is independent of G(i/0) proteins stimulation, but results from the activation of several potassium channels, K(Ca), K(ATP), and K(v).

Beta-2 adrenergic receptor variants are associated with subcutaneous fat accumulation in response to long-term overfeeding

OBJECTIVE

The effects of alpha-2A (A2A)-, beta-2 (B2)- and beta-3 (B3)-adrenergic receptor (ADR) gene polymorphisms on adiposity, fat distribution and plasma insulin and leptin changes in response to long-term overfeeding were explored.

METHODS

Twenty four men (mean (+/-s.d.) age 21+/-2 y) who constituted 12 pairs of identical twins ate a 4.2 MJ/day energy surplus, 6 days a week, for a period of 100 days. Total body fat was assessed by hydrodensitometry and total subcutaneous fat by the sum of eight skinfolds. Abdominal fat areas were measured by computerized tomography (CT). Plasma glucose and insulin during fasting and in response to an oral glucose tolerance test (OGTT) were assayed. The insulin and glucose areas were computed using the trapezoidal method. Plasma leptin was measured with an enzyme-linked immunosorbent assay. The ADR polymorphisms were identified by PCR or Southern blot technique.

RESULTS

The ADRB2 Gln27Gln genotype (n=10) was associated with a larger gain (percentage change) in weight (P<0.001) and total subcutaneous (P<0.005) fat than the Glu27Glu/Gln27Glu genotype (n=14). In addition, overfeeding induced greater increases in the insulin areas under the curve during the OGTT and the fasting plasma level of leptin (P<0.01 and <0.03, respectively) among Gln27Gln than in the Glu27Glu/Gln27Glu subjects. The body composition and metabolic changes among the ADRB2 BanI 3.7/3.4 kb subjects (n=10) were similar to those of Gln27Gln subjects. ADRA2A DraI (n=4) 6.3/6.3 kb subjects experienced a decrease (-8%) while 6.7/6.3 kb subjects (n=20) registered an increase (+10%; P=0.017) of OGTT glucose area after the 100-day caloric surplus. The four carriers of the ADRB3 variant (Trp64Arg) experienced the same magnitude of changes as the 20 homozygotes for the Trp allele. In general, comparisons based on the 24 subjects considered as unrelated men and the mean values for each of the 12 pairs yielded similar results.

CONCLUSION

The ADRB2 Gln27Gln subjects gained more weight and total subcutaneous fatness and also experienced a greater increase in insulin resistance than Glu27Glu/Gln27Glu subjects with exposure to long-term overfeeding. Similar differences were observed between carriers and non-carriers of the ADRB2 3.7/3.4 kb BanI variant. Genetic variation at the ADRB2 locus could thus be one of the factors responsible for the large inter-individual differences observed in the response to long-term alterations in energy balance and should be further investigated.

Effect of SR59230A on atypical beta-adrenoceptor mediating relaxation in the Guinea Pig gastric fundus

The purpose of the present study was to clarify whether atypical beta-adrenoceptors which presented in the guinea pig gastric fundus are beta(3)-adrenoceptors or putative beta(4)-adrenoceptors. In the presence of both the selective beta(1)-adrenoceptor antagonist atenolol (10(-4) mol/l) and the selective beta(2)-adrenoceptor antagonist butoxamine (10(-4) mol/l), the selective beta(3)-adrenoceptor antagonist SR59230A caused a concentration-dependent rightward shift of the concentration-response curve to catecholamines (isoprenaline, noradrenaline and adrenaline) and beta(3)-adrenoceptor agonists (BRL37344 and CGP12177A) in the guinea pig gastric fundus. Schild plot analyses of SR59230A against these agonists gave pA(2) values of 7.35 +/- 0.03 (isoprenaline), 7.26 +/- 0.04 (noradrenaline), 7.26 +/- 0.05 (adrenaline), 7.79 +/- 0.03 (BRL37344) and 6.74 +/- 0.03 (CGP12177A), respectively, and all Schild slopes were not significantly different from unity. These results suggest that atypical beta-adrenoceptors mediating relaxant responses of these agonists in the guinea pig gastric fundus are beta(3)-adrenoceptors rather than putative beta(4)-adrenoceptors.

Beta3-adrenoceptors in the cardiovascular system

beta-Adrenoceptors of the beta1 and beta2 subtypes classically mediate the effects of catecholamines on the contractility of cardiac muscle and the relaxation of vascular smooth muscle. Since the molecular characterization of the beta3-adrenoceptor in 1989, most studies of this adrenoceptor subtype have focused on its control of lipolysis in adipose tissues. However, more recent studies have investigated the involvement of beta3-adrenoceptors in the physiological control of cardiac and vascular contractility. In this article, the pharmacological and molecular evidence that supports the functional role of beta3-adrenoceptors in cardiovasculartissues of various species, including humans, will be discussed. These data might provide new insights into our understanding of the abnormal responsiveness of the cardiovascular system to catecholamines in heart failure and its treatment with beta3-adrenoceptor antagonists.

The negative inotropic action of catecholamines: Role of β3-adrenoceptors

There is now evidence for the involvement of four β-adrenoceptor populations in the regulation of cardiac function by catecholamines. β1- and β2-adrenoceptor stimulation classically produces an increase in contractility. A fourth β-adrenoceptor, as yet uncloned and designated provisionally as a β4-adrenoceptor, also mediates a positive inotropic effect. β3-adrenoceptors, which had been cloned at the end of the eighties, has been extensively studied as a potential target for antiobesity and antidiabetic drugs. Its characterization in the heart has opened new fields of investigations for the understanding of the cardiac adrenergic regulation. This review describes the cardiac electrical and mechanical effects induced by β3-adrenoceptor stimulation in different species (including human), as well as the signaling pathway. It also analyzes the role of these receptors in the abnormal responsiveness of catecholamines in heart failure.Key words: beta-adrenoceptor, heart, contractility, signaling pathway, heart failure.

Beta 3-adrenoceptor stimulation induces vasorelaxation mediated essentially by endothelium-derived nitric oxide in rat thoracic aorta

1. The relaxant effects of isoprenaline may result from activation of another beta-adrenoceptor subtype in addition to beta1 and beta2. This study evaluated the role of a third beta-adrenoceptor subtype, beta3, in beta-adrenoceptor-induced relaxation of rat thoracic aorta by isoprenaline. 2. Isoprenaline produced a concentration-dependent relaxation of phenylephrine pre-contracted rings of the thoracic aorta (pD2=7.46+/-0.15; Emax=85.9+/-3.4%), which was partially attenuated by endothelium removal (Emax=66.5+/-6.3%) and administration of the nitric oxide (NO) synthase inhibitor, L-NG-monomethyl arginine (L-NMMA) (Emax=61.3+/-7.9%). 3. In the presence of nadolol, a beta1- and beta2-adrenoceptor antagonist, isoprenaline-induced relaxation persisted (Emax=55.6+/-5.3%), but occurred at higher concentrations (pD2=6.71+/-0.10) than in the absence of nadolol and lasted longer. 4. Similar relaxant effects were obtained with two beta3-adrenoceptor agonists: SR 58611 (a preferential beta3-adrenoceptor agonist), and CGP 12177 (a partial beta3-adrenoceptor with beta1- and beta2-adrenoceptor antagonistic properties). SR 58611 caused concentration-dependent relaxation (pD2=5.24+/-0.07; Emax=59.5+/-3.7%), which was not modified by pre-treatment with nadolol but antagonized by SR 59230A, a beta3-adrenoceptor antagonist. The relaxation induced by SR 58611 was associated with a 1.7 fold increase in tissue cyclic GMP content. 5 Both relaxation and the cyclic GMP increase induced by SR 58611 were greatly reduced by endothelium removal and in the presence of L-NMMA. 6 We conclude that in the rat thoracic aorta, beta3-adrenoceptors are mainly located on endothelial cells, and act in conjuction with beta1- and beta2-adrenoceptors to mediate relaxation through activation of an NO synthase pathway and subsequent increase in cyclic GMP levels.

Innervation of mammalian white adipose tissue: implications for the regulation of total body fat

We review the extensive physiological and neuroanatomical evidence for the innervation of white adipose tissue (WAT) by the sympathetic nervous system (SNS) as well as what is known about the sensory innervation of this tissue. The SNS innervation of WAT appears to be a part of the general SNS outflow from the central nervous system, consisting of structures and connections throughout the neural axis. The innervation of WAT by the SNS could play a role in the regulation of total body fat in general, most likely plays an important role in regional differences in lipid mobilization specifically, and may have a trophic affect on WAT. The exact nature of the SNS innervation of WAT is not known but it may involve contact with adipocytes and/or their associated vasculature. We hypothesize that the SNS innervation of WAT is an important contributor to the apparent "regulation" of total body fat.

Alterations in dynamic heart rate control in the beta 1-adrenergic receptor knockout mouse

beta 1-Adrenergic receptors (beta 1-ARs) are key targets of sympathetic nervous system activity and play a major role in the beat-to-beat regulation of cardiac chronotropy and inotropy. We employed a beta 1-AR gene knockout model to test the hypothesis that beta 1-AR function is critical for maintenance of resting heart rate and baroreflex responsiveness and, on the basis of its important role in regulating chronotropy and inotropy, is also required for maximal exercise capacity. Using an awake unrestrained mouse model, we demonstrate that resting heart rate and blood pressure are normal in beta 1-AR knockouts and that the qualitative responses to baroreflex stimulation are intact. Chronotropic reserve in beta 1-AR knockouts is markedly limited, with peak heart rates approximately 200 beats/min less than wild types. During graded treadmill exercise, heart rate is significantly depressed in beta 1-AR knockouts at all work loads, but despite this limitation, there are no reductions in maximal exercise capacity or metabolic indexes. Thus, in mice, the beta 1-AR is not essential for either maintenance of resting heart rate or for maximally stressed cardiovascular performance.

alpha1-Adrenergic stimulation potentiates the thermogenic action of beta3-adrenoreceptor-generated cAMP in brown fat cells

The relationship between cAMP levels and thermogenesis was investigated in brown fat cells from Syrian hamsters. Irrespective of whether the selective beta3-, beta2-, and beta1-agonists BRL 37344, salbutamol, and dobutamine or the physiological agonist norepinephrine was used to stimulate the cells, increases in cAMP levels were mediated via the beta3-receptor, as were the thermogenic effects. However, the relationship "thermogenesis per cAMP" was much lower for agents other than norepinephrine. Similarly, forskolin, although more potent than norepinephrine in elevating cAMP, was less potent in inducing thermogenesis. The selective alpha1-agonist cirazoline was in itself without effect on cAMP levels or thermogenesis, but when added to forskolin-stimulated cells, potentiated thermogenesis, up to the norepinephrine level, without affecting cAMP. This potentiation could not be inhibited by chelerythrine, but could be mimicked by Ca2+ ionophores. It was apparently not mediated via calmodulin-dependent protein kinase and was not an effect on mitochondrial respiratory control. The ability of all cAMP-elevating agents to induce thermogenesis in brown fat cells has earlier been interpreted to mean that it is only through the beta-receptors and the resulting increase in cAMP levels that thermogenesis is induced. However, it is here concluded that the thermogenic response to norepinephrine involves two interacting parts, one mediated via beta-receptors and cAMP and the other via alpha1-receptors and increases in cytosolic Ca2+ levels.

In vivo resistance of lipolysis to epinephrine. A new feature of childhood onset obesity

A decreased mobilization of triglycerides may contribute to fat accumulation in adipocytes, leading to obesity. However, this hypothesis remains to be proven. In this study, epinephrine-induced lipid mobilization was investigated in vivo in nine markedly obese children (160+/-5% ideal body weight) aged 12.1+/-0.1 yr during the dynamic phase of fat deposition, compared with six age-matched nonobese children. As an in vivo index of lipolysis, we measured glycerol flux using a nonradioactive tracer dilution approach, and plasma free fatty acid concentrations. In the basal state, the obese children had a 30% lower rate of glycerol release per unit fat mass than the lean children. To study the regulation of lipolysis, epinephrine was infused stepwise at fixed doses of 0.75 and then 1. 50 microg/min in both groups. In lean children, glycerol flux and plasma free fatty acid increased to an average of 249-246% of basal values, respectively, in response to a mean plasma epinephrine of 396+/-41 pg/ml. The corresponding increase was only 55-72% in the obese children, although their mean plasma epinephrine reached 606+/-68 pg/ml. All obese and nonobese children, except an Arg64Trp heterozygote, were homozygotes for Trp at position 64 of their beta3-adrenoreceptor. The resistance of lipolysis to epinephrine showed no relationship with the Arg64 polymorphism of the beta3-adrenoreceptor gene. In summary, in vivo lipolysis, which mostly reflects the mobilization of lipid stores from subcutaneous adipose tissue, shows a decreased sensitivity to epinephrine in childhood onset obesity. Since our study was carried out in obese children during the dynamic phase of fat accumulation, the observed resistance to catecholamines might possibly be causative rather than the result of obesity.

Fat cells

The adipocyte is a metabolically active cell that functions to store energy for times of energy deprivation or enhanced need. Obesity is characterized by increased lipid accumulation and turnover compared with the nonobese state. Both triglyceride synthesis and lipolysis are regulated metabolic processes in the adipocyte. Current research on the metabolic activities of the human adipocyte focus on plasma triglyceride hydrolysis and uptake of fatty acids by LPL, esterification of these fatty acids, and the subsequent triglyceride breakdown by hormone-sensitive lipase in response to stimulation of adrenergic receptors. These topics are discussed in relationship to the development of obesity.

Endogenous beta 3- but not beta 1-adrenergic receptors are resistant to agonist-mediated regulation in human SK-N-MC neurotumor cells

Although there is considerable interest in the regulation of the different beta-adrenergic receptor (AR) subtypes, most previous studies have utilized stably transfected cells expressing recombinant receptors under the control of viral promoters. Human SK-N-MC neurotumor cells appear to be novel, since they express both endogenous beta 1AR and beta 3AR based on radioligand binding and on functional response. Saturation binding of either the hydrophilic ligand (-)-[3H]CGP-12177 or the more hydrophobic (-)-[125I]iodocyanopindolol indicated the presence of two populations of binding sites with high and low affinities. With either ligand, the beta 1AR antagonist CGP-20712A preferentially inhibited binding to the high-affinity sites. This is consistent with the latter representing beta 1AR whereas the low-affinity sites represent beta 3AR. Both subtypes appeared to be functional on the basis of isoproterenol stimulation of cyclic adenosine monophosphate (cAMP) in intact cells and adenylyl cyclase activity in cell membranes in the absence and presence of CGP-20712A. SK-N-MC-IXC cells, derived by twice subcloning the parental cells, also expressed both beta AR subtypes, indicating that they co-exist in the same cell. SK-N-MC cells exposed to isoproterenol exhibited a rapid sequestration and a slower downregulation of beta 1AR. The latter subtype also underwent desensitization, as indicated by a rightward shift to less sensitivity in the EC50 for isoproterenol stimulation of adenylyl cyclase activity. In contrast, the beta 3AR subtype was resistant to agonist-mediated sequestration, downregulation, and desensitization. Thus, when endogenously expressed in the same cell line, human beta 1AR and beta 3AR display differences in their ability to be regulated by agonist.

Triiodothyronine regulates beta 3-adrenoceptor expression in 3T3-F442A differentiating adipocytes

The effect of thyroid hormones on the beta 3-adrenoceptor expression was studied in the preadipose 3T3-F442A cell line. As assessed by molecular and pharmacological analyses, triiodothyronine addition to differentiating 3T3-F442A cells caused a 2.3-fold increase in beta 3-adrenoceptor mRNA levels, which was correlated with a parallel induction of beta 3-adrenoceptor number and of beta 3-adrenoceptor coupling to the adenylate cyclase system. Nuclear transcription experiments showed that triiodothyronine did not significantly alter the transcription rate of the beta 3-adrenoceptor gene. By contrast, the hormone increased by 36% the half-life of beta 3-adrenoceptor mRNA. Triiodothyronine exhibited a discrete effect on beta 3-adrenoceptor expression when added to mature 3T3-F442A adipocytes. This study indicates that thyroid hormones exert a differentiation-dependent and post-transcriptional regulation of beta 3-adrenoceptor expression in adipocytes.

Triiodothyronine and amiodarone effects on beta 3-adrenoceptor density and lipolytic response to the beta 3-adrenergic agonist BRL 37344 in rat white adipocytes

The beta-adrenergic effects of catecholamines are potentiated by thyroid hormones in adipose tissue. Amiodarone (AM) is structurally similar to thyroid hormones and was used to explore the mechanism of the triiodothyronine (T3) effect on beta-adrenergic receptors (beta-ARs) in adipose tissue. AM decreases the expression of some T3 sensitive genes in various tissues and antagonizes the effect of T3 on its nuclear receptors. In this study, the T3, AM and AM + T3 effects on the beta 1- and beta 3-AR density were assessed on rat white adipocytes by radioligand binding using [3H]CGP 12177 after characterization of these subtypes by displacement of [3H]CGP 12177 binding by isoproterenol, BRL 37344 and noradrenaline. BRL 37344 was used to study beta 3-AR lipolysis. White adipocytes from hyperthyroid rats had increased responsiveness (Emax x 2) and sensitivity (+ 38%) to BRL 37344, while those given AM alone had decreased values. Moreover, AM antagonized the T3 effect on lipolysis. The beta 1-binding characteristics (receptor density [Bmax]: 45 +/- 4 fmol/mg of proteins; dissociation factor [Kd]: 0.96 +/- 0.10 nM) were not modified by either compound. Finally, T3 significantly increased beta 3-AR density (587 +/- 69 versus 363 +/- 25 fmol/mg of proteins) and Kd (38 +/- 2 versus 23 +/- 3 nM), while AM alone had no effect and did not antagonize the T3 effect on beta 3-AR number. In conclusion, the hyperthyroid state in the rat potentiated the lipolytic response of white adipocytes to a specific beta 3-agonist and increased the beta 3-AR density without changing in beta 1-AR number and affinity. Furthermore, the lack of antagonism between AM and T3 on beta 3-AR expression suggests that T3 does not work directly on the beta 3-AR gene. Moreover, AM induced a functional tissular hypothyroid-like effect and its antilipolytic effect probably occurred at a postreceptor level.

In situ assessment of the role of the beta 1-, beta 2- and beta 3-adrenoceptors in the control of lipolysis and nutritive blood flow in human subcutaneous adipose tissue

1. The involvement of beta 1-, beta 2- and beta 3-adrenoceptors in the control of lipolysis and nutritive blood flow was investigated in abdominal subcutaneous adipose tissue of healthy young adults by use of an in situ microdialysis technique. 2. Dialysis probes were infused either with isoprenaline (non-selective beta-adrenoceptor agonist), CGP 12,177 (selective beta 3-adrenoceptor agonist having beta 1-/beta 2-antagonist properties), dobutamine (selective beta 1-adrenoceptor agonist) or terbutaline (selective beta 2-adrenoceptor agonist). The recovery of each probe used for perfusion was calculated by an in vivo calibration method. The local blood flow was estimated through the measurement of the escape of ethanol infused simultaneously with the drugs included in the probe. 3. Isoprenaline infusion at 0.01 microM had a weak effect while higher concentrations of isoprenaline (0.1 and 1 microM) caused a rapid, sustained and concentration-dependent increase of glycerol outflow; the maximum increase was 306 +/- 34% with 1 microM. Isoprenaline also increased the nutritive blood flow in adipose tissue; a significant effect appeared at 0.1 microM isoprenaline and was greater at 1 microM. 4. CGP 12,177 (10 and 100 microM) increased the glycerol concentration in the dialysate (128 +/- 8 and 149 +/- 12%, respectively) and nutritive blood flow. Terbutaline and dobutamine (100 microM) both provoked rapid and similar increases in glycerol outflow (252 +/- 18 and 249 +/- 18%, respectively). Both, terbutaline and dobutamine increased nutritive blood flow. 5. It is concluded that beta 1- and beta 2-adrenoceptor subtypes are both mainly involved in the mobilization of lipids and in the control of nutritive blood flow. beta 3-Adrenoceptors play a weaker role in the control of lipolysis and nutritive blood flow in human subcutaneous abdominal adipose tissue.

Insulin sensitizes beta-agonist and forskolin-stimulated lipolysis to inhibition by 2',5'-dideoxyadenosine

In isolated rat adipocytes incubated in the absence of insulin, 2',5'-dideoxyadenosine blocked the increase in total adenosine 3',5'-cyclic monophosphate (cAMP) accumulation due to beta 1- or beta 3-catecholamine agonists and forskolin without affecting their stimulation of lipolysis. The inhibition of cAMP accumulation by 2',5'-dideoxyadenosine was not reflected in the total cytosolic cAMP-dependent protein kinase A activity, suggesting that the inhibition of cAMP occurred in cellular compartments distinct from those involved in the regulation of bulk protein kinase A activity. However, there was a good correlation between effects of lipolytic agents on cytosolic protein kinase A activity in fat cell extracts and lipolysis. Furthermore, it was possible to see an inhibition of the increase due to beta-agonists in cAMP accumulation, protein kinase A activity, and lipolysis by 2',5'-dideoxyadenosine in the presence of insulin. These data suggest that the readily measurable accumulation of cAMP seen with catecholamines in the absence of insulin is in a compartment separate from that involved in protein kinase A activation.