Why do adipocytes make the beta 3 adrenergic receptor?

Trigonelline induces browning in 3T3-L1 white adipocytes

Trigonelline, a major alkaloid component of fenugreek, has been demonstrated to have several biological activities, including antidiabetic and anticancer effects. This study aimed to examine the possible application of trigonelline as an anti-obesity compound based on an investigation of its enhancement of lipid catabolism and induction of browning in white adipocytes. Trigonelline induces browning of 3T3-L1 white adipocytes by enhancing the expressions of brown-fat signature proteins and genes as well as beige-specific genes, including Cd137, Cited1, Tbx1, and Tmem26. Trigonelline also improves lipid metabolism in white adipocytes by decreasing adipogenesis and lipogenesis as well as promotes lipolysis and fatty acid oxidation. Moreover, trigonelline increases the expression of Cox4, Nrf1, and Tfam genes that are responsible for mitochondrial biogenesis. Mechanistic studies revealed that the browning effect of trigonelline in 3T3-L1 white adipocytes is mediated by activating β3-AR and inhibiting PDE4, thereby stimulating the p38 MAPK/ATF-2 signaling pathway. Considering its high bioavailability in humans and the results of this study, trigonelline may have potential as an anti-obesity compound.

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.

Unlock the Thermogenic Potential of Adipose Tissue: Pharmacological Modulation and Implications for Treatment of Diabetes and Obesity

Brown adipose tissue (BAT) is considered an interesting target organ for the treatment of metabolic disease due to its high metabolic capacity. Non-shivering thermogenesis, once activated, can lead to enhanced partitioning and oxidation of fuels in adipose tissues, and reduce the burden of glucose and lipids on other metabolic organs such as liver, pancreas, and skeletal muscle. Sustained long-term activation of BAT may also lead to meaningful bodyweight loss. In this review, we discuss three different drug classes [the thiazolidinedione (TZD) class of PPARγ agonists, β3-adrenergic receptor agonists, and fibroblast growth factor 21 (FGF21) analogs] that have been proposed to regulate BAT and beige recruitment or activation, or both, and which have been tested in both rodent and human. The learnings from these classes suggest that restoration of functional BAT and beige mass as well as improved activation might be required to fully realize the metabolic potential of these tissues. Whether this can be achieved without the undesired cardiovascular side effects exhibited by the TZD PPARγ agonists and β3-adrenergic receptor agonists remains to be resolved.

Angiotensin II Stimulates Sympathetic Neurotransmission to Adipose Tissue

Angiotensin II (AngII) facilitates sympathetic neurotransmission by regulating norepinephrine (NE) synthesis, release, and uptake. These effects of AngII contribute to cardiovascular control. Previous studies in our laboratory demonstrated that chronic AngII infusion decreased body weight of rats. We hypothesized that AngII facilitates sympathetic neurotransmission to adipose tissue and may thereby decrease body weight. The effect of chronic AngII infusion on the NE uptake transporter and NE turnover was examined in metabolic (interscapular brown adipose tissue, ISBAT; epididymal fat, EF) and cardiovascular tissues (left ventricle, LV; kidney) of rats. To examine the uptake transporter saturation isotherms were performed using [³H]nisoxetine (NIS). At doses that lowered body weight, AngII significantly increased ISBAT [³H]NIS binding density. To quantify NE turnover, alpha-methyl-para-tyrosine (AMPT) was injected in saline-infused, AngII-infused, or saline-infused rats that were pair-fed to food intake of AngII-infused rats. AngII significantly increased the rate of NE decline in all tissues compared to saline. The rate of NE decline in EF was increased to a similar extent by AngII and by pair feeding. In rats administered AngII and propranolol, reductions in food and water intake and body weight were eliminated. These data support the hypothesis that AngII facilitates sympathetic neurotransmission to adipose tissue. Increased sympathetic neurotransmission to adipose tissue following AngII exposure is suggested to contribute to reductions in body weight.

Effect of β(3)-adrenergic receptor gene polymorphism on body weight change in middle-aged, overweight women

OBJECTIVES

To examine the effects of β(3)-adrenergic receptor gene polymorphism on body weight change during a weight reduction program for middle-aged, overweight women with careful consideration of their energy intake and expenditure.

METHODS

DESIGN

Intervention study of weight reduction for 12 weeks in a community setting.

SUBJECTS

Eighty overweight middle-aged women who completed the individualized lifestyle modification program.

MEASUREMENTS

β(3)-adrenergic receptor gene polymorphism was identified by polymerase chain reaction and consecutive restriction fragment-length polymorphism analysis. Anthropometrical parameters, lifestyle factors, blood lipid and glucose levels, physical activity level and energy intake were measured before and at the end of the program.

RESULTS

The numbers of subjects with the Trp64Trp, Trp64Arg, and Arg64Arg genotypes were 45, 30 and 5, respectively. Baseline characteristics among subjects with the 64Arg allele had significantly smaller decrease in body weight and energy intake than those without the 64Arg allele. The change of other clinical characteristics did not differ between the two groups. After adjusting for the %change of energy intake, the %change of body weight did not differ between the two groups.

CONCLUSION

The 64Arg allele of the β(3)-AR gene is not likely to be the factor determining the difficulty in losing body weight in Japanese middle-aged, overweight women. Lifestyle factors, such as the decrease in energy intake, might mask the effect of the 64Arg allele on body weight loss. Specific considerations for the management of energy intake would be needed to promote body weight loss for those with the 64Arg allele.

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.

beta3-adrenergic receptor induction of adipocyte inflammation requires lipolytic activation of stress kinases p38 and JNK

Activation of beta-adrenergic receptors (AR) in adipocytes triggers acute changes in metabolism that can alter patterns of gene expression. This work examined the mechanisms by which activation of hormone sensitive lipase (HSL) induces expression of inflammatory cytokines in adipocytes in vivo and model adipocytes in vitro. beta3-AR activation in mice triggered expression of inflammatory genes CCL2, IL-6, and PAI-1, as well as endoplasmic reticulum (ER) stress markers GRP78 and CHOP. Pharmacological inhibition of HSL blocked induction of inflammatory genes, but not ER stress markers. Promoting intracellular accumulation of free fatty acids (FFAs) in 3T3-L1 adipocytes increased expression of inflammatory cytokines, whereas inhibiting ceramide synthesis partly blocked PAI-1 expression, but not IL-6. Induction of inflammatory markers in vivo and in vitro was preceded by phosphorylation of p38 and JNK, and inhibition of HSL prevented activation of these kinases. Experiments with pharmacological inhibitors of specific MAP kinases demonstrated the importance of p38 MAPK as a mediator of lipolysis-induced inflammation in vivo and in vitro. Together, these results demonstrate that FFAs liberated by HSL activate p38 and JNK, and p38 mediates pro-inflammatory cytokine expression in adipose tissue.

Prolonged treatment with the beta3-adrenergic agonist CL 316243 induces adipose tissue remodeling in rat but not in guinea pig: 1) fat store depletion and desensitization of beta-adrenergic responses

Beta3-adrenergic agonists have been considered as potent antiobesity and antidiabetic agents mainly on the basis of their beneficial actions discovered twenty years ago in obese and diabetic rodents. The aim of this work was to verify whether prolonged treatment with a beta3-adrenergic agonist known to stimulate lipid mobilisation, could promote desensitization of beta-adrenergic responses. Wistar rats and guinea pigs were treated during one week with CL 316243 (CL, 1 mg/kg/d) by implanted osmotic minipumps. In control animals, beta3-adrenergic agonists were lipolytic in rat but not in guinea pig adipocytes. CL-treatment did not alter body weight gain in both species, but reduced fat stores in rats. Lipolysis stimulation by forskolin was unmodified but responses to beta1-, beta2- and beta3-agonists were reduced in visceral or subcutaneous white adipose tissues of CL-treated rats. Similarly, the beta3-adrenergic-dependent impairment of insulin action on glucose transport and lipogenesis in rat adipocytes was diminished after CL-treatment. In rat adipocytes, [125I]ICYP binding and beta3-adrenoceptor mRNA levels were reduced after sustained CL administration. These findings show that CL 316243 exerts (beta3-adrenergic lipolytic and antilipogenic effects in rat adipocytes. These actions, which are likely involved in the fat depletion observed in rat, also lead to the desensitization of all beta-adrenergic responses. Therefore this desensitization, together with the lack of slimming action in guinea pig, seriously attenuates the usefulness of beta3-agonists as antiobesity agents, and may explain why such agonists have not been conducted to a widespread clinical use.

Prolonged treatment with the beta3-adrenergic agonist CL 316243 induces adipose tissue remodeling in rat but not in guinea pig: 2) modulation of glucose uptake and monoamine oxidase activity

Beta3-adrenergic agonists are well-recognited to promote lipid mobilisation and adipose tissue remodeling in rodents, leading to multilocular fat cells enriched in mitochondria. However, effects of beta3-adrenergic agonists on glucose transport are still controversial. In this work, we studied in white adipose tissue (WAT) the influence of sustained beta3-adrenergic stimulation on the glucose transport and on the mitochondrial monoamine oxidase (MAO) activity. As one-week administration of CL 316243 (CL, 1 mg/kg/d) induces beta-adrenergic desensitization in rat but not in guinea pig adipocytes, attention was paid to compare these models. When expressing glucose uptake as nmoles of 2-deoxyglucose/100 mg cell lipids, maximally stimulated uptake was increased in adipocytes of WAT from treated rats but not from treated guinea pigs. However, basal hexose uptake was also increased in CL-treated rats and, as a consequence, the dose-dependent curves for insulin stimulation were similar in control and CL-treated rats when expressed as fold increase over basal. Insulin-induced lipogenesis was unchanged in rat or guinea pig adipocytes after CL-treatment. The glucose carriers GLUT4 and corresponding mRNA were increased in subcutaneous WAT or in brown adipose tissue (BAT) but not in visceral WAT or muscles of CL-treated rats. There was an increase of MAO activity in WAT and BAT, but not in liver, of CL-treated rats while no change was detected in guinea pigs. These findings show that only rat adipocytes, which are beta3-adrenergic-responsive, respond to chronic beta3-AR agonist by an increase of GLUT4 content and MAO activity, despite a desensitization of all beta-adrenoceptor subtypes.

Adipose targets for obesity drug development

The prevalence of obesity is increasing rapidly in most parts of the world and effective therapeutic drugs are urgently needed. The discovery of leptin in 1994 initiated a new understanding of adipose tissue function, and adipose tissue is now known to not only store and release fatty acids, but also to produce a wealth of factors that have an impact on the regulation of body weight and blood glucose homeostasis. Also, adipocytes express proteins that engage signalling pathways playing important roles in fuel substrate and energy metabolism. These proteins constitute a diverse array of adipose target candidates for the development of drugs to treat obesity. Some of these potential targets have been validated and are now in drug development stages, providing hope that the current obesity epidemic can be addressed by effective drug treatments in the near future.

Protein kinase C attenuates β-adrenergic receptor-mediated lipolysis, probably through inhibition of the β1-adrenergic receptor system

Lipolysis in rat white adipocytes is stimulated by beta-adrenergic agonists. Phorbol 12-myristate 13-acetate (PMA) attenuated the receptor-mediated lipolysis by causing a shift of the dose-response curve to the higher concentrations of norepinephrine and isoproterenol. Although the adipocytes possess beta1-, beta2-, and beta3-adrenergic receptor subtypes, the effect of PMA was observed only when a beta1-agonist (dobutamine) was used. No lipolysis-attenuating effect of PMA was found when cells were exposed to a beta2-agonist (procaterol) and beta3-agonists (BRL 37344 and CL 316243), or to forskolin and 8-bromo cAMP. CGP 20712A (beta1-antagonist) efficiently inhibited lipolysis by norepinephrine, isoproterenol, and dobutamine, but did not affect lipolysis by the beta2- and beta3-agonists. ICI 118551 (beta2-antagonist) had no significant effect on lipolysis by the beta-agonists examined. CGP 20712A abolished the lipolysis-attenuating effect of PMA, but ICI 118551 did not. The protein kinase C (PKC) inhibitors, GF 109203X or Gö 6976, suppressed the effect of PMA. Pretreatment of adipocytes with PMA for 6 h caused downregulation of conventional and novel PKCs in association with a decrease in the lipolysis-attenuating effect of PMA. These results indicate that conventional and novel PKCs attenuate lipolysis mediated by beta-adrenergic receptors, probably through inhibition of the beta1-adrenergic receptor system.

Metabolic and cellular plasticity in white adipose tissue I: effects of beta3-adrenergic receptor activation

Selective agonists of beta(3)-adrenergic receptors (Adrb3) exhibit potent anti-diabetes properties in rodent models when given chronically, yet the mechanisms involved are poorly understood. A salient feature of chronic Adrb3 activation is pronounced remodeling of white adipose tissue (WAT), which includes mitochondrial biogenesis and elevation of metabolic rate. To gain insights into potential mechanisms underlying WAT remodeling, the time course of remodeling induced by the Adrb3 agonist CL-316,243 (CL) was analyzed using histological, physiological, and global gene profiling approaches. The results indicate that continuous CL treatment induced a transient proinflammatory response that was followed by cellular proliferation among stromal cells and multilocular adipocytes. CL treatment strongly fragmented the central lipid storage droplet of mature adipocytes and induced mitochondrial biogenesis within these cells. Mitochondrial biogenesis was correlated with the upregulation of genes involved in fatty acid oxidation and mitochondrial electron transport activity. The elevated catabolic activity of WAT was temporally correlated with upregulation of peroxisome proliferator-activated receptor-alpha and its target genes, suggesting involvement of this transcription factor in coordinating the gene program that elevates WAT catabolic activity.

Molecular characterization of pharmacological properties and selectivity of SWR-0315NA for beta3-adrenoceptors

The pharmacological properties of SWR-0315NA, (E,Z)-[4[[1-[2-[(3-phenoxy-2-hydroxy propyl)]amino]ethyl]-1-propenyl]phenoxy]acetic acid sodium, were compared with those of (-)-isoproterenol. In the radioligand binding studies of [(125)I]iodocyanopindolol with COS-7 cell membranes that transiently expressed human beta-adrenoceptor (beta-AR) subtypes, SWR-0315NA exhibited 1-fold and 2-fold greater binding affinities for beta(3)-AR than those for beta(1)- and beta(2)-ARs, respectively. The maximal stimulatory effects of SWR-0315NA on cAMP accumulation in CHO cells expressing all the beta-AR subtypes were 79%, 3% and 93% for beta(1)-, beta(2)- and beta(3)-ARs of those produced by (-)-isoproterenol, respectively. SWR-0315NA has 26.3-fold and more than 630-fold greater selectivity for beta(3)-AR than those for beta(1)- and beta(2)-ARs in potency, respectively. These results indicate that although SWR-0315NA has lower binding selectivity towards beta-AR subtypes, it is a selective agonist with high intrinsic activity for beta(3)-AR as compared with (-)-isoproterenol.

Short-chain fatty acids stimulate leptin production in adipocytes through the G protein-coupled receptor GPR41

Leptin is an adipose-derived hormone that regulates a wide variety of physiological processes, including feeding behavior, metabolic rate, sympathetic nerve activity, reproduction, and immune response. Circulating leptin levels are tightly regulated according to energy homeostasis in vivo. Although mechanisms for the regulation of leptin production in adipocytes are not well understood, G protein-coupled receptors may play an important role in this adipocyte function. Here we report that C2-C6 short-chain fatty acids, ligands of an orphan G protein-coupled receptor GPR41, stimulate leptin expression in both a mouse adipocyte cell line and mouse adipose tissue in primary culture. Acute oral administration of propionate increases circulating leptin levels in mice. The concentrations of short-chain fatty acids required to stimulate leptin production are within physiological ranges, suggesting the relevance of this pathway in vivo.

White adipose tissue contributes to UCP1-independent thermogenesis

Beta3-adrenergic receptors (AR) are nearly exclusively expressed in brown and white adipose tissues, and chronic activation of these receptors by selective agonists has profound anti-diabetes and anti-obesity effects. This study examined metabolic responses to acute and chronic beta3-AR activation in wild-type C57Bl/6 mice and congenic mice lacking functional uncoupling protein (UCP)1, the molecular effector of brown adipose tissue (BAT) thermogenesis. Acute activation of beta3-AR doubled metabolic rate in wild-type mice and sharply elevated body temperature and BAT blood flow, as determined by laser Doppler flowmetry. In contrast, beta3-AR activation did not increase BAT blood flow in mice lacking UCP1 (UCP1 KO). Nonetheless, beta3-AR activation significantly increased metabolic rate and body temperature in UCP1 KO mice, demonstrating the presence of UCP1-independent thermogenesis. Daily treatment with the beta3-AR agonist CL-316243 (CL) for 6 days increased basal and CL-induced thermogenesis compared with naive mice. This expansion of basal and CL-induced metabolic rate did not require UCP1 expression. Chronic CL treatment of UCP1 KO mice increased basal and CL-stimulated metabolic rate of epididymal white adipose tissue (EWAT) fourfold but did not alter BAT thermogenesis. After chronic CL treatment, CL-stimulated thermogenesis of EWAT equaled that of interscapular BAT per tissue mass. The elevation of EWAT metabolism was accompanied by mitochondrial biogenesis and the induction of genes involved in lipid oxidation. These observations indicate that chronic beta3-AR activation induces metabolic adaptation in WAT that contributes to beta3-AR-mediated thermogenesis. This adaptation involves lipid oxidation in situ and does not require UCP1 expression.

Fatty acid mobilization from adipose tissue during exercise

By far the largest energy reserve in the human body is adipose tissue triglycerides, and these reserves are an important source of fuel during prolonged endurance exercise. To use this rich source of potential energy during exercise, adipose tissue triglycerides must first be hydrolyzed and the resultant fatty acids delivered to the working muscles. The aims of this review are to describe how exercise alters lipid mobilization from adipose tissue, to identify alternative sources of lipids and to discuss some of the key factors regulating fatty acid mobilization, uptake and oxidation during exercise. The impact of understanding factors involved in the coordinated regulation of lipid mobilization and oxidation during exercise goes far beyond its relevance for endurance exercise performance. A better understanding of the regulation of these processes will facilitate the development of more effective treatment modalities for obesity-related metabolic disorders.

Beta-adrenergic receptor subtypes that mediate ractopamine stimulation of lipolysis

Ractopamine HCl is an beta-adrenergic receptor (betaAR) ligand that was recently approved for use in swine to enhance carcass leanness. The RR stereoisomer of ractopamine is the most active of the four stereoisomers exhibiting the highest affinity and signaling response. The RR isomer exhibits selective activation of the porcine beta2AR, which might limit the lipolytic response to ractopamine because the betaAR is the predominant subtype in swine adipocytes and may mediate most of the lipolytic response. Therefore, we determined the betaAR subtypes that mediate the lipolytic response to ractopamine in swine adipocytes. In order to confirm the predominant role of the beta1AR in porcine adipocytes, isoproterenol-stimulated lipolysis was inhibited by increasing doses of subtype-selective antagonists. Inhibition curves were biphasic using beta1AR antagonists (CGP 20712A and bisoprolol) and curve analysis indicated that both beta1AR an beta2AR contributed to lipolysis with 50 to 60% of the response coming from the beta1AR. Inhibition with the beta2AR antagonist clenbuterol revealed only one class of betaAR that closely approximated the kinetics of the beta1AR. When the RR isomer of ractopamine was the lipolytic agent, similar results to isoproterenol were observed, except that the estimated contribution of the beta1AR was 38%. That beta2AR antagonists did not detect a contribution of the beta2AR to lipolysis may indicate that the beta1AR masked the response to the beta2AR. Dose titration with the RR isomer in the presence of a saturating concentration of beta1AR or beta2AR antagonists indicated that each subtype was present in sufficient quantities to stimulate lipolysis near maximally. Data indicate that both the beta1AR and beta2AR are functionally linked to lipolysis in swine adipocytes and that ractopamine activates each subtype. The RR isomer of ractopamine stimulated adenosine 3',5'-cyclic phosphate accumulation with equal efficacy to isoproterenol through the cloned porcine beta2AR, but was only 35% as efficacious through the cloned porcine beta1AR. These data confirm the beta2AR selectivity of the RR stereoisomer, but suggest the partial agonism through the beta1AR is sufficient to activate lipolysis through both subtypes in swine adipocytes.

Leptin: metabolic control and regulation

Leptin, a protein released from adipose tissue, is being recognized to play an integral role in endocrine regulation of metabolism. While it is clearly evident that leptin is decreased during caloric restriction, the response of leptin to other types of stress has been plagued by conflicting data. With hypoglycemia stress, the literature may conflict because experimentally hypoglycemia is induced with infusion of insulin, an endocrine factor that can increase leptin levels. With exercise, leptin's response may depend on duration and intensity of exercise. While it has been clearly shown that the sympathetic nervous system (SNS) inhibits leptin secretion in a variety of experimental modes, the hypothalamic-pituitary-adrenal (HPA) axis may stimulate leptin secretion. This creates a paradox of leptin regulation during stress since both systems are activated with stress. If the SNS inhibition overrides the HPA axis' activation of leptin secretion, leptin's role during stress may be to allow a shifting of fuel consumption towards carbohydrate utilization. In type 1 diabetes mellitus, autonomic dysfunction may prevent the fall in leptin during stress. Although obesity is associated with type 2 diabetes mellitus, patients may have decreased leptin levels, especially when glucose is poorly controlled. This may contribute to further obesity and worsening of the disease. The purpose of this review to is critically analyze the literature regarding the impact of different types of stress on leptin secretion, the function of leptin during stress, and the role of leptin in the pathophysiology of diabetes.

Binding and functional affinity of some newly synthesized phenethylamine and phenoxypropanolamine derivatives for their agonistic activity at recombinant human beta3-adrenoceptor

Beta(3)-adrenoceptor is the predominant beta-adrenoceptor in adipocytes and has drawn much attention during the investigation for anti-obesity and antidiabetes therapeutics. Thirteen new compounds have been evaluated for their potencies and efficacies as beta(3)-adrenoceptor agonists on human beta(3)-adrenoceptor expressed in COS-7 and Chinese hamster ovary (CHO) cells using radioligand binding assay and cyclic AMP (cAMP) accumulation assay. Phenoxypropanolamine derivatives, SWR-0334NA (([E)-[4-[5-[(3-phenoxy-2-hydroxypropyl)amino]-2-pentene-3-yl] phenoxy]acetic acid sodium salt), SWR-0335SA ((E)-[4-[5-[(3-phenoxy-2-hydroxypropyl)amino]-2-pentene-3-yl] phenoxy] acetic acid ethanedioic acid), SWR-0342SA (S-(Z)-[4-[[1-[2-[(2-hydroxy-3-phenoxypropyl)]amino] ethyl]-1-propenyl]phenoxy] acetic acid ethanedioic acid), SWR-0348SA-SITA ((E)-[4-[5-[(3-phenoxy-2-hydroxypropyl)amino]-2-hexene-3-yl] phenoxy]acetic acid ethanedioic acid) and SWR-0361SA ((E)-N-methyl[4-[5-[(3-phenoxy-2-hydroxypropyl)amino]-2-pentene-3-yl]phenoxy]acetoamide ethanedioic acid) showed higher agonistic activity for the beta(3)-adrenoceptor. Among the compounds tested, SWR0334NA exhibited full agonist activity (%E(max) = 100.26) despite its lower binding affinity (pK(I) = 6.11). Compounds SWR-0338SA ((E)-[4-[5-[(2-phenyl-2-hydroxyethyl)amino]-2-pentene-3-yl] phenoxy]acetic acid ethanedioic acid), SWR-0339SA (S-(E)-[4-[5-[(3-phenoxy-2-hydroxypropyl)amino]-2-pentene-3-yl] phenoxy] acetic acid ethanedioic acid), SWR-0345HA ((E)-2-methyl-3-[4-[2-(2-phenyl-2-hydroxyethyl-amino)ethoxy] phenyl]-2-propenoic acid ethyl ester hydrochloride), SWR-0358SA ((E)-(2-methoxyethyl)-[4-[5-[(3-phenoxy-2-hydroxypropyl) amino]-2-pentene-3-yl]phenoxy]acetoamide ethanedioic acid) and SWR-0362SA ((E)-1-[[[4-[5-[(3-phenoxy-2-hydroxypropyl)amino]-2-pentene-3-yl]phenoxy]acetyl]carbonyl]piperidine ethanedioic acid) had moderate agonistic activity and were phenethylamine and phenoxypropanolamine derivatives. Compounds SWR-0065HA ([4-[2-[3-[[(3,4-dihydro-4-oxo-[1,2,4]-triazino(4,5-a)indol)-lyl]oxy]-2-hydroxypropylamino]ethoxy]phenyl]acetic acid methyl ester hydrochloride), SWR-0098NA ((E)-[4-[3-[(2-phenyl-2-hydroxyethyl)amino]-1-butenyl] phenoxy]acetic acid sodium salt) and SWR-0302HA ([4-[[4-[2-(3-chlorophenoxy-2-hydroxypropyl)amino]-E-2-butenyl]oxy]phenoxy]acetic acid hydrochloride) had very low binding affinity towards beta(3)-adrenoceptors and they did not induce cAMP accumulation. We concluded that compounds SWR-0334NA, SWR-0335SA, SWR-0342SA, SWR-0348SA-SITA and SWR-0361SA were potential agonists of human beta(3)-adrenoceptor. Further investigation on their selectivity towards beta(3)-adrenoceptor could be useful for the exploration of the physiological properties of the beta(3)-adrenoceptor.

Role of beta3-adrenergic receptors in the action of a tumour lipid mobilizing factor

Induction of lipolysis in murine white adipocytes, and stimulation of adenylate cyclase in adipocyte plasma membranes, by a tumour-produced lipid mobilizing factor, was attenuated by low concentrations (10(-7)--10(-5)M) of the specific beta3-adrenoceptor antagonist SR59230A. Lipid mobilizing factor (250 nM) produced comparable increases in intracellular cyclic AMP in CHOK1 cells transfected with the human beta3-adrenoceptor to that obtained with isoprenaline (1 nM). In both cases cyclic AMP production was attenuated by SR59230A confirming that the effect is mediated through a beta3-adrenoceptor. A non-linear regression analysis of binding of lipid mobilizing factor to the beta3-adrenoceptor showed a high affinity binding site with a Kd value 78 +/- 45 nM and a B(max) value (282 +/- 1 fmole mg protein(-1)) comparable with that of other beta3-adrenoceptor agonists. These results suggest that lipid mobilizing factor induces lipolysis through binding to a beta3-adrenoceptor.

Characterization of RII(beta) and D-AKAP1 in differentiated adipocytes

A-kinase anchoring proteins (AKAPs) have been proposed to regulate cAMP-dependent signaling in the cell by targeting RII subunits of protein kinase A (PKA) to specific subcellular compartments. RII(beta) is the predominant PKA subtype in adipose tissue. In gel overlay assays of C3H/10T1/2 adipocytes and adipose tissue, RII(beta) bound to several proteins including a prominent 132-kDa band, which was strongly induced upon differentiation of C3H/10T1/2 cells into adipocytes. Immunoblotting and nuclease protection analysis of C3H/10T1/2 cellular extracts identified this band as D-AKAP1/S-AKAP84, a putative AKAP. Immunocytochemical analysis of C3H/10T1/2 adipocytes revealed that most of D-AKAP1/S-AKAP84, but not RII(beta), was colocalized with a mitochondrial-selective dye, MitoTracker red. These findings were further confirmed in studies where D-AKAP1/ S-AKAP84, but not RII(beta), were localized in purified mitochondria made from C3H/10T1/2 adipocytes. Moreover, D-AKAP1, which is upregulated after differentiation, did not recruit RII(beta) to membrane fractions enriched in mitochondria. These results demonstrate that D-AKAP1/S-AKAP84 does not interact with PKA in differentiated C3H/10T1/2 adipocytes under the conditions tested.

Central leptin regulates the UCP1 and ob genes in brown and white adipose tissue via different beta-adrenoceptor subtypes

The three known subtypes of beta-adrenoreceptors (beta(1)-AR, beta(2)-AR, and beta(3)-AR) are differentially expressed in brown and white adipose tissue and mediate peripheral responses to central modulation of sympathetic outflow by leptin. To assess the relative roles of the beta-AR subtypes in mediating leptin's effects on adipocyte gene expression, mice with a targeted disruption of the beta(3)-adrenoreceptor gene (beta(3)-AR KO) were treated with vehicle or the beta(1)/beta(2)-AR selective antagonist, propranolol (20 microgram/g body weight/day) prior to intracerebroventricular (ICV) injections of leptin (0.1 microgram/g body weight/day). Leptin produced a 3-fold increase in UCP1 mRNA in brown adipose tissue of wild type (FVB/NJ) and beta(3)-AR KO mice. The response was unaltered by propranolol in wild type mice, but was completely blocked by this antagonist in beta(3)-AR KO mice. In contrast, ICV leptin had no effect on leptin mRNA in either epididymal or retroperitoneal white adipose tissue (WAT) from beta(3)-AR KOs. Moreover, propranolol did not block the ability of exogenous leptin to reduce leptin mRNA in either WAT depot site of wild type mice. These results demonstrate that the beta(3)-AR is required for leptin-mediated regulation of ob mRNA expression in WAT, but is interchangeable with the beta(1)/beta(2)-ARs in mediating leptin's effect on UCP1 mRNA expression in brown adipose tissue.

Regulation of leptin release and lipolysis by PGE2 in rat adipose tissue

The role of eicosanoids formed by adipose tissue from rats was examined in the presence of the specific cyclooxygenase-2 inhibitor NS-398. This agent totally blocked the release of prostaglandin E2 (PGE2) by rat adipose tissue over a 24-h incubation in primary culture. The final concentration of PGE2 after 24 h was 12 nM, and half-maximal inhibition of PGE2 formation required 35 nM NS-398. While inhibition of PGE2 formation by NS-398 had no effect on basal leptin release or lipolysis, it enhanced the lipolytic action of 10 nM isoproterenol by 36%. The in vivo administration of PGE2 doubled serum leptin. PGE2 also directly stimulated leptin release by rat adipose tissue incubated in the presence of 25 nM dexamethasone, which inhibited endogenous PGE2 formation by 94%. The inhibition of lipolysis as well as the stimulation of leptin release by PGE2 were mimicked by N6-cyclopentyladenosine (CPA). These data indicate that exogenous PGE2 can stimulate leptin release by adipose tissue when the basal formation of PGE2 is blocked by dexamethasone. However, while the endogenous formation of PGE2 does not appear to regulate basal lipolysis or leptin release, it may play a role in the activation of lipolysis by catecholamines.

Biochemical and functional characterization of 1-benzyl substituted trimetoquinol affinity analogs on rat and human beta-adrenoceptors

The site of interaction for the 1-(3',4',5'-trimethoxybenzyl) group of trimetoquinol (TMQ) with beta-adrenoceptors (beta-ARs) is important for the rational design of highly potent and beta3-AR-selective analogs. 1-Benzyl ring-substituted TMQ analogs were evaluated for binding affinities and biochemical activities (cyclic AMP accumulations) in Chinese hamster ovary (CHO) cells expressing the rat and human beta3-AR, and for functional activities on isolated rat tissues. Binding affinities (K1 approximately 0.055 to 1.5 microM) for the rat beta3-AR and potencies for adenylyl cyclase activation (K(act) approximately 0.43 to 2;5 nM) of the 3'-monoiodo or 3',5'-diiodo derivatives with 4'-isothiocyanato-, 4'-amino, 4'-acetamido, or 4'-alpha-haloacetamido substitutions were higher than those of (-)-isoproterenol, and comparable to those of BRL 37344 [(+/-)-(R*,R*-[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]ph enoxy]-acetic acid sodium]. A similar rank order of binding affinities (K(i) approximately 0.11 to 2.5 microM) and potencies (K(act) approximately 0.45 to 9.5 nM) was obtained for TMQ analogs on the human beta3-AR. The 4'-acetamido and 4'-alpha-chloroacetamido analogs of 3',5'-diiodoTMQ were more potent than (-)-isoproterenol in rat atria (beta1-AR) and rat trachea (beta2-AR) and exhibited partial agonist activities, whereas full agonist activities were observed in rat esophageal smooth muscle (EC50 approximately 2-8 nM, beta3-AR). 4'-alpha-Chloroacetamido-3',5'-diiodoTMQ-mediated chronotropic responses in atria were sustained and resistant to washout. Further, the 4'-alpha-chloroacetamido and 4'-alpha-bromoacetamido analogs of 3',5'-diiodoTMQ demonstrated significant concentration-dependent irreversible binding to the rat beta3-AR. Reversible beta-AR agonists such as (-)-isoproterenol, BRL 37344, and 4'-acetamido-3',5'-diiodoTMQ or nucleophilic 1-amino acids (lysine, glutathione, cysteine) did not protect against this irreversible binding. Thus, the lipophilic 1-benzyl ring of TMQ analogs interacts with a hydrophobic region of the beta-AR that may represent an exo-site or an allosteric binding site.

Insulin resistance and type 2 diabetes mellitus: its relationship with the beta 3-adrenergic receptor

The beta 3 subtype of adrenaline and noradrenaline receptors has been extensively characterized at structural and functional levels. Ligand binding and adenyl cyclase activation studies have helped to define their unique beta-adrenergic profile. Humans, other larger mammals, and rodents share most of the characteristic beta 3-adrenergic receptor properties, although obvious species-specific differences have been identified. Most studies in animal models have shown a distinct beta 3-adrenergic receptor activity that results in an increase in energy expenditure, decrease of fat mass (especially of intra-abdominal fat), and increased glucose disposal efficiency. It is of interest that mild weight increase was shown to develop in female but not male mice, in whom the beta 3-adrenergic receptor gene was disrupted. Recently, the incidence of a naturally occurring variant of the human beta 3-adrenergic receptor was shown to correlate with hereditary obesity in Pima Indians and Japanese individuals. In Western obese patients, this phenotype increased the capacity to gain weight and develop type 2 diabetes mellitus. Studies of humans with the Trp64Arg variant have shown controversial results. Many studies have failed to show any effect in heterozygous male subjects, and only modest effects in homozygous male subjects. In women, several studies have shown modest-to-significant effects regarding weight gain, intra-abdominal fat, and decreased insulin sensitivity in heterozygous and homozygous women. Other studies have failed to show any effect in heterozygous females. Disruptions in the activity of the beta 3-adrenergic receptor in the homozygous male and the heterozygous or homozygous female appear to have a profound effect in animal models, but a limited consequence in human physiology. Association with obesity or diabetes in humans is still controversial. This difference between animal and human models may be explained by the different quantity and distribution of metabolically active brown adipose tissue in the two.

Differential regulation of functional responses by beta-adrenergic receptor subtypes in brown adipocytes

Brown adipose tissue contains both beta(1)- and beta(3)-adrenergic receptors (beta-ARs), and whereas both receptor subtypes can activate adenylyl cyclase, recent studies suggest that these subtypes have different pharmacological properties and may serve different signaling functions. In this study, primary brown adipocyte cultures were used to determine the role of beta-AR subtypes in mediating lipolysis and uncoupling protein-1 (UCP1) gene expression, elicited by the physiological neurohormone norepinephrine (NE). NE increased both lipolysis and UCP1 mRNA levels in brown adipocyte cultures; the beta(1)-receptor-selective antagonist CGP-20712A strongly antagonized the increase in UCP1 gene expression but had little effect on lipolysis. The beta(3)-receptor-selective agonist CL-316243 (CL) also increased lipolysis and UCP1 mRNA levels, yet CL was more potent in stimulating lipolysis than UCP1 gene expression. NE also increased the phosphorylation of cAMP response element-binding protein (CREB) and perilipin (PL), both of which are protein kinase A substrates that are differentially targeted to the nucleus and lipid droplets, respectively. beta(1)-receptor blockade inhibited NE-stimulated phosphorylation of CREB but not PL. The results suggest that beta-AR subtypes regulate different physiological responses stimulated by NE in brown adipocyte cultures in part by differentially transducing signals to subcellular compartments.

Differential expression of a novel seven transmembrane domain protein in epididymal fat from aged and diabetic mice

To identify novel seven transmembrane domain proteins from 3T3-L1 adipocytes, we used PCR to amplify 3T3-L1 adipocyte complementary DNA (cDNA) with primers homologous to the N- and C-termini of pancreatic glucagon-like peptide-1 (GLP-1) receptor. We screened a cDNA library prepared from fully differentiated 3T3-L1 adipocytes using a 500-bp cDNA PCR product probe. Herein describes the isolation and characterization of a 1.6-kb cDNA clone that encodes a novel 298-amino acid protein that we termed TPRA40 (transmembrane domain protein of 40 kDa regulated in adipocytes). TPRA40 has seven putative transmembrane domains and shows little homology with the known GLP-1 receptor or with other G protein-coupled receptors. The levels of TPRA40 mRNA and protein were higher in 3T3-L1 adipocytes than in 3T3-L1 fibroblasts. TPRA40 is present in a number of mouse and human tissues. Interestingly, TPRA40 mRNA levels were significantly increased by 2- to 3-fold in epididymal fat of 24-month-old mice vs. young controls as well as in db/db and ob/ob mice vs. nondiabetic control littermates. No difference in TPRA40 mRNA levels was observed in brain, heart, skeletal muscle, liver, or kidney. Furthermore, no difference in TPRA40 expression was detected in brown fat of ob/ob mice when compared with age-matched controls. Taken together, these data suggest that TPRA40 represents a novel membrane-associated protein whose expression in white adipose tissue is altered with aging and type 2 diabetes.

Nutrition in the intensive care unit

Nutritional support has become a routine part of the care of the critically ill patient. It is an adjunctive therapy, the main goal of which is to attenuate the development of malnutrition, yet the effectiveness of nutritional support is often thwarted by an underlying hostile metabolic milieu. This requires that these metabolic changes be taken into consideration when designing nutritional regimens for such patients. There is also a need to conduct large, multi-center studies to acquire more knowledge of the cost-benefit and cost effectiveness of nutritional support in the critically ill.

Mechanisms regulating adipocyte lipolysis

Mechanisms regulating adipocyte lipolysis are reviewed in three stages. The first stage examines plasma membrane hormone receptors and G-proteins. The primary regulators of adipose tissue lipolysis, the catecholamines, bind to the alpha 2, beta 1, beta 2, and beta 3 adrenergic receptors. The alpha 2 receptor couples with Gi-proteins to inhibit cyclic AMP formation and lipolysis, while the beta receptors couple with Gs-proteins to stimulate cyclic AMP formation and lipolysis. The beta 1 receptor may mediate low level catecholamine stimulation, while the beta 3 receptor, which is activated by higher levels of catecholamines, may deliver a more sustained signal. The second stage examines the regulation of cyclic AMP, the intracellular messenger that activates protein kinase A. Adenylyl cyclase synthesizes cyclic AMP from ATP and is regulated by the G-proteins. Phosphodiesterase 3B hydrolyzes cyclic AMP to AMP and is activated and phosphorylated by both insulin and the catecholamines norepinephrine and epinephrine. The third stage focuses on the rate-limiting enzyme of lipolysis, hormone-sensitive lipase (HSL). This 82 to 88 kDa protein is regulated by reversible phosphorylation. Protein kinase A activates and phosphorylates the enzyme at 2 sites, and 3 phosphatases have been implicated in HSL dephosphorylation. The translocation of HSL from the cytosol to the lipid droplet in response to lipolytic stimulation may be facilitated by a family of lipid-associated droplets called perilipins that are heavily phosphorylated by protein kinase A and dephosphorylated by insulin. As the mechanisms regulating adipocyte lipolysis continue to be uncovered, we look forward to the challenges of integrating these findings with research at the in situ and in vivo levels.

Role of beta1- and beta3-adrenoceptors in the regulation of lipolysis and thermogenesis in rat brown adipocytes

To evaluate the physiological functions of beta1-, beta2-, and beta3-adrenoceptors (ARs) in brown adipose tissue, the lipolytic and respiratory effects of various adrenergic agonists and antagonists were studied in rat brown adipocytes. The beta-agonists stimulated both lipolysis and respiration (8-10 times above basal levels), with the following order of potency (concentration eliciting 50% of maximum response): CL-316243 (beta3) > BRL-37344 (beta3) > isoproterenol (mainly beta1/beta2) > norepinephrine (NE; mainly beta1/beta2) > epinephrine (mainly beta1/beta2) >> dobutamine (beta1) >> procaterol (beta2). Schild plot coefficients of competitive inhibition experiments using ICI-89406 (beta1 antagonist) revealed that more than one type of receptor mediates NE action. It is concluded from our results that 1) NE, at low plasma levels (1-25 nM), stimulates lipolysis and respiration mainly through beta1-ARs, 2) NE, at higher levels, stimulates lipolysis and respiration via both beta1- and beta3-ARs, 3) beta2-ARs play only a minor role, and 4) beta3-ARs may represent the physiological receptors for the high NE concentrations in the synaptic cleft, where the high-affinity beta1-ARs are presumably desensitized. It is also suggested that lipolysis represents the flux-generating step regulating mitochondrial respiration.

Beta3-adrenergic receptors on white and brown adipocytes mediate beta3-selective agonist-induced effects on energy expenditure, insulin secretion, and food intake. A study using transgenic and gene knockout mice

beta3-Adrenergic receptors (beta3-ARs) are expressed predominantly on white and brown adipocytes, and acute treatment of mice with CL 316,243, a potent and highly selective beta3-AR agonist, produces a 2-fold increase in energy expenditure, a 50-100-fold increase in insulin levels, and a 40-50% reduction in food intake. Recently, we generated gene knockout mice lacking functional beta3-ARs and demonstrated that each of these responses were mediated exclusively by beta3-ARs. However, the tissue site responsible for producing these actions is unknown. In the present study, genetically engineered mice were created in which beta3-ARs are expressed exclusively in white and brown adipocytes (WAT+BAT-mice), or in brown adipocytes only (BAT-mice). This was accomplished by injecting tissue-specific beta3-AR transgenic constructs into mouse zygotes homozygous for the beta3-AR knockout allele. Control, knockout, WAT+BAT, and BAT-mice were then treated acutely with CL, and the effects on various parameters were assessed. As previously observed, all effects of CL were completely absent in gene knockout mice lacking beta3-ARs. The effects on O2 consumption, insulin secretion, and food intake were completely rescued with transgenic re-expression of beta3-ARs in white and brown adipocytes (WAT+BAT-mice), demonstrating that each of these responses is mediated exclusively by beta3-ARs in white and/or brown adipocytes, and that beta3-ARs in other tissue sites were not required. Importantly, transgenic re-expression of beta3-ARs in brown adipocytes only (BAT-mice) failed to rescue, in any way, CL-mediated effects on insulin levels and food intake and only minimally restored effects on oxygen consumption, indicating that any effect on insulin secretion and food intake, and a full stimulation of oxygen consumption required the presence of beta3-ARs in white adipocytes. The mechanisms by which beta3-AR agonist stimulation of white adipocytes produces these responses are unknown but may involve novel mediators not previously known to effect these processes.

Selective up-regulation of alpha1a-adrenergic receptor protein and mRNA in brown adipose tissue by neural and beta3-adrenergic stimulation

Previous studies have shown that neural stimulation of brown adipose tissue (BAT) reorganizes the expression and activity of signaling proteins in the beta-adrenergic adenylyl cyclase pathway. Cold stress increases neural stimulation of BAT and increases alpha1-adrenergic receptor number; however, the alpha1 receptor subtype involved and the mechanism of up-regulation by cold stress have not been determined. Using reverse transcription/polymerase chain reaction analysis and nuclease protection assay, BAT was demonstrated to express mRNAs encoding alpha1a and alpha1d, but not alpha1b, receptors. Parallel pharmacologic studies of BAT membranes and recombinant alpha1a and alpha1d receptors expressed in COS-7 cells demonstrated that alpha1a receptors predominate in BAT. Exposure of rats to 4 degrees for 4 days increased alpha1a receptors and mRNA in BAT but did not alter expression of alpha1d receptors or mRNA. The induction of alpha1a receptor and mRNA level by cold stress was prevented by selective surgical denervation of BAT. Furthermore, alpha1a receptor and mRNA expression could be induced in warm-adapted rats by infusions of the selective beta3-adrenergic receptor agonist CL 316,243. These data indicate that neural activation of beta3-adrenergic receptors is an important determinant of alpha1a adrenergic receptor expression in BAT.

Down-regulation of beta3 adrenoreceptor gene expression in brown fat cells is transient and recovery is dependent upon a short-lived protein factor

The regulation of the expression of the beta3 adrenoreceptor gene was examined in the brown adipose tissue of intact mice and in murine brown fat primary cell cultures. Both in vivo and in vitro, high levels of beta3 receptor mRNA were observed. Acute cold exposure of mice resulted in a marked and rapid down-regulation of beta3 gene expression; this down-regulation was, however, transient. Similarly, in brown fat cell cultures, norepinephrine addition led to down-regulation of beta3 gene expression, with a lag phase of 30 min and with an apparent half-life of beta3 mRNA of approximately 30 min. This down-regulation was stimulated via the beta3 receptors themselves and mediated via cAMP; the apparent affinity of norepinephrine was extremely high (<1 nM). The degradation rate after actinomycin was identical to that after norepinephrine and was not affected by the presence of norepinephrine; thus, the down-regulation was due to cessation of transcription but not to an increased rate of degradation. Notably, inhibition of protein synthesis by cycloheximide also led to down-regulation. The norepinephrine-induced down-regulation was transient; spontaneous recovery occurred after approximately 18 h and was not due to depletion of adrenergic agent. Recovery did not occur in the presence of cycloheximide. After recovery, the cells showed a functional desensitization of the down-regulation process itself (EC50 now approximately 10 nM). It is concluded that a down-regulated state cannot explain the functional desensitization of beta3 adrenergic responsiveness observed in brown fat cells isolated from cold-acclimated animals (i.e. physiologically chronically adrenergically stimulated brown fat cells); since the beta3 receptor is not subject to desensitization via phosphorylation processes, no satisfactory explanation for the functional desensitization exists as yet. A model is presented for the down-regulation/recovery process, involving the participation of a phosphorylatable short-lived transcription factor.

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.

Respective degree of expression of beta 1-, beta 2- and beta 3-adrenoceptors in human brown and white adipose tissues

1. The possible existence of a beta 3-adrenoceptor in human brown and white adipose tissues was investigated by mRNA expression and binding studies. 2. The relative amounts of beta 1-, beta 2- and beta 3-adrenoceptor mRNA, as determined by total RNA Northern blot analysis in newborn brown adipose tissue, were 28, 63 and 9% respectively of the total beta-adrenoceptor mRNA. 3. The beta 1/beta 2-adrenoceptors of human brown adipose tissue plasma membranes were characterized using [3H]-CGP 12177 as a ligand. Their Kd and Bmax values were 1.9 nM and 156 fmol mg-1 of membrane proteins, respectively. The beta 3-adrenoceptor was characterized by use of the new specific radioligand [3H]-SB 206606. The binding of this ligand was stereospecifically displaced by the active R,R- or the inactive S,S-enantiomer of BRL 37344 up to a concentration of about 10 microM. The Kd and Bmax values of the brown adipose tissue membrane beta 3-adrenoceptors were 87 nM and 167 fmol mg-1 of proteins, respectively. A low affinity [3H]-CGP 12177 binding site population was also detected in these membranes. 4. In human omental white adipose tissue, no beta 3-adrenoceptor mRNA could be detected in total RNA Northern blots and the beta 1-and beta 2-adrenoceptor mRNAs represented 9 and 91%, respectively of the total beta-adrenoceptor mRNA, and no specific binding of [3H]-SB 206606 could be measured.

Biphasic effects of the beta-adrenoceptor agonist, BRL 37344, on glucose utilization in rat isolated skeletal muscle

1. The effects of the selective beta 3-adrenoceptor agonist, BRL 37344 (BRL) on glucose uptake and phosphorylation (i.e. glucose utilization; GU) and glycogen synthesis in rat isolated soleus and extensor digitorium longus (EDL) muscle preparations in vitro were investigated by use of 2-deoxy-[3H]-glucose (GU) and [U-14C]-glucose (glycogen synthesis). 2. Low concentrations of BRL (10(-11)-10(-9) M) significantly increased GU, with maximal increases of 30% in soleus and 24% in EDL at 10(-11) M. Neither the selective beta 1-adrenoceptor antagonist, atenolol (10(-8)-10(-6) M), nor the selective beta 2-adrenoceptor antagonist, ICI 118551 (10(-8)-10(-6) M) had any effect on the stimulation of GU induced by 10(-11) M BRL. 3. High concentrations of BRL (10(-6)-10(-5) M) caused significant inhibition (up to 30%) of GU in both soleus and EDL muscles. The inhibition of 10(-6) M BRL was blocked completely by 10(-6) and 10(-7) M ICI 118551 in soleus, and by 10(-6)-10(-8) M ICI 118551 in EDL; atenolol (10(-8)-10(6) M) had no effect. 4. Another selective beta 3-adrenoceptor agonist, CL 316,243, also caused a significant stimulation of muscle GU, with maximal increases of 43% at 10(-9) M in soleus and 45% at 10(-10) M in EDL. The stimulation of GU declined with further increases in the concentration of CL 316,243, but no inhibition of GU was seen, even at the highest concentration (10(-5) M) tested. 5. BRL at 10(-5) M inhibited completely insulin-stimulated glycogen synthesis in both soleus and EDL, but this inhibitory effect of BRL was abolished by 10(-6) M ICI 118551. BRL at 10(-11) M (with or without 10(-6) M ICI 118551) had no effect on insulin-stimulated glycogen synthesis. 6. It is concluded that: (i) low (< nM) concentrations of BRL stimulate GU via an atypical beta-adrenoceptor that is resistant to conventional beta 1-adrenoceptor and beta 2-adrenoceptor antagonists; (ii) the stimulation of GU is negated by the activation of beta 2-adrenoceptors that occurs at higher (> nM) concentrations of BRL; (iii) inhibition of GU via beta 2-adrenoceptor activation is associated with inhibition of glycogen synthesis, possibly due to activation of glycogenolysis; (iv) the opposing effects of beta 2-adrenoceptor and atypical beta-adrenoceptor activation on GU suggest that in skeletal muscle these adrenoceptors are linked to different post-receptor pathways.

Of mice and women: the beta 3-adrenergic receptor leptin and obesity

The metabolic response of adipose tissue to stimuli leading to lipid mobilization is important in determining the direction of metabolism and the degree to which adipose tissue can store lipids and release fatty acids in times of need. The lipolytic machinery is controlled by the activity of hormone-sensitive lipase, which in turn is controlled by the cellular levels of cAMP. The production of cAMP is abnormal in the adipose tissue of some animal models of obesity. In the ob/ob mouse, the defective cAMP production has been associated with deficient levels of some of the isoforms of the guanine nucleotide transducing G-proteins and also with the low expression and functionality of the beta 3-adrenergic receptor (beta 3-AR). The recent discovery of the ob gene product leptin calls into question the role of the ob gene in the regulation of the cAMP cascade in adipose tissue. The importance of the beta 3-AR and leptin in regulating human adipose tissue metabolism remains to be clarified.