Transgenic Mice Overexpressing the 1-Adrenergic Receptor in Adipose Tissue Are Resistant to Obesity

Adrenoceptors in white, brown, and brite adipocytes

Adrenoceptors play an important role in adipose tissue biology and physiology that includes regulating the synthesis and storage of triglycerides (lipogenesis), the breakdown of stored triglycerides (lipolysis), thermogenesis (heat production), glucose metabolism, and the secretion of adipocyte-derived hormones that can control whole-body energy homeostasis. These processes are regulated by the sympathetic nervous system through actions at different adrenoceptor subtypes expressed in adipose tissue depots. In this review, we have highlighted the role of adrenoceptor subtypes in white, brown, and brite adipocytes in both rodents and humans and have included detailed analysis of adrenoceptor expression in human adipose tissue and clonally derived adipocytes. We discuss important considerations when investigating adrenoceptor function in adipose tissue or adipocytes. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Disruption of beta3 adrenergic receptor increases susceptibility to DIO in mouse

The brown adipose tissue (BAT) mediates adaptive changes in metabolic rate by responding to the sympathetic nervous system through β-adrenergic receptors (AR). Here, we wished to define the role played by the ARβ₃ isoform in this process. This study focused on the ARβ₃ knockout mice (ARβ₃KO), including responsiveness to cold exposure, diet-induced obesity, intolerance to glucose, dyslipidaemia and lipolysis in white adipose tissue (WAT). ARβ₃KO mice defend core temperature during cold exposure (4°C for 5 h), with faster BAT thermal response to norepinephrine (NE) infusion when compared with wild-type (WT) mice. Despite normal BAT thermogenesis, ARβ₃KO mice kept on a high-fat diet (HFD; 40% fat) for 8 weeks exhibited greater susceptibility to diet-induced obesity, markedly increased epididymal adipocyte area with clear signs of inflammation. The HFD-induced glucose intolerance was similar in both groups but serum hypertriglyceridemia and hypercholesterolemia were less intense in ARβ₃KO animals when compared with WT controls. Isoproterenol-induced lipolysis in isolated white adipocytes as assessed by glycerol release was significantly impaired in ARβ₃KO animals despite normal expression of key proteins involved in lipid metabolism. In conclusion, ARβ₃ inactivation does not affect BAT thermogenesis but increases susceptibility to diet-induced obesity by dampening WAT lipolytic response to adrenergic stimulation.

The effects of early under-nutrition on the development of wBAT and obesity

A population of brown adipocytes emerges in white fat depots at weaning. The function of these adipocytes is not known, but at this late stage of development it is unlikely they are essential for body temperature regulation experienced during the cold stress at birth. A dietary protocol of under-nutrition during the perinatal period, causing hypoinsulinemia, hypoleptinemia and hypoglycemia, led to an 85% reduction in expression of brown fat biomarkers and genes encoding the components of the respiratory complex, the TCA cycle and fatty acid oxidation. Suppression of wBAT in 21-day-old mice showed no significant effect on diet-induced obesity or cold tolerance in adult mice. Analysis of gene expression indicated that capacity to induce the brown fat phenotype was normal. This suggests that the brown adipocytes in white fat of 21-day-old mice are highly plastic and able to recover from severe malnutrition or that a new population of brown adipocytes is induced de novo in adult mice.

β(1) Adrenergic receptor is key to cold- and diet-induced thermogenesis in mice

Brown adipose tissue (BAT) is predominantly regulated by the sympathetic nervous system (SNS) and the adrenergic receptor signaling pathway. Knowing that a mouse with triple β-receptor knockout (KO) is cold intolerant and obese, we evaluated the independent role played by the β(1) isoform in energy homeostasis. First, the 30  min i.v. infusion of norepinephrine (NE) or the β(1) selective agonist dobutamine (DB) resulted in similar interscapular BAT (iBAT) thermal response in WT mice. Secondly, mice with targeted disruption of the β(1) gene (KO of β(1) adrenergic receptor (β(1)KO)) developed hypothermia during cold exposure and exhibited decreased iBAT thermal response to NE or DB infusion. Thirdly, when placed on a high-fat diet (HFD; 40% fat) for 5 weeks, β(1)KO mice were more susceptible to obesity than WT controls and failed to develop diet-induced thermogenesis as assessed by BAT Ucp1 mRNA levels and oxygen consumption. Furthermore, β(1)KO mice exhibited fasting hyperglycemia and more intense glucose intolerance, hypercholesterolemia, and hypertriglyceridemia when placed on the HFD, developing marked non-alcoholic steatohepatitis. In conclusion, the β(1) signaling pathway mediates most of the SNS stimulation of adaptive thermogenesis.

β(1)-adrenergic receptor polymorphisms and response to β-blockade in the African-American study of kidney disease and hypertension (AASK)

BACKGROUND

This study focuses on the relationship between β(1)-adrenergic receptor (ADRB1) polymorphisms and blood pressure response to the β-blocker metoprolol among African Americans with early hypertensive nephrosclerosis.

METHODS

Participants from the African-American Study of Kidney Disease and Hypertension (AASK) trial were genotyped for ADRB1 polymorphisms: Ser49Gly and Arg389Gly. Cox proportional hazards models were used to determine the relationship between ADRB1 polymorphisms and time to reach a mean arterial pressure (MAP) of ≤107 mm Hg in the first year after randomization, adjusted for other predictors of blood pressure response.

RESULTS

In the Ser49Gly model, Ser49/Gly49 individuals were less responsive compared to Ser49/Ser49 only among the more obese (body mass index (BMI) ≥39 kg/m(2)) participants (P < 0.05 for genotype × BMI interaction). The hazard ratio (HR) with a BMI of 39 kg/m(2) was 0.68 (95% confidence interval (CI) 0.46-0.99). In the Arg389Gly model, participants with Arg389 were less likely to respond to metoprolol: HR: 0.68 (95% CI 0.50-0.93). In addition, women were less responsive to metoprolol compared to men: HR: 0.78 (95% CI 0.60-0.995).

CONCLUSIONS

Ser49/Gly49 was predictive of blood pressure response to metoprolol only among more obese African Americans with early hypertensive nephrosclerosis. In contrast to other studies suggesting increased short-term responsiveness to β-blockers with Arg389, Arg389 individuals were less responsive in this study analyzing blood pressure over a 1-year period. This may be partly explained by decreased agonist-promoted desensitization with Arg389. However, gender, physiological adaption to stress, interactions between genes and between genes and the environment, as well as study in other patient populations need to be considered.

Induction of subcutaneous adipose proliferation by olanzapine in rodents

Weight gain induced by atypical antipsychotics causes a serious health concern in the treatment of schizophrenic patients. In the present study chronic treatment of female Wistar rats with olanzapine caused weight gain, but limited effect on food intake. A dramatic drug-induced morphological change of the subcutaneous adipose tissue was observed, i.e. development of a pinkish coloration with the appearance of a "fish egg"-like texture. Histological examination revealed a massive increase in the proliferation of undifferentiated adipocytes. Such proliferation was detected as early as the third day after olanzapine treatment. The changes progressed in a time- and dose-dependent manner. The proliferation of adipose tissue was detected in rats treated with olanzapine independent of increases in weight gain. Protein profiles of the adipose tissue were also altered by olanzapine. These results suggest that olanzapine-induced weight gain may be not solely due to an effect on behavioural satiety. The potential involvement of adipose neuronal input and proliferation are discussed.

The genetics of brown adipose tissue

Brown adipose tissue is highly differentiated and has evolved as a mechanism for heat production based upon uncoupling of mitochondrial oxidative phosphorylation. Additionally, large amounts of lipid can be stored in the cells to provide fuel necessary for heat production upon adrenergic stimulation from the central nervous system, and a highly developed vascular system evolved to rapidly deliver heat to vital organs. For unknown reasons, the development of brown adipocytes has two independent pathways: one originates from muscle progenitor cells in the fetus and leads to a fully functional cell at birth (interscapular-type brown fat), while the other transiently emerges in traditional white fat depots at weaning, regresses, and then can be induced in adult mice upon adrenergic stimulation. No genetic variants have been found for interscapular fat, but naturally occurring alleles at eight genetic loci in mice lead to over 100-fold variation for brown adipocytes in white fat upon adrenergic stimulation. The ability to activate this potential for energy expenditure is of great interest in obesity research.

Mutations in the beta1 adrenergic receptor gene and massive obesity in Japanese

Catecholamines strongly promote lipolysis and thermogenesis, and play a central role in the regulation of body fat content. The beta1 adrenergic receptor (BAR-1) is a major mediator of catecholamine-induced lipolysis and thermogenesis. To explore whether mutations in the BAR-1 gene contribute to morbid obesity in Japanese, we scanned for mutations in the coding sequence of the gene in 50 morbid obese [body mass index (BMI)>==35.0kg/m(2); 99.7th percentile] Japanese subjects. Direct DNA sequencing was performed following polymerase chain reaction (PCR) amplification. Two common polymorphisms, Gly49Arg and Arg389Ser, were detected in these subjects. The frequencies of these polymorphisms, as determined by PCR-restriction fragment length polymorphism (RFLP) analysis, showed no significant difference between 180 severely obese subjects (BMI>==30.0kg/m(2); 97th percentile) and 132 control (BMI<25.0kg/m(2)) subjects. This study represents the first investigations of genetic variations of BAR-1 in relationship to morbid obesity and suggests mutations in the BAR-1 coding sequence is not likely a major cause of morbid obesity at least in Japanese.

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.

Transgenic animal models for the study of adipose tissue biology

The traditional view of adipose tissue as a passive energy reservoir has changed. Adipose tissue is a complex, highly active metabolic and endocrine organ. With obesity as an increasingly important public health threat, a major development in the understanding of adipose tissue biology has come with observations in different biological spheres including whole-body physiology and application of transgenic animal models. Scientific progress has been made with the identification of several genes in spontaneous monogenic animal models of obesity, and in understanding the molecular mechanisms underlying phenotypes of altered body weight, adiposity and fat distribution by creating transgenic and knockout animal models. Mouse phenotypes resulting from inactivation or overexpression of molecules responsible for the regulation of adipose tissue metabolism have led to novel concepts in the understanding of adipocyte biology and development of obesity. This review presents an overview of transgenic animal models for the study of adipose tissue biology.

Lipoatrophy revisited

The lipoatrophy syndromes are a heterogeneous group of syndromes characterized by a paucity of adipose tissue. Severe lipoatrophy is associated with insulin-resistant diabetes mellitus (DM). The loss of adipose tissue can have a genetic, immune, or infectious/drug-associated etiology. Causative mutations have been identified in patients for one form of partial lipoatrophy--Dunnigan-type familial partial lipodystrophy. Experiments using lipoatrophic mice demonstrate that the diabetes results from the lack of fat and that leptin deficiency is a contributing factor. Thiazolidinedione therapy improves metabolic control in lipoatrophic patients; the efficacy of leptin treatment is currently being investigated.

Transgenic study of energy homeostasis equation: implications and confounding influences

Recently novel molecular mediators and regulatory pathways for feeding and body weight regulation have been identified in the brain and the periphery. Mice lacking or overexpressing these mediators or receptors have been produced by molecular genetic techniques, and observations on mutant mice have shed new light on the role of each element in the homeostatic loop of body weight regulation. However, the interpretation of the phenotype is under the potential influence of developmental compensation and other genetic and environmental confounds. Specific alterations of the mediators and the consequences of the altered expression patterns are reviewed here and discussed in the context of their functions as suggested from conventional pharmacological studies. Advanced gene targeting strategies in which genes can be turned on or off at desired tissues and times would undoubtedly lead to a better understanding of the highly integrated and redundant systems for energy homeostasis equation.

Decreased fatty acid synthesis due to mitochondrial uncoupling in adipose tissue

Synthesis of fatty acid (FA) in adipose tissue requires cooperation of mitochondrial and cytoplasmic enzymes. Mitochondria are required for the production of ATP and they also support the formation of acetyl-CoA and NADPH in cytoplasm. Since cellular levels of all these metabolites depend on the efficiency of mitochondrial energy conversion, mitochondrial proton leak via uncoupling proteins (UCPs) could modulate FA synthesis. In 3T3-L1 adipocytes, 2,4-dinitrophenol depressed the synthesis of FA 4-fold while increasing FA oxidation 1. 5-fold and the production of lactate 14-fold. Inhibition of FA synthesis in 3T3-L1 adipocytes was proportional to the decrease in mitochondrial membrane potential. FA synthesis from D-[U-(14)C] glucose was reduced up to fourfold by ectopic UCP1 in the white fat of transgenic aP2-Ucp1 mice, reflecting the magnitude of UCP1 expression in different fat depots and the reduction of adiposity. Transcript levels for lipogenic enzymes were lower in the white fat of the transgenic mice than in the control animals. Our results show that uncoupling of oxidative phosphorylation depresses FA synthesis in white fat. Reduction of adiposity via mitochondrial uncoupling in white fat not only reflects increased energy expenditure, but also decreased in situ lipogenesis.

Transgenic mice lacking white fat: models for understanding human lipoatrophic diabetes

The human disease lipoatrophic (or lipodystrophic) diabetes is a rare syndrome in which a deficiency of adipose tissue is associated with Type 2 diabetes. This disease is an interesting contrast to the usual situation in which diabetes is associated with obesity, an excess of fat. Aside from obesity, patients with lipodystrophic diabetes have the other features associated with Metabolic Syndrome X, including hypertension and dyslipidemia. The contrast between diabetes with a lack of fat and diabetes with an excess of fat provides an opportunity to study the mechanisms causing Type 2 diabetes and its complications. Recently, three laboratories have produced transgenic mice that are deficient in white adipose tissue. These mice have insulin resistance and other features of lipoatrophic diabetes, and are a faithful model for the human disease. Here we review the different murine models of fat ablation and compare the murine and human diseases, addressing the questions: Is the lack of fat causative of the diabetes, and if so by what mechanism? How could the other clinical features be explained mechanistically? And finally, what can be gleaned about insight into treatment options?

Life without white fat: a transgenic mouse

We have generated a transgenic mouse with no white fat tissue throughout life. These mice express a dominant-negative protein, termed A-ZIP/F, under the control of the adipose-specific aP2 enhancer/promoter. This protein prevents the DNA binding of B-ZIP transcription factors of both the C/EBP and Jun families. The transgenic mice (named A-ZIP/F-1) have no white adipose tissue and dramatically reduced amounts of brown adipose tissue, which is inactive. They are initially growth delayed, but by week 12, surpass their littermates in weight. The mice eat, drink, and urinate copiously, have decreased fecundity, premature death, and frequently die after anesthesia. The physiological consequences of having no white fat tissue are profound. The liver is engorged with lipid, and the internal organs are enlarged. The mice are diabetic, with reduced leptin (20-fold) and elevated serum glucose (3-fold), insulin (50- to 400-fold), free fatty acids (2-fold), and triglycerides (3- to 5-fold). The A-ZIP/F-1 phenotype suggests a mouse model for the human disease lipoatrophic diabetes (Seip-Berardinelli syndrome), indicating that the lack of fat can cause diabetes. The myriad of consequences of having no fat throughout development can be addressed with this model.