Nicotine induces uncoupling protein 1 in white adipose tissue of obese mice

The controversial effect of smoking and nicotine in SARS-CoV-2 infection

The effects of nicotine and cigarette smoke in many diseases, notably COVID-19 infection, are being debated more frequently. The current basic data for COVID-19 is increasing and indicating the higher risk of COVID-19 infections in smokers due to the overexpression of corresponding host receptors to viral entry. However, current multi-national epidemiological reports indicate a lower incidence of COVID-19 disease in smokers. Current data indicates that smokers are more susceptible to some diseases and more protective of some other. Interestingly, nicotine is also reported to play a dual role, being both inflammatory and anti-inflammatory. In the present study, we tried to investigate the effect of pure nicotine on various cells involved in COVID-19 infection. We followed an organ-based systematic approach to decipher the effect of nicotine in damaged organs corresponding to COVID-19 pathogenesis (12 related diseases). Considering that the effects of nicotine and cigarette smoke are different from each other, it is necessary to be careful in generalizing the effects of nicotine and cigarette to each other in the conducted researches. The generalization and the undifferentiation of nicotine from smoke is a significant bias. Moreover, different doses of nicotine stimulate different effects (dose-dependent response). In addition to further assessing the role of nicotine in COVID-19 infection and any other cases, a clever assessment of underlying diseases should also be considered to achieve a guideline for health providers and a personalized approach to treatment.

Anatabine, Nornicotine, and Anabasine Reduce Weight Gain and Body Fat through Decreases in Food Intake and Increases in Physical Activity

Obesity is a leading cause of preventable death in the United States. Currently approved pharmacotherapies for the treatment of obesity are associated with rebound weight gain, negative side effects, and the potential for abuse. There is a need for new treatments with fewer side effects. Minor tobacco alkaloids (MTAs) are potential candidates for novel obesity pharmacotherapies. These alkaloids are structurally related to nicotine, which can help reduce body weight, but without the same addictive potential. The purpose of the current study was to examine the effects of three MTAs (nornicotine, anatabine, and anabasine) and nicotine on weight gain, body composition, chow intake, and physical activity. We hypothesized that the MTAs and nicotine would reduce weight gain through reductions in chow intake and increases in physical activity. To test this, male Sprague Dawley rats were housed in metabolic phenotyping chambers. Following acclimation to these chambers and to (subcutaneous (sc)) injections of saline, animals received daily injections (sc) of nornicotine, anabasine, anatabine, or nicotine for one week. Compared to saline-injected animals that gained body weight and body fat during the treatment phase, injections of nornicotine and anatabine prevented additional weight gain, alongside reductions in body fat. Rats receiving anabasine and nicotine gained body weight at a slower rate relative to rats receiving saline injections, and body fat remained unchanged. All compounds reduced the intake of chow pellets. Nornicotine and nicotine produced consistent increases in physical activity 6 h post-injection, whereas anabasine’s and anatabine’s effects on physical activity were more transient. These results show that short-term, daily administration of nornicotine, anabasine, and anatabine has positive effects on weight loss, through reductions in body fat and food intake and increases in physical activity. Together, these findings suggest that MTAs are worthy of further investigations as anti-obesity pharmacotherapies.

Recent developments in natural products for white adipose tissue browning

Excess accumulation of white adipose tissue (WAT) causes obesity which is an imbalance between energy intake and energy expenditure. Obesity is a serious concern because it has been the leading causes of death worldwide, including diabetes, stroke, heart disease and cancer. Therefore, uncovering the mechanism of obesity and discovering anti-obesity drugs are crucial to prevent obesity and its complications. Browning, inducing white adipose tissue to brown or beige (brite) fat which is brown-like fat emerging in WAT, becomes an appealing therapeutic strategy for obesity and metabolic disorders. Due to lack of efficacy or intolerable side-effects, the clinical trials that promote brown adipose tissue (BAT) thermogenesis and browning of WAT have not been successful in humans. Obviously, more specific means still need to be developed to activate browning of white adipose tissue. In this review, we summarized seven kinds of natural products (alkaloids, flavonoids, terpenoids, long chain fatty acids, phenolic acids, else and extract) promoting white adipose tissue browning which can ameliorate the metabolic disorders, including obesity, dislipidemia, insulin resistance and diabetes. Since natural products are important drug sources and the browning property plays a significant role in not only obesity treatment but also in type 2 diabetes (T2DM) improvement, natural products of inducing browning may be an irreplaceable drug discovery orientation for obesity, diabetes and even other metabolic disorders.

Nicotine' actions on energy balance: Friend or foe?

Obesity has reached pandemic proportions and is associated with severe comorbidities, such as type 2 diabetes mellitus, hepatic and cardiovascular diseases, and certain cancer types. However, the therapeutic options to treat obesity are limited. Extensive epidemiological studies have shown a strong relationship between smoking and body weight, with non-smokers weighing more than smokers at any age. Increased body weight after smoking cessation is a major factor that interferes with their attempts to quit smoking. Numerous controlled studies in both humans and rodents have reported that nicotine, the main bioactive component of tobacco, exerts a marked anorectic action. Furthermore, nicotine is also known to modulate energy expenditure, by regulating the thermogenic activity of brown adipose tissue (BAT) and the browning of white adipose tissue (WAT), as well as glucose homeostasis. Many of these actions occur at central level, by controlling the activity of hypothalamic neuropeptide systems such as proopiomelanocortin (POMC), or energy sensors such as AMP-activated protein kinase (AMPK). However, direct impact of nicotine on metabolic tissues, such as BAT, WAT, liver and pancreas has also been described. Here, we review the actions of nicotine on energy balance. The relevance of this interaction is interesting, because considering the restricted efficiency of obesity treatments, a possible complementary approach may focus on compounds with known pharmacokinetic profile and pharmacological actions, such as nicotine or nicotinic acetylcholine receptors signaling.

There and Back Again: Leptin Actions in White Adipose Tissue

Leptin is a hormone discovered almost 30 years ago with important implications in metabolism. It is primarily produced by white adipose tissue (WAT) in proportion to the amount of fat. The discovery of leptin was a turning point for two principle reasons: on one hand, it generated promising expectations for the treatment of the obesity, and on the other, it changed the classical concept that white adipose tissue was simply an inert storage organ. Thus, adipocytes in WAT produce the majority of leptin and, although its primary role is the regulation of fat stores by controlling lipolysis and lipogenesis, this hormone also has implications in other physiological processes within WAT, such as apoptosis, browning and inflammation. Although a massive number of questions related to leptin actions have been answered, the necessity for further clarification facilitates constantly renewing interest in this hormone and its pathways. In this review, leptin actions in white adipose tissue will be summarized in the context of obesity.

Central nicotine induces browning through hypothalamic κ opioid receptor

Increased body weight is a major factor that interferes with smoking cessation. Nicotine, the main bioactive compound in tobacco, has been demonstrated to have an impact on energy balance, since it affects both feeding and energy expenditure at the central level. Among the central actions of nicotine on body weight, much attention has been focused on its effect on brown adipose tissue (BAT) thermogenesis, though its effect on browning of white adipose tissue (WAT) is unclear. Here, we show that nicotine induces the browning of WAT through a central mechanism and that this effect is dependent on the κ opioid receptor (KOR), specifically in the lateral hypothalamic area (LHA). Consistent with these findings, smokers show higher levels of uncoupling protein 1 (UCP1) expression in WAT, which correlates with smoking status. These data demonstrate that central nicotine-induced modulation of WAT browning may be a target against human obesity.

Nicotine reduces food intake and increases energy expenditure in brown adipose tissue. Here the authors show that nicotine also induces white adipose tissue browning via central kappa opioid receptor action.

Small molecules for fat combustion: targeting thermosensory and satiety signals in the central nervous system

Brown adipose tissue (BAT) dissipates fatty acids as heat to maintain body temperature in cold environments. The existence of BAT and beige cells in human adults supplies a promising weight-reduction therapy. The central thermogenic regulation descends through an excitatory neural pathway from the hypothalamus, medullar and spine towards BAT. This sympathoexcitatory thermogenic circuit is controlled by GABAergic (γ-aminobutyric acid) signaling from the thermoregulatory center in the preoptic area and the satiety center in the ventromedial nucleus of the hypothalamus. This review summarizes recent research progresses in thermogenic regulators targeting thermosensory and satiety signals in the central nervous system, and speculates on their potential as antiobesity agents.

An immune-beige adipocyte communication via nicotinic acetylcholine receptor signaling

Beige adipocytes have been recently shown to regulate energy dissipation when activated, and help organisms defend against hypothermia and obesity. Prior reports indicate beige-like adipocytes exist in adult humans and may present novel opportunities to curb the global epidemic in obesity and metabolic illnesses. In an effort to identify unique features of activated beige adipocytes, we uncovered that the cholinergic receptor nicotinic alpha 2 subunit (Chrna2) is induced in subcutaneous fat during the activation of these cells, and that acetylcholine-producing immune cells within this tissue regulate this signaling pathway via paracrine mechanisms. CHRNA2 functions selectively in uncoupling protein 1 (Ucp1)⁺ beige adipocytes, increasing thermogenesis through a cAMP and PKA pathway. Furthermore, this signaling via CHRNA2 is conserved and present in human subcutaneous adipocytes. Inactivation of Chrna2 in mice compromises the cold-induced thermogenic response selectively in subcutaneous fat and exacerbates high-fat diet-induced obesity and associated metabolic disorders, indicating that even partial loss of beige fat regulation in vivo leads to detrimental consequences. Our results reveal a beige-selective immune-adipose interaction mediated through CHRNA2 and identify a novel function of nicotinic acetylcholine receptors (nAChRs) in energy metabolism. These findings may lead to identification of therapeutic targets to counteract human obesity.

Lifestyle Practices and Obesity in Nepalese Youth: A Cross-sectional Study

Introduction

Understanding the lifestyle factors associated with obesity is critical to create a successful intervention that would prevent or reduce the obesity beforehand. However, these factors have not been assessed among Nepalese youths thus far. This study aims to determine the prevalence of obesity and to explore the potential lifestyle risk factors in young university students of Nepal.

Methods

We included in the study 384 young students aged between 17 and 24 years, pursuing medicine at Tribhuvan University, Institute of Medicine, in this cross-sectional study. A self-administered questionnaire to collect information about age, sex, smoking, alcohol consumption, meat consumption, fast-food consumption, and sedentary lifestyle was employed. Anthropometric measurements were taken to calculate body mass index (BMI) and waist-to-hip ratio (WHR).

Results

This study revealed that the current, episodic heavy alcohol consumers, current cigarette smokers, and individuals with a sedentary lifestyle had a statistically significant higher BMI and WHR as compared to age and gender-matched healthy subjects. Meat consumers as well had a statistically significant higher BMI. However, there has been no statistically significant difference in BMI and WHR in those who consume fast food from those who don’t.

Conclusion

Our study shows a high prevalence of obesity among young university students of Nepal, making it necessary to develop effective preventive measures to reduce their exposure to the risk factors associated with obesity. Early interventions to encourage lifestyle changes can be a worthwhile and effective strategy to prevent and/or reduce the risks for the development of cardiovascular diseases (CVD) and other comorbidities.

Effects of nicotine on homeostatic and hedonic components of food intake

Chronic tobacco use leads to nicotine addiction that is characterized by exaggerated urges to use the drug despite the accompanying negative health and socioeconomic burdens. Interestingly, nicotine users are found to be leaner than the general population. Review of the existing literature revealed that nicotine affects energy homeostasis and food consumption via altering the activity of neurons containing orexigenic and anorexigenic peptides in the brain. Hypothalamus is one of the critical brain areas that regulates energy balance via the action of these neuropeptides. The equilibrium between these two groups of peptides can be shifted by nicotine leading to decreased food intake and weight loss. The aim of this article is to review the existing literature on the effect of nicotine on food intake and energy homeostasis and report on the changes that nicotine brings about in the level of these peptides and their receptors that may explain changes in food intake and body weight induced by nicotine. Furthermore, we review the effect of nicotine on the hedonic aspect of food intake. Finally, we discuss the involvement of different subtypes of nicotinic acetylcholine receptors in the regulatory action of nicotine on food intake and energy homeostasis.

Browning of white adipose tissue: lessons from experimental models

Beige or brite (brown-in-white) adipocytes are present in white adipose tissue (WAT) and have a white fat-like phenotype that when stimulated acquires a brown fat-like phenotype, leading to increased thermogenesis. This phenomenon is known as browning and is more likely to occur in subcutaneous fat depots. Browning involves the expression of many transcription factors, such as PR domain containing 16 (PRDM16) and peroxisome proliferator-activated receptor (PPAR)-γ, and of uncoupling protein (UCP)-1, which is the hallmark of thermogenesis. Recent papers pointed that browning can occur in the WAT of humans, with beneficial metabolic effects. This fact indicates that these cells can be targeted to treat a range of diseases, with both pharmacological and nutritional activators. Pharmacological approaches to induce browning include the use of PPAR-α agonist, adrenergic receptor stimulation, thyroid hormone administration, irisin and FGF21 induction. Most of them act through the induction of PPAR-γ coactivator (PGC) 1-α and the consequent mitochondrial biogenesis and UCP1 induction. About the nutritional inducers, several compounds have been described with multiple mechanisms of action. Some of these activators include specific amino acids restriction, capsaicin, bile acids, Resveratrol, and retinoic acid. Besides that, some classes of lipids, as well as many plant extracts, have also been implicated in the browning of WAT. In conclusion, the discovery of browning in human WAT opens the possibility to target the adipose tissue to fight a range of diseases. Studies have arisen showing promising results and bringing new opportunities in thermogenesis and obesity control.

The ever-expanding myokinome: discovery challenges and therapeutic implications

Exercise reduces the risk of a multitude of disorders, from metabolic disease to cancer, but the molecular mechanisms mediating the protective effects of exercise are not completely understood. The realization that skeletal muscle is an endocrine organ capable of secreting proteins termed 'myokines', which participate in tissue crosstalk, provided a critical link in the exercise-health paradigm. However, the myokine field is still emerging, and several challenges remain in the discovery and validation of myokines. This Review considers these challenges and highlights some recently identified novel myokines with the potential to be therapeutically exploited in the treatment of metabolic disease and cancer.

Modification of genetic influences on adiposity between 36 and 63 years of age by physical activity and smoking in the 1946 British Birth Cohort Study

Background:

Previous studies reporting on the interaction between physical activity and genetic susceptibility on obesity have been cross-sectional and have not considered the potential influences of other lifestyle behaviours. The aim of this study was to examine modification of genetic influences on changes across age in adiposity during mid-adulthood by physical activity and smoking.

Methods:

The sample comprised 2444 participants who were genotyped for 11 obesity variants and had body mass index (BMI), waist circumference-to-height ratio (WHtR), physical activity and smoking measures at 36, 43, 53 and 60–64 years of age. A genetic risk score (GRS) comprising the sum of risk alleles was computed. Structural equation models investigated modification of the longitudinal GRS associations by physical activity (active versus inactive) and smoking (non-smoker versus smoker), using a latent linear spline to summarise BMI or WHtR (multiplied by 100) at the age of 36 years and their subsequent rates of change over age.

Results:

Physical activity at the age of 36 years attenuated the GRS associations with BMI and WHtR at the same age (P-interaction 0.009 and 0.004, respectively). Further, physical activity at the age of 53 years attenuated the GRS association with rate of change in BMI between 53 and 63 years of age (by 0.012 kg m⁻² per year (95% confidence interval (CI): 0.001, 0.024), P-interaction 0.004). Conversely, smoking at the age of 43 years showed a trend towards augmenting the GRS association with rate of change in WHtR between 43 and 63 years of age (by 0.012 (95% CI: 0.001, 0.026), P-interaction 0.07). Estimated GRS effect sizes were lowest at all ages in the healthiest group (e.g., active non-smokers).

Conclusions:

Healthy lifestyle behaviours appeared to attenuate the genetic influence on changes across age in BMI and central adiposity during mid-adulthood. An active lifestyle and not smoking may have additive effects on reducing the genetic susceptibility to obesity in adults.

Joint association of nicotinic acetylcholine receptor variants with abdominal obesity in American Indians: the Strong Heart Family Study

Cigarette smoke is a strong risk factor for obesity and cardiovascular disease. The effect of genetic variants involved in nicotine metabolism on obesity or body composition has not been well studied. Though many genetic variants have previously been associated with adiposity or body fat distribution, a single variant usually confers a minimal individual risk. The goal of this study is to evaluate the joint association of multiple variants involved in cigarette smoke or nicotine dependence with obesity-related phenotypes in American Indians. To achieve this goal, we genotyped 61 tagSNPs in seven genes encoding nicotine acetylcholine receptors (nAChRs) in 3,665 American Indians participating in the Strong Heart Family Study. Single SNP association with obesity-related traits was tested using family-based association, adjusting for traditional risk factors including smoking. Joint association of all SNPs in the seven nAChRs genes were examined by gene-family analysis based on weighted truncated product method (TPM). Multiple testing was controlled by false discovery rate (FDR). Results demonstrate that multiple SNPs showed weak individual association with one or more measures of obesity, but none survived correction for multiple testing. However, gene-family analysis revealed significant associations with waist circumference (p = 0.0001) and waist-to-hip ratio (p = 0.0001), but not body mass index (p = 0.20) and percent body fat (p = 0.29), indicating that genetic variants are jointly associated with abdominal, but not general, obesity among American Indians. The observed combined genetic effect is independent of cigarette smoking per se. In conclusion, multiple variants in the nAChR gene family are jointly associated with abdominal obesity in American Indians, independent of general obesity and cigarette smoking per se.

Nicotine improves obesity and hepatic steatosis and ER stress in diet-induced obese male rats

Nicotine, the main addictive component of tobacco, promotes body weight reduction in humans and rodents. Recent evidence has suggested that nicotine acts in the central nervous system to modulate energy balance. Specifically, nicotine modulates hypothalamic AMP-activated protein kinase to decrease feeding and to increase brown adipose tissue thermogenesis through the sympathetic nervous system, leading to weight loss. Of note, most of this evidence has been obtained in animal models fed with normal diet or low-fat diet (LFD). However, its effectiveness in obese models remains elusive. Because obesity causes resistance towards many factors involved in energy homeostasis, the aim of this study has been to compare the effect of nicotine in a diet-induced obese (DIO) model, namely rats fed a high-fat diet, with rats fed a LFD. Our data show that chronic peripheral nicotine treatment reduced body weight by decreasing food intake and increasing brown adipose tissue thermogenesis in both LFD and DIO rats. This overall negative energy balance was associated to decreased activation of hypothalamic AMP-activated protein kinase in both models. Furthermore, nicotine improved serum lipid profile, decreased insulin serum levels, as well as reduced steatosis, inflammation, and endoplasmic reticulum stress in the liver of DIO rats but not in LFD rats. Overall, this evidence suggests that nicotine diminishes body weight and improves metabolic disorders linked to DIO and might offer a clear-cut strategy to develop new therapeutic approaches against obesity and its metabolic complications.

Chronic exposure to nicotine enhances insulin sensitivity through α7 nicotinic acetylcholine receptor-STAT3 pathway

This study was to investigate the effect of nicotine on insulin sensitivity and explore the underlying mechanisms. Treatment of Sprague-Dawley rats with nicotine (3 mg/kg/day) for 6 weeks reduced 43% body weight gain and 65% blood insulin level, but had no effect on blood glucose level. Both insulin tolerance test and glucose tolerance test demonstrated that nicotine treatment enhanced insulin sensitivity. Pretreatment of rats with hexamethonium (20 mg/kg/day) to antagonize peripheral nicotinic receptors except for α7 nicotinic acetylcholine receptor (α7-nAChR) had no effect on the insulin sensitizing effect of nicotine. However, the insulin sensitizing effect but not the bodyweight reducing effect of nicotine was abrogated in α7-nAChR knockout mice. Further, chronic treatment with PNU-282987 (0.53 mg/kg/day), a selective α7-nAChR agonist, significantly enhanced insulin sensitivity without apparently modifying bodyweight not only in normal mice but also in AMP-activated kinase-α2 knockout mice, an animal model of insulin resistance with no sign of inflammation. Moreover, PNU-282987 treatment enhanced phosphorylation of signal transducer and activator of transcription 3 (STAT3) in skeletal muscle, adipose tissue and liver in normal mice. PNU-282987 treatment also increased glucose uptake by 25% in C2C12 myotubes and this effect was total abrogated by STAT3 inhibitor, S3I-201. All together, these findings demonstrated that nicotine enhanced insulin sensitivity in animals with or without insulin resistance, at least in part via stimulating α7-nAChR-STAT3 pathway independent of inflammation. Our results contribute not only to the understanding of the pharmacological effects of nicotine, but also to the identifying of new therapeutic targets against insulin resistance.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

Associations of smoking and smoking cessation with CT-measured visceral obesity in 4656 Korean men

OBJECTIVES

Although obesity is shown to be less common among current smokers than never smokers, the association between visceral obesity and smoking remains uncertain.

METHODS

For this cross-sectional analysis, we recruited 4656 Korean men of 19 to 79 years who received a regular checkup at a health examination center between 2008 and 2010. Computed tomography was performed to measure the area of visceral and subcutaneous adipose tissue (VAT and SAT). We compared the mean VAT by multiple regression analysis across smoking status after adjusting for confounders.

RESULTS

Both current and former smokers had more mean VAT than never smokers. Current smokers who consumed more than 20 cigarettes per day had 11% higher mean VAT than never smokers (P<0.01). Longer smoking duration, higher daily cigarette consumption before quitting, and shorter abstinence duration among ex-smokers were associated with increasing mean VAT (all P for trend<0.01). The mean VAT in former smokers was highest within 2 years of abstinence. There was no significant difference of mean VAT between ex-smokers with >20 years of abstinence duration and never smokers.

CONCLUSION

Both current and former smoking is associated with increased VAT. The risk of visceral obesity is proportional to the degree of exposure to cigarette smoking.

Cigarette smoking and brain regulation of energy homeostasis

Cigarette smoking is an addictive behavior, and is the primary cause of cardiovascular and pulmonary disease, and cancer (among other diseases). Cigarette smoke contains thousands of components that may affect caloric intake and energy expenditure, although nicotine is the major addictive substance present, and has the best described actions. Nicotine exposure from cigarette smoke can change brain feeding regulation to reduce appetite via both energy homeostatic and reward mechanisms, causing a negative energy state which is characterized by reduced energy intake and increased energy expenditure that are linked to low body weight. These findings have led to the public perception that smoking is associated with weight loss. However, its effects at reducing abdominal fat mass (a predisposing factor for glucose intolerance and insulin resistance) are marginal, and its promotion of lean body mass loss in animal studies suggests a limited potential for treatment in obesity. Smoking during pregnancy puts pressure on the mother's metabolic system and is a significant contributor to adverse pregnancy outcomes. Smoking is a predictor of future risk for respiratory dysfunction, social behavioral problems, cardiovascular disease, obesity, and type-2 diabetes. Catch-up growth is normally observed in children exposed to intrauterine smoke, which has been linked to subsequent childhood obesity. Nicotine can have a profound impact on the developing fetal brain, via its ability to rapidly and fully pass the placenta. In animal studies this has been linked with abnormal hypothalamic gene expression of appetite regulators such as downregulation of NPY and POMC in the arcuate nucleus of the hypothalamus. Maternal smoking or nicotine replacement leads to unhealthy eating habits (such as junk food addiction) and other behavioral disorders in the offspring.

Nicotine induces negative energy balance through hypothalamic AMP-activated protein kinase

Smokers around the world commonly report increased body weight after smoking cessation as a major factor that interferes with their attempts to quit. Numerous controlled studies in both humans and rodents have reported that nicotine exerts a marked anorectic action. The effects of nicotine on energy homeostasis have been mostly pinpointed in the central nervous system, but the molecular mechanisms controlling its action are still not fully understood. The aim of this study was to investigate the effect of nicotine on hypothalamic AMP-activated protein kinase (AMPK) and its effect on energy balance. Here we demonstrate that nicotine-induced weight loss is associated with inactivation of hypothalamic AMPK, decreased orexigenic signaling in the hypothalamus, increased energy expenditure as a result of increased locomotor activity, increased thermogenesis in brown adipose tissue (BAT), and alterations in fuel substrate utilization. Conversely, nicotine withdrawal or genetic activation of hypothalamic AMPK in the ventromedial nucleus of the hypothalamus reversed nicotine-induced negative energy balance. Overall these data demonstrate that the effects of nicotine on energy balance involve specific modulation of the hypothalamic AMPK-BAT axis. These targets may be relevant for the development of new therapies for human obesity.

TGF-β/Smad3 Signaling Regulates Brown Adipocyte Induction in White Adipose Tissue

Recent identification of active brown fat reserves in adult humans has re-stimulated interest in the role of brown adipocytes in energy homeostasis. In addition, there is accumulating evidence to support the concept of an alteration in energy balance through acquisition of brown fat features in traditional white fat depots. We recently described an important role played by the TGF-β/Smad3 signaling pathway in modulating the appearance of brown adipocytes in traditional white fat, and its implications to thermogenesis, mitochondrial energetics, energy expenditure, and protection from diabetes and obesity. Here we review the data supporting this phenomenon and put into perspective the promise of conversion of white fat to a brown fat state as a potential therapeutic option for obesity and diabetes.

A new era for brown adipose tissue: New insights into brown adipocyte function and differentiation

Until quite recently, brown adipose tissue was considered of metabolic significance only in small mammals and human newborns, since it was thought to disappear rapidly after birth in humans. However, nowadays this tissue is known to play a role in the regulation of energy balance not only in rodents, but also in humans. In this review we highlight new features regarding brown adipose tissue origin and function and revise old paradigms about brown adipocyte differentiation.

Perilipin overexpression in white adipose tissue induces a brown fat-like phenotype

BACKGROUND

Perilipin A (PeriA) exclusively locates on adipocyte lipid droplets and is essential for lipid storage and lipolysis. Previously, we reported that adipocyte specific overexpression of PeriA caused resistance to diet-induced obesity and resulted in improved insulin sensitivity. In order to better understand the biological basis for this observed phenotype, we performed additional studies in this transgenic mouse model.

METHODOLOGY AND PRINCIPAL FINDINGS

When compared to control animals, whole body energy expenditure was increased in the transgenic mice. Subsequently, we performed DNA microarray analysis and real-time PCR on white adipose tissue. Consistent with the metabolic chamber data, we observed increased expression of genes associated with fatty acid β-oxidation and heat production, and a decrease in the genes associated with lipid synthesis. Gene expression of Pgc1a, a regulator of fatty acid oxidation and Ucp1, a brown adipocyte specific protein, was increased in the white adipose tissue of the transgenic mice. This observation was subsequently verified by both Western blotting and histological examination. Expression of RIP140, a regulator of white adipocyte differentiation, and the lipid droplet protein FSP27 was decreased in the transgenic mice. Importantly, FSP27 has been shown to control gene expression of these crucial metabolic regulators. Overexpression of PeriA in 3T3-L1 adipocytes also reduced FSP27 expression and diminished lipid droplet size.

CONCLUSIONS

These findings demonstrate that overexpression of PeriA in white adipocytes reduces lipid droplet size by decreasing FSP27 expression and thereby inducing a brown adipose tissue-like phenotype. Our data suggest that modulation of lipid droplet proteins in white adipocytes is a potential therapeutic strategy for the treatment of obesity and its related disorders.

Long-term cigarette smoke exposure increases uncoupling protein expression but reduces energy intake

The appetite suppressing effect of tobacco is a major driver of smoking behaviour; however few studies have addressed the effects of chronic cigarette smoke exposure (SE) on appetite, body weight and metabolic markers. We compared the effects of SE to equivalent food restriction (pair-fed, PF), against sham-exposure, on body weight, adiposity, cytokines, and levels of uncoupling proteins (UCP) and brain neuropeptide Y (NPY) in male Balb/C mice. SE rapidly induced anorexia, and after 12 weeks, SE and PF groups were lighter than control animals (23.9+/-0.2, 25.5+/-0.5, 26.8+/-0.4 g respectively, P<0.05). White fat (WAT) masses were reduced by both SE and PF. Plasma leptin and insulin were reduced in SE mice; insulin was further reduced by PF. Brown fat UCP1 and 3 mRNA were increased in SE animals relative to PF animals, possibly promoting thermogenesis. WAT mRNA expression of the inflammatory cytokine, TNFalpha was doubled by SE, while IL-6 was reduced by both PF and SE. Hypothalamic NPY content was increased by SE (89.3+/-2.8 vs. 75.9+/-2.4 ng control, P<0.05), and more by PF (100.7+/-3.4 ng, P<0.05 compared to both groups), suggesting disinhibition due to reduced adipose derived leptin. In contrast to equivalent food restriction, cigarette smoke exposure reduced body weight and total hypothalamic NPY, and increased thermogenesis and markers of inflammation. The suppressed hypothalamic NPY and increased UCPs may contribute to the spontaneous hypophagia and extra weight loss in SE animals. These findings contribute to our understanding of weight loss in smoking-related lung disease, suggesting a greater impact than that due to anorexia alone.

Consequences of smoking for body weight, body fat distribution, and insulin resistance

Our aim was to critically evaluate the relations among smoking, body weight, body fat distribution, and insulin resistance as reported in the literature. In the short term, nicotine increases energy expenditure and could reduce appetite, which may explain why smokers tend to have lower body weight than do nonsmokers and why smoking cessation is frequently followed by weight gain. In contrast, heavy smokers tend to have greater body weight than do light smokers or nonsmokers, which likely reflects a clustering of risky behaviors (eg, low degree of physical activity, poor diet, and smoking) that is conducive to weight gain. Other factors, such as weight cycling, could also be involved. In addition, smoking increases insulin resistance and is associated with central fat accumulation. As a result, smoking increases the risk of metabolic syndrome and diabetes, and these factors increase risk of cardiovascular disease. In the context of the worldwide obesity epidemic and a high prevalence of smoking, the greater risk of (central) obesity and insulin resistance among smokers is a matter of major concern.

Effects of adolescent nicotine and SR 147778 (Surinabant) administration on food intake, somatic growth and metabolic parameters in rats

Tobacco smoking and obesity are worldwide important health problems with a growing impact in adolescent and young adults. One of the consequences of nicotine withdrawal is an increase in body weight that can act as a risk factor to relapse. Experimental therapies with a cannabinoid receptor antagonist have been recently proposed for both cigarette smoking and complicated overweight. In the present study, we aimed to investigate metabolic and hormonal effects of chronic nicotine treatment (during treatment and in abstinence) in an animal model of adolescence as well as to address the pharmacological effects of the novel selective CB1 cannabinoid receptor antagonist, SR 147778 (Surinabant). Adolescence (postnatal days 37-44) and/or post-adolescence (postnatal days 45-59) administration of Surinabant reduced body weight gain, as well as plasma glucose levels and triglycerides. The drug also reduced insulin and leptin secretion, and increased adiponectin and corticosterone levels. The effects showed sexual dimorphisms and, in general, were more pronounced in females. Chronic exposure to nicotine (0.8 mg/kg), from postnatal days 30-44 did not result in overt effects on food intake or body weight gain. However, it altered certain responses to the administration of Surinabant, both when the two drugs were given simultaneously and when Surinabant was administered during the post-adolescence period, along nicotine withdrawal. The present results indicate that the endogenous cannabinoid system is active as a metabolic modulator during adolescence and that nicotine exposure can induce long-lasting effects on metabolic regulation, altering cannabinoid modulation of energy expenditure and metabolism.

Relationships between cigarette smoking, body size and body shape

OBJECTIVE

To define relationships between smoking status, body mass index (BMI), waist and hip circumferences (WC, HC) and waist to hip ratio (WHR).

DESIGN

Further analysis of the cross-sectional Scottish Health Survey 1998 data.

SUBJECTS

Nationally representative sample of 9047 adults aged 16-74 y.

RESULTS

Body mass index (BMI) was lower in current smokers and higher in ex-smokers (P<0.001) when compared with nonsmokers in the survey population as a whole. After adjustment for confounding factors (age, social class, physical activity and alcohol intake), these differences still remained. However, examination of age categories showed no such differences in BMI between current smokers and nonsmokers in men aged 16-24 y or women aged below 55 y. In the age category 16-24 y, prevalence of cigarette smoking was highest at 51% (men) and 43% (women) in obese subjects and lowest at 35% (men) and 33% (women) in people with BMI of 25-30 kg/m(2). For women current smokers, mean WC and WHR were higher and HC was lower compared with nonsmokers (P<0.001). In men, only HC was lower in current smokers compared with nonsmokers for the entire sample (P<0.001).

CONCLUSION

Cigarette smoking is associated with a lower BMI in adults over 24 y particularly in men, but not in younger people. In women, smoking is linked to the development of central adiposity. The gender-related central adiposity of men is not further increased by smoking, but a lower HC could suggest a reduction in muscle mass.

Effect of short-term cigarette smoke exposure on body weight, appetite and brain neuropeptide Y in mice

Although nicotinic receptors have been demonstrated in hypothalamic appetite-regulating areas and nicotine administration alters food intake and body weight in both animals and humans, the mechanisms underlying the effects of smoking on appetite circuits remain unclear. Conflicting effects of nicotine on the major orexigenic peptide, neuropeptide Y (NPY), have been observed in the brain, but the effects of smoking are unknown. Thus, we aimed to investigate how cigarette smoking affects body weight, food intake, plasma leptin concentration, hypothalamic NPY peptide, adipose mass and mRNA expression of uncoupling proteins (UCP), and tumor necrosis factor (TNF) alpha. Balb/C mice (8 weeks) were exposed to cigarette smoke (three cigarettes, three times a day for 4 consecutive days) or sham exposed. Body weight and food intake were recorded. Plasma leptin and brain NPY were measured by radioimmunoassay. UCPs and TNF alpha mRNA were measured by real-time PCR. Food intake dropped significantly from the first day of smoking, and weight loss became evident within 2 days. Brown fat and retroperitoneal white fat masses were significantly reduced, and plasma leptin concentration was decreased by 34%, in line with the decreased fat mass. NPY concentrations in hypothalamic subregions were similar between two groups. UCP1 mRNA was decreased in white fat and UCP3 mRNA increased in brown fat in smoking group. Short-term cigarette smoke exposure led to reduced body weight, food intake, and fat mass. The reduction in plasma leptin concentration may have been too modest to increase NPY production; alternatively, change in NPY or its function might have been offset by nicotine or other elements in cigarette smoke.

Overlapping and Interactive Pathways Regulating Appetite and Craving

Multidisciplinary research in recent years has delineated the hypothalamic hardcore wiring that encodes appetitive drive. The appetite regulating network (ARN) consisting of distinct orexigenic and anorexigenic circuitries operates in the arcuate nucleus-paraventricular nucleus axis of the hypothalamus to propagate and relay the appetitive drive, and is subject to modulation by excitatory and inhibitory messages from the lateral hypothalamus and ventromedial nucleus, respectively. Reciprocal afferent humoral signals, comprised of anorexigenic leptin from white adipose tissue and orexigenic ghrelin from stomach, to the ARN integrate the moment-to-moment regulation of energy homeostasis. Various loci in the ARN and afferent hormonal feedback circuitry in the rodent brain are important for food craving elicited by drugs of abuse. This convergence of neurochemical and hormonal signaling has now paved the way to address the fundamental question of whether cellular and molecular events that underlie the appetitive drive in response to diminished energy stores in the body are akin to drug craving during withdrawal in humans.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Nicotine treatment decreases food intake and body weight via a leptin-independent pathway in Psammomys obesus

It has been reported previously that leptin may be involved in nicotine's ability to reduce body weight. Our aim was to investigate whether the anorexic action of nicotine is related to the actions of leptin by utilizing lean leptin-sensitive and obese leptin-resistant Psammomys obesus. Lean and obese P. obesus were assigned to receive nicotine sulphate at 6, 9 or 12 mg/day or saline (control) for 9 days (n = 6-10 in each group), administered using mini-osmotic pumps. Food intake, body weight, plasma leptin concentrations, plasma insulin and blood glucose were measured at baseline and throughout the study period. Nicotine treatment reduced food intake by up to 40% in lean and obese P. obesus. Plasma leptin levels fell significantly only in lean nicotine-treated animals, whereas no changes were observed in obese nicotine-treated animals. However, both lean and obese nicotine-treated animals had similar reductions in body weight. Our results show that nicotine has dramatic effects on food intake and body weight, however, these changes appear to be independent of the leptin signalling pathway.

Impaired noradrenaline-induced lipolysis in white fat of aP2-Ucp1 transgenic mice is associated with changes in G-protein levels

In vitro experiments suggest that stimulation of lipolysis by catecholamines in adipocytes depends on the energy status of these cells. We tested whether mitochondrial uncoupling proteins (UCPs) that control the efficiency of ATP production could affect lipolysis and noradrenaline signalling in white fat in vivo. The lipolytic effect of noradrenaline was lowered by ectopic UCP1 in white adipocytes of aP2-Ucp1 transgenic mice, overexpressing the UCP1 gene from the aP2 gene promoter, reflecting the magnitude of UCP1 expression, the impaired stimulation of cAMP levels by noradrenaline and the reduction of the ATP/ADP ratio in different fat depots. Thus only subcutaneous but not epididymal fat was affected. UCP1 also down-regulated the expression of hormone-sensitive lipase and lowered its activity, and altered the expression of trimeric G-proteins in adipocytes. The adipose tissue content of the stimulatory G-protein alpha subunit was increased while that of the inhibitory G-protein alpha subunits decreased in response to UCP1 expression. Our results support the idea that the energy status of cells, and the ATP/ADP ratio in particular, modulates the lipolytic effects of noradrenaline in adipose tissue in vivo. They also demonstrate changes at the G-protein level that tend to overcome the reduction of lipolysis when ATP level in adipocytes is low. Therefore, respiratory uncoupling may exert a broad effect on hormonal signalling in adipocytes.

Modulation of lipid metabolism by energy status of adipocytes: implications for insulin sensitivity

It is becoming evident that insulin resistance of white adipose tissue is a major factor underlying the cardiovascular risk of obesity. Impaired fat storage rather than altered glucose metabolism in adipocytes probably contributes to development of insulin resistance in muscle and other tissues, in particular via increased delivery of nonesterified fatty acids into circulation. Lipid metabolism of adipose tissue is affected by the energy status of fat cells. In vitro experiments indicated the dependence of both lipogenesis and lipolysis on ATP levels in adipocytes. Thus, respiratory uncoupling in adipocytes that results in stimulation of energy dissipation and depression of ATP synthesis may contribute to the control of lipid metabolism, adiposity, and insulin sensitivity. This notion is supported by the expression of UCPs in adipocytes, for example, UCP2, UCP5, as well as some protonophoric anion transporters, and by induction of UCP1 and UCP3 in white fat by pharmacological treatments that reduce adiposity. A negative correlation between expression of UCPs in adipocytes and accumulation of white fat was also found. Expression of UCP1 from the adipose-specific promoter in the aP2-Ucp1 transgenic mice mitigated obesity induced by genetic or dietary factors. The obesity resistance, accompanied by respiratory uncoupling in adipocytes and increased energy expenditure, resulted from ectopic expression of UCP1 in white, but not brown fat. Probably due to depression of the ATP/ADP ratio, both fatty acid synthesis and lipolytic action of norepinephrine in adipocytes of transgenic mice were relatively low. Expression of regulatory G-proteins, which are essential for both catecholamine and insulin signaling in adipocytes, was also altered by ectopic UCP1. These results support the role of protonophoric proteins in adipocytes in the control of adiposity and insulin sensitivity. Antidiabetic effects of thiazolidinediones, fibrates, beta(3)-adrenoreceptor agonists, dietary n-3 PUFAs, and leptin may be explained at least partially by their effects on the energy and hence also the lipid metabolism of fat cells.

Expression of the uncoupling protein 1 from the aP2 gene promoter stimulates mitochondrial biogenesis in unilocular adipocytes in vivo

Mitochondrial uncoupling protein 1 (UCP1) is a specific marker of multilocular brown adipocytes. Ectopic UCP1 in white fat of aP2-Ucp1 mice mitigates development of obesity by both, increasing energy expenditure and decreasing in situ lipogenesis. In order to further analyse consequences of respiratory uncoupling in white fat, the effects of the ectopic UCP1 on the morphology of adipocytes and biogenesis of mitochondria in these cells were studied. In subcutaneous white fat of both aP2-Ucp1 and young control (5-week-old) mice, numerous multilocular adipocytes were found, while they were absent in adult (7- to 9-month-old) animals. Only unilocular cells were present in epididymal fat of both genotypes. In both fat depots of aP2-Ucp1 mice, the levels of the UCP1 transcript and UCP1 antigen declined during ageing, and they were higher in subcutaneous than in epididymal fat. Under no circumstances could ectopic UCP1 induce the conversion of unilocular into multilocular adipocytes. Presence of ectopic UCP1 in unilocular adipocytes was associated with the elevation of the transcripts for UCP2 and for subunit IV of mitochondrial cytochrome oxidase (COX IV), and increased content of mitochondrial cytochromes. Electron microscopy indicated changes of mitochondrial morphology and increased mitochondrial content due to ectopic UCP1 in unilocular adipocytes. In 3T3-L1 adipocytes, 2,4-dinitrophenol increased the levels of the transcripts for both COX IV and for nuclear respiratory factor-1. Our results indicate that respiratory uncoupling in unilocular adipocytes of white fat is capable of both inducing mitochondrial biogenesis and reducing development of obesity.

Nicotine infusion alters leptin and uncoupling protein 1 mRNA expression in adipose tissues of rats

We attempted to clarify whether leptin and uncoupling protein 1 (UCP1) are involved in the action of nicotine on the energy balance. Male Wistar rats were infused subcutaneously with nicotine (12 mg x kg(-1) x day(-1)) for 4 or 14 days. At the end of the 4-day period, the plasma concentrations of leptin of the nicotine-treated and pair-fed rats were lower than those of the freely fed rats, although the levels of leptin mRNA expression in various white adipose tissues did not differ among the three groups. At the end of the 14-day nicotine infusion period, plasma concentrations of leptin were higher, and leptin mRNA expression in the omentum and epididymal and retroperitoneal adipose tissues was stronger in the nicotine-treated rats than in the pair-fed and freely fed rats. UCP1 mRNA expression in the brown adipose tissue of nicotine-treated was stronger than that of the pair-fed rats. These results suggest that continuous nicotine infusion differentially affects the synthesis and secretion of leptin according to the duration of infusion and stimulates UCP1 mRNA expression, probably in a manner independent of leptin.

Time-dependent effects of a high-energy-yielding diet on the regulation of specific white adipose tissue genes

White adipose tissue development is regulated by many factors, including the energy content of food and the genetic background. Nevertheless, little is known about possible differential effects of high-fat palatable diets when fed for short or long-time periods. Thus, the expression of certain genes involved with lipid metabolism (peroxisome proliferator-activated receptor gamma, PPARgamma2; retinoic receptors; fatty acid binding protein, aP2 and uncoupling proteins, UCP) may be affected by those dietary manipulations (high-energy-yielding diet and time duration of feeding). High-fat feeding for 8 days decreased mRNA UCP3 levels compared to control fed animals, while feeding for 30 days increased them over controls. Similar findings occurred for PPARgamma2 and aP2. Furthermore, statistically significant associations were found among PPARgamma2, aP2 and UCP3 mRNA levels. These data suggest a physiological time-dependent response seeking to prevent excessive fat deposition when animals are fed for short-term with a high amount of dietary fat, which was followed by an adaptive period to the high-energy content of diet throughout a coregulation among certain lipid metabolism related genes: PPARgamma2, aP2, UCP3.

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.

Regulation of feeding-associated peptides and receptors by nicotine

Although numerous epidemiological studies have provided convincing evidence for the inverse association between tobacco smoking and body weight, the molecular mechanisms underlying this relationship are not well-understood. Nicotine, as a potent secretagogue, could be expected to influence the levels and expression of many classes of neurotransmitters, as well as of cell-membrane constituents linked to neurotransmission, including signal transducers and related effectors. A potentially major group of candidate molecules that could be involved in feeding-related actions of nicotine are the numerous neuropeptides and peptide hormones shown in the past two decades to regulate food intake and energy expenditure. These could include neuropeptide Y (NPY), orexins, leptins, and uncoupling proteins (UCPs). Some of these peptides were already shown to respond to nicotine treatment in terms of regulation of levels and of activity at the level of cell-membrane receptors. The primary objective of this review is to summarize our current understanding of the regulatory effects of nicotine on the food intake and energy expenditure as related to the expression levels of leptin, NPY, orexin, uncoupling proteins, and of NPY and orexin receptors.