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.

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. Diabetes 61: [807][808][809][810][811][812][813][814][815][816][817]2012 N icotine, the main addictive component of tobacco, shows anorexigenic properties and promotes body weight reduction in humans and rodents (1)(2)(3)(4). In keeping with these points, smoking cessation is associated with hyperphagia and weight gain in over 70% of people who attempt to do so. In fact, in many cases, the prospect of weight gain itself has acted as an incentive for persevering in the smoking habit (1,2,4). Therefore, it is clear that smoking leads to a state of negative energy balance and one that might be exploited for therapeutic purposes if its molecular mechanisms were better understood.
An important mediator of all of these effects is hypothalamic AMP-activated protein kinase (AMPK), a cellular gauge that controls whole-body energy balance and is activated in conditions of low energy (14,15). When activated, AMPK optimizes energy utilization by increasing energyproducing and reducing energy-wasting processes at the whole-body level. In the hypothalamus, AMPK activation drives feeding by modulating mitochondrial fatty acid oxidation and intracellular levels of reactive oxygen species. This untimely regulates neuropeptide expression in the ARC (16)(17)(18) and also suppresses EE and thyroid-induced thermogenesis in the BAT (7,8). Activated AMPK is also an important regulator of fatty acid biosynthesis, suppressing de novo lipogenesis by phosphorylation and inactivation of acetyl-CoA carboxylase (ACC) (7,8,14,15,17,18).
Whether the effects of nicotine on energy balance are mediated by modulation of hypothalamic AMPK is currently unknown. However, there are some clues that might indicate the existence of such an action. Nicotine coregulates most of the processes controlled by AMPK. For instance, chronic nicotine exposure in rats reduces the expression of NPY (19) and consistently blunts NPYinduced hyperphagia (20). Nicotine withdrawal also increases hypothalamic NPY and AgRP expression, mechanisms that contribute to increased feeding after nicotine cessation (20,21). Current evidence has demonstrated that nicotine also upregulates the expression of POMC (22) and decreases feeding through activation of POMC neurons (3). Besides its actions on hypothalamic feeding circuits nicotine also modulates peripheral metabolism in rats, increasing BAT thermogenesis through activation of the sympathetic nervous system (SNS) and increased expression of uncoupling protein 1 (UCP1) (23,24).
Based on this evidence, we hypothesized that the effect of nicotine on energy homeostasis could be mediated at least in part by hypothalamic AMPK. The aim of this study was to investigate the effect of nicotine on hypothalamic AMPK and its impact on feeding and physiological parameters modulating energy balance such as EE, LA, BAT function, and nutrient partitioning. Our data confirm the existence of a previously unknown link between nicotineinduced negative energy balance and hypothalamic AMPK.

RESEARCH DESIGN AND METHODS
Animals. We used adult male Sprague-Dawley rats (9-11 weeks old; Animalario-General USC). Rats were housed in a temperature-controlled (22°C) room, with a 12-h light/dark cycle (7:00 to 19:00). The experiments were performed in agreement with the International Law on Animal Experimentation and approved by the USC Bioethics Committee and the Ministry of Science and Innovation of Spain (PS09/01880). For all of the experimental groups and analytic methods, we used 9-16 rats per group. Nicotine treatment. Nicotine (nicotine hydrogen tartrate salt; Sigma-Aldrich, St. Louis, MO) or vehicle (saline) was given subcutaneously (2 mg/kg per 12 h) in a total volume of 100 mL (25). Rats received a nicotine or vehicle injection every 12 h in both subchronic (48 h) and chronic (17 days) experiments. To validate the EE data, all rats were matched for body weight before treatment began. To control nicotine-induced respiratory depression (RD), rats were treated with nicotine and symptoms of RD were visually controlled from 2 min before to 12 h after treatment. For the antagonism of a3b4 nicotinic acetylcholine receptors, rats were treated previously (30 min) intraperitoneally with mecamylamine (mecamylamine hydrochloride, 1 mg/kg; Sigma-Aldrich) as described recently (3) and subcutaneously with nicotine during 48 h. Intracerebroventricular treatment. Intracerebroventricular cannulae were stereotaxically implanted under ketamine/xylazine anesthesia in the lateral ventricle (coordinates 1.2 mm lateral to sagittal suture and 1.0 mm posterior to bregma), as described previously (8,18,26), and demonstrated to be correctly located by methylene blue staining. Rats received either a single administration of the AMPK activator AICAR (Sigma-Aldrich; 5 mg, dissolved in 5 mL of saline) or vehicle (5 mL saline) (8) at 19:00. Stereotaxic microinjection of adenoviral expression vectors. Rats were placed in a stereotaxic frame (David Kopf Instruments, Tujunga, CA) under ketamine/xylazine anesthesia. The ventromedial nucleus of the hypothalamus (VMH) was targeted bilaterally using a 25-gauge needle (Hamilton, Reno, NV) connected to a 1-mL syringe. The injection was directed to stereotaxic coordinates 2.3/3.3 mm posterior to the bregma (two injections were performed in each VMH), 60.6 mm lateral to midline and 10.2 mm below the surface of the skull (8,18). Adenoviral vectors (green fluorescent protein [GFP] or AMPKa-CA; Viraquest, North Liberty, IA) were delivered at a rate of 200 nL/min for 5 min (1 mL/injection site) (8,18). Conditioned taste aversion. Five days before the test, rats were allowed 2-h daytime access to water. On the day of the experiment, rats were given access to 0.15% sodium saccharin rather than water for 30 min. Next, one experimental group was injected intraperitoneally with 0.15 mol/L lithium chloride (LiCl) in saline and subcutaneously with saline (vehicle). The second experimental group was injected intraperitoneally with saline and subcutaneously with nicotine. A third experimental group was injected intraperitoneally and subcutaneously with saline. Twenty-four hours later, rats were given a 2-h access to a two-bottle choice test of 0.15% saccharin versus water. Conditioned taste aversion (CTA) was expressed as percent saccharin preference ratio [100 3 saccharin intake/(saccharin intake + water intake)]. A saccharin preference ratio ,50% was considered as a signal of CTA (26). Temperature measurements. Body temperature was recorded with a rectal probe connected to digital thermometer (BAT-12 Microprobe-Thermometer; Physitemp, Clifton, NJ). Skin temperature surrounding BAT was recorded with an infrared camera (E60bx: Compact-Infrared-Thermal-Imaging-Camera; FLIR, West Malling, Kent, U.K.) and analyzed with a specific software package (FLIR-Tools-Software; FLIR). We used eight animals per group, and for each animal, three or four pictures were analyzed. The skin temperature surrounding BAT for one particular animal was calculated as the average temperature recorded by analyzing those pictures. In situ hybridization. Coronal brain sections (16 mm) were probed as published (8,18,26). We used specific oligonucleotides for AgRP, CART, fatty acid synthase (FAS), NPY, and POMC detection (Supplementary Table 1). Anatomical regions were analyzed using the rat brain atlas (27). Sections were scanned, and the hybridization signal was quantified by densitometry using ImageJ-1.33 (National Institutes of Health, Bethesda, MD) (8,18,26). We used between 16 and 20 sections for each animal (4 to 5 slides with four sections per slide). The mean of these 16-20 values was used as the densitometry value for each animal. Enzymatic assays. FAS activity was performed as described previously (8,18). Western blotting. The hypothalamus, defined by the posterior margin of the optic chiasm and the anterior margin of the mammillary bodies (to the depth of ;2 mm), was dissected out and immediately homogenated on ice to preserve phosphorylated protein levels (8,18,26). Hypothalamic total protein lysates were subjected to SDS-PAGE, electrotransferred on a polyvinylidene fluoride membrane, and probed with the following antibodies: ACC, pACC-Ser 79 , AMPKa1, and AMPKa2 (Upstate Biotechnology, Temecula, CA); FAS (BD Biosciences, Franklin Lakes, NJ), pAMPK-Thr 172 , and phospho-liver kinase B1 (pLKB1)-Ser 428 (Cell Signaling, Danvers, MA); PP2Ca and Ca 2+ /calmodulindependent protein kinase kinase a and b (CaMKKa and CaMKKb) (Santa Cruz Biotechnology, Santa Cruz, CA); and b-actin (Sigma-Aldrich) as described previously (8,18,26). Values were expressed in relation to b-actin levels.
Real-time quantitative PCR. We performed real-time PCR (TaqMan; Applied Biosystems, Carlsbad, CA) as described previously (8,26) using specific sets of primer probes (Supplementary Table 2). Values were expressed in relation to hypoxanthine-guanine phosphoribosyltransferase levels. EE, LA, respiratory quotient, and nuclear magnetic resonance analysis. During the 48-h experiment and the 17-day experiment, rats were analyzed for EE, respiratory quotient (RQ), and LA using a calorimetric system (LabMaster; TSE Systems, Bad Homburg, Germany). This system is an open-circuit instrument that determines O 2 consumption, CO 2 production, and RQ (VCO 2 /VO 2 ) (12,28). Rats were placed for adaptation for 1 week before starting the measurements. For the measurement of body composition, we used the nuclear magnetic resonance imaging (Whole Body Composition Analyzer; EchoMRI, Houston, TX) (8,12,28). Statistical analysis. Data are expressed as mean 6 SEM. Statistical significance was determined by Student t tests or ANOVAs and post hoc two-tailed Bonferroni test. P , 0.05 was considered significant.

RESULTS
Nicotine treatment induced negative energy balance by decreasing feeding and increasing EE. Nicotinetreated rats exhibited a marked reduction in body weight (Fig. 1A) associated with reduced food intake over 24 and 48 h of treatment (Fig. 1B). We evaluated whether the anorectic effect was a genuine action of nicotine or a secondary effect of the drug. It has been reported that nicotine induces RD in neonatal and anesthetized rats (29,30). Nicotine-treated rats showed a significant short-term RD after the injection (RD-vehicle-treated: nondetected; RDnicotine-treated: 5.0 6 1.4 min; P , 0.001). Ten minutes after nicotine administration just 5% of the rats showed symptoms of RD; 15 min after injection those symptoms were completely abolished (Fig. 1C). These data suggest that an effect of RD on 12-h feeding patterns is unlikely.
We further evaluated whether the anorectic effect of nicotine was associated with an aversive response by performing a CTA experiment. In this type of experiment a saccharin preference ratio ,50% was considered a marker of CTA, since it indicates that rats prefer to drink water after an aversive response to saccharine (26). LiCl was used as a positive control, and as expected, LiCl-treated rats drank significantly less saccharin solution (LiCl paired flavor; Fig.  1D), indicating that they had developed CTA (26). Vehicle and nicotine-treated rats had a similar saccharine preference consumption ratio, and it is noteworthy that this was .50% (Fig. 1D). Supporting the absence of stress, illness, or malaise associated with the nicotine treatments, we did not observe reduced LA (Fig. 1G), changes in skin aspect, stool consistency, or obvious abnormal behavior such as was evident in the LiCl-treated animals (data not shown) (26).
We next evaluated whether nicotine treatment induced a change in the other side of the energy balance equation. Nicotine-treated rats showed a significant increase in body temperature (Fig. 1E) associated with higher EE (Fig. 1F; area under the curve: vehicle: 100 6 2.4; nicotine: 132 6 9.1; P , 0.01), increased LA (Fig. 1G), and reduced RQ (Fig. 1H) compared with vehicle-treated rats. Overall, these data indicate that nicotine treatment induced negative energy balance, including decreased feeding, increased EE, and increased lipid oxidation. Nicotine treatment reduced AgRP and NPY and increased POMC expression in the ARC and decreased hypothalamic AMPK activation. Consistent with their hypophagic state, expression of orexigenic neuropeptides AgRP and NPY decreased and POMC expression increased ( Fig. 2A and B) in the ARC of nicotine-treated rats. No significant changes were detected in CART mRNA expression in the ARC (Fig. 2B). Nicotine treatment also reduced FAS mRNA expression (Fig. 2C) and activity (Fig. 2D) in the VMH. Recent data have demonstrated that inhibition of this enzyme in this hypothalamic nucleus induces anorexia (26). In keeping with the mRNA and activity data, FAS protein expression was reduced in the hypothalamus of nicotine-treated rats (Fig. 2E).
We next focused on other central components that could mediate the effects of nicotine on energy balance. Nicotine-treated rats showed a marked decrease in phosphorylation of hypothalamic AMPKa (pAMPKa) and its molecular target ACCa (pACCa) compared with vehicletreated rats (Fig. 2E). The reduction in pAMPKa levels was associated with unaltered protein concentrations of the nonphosphorylated isoforms of AMPKa1 and AMPKa2. ACCa levels were reduced in the hypothalamus of nicotinetreated rats (Fig. 2E). AMPK is regulated in a tissue-specific fashion, normally opposite between brain and peripheral tissues. For example, AMPK activation promotes fatty acid oxidation and glucose uptake in skeletal muscle, whereas conversely, hepatic AMPK activity inhibits fatty acid and cholesterol synthesis. In the hypothalamus AMPK activation elicits feeding and inhibits EE (8,11,(14)(15)(16)(17)(18). Thus, we analyzed the effect of nicotine on the AMPK pathway in liver and skeletal muscle. Our data show that 48-h nicotine treatment did not induce changes in this metabolic pathway either in liver ( Supplementary Fig. 1A) or muscle ( Supplementary Fig. 1B).
AMPK is activated by phosphorylation of upstream kinases. The main upstream AMPK kinases are the tumor suppressor liver kinase B1 (LKB1) and Ca 2+ /calmodulin-dependent protein kinase kinase a and b (CaMKKa and CaMKKb), which phosphorylate AMPK at Thr 172 (14,15). AMPK phosphorylation levels are also regulated by protein phosphatase-2Ca (PP2Ca) (14,15). Our data show that CaMKKb (but not CaMKKa) protein levels were reduced and PP2Ca increased in the hypothalamus of nicotine-treated rats (Fig. 2E), whereas no changes were detected in the phosphorylation levels of LKB1 (pLKB1; Fig. 2E). Nicotine administration increased BAT temperature and the expression of thermogenic markers in BAT. Recent evidence has linked inhibition of hypothalamic AMPK to activation of BAT thermogenesis (7,8). To further characterize the mechanisms mediating nicotine-induced negative energy balance, we examined the effects of nicotine on BAT temperature. Our data showed that nicotinetreated rats displayed a marked and significant increase in skin temperature surrounding interscapular BAT (Fig. 2F). In keeping with this effect, nicotine treatment for 48 h increased the expression of thermogenic markers such as UCP1 and peroxisome proliferator-activated receptor g coactivator 1a and -b in BAT (Fig. 2G). Nicotine action on hypothalamic AMPK pathway is mediated by a3b4-nicotinic acetylcholine receptors. Current data have demonstrated that nicotine-induced activation of hypothalamic a3b4-nicotinic acetylcholine receptors leads to activation of POMC neurons and decreased feeding (3). Therefore, we aimed to investigate whether these receptors were also mediating the actions of nicotine on AMPK. Our data show that cotreatment with mecamylamine, an antagonist of a3b4 receptors (3,30,31), blunted nicotine-induced weight loss (Fig. 3A) anorexia (Fig. 3B) and inhibition of AMPK (Fig. 3C). Overall, these results indicated that a3b4 receptors mediated the effects of nicotine on hypothalamic AMPK. Nicotine withdrawal restored energy balance, hypothalamic AMPK pathway, and the thermogenic program in BAT. We next investigated the effect of nicotine withdrawal on energy balance. First, we confirmed that chronic nicotine administration for 8 days resulted in marked negative energy balance, as demonstrated by reduced body weight (Fig. 4A), decreased feeding (Fig. 4B), and lowered fat mass (Fig. 4C; without affecting lean mass). At the same time we observed increased EE ( Fig. 4D; area under the curve: vehicle: 100 6 4.1; nicotine: 121 6 6.2; withdrawal: 103.9 6 2.1; P , 0.05 vehicle vs. nicotine; P , 0.05 nicotine vs. withdrawal), elevated LA (Fig. 4E), and decreased RQ (Fig. 4F). Withdrawal of nicotine treatment shifted all of the above parameters toward the level of vehicle-treated rats.
In keeping with restoration of normal feeding and EE, nicotine withdrawal also elevated the decreased hypothalamic protein levels of the AMPK pathway (Fig. 5A) and reversed the elevated mRNA expression of BAT thermogenic markers (Fig. 5B). Overall, these data suggest that nicotine withdrawal restored energy balance by normalizing feeding, EE/thermogenesis, and lipid mobilization, potentially via normalization of AMPK activity. Activation of hypothalamic AMPK reversed nicotineinduced negative energy balance. To establish the dependence of the actions of nicotine on AMPK, we investigated whether pharmacological or genetic activation of hypothalamic AMPK could reverse nicotine-induced effects on food intake and EE. First, intracerebroventricular administration of the AMPK activator AICAR reversed the hypophagia observed in rats treated with nicotine for 48 h (Fig. 6A). Second, we aimed to identify the hypothalamic nucleus where nicotine exerted its actions on AMPK. To elucidate the contribution of AMPK in the VMH to the effects of nicotine on neuropeptide expression and the thermogenesis in BAT, adenoviruses encoding either constitutively active AMPKa (AMPKa-CA) with GFP, or control virus expressing GFP alone (8,18), were injected stereotaxically into the VMH of nicotine-treated rats. Infection efficiency was assessed by 1) expression of GFP in the VMH, 2) increased hypothalamic protein concentration of pACCa, and 3) decreased hypothalamic malonyl-CoA levels (data not shown) (8,18). Overexpression of AMPKa-CA in the VMH was accompanied by increased body weight (Fig. 6B) and food intake (Fig. 6C) in nicotine-treated rats. AMPKa-CA administration also restored expression levels of AgRP, NPY, and POMC in the ARC (Fig. 6D and E) and mRNA expression of BAT thermogenic markers (Fig. 6F). These results are in agreement with the initial observation that nicotine impairs the function of AMPK and that this is associated with decreased orexigenic signaling (AgRP and NPY) and increased anorexigenic signaling (POMC) in the hypothalamus and reduced activation of the thermogenic program in BAT.

DISCUSSION
This study identifies a previously unknown link between nicotine-induced negative energy balance and hypothalamic AMPK. More specifically, the inactivation of hypothalamic AMPK caused by nicotine alters the physiological parameters of energy balance. This ultimately decreases feeding behavior, stimulates EE (by triggering both physical activity and thermogenesis in BAT), and drives an increase in lipid oxidation.
The current therapeutic arsenal to treat obesity is very limited. Besides lifestyle interventions focusing on diet, exercise, or behavioral changes (32,33) there is a paucity of available pharmacological treatments (34). For this reason, and given the urgency of the problem, we decided to focus on current situations where there is evidence of interventions leading to weight loss. One of these is smoking, such that there is no doubt that nicotine promotes weight loss and, conversely, those individuals who stop smoking gain weight. Extensive epidemiological studies have shown a strong relationship between smoking and body weight, with nonsmokers weighing more than smokers at any age (1,4,35,36). In keeping with this, nicotine replacement therapy and in particular the nicotine gum, has proved to be effective in delaying postcessation weight gain (1,37).
It is well known that nicotine, the main addictive constituent of tobacco, is the major component linking smoking to negative energy balance. Nicotine exerts anorectic actions and elicits weight loss in humans and rodents (1,2,4,35,36). Several hypotheses have been put forward. Data showing that the peripheral nicotine administration leads to increased expression of UCP1 in BAT (and white adipose tissue) were interpreted as a mechanism by which nicotine influences body weight homeostasis by acting at peripheral level in nonneural tissues (23,38). However, it is generally accepted that despite the many effects exerted by nicotine at peripheral level, the main effect of nicotine on feeding homeostasis is exerted at a central level. Data gleaned in this regard have led to the assumption that the main hypothalamic effects of nicotine are exerted on NPY (19)(20)(21)39,40) and POMC neurons (3,22) in the ARC. In light of the aforementioned findings we decided to focus our attention in the VMH, since data have involved this nucleus in the context of AMPK-driven biological effects (7,8,18). AMPK controls feeding by directly regulating neuropeptide expression in the ARC (16)(17)(18) and also regulates EE by modulating thermogenesis in BAT via the SNS (7,8,41). In this study we aimed to investigate whether nicotine-induced negative energy balance was mediated by specific modulation of the hypothalamic AMPK pathway. Our findings show that nicotine decreases body weight through its combined effects of hypophagia and increased thermogenesis in BAT, in addition to driving elevated locomotor activity and increased lipid mobilization (demonstrated by a lower RQ). In keeping with former reports (3,(19)(20)(21)(22)39,40), nicotine also decreased the expression of ARC orexigenic neuropeptides such as AgRP and NPY and increased POMC expression. Our data indicate that at the same dose that caused anorexia, nicotine inhibited the hypothalamic AMPK pathway. This effect, as well as the aforementioned actions, was reversed by nicotine cessation and antagonism of a3b4 nicotinic acetylcholine receptors, which have recently been demonstrated to be involved in the action of nicotine in the hypothalamus (3). Together, these results suggest the existence of a mechanistic link between hypothalamic AMPK function and nicotineinduced changes in energy homeostasis. To specifically address this hypothesis we investigated whether activation of hypothalamic AMPK could prevent the anorexigenic and thermogenic effects of nicotine. We first showed that intracerebroventricular administration of the AMPK activator AICAR reversed nicotine-induced anorexia. Secondly, overexpression of AMPKa-CA in the VMH reversed the anorectic and thermogenic effects observed in nicotinetreated rats. The VMH is considered as a key hypothalamic nucleus controlling both sides of the energy balance equation (feeding and EE) (41)(42)(43), and recent evidence has demonstrated that AMPK in this nucleus plays a major role in its ability to do so (8,18,41). In addition, our data demonstrate that genetic overactivation of AMPK in the VMH totally reversed the nicotine-induced anorexia and weight loss. AMPK-mediated normalization of energy balance was associated with normalized mRNA levels of AgRP, NPY, and POMC in the ARC, as well as reduced or restored levels of thermogenic markers in BAT increased previously by nicotine treatment. Overall, our results indicate that nicotine impairs AMPK function in the hypothalamus and that this effect is essential for the weight loss, hypophagia, and increased BAT-mediated energy dissipation. The mechanism connecting the nicotine-induced changes in the hypothalamus that stimulates the thermogenic program in the BAT is likely mediated via activation of the a3b4-nicotinic acetylcholine receptors (3) and through the SNS. This is supported by recent evidence showing that AMPK inhibition stimulates thermogenic program in the BAT through the SNS (7,8,41) and that nicotine increases BAT thermogenesis and UCP1 expression in rats through SNS activation (7,8,23,24,38,41).
Our data support the pathophysiological relevance of the hypothalamic AMPK-BAT axis in the control of energy balance (7,8,41) and demonstrate that the well-established effects of nicotine on body weight are mediated through this mechanism. These results also raise the question of its suitability as a therapeutic target for the treatment of human obesity. Up to now this was considered unlikely because of the well-known addictive actions of nicotine. Our data, however, show that its effects on increasing EE are mediated by a specific hypothalamic nuclei, the VMH (8). VMH neurons play a minor role in the rewarding properties of nicotine (31), indicating that by targeting this nicotineactivated pathway it may be feasible to preserve nicotine's effect on body weight without developing addiction. Further work addressing this issue is clearly merited, since it has also been demonstrated that peripheral AMPK is a suitable target for human disease using, for example, metformin and thiazolidinediones to improve insulin sensitivity (14,15). Current evidence has highlighted the importance of BAT in adult humans and its potential as an alternative to the existing strategies to solely target food intake (44)(45)(46)(47), which have seen limited success (34). Taken together, our data support drug-induced BAT activation as a robust strategy to promote weight loss.
In summary, we show for the first time that nicotine inactivates hypothalamic AMPK and this effect mediates decreased food intake and BAT activation, likely via the SNS (23,24,38). Whether nicotine administration or smoking activates the AMPK-BAT axis in humans remains to be determined, but it is well known that EE is increased in smokers and that this effect may be mediated in part by FIG. 5. Effects of nicotine withdrawal hypothalamic AMPK pathway and BAT thermogenic program. Western blot autoradiographic images (left panel) and hypothalamic protein levels of the different proteins of the AMPK pathway (right panel) (A) and thermogenic markers (B) in the BAT of vehicle, nicotine, and nicotine withdrawal rats. Dividing lines show spliced bands. *P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle; ##P < 0.01, ###P < 0.001 nicotine vs. withdrawal. HPRT, hypoxanthine-guanine phosphoribosyltransferase; PGC1, peroxisome proliferator-activated receptor g coactivator 1. All data are expressed as mean 6 SEM.
FIG. 6. Effects of hypothalamic AMPK activation on nicotine actions on energy balance, neuropeptides, and BAT thermogenic program. A: Food intake in rats treated with nicotine and the AMPK activator AICAR. Body weight change (B); daily food intake (C); in situ hybridization autoradiographic images (D); AgRP, NPY, and POMC mRNA levels in the ARC (E); and thermogenic markers (F) in the BAT of rats treated with vehicle or nicotine and stereotaxically treated with GFP-expressing adenoviruses or GFP plus AMPK constitutively active (AMPKa-CA) adenoviruses are shown. *P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle or vehicle GFP; #P < 0.05, ##P < 0.01, ###P < 0.001 nicotine vehicle vs. nicotine AICAR or nicotine GFP vs. nicotine AMPKa-CA. 3V, third ventricle; PGC1, peroxisome proliferator-activated receptor g coactivator 1; HPRT, hypoxanthineguanine phosphoribosyltransferase. All data are expressed as mean 6 SEM. the SNS (48). Thus, it is tempting to speculate that negative energy balance induced by cigarette smoking may be mediated by the specific activation of the hypothalamic AMPK-BAT axis, a possibility that will require further investigation. Although nicotine is not considered the most harmful component of cigarette smoke, and in fact has been reported to have potentially beneficial effects on mental health (49,50), our results should not be construed as supporting continued tobacco use as a treatment option. It is noteworthy that our observation provides new insights into the molecular actions of nicotine and suggest that evaluating candidate drugs modulating the hypothalamic AMPK-BAT axis (7,8,41) may provide a more targeted approach to develop new therapeutic targets. These targets may be relevant not just for the treatment of obesity, but also for smoking cessation in humans.