OBJECTIVE—Cannabinoid type 1 (CB1) receptor blockade decreases body weight and adiposity in obese subjects; however, the underlying mechanism is not yet fully understood. Nitric oxide (NO) produced by endothelial NO synthase (eNOS) induces mitochondrial biogenesis and function in adipocytes. This study was undertaken to test whether CB1 receptor blockade increases the espression of eNOS and mitochondrial biogenesis in white adipocytes.
RESEARCH DESIGN AND METHODS—We examined the effects on eNOS and mitochondrial biogenesis of selective pharmacological blockade of CB1 receptors by SR141716 (rimonabant) in mouse primary white adipocytes. We also examined eNOS expression and mitochondrial biogenesis in white adipose tissue (WAT) and isolated mature white adipocytes of CB1 receptor–deficient (CB1−/−) and chronically SR141716-treated mice on either a standard or high-fat diet.
RESULTS—SR141716 treatment increased eNOS expression in cultured white adipocytes. Moreover, SR141716 increased mitochondrial DNA amount, mRNA levels of genes involved in mitochondrial biogenesis, and mitochondrial mass and function through eNOS induction, as demonstrated by reversal of SR141716 effects by small interfering RNA–mediated decrease in eNOS. While high-fat diet–fed wild-type mice showed reduced eNOS expression and mitochondrial biogenesis in WAT and isolated mature white adipocytes, genetic CB1 receptor deletion or chronic treatment with SR141716 restored these parameters to the levels observed in wild-type mice on the standard diet, an effect linked to the prevention of adiposity and body weight increase.
CONCLUSIONS—CB1 receptor blockade increases mitochondrial biogenesis in white adipocytes by inducing the expression of eNOS. This is linked to the prevention of high-fat diet–induced fat accumulation, without concomitant changes in food intake.
Adipose tissue is not merely an energy store but rather an endocrine organ playing an important role in fuel metabolism (1,2). Recent studies have demonstrated that mitochondrial biogenesis increases during adipocyte differentiation (3), and this is presumably due at least in part to the presence of the endothelial nitric oxide (NO) synthase (eNOS). In fact, eNOS protein is not expressed in either undifferentiated 3T3-L1 cells or preadipocytes, whereas its expression markedly increases with adipocyte differentiation (4). It is noteworthy that NO increases mitochondrial biogenesis in white adipocytes via cyclic guanosine monophosphate (cGMP)-dependent pathways, including peroxisome proliferator–activated receptor γ coactivator-1α (PGC-1α) gene expression (5,6). Consistently, mitochondrial biogenesis is reduced in fat of eNOS-null mutant mice (eNOS−/−), with decreased energy expenditure and increased body weight (5,6). This suggests that the eNOS-dependent mitochondrial biogenesis is relevant to adipocyte maturation and fuel metabolism.
Cannabinoid type 1 (CB1) receptors participate in the physiological modulation of many central and peripheral functions related to the control of energy metabolism (7). They are the most abundant G protein–coupled receptors expressed in the brain, where CB1 receptor activation promotes feeding and modulates the rewarding properties of food (7–9). CB1 receptors also control metabolic functions by acting on peripheral organs, including white adipose tissue (WAT) (7–9). Notably, in adipocytes, CB1 receptor expression increases with cell differentiation (10,11). Moreover, CB1 receptor–deficient mice (CB1−/−) are lean (12) and resistant to a high-fat diet (13). Similarly, the selective CB1 receptor antagonist SR141716 (rimonabant) persistently reduces body weight during the treatment of obese animals, although food intake renormalizes after an initial 1- to 2-week weight reduction (14,15), suggesting that CB1 receptor blockade stimulates fat metabolism, leading to a decreased fat content.
We now demonstrate a prominent role for the CB1 receptors to modulate eNOS gene expression and mitochondrial biogenesis in white adipocytes. Collectively, these data suggest that targeting the endocannabinoid system improves mitochondrial function, implying a novel, unrecognized mechanism of action for the antiobesity drug rimonabant.
RESEARCH DESIGN AND METHODS
White fat precursor cells were enzymatically isolated from epididymal WAT of wild-type C57BL/6J mice and cultured as described (16). SR141716 [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide, hydrochloride, contract no. N01-MH-32005 WA 08928.019, National Institute of Mental Health no. S-705, Chemical Synthesis and Drug Supply Program, National Institute of Mental Health; RTI)] stock solutions (50 mmol/l in DMSO) were diluted in the culture medium to the required final concentrations containing up to 0.02% DMSO. At days 3–4 from seeding, cells were exposed to the culture medium containing SR141716 (0.1–10 μmol/l) for 72 h or different periods of time when specified. The medium was discarded, the wells were washed twice with 2 ml ice-cold PBS (Sigma-Aldrich), and the cells were harvested as reported below for the different assays.
eNOS small interfering RNA.
White adipocytes were transfected at days 3–4 after seeding with 100 nmol/l eNOS small interfering RNA (siRNA) SMARTpool (Dharmacon, Lafayette, CO) or siCONTROL nontargeting siRNA using Dharmafect 3 transfection reagent. Cells were treated for 72 h with 1 μmol/l SR141716, and RNA and protein were harvested. Efficacy of transfection was determined using siGLO-RISC–free nontargeting siRNA and estimation of siRNA uptake by fluorescence detection (absorbance/emission 557/570 nm).
Animals and treatments.
Animals were treated according to protocols approved by Milan and Bologna University institutional animal care and use committee. Eight-week-old male CB1−/− mice (17) and their wild-type littermates (n = 10 per group) were fed either a standard mouse diet (8% kcal fat, 19% kcal protein, and 73% kcal carbohydrate) or a high-fat diet (D12492, 60% kcal fat, 20% kcal protein, and 20% kcal carbohydrate; Research Diets, New Brunswick, NJ) for 12 weeks. Moreover, 4-week-old male C57BL/6J mice (Harlan Nossan) were fed either a standard diet or high-fat diet for 6 weeks before treatments. While fed on the standard or high-fat diet, these mice (n = 10 per group) were further treated with either vehicle or SR141716 (at 10 mg/kg orally in distilled water with 0.1% Tween 80) for 11 weeks. Body weight and food intake were recorded weekly. Adiposity (wet weight of visceral and subcutaneous fat), cumulative food intake (for the time of the experiment), and feed efficiency were measured (5). On the day of the experiments, animals were killed by cervical dislocation and epididymal WAT was immediately isolated, frozen in liquid nitrogen, and stored at −80°C before processing for mRNA, protein, and mtDNA analysis or citrate synthase and AMP-activated protein kinase (AMPK) activity.
Isolation of mouse mature adipocytes.
Wild-type and CB1−/− mice were fed a standard or high-fat diet (n = 6 per group), and wild-type mice either on standard or high-fat diet were treated with either vehicle or SR141716 (n = 6 per group) as described above. Mice were killed and epididymal WAT excised. Fat pads from two mice were pooled, and mature adipocytes were acutely isolated in Hank's balanced salt solution containing 4% BSA and 1.5 mg/ml collagenase (Calbiochem) as described (18).
RNA was isolated from tissue or cells using the RNeasy Lipid Tissue Mini Kit (Qiagen). cDNA was synthesized using iScript cDNA Synthesis Kit (Bio-Rad Laboratories). Primers were designed using Beacon Designer 2.6 software from Premier Biosoft International. Triplicate PCR were carried out with the intercalating dye SybrGreen as previously described (5,16). PCR cycles were programmed on an iCycler iQ Real-Time PCR detection system (Bio-Rad Laboratories). The cycle number at which the various transcripts were detectable (threshold cycle [CT]) was compared with that of β-actin, referred to as ΔCT. The gene relative level was expressed as 2−(ΔΔCT), in which ΔΔCT equals ΔCT of either the knockout or drug-treated mice (or treated cells) minus ΔCT of the control mice (or untreated cells).
Protein extracts were obtained by harvesting cultured adipocytes in M-PER Mammalian Protein Extraction Reagent or homogenizing WAT tissue in T-PER Tissue Protein Extraction Reagent (both from Pierce), as indicated by the manufacturer, in the presence of 1 mmol/l NaVO4, 10 mmol/l NaF, and a cocktail of protease inhibitors (Sigma-Aldrich). Protein content was determined by the bicinchoninic acid protein assay (Pierce), and 50 μg of proteins were run on SDS-PAGE under reducing conditions. The separated proteins were then electrophoretically transferred to a nitrocellulose membrane (Pierce). Proteins of interest were revealed with specific antibodies: anti-eNOS (Transduction Labs), anti–cytochrome c oxidase (COX IV) (Molecular Probes), anti–cytochrome c (BD Bioscience), each one at 1:500 dilution; anti–phospho-AMPKα (Thr172) and anti-AMPKα (both at 1:1,000 dilution; Cell Signaling Technology); and anti–β-actin (1:10,000; Sigma-Aldrich). The immunostaining was detected using horseradish peroxidase–conjugated anti-rabbit or anti-mouse immunoglobulin for 1 h at room temperature. After the visualization of phospho-AMPKα, filters were stripped with the Re-Blot Western blot recycling kit (Chemicon International) and further used for the visualization of total AMPKα. Bands were revealed by the SuperSignal Substrate (Pierce) and quantitated by densitometry using a Quick Image densitometer (Canberra Packard) and a Phoretix 1D, version 3.0, software image analyser.
Fluorescence-activated cell sorting analysis.
White adipocytes were treated or not with 1 μmol/l SR141716 for 72 h. Cells (∼1 × 106 per sample) were harvested by trypsinization, fixed with 4% paraformadehyde in PBS for 10 min, and incubated 1 h at room temperature in a permeabilization buffer containing 1% BSA, 0.1% saponin, and either anti–COX IV or anti–cytochrome c antibody. After incubation with labeled secondary antibody, the fluorescence was analyzed by flow cytometry (FACScan; Becton Dickinson, Sunnyvale, CA) and Cell Quest software (Becton Dickinson). Data are expressed as relative fluorescence intensity of the mean fluorescent signal versus unstained samples.
Mitochondria were isolated from tissue and cells (19). The mitochondrial DNA (mtDNA) was extracted and analyzed as described (5,16). Briefly, an aliquot of mtDNA was loaded on ethidium bromide–stained agarose gel (1.2%) and quantified using the QuickImage densitometer (Packard). To determine mtDNA levels, the signal intensities were normalized to cell numbers in experiments on cultured cells or to protein content in experiments using tissues from wild-type and knockout mice. Alternatively, mtDNA copy number was measured by means of quantitative PCR from the cytochrome C oxidase II mtDNA gene compared with the uncoupled protein-2 nuclear gene in WAT of CB1−/− and wild-type mice as described (20).
White adipocytes were harvested by trypsinization, centrifuged at 500g for 5 min at 4°C, and resuspended in Hank's balanced salt solution. Cell samples (4 × 106/ml) were analyzed at 37°C in a gas-tight vessel equipped with a Clark-type oxygen electrode (Rank Brothers) connected to a chart recorder. Cellular oxygen consumption was measured as described (5,16). The oxygen electrode was calibrated, assuming the concentration of oxygen in the incubation medium at 37°C to be 200 μmol/l. Protein content in both cell and tissue samples was determined by the bicinchoninic acid protein assay (Pierce).
The whole amount of ATP in cells was measured by using the ATP determination kit from Molecular Probes. Cells were harvested by trypsinization and centrifuged at 500g for 5 min at 4°C. ATP was extracted by incubating cell pellets with 1% trichloroacetic acid per 4 mmol/l EDTA solution for 10 min on ice. Cell extracts were centrifuged at 12,000g for 10 min at 4°C and used for ATP determination as indicated by the manufacturer.
White adipocytes seeded onto glass coverslips were incubated with medium containing 200 nmol/l Mitotracker red 580 (Invitrogen). After incubation for 30 min at 37°C, coverslips were rinsed two times with medium and two times with PBS and fixed at room temperature for 10 min in 4% paraformaldehyde in PBS. Coverslips were mounted onto glass slides and were then observed with a Bio-Rad confocal microscope MRC1024.
Citrate synthase activity.
The activity was measured spectrophotometrically at 412 nm at 30°C in either tissue or whole-cell extracts (21). Tissue or cell homogenates were added to buffer containing 0.1 mmol/l 5,5-dithio-bis-(2-nitrobenzoic) acid, 0.5 mmol/l oxaloacetate, 50 μmol/l EDTA, 0.31 mmol/l acetyl CoA, 5 mmol/l triethanolamine hydrochloride, and 0.1 mol/l Tris-HCl, pH 8.1. Citrate synthase activity was expressed as nanomoles citrate produced per minute per milligram of protein.
Adipocytes were scraped in ice-cold lysis buffer (22) in the presence of 1 mmol/l NaVO4, 10 mmol/l NaF, and protease inhibitors (Sigma-Aldrich). Epididymal fat pads from wild-type or CB1−/− mice were homogenized in ice-cold lysis buffer. AMPK activity was determined on protein samples immunoprecipitated with anti-AMPKα (Cell Signaling Technology) as described (22). The immunocomplexes were collected and washed with AMPK reaction buffer (22). AMPK immunoprecipitates were incubated with 20 μl of AMPK reaction buffer, 5 μl of SAMS substrate peptide (1 mmol/l), and 2 μCi of [γ-32P]ATP (MP Biomedicals) for 30 min at 37°C. At the end of the incubation, an aliquot was removed and spotted onto Whatman P81 paper. The P81 paper was washed three times with 0.75% (vol/vol) phosphoric acid and once with acetone, and radioactivity was determined by scintillation counting.
Raw data from each experiment were analyzed using either an ANOVA with Newman-Keuls’ multiple comparison post hoc test or t test.
CB1 receptor antagonist increases eNOS expression and mitochondrial biogenesis in white adipocytes.
CB1 receptor antagonist SR141716 affects eNOS expression in mouse white adipocytes in culture. In fact, the drug significantly increased eNOS mRNA (Fig. 1A) and protein levels (Fig. 1B) in a dose-dependent manner (0.1–10 μmol/l for 72 h).
The effect of SR141716 was further tested on several markers of mitochondrial biogenesis. The amount of mtDNA (a marker of mitochondrial content) and the mRNA levels of PGC-1α, nuclear respiratory factor-1 (NRF-1), and mtDNA transcription factor A (Tfam) (three master regulators of mitochondrial biogenesis) (23,24) were higher in 1 μmol/l SR141716-treated than in vehicle-treated cells (Fig. 2A). Protein levels of COX IV and cytochrome c (two mitochondrial proteins involved in oxidative phosphorylation) were also increased by SR141716 treatment as assessed either by immunoblot (Fig. 2B) or by flow cytometry (mean fluorescence intensity ± SE of SR141716- vs. vehicle-treated cells: 20.1 ± 3.4 vs. 9.6 ± 2.3 for COX IV and 15.6 ± 3.2 vs. 9.9 ± 1.9 for cytochrome c; n = 3 experiments; P < 0.001). Both activity of citrate synthase and oxygen consumption were markedly increased by SR141716 treatment (Fig. 2C and D). Consistently, the ATP levels in the SR141716-treated cells were significantly higher than those in the vehicle-treated cells (1.73 ± 0.19 vs. 1.34 ± 0.16; n = 3 experiments, P < 0.05). Finally, white adipocytes treated with SR141716 showed a significant increase of Mitotracker red signal, indicating the increase of mitochondrial mass compared with vehicle-treated cells (Fig. 2E). Altogether, these findings demonstrate that pharmacological blockade of the CB1 receptor increases eNOS expression and mitochondrial biogenesis and function in white adipocytes.
SR141716 effects on mitochondrial biogenesis are eNOS dependent.
Coregulation of eNOS expression and mitochondrial biogenesis in the SR141716-treated adipocytes suggests that these processes might be mechanistically linked. To assess whether eNOS expression is relevant in the drug-induced mitochondrial biogenesis, white adipocytes were transfected with siRNA against eNOS or a scrambled RNA sequence. siRNA against eNOS reduced basal eNOS mRNA and protein levels by 82 and 85%, respectively, in WAT cells (Fig. 1C and D), with a moderate, but statistically significant, decrease of all measured parameters of mitochondrial biogenesis in vehicle-treated cells (Fig. 2A–E). Importantly, however, siRNA against eNOS strongly decreased mitochondrial biogenesis induced by 1 μmol/l SR141716 treatment, indicating that the effect of SR141716 depends, at least in part, on the presence of eNOS (Fig. 2A–E).
Mitochondrial biogenesis is increased in WAT of CB1−/− mice.
To assess whether the CB1 receptor blockade increases mitochondrial biogenesis in WAT in vivo, we examined the eNOS expression and markers of mitochondrial biogenesis in epididymal fat pads derived from 8-week-old male CB1−/− mice and their wild-type littermates either on a standard or high-fat diet for 12 weeks. As expected (16), a high-fat diet reduced eNOS expression and mitochondrial biogenesis in wild-type mice (Fig. 3A–D). eNOS mRNA levels in WAT of CB1−/− mice were significantly higher than those of wild-type mice on both the standard and high-fat diets (Fig. 3A). Furthermore, CB1−/− mice displayed increased mRNA levels of PGC-1α and Tfam (Fig. 3A), COX IV, and cytochrome c (Fig. 3C); increased mtDNA amounts (Fig. 3B); and increased citrate synthase activity (Fig. 3D) compared with wild-type littermates. These increments were evident in animals either on standard or high-fat diets. To confirm that the mitochondrial biogenesis induced by CB1 gene deletion occurs in white adipocytes in vivo, mature white adipocytes were isolated from the epididymal fat pad of CB1−/− and wild-type littermates after standard or high-fat diets. Although we do not rule out that other cell types present in whole WAT (i.e., endothelial cells, macrophages, or others) may be involved, we found that CB1 gene deletion increases eNOS expression and mitochondrial biogenesis and function in white adipocytes (online appendix Fig. 1 [available at http://dx.doi.org/10.2337/db07-1623]).
As previously described (12), CB1−/− mice on the standard diet had a lower body weight and adiposity (online appendix Fig. 2A) than wild-type controls, having showed a significantly reduced body weight compared with wild-type mice throughout the whole period of observation (data not shown). Most importantly, CB1−/− mice maintained a leaner phenotype under the high-fat diet as evidenced by lower body weight and adiposity (online appendix Fig. 2A and B) (13). Cumulative food intake was comparable between the two groups of mice on standard or high-fat diet (online appendix Fig. 2C), although CB1−/− mice showed a smaller feed efficiency (weight gain/food intake, an indirect measurement of increased energy expenditure) than their wild-type littermates (online appendix Fig. 2D).
Defective mitochondrial biogenesis is restored in WAT of high-fat diet–fed SR141716-treated mice.
CB1 receptor effects on mitochondrial biogenesis were further confirmed in vivo by a long-term treatment with the CB1 receptor antagonist. Four-week-old male mice were put on either standard or high-fat diet. Starting at week 10, standard diet–and high-fat diet–fed mice were treated either with SR141716 (10 mg · kg−1 day−1 orally) or vehicle for 11 weeks (13–15). Consistent with the data from CB1−/− mice, eNOS mRNA and protein levels and mitochondrial biogenesis were significantly higher in WAT of the SR141716-treated high-fat diet–fed mice than in those of vehicle-treated high-fat diet–fed mice, although as expected (16), they were lower in the vehicle-treated high-fat diet–fed mice than in the vehicle-treated standard diet–fed mice (Fig. 4). These results suggest that SR141716 treatment amends the high-fat diet–induced impairment of the NO generating system, restoring the mitochondrial function at levels observed in wild-type mice on a standard diet. Treatment with SR141716 induced only a slight, but not statistically significant, increase of eNOS expression and mitochondrial biogenesis in mice on the standard diet (Fig. 4). Superimposable results were obtained in isolated mature white adipocytes from SR141716-treated mice (online appendix Fig. 3).
Interestingly, the mean body weight and adiposity of the SR141716-treated mice were markedly lower compared with that of vehicle-treated mice on the high-fat diet (online appendix Fig. 4A and B). Cumulative food intake was comparable between the drug- and vehicle-treated animals on a high-fat diet (online appendix Fig. 4C), whereas SR141716-treated mice showed a reduced feed efficiency compared with their vehicle-treated high-fat diet–fed controls (online appendix Fig. 4D). Only slight, but not statistically significant, decreases in body weight and adiposity were seen in the SR141716-treated mice on a standard diet (online appendix Fig. 4A and B).
CB1 receptor blockade stimulates AMPK phosphorylation and activity.
The AMPK cascade has been described to activate eNOS and NO production (25). Thus, AMPK might be involved in the effect of CB1 receptor blockade on eNOS-mediated mitochondrial biogenesis. In fact, treatment of cultured white adipocytes with 1 μmol/l SR141716 induced a rapid and persistent AMPK phosphorylation (Fig. 5A) and significantly increased AMPK activity (Fig. 5B). Moreover, in line with previous reports (26), we found that AMPK activity was lower in the WAT of wild-type mice fed a high-fat diet compared with those fed a standard diet. Noticeably, while CB1 receptor deletion did not affect AMPK phosphorylation or activity in WAT of standard diet–fed mice (Fig. 5C and D), both AMPK phosphorylation (Fig. 5C) and activity (Fig. 5D) were normalized in the WAT of high-fat diet–fed CB1−/− mice, up to the levels measured in wild-type standard diet–fed mice.
Taken together, these findings demonstrate that the CB1 receptor blockade in vivo concomitantly activates AMPK cascade and upregulates eNOS expression and mitochondrial biogenesis, which are impaired by a high-fat diet in WAT, and these processes may be involved in the control of adiposity and body weight of animals on a high-fat diet.
The present study demonstrates that genetic and pharmacological blockade of the cannabinoid CB1 receptor increases mitochondrial biogenesis in white adipocytes. Treatment of adipocytes with SR141716, a selective antagonist of CB1 receptors (14,15,27), increases mitochondrial biogenesis genes, including PGC-1α and Tfam. This possibly implies that SR141716 is antagonizing an endocannabinoid tone present in cultured fat cells. It has been recently reported by us and others (28,29) that rodent and human adipocytes produce and release endocannabinoids, including 2-arachidonoylglycerol, anandamide, and other cannabimimetic compounds. Thus, our results suggest that SR141716 increases mitochondrial biogenesis by antagonizing the endocannabinoids produced in cell culture. In turn, this strongly implies that the CB1 receptor agonists might downregulate the eNOS-dependent mitochondrial biogenesis in white adipocytes. Consistently, cannabinoids, including Δ(9)-tetrahydrocannabinol, inhibit the respiratory enzyme system in brain, liver, skeletal muscle, and heart (30) and, consequently, reduce the rates of oxygen consumption in rodents (31,32).
We have recently found that eNOS-dependent NO production induces mitochondrial biogenesis in different cell types, including white adipocytes, by cGMP-dependent pathways and that the newly synthesized mitochondria are functionally active, ensuing high oxygen consumption and ATP production (5,6,33). This suggests that the SR141716-dependent increase in eNOS expression is mechanistically linked to the mitochondrial biogenesis induced by the CB1 receptor blockade. This was confirmed by the observation that gene silencing of eNOS by RNAi markedly impairs the SR141716 stimulatory effects on mitochondrial biogenesis and function in cultured adipose cells.
We observed that genetic CB1 receptor blockade in vivo increases eNOS expression and mitochondrial biogenesis both in whole WAT and isolated mature white adipocytes, and this is accompanied by prevention of high-fat diet–induced fat accumulation. Most importantly, the pharmacological CB1 receptor blockade fully counteracts the downregulation of eNOS-dependent mitochondrial biogenesis due to a high-fat diet in mice. This might indicate a novel, previously unrecognized mechanism of action of SR141716 as an antiobesity drug. Not only would it transiently suppress food intake (34,35) and reduce lipogenesis (12), but it also would increase the oxidative metabolism in white adipocytes by counteracting the inhibitory effects of endocannabinoids, whose levels are increased in fat tissues of obese rodents and humans (28). Thus, we propose that the overactivity of endocannabinoids might contribute to downregulate mitochondrial biogenesis in white adipocytes of obese mice and that the blockade of adipose CB1 receptor might revert this process, thus preventing the high-fat diet–induced increase in adiposity and body weight gain.
There is an open question regarding the mechanisms that link CB1 receptor blockade to the eNOS expression. Furthermore, it remains to be clarified how the recovery of mitochondrial biogenesis and function in WAT could counteract the development of obesity. We found that SR141716 enhances AMPK activity in cultured white adipocytes and that the WAT AMPK activity, which is reduced by a high-fat diet, is normalized in WAT from high-fat diet–fed CB1−/− mice to the levels measured in wild-type standard-diet–fed mice. Since AMPK is known to enhance NO production by activating eNOS (25), the AMPK cascade might operate upstream of the NO generating system, possibly contributing to the CB1 blockade–mediated mitochondrial biogenesis. Accordingly, induction of AMPK activity has been found to increase mitochondrial content in adipocytes (36,37). Thus, the enhanced β-oxidation of free fatty acids elicited by AMPK activation (36,38) and by eNOS-dependent mitochondrial biogenesis (39) might functionally link SR141716 treatment to its antiobesity effects.
The mechanisms that link eNOS-derived NO to the gene expression machinery involved in mitochondrial biogenesis also remain to be elucidated. Studies of eNOS−/− mice have demonstrated an obligatory role of eNOS in mitochondrial biogenesis (5,6). Noticeably, eNOS, NO, and its intracellular messenger cGMP are located in both the cytoplasm and the nucleus, and several transcription factors possess heme moieties that can bind NO (6), but further investigation is needed to understand whether eNOS-mediated responses involve the nuclear NO/cGMP.
It is now widely accepted that obesity is causally linked to a chronic, low-grade inflammation of visceral adipose tissue, characterized by infiltrating macrophages, abnormal cytokine production, and activation of a network of inflammatory signaling pathways, including tumor necrosis factor-α and inducible NOS (iNOS) (40,41). It has been observed that upregulation of iNOS (which is induced in inflammatory conditions) often correlates with a downregulation of eNOS (16). Corroborating this, tumor necrosis factor-α increases iNOS expression in different cells and tissues, including fat (42) and inhibits eNOS expression, mitochondrial function, and energy production in WAT (16). Interestingly, SR141716 has been shown to reduce plasma tumor necrosis factor-α levels in inflammatory conditions (43). Thus, we can speculate that the anti-inflammatory effects of SR141716 could help counteracting the increase of iNOS expression and the reduction of eNOS expression and mitochondrial biogenesis in high-fat diet–induced obesity.
Endogenous cannabinoids and CB1 agonists promote vasodilation and decrease blood pressure, particularly in rodent models of hypertension (44,45). A variety of putative mechanisms could explain these effects, including the increased production of vasodilator NO in vessels. Although we have not analyzed the possible effects of SR141716 on eNOS expression in the vasculature, a recent analysis of the impact of rimonabant on blood pressure in the pooled population from four large trials with similar design (the Rimonabant-In-Obesity Program) demonstrates that there is no effect of rimonabant on blood pressure beyond the modest reduction mediated by weight loss (46).
In conclusion, our results demonstrate that mitochondrial biogenesis and function, which are impaired by a high-fat diet, are restored to normal levels in white adipocytes by the CB1 receptor antagonist SR141716, and this is accompanied by prevention of adiposity and body weight increase in high-fat–fed obese mice. In view of the present findings, targeting eNOS expression and mitochondrial biogenesis might offer therapeutic gains and simplify the drug approach to clinical situations as complex as obesity and related disorders.
Published ahead of print at http://diabetes.diabetesjournals.org on 13 May 2008.
U.P. has been a consultant for, has served on an advisory board for, and has received honoraria from sanofi-aventis. R.P. has received honoraria from sanofi-aventis.
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This work was supported by grants from the Ministero della Salute (to E.N. and M.O.C.); the Cofinanziamento 2005 Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale from the Ministero dell'Istruzione, dell'Università e della Ricerca (to E.N., MOC., and R.V.); the Diabesity EC-FP6 (contract no. LSHM-CT-2003-503041) (to U.P.); Fondazione Cassa di Risparmio, Bologna, Italy (to C.C., R.P., and U.P.); Avenir Program of Institut National de la Santé et de la Recherche Médicale, in partnership with the Fondation Bettencourt Schueller (to G.M.); and the European Federation Studies of Diabetis (EFSD/Sanofi-Aventis European Programme) (to G.M. and B.L.).
We thank Clara De Palma for fluorescence-activated cell sorting analysis and Emilio Clementi for oxygen consumption measurements.