Oleoylethanolamide enhances β-adrenergic-mediated thermogenesis and white-to-brown adipocyte phenotype in epididymal white adipose tissue in rat

Despite initial expectations, data obtained over the last two decades have shown an almost complete failure of traditional pharmacological-based monotherapies for the treatment of obesity. To try to overcome this situation, the attention is now being focused on the development of polytherapies that, by exerting their effects in different biological process such as food intake and energy expenditure, will have a greater chance of success.

β3-adrenergic receptor, a G-protein-coupled receptor abundantly expressed in adipose tissues, is a potent target for anti-obesity and anti-diabetic drug therapy. Mice lacking the β-adrenergic receptors develop massive obesity (Bachman et al., 2002), whereas β3-adrenergic agonists elicit potent effects on energy homeostasis by suppressing food intake, and promoting body fat loss, lipid oxidation, oxygen consumption and mitochondrial biogenesis, as well as improving insulin sensitivity and glucose tolerance (Arch and Wilson, 1996; Granneman et al., 2005; Lowell and Spiegelman, 2000; White et al., 2004). Specifically, β3-adrenergic activation stimulates the expression of uncoupling protein-1 (UCP1), a mitochondrial molecule involved in cold-induced nonshivering thermogenesis as well as diet-induced thermogenesis, by uncoupling the respiratory chain in specific adipocytes in both brown (BAT) and white (WAT) adipose tissues (Dallner et al., 2006; Himms-Hagen et al., 2000; Klein et al., 2000; Lowell and Spiegelman, 2000). Thus, chronic β3-adrenergic activation in WAT induces catecholamine-mediated lipolysis, reduces leptin expression in WAT and leptin levels in plasma (Kumar et al., 1999), and elevates adiponectin expression in WAT and adiponectin levels in plasma (Zhang et al., 2002).

On the basis of this pharmacological profile, several clinical trials tried to develop anti-obesity strategies based on β3-adrenergic agonists. The lack of effectiveness in human obesity or the induction of cardiac effects due to the lack of selectivity towards β3-adrenergic receptors led to the withdrawal of these therapies. However, a potentiation of the β3-adrenergic-agonist-induced anti-obesity actions by combinational therapy has now appeared as a new strategy to rescue this type of therapy. As an example, the inhibition of mitogen-activated protein kinase (MAPK) potentiates the effect of β3-adrenergic activation on UCP1-mediated energy dissipation in BAT of fatty acids released from WAT (Inokuma et al., 2006; Klein et al., 2000). In this regard, the stimulation of the transcription factor peroxisome proliferator-activated receptor (PPAR)α, a modulator of appetite and lipid metabolism (Rodríguez de Fonseca et al., 2001), by the natural ligand oleoylethanolamide inhibits insulin-receptor-dependent stimulation of MAPK pathways, providing a frame for a combinational therapy with β3-adrenergic receptor agonists (Martínez de Ubago et al., 2009).

In the absence of β3-adrenergic stimulation, differentiation of adipocyte progenitors into white adipocytes in the WAT was dramatically promoted by high-fat feeding (Lee et al., 2012). In contrast, cold exposure or pharmacological activation of β3-adrenergic receptors induces the appearance of brown fat-like (‘brite’) adipocytes in WAT (Cousin et al., 1992; Ishibashi and Seale, 2010; Petrovic et al., 2010), suggesting a mechanism for adipocyte progenitors to promote WAT remodelling. Several data indicate that this process occurs as the result of de novo differentiation of stem cells or committed brown preadipocytes (Macotela et al., 2012), or through direct transformation of adult cells via physiological reversible transdifferentiation, which involves genetic reprogramming and tissue reorganization with changes in the density of capillaries and parenchymal nerve fibers (Granneman et al., 2005; Himms-Hagen et al., 2000; Murano et al., 2009). Several transcription factors and coregulators, including PRD1-BF1-RIZ1 homologous domain containing 16 (PRDM16), fibroblast growth factor (FGF)-21, PPARγ coactivator 1α (PGC-1α) and prostaglandins, have been proposed to induce a BAT-specific gene expression profile in response to adrenergic stimulation (Seale et al., 2007; Seale et al., 2008; Seale et al., 2011; Vegiopoulos et al., 2010). Thus, PRDM16 has been identified as a transcriptional coactivator responsible for determining the BAT lineage and has been reported to promote the induction of the thermogenic programme in subcutaneous WAT (Seale et al., 2011). The expression of fibroblast growth factor 21 (FGF21), promoted by β3-adrenergic activation via a p38 MAPK mechanism in BAT, induces higher energy expenditure and body temperature, and triggers a lowering of glucose and triglyceride levels, improved insulin sensitization and enrichment of brown adipocytes (Coskun et al., 2008; Hondares et al., 2011a; Kharitonenkov et al., 2005). Moreover, PPARα is upregulated by the β3-adrenergic receptor agonist CL316243 (Li et al., 2005) and is a critical regulator of thermogenic key components, including PRDM16, FGF21, PGC-1α and UCP1, in BAT (Badman et al., 2007; Chartoumpekis et al., 2011; Hondares et al., 2011a; Hondares et al., 2011b; Villarroya et al., 2007). Thus, PPARα and/or PGC-1α target genes are involved in fatty-acid catabolism, including cellular uptake and mitochondrial and peroxisomal β-oxidation (Mandard et al., 2004), as well as suppression of proinflammatory signalling in adipose tissue (Li et al., 2005).

In the present study, we analyze the potentiation of feeding inhibition, weight loss, thermogenesis and lipid oxidation observed when β3-adrenergic and PPARα receptors are coactivated by the selective adrenergic agonist CL316243 and the natural PPARα receptor ligand OEA, respectively. Because the appearance of ‘brite’ adipocytes within WAT depots is associated with improved metabolic phenotypes, we also explored the expression of mitochondrial (Cox4i1, Cox4i2) and BAT (Prdm16, Fgf21) factors in epididymal WAT (eWAT) as markers of white-to-brown metaplasia.

The acute administration of CL316243, BRL37344 and ICI215,001 at 0.1, 0.5 and 1 mg/kg body weight induced significant dose-response reductions of cumulative food intake over 24 hours in rats that had been previously food-deprived for 24 hours (supplementary material Fig. S1A–C). The acute administration of ZD7114, ZD2079 and CGP12177 did not produce any effect or induce an increase of food intake at discrete doses and times over 24 hours (supplementary material Fig. S1D–F). CL316243 at 1 mg/kg was the most effective β3-adrenoceptor agonist and had the most potent dose-response effect on reducing cumulative food intake over time (P<0.001 after 24 hours), and this drug and dose were selected for the repeated-treatment experiment.

Over the 6 days of repeated treatment with vehicle, CL316243 (1 mg/kg), OEA (5 mg/kg) and CL316243+OEA (CL+OEA), we monitored cumulative food intake (Fig. 1A; supplementary material Fig. S2). ANOVA analysis showed treatment effects of CL316243, OEA and CL+OEA on cumulative food intake [CL: F(1,182)=48.58, P<0.0001; OEA: F(1,182)=18.13, P<0.0001; CL+OEA: F(1,182)=96.69, P<0.0001]. In all cases, we detected a very significant time effect on cumulative food intake [F(12,182)>1313, P<0.0001]. OEA did not produce any reduction in cumulative food intake until 2 days of treatment (P<0.05), and this reduction disappeared in the 6th day of treatment. Regarding the effect of CL316243, we observed a decrease in cumulative food intake from 2 hours to 4 days of treatment, being very prominent within the first 48 hours (P<0.001). Post hoc test indicated that the cumulative food intake reduction was more significant and lasting when CL316243 and OEA were co-administered (P<0.001 from 6 hours to 5 days; P<0.05 after 6 days) than when each was independently administered.

We also monitored the body weight gain over the 6 days of treatment (Fig. 1B). ANOVA indicated that there were treatment effects of CL316243, OEA and CL+OEA on body weight gain [CL: F(1,70)=37.11, P<0.0001; OEA: F(1,70)=13.64, P<0.0001; CL+OEA: F(1,70)=44.08, P<0.0001]. In all cases, we detected a very significant time effect on body weight gain [F(4,70) >4.12, P<0.005]. OEA induced a slight reduction of body weight gain, being only significant in days 2 and 5 (P<0.05). CL316243 showed a decrease in body weight gain from day 2 to day 5, being more prominent within the first 72 hours of treatment (P<0.01). Consistent with food intake, the reduction of body weight gain was more significant and lasting when CL316243 and OEA were co-administered (P<0.01 from 2 to 5 days of treatment) than when CL316243 and OEA were independently administered.

We analyzed the change of fat mass and non-fat mass (lean mass) after the repeated treatment of CL316243 and OEA for 6 days (Fig. 1C,E). We detected significant reductions of fat mass after CL316243 (P<0.01) or OEA (P<0.05) administrations compared with vehicle-treated rats. The fat mass reduction was enhanced after the combined treatment of CL+OEA (P<0.001) (Fig. 1C). Interestingly, the ratio of the final fat:non-fat mass confirmed the effects of CL+OEA treatment on fat mass reduction (P<0.001) (Fig. 1D).

We next evaluated whether CL316243, OEA and CL+OEA treatments induced changes in the other side of the energy-balance equation. CL316243-, OEA- and CL+OEA-treated rats showed significant increases in cumulative energy expenditure (EE) (P<0.001) (Fig. 2A–F) and significant decreases in the respiratory quotient (RQ) (P<0.001) (Fig. 2G–L). When the area under the curve (AUC) was analyzed, we detected that only the combination of CL+OEA treatment induced significant EE increase (P<0.01) (Fig. 2D–F) and RQ decrease (P<0.05) (Fig. 2J–L) compared with vehicle-treated rats. Both effects of CL+OEA on EE and RQ were mainly produced during the light phase. Cumulative locomotor activity (LA) was not modified after CL+OEA treatment (supplementary material Fig. S3). When the AUC was analyzed, a significant LA increase was detected after OEA treatment (P<0.05) that was blocked by CL316243 (P<0.05) during the dark phase.

Whereas OEA did not affect the temperature surrounding interscapular BAT, CL316243 induced an increased temperature (Fig. 3A–E). However, co-administration of CL+OEA showed a more potent increase in the temperature surrounding interscapular BAT compared with that of vehicle-treated rats (P<0.01 and P<0.001, respectively) (Fig. 3A–E). Consistently, only the increase of BAT temperature after the co-administration of CL+OEA was accompanied with an increase in the amount of BAT weight (P<0.05) (Fig. 3F).

To obtain information about the regulators mediating CL+OEA-induced negative energy balance and BAT temperature, first we examined the morphology of the white and brown adipocytes, and then we analyzed the expression of thermogenic markers such as PPARα and UCP1 in eWAT and BAT. Haematoxylin and eosin staining showed clear differences in the morphology of the white and brown adipocytes in a treatment-dependent manner (supplementary material Fig. S4A–B′). In the white adipocytes, CL316243 and CL+OEA treatments produced a dramatic reduction in the size of the monolocular lipid droplet and, as a consequence, an increase of the cytoplasm appearance (supplementary material Fig. S4A–D). In the brown adipocytes, CL316243 induced a higher fragmentation of the multilocular lipid droplets, whereas OEA and CL+OEA treatments produced almost total removal of lipid droplets (supplementary material Fig. S4A′–D′). The dramatic change after CL+OEA treatment made the morphology and limits of the brown adipocytes unrecognizable (supplementary material Fig. S4D,D′).

In keeping with these effects, CL316243 and CL+OEA treatments for 6 days increased the expression of PPARα and UCP1 immunofluorescence in eWAT (P<0.001 and P<0.05, respectively) (supplementary material Fig. S4E,J). In eWAT and BAT, we observed an increase of PPARα immunofluorescence after CL316243 and CL+OEA treatments (P<0.05 and P<0.01, respectively) (supplementary material Fig. S4E′), but only after CL+OEA coadministration did we detect an increase of UCP1 immunofluorescence in BAT (P<0.05) (supplementary material Fig. S4J′).

The effect of CL+OEA treatment on the increase of PPARα and UCP1 immunofluorescences in eWAT was confirmed after the analysis of the gene and protein expressions by quantitative realtime reverse transcription polymerase chain reaction (RT-qPCR) and western blot techniques. Thus, increased levels of UCP1 mRNA, but not PPARα, were detected in the BAT only after CL316243 and CL+OEA treatments (P<0.01 and P<0.05, respectively) (Fig. 4A,B). Increased levels of PPARα and UCP1 mRNA were observed in the eWAT of CL316243-and CL+OEA-treated rats (P<0.05) (Fig. 4C,D). Moreover, CL+OEA co-administration also produced increased protein levels of PPARα and UCP1 in WAT (P<0.05) compared with vehicle-treated rats (Fig. 4E,F).

We evaluated whether CL316243, OEA and CL+OEA effects on PPARα and UCP1 were conducted by a p38 MAPK-dependent activation. Decreased protein levels of p38 MAPK was observed in the WAT of CL316243-treated rats (P<0.001), but this effect was blocked when CL+OEA was co-administered (Fig. 4G). Interestingly, p38 MAPK phosphorylation was also reduced in the WAT of CL316243-treated rats (P<0.01), but this effect was enhanced when CL+OEA was co-administered (P<0.001) (Fig. 4G).

To obtain information about the regulators promoting CL+OEA-induced BAT-like features in eWAT, we analyzed the expression of mitochondrial markers such as the cytochrome c oxidase subunit 4 isoform 1 and 2 (Cox4i1 and Cox4i2), and BAT markers such as FGF21 and PRDM16. CL+OEA co-administration induced increased expression of Cox4i1, Cox4i2, Fgf21 and Prdm16 mRNA in eWAT (all at P<0.05) (Fig. 6A–D). The increase of Prdm16 gene expression in eWAT after CL+OEA treatment was confirmed when protein levels of PRDM16 were analyzed (P<0.05) (Fig. 5E,F).

To investigate whether the reduction of fat mass and droplets in WAT involves an alteration in lipid and cholesterol metabolism, we analyzed the gene expression of factors related to fatty acid synthesis such as Fasn and Scd1, β-oxidation such as Cpt1b and Acox1, and cholesterol synthesis such as Insig1, Insig2, Srebf1, Srebf2 and Hmgcr. The WAT of CL316243-treated rats showed increased levels of Cpt1b and Acox1 mRNA (P<0.05 and P<0.01, respectively) (Fig. 6C,D) and a decreased level of Insig2 mRNA (P<0.05) (Fig. 6F) compared with vehicle-treated rats. OEA-treated rats showed an increased level of Acox1 mRNA (P<0.05) (Fig. 6D) but decreased levels of Scd1, Insig2 and Srebf2 (P<0.05) (Fig. 6B,F,H). The co-administration of CL+OEA produced increased expressions of Fasn, Cpt1b, Acox1, Srebf1 and Hmgcr mRNA (all at P<0.05, except Hmgcr at P<0.01) (Fig. 6A,C,D,G,I) but decreased expressions of Scd1 and Insig1 mRNA (P<0.05) (Fig. 6B,E) compared with vehicle-treated rats.

To characterize the metabolic state in the plasma of CL316243-, OEA- and CL+OEA-treated rats, we evaluated the plasma levels of relevant metabolites such as glucose, triglycerides, total cholesterol, high-density lipoprotein (HDL)-cholesterol and non-essential fatty acids (NEFAs), several metabolic hormones such as insulin and the free (unbound) and active thyroid hormones free triiodothyronine (fT3) and free thyroxine (fT₄), and several adipokines such as leptin, adiponectin, interleukin-6 (IL-6), interleukin-1β (IL-1β) and tumour necrosis factor-α (TNF-α). CL316243-treated rats showed reduced plasma levels of NEFAs (P<0.05) and a lowered insulin resistance (P<0.05) after using the homeostatic model assessment (HOMA-IR) (Table 1). In contrast, CL316243 induced a significantly higher fT₃:fT₄ ratio (P<0.001) (Table 1). The plasma of OEA-treated rats showed reduced levels of total cholesterol, but higher levels of insulin and fT₃:fT₄ ratio compared with vehicle-treated rats (all at P<0.05) (Table 1). The co-administration of CL+OEA induced significant increases of glucose (P<0.01), HDL-cholesterol and fT3:fT4 ratio (P<0.05), but significant decreases of triglycerides (P<0.01), total cholesterol (P<0.01), NEFAs (P<0.05) and HOMA-IR (P<0.05) compared with the vehicle group (Table 1).

Regarding adipokine levels, CL316243 increased the plasma levels of IL-6 (P<0.05), and OEA increased the plasma levels of adiponectin (P<0.05), but CL+OEA reduced the plasma levels of leptin (P<0.01) and TNF-α (P<0.05) (Table 1).

Considering the energy-dissipating ability of UCP1 by β3-adrenergic stimulation, the enhancement of its expression and activation by transcription factors and coactivators can be a promising strategy for anti-obesity and anti-diabetic drug therapy. A combinatorial therapy based on the association of β3-adrenergic receptor stimulation with the natural PPARα receptor ligand OEA was developed on the basis of potential convergent effects of OEA with the adrenergic mechanisms in the adipose tissue. Thus, the aim of the present study was to determine whether PPARα activation potentiates the effect of β3-adrenergic stimulation on energy balance in association with UCP1-mediated thermogenesis, white-to-brown adipocyte metaplasia and reduced fat content. We examined several factors that are crucial for the regulation of thermogenesis, fatty acid β-oxidation and lipid metabolism, as well as the promotion of mitochondrial and BAT-like features in eWAT. The principal findings in the present study were as follows (Fig. 7): (1) CL316243 and OEA potentiated the reduction of food intake and body weight gain and increased energy expenditure. (2) The reduced body weight was accompanied with decreased fat mass and lowered levels of triglycerides, cholesterol, NEFAs, leptin and TNF-α in plasma. (3) All these effects can be linked with the overexpression of the thermogenic factors UCP1 and PPARα in eWAT and BAT, which, in turn, decreases p38 MAPK phosphorylation in eWAT. (4) These improved metabolic phenotypes concurred with mitochondrial biogenesis and the appearance of brown fat-like phenotypes in the white adipocytes: the mitochondrial (Cox4i1, Cox4i2) and BAT (Fgf21, Prdm16) genes were overexpressed in eWAT of CL+OEA-treated rats. (5) Finally, regarding the mitochondrial β-oxidation of fatty acids and lipid metabolism, the enhancement of Cpt1b and Acox1 was accompanied with an upregulation of de novo lipogenesis and a reduction of the unsaturated-fatty-acid-synthesis enzyme Scd1 in eWAT (Fig. 7).

Our results indicated that there were gene and protein expression changes in eWAT based on three levels of activation: (1) specific changes in PPARα, Ucp1 and Cpt1b mRNA levels and phospho-p38 MAPK protein levels in eWAT based on β-adrenergic activation. Regarding this issue, three facts should be noted: (a) the increased gene expressions of PPARα, Ucp1 and Cpt1b were enhanced with the co-administration of the PPARα agonist OEA, and (b) the increased protein levels of PPARα and UCP1 were only detected after PPARα and β-adrenergic coactivation. (c) The increased level of p38 MAPK phosphorylation in eWAT by β-adrenergic stimulation was also enhanced by PPARα coactivation. These results suggest that, in some way, PPARα activation produced a synergic effect downstream on the β-adrenergic pathway in the adipose tissue. (2) Specific changes in Scd1 mRNA levels based on PPARα activation. (3) Synergic changes based on the coactivation of β-adrenergic and PPARα receptors that mainly produced an overexpression of the mitochondrial (Cox4i1, Cox4i2) and BAT (Fgf21, Prdm16) genes, and the β-oxidation (Acox) and the lipid metabolism (Fasn, Srebf1 and Hmgcr) genes in eWAT. Altogether, the three levels of activation produced convergent effects of OEA with the adrenergic mechanisms in the adipose tissue.

The involvement of UCP1 in adrenergically induced thermogenesis demonstrates that UCP1 plays a significant role in the control of energy expenditure; its dysfunction contributes to the development and maintenance of obesity (Feldmann et al., 2009; Inokuma et al., 2006; Ricquier and Bouillaud, 2000). Our data indicated that PPARα and β3-adrenergic coactivation potentiated the decrease of body weight through a decrease in food intake and increased energy expenditure. According to the increased energy expenditure and decreased fat mass, we found elevated interscapular temperature, which was explained by stimulated UCP1 expression in eWAT and BAT. Furthermore, we also detected a reduced insulin resistance and lowered levels of triglycerides, cholesterol and NEFAs in CL+OEA-treated rats. Body composition and histological data indicated that the decrease in fat mass, the reduction of fat depots and/or the change from monolocular to multilocular lipid droplets in the white adipocytes were correlated with decreased levels of leptin and TNF-α in plasma of CL+OEA-treated rats. Moreover, CL+OEA treatment seemed to reduce dramatically the levels of multilocular lipid droplets that characterize brown adipocytes in BAT. Previous studies reported that leptin secretion is positively correlated with adiposity (Skurk et al., 2007), and that TNF-α action is implicated in inflammatory response and insulin resistance, at least in obesity (Zhang et al., 2002). Regarding eWAT, these results agree with the fact that CL316243 stimulated PPARα expression and fatty acid β-oxidation (Cpt1b and Acox1) and reduced the expression of Scd1, a critical control point of lipid partitioning, obesity development and diet-induced hepatic insulin resistance (Gutiérrez-Juárez et al., 2006; Jiang et al., 2005). Interestingly, this metabolic response was potentiated by the lipolytic effect of OEA (Guzmán et al., 2004; Rodríguez de Fonseca et al., 2001), whereby this elevated capacity for β-oxidation within WAT contributes to insulin-sensitizing effects (lowering insulin resistance) and reduced proinflammatory signalling (lowering TNF-α level in plasma). Taken together, all these effects clearly contribute to anti-obesity actions.

Improved metabolic rates are associated with the appearance of brown fat-like (‘brite’) phenotypes within eWAT depots. UCP1 expression and uncoupling respiration is highly induced by transcription factors related to brown fat-like multilocular adipocytes such as PRDM16, FGF21 and/or PPARα and PGC-1α, among others (Seale et al., 2007; Seale et al., 2011; Hondares et al., 2011a; Hondares et al., 2011b). Thus, transgenic expression of PRDM16 in white adipocytes stimulates the formation of brown fat-like adipocytes (Seale et al., 2007). Our findings strongly indicate that β3-adrenergic stimulation of UCP1 expression in eWAT and BAT seems not to require the participation of PPARα, but UCP1 expression can be enhanced after the administration of the PPARα ligand OEA. This effect of the β3-adrenergic receptor controlling UCP1 expression can be regulated by p38 MAPK mechanisms (Klein et al., 2000), because phosphorylated levels of p38 MAPK were reduced in eWAT of CL316243-treated rats, with this effect being more prominent after CL+OEA treatment. In contrast, the coactivation of PPARα and β3-adrenergic receptors is most likely required for the expression of PRDM16, FGF21 and the mitochondrial factors Cox4i1 and Cox4i2 in eWAT. These features seem to be related to the promotion of transdifferentiation towards brown fat-like adipocytes, including mitochondrial biogenesis, in eWAT (Himms-Hagen et al., 2000; Villarroya et al., 2007). However, chronic and systemic administration of these selective β3-adrenergic and PPARα agonists seems to be insufficient to induce PRDM16 and FGF21 expression in BAT, suggesting that either participation of additional coactivators such as PGC-1α, highly expressed in B AT (Barbera et al., 2001), or the stimulus driven by cold acclimatization, high-fat feeding or higher free-fatty-acid mobilization from triglyceride stores, could be necessary in this process (Chartoumpekis et al., 2011; Seale et al., 2011). The latter hypothesis is in harmony with the fact that there is differential sensitivity in sympathetic stimulation of WAT and BAT. Thus, BAT-specific β3-adrenergic transgenic re-expression into β3-adrenergic knockout mice failed to rescue CL316243-mediated effects on food intake and minimally restored effects on oxygen consumption, indicating that a full stimulation required the presence of β3-adrenergic receptors in white adipocytes (Grujic et al., 1997).

Besides the peripheral β3-adrenergic mechanisms on sympathetic activity, there is a role for β3-adrenergic receptors in the central control of feeding. Intracerebroventricular administration of β3-adrenergic agonists causes a dose-dependent decrease in food intake (Tsujii and Bray, 1992) and affects neurons within specific hypothalamic nuclei such as paraventricular, lateral, ventromedial and dorsal hypothalamic nuclei (Castillo-Meléndez et al., 2000). Interestingly, OEA, by its ability to engage PPARα (Fu et al., 2003), is involved in the peripheral regulation of feeding. OEA can activate sensory vagus fibers, which in turn inhibit feeding by the discrete activation of the paraventricular hypothalamic nucleus and the nucleus of the solitary tract. However, OEA does not affect food intake when injected into the brain ventricles (Rodríguez de Fonseca et al., 2001; Schwartz et al., 2008). Further analysis is needed to find direct evidence of β3-adrenergic activation in the CNS. However, we observed some CL316243-mediated effects that can be indirectly related with a possible central action. Thus, the dose- and time-related decrease in food intake after intraperitoneal (i.p.) CL316243 administration was similar to those described in previous studies after ventricular infusions of β3-adrenergic agonists (Tsujii and Bray, 1992), suggesting similar sympathetic and/or central β3-adrenergic activation. The changes in food intake are closely related by the central control of appetite and feeding behaviour processes (affective and mnemonic aspect of eating) in the hypothalamus and hippocampus. Additionally, the alterations of leptin and TNFα levels in plasma can affect the central regulation of energy balance.

The observation that UCP1-mediated thermogenesis through β3-adrenergic activation is required for maximal stimulation of energy expenditure have important implications for the treatment of human obesity. Given that BAT has traditionally been considered to show discrete physiological relevance in humans (Cannon and Nedergaard, 2004; Virtanen et al., 2009), it is reasonable to anticipate that the effect of β3-adrenergic agonists on energy expenditure in humans will be less efficient than might be expected. However, several data point to a higher physiological relevance of β3-adrenergic stimulation when human white adipocytes acquire functional features (UCP1) of brown fat-like phenotypes (Cinti, 2009; Oberkofler et al., 1997; Tiraby et al., 2003). Whether the proposed combinational therapy tested in the present study works in human adipose tissue remains to be elucidated. It might be important to know whether human adipose tissue will be sensitized to the anti-obesity actions of β3-adrenergic agonists by the actions of OEA and PPARα receptor agonists.

In summary, the present results support the utility of β3-adrenergic receptor agonist-based combinational therapies for future therapeutic strategies to treat human obesity. The present study demonstrates that this combinational therapy promotes adipocyte remodelling in eWAT. This study not only support the roles of co-stimulation of β3-adrenergic and PPARα receptors on the induction of white-to-brown adipocyte phenotypes, but also set the place for the utility of new regulators such as PRDM16 for the development of new therapeutics of complicated obesity. In any event, our data showing that combined administration of OEA and β3-adrenergic agonists show clear-cut effects in body weight, body composition and different metabolic markers open up the avenue for translational studies in their therapeutic use in obesity.

All experimental procedures with animals were performed in compliance with the European Communities directive of 24 November 1986 (86/609/ECC) and Spanish legislation (BOE 252/34367-91, 2005) regulating animal research. Research procedures included in the present study were approved by the Research and Bioethics Committee of University of Santiago de Compostela and Hospital Carlos Haya. For all the analytic methods, we used eight rats per experimental group.

Adult male Sprague-Dawley rats that weighed ~300 g at the beginning of the experiments (Animal House, University of Santiago de Compostela) were used in this study. All animals were experimentally naïve, and they were individually housed in controlled room conditions (temperature: 22±2°C; humidity: 40±5%; 12-hour light-dark cycle, lights on at 8:00 am) with free access to food and tap water. The cumulative food intake by each rat and their body weight gain were monitored throughout the experiments.

To select the most effective β3-adrenoceptor agonist for the chronic study, several drugs were tested in a dose-response study. The potent and highly selective β3 agonists CL316243 disodium salt {5-[(2R)-2-[[(2R)-2-(3-chlorophenyl)-2-hydroxyethyl]amino]propyl]-1,3-benzodioxole-2,2-dicarboxylic acid, disodium salt} (cat. no. 1499, Tocris Bioscience, Bristol, UK) and BRL37344 sodium salt {(R*,R*)-(±)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid, sodium salt} (cat. no. 0948, Tocris Bioscience), and the less potent β3 agonists ICI215,001 hydrochloride {(S)-4-[2-hydroxy-3-phenoxypropylaminoethoxy] phenoxyacetic acid hydrochloride} (cat. no. 0929, Tocris Bioscience), ZD7114 hydrochloride {(S)-4-[2-hydroxy-3-phenoxypropylaminoethoxy]-N-(2-methoxyethyl) phenoxyacetamide hydrochloride} (cat. no. 0930, Tocris Bioscience) and ZD2079 {4-[2-[[(2R)-2-hydroxy-2-phenylethyl]amino]ethoxy]-benzeneacetic acid hydrochloride} (cat. no. 2154, Tocris Bioscience) were used. We also tested the partial β3 agonist CGP12177 {4-[3-[(1,1-dimethylethyl)amino]2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one hydrochloride} (cat. no. 1134, Tocris Bioscience). All these drugs were dissolved in sterile saline (0.9% NaCl) and administered i.p. at doses of 0.1, 0.5 and 1 mg/kg body weight. The natural PPARα agonist oleylethanolamide [OEA, (9Z)-N-(2-hydroxyethyl)-9-octadecenamide] (cat. no. 1484, Tocris Bioscience) was dissolved in 5% Tween 20 and sterile saline and administered i.p. at a dose of 5 mg/kg body weight.

For acute dose-response treatment, rats received one i.p. injection of either vehicle (1 ml/kg body weight of 5% Tween 20 in sterile saline) or the β3 agonists CL316243, BRL37344, ICI215,001, ZD7114, ZD2079 and CGP12177 at 0.1, 0.5 and 1 mg/kg body weight. For repeated treatment, rats received a daily i.p. injection either of vehicle (1 ml/kg body weight of 5% Tween 20 in sterile saline), CL316243 at 1 mg/kg body weight and/or OEA at 5 mg/kg body weight over 6 days. Food and water remained unchanged and ad libitum. Finally, we generated four experimental groups for acute treatment with either of the above β3 agonists (n=8): vehicle, 0.1 mg/kg, 0.5 mg/kg and 1 mg/kg body weight; and four experimental groups for repeated treatment with CL316243 and OEA (n=8): vehicle, CL316243 1 mg/kg, OEA 5 mg/kg body weight and the combination of CL316243 and OEA.

After one administration of either of the above β3 agonists at 0.1, 0.5 or 1 mg/kg body weight (acute dose-response treatment), the cumulative food intake was measured over a time course of 0.5, 1, 2, 4, 6, 8, 12 and 24 hours in rats previously food-deprived for 24 hours. The optimal β3 agonist and dose at which treatment would be more effective on food intake changes were selected for the repeated treatment experiment. During repeated treatment of CL316243 at 1 mg/kg and OEA at 5 mg/kg body weight (Rodríguez de Fonseca et al., 2001; White et al., 2004), we measured the cumulative food intake and the body weight gain every day during the 6 days of treatment.

During 48 hours from the fourth day of the repeated treatment experiment, rats were analyzed for energy expenditure (EE, kcal/kg lean mass), respiratory quotient (RQ, VCO₂/VO₂), food intake and locomotor activity (LA) using a calorimetric system (LabMaster, TSE System, Bad Homburg, Germany) (Imbernon et al., 2013). This system is an open-circuit instrument that determines: (1) the energy consumed by the amount of caloric intake (kilocalories) along time (hours) and normalized by the lean mass (kilograms); (2) the ration between the CO₂ production and O₂ consumption (VCO₂/VO₂); and (3) the total horizontal locomotion. Previously, all rats were acclimated to the experimental room and habituated to the system for 48 hours before starting the measurements.

We analyzed the variation of the amount of fat mass and non-fat mass (lean mass) before and after the 6-day treatment of CL316243 and OEA using a nuclear magnetic resonance imaging (Whole Body Composition Analyzer, EchoMRI, Houston, TX) (Imbernon et al., 2013).

Interscapular temperature surrounding BAT was recorded with an infrared camera (Compact-Infrared-Thermal-Imaging Camera E60bx, FLIR, West Malling, Kent, UK) and analyzed with a specific software package (FLIR Tools Software). For each animal/group (n=8), three or four pictures were taken and analyzed. The temperature surrounding BAT for one particular animal was calculated as the average temperature in a defined interscapular area (2 cm Ø) recorded by analyzing those pictures.

Animals from the four repeated treatment experimental groups (n=8; vehicle, CL316243, OEA and CL316243+OEA) were killed by decapitation 2 hours after the last dose of treatment in a separate room from the other experimental animals. Blood samples were briefly collected and centrifuged (1000g for 10 minutes at 4°C), and all plasma samples were frozen at −80°C for biochemical and hormonal analysis. White (epididymal fat, eWAT) and brown (BAT) adipose tissues and liver were dissected out. Part of each sample was fixated with 4% paraformaldehyde in 0.1 M phosphate buffered saline (PBS) by immersion until immunohistochemical analysis. The remaining of each sample was briefly frozen at −80°C until RT-qPCR and western blot analyses.

Blood samples from rats treated with vehicle, CL316243, OEA and CL+OEA for 6 days were collected into tubes containing EDTA-2Na (1 mg/ml blood). The samples were immediately centrifuged, and the plasma aliquoted and stored at −80°C until the determination of biochemical parameters. The following metabolites, metabolic hormones and adipokines were measured in plasma: glucose, triglycerides, total cholesterol, high-density lipoprotein (HDL)-cholesterol, NEFAs, insulin, fT₃, fT₄, leptin, adiponectin, IL-6, IL-1β and TNF-α. The metabolites were analyzed using commercial kits according to the manufacturer’s instructions, and a Hitachi 737 Automatic Analyzer (Hitachi Ltd, Tokyo, Japan). The insulin levels were measured using a commercial rat insulin ELISA kit (cat. no. 10-1113-01, Mercodia, Sweden). The free (unbound) and active thyroid hormones were measured using commercial Advia Centaur® fT₃ and fT₄ Ready Packs assays for direct chemiluminescence in an Advia Centaur® XP immunoassay system (Siemens, Erlangen, Germany). The adipokines were analyzed using a commercial rat adipocyte Millipex™ Map kit (cat. no. RADPCYT-82K, Millipore, Billerica, MA) and a Luminex 100™ IS v.1.7 system (Luminex, Austin, TX).

We performed fat extraction as was described previously (Alonso et al., 2012). Total lipids were extracted from frozen liver samples with chloroform-methanol (2:1, v/v) and butylated hydroxytoluene (0.025%, w/v) according to the Bligh and Dyer method. After two centrifugation steps (2800g, 4°C for 10 minutes), the lower phase containing lipids was extracted with a Pasteur pipette. Nitrogen was used to dry each sample, and the liver fat content was expressed as a percentage of the tissue weight (supplementary material Table S1).

We performed real-time PCR (TaqMan, Applied Biosystem, Carlsbad, CA) as described previously (Crespillo et al., 2011a; Decara et al., 2012) using specific sets of primer probes (supplementary material Tables S2, S3). Briefly, tissue portions of eWAT and BAT (~100 mg) were homogenized on ice and RNA was extracted following the Trizol® method according to the manufacturer’s instructions (Gibco BRL Life Technologies, Baltimore, MD). RNA samples were isolated with RNAeasy minelute cleanup-kit including digestion with DNase I column (Qiagen, Hilden, Germany). After reverse transcript reaction from 1 μg of WAT and BAT mRNA, quantitative realtime reverse transcription polymerase chain reaction (RT-qPCR) was performed in a CFX96TM Real-Time PCR Detection System (Bio-Rad, Hercules, CA) and the FAM dye label format for the TaqMan® Gene Expression Assays (Applied Biosystems). Melting curves analysis was performed to ensure that only a single product was amplified. After analyzing several control genes, values obtained from eWAT samples were normalized in relation to glyceraldehyde 3-phosphate dehydrogenase (Gapdh) levels, whereas values obtained from BAT samples were normalized in relation to beta-glucuronidase (GusB) levels.

We performed western blotting as described previously (Crespillo et al., 2011b) using specific antibodies (supplementary material Table S4). Tissue portions of eWAT (~100 mg) were homogenized on ice to preserve protein levels. Total protein lysates were subjected to SDS-PAGE on 10% (w/v) SDS gels, electrotransferred on nitrocellulose membranes and probed with the following antibodies: anti-PPARα (cat. no. 20R-PR021, Fitzgerald, Acton, MA), anti-UCP1 (cat. no. sc-6528, Santa Cruz Biotechnology, Santa Cruz, CA), anti-p38 MAPK (cat. no. ab7952-1, Abcam, Cambridge, UK), anti-phospho-p38 MAPK (cat. no. ab32557, Abcam), anti-PRDM16 (cat. no. ab106410, Abcam), anti-β-actin (cat. no. A5316, Sigma, St Louis, MO) and anti-γ-adaptin (cat. no. A36129, BD Biosciences, Franklin Lakes, NJ). Values were expressed in relation to β-actin or γ-adaptin depending on the molecular weight.

We performed immunofluorescence as described previously (Crespillo et al., 2011a) using specific antibodies (supplementary material Table S4). Paraffined tissue microarray blocks (Manual Tissue Arrayer MTA-1, Beecher Instruments, Inc., Sun Prairie, WI) of eWAT were analyzed for the presence and quantification of PPARα receptor and UCP1 by immunofluorescence and densitometry. The primary antibodies were: anti-PPARα (diluted 1:100, cat. no. 20R-PR021, Fitzgerald) and anti-UCP1 (diluted 1:100, cat. no. sc-6528, Santa Cruz).

Digital high-resolution microphotographs of the eWAT were taken under the same optimized conditions of high-sensitivity fluorescence detection by an Olympus BX41 microscope equipped with an Olympus DP70 digital camera (Olympus Europa GmbH, Hamburg, Germany) and an Olympus X-Cite fluorescence system (X-Cite series 120Q, Olympus). Quantifications of immunofluorescence were carried out by measuring densitometry of the images obtained from replicates and samples: two replicates per sample, eight samples per group and four groups, using the analysis software ImageJ 1.38x (National Institutes of Health, Bethesda, MA).

All data are represented as mean±s.e.m. (standard error of the mean) of at least eight determinations per experimental group (n=8). Kolmogorov-Smirnov normality tests indicated that all data followed a Gaussian distribution (P>0.1), so we selected a parametric statistical test. Differences between the treatments were analyzed by ANOVAs and post-hoc two-tailed Bonferroni test. P<0.05 was considered significant.