- AMPK
AMP-activated protein kinase
- WAT
white adipose tissue
Adipokines, obesity and inflammation
Obesity represents an increasing problem of health care. It leads to several chronic morbidities including type 2 diabetes, dyslipidaemia, atherosclerosis and hypertension, which are major components of the metabolic syndrome(Reference Sethi and Vidal-Puig1). Obesity also predisposes individuals to an increased risk of developing non-alcoholic fatty liver disease and certain cancers(Reference Gurevich-Panigrahi, Panigrahi and Wiechec2). Furthermore, obesity and impaired immune function have been described in both human subjects and genetically obese rodents, supporting a link between adipose tissues and immunocompetent cells(Reference Marti, Marcos and Martinez3).
Adipose tissues play crucial roles in the development of obesity, with white adipose tissue (WAT) functioning as an energy storage organ and brown adipose tissue as an energy consumption organ. Several studies have suggested the importance of WAT metabolism and WAT-derived factors (fatty acids and adipokines) in the development of obesity and systemic insulin resistance, the key event in the pathophysiology of the metabolic syndrome(Reference Sethi and Vidal-Puig1).
WAT is an important secretory organ which produces a number of molecules that putatively play critical roles in fuel homeostasis and contribute to maintain metabolic control. These bioactive molecules, generally termed ‘adipokines’, include leptin, adiponectin, TNFα, IL-6, omentin, visfatin and apelin, among others(Reference Scherer4). In fact, the number of adipokines has enlarged considerably during the last few years and nowadays more than 50 have been identified(Reference Trayhurn5). These adipokines are involved in the physiological regulation of fat storage, adipogenesis, energy metabolism, food intake and also play an important role in metabolic disorders. Adipokines may exert their physiological functions in WAT locally (autocrine/paracrine) and systemically (endocrine) and, in addition, WAT also expresses a high number of important receptors leading to the interaction between different organs and tissues involved in energy homeostasis, such as central nervous system, liver, skeletal muscle and pancreas(Reference Trayhurn5). Thus, WAT participates in a wide range of biological processes including the regulation of energy metabolism and insulin sensitivity.
Indeed, the development of obesity and accompanying comorbidities is associated with an altered function of the adipocytes, especially concerning the synthesis and secretion of adipokines. Thus, obesity is frequently associated with increased levels of leptin and apelin, while adiponectin and omentin concentrations are decreased, being closely related to insulin resistance(Reference Havel6–Reference de Souza Batista, Yang and Lee8). Moreover, hypertrophic WAT secretes various pro-inflammatory adipocytokines including TNFα and IL-6(Reference Trayhurn and Wood9). In fact, obesity has been associated with systemic low-grade inflammation, which has been suggested to play an important role in the pathogenesis of obesity-related disorders, such as insulin resistance and atherosclerosis(Reference Das10).
In this review, we provide an overview of recent advances in the understanding of adipose tissue and its secreted bioactive peptides termed ‘adipokines’ focusing on adiponectin, leptin and pre-B cell colony-enhancer factor/visfatin, three adipokines with immune modulatory properties that directly regulate metabolism and insulin sensitivity and their regulation takes place by n-3 PUFA.
n-3 PUFA in the prevention of obesity and linked disorders
A growing body of evidence suggests that the amount and type of fat(Reference Jump and Clarke11, Reference Bray, Lovejoy and Smith12) included in the diet contribute to the development of obesity and insulin resistance. Specifically, diets high in saturated fat promote obesity and insulin resistance in rodents(Reference Kraegen, Clark and Jenkins13), whereas diets high in fish oil appear to prevent or attenuate the development of these diseases(Reference Storlien, Kraegen and Chisholm14). In fact, there are many evidences suggesting that the intake of n-3 PUFA, namely DHA (22:6n-3) and EPA (20:5n-3), produce some benefits on CVD markers, insulin resistance, obesity and serum lipids both in rodents(Reference Ruzickova, Rossmeisl and Prazak15–Reference Perez-Echarri, Perez-Matute and Marcos-Gomez19) and human subjects(Reference Kunesova, Braunerova and Hlavaty20). During the last few years, research focused on the study of the mechanisms underlying the beneficial effects of marine n-3 PUFA consumption. Several studies have indicated that n-3 PUFA exert their hypolipidaemic and anti-obesity effects by co-ordinately suppressing new fatty acid synthesis and by inducing fatty acid oxidation in different tissues, such as liver, skeletal muscle and WAT(Reference Flachs, Horakova and Brauner16, Reference Perez-Echarri, Perez-Matute and Marcos-Gomez19). Furthermore, it was demonstrated that n-3 PUFA are important regulators of gene expression acting through the PPAR and sterol regulatory element-binding protein pathways(Reference Takahashi, Tsuboyama-Kasaoka and Nakatani21, Reference Kim, Takahashi and Ezaki22), which are two critical transcriptional factors involved in β-oxidation (PPARα) and lipogenesis, respectively.
Moreover, it has been suggested that n-3 PUFA could prevent insulin resistance linked to obesity by the prevention of the decreased expression of GLUT4 in both skeletal muscle and adipose tissue(Reference Delarue, LeFoll and Corporeau23, Reference Peyron-Caso, Fluteau-Nadler and Kabir24) and by regulating both the activity and expression of liver glucose-6-phosphatase, which could explain the protective effect with respect to the excessive hepatic glucose output induced by a high-fat diet(Reference Delarue, LeFoll and Corporeau23).
Furthermore, n-3 PUFA have been widely reported to have anti-inflammatory effects in a range of chronic inflammatory conditions including rheumatoid arthritis and Crohn's disease(Reference Ruggiero, Lattanzio and Lauretani25). Treatment of obese subjects with n-3 PUFA in a clinical setting has been shown to reduce circulating levels of both pro-inflammatory cytokines and acute phase proteins(Reference White and Marette26). The beneficial actions of n-3 PUFA were initially believed to be mediated by a decrease in the production of classic inflammatory mediators, such as arachidonic acid-derived eicosanoids and inflammatory cytokines(Reference Calder27). However, in recent years, n-3 PUFA have been demonstrated to serve as substrates for the conversion to a novel series of lipid mediators designated resolvins and protectins(Reference Serhan, Hong and Gronert28) which have been proposed to mediate between the protective and beneficial actions underlying the effects of n-3 PUFA(Reference Serhan, Chiang and Van Dyke29). The role of these lipid mediators in the beneficial effects of n-3 PUFA in obesity and its linked disorders has been demonstrated in a recent study in ob/ob mice, showing that increased intake of n-3 PUFA not only inhibited the formation of eicosanoids derived from the n-6 PUFA arachidonic acid, but also increased the generation of protective n-3 PUFA-derived lipid mediators (protectins and resolvins), which mimicked the insulin-sensitizing and antisteatotic effects exerted by n-3 PUFA. Interestingly, the effects of resolvins and protectins appeared to be more potent than their n-3 precursors(Reference Gonzalez-Periz, Horrillo and Ferre30).
Furthermore, several recent studies have suggested that the improvements in lipid metabolism and prevention of obesity and diabetes described for n-3 PUFA partially result from the modulation of metabolism and secretory functions of adipose tissue(Reference Flachs, Rossmeisl and Bryhn31). In fact, n-3 PUFA have been shown to ameliorate low-grade inflammation in adipose tissue associated with obesity(Reference Todoric, Loffler and Huber32, Reference Pérez-Echarri, Pérez-Matute and Marcos-Gómez33) and up-regulate mitochondrial biogenesis and induce beta-oxidation in WAT in mice(Reference Flachs, Horakova and Brauner16). Moreover, the ability of n-3 PUFA to regulate adipokine gene expression and secretion has been observed both in vitro and in vivo in rodents(Reference Pérez-Matute, Pérez-Echarri and Martínez17, Reference Pérez-Matute, Marti and Martínez34) and human subjects(Reference Itoh, Suganami and Satoh35).
Leptin
Leptin is a hormone primarily secreted by WAT, although it is also produced by several other tissues, such as placenta, stomach, bone marrow and brain(Reference Marti, Berraondo and Martinez36). Leptin is involved in the regulation of food intake, energy expenditure, body fat storage and insulin signalling(Reference Havel6). In leptin-sensitive subjects, short-term increases in circulating leptin in response to feeding promote satiety. The key role of the leptin system in regulating body fat in animals and human subjects is demonstrated by severe hyperfagia and obesity caused by leptin deficiency(Reference Zhang, Proenca and Maffei37, Reference Montague, Farooqi and Whitehead38). However, circulating leptin concentrations are correlated with adiposity in human subjects(Reference Havel, Kasim-Karakas and Mueller39) and animals(Reference Ahren, Mansson and Gingerich40), suggesting that resistance to leptin action develops with chronic overfeeding and obesity(Reference Lee and Fried41). Increased sensations of hunger during dieting are related to the magnitude of decreases in leptin(Reference Keim, Stern and Havel42). In addition, it was demonstrated that the normal compensatory decreases of energy expenditure and thyroid axis function in response to energy-restricted diet in human subjects were prevented by low-dose leptin replacement. Thus, although leptin administration may not promote substantial weight loss in obese leptin-resistant subjects, leptin therapy may be useful to maintain the weight loss achieved by dieting(Reference Rosenbaum, Murphy and Heymsfield43).
Leptin also participates in the regulation of the activity of the reproductive(Reference Caprio, Fabbrini and Isidori44) and immune system(Reference La Cava and Matarese45). In fact, leptin administration corrects many of the neuroendocrine, reproductive, metabolic and immune system deficits associated with leptin deficiency(Reference Farooqi, Matarese and Lord46). With regard to the role of leptin in inflammation and immunity, the expression of leptin is increased in conditions that are associated with the release of pro-inflammatory cytokines, such as acute inflammatory conditions during sepsis(Reference Sarraf, Frederich and Turner47). In human adipose tissue, TNFα and IL-6 increases leptin mRNA only when added together with dexamethasone. It has been suggested that the increase in local cortisol and inflammatory cytokines in adipose tissue may contribute to higher leptin mRNA levels in obese subjects and to higher leptin levels observed after endotoxin administration(Reference Lee and Fried41).
Congenital deficiency of leptin has been associated with increased frequency of infection and related mortality and it was hypothesized that a low concentration of serum leptin might contribute to increased susceptibility to infection(Reference La Cava and Matarese45). Several studies have investigated the effects of leptin on innate and adaptive immune responses(Reference Matarese, Leiter and La Cava48). In innate immunity, the ability of leptin to up-regulate the phagocytic function of macrophages/monocytes in mice and human subjects has been described. In macrophages, leptin increases the secretion of pro-inflammatory cytokines, such as TNFα (early), IL-6 (late) and IL-12(Reference Shen, Sakaida and Uchida49, Reference Santos-Alvarez, Goberna and Sanchez-Margalet50). Furthermore, leptin stimulates neutrophil chemotaxis and the production of reactive oxygen species by these cells and regulates natural killer-cell differentiation, proliferation, activation and cytotoxicity(Reference Zarkesh-Esfahani, Pockley and Wu51, Reference Zhao, Sun and You52). The most evident effects of leptin on the modulation of adaptive immune responses have been shown in leptin-deficient mice (ob/ob) and human subjects, which exhibited immune abnormalities, such as thymic hypotrophy and reduced secretion of inflammatory cytokines, in parallel with metabolic disturbances(Reference Howard, Lord and Matarese53). These alterations are reversed by the administration of recombinant leptin(Reference Farooqi, Matarese and Lord46).
Furthermore, patients with autoimmune diseases have shown high serum levels of leptin, suggesting that this adipokine might be either a contributing factor or a marker of disease activity. It has also been observed that leptin administration accelerates autoimmune diabetes in female non-obese diabetic mice(Reference Matarese, Leiter and La Cava48). In summary, leptin has pro-inflammatory effects. This could be detrimental in many animal models of inflammatory and autoimmune disease, but might be protective in several infectious disease settings(Reference Tilg and Moschen54).
Regulation of leptin by n-3 PUFA
Several studies from our group and others have demonstrated the ability of dietary n-3 PUFA to modulate leptin gene expression and secretion both in vitro and in vivo. Thus, in vitro studies with EPA (0·1–200 μl) showed the ability of this fatty acid to stimulate in a dose-dependent manner leptin mRNA expression and leptin secretion in 3T3-L1 cells(Reference Murata, Kaji and Takahashi55) and in primary rat adipocytes(Reference Pérez-Matute, Marti and Martínez34). Little is still known about the mechanisms underlying this stimulatory action of EPA on leptin. Insulin-stimulated glucose metabolism has been described as a major determinant of leptin production(Reference Mueller, Gregoire and Stanhope56–Reference Moreno-Aliaga, Stanhope and Havel58). EPA increases glucose utilization, decreases anaerobic metabolism of glucose to lactate and increases glucose oxidation. Moreover, the ability of EPA to increase leptin production was found to be highly correlated with its effects to decrease anaerobic glucose metabolism to lactate(Reference Pérez-Matute, Marti and Martínez34). However, Cammisotto et al.(Reference Cammisotto, Gelinas and Deshaies59) reported an inhibitory effect of EPA (1 mm) on insulin-stimulated leptin secretion in white adipocytes. The disparity may be related to differences in the duration of the cultures (short v. long), the type of culture system employed and the higher concentration of insulin and EPA used in this study.
Several in vivo studies in rats and mice have reported that prolonged intake of diets high in n-3 PUFA resulted in significant decreases in plasma leptin(Reference Raclot, Groscolas and Langin60, Reference Reseland, Haugen and Hollung61), which are likely to be secondary to decreases observed in WAT mass. However, a study of our group observed that the administration of highly purified EPA (1 g/kg) during 35 d significantly decreased the leptin circulating levels in lean rats fed on a standard diet, while a significant increase of the adipokine was observed in overweight rats treated with fatty acid. Similarly, leptin gene expression in epididymal fat showed the same pattern as circulating levels(Reference Pérez-Matute, Pérez-Echarri and Martínez17). This is in agreement with the study of Peyron-Caso et al.(Reference Peyron-Caso, Taverna and Guerre-Millo62), which also described an increase in leptin concentrations in rats fed an n-3 PUFA-enriched diet. In addition, Rossi et al.(Reference Rossi, Lombardo and Lacorte63) observed that dietary fish oil positively regulates the plasma leptin levels in sucrose-fed, insulin-resistant rats. Taken together, these data suggest that n-3 PUFA actions on leptin seem to be dependent on diet composition and the physiological and metabolical status of animals, which could be important to take into account when considering supplementation with n-3-enriched products. Studies about the effects of n-3 PUFA on leptin levels in human subjects have shown that the inclusion of either lean (150 g cod, three times per week) or fatty fish (150 g salmon, three times daily), or six fish oil capsules (approximately 3 g/d containing EPA+DHA) as part of an energy-restricted diet resulted in approximately 1 kg more weight loss, which was accompanied by a decrease in fasting insulin and leptin levels(Reference Ramel, Parra and Martinez64).
Adiponectin
Adiponectin is a hormone mainly secreted by adipose tissue and to a small degree is also produced by cardiac myocytes, muscle cells and endothelial cells with important metabolic effects(Reference Havel6). Adiponectin exists both as a full-length protein, and a proteolytic cleavage fragment (globular adiponectin). Full-length adiponectin is a trimer (low molecular weight adiponectin) that forms hexamers (medium molecular weight adiponectin), which can further oligomerize to form polymers (high molecular weight form). Bioactivity studies have suggested that high molecular weight rather than low molecular weight are the functional components of adiponectin as it relates to the regulation of insulin sensitivity(Reference Pajvani, Hawkins and Combs65). Plasma adiponectin concentrations are decreased in obesity and weight loss leads to an increase in the adiponectin circulating level(Reference Bruun, Lihn and Verdich66). Furthermore, intracerebroventricular administration of adiponectin decreased body weight mainly by stimulating energy expenditure, without affecting food intake in mice(Reference Qi, Takahashi and Hileman67). Circulating levels of adiponectin have been positively associated with whole-body insulin sensitivity(Reference Yamauchi, Kamon and Waki68). In fact, low adiponectin levels have been shown to precede and predict the onset of insulin resistance, type 2 diabetes and CVD(Reference Yamauchi, Kamon and Waki68, Reference Kumada, Kihara and Sumitsuji69). In this context, it has been observed that adiponectin knockout mice are more sensitive to diet-induced insulin resistance(Reference Kubota, Terauchi and Yamauchi70, Reference Maeda, Shimomura and Kishida71). On the contrary, increasing endogenous adiponectin levels lead to improved insulin sensitivity(Reference Combs, Pajvani and Berg72) as it reverses the insulin resistance associated with both lipoatrophy and obesity(Reference Yamauchi, Kamon and Waki68). In this context, it has also been observed that treatment with thiazolidinedione drugs, which are insulin sensitizers, increases the plasma adiponectin levels(Reference Combs, Wagner and Berger73). Moreover, it has been suggested that adiponectin may mediate some cardioprotective and insulin-sensitizing effects through its anti-inflammatory properties. Thus, adiponectin has been shown to inhibit endothelial NF-κB signalling(Reference Ouchi, Kihara and Arita74) and markedly reduce the phagocytic activity and TNFα production in cultured macrophages(Reference Yokota, Oritani and Takahashi75). Adiponectin also induces the production of anti-inflammatory cytokines IL-10 and IL-1RA in human leucocytes(Reference Wolf, Wolf and Rumpold76). Moreover, adiponectin-deficient mice have increased the levels of TNFα mRNA in adipose tissue and higher TNFα circulating levels(Reference Maeda, Shimomura and Kishida71). In addition, pro-inflammatory cytokines, such as TNFα and IL-6, inhibit adiponectin gene expression and secretion in adipocytes(Reference Fasshauer, Kralisch and Klier77), suggesting that the pro-inflammatory state associated with obesity could contribute to the decreased levels of adiponectin observed in obese subjects. In turn, this lower synthesis of adiponectin might lead to dysregulation of the controls that inhibit the production of pro-inflammatory cytokines, resulting in an overwhelmingly pro-inflammatory state(Reference Tilg and Moschen54).
Regulation of adiponectin by n-3 PUFA
During the last few years, several studies suggested that the insulin-sensitizing properties of dietary fish oils could be related to the increase in circulating levels of adiponectin both in rodents and human subjects. The study of Flachs et al.(Reference Flachs, Mohamed-Ali and Horakova78) observed that feeding mice with a high-fat diet enriched with EPA/DHA concentrate (6% EPA, 51% DHA) for 5 weeks leads to elevated systemic concentrations of adiponectin and suggested that this increase could explain, to some extent, the anti-diabetic properties of these n-3 PUFA. Rossi et al.(Reference Rossi, Lombardo and Lacorte63) also found that dietary fish oil positively regulates the plasma adiponectin levels in sucrose-fed, insulin-resistant rats. A recent study by Gonzalez-Periz et al.(Reference Gonzalez-Periz, Horrillo and Ferre30) reported increased adipose adiponectin mRNA levels in ob/ob mice receiving a diet enriched with n-3 PUFA for 5 weeks. Moreover, a study of our group demonstrated that the ability of adipocytes to produce adiponectin was significantly increased by the administration of highly purified EPA ethyl ester in both lean and high-fat-induced overweight rats. These changes in adiponectin were inversely related to the homeostatic model assessment index, a marker of insulin resistance, suggesting that the EPA-stimulation of adiponectin could contribute to the insulin-sensitizing properties of this n-3 fatty acid(Reference Pérez-Matute, Marti and Martínez34). Furthermore, a 3-month treatment with EPA (1·8 g daily) in human obese subjects has also been shown to increase adiponectin secretion(Reference Itoh, Suganami and Satoh35).
Regarding the mechanisms involved in the stimulatory action of n-3 PUFA on adiponectin, Neschen et al.(Reference Neschen, Morino and Rossbacher79) found that the up-regulation of adiponectin secretion by fish oil in vivo is mediated by a PPARγ-dependent and PPARα-independent manner in mice epididymal fat. However, these authors also found that in clear contrast to what was observed after in vivo administration, n-3 fatty acids failed to stimulate adiponectin mRNA expression in 3T3-L1 adipocytes. Moreover, we found that long-term exposure of primary cultured adipocytes to EPA (200 μm) significantly decreased adiponectin gene expression and protein secretion and reduced PPARγ mRNA levels, suggesting that the inhibition of adiponectin by EPA is likely to be secondary to the down-regulation of this transcription factor(Reference Lorente-Cebrián, Pérez-Matute and Martínez80). These features suggest that the stimulation of adiponectin secretion observed after n-3 PUFA administration to rodents and human subjects involves an indirect mechanism or that they require in vivo metabolic processing to do so(Reference Neschen, Morino and Rossbacher79).
In addition, using an in vitro co-culture of adipocytes and macrophages, Itoh et al.(Reference Itoh, Suganami and Satoh35) showed that EPA (50–200 μm) reversed the co-culture-induced decrease in adiponectin secretion at least in part through down-regulation of TNFα in macrophages. These investigators suggested that in vivo EPA could increase adiponectin secretion at least partly by interrupting the vicious cycle created by adipocytes and macrophages in human obese subjects as in the in vitro co-culture experiments.
AMP-activated protein kinase (AMPK) is a fuel-sensing enzyme that acts as a gatekeeper of energy balance by regulating glucose and lipid homeostasis in adipose, liver and muscle tissues(Reference Long and Zierath81). Furthermore, the anti-diabetic efficacy of some insulin sensitizers, such as metformin and glitazones, involves the activation of AMPK(Reference Fryer, Parbu-Patel and Carling82). AMPK activation has been shown to stimulate the production of adiponectin by adipocytes(Reference Lihn, Jessen and Pedersen83). It has been suggested that AMPK activation could be involved in n-3 PUFA-induced improvements on insulin sensitivity(Reference Flachs, Rossmeisl and Bryhn31). A recent study of our group has demonstrated that EPA (100–200 μm) strongly stimulates AMPK phosphorylation in 3T3-L1 adipocytes(Reference Lorente-Cebrian, Bustos and Marti84). Moreover, two recent trials have described the ability of n-3 PUFA to activate AMPK in vivo (Reference Gonzalez-Periz, Horrillo and Ferre30, Reference Kopecky, Rossmeisl and Flachs85). Thus, Kopecky et al.(Reference Kopecky, Rossmeisl and Flachs85) showed that both α1 AMPK and phosphorylated AMPK contents increase significantly in mice fed a high-fat diet with 44% of its lipid replaced by n-3 long-chain PUFA concentrate EPAX 1050 TG for 5 weeks. Gonzalez-Periz et al.(Reference Gonzalez-Periz, Horrillo and Ferre30) observed that the AMPK activity was significantly increased in ob/ob mice receiving DHA at a dose of 4 μg/g body weight every 12 h for 4 d. These observations have led to suggest that AMPK activation could be a potential mechanism underlying the stimulatory effects of n-3 PUFA on adiponectin levels. Future studies are necessary to better characterize the mechanism involved in the n-3 PUFA stimulatory effect on adiponectin in vivo.
Pre-B cell colony-enhancer factor/visfatin
Visfatin was identified as an adipokine that was highly expressed in visceral fat of human subjects and rodents, whose plasma circulating levels were positively correlated with the size of visceral fat depots(Reference Fukuhara, Matsuda and Nishizawa86). Besides, this adipokine seemed not to be correlated with subcutaneous fat depots and consequently, it was termed visfatin (from visceral fat). This adipocytokine was firstly isolated more than 10 years ago from lympocytes as a 52-kDa protein called pre-B cell colony-enhancer factor(Reference Samal, Sun and Stearns87), because it favoured the development of lymphocyte B colonies, inhibited apoptosis in neutrophils and was also linked to several inflammatory diseases(Reference Tilg and Moschen88). Moreover, this adipokine, which has also been named nicotinamide phosphoribosyltranferase, was found to catalyse the first step in NAD biosynthesis from nicotinamide(Reference Revollo, Grimm and Imai89). In addition to visceral fat, visfatin/pre-B cell colony-enhancer factor/nicotinamide phosphoribosyltranferase is also expressed in bone marrow, liver, muscle and kidney among other tissues, where it is involved in a wide variety of functions including reproduction and immunity(Reference Luk, Malam and Marshall90).
Visfatin and inflammation
Several studies have demonstrated that visfatin induces pro-inflammatory cytokine production, such as TNFα, IL-1β, IL-6, in CD14+ monocytes(Reference Tilg and Moschen88, Reference Moschen, Kaser and Enrich91). When administered to mice, the murine pre-B cell colony-enhancer factor/visfatin significantly increased the level of circulating IL-6(Reference Moschen, Kaser and Enrich91). Moreover, visfatin promotes macrophage survival, which could affect the balance of macrophage survival and death in the setting of obesity, which in turn could play important roles in obesity-associated diseases(Reference Li, Zhang and Dorweiler92). Several studies now support the evidence that visfatin is primarily a pro-inflammatory cytokine, as its serum/plasma levels are increased in various inflammatory disorders(Reference Tilg and Moschen88). Thus, recent research has shown that visfatin may be a direct contributor to vascular inflammation(Reference Romacho, Azcutia and Vazquez-Bella93) and that plasma visfatin levels are significantly higher in chronic coronary artery diseases and acute coronary syndromes(Reference Liu, Qiao and Yuan94). However, the role played by TNFα, IL-6 and other pro-inflammatory cytokines in visfatin production by adipocytes is still controversial. Indeed, while some studies have observed that TNFα and IL-6 inhibit visfatin synthesis in 3T3-L1 adipocytes(Reference Kralisch, Klein and Lossner95, Reference Kralisch, Klein and Lossner96), an assay performed in human adipocytes has shown that treatment with TNFα induces an up-regulation in visfatin production(Reference Hector, Schwarzloh and Goehring97).
Role of visfatin on obesity and insulin resistance
The role of visfatin in obesity and linked metabolic disorders remains unclear. Thus, in KKAy mice, an experimental model for obesity and type 2 diabetes, plasma visfatin was significantly increased during obesity development. This was also accompanied by the enhancement of visfatin mRNA expression in visceral adipose tissue, but not in the liver and subcutaneous adipose tissue. Moreover, it was also described that mice fed a high-fat diet showed increased plasma visfatin concentrations, which were also associated with increases of visfatin mRNA levels in visceral mesenteric fat(Reference Fukuhara, Matsuda and Nishizawa86). However, Kloting et al.(Reference Kloting and Kloting98) reported no significant change in relative visfatin gene expression in adipocytes of subcutaneous and epididymal fat depots in Wistar–Ottawa–Karlsburg W rats, a model of polygenic metabolic syndrome.
Heterozygous visfatin+/− mice were not obese and their insulin circulating levels were not significantly different to wild-type mice. Glucose plasma levels were higher in visfatin+/− mice in fasting and after re-feeding, but glucose tolerance tests did not show any significant difference between both groups.
On the other hand, several studies have suggested that up-regulation of visfatin could be a potential mechanism mediating between the effects of PPAR (PPARα, PPARγ and PPARδ) agonists on insulin sensitivity in Otsuga Long–Evans Tokushima fatty rats and Wistar rats fed a high-fat diet(Reference Choi, Ryu and Lee99, Reference Choi, Lee and Yoo100).
The role of visfatin in human obesity is controversial. While some studies have observed a positive correlation between visfatin plasma levels and visceral obesity(Reference Fukuhara, Matsuda and Nishizawa86, Reference Filippatos, Derdemezis and Kiortsis101, Reference Haider, Holzer and Schaller102), others have argued against these data. In fact, Berndt et al.(Reference Berndt, Kloting and Kralisch103) did not find any correlation between visfatin plasma levels and visceral fat mass in obese subjects. Hammarstedt et al.(Reference Hammarstedt, Pihlajamaki and Rotter Sopasakis104) also reported no association between circulating visfatin levels and fat abdominal content and/or with waist:hip ratio in people with diabetes. On the other hand, other studies have found an inverse relationship between visfatin and obesity development. A recent study showed that plasma visfatin was negatively correlated with visceral fat in human subjects genetically predisposed to obesity(Reference Wang, van Greevenbroek and Bouwman105). Besides, they associated visfatin circulating levels with a beneficial lipid profile in non-diabetic subjects. Moreover, Pagano et al.(Reference Pagano, Pilon and Olivieri106) reported that while visfatin mRNA expression in visceral adipose tissue was enhanced in obesity, visfatin circulating and gene expression levels in subcutaneous adipose tissue were significantly decreased in obese subjects.
Further studies related to weight loss in obese subjects with changes in circulating visfatin levels have not provided concluding evidence. Thus, while some trials described enhancements on plasma visfatin levels after a decrease of fat depots and weight loss(Reference Krzyzanowska, Mittermayer and Krugluger107, Reference Garcia-Fuentes, Garcia-Almeida and Garcia-Arnes108), others found a significant reduction in visfatin levels after massive weight loss following bariatric surgery(Reference Haider, Schindler and Schaller109, Reference Manco, Fernandez-Real and Equitani110).
Several human studies investigating visfatin and insulin-resistance have demonstrated a positive correlation between increased visfatin concentrations with type 2 diabetic and obese subjects(Reference Chen, Chung and Chang111, Reference Zahorska-Markiewicz, Olszanecka-Glinianowicz and Janowska112). However, others did not find any correlation between visfatin plasma levels and several parameters of insulin resistance, such as the homeostatic model assessment index, suggesting that visfatin is not related to insulin resistance in human subjects(Reference Berndt, Kloting and Kralisch103, Reference Pagano, Pilon and Olivieri106). In this context, and contrary to observations in rodents, Hammarstedt et al.(Reference Hammarstedt, Pihlajamaki and Rotter Sopasakis104) reported that neither gene expression nor visfatin circulating levels are regulated by thiazolidinediones in human subjects, suggesting that visfatin is not involved in the insulin-sensitizing properties of these drugs.
Another conflicting point regarding visfatin actions is the potential insulin-mimetic properties of this adipokine initially described by Fukuhara et al.(Reference Fukuhara, Matsuda and Nishizawa86). However, these actions of visfatin are in doubt, because such findings have not been corroborated by other investigators(Reference Revollo, Grimm and Imai89, Reference Fukuhara, Matsuda and Nishizawa113).
Regulation of visfatin by n-3 PUFA
A previous study by our group showed that the oral supplementation of EPA ethyl ester (1 g/kg) during 35 d was able to prevent the decrease of visfatin gene expression observed in high fat diet-induced obese rats. Moreover, we found an inverse relationship with the homeostatic model assessment index, suggesting that the insulin-sensitizing effects of EPA could be related to its stimulatory action on visfatin gene expression in visceral fat(Reference Perez-Echarri, Perez-Matute and Marcos-Gomez18). In a recent trial, we demonstrated a direct stimulatory effect of EPA (200 μm) on both visfatin gene expression and protein secretion in primary rat and 3T3-L1 adipocytes, suggesting that the up-regulation of visfatin gene expression in visceral adipose tissue observed after in vivo EPA administration was not only due to the reducing effects of EPA treatment on the size of this fat depot, but also by a direct transcriptional up-regulation of visfatin gene by this n-3 PUFA(Reference Lorente-Cebrian, Bustos and Marti84).
Other studies have reported the ability of dietary fatty acids to modulate visfatin gene expression. In contrast to EPA, palmitate and oleate (0·125–1 mm) down-regulated visfatin mRNA gene expression in 3T3-L1 adipocytes(Reference Wen, Wang and Wu114). Moreover, this down-regulation of visfatin was mentioned as a potential mechanism to directly induce insulin resistance by oleate and palmitate in vitro (Reference Wen, Wang and Wu114). In this context, it has also been shown that a synthetic mixture including stearic, oleic, linoleic, linolenic and arachidonic acid normalized the increase in visfatin release induced by treatment with the insulin-sensitizing PPARγ agonist rosiglitazone in human-isolated adipocytes(Reference Haider, Mittermayer and Schaller115). These findings suggest a differential regulation of visfatin depending on the type of dietary fat and support our hypothesis that visfatin up-regulation by EPA could be another mechanism by which n-3 PUFA may improve insulin sensitivity. An interesting finding of our study was that the stimulatory effect of EPA on visfatin secretion in adipocytes involved the AMPK activation pathway(Reference Lorente-Cebrian, Bustos and Marti84).
Conclusion
Low-grade inflammation has been identified as a key factor in the development of metabolic syndrome features affecting obese subjects, leading to type 2 diabetes and CVD. In obesity, the expanding adipose tissue makes a substantial contribution to the development of obesity-linked inflammation via dysregulated secretion of pro-inflammatory cytokines, chemokines and adipokines and the reduction of anti-inflammatory adipokines, such as adiponectin. In this context, n-3 PUFA have been shown to prevent and/or ameliorate inflammation in key metabolic organs including adipose tissue, liver and muscle. Indeed, the n-3 PUFA EPA and DHA have been widely reported to have protective effects in a range of chronic inflammatory conditions including obesity, insulin resistance and CVD. From the present review, it can be concluded that these beneficial properties of n-3 PUFA partially result from the modulation of WAT metabolism and the secretion of bioactive adipokines (such as leptin, adiponectin and visfatin) that directly regulate nutrient metabolism and insulin sensitivity. Taking into account the beneficial actions of n-3 PUFA, several government and health organizations worldwide have promoted n-3 PUFA consumption, and in general, recommendations for prevention are lower than for treatment of diseases. High quality and purity n-3 PUFA supplements have been proposed to get therapeutically significant doses in patients with different pathologies. However, questions have been raised about the recommendation of eat more fish or take fish oil supplements as a source of n-3 PUFA for diseases prevention(Reference Lichtenstein, Appel and Brands116–Reference Weber, Selimi and Huber120).
Acknowledgements
This work was supported by the Spanish Ministry of Science and Innovation (AGL2006–04716/ALI and AGL2009-10873), Department of Education of Navarra Government, and ‘Línea Especial de Investigación: Nutrición, Salud y Obesidad’ from the University of Navarra (LE/97). S. L.-C. was supported by a pre-doctoral grant ‘FPU’ from the Spanish Ministry of Education and Science. The authors declare no conflict of interest. All the authors have contributed in the preparation of the manuscript.
