Obesity and Its Metabolic Complications: The Role of Adipokines and the Relationship between Obesity, Inflammation, Insulin Resistance, Dyslipidemia and Nonalcoholic Fatty Liver Disease

Abstract: Accumulating evidence indicates that obesity is closely associated with an increased risk of metabolic diseases such as insulin resistance, type 2 diabetes, dyslipidemia and nonalcoholic fatty liver disease. Obesity results from an imbalance between food intake and energy expenditure, which leads to an excessive accumulation of adipose tissue. Adipose tissue is now recognized not only as a main site of storage of excess energy derived from food intake but also as an endocrine organ. The expansion of adipose tissue produces a number of bioactive substances, known as adipocytokines or adipokines, which trigger chronic low-grade inflammation and interact with a range of processes in many different organs. Although the precise mechanisms are still unclear, dysregulated production or secretion of these adipokines caused by excess adipose tissue and adipose tissue dysfunction can contribute to the development of obesity-related metabolic diseases. In this review, we focus on the role of several adipokines associated with obesity and the potential impact on obesity-related metabolic diseases. Multiple lines evidence provides valuable insights into the roles of adipokines in the development of obesity and its metabolic complications. Further research is still required to fully understand the mechanisms underlying the metabolic actions of a few newly identified adipokines. Keywords: obesity; inflammation; insulin resistance; dyslipidemia; nonalcoholic fatty liver disease; adipose tissue; adipokine OPEN ACCESS Int. J. Mol. Sci. 2014, 15 6185 1. Introduction The worldwide prevalence of obesity and its metabolic complications have increased substantially in recent decades. According to the World Health Organization, the global prevalence of obesity has nearly doubled between 1980 and 2008, and more than 10% of the adults aged 20 and over is obese in 2008 [1]. Projections based on the current obesity trends estimate that there will be 65 million more obese adults in the USA and 11 million more obese adults in the UK by 2030, consequently accruing an additional 6–8.5 million cases of diabetes, 5.7–7.3 million cases of heart disease and stroke for USA and UK combined [2]. The increased prevalence in obesity is also associated with increasing prevalence of nonalcoholic fatty liver disease (NAFLD). Among the Americas, the prevalence of NAFLD is highest in the USA, Belize and Barbados and Mexico, which have a high prevalence of obesity [3]. Obesity, especially abdominal obesity, is one of the predominant underlying risk factors for metabolic syndrome [4]. Obesity increases the risk of developing a variety of pathological conditions, including insulin resistance, type 2 diabetes, dyslipidemia, hypertension and NAFLD (Figure 1). Accumulating evidence suggests that chronic inflammation in adipose tissue may play a critical role in the development of obesity-related metabolic dysfunction [5–7]. Figure 1. Concept of metabolic syndrome. Adipose tissue has been recognized as an active endocrine organ and a main energy store of the body [8]. Excess adiposity and adipocyte dysfunction result in dysregulation of a wide range of adipose tissue-derived secretory factors, referred to as adipokines, which may contribute to the development of various metabolic diseases via altered glucose and lipid homeostasis as well as inflammatory responses [9,10]. In addition, excess fat accumulation promotes the release of free fatty acids into the circulation from adipocytes, which may be a critical factor in modulating insulin sensitivity [11,12]. However, plasma free fatty acid levels do not increase in proportion to the amount Int. J. Mol. Sci. 2014, 15 6186 of body fat, since their basal adipose tissue lipolysis per kilogram of fat is lower in obese subjects than in lean subjects [13]. This finding has been supported by other studies of adipocytes from obese subjects [14,15] and it was associated with down-regulation of hormone sensitive lipase and adipose triglyceride lipase, key enzymes involved in intracellular degradation of triglycerides [14,16–18]. Thus, Karpe et al. [19] have recently suggested that the link between circulating free fatty acid levels and insulin sensitivity in vivo is needed to further elucidate this complicated relationship. In this review, we will first discuss the critical role of adipose tissue for health and as a repository of free fatty acids. We will also review how the dysregulation of free fatty acids and inflammatory factors released by enlarged adipose tissue is associated with the pathogenesis of metabolic syndrome (insulin resistance, dyslipidemia and NAFLD). In particular, we will focus on the imbalance of pro-inflammatory and anti-inflammatory molecules secreted by adipose tissue which contribute to metabolic dysfunction. 2. Function of Adipose Tissue Adipose tissue is the major site for storage of excess energy in the form of triglycerides, and it contains multiple cell types, including mostly adipocytes, preadipocytes, endothelial cells and immune cells. During positive energy balance, adipose tissue stores excess energy as triglycerides in the lipid droplets of adipocytes through an increase in the number of adipocyte (hyperplasia) or an enlargement in the size of adipocytes (hypertrophy) [20]. The number of adipocytes is mainly determined in childhood and adolescence and remains constant during adulthood in both lean and obese subjects, even after marked weight loss [21]. Hence, an increase in fat mass in adulthood can primarily be attributed to hypertrophy. However, recent study has reported that normal-weight adults can expand lower-body subcutaneous fat, but not upper-body subcutaneous fat, via hyperplasia in response to overfeeding [22], suggesting hyperplasia of adipocytes can also occur in adulthood. Although overall obesity is associated with metabolic diseases, adipose tissue dysfunction caused by hypertrophy has been suggested to play an important role in the development of metabolic diseases such as insulin resistance [23–25]. In contrast to positive energy balance states, when energy is needed between meals or during physical exercise, triglycerides stored in adipocytes can be mobilized through lipolysis to release free fatty acids into circulation and the resulting free fatty acids are transported to other tissues to be used as an energy source. It is generally accepted that free fatty acids, a product of lipolysis, play a critical role in the development of obesity-related metabolic disturbances, especially insulin resistance. In obesity, free fatty acids can directly enter the liver via the portal circulation, and increased levels of hepatic free fatty acids induce increased lipid synthesis and gluconeogenesis as well as insulin resistance in the liver [26]. High levels of circulating free fatty acids can also cause peripheral insulin resistance in both animals and humans [26,27]. Moreover, free fatty acids serve as ligands for the toll-like receptor 4 (TLR4) complex [28] and stimulate cytokine production of macrophages [29], thereby modulating inflammation of adipose tissue which contributes to obesity-associated metabolic complications. However, circulating free fatty acid concentrations do not increase in proportion to fat mass and do not predict the development of metabolic syndrome [30–33], although many studies suggest a relationship between the release of free fatty acids from adipose tissue and obesity-related metabolic disorders. Int. J. Mol. Sci. 2014, 15 6187 Adipose tissue also has a major endocrine function secreting multiple adipokines (including chemokines, cytokines and hormones) (Figure 2). Many of the adipokines are involved in energy homeostasis and inflammation, including chemokines and cytokines. In the obese state, the adipocyte is integral to the development of obesity-induced inflammation by increasing secretion of various pro-inflammatory chemokines and cytokines [34,35]. Many of them, including monocyte chemotactic protein (MCP)-1, tumor necrosis factor (TNF)-α, interlukin (IL)-1, IL-6 and IL-8, have been reported to promote insulin resistance [36–39]. Moreover, the macrophage content of adipose tissue is positively correlated with both adipocyte size and body mass, and expression of pro-inflammatory cytokines, such as TNF-α, is mostly derived from macrophages rather than adipocytes [40]. Along with the increased number of macrophages in adipose tissue, obesity induces a phenotypic switch in these cells from an anti-inflammatory M2 polarization state to a pro-inflammatory M1 polarization state [41]. The accumulation of M1 macrophages in adipose tissue has been shown to result in secretion of a variety of pro-inflammatory cytokines and chemokines that potentially contribute to obesity-related insulin resistance [5,42]. In contrast, M2-polarized macrophages participate in remodeling of adipose tissue, including clearance of dead or dying adipocytes and recruitment and differentiation of adipocyte progenitors [43]. Decreased adipose macrophage infiltration or macrophage ablation reduces expression of inflammatory cytokines in adipose tissue and improves insulin sensitivity in diet-induced obese mice [44–47]. Furthermore, weight loss decreases macrophage infiltration and pro-inflammatory gene expression in adipose tissue in obese subjects [48,49]. In addition to M1 macrophages, levels of multiple pro-inflammatory immune cells, such as interferon (IFN)-γ + T helper type 1 cells and CD8+ T cells, are increased in adipose tissue in obesity [50]. In contrast, secretion of insulin-sensitizing adiponectin is reduced in obese subjects [51]. 3. Obesity and Insulin Resistance Insulin resistance is an integral feature of metabolic syndrome and is a major predictor of the development of type 2 diabetes [52]. It has long been recognized that obesity is associated with type 2 diabetes, and the major basis for this link is the ability of obesity to induce insulin resistance. Insulin resistance is defined as the decreased ability of tissues to respond to insulin action. Adipose tissue is one of the insulin-responsive tissues, and insulin stimulates storage of triglycerides in adipose tissue by multiple mechanisms, including promoting the differentiation of preadipocytes to adipocytes, increasing the uptake of glucose and fatty acids derived from circulating lipoproteins and lipogenesis in mature adipocytes, and inhibiting lipolysis [53]. The metabolic effects of insulin are mediated by a complex insulin-signaling network (Figure 3). Insulin signaling is initiated when insulin binds to its receptor on the cell membrane, leading to phosphorylation/activation of insulin receptor substrate (IRS) proteins that are associated with the activation of two main signaling pathways: the phosphatidylinositol 3-kinase (PI3K)-AKT/protein kinase B (PKB) pathway and the Ras-mitogen-activated protein kinase (MAPK) pathway. The PI3K-AKT/PKB pathway is important for most metabolic actions of insulin. IRS-1, which is phosphorylated by the insulin receptor, activates PI3K by binding to its SH2 domain. PI3K generates phosphatidylinositol-(3,4,5)-triphosphate, a lipid second messenger, which activates several phosphatidylinositol-(3,4,5)-triphosphate-dependent serine/threonine kinases, including AKT/PKB. Ultimately, these signalling events result in the Int. J. Mol. Sci. 2014, 15 6188 translocation of glucose transporter 4 to the plasma membrane, leading to an increase in adipocyte glucose uptake. The MAPK pathways are not implicated in mediating metabolic actions of insulin but rather in stimulating mitogenic and growth effects of insulin. In the adipose tissue, insulin also has an anti-lipolytic effect, whereby the activation of PI3K stimulates phosphodiesterase-3 so that more adenosine 3',5'-cyclic monophosphate is hydrolyzed in adipocytes, which in turn limits the release of fatty acids from adipocytes. In addition, the transcription factors, including adipocyte determination and differentiation factor 1/sterol regulatory element-binding protein-1c (SREBP1-c), regulate the expression of multiple genes that are responsible for adipocyte differentiation, lipogenesis and fatty acid oxidation. Figure 2. Secretion of inflammatory adipokines from adipose tissue in obese state. In obese state, the enlarged adipose tissue leads to dysregulated secretion of adipokines and increased release of free fatty acids. The free fatty acids and pro-inflammatory adipokines get to metabolic tissues, including skeletal muscle and liver, and modify inflammatory responses as well as glucose and lipid metabolism, thereby contributing to metabolic syndrome. In addition, obesity induces a phenotypic switch in adipose tissue from anti-inflammatory (M2) to pro-inflammatory (M1) macrophages. On the other hand, the adipose production of insulin-sensitizing adipokines with anti-inflammatory properties, such as adiponectin, is decreased in obese state. The red arrows indicate increased (when pointing upward) or decreased (when pointing downward) responses to obesity. ANGPTL, angiopoietin-like protein; ASP, acylation-stimulating protein; IL, interleukin; MCP-1, monocyte chemotactic protein; NAFLD, nonalcoholic fatty liver disease; PAI-1, plasminogen activator inhibitor-1; RBP4, retinol binding protein 4; SAA, serum amyloid A; SFRP5, secreted frizzled-related protein 5; TGF-β, Transforming growth factor-β; TNF-α, tumor necrosis factor-α. Int. J. Mol. Sci. 2014, 15 6189 Figure 3. Schematic view of insulin signaling pathway in adipose tissue. Binding of insulin to its receptor on adipocytes triggers the phosphorylation and activation of insulin receptor substrate, which forms a docking site for phosphatidylinositol 3-kinase (PI3K) at the membrane. When docked, PI3K converts phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 3,4,5-trisphosphate, a second messenger that activates phosphoinositide-dependent protein kinase 1 and recruits Akt (also known as protein kinase B, PKB) to the cell membrane. Consequently, PI3K-AKT/PKB signaling pathway regulates metabolic processes. The red arrows indicate up-regulation (when pointing upward) or down-regulation (when pointing downward) in response to PI3K-AKT/PKB signaling pathway. The Ras-mitogen-activated protein kinase pathway leads to the activation of genes which are involved in cell growth, thereby promoting inflammation and atherogenesis. IRS-1, insulin receptor substrate; MAPK, mitogen-activated protein kinase; PDK, phosphoinositide-dependent protein kinase 1; PI3K, phosphatidylinositol 3-kinase; PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol 3,4,5-trisphosphate; PKB, protein kinase B. Evidence has suggested a role for adipose tissue in the development of insulin resistance. As discussed in the preceding text, free fatty acids and various adipokines released from adipose tissue have been involved in abnormal insulin signaling. It has been suggested that fatty acids and their metabolites, such as acyl-coenzyme A, ceramides and diacyglycerol, can impair insulin signaling by promoting protein kinases such as protein kinase C, MAPK, c-Jun N-terminal kinase (JNK), and the inhibitor of nuclear factor κB kinase β [54]. Saturated fatty acids, but not unsaturated fatty acids, induce the synthesis of ceramide, and inhibition of ceramide synthesis ameliorates saturated fatty acids-induced insulin resistance [55]. TNF-α also promotes ceramide accrual by activating sphingomyelinase, an enzyme that catalyzes the hydrolysis of sphingomyelin to ceramide [56], and ceramide mediates TNF-α-induced insulin resistance in adipocytes [57]. Haus et al. [58] reported that Int. J. Mol. Sci. 2014, 15 6190 plasma ceramide levels are elevated in obese subjects with type 2 diabetes and it contributes to insulin resistance by activating inflammatory mediators, such as TNF-α. Thus, ceramide has been regarded as mediator linking several metabolic stresses (i.e., TNF-α and saturated fatty acids, but not unsaturated fatty acids) to the induction of insulin resistance [55,57], although the role of TNF-α in insulin resistance is somewhat controversial [59]. Obese subjects had greater whole body free fatty acids rates of appearance in plasma compared with lean subjects [60], and a sustained reduction in plasma free fatty acids levels after treatment of lipolysis inhibitor was associated with an improvement of insulin sensitivity in diabetic obese subjects [61]. The anti-lipolysis drug also decreased fasting plasma free fatty acids levels in lean control, obese nondiabetic, obese subjects with impaired glucose tolerance, and the lowering of plasma free fatty acids levels improved insulin resistance and glucose tolerance in obese subjects, regardless of the degree of their preexisting insulin resistance [62]. Recently, Girousse et al. [63] reported that a decrease in adipose tissue lipolysis improved insulin tolerance and glucose metabolism without altering fat mass. Obesity-induced increases in lipolysis not only increases local extracellular lipid concentrations but also derives accumulation of macrophages in adipose tissue [64], which is associated with systemic hyperinsulinemia and insulin resistance in obese subjects [65]. In fact, macrophage recruitment was increased with fat mass [54], and the phenotype of adipose macrophages and recruitment of macrophages and other immune cells to the adipose tissue play important roles in the development of obesity-related insulin resistance [66]. Obesity-induced insulin resistance is also associated with increased secretion of cytokines and other bioactive substances from adipose tissue as well as the number of adipose macrophages. In the adipose tissue of obese humans and animals, there are a large number of macrophages infiltrations, and this recruitment is linked to the pathogenesis of obesity-induced inflammation and insulin resistance [5,40]. The production of most inflammatory factors by adipose tissue is also increased in the obese state and promotes obesity-linked metabolic diseases [67,68]. Adipocytes and immune cells (primarily macrophages) in the adipose tissue are the primary sources of many inflammatory proteins [67,68]. There are two types of inflammatory proteins: pro-inflammatory and anti-inflammatory. A number of pro-inflammatory proteins, including MCP-1, TNF-α, IL-6, IL-18, leptin, resistin, plasminogen activator inhibitor (PAI)-1, visfatin, retinol binding protein 4 (RBP4) and angiopoietin-like protein 2 (ANGPTL2), are described in more detail in the following text. Additionally, we briefly discuss the metabolic properties of two anti-inflammatory adipokines, adiponectin and secreted frizzled-related protein 5 (SFRP5). There are discrepancies between preclinical studies and clinical trials regarding some adipokines, including TNF-α, resistin and SFRP5. Although the cause of discrepancy between preclinical studies and clinical trials is unclear, it may be due to a number of factors including the discrepancies on the species (e.g., difference in the tissue compositions and gene profiles between animals and humans), outcome measures, pre-morbid conditions and treatment methods. In addition, considering the wide spectrum of pro- and anti-inflammatory adipokines, which are altered in obesity, it is likely that crosstalk of many adipokines rather than a single adipokine in adipose tissue and other tissues may be involved in the metabolic dysregulation. Further studies are still required to clarify their roles in various conditions.