Systemic Effects of White Adipose Tissue Dysregulation and Obesity‐Related Inflammation (2010)

Over the past 20 years, there has been an alarming increase in the prevalence of overweight and obesity ((1)). Current estimates indicate that ∼66% of the US population is overweight with >30% obese, a state of affairs that raises numerous health and fiscal concerns. With regard to the former, recent research has established that obesity significantly increases the risk of chronic diseases such as type 2 diabetes (T2D), cardiovascular disease, nonalcoholic fatty liver disease (NAFLD), colon cancer, and obstructive sleep apnea ((2)). Fiscally, obesity-related illness encompassed 10% of all health-care costs in the United States in 2008 ((1),(3)), a rate that is anticipated to grow if trends in childhood obesity continue on their current trajectory. If the epidemic in obesity is not effectively dealt with, this reality forewarns of very serious health and financial problems for the United States. The problems associated with obesity develop initially as a problem related to energy storage, thereby placing WAT at the front lines. Although WAT's principal function deals with energy storage, it serves other important homeostatic functions as well. WAT acts to buffer the amount of circulating fatty acids (FAs) that occurs during the postprandial period, protecting peripheral tissues from excessive lipid accumulation ((4)), and most prominent energy-storing cells in WAT, adipocytes, secrete hormones called adipokines that affect a diverse array of local and systemic functions. WAT also consists of a stromal vascular fraction comprised of preadipocytes, endothelial cells, fibroblasts, macrophages, monocytes, and lymphocytes, which also holds important metabolic and immunological roles. To gain greater perspective on the importance of WAT for normal metabolic function, researchers developed the A-ZIP/F-1 mouse, a model almost entirely deficient in WAT ((5),(6)). Mice without fat experience a severe form of insulin resistance (IR), similar to humans born with genetic fat deficiency. The IR is ameliorated if WAT is transplanted into the body ((5),(6)). The A-ZIP/F-1 mouse exemplifies the importance of WAT in the maintenance of systemic insulin sensitivity, though it is still unclear how precisely WAT regulates systemic insulin response and glucose homeostasis. Differentiating the relative importance of WAT's energy storage vs. endocrine functions has become a complex task, and both of these functions appear to be involved WAT's systemic effects. The objective of this review is to describe how obesity-induced imbalances in WAT can lead to significant systemic consequences. A familiar outcome of WAT dysfunction is the initiation of a local inflammatory response in the various peripheral tissues, a response which can feedback on itself and perhaps potentiates a number of local metabolic disorders (Figure 1). The relation between white adipose tissue (WAT) dysfunction and the development of a local inflammatory response in peripheral tissues. WAT dysfunction, characterized by adipocyte hypertrophy and a heightened stress response, causes a recruitment of immune cells to WAT. Immune cells become the principal mediators of inflammation and the further immune cell recruitment to WAT, generating a positive feedback cycle of chronic inflammation that contributes to tissue dysfunction. The compounds released by dysfunctional WAT, such as nonesterified fatty acids (NEFA) and proinflammatory cytokines, initiate the dysfunction of peripheral tissues, including skeletal muscle, liver, pancreas, and kidney. Similar to WAT, lipid- or cytokine-induced stress may lead to the infiltration or activation of immune cells and, subsequently, to chronic, self-perpetuating inflammatory response. This immune-cell-driven, local inflammatory response may be a key factor in the dysfunction of peripheral tissues in obesity. One explanation tying the energy-storing and endocrine functions together involves the deleterious effects of adipocyte hypertrophy. Adipocytes are well-known for their pliable nature and have the ability to expand to very large sizes. Human adipocytes can grow to ∼20-fold in diameter and several thousand-fold in volume ((7)). There appears to be a breaking point, however, when adipocytes become too large to function effectively mainly due to variety of stress signals released by the expanding cell. One source of these signals is thought to be endoplasmic reticulum (ER) stress ((8)). Nutrient excess is a well-known activator of ER stress, and when a certain threshold is passed a signaling cascade known as the unfolded protein response is initiated. The unfolded protein response triggers the activation of numerous stress and inflammatory pathways and the production of cytokines and chemokines, all of which have been shown to antagonize the insulin pathway ((9)). There is also evidence that adipocyte stress signals may result from a hardening of its extracellular matrix. Extracellular matrix components such as collagen VI are upregulated in the obesity and act to increase the mechanical rigidity of adipocytes ((10)), thereby making it more difficult to expand. In mice deficient in collagen VI, adipocytes are able to enlarge "stress-free" during periods of positive energy balance, and resultantly these mice have a significantly improved inflammatory profile ((10)). Because of the stress hypertrophic adipocytes are under, the cells are largely dysfunctional. The cells show a poor ability to take up and release FFA appropriately, and overall WAT from obese individuals demonstrates a net efflux of FAs ((11)). Thus, peripheral tissues, including SM, liver, pancreas, heart, kidney, and blood vessels, must increase their lipid intake to compensate, exposing them to the potentially harmful affects of lipotoxic compounds. The adipokine profile of WAT of obese individuals is also remarkably different in comparison to lean individuals. The proportion of proinflammatory adipokines and cytokines, such as leptin, interleukin-6 (IL-6), IL-8, monocyte chemoattractant protein-1 (MCP-1), resistin, retinol-binding protein-4, and interferon γ inducible protein-10 increases significantly while adiponectin, the main anti-inflammatory compound secreted by WAT, decreases ((12)). Adipokines play central roles in the regulation of food intake, insulin action, lipid and glucose metabolism, angiogenesis and vascular remodeling, regulation of blood pressure, and coagulation ((13)), and therefore alterations can have significant consequences. Under obese conditions, stressed adipocytes begin recruiting immune cells into the stromal vascular fraction. In 2003, Weisberg et al. ((14)) and Xu et al. ((15)) demonstrated that macrophages are increased in WAT of obese mice and humans, and recently Nishimura et al. discovered that CD8+ T cells precede and contribute to the macrophage influx ((16)). The physiological role of infiltrating adipose tissue macrophages is thought to be debris clearing in nature. Once extravasated into the tissue, macrophages localize around dead adipocytes and form crown-like structures that can persist for long lengths of time ((17)). The F4/80+CCR2+CD11c+ macrophages recruited to WAT are significantly more inflammatory than resident adipose tissue macrophages from lean individuals ((18),(19),(20)) and are responsible for the majority of WAT proinflammatory cytokine and chemokine secretion. Eventually, a positive feedback cycle is formed whereby infiltrating macrophages recruit and sustain the presence of new macrophages, thus resulting in further propagation of inflammation ((21),(22)). The remainder of this article will review the systemic direct and indirect effects of obesity and adipocyte hypertrophy-related manifestations on important regions in the body. Proper maintenance of blood glucose levels relies on concerted regulation of a number of metabolically important tissues. As a tissue responsible for ∼80% of insulin-stimulated glucose disposal, SM is a region that, when compromised, can create numerous complications for the remaining tissues in the body. Much of investigation on SM IR has focused on lipid-induced effects, but SM inflammation is beginning to enter the dynamic. Numerous studies have linked to the development of SM IR with nonesterified FA (NEFA) overexposure. Insulin signaling in SM and cultured myocytes is inhibited by NEFA overexposure ((23),(24)), and increasing FA levels in rodents and humans to high levels abolishes insulin sensitization of the insulin receptor substrate-1/phosphoinositide kinase-3 insulin signaling pathway ((23),(24),(25)). The inhibition of SM insulin response is preceded in vivo by an upregulation in SM FA transporters and increase in intramyocellular fat content ((26),(27)). FA transporters are required for long-chain FAs (LCFAs) such as palmitate to enter the myocyte. FA translocase (FAT/CD36), plasma membrane-associated FA-binding protein (FABPpm), and FABP1–6 are all enhanced in the plasma membrane under obese conditions ((27),(28)). FAs entering the myocyte are immediately acted upon by acyl-CoA synthetase, which attaches an acyl-coenzyme A (acyl-CoA) moiety to form an FA-acyl-CoA. The FA-acyl-CoA can migrate toward the mitochondria to be oxidized or is utilized in the production of intramyocellular lipid ((29)). The entry of LCFAs into the mitochondria requires further modification from the protein, carnitine palmitoyltransferase-1, making it an essential point in allosteric regulation of β-oxidation ((29)). The increased presence of FAs promotes FA oxidation, in part because FAs directly activate genes necessary for β-oxidative processes. FAs and their lipid derivatives function as ligands for the peroxisome proliferator-activated receptor (PPAR) family of nuclear receptors, which include PPARs α, γ, and δ ((30),(31)). PPAR-γ activation in the myocyte promotes FA storage and glucose utilization, whereas activation of PPARs α and δ, the two more abundant PPARs in SM, increase the production of pro-oxidative proteins ((32)). The increase in FA abundance and oxidation in the myocyte ultimately affects insulin-stimulated glucose transport into the cell. As confirmed by magnetic resonance spectroscopy, glucose transport is the rate-controlling step for insulin-stimulated glucose disposal into SM ((33)). One theory underlying the connection between FA overabundance and IR purports that mitochondria in obese and T2D patients become dysfunctional under such chronic conditions. The dysfunctional mitochondria are less able to perform their oxidative capacities, resulting in the eventual build-up of intramyocellular fat and LCFA-CoAs. LCFA-CoAs can be converted into ceramide and diacylglycerol, noted in vitro activators of stress kinase pathways protein kinase C and IκB kinase-β (IKKβ) that inhibit insulin by phosphorylating serine residues of insulin receptor substrates 1 and 2 ((33)). Patients with T2D have ∼30% fewer mitochondria and significantly reduced mitochondrial density and mass than their healthy counterparts, key pieces of evidence supporting mitochondrial dysfunction ((34),(35)). Insulin-resistant offspring of T2D patients also have decreased mitochondrial density ((36)), a finding which suggests that reduced mitochondrial density may be a primary cause of pathology in some individuals. Despite these associations, evidence supporting a direct association between mitochondrial dysfunction and IR has been conflicting. In support, overweight/obese individuals undergoing a 4-month intervention of physical activity and food restriction have increased mitochondrial size and electron transport chain activity alongside improvements in insulin sensitivity ((37)). Treatment with the antidiabetic drug rosiglitazone for 8 weeks, however, enhances SM insulin sensitivity without significantly altering mitochondrial function ((38)). Also, dietary restriction in the absence of physical activity improves SM IR without significantly altering mitochondrial metabolism ((39)). Rather, it may be the case that a deficiency in insulin signaling adversely affects mitochondrial metabolism. In support of this latter possibility, studies have shown that administering insulin increases mitochondrial ATP production in healthy individuals, but not in those with T2D ((40)). Muscle mitochondrial protein content is also enhanced by insulin treatment in healthy, but not insulin-resistant, individuals ((40),(41)). A separate theory put forward by Koves et al. and Muoio contends that SM IR is more closely related to lipid-induced mitochondrial stress rather than mitochondrial dysfunction ((42)). This stress occurs when the increase in β-oxidation is not matched by a concomitant increase in tricarboxylic acid cycle genes in the mitochondria and results in the release of incompletely oxidized intermediates, such as acylcarnitines, which activate the stress kinase pathways ((42)). In support of this theory, mice lacking the enzyme malonyl-CoA decarboxylase, an enzyme which increases the rate of β-oxidation by relieving malonyl-CoA-mediated inhibition of carnitine palmitoyltransferase-1, are protected from high-fat-diet-induced glucose intolerance despite a high presence of LCFA-CoAs in the cytosol ((42)). Studies investigating the effect of exercise intervention in T2D patients and obese Zucker rats also support the assertion that high levels of intramuscular fat and LCFA-CoAs are not directly responsible for IR in SM ((43),(44)). The theory put forward by Koves and Muoio also suggest an explanation for the so-called "athlete's paradox," a term used to describe the observation that exercise-trained athletes have enhanced insulin sensitivity despite having intramuscular fat levels similar to that of obese and T2D patients ((45)). The latter groups, of course, tend to be more prone to IR. The authors have suggested greater levels of the mitochondrial flux protein PPAR-γ coactivator-1α brought about by training may place athletes in a better position to respond to an increased presence in intramuscular fat than untrained individuals by tightening the link between oxidation and the tricarboxylic acid cycle ((42)). It was recently determined, however, that overexpression of PPAR-γ coactivator-1α does not correct obesity-induced imbalances in FA oxidation or mitochondrial DNA content ((46)), a finding which suggests that other than PPAR-γ coactivator-1α are involved. An alternative explanation is that muscles from exercise-trained athletes are better able to divert potentially toxic lipid substrates into more innocuous, less oxidizing forms than untrained individuals. Endurance athletes have been shown to express greater levels of the triglyceride (TG)-synthesis protein diacylglycerol acyltransferase-1, an enzyme which catalyzes the final step in TG synthesis ((47)). Liu et al. have shown that transgenic overexpression of diacylglycerol acyltransferase-1 in mice mimics the insulin-sensitizing effects of exercise in muscle ((47)), lending significant credence to this possibility. These findings do show a definitive role for NEFA overexposure and their transport into SM in the development of SM IR, though the mechanisms by which they lead to dysfunction still require greater elucidation. Adipokines have also been shown to mediate important aspects of SM metabolism function, and therefore the significant change in adipokine secretion that occurs during obesity may play a role in the tissue's dysfunction. SM also develops resistance to both adiponectin and leptin during the course of obesity ((26),(48)). The reductions in adiponectin activity following high-fat feeding occur in as little as 3 days and precede the increase in FA transport (2 weeks) and decrease in insulin activity (4 weeks) ((49)). Some of this resistance may be attributable to decreases in the adiponectin receptors AdipoR1/R2 but findings that SM ((50)), but the findings that AdipoR1 protein content is similar among lean and obese humans detracts from this possibility ((51),(52)). For leptin, the protein suppressor of cytokine signaling-3 has been linked with the diminished activity of its receptor ((53)). Suppressor of cytokine signaling-3 prevents leptin-induced activation of adenosine monophosphate-activated kinase (AMPK) by inhibiting STAT3, a main signaling component ((53),(54)). Both leptin and adiponectin protect SM from lipotoxicity and increase the activity of AMPK, a simulator of FA oxidation ((55)). The pro-oxidative effects of leptin have been shown to be effective in muscle of lean, but not obese, humans ((56)), an observation that has been attributed to leptin resistance. Fuentes et al. recently demonstrated that the 170-kDa (OB-R170) receptor, the main receptor linked to leptin's intracellular signaling ((57)), is significantly decreased in muscles of obese individuals, as too was leptin signaling, as reflected by lower levels of phosphorylated Tyr705-STAT3, Thr172-AMPKα, and Ser221-ACCβ ((48)). Adiponectin has been shown to enhance FA oxidation by inducing mitochondrial biogenesis and PPAR-α-responsive gene expression in SM ((58),(59)). Mice lacking adiponectin are born with pre-existing mitochondrial dysfunctions ((59)), adding support to the notion that its decline is related to the mitochondrial stress observed in obese patients. Both leptin and adiponectin are also implicated in decreasing FA transport abundance in the plasma membrane ((26)). In lean rats, plasma membrane concentrations of FAT/CD36 and FABPpm decline significantly following treatment with exogenous leptin ((60)). Resistin is another adipose-derived factor that has been shown to influence SM substrate metabolism and insulin sensitivity. Like leptin, resistin is increased in genetic and diet-induced models of obesity ((61)), though its effect on FA oxidation are opposed to that of leptin. Treatment of L6 myotubes with resistin significantly attenuates AMPK activation and FA oxidation and causes an increase in intramuscular lipid content within 24 h ((62)). Resistin also significantly impairs insulin signaling in cultured myotubes and in isolated rat soleus muscles ((63),(64)), though the presence of palmitate appears requisite for its effects on oxidation and insulin sensitivity ((64)). Interestingly, resistin's insulin-antagonizing effects in the latter study did not involve an increase in ceramide levels, though reducing ceramide levels still mitigated its effect on glucose transport and insulin signaling. Today, resistin's role in pathogenesis of SM IR in obesity remains complex and somewhat undefined. One possibility why researchers have been unable to explain SM IR solely in terms of lipids, lipid derivatives, mitochondrial irregularities, or adipokines is that these factors are only part of the overall picture. Integrating knowledge about immune cells, cytokines, and adipokines is essential for a comprehensive understanding of SM dysfunction. For instance, in regard to proinflammatory cytokine involvement, it has been shown that tumor necrosis factor-α (TNF-α) levels are increased in SM of obese humans ((65)), that acute TNF-α infusion decreases SM insulin sensitivity ((66)), and that in vitro exposure of proinflammatory cytokines to myotubes impairs glucose transport and insulin action ((67)). Acute treatment with the anti-inflammatory cytokine IL-10 protects SM from IL-6- and lipid-induced IR ((68)). Interestingly, although exogenous treatment of IL-6 has been shown to have insulin-antagonizing effects, it is also produced in high amounts by exercising muscle and may also induce anti-inflammatory effects under certain conditions. IL-6 has been shown to suppress TNF-α in exercise conditions ((69)), and transient increases in plasma IL-6 have been shown to increase the production of anti-inflammatory cytokines IL-1ra and IL-10 ((70)). However, the conditions and environment in obesity differ from that of exercise. In the former, inflammation has been shown to be predominantly immune-cell derived, rather than emanating from distressed myocytes. Deletion of IKKβ in myocytes does not affect FA-induced IR ((71)), whereas deletion of IKKβ in myeloid cells protects the mouse from high-fat-diet-induced IR ((72)). Although it is possible that adipose tissue macrophages exert a distal affect on SM insulin response, there is now strong evidence of a local immune response in SM of obese mice and SM in obese mice and humans and are associated with increased and chemokine have similar results in as well. mice high-fat for and days and obese mice significantly greater immune cell infiltration in the muscles than mice 1). The of macrophages also significantly enhanced in mice high-fat days (Figure of obesity on skeletal muscle macrophage Mice or high-fat for or Muscle from into pieces and to adipose in with type 2 for and a SM immune cell fraction was by exposing cells to a and the with cells with a with and with and with a and with are as with an are significantly different The mechanisms underlying SM macrophage infiltration are though myocyte stress perhaps incompletely oxidized FA intermediates, oxidative stress, are thought to be involved. that the local immune response in SM, as as in other tissues, from the of factors in including NEFA adipokine and systemic The liver is responsible for ∼80% of glucose production and holds other key roles in lipid and protein metabolism, metabolism, and lipid content and IR are associated with increased size and abundance of which contribute to cardiovascular disease and Both lipid accumulation and inflammation have been to play significant roles in the dysfunction of this tissue, which because of its diverse array of metabolic can significant systemic consequences. Similar to SM, the liver increases its and synthesis of fat during the of obesity and obese patients genes involved in lipid storage and FA metabolism, including acyl-CoA long-chain family and which obesity-induced liver nonalcoholic and is also directly and associated with IR with this and evidence that mice lacking liver are protected from obesity and obesity-induced IR it is still unclear or not liver insulin sensitivity. Some studies that TG in the liver may be FA-induced by FAs have been shown to buffer the effects of palmitate in various cell treatment of palmitate to liver cells impairs IR without increasing TG whereas the FA acts to increase intracellular TG accumulation without insulin signaling One possibility is that chronic lipid infiltration stress kinase pathways and nuclear factor inflammation in can and significantly dysfunction in is a source of such stress as its development that of IR in rats of is also increases in of and do not in mice lacking a key component of the signaling mice deficient in IKKβ are immune from though not IR ((72)). To the importance of in the dysfunction, et al. IKKβ in lean mice to similar to that of obese The transgenic mice developed both and SM IR without and increased plasma concentrations and TNF-α and IL-6 liver production The activation of macrophages, the resident macrophage population in the liver, to play a in SM IR In with these cell and with significantly improves glucose and insulin signaling Also, the development of and IR following a high-fat or is in mice with cell The effects of leptin and adiponectin have been in the Both compounds FA oxidation AMPK has also been shown to be in the liver and cells effects appear to be on activation of a noted of Studies the effect of adiponectin in various models of have shown it to be adiponectin treatment significantly improves and in and mice other forms of liver including those by and a are ameliorated with adiponectin treatment or with its deficiency Adiponectin treatment improves and in an model of nonalcoholic in part inhibition of TNF-α and leptin, adiponectin IL-6 and chemokine and protects from cell thereby an anti-inflammatory adipose-derived factors have also been associated with the development of and IR, though for number of these compounds there is One such is the recently adipokine which has been associated with obesity and It remains however, its increase is by or its increase is to The has also been shown to have a proinflammatory effect on macrophages the receptor though its has been shown to have anti-inflammatory effects is another compound that has been to be implicated in obesity-induced IR and though there are on its effects as is a transport protein for A that is secreted by as as effect on is thought to be linked to its upregulation of enzyme in mice although some show positive of retinol-binding levels and in obese or T2D patients have at all Thus, although the cause of inflammation remains there are that activation of resident macrophages and alterations in adipokines secreted from WAT may be key to the that occurs in this As the of the is a important tissue in the maintenance of glucose homeostasis. Insulin is produced by the of the of and these cells become dysfunctional and in the obese state In T2D does not occur in the absence of making the understanding of how cells become a very important of the of diet-induced obesity the environment is glucose levels are for periods of time due to the of systemic IR and levels of are also In the healthy transient glucose and are important to and both insulin action also as ligands for and γ, which have both been shown to protect the to the in has been mass and increase to for the peripheral IR and insulin secretion is increased to blood glucose concentrations It has been suggested that a