Control of lipid storage and cell size between adipocytes by vesicle-associated glycosylphosphatidylinositol-anchored proteins

Adipose tissue mass in mammals is expanding by increasing the average cell volume as well as the total number of the adipocytes. Up-regulation of lipid storage in fully differentiated adipocytes resulting in their enlargement is well documented and thought to be a critical mechanism for the expansion of adipose tissue depots during the growth of both lean and obese animals and human beings. A novel molecular mechanism for the regulation of lipid storage and cell size in rat adipocytes has recently been elucidated for the physiological stimuli, palmitate and hydrogen peroxide, the anti-diabetic sulfonylurea drug, glimepiride, and insulin-mimetic phosphoinositolglycans. It encompasses (i) the release of small vesicles, so-called adiposomes, harbouring the glycosylphosphatidylinositol-anchored (c)AMP-degrading phosphodiesterase Gce1 and 5'-nuceotidase CD73 from large donor adipocytes, (ii) the transfer of the adiposomes and their interaction with detergent-insoluble glycolipid-enriched microdomains of the plasma membrane of small acceptor adipocytes, (iii) the translocation of Gce1 and CD73 from the adiposomes to the intracellular lipid droplets of the acceptor adipocytes and (iv) the degradation of (c)AMP at the lipid droplet surface zone by Gce1 and CD73 in the acceptor adipocytes. In concert, this sequence of events leads to up-regulation of esterification of fatty acids into triacylglycerol and down-regulation of their release from triacylglycerol. This apparent mechanism for shifting the triacylglycerol burden from large to small adipocytes may provide novel strategies for the therapy of metabolic diseases, such as type 2 diabetes and obesity.     

Adipocyte Fetuin-A Contributes to Macrophage Migration into Adipose Tissue and Polarization of Macrophages

Macrophage infiltration into adipose tissue during obesity and their phenotypic conversion from anti-inflammatory M2 to proinflammatory M1 subtype significantly contributes to develop a link between inflammation and insulin resistance; signaling molecule(s) for these events, however, remains poorly understood. We demonstrate here that excess lipid in the adipose tissue environment may trigger one such signal. Adipose tissue from obese diabetic db/db mice, high fat diet-fed mice, and obese diabetic patients showed significantly elevated fetuin-A (FetA) levels in respect to their controls; partially hepatectomized high fat diet mice did not show noticeable alteration, indicating adipose tissue to be the source of this alteration. In adipocytes, fatty acid induces FetA gene and protein expressions, resulting in its copious release. We found that FetA could act as a chemoattractant for macrophages. To simulate lipid-induced inflammatory conditions when proinflammatory adipose tissue and macrophages create a niche of an altered microenvironment, we set up a transculture system of macrophages and adipocytes; the addition of fatty acid to adipocytes released FetA into the medium, which polarized M2 macrophages to M1. This was further confirmed by direct FetA addition to macrophages. Taken together, lipid-induced FetA from adipocytes is an efficient chemokine for macrophage migration and polarization. These findings open a new dimension for understanding obesity-induced inflammation.     

Adipose tissue,the immune system and exercise fatigue: how activated lymphocytes compete for lipids

Adipose depots that contain lymph nodes, and probably intermuscular fat in skeletal and cardiac muscle, are specialized to provision adjacent tissue in a paracrine mode. Perinodal adipocytes respond selectively to various cytokines and incorporate proportionately more polyunsaturated fatty acids. Lipolysis in the adipocytes of node-containing depots can be stimulated via inflammation of the enclosed lymph nodes. Repeated immune stimulation elicits properties characteristic of perinodal adipocytes in those elsewhere in the same depot, and hours later in other node-containing depots, but not in nodeless depots. Such site-specific properties of adipose tissue enable partitioning of dietary and metabolic supplies of fatty acids between competing tissues. Local interactions emancipate the peripheral immune system from competing with other tissues for lipids during immune responses, and may be especially important during periods of high demand, such as strenuous exercise. Biopsies of subcutaneous adipose tissue from sites remote from lymph nodes do not adequately represent the composition of fatty acids available to the immune system in situ, and perhaps that supplied to other tissues. Intermuscular fat in skeletal and cardiac muscle may also indicate paracrine relationships between adipocytes and "end-user" tissues. The concept of paracrine interactions between certain adipocytes and "user" tissue may account for the widespread contiguity between these tissues in vivo.     

LXR is crucial in lipid metabolism

Liver X receptors (LXRalpha and LXRbeta) are members of the nuclear receptor superfamily and are activated by oxysterols and intermediates in the cholesterol synthetic pathway. The pivotal role of LXRs in the metabolic conversion of cholesterol to bile acids is well established. Analysis of gene expression in LXRalpha and LXRbeta deficient mice have confirmed that LXR regulates a number of target genes involved in both cholesterol and fatty acid metabolism in liver, macrophages and intestine. The observation that LXRalpha is responsive to fatty acids and is expressed in metabolic tissues suggests that it also plays a general role in lipid metabolism. Adipose tissue is the main storage site for fat in the body and plays a crucial role in overall lipid handling. Both LXRalpha and LXRbeta are expressed and activated by endogenous and synthetic ligands, which lead to lipid accumulation into adipocytes. This indicates an important regulatory role of LXR in several metabolic signaling pathways in the adipose tissue, such as glucose uptake and de novo fatty acid synthesis. Here, we review recent studies that provide new insights into the mechanisms by which LXRs act to influence fatty acid synthesis in liver and adipose tissue.     

Macrophages block insulin action in adipocytes by altering expression of signaling and glucose transport proteins

Obesity leads to a proinflammatory state with immune responses that include infiltration of adipose tissue with macrophages. These macrophages are believed to alter insulin sensitivity in adipocytes, but the mechanisms that underlie this effect have not been characterized. We have explored the interaction between macrophages and adipocytes in the context of both indirect and direct coculture. Macrophage-secreted factors blocked insulin action in adipocytes via downregulation of GLUT4 and IRS-1, leading to a decrease in Akt phosphorylation and impaired insulin-stimulated GLUT4 translocation to the plasma membrane. GLUT1 was upregulated with a concomitant increase in basal glucose uptake. These changes recapitulate those seen in adipose tissue from insulin-resistant humans and animal models. TNF-alpha-neutralizing antibodies partially reversed the insulin resistance produced by macrophage-conditioned media. Peritoneal macrophages and macrophage-enriched stromal vascular cells from adipose tissue also attenuated responsiveness to insulin in a manner correlating with inflammatory cytokine secretion. Adipose tissue macrophages from obese mice have an F4/80(+)CD11b(+)CD68(+)CD14(-) phenotype and form long cellular extensions in culture. Peritoneal macrophages take on similar characteristics in direct coculture with adipocytes and induce proinflammatory cytokines, suggesting that macrophage activation state is influenced by contact with adipocytes. Thus both indirect/secreted and direct/cell contact-mediated factors derived from macrophages influence insulin sensitivity in adipocytes.     

Adipose tissue: a key target for diabetes pathophysiology and treatment?

Excess adipose tissue brings with it a number of adverse consequences, many of which may stem from the development of insulin resistance. An emerging view is that inflammatory changes occurring in expanding adipose tissue are associated with the secretion of peptide and other factors that can adversely affect metabolic processes in other key insulin-target tissues, especially liver and skeletal muscle. However, there is still a commonly-expressed view that the adverse changes in other tissues are ultimately due to an excess of fatty acids, liberated by a metabolically-challenged adipose tissue. Our own studies of adipose tissue metabolism and physiological function (especially blood flow) IN VIVO suggest that these two views of adipose tissue function may be closely linked. Enlarged adipocytes are less dynamic in their responses, just as 'enlarged adipose tissue' is less dynamic in blood flow regulation. Adipocytes seem to be able to sense the appropriate level of fat storage. If the normal mechanisms regulating adipocyte fat storage are interfered with (either in genetically-modified animals or by increasing the size of the adipocytes), then perhaps some sort of cellular stress sets in, leading to the inflammatory and endocrine changes. Some evidence for this comes from the effects of the thiazolidinediones, which improve adipose tissue function and in parallel reduce inflammatory changes.     

Dietary n-3 polyunsaturated fatty acids modify fatty acid composition in hepatic and abdominal adipose tissue of sucrose-induced obese rats

The fatty acid profile of hepatocytes and adipocytes is determined by the composition of the dietary lipids. It remains unclear which fatty acid components contribute to the development or reduction of insulin resistance. The present work examined the fatty acid composition of both tissues in sucrose-induced obese rats receiving fish oil to determine whether the effect of dietary (n-3) polyunsaturated fatty acids (PUFAs) on the reversion of metabolic syndrome in these rats is associated to changes in the fatty acid composition of hepatocyte and adipocyte membrane lipids. Animals with metabolic syndrome were divided into a corn–canola oil diet group and a fish oil diet group, and tissues fatty acids composition were analyzed after 6 weeks of dietary treatment. Fatty acid profiles of the total membrane lipids were modified by the fatty acid composition of the diets fed to rats. N-3 PUFAs levels in animals receiving the fish oil diet plus sucrose in drinking water were significantly higher than in animals under corn–canola oil diets. It is concluded that in sucrose-induced obese rats, consumption of dietary fish oil had beneficial effects on the metabolic syndrome and that such effects would be conditioned by the changes in the n-3 PUFAs composition in hepatic and adipose tissues because they alter membrane properties and modify the type of substrates available for the production of active lipid metabolites acting on insulin resistance and obesity.     

Adipose tissue and adipocytes support tumorigenesis and metastasis

Adipose tissue influences tumor development in two major ways. First, obese individuals have a higher risk of developing certain cancers (endometrial, esophageal, and renal cell cancer). However, the risk of developing other cancers (melanoma, rectal, and ovarian) is not altered by body mass. In obesity, hypertrophied adipose tissue depots are characterized by a state of low grade inflammation. In this activated state, adipocytes and inflammatory cells secrete adipokines and cytokines which are known to promote tumor development. In addition, the adipocyte mediated conversion of androgens to estrogen specifically contributes to the development of endometrial cancer, which shows the greatest relative risk (6.3-fold) increase between lean and obese individuals. Second, many tumor types (gastric, breast, colon, renal, and ovarian) grow in the anatomical vicinity of adipose tissue. During their interaction with cancer cells, adipocytes dedifferentiate into pre-adipocytes or are reprogrammed into cancer-associated adipocytes (CAA). CAA secrete adipokines which stimulate the adhesion, migration, and invasion of tumor cells. Cancer cells and CAA also engage in a dynamic exchange of metabolites. Specifically, CAA release fatty acids through lipolysis which are then transferred to cancer cells and used for energy production through β-oxidation. The abundant availability of lipids from adipocytes in the tumor microenvironment, supports tumor progression and uncontrolled growth. Given that adipocytes are a major source of adipokines and energy for the cancer cell, understanding the mechanisms of metabolic symbiosis between cancer cells and adipocytes, should reveal new therapeutic possibilities. This article is part of a Special Issue entitled Lipid Metabolism in Cancer.     

Ceramides in insulin resistance and lipotoxicity

Obesity predisposes individuals to the development of insulin resistance in skeletal muscle and the liver, and researchers have recently proposed two mechanisms by which excess adiposity antagonizes insulin action in peripheral tissues. First, when adipocytes exceed their storage capacity, fat begins to accumulate in tissues not suited for lipid storage, leading to the formation of specific metabolites that inhibit insulin signal transduction. Second, obesity triggers a chronic inflammatory state, and cytokines released from either adipocytes or from macrophages infiltrating adipose tissue antagonize insulin action. The sphingolipid ceramide is a putative intermediate linking both excess nutrients (i.e. saturated fatty acids) and inflammatory cytokines (e.g. tumor necrosis factor-alpha, TNFalpha) to the induction of insulin resistance. Moreover, ceramide has been shown to be toxic in a variety of different cell types (e.g. pancreatic beta-cells, cardiomyocytes, etc.), and review of the literature reveals putative roles for the sphingolipid in the damage of cells and tissues which accompany diabetes, hypertension, cardiac failure, atherosclerosis, etc. In this review, I will evaluate the contribution of ceramides in the development of insulin resistance and the complications associated with metabolic diseases.     

Lipidomics of brown and white adipose tissue: Implications for energy metabolism

Adipose tissue exerts multiple vital functions that critically maintain energy balance, including storing and expending energy, as well as secreting factors that systemically modulate nutrient metabolism. Since lipids are the major constituents of the adipocytes, it is unsurprising that the lipid composition of these cells plays a critical role in maintaining their functions and communicating with other organs and cells. In both positive and negative energy balance conditions, lipids and free fatty acids secreted from adipocytes exert either beneficial or detrimental effects in other tissues, such as the liver, pancreas and muscle. The way the adipocytes communicate with other organs tightly depends on the nature of their lipidome composition. Notwithstanding, the lipidome composition of the adipocytes is affected by physiological factors such as adipocyte type, gender and age, but also by environmental cues such as diet composition, thermal stress and physical activity. Here we provide an updated overview on how the adipose tissue lipidome profile is shaped by different physiological and environmental factors and how these changes impact the way the adipocytes regulate whole-body energy metabolism.     

The effect of dietary carbohydrate on genes for fatty acid synthase and inflammatory cytokines in adipose tissues from lean and obese subjects

BACKGROUND: Hepatic de novo lipogenesis (DNL) is markedly stimulated in humans by low-fat diets enriched in simple sugars. However, the dietary responsiveness of the key enzyme controlling DNL in human adipose tissue, fatty acid synthase (FAS), is uncertain. HYPOTHESIS: Adipose tissue mRNA for FAS is increased in lean and obese subjects when hepatic DNL is elevated by a eucaloric, low-fat, high-sugar diet. DESIGN: Twelve lean and seven obese volunteers were given two eucaloric diets (10% vs. 30% fat; 75% vs. 55% carbohydrate; sugar/starch 60/40) each for 2 weeks by a random-order cross-over design. FAS mRNA in abdominal and gluteal adipose tissues was compared to hepatic DNL measured in serum by isotopic and nonisotopic methods. Adipose tissue mRNA for tumor necrosis factor-alpha and IL-6, which are inflammatory cytokines that modulate DNL, was also assayed. RESULTS: The low-fat high-sugar diet induced a 4-fold increase in maximum hepatic DNL (P<.001) but only a 1.3-fold increase in adipose tissue FAS mRNA (P=.029) and no change in cytokine mRNA. There was a borderline significant positive correlation between changes in FAS mRNA and hepatic DNL (P=.039). Compared to lean subjects, obese subjects had lower levels of FAS mRNA and higher levels of cytokine mRNA (P<.001). CONCLUSIONS: The results suggest that key elements of human adipose tissue DNL are less responsive to dietary carbohydrate than is hepatic DNL and may be regulated by diet-independent factors. Irrespective of diet, there is reduced expression of the FAS gene and increased expression of cytokine genes in adipose tissues of obese subjects.     

TNF-α mediates the stimulation of sclerostin expression in an estrogen-deficient condition

Obesity consists in fat accumulation leading to increase in adipose cells number and size. Adipocyte membrane biophysical properties are critical to maintain cellular viability in metabolically healthy obesity. This study investigated the effect of the genetic background and dietary protein restriction on fat tissue lipid composition, adipocyte membrane fluidity and water permeability using the pig as experimental model. Twenty-four male pigs from distinct genotypes, lean and obese, were fed on normal and reduced protein diets within a 2 × 2 factorial arrangement (two genotypes and two diets). Backfat thickness was twofold higher in obese than in lean pigs but unrelated to dietary protein level. In contrast, total fatty acids in the subcutaneous adipose tissue were dependent on both breed and diet, with increased lipid content promoted by the fatty genotype and by the restriction of dietary protein. Adipose membranes isolated from obese pig's subcutaneous fat tissue showed higher permeability to water, in line with an increased fluidity. Moreover, the reduced content of dietary protein influenced positively the fluidity of adipose membranes. Neither genotype nor diet affected total cholesterol concentration in the adipose membranes. Membrane-saturated fatty acids' content was influenced by genotype, while membrane-polyunsaturated fatty acids, particularly from the n-6 family, was influenced by diet. The ratio of oleic (18:1c9)/linoleic (18:2n-6) acids was positively correlated with membrane fluidity. All together, these findings reinforce the genetic background as a determinant player on adipose membrane biophysical properties and point to the dietary protein level as an important factor for subcutaneous lipid deposition as well as for regulation of membrane function, factors that may have impact on human obesity and metabolic syndrome.     

Imaging immune and metabolic cells of visceral adipose tissues with multimodal nonlinear optical microscopy

Visceral adipose tissue (VAT) inflammation is recognized as a mechanism by which obesity is associated with metabolic diseases. The communication between adipose tissue macrophages (ATMs) and adipocytes is important to understanding the interaction between immunity and energy metabolism and its roles in obesity-induced diseases. Yet visualizing adipocytes and macrophages in complex tissues is challenging to standard imaging methods. Here, we describe the use of a multimodal nonlinear optical (NLO) microscope to characterize the composition of VATs of lean and obese mice including adipocytes, macrophages, and collagen fibrils in a label-free manner. We show that lipid metabolism processes such as lipid droplet formation, lipid droplet microvesiculation, and free fatty acids trafficking can be dynamically monitored in macrophages and adipocytes. With its versatility, NLO microscopy should be a powerful imaging tool to complement molecular characterization of the immunity-metabolism interface.     

The action of D-dopachrome tautomerase as an adipokine in adipocyte lipid metabolism

Adipose tissue is a critical exchange center for complex energy transactions involving triacylglycerol storage and release. It also has an active endocrine role, releasing various adipose-derived cytokines (adipokines) that participate in complex pathways to maintain metabolic and vascular health. Here, we found D-dopachrome tautomerase (DDT) as an adipokine secreted from human adipocytes by a proteomic approach. DDT mRNA levels in human adipocytes were negatively correlated with obesity-related clinical parameters such as BMI, and visceral and subcutaneous fat areas. Experiments using SGBS cells, a human preadipocyte cell line, revealed that DDT mRNA levels were increased in an adipocyte differentiation-dependent manner and DDT was secreted from adipocytes. In DDT knockdown adipocytes differentiated from SGBS cells that were infected with the adenovirus expressing shRNA against the DDT gene, mRNA levels of genes involved in both lipolysis and lipogenesis were slightly but significantly increased. Furthermore, we investigated AMP-activated protein kinase (AMPK) signaling, which phosphorylates and inactivates enzymes involved in lipid metabolism, including hormone-sensitive lipase (HSL) and acetyl-CoA carboxylase (ACC), in DDT knockdown adipocytes. The AMPK phosphorylation of HSL Ser-565 and ACC Ser-79 was inhibited in DDT knockdown cells and recovered in the cells treated with recombinant DDT (rDDT), suggesting that down-regulated DDT in adipocytes brings about a state of active lipid metabolism. Furthermore, administration of rDDT in db/db mice improved glucose intolerance and decreased serum free fatty acids levels. In the adipose tissue from rDDT-treated db/db mice, not only increased levels of HSL phosphorylated by AMPK, but also decreased levels of HSL phosphorylated by protein kinase A (PKA), which phosphorylates HSL to promote its activity, were observed. These results suggested that DDT acts on adipocytes to regulate lipid metabolism through AMPK and/or PKA pathway(s) and improves glucose intolerance caused by obesity.     

Selective mobilization of fatty acids from adipose tissue triacylglycerols

Adipose tissue triacylglycerols represent the main storage of a wide spectrum of fatty acids differing by molecular structure. The release of individual fatty acids from adipose tissue is selective according to carbon chain length and unsaturation degree in vitro and in vivo in animal studies and also in humans. The mechanism of selective fatty acid mobilization from white fat cells is not known. Lipolysis is widely reported to work at a lipid-water interface where only small amounts of substrate are available. A preferential hydrolysis of a small triacylglycerol fraction enriched in certain triacylglycerol molecular species at the lipid-water interface and enzymological properties of hormone-sensitive lipase could explain the selective mobilization of fatty acids from fat cells. This selectivity could affect the individual fatty acid supply to tissues.     

Lipid-Overloaded Enlarged Adipocytes Provoke Insulin Resistance Independent of Inflammation

In obesity, adipocyte hypertrophy and proinflammatory responses are closely associated with the development of insulin resistance in adipose tissue. However, it is largely unknown whether adipocyte hypertrophy per se might be sufficient to provoke insulin resistance in obese adipose tissue. Here, we demonstrate that lipid-overloaded hypertrophic adipocytes are insulin resistant independent of adipocyte inflammation. Treatment with saturated or monounsaturated fatty acids resulted in adipocyte hypertrophy, but proinflammatory responses were observed only in adipocytes treated with saturated fatty acids. Regardless of adipocyte inflammation, hypertrophic adipocytes with large and unilocular lipid droplets exhibited impaired insulin-dependent glucose uptake, associated with defects in GLUT4 trafficking to the plasma membrane. Moreover, Toll-like receptor 4 mutant mice (C3H/HeJ) with high-fat-diet-induced obesity were not protected against insulin resistance, although they were resistant to adipose tissue inflammation. Together, our in vitro and in vivo data suggest that adipocyte hypertrophy alone may be crucial in causing insulin resistance in obesity.     

Adipolin/C1qdc2/CTRP12 protein functions as an adipokine that improves glucose metabolism

Obesity is a major risk factor for the development of insulin resistance and type 2 diabetes. Adipose tissue secretes various bioactive molecules, referred to as adipokines, whose dysregulation can mediate changes in glucose homeostasis and inflammatory responses. Here, we identify C1qdc2/CTRP12 as an insulin-sensitizing adipokine that is abundantly expressed by fat tissues and designate this adipokine as adipolin (adipose-derived insulin-sensitizing factor). Adipolin expression in adipose tissue and plasma was reduced in rodent models of obesity. Adipolin expression was also decreased in cultured 3T3-L1 adipocytes by treatment with inducers of endoplasmic reticulum stress and inflammation. Systemic administration of adipolin ameliorated glucose intolerance and insulin resistance in diet-induced obese mice. Adipolin administration also reduced macrophage accumulation and proinflammatory gene expression in the adipose tissue of obese mice. Conditioned medium from adipolin-expressing cells diminished the expression of proinflammatory cytokines in response to stimulation with LPS or TNFα in cultured macrophages. These data suggest that adipolin functions as an anti-inflammatory adipokine that exerts beneficial actions on glucose metabolism. Therefore, adipolin represents a new target molecule for the treatment of insulin resistance and diabetes.     

Aquaporin adipose, a putative glycerol channel in adipocytes

Adipose tissue is a major site of glycerol production in response to energy balance. However, molecular basis of glycerol release from adipocytes has not yet been elucidated. We recently cloned a novel member of the aquaporin family, aquaporin adipose (AQPap), which has glycerol permeability. The current study was designed to examine the hypothesis that AQPap serves as a glycerol channel in adipocytes. Adipose tissue expressed AQPap mRNA in high abundance, but not the mRNAs for the other aquaglyceroporins, AQP3 and AQP9, indicating that AQPap is the only known aquaglyceroporin expressed in adipose tissue. Glycerol release from 3T3-L1 cells was increased during differentiation in parallel with AQPap mRNA levels and suppressed by mercury ion, which inhibits the function of AQPs, supporting AQPap functions as a glycerol channel in adipocytes. Fasting increased and refeeding suppressed adipose AQPap mRNA levels in accordance with plasma glycerol levels and oppositely to plasma insulin levels in mice. Insulin dose-dependently suppressed AQPap mRNA expression in 3T3-L1 cells. AQPap mRNA levels and adipose glycerol concentrations measured by the microdialysis technique were increased in obese mice with insulin resistance. Accordingly, negative regulation of AQPap expression by insulin was impaired in the insulin-resistant state. Exposure of epinephrine translocated AQPap protein from perinuclear cytoplasm to the plasma membrane in 3T3-L1 adipocytes. These results strongly suggest that AQPap plays an important role in glycerol release from adipocytes.