Adipose Tissue: Not Just FatIntroduction to Adipose Tissue: WAT and BAT
Regulation of Adipogenesis
Regulation of Lipid Metabolism in Adipocytes
Table of Adipose Tissue Hormones and Cytokines
Leptin
Adiponectin
Resistin
Inflammatory Functions of Adipose Tissue
Metabolic Functions of Brown Adipose Tissue, BAT

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Introduction to Adipose TissueAdipose tissue is not merely an organ designed to passively store excess carbon in the form of fatty acids esterified to glycerol (triacylglycerols). Mature adipocytes synthesize and secrete numerous enzymes, growth factors, cytokines and hormones that are involved in overall energy homeostasis. Many of the factors that influence adipogenesis are also involved in diverse processes in the body including lipid homeostasis and modulation of inflammatory responses. In addition, a number of proteins secreted by adipocytes play important roles in these same processes. In fact recent evidence has demonstrated that many factors secreted from adipocytes are pro-inflammatory mediators and these proteins have been termed adipocytokines or adipokines. There are currently over 50 different adipokines recognized as being secreted from adipose tissue. These adipokines are implicated in the modulation of a range of physiological responses that globally includes appetite control and energy balance. Specific metabolic processes regulated by adipose tissue include lipid metabolism, glucose homeostasis, inflammation, angiogenesis, hemostasis (regulation of blood coagulation), and blood pressure.The major form of adipose tissue in mammals (commonly referred to as "fat") is white adipose tissue, WAT. Specialized adipose tissue that is primarily tasked with thermogenesis, especially in the neonate, is brown adipose tissue, BAT. BAT is so-called because it is darkly pigmented due to the high density of mitochondria rich in cytochromes. BAT specializes in the production of heat (adaptive thermogenesis) and lipid oxidation.histology of white and brown adipose tissue Histological section of adipose tissue demonstrating distinctive morphology of WAT and BAT. White adipocytes occupy the left side of the image and brown adipocytes the right side. As described below white adipocytes are generally rounded with over 90% of the cell volume taken up by a single fat droplet. The few small mitochondria and the nucleus are compressed to the very edge of the white adipocyte. The brown adipocytes are smaller in overall size, polygonal in shape, contain several small lipid droplets and high numbers of large mitochondria which imparts the brown color to these cells. Image from Junqueiras Basic Histology, 12th ed. by Anthony L. Mescher, McGraw-Hill Professional Division, reproduced with permission.WAT is composed of adipocytes held together by a loose connective tissue that is highly vascularized and innervated. White adipocytes are rounded cells that contain a single large fat droplet that occupies over 90% of the cell volume. The mitochondria within white adipocytes are small and few in number. The mitochondria and nucleus of the white adipocyte is squeezed into the remaining cell volume. Molecular characteristics of white adipocytes include expression of leptin but no expression of uncoupling protein 1, UCP1 (designated UCP1, leptin+). Brown adipocytes are smaller in overall size compared to white adipocytes. Brown adipocytes are polygonal in shape and contain numerous large mitochondria packed with cristae. Whereas, white adipocytes contain a single large fat droplet, brown adipocytes contain several small lipid droplets. Brown adipocytes are molecularly UCP1+ and leptin. BAT is primarily visceral with highest concentrations around the aorta. BAT is highy vascularized and contains a very high density of noradrenergic nerve fibers.In addition to adipocytes, WAT contains macrophages, leukocytes, fibroblasts, adipocyte progenitor cells, and endothelial cells. The presence of the fibroblasts, macrophages, and other leukocytes along with adipocytes, accounts for the vast array of proteins that are secreted from WAT under varying conditions. The highest accumulations of WAT are found in the subcutaneous regions of the body and surrounding the viscera (internal organs of the chest and abdomen).Although WAT can be found associated with numerous organs its functions are more than just insulation of the organ and a ready reservoir of fat for energy production. Depending on its location WAT serves specialized functions. The WAT associated with abdominal and thoracic organs (excluding the heart), the so-called visceral fat, secretes several inflammatory cytokines and is thus involved in local and systemic inflammatory processes. WAT associated with skeletal muscle secretes free fatty acids, interleukin-6 (IL-6) and tumor necrosis factor- (TNF) and as a consequence plays a significant role in the development of insulin resistance. Cardiac tissue associated WAT secretes numerous cytokines resulting in local inflammatory events and chemotaxis that can result in the development of atherosclerosis and systolic hypertension. Kidney associated WAT plays a role in sodium reabsorption and therefore can affect intravascular volume and hypertension.The major focus of this discussion will be on the biological activities associated with WAT, however, discussion of BAT is included at the end of this page. WAT serves many functions including insulating the viscera, storing excess carbon energy in the form of triacylglycerols and mediating glucose homeostasis. WAT also plays important roles as an endocrine/immune organ by secreting adipokines that includes inflammatory cytokines, complement-like factors, chemokines, and acute phase proteins. The endocrine functions of WAT regulate appetite, energy metabolism, glucose and lipid metabolism, inflammatory processes, angiogenesis, and reproductive functions. back to the top

Regulation of AdipogenesisThe process of adipocyte differentiation from a precursor preadipocyte to a fully mature adipocyte follows a precisely ordered and temporally regulated series of events. Adipocyte precursor cells emerge from mesenchymal stem cells (MSCs) that are themselves derived from the mesodermal layer of the embryo. The plutipotent MSCs receive extracellular cues that lead to the commitment to the preadipocyte lineage. Preadipocytes cannot be morphologically distinguished from their precursor MSCs but they have lost the ability to differentiate into other cell types. This initial step in adipocyte differentiation is referred to as determination and leads to proliferating preadipocytes undergoing a growth arrest. This initial growth arrest occurs coincident with the expression of two key transcription factors, CCAAT/enhancer binding protein alpha (C/EBP) and peroxisome proliferator-activated receptor gamma (PPAR). Following the induction of these two critical transcription factors there is a permanent period of growth arrest followed by expression of the fully differentiated adipocyte phenotype. This latter phase of adipogenesis is referred to as terminal differentiation.Although PPAR and C/EBP are the most important factors regulating adipogenesis additional transcription factors are known to influence this process. These additional factors include sterol-regulated element binding protein 1c (SREBP1c, also known as ADD1 for adipocyte differentiation-1), signal transducers and activators of transcription 5 (STAT5), AP-1 and members of the Krppel-like factor (KLF4, KLF5, and KLF15) family as well as C/EBP beta () and C/EBP delta (). More information about the roles of PPAR and SREBP in metabolic homeostasis can be found in the PPAR page as well as the Cholesterol page. Although these numerous transcription factors have been shown to influence overall adipogenesis, either positively or negatively, PPAR is the only one that is necessary for adipogenesis to take place. In fact, in the absence of PPAR, adipocyte differentiation fails to occur and as yet no factor has been identified that can rescue adipogenesis in the absence of PPAR. In spite of this fact, expression of PPAR does not commence during the initial activation of adipocyte differentiation but only after the responses elicited by STAT5, KLF4, KLF5, AP-1, SREBP1c, and C/EBP and C/EBP are exerted.PPAR was originally identified as being expressed in differentiating adipocytes and as indicated above it is now recognized as a master regulator of adipogenesis. PPAR was identified as the target of the thiazolidinedione (TZD) class of insulin-sensitizing drugs. The mechanism of action of the TZDs is a function of the activation of PPAR and the consequent induction of genes necessary for differentiation of adipocytes. The human PPAR gene (symbol PPARG) is located on chromosome 3p25 spanning over 100kb and composed of 9 exons encoding two biologically active isoforms as a consequence of alternative mRNAs and translational start codon usage. The principal protein products of the PPARG gene are identified as PPAR1 and PPAR2. PPAR1 is encoded for by exons A1 and A2 then common exons 1 through 6. PPAR2 is encoded by exon B and common exons 1 through 6. PPAR2 is almost exclusively expressed in adipocytes. Like all nuclear receptors the PPAR proteins contain a DBD and a LBD. In addition, like PPAR, the PPAR proteins contain a ligand-dependent activation function domain (identified as AF-2) and a ligand-independent activation function domain (identified as AF-1). The AF-2 domain resides in the LBD and the AF-1 domain is in the N-terminal region of the PPAR proteins. PPAR2 protein contains an additional 30 N-terminal amino acids relative to PPAR1 and these additional amino acids confer a 56-fold increase in the transcription-stimulating activity of AF-1 when compared to the same domain in the PPAR1 protein. Expression of PPAR1 is nearly ubiquitous. PPAR2 is expressed near exclusively in white adipose tissue (WAT) where it is involved in lipid storage and in BAT where it is involved in energy dissipation.As indicated above, during adipocyte differentiation several upstream genes are required for the activation of the PPARG gene. These include C/EBP and C/EBP, SREBP-1c, KLF5, KLF15, zinc-finger protein 423 (Zfp423), and early B-cell factor (Ebf1). In the process of adipocyte differentiation PPAR activates nearly all of the genes required for this process. These genes include aP2 which is required for transport of free fatty acids (FFAs) and perilipin which is a protein covering the surface of mature lipid droplets in adipocytes. Additional genes regulated by PPAR that are involved in lipid metabolism or glucose homeostasis include lipoprotein lipase (LPL), acyl-CoA synthase (ACS), acetyl-CoA acetyltransferase 1 (ACAT1), several phospholipase A (PLA) genes, adiponectin, the gluconeogenic enzyme PEPCK, and glycerol-3-phosphate dehydrogenase (GPD1). PPAR also functions in macrophage lipid metabolism by inducing the expression of the macrophage scavenger receptor, CD36. The CD36 receptor is also known as fatty acid translocase (FAT) and it is one of the receptors responsible for the cellular uptake of fatty acids. The role of SREBP-1c in activation of adipocyte differentiation is thought to be the result of this transcription factor initiating the expression of genes that, as part of their activities, generate PPAR ligands. This fact explains the necessity for SREBP expression to precede that of PPAR. In spite of this fact it has been shown that mice lacking SREBP-1 do not display significant reductions in the amount of WAT. However, levels of SREBP-2 are increased in these animals indicating that this may be a compensatory mechanism. Although loss of SREBP-1 expression does not result in a significant deficit in adipose tissue development, ectopic overexpression of SREBP-1c does enhance the adipogenic activity of PPAR.The C/EBP family of transcription factors were among the first to be shown to play a role in overall adipocyte differentiation. The three members of the family (C/EBP, C/EBP and C/EBP) are highly conserved basic-leucine zipper containing transcription factors. The importance of these factors in adipogenesis has been demonstrated in knockout mouse models. for example whole body disruption of C/EBP expression results in death shortly after birth due to liver defects, hypoglycemia, and failure of WAT or BAT accumulation. Using knockout mice it has been determined that the roles of C/EBP and C/EBP are exerted early in the process of adiopcyte differentiation whereas those of C/EBP are required later. In fact, expression of C/EBP is induced late in adipogenesis and is most abundant in mature adipocytes. The expression of both C/EBP and PPAR is, in part, regulated by the actions of C/EBP and C/EBP. One of the major effects of the expression of C/EBP in adipocytes is enhanced insulin sensitivity of adipose tissue. This later fact is demonstrated by the fact that C/EBP knockout does not abolish adipogenesis but the WAT is not sensitive to the actions of insulin.The general model of transcription factor activation of adipogenesis indicates that AP-1, STAT5, KLF4, and KLF5 are activated early and result in the transactivation of C/EBP and C/EBP. These latter two factors in turn activate the expression of SREPB-1 and KLF15 which leads to the activation of PPAR and C/EBP. It is important to keep in perspective that it is not only transcription factor activation of adipocyte precursors that controls adipogenesis. There is also a balance exerted at the level of transcription factor-mediated inhibition of adipogenesis. Some of the factors that are anti-adipogeneic include members of the Krppel-like factor family, KLF2 and KLF3. GATA2 and GATA3 also exert anti-adipogenic activity. GATA factors are so-called because they bind DNA elements that contain a core GATA sequence. Two of the interferon regulatory factor family of transcription factors, IRF3 and IRF4, oppose the process of adipogenesis as well.The changes in the pattern of expression of transcription factors that control the overall process of adipogenesis is associated with changes in chromatin dynamics. These changes in chromatin dynamics involve both histone protein methylation and DNA methylation events. The chromatin in pluripotent cells displays a highly dynamic nature with a high level of decondensed DNA. Once differentiation is induced there is a change in the overall pattern of methylated genes. Lineage-specific genes are demethylated whereas pluripotency genes are methylated resulting in transcriptional activation and silencing, respectively. As the process of adipocyte differentiation proceeds the genes encoding PPAR and C/EBP are observed to be repositioned into the interior of the nucleus coincident with their increased rates of transcription. Since MSCs can be induced to differentiate into bone and muscle, as well as adipocyte, it is necessary that adipocyte differentiation genes such as PPAR and C/EBP be silenced if the induced pathway is to bone or muscle.Associated with transcriptional silencing are protein complexes termed co-repressors and associated with transcriptional activation are complexes termed co-activators. When MSCs are induced down the bone lineage the histone 3 proteins in the PPAR promoter region are methylated on lysine 9 (identified as H3K9) by a co-repressor complex that includes the histone methyltransferase SETDB1 and the associated proteins NLK (Nemo-like kinase) and CHD7 (chromodomain helicase DNA binding protein-7). In addition to silencing of the PPAR promoter, the activity of the PPAR protein on its target genes is also restricted by association with co-repressor complexes. In preadipocytes PPAR activity is repressed by association with pRB and HDAC3 (histone deacetylase 3). The induction of differentiation results in the phosphorylation of pRB which leads to its release from the repressive complex. This in turn results in the recruitment of histone acetyltransferases (HATs) and the co-activator protein CBP/p300 (CBP is CREB binding protein, where CREB is cAMP-response element-binding protein) to the PPAR complex resulting in activation of PPAR target gene transcription.Numerous experiments have begun to define the large array of histone modifications that regulate the expression of genes involved in overall adipogenesis particularly the expression of PPAR. These histone modifying complexes include HATs, HDACs, histone methyltransferases (HMTs), and histone demethylases (HDMs). The general consequences of activation of HATs and HMTs is the activation of PPAR expression and/or enhancement of PPAR activity at its target gene promoters. Conversely, as expected, HDAC activation results in inhibited PPAR activity at its target gene promoters.back to the top

Regulation of Lipid Metabolism in AdipocytesThe triacylglycerols (TAG) found in WAT represent the major energy reserves of the body. The pool of TAG is in a constant state of flux that is regulated by food intake and fasting and the consequences of those dietary states on the levels of pancreatic hormones. In addition, adipose tissue fat pools change as a result of other hormonal fluctuations, inflammatory processes, and pathophysiology. The overall biochemistry of TAG metabolism is covered in the Lipid Synthesis page and the Fatty Acid Oxidation page. The aim of this section is to discuss in more detail the enzyme activities that regulate overall adipose tissue TAG homeostasis as well as the physiological and hormonal regulation of these processes.It was originally believed that the liberation of fatty acids from adipose tissue TAG stores was triggered exclusively via hormonal activation of hormone-sensitive lipase (HSL). However, when HSL-null mice were generated it was discovered that the process involves additional adipocyte HSL-independent TAG lipase activities. Subsequent research has led to the identification of at least five adipose tissue TAG lipases in addition to HSL. HSL has demonstrated activity with a wide variety of substrates that includes TAG, diacylglycerols (DAG), and cholesterol esters (CEs). When assayed in vitro the activity of HSL is at least 10-fold higher against DAG than TAG. When acting on TAG or DAG the activity of HSL is greatest against fatty acids that are in the sn-1 or sn-3 position of the glycerol backbone. Until recent experiments in knock-out mice demonstrated otherwise, HSL was believed to be the principle enzyme involved in adipocyte TAG and DAG hydrolysis as well as the primary neutral cholesteryl ester hydrolase (NCEH) activity.Although HSL-null mice still exhibit TAG hydrolase activity, results from studies in these mice indicate that HSL-mediated lipolysis is a significant contributor to overall fatty acid liberation from adipocytes. In mice lacking HSL there is a reduced level of circulating free fatty acids and TAGs as well as reduced hepatic storage of TAG. These results indicate that in the absence of HSL there is insufficient adipose tissue lipolysis to support the normal cellular demands for energy from fatty acids nor for adequate VLDL synthesis in the liver. Results of studies on the role of HSL in overall adipose tissue lipolysis demonstrate that it is not strictly required for the initiation of TAG hydrolysis as originally thought. However, in HSL-null mice there is an accumulation of DAGs indicating that the critical role for HSL is in the liberation of fatty acid from DAG which in turn generates monoacylglycerols (MAGs). The rate of fatty acid release from DAGs is on the order of 10- to 30-times that the rate of release from TAG. To date the only DAG lipase identified in adipose tissue is HSL.Further understanding of the role of HSL in overall adipose tissue lipolysis came from the identification of an additional TAG lipase that was originally termed desnutrin. The overall structure of desnutrin indicates that it contains typical domains found in many other lipases. Subsequent to the identification of desnutrin another lipase was characterized and called adipose triglyceride lipase (ATGL). Desnutrin and ATGL are the same protein so it is often designated desnutrin/ATGL. The desnutrin/ATGL gene is expressed predominantly in adipose tissue but also at much lower levels in cardiac and skeletal muscle and the testes. The intracellular location of desnutrin/ATGL is the cytosol as well as in tight association with lipid droplets. The activity of desnutrin/ATGL is specific for TAG as evidenced in cell culture experiments where over-expression of the gene results in increased free fatty acid release with no effect on phospholipid stores. In addition, desnutrin/ATGL has limited activity against DAG since in these and similar in vitro experiments there is a significant accumulation of DAG compared to the same types of experiments carried out with HSL.Expression of desnutrin/ATGL is under the influence of dietary status. In fasting animals the level of desnutrin/ATGL increases and then declines following re-feeding. This dietary regulation of desnutrin/ATGL suggests that it may play a contributory role in the development of obesity, a hypothesis supported by the fact that in genetically obese mice (ob/ob and db/db) the level of desnutrin/ATGL expression is reduced. When experiments are performed that artificially reduce the level of desnutrin/ATGL RNA or protein there is a significant drop in the level of free fatt acid release. Demonstrating a synergy between HSL and desnutrin/ATGL activity, in cells where both enzymes are reduced there is an additive level of reduction in free fatty acid release. A critical role for desnutrin/ATGL in TAG hydrolysis in tissues other than adipose tissue was shown by results of desnutrin/ATGL knock-out in mice. Thesse animals died at around 12-weeks of age due to increased ectopic fat stores particularly in the heart. In addition, total lipase activities in several tissues in addition to WAT and BAT were altered in the desnutrin/ATGL-null mice. These data point to a critical role for desnutrin/ATGL in TAG hydrolysis and fatty acid release not only from adipose tissue but also from tissues such as the heart, skeletal muscle, and testes.In addition to HSL and desnutrin/ATGL, adipose tissue expresses a number of other TAG hydrolases. Adipose tissue microsomes contain a non-HSL TAG lipase that is identified as triacylglycerol hydrolase (TGH, also called carboxylesterase 3). TGH contains typical lipase motifs and displays catalytic activity against long-, medium-, and short-chain TAGs as well as neutral cholesteryl esters. However, TGH does not hydrolyze phospholipids . Expression of TGH is seen predominantly in the liver where its primary functions are to mobilize intracellular TAG stores and participate in the synthesis of TAG-rich VLDLs. TGH expression is also seen in adipocytes and the level of its expression increases dramatically when preadipocytes differentiate into mature adipocytes. The adipose tissue regulation of TGH expression is effected, in part, via the action of C/EBP. A related protein, identified as TGH-2, has also been found predominantly expressed in the liver but is also present in adipose tissue and kidney.There is another interesting protein expressed predominantly in adipose tissue with a significant degree of homology to desnutrin/ATGL. This protein is called adiponutrin. Whereas, adiponutrin shows TAG lipase activity when assayed in vitro, when it is over-expressed in cells it has no effect on TAG hydrolysis. In addition, whereas desnutrin/ATGL (as well as most other lipases) expression is increased in the fasting state and decreased following re-feeding, adiponutrin expression exhibits the opposite pattern. In fasted animals adiponutrin mRNA is essentially undetectable and its levels increase dramatically in the re-fed state. It appears that although this enzyme is a member of the lipase family of enzymes it plays an anabolic rather than a catabolic role in adipocyte lipid metabolism.The final step in the complete hydrolysis of TAGs occurs when glycerol and the last fatty acid are released from MAGs by MAG lipase. This enzyme possesses no catalytic activity towards TAGs or DAGs nor cholesteryl esters. Numerous other proteins in addition to the lipases are involved in overall TAG homeostasis in adipose tissue. Several of these other proteins are associated with the lipid droplets inside the cell such as the perilipins, adipose fatty acid-binding protein (aFABP), and caveolin-1. Additional proteins important in overall TAG metabolism include aquaporin 7 (a water and glycerol transport protein) and lipotransin. The perilipins play a role in restricting access of TAG lipases to substrates in order to prevent unrestrained hydrolysis in the un-stimulated state. The role of aFABP is to carry free fatty acids from the fat droplet to the plasma membrane where they can be released to the plasma. The glycerol that is released from TAGs is exported via the action of aquaporin 7 as shown by experiments in mice lacking expression of this gene. These mice release free fatty acids upon stimulation of adipose tissue with catecholamines but no glycerol is released. The role of lipotransin is believed to be in shuttling HSL from the cytosol to the lipid droplet upon stimulation of adipocytes.Whether adipose tissue stores fatty acids as TAGs or releases them for energy production by other tissues is dependent upon the dietary, hormonal and physiological status of the organism. The primary mechanism for the stimulation of adipose tissue TAG hydrolysis is discussed in the fatty acid oxidation page. In brief, catecholamines such as epinephrine and norepinephrine, as well as the pancreatic hormone glucagon, bind to their cognate receptors on adipocytes triggering activation of adenylate cyclase resulting in increased levels of cAMP. In turn the cAMP activates PKA which then phosphorylates and activates HSL. Of significance is the fact that activation of PKA in HSL-null mice also results in enhanced TAG hydrolysis albeit at a level much lower than in the presence of active HSL. This indicates that there are PKA-mediated events in adipose tissue TAG lipolysis that are distinct from the classic HSL-mediated process.The primary change in adipose tissue TAG lipolysis following feeding is exerted via the actions of insulin. The cAMP-dependent changes that occur in response to insulin binding are effected by activation of phosphodiesterase 3B which hydrolyzes cAMP rendering PKA much less active. The activation of phosphodiesterase 3B occurs via PKB/Akt-mediated phosphorylation which itself is activated following insulin binding its receptor. The principle cAMP-independent mechanism for insulin-mediated reduction in TAG lipolysis is due to the stimulation of protein phosphatase-1 which removes the phosphate from HSL rendering it much less active. The activity of HSL is also affected via phosphorylation by AMPK. In this case the phosphorylation inhibits the enzyme. Inhibition of HSL by AMPK may seem paradoxical since the release of fatty acids stored in TAGs would seem necessary to promote the production of ATP via fatty acid oxidation and the major function of AMPK is to shift cells to ATP production from ATP consumption. This paradigm can be explained if one considers that if the fatty acids that are released from TAGs are not consumed they will be recycled back into TAGs at the expense of ATP consumption. Thus, it has been proposed that inhibition of HSL by AMPK mediated-phosphorylation is a mechanism to ensure that the rate of fatty acid release does not exceed the rate at which they are utilized either by export or oxidation.back to the top

Table of Adipose Tissue Hormones and CytokinesAdipose tissue produces and releases a vast array of protein signals including growth factors, cytokines, chemokines, acute phase proteins, complement-like factors, and adhesion molecules. The Table below describes several of the adipocyte proteins in more detail with leptin, adiponectin, and resistin discussed in greater detail in the following sections. The proteins of the various signaling processes are listed below. Not all WAT secretes the same adipokines as is evident from studies of differences in adipose tissue function in various anatomical regions of the body as described above. This is most significant when considering the clinical risks associated with increased adipose tissue mass. For example, increases in visceral WAT, even when subcutaneous fat depots are not increased, carry a greater metabolic risk for insulin resistance and diabetes and cardiovascular disease.Growth and angiogenic factors: fibroblast growth factors (FGFs), insulin-like growth factor-1 (IGF-1), hepatocyte growth factor (HGF), nerve growth factor (NGF), vascular endothelial cell growth factor (VEGF), transforming growth factor- (TGF), angiopoietin-1, angiopoietin-2, tissue factor (TF, factor 3)Cytokines: IL-1, IL-4, IL-6, IL-8, IL-10, IL-18, TNF, macrophage migration inhibitory factor (MIF)Complement-like factors: adipsin, adiponectin, acylation stimulating protein (ASP)Adhesion molecules and ECM components: 2-macroglobulin, vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), collagen I, collagen III, collagen IV, collagen VI, fibronectin, matrix metalloproteinase 1 (MMP1), MMP7, MMP9, MMP10, MMP11, MMP14, MMP15, lysyl oxidaseAcute phase proteins: C-reactive protein (CRP), serum amyloid A3 (SAA3), plasminogen activator inhibitor-1 (PAI-1), haptoglobinChemokines: chemerin, monocyte chemotactic protein-1 (MCP-1), macrophage inhibitory protein-1 alpha (MIP-1), regulated upon activation, normally T cell expressed and secreted (RANTES)Metabolic processes: adipocyte fatty acid binding protein (aP2), apolipoprotein E (apoE), resistin, omentin, vaspin, apelin, retinol binding protein 4 (RBP4), visfatin, leptinOther processes: COX pathway products PGE2 and PGI2 (prostacyclin), nitric oxide synthase pathway, renin-angiotensin system, tissue inhibitors of metalloproteinases (TIMPs)In addition to these secreted factors adipose tissue produces several plasma membrane and nuclear receptors that can trigger changes in adipose tissue function. Plasma membrane receptors include those for insulin, glucagon, growth hormone, adiponectin, gastrin, and angiotensin-II. Nuclear receptors include peroxisome proliferator-activated receptor- (PPAR), estrogens, androgens, vitamin D, thyroid hormone, progesterone, and glucocorticoids.

FactorPrincipal SourceMajor Action

adiponectin
also called adipocyte complement factor 1q-related protein (ACRP30), and adipoQ adipocytessee below

adipsin (also called complement factor D)adipocytes, liver, monocytes, macrophagesrate limiting enzyme in complement activation

apelinadipocytes, vascular stromal cells, heartlevels increase with increased insulin, exerts positive hemodynamic effects, may regulate insulin resistance by facilitating expression of BAT uncoupling proteins (e.g. UCP1, thermogenin)

chemerinadipocytes, livermodulates expression of adipocyte genes involved in glucose and lipid homeostasis such as GLUT4 and fatty acid synthase (FAS); potent anti-inflammatory effects on macrophages expressing the chemerin receptor (chemokine-like receptor-1, CMKLR1)

C-reactive protein (CRP)hepatocytes, adipocytesis a member of the pentraxin family of calcium-dependent ligand binding proteins; assists complement interaction with foreign and damaged cells; enhances phagocytosis by macrophages; levels of expression regulated by circulating IL-6; modulates endothelial cell functions by inducing expression of various cell adhesion molecules, e.g. ICAM-1, VCAM-1, and selectins; induces MCP-1 expression in endothelium; attenuates NO production by downregulating NOS expression; increase expression and activity of PAI-1

IL-6adipocytes, hepatocytes, activated Th2 cells, and antigen-presenting cells (APCs)acute phase response, B cell proliferation, thrombopoiesis, synergistic with IL-1 and TNF on T cells

leptinpredominantly adipocytes, mammary gland, intestine, muscle, placentasee below

monocyte chemotactic protein-1 (MCP-1)leukocytes, adipocytesis a chemokine defined as CCL2 (C-C motif, ligand 2); recruits monocytes, T cells, and dendritic cells to sites of infection and tissue injury

omentinvisceral stromal vascular cells of omental adipose tissuethe omentum is one of the peritoneal folds that connects the stomach to other abdominal tissues, enhances insulin-stimulated glucose transport, levels in the blood inversely correlated with obesity and insulin resistance

plasminogen-activator inhibitor-1 (PAI-1)adipocytes, monocytes, placenta, platelets, endometriumsee the Blood Coagulation page for more details

resistinadipocytes, spleen, monocytes, macrophages, lung, kidney, bone marrow, placentasee below

TNFprimarily activated macrophages, adipocytesinduces expression of other autocrine growth factors, increases cellular responsiveness to growth factors and induces signaling pathways that lead to proliferation

vaspinvisceral and subcutaneous adipose tissueis a serine protease inhibitor, levels decrease with worsening diabetes, increase with obesity and impaired insulin sensitivity

visfatin; also called pre-B cell-enhancing factor (PBEF);

reported to be the extracellular version of the enzyme nicotinamide phosphoribosyltransferase (Nampt or eNampt), however, the original paper claiming this has been retractedvisceral white adipose tissueconflicting results relative to insulin receptor binding but blocking insulin receptor signaling interferes with effects of eNampt; changes in eNampt activity occur during fasting and positively regulate the activity of the NAD+-dependent deacetylase SIRT1 leading to alterations in gene expression

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LeptinLeptin is 16kDa peptide whose central function is the regulation of overall body weight by limiting food intake and increasing energy expenditure. However, leptin is also involved in the regulation of the neuroendocrine axis, inflammatory responses, blood pressure, and bone mass. The human leptin gene is the homolog of the mouse "obese" gene (symbol OB) that was originally identified in mice harboring a mutation resulting in a severely obese phenotype. Leptin-deficient (ob/ob) and leptin receptor-deficient (db/db) mice exhibit numerous disruption in energy, hormonal, and immune system balance. These mice are obese, display hormonal imbalances, have defects in thermoregulation, have hematopoietic defects and are infertile. Levels of leptin increase in the serum in obese individuals and drop during weight loss. There is a direct correlation between the amount of body fat an individual carries and the circulating levels of leptin. Leptin activates the anorexigenic axis (appetite suppression) in the arcuate nucleus (ARC) of the hypothalamus by increasing the frequency of action potentials in the hypothalamic POMC neurons by depolarization through a nonspecific cation channel and by reduced inhibition by local orexigenic neuropeptide-Y (NPY) neurons. Leptin functions by binding to its receptor which is a member of the cytokine receptor family. Leptin and its receptors possess structural similarities to the IL-6 family of cytokines and the class I cytokine receptor family. The leptin receptor mRNA is alternatively spliced resulting in six different products. The leptin receptors are named Ob-R, OB-Rb, OB-Rc, Ob-Rd, Ob-Re, and Ob-Rf. The OB-Rb mRNA encodes the long form of the leptin receptor (also called LEPR-B) and is expressed primarily in the hypothalamus but is also expressed in cells of the innate and adaptive immune systems as well as in macrophages. The other receptor subtypes are expressed in numerous tissues including muscle, liver, kidney, adrenal glands, leukocytes, and vascular endothelium. Activation of the receptor leads to increased phosphatidylinositol-3-kinase (PI3K) and AMPK activity via activation of the Jak/STAT signaling pathway. One effect of the activation of the Jak/STAT pathway is activation of suppressor of cytokine signaling 3 (SOCS3) which then inhibits leptin signaling in a negative feed-back loop. Leptin binding its receptor also results in the activation of mTOR both in the hypothalamus and in peripheral tissues. The role of mTOR in the regulation of protein synthesis is covered in both the Protein Synthesis page as well as in the Insulin Functions page. The role of leptin in the activation of mTOR function is an important factor in the ability of leptin to activate macrophages. An interesting aspect of the role of leptin in mTOR function is that within mature adipocytes leptin synthesis itself is dependent on mTOR activation. Given that leptin levels rise in the serum of obese individuals and that leptin interaction with macrophages leads to increased macrophage inflammatory processes, it is not surprising that there is a direct correlation between leptin levels and the development of atherosclerosis.Signal transduction pathways activated by leptin receptor When leptin binds to its receptor (LEPR-B) the receptor undergoes a conformational change that activates the receptor-associated Jak2 tyrosine kinase. Activated Jak2 will autophosphrylate itself as well as phosphorylate the tyrosine (Y) residues in LEPR-B at positions 985, 1077, and 1138. Phosphorylated Y-985 serves as a docking site for SHP2 (SH2 domain containing protein tyrosine phosphatase, also called PTP1D). The gene that encodes SHP2 is identified as PTPN11. Phosphorylated Y-1077 serves as a docking site for STAT5 (signal transduction and activation of transcription 5). Phosphorylated Y-1138 serves as a docking site for STAT3. When SHP2, STAT5, and STAT3 bind to phosphorylated LEPR-B they themselves are activated by Jak2-mediated phosphorylation. Activated SHP2 in turn activates the ERK1/2 (extracellular-regulated kinase 1/2) signal pathway that results in increasd transcription of the EGR-1 gene. Activated STAT3 in turn activates the transcription of SOCS3 (suppressor of cytokine signaling 3). SOCS3 will then interact with Y-985 and attenuate signaling from SHP2 as well as interact with Jak2 and attenuate its tyrosine kinase activity resulting in a negative feed-back loop.Leptin expression is under complex control and a number of transcription factor binding sites have been identified in the promoter region of the leptin gene. Leptin levels are higher in age and weight-matched females compared with males. This is partially due to the inhibition of leptin expression by androgens and the stimulation of expression by estrogens. Leptin expression has been shown to be increased by sex steroids, glucocorticoids, cytokines, and toxins released during acute infection. The sympathetic nervous system triggers a reduction in circulating leptin levels via the release of catecholamines. This effect of catecholamines has been shown to be due to activation of -adrenergic receptor signaling.In addition to effects on appetite exerted via central nervous system functions, leptin is also known to exert effects on inflammatory processes. Leptin modulates peripheral T cell function leading to increased levels of T helper cell type 1 cytokines. In addition leptin reduces thymocyte apoptosis and increases thymic cellularity. These result correlate well with observations demonstrating a reduced capacity for immunologic defense when leptin levels are low. However, too much leptin is not beneficial as high concentrations can result in an abnormally strong immune response which predisposes an individual to autoimmune phenomena. Acute stimulation with pro-inflammatory cytokines results in increased serum levels of leptin, whereas, chronic stimulation by IL-1, IL-6, or TNF leads to reduced levels of serum leptin. back to the top

AdiponectinAdiponectin was independently isolated by four different laboratories leading to different names. However, adiponectin is considered the standard name for this adipose tissue-specific protein. Other names include adipocyte complement related protein of 30kDa (ACRP30) because of its homology to complement factor 1q (C1q), adipoQ, gelatin-binding protein 28kDa (BGP28), and adipocyte most abundant gene transcript 1 (apM1). The major biological actions of adiponectin are increases in insulin sensitivity and fatty acid oxidation. The adiponectin gene is located on chromosome 3q27.3 spanning 16 kbpand composed of 3 exons. The adiponectin gene does not contain a typical typical TATA-box promoter element upstream of the transcriptional start site. Adiponectin contains a C-terminal globular domain which harbors the homology to C1q and an N-terminal collagen-like domain. The globular domain allows for a homotrimeric association of the protein forming the functional structure of the protein. The association of the subunits is such that two trimeric globular domains interact with a single stalk of collagen domains formed from two trimers. This complex structure is similar to the TNF superfamily of proteins despite there being no amino acid sequence homology between adiponectin and TNF proteins. Within the circulation, adiponectin exists in both a full-length form as well as a globular form that is the result of proteolytic cleavage of the full-length protein. The hormone forms complex structures such that it can be found as a trimer, hexamer, and as the high molecular weight oligomer. In addition to the complex structure, adiponectin is glycosylated, a modification that is essential to its activity. Adiponectin activity is inhibited by adrenergic stimulation and glucocorticoids. Expression and release of adiponectin is stimulated by insulin and inhibited by TNF-. Conversely, adiponectin exerts inflammatory modulation by reducing the production and activity of TNF- and IL-6. Unlike leptin, levels of adiponectin are reduced in obese individuals and increased in patients with anorexia nervosa. There are sex-related differences in adiponectin levels as well similar to that seen for leptin where age and weight matched males have lower levels than females. In patients with type 2 diabetes, levels of adiponectin are significantly reduced. Adiponectin functions by interaction with specific cell-surface receptors and at least two receptors have been identified. AdipoR1 is expressed at highest levels in skeletal muscle and AdipoR2 in primarily expressed in the liver. Expression of adiponectin receptors is also seen in various regions of the brain. AdipoR1 is highly expressed in the medial prefrontal cortex, hippocampus, and the amygdala. Whereas, AdipoR2 expression in the brain is restricted to the hippocampus and specific hypothalamic nuclei. AdipoR1 is a high-affinity receptor for globular adiponectin and has low affinity for the full-length adiponectin. In contrast, AdipoR2 has intermediate affinity for globular and full-length adiponectin. Although these receptors contain seven transmembrane domains typical of the serpentine family of G-protein coupled receptors (GPCRs), they are structurally distinct from the GPCR class. The AdipoRs stimulate the phosphorylation and activation of AMPK. The adiponectin-mediated activation of AMPK results in increased glucose uptake, increased fatty acid oxidation, increased phosphorylation and inhibition of acetylCoA carboxylase (ACC) in muscle. In the liver the result is reduced activity of gluconeogenic enzymes and glucose output.Adiponectin also plays an important role in hemostasis by suppressing TNF-mediated inflammatory changes in endothelial cell responses and inhibiting vascular smooth muscle cell proliferation. Activation of AMPK activity in endothelial cells results in increased fatty acid oxidation and activation of endothelial NO synthase (eNOS).back to the top

ResistinResistin is a 12 kDa protein that was originally identified in mice in a screen for genes suppressed by an agonist of the peroxisome proliferator-activated receptor- (PPAR). The name resistin derives from the original observation that this protein induced insulin resistance in mice. Resistin belongs to a family of four proteins referred to as FIZZ proteins for "found in inflammatory zone". Resistin is thus, also referred to as FIZZ3. Although resistin is expressed in adipocytes, in humans it appears that macrophages may be the most important source of the protein.In mice resistin expression increases during adipocyte differentiation and levels of resistin increase in diet-induced obesity. Reduction in resistin levels is associated with increased AMPK activity in the liver which leads to decreased expression of gluconeogenic enzymes and consequent reduction in hepatic glucose production. Conversely, elevation in resistin levels is associated with increased hepatic glucose production and glucose intolerance. Whether these same responses to resitin are evident in human is still under investigation. What is known is that overexpression of resistin in human heptocytes impairs insulin-stimulated glucose uptake and glycogen synthesis. Part of the mechanism for impaired glycogen synthesis is that resistin decreases the expression of one of the insulin receptor substrates (IRS-2) which is involved in the activation of PI3K. The PI3K-activated signal pathway leads to the phosphorylation and inhibition of glycogen synthase kinase 3 (GSK3). Un-phosphorylated GSK3 would normally phosphorylate and inhibit glycogen synthase activity. Loss of this pathway then leads to a higher rate of glycogen synthase inhibition by GSK3-mediated phosphorylation.Resistin also exerts effects on the immune system and the vasculature. Resistin modulates endothelial cell function by enhancing expression of the cell adhesion molecule VCAM-1 and the chemoattractant MCP-1. Resistin has also been shown to exert a pro-inflammatory effect on smooth muscle cells.back to the top

Inflammatory Functions of Adipose TissueThe significance of inflammatory responses elicited via secretion of adipose tissue-derived (WAT) cytokines relates to the fact that their production and secretion is increased in obese individuals. There is a growing body of evidence that demonstrates a direct link between the changes in adipose tissue function in obesity and the development of type 2 diabetes and the metabolic syndrome. One key change in adipose tissue during obesity is an increase in the percentage of macrophages resident within the tissue. Macrophages are a primary source of pro-inflammatory cytokines secreted by adipose tissue. The primary adipokine responsible for this infiltration is monocyte chemotactic protein-1 (MCP-1).As the level of macrophages increases in adipose tissue the level of pro-inflammatory cytokine secretion by the tissue increases. Circulating levels of both TNF- and IL-6 increase as adipose tissue expands in obesity and these changes are directly correlated with insulin resistance and the development of type 2 diabetes. As indicated above adiponectin plays an important anti-inflammatory role by suppressing the production of both TNF- and IL-6. However, as the level of macrophage infiltration increases in obesity the increased secretion of TNF- results in suppression of adiponectin production and secretion. Adipose tissue-derived IL-6 accounts for approximately 30% of the circulating level of this pro-inflammatory cytokine. Visceral WAT has been shown to secrete a higher percentage of the circulating IL-6 than subcutaneous WAT and this fact correlates with the negative effects of a pro-inflammatory status (as is the case with obesity) on the organs. As WAT density increases there is an associated increase in IL-6 secretion which is correlated to an increase in the circulating levels of acute-phase proteins such as CRP. In addition to the effects of TNF- on adiponectin production the cytokine also directly affects insulin sensitivity by inhibiting insulin receptor signaling. This effect of TNF- is the result of increased insulin receptor serine phosphorylation. TNF- (as well as IL-6) trigger the release of pro-inflammatory cytokines such as JUN N-terminal kinase (JNK) and nuclear factor kappa B (NFB). Activation of JNK leads to the insulin receptor serine phosphorylation as well as insulin receptor substrate (IRS) serine phosphorylation. Both of these TNF--mediated serine phosphorylations lead to impaired insulin receptor signaling. JNK and NFB activate pro-inflammatory genes which results in a self-perpetuating cycle of increased inflammatory cytokine release. TNF- also decreases endothelial nitric oxide synthase (eNOS) resulting in decreased levels of NO as well as decreased expression of mitochondrial oxidative phosphorylation genes. This leads increased oxidative stress, accumulation of reactive oxygen species (ROS), and increased endoplasmic reticulum stress. The responses of the cell to the increased oxidative stress is further increases in NFkB releases and thus, increased inflammatory processes. The effects of adipocyte-derived TNF- and IL-6 demonstrates a clear correlation between obesity and a pro-inflammatory role for adipocytes.Although lymphocytes (T-cells and B-cells) are not constituents of adipose tissue they are physically associated within the lymph nodes. Lymph tissue is surrounded by pericapsular adipose tissue which increases in density with increasing obesity. This close association allows for 2-way paracrine interactions between the lymph and adipose tissues. One important interaction between lymph tissue and WAT involves leptin. As indicated above, leptin plays a major role in the regulation of appetite and energy balance and the circulating levels of leptin increases as adipose tissue mass increases. Leptin has also been shown to modulate T-cell function, thus demonstrating a pro-inflammatory role for leptin. Leptin protects T-cells from apoptosis and enhances the switching of T-cells to a Th1 response. Pro-inflammatory cytokine production and release from T-cells is increased as a result of leptin action. Leptin induces a number of signal transduction pathways in immune and endothelial cells as outlined above. Leptin effects on the vascular endothelium are also pro-inflammatory. Expression of adhesion molecules is increased by leptin binding its receptor on endothelial cells. This results in an increased ability of neutrophils and other leukocytes to adhere to the endothelium leading to increased local inflammatory processes.back to the top

Metabolic Functions of Brown Adipose Tissue, BATIt was originally thought that BAT was present in humans only during the neonatal period. However, recent evidence has demonstrated that adults retain some metabolically active BAT deposits that respond to cold and sympathetic nervous system activation. The most studied regulator of the action of BAT is norepinephrine.Within BAT, norepinephrine interacts with all three types of adrenergic receptors (1, 2, and ) each of which activates distinct signaling pathways in the brown adipocyte. With respect to the -adrenoreceptors, the 3 subtype is the most significant, 1 receptors are expressed in brown preadipocytes but not mature adipocytes and 2 receptors are expressed in BAT but not brown adipocytes themselves. Of significance to the role of BAT in thermogenesis and the major role of 3-adrenoreceptors is the fact that these receptors are not subject to desensitization as are the 1 and 2 receptors. However, this is not to say that continuous adrenergic stimulation has no negative effect in BAT. Indeed, the level of 3 receptor expression is downregulated during continuous adrenergic stimulation. However, this effect is transient and the mRNA rapidly re-accumulates after termination of the stimulatory signal.Signal transduction events triggered by adrenergic stimulation of BAT are effected via the activation of Gs type G-proteins. Gs proteins couple to the activation of adenylate cyclase resulting in increased cAMP production and activation of PKA. For more information on the types of G-proteins visit the Signal Transduction page. Activation of HSL leads to release of free fatty acids which are taken up by the mitochondria similarly as they would be for purposes of oxidation, however, in BAT they interact with and activate the proton gradient uncoupling activity of uncoupling protein 1 (UCP1, also known as thermogenin). Uncoupling the proton gradient releases the energy of that gradient as heat. The thermogenic function of BAT is outlined in the Figure below. In addition to stimulating heat production in BAT, norepinephrine promotes the proliferation of brown preadipocytes, promotes the differentiation of mature brown adipocytes, inhibits apoptosis of brown adipocytes, and regulates the expression of the UCP1 gene.Hormonal generation of heat in brown fatHormonal generation of heat in brown fatInterestingly, norepinephrine stimulation of 2-adrenergic receptors on BAT has the opposite effect to 3 receptor activation. This is due to the fact that the 2 receptors are coupled to Gi type G-proteins whose activation results in inhibition of adenylate cyclase. The precise physiological significance of this parallel response of BAT to norepinephrine is as yet not fully appreciated. It is unclear how, or when, BAT balances the thermogenic stimulatory actions of 3 receptors with the inhibitory actions of 2 receptors but it may allow the cells to modulate their responses to norepinephrine under varying physiological conditions.Similar to the endocrine role of WAT, BAT also synthesizes and secretes numerous factors. However, due to the relatively small overall size of brown fat compared to white fat the role of factors secreted from BAT serve many autocrine and paracrine roles. This is not to say that BAT doesn't have a potential endocrine role it is just not as well established as the endocrine role of WAT.Several proteins produced from BAT serve autocrine functions such as adipsin, fibroblast growth factor 2 (FGF2), insulin-like growth factor-1 (IGF-1), prostaglandins, and adenosine. Adipsin (also known as complement factor D) cleaves the complement protein C3 into C3a and C3b. C3a is inactivated to C3desArg which is more commonly referred to as acylation-stimulating protein (ASP). ASP stimulates TAG synthesis and glucose uptake in WAT. The level of C3 expression is much higher in WAT than in BAT and ASP has not been found in BAT so the precise function of adipsin in BAT is not clear although it could be functional when BAT is in a thermogenically inactive state and is functioning in an anabolic state to store TAGs. Acute and chronic cold exposure results in increased FGF2 expression in BAT as does norepinephrine. The FGF receptor 1 (FGFR1) gene is also expressed in BAT and the result of FGF2 activation of the receptor is increased brown adipocyte cell density. Similarly, during cold exposure the levels of IGF-1 mRNA increase and IGF-1 receptors are highly expressed on brown adipocytes. IGF-1 action can prevent TNF--induced apoptosis in brown fat.Paracrine factors synthesized by BAT include nerve growth factor (NGF), vascular endothelial cell growth factor (VEGF), angiotensinogen, and NO. The secretion of NGF occurs primarily from proliferating brown preadipocytes and in this capacity is believed to promote sympathetic innervation of the tissue which in turn permits increased norepinephrine stimulation of the cells in BAT. Expression of VEGF is high in proliferating and mature brown adipocytes and the VEGF receptors, FLK-1 and FLK-4, are expressed in BAT. The expression of VEGF in BAT may promote and maintain the high level of vascularization in this tissue. Norepinephrine stimulation and cold stress both result in increased levels of VEGF expression in BAT. Both inducible nitric oxide synthase (iNOS) and endothelial NOS (eNOS) are expressed in BAT. In addition to its expression in the endothelial cells of BAT, eNOS expression is seen in brown adipocytes. Norepinephrine stimulation of BAT, as well as brown adipocytes in culture, results in increase NO production. Within brown adipocytes the production of NO may lead to inhibition of mitochondrial oxidation. Within BAT the production of NO likely promotes rapid increases in blood flow.back to the top

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Last modified: February 7, 2013