模板综述-白色脂肪组织的存在

The International Journal of Biochemistry &Cell Biology 56(2014)123–132Contents lists available at ScienceDirectThe International Journal of Biochemistry&CellBiologyj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /b i o c elReviewMaintenance of white adipose tissue in man ଝMervi T.Hyvönen,Kirsty L.Spalding ∗Department of Cell and Molecular Biology,Karolinska Institute,Berzelius väg 35,Stockholm 171-77,Swedena r t i c l ei n f oArticle history:Received 14April 2014Received in revised form 30August 2014Accepted 9September 2014Available online 20September 2014Keywords:Preadipocytes AdipocytesAdipocyte progenitors Adipocyte turnover Obesitya b s t r a c tObesity is increasing in an epidemic manner in most countries and constitutes a public health problem by enhancing the risk for diseases such as diabetes,fatty liver disease and atherosclerosis.Together these diseases form a cluster referred to as the metabolic syndrome.Despite the negative health consequences associated with excess adipose tissue,very little is known about the origin and maintenance of white adipose tissue in man.In this review we discuss what is known about the turnover of adult human adipocytes and their precursors,as well as adipose tissue heterogeneity,plasticity and developmental origins.The focus of this review is human tissue,however in many cases human data are missing and are inferred from animal studies.As such,reference to animal studies are made where human data is not available.This article is part of a directed issue entitled:Regenerative Medicine:the challenge of translation.©2014Elsevier Ltd.All rights reserved.Contents 1.Introduction .........................................................................................................................................1242.Adipocyte turnover ..................................................................................................................................1242.1.Adipocyte number in man ...................................................................................................................1242.2.Adipocyte death .............................................................................................................................1253.Adipose tissue heterogeneity .......................................................................................................................1254.Plasticity of adipocytes and their precursors .......................................................................................................1264.1.Adipose tissue-derived stem/stromal cells..................................................................................................1264.2.Dedifferentiated adipocytes (dedifferentiated fat (DFAT)cells)............................................................................1274.3.White-to-brite adipocyte transdifferentiation ..............................................................................................1275.Developmental origins of adipocytes and adipocyte precursors ...................................................................................1275.1.Ectodermal origin ............................................................................................................................1275.2.Mesenchymal origin .........................................................................................................................1285.3.Endothelial origin ............................................................................................................................1295.4.Hematopoietic origin ........................................................................................................................1295.5.Origins of brite adipocytes...................................................................................................................1306.Conclusions ..........................................................................................................................................130References ...........................................................................................................................................130Abbreviations:ADSC,adipose tissue-derived stromal/stem cell(s);ASC,adipose-tissue stromal/stem cell(s);BAT,brown adipose tissue;C/EBP,CCAAT/enhancer-binding protein;DFAT cells,dedifferentiated fat cells;EGFP,enhanced green fluorescent protein;FABP4,fatty acid binding protein 4(aP2);GLUT4,glucose transporter 4;GFP,green fluorescent protein;HSL,hormone sensitive lipase;iPS,induced pluripotent stem cell;LPL,lipoprotein lipase;PDGFR,platelet-derived growth factor receptor;PPAR ␥,peroxisome proliferator-activated receptor ␥;SA-ASC,supraadventitial-adipose stromal cells;scWAT,subcutaneous white adipose tissue;SVF,stromal–vascular fraction;TNF-␣,tumor necrosis factor ␣;UCP1,uncoupling protein 1;vWAT,visceral white adipose tissue;WAT,white adipose tissue;scWAT,subcutaneous white adipose tissue;YFP,yellow fluorescent protein.ଝThis article is part of a directed issue entitled:Regenerative Medicine:the challenge of translation.∗Corresponding author.E-mail address:kirsty.spalding@ki.se (K.L.Spalding)./10.1016/j.biocel.2014.09.0131357-2725/©2014Elsevier Ltd.All rights reserved.124M.T.Hyvönen,K.L.Spalding/The International Journal of Biochemistry&Cell Biology56(2014)123–1321.IntroductionTwo general types of adipose tissue exist in humans,white (WAT)and brown(BAT).White adipocytes store triglycerides and cholesterol in a single large lipid droplet(unilocular appear-ance),while brown adipocytes are present mainly in infants, contain several smaller lipid droplets(multilocular appearance) and oxidize fatty acids for heat production(non-shivering ther-mogenesis).WAT is a highly dynamic tissue,i.e.capable of rapidly changing its mass according to the body’s energy sta-tus.Although mature adipocytes constitute the majority of WAT mass/volume,they account for less than20%of the total cells in WAT(Eto et al.,2009).The other cells,collectively referred to as the stromal–vascular fraction(SVF),are a heterogeneous population of endothelial cells,macrophages,fibroblasts,stem cells and lymphocytes.Although once considered merely as a site of passive energy storage,WAT is now recognized as an important endocrine organ,secreting adipokines,which regulate important physiological functions such as appetite,energy expen-diture,insulin sensitivity,inflammation and coagulation(Hauner, 2005).WAT is necessary for normal human physiology.Lipodystro-phies,characterized by loss and/or redistribution of fat in certain depots,are associated with insulin resistance,dyslipidemia,car-diovascular disease,and high blood pressure–i.e.the metabolic syndrome(Capeau et al.,2005).On the other hand,in most individuals obesity leads to metabolic syndrome,type2dia-betes and some types of cancer(Pedersen,2013;De Pergola and Silvestris,2013).To add further complexity,WAT is not the same in all body depots.An increased amount of visceral adi-pose tissue(vWAT)results in a much higher risk for the metabolic syndrome than an increased amount of subcutaneous adipose tissue(scWAT)(Patel and Abate,2013).In fact,increased subcu-taneous fat deposition might even protect against the metabolic syndrome(Snijder et al.,2003).In addition,the morphology of WAT has a clear clinical relevance.Adipocyte hypertrophy (increased cell size)is associated with decreased insulin sensitiv-ity,even in lean subjects(Arner et al.,2010).Moreover,adipocyte hypertrophy is an independent risk factor for developing type 2diabetes,and individuals with hypertrophic obesity have a poorer metabolic profile than those with hyperplastic(increased number of cells)obesity(Arner et al.,2010;Hoffstedt et al., 2010).Recently a third type of adipocyte has been characterized and termed‘brite’,for its‘br own-i n-whi te’phenotype(Petrovic et al.,2010).Brite cells(also referred to as beige,inducible, recruitable,brown-like or paucilocular cells)are interspersed in white adipose tissue and can activate UCP-1mediated thermo-genic activity following cold induction or administration of agonists of the␤-adrenergic receptor or peroxisome proliferator-activated receptor-␥(PPAR␥)(Ohno et al.,2012;Wu et al.,2012;Lee et al., 2012,reviewed in Giordano et al.,2014;Lee and Cowan,2013; Rosenwald and Wolfrum,2014).Based on their initial character-ization these cells are collectively referred to as brite here,unless discussing a specific body of work where the authors have chosen a different term.Wolfrum and colleagues recently showed that there is a direct interconversion between brite adipocytes and mature white adipocytes,demonstrating that brite-stimulated cell expres-sion is an inducible and reversible event(Rosenwald et al.,2013). Given the energy-dissipating nature of brown and brite adipocytes, much interest is focused on targeting cells that can be induced to become brite as a means of increasing energy expenditure and com-bating obesity(van der Lans et al.,2013;van Marken Lichtenbelt et al.,2009;Virtanen et al.,2009,reviewed in Harms and Seale, 2013;Lee and Cowan,2013).2.Adipocyte turnover2.1.Adipocyte number in manAdipocyte number in WAT in man is remarkably stable in adult-hood.Weight loss,resulting in a significant reduction in adipose tissue,occurs as a result of a decrease in the average size of adipocytes and not due to a decrease in cell number(Salans et al., 1971;Björntorp et al.,1975;Häger et al.,1978;Spalding et al., 2008).For many years such observations supported the notion that adipocytes represent a terminally differentiated,non-turning over population of cells that respond to changes in the energy balance by increasing or decreasing their lipid content.Recent studies,how-ever,have shown that adipocytes in adult humans turn over,at rates ranging from10%(Spalding et al.,2008)to100%(Strawford et al.,2004)per year.Differences in adipocyte turnover estimates most likely reflect differences in the methods used to measure cell turnover.The14C labeling method of Spalding et al.(2008),retro-spectively determines the age of cells across the entire lifespan of an individual,which is in contrast to Strawford et al.(2004)who use short-term labeling studies,administering deuterated water for pulse periods to label newly synthesized DNA.Radiocarbon dat-ing and heavy water labeling are thus complementary approaches suitable for detecting cell turnover in the older,mature adipocyte fraction(14C)as well as the faster turnover of the proliferating fraction(heavy water).This data suggests that most proliferating pre-adipocytes do not permanently join the mature adipocyte pop-ulation(Spalding et al.,2008).Such a dynamic turnover of adipocytes raises the question as to why,when one significantly adjusts the energy balance in favor of weight loss(decreased calorie intake,increased energy expen-diture)does the body continue to produce the same number of new adipocytes per year?It seems that adipocyte number,at least in the case of weight loss,is tightly regulated and independent of the energy balance.One theory explaining the tight control of adipocyte number in humans is the“thrifty genotype”theory.This theory proposes that humans have undergone a positive selec-tion for genes that favor energy storage,as part of an evolutionary adaption to survive times of famine and promote energy storage for times of need(Need,1999;Sellayah et al.,2014).This,com-bined with a sedentary lifestyle and a diet high in saturated fats and refined carbohydrates,contributes to the obesity epidemic we see today.Whilst adipocyte number seems to be tightly regulated in weight-stable adults and during weight loss,which may have some evolutionary advantage,it is less clear how adipocyte num-ber is affected in obesity.Much needed longitudinal studies,where individuals are followed with and without weight gain over many years,is sorely missing from the literature.Short-term studies of weight gain,however,provide some insight.Salans et al.(1971) looked at short-term weight gain(3–4months)in lean men and reported a significant increase in adipocyte size following weight gain,however no increase in adipocyte number.Adipocyte number was calculated by dividing total body fat by the average amount of fat per cell,taken from femoral,abdominal and tricep adipose biopsies(Salans et al.,1971).Since regional information on fat mass was not known,fat cell number for individual depots was not able to be calculated and was instead calculated using an aver-age from all three depots.Recently Jensen and colleagues,using dual-energy X-ray absorptiometry(DEXA)and computed tomogra-phy(CT)imaging to determine the size of abdominal and femoral adipose depots,showed regional differences in adipose cellular-ity following diet-induced weight gain(Tchoukalova et al.,2010). In this study short-term weight gain(8weeks)in female subjects demonstrated a significant increase in adipocyte number in the femoral sc depot.Interestingly,no significant change in adipocyteM.T.Hyvönen,K.L.Spalding/The International Journal of Biochemistry&Cell Biology56(2014)123–132125number was seen in the abdominal sc adipose tissue,suggesting a region-specific regulation of fat cell number.Lipectomy studies in animals(for review see Mauer et al.,2001)and humans(Finzi, 2003;Yun et al.,2003;Frew et al.,2005;Van der Lei et al.,2007) demonstrate a compensatory recovery of body fat,suggesting the existence of a body fat regulatory system.In nearly all species stud-ied,regrowth at the excision site does not occur,rather adiposity levels are restored(or partially restored)via compensatory hyper-trophy,without accompanying weight gain(Mauer et al.,2001). In studies by Scarborough and Bisaccia(Bisaccia and Scarborough, 1990;Scarborough and Bisaccia,1991)it was observed that40% of women who have abdominal liposuction,increase one or more cup sizes.Multiple studies have since confirmed these results, demonstrating that abdominal fat excision increases breast size and that this increase in breast size is positively correlated to the amount of abdominal adipose tissue removed(Finzi,2003;Yun et al.,2003;Frew et al.,2005;Van der Lei et al.,2007).Whilst this indirectly supports the lipostatic theory,that is that total adipos-ity is centrally regulated,extensive abdominal liposuction results in peripheral changes in the estrogen-to-androgen ratio(with a relative increase in estrogen effect),providing an alternative expla-nation for increases in breast size.2.2.Adipocyte deathImmunohistochemical staining of macrophage markers in WAT from insulin-resistant individuals identify“crown-like structures”, which are dead/dying adipocytes surrounded by macrophages. Necrosis-like cell death and associated recruitment of macrophages is increased up to30-fold in obese humans and mice,and has been implicated in the metabolic complications of obesity(Cinti et al., 2005).As opposed to apoptotic cell death,where cells die in a con-trolled non-inflammatory manner,necrosis involves macrophage proliferation and recruitment with a subsequent inflammatory response.Hypertrophic obesity is associated with an inflammatory response,characterized by high serum IL-6,TNF-␣and C-reactive protein(CRP)levels and low adiponectin levels(Bahceci et al., 2007).The rate of adipocyte death is positively correlated with adipocyte size in obese mice and humans and in hormone-sensitive lipase-deficient(HSL−/−)mice,a model of adipocyte hypertrophy without increased adipose mass(Cinti et al.,2005).WAT of HSL−/−mice exhibit a15-fold increase in necrotic-like adipocyte death and formation of crown-like structures,coincident with increased inflammatory gene expression.Recent studies in mice suggest that the polarization sig-nature of macrophages recruited to adipose tissue determines subsequent metabolic consequences(Lee et al.,2013).Classically polarized macrophages(M1)appear to induce insulin resistance by triggering local and systemic inflammatory signaling,whereas alternatively activated(M2)macrophages enhance insulin sensitiv-ity via PPAR␥-signaling(Heilbronn and Campbell,2008).Adipose tissue macrophages not only arise from the recruitment of blood monocytes,but recently it has been shown that local macrophage proliferation contributes significantly to obesity-induced increases in macrophage number(Amano et al.,2014;Jenkins et al.,2011; Hashimoto et al.,2013;Qiu et al.,2014;Yona et al.,2013).Haase et al.(2014)demonstrate that local macrophage proliferation in crown-like structures leads to an increase in M2macrophages during obesity-induced inflammation.These studies suggest a dif-ferent recruitment mechanism of classically activated(M1)and alternatively activated(M2)macrophages in obesity.Macrophages have also recently been implicated in the activa-tion of brite fat in mice.Chawla and colleagues show that acute cold stress results in M2activation in WAT,which results in the local secretion of catecholamines that induce brite-fat activation (Nguyen et al.,2011).Spiegelman and colleagues demonstrate that meteorin-like(Metrnl),a hormone secreted following exer-cise in skeletal muscle and cold exposure in adipose tissue,leads to the recruitment of eosinophils into adipose tissue(Rao et al., 2014).Eosinophils are the major source of cytokines IL-4and IL-13,shown previously by Nguyen et al.(2011)to stimulate the induction of beige fat thermogenic genes.Chawla and col-leagues(Qiu et al.,2014)simultaneously report that M2activated macrophages,recruited to cold-stressed WAT(Nguyen et al.,2011), are the core integrators of eosinophil and IL-4and IL-13regu-lated beige fat development.Blocking IL-4production in scWAT (using4get- dblGATA mice,which lack eosinophils)results in the failure to cold-induce a thermogenic gene expression profile in scWAT.IL-4and IL-13signalling via the IL-4R␣in macrophages (the receptor which mediates the known biological effects of IL-4/13)was shown to be required for the development of func-tional beige/brite fat in cold-housed animals(Qiu et al.,2014).The generation of brite adipocytes from progenitors in response to␤3-adrenergic receptor signaling has also been shown to be preceded by white adipocyte death and subsequent removal by M2-polarized macrophages(Lee et al.,2013).The recruited macrophages secreted osteopontin,which acts as a chemokine for brite adipocyte precur-sors.These studies demonstrate the role of alternatively activated macrophages in the development and activity of brite/beige fat. 3.Adipose tissue heterogeneityDistinct WAT depots possess dramatic heterogeneity,differ-ing substantially in their gene expression profiles,cell size and response to physiological factors such as hormones(Gil et al.,2011). Depot heterogeneity is apparent,for example,in most lipodys-trophies,which are characterized by loss of fat in certain body locations,while fat in other areas is unaffected or even expanded (Capeau et al.,2005).Differences between vWAT and scWAT have been recognized for a long time and research is ongoing in this field due to the well-known relationship between increased vis-ceral adiposity and insulin resistance.Although visceral adipocytes are smaller than subcutaneous adipocytes,they exhibit higher rates of fatty acid turnover and lipolysis,and are less responsive to the antilipolytic effect of insulin(Engfeldt and Arner,1988),leading to a greater release of free fatty acids into the circulation.They also secrete more inflammatory factors such as tumor necrosis factor␣(TNF-␣)and leptin(Wronska and Kmiec,2012).An inter-esting study with inducible labeling of mature adipocytes using the AdipoChaser mouse indicates that epididymal(visceral)fat responds to a high-fat diet preferentially by inducing adipogen-esis from precursors,whereas subcutaneous fat initially undergoes hypertrophy of existing adipocytes(Wang et al.,2013).Thisfind-ing is in agreement with the notion that adipocytes from visceral depots have a lower capacity to enlarge than those from subcuta-neous depots.Regional differences in WAT composition and function also include variations in the SVF population,vasculature and inner-vation.Tchkonia et al.(2001)showed that cloned human preadipocytes from subcutaneous depots have a higher adipogenic capacity than omental preadipocytes from the same individual.The group also identified920transcripts that differed among the cloned preadipocytes from abdominal subcutaneous,mesenteric and omental depots(Tchkonia et al.,2007).In later experiments they showed that the expression of adipogenic transcription factors per-oxisome proliferator-activated receptor␥2(PPAR␥2)and CCAAT-enhancer binding protein␣(C/EBP␣)is greater in abdominal subcutaneous tissue than femoral,in lean and obese adult women (Tchoukalova et al.,2010;for review see Tchkonia et al.,2013). It appears that depot-specific differences in preadipocyte pheno-type are established early during development.Gene expression126M.T.Hyvönen,K.L.Spalding/The International Journal of Biochemistry&Cell Biology56(2014)123–132analyses have revealed changes in genes that regulate early embry-onic development such as homeotic genes(Gesta et al.,2006; Cartwright et al.,2010;Yamamoto et al.,2010).These genetic dif-ferences seem to be maintained for several passages and are not affected by nutritional status,suggesting that each depot has its own unique gene expression signature.Interestingly,studies by Roncari et al.(1981)indicated that omental preadipocytes isolated from massively obese subjects had higher capacity to proliferate and differentiate in vitro than cells from lean subjects,whereas others reported that there was no difference in the preadipocyte replication and differentiation rates between moderately obese and lean subjects(Pettersson et al.,1985;Hauner et al.,1988).These and other studies(see Farooqi,2006for a review)show that some individuals have a high genetic predisposition to develop massive obesity,especially the early-onset type.Adipocytes and their precursors possess heterogeneity also within the same depots.Mice having WAT-specific knockout of the insulin receptor(FIRKO)or hormone-sensitive lipase(HSL)exhibit two distinct populations of adipocytes,small and large,instead of a normal Gaussian distribution(Blüher et al.,2002;Fortier et al., 2005).The presence of at least two adipocyte populations is also evident in mice treated with the PPAR␥ligand,pioglitazone(de Souza et al.,2001).Gene expression arrays and proteomic profiling indicates that small and large adipocytes of both FIRKO and wild-type mice differ dramatically in the expression of genes involved in lipogenesis,lipolysis and inflammation.The transcriptome and proteome of large and small adipocytes from the same depot also differ in humans(Jernås et al.,2006).Moreover,cloning of human preadipocytes from a single depot revealed two distinct clones, with one displaying higher replication and adipogenic capacity and decreased sensitivity to apoptosis(Tchkonia et al.,2005,2006). These clones retained most of their properties even after40pop-ulation doublings.Collectively thesefindings suggest that at least two different lineages give rise to white adipocytes.4.Plasticity of adipocytes and their precursors4.1.Adipose tissue-derived stem/stromal cellsSVF cells from WAT were isolated for thefirst time by Rodbell in 1964using proteolytic enzymes and centrifugation(Rodbell,1964). Nowadays there are several commercial automated devices for the isolation and concentration of SVF cells from WAT that can be achieved within2h(Aronowitz and Ellenhorn,2013).The WAT SVF is a rich source of stem cells that are capable of differentiating into all mesenchymal lineages(adipogenic,osteogenic,chondrogenic, myogenic and cardiomyogenic)as well as angiogenic,epithelial, hepatic and neurogenic(Zuk et al.,2001;Gimble et al.,2007).The existence of multipotent cells in WAT is also supported by the exist-ence of progressive osseous heteroplasia,a disease where ectopic bone forms within adipose tissue(Shore and Kaplan,2010).In 2009,Sun and colleagues generated induced pluripotent stem cells (iPS)by viral transduction of human adipose-derived stem/stromal cells,finding it to be faster and more efficient than induction of pluripotency in humanfibroblasts(Sun et al.,2009).In the litera-ture,many terms are used for these cells,such as adipose-derived stem/stromal cells(ADSC)and adipose stem/stromal cells.How-ever,since ADSCs are obtained by plating and selecting for plastic-adherent SVF cells,they are a heterogeneous population and display little adipogenic potential when transplanted into mice(Zheng et al.,2006).For human ASCs,stimulation of adipogenesis in vitro is needed before they are able to form WAT in vivo(Hong et al.,2006).Regardless of some limitations,WAT represents an attrac-tive source of adult human stem cells for regenerative therapy due to its abundance,surgical accessibility and high content of Table1Some published clinical trials of adipose tissue-derived stromal cells.Trial acronymand referenceDisease/cause Trial name/purposeAdipoCellKim et al.(2011)Depressed scar“Safety and efficacy of autologouscultured adipocytes in patientswith depressed scar”APOLLOHoutgraafet al.(2012)Acute myocardialinfarction“A Randomized Clinical Trial ofAdiPOse-derived Stem ceLLs in theTreatment of Patients WithST-elevation myOcardialInfarction”Ra et al.(2011)Spinal cord injury“Autologous Adipose Derived MSCsTransplantation in Patient WithSpinal Cord Injury”ANTG-ASC-210Garcia-Olmoet al.(2009)FATT1Herreroset al.,2012Complex perianalfistula“Clinical Trials of AutologousCultured Adipose-derived StemCells on Complex Fistula““Fistula Advanced Therapy Trial1”RESTORE-2Pérez-Canoet al.(2012)Partial mastectomy“A clinical evaluation of adiposederived Regenerative cells in thetreatment of patients with brEastdeformities post Segmental breastresection(lumpectomy)with Orwithout Radiation therapy.A phaseIV post market study–theRESTORE2trial”Kølle et al.(2013)Radicalmastectomy“Lipofilling with MSC enriched fat,a permanent autologousfiller?”Thesleff et al.(2011)Cranial injury Cranioplasty with adipose-derivedstem cells and biomaterial:a novelmethod for cranial reconstruction Cervelli et al.(2012)Rombergsyndrome,hemifacial atrophy,chemical injuries,burn sequelaePlatelet-rich plasma greatlypotentiates insulin-inducedadipogenic differentiation ofhuman adipose-derived stem cellsthrough a serine/threonine kinaseAkt-dependent mechanism andpromotes clinical fat graftmaintenanceHULPVASMarino et al.(2013)Chronic ulcers ofthe lower limbs“Clinical trial phase IIa to safetytreating critical ischemianon-revascularizable lower limbby mesenchymal stem cells.”multipotential progenitor cells.While mesenchymal stem cells comprise only a minor fraction of all nucleated cells in the bone marrow(0.0001–0.01%)(Pittenger et al.,1999),human adipose tis-sue contains around100,000stem cells per gram(Sen et al.,2001). Moreover,graft rejection is not a concern,because the cells can be isolated from the same person they are administered to.Several clinical trials are in progress around the world(Table1).As clini-cal applications are increasingly reported,there is growing concern that clinical practices have not been validated by adequate scientific evidence.The use of ADSCs has also raised the question of whether the transplanted cells could promote cancer development.Several studies have been published,with some reports demonstrating that transplanted ADSC’s promote tumor growth and others that they do not(Akimoto et al.,2013;Kølle et al.,2013;Nowicka et al.,2013; Ra et al.,2011;Rowan et al.,2014).Another drawback in the clini-cal use of ADSCs might be that they primarily undergo osteogenic and chondrogenic differentiation in vivo(Zheng et al.,2006),which could result,for example,in calcification in target tissues.Indeed, one study found that the use of ADSCs in an autologous lipoinjec-tion was associated with cyst formation and tissue calcification in 4of70patients(Yoshimura et al.,2008).However,another study where SVF purified ADSC’s were co-transplanted with adipose tis-sue in breast augmentation surgery,reported no calcification in surgical follow up(Wang et al.,2014).Achieving consistent and。