j ourna l homepage: www.e lsev ie r .com/ locate /bbad is
Review
The role of JAK–STAT signaling in adipose tissue function☆,☆☆,★
Allison J. Richard a, Jacqueline M. Stephens a,b,⁎a Adipocyte Biology Lab, Pennington Biomedical Research Center, USAb Department of Biological Sciences, LSU, Baton Rouge, LA 70808, USA
☆ This is an open-access article distributed under the tAttribution-NonCommercial-Share Alike License, which perbution, and reproduction in any medium, provided thecredited.☆☆ This article is part of a Special Issue entitled: MoHealth and Disease.★ This work was supported by grant R01DK52968 f
Health to J.M.S.⁎ Corresponding author at: Louisiana State University
ences, 202 Life Sciences Bldg., Baton Rouge, LA 70803, US+1 225 578 2597.
Article history:Received 23 January 2013Received in revised form 20 May 2013Accepted 22 May 2013Available online 2 June 2013
Keywords:Brown and white adiposeJanus kinaseImmune cellCytokineTyk2Obesity
Adipocytes play important roles in lipid storage, energy homeostasis and whole body insulin sensitivity. TheJAK–STAT (Janus Kinase–Signal Transducer and Activator of Transcription) pathway mediates a variety of phys-iological processes including development, hematopoiesis, and inflammation. Although the JAK–STAT signalingpathway occurs in all cells, this pathway canmediate cell specific responses. Studies in the last two decades haveidentifiedhormones and cytokines that activate the JAK–STAT signaling pathway. These cytokines and hormoneshave profound effects on adipocytes. The content of this review will introduce the types of adipocytes and im-mune cells that make up adipose tissue, the impact of obesity on adipose cellular composition and function,and the general constituents of the JAK–STAT pathway and how its activators regulate adipose tissue develop-ment and physiology. A summary of the identification of STAT target genes in adipocytes reveals how these tran-scription factors impact various areas of adipocyte metabolism including insulin action, modulation of lipidstores, and glucose homeostasis. Lastly, we will evaluate exciting new data linking the JAK–STAT pathway andbrown adipose tissue and consider the future outlook in this area of investigation. This article is part of a SpecialIssue entitled: Modulation of Adipose Tissue in Health and Disease.
Obesity is a condition of excess adipose tissue and is the mostcommon metabolic disorder in the industrialized world. In the USalone, it affects 154.7 million individuals over the age of 20, whichis approximately 25% of the adult population. This obesity epidemichas been a prelude to increases in chronic diseases. Obese individuals,particularly those with excess abdominal adipose tissue, have an ele-vated risk of developing Type 2 diabetes mellitus (T2DM), cardiovas-cular disease, and hypertension. During obesity, the production ofinflammatory cytokines and reactive oxygen species within adiposetissue increases as well as ectopic lipid deposition in liver or skeletalmuscle (reviewed in [58]). These consequences reflect potential caus-ative links between adipose tissue dysfunction and insulin resistance.
erms of the Creative Commonsmits non-commercial use, distri-original author and source are
dulation of Adipose Tissue in
rom the National Institutes of
, Department of Biological Sci-A. Tel.: +1 225 763 2648; fax:
blished by Elsevier B.V. All rights re
However, the exact nature of this relationship is still poorly under-stood and the subject of intense investigation. Hence, understandingadipose tissue biology is highly relevant in elucidating the pathogen-esis and treatment of metabolic diseases like T2DM.
Adipocytes are highly specialized lipid storage cells that play a keyrole in modulating energy balance and nutrient flux in vertebrates.They provide a storage reservoir for energy in the form of lipid, whichis stored as a single or multiple droplet(s) that give adipocytes theircharacteristic roundedmorphological appearance. Adipocytes also pro-duce and secrete numerous enzymes, hormones, cytokines, and growthfactors that modulate appetite, lipid and glucose homeostasis, insulinsensitivity, inflammation, blood vessel formation, and overall energyhomeostasis [3]. Several of these secreted factors, such as leptin, prolac-tin, interleukin-6, and cardiotrophin-1, activate the JAK–STAT pathwayand are mentioned in this review. In the context of this review, we alsodiscuss the STAT1-mediated transcriptional regulation of lipoprotein li-pase, an enzyme secreted from adipocytes.
The two classical types of fat cells that have been widely studied in-clude white and brown adipocytes. White adipocytes are important inenergy storage and have three main functions — they sequester and re-lease lipid, take up glucose in response to insulin, and secrete paracrineand endocrine factors. Brown adipocytes are predominantly classifiedby their high content of mitochondria containing uncoupling protein-1(UCP-1) and contribute to energy expenditure. UCP-1 uncouples theelectron transport chain from energy production and results in the re-lease of potential energy obtained from food as heat. As a result, brownadipocytes play an important role in adaptive thermogenesis and are
essential for non-shivering thermogenesis in response to cold orβ3-adrenergic stimulation [23,75]. We will review two recent high im-pact studies that link the JAK–STAT signaling pathway to brown adipo-cyte differentiation and adaptive thermogenesis and mark the infancyof our understanding of JAK–STAT signaling in brown adipose tissue(BAT).
Expansion of adipose tissue occurs through both increases in thesize and number of the adipocyte population. New, mature adipo-cytes arise via differentiation of progenitor cells within adipose tissue.Evidence exists suggesting that white and brown adipocytes derivefrom different types of mesenchymal progenitor cells [78]. However,innovative studies examining the development of brown-like adipo-cytes within white adipose tissue (WAT) recently have challengedthis concept [74]. The signaling factors regulating the transition ofmesenchymal progenitor cells to committed preadipocytes are poorlydefined. Nonetheless, significant advances towards an understandingof adipose tissue biology have been made by studying the function oftranscription factors, which regulate differentiation of committedpreadipocytes, and are involved in the modulation of adipocytegene expression. Fat cell differentiation, known as adipogenesis, pro-ceeds as a highly coordinated and temporally defined series of eventsthat involves the regulated expression of numerous transcription fac-tors (reviewed in [75,104]). Several laboratories have investigatedthe role of STATs (Signal Transducers and Activators of Transcription)in adipocyte development and function. Additionally, studies showthat many STAT activators play a critical role in the regulation of ad-ipocyte gene expression and exhibit differential expression in condi-tions of obesity and/or insulin resistance [13,75].
1.2. Other AT cell types
In addition to adipocytes, immune cells significantly contribute tothe cellular composition of adipose tissue. Their presence within adi-pose tissue is regulated by obesity and metabolic dysfunction. The pur-pose of these immune cells and their relationship to metabolicdysfunction within obese adipose tissue is the subject of intense inves-tigation and debate. Whether their presence is a cause or consequencewith regard to insulin resistance is unknown, and both hypotheseshave been proposed. Some types of immune cells, such asmacrophages,increase in obese adipose tissue, and are associated with inflammationand metabolic disease. Yet the levels of eosinophils, which are anti-inflammatory and associatedwith healthy adipose tissue, decrease dur-ing obesity and insulin resistance (reviewed in [77]). Many studies sug-gest that adipose tissue macrophages (ATMs) are associated withinsulin resistance in a manner that is dependent upon their activationstatus. Yet, more recent studies suggest that ATMs may have house-keeping functions in adipose tissue and may serve physiological rolesin modulating lipid flux in adipocytes [47]. Interestingly, the JAK–STATpathwaywas first identified and characterized as the result of immuno-logical studies focused on understanding the signal transduction path-way utilized by interferon gamma (IFNγ) (reviewed in [84]).
Interest in adipose tissue immune cells has prompted recent studiesexamining the role of JAK–STAT activators and signaling in adipose tis-sue immune cells. Several cytokines that are activators of the JAK–STATpathway are produced from immune cells, preadipocytes, and adipo-cytes within adipose tissue and have paracrine and endocrine effectson other cells with important functions in regulating metabolism andenergy balance. Little is known regarding the complex interplay ofJAK–STAT signaling between adipose tissue cells, but activators of thispathway have been shown to regulate development and function ofboth immune cells and adipocytes.
1.3. JAK–STAT signaling pathway
The STAT family of mammalian transcription factors is comprisedof seven members (STATs 1–4, 5A, 5B, and 6) that have cell and
tissue-specific distribution that influences their specificity and function[76]. STATs 5A and 5B, although highly homologous, are transcribedfrom different genes. While the expression level of STAT5A relative toSTAT5B is tissue specific, the STAT5 proteins typically share similar pat-terns of tissue-dependent gene expression. Intriguingly, they have beenshown to exhibit both redundant and non-redundant functions [94].STATs are predominantly activated by phosphorylation of one tyrosineresidue near the C-terminus that is catalyzed by a Janus Kinase (JAK).Members of the JAK family include JAKs 1–3 and Tyk2. The JAK–STATpathway is present in all cells, mediates the action of numerous cyto-kines, growth factors, and hormones, and regulates diverse biologicalfunctions, including immune responses, energy expenditure, and cellu-lar differentiation. Under basal conditions, STATs are largely inactiveand localized to the cytoplasm. Upon ligand binding to a membrane-bound receptor, the receptor-associated JAKs become activated andphosphorylate tyrosine residues within the receptor, which then directrecruitment of specific STATs. STATs bind the activated receptor viatheir SH2 domains and become JAK substrates. Tyrosine phosphoryla-tion of STATs results in the formation of homo- or hetero-dimers thattranslocate to the nucleus where they regulate transcription of specifictarget genes.
This review provides in depth coverage of the literature that relatesto the role of JAK–STAT signaling in adipogenesis. We also address theability of STATs to modulate fat cell function via transcriptional regula-tion of adipocyte-specific gene targets in response to activator stimula-tion. Additionally, we explore knockout studies of JAK–STAT activatorsinmice. These studies suggest that JAK–STAT signaling in adipose tissueplays an important role in paracrine communication between adipo-cytes and AT immune cells that might influence the pathogenesis ofobesity. Lastly, we highlight novel studies regarding JAK–STAT signalingin brown adipose tissue.
2. Regulation of adipogenesis by STAT proteins
The first studies on the modulation of STATs during adipocyte devel-opment were performed over fifteen years ago and demonstrated thatprotein levels of STATs 1, 3, 5A and 5B increased during 3T3–L1 fat celldifferentiation, providing the first suggestion that these STAT proteinsmay play a role in the transcriptional control of adipogenesis [86]. Fiveyears later, studies in subcutaneous human primary adipocytes con-firmed the up regulation of STATs 3 and 5 during differentiation [32].However, the pattern of STAT1 protein expression during human [32]and murine [86] adipogenesis differed, suggesting species-specificregulation. Decreased STAT1 expression during adipogenesis of humanadipocytes indicates that it does not promote human fat cell differentia-tion. There are few studies that examine the role of STATs 1 and 3 in thetranscriptional control of adipogenesis. However, substantial in vitro andin vivo evidence from over a dozen independent laboratories supportsthe hypothesis that STAT5 promotes fat cell differentiation in mouseand man.
2.1. The role of STAT5 proteins in adipocyte development
Studies of transgenic mice containing knockouts or deletions ofthe STAT proteins have been critical in obtaining an understandingof the function of these proteins in vivo. Deletion of STAT5A,STAT5B, or both STAT5 proteins in genetically modified mice resultsin impaired adipose tissue development with the double knockoutmice having fat pads only 20% of the normal size [94]. Since theseare non-inducible whole-body deletions of STAT5, it is unclear if thereduced adipose tissue is related to developmental deficiencies. How-ever, a recent study provides evidence that STAT5 proteins can pro-mote adipocyte development in vivo in a mature animal. Fibroblastswere genetically engineered to express STAT5A and injected intoathymic mice. STAT5A-expressing fibroblasts conferred the formationof ectopic fat pads and demonstrated that STAT5A is physiologically
capable of regulating adipose tissue development in vivo [89]. A bet-ter understanding of the ability of STAT5 to modulate endogenousfat cell differentiation awaits the creation of transgenic mice inwhich STAT5 is conditionally deleted or knocked out of only adipo-cytes or adipose tissue. To date, a role of STAT5 in the developmentof brown adipose tissue has not been explored.
Although in vivo studies are limited, numerous laboratories have in-dependently demonstrated pro-adipogenic activity of STAT5 proteinsusing multiple murine and human non-precursor and preadipocyte celltypes in culture. These studies have led to insights into the involvementof STAT5 proteins in fat cell differentiation and the mechanisms bywhich they promote adipogenesis, and the findings are summarized inFig. 1. In differentiating murine preadipocytes, the protein levels of bothSTATs 5A and 5B are increased and tightly coupled to the developmentof the lipid-bearing cellular phenotype and elevated expression ofwell-studied adipogenic transcription factors, including C/AAAT enhanc-er binding protein α (C/EBPα) and peroxisome proliferator-activatorreceptor γ (PPARγ) [88]. Furthermore, ectopic expression of STAT5A innon-precursor cells sufficiently induces fat cell differentiation [27,87]. In-terestingly, STAT5B was unable to promote adipogenesis in non-precursor cells, but it did enhance STAT5A-induced fat cell differentia-tion, indicating distinct roles for STATs 5A and 5B in regulatingadipogenesis [27]. RNA interference studies support a supplementaryrole of STAT5B in fat cell differentiation [38]. Studies using antisense
Fig. 1. Roles of the JAK–STAT5 signaling pathway in adipose tissue. The JAK–STAT5 signalingfunction of preadipocytes, adipocytes, and macrophages in adipose tissue. Substantial evideadipogenesis. In mature adipocytes, multiple STAT5 target genes have been identified, demStudies also indicate that JAK–STAT5 signaling is important in the recruitment and develop
Stat5 oligonucleotides as well as constitutively active and dominant-negative STAT5 constructs have demonstrated that STAT5 proteins me-diate the pro-adipogenic activity of growth hormone on preadipocytes[39,79,108].
In several preadipocyte model systems, growth hormone (GH)-activated STAT5 proteins have been shown to induce PPARγ expres-sion suggesting that STAT5 can promote adipocyte differentiation byregulating PPARγ [39]. This is supported by data showing thatSTAT5 can directly bind and transactivate the PPARγ promoter[39,55,100]. Many transcription factors have profound effects on adi-pocyte development, but PPARγ is a critical transcriptional regulatorthat is absolutely required for fat cell differentiation [6,61]. While theevidence suggests that STAT5 proteins regulate PPARγ expression, it isplausible that STAT5 also regulates the expression of proteins responsi-ble for making PPARγ ligands or other proteins important in the deve-loping and mature adipocyte.
STAT5 proteins are specifically activated by tyrosine phosphorylationalmost immediately following induction of adipogenesis in 3T3–L1 cells[6,61]. Interestingly, cooperative binding of C/EBPβ and STAT5A occursduring a very early stage of adipogenesis and suggests that STAT5A is in-volved in chromatin remodeling and priming of regulatory sites forsubsequent binding by other transcription factors [82]. Intriguingly, inhuman bone marrow-derived stromal cells induced to undergoadipogenesis, PPARγ also binds to the STAT5A promoter while C/EBPα
pathway is activated by GH, PRL, and GM-CSF, and it regulates the development and/ornce supports that STAT5 proteins are activated in preadipocytes and that they promoteonstrating a role for STAT5 in modulating key physiological properties of adipocytes.ment of adipose tissue macrophages.
and C/EBPβ bind to the STAT5B promoter region [38]. Thus, there exists acomplex interplay of STAT5 proteins with other adipogenic transcrip-tional regulators to orchestrate the stages of differentiation leading tothe biochemical and morphological changes associated with the maturelipid-laden fat cell.
Although the majority of studies suggest that STAT5 proteins pro-mote adipogenesis, there is some confounding data. For example,STAT5 activators such asGH and oncostatinM (OSM) can repress adipo-cyte differentiation in cell culture models of precursor cells from mice[57], rats [70], and humans [83]. This anti-adipogenic activity of STAT5can likely be attributed to specifics of OSM signaling [57], possibly viamodulation of post-translational modifications of STAT5 and/or itsinteracting protein partners. For studies inwhich primary preadipocytes[70,83] or mouse embryonic fibroblasts [57] – versus committedpreadipocyte cell lines – were used to assess adipogenic potential, thedevelopmental stage of the cell and how far it is committed to developinto an adipocyte might be responsible for the opposing effects ofSTAT5 on adipogenesis. Other factors could include species-specific dif-ferences in rat primary preadipocytes in response to GHor pre-exposureof the primary preadipocytes to GH in vivo [70]. Nonetheless, STAT5proteins, particularly STAT5A, play an important role in regulatingadipogenesis, and the preponderance of evidence suggests that theyare pro-adipogenic in most model systems.
2.2. The role of STAT1 in adipocyte development
STAT1 expression is induced during adipocyte development inmouse cells [86]. However, mice with a targeted disruption of theStat1 gene do not exhibit differences in weight gain, and with the ex-ception of IFN-dependent responses, biologic responses to other cyto-kines were not defective [56]. Interestingly, IFNγ inhibits adipogenesisof SGBS (Simpson–Golabi–Behmel syndrome) human fat cells [53]and rodent preadipocytes [30,40]. Although the direct role of STAT1 inthe anti-adipogenic action of IFNγ was not investigated, experimentsusing pharmacological inhibitors indicate that the JAK–STAT1 pathwayplays a central role in the ability of IFNγ to induce insulin resistance, de-crease triglyceride stores, and down-regulate expression of lipogenicgenes in mature human fat cells [53]. IFNγ also activates JAK2–STAT3[4,53,85]. However, specific inhibition of JAK2 did not block IFNγ effectson fat cell development and physiology, and leptin-induced activationof JAK2–STAT3 failed to substantially decrease adipocyte differentiationand lipid accumulation [53]. Thus, it was concluded that JAK1–STAT1primarily mediated the substantial IFNγ-induced modulation ofhuman adipocyte functions [53].
IFNγ-null mice fed a high fat diet have smaller adipocytes in visceralWAT than wild-type mice [65]. Interestingly, there were no differencesin body weight [65] similar to the STAT1 null mice on chow diet [56].Together the presence of smaller adipocytes and lack of change inbody weight in the IFNγ knockout mice suggest an increase in fat celldifferentiation and indicate that IFNγ might play an inhibitory role inadipogenesis in vivo. An increase in adipocyte cell number in theIFNγ-null mice would have better supported a claim of increasedadipogenesis; however, this parameter is difficult to measure and wasnot assessed in the study.
Knockout of IFNγ also decreased the size of the natural killer (NK)cell population in visceral but not subcutaneous WAT, while it shiftedthe activation phenotype of ATMs from M1 to M2 in both depots [65].M1-type ATMs produce inflammatory cytokines that are correlatedwith the development of insulin resistance in obesity. Amultitude of re-ports demonstrate an association between immune cell infiltration invisceral but not subcutaneous adipose tissue during obesity. Thus, it isnot surprising that knockout of IFNγ results inmore substantial changesin the population of immune cells in visceralWAT. Accordingly, it is pos-sible that IFNγ indirectly modulates adipogenesis via its effects on thecomposition of immune cells within adipose tissue. The studies ofIFNγ-null mice indicate that IFNγ plays a role during obesity in the
regulation of inflammation and insulin sensitivity through several prob-able mechanisms such as modulating adipogenesis and influencing thesize and composition of the AT immune cell population. However, therole of JAK–STAT1 signaling inmediating these effects of IFNγ in obesityremains to be elucidated.
The ability of IFNγ and JAK–STAT1 signaling to regulate fat cell dif-ferentiation has also been studied in the context of crosstalk with thehedgehog signaling pathway. The hedgehog signaling pathway consti-tutes an ancestral developmental process important in the regulationof stem cell differentiation during embryonic development and adulttissue homeostasis [7,37,98]. Sonic hedgehog (Shh) is the most widelystudied homolog of the three mammalian hedgehog proteins. Shh sig-naling specifically blocks adipogenesis in white, but not brown adiposetissue [69,92]. IFNγ inhibits fat cell differentiation in the absence of Shhsignaling [30,40,96]. However, when the Shh pathway is activated invarious preadipocyte model systems, IFNγ blocks Shh signaling andrescues adipogenesis via a JAK–STAT1-dependent mechanism [96].IFNγ-mediated inhibition of Shh signaling did not occur in Stat1−/−mouse embryonic fibroblasts (MEFs), indicating that the crosstalk de-pends on Stat1. Thus, STAT1 appears to regulate adipocyte differentia-tion via crosstalk with the Shh signaling pathway.
Collectively, these findings suggest that STAT1 may not play a majorrole in adipocyte differentiation and adipose tissue development under“normal” conditions in lean subjects. However, in the context of highfat diet (HFD)-induced obesity when IFNγ levels are elevated as a resultof increased numbers of activated IFNγ-producing immune cells infiltrat-ing visceral adipose tissue, JAK–STAT1 signaling may play an inhibitoryrole in the control of adipocyte differentiation. Further studies using ge-neticmanipulation of Stat1 expression in adipocytes are needed to betterelucidate the role of STAT1 in the control of adipogenesis in vivo. Addi-tionally, it would be interesting to examine the phenotype of adipose tis-sue in Stat1 null mice following the development of diet induced obesity.Based on the available data, it seems that cross-talk with other signalingpathways and the inflammatory state of adipose tissue are important fac-tors modulating the ability of STAT1 to regulate adipogenesis.
2.3. Role of STAT3 in the regulation of adipogenesis
The JAK2–STAT3 pathway is activated early during adipogenesis[18,101,109] and is involved in achievingmaximal adipocyte differenti-ation potentially through modulation of C/EBPβ transcription [109]. Asdemonstrated by in vitro experiments, adipogenesis is suppressed byselective inhibition of JAK2 or STAT3, siRNA-induced knock-down ofSTAT3, or overexpression of dominant-negative STAT3 [101]. PIAS3,protein inhibitor of activated STAT3, is constitutively expressed in3T3–L1 cells, and its activation represses STAT3 activity and inhibitsfat cell differentiation [16]. In addition, a PPARγ synthetic agonist res-cues adipogenesis following RNAi-induced knock-down of STAT3,suggesting that STAT3 regulation of adipocyte differentiation occurs up-stream of PPARγ activation [101]. Collectively, these studies indicatethat activation of the JAK2–STAT3 signaling pathway plays a role inthemodulation of adipogenesis. Additional studies have shown that ac-tivation of STAT3may promote fat cell differentiation viamodulation ofmitotic clonal expansion, a proliferative phase that occurs immediatelyfollowing induction of adipogenesis and is necessary for differentiationof 3T3–L1 fat cells [11,18,51].
Since germ-line deletion of STAT3 is embryonic lethal [44,49], stud-ies are lacking to demonstrate that STAT3 modulates adipocyte differ-entiation in vivo. However, there is a transgenic mouse model whereSTAT3 expressionwas knocked outwith use of aP2 Cre. The aP2 proteinis a lipid binding protein that is highly expressed in fat cells and is likelythe most abundantly expressed protein in mature adipocytes. The pri-mary phenotype of the aP2 Cre-driven STAT3 knockout mouse was in-creased weight and increased adipose tissue mass, associated withadipocyte hypertrophy [10]. These studies suggest that STAT3 contrib-utes to body weight homeostasis. However, since aP2 can also be
expressed inmacrophages [28,29,67], brain cells, andmouse embryoniccells [97], it is unclear if these observations are solely mediated by thelack of STAT3 in adipocytes. Furthermore, since aP2-driven deletion ofSTAT3 does not occur until late adipogenesis, the observed adipocytehypertrophy is not likely a result of direct effects of STAT3 on fat cell dif-ferentiation. Overall, evidence suggests that STAT3 is capable of modu-lating adipogenesis. However, further investigations probing JAK–STAT3 signaling at various stages of adipogenesis in vitro and in vivoare necessary to better understand the ability of STAT3 to regulate adi-pocyte and adipose tissue development.
3. Role of JAKs in adipocyte development and function
Although STATs have been fairly well studied in adipocytes, thereareminimal studies focusing on JAK expression, activation, and functionin fat cells and adipose tissue. JAK kinases are largely controlled by tyro-sinephosphorylation, rather than by expression levels. The ubiquitouslyexpressed JAKs 1 and 2 are present at similar levels in preadipocytesand adipocytes [88], and they are expressed in adipose tissue in vivo[33]. There is some evidence that Tyk2 and JAK3 are expressed in adi-pose tissue [19,33]. As indicated earlier, adipose tissue is comprised ofmany cell types and there is no data to suggest that these two JAK familymembers are expressed in white adipocytes.
Both preadipocytes and adipocytes are responsive to hormones andgrowth factors which activate JAKs 1 and 2, including GH, prolactin (PRL),IFNγ, leukemia inhibitory factor (LIF), OSM, cardiotrophin-1 (CT-1), and cil-iary neurotrophic factor (CNTF) [4,24,39,53,66,85,90,110-112]. Currently,there is no evidence that JAKs play a STAT-independent role inmodulatingadipocyte differentiation in white adipose tissue. However, severalcytokines that inhibit adipogenesis, including IFNγ [30,40], OSM[57,83], and neuropoietin (NP) [104], are potent activators of JAK ki-nases. To date, only JAKs 1 and 2 have been detected in white adipo-cytes and their roles are solely attributed to their ability to beactivated by cytokines and confer STAT activation. There is one ex-ception, however, in which JAK2 was shown to physically associatewith aP2 in adipocytes [95]. The unphosphorylated form of JAK2has been shown to interact with aP2 and the results of this study sug-gest that ligand-bound aP2 decreases JAK2 signaling [95]. Overall,there is a paucity of data regarding the role of JAKs in adipocytes.Hence, additional studies will be required to further elucidate theSTAT-dependent and/or independent functions of these kinases infat cells.
4. STAT target genes in preadipocytes and adipocytes
The regulation of tissue-specific genes has been shown to be a phys-iological role of STAT proteins in a variety of cell types, including adipo-cytes. To date, specific target genes have been identified for STATs, 1, 5A,and 5B, but not for STATs 3 and 6, in adipocytes (Table 1). The STAT tar-get genes elucidated in fat cells code for proteins that regulate adipocytedevelopment, insulin action, and lipid and carbohydratemetabolism. Assummarized in Fig. 1, many laboratories around the world have showna role for STAT5 proteins in adipocyte and adipose tissue development
able 1TAT target genes in preadipocytes and adipocytes.
in vitro and in vivo. Hence, it is not surprising that STAT5 can directlybind the PPARγ3 promoter [55] and can transactivate the PPARγ2 andPPARγ3 promoters [39,55]. PPARγ is a STAT5 target gene during adipo-cyte development and its modulation by STAT5 likely plays a role in theability of STAT5 to promote adipocyte differentiation in vitro and in vivo.Studies have shown that PPARγ is also a STAT1 target gene in adipo-cytes. In 3T3–L1 adipocytes, STAT1 homodimers bind to an IFNγresponsive site within the PPARγ2 promoter and suggest that IFNγ-induced repression of PPARγ2 transcription [99] is mediated by the di-rect action of STAT1 on the PPARγ2 promoter [34]. Of note, a dominantnegative mutation in PPARγ and IFNγ signaling have been associatedwith the development of insulin resistance [5,53,99]. Consequently,STAT1 likely mediates the ability of IFNγ to induce insulin resistance[46,53,80,99] and inhibit adipogenesis [30,40] via transcriptional repres-sion of PPARγ. An IFNγ-sensitive binding site for STAT1 was also discov-ered in the murine lipoprotein lipase (LPL) promoter [35]. LPL is therate-limiting enzyme that catalyzes the hydrolysis of serum triglyceridesfrom lipoproteins into free fatty acids for uptake and storage in adiposetissue. IFNγ-activated STAT1 binds to the LPL promoter in a mannerthat is consistent with IFNγ-induced repression of LPL expression and in-hibition of LPL activity in murine adipocytes [20,30]. While STAT3 is alsoinduced in response to IFNγ, STAT1 is amore robustmediator of IFNγ sig-naling in murine and human adipocytes [4,53,85]. In the described stud-ies, STAT3 was unable to bind to the STAT1 binding sites within thePPARγ promoter [34], and LIF, a potent STAT3 activator, did not conferbinding of STAT3 to the IFNγ sensitive region of the LPL promoter [35].
Since STAT5 proteins are activated early during adipocyte differenti-ation and have been shown to play such a key role in adipocyte devel-opment, it is not surprising that most studies have focused on thefunctions of STAT5 proteins in mature adipocytes. The promoter foracyl CoA oxidase (AOX), the rate limiting enzyme in peroxisomal fattyacid β-oxidation, contains a STAT5 binding site that modulates itsgene expression in fat cells [12]. Transfection studies have shown thatthe promoter activity of aP2, an abundantly expressed lipid binding pro-tein in fat cells, can be activated by STAT5 [60]. Other studies haveshown that STAT5 mediates the inhibition of aP2 expression in rat pri-mary preadipocytes [70]. This was the first study to suggest thatSTAT5 proteins could act as transcriptional repressors. Since that time,our own research has revealed that STAT5A can act as a transcriptionalrepressor in adipocytes. A STAT5A binding site in the murine fatty acidsynthase (FAS) promoter mediates the repression of FAS transcriptionthat occurs with growth hormone (GH) or prolactin (PRL) treatment[36]. FAS catalyzes the production of long chain fatty acids and is a cru-cial enzyme involved in de novo lipogenesis. In addition to modulationof genes associated with lipid metabolism such as AOX and FAS,STAT5 can also increase the transcription of pyruvate dehydrogenase ki-nase (PDK)-4, a known regulator of glycolysis that is highly induced inadipocytes by PRL or GH in a STAT5 dependent manner [103]. Underthese conditions, insulin resistance accompanies the induction ofPDK4. It is well known that PRL and GH are important modulators oflipid metabolism and are also potent inducers of STAT5 in adipocytes[4,60]. Hence, many of the metabolic actions of these anterior pituitaryhormones could be controlled by direct modulation of target genes bySTAT5 (Table 1).We also have data to indicate that STAT5 proteins reg-ulate the expression of adiponectin, an important adipocyte hormonethat modulates insulin sensitivity. In summary, STAT5 transcriptionallyregulates the expression of AOX, aP2, FAS, PDK4, and adiponectin —
proteins that modulate lipid and glucose metabolism and insulin sensi-tivity in fully differentiated fat cells (Fig. 1). Although relatively fewSTAT5 target genes have been identified in adipocytes, it is logical topredict that other STAT5A target genes that play a role in lipid or glucosemetabolism will be identified.
STAT3 is abundantly expressed in adipocytes [32,86] and mediatesthe action of numerous cytokines in fat cells. However, with the ex-ception of C/EBPβ as a potential STAT3 gene target activated early inthe adipogenic program [109], to date no adipocyte-specific direct
target genes have been identified for STAT3 (Table 1). AlthoughSTAT6 is equivalently expressed in preadipocytes and throughoutfat cell differentiation [86], only IL-4 has been shown to activate thistranscription factor in 3T3–L1 preadipocytes but not in adipocytes[17]. Thus, activators, functions, and gene targets of STAT6 in bothpreadipocytes and adipocytes remain to be elucidated. Overall, thereis almost nothing known about the identity of STAT3 and STAT6 tar-get genes in adipocytes.
5. JAK–STAT signaling in AT immune cells
The JAK–STAT pathway was first elucidated from immunologicalstudies in the early 1990s that were investigating cell-specific IFNγ-responsive gene transcription [84]. In the past two decades, investiga-tors have generated a wealth of information regarding activators andfunctions of the JAK–STAT pathway in immune cells, inflammation,and inflammatory disorders [68,84]. Research over the past decadehas established AT as a bona fide endocrine organ comprised of a hetero-geneous population of preadipocytes, adipocytes, endothelial cells, con-nective tissue, and immune cells.Multiple lines of evidence suggest thatalterations in the phenotype and/or number of AT-resident immunecells are associated with the development of insulin resistance inobese mice and humans [77]. Currently, there are only a few studiesthat examine activators and functions of the JAK–STAT signaling path-way in the context of AT and immunometabolism, an emerging fieldof investigation linking immunology and metabolism [52,77]. In thissection, we discuss the ability of two known JAK–STAT activators,GM-CSF (granulocyte-macrophage colony-stimulating factor) andIFNγ, to modulate adipose tissue function.
STAT5 plays a role in the differentiation of myeloid cells and activa-tion of macrophages [106]. GM-CSF is a proinflammatory cytokine thatsignals via the JAK2–STAT5 signaling pathway [21,59,73,102]. GM-CSFknockout mice fed a HFD have increased adiposity and adipocyte size[42]. Analysis of these mice suggests that the presence of GM-CSF posi-tively correlates with the relative number of macrophages within themesenteric fat and the relative expression of GM-CSF differs amongthe fat depots. In the absence of GM-CSF, the number of ATMs inmesen-teric fat declined and was accompanied by decreased expression ofpro-inflammatory cytokines in mice fed a high fat diet. Additionally,GM-CSF null mice were protected from HFD-induced insulin resistancedespite increased adiposity [42]. The exact mechanisms underlying thecorrelation between increased adiposity, adipose tissue dysfunction,and insulin resistance are active areas of investigation. Other exam-ples [41,43], including these findings with the GM-CSF knockout mice[42], seem to rule out increased adipocyte size as the causative factorin the relationship between AT dysfunction and insulin resistance. Cur-rently, it is not knownwhether STAT5mediates these effects of GM-CSFin AT, as both STATs 1 and 3 also can be activated by this cytokine [9].Interestingly, STAT5 signaling in hypothalamic nuclei of the brain hasbeen implicated in the ability of GM-CSF to regulate food intake andbody adiposity [48]. Overall, these data indicate that GM-CSF plays animportant role in AT to recruit and activate macrophages that contrib-ute to AT inflammation, and these actions of GM-CSF are likely mediat-ed by the JAK–STAT signaling pathway (Fig. 1).
IFNγ is produced from both natural killer (NK) cells [64] and Tcells [22,71,91,107] present in adipose tissue. IFNγ can inhibit the dif-ferentiation of preadipocytes [30,40], induce insulin resistance in ma-ture adipocytes [53,99], and decrease PPARγ expression by targetingthis nuclear receptor to the ubiquitin proteasome system for degrada-tion in adipocytes [26]. It is highly likely that the production of IFNγfrom infiltrated immune cells acts in a paracrine fashion on adjacentadipocytes to result in insulin resistance. Interestingly, deletion ofIFNγ in transgenic mice shifted the activation of ATMs in visceralWAT toward an alternatively activated phenotype that was associat-ed with decreased production of inflammatory cytokines and im-proved insulin sensitivity [65].
Numerous other JAK–STAT activators are produced in adipose tissueand likely act in a paracrine manner to regulate the development andfunction of immune cells and adipocytes. For instance, IL-1 and IL-6are secreted fromATMs [54]. Themajority of IL-4 in adipose tissue is re-leased from eosinophils, and as discussed in the next section signals viaJAK–STAT6 tomodulate the function of BAT andWAT [63]. Prolactin andleptin, although they also act as endocrine hormones, have direct ac-tions on immune and fat cells in adipose tissue [2,24,25,81].
In addition to macrophages, T cells, and NK cells that have alreadybeen mentioned, recent prominent studies have also examined theroles of B cells [15,105], mast cells [1,50], and neutrophils [93] inmediat-ing insulin resistance associated with obesity. B cells, mast cells andneutrophils have all been shown to increase in number and/or shift acti-vation status within adipose tissue upon feeding mice HFD [1,93,105].Using genetic knockout, immuno-neutralization, or pharmacological in-activation techniques, loss of any of these immune cells or their directmediators, such as elastase for neutrophils, was associated withimproved insulin sensitivity. Similarly, attempts to increase B cells,neutrophils, or mast cells correlate with increased insulin resistance[50,93,105], thus establishing these immune cells as modulators of insu-lin resistance. To date, no studies have examined the role of the JAK–STAT pathway inmediating the action of B cells or neutrophils in adiposetissue. However, it was shown that mast cells promote angiogenesis inAT in an IL-6 and IFNγ-dependent manner [50]. Activation of the JAK–STAT pathwayswas not assessed in these studies; therefore, future stud-ies are needed to evaluate the role of this pathway inmast cell action. Asthe immunometabolism field continues to develop, we propose that fu-ture studies of JAK–STAT signaling within adipose tissue will be an excit-ing new area of research that will lead to additional insights into themechanism(s) by which AT immune cells communicate with adipocytesand/or other AT immune cells to modulate insulin sensitivity.
6. Emerging roles of the JAK–STAT pathway in brown adiposetissue
The last few years have been accompanied by a large increase instudies on BAT. Although originally thought to be absent in adulthumans, there is now evidence to support the presence of BAT andits association with metabolic health [14,62]. In addition, many trans-genic mouse models have been generated that strongly suggest thatincreased BAT mass is associated with increases in energy expendi-ture that are associated with improvements in metabolic health [8].The opposite is also true, and mice lacking BAT are obese and havemetabolic impairments [31,45]. In the last few months, two high im-pact studies have linked the JAK–STAT signaling pathway to BAT, andthese findings are summarized in Fig. 2. It is largely accepted thatmacrophages in WAT play a role in the pathogenesis of insulin resis-tance and T2DM (reviewed in [72]). It is not clear whether macro-phages in BAT are also important, and this topic is controversial.However, a recent study clearly demonstrates the presence of AAMs(alternatively activated macrophages) in BAT [63]. These macro-phages are sensitive to IL-4, which acts via the JAK–STAT6 signalingpathway. The activation of this pathway results in the production ofnorepinephrine (NE). Among other physiological effects, NE can activatelipolysis in WAT. The hypothesis developed from these recent studiessuggests that NE produced from alternatively activated macrophages inBAT acts on WAT to modulate free fatty acid levels (Fig. 2). The use ofIL-4 receptor null mice and STAT6 null mice reveals that these proteinsare necessary for adaptive thermogenesis [63].
Another study linking BAT with the JAK–STAT pathway providesevidence that the JAK kinase Tyk2 plays an important role in BAT de-velopment. In vitro studies strongly provide evidence that Tyk2 pro-motes adipogenesis. Moreover, Tyk2 null mice are obese and haveimpaired glucose tolerance. The phenotype of these mice can be res-cued by aP2 driven expression of a constitutively active STAT3 protein[19]. In summary, this study suggests that the Tyk2–STAT3 signaling
Fig. 2. JAK–STAT signaling in white and brown adipose tissues. Different transcriptional programs control the adipogenesis of white and brown adipocytes, and different JAK andSTAT members have been implicated in these programs. Cytokine and hormone activators of the JAK–STAT pathway are produced and utilized by various cell types present withinadipose tissue to modulate the physiological function of WAT and BAT.
pathway plays a role in regulating BAT development (Fig. 2). Theseare the first two studies to indicate the potential importance of theJAK–STAT signaling pathway in regulating the production and func-tion of BAT.
7. Conclusions and outlook
In conclusion,modulation of the JAK–STAT pathway can regulate ad-ipocyte development and function. An emerging area of investigationlinking JAK–STAT signaling and adipocyte function will clearly be stud-ies that examine the interplay of immune cells and adipocytes in bothbrown and white adipose tissues. Additional studies in both culturedadipocytes and in adipose tissue will be needed to reveal comprehen-sive roles of the JAK–STAT family members in adipocytes, obesity, andinsulin resistance. Although tyrosine phosphorylation is critical for ca-nonical STAT activation, other covalent modifications such as serinephosphorylation, acetylation, methylation and sumoylation can alsooccur (reviewed in [84]), and studies of these STAT modifications arein their infancy. Moreover, the non-canonical mechanisms and func-tions of JAKs and STATs in adipocytes have not been well studied. Re-cent investigations indicate that STATs can participate in chromatinorganization and mitochondrial respiration in ways that are indepen-dent of transcriptional regulation. All of these topics will likely be in-tense areas of investigation in adipocyte biology in the near futureand will hopefully lead to the identification of new therapeutic targetsfor metabolic diseases.
References
[1] M.M. Altintas, A. Azad, B. Nayer, G. Contreras, J. Zaias, C. Faul, J. Reiser, A. Nayer,Mast cells, macrophages, and crown-like structures distinguish subcutaneousfrom visceral fat in mice, J. Lipid Res. 52 (2011) 480–488.
[2] S. Ambati, H.K. Kim, J.Y. Yang, J. Lin, M.A. Della-Fera, C.A. Baile, Effects of leptinon apoptosis and adipogenesis in 3T3–L1 adipocytes, Biochem. Pharmacol. 73(2007) 378–384.
[3] V.G. Athyros, K. Tziomalos, A. Karagiannis, P. Anagnostis, D.P. Mikhailidis, Shouldadipokines be considered in the choice of the treatment of obesity-relatedhealth problems? Curr. Drug Targets 11 (2010) 122–135.
[4] J.P. Balhoff, J.M. Stephens, Highly specific and quantitative activation of STATs in3T3–L1 adipocytes, Biochem. Biophys. Res. Commun. 247 (1998) 894–900.
[5] I. Barroso, M. Gurnell, V.E. Crowley, M. Agostini, J.W. Schwabe, M.A. Soos, G.L.Maslen, T.D. Williams, H. Lewis, A.J. Schafer, V.K. Chatterjee, S. O'Rahilly,Dominant negative mutations in human PPARgamma associated with severeinsulin resistance, diabetes mellitus and hypertension, Nature 402 (1999)880–883.
[6] J.E. Baugh Jr., Z.E. Floyd, J.M. Stephens, The modulation of STAT5A/GR complexesduring fat cell differentiation and in mature adipocytes, Obesity (Silver Spring)15 (2007) 583–590.
[8] O. Boss, S.R. Farmer, Recruitment of brown adipose tissue as a therapy forobesity-associated diseases, Front. Endocrinol.(Lausanne) 3 (2012) 14.
[9] M.F. Brizzi, M.G. Aronica, A. Rosso, G.P. Bagnara, Y. Yarden, L. Pegoraro,Granulocyte-macrophage colony-stimulating factor stimulates JAK2 signalingpathway and rapidly activates p93fes, STAT1 p91, and STAT3 p92 in polymor-phonuclear leukocytes, J. Biol. Chem. 271 (1996) 3562–3567.
[10] E.R. Cernkovich, J. Deng, M.C. Bond, T.P. Combs, J.B. Harp, Adipose-specific dis-ruption of signal transducer and activator of transcription 3 increases bodyweight and adiposity, Endocrinology 149 (2008) 1581–1590.
[11] E.R. Cernkovich, J. Deng, K. Hua, J.B. Harp, Midkine is an autocrine activator ofsignal transducer and activator of transcription 3 in 3T3–L1 cells, Endocrinology148 (2007) 1598–1604.
[12] A.A. Coulter, J.M. Stephens, STAT5 activators modulate acyl CoA oxidase (AOX)expression in adipocytes and STAT5A binds to the AOX promoter in vitro,Biochem. Biophys. Res. Commun. 344 (2006) 1342–1345.
[14] A.M. Cypess, S. Lehman, G. Williams, I. Tal, D. Rodman, A.B. Goldfine, F.C. Kuo,E.L. Palmer, Y.H. Tseng, A. Doria, G.M. Kolodny, C.R. Kahn, Identification and im-portance of brown adipose tissue in adult humans, N. Engl. J. Med. 360 (2009)1509–1517.
[15] J. Defuria, A.C. Belkina, M. Jagannathan-Bogdan, J. Snyder-Cappione, J.D. Carr,Y.R. Nersesova, D. Markham, K.J. Strissel, A.A. Watkins, M. Zhu, J. Allen, J.Bouchard, G. Toraldo, R. Jasuja, M.S. Obin, M.E. McDonnell, C. Apovian, G.V.Denis, B.S. Nikolajczyk, B cells promote inflammation in obesity and type 2 dia-betes through regulation of T-cell function and an inflammatory cytokine pro-file, Proc. Natl. Acad. Sci. U. S. A. 110 (2013) 5133–5138.
[16] J. Deng, K. Hua, E.J. Caveney, N. Takahashi, J.B. Harp, Protein inhibitor of activatedSTAT3 inhibits adipogenic gene expression, Biochem. Biophys. Res. Commun.339 (2006) 923–931.
[17] J. Deng, K. Hua, S.S. Lesser, A.H. Greiner, A.W. Walter, M.B. Marrero, J.B. Harp, In-terleukin-4 mediates STAT6 activation in 3T3–L1 preadipocytes but not adipo-cytes, Biochem. Biophys. Res. Commun. 267 (2000) 516–520.
[18] J. Deng, K. Hua, S.S. Lesser, J.B. Harp, Activation of signal transducer and activatorof transcription-3 during proliferative phases of 3T3–L1 adipogenesis, Endocri-nology 141 (2000) 2370–2376.
[19] M. Derecka, A. Gornicka, S.B. Koralov, K. Szczepanek, M. Morgan, V. Raje, J. Sisler,Q. Zhang, D. Otero, J. Cichy, K. Rajewsky, K. Shimoda, V. Poli, B. Strobl, S.Pellegrini, T.E. Harris, P. Seale, A.P. Russell, A.J. McAinch, P.E. O'Brien, S.R.Keller, C.M. Croniger, T. Kordula, A.C. Larner, Tyk2 and stat3 regulate brown ad-ipose tissue differentiation and obesity, Cell Metab. 16 (2012) 814–824.
[20] W. Doerrler, K.R. Feingold, C. Grunfeld, Cytokines induce catabolic effects in cul-tured adipocytes by multiple mechanisms, Cytokine 6 (1994) 478–484.
[21] S.E. Doyle, J.C. Gasson, Characterization of the role of the human granulocyte-macrophage colony-stimulating factor receptor alpha subunit in the activation ofJAK2 and STAT5, Blood 92 (1998) 867–876.
[22] C. Duffaut, A. Zakaroff-Girard, V. Bourlier, P. Decaunes, M. Maumus, P. Chiotasso, C.Sengenes, M. Lafontan, J. Galitzky, A. Bouloumie, Interplay between human ad-ipocytes and T lymphocytes in obesity: CCL20 as an adipochemokine and Tlymphocytes as lipogenic modulators, Arterioscler. Thromb. Vasc. Biol. 29(2009) 1608–1614.
[23] S.R. Farmer, Molecular determinants of brown adipocyte formation and func-tion, Genes Dev. 22 (2008) 1269–1275.
[24] D. Fleenor, R. Arumugam, M. Freemark, Growth hormone and prolactinreceptors in adipogenesis: STAT-5 activation, suppressors of cytokine sig-naling, and regulation of insulin-like growth factor I, Horm. Res. 66 (2006)101–110.
[25] D.J. Flint, N. Binart, J. Kopchick, P. Kelly, Effects of growth hormone and prolactinon adipose tissue development and function, Pituitary 6 (2003) 97–102.
[26] Z.E. Floyd, J.M. Stephens, Interferon-gamma-mediated activation and ubiquitin-proteasome-dependent degradation of PPARgamma in adipocytes, J. Biol. Chem.277 (2002) 4062–4068.
[27] Z.E. Floyd, J.M. Stephens, STAT5A promotes adipogenesis in nonprecursor cellsand associates with the glucocorticoid receptor during adipocyte differentiation,Diabetes 52 (2003) 308–314.
[28] M. Furuhashi, R. Fucho, C.Z. Gorgun, G. Tuncman, H. Cao, G.S. Hotamisligil,Adipocyte/macrophage fatty acid-binding proteins contribute to metabolic de-terioration through actions in both macrophages and adipocytes in mice, J.Clin. Invest. 118 (2008) 2640–2650.
[29] M. Furuhashi, G. Tuncman, C.Z. Gorgun, L. Makowski, G. Atsumi, E. Vaillancourt,K. Kono, V.R. Babaev, S. Fazio, M.F. Linton, R. Sulsky, J.A. Robl, R.A. Parker, G.S.Hotamisligil, Treatment of diabetes and atherosclerosis by inhibitingfatty-acid-binding protein aP2, Nature 447 (2007) 959–965.
[30] F. Gregoire, B.N. De, N. Hauser, H. Heremans, D.J. Van, C. Remacle, Interferon-gamma and interleukin-1 beta inhibit adipoconversion in cultured rodentpreadipocytes, J. Cell. Physiol. 151 (1992) 300–309.
[31] A. Hamann, J.S. Flier, B.B. Lowell, Decreased brown fat markedly enhances sus-ceptibility to diet-induced obesity, diabetes, and hyperlipidemia, Endocrinology137 (1996) 21–29.
[32] J.B. Harp, D. Franklin, A.A. Vanderpuije, J.M. Gimble, Differential expression ofsignal transducers and activators of transcription during human adipogenesis,Biochem. Biophys. Res. Commun. 281 (2001) 907–912.
[33] G. Hellgren, K. Albertsson-Wikland, H. Billig, L.M. Carlsson, B. Carlsson, Growthhormone receptor interaction with Jak proteins differs between tissues, J. Inter-feron Cytokine Res. 21 (2001) 75–83.
[34] J.C. Hogan, J.M. Stephens, The identification and characterization of a STAT 1binding site in the PPARgamma2 promoter, Biochem. Biophys. Res. Commun.287 (2001) 484–492.
[35] J.C. Hogan, J.M. Stephens, STAT 1 binds to the LPL promoter in vitro, Biochem.Biophys. Res. Commun. 307 (2003) 350–354.
[36] J.C. Hogan, J.M. Stephens, The regulation of fatty acid synthase by STAT5A, Dia-betes 54 (2005) 1968–1975.
[37] P.W. Ingham, A.P. McMahon, Hedgehog signaling in animal development: para-digms and principles, Genes Dev. 15 (2001) 3059–3087.
[38] H.S. Jung, Y.J. Lee, Y.H. Kim, S. Paik, J.W. Kim, J.W. Lee, Peroxisome proliferator-activated receptor gamma/signal transducers and activators of transcription 5Apathway plays a key factor in adipogenesis of human bone marrow-derived stro-mal cells and 3T3–L1 preadipocytes, Stem Cells Dev. 21 (2012) 465–475.
[39] M. Kawai, N. Namba, S. Mushiake, Y. Etani, R. Nishimura, M. Makishima, K. Ozono,Growth hormone stimulates adipogenesis of 3T3–L1 cells through activation of theStat5A/5B-PPARgamma pathway, J. Mol. Endocrinol. 38 (2007) 19–34.
[40] S. Keay, S.E. Grossberg, Interferon inhibits the conversion of 3T3–L1 mouse fi-broblasts into adipocytes, Proc. Natl. Acad. Sci. U. S. A. 77 (1980) 4099–4103.
[41] T. Khan, E.S. Muise, P. Iyengar, Z.V. Wang, M. Chandalia, N. Abate, B.B. Zhang, P.Bonaldo, S. Chua, P.E. Scherer, Metabolic dysregulation and adipose tissue fibro-sis: role of collagen VI, Mol. Cell. Biol. 29 (2009) 1575–1591.
[42] D.H. Kim, D. Sandoval, J.A. Reed, E.K. Matter, E.G. Tolod, S.C. Woods, R.J. Seeley,The role of GM-CSF in adipose tissue inflammation, Am. J. Physiol. Endocrinol.Metab. 295 (2008) E1038–E1046.
[43] J.Y. Kim, E. van deWall,M. Laplante, A. Azzara,M.E. Trujillo, S.M. Hofmann, T. Schraw,J.L. Durand, H. Li, G. Li, L.A. Jelicks,M.F.Mehler, D.Y. Hui, Y. Deshaies, G.I. Shulman, G.J.Schwartz, P.E. Scherer, Obesity-associated improvements in metabolic profilethrough expansion of adipose tissue, J. Clin. Invest. 117 (2007) 2621–2637.
[44] T. Kisseleva, S. Bhattacharya, J. Braunstein, C.W. Schindler, Signaling throughthe JAK/STAT pathway, recent advances and future challenges, Gene 285(2002) 1–24.
[45] S. Klaus, H. Munzberg, C. Truloff, G. Heldmaier, Physiology of transgenic micewith brown fat ablation: obesity is due to lowered body temperature, Am. J.Physiol. 274 (1998) R287–R293.
[46] V.A. Koivisto, R. Pelkonen, K. Cantell, Effect of interferon on glucose toleranceand insulin sensitivity, Diabetes 38 (1989) 641–647.
[47] A. Kosteli, E. Sugaru, G. Haemmerle, J.F. Martin, J. Lei, R. Zechner, A.W. FerranteJr., Weight loss and lipolysis promote a dynamic immune response in murineadipose tissue, J. Clin. Invest. 120 (2010) 3466–3479.
[48] J.Y. Lee, H. Muenzberg, O. Gavrilova, J.A. Reed, D. Berryman, E.C. Villanueva, G.W.Louis, G.M. Leinninger, S. Bertuzzi, R.J. Seeley, G.W. Robinson, M.G. Myers, L.Hennighausen, Loss of cytokine-STAT5 signaling in the CNS and pituitarygland alters energy balance and leads to obesity, PLoS One 3 (2008) e1639.
[49] D.E. Levy, J.E. Darnell Jr., Stats: transcriptional control and biological impact, Nat.Rev. Mol. Cell Biol. 3 (2002) 651–662.
[50] J. Liu, A. Divoux, J. Sun, J. Zhang, K. Clement, J.N. Glickman, G.K. Sukhova, P.J. Wolters,J. Du, C.Z. Gorgun, A. Doria, P. Libby, R.S. Blumberg, B.B. Kahn, G.S. Hotamisligil, G.P.Shi, Genetic deficiency and pharmacological stabilization of mast cells reducediet-induced obesity and diabetes in mice, Nat. Med. 15 (2009) 940–945.
[51] F. Machinal-Quelin, M.N. Dieudonne, M.C. Leneveu, R. Pecquery, Y. Giudicelli,Proadipogenic effect of leptin on rat preadipocytes in vitro: activation of MAPK andSTAT3 signaling pathways, Am. J. Physiol. Cell Physiol. 282 (2002) C853–C863.
[52] D. Mathis, S.E. Shoelson, Immunometabolism: an emerging frontier, Nat. Rev.Immunol. 11 (2011) 81.
[53] F.C. McGillicuddy, E.H. Chiquoine, C.C. Hinkle, R.J. Kim, R. Shah, H.M. Roche, E.M.Smyth, M.P. Reilly, Interferon gamma attenuates insulin signaling, lipid storage,and differentiation in human adipocytes via activation of the JAK/STAT pathway,J. Biol. Chem. 284 (2009) 31936–31944.
[54] F.C. McGillicuddy, K.A. Harford, C.M. Reynolds, E. Oliver, M. Claessens, K.H. Mills,H.M. Roche, Lack of interleukin-1 receptor I (IL-1RI) protects mice from high-fatdiet-induced adipose tissue inflammation coincident with improved glucose ho-meostasis, Diabetes 60 (2011) 1688–1698.
[55] A. Meirhaeghe, L. Fajas, F. Gouilleux, D. Cottel, N. Helbecque, J. Auwerx, P.Amouyel, A functional polymorphism in a STAT5B site of the human PPARgamma 3 gene promoter affects height and lipid metabolism in a French popu-lation, Arterioscler. Thromb. Vasc. Biol. 23 (2003) 289–294.
[56] M.A. Meraz, J.M. White, K.C. Sheehan, E.A. Bach, S.J. Rodig, A.S. Dighe, D.H.Kaplan, J.K. Riley, A.C. Greenlund, D. Campbell, K. Carver-Moore, R.N. DuBois,R. Clark, M. Aguet, R.D. Schreiber, Targeted disruption of the Stat1 gene inmice reveals unexpected physiologic specificity in the JAK–STAT signaling path-way, Cell 84 (1996) 431–442.
[57] Y. Miyaoka, M. Tanaka, T. Naiki, A. Miyajima, Oncostatin M inhibits adipogenesisthrough the RAS/ERK and STAT5 signaling pathways, J. Biol. Chem. 281 (2006)37913–37920.
[58] B. Mlinar, J. Marc, New insights into adipose tissue dysfunction in insulin resis-tance, Clin. Chem. Lab. Med. 49 (2011) 1925–1935.
[59] A.L. Mui, H. Wakao, A.M. O'Farrell, N. Harada, A. Miyajima, Interleukin-3,granulocyte-macrophage colony stimulating factor and interleukin-5 transducesignals through two STAT5 homologs, EMBO J. 14 (1995) 1166–1175.
[60] R. Nanbu-Wakao, Y. Fujitani, Y. Masuho, M. Muramatu, H. Wakao, Prolactin en-hances CCAAT enhancer-binding protein-beta (C/EBP beta) and peroxisomeproliferator-activated receptor gamma (PPAR gamma) messenger RNA expres-sion and stimulates adipogenic conversion of NIH-3T3 cells, Mol. Endocrinol.14 (2000) 307–316.
[61] R. Nanbu-Wakao, Y. Morikawa, I. Matsumura, Y. Masuho, M.A. Muramatsu, E.Senba, H. Wakao, Stimulation of 3T3–L1 adipogenesis by signal transducer andactivator of transcription 5, Mol. Endocrinol. 16 (2002) 1565–1576.
[62] J. Nedergaard, T. Bengtsson, B. Cannon, Unexpected evidence for active brownadiposetissue in adult humans, Am. J. Physiol. Endocrinol. Metab. 293 (2007) E444–E452.
[63] K.D. Nguyen, Y. Qiu, X. Cui, Y.P. Goh, J. Mwangi, T. David, L. Mukundan, F. Brombacher,R.M. Locksley, A. Chawla, Alternatively activated macrophages produce catechol-amines to sustain adaptive thermogenesis, Nature 480 (2011) 104–108.
[64] R.W. O'Rourke, M.D. Metcalf, A.E. White, A. Madala, B.R. Winters, I.I. Maizlin, B.A.Jobe, C.T. Roberts Jr., M.K. Slifka, D.L. Marks, Depot-specific differences in inflam-matory mediators and a role for NK cells and IFN-gamma in inflammation inhuman adipose tissue, Int. J. Obes. (Lond.) 33 (2009) 978–990.
[66] V. Ott, M. Fasshauer, A. Dalski, H.H. Klein, J. Klein, Direct effects of ciliaryneurotrophic factor on brown adipocytes: evidence for a role in peripheral reg-ulation of energy homeostasis, J. Endocrinol. 173 (2002) R1–R8.
[67] P.D. Pelton, L. Zhou, K.T. Demarest, T.P. Burris, PPARgamma activation inducesthe expression of the adipocyte fatty acid binding protein gene in human mono-cytes, Biochem. Biophys. Res. Commun. 261 (1999) 456–458.
[68] E. Pfitzner, S. Kliem, D. Baus, C.M. Litterst, The role of STATs in inflammation andinflammatory diseases, Curr. Pharm. Des. 10 (2004) 2839–2850.
[69] J.A. Pospisilik, D. Schramek, H. Schnidar, S.J. Cronin, N.T. Nehme, X. Zhang, C.Knauf, P.D. Cani, K. Aumayr, J. Todoric, M. Bayer, A. Haschemi, V. Puviindran, K.Tar, M. Orthofer, G.G. Neely, G. Dietzl, A. Manoukian, M. Funovics, G. Prager, O.Wagner, D. Ferrandon, F. Aberger, C.C. Hui, H. Esterbauer, J.M. Penninger, Dro-sophila genome-wide obesity screen reveals hedgehog as a determinant ofbrown versus white adipose cell fate, Cell 140 (2010) 148–160.
[70] H.E. Richter, T. Albrektsen, N. Billestrup, The role of signal transducer and activa-tor of transcription 5 in the inhibitory effects of GH on adipocyte differentiation,J. Mol. Endocrinol. 30 (2003) 139–150.
[71] V.Z. Rocha, E.J. Folco, G. Sukhova, K. Shimizu, I. Gotsman, A.H. Vernon, P. Libby,Interferon-gamma, a Th1 cytokine, regulates fat inflammation: a role for adap-tive immunity in obesity, Circ. Res. 103 (2008) 467–476.
[72] G.R. Romeo, J. Lee, S.E. Shoelson, Metabolic syndrome, insulin resistance, and rolesof inflammation–mechanisms and therapeutic targets, Arterioscler. Thromb. Vasc.Biol. 32 (2012) 1771–1776.
[73] R.L. Rosen, K.D. Winestock, G. Chen, X. Liu, L. Hennighausen, D.S. Finbloom,Granulocyte-macrophage colony-stimulating factor preferentially activates the94-kD STAT5A and an 80-kD STAT5A isoform in human peripheral blood mono-cytes, Blood 88 (1996) 1206–1214.
[74] J. Sanchez-Gurmaches, C.M. Hung, C.A. Sparks, Y. Tang, H. Li, D.A. Guertin, PTENloss in the Myf5 lineage redistributes body fat and reveals subsets of white ad-ipocytes that arise from Myf5 precursors, Cell Metab. 16 (2012) 348–362.
[75] K. Sarjeant, J.M. Stephens, Adipogenesis, Cold Spring Harb, Perspect. Biol. 4 (2012).[76] C. Schindler, J.E. Darnell Jr., Transcriptional responses to polypeptide ligands: the
JAK–STAT pathway, Annu. Rev. Biochem. 64 (1995) 621–651.[77] H.S. Schipper, B. Prakken, E. Kalkhoven, M. Boes, Adipose tissue-resident im-
[78] P. Seale, B. Bjork, W. Yang, S. Kajimura, S. Chin, S. Kuang, A. Scime, S. Devarakonda,H.M. Conroe, H. Erdjument-Bromage, P. Tempst, M.A. Rudnicki, D.R. Beier, B.M.Spiegelman, PRDM16 controls a brown fat/skeletal muscle switch, Nature 454(2008) 961–967.
[79] C.A. Shang, M.J. Waters, Constitutively active signal transducer and activator oftranscription 5 can replace the requirement for growth hormone in adipogenesisof 3T3–F442A preadipocytes, Mol. Endocrinol. 17 (2003) 2494–2508.
[80] T. Shiba, N. Higashi, Y. Nishimura, Hyperglycaemia due to insulin resistancecaused by interferon-gamma, Diabet. Med. 15 (1998) 435–436.
[81] C.A. Siegrist-Kaiser, V. Pauli, C.E. Juge-Aubry, O. Boss, A. Pernin, W.W. Chin, I.Cusin, F. Rohner-Jeanrenaud, A.G. Burger, J. Zapf, C.A. Meier, Direct effects of lep-tin on brown and white adipose tissue, J. Clin. Invest. 100 (1997) 2858–2864.
[82] R. Siersbaek, R. Nielsen, S. John, M.H. Sung, S. Baek, A. Loft, G.L. Hager, S.Mandrup, Extensive chromatin remodelling and establishment of transcriptionfactor ‘hotspots’ during early adipogenesis, EMBO J. 30 (2011) 1459–1472.
[83] H.Y. Song, E.S. Jeon, J.I. Kim, J.S. Jung, J.H. Kim, Oncostatin M promotes osteogen-esis and suppresses adipogenic differentiation of human adipose tissue-derivedmesenchymal stem cells, J. Cell. Biochem. 101 (2007) 1238–1251.
[84] G.R. Stark, J.E. Darnell Jr., The JAK–STAT pathway at twenty, Immunity 36 (2012)503–514.
[85] J.M. Stephens, S.J. Lumpkin, J.B. Fishman, Activation of signal transducers and ac-tivators of transcription 1 and 3 by leukemia inhibitory factor, oncostatin-M, andinterferon-gamma in adipocytes, J. Biol. Chem. 273 (1998) 31408–31416.
[86] J.M. Stephens, R.F. Morrison, P.F. Pilch, The expression and regulation of STATs dur-ing 3T3–L1 adipocyte differentiation, J. Biol. Chem. 271 (1996) 10441–10444.
[87] W.C. Stewart, J.E. Baugh Jr., Z.E. Floyd, J.M. Stephens, STAT 5 activators can re-place the requirement of FBS in the adipogenesis of 3T3–L1 cells, Biochem.Biophys. Res. Commun. 324 (2004) 355–359.
[88] W.C. Stewart, R.F. Morrison, S.L. Young, J.M. Stephens, Regulation of signal trans-ducers and activators of transcription (STATs) by effectors of adipogenesis: coordi-nate regulation of STATs 1, 5A, and 5B with peroxisome proliferator-activatedreceptor-gamma and C/AAAT enhancer binding protein-alpha, Biochim. Biophys.Acta 1452 (1999) 188–196.
[89] W.C. Stewart, L.A. Pearcy, Z.E. Floyd, J.M. Stephens, STAT5A expression in Swiss3T3 cells promotes adipogenesis in vivo in an athymic mice model system, Obe-sity (Silver Spring) 19 (2011) 1731–1734.
[90] D.J. Story, J.M. Stephens, Modulation and lack of cross-talk between signal trans-ducer and activator of transcription 5 and Suppressor of cytokine signaling-3 in
insulin and growth hormone signaling in 3T3–L1 adipocytes, Obesity (SilverSpring) 14 (2006) 1303–1311.
[91] K.J. Strissel, J. DeFuria, M.E. Shaul, G. Bennett, A.S. Greenberg, M.S. Obin, T-cellrecruitment and Th1 polarization in adipose tissue during diet-induced obesityin C57BL/6 mice, Obesity (Silver Spring) 18 (2010) 1918–1925.
[92] J.M. Suh, X. Gao, J. McKay, R. McKay, Z. Salo, J.M. Graff, Hedgehog signaling playsa conserved role in inhibiting fat formation, Cell Metab. 3 (2006) 25–34.
[93] S. Talukdar, d.Y. Oh, G. Bandyopadhyay, D. Li, J. Xu, J. McNelis, M. Lu, P. Li, Q. Yan,Y. Zhu, J. Ofrecio, M. Lin, M.B. Brenner, J.M. Olefsky, Neutrophils mediate insulinresistance in mice fed a high-fat diet through secreted elastase, Nat. Med. 18(2012) 1407–1412.
[94] S. Teglund, C. McKay, E. Schuetz, J.M. van Deursen, D. Stravopodis, D. Wang, M.Brown, S. Bodner, G. Grosveld, J.N. Ihle, Stat5a and Stat5b proteins have essentialand nonessential, or redundant, roles in cytokine responses, Cell 93 (1998)841–850.
[95] B.R. Thompson, A.M. Mazurkiewicz-Munoz, J. Suttles, C. Carter-Su, D.A. Bernlohr,Interaction of adipocyte fatty acid-binding protein (AFABP) and JAK2: AFABP/aP2as a regulator of JAK2 signaling, J. Biol. Chem. 284 (2009) 13473–13480.
[96] J. Todoric, B. Strobl, A. Jais, N. Boucheron, M. Bayer, S. Amann, J. Lindroos, R.Teperino, G. Prager, M. Bilban, W. Ellmeier, F. Krempler, M. Muller, O.Wagner, W. Patsch, J.A. Pospisilik, H. Esterbauer, Cross-talk betweeninterferon-gamma and hedgehog signaling regulates adipogenesis, Diabetes60 (2011) 1668–1676.
[97] S. Urs, A. Harrington, L. Liaw, D. Small, Selective expression of an aP2/Fatty AcidBinding Protein 4-Cre transgene in non-adipogenic tissues during embryonicdevelopment, Transgenic Res. 15 (2006) 647–653.
[98] M. Varjosalo, J. Taipale, Hedgehog: functions and mechanisms, Genes Dev. 22(2008) 2454–2472.
[99] K.J. Waite, Z.E. Floyd, P. Arbour-Reily, J.M. Stephens, Interferon-gamma-inducedregulation of peroxisome proliferator-activated receptor gamma and STATs inadipocytes, J. Biol. Chem. 276 (2001) 7062–7068.
[100] H. Wakao, R. Wakao, A. Oda, H. Fujita, Constitutively active Stat5A and Stat5Bpromote adipogenesis, Environ. Health Prev. Med. 16 (2011) 247–252.
[101] D. Wang, Y. Zhou, W. Lei, K. Zhang, J. Shi, Y. Hu, G. Shu, J. Song, Signal transducer andactivator of transcription 3 (STAT3) regulates adipocyte differentiation viaperoxisome-proliferator-activated receptor gamma (PPARgamma), Biol. Cell 102(2010) 1–12.
[102] S. Watanabe, T. Itoh, K. Arai, JAK2 is essential for activation of c-fos andc-myc promoters and cell proliferation through the human granulocyte-macrophage colony-stimulating factor receptor in BA/F3 cells, J. Biol.Chem. 271 (1996) 12681–12686.
[103] U.A. White, A.A. Coulter, T.K. Miles, J.M. Stephens, The STAT5A-mediated induc-tion of pyruvate dehydrogenase kinase 4 expression by prolactin or growth hor-mone in adipocytes, Diabetes 56 (2007) 1623–1629.
[105] D.A. Winer, S. Winer, L. Shen, P.P. Wadia, J. Yantha, G. Paltser, H. Tsui, P. Wu, M.G.Davidson, M.N. Alonso, H.X. Leong, A. Glassford, M. Caimol, J.A. Kenkel, T.F.Tedder, T. McLaughlin, D.B. Miklos, H.M. Dosch, E.G. Engleman, B cells promoteinsulin resistance through modulation of T cells and production of pathogenicIgG antibodies, Nat. Med. 17 (2011) 610–617.
[106] W. Xiao, H. Hong, Y. Kawakami, C.A. Lowell, T. Kawakami, Regulation ofmyeloproliferation and M2 macrophage programming in mice by Lyn/Hck,SHIP, and Stat5, J. Clin. Invest. 118 (2008) 924–934.
[107] H. Yang, Y.H. Youm, B. Vandanmagsar, A. Ravussin, J.M. Gimble, F. Greenway,J.M. Stephens, R.L. Mynatt, V.D. Dixit, Obesity increases the production ofproinflammatory mediators from adipose tissue T cells and compromises TCRrepertoire diversity: implications for systemic inflammation and insulin resis-tance, J. Immunol. 185 (2010) 1836–1845.
[108] S.J. Yarwood, E.M. Sale, G.J. Sale, M.D. Houslay, E. Kilgour, N.G. Anderson, Growthhormone-dependent differentiation of 3T3–F442A preadipocytes requires Januskinase/signal transducer and activator of transcription but not mitogen-activatedprotein kinase or p70 S6 kinase signaling, J. Biol. Chem. 274 (1999) 8662–8668.
[109] K. Zhang, W. Guo, Y. Yang, J. Wu, JAK2/STAT3 pathway is involved in the earlystage of adipogenesis through regulating C/EBPbeta transcription, J. Cell.Biochem. 112 (2011) 488–497.
[110] S. Zvonic, P. Cornelius, W.C. Stewart, R.L. Mynatt, J.M. Stephens, The regulationand activation of ciliary neurotrophic factor signaling proteins in adipocytes, J.Biol. Chem. 278 (2003) 2228–2235.
[111] S. Zvonic, J.C. Hogan, P. Arbour-Reily, R.L. Mynatt, J.M. Stephens, Effects ofcardiotrophin on adipocytes, J. Biol. Chem. 279 (2004) 47572–47579.
[112] S. Zvonic, D.J. Story, J.M. Stephens, R.L. Mynatt, Growth hormone, but not insulin,activates STAT5 proteins in adipocytes in vitro and in vivo, Biochem. Biophys.Res. Commun. 302 (2003) 359–362.
