METABOLIC DISEASE Hyaluronan in adipose tissue: … › content › scitransmed › 8 › 323 › 323...METABOLIC DISEASE Hyaluronan in adipose tissue: Beyond dermal filler and therapeutic

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Hyaluronan in adipose tissue: Beyonddermal filler and therapeutic carrierYi Zhu,1,2 Clair Crewe,1 Philipp E. Scherer1,3*

Adipose hyaluronan is increasingly recognized as an active player in adipose tissue fibrosisand metabolic dysfunction. However, this role poses as many challenges as opportunitiesfor therapeutic targeting of adipose tissue dysfunction during nutrient oversupply.

THE EXTRACELLULAR MATRIX OFADIPOSE TISSUEThe extracellular matrix (ECM) is an integralcomponent for the process of adipogenesisand adipose tissue homeostasis however, ex-cessive production of ECM components canresult in local adipose tissue fibrosis as well,leading to adipocyte dysfunction. By actingas a scaffold for cell migration, a reservoirfor cytokines and growth factors, and a bind-ing site for various cellular receptors, the ECMmodulates adipose tissue metabolism, im-mune responses, and cell behavior. The ECMis maintained and expanded by the adipo-cytes themselves as well as resident stromalcells, such as fibroblasts, which secrete ECMproteins, proteoglycans, and nonproteoglycanpolysaccharides, along with a host of enzymesthat control modifications and degradation ofthese structures. The high level of activity be-tween buildup and breakdown allows theECM to provide structural support while alsomaintaining the capacity to undergo dramaticremodeling.

Rapid tissue expansion during obesity al-ters this balance and induces local tissue hy-poxia and activation of HIF1a. When adiposetissue expansion exceeds the HIF1a-inducedangiogenic program, an alternate HIF1a-mediated transcriptional program is inducedthat enhances synthesis of ECM collagen pro-teins and enzymes involved in collagen cross-linking and stabilization. Hypoxic adipocytesbecome dysfunctional and prompt the infiltra-tion of macrophages, neutrophils, lymphocytes,and mast cells by secreting various adipokines.These changes give rise to a local proinflam-

1Touchstone Diabetes Center, Department of InternalMedicine, University of Texas Southwestern MedicalCenter, Dallas, TX 75390, USA. 2LIFA Diabetes, Lilly ResearchLaboratories, Division of Eli Lilly and Company, Indianapolis,IN 46285, USA. 3Department of Cell Biology, University ofTexas Southwestern Medical Center, Dallas, TX 75390,USA.*Corresponding author. E-mail: [email protected]

matory microenvironment, further exacerbat-ing the accumulation of fibrotic proteins inadipose tissue. In humans, adipose tissue fibro-sis, as quantified by total tissue hydroxyproline,or histologically by trichrome or picrosiriusred staining, is inversely associated with theoverall metabolic fitness of the individual.We have recently discussed the general impli-cations of fibrosis for the pathophysiology ofadipose tissue (1).

HYALURONIC ACID: HIGHLYABUNDANT, HIGHLY NEGLECTEDThe ECM proteins collagen and fibronectinhave been widely studied for their roles inobesity-associated adipose tissue dysfunction,but much less is known about the participa-tion of other macromolecules such as proteo-glycans and nonproteoglycan polysaccharides.Of particular recent interest is hyaluronic acid(HA, also known as hyaluronan). Historically,HA has been understudied as an ECM com-ponent in the context of obesity, mostly becauseof the lack of easily accessible assay protocolsand histological methods for its visualization.HA is a nonsulfated glycosaminoglycan(GAG) polymer consisting of repeating di-saccharide units of D-glucuronic acid (GlcUA)and N-acetyl-D-glucosamine (GlcNAc) and isenergetically stable, with high abundance inconnective, epithelial, and dermal tissues. HAis synthesized at the level of the plasma mem-brane by hyaluronan synthases (HAS1 toHAS3), with HAS2 being the major isoformin adult adipose tissues; HA degradation ismediated by hyaluronidases (HYAL1-4, PH20,and HYALP1) (Fig. 1). HA polymers varywidely in size, ranging from kilodaltons tomegadaltons, and each isoform of HA syn-thase and hyaluronidase displays enzymaticspecificity toward HA within a given molec-ular weight range. HAs are hydrophilic andinfluence the hydration and biomechanicalproperties of many tissues, including adiposetissue. Additionally, successful morphogene-sis commonly relies on the physical proper-

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ties of HA, which regulate the interaction ofHA with many proteoglycans that are impor-tant for ECM maturation (2).

HYALURONIC ACID: FROM A NEWGENERATION OF DERMAL FILLERS TOPOTENTIALLY A NEW GENERATION OFTHERAPEUTIC CARRIERSThe cosmetic industry has long used the hy-drophilic and nonimmunogenic properties ofHA for the development of cosmetic dermalfillers with no adverse side effects. Unlike pre-vious generations of fillers, HA can be in-jected into deeper tissue layers of the face tobring a subtle, yet definitive, rejuvenation rath-er than just simply “filling” wrinkles or scars(3). HA fillers also last longer owing to slowerabsorption. Recently, HA has also been studiedas a potential therapeutic carrier for humanadipose-derived stem cell (hASC) transplan-tation. A HA gel containing hASCs promotedin vivo growth of new adipocytes, acting as along-lasting soft tissue filler, although the oc-currence of bona fide adipogenesis and adi-pose progenitor recruitment needs furtherverification (4). This technique has also beenapplied to a promising therapeutic strategy incombating metabolic syndrome. Transplanta-tion of adipose tissue–derived multipotent stemcells (ADMSCs) with HA-based hydrogelshas led to in vivo differentiation of lipid-accumulating, UCP1-expressing beige adiposetissue (5). Implant recipient mice exhibitedenhanced respiration rates and improved glu-cose homeostasis. This study demonstratedthe therapeutic potential of this potentiallytranslatable approach for humans. HA hasalso been used in delivering many other U.S.Food and Drug Administration–approved drugs.For example, a relevant application in thecontext of whole-body metabolism is the useof HA in oral delivery of insulin. An HA-insulin complex was prepared in the labora-tory and was shown to be effective after oraladministration in lowering blood glucose indiabetic rats (6, 7), although testing the effica-cy in humans is pending.

HYALURONIC ACID: BEYOND MERESTRUCTURAL COMPONENT IN THEADIPOSE TISSUEHA function stretches beyond inert structuralcarrier properties. It also binds to many ECMproteins and cell membrane receptors to ac-tivate downstream signaling pathways that af-fect cell migration, apoptosis, tumorigenesis,and inflammation (Fig. 1) (8, 9). Recent studieshave provided evidence that HA-mediated

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signaling is altered in major tissues in obesi-ty. Total HA content is increased in insulin-resistant skeletal muscle and adipose tissuein a mouse model of diet-induced obesity(DIO) (10) possibly through multiple mecha-nisms (11). Treatment of these mice with aserum-stable, recombinant hyaluronidasePH20 (PEGPH20) reduced HA accumulationand preserved whole-body insulin sensitivity(10). Interestingly, treatment with PEGPH20resulted in up to 35% reduction of adiposetissue mass, with simultaneous reduction ofadipocyte size (10). The mechanism for thePEGPH20-mediated reduction in adiposetissue mass is unknown, although a recent studymay offer insight. Ji et al. demonstrated thatHA is a positive regulator of adipogenesis (12).During adipogenesis, HA synthesis is in-creased, whereas experimental inhibition ofHA synthesis in 3T3-L1 adipocytes results insuppressed peroxisome proliferator–activatedreceptor g (PPARg) and CCAAT/enhancerbinding protein a (C/EBPa) expression, whichare critical mediators for adipogenesis as wellas lipid droplet formation and accumulation.Because adipogenesis and lipid deposition are

important elements of adipose tissue expan-sion, pharmacological modulation of HAlevels may offer an opportunity to controlfat mass gain. HA-mediated signaling inDIO mice may also promote inflammation,a hallmark of late-stage adipose tissue dys-function and possible cause of adipose tissuefibrosis.

There is also a likely connection betweenHA and excessive ECM accumulation. Clini-cally, subcutaneous adipose tissue fibrosis isthe major negative predictor for bariatricsurgery–mediated weight loss (13). However,the interplay between fibrosis and hyaluronancontent was not investigated further in thisparticular study.

Patient-matched dermal and oral mucosalfibroblasts used as models of scarred versusscar-free healing showed that HA expressionwas much higher in dermal fibroblasts, inwhich HA was implicated in tumor growthfactor–b1 (TGF-b1)–mediated induction ofproliferation and fibrotic protein deposition(14), highlighting the involvement of HA infibroblast proliferation and TGF-b1–mediatedfibrosis. Kang et al. showed that PEGPH20

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decreased gene expression of proinflamma-tory markers in adipose tissue, whereas theexpression of the anti-inflammatory markersand total macrophage markers was unchanged.The authors concluded that macrophageswith the classical activation state (M1) weredecreased by PEGPH20 treatment and thus re-sulted in a decreased inflammatory profile inadipose tissue during high-fat diet (HFD) ex-posure. This effect is suspected to be modu-lated through CD44, the major cell-surfaceHA-binding protein. HA binds to CD44, trig-gering phosphorylation of the CD44 cyto-plasmic tail and activation of downstreamsignaling cascades, regulating inflammation,T cell recruitment, and activation. CD44-deficient mice exhibit a substantially reducedWAT-associated inflammation but an in-creased lipid accumulation during an HFDchallenge (15), which complicates the inter-pretation of the role of the interaction be-tween HA and CD44 in WAT during DIO.Expression levels of several collagen geneswere greatly diminished in CD44-deficientmice, suggesting HA-CD44 interactions maypromote a buildup of collagen and lead to thedevelopment of fibrosis.

HYALURONIC ACID: FUNCTIONALIMPLICATIONS OF SIZEA major question remaining is whether thesize distribution of HA is important for theregulation of adipogenesis. Interestingly, bothHAS2 and HYAL2 are up-regulated duringadipogenesis. The net result is an overall in-crease in HA production, associated with ahigh degree of HA turnover. The HYAL2 en-zyme hydrolyzes only HA of high molecularmass, yielding intermediate-sized HA frag-ments of ~20 kilodaltons, which can be fur-ther hydrolyzed to small oligosaccharides byPH20. Therefore, it is possible that inductionofHAS2 andHYAL2 leads to a net increase ofboth high-molecular-weight and intermediate-molecular-weight HAs. Yet, how the potentialsize distribution of HA polymers affects adi-pogenic signaling remains to be determined.Smaller HA fragments produced by hyalur-onidases can induce angiogenesis, an impor-tant component of adipose tissue expansion.However, a recent study showed medium-molecular-weight HA inhibits adipogenesisin cultured 3T3-L1 cells (16), further compli-cating the roles of different molecular weightHAs in adipogenesis. Furthermore, HA in-teracts with collagen VI and promotes its as-sembly in vitro (17). Whether this process isphysiologically relevant in vivo and whether

Adipocyte

Extracellularmatrix

Col VI

Eosinophil

Macrophage

Hyaluronic acid

Aggrecan

Proteinases

Collagen I fibrils

Hyal2 enzymecleavage sites

Has2

Fig. 1. Illustration depicting an adipocyte and its extracellular matrix focusing on hyaluronicacid. Collagen I fibrils form the main structural component and provide the physical support of the

adipose tissue. Collagen VI is the major isoform of collagen that surrounds each adipocyte. HAs aresynthesized by HAS2 and exported during the synthesis. HA fibrils provide an anchor for the proteo-glycan core protein aggrecan. HYAL2 processes HAs into small fragments, which have a differentbinding dynamics compared with high-molecular-weight HA, and may promote angiogenesis andattract macrophages, eosinophils, and other cells.

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it is involved in pathological collagen VI dep-osition inWAT needs to be investigated fur-ther. Last, it is unknown whether there is areciprocal interaction between HA and fibro-sis. We therefore need to be careful to pay at-tention to the possibility that the changes inHA may be secondary to changes in fibrosis.

OUTLOOKHA has been extracted from rooster combsand studied for many decades, but the molec-ular regulation of its synthesis and degrada-tion in adipose tissue and its physiologicaland pathological roles in adipose tissue ex-pansion are still largely unknown. In orderto use HA as a pharmacological target so asto reduce adipose tissue fibrosis and metabolicdisease, many questions remain to be an-swered: Is there an optimal size distributionof HA in adipose tissue? Is this profile alteredin obese patients through changes in the ex-pression of synthesis and/or degradation en-zymes? Does HA accumulation in obesityplay a causative role in the development offibrosis? Answering these questions will helpus assess whether hyaluronidase-based inter-ventions aimed at a reduction of adipose HAcontent can lead to an improvement in themetabolic profile in obese individuals. Withthe advancement of transgenic animal tech-niques, we can start to dissect these pathwaysin adipose tissue itself and also elucidate apossible crosstalk between multiple metaboli-cally active tissues. Hopefully, these preclin-

ical studies will pave the way for clinicallyapplicable approaches that target HA turn-over, with the goal to ameliorate metabolicdisease sequelae.

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3. R. G. Glogau, Fillers: From the past to the future. Semin.Cutan. Med. Surg. 31, 78–87 (2012).

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Funding: P.E.S. is funded by U.S. National Institutes of Healthgrants R01-DK55758, R01-DK099110, and P01-DK088761 aswell as a grant from the Cancer Prevention and Research In-stitute of Texas (CPRIT RP140412). Y.Z. is funded by a Lilly Inno-vation Fellowship Award (LIFA). Competing interests: Theauthors declare they have no competing interests.

10.1126/scitranslmed.aad6793

Citation: Y. Zhu, C. Crewe, P. E. Scherer, Hyaluronan inadipose tissue: Beyond dermal filler and therapeutic carrier.Sci. Transl. Med. 8, 323ps4 (2016).

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