TítuloSerum levels of the adipomyokine irisin in patients with chronic kidney disease

RevistadelaSociedadEspañoladeNefrología

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Original

article

Serum

levels

of

the

adipomyokine

irisin

in

patients

with

chronic

kidney

disease

夽

Ana

Rodríguez-Carmona

_,

_Miguel

_Pérez

_Fontán

_,

_Susana

_Sangiao

_Alvarellos

_,

Teresa

García

Falcón

_,

_María

_Lara

_Pena

_Bello

_,

_Andrés

_López

_{Mu ˜niz}

_,

Fernando

Cordido

a_{ServiciodeNefrología,HospitalUniversitariodeACoru ˜na,LaCoru ˜na,Spain}

b_{InstitutodeInvestigaciónBiomédica(INIBIC),UniversidaddeACoru ˜na,LaCoru ˜na,Spain} c_{ServiciodeEndocrinología,HospitalUniversitariodeACoru ˜na,LaCoru ˜na,Spain}

d_{DepartamentodeMedicina,FacultaddeCienciasdelaSalud,UniversidaddeACoru ˜na,LaCoru ˜na,Spain}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received30July2015 Accepted11May2016

Keywords: Irisin

Chronickidneydisease Glomerularfiltrationrate Peritonealdialysis Haemodialysis Bicarbonate

a

b

s

t

r

a

c

t

Background:Irisinisanadipomyokinewithclaimedanti-obesityandanti-diabeticeffects. Thishormonehasbeeninsufficientlystudiedinpatientswithadvancedchronickidney disease(CKD).

Objective:Toperformanexploratoryanalysisofserumirisinlevelsinpatientsundergoing differentCKDtreatments.

Method:Followingacross-sectionaldesign,weestimatedserumlevelsofirisinin95patients withCKDmanagedconservatively(advancedCKD),withperitonealdialysis(PD)orwith haemodialysis,andcomparedourfindingswithacontrolgroupof40healthyindividuals. Weinvestigatedthecorrelationsbetweenserumirisinanddemographic,clinical,body com-positionandmetabolicvariables.

Results:IrisinlevelswerelowerinalltheCKDgroupsthaninthecontrolgroup.The uni-variateanalysisrevealedlimitedcorrelationsbetweenirisin,ontheonehand,andfat(but notlean)mass,glomerularfiltrationrate(GFR)andplasmaalbuminandbicarbonate,on theother.ThemultivariateanalysisconfirmedthatadvancedCKDpatientsmanaged con-servatively(difference111.1ng/mL),withPD(25.9ng/mL)orhaemodialysis(61.4ng/mL)(all p<.0005)presentedloweririsinlevelsthanthecontrolgroup.Furthermore,PDpatients pre-sentedhigherserumlevelsofirisinthanthoseonhaemodialysis(difference39.4ng/mL, p=.002) or those managedconservatively (24.4ng/mL, p=.036). Themultivariate analy-sisalsoidentifiedplasma bicarbonate(B=3.90per mM/l,p=.001) andGFR(B=1.89per mL/min,p=.003)asindependentpredictorsofirisin levels.Conversely,noadjusted cor-relationbetweenirisinandbodycompositionmarkerswasfound.

DOIoforiginalarticle:

http://dx.doi.org/10.1016/j.nefro.2016.05.019.

夽_{Pleasecitethisarticleas:Rodríguez-CarmonaA,PérezFontánM,SangiaoAlvarellosS,GarcíaFalcónT,PenaBelloML,LópezMu ˜nizA,}

etal.Nivelesséricosdelaadipomioquinairisinaenpacientesconenfermedadrenalcrónica.Nefrologia.2016;36:496–502.

∗ _{Correspondingauthor.}

E-mailaddress:Miguel.Perez.Fontan@sergas.es(M.PérezFontán).

Conclusions:SerumirisinlevelsarelowinpatientswithCKDandshowaconsistent corre-lationwithGFRandplasmabicarbonatelevels.PDpatientspresenthigherlevelsofirisin thanthosemanagedconservativelyorwithhaemodialysis.Ourstudyconfirmsageneral inconsistencyoftheassociationbetweenserumirisinlevels,ontheonehand,andbody compositionandmetabolicmarkers,ontheother.

©2016SociedadEspa ˜noladeNefrolog´ıa.PublishedbyElsevierEspa ˜na,S.L.U.Thisisan openaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/ by-nc-nd/4.0/).

Niveles

séricos

de

la

adipomioquina

irisina

en

pacientes

con

enfermedad

renal

crónica

Palabrasclave: Irisina

Enfermedadrenalcrónica Filtradoglomerular DiálisisPeritoneal Hemodiálisis Bicarbonato

r

e

s

u

m

e

n

Antecedentes:Lairisinaesunaadipomioquinaconposiblesefectosantiobesidady antidia-béticos.Estahormonahasidoinsuficientementeestudiadaenpacientesconenfermedad renalcrónica(ERC)avanzada.

Objetivo:Realizarunanálisisexploratoriodelosnivelesséricosdeirisinaenpacientescon diferentesmodalidadesdetratamientodelaERC.

Método:Segúndise ˜notransversal,estimamosnivelesdeirisinaen95pacientesconERC manejadosconservadoramente(ERCA), condiálisisperitoneal (DP) oconhemodiálisis, comparándolosconungrupocontrolde40individuossanos.Tambiéninvestigamoslas correlacionesentreirisinaséricayvariablesdemográficas,clínicas,metabólicasyde com-posicióncorporal.

Resultados:Losnivelesdeirisinafueronmásbajosencualquiergrupodepacientesqueen loscontroles.Elanálisisunivariantedesvelócorrelacionesmoderadasentreirisina,porun lado,ymasagrasa(peronomagra),filtradoglomerular(GFR)yalbúminaybicarbonato plas-mático,porotro.ElanálisismultivarianteconfirmóquelospacientesconERCA(diferencia 111,1ng/mL),enDP(25,9ng/mL)ohemodiálisis(61,4ng/mL)(todosp<0,0005)presentaban nivelesajustadosmásbajosdeirisinaqueloscontroles.Asimismo,lospacientesenDP pre-sentabannivelesmásaltosdelahormonaquelosdehemodiálisis(diferencia39,4ng/mL; p=0,002)oERCA(24,4ng/mL;p=0,036).Elanálisismultivariantetambiénidentificó bicar-bonatoplasmático(B=3,90pormM/L;p=0,001)yGFR(B=1,89pormL/min;p=0,003)como predictoresindependientesdelosnivelesdeirisina.Porelcontrario,noobservamos cor-relaciónajustadaentreirisinaymarcadoresdecomposicióncorporal.

Conclusiones:LosnivelesdeirisinasonbajosenpacientesconERC,ymuestrancorrelación consistenteconGFRybicarbonatoplasmático.LospacientesenDPpresentannivelesmás altosdeirisinaquelosmanejadosconservadoramenteoconhemodiálisis.Nuestroestudio confirmaunainconsistenciageneralenlosanálisisdeasociaciónentreirisinasérica,por unlado,ymarcadoresmetabólicosydecomposicióncorporal,porotro.

©2016SociedadEspa ˜noladeNefrolog´ıa.PublicadoporElsevierEspa ˜na,S.L.U.Esteesun art´ıculoOpenAccessbajolalicenciaCCBY-NC-ND(http://creativecommons.org/licenses/ by-nc-nd/4.0/).

Introduction

In recent years, muscle tissue has been emerging as an importantendocrineorgan,involvedinregulatingmany phys-iologicalprocesses.1–3_Irisin4_{isanadipomyokinethatappears}

to participate in regulating energy metabolism and other physiologicalprocesses. Itsbest-known and probably most importantfunctionistobreakdownbodyfat,thereby con-vertingitintobrownadiposetissue.4_{Themainconsequence}

isanincreaseinenergyexpenditure,causedbythe thermo-genicnatureofbrownfat.Thisphenomenoncontributesto theregulationofbodytemperatureasanimmediateeffectbut otherpotentialconsequencesincludeweightlossand there-foreadecreaseinplasmalevelsaswellasanimprovement

ininsulintolerance.Therefore,irisincouldhaveabeneficial effectagainstobesityandasanantidiabeticagent.5,6

Theeffectsofchronickidneydisease(CKD)onthe synthe-sisandsecretionofirisin,anditsinfluenceonthecomplex metabolicdisordersofCKDpatients,havenotbeen investi-gatedsufficiently.Somestudieshavereportedrelativelylow levelsofserumirisininpatientswithadvancedCKD,7–9_but

the comparative effectofdifferent treatment methodshas notbeenevaluatedadequately.Inaddition,thecorrelations observed between irisin and different clinical, biochemical andmetabolicmarkersinthesepatientsarecontroversial.

differentdemographic, clinical,biochemical, metabolicand bodycompositionmarkers.

Population

and

method

Following a cross-sectional design, we investigated serum irisin levels inasample taken consecutivelyfrom a set of advancedCKDpatientsmanagedconservatively(ACKD),with peritonealdialysis(PD)orwithhaemodialysis(HD)atour cen-tre.Weusedthefollowinginclusion/exclusioncriteria:

1. Agedbetween18and80.

2. CKD, with estimated glomerular filtration rate (GFR) <30mL/min.

3. Nosignificantclinicaleventsforatleast3monthspriorto inclusion.

4. Fullcapacitytounderstandtheobjectivesandconditions ofthestudy.

5. Writteninformedconsent.

Theprimarystudyvariablewasserumirisin.We investi-gatedthecorrelationofthisvariablewiththefollowing:

(a) Demographic data: age, gender, diabetes, comorbid-ity (Charlson Comorbidity Index), treatment method (ACKD, PD, HD), duration of dialysis, blood pressure (at the time of sampling) and treatment with statins, renin–angiotensin–aldosterone system (RAAS) antago-nistsanderythropoiesis-stimulatingfactors(ESAs). (b) Body composition: body weight, body mass index

(weight/height2_{), total body water, intra-/extracellular}

waterratio, overhydration,and body fatand lean body mass (corrected for body surface area [BSA]). The last 5 parameters were estimated using a multifrequency bioimpedanceanalysis(BIA)device(BCM,Fresenius, Hei-delberg,Germany).

(c) Laboratory: GFR(mean renal urea and creatinine clear-ances), proteinuria, haemoglobin, albumin, cholesterol, triglycerides,uricacid,bicarbonate,calcium,phosphate (automatedanalyser),parathyroidhormone(PTH)(IRMA, Nichols Inst., San Juan Capistrano, USA), interleukin 6 (ELISA, Abcam, Cambridge, UK) and C-reactive protein (CRP)(Immunoturbidimetry,Roche,Mannheim,Germany). (d) Hormonal:bloodglucose,C-peptide(Chemiluminescence, Immulite 2000, Siemens, Munich, Germany), insulin (RIA, CIS Bio Int. Cedex, France), HOMA calculator (by Matthews),10 _{growth hormone (RIA, Nichols), IGF1}

(Immunoassay, Nichols), leptin (ELISA, R&D Systems, Minneapolis, USA) and 25-OH vitamin D test (Chemi-luminescence,Centaur,Siemens,Erlangen,Germany). (e) Toselectthemethodforestimatingirisinlevels,we

con-ductedapreliminaryanalysisofthecorrelationbetween 2commerciallyavailableELISAkits(EK-067-29byPhoenix Pharmaceuticals,Burlingame,USAAnd AG-45A-0046by AdipogenInternational,SanDiego,USA).Onaverage,the EK-067-29kitprovidedvalues41%higherthanthe AG-45A-0046kit.TheAG-45A-0046kitwasselectedforthegreater internalconsistencyoftheestimates,whichweremadein duplicate.Bloodsampleswereplacedinserumseparator

tubesandcentrifugedat3000×gfor15minat4◦C.The serumwasstoredat−80◦Cuntilanalysis.

Thevariablesarepresentedasmeans(±standarddeviation [SD])exceptforabnormaldistributions(medianwith ampli-tude).UnivariatecomparisonsweregeneratedbyANOVAand Kruskal–Wallis and distribution tests 2_{. Univariate}

corre-lations were estimated using Spearman’s rank correlation coefficient. The adjustedcorrelationsbetween serum irisin and different variables were investigated using multiple regressionmodels(stepwise).Weusedarandomsampleof40 healthyindividuals,adjustedforage,genderandbodymass indexasareferencecontrolgroup.WeusedSPSS19.0software fordatamanagement.

Results

Thenumberofpatientsaskedtotakepartinthestudywas 105 patients. Seven patients refused to participate and 3 wereexcludedforhavingdevelopedclinicaleventsduringthe screeningphase.Thus,thestudygroupwascomposedof95 patients.Themaindemographicandbodycomposition vari-ablesareshowninTable1,whilethebiochemicalvariablesand those concerningtreatmentsareshowninTable2.Patients onHDhadalongeraveragetimeondialysis thanthoseon PD,aswellaslowervaluesforbodyweightandbodyfat,and ahighermeanlevelofoverhydrationthantheother2study groups(Table1).ThedifferencesobservedforGFRvalueswere asanticipated(Table2).Thehealthycontrolgroupconsisted of20menand20women,aged56.6±8.7andwithabodymass indexof27.6±4.6kg/m2_.

Table 3 shows the main hormonal variables analysed. Serum irisin levels were lower in any of the groups of patients than inthe healthycontrols. Inaddition, compar-isonsbetweenthegroupsofpatientsrevealedlowerlevelsof adipomyokineinpatientsonHDthaninthosetreatedwithPD (Fig.1).Amongpatients,irisinlevelsweresimilarin diabet-ics(108.0±32.4ng/mL)andnon-diabetics(113.3±45.4ng/mL) (p=0.77).Table4showsthemainunivariatecorrelationsfor irisin. A moderate direct association with body fat (BIA), diastolic blood pressure and GFR was observed. Themost significant association was observed between serum con-centration of irisin and bicarbonate, which was relatively consistentforthedifferentgroups,butitdidnotreach statisti-calsignificanceinPDpatients.Conversely,irisinlevelsshowed noassociationwithtreatmentwithstatins,RAASantagonists orESAs.Nordidweobserveconsistentassociationsbetween irisin levels and theserum concentrationofthe hormones measured(Table4).

Table1–Demographicvariablesandbodycomposition.

ACKD Peritonealdialysis HD p

N 30 35 30

Age(years) 57.0(12.4) 60.8(12.3) 56.1(13.3) 0.29

Gender(%male) 60.0 61.1 50.0 0.62

Diabetes(%) 10(33.3) 16(44.4) 12(40) 0.87

CharlsonComorbidityIndex 4.5(1.8) 5.3(2.0) 5.4(1.9) 0.11 Bodyweight(kg) 78.1(12.5) 75.5(11.5) 64.7(13.7) 0.0005a

Bodymassindex(kg/m2_{) 28.6(4.3) 28.2(3.9) 24.6(3.7) 0.001}a

SystolicBP(mmHg) 142.9(18.1) 144.1(19.3) 141.9(23.5) 0.91

DiastolicBP(mmHg) 82.6(11.4) 83.0(13.0) 79.1(13.5) 0.16

Timeondialysis(months) N/A 29.8(29.2) 60.7(52.3) 0.0005a

Totalbodywater(L)(BIA) 34.8(6.2) 34.3(6.9) 31.3(7.4) 0.10

Intra-/extracellularwater(BIA) 0.92(0.11) 0.97(0.14) 0.98(0.13) 0.12

Bodyfat(kg)b_{(BIA) 15.4(5.4) 15.0(4.7) 11.5(85.4) 0.008}a

Leanbodymass(kg)b_{(BIA) 13.6(5.9) 12.5(2.7) 11.7(3.2) 0.20}

Overhydration(L)(BIA) 0.70(1.15) 1.40(1.72) 1.83(2.37) 0.059

BIA:multifrequencybioimpedance;ACKD:advancedchronickidneydisease(estimatedGFR<30mL/min);HD:haemodialysis;N/A:not appli-cable;BP:bloodpressure.

ComparisonsusingANOVAandanalysis2_.

a _{SignificantdifferencebetweenHDandtheother2groups(Scheffé’smethod).}

b _{Correctedforbodysurfacearea.}

Table2–Laboratoryandtreatmentvariables.

ACKD Peritonealdialysis HD p

N 30 35 30

GFR(mL/m) 18.4(8.3)a _{5.8(4.3) 0.9(1.3) 0.0005}

Proteinuria(g/24h) 1.8(1.8)a _{1.1(1.4) 0.2(0.4) 0.0005}

Haemoglobin(g/dL) 11.3(1.7) 11.6(1.4) 11.5(1.6) 0.77

ESFs(%) 8(26.7)a _{22(61.1) 27(90.0) 0.0005}

Albumin,plasma(g/dL) 4.0(0.4)a _{3.7(0.4) 3.7(0.7) 0.007}

Cholesterol,plasma(mg/dL) 161.6(40.2) 155.9(44.4) 139.1(28.5)a _0.0005

HDLcholesterol(mg/dL) 42.3(10.9) 41.1(14.7) 43.3(11.1) 0.50

LDLcholesterol(mg/dL) 90.9(38.9) 93.8(39.8) 88.8(35.5) 0.79

Triglycerides,plasma(mg/dL) 146(60.445)a _{106(49.233) 108(33.242) 0.020}

Uricacid,plasma(mg/dL) 7.6(1.5)a _{5.9(1.1) 6.2(1.2) 0.0005}

Bicarbonate,plasma(mM/L) 23.2(4.6) 28.1(3.5)a _{24.5(2.6) 0.0005}

Calcium,plasma(mg/dL) 9.3(0.5) 9.0(0.6) 9.1(0.8) 0.42

Phosphate,plasma(mg/dL) 4.8(1.1) 4.8(0.9) 4.9(1.2) 0.94

PTH,plasma(pg/mL) 306.8(194.6) 309.8(220.8) 438.0(370.4) 0.10

Interleukin6,plasma(pg/mL) 1.1(0.5;31.0) 2.8(0.5;35.0) 3.0(1.0;66.0) 0.29

CRP,plasma(mg/dL) 0.33(0.10;11.30) 0.45(0.11;6.40) 0.50(0.10;12.40) 0.15

RAASantagonists(%) 15(50.0) 15(41.7) 8(26.7) 0.17

Statins(%) 25(83.3) 26(72.2) 19(63.3) 0.22

Valuesindicatemeans(SD)excepttriglycerides,CRPandinterleukin1,expressedasamedian(range),ANOVA.

ACKD:advancedchronickidneydisease;ESFs:erythropoiesis-stimulatingfactors;GFR:glomerularfiltrationrate;HD:haemodialysis;HDL: high-densitylipoprotein;LDL:low-densitylipoprotein;CRP:C-reactiveprotein;PTH:parathyroidhormone;RAAS:renin–angiotensin–aldosterone system.

a _{Significantdifferencecomparedwiththeother2groups(Scheffé’smethod).}

thosetreatedwithHD.Thesedifferencespersistedafter adjus-tingforbodyfatandleanbodymass(BIA).

Discussion

Irisin is a degradation peptide made of 112 amino acids, resultingfromfibronectintypeIIIdomain-containingprotein5 whenbrokendownbyproteases.4_{Itwasinitiallycategorised}

asamyokine,asitwasconsideredtobeessentially synthe-sisedinskeletal andcardiacmuscle.Butmorerecentwork

indicatesthatfat11,12_{and,tovaryingdegrees,otherperipheral}

tissues5 _{arealsoabletosecretethisfactor.Irisinmaintains}

aremarkablemolecularhomologyacrossdifferentspeciesof mammals.13

Themechanismsthatregulatethesynthesisand subse-quentsecretionofirisinhavenotbeenfullyelucidated.The initialbeliefthat exerciseand physicalconditioning stimu-latethesynthesisandsecretionofirisinhasbeenconfirmed bymost,thoughnotallsubsequentinvestigations.5_Bothcold

Table3–Hormonalvariables.

ACKD Peritonealdialysis HD Healthycontrols p

Bloodglucose(mg/dL) 98.4(37.8) 114.0(37.1) 101.3(37.3) 95.5(13.4) 0.11 C-peptide,plasma(ng/mL) 4.6(3.7)a _{9.5(6.7) 9.0(5.1) – 0.001}

Insulin,plasma(mcIU/mL) 20.5(19.8) 21.1(17.7) 13.4(6.6) 7.1(5.0)e _0.0005

HOMAindex 3.65(2.30) 5.21(2.51)b _{3.46(2.50) 1.67(1.40)}f _0.011

GH,plasma(ng/mL) 0.52(0.02;13.80) 0.83(0.11;13.54) 2.25(0.28;23.71)c _{0.47(0.05;9.00)}g _0.0005

IGF1,plasma(mg/dL) 209.6(93.2) 260.6(129.5) 199.3(89.7) 139.0(67.1)e _0.0005

Leptin,plasma(pg/mL) 141.2(105.3) 161.1(106.1) 74.9(80.0)c _26.6(12.7)f _0.004

25-OHvit.D,plasma(ng/mL) 19.2(12.8) 17.8(10.4) 11.5(6.2)c _{– 0.010}

Irisin,serum(ng/mL) 109.1(34.3) 132.2(45.3) 95.1(41.2)d _215.6(53.1)f _0.0005

ACKD:advancedchronickidneydisease;HD:haemodialysis;IGF1:insulin-likegrowthfactor.

Thevaluesindicatethemeans(SD)exceptGH(growthhormone),expressedasamedian(range),ANOVA. a _{p=0.01vs.PDandHD.}

b _{p<0.05PDvs.ACKDandHD.} c _{p<0.05HDvs.ACKDandPD.}

d _{p=0.006PDvs.HD(Scheffé’smethod),otherdifferencesamongstpatientsNS.} e _{p<0.01vs.ACKDandPD.}

f _{p<0.001vs.patients(anygroup).} g _{p=0.003vs.HD.}

inmenthaninwomen,andare elevatedinobesepatients, withadirectcorrelationwithbodyfatandbodymassindex,6

althoughtheseassociationshavenotbeenconfirmedbyother studies.5 _{It has been hypothesised that elevated levels of}

irisininobesepatientswouldbeacompensatorymechanism fortheprogressiveaccumulationofbodyfat6 _andalsothat

irisincouldplayaroleinimprovingcarbohydratemetabolism observedafterweightlossinobesepatientswithmetabolic syndrome.14_{Finally,somestudieshavereportedrelativelylow}

levelsofirisinintype2diabetics15,16_{andpatientswithfatty}

liverdisease.6

The effect of renal disease on serum irisin levels has not been adequately investigated. Irisin levels could maintainaninversecorrelationwithgradeofproteinuriain type2diabetes.17_Wenetal.7_{observedirisinlevelsupto60%}

lowerin38non-diabeticpatientswithCKDthanin19healthy controlsadjustedforageandgender.Theauthorsindicated thatsarcopeniaandindoxylsulfate-inducedinhibitionofthe expressionoffibronectintypeIIIdomain-containingprotein 5(FNDC5) inmusclecells could explainthis phenomenon. A cursoryexploratory analysisofselectedbiochemical val-ues showed only a slight direct correlation with plasma

Table4–Univariatecorrelationsbetweenserumirisinandmainvariables.

All ACKD Peritonealdialysis HD

Age 0.09(0.42) 0.35(0.08) −0.07(0.71) −0.01(0.95) Gender 0.15(0.18) 0.13(0.54) 0.11(0.57) 0.19(0.36) CharlsonComorbidityIndex 0.025(0.83) 0.36(0.08) −0.21(0.21) 0.14(0.51) Bodymassindex 0.14(0.21) 0.09(0.66) −0.03(0.90) 0.09(0.66) LeanbodymassbyBSA(BIA) −0.12(0.29) 0.001(0.99) −0.23(0.26) −0.25(0.23) BodyfatbyBSA(BIA) 0.21(0.07) 0.10(0.52) 0.10(0.64) 0.12(0.55) GFR 0.21(0.07) 0.40(0.05) 0.15(0.35) 0.37(0.07) Proteinuria 0.00(0.99) −0.04(0.84) 0.13(0.52) 0.31(0.12) Albumin(plasma) −0.25(0.030) −0.15(0.45) 0.15(0.46) 0.12(0.56) Bicarbonate(plasma) 0.44(0.0005) 0.46(0.02) 0.28(0.16) 0.72(0.001)

C-reactiveprotein(plasma) −0.15(0.20) −0.15(0.49) −0.30(0.13) −0.03(0.90) Interleukin6(plasma) −0.16(0.20) −0.18(0.42) −0.28(0.19) 0.08(0.72) Bloodglucose 0.01(0.94) −0.03(0.90) −0.29(0.14) 0.04(0.86) Insulin(plasma) 0.10(0.39) −0.39(0.04) 0.11(0.60) 0.21(0.30) HOMAindex −0.24(0.20) −0.37(0.05) −0.06(0.78) 0.11(0.59) C-peptide(plasma) −0.05(0.67) −0.38(0.05) −0.06(0.77) 0.11(0.61) GH(plasma) −0.12(0.28) −0.06(0.80) −0.14(0.49) 0.07(0.73) IGF1(plasma) −0.04(0.73) −0.47(0.019) −0.10(0.70) 0.08(0.68) Leptin(plasma) 0.18(0.23) 0.14(0.52) 0.16(0.43) 0.12(0.58)

BIA:bioimpedance;GFR:glomerularfiltrationrate;GH:growthhormone;IGF1:insulin-likegrowthfactor;BSA:bodysurfacearea.

ThevaluesindicateSpearman’srankcorrelationcoefficients(p).Othervariables,includingbloodpressure(systolic,diastolicandmean),uric acid,cholesterol,HDLcholesterol,LDLcholesterol,triglycerides,haemoglobin,calcium,phosphate,PTHandvitaminD,withnosignificant values.

Table5–Correlationsadjustedbetweenserumirisinandmainvariables.

Variable B 95%CI p

Diabetesmellitus −0.52 −11.69,10.73 0.93

Bodymassindex(kg/m2_{) 1.24 −1.02,3.50 0.28}

Bodymass(kg)(BIA) 1.24 −0.80,3.29 0.23

Leanbodymass(kg)(BIA) −0.27 −2.83,2.30 0.84

GFR(mL/min) 1.89 0.65,3.13 0.003

Proteinuria(g/24h) 1.72 −5.70,9.15 0.64

Cholesterol,plasma(mg/dL) 0.13 −0.10,0.37 0.29

HDLcholesterol,plasma(mg/dL) 0.66 −0.41,1.74 0.28

LDLcholesterol,plasma(mg/dL) 0.02 −0.32,0.36 0.90

Triglycerides,plasma(mg/dL) 0.00 −0.15,0.16 0.95

Bicarbonate,plasma(mM/L) 3.90 1.79,6.01 0.001

Interleukin6,plasma(pg/mL) −0.56 −1.41,0.30 0.20

C-reactiveprotein,plasma(mg/dL) −1.14 −5.37,3.10 0.59

HOMAindex 1.03 −1.80,3.87 0.47

Treatmenttype

PDvs.ACKD 24.40 1.67,47.14 0.036

PDvs.haemodialysis 39.39 15.37,63.40 0.002

Haemodialysisvs.ACKD −6.91 −17.58,3.76 0.20

BIA:bioimpedance;PD:peritonealdialysis;ACKD:chronickidneydiseasemanagedconservatively;GFR:glomerularfiltrationrate;CI:confidence interval.

Multipleregressionanalysis(stepwise).Allpatients.Variablesindependentlyassociatedwithserumirisinareinbold.Thebestmodelincludes significantvariables,adjustedforageandgender.Thecoefficientsoftheothervariables(notsignificant)representtheindividualcorrelation ofeachvariablewithirisinafteradjustingforthebestmodel.

levelsofHDLcholesterol.Inanother study8 _{of532patients}

withCKD(including169onHD)therewasadirect correla-tion(r-adjusted=0.277)betweenirisinlevelsandGFR.Inthis study,irisinlevelsweresimilarindiabeticsandnon-diabetics, higher in women than in men, and showed a significant adjustedassociationwithage(inverse),HDLcholesteroland LDLcholesterol,andHOMAindex(direct).Finally,Liuetal.9

studied365patientswithtype2diabetesanddifferentlevels ofGFR,andconfirmedtheexistenceofaninverseassociation betweenGFRandserumirisin.Thisstudyalsodetectedan inversecorrelationbetweenageandserumirisinthat, remark-ably,seemedtodisappearinpatientswithlowerlevelsofGFR.

400.0

ACKD PD

*

**

Haemodialysis

Serum irisin (ng/mL)

Controls 300.0

200.0

100.0

0.0

Fig.1–Serumirisinlevelsbytreatmentmethodforkidney disease.PD:peritonealdialysis;ACKD:advancedchronic kidneydisease.*p=0.006peritonealdialysisvs.

haemodialysis.**p<0.001vs.anyothergroup.

Finally,thisanalysisdidnotdetectanyassociationwith gen-der,but showedtrends,whichwere notveryconsistent,in correlationbetweenirisinaemiaandleanbodymassandbody fat.

Overall,attemptstocharacteriseirisinsecretionpatterns and their correlation with clinical and metabolic markers havebeenunsuccessful,andtheseinconsistenciesalsoaffect patientswithCKD.Thereareseveralpossibleexplanationsfor thediscrepanciesobserved.First,irisinisusuallymeasured byELISA,butsignificantvariationshavebeenobservedinthe estimatedvalueswhendifferentkitsonthemarketareused.18

Thelimitationsofknowledgeonthesynthesisandsecretionof irisingreatlyhinderthesearchtoexplainthesediscrepancies, andhassofarpreventednormalrangesofserumirisinlevels frombeingestablished.Inconsistenciesintheestimatesalso helptoexplainthemanycontradictionsregardingcorrelations betweenirisinaemia,ontheonehand,andvariablessuchas, forexample,bodyfatorleanbodymass,insulinresistanceand lipidpatternsontheotherhand.Moreover,irisinlevelshave beeninvestigatedinverydiversecontexts,whichadds confu-siontotheoverallanalysis.Forexample,diabetesorobesity couldsignificantlyalterthephysiologicalbehaviourofirisin. Overall,itisclearthatmoreresearchisneededtoclarifythe significanceofirisininhealthanddisease.

study.InthecaseofACKDpatients,thesignificantlyhigher levelofGFRallowedthisgrouptomaintainrawirisinlevels comparabletothoseobservedinpatientson PD.Moreover, serumbicarbonatelevelswerehigherinPDpatientsthanin anyother groupofpatients(Table2),and thisvariablewas aconsistentpredictorofirisinaemia.Toourknowledge,this associationhasnotbeenpreviouslyinvestigated,andwehave noclearexplanationforit,althoughmetabolicacidosishas been mentioned as a determinantbehind the secretion of otheradipokines.19

Thisstudyhassignificantlimitations,includingits cross-sectional, exploratory design, which does not allow for investigatingcausality.Thestatisticalpowerofthesampleis alsolimited,whichmightexplainsomeinconsistenciesinthe results,althoughthisproblemwasobservedinstudieswith largersamples.

Insummary,ourstudyconfirmsthatserumlevelsofirisin arerelativelylowinpatientswithCKD,andthereisadirect association between GFR and levels of this adipomyokine. Inaddition, our dataindicate thatirisinaemia maintains a consistentandindependentrelationshipwithplasma bicar-bonatelevelsinpatientswithCKD.PatientsonPDhavehigher levelsofirisinthanpatientswithACKDorthosetreatedwith HD. Finally, ourdata confirman essentialinconsistency in associations between serum irisin, on the one hand, and metabolicmarkersorbodycompositionontheother hand, asobservedinpreviousstudies.Furtherstudiesareneeded tocharacterisethesignificanceofabnormallevelsofirisinin CKD.

Funding

ThisstudywaspartiallyfundedbygrantsfromtheCarlosIII HealthInstituteHealthResearchFundPI10/00088,PI13/00322 (the European Union’s ERDF) and the “Xunta de Galicia” IN845B-2010/187,10CSA916014PR,CN2012/312.

Conflicts

of

interest

Theauthorshavenoconflictsofinteresttodeclare.

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