Apelin in reproductive physiology and pathology of different species : a critical review

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Hindawi

InternationalJournalofEndocrinologyVolum e2018,ArticleID9170480,12pageshttps://doi .org/10.1155/2018/9170480

ReviewA r t i c l e

Apelini n R e p r o d u c tiveP h y s i o logya n d P a t h o l o g y o f D i f f e r e n t Species:ACriticalReview

PatrycjaKurowska,

^{1A l i x}

Barbe,

^{2M a r t a}

Różycka,

1J u s t y n a

Chmielińska,

1J o e l l e

Dupont ,

²

and

AgnieszkaRak

¹

1DepartmentofPhysiologyandToxicologyofReproduction,InstituteofZoologyandBiomedicalResearch,JagiellonianUniversityinKrakow,3 0-387Krakow,Poland

2INRA,UnitéPhysiologiedelaReproductionetdesComportements,37-380Nouzilly,France Correspondences h o u l d b e a d d r e s s e d t o A g n i e s z k a R a k ; a g n i e s z k a . r a k

@ u j . e d u . p l Received4January2018;Accepted2April2018;Published6June2018Academi cEditor:LudwikK.Malendowicz

Copyright©2018PatrycjaKurowskaetal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLice nse,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.

Apelinh a s be e n i so l a t e df ro m t he b o v i n e st om a c h e x t r a c t sa sa n e n d o g e n o u s l i g a n d of t he pr e v i o us l y or ph a n re c e p t o rA PJ. Expressionoftheapelinergicsystem(apelinandAPJ)wasdescribedinmanyorganswherepleiotropiceﬀectslikeregulationo ffoodintake,bodyweight,orcardiovascularandimmunefunctionweredescribed.Recentstudieshaveshownthatapelin alsoplaysa n i m p o r t a n t r o l e i n t h e r e g u l a t i o n o f f e m a l e a n d m a l e r e p r o d u c t i o n . S o m e d a t a s h o w e d t h a t t h e g e n e a n d p r o t e i n o f apelin/APJa r e e x p r e s s e d i n t h e h y p o t h a l a m i c - p i t u i t a r y -

g o n a d ( H P G ) a x i s t i s s u e . T h u s , a p e l i n i s s y n t h e s i z e d l o c a l l y i n t h e hypothalamus,pituitary,ovaries,andtestisof manyspeciesandhasautocrineand/orparacrineeffects.Mostresearchindicatesthatapelinhasaninhibitoryeffectongonad otropin secretionandparticipatesin thedirec tregulationofsteroidogenesis, cellproliferation,andapoptosisingona ds.Thearticlesummarizesalsoresultsofaseriesofrecentstudiesontheeffectofapelinonreproduction pa th ol og y,l ik e pol yc yst i c ov a r i a n syndrom e , e ndom e t ri osi s, a nd ov a r i a n c ancer. Ma ny of t h e s e p a t ho l o gi e s ares t i l l i n c r i t i c a l ne e d o f t h e r a p e u t i c i nt e r ve nt i o n, a n d r e c e n t s t u d i e s h a v e f o u n d t ha t a p e l i n c a n be t a r g e t s i n r e p r o d u c t i v e path ologicalstates.

1. Introduction

Theh o r m o n a l i n t e r a c t i o n s o f t h e h y p o t h a l a m i c–

pituitary–

gonadal(HPG)axisareaccountableforaproperphysiologyofbot hfemaleandmalereproduction.Itisofimportancetohaveknowl edgeofnewregulators/hormonescontrollingreproduction.Iti swellknownthatadiposetissueisimpli-

catedinthesecretionofseveralhormonessuchasadiponec- tin,r e s i s t i n , l e p t i n , v i s f a tin,a n d a p e l i n c a l l e d a d i p o k i nes“adiposet i s s u e -

d e r i v edh o r m o n e s .”Th e re i s e v i d e n c e t h a t theincre asedproductionofadipokinesmighthaveastronglinktoins ulinresistance,metabolicsyndrome,andobesity[1].Ape linisaregulatorypeptide,identiﬁedasanendoge-

nousligandoftheapelinreceptornamedAPJ[2].Recently,thea p e l i n e r g i c ( a p e l i n a n d A P J ) s y s t e m w a s f o u n d i n t h e HPGaxisandapelinhasbeenextensivelydescribedasaben-

eﬁcialfactorcontrollingreproductionbothinfemalesandin

males.Theintentionofthispaperistoreviewcurrentknowl-

edgeconcerningtheexpressionofapelin/APJintissueoftheHPGaxisa ndphysiologicalaspectsofapelinonthephysiol-

ogyofbothfemaleandmalereproduction.Itwillalsodescribea pe l i n l i n kedwi t h r e p r o d u c t i o n d y s f u n c t i o n s l i k e inf ertility,polycysticovariansyndrome(PCOS),endometriosis,andovariancancer.Manyofthesepathologiesarestillincrit icalneedoftherapeuticintervention,andrecentstudieshavefoundthat apelincanbetargetsinpathologicalstates.Therefore,apeli nactivitycouldbeappliedinthefutureinthetreatmentof manydiseasesofthereproductivesystem.

2. Apelin:S tr u c t u re , E x p re s s i o n , a n d F un cti on

2.1. StructureofApelin.Apelinhasbeenisolatedfromthe bovines t o m a c h e x t r a c t s a s a n e n d o genousl i g a n d o f t hepreviouslyo r p h a n r e c e p t o r A P J ( p u t a t i v e r e c e p t o r p r o t e i n

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Activeforms

2 InternationalJournalofEndocrinology

Preproapelin(77aminoacids)

Endopeptidases

Proapelin(55aminoacids)

Pro Gly

Pro

GlyAsnArg Ser Gl

y ArgPro Gln ValLeu

N ACE2

Trp Gln GlyGly ArgArg

N

LysPheArgArgGlnArg

Pro

Pr o

ArgLeuSerHisLys

Gly

ProMetProPhe C

Apelin-36

Apelin-17

GlnArg N

Pr o

Pro

ArgLeuSerHisLys

Gly

ProMetProPhe C

Pyr-apelin-13

Apelin-13 Figure1:Aminoacidsequenceofnativeapelinandapelinisoformstructure.ACE2:angiotensinI-convertingenzyme2.

relatedtotheangiotensinreceptorATl),whichisaGprotein- coupledr e c e p t o r [ 2 ] . H um an a p e l i n i se nc od ed b y the APLNgenelocatedonchromosomeXq25-

26[2].Thispeptidehasa77-amino-

acidpreproapelinprecursorandexistsinmultiplemolecular formswithdiﬀerentbiologicalactivities.Nativepreproapel in,asaresultofenzymatichydrolysis,i s t r a n s f o r m e d i n t o a c t i v e f o r m s : a p e l i n - 3 6 (preproapelin-42–7),-

17(preproapelin 61 –77),and-13(preproapelin-65–

77)a n d p y r o g l u t a m a t e - a p elin-13( p y r - apelin-13) (Figure1)[2,3].Shorterformsofapelin(apelin-

13)showmuchhigherbiologicalpotencythanlongerformsdo(ap elin-36);thus,apelin-13hasbeenusedformanydiﬀer-

enti nvi t ro a nd i n v i v o expe rimentst o i nv es t i g a tesever alphysiologicalfunctionsofapelin[2].Additionally,pyr- apelin-13a n d a p e l i n -

1 7 s h o w a c o n s e r v edb i n d i n g t o t h e angiotensin- convertingenzyme2(ACE2)catalyticsiteandhumanA C E 2 c a n c l e a v e p y r - a p e l i n - 1 3 a n d a p e l i n - 1 7 [ 4 ] . Pyr- apelin-

13isamajorisoforminhumantissues,forexample,incardia ctissuefrompatientswithcoronaryarterydisease[5],an dtheplasmarangesfrom7.7to

23.3pg/ml[6].Moreover, pyr-apelin-13, apel i n- 13,and apelin-

36havesimilareﬃcacyandpotencyincardiovasculartissuesofhuma ns[5].

TheN-

terminalregionofapelinisrichinhydrophobicaminoaci ds,indicatingthattheserepresentsecretorysignalsequences, whiletheC-

terminalregionhasasequenceof23aminoacids.Itisconservedandcriti calforbiologicalactivity[2,7].Bovine,human,rat,andmousepre proapelinprecur-sorshave76–

95%homology.Theendogenousformofthese

proteinsi s a d i m e r l i n k e d b y a d i s u l ﬁdeb o nd[6 ] . M a t u r e f ormsofapelindonothavecysteineresidues,andtheyareprob ablyo n l y m o n o m e r i c proteins[ 7 ] . Ino r d e r t o binda pelint o i t s r e c e p tor,i t i s n e c e s s aryt o h a v e a 1 3 - a m i n o -

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InternationalJournalofEndocrinology 3 acidCterminus,whichisobservedintheinapelin-

36andpyr-apelin-13[8].

2.2. Expressionof A p e l i n . A p e l i n e xp re ss i o n ( m R N Aa nd protein)wasdetectedinvarioustissuesandorganss uchasstomach,b r a i n , h e a r t , l u n g , u t e r u s , a n d o v a r y ( F i g u r e 2 ) [8–

10].Additionally,literaturedataalsodocumentedapelinlocali zationintheendotheliaofsmallarteriesinmanyorganssuchaslung,s pleen,liver,pancreas,andadiposetissuesinrats[3,11].Ex pressionofapelinincreasesduringadipocytediﬀerentiation,a nditsproductionisregulatedbyseveralfac-

torss u c h g r o wthh o r m o n e ( G H ) o r t u m o r n e c r o s i s f a c t o r (TNF-

α)andinsulinwhichincreasedapelinproductionbyadip ocytes[12].

2.3. FunctionofApelin.Theapelinsignalingpathwayplaysaroleint hecentralandperipheralregulationofthecardiovas-

cularsystem,suchasbloodpressureandbloodﬂow,inwaterandfood intake,energymetabolism,andpossiblyimmunefunct ion(Figure2) [10, 13]. Apelincausesendothelium-

dependentvasorelaxationbytriggeringthereleaseofnitri coxide(NO),anditincreasesmyocardialcontractility[3,14].M oreover,i t i s r e p o r t e d t h a t a p e l i n i s apot ent a n g i o g e n i c factorinducingendothelialcellproliferation,migr ation,andt h e d e v e l o p m e n t o f b l o o d v e s s e l s i n a n i n v i v os t u d y [ 1 4 , 1 5 ] . A P J m R N A e x p r e s s i o n w a s d e t e c t e d i n a r easo f theb r a i n c r i t i c a l f o r t h e c o n t r o l o f ﬂuidh o m e o s t a sis,s o apelinm a y p l a y a r o l e i n t h e r e g u l a t i o n o f w a t e r b a l a n c e [7].Levelsofapelinan dAPJmRNAincreaseinwhiteadi-

posetissueandplasmawithobesitythanincontrolsubjects.How ever,obesityhastobeassociatedwithhyperinsulinemia[ 1 2 , 1 6 ] , s o i t m a y b e t h e m a i n c a u s e f o r t h e r i s e i n t h e expr essionofapelin.Ontheotherhand,apelininhibits

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↓Muscletone

↑Contractility

↑ACTHrealase

↑Vasopresinrealase Heart

↑Corticosteroidrealase

↓Polycistickidneydiseases

↓Renalischemia

Brain

Kidney

↑Preeclampsia

development Apelinexpressionandfunction

↑Fas-inducedapoptosis

↑Formationofhepaticﬁbrosis

↓Liverregeneration Placenta

Pancreas

Adiposetissue

Liver

Bloodvessels

↓Insulinrealase

↑Glucoseuptake ↑Angiogenesis

↑Cholecystokinin

release ↑Insulinsensitivity ↑Bloodveselenlargement

Figure2:Apelinexpressionandfunctionintheorganism.ACTH:adrenocorticotropichormone;PRL:prolactin;LH:luteinizinghormone;FSH:follicle- stimulatinghormone.

insulinrelease[17].DataofHeinonenetal.

[16]showedapositivec o r r e l a t i o n b e t w e e n t h e l e v e l o f a p e l i n i n p l a s maandthebodymassindex(BMI).Further more,researchstudiesb a s e d o n y o ungf e m a l e s w i t h e a t i n g d i s o r d e r s showedt h e h i g h e stl e v e l o f a p e l i n i n t h e g r o u p o f o b e s e patients[17]. Apelinserumlevelsarerela tedtothenutri-

tionalstatusandparallelinsulinplasmalevelsinmicean dhumans[12,18].Furthermore,apelinplasmaconcentrationsar eincreasedinobese[16]andtype2diabeticsubjects[19]asw e l l a s i n h y p erinsulinemico b e s e m i c e [ 1 2 ] . I n m i c e , apel ininhibitedglucose-stimulatedinsulinsecretioninpan- creaticislets[20],suggestingalinkwithglucosehomeosta- sis.Recently,a14-

dayapelintreatmentinmicewasshowntor e g u l atea d ipos itya n d toi n c r e a s e uncouplingproteinexpression[21],su ggestingaroleofapelininenergymetabolism.L i t e r a t u r e d atadocumentedalsothata p e l i n hasa n t i -

i n flammatoryeffectso n t her e l e a seo f i n flamma- torymediators[22].Italsoinhibitsreleaseofreactiveoxyg ens p e c i e s ( R O S ) i n a d i p o c y t e s a n d p r o m o t e s a n exp ressionofantioxidativeenzymes[23].Additionally,apel inmayplayanimportantroleinlymphatictumorprogressi on,b e c a u s e i t s o v e r e x p r e s s i o n w a s p r o v e d i n r a t m alignantcells[24].

3. CharacteristicofApelinReceptor:APJ

3.1. ApelinReceptor(APJ).Thisreceptorisencodedbyth eA P L N R ( a l s o k n o w n a s A G T R L 1 , A P J R , A P J , a n d FLJ90771)gene[25].APJisaclassGprotein-

coupledrecep-

tor(GPCR)identiﬁedin1993,anditsstructureshowshigh

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homology(40–

50%inthetransmembraneregion)withangiotensinIIrece ptortypeAT1,butangiotensinIIisunablet o a t t acht o t h i s r e c e p t o r [ 2 6 ] . T h e e x a c t l o c a t i o n ofthisgenew asalsodeterminedformiceonchromosome2E1andforra tsonchromosome3q24[27].Boththestruc-

tureandfunctioningofthehumangenepromoterAPJhave notbeenfullyunderstood[7].APJhasahigh(90%)similarityb etweenhuman,rat,andmouse[28,29]andabout5 0 % b e t w e e n m a n a n d r o y a l m a c a q u e , c o w , f r o g , andze braﬁshDaniorerio[7].

APJ,d u e t o t h e d ifferenta ffinityf o r v a r i o u s f o r m s o f apelinandcointeractionwithdifferentG(Gα,Gβ,and Gγ)

proteins,i nt e r actsw i t h a c t i v ationo f m a n y s i g n a l i n g p a t h - ways[2]

(Figure3), therebycausingvariouseﬀectsin thebody.Inearl yexperiments,apelin-13hasbeenobservedto

inhibitforskolin’sstimulatingeﬀecton3′,5′-

cyclicadenosinemonophosphate(cAMP)bybindingAPJtothe Gi/oprotein[2].ThesestudieshavebeenconfirmedbyHabataan dcoau-thors[6],whoprovedthatbothapelin-13andapelin- 36arenotc a p a bleo f g e n e r atingc a l c i u m ( C a²⁺)m o b i l i z a t i o n i n Chinesehamsterovary(CHO)cells.Thediff erenteffectsofbothoftheseapelin isoformsare observ edin neuronsandinthehumanembryonickidneycellline(H EK-

293),wherebothisoformsincreaseCa^2+levels[30].APJcanalsoa ctviaGαi1andGαi2proteinstoinhibitadenylatecyclasei nrats[ 3 1 ] . I n t u r n , t heC H O a n d t h e H E K-

293c e l l l i n e s b i n d apelinwiththeAPJreceptorviaGαi2and thenconsequentlyactivatetheextracellularsignal-

regulatedkinase(ERK1/2)pathway[ 3 2 ] . A d d itionally,a p e l i n b i n d i n g A P J a c t i v a t e d phosphorylationo f p h o s phoinositide3 - k i n a s e ( P I 3 K ) a n d

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Extracellularspace Apelin

Cytoplasm

PKC

Ga/Gf3/Gy

PI3K AMPK

AC

ATP

cAMP

PKA

RAS Raf-1

MEK

Akt

mTOR

JNK

SP-1

ERK

c-JUN

p70S6K eNOS

c-FOS Nuclei

Figure3:PathwaysofapelinsignalingafterconnectionwithAPJ.PKC:proteinkinaseC:MEK-ERKactivatorkinase;ERK:extracellularsignal- regulatedkinase;JNK:c-JunN-terminalkinases;SP-1:speciﬁcityprotein1;PI3K:phosphoinositide3-

kinase;Akt:proteinkinaseB;mTOR:mammaliantargetofrapamycinkinase;p70S6K:ribosomalS6kinase;eNOS:endothelialNOS;AMPK:5′-AMP- activatedkinase:AC:adenylcyclase;PKA:proteinkinaseA;ATP:adenosinetriphosphate;cAMP:cyclicadenosinemonophosphate;c- JUN:transcriptionfactorc-JUN;c-FOS:transcriptionfactorc-FOS.

proteinkinaseB(Akt),whichplayanimportantroleincellproli ferationo r a p o p t o sis.A p e l i n p h o s phorylatesa l s o t h e ribosomalS6kinase(p70S6K)inhumanumbilicalveincells(HUVE C),therebypromotingtheproliferationofthesecells[31].APJsignal ingchangesthelevelofROS,sothatapelinwithAPJcanstimul atecatalaseproductionandinhibittheproductiono f h y d r o g e n p e r o x i d e , t h u s p r o t e c tinga g a i n stcardiac hypertrophy[33]. Inaddition,apelin,byreducingROSp r o d u c tiona n d a c t i v a t i n g t h e a c t i n k i n a s e , p r o t e c t s mouseneuronsfromcelldeath[34].Oneformofapelin,ape-lin- 13,throughkinase5′AMP-

activatedkinase(AMPK)phosphorylation,lowerstheprocess ofmouseneuronalapo-

ptosisafterstroke.StudiesonAPJKOknockoutmicehaves hownthatapelin-13bybindingwithAPJnegativelyregu- latesAMPK,whichlowersthelipolysisprocess,thehydro-

lyticd e g r a d a t i o n o f t r i g l y c e r i d e i n a d i p o s e t i s s u e t o f a t t y acidsandglycerol[35,36].

Genea n d p r o t e i n e x p r e s s i o n o f A P J h a s b e e n d e m o n-stratedinseveraltissuesincludingthebrain,ovary,kidney,

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pancreas,breast,andheart.Moreover,inhumans,expressionofAPJ washighinthehumanbrainandspleenandslightlylowerinth eovaryandplacenta.Incontrast,inthecaseofratandmou se,thehighestAPJexpressionwasobservedintheheartcells [7].APJexpressionsareregulatedbymanyfactors,forexample,e strogens,insulin,cAMP,andCCAAT-

(C/EBP-)b i n d i n g p r o t e i n , a n d s t r o n g b r a i n s t r e s s s i g n i ﬁ-

cantlystimulatesAPJsecretionbyadiposetissuecells[37].

3.2.ELABELA/ToddlerasaLigandofAPJ.Therecentdiscov- eryo f a n e w e n d o g e n o u s p e p t i d e l i g a n d f o r A P J , c u r rentlyknownasbothToddler[38] andELABELA[39], follo wedscreenst o d i s c o v e r s i g n a l s r e g u l a t i n g e a r l y d e v e l opment.Althoughch aracterized in zebraﬁsh,ah ig hdegr eeofconser-

vationo f t h e E L A B E L A / T o d d lerg e n e i n v e r t e b rat es p e c i e s includinghumansimplieslikelihoodofsimilarimp ortanceinh u m a n d e v e l opment,b u t t h i s h a s y e t t o b e s h o w n . L i keapelin,t h i s p e p t i d e e x i s t s i n m u l t i p l e e n d o genousi s o f o r m s [ 4 0 ] . E L A B E L A / T o d d lers i g n a l i n g i s m o t o g e n i c , a n d i t s

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absenceoroverproductionreducesthemovementofmesen- dodermalz e b r a fishc e l l s d u r ingg a s t r u l a t i o n , i n h i b i t i n g properdevelopment[38].Moreover,inELABELA/To ddlerKOk n o c k o u t zebrafish,thecellsoft h e endodermha veimpairedd ifferentiationp o t e n t i ala n d e m b r y o s e x h i b i t stuntedo r c o m pletelya b s e n t h e a r t d e v e l op ment.T h i s mirrorsthephenotypeobservedintargeteddeleti onofAPJ(APJKO)embryos[39].ApelinKOembryos,o ntheotherhand,donothavethisphenotype.Systemicad ministrationofELABELA/ToddlerinELABELA/Toddler KOzebrafishrescuestheotherwiseaberrantphenotype[38].

Receptora c t i v a t i o n s t u d i e s r e v e a l edt h a t t h e z e b r a ﬁshToddler-

21peptideactsbybindingAPJandinducingrecep-

torinternalization[38].Moreover,theexpressionprofilesofELA BELA/Toddlerandapelindifferduringzebrafishdevel-

opment[38].Inparticular, duringgastrulationELABE LA/Toddlerishighlyexpressed,whereasapelinexpressi onremainsl ow . F oll o wingt hispe ri od ,h ow ev e r, E L A B E L A / Toddlerexpressiondropssharplyandapelinlevelsbegi ntorisesteadily.AlltheseﬁndingsindicatethatELABELA/Tod- dlerisadevelopmentallycriticalAPJligandwhosesignaling

behaviordiﬀerssigniﬁcantlyfromthatofapelin.Theexacti ntracellularsignalingmechanism(s)ofELABELA/Toddlerre mainsunknown.ELABELA/ToddlerbyactivatedGpro- tein-andβ-arrestin-

dependentpathwaysactsinthehumanheart.Moreover, a pel i n acti ngon cardiac contracti li ty and vasodilatation ininvitroexperimentsinratheart[41].Anotherteamdiscover edthatELABELA/Toddlerincreasescardiaccontractilityi n an E R K 1 / 2 -dependentmannerinadultrathearts[42].

4. PhysiologyandPathologyofApelinintheHy pothalamus–PituitaryAxis

4.1. ExpressionandEﬀectofApelinontheHypothalamus–

Pituitary–

Axis.T h e c e n t r a l n e r v o u s s y s t e m , e s p e c i allyi n th eh y p o t h a l a m u s a n d p i t u i t a r y , c o n t a i n s p r i m a ry s i t e s o f apelinaction.Theapelinergicneuronswereﬁrstlyob servedinthecentralnervoussystemofratsusingtheimmunohisto- chemistrymethod[43],indicatingthetopographicaldistri- butionofapelinergicneuronssuggestingmultiplerolesf orapelininthecontrolofbehaviors,pituitaryhormonerelease,andc ircadianrhythms.ApelinandAPJgeneexpressionwasobservedint hehypothalamicsupraopticnucleusandinthemagnocellular andparvocellularpartsoftheparaventricularnucleus(PVN)inra ts[43].Inthehypothalamus,apelinergicnerveﬁbersweredetecte dintheperiventricular,suprachias-

matic,v e n t r o m e d i a l , d o r s o m e d i a l,n u c l e i c , a n d r e t r o c hias-

matica r e a s . T h e i m m u noﬂuorescencem e t h o d s h o w s t h a t apelin-

immunoreactiveneuronalcellbodieswerelocalizedthroug houtt h e r o s t r o c a u d a l e x t e n t o f t h e m o u s e a c t i v i t y - regulatedcytoskeleton-

associatedprotein(Arc).Moreover,apelinlocalizedwith proopiomelanocortin(POMC)andweaklywi t h n eu ro pe p t i de Y (NP Y). B y i m m un ohi st o -

chemistryusinginsituhybridization,APJispresentinArcPOM Cneurons.Apelin/APJmRNAwasalsodetectedint h e a n t e r i o r a n d p o s t e r i orp i t uitarya n d i n i n t e r m e d i a t e lo

besoftheratpituitary[29]. Moreover,Reauxetal.

[43]usingi m m u noﬂuorescences t a i n i n g d i s c o v e r e d t h a t a p e l i n

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isc o e x p r e s s e d i n t h e a n t e r i o r p i t u i t a r y w i t h c o r t i c o t r o p h s ands o m a t o t r o p hsu s i n g r a t s a s m o d e l .

Thehypothalamiclocalizationofapelinﬁbersandrecep- torss u g gestsa n i n v o l v e m e n t o f a p e l i n i n t h e c o n t r o l o f hormonerelease[44].Inanexvivoperifusionsyst emofratanteriorp i t u i t a r i e s , a p e l i n -

1 7 s i g n i ﬁcantlyi n c r e a s e d b a s a l adrenocorticotropic hormone(ACTH)release[45]. More-

over,r e s u l t s o f t h e p e r i f usiont e c h n i q u e f o r h y p o th alamicexplantsh a v e b e e n d e m o n s t r a t e d t h a t a p e l i n - 1 7 i n c r e a s e d

α-melanocyte-stimulatingh o r m one( α- MSH)r e l e a s e , s u g -

gestingthatapelinreleasedsomatodendriticallyoraxonally fromP O MCn e u r o n s m a y s t i m u l ateα -

MSHr e l e a s e i n a n autocrinemanner[46].Inthehypothalam us,apelinmaybeinvolvedalsoinfoodintake;inrats,apelin- 13intracerebro-

ventricular(icv)injectionincreasedfoodintakebyinhibitedcoca ine-a n d a m p h e tamine-

regulatedt r a n s c r i p t ( C A R T ) mRNAexpressionandser otoninsecretionandbyincreasedorexinmRNAexpressioninth ehypothalamus[47].Chronicicvinfusionofapelininthemouse hypothalamusincreasedalsot h e e x p r essiono f p r o i nﬂam matoryf a c t o rs,a s s ociatedwithhigherlevelsofinterleukin- 1betainplasma[48].

Apelin-13inthePVNincreasedc-

Fosexpression[49]andsecretionofbothplasmaACTHand corticosterone(CORT)

[50,51].Moreover,icvadministrationofpyr-apelin-

13wasusedt o i n d i c a t e w h e r e t h e p o s t r a n s l a t i o n m o d iﬁcationoccursa n d s h o w e d a p e l i n -

1 3 d e c r e a s i n g p r o l a c t i n ( P R L ) , luteinizinghormo ne(LH),andfollicle-

stimulatinghormone(FSH)levels[50].Aninvitrostudydocume ntedthatapelin-13increasedthereleaseofcorticotropin- releasinghormone(CRH)andvasopressin(AVP)fromhypot halamicexplants,withnoeﬀectonNPYrelease[44,50],suggestin gthatapelinmayplayanimportantroleinthehypothalamicregulation ofwaterintakeandendocrine axes.Newsonetal.

[52]using

APJKOmicehadestablishedaroleforAPJintheintegrationofneuroen docrineresponsestoacutestressandhaddemon-

strateda g e n d e r -

s p e c iﬁcf u n c t i ono f a p e l i n i n p e r i p h eralimmun ea c t i v a t i o n o f t h e h y p o thalamus–pituitary–

adrenalaxis[52].Moreover,Tobinetal.

[53]documentedthatapelin-

13a d m i ni s t r a t i o n o nt o t hehypot hal am i c su pr a opti cnucleusi n c r e asedt h e fi ringr a t e s o f v a s o p r e s s i n c e l l s b u t hadnoeffectonthefiringrateofoxytocinneurons,sug gest-ingalocalautocrinefeedbackactionofapelinonmagnocel- lularvasopressinneurons.

Anicvadministrationofapelin-13producedadose- andtime-

relateda n t i n o c i c e p t i v e effect;t h i s effectw a s s i g n i fi-

cantlya n t a g o n i z e d b y t h e A P J r e c e p t o r a n t a g o n i st a p e l i n - 13,i n d i c a t i n g a n A P J r e c e p t o r -

m e d i a tedm e c h a n i sm[ 54].Apelin-

13isalsoinvolvedintheautophagysuppressionofneural cells;thus,itattenuatestraumaticbraininjury[55].Inl

act at i ng r at s, ap el i n m o d ul a t e s t h ea ct i v i t yo f oxyt o ci n neurons;t h e a c t i v i t y i s i n h i b i t e d b y a d i r e c t a c t i o n o f t h e apelinonitsreceptor,expressedbytheseneurons[56].

5. PhysiologyandPathologyofApelininthe Ovary

5.1. ExpressionandFunctionofApelin/APJintheOvary.Theapeliner gicsystemwasfoundintheovaryofmanyspecieslikebovin e,rhesusmonkey,porcine,andhuman(Table1)

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[13,57–61].Shimizuetal.

[62]demonstratedthatinbovinefolliclestheexpressi onof ap elinmRNAwas notfound ingranulosacells(Gc), while theAPJgenewasincreased inGcofestrogen-

inactivedominantfollicles.Additionally,theexpressionofapelin mRNAincreasedinthecacells(Tc)ofestrogen-

inactivedominantfolliclesbutAPJexpression inTcincre asedwithfolliclegrowth[62].Invitroexperimentsofbovineov ariancellsshowedthatseveralfactorsregulatedapelin/APJexpres sion;forexample,progesterone(P4)andFSHstimulatedth eexpressionofAPJmRNAintheculturedGc,whileLHinducedth eexpressionofapelinandAPJinculturedTc[62]. Inthenextst udy,Schilﬀarthe t a l .

[ 1 3 ] observedt h a t i n t h e b o v i n e o v a r y , t h e e x p r e s s i o n l e v e l o f apelind u r i n g t h e o e s t r o u s c y c lew a s s i g n i ﬁcantlyh i g h e r comparedt o t heo n e d u r i n g p r e g n a n c y . M o r e o v e r , a p e l i n mRNAwashighduringthecyclean ddecreasedaftercorpusluteum(CL)regression,whileinovarianfoll iclestheexpres-

sionofapelin/APJwassigniﬁcantlyupregulatedinfolliclesw ithanestradiol (E2)concentra tionofmorethan5ng/ml,su ggestingt h a t t hea p e l i n / A P J s y s t e m i s i n v o l v edi n t h e mechanismregulatingangiogenesisduringfollicl ematura-

tiona s w e l l a s d u r ingC L f o r m a t i o n a n d f u n c t i o n i n t h e bovineovary[13].Ourlastdatademonstratedthatth eexpressionofbothapelinandAPJinbovinegranulosaando ocytessigniﬁcantlyincreasedwithovarianfolliclesizewhereas itwassimilarinthecainterstitialcells[59].Further-

more,invitroexperimentsshowedthatinsulin-

likefactorI(IGF1)i n c r e a s e d a p e l i n e x p r e s s i o n , w h e r e a s i t d e c r e a s e d themRNAexpressionofAPJ[59].Inthe porcineovary,ape-

linconcentrationinthefollicularﬂuidandexpressionofbothapelinan dAPJincreasedwithfolliculargrowth;thegreatestvalueswerefoun dinlargefollicles[61].Immunohistochem-

istryr e v e a l e d t h e p o s i t i v e s t a i n i n g f o r a p e l i n a n d A P J i n membranesofporcineGc,thaninTc;additionally,ast rongexpressionofapelininoocytesandAPJinthezonapellucidawas observed[61]. SimilarasinbovineCL,ourdataalsodo cumentedt h a t i n p o r c i n e C L , a p e l i n / A P J i s d e p e n d e n t ontheCLgrowthanddevelopmentphase;apelinexpressio nwassimilarinearlyand middleCLandthendecreased inregressingC L [ 6 3 ] . M o r e o v e r , l o c a l izationo f a p e l i n w a s foundinthecytoplasmoflutealcellsinallstagesofCLdevel- opment,whilethestrongestAPJstainingwasfoundinmid- dlecells[63].Rocheetal.

[58]demonstratedapelinandAPJatthegeneandproteinlevelsalsoin humanovariancellsandgranulosac e l l l i n e s ( K G N ) . T h e s e a u t h o r s d e m o n s t r a t e d higherimmunolocali zationofAPJinhumanGc,cumulus,andoocyteascompa redtoTc.Thehighexpressionisalsodemonstratedinprimar y,medium,andmaturefollicles;apelin/APJisexpressedinthec ytoplasmandnucleiofGc[58].

Thepresenceofapelin/APJ(Table1)invariousovariancells anditschangeduringovarianfolliclesandCLdevelop-

ments u g g e s tsa p o t e n t i a l r o l e o f a p e l i n i n t hec o n t r o l o f severalaspectsofovariancellfunctionsuchasfolliculogen- esis,steroidhormonesecretion,proliferation, orapopto sis.Inv i t r o s t u d i e s i n d i c a t e t hata p e l i n m a y d i r e c t l y r e g u l a t e steroidogenesisinovariancells.Apelinbyact ivationoftheAPJreceptorcausesastatisticallysigniﬁcantinc

reaseinP4andE2secretionand3β- hydroxysteroiddehydrogenase/

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Δ^5–

4i s o m e rase(3βHSD)proteinlevelbothinprimarycellcul turesa n d i n I G F I -

inducedh u m a n a n d p o r cineo v a r i a n cells[ 5 8 , 5 9 , 6 1 ] . A s a m o l e c u larm e c h a n i smo f a p e l i n action o n t h e s t e r o i d s y n t h e s i s p r o c e s s a u t h o r s c o n s i d er edactivationoftheserine–threoninekinase,mitogenacti- vatedp r o t e i n k i n a s e ( M A P K 3 ) a n d A M P K k i n a s e p a t h -

ways[58]. Similarresultshavebeenobtainedini n v i t r o studi eso f bovineovariancells,whichshowthatapelinstimulatesP 4 p r o d u c t i o n a n d p r o l i f e r a t i o n o f t h e s e c el lsbyactivatingAktkinase[59].Inaddition,theauthors demonstrateda ni nh i bi t o ry eﬀecto f apelino n t he inv i t r o m a t u r a t i o n o f b o v i n e o o c y t e s a n d t h e r e l e a s e o f P4bycumulusc el l s , indicat ing t hedirectrol e ofthisadi- pokineinthematurationofoocytes.Shuangetal.

[64]showedthatapelinstimulatesproliferationandinhib itstheprocessofa p o p t o s i s i n r a t G c b y a c t i v a t i n g t h e A k t kinasep a t h w a y . I n a d dition,S h i m i z u e t a l .

[ 6 2 ] s u g g e s t involvemento f a p e l i n i n f o l l i c u l a r a t r e s i a i n d u c e d b y G c apoptosisd u r ingb o v i n e f o l l i c u l a r b e c a u s e t h e y h a v e demonstratedhighexp ressionoftheAPJreceptorinatreticbovinefollicles.

ApelinisalsoaregulatoroftheCLluteolysisprocess[57].InthemiddleC L(sensitivetoPGF2α),atransientincreaseinbloodﬂowassoci a t ed w i t h t hest i m ul at i on o fe nd ot he l i um nitricoxide(

eNOS)wasobserved,whichistheﬁrstsignalthatinitiatesl uteolysis[65].ApelinactivatestheeNOSpath-

waythroughstimulationofnitricoxideproduction,resultinginthee xpansionofbloodvessels[3].Anothermechanismtoexplainthelut eolyticeﬀectofapelinisCLapoptosis.Apelinisoneofthefactor sthatslowdowntheprocessofovarianapoptosis.Ontheot herhand,apelininducestheexpressionoftheantiapoptoticB- celllymphoma2(Bcl-

2)protein,whiledecreasingproapop toticBaxproduc tionfur t herbl oc ks t hereleaseofcytochromecandactivatesthecaspase- 3apoptosisexecutiveenzymeresultinginapoptosissuppr essioninosteoblastcells[66].

5.2. ApelinandOvarianPathology.PCOSisthemostco mmoncauseofinfertilityduetolackofovulation.Thiss yndromew a s ﬁ rstd e s c r i b e d b y S teina n d L e v e n t h a l i n 1935.Theydescribedwomen withexcessive h air,obesity,andovariescoveredwithcysts.Itisthemainendocrino pathyofreproductive-agewomen.PCOSalsobindstoinsulinresis- tance,w h i c h r e s ultsi n h y p e r i n s ulinism,w h i c h aﬀect st heproductionofandrogensbytheovariesandadrenalglan ds.Therearealsochangesinthelipidandcarbohydratee con-omy,whichinturnleadstodiabetestype2andcardiovascu- larandbiliarytractdiseases. Increasedriskofendomet rialcanceranddiabeticpregnancies,preeclampsiaduring preg-

nancy,o r v e n o ust hrombosisarealsos y m p t o m s oft h i s condition[67].GeneticfactorsresponsibleforPCOSpatho- genesisa r e m u t a tionsi n t h e g e n e s r e s ponsiblef o r s t e r o i d hormonesynthesis,regulation ofgonadotropi ns, andthoseassociatedwiththepathwayforweightregulation.

Environ-

mentalfactorscanalsobeclassiﬁedasobesity,occurringin50

%ofpatients,resultingindisordersofimplantation,cycle,ovulation, andmiscarriage[68].TheresultsofRoche’setal.

[58]datacomparedtheexpressionofapelinandAPJinGcfro mobeseornonobesepatientswithorwithoutPCOS.They

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Species Apelin/APJex Granulosa Theca

pression

Oocyte CL References EﬀectofapelinonovarianfunctionSteroido

genesis Proliferation Apoptosis Apelindoses Reference

Human +/+ +/+ +/+ ns [58] ↑P4,↑E2 ns ↓ 10⁻⁹M

[58]

Pig +/+ +/+ +/+ +/+ [61] ↑P4,↑E2 ns ns 0.02,0.2,2,and20ng/ml [61]

Bovine −/+ +/+ +/+ +/+ [13,57,59,62] ↑P4 ↑ ns 10⁻⁹M,

10⁻⁸M,

and10⁻⁶M [59]

Rat ns ns ns ns — ns ↑ ↓ 10^–8mol/l

[64]

Rhesusmonkey +/+ +/+ ns +/+ [60] ns ns ns ns —

In terna tio nalJ ou rnal of End ocrin ol

7 Table1:Apelin/APJexpressionintheovaryofmanyspeciesanddirecteﬀectsofapelinonovarianfunction.

s

+:present;−doesnotexist;ns:nostudy;↑:increase;↓:decrease;P4:progesterone;E2:estradiol.

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Physiology Pathology

[58,66]↑LuteolysiseN

OSpathwayactivation PCOS[70−71]

inconsistent:

↑↓

[61,65]↓Apoptosis

↑BCL-2,↓Bax

[60,62,65]↑SteroidegenesisActivati onofAPJandAkt/MAPK/AMPK

Endometriosis[78]

↑Angiogenesis

Ovariancancer[80]

↑Proliferation

[60,65]↑Proliferation

PI3K/Aktactivation ↓Bloodvesseldevelopment

Figure4:Apelineﬀectonovarianphysiologyandpathology.eNOS:endotheliumnitricoxide;Bcl-2:B-

celllymphoma2;APJ:apelinreceptor;Akt:p r o t e i n k i n a s e B ; M A P K : m i t o g e n - a c t i v a t e d p r o t e i n k i n a s e s ; A M P K : 5′AMP- activatedk i n a s e ; P I 3 K : p h o s p h o i n o s i t i d e 3 - k i n a s e ; PCOS:polycysticovarysyndrome.

observedthatapelinandAPJmRNAexpressionisincreasedinPCO Spatients,anditwashigherinobesepatients[58],suggesti ngtheroleofapelinasamarkerofPCOSpathogen-

esis( F i g u r e 4).Moreover,higherlevelsofa p e l i n - 1 3 infollicularﬂuidinobesewomencomparedtononobese womeninboththePCOSandnon-

PCOSgroupswasobserved[58].However,thepublisheddata comparisonofserumapelinlevelsinPCOSandnon-

PCOSwomenisincon-

clusive.Someauthorspointtoitsconsiderableelevationinse rumP C O S [ 6 9 –74]. DataofSunetal.[72] indicateda weight- dependentincreaseintheconcentrationofapelininobesewo menwithPCOScomparedtoPCOS-

deﬁcientwomen.ApelinwasfoundtobehigherinPCOSpatients byGörenetal.

[70]butwithoutasigniﬁcantcorrelationwit hhomeostat icm o d ela s s e s s m e n t ( H O M A-

IR).O l s z a n e c k a - Glinianowiczetal.

[73]reportedaninverseassociationbetweenapelinandglucose, insulin,andHOMA-

IRvalues,supportingt h e r o l e o f a p e l i n i n t h e r e g u l a t i o n o f i n s u l i n sensitivity.A p e l i n l e v e l s w e r e h i g h e r i n n o n o b e s e P COSpatients,s u g g e s tinga c o m p e n s a t o r y m e c h a n i smf o r m e t a -

bolicconsequencesofinsulinresistance.Comparativeresultsofstudiess howinglowerserumapelinlevelsinPCOShavebeeno b t ained b y s e v e r a l a u t h o r s [ 7 3 –

7 6 ] . D iﬀerentf r o m Cekmezetal.’sstudy[6 9 ], lower se r umco nc en t r a t i on s o f apelinwerefoundinPCOSsubjectsb yAltinkayaetal.

[75]witha p o s i t i v e c o r r e l a t i o n w i t h B M I , i n s u l i n , H O M A -

I R , triglyceride,a n d f r e e t e s t o s t e rone,s p e c u l a tingt h a t a p e l i n canbeusedasamarkerforinsulinsensitivity.Co nversely,Sunetal.

[72]observeda signiﬁcantlye n h a n ceda p e l i n concentrat

ioninPCOSpatientswithapositiveassociationwithB M I a n d H O MA-IR;t r e a t mentw i t h d r o s p i r e n o n e–

ethinylestradiolplusmetforminimprovedinsulinresistancean dapelinlevels.Discrepantﬁndingsamongthepublished

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InternationalJournalofEndocrinology 9 studiesm a y b e a t t r ibutedt o t h e d iﬀerencesi n e t h n

icity,age,s t u d y d e s i g n , s a m p l e s i z e , g e n e t i c c h a r a c t e r i s t i c s o f populations,a n d a s s e s smentm e t h odology,d efiningP C O S definitions;thedifferenceinthetes twasusedtoanalyzetheconcentrationofdifferentapelinisofor ms.

Anotherovarianpathologythathasbeenrecentlylinked toapelinactionisendometriosis.Endometriosisisadiseasewh ichischaracterizedbythesurvivalandgrowthofendome-

trialtissueoutsidetheuterusprimarilyinthepelvicarea.Itisoneofthe mostcommongynecologicaldiseaseswithupto10%o f w o m e n i n t h e U S A s uﬀeringf r o m i t s s y m p t o m s wh ichi n cludei n f e r t i l i t y a n d s ev er e p e l v i c p a i n [ 7 7 ] . T h i s diseaseishighlyestrogen-

dependentandisaccompaniedbyamajorinﬂammatoryrespons e.Apartfromsurgicalremovalofe nd om et ri o ticl es i o ns , t h e m ai nt h er apeutica p p r o a c h i s continuoustreatmentwith progestinstoinhibittheprolifer-

ationofthisectopictissuewhichisnotalwayseﬀective[77].Theref ore,investigationofsteroidhormonesignalinginthisdiseasei s c r i t icalt o i d e n t i f y i n g newtherapeutictargets.Apelin mightbeafactor playingaroleintheendometrial regen erationviaangiogenesis.Ozkanetal.

[77] usingtheimmunohistochemistrym et h oda nd i m m un o a ss ay d e t e c t e d apelini n t h e e u t o p i c a n d e c t o p i c e n d o m e t r i u m o f w o m e n witho r / a n d w i t h o u t e n d o m e t r i o sis.A p e l i n c o n c e ntrationsincreasedduringt hesecretory phase anddecreased duringproliferativep hasesofeutopicandectopicendometrialtis-

sue.Moreov er,thehigher im m u noreactivityofapeli n was observedintheendometriuminthesecretoryphase andinglandularcellsofbotheutopicandectopicendometria ltissues,suggestingthatincreasedlocalapelinconcentrati onmayindicateaparacrinefunctionontheendometriu m[77].Additionally,apelincausesendothelium- dependentvasorelaxationbytriggeringthereleaseofnitri coxideand

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isapotentangiogenicfactorinducingendothelialcell(EC) migration,proliferation,andbloodvesselinvivodevelop -

ment,i n d i c a t i n g i t s eﬀectsa s a c h e m o a t tractantf o r e n d o - thelialcellgrowth[77].

Recentdataindicatetherelationshipbetweenapelinandovar iancancer. Ovarian tumors,thesecondmostcommonty peof g y n e c o l o g i calm al i gn an cy [ 7 8 ] , a r e h e t e r o g e n e o u s neoplasmsc l a s s i ﬁedi n t o t h r e e m a j o r c a t e g o r i e s , n a m e l y , epithelialo v a r i a n t u m o r s , s e x c o rd-stromalt u m o r s ( e . g . ,

granulosac e l l t u m o rs),a n d g e r m c e l l t u m o rs.E p i t h e l i altumorsa c c o u n t f o r 8 0 % t o 9 0 % o f o v a r i a n m a l i g nancies,whereasGctumorsaccountfor1%to2%ofovaria nmalig-

nanciesi n t h e U S A a n d E u r o p e . D a t a o f H oﬀmanne t a l .

[ 7 9 ] d o c u m e n t e d t h e e x p r e s s i o n o f a p e l i n / A P J i n d i ﬀerentovariancelllines;theyobservedthattheAPJexpressionlevelwashi gherinepithelialcancercellsthaninGctumor,whereastherever sewastrueforapelinexpressionandsecre-

tion.Additionally,thesedataindicatethatapelinstimulatedO VCAR-

3cellproliferationandsuggestitsmitogenicactioninovarianepithelialc ancercells.Furthermore,recentstudiesreportt h a t a p e l i n s t i m u latesc a n c e r c e l l m i g r a t i o n i n t h e

lung,oralcavity,andcolon[80,81].

6. PhysiologyandPathologyofApelininthe Testis

Toourknowledge,thereisonepublisheddatademonstratingtheeﬀectof apelinonmalereproduction.Infusionofapelin-

13inmaleratssigniﬁcantlysuppressedLHreleasecomparedwiththev ehiclevalues,whilelevelsofFSHdidnotsigniﬁ-

cantlyd iﬀera m o n g t h e g r o u p s [ 8 2 ] . F u r t h e r m o r e , s e r u m testosteronelevelsintheapelin-

13groupwerestatisticallylowert hani n t h e c o n t rolg r o u p ; h i s t o l o g i cale x a m i n a t i o n showedt h a t i n f u s i o n o f a p e l i n -

1 3 s i g n i ﬁcantlyd e c r e a s e d thenumberofLeydigcells,su ggestingthatapelinmayplayar o l e i n t hec e n t r a l r e g u l a t i o n a n d d e creaset e s t o s t e r o n e releasebysuppressin gLHsecretion.Finally,theseauthorsconcludedthattheag onistofAPJmaybeausefuldrugforpharmaceuticalsinthetrea tmentofmaleinfertility[82].

7. SummaryandConclusion

Insum m a ry , t h e ap el i n er gi c ( a p e l i n an d A P J ) sy st e m w a s foundinthehypothalamus,pituitary,ovaries,and testisofmanyspeciesandhasautocrineand/orparacrine effectsoncontrolreproductionbothinfemaleandinmalere gulationoftheirphysiology.Mostresearchindicatesthata pelinhasani n h i b i t o r y effecton g o n a d o t r o p i n an d P R L s e c r e t i o n i n females,whileinmalerats,aninhibitoryeffecto fapelinonLHandt estosteronewas observed i ninvivoe xperiments.Apelinal sopart icipat es i n thedirectregul a tionofovarian physiology;i t w a s c l e a r l y d o c u m e n t e d t h a t a p e l i n h a s a stimulatoryeffecto n s t e r o i d o g enesisa n d p r o l i f e r a t i o n b u t aninhibitoryactiononcellapop tosisbyactivationonseveralkinasepathwayssuchasAMPK,E

RK,andAkt.Basedonavailabled a t a , w e sp ec ul at ed t h a t a p e l i n h asa co nn e ctionwiths u c h d y s f u n c t i o n s l i k e P C O S , e n d o m e t r i o sis,a n d mitogenicactioninovariancancer .Manyofthesepatholo-

giesarestillincriticalneedoftherapeuticintervention,and

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recents t u d i esh a v e f o u n d t h a t a p e l i n c a n b e a t a r g e t i n pathologicals t a t e s . T h e r e f o r e , a p e l i n a c t i v i t y c o u l d b e appliedi n t h e f u t u r e i n t h e t r e a t m e nto f m an y d i s e a s e s o f thereproductivesystem.

ConflictsofInterest

Theauthorsdeclarenoconﬂictofinterestregardingthepub- licationofthisarticle.PatrycjaKurowskaandAgnieszkaRakwereBGF scholarsoftheFrenchEmbassyinPoland2017.

Acknowledgment s

Thisw o rkw a s s u p p o r t e d b y t h e M i n i stryo f S c i e n c e a n d HigherEducationforthePHCprojectunderthebilat- eralPolish-FranceAgreement “POLONIUM”(2016–

2017)betweenAgnieszkaRak and Joell eDupont, as w ellasthe FrenchGovernmentandtheFrenchEmbassyinPol andandR e g i o n C e ntrei n F r a n c e ( P R E V A D I P r o j ectn o . 32000820).

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