ContentslistsavailableatScienceDirect
Pharmacological
Research
j ou rn a l h o m epa g e :w w w . e l s e v i e r . c o m / l o c a t e / y p h r s
Review
Recent
advances
on
natriuretic
peptide
system:
New
promising
therapeutic
targets
for
the
treatment
of
heart
failure
Silvia
Del
Ry
a,∗,
Manuela
Cabiati
a,
Aldo
Clerico
baCNRInstituteofClinicalPhysiology,LaboratoryofCardiovascularBiochemistry,Pisa,Italy
bScuolaSuperioreSant’AnnaandDepartmentofLaboratoryMedicine,FondazioneG.MonasterioCNR-RegioneToscana,Pisa,Italy
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received27June2013
Receivedinrevisedform31July2013 Accepted16August2013
Keywords:
Natriureticpeptides Cardiacendocrinefunction BNP
CNP Heartfailure
a
b
s
t
r
a
c
t
Sincethediscoveryoftheinfluenceoftheendocrinesystemoncardiacendocrinefunction30years ago,anincreasingnumberofexperimentalandclinicalstudieshaveconsolidatedendocrinefunction ofhumanheartasbeingarelevantcomponentofacomplexnetworkincludingendocrine,nervousand immunesystems.Manyaspects,however,stillremainunclearastotheproduction,secretionand periph-eraldegradationpathwaysofB-andC-typenatriureticpeptides.Inparticular,thehypothesisthatthe circulatingplasmapoolofthepro-hormonecanfunctionasprecursoroftheactivepeptidehormoneisyet tobefullydemonstrated.Accordingtorecentstudies,peripheralprocessingofcirculatingpro-hormone likelyundergoesregulationpathwayswhichseemtobeimpairedinpatientswithheartfailure.This wouldopennewperspectivesalsointhetreatmentofheartfailure,andidentifynovelpharmacological targetsfordrugsinducingand/ormodulatingthematurationofthepro-hormoneintoactivehormone.
© 2013 Elsevier Ltd. All rights reserved.
Contents
1. Introduction... 190
2. NPhormones:anintegratedregulatorysystem... 191
3. RecentinsightsonproductionandperipheraldegradationofB-typenatriureticpeptides... 193
3.1. Pathophysiological,clinicalandpharmacologicalconsiderations... 193
4. Recentinsightsonproduction,structureandreleaseofC-typenatriureticpeptide... 195
4.1. ClinicalrelevanceofC-typenatriureticpeptideinheartfailure... 195
5. Pharmacologicalandclinicalperspectivesandconclusion... 196
References... 197
1. Introduction
Thecardiacendocrinefunctionwasfirstdiscovered30 years
ago[1,2].Sincethen,anincreasingnumberofexperimentaland
clinicalstudieshavebeenperformedonthetopic,anda recent
searchonthisfieldthekeywords“cardiacnatriureticpeptides”
onPubMedperformedinMay2013producedover13,900results,
(lastaccessMay2013).Overall,manyadvanceshavebeenmade
inthefield,consolidatingtheendocrinefunctionofhumanheart
asbeingarelevantcomponent ofa complexnetworkincluding
∗Correspondingauthorat:LaboratoryofCardiovascularBiochemistry,Institute ofClinicalPhysiology–NationalResearchCouncil,ViaGiuseppeMoruzzi1,56124 Pisa,Italy.Tel.:+390503152793;fax:+390503152166.
E-mailaddress:[email protected](S.DelRy).
endocrine,nervousandimmunesystems.Yet,manykeyaspects
haveremainedunsolved.
The first real breakthrough in research dates back to 1981
when deBoldet al. [1]reportedintravenous injection of atrial
extractspromotingarapidand massivediuresisand natriuresis
inrats.Followingthatseminalstudy,afamilyofnatriureticand
vasodilatorpeptideswassoonisolated,purifiedandwithinafew
yearswaschemicallyidentifiedinhumantissues[3–5].Todate,
researchhasidentified,withinthisfamilyofpeptides,anumber
ofdifferentcomponents:theatrialnatriureticpeptide(ANP)and
the brain (or B-type) natriuretic peptide (BNP), predominantly
producedbycardiomyocytes;theC-typenatriureticpeptide(CNP)
predominantly produced by the endothelium; and urodilatin
produced byrenal tubular cells by thesame gene of ANP, and
whichisfoundonlyinurine[3–5].Mostrecently,anewpeptide,
namedDendroaspisnatriureticpeptide(DNP)hasbeenidentified
1043-6618/$–seefrontmatter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.phrs.2013.08.006
Table1
SomecommonclinicalconditionscharacterizedbyalteredplasmaBNPcirculating levels,comparedtohealthysubjects.
Diseases BNPlevels
(A)Cardiacdisease
Heartfailure Greatlyincreased
Acutemyocardialinfarction(first2–5days) Greatlyincreased Leftventricularhypertrophywithfibrosis Increased Cardiacischemia/hypoxia Increased (B)Pulmonarydisease
Acutedyspnea Increased
Pulmonaryembolism Increased Obstructivepulmonarydisease Increased (C)Endocrinedisease
Hyperthyroidism Increased
Hyporthyroidism Increased
Cushing’ssyndrome Increased Primaryhyperaldosteronism Increased (D)Livercirrhosiswithascites Increased (E)Renalfailure(acuteorchronic) Greatlyincreased (F)Paraneoplasticsyndrome Normalorincreased (G)Subarachnoidhemorrhage Increased (H)Useofcardiotoxicdrugs Increased
(I)Acutesepsis Increased
(J)Chronicinflammatorydisease(cardiac impairment)
Increased
in mammalian plasma, but its pathophysiological relevance in
humansisstillunclear[6].
Allnatriureticpeptides(NP)shareadirectdiuretic,natriuretic
andvasodilatoreffectsandaninhibitoryandprotectiveactionon
inflammatoryprocessesofthemyocardium,theendotheliumand
ofsmoothmusclecells,thusmodulatingcoagulationand
fibrino-lysispathways,andinhibitingplateletactivation[5–10].
A conspicuous number of experimental and clinical studies
demonstrated that production and secretion of NP are closely
and subtly regulated bymechanical, chemical, neuro-hormonal
andimmunologicalfactors[1,3].PlasmaconcentrationofNPcan
be considered as a sensitive index of the perturbation of the
homeostaticsystemsregulatingcardiac functionand
cardiovas-cularhemodynamics[3].GiventheimportantrolesoftheBNPin
bothcardiovascularphysiologyandpathology,itisnotsurprising
thatplasmaBNPlevelsareincreasedinbothcardiovascularand
extra-cardiovasculardiseases(Table1).
TheclinicalinterestontheNPsystemhasenormouslyincreased
afterseveralmeta-analysesandinternationalguidelines
recom-mendedmeasuringspecificB-type-relatednatriureticpeptidesfor
diagnosis,riskstratification,andfollow-upofpatientswithheart
failure[11–18].Recentstudies,however,arerevealingthatmany
B-typenatriureticpeptidesmeasuredinpatientswithcardiovascular
diseaseactuallylackbiologicalactivity[2,5].Itappearsratherthat
plasmaofpatientswithheartfailurecarriesinsteadlargeamounts
ofitspro-hormonepeptide(i.e.,proBNP)[19–28]suggestinga
pos-siblepathophysiologicalroleofcirculatingproBNPlevelsinsuch
patients[2,5,29,30].Inparticular,someauthorshavehypothesized
thattheperipheralprocessingofcirculatingproBNPmaybe
sub-jecttosomeregulatoryprocessthatisimpairedinpatientswith
heartfailure.Ifthisweretobeconfirmed,itwouldobviouslyset
newperspectivesinthepharmacologicaltreatmentofheartfailure
[2,5,29,30].
Atthesametime,recentstudiessuggestthatCNPandits
spe-cificreceptor,NPR-B,mayplayaveryimportantroleinregulating
cardiac hypertrophy and remodelingin patients with systemic
arterialhypertension,heartfailure,andacutemyocardial
infarc-tion[31,32].ThepathophysiologicalroleofCNPoncardiovascular
diseaseprovidesarationaleforthetherapeuticpotentialof
phar-macologicalinterventionstargetedalsotoCNPsystem[33].
Here,wewillreviewindetailsomerecentandstilldebated
find-ingsregardingtheroleofNPsystemoncardiovasculardiseases.
Table2
Biologicalfactorssuggestedtostimulateorinhibittheproduction/secretionofANP andBNPinvivoorincellcultureofcardiomyocytes(modifiedfromRef.[2,3,5]).
Suggestedfactors Suggestedmeanintra-cellular signalingmechanism
Stimulating
Mechanicalstrain TranscriptionfactorNF-kB activatedbyp38MAPK AngiotensinII TranscriptionfactorNF-kB
activatedbyp38MAPK Endothelin-1 TranscriptionfactorNF-kB
activatedbyp38MAPK a-Adrenergicagents TranscriptionfactorNF-kB
activatedbyp38MAPK Argininevasopressin Ca2+influxandPKC
Cytokines(includingIL-1,IL-6,TNF-a) TranscriptionfactorNF-kB activatedbyp38MAPK Growthfactors(suchasbFGF) MAPK
Prostaglandins(suchasPGF2aandPGD2) PLC,IP3,PKC,andMLCK pathway
Lipolysaccharide(LPS) TranscriptionfactorNF-kB activatedbyp38MAPK ChromograninB TranscriptionfactorNF-kBand
IP3/Ca2+influx
Thyroidhormones Thyroidhormoneregulatory element
Corticosteroids Glucocorticoidresponsive element
Estrogens Notyetdetermined
Inhibiting
Androgens Nitricoxide(NO)
Notyetdetermined TranscriptionfactorJMJ bFGF:basicfibroblasticgrowthfactor;IGF:insulin-likegrowthfactor,IL: inter-leukin; IP3: inositol 3-phosphate; JMJ: jumonji domain-containing protein; MLCK:myosinlightchainkinase;MAPK:mitogenactivatedproteinkinase;PG: prostaglandin;PKC:proteinkinaseC;PLC:phospholipaseC;TNF:tumornecrosis factor-a·
Firstwewillconsider(i)therecentevidenceonthealteredBNP
productionandperipheralmetabolisminpatientswithcardiac
dis-ease,andthen(ii)therecentstudiesconcerningtheroleofthe
CNPsystemoncardiovasculardisease.Thesefindingswillbethen
(iii)setintocontextthroughacomprehensivevisionontheroleof
endocrinecardiacfunctionandofthenetworkofNPhormones;and
(iv)consideredfortheirclinicalandpharmacologicalimplications.
2. NPhormones:anintegratedregulatorysystem
NPhormones constituteawell-integrated regulatorysystem
andshareasimilarstructuralconformation;theyarecharacterized
byapeptideringwithacysteinebridge,whichiswellpreserved
throughoutthephylogeneticevolution,beingtheportionofthe
peptidechainthatbindstothespecificreceptor(Fig.1).Members
oftheNPfamilyarepresentinbothplantandanimalkingdoms
[34–38].Inparticular,CNPistheancestralprecursoroftheNP
fam-ilyandisthemoststructurallyconserved(Fig.1)[34,37];itisquite
probablethattandemduplicationofanancestralCNPgeneearlyin
animalevolution(probablyinfish)thenoriginatedANPandBNP
[39].Indeed,consideringtheroleofNPinfundamentalbiological
functions,suchastheregulationofsoluteandfluidhomeostasisand
cardiovascularfunction,theNPsystemmusthaveappearedearly
inphylogenesis,potentiatingitsbiologicalactionthroughmultiple
tandemduplicationsofspecificgenesthroughouttheevolutionof
vertebrates.
Overall,theproductionofANP,BNPandCNPisregulatedby
sim-ilar(atleastinpart)intracellularpathways(Fig.2andTable2).Both
ANPandBNPgenesareregulatedbythesametranscriptional
fac-tors,includingp38mitogen-activatedproteinkinase(MAPK)[3,5].
Inparticular,p38MAPKactivatesthetranscriptionfactorNF-kBand
Fig.1.Natriureticpeptideancestralgeneevolutionfromamphibiantomammals(modifiedbyRef.[34]).
biologicalfactorsregulatingtheproduction/secretionofANPand
BNPinvivoorincellcultureofcardiomyocytesaresummarized
inTable2.Thesametranscriptionalfactorsandneuro-endocrine
effectorsarelikelytoalsoregulateCNPproductioninseveraltissues
[40–43].
SimilaritiesalsoconcerntheactionofANP,BNPandCNP:they
activate3 specificcomponentsof theguanylylcyclase receptor
family that catalyze the conversion of guanosine triphosphate
tocyclicguanosinemonophosphate(cGMP)andpyrophosphate
[4,44](Fig.2).Intracellular cyclicGMPis thesecondmessenger
of the guanylyl cyclase receptor family that modulates several
important functions in mammals: platelet aggregation,
neuro-transmission,sexualarousal,gutperistalsis,bloodpressure,long
bone growth,intestinalfluid secretion, lipolysis, cardiac
hyper-trophy and oocyte maturation [4,44]. ANP, BNP and CNP can
bindand activateallthe3guanylyl cyclasereceptors,although
withdifferentspecificity(Figs.2and3).Asaresult,theNP
hor-mones share a similarspectrum of biological actions, although
therearesomedifferencesinbiologicalpotencyamongANP,BNP
andCNP.
Fig.2. InteractionofANP,BNP,andCNPwiththeirspecificreceptorNPR-A(NPR-1,GC-A)NPR-B(NPR-2,GC-B)andNPR-C(NPR-3).NPR-AandNPR-Baretrans-membrane containinganequallylargeintracellulardomainconsistingofakinasehomologydomain,dimerizationdomain,andcarboxyl-terminalguanylylcyclasedomain.Differently, NPR-Conlycontainsanintracellulardomainandlacksguanylylcyclaseactivity.
Fig.3. Schematicmodelofthestructureofthenatriureticpeptidereceptor sub-types(NPR-A/GC-A;NPR-B/GC-BandNPR-C)andnatriureticpeptidespresencein endothelialcellsandcardiomyocytesbeforebeingreleasedintobloodvessels.
3. Recentinsightsonproductionandperipheral
degradationofB-typenatriureticpeptides
Inhumancardiomyocytes,BNPissynthesizedasa134-amino
acidprecursorpeptide(preproBNP)andissubsequentlyprocessed
toforma 108-aminoacidpropeptide (proBNP). Partof proBNP
is O-glycosylated within the Golgi apparatus [5,45–48] and is
secreted into the circulation, whereas a remaining part that is
notO-glycosylatedmayundergoenzymaticcleavageby
cardiomy-ocyteproteinconvertases(such ascorinand furin)toformthe
76-aminoacidN-terminalpeptide(i.e.,NT-proBNP)andthe
biolog-icallyactive32-aminoacidC-terminalpeptide(i.e.,BNP)(Fig.4)[5].
Asmallerpartofintactpro-hormoneisnorglycosylatedorcleaved,
andcanbefoundintocirculationinitsintactformasproBNP.
Glyco-sylationonthethreonylresidueinposition71(Thr71)mayregulate
pro-hormonecleavagebyeitherblockingorguiding
endoprote-olyticenzymes[5,48].
Recent studies indicate that the prevalent circulating form
of B-type natriuretic peptides is not, however, thebiologically
activepeptideBNP.Indeed,chromatographicproceduresonplasma
of experimental animals and patients with heart failure
evi-dence a largenumber of circulatingproBNP-derived fragments
[5,23,24,28,48–50].Moreover,plasmaproBNPandNT-proBNP(and
probablyalsoothershorterpeptidesderivedfromthesepeptides)
arefoundinbothglycosylatedandnon-glycosylatedform
(espe-ciallyinpatientswithheartfailure)[5,24,25].Somestudiessuggest
thatproBNPmaybethemajorB-typeNPimmunoreactiveform
inthehumanblood,especiallyinpatientswithcongestiveheart
failure[22–24,28].Inparticular,Seferianetal.[46]haverecently
reportedthattheplasmapoolofendogenousNT-proBNPcontains
asmallportion(about5%)ofnonglycosylatedorincompletely
gly-cosylatedprotein.
3.1. Pathophysiological,clinicalandpharmacological
considerations
ComparedtoNT-proBNPandproBNP,theactivepeptideBNPhas
ashorterplasmahalf-life(about15–20minvs1ormorehours)and
ispresentinplasmaatlowerconcentration,especiallyinpatients
withheartfailure(Table3andFig.5).Indeed,a50-foldprogressive
increaseinmedianBNPlevelanda150-foldincreaseinmedian
Fig.4.Natriureticpeptidegenename,chromosomelocalization,processing,structureandexpression.Thefinalaminoacidsequenceandstructureofthematurepeptides alongwiththemajordegradationproductareshownonthebottom.Thesitesofcleavageareindicatedwithscissors.BNP,andCNParemainlyexpressedintheindicated tissuesaspre-pro-hormones.Thesignalsequencesarecleavedtoformpro-BNPandpro-CNP.Thepeptidesarefurtherproteolyticallyprocessedtoformmaturepeptides (modifiedfromRef.[4]).
Table3
BiochemicalandphysiologicalcharacteristicsofBNP,NT-proBNPandproBNPpeptides.
BNP NT-proBNP proBNP
Molecularmass 3462Da 8457Da* 11,900Da*
Aminoacids 32 76 108
Biologicalfunction Activehormone Inactive Pro-hormone
Halflife 15–20min >60min >60min
Glycosylation Notglycosylated Highlyglycosylatedinvivo Highlygllycosylatedinvivo
Normalrange** 1–50ng/L 4–200ng/L 40–100ng/L
*Themolecularmass(MM)ofNT-proBNPandproBNPdependstothedegreeofglycosylationofthepeptide;inthetablearereportedtheMMofnotglycosylatedpeptides. **TherangevaluesofplasmaBNP,NT-proBNPandproBNPconcentrationsinhealthysubjectsarestronglydependenttotheimmunoassaymethodsusedforthe
measure-ment,aswellastoage,genderandBMIvalues[12,22,30,45].Therefore,thevaluesreportedinthetableshouldbeconsideredonlyasindicative.
Fig.5.BNPandNT-proBNPlevelsinhealthysubjectsandpatientswithdifferent clinicalseverityofheartfailure.Toppanel:plasmaBNPandNT-proBNPincrement ofmedianlevelin820patientswithstageA–Dofheartfailure,ascomparedto 182healthysubjectsmedianconcentration.Bottompanel:plasmaBNPand NT-proBNPincrementofmedianlevelin479stageCandDpatientsgroupedaccording toNYHAclass,ascomparedto182healthysubjectsmedianconcentration.Thedata reportedinthefigureclearlydemonstratethatpatientswithheartfailureshow higherlevelsoftheinactiveNT-proBNPthanthebiologicallyactivehormoneBNP. Moreover,thesedataindicatethatbothplasmaBNPandNT-proBNPlevels progres-sivelyincreaseaccordingtotheseverityofheartfailure.Datafromourlaboratory (modifiedfromRefs.[51,52]).
NT-proBNPlevelareobservedfromhealthysubjectstopatients
withheart failure in NYHA class IV/stage D (Fig.5)[51]. Data
reportedinFig.5indicatethatasymptomaticpatientsinstageB
(i.e.,symptomaticpatientsatriskfordevelopingheartfailurewith
structuralcardiacinvolvement)[18],onaverage,havehigherBNP
andNT-proBNPlevelsthanapparentlyhealthysubjects.
Further-more,symptomaticpatientswithmoderatesystolicheartfailure,
suchasthoseinclassNYHAclassI,showalsosignificantlyhigher
levelsofNPthanapparentlyhealthysubjects(Fig.5).These
find-ingsrepresenttherationaleforevaluatingthediagnosticaccuracy
ofBNPandNT-proBNPlevelsinscreeningprogramsincluding
gen-eralpopulationorprimarycarepatients[51,52].Moreover,some
recentstudiesreportedthatpatientwithheartfailurehavealso
greatlyincreasedcirculatingproBNPlevelscomparedtohealthy
subjects[22,26–28].Froma pathophysiologicalpointof view,a
bluntednatriureticresponseafterpharmacologicaldosesofANP
andBNPhasbeenobservedinexperimentalmodelsandinpatients
withchronicheartfailure,suggestingaresistancetothebiological
effectsofNP,mainlytonatriuresis[3].Asmentionedabove[3],
resistancetothebiologicalactionofNPcanbeattributedtoatleast
threekindsofdifferentcauses/mechanisms,actingatpre-receptor,
receptor,andpost-receptorlevel,respectively.
Basedonrecentfindings[19–28,49,50,53–55],resistanceatthe
pre-receptorlevelinpatientswithheartfailurecouldbeexplained
byaninsufficientpost-translationmaturationofthebiosynthetic
precursorsof B-type natriureticpeptide system.Accordingly, it
is theoretically conceivable that the active hormone (i.e., BNP)
couldbe produced even in vivo from thecirculating precursor
proBNP1-108byplasmaenzymedegradation[19–28,49,50,53–55].
Indeed,thesolubleformofcorin,atransmembraneserineprotease
abletocleaveproBNP,isalsocapableofprocessingthe
circulat-ingintactprecursorofnatriuretichormones[53,54].Dongetal.
[54]recentlyconfirmedthatsolublecorinisdetectableinhuman
blood. Furthermore, theseauthors report that levels of plasma
corin aresignificantly lowerin patients withheartfailure than
inhealthycontrols,andthatthereductioninplasmaenzymeis
correlated tothe severityof thedisease [54]. Finally, Semenov
etal.demonstratedthatsyntheticorrecombinanthumanproBNP
can beprocessed to active BNPin the circulating blood, when
injectedinthefemoralveinofrats[55].Ontheotherhandrecent
resultsindicatethatproBNPmaybindandalsoactivatethe
spe-cificreceptorofNP(i.e.,theguanylylcyclase-A,alsonamedNPR-A),
althoughwithreducedefficacycomparedtoANPandBNP[56].As
aresult,proBNPshouldbeconsideredtobea lowefficacy
ago-nist compared tobiologically active hormone BNP,in this way
contributingtothereducedNPresponseinpatientswithheart
fail-ure.Indeed,reduceddegradationofproBNPmaycontributetothe
increasedproBNP:BNPratioobservedinpatientswithcongestive
heartfailure,consequentlyexplaining(almostinpart)the
periph-eralresistancetobiologicalactionofNP[57].
PeripheralprocessingofcirculatingproBNPseemstobe
sub-mitted to regulatory rules that are impaired in patients with
Fig.6.(a)DistributionofCNPandNT-pro-CNPplasmalevelsinchronicheartfailurepatientsasafunctionofdiseaseseverity.Eachboxconsistsof5horizontallinesdisplaying the10th,25th,50th(median),75th,and90thpercentilesofthevariable.Allvaluesabovethe90thpercentileandbelowthe10thpercentileareplottedseparately(modified byRef.[76]).(b)CNPmRNAexpressionmeasuredbyRealTimePCRinhumanwholebloodofHFpatientsasafunctionofclinicalseverity(modifiedbyRef.[90]).
open new perspectives even in the treatment of heart failure
[2,20,21,29,30,58],andcouldidentifynovelpharmacological
tar-getsfordrugsinducingand/ormodulatingthematurationofthe
prohormoneintoactivehormone(i.e.,BNP)[2,29,58].Inconclusion,
thesestudies,takentogether,stronglyindicatethattheprevalent
amountofthecirculatingB-typenatriureticpeptidesinpatients
withheartfailurelacksbiologicalactivity.Theseresultsopenthe
questionaboutthepossibleregulationoftheperipheralprocessing
ofcirculatingproBNP.Based onthis hypothesis,severalauthors
suggestedthebiologicallyactiveBNPcanbeproducedalsoinblood
fromproBNPplasmapool.Accordingly,plasmaproBNPshouldbe
consideredtobealsoa circulatingprecursor ofthebiologically
activehormone,BNP[2,5,20,25,30,58].Inotherwords,BNPmay
beproducedfromtheprecursorproBNPinbothcardiomyocytes
andblood.
4. Recentinsightsonproduction,structureandreleaseof
C-typenatriureticpeptide
First isolated from porcine brain in 1990 [59] CNP, is the
mostexpressednatriureticpeptideinthebrain;itisalsowidely
expressedthroughoutthevasculatureandisfoundinparticularly
highconcentrationsintheendothelium,whereitplaysaroleinthe
localregulationofvasculartone[60–62].Itisstoredinside
endothe-lialcellsandisabletoinducevasorelaxationbyhyperpolarization,
suggestingthatCNPmaybeanendothelium-derived
hyperpolar-izingfactor(EDHF),participatingintheparacrineactionofother
endothelialvasorelaxantmediators,suchasnitricoxide(NO)and
prostacyclin[63–65].Recently,CNPexpressionwasalsofoundin
cardiomyocytesatbothgeneandproteinlevels[66].Thehuman
geneencodingforCNP,NPPC(GeneID4880),islocatedon
chromo-some2andencodesapolypeptideof126aminoacids,witha23
aminoacidsignalsequencefollowedbya103aminoacidproCNP
[67,68].PreproCNPshowsremarkablehomologybetweenspecies
andthecirculating22aminoacidcarboxylterminalformis
abso-lutelyidenticalinchimpanzees,dogs,mice,andratsspecies[68]
(Fig.1).
ProcessingofproCNPtoitsmatureformmayoccurthroughthe
actionoftheintracellularserineendoprotease,furin.Invitro,furin
cleavesthe103aminoacidproCNPintoa53aminoacid
carboxyl-terminal biologicallyactive peptide(CNP-53), thatis themajor
activeformofCNPattissuelevel[69,70](Fig.4).Inthesystemic
cir-culation,ashorter22aminoacidformdominates(CNP-22)butthe
proteaseresponsibleforthiscleavageisnotknown.Importantly,
CNP-53andCNP-22appeartobindandactivatetheirspecific
recep-tor,NPR-B,equallywell[70,71].CNPisnotstoredingranulesand
itssecretionisincreasedbygrowthfactors[72,73]andshearstress
[74].TheclearanceofCNP-22inhumanplasmaisveryrapid,with
acalculatedhalf-lifeof2.6min[75,76].
4.1. ClinicalrelevanceofC-typenatriureticpeptideinheart failure
The involvement of CNP in cardiovascular disease hasbeen
recentlytakingonanincreasingly importantrole.Several
stud-ies carried out in humans have reported increased NT-proCNP
[77,78] and CNP plasma levels in patients with chronic heart
failure compared tohealthy subjects[77,79–83].Data reported
in Fig. 6a indicate a progressive plasma increase of both CNP
progressiverisein CNPand NT-proCNP plasmavalues withthe
worsening of symptoms [77,79–81] and (ii) the negative
rela-tion between plasma CNP values and left ventricular ejection
fraction [81], such increase in secretion appears to be related
to the severity of the disease. Moreover, this increase in CNP
plasmaconcentrations in paralleltoworsening clinicalseverity
couldbeacompensatoryeffectin responsetocardiac
dysfunc-tion;togetherwiththeothernatriureticpeptidesANPandBNP,
CNPprobablycontributestoneuro-hormonalactivation(whichisa
hallmarkofchronicheartfailure)counteractingthepredominance
of vasoconstrictor and sodium retentive systems (sympathetic,
rennin–angiotensin–aldosterone,vasopressinandendothelin
sys-tems)[81,82].
Recentstudies indicate that theproduction of C-type
natri-ureticpeptides,especially inpatientswithheartfailuredirectly
involvestheheartand,namely,themyocardium[79,84–88].Infact,
immunohistochemistryandradioimmunoassaytechniqueswithin
themyocardiumdemonstratethatCNPcardiacexpressionistwoto
threetimeshigherinpatientswithchronicheartfailurecompared
tocontrols[66];inparticular,CNPlevelsinpatientswithchronic
heartfailurearesignificantlyincreasedinthecoronarysinus,as
comparedtotheaorta[84,85].
Fromapathophysiologicalpointofview,CNPalsocarriesoutan
importantautocrineandparacrinefunctioninpathophysiological
cardiacremodeling[84,85].
Studiescarriedoutbybimoleculartechniquebothinanimals
model[86,88]thatinhumanleukocytes[89,90]permittedtodetect
themRNAexpressionofCNPandNPR-Binnormalmyocardial
tis-sueaswellasinhumanwholebloodsamplesofhealthysubjects.
Thestudiesonminipigshavereportedmyocardialalterationsin
theexpression of CNP and NPR-B in normal and failing hearts
[86–88],moreoveranincreaseofCNPandNPR-Btranscriptomic
profilingwasobservedinhuman wholeblood samplesof heart
failurepatients[89]andinarecentpaper[90]originaldata
rel-ativetoCNPmRNAincreaseasafunctionofclinicalseverityin
parallelwithadown-regulationofNPR-Bexpressioninpatients
withworseningofsymptomswasreported(Fig.6b).HenceCNP
couldhaveabehaviorsimilartoBNP,underliningtheimportanceof
theendocrineroleofmyocardiuminthegenerationofCNPduring
earlyleftventriculardysfunction[86–88].Inparticularthestudies
showedaCNPproteinandgeneexpressiondetectedintissueof
normalandfailinghearts,alongwithanincreasedmyocardialCNP
synthesisinthepresenceoftheNPR-Bdown-regulationinfailing
heart[86].ThepresenceofCNPincardiactissuewouldsuggesta
possiblesynergisticeffectwiththeothernatriureticcardiac
pep-tides,ANPandBNP[91].Moreover,recentstudieshavefoundCNP
expressionincardiomyocytes,bothatthegeneandprotein
lev-els,aswellasNPR-BmRNAexpression[66].Thisco-localization
ofthepeptideanditsspecificreceptorNPR-Bincardiomyocytes
suggeststhehearttobeactivelyinvolvedintheproductionofCNP,
reinforcingitsinvolvementincardiovascularpathophysiology.
Taken together,thesestudies indicate thatCNP is produced
intheheartduringcardiacfailure,whereitmayelicitimportant
compensatoryphysiologicalconsequences inventricular
remod-eling. Positive effects of CNP oncardiac contractility,lusitropy
and dromotropy have been also described in several studies
usingintactanimals,isolatedhearts,cardiacmusclepreparations
andisolatedcardiomyocytes[92].Atpresent,however,evidence
toward cardioprotective effects of CNPis controversial. Studies
havereportedthatactivationoftheCNP/NPR-Bpathway
follow-ingmyocardialinfarctionpromotesapoptosisofcardiomyocytes
[93]andmediatesanti-proliferativeandanti-hypertrophiceffects
inadultcardiaccells [32,94]; furthermore,in infarctremodeled
myocardium,expressionofCNPislocallyincreasedinthepresence
ofadensecapillarynetwork[95].Inrats,invivoadministrationof
CNPimprovescardiacfunctionandattenuatescardiacremodeling
aftermyocardialinfarction.Itishypothesizedthatthiseffecton
car-diacremodelingisduetoCNPanti-fibroticandanti-hypertrophic
actions,whichwouldidentifythispeptideasausefulnovel
anti-remodelingpharmacologicalagent[94,96].Inaddition,giventhe
inhibitoryeffects ofCNPoncardiacfibrosisand hypertrophyin
vitro,CNPcouldoperateagainsttheprogression oflate cardiac
remodelingaftermyocardialinfarction[96].
Finally,CNPappearstoalsobepotentiallyinvolvedintherenal
natriureticpeptidesystem,whichparticipatesintheregulationof
sodiumandwaterbalanceandtherenalcirculation[84].Infact,in
patientswithheartfailurealsotherenalCNPsystemisactivated
andurinaryexcretionofCNPissignificantlyincreased[97].
5. Pharmacologicalandclinicalperspectivesand
conclusion
Despitetheremarkableadvancesintheknowledgeoncardiac
endocrine functionand theNP network, which havebeen
nar-rowingfocustowardspecificpharmacologictargets,therearestill
manyaspectsthatneedtobesolved.
Infirstinstance,theproduction,secretionandperipheral
degra-dationpathwaysofbothB-andC-typenatriureticpeptides,still
needtobeconfirmed, clarifyingin particularthehypothesis of
whetherthecirculatingplasmapoolofthepro-hormonecan
actu-allyfunctionasprecursoroftheactivepeptidehormone.
AnotherimportantunsolvedquestionconcernstheuseofNP
astherapeuticagentsinheartfailure[98–100].Hospitalizationfor
heartfailureisaclinicalentityassociatedwithhighpostdischarge
morbidityandmortality[100,101].Unfortunately,fewtherapies
areavailabletoimproveoutcomesinpatientswiththiscondition
[100].Inthefirstyearsofthenewcentury,theanti-fibroticand
remodelingeffectsofNPsuggestedtheuseofsomeanalogsofBNP
(likeasNesiritide,arecombinanthumanBNP)astherapeuticsin
heartfailure[98–100].Theinitialenthusiasm fortheparenteral
(IVinfusion)administrationofNPanalogswassoonattenuatedby
theappearanceofsomeadverseeffects(suchassevere
hypoten-sion) onsurvival and renalfunction[98–100].However,apilot
study[102]hasrecentlysuggestedthatsubcutaneous
administra-tionofBNP(insteadofIVinfusion)mayrepresentsanovel,safe,
andefficaciousproteintherapeuticstrategyinpatientswithheart
failure.
Althoughseveralclinicaltrialshavedocumentedthebenefits
andrisksoftheuseofsyntheticANP(Anaritide),BNP(Nesiritide)
andurodilatin(Ularitide)inpatientswithcardiovascularandrenal
diseases[103,104],toourknowledge,onlyfewclinicalstudieshave
beenreportedontheuseofsomeCNPanalogsforthetreatmentof
heartfailure.Aninnovativeapproachmaybetocreate“designer”
chimericpeptides,whichholdthepossibilitytoextendboththe
application and therapeuticbenefits possiblewitha natriuretic
peptidebasedapproach[105].Forexample,CD-NPisachimeric
peptideconsistingofCNPandpartofDNP[105–107].Thispeptide
wasshowntopossesscyclicguanosinemonophosphate-activating,
natriuretic,andaldosteronesuppressingpropertieswithout
induc-ingexcessivehypotensionwhengiventohealthysubjects[105].
Thispreliminarystudymaypavethewayforfurtherclinicaltrials.
The administration of some oral agents, which are able to
modulatetheproductionand/ordegradationofactivehormones
in thecirculation, may have somebeneficial effects oncardiac
remodeling without affecting cardiovascular hemodynamics. In
particular,inhibitionoftheneutralendopeptidasehadbeen
inves-tigatedasapotentialnoveltherapeuticapproachbecauseofits
abilitytoincreasetheplasmaconcentrationsofNPincombination
withothercardiovascularagents,suchasangiotensin-converting
enzyme inhibitors,and antagonistsof the angiotensin receptor
theNPsystemandthenetworkincludingendocrine,nervousand
immunesystems.
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