ContentslistsavailableatScienceDirect
Seminars
in
Immunology
jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / y s m i m
Review
Gaseous
mediators
in
resolution
of
inflammation
John
L.
Wallace
a,∗,
Angela
Ianaro
b,
Kyle
L.
Flannigan
c,
Giuseppe
Cirino
baDepartmentofPhysiology&Pharmacology,UniversityofCalgary,Calgary,Alberta,Canada bDepartmentofExperimentalPharmacology,UniversityofNaples,Naples,Italy
cInstituteforBiomedicalSciences,GeorgiaStateUniversity,Atlanta,GA,USA
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received31March2015 Accepted1May2015 Keywords: Hydrogensulfide Nitricoxide Carbonmonoxide Anti-inflammatory Cytoprotectiona
b
s
t
r
a
c
t
Therearenumerousgaseoussubstancesthatcanactassignalingmolecules,butthebestcharacterized ofthesearenitricoxide,hydrogensulfideandcarbonmonoxide.Eachhasbeenshowntoplay impor-tantrolesinmanyphysiologicalandpathophysiologicalprocesses.Thisarticleisfocusedontheeffects ofthesegasotransmittersinthecontextofinflammation.Thereisconsiderableoverlapintheactions ofnitricoxide,hydrogensulfideandcarbonmonoxidewithrespecttoinflammation,andthese medi-atorsappeartoactprimarilyasanti-inflammatorysubstances,promotingresolutionofinflammatory processes.Theyalsohaveprotectiveandpro-healingeffectsinsometissues,suchasthegastrointestinal tractandlung.Overthepasttwodecades,significantprogresshasbeenmadeinthedevelopmentofnovel anti-inflammatoryandcytoprotectivedrugsthatreleaseofoneormoreofthesegaseousmediators.
©2015ElsevierLtd.Allrightsreserved.
1. Introduction
Sincetheidentificationofnitricoxide(NO) asthesubstance accountingfortheactionspreviouslyattributedto ‘endothelium-derivedrelaxingfactor’[1],therehasbeenaburstofresearchinto severalgaseousmediators.NO,hydrogensulfide(H2S)andcarbon
monoxide(CO)havebeenthemoststudiedgaseousmediatorsover thepastthreedecades,andattemptshavebeenmadetodevelop novelanti-inflammatorydrugsbasedondeliveryofoneormore ofthesegaseousmediators[2].Allthreeofthesemediatorshave verylowmolecularweights,shorthalf-lives,andcanfreelydiffuse acrossmembranes.Theydonothavespecificreceptors,perse,but caninteractwitharangeofproteinsandgenestoproduceawide rangeofeffects.ParticularlyinthecaseofNOandH2S,suppression
ofsynthesiscanprofoundlyaltercellandtissuefunction.
Thepotentialfortheactionsofthesemediatorstobeexploited inthedesignoftherapeuticsisconsiderable.Ofcourse,drugsthat release NOhavebeenin useforover a century(e.g., nitroglyc-erin).Developmentofdrugsbasedonstimulationorsuppression ofH2SorCOproductionhasoccurredmainlyinthepastdecade.
Inthisarticle,wereviewsomeofthekeyactionsofthesegaseous mediatorswithrespecttotheirimpactoninflammatoryprocesses.
∗ Correspondingauthorat:15PrinceArthurAvenue,Toronto,OntarioM5R1B2, Canada.Tel.:+19055156132.
E-mailaddress:altapharm@hotmail.com(J.L.Wallace).
Wealsoreviewsomeofthedrugsindevelopmentthatareaimto exploittheanti-inflammatoryactionsofthesemediators.
2. Nitricoxide
2.1. Theimportanceofl-arginine-nitricoxidepathway
Theacuteinflammatoryresponseisaself-limitingprocessthat normallyresultsin restorationof tissuehomeostasis. Persistent inflammatorystimuliorderegulationofmechanismsofthe resolu-tionphaseresultsinchronicinflammation,recognizedtobeakey underlyingfactorintheprogressionofarangeofdiseases, includ-ingatherosclerosis,arthritis,cancerandchronicneurodegenerative diseases.AlterationsinNOsynthesisbyendogenoussystemscan influencetheseinflammatoryprocesses.
The inorganic free radical, NO was first identified as an endothelium-derivedendogenousmessengerresponsibleforthe regulationofvasculartone[3,4].However,sincethenithasbecome clear that NO is the signalingmolecule responsible for several diversephysiologicalandpathophysiologicalprocesses.NOis pro-ducedbythreedifferentformsofNOS,namelynNOS(NOS1),eNOS (NOS3),andiNOS(NOS2)[5].
Arathersimple,notfullycorrectbuttraditionallyuseful classifi-cationdiscriminatesinducibleversusconstitutivelyexpressedNOS isoforms,whichapproximateslowversushighproductionrates ofendogenouslygeneratedNO[5,6].Thus,it hasbeengenerally assumed thatnNOSand eNOSarecriticalfor anormal physiol-ogy,whereas iNOS is associated with injury. NOSisoformsnot http://dx.doi.org/10.1016/j.smim.2015.05.004
onlyproduceNO,theprimaryreactionproduct,butalsoa num-berofspecies resultingfromoxidation,reduction, oradduction ofNOin physiological milieus,thereby generatingvarious clin-icalspeciessuchas,S-nitrosothiols,peroxynitrite(ONOO ), and transitionmetaladducts[7].TheclassicalpathwaybywhichNO exertsmanyofitsactionsisviaactivationoftheenzymesoluble guanylatecyclase(sGC)[8]andresultantconversionofguanosine 5-triphosphate(GTP)tothesecondmessenger3,5-cyclic guano-sinemonophosphate(cGMP)[9].However,severalstudieshave establishedthatNOcanalsoactviacGMP-independentpathways invarioussystems,particularlyduringtheinhibitionofplatelet aggregationandregulationofinflammatorycellapoptosis[10–13]. Inaddition,NOmodulatestranscription/translationindirectlyby affectingsignalingpathways,suchasmitogenactivatedprotein kinases,G-proteins,theRaspathway,glyceraldehyde dehydroge-naseorphosphatidylinositol-3kinase(PI3K)[14].
2.2. NOapoptosisandresolutionofinflammation
Progressionofinflammatoryconditionsdependsnotonlyupon the recruitment and activation of inflammatory cells but also upon their subsequent removal from the inflammatory milieu. Apoptosis, or programmed cell death, can be considered as a “removal”mechanism,leadingtoresolutionofinflammation, char-acterizedbya seriesofmorphologicalandbiochemical features [15].Ininflammation-baseddiseasesapoptosisisafundamental processregulatinginflammatory cellsurvival and it iscritically involvedinensuringthesuccessfulresolutionofaninflammatory response[15].Dysregulationof apoptosisand phagocytic clear-ancemechanismscanhavedrasticconsequencesonresolutionof inflammatoryprocesses.Hence,apoptosisrepresentsamechanism toremovepotentiallydamagingpro-inflammatorycellsfromthe siteofinflammationandisthereforecriticaltothesuccessful reso-lutionofinflammation.Duringthisprocess,activatedinflammatory cellsgeneratereactiveoxygenandnitrogenspecies,includingNO. InthiscontextitisofparticularinteresttheabilityofNOtoregulate apoptosisofinflammatorycells[16].Whatmakescomplexto eval-uatetheinvolvementofNOintheresolutionphaseisthatNOhas bothpro-apoptoticandanti-apoptoticproperties[17–19]. How-ever,itisknownthatlowerconcentrationsofNOproducedbythe eNOSandnNOSarecytoprotective,whilstsupraphysiological con-centrationsproducedbytheiNOStriggercelldeath.Theseapparent oppositeeffectsareexplained,atleastinpart,bythefreeradical natureofNOandhenceitschemicalinteractionwithotherradicals presentinthemilieutoformvariousNO-relatedspeciesinvivo. Thepro-oranti-apoptoticeffectsofNOmaythusbecritically gov-ernedbythespecificNO-relatedspeciesgenerated.Anexampleis representedbytheONOO speciesthatcouldaccountforthe apop-toticresolvingprocess[20].However,thepreciseroleofONOO ininflammatorycellapoptosisisstillnotclearlydefined.Thus,it isintuitivethattheabilityofNOtoinduceapoptosisis particu-larlyrelevantduringtheresolutionphaseofinflammation.Several studieshavedemonstratedthatactivatedmacrophages infiltrat-ingmurinetumorsinduceapoptosisviaaNO-dependentpathway inbothactivatedanti-tumorTcellsandinthetumorcells them-selves[21,22].Thus,itappearsthatmacrophagesinduceapoptosis ofnearbycellsthroughNOthatinturnenhancestheclearanceof apoptoticcellstherebypromotingtheresolutionphaseof inflam-mation.
Itiswidelyacceptedthatinflammationdrivesdevelopmentof somecancersthatadapttoandusetheoxidant-rich microenvi-ronment[23,24].This latter phenomenonprovides a persistent andself-perpetuatingoxidativestresscomposedofbothreactive nitrogenspeciesandreactiveoxygenspecies[25].Amongthe crit-icaloxidantsourcesNOplaysamajorroleinoxidativestressin melanomaandothercancers[25–29].
AlsoincancertheeffectofNOisdependentuponthe concen-trationsince it hasbeenshownthat angiogenesis,proliferation and metastasis can normally be stimulated by lower levels of NO(<100nM),whilehigherconcentrationsofNO(>400–500nM) promotecytotoxicityandcellapoptosis[30,31].Thus,NOdonors havebeenproposedasanoveltherapytovariouscancers[32,33]. The mostinteresting results have beenobtainedby the useof nitricoxide-releasingnon-steroidalanti-inflammatorydrugs (NO-NSAIDs).Infact,severalstudieshavefoundalinkbetweenNSAIDs useanddecreasedriskofcolorectalcancer[34–36],suggestinga possiblechemopreventiverole.NO-NSAIDshavebeenshowntobe moreeffectiveintheinhibitionofcancercellgrowthandmetastasis thantheparentdrugalone[33,37].
Atherosclerosisisanotherdiseaseswheretheconceptof reso-lutionofinflammationcanbeappliedwithregardtoNO.Itisnow widelyrecognizedthattheinflammatorycomponentof atheroscle-rosiscontributestoplaqueformation[38–40].Plaquegrowthand developmentaredrivenbyinflammatorycells,inparticular mono-cytes and macrophages, representing the major driving force. Becauseapoptoticcellsareingestedbyphagocyteswithout initiat-inganyfurtherpro-inflammatoryresponse,ithasbeensuggested thatapoptosismayrepresentamechanismtoregresstheplaque. ThereforethisprocesstriggeredbyNOcontributestoresolutionof inflammation[41,42].
InconclusionNOplaysaroleintheresolutionofinflammation butitsactivitydependsupontheconcentrationofNOinthelocal environment,thetimingofadministrationortherouteof admin-istration,aswellastheNOSisoformtargeted[43,44].
3. Hydrogensulfide
3.1. H2Sasamediatorofinflammation
Kimuraandcolleagueswerethefirsttoidentifyphysiological roles for H2S through their studies of its actions in the
ner-vous system[45,46]. Severalyears, later, Wangand colleagues demonstratedtheH2Swasapotentendogenousvasorelaxant[47].
Together,thesestudiesstimulatedaburstofresearchintotherole ofthisgaseousmediatorinmanycellsandorgansystems[47,48]. Zanardoet al. provided keyevidence suggesting a rolefor H2S
animportantendogenous anti-inflammatoryandpro-resolution mediator[49].TheyreportedthatH2Sexertedpotentinhibitory
effectsonleukocyteadherencetothevascularendothelium.Using intravital microscopy to examine the mesenteric microcircula-tioninrats,theydemonstratedthatadministrationofH2S(using
donors suchas Na2S, NaHS and Lawesson’s reagent) markedly
andpotentlysuppressedleukocyteadherenceofleukocytestothe vascularendotheliumandpreventedextravasationofleukocytes [49].They alsoobservedthattheinhibition of endogenousH2S
synthesisresultedinaveryfastinductionofleukocyteadhesion tothevascular endothelium.In modelsof carrageenan-induced sub-dermal inflammation, H2S donors were found to suppress
leukocyteinfiltrationandedemaformation[49,50].Inhibitionof leukocyte-endothelialadhesionbyH2Sisaconsequenceof
sup-pressionoftheexpressionofcelladhesionmoleculesonboththe endothelium(e.g.,intercellularadhesionmolecule(ICAM)-1and P-selectin)andontheleukocyte(lymphocytefunction-associated antigen (LFA)-1) [49]. Severalother groups have also reported inhibitory effects of H2S onthese adhesionmolecules [51–54].
ConsistentwiththefindingsofZanardoetal.[49],micethatare heterozygous for the gene for cystathionine -synthase (CBS), oneofthemajorenzymesforsynthesis ofH2S, exhibitreduced
leukocyte-rolling velocity, increased vascular permeability, and increasednumbersofadherentleukocytesinmesentericvenules [55].
InadditiontoH2Spotentlyinhibitingleukocyteadhesiontothe
vascularendothelium,numerousstudieshaveshownthatH2Scan
directlyinfluenceawiderangeofleukocytefunctions.For exam-ple,arecentstudydemonstratedthatH2Sinhibitstheactivityofthe
granulocyteenzymemyeloperoxidase(MPO),whichhadbeen iso-latedfromhumansandrats[56].MPOcatalyzestheconversionof hydrogenperoxidetohypochlorousacid.Thelatterhaspowerful bactericidaleffects,butalsohasthecapacitytocausesignificant damagetohosttissue.H2Shasbeenshowntobeanavid
scav-engerothergranulocyte-derivedcytotoxicsubstances,including peroxynitrite[57],superoxideanion[58],hypochlorousacid[59] andhydrogenperoxide[60].
Two of the most critical processes in resolution of inflam-mationareinductionofneutrophilapoptosisandstimulationof macrophages tophagocytoseapoptotic neutrophils(i.e.,driving macrophagestoa“M2”phenotype)[61].Aninabilitytoclear neu-trophilscontributes significantlytothedevelopmentofchronic inflammation. H2S promotes neutrophil apoptosis [62]. Dufton
et al. [61] demonstrated that application of H2S to human or
murinemacrophagesincreasedchemotaxsisofmacrophagesand therate of phagocytosis of thebacteriumEscherichia coli.Both arestimulated byH2S, resulting in a reductionof the
accumu-lationofinflammatorycells[61].ExposuretoH2Salsorendered
macrophageshyporesponsivetoinflammatorystimulisuchas bac-terialendotoxinorTNF-␣.
Some anti-inflammatory actions of H2S are mediated by
annexin-A1,anotherpro-resolutionmediator [52].Treatmentof isolatedhumanneutrophilswithanH2S-releasingagent(NaHS)
resultedin translocationof annexin-A1fromthecytosoltothe plasmamembrane.Thisisaneventthatisrequiredforthe initi-ationofannexin-A1signaling.Instudiesinmice,treatmentwith anH2SdonorinhibitedIL-1-stimulatedgranulocyteadhesionand
extravasationinmesentericvenules,aswellasdown-regulating endotoxin-inducedexpressionofCOX-2andinducibleNOsynthase inmacrophages[52].Whensimilarexperimentswereperformed inannexin-A1-deficientmice,bothoftheseeffectswereabsent. Moreover,in theannexin-1deficient micethere wasincreased expressionofCBSand cystathionine␥-lyase(CSE)ina rangeof tissues,furtherdemonstratingalinkbetweenannexin-A1andH2S
inpromotingresolutionofinflammation[52].
Many key mediatorsof inflammationare regulatedto some degreebyH2S.ThisrepresentsanothermeansthroughwhichH2S
regulatesprocessessuchasleukocyteadhesionandemigration.In theGItract,forexample,H2Shasbeenshowntoplayacrucialrolein
promotingtheconstitutiveexpressionofcyclooxygenase(COX)-2 andofprostaglandinsynthesis[63,64].Thisisimportantfor main-tenanceofmucosalintegrity,butalsoforrapidresponsestoinjury. Inthiscontext,H2Sactstomaintainphysiologicalexpressionof
COX-2.However,inothercircumstances,suchaswhen thereis chronicinflammationofthetissue,H2Scandampen
inflammation-associatedup-regulationofCOX-2expressionandofanumberof otherpro-inflammatorycytokines,therebycontributingto reso-lution ofinflammation [65].For example, H2Shasbeen shown
todown-regulatetheexpressionofarangeofpro-inflammatory cytokines (e.g., IL-1, tumor necrosis factor (TNF)␣, interferon (IFN)␥,IL-12,IL-23,etc.).Incontrast,H2Sdoesnotsuppress,and
insomecircumstanceselevates,theexpressionofIL-10,a power-fulanti-inflammatorycytokine[65,66].TheabilityofH2Stoaffect
theexpressionofsomany pro-inflammatorycytokinesis likely duetoitsmodulatingeffectsonnuclearfactor kappa-light-chain-enhancerofactivatedBcells(NF-B)activity[67].Asdescribed inmore detail below,IL-10alsoplaysan importantrole inthe regulationofH2Ssynthesis[68].
In addition to its role in inflammation, COX-2-derived prostaglandin synthesis is an essential element of GI mucosal defence and repair [63], and its modulation by H2S is very
important. For example, if healthy rats are treated with phar-macological inhibitors of H2S synthesis, there is a significant
reduction of the expression of COX-2, a concomitant decrease in mucosalprostaglandinE2 synthesis,and thedevelopmentof
mucosalinflammationintheabsenceofanydetectabletissueinjury [69,70].Ontheotherhand,administrationofanH2Sdonor(diallyl
disulfide)resultedinamarked,dose-dependentelevationofCOX activityinthesmallintestine,aswellasareductioninmucosal inflammation[71].
3.2. H2Sinpromotionofrepair
Inadditiontoitsanti-inflammatoryeffects,thereisconsiderable evidencethatH2Scontributessignificantlytopromotingrepairto
tissueinjuryandrestorationoftissuefunction.Someofthe earli-eststudiesoftheroleofH2Sinresolutionoftissueinjuryutilized
modelsofinflammationandinjuryintheGItract[70,72,73].For example,administrationofH2Sdonorsorwithl-cysteine
(precur-sorforH2Ssynthesis)markedlyaccelerated healingofulcersin
thestomachofrats,whilesuppressionofH2Ssynthesisresulted
inamarkeddelayinhealing[73].EndogenousH2Ssynthesiswas
markedlyelevated,aswastheexpressionoftwoofthekeyenzymes forH2SsynthesisCSEandCBS,particularlyalongtheulcer
mar-gin,wherere-epithelializationismostactive[73].Thisisconsistent withthepowerfulangiogeniceffectsofH2S[74].Interestingly,
non-steroidalinflammatorydrugs(NSAIDs)areknowtoimpairgastric ulcerhealing,buttreatmentofmicethathadgastriculcerswith anH2S-releasingNSAID(ATB-346)resultedinamarked
accelera-tionofhealing[75].Thisoccurreddespitethecompoundmarkedly suppressinggastricprostaglandinsynthesis,whichisbelievedto beresponsibleforCOX-2-drivenangiogenesisandrepair.Thus,H2S
appearstohaveadditionalpro-healingmechanismstothose medi-atedviainductionofCOX-2expression.
3.3. RapidH2Sresponsetoinjury
H2Sactsasa‘rescuemolecule’inmanycircumstances,helping
topreservetissueintegrityandfunctionwhilerepairoccurs.It addi-tiontotherapidup-regulationoftheenzymesthatproduceH2S
froml-cysteine,whichdrivetissuerepair,H2Scanmarkedlyreduce
tissueinjuryassociatedwithhypoxia/anoxiabypreserving mito-chondrialfunction.H2Scan‘drive’mitochondrialrespiration,and
thegenerationofadenosinetriphosphate,particularlyin circum-stanceswhereoxygenlevelsareverylow,suchasduringischemia [76–78].ThereissubstantialevidencefromstudiesoftheGItract demonstratingthis‘rescue’roleofH2S.Indeed,theepithelialcells
oftheGItracthavebeenreportedtobethemostefficientatusing H2StodrivemitochondrialATPproduction[76–78].
Asmentionedbove,whencolonicinflammationisinducedin animalmodels,thereisaveryrapidandprofoundincreaseinH2S
synthesis,andup-regulationofthekeyenzymesforH2S
synthe-sis[70].TheseincreasesinH2Ssynthesisandenzymeexpression
occurspecificallyatthesitesoftissueinjury–thereisnochangein theimmediatelyadjacentinflamed,butnotdamagedtissue[79]. Moreover,thereisadown-regulationofoxidationofH2Satthese
samesitesofdamage[79].Ofcourse,thenetresultoftheelevated synthesisanddecreaseddegradationofH2Swouldbegreaterlocal
concentrationsofthismediator,whichdrivestissuerepair. Admin-istrationofinhibitorsofH2Ssynthesisinmodelsofcolitisresults
inadramaticincreaseintheseverityofdisease;indeed,insome studies,colonicperforationanddeathoccurred[70].Suppression of H2Ssynthesis inratswithcolitis wasalsoassociated witha
markedthickeningofthewallofthecolon,mainlyattributedto hyperplasiaofthelayersofmuscle.AdministrationofH2S-releasing
drugs,suchasNaHSorLawesson’sreagent,promotesresolutionof colitis[66,70].Thesedrugsalsoreducedthethickeningofcolonic
muscle,andreducedthelevelsofTNF␣incolonictissue[70]. Sim-ilarresultshavealsobeendemonstratedinamodelofintestinal ulceration[71],furtherhighlightingtheabilityofH2Stopromote
tissuerepair.
AnotherdemonstrationofaclinicallyrelevantroleforH2Sin
drivingresolutionofinjuryandinflammationinthedigestivetract involvedtheinductionofhyperhomocysteinemia,inducedby feed-ingavitaminB-deficientdiettorats[68].Approximatelyone-third ofpatientswithinflammatory boweldisease(IBD) exhibit vita-minBdeficiency,whichcanleadtohyperhomocysteinemia,and thereis clearevidencethatthis conditionsignificantlyworsens theseverityofIBD[80–82].Inratswithdiet-induced hyperhomo-cysteinemia,weobservedthattherewassignificantimpairment ofcolonicH2Ssynthesis,andanabsenceoftheup-regulationof
CSEexpression typically seen in thecolon when it is inflamed [70].Thesechanges wereobservedinthree differentmodelsof experimentalcolitis.Moreovertreatmentwithdiallyldisulfide,an H2S-donatingmolecule, normalized levelsofH2Ssynthesis and
significantlyreducedthediseaseseverityintheratswith hyper-homocysteinemia[68].
Further mechanisms that may contribute to the beneficial effectsofH2SinpromotingGImucosalintegrityandrepairinclude
therecentlydemonstratedstimulatoryeffectsofthismediatoron gastricbicarbonatesecretion[83],andonmucussecretion[84].The lattercanpromoteastrongerbarrierfunctionoftheGImucosa, therebylimitingexposuretoluminalbacteria[84].Suppressionof colonicH2Ssynthesisinmicewithcolitismarkedlyreducedmucus
granuleformationinthecolon,inparallelwithanexacerbation ofthedisease.H2Shasalsobeenshowntohavedirectinhibitory
effectsonthegrowthofdifferentbacterialstrainsin thelumen ofthegut[84].Togetherthese effectsof H2Slimit exposureof
inflamedsitesintheGItracttoluminalbacteriaandmayinpart explainthepro-resolutioneffectsofH2S.
3.4. H2S-basedanti-inflammatorytherapeutics
AswasthecaseforNO,soonafterreportsofthephysiological importanceofH2Shadbeenreported,severalgroupsbegantotry
todevelopnoveltherapeuticsbasedonreleaseofH2Sorinhibition
ofitssynthesis[85–88].Belowisasummaryofsomeofthedrugs designedtotargetinflammatoryprocesses.
3.4.1. Inflammatoryboweldisease
TheincidenceofIBDhasbeenincreasingsteadilyoverthepast 5decades,andthepathogenesisofthisgroupofdisorders(which includes Crohn’s disease and ulcerative colitis) remains poorly understood.BiologicaltherapiestargetingcytokinessuchasTNF␣a havebeenverycommerciallysuccessfuloverthepast20years,but theyareexpensive,cantriggersignificantadverseeventsandhave ahighfailurerate.Thus,thereisstillaneedforsafe,affordable med-icationsforreducinginflammationandpromotingrepairoftissue injury.Mesalamine(5-aminosalicylicacid)isthefirst-linetherapy forIBD.Itistheactivecomponentofsulfasalazine,whichwas devel-opedfortreatmentofarthritis,thenfoundtobeeffectiveinpatients withIBD.Mesalamineisarelativelyweakanti-inflammatorydrug, sohighdoseshavetobeadministered,andtypicallyin formula-tionsthatpreventabsorptionintheupperGItract.Ontheother hand,mesalamineisverysafe,withaverylowincidenceoflargely mileadverse effects.While sometimesreferredtoasanNSAID, mesalamineactuallyhasveryweakinhibitoryactivityonCOX,and thatactivityisveryunlikelytocontributetoitsbeneficialeffectsin patientswithIBD.Itsanti-oxidantactionshavebeensuggestedto accountformosttoitsbeneficialeffects[89].
AdrugdevelopedbyAntibeTherapeutics,ATB-429,isa deriva-tive of mesalamine that releases H2S. It exhibits significantly
enhanced anti-inflammatory, pro-resolution and pro-healing
effectsinrodentmodelsofcolitis[66].Ithasbeenshowntobevery effectivewhengivenorallyorintrarectally,withnegligible absorp-tionofthedrugitself,orofmesalamineliberatedfromthedrug.The lowlevelofabsorptionofATB-429raisesthepossibilityofitbeing usedtotreatinflammatoryconditionsthroughouttheGItract,with noneedforthetypesofformulationsthathavebeenemployedin commercialpreparationsofmesalaminetopreventitsabsorption. Independentofthebenefitsassociatedwithreducedabsorption, ATB-429 hasbeen shown to besignificantly more potent than mesalamineatreducingGIinflammationinratandmouse mod-elsofcolitis[66].Moreover,ATB-429producedanti-inflammatory actionsnotseenwithequimolardosesofmesalamine,including inhibitionoftissueexpressionofanumberofpro-inflammatory cytokinesand chemokines(IL-1,IL-8,IL-12,TNF␣,RANTES),but sparingoftheexpressionofIL-10[66].Unlikemesalamine, ATB-429alsoexhibitedsignificantanti-nociceptiveeffectsinaratmodel ofvisceralpain[90].
3.4.2. Acuteandchronicpain
AmongthemostcommonlyusedclassesofdrugsareNSAIDs: nonsteroidalanti-inflammatorydrugs.Theyareusedonachronic basis by hundreds of millions of people suffering from disor-derssuchas osteoarthritis,rheumatoid arthritis andankylosing spondylitis.Theyarewidelyusedanacutebasisfortreatmentofa rangeofdisorderscharacterizedbyseverepain(e.g.,sportsinjuries, dentalpain,gout,dysmenorrhea,post-surgicaltrauma,etc.).The greatestlimitationtotheuseofthisclassofdrugsistheir propen-sitytocauseulcerationandbleedingintheGItract[91].Thiscarries anenormouseconomicburdentohealthcaresystems,particularly whentheinjuryoccursinthedistalsmallintestine,whereitis diffi-culttodetect,andforwhichnopreventativeorcurativetreatments areavailable[92,93].H2S-releasingNSAIDshavebeendeveloped
byanumberofgroups,someofwhicharefocusingprimarilyon theiruseasanti-inflammatoryandanalgesicagents,andsomeof whicharefocusingontheirpotentialutilityaschemopreventative agentsforvarioustypesof cancerand/orforneurodegenerative diseases.ThemajoradvantageoftheH2S-releasingNSAIDsistheir
greatlyreducedpropensitytocauseulcerationandbleedinginthe GItract[50,75,94–97].Extensivepre-clinicalstudiesdemonstrate thatH2S-NSAIDsareatleastaseffectiveandpotentastheparent
NSAIDsinmodelsofinflammation/pain[50,75,94–96].Thesedrugs alsoinhibittheactivityofCOX-1andCOX-2withsimilarpotency astheparentNSAIDs[50,75,94–96].
TheGIsafetyofH2S-NSAIDshasbeenextensivelystudied.In
healthyrats,ATB-346wasfoundtoproducenegligiblehemorrhagic damageinthestomach,whilenaproxencausedextensiveinjury [75].Withrepeateddosingover severaldays, naproxeninduces extensiveulcerationandbleedinginthesmallintestine.However, nodamagewasobservedwhenATB-346wasadministeredinthe samemanneratequimolardoses[75].GreatlyenhancedGIsafety wasalsoobservedinanimalmodelsthatattemptedtomimicthe clinicalscenario,wherepatientswitharthritishaveco-morbities (obesity, diabetes, old age, concomitant useof anti-coagulants, etc.)that substantiallyincreasestheirrisk ofdeveloping signifi-cantGIbleeding.Inallsuchmodels,ATB-346sparedtheanimalof ulcerationandbleeding,whilenaproxencausedsignificantdamage [75,94].
4. Carbonmonoxide
COisproducedviatheenzymehemeoxygenase,whichexists asconstitutive(HO-2)andaninducible(HO-1)isoforms.Thelatter enzymeactsasasensorofcellularstress,andtheCOitgenerates canlimittissueinjury[98].Approachestoexploitthesebeneficial actionsofCOinaclinicalsettingincludetheuseofCOasaninhaled
gasandofCO-releasingmolecules(oftenreferredtoas“CO-RMs”or “CORMs”),whichcanbeadministeredorally.Therehavealsobeen attemptstodevelopsmallmoleculesthatcanup-regulateHO-1 expression,therebyfacilitatingmoreCOproduction.LikeNOand H2S,COexhibitscellandtissueprotectionthroughanti-apoptotic,
anti-inflammatory,andanti-proliferativeeffects[98].Otherthan theknowninteractionofCOwithtransitionmetals,themolecular targetsofCOarelargelyunknown[2],thoughithasbeensuggested thationchannelsarelikelytargets[99].
CO-RMs have been shown to exert powerful cytoprotective andanti-inflammatoryactions,likelyattributableinparttotheir effectsonmetalcarbonylsthatcaninfluenceoxidativestress,redox signalingandcellularrespiration[98].Thesemoleculeshavebeen showntobeeffectiveinawiderangeofanimalmodels, includ-ingpancreatitis,hepaticischemia-reperfusion,colitis, cutaneous woundhealing,neuropathicpainandosteoarthritis[98].LikeNO and H2S, CO cansuppressleukocyte adherencetothe vascular
endotheliumandreducepro-inflammatorycytokineexpressionby inhibitingNF-B [100–103].Alsoinlinewiththeactionsofthe othertwomajorgaseousmediators,COhasbeenshowntoexert particularlypotentprotectiveeffectsintheGItract[104].
InhalationofCOisbeingevaluatedasameanstoreduce inflam-mation. Chiang et al. [105]reported that inhaled CO markedly reducedneutrophilinfiltrationintotheperitoneumofmice,and increasedhemeoxygenaseactivity.Theyalsoshowedadramatic shiftintheproductionofpro-resolvingmediators:increased pro-ductionofresolvinD1 andmaresin1,anddecreasedproduction
of leukotriene B4. The phagocytic activity of macrophages was
also significantly enhanced by treatment with CO. This work wasextendedtoamodelofbacterialpneumoniainbaboonsby Dallietal. [106].Theyobserved thattheplasma and leukocyte lipidmediatorprofilesinbaboonswithpneumoniawere signifi-cantlyshifted,withareductioninthepro-resolutionphenotype. However, this could be partially reversed by inhalation of CO. CO hasshown therapeutic potentialin animal modelsof acute lunginjury,includingthoseinvolvingendotoxinchallenge, oxida-tivelunginjury,ischemia-reperfusioninjury,pulmonaryfibrosis, ventilator-inducedlunginjury,andlungtransplantation[98,107]. InhaledCOispresentlybeingassessedinclinicaltrialsto deter-mineifitcanimprovelungfunctionandsurvivalinacriticalcare setting.
5. Futuredirections
ThediscoverythatagassuchasNOcouldstimulateanenzyme andcontributetothebodyhomeostasistriggeredare-thinkingof manybiologyconcepts.Asdescribedinthisreview,anothertwo gasotransmitters,COandH2S,havebeendiscoveredthatalso
con-tributetoseveralpathophysiologicalprocesses.Inparticular,H2S
hasbeenshowntosharemanyfeatureswithNO.Thesegasescan contributetoseveraldifferentphasesofresolutionofinflammation as summarized in this review of the relevant literature. How-ever,stillthereareseveralissuesthatneedtobeclarifiedthrough moredetailedand focusedstudies.Increasingtheknowledgeof how these gases contribute to the resolution of inflammation and gettingmore insights intothe cross-talkamong them will clarifythemolecularbasisofresolutionindifferent inflammatory-based diseases allowing the development of new therapeutic approaches.
Disclosures
Drs.WallaceandCirinoarefoundersofAntibeTherapeuticsInc., whichisdevelopinghydrogensulfide-releasingdrugs.
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