The AH Receptor in Biology and Toxicology

555 Pages·2011·0.46 MB·English

by  Raimo Pohjanvirta

52 OLEROFONECHAPERTEINSOPRINAHRFUNCTION
this sensitizationfectefhas been observed with other AHR cess, under both basal and ligand-stimulated conditions
ligands, a study on the fectef of XAP2 in the context of [33,60,65,69].Theseationsobservsupporttheconceptthat
ferentdif classes of ligands, for example, antagonists and XAP2can alter AHR-mediatedtranscription.First,bylim-
elnovveselectiAHRligands,hasyettobeundertaken.The itingtranslocation,XAP2canbeedclassi(cid:147)asaninhibitorof
increasedligandvenessresponsiinthepresenceofXAP2has AHR action.,verselyContheutionredistribofunstimulated
been edattribut to enhanced receptor stability rather than a AHRfromthenuclearcompartmentintothecytoplasmwill
changeinreceptorconformation.Thishypothesis wassub- resultinatedveleoplasmiccytvelsleofAHR,whichislikely
sequentlycon(cid:147)rmedinmammaliansystems,whichdemon- to increase the ligand vitysensitiin anyvengicell. Despite
strated higher svelle of cytoplasmic AHR in COS-1 cells theseferencesdifofinterpretation,XAP2clearlyin(cid:148)uences
transfected with XAP2 when compared to cells not trans- the localization of the AHR and a number of potential
fectedwithXAP2[12,59,62].Furthermore,theincreasein mechanisms ehav been proposed to explain this phenome-
AHR protein elicited by XAP2 was demonstrated to be non. The immunophilins present in the glucocorticoid
speci(cid:147)c for XAP2, since immunophilins such as FKBP52 receptor ehav also been wnsho to in(cid:148)uence receptor local-
failed to mediate a vepositi in(cid:148)uence on AHR svelle [46]. ization and transport through a mechanism that vesvolin
Datastexithat discredit the notion that XAP2 can increase the peptidylprolyl isomerase domain of FKBP52, forming
ligandvenessresponsithroughaconformationalchange[63]. a transport complex with microtubules to facilitate TP-A
AHR ligand ssveneresponsi has been wnsho to exhibit a dependent nuclear utionredistrib [71]. A complementary
marked degree of species ,dependency with mouse AHR mechanism volvingin XAP2 and AHR has largely been
beingvefold(cid:147)morevesensitithanitshumanhomologtomost discounted,duetotheationobservthatchemicalmicrotubule
butnotallAHRagonists[25,64].,verweHochimericmouse/ disruption fails to ertlyov in(cid:148)uence ligand-induced AHR
humanreceptorsfailtoexchangeligand,vitysensitisuggest- translocation [33]. It has also been speculated that binding
ingthatligandvenessresponsiisinherenttothereceptoritself of XAP2 to the AHR cytoplasmic xcomple may result in
ratherthanaconsequenceofconformationalchangesfacil- physicalmaskingoraconformationalchangeofthenuclear
itated by the XAP2 chaperone complex [63, 65]. Interest- localization signal, thus ventingpre translocation. Such a
,inglytheinitialpuri(cid:147)cationofAHRproteinwashamperedin mechanism seems ,unlikely since an antibody directed
partbyitsinstabilityandacutevitysensititoheatstress.This against the nuclear localization signal is not blocked when
vitysensiti to temperature can be whatsome ercomeov by AHRisboundtoXAP2[33].ranslocationToflargemolec-
incorporationofXAP2intotheAHRcorecomplex,support- ular weight and multimeric escomplex containing nuclear
ing a role of XAP2 in stabilizing the AHR complex [62]. localization signals across the nuclear opevelen is highly
Furthervestigationinhasindicatedthatthestabilizingfectef regulated, vingvolin multiple adaptor proteins [72]. The
ofXAP2,incombinationwithHSP90andp23,maybedueto AHRcomplexhasbeenwnshotointeractwithcomponents
XAP2protectingAHRfromproteolyticadationdegrthrough of this transport machinery and suggest a mechanism by
inhibitionofAHRubiquitinationbyE3ligases,suchasthat which XAP2 can in(cid:148)uence AHR compartmentalization
observed with carboxyl terminus of Hsc70-interacting pro- [33, 73]. Immunoprecipitation and GST npull-dow studies
tein(CHIP),althoughtheroleofCHIPinAHRadationdegr using the mouse AHR demonstrate a physical interaction
has only been demonstrated inan in vitro context[66(cid:133)68]. with importin b , a member of the karyopherin family of
In addition to stabilizing the AHR, studies utilizing adaptor proteins. Importin a interacts with basic arginine-
(cid:148)uorescently tagged AHR identi(cid:147)ed XAP2 as a factor in- rich nuclear localization signals such as that carried by the
vedvol in the cytoplasmic retention of the AHR. Although AHR.Subsequentinteractionofthiscomplexwithimportin
viouslypre considered primarily cytoplasmic in the unli- b andGDP-boundRanfacilitatestransportacrossthenuclear
gandedstate,theAHRhasactuallybeenwnshotoundergo velope.enInclusionofXAP2intheAHRcomplexappearsto
dynamic nucleocytoplasmic shuttling in the absence of a diminishtheassociationofAHR,throughitsnuclearlocal-
ligand,resultinginafusedifoplasmiccytandnuclearstaining ization signal, with importin b and thus is likely to limit
whenvisualizedbyimmuno(cid:148)uorescenceortheuseof(cid:148)uo- transportintothenucleus,whichwouldexplaintheobserved
rescently tagged AHR constructs [33, 69]. reatmentT with cytoplasmicretentionofAHR[33].ThenatureoftheXAP2-
AHR ligands, for example, TCDD, prompts a marked and mediated inhibition of importin b binding has yet to be
rapidionredistributofAHRintothenucleus,typicallywithin elucidated, ,but as mentioned ,earlier it does not seem to
1h, consistent with its role as a transcription .factor Intra- evolvinblockade ofthe nuclear localization sequence.
cellularementmovoftheAHRthroughveconstitutinucleo- In conclusion, XAP2 represents a whatsome enigmatic
oplasmiccyt shuttling or ligand-induced translocation is regulator of AHR function. First, XAP2 increases AHR
broughtaboutthroughtheactionofastrongbipartitenuclear protein ,stability prompting an apparent enhancement of
localization and export signals within the AHR [29, 70]. ligand,vitysensitiwhich leads to increased AHR-mediated
XAP2xpressionereovstudiesinCOS-1cellsehavwnshothat transcriptionofappropriatetargetgeneswingfolloexposure
XAP2 can markedly diminish the AHR translocation pro- toAHRagonists.,verweHoincorporationofXAP2intothe
NCOMPOSITIOOFTHEDEDUNLIGANAHRCORECOMPLEX 53
TABLE3.1 XAP2-InteractingProteins
ProteinsReportedtoInteractwithXAP2
Protein Comments References
HepatitisBvirusXprotein InteractionwithXAP2identi(cid:147)edthroughyeasttwo-hybrid.assayXAP2 [76(cid:133)78,80,81]
inhibitsationvtransactipotentialofHBxandmayin(cid:148)uencehepatitisB
pathology
Heatshockprotein90 XAP2undergoesevcooperatibindingtobothAHRandHSP90toformthe [45(cid:133)47]
matureandstable9ScytoplasmicAHRcorecomplex
Arylhydrocarbonreceptor AssociationofXAP2intotheAHRcomplexwithHSP90andp23stabilizes [12,45(cid:133)48,53,59]
AHRandorsfavasmiccytoplretentionofAHRtolimitAHR-dependent
transcription.,erselyvConstabilizationofAHRalsoappearstoincrease
theligandvitysensitiofAHR,wingallogreatertranscriptionofAHR-
dependentgettargenes
Glucocorticoidreceptor BindingofXAP2toGRrequiresHSP90andservestolimitGRvenessresponsi [84]
toglucocorticoidligands,possiblythroughstabilizationandoplasmiccyt
retention
Peroxisome-proliferator SimilartoGR,theassociationofXAP2withARPP a andHSP90limits [83]
atedvactireceptoralpha ationvtransactiofARPP a targetgenessionexpre
Thyroidhormonereceptor b 1 XAP2enhancesthetranscriptionalvityactiofTR b 1utbnotTR b 2throughan [86,87]
(TR b 1) unidenti(cid:147)edmechanism
Phosphodiesterase2A(PDE2A) InteractionwithXAPincreasesthephosphodiesterasevityactiofPDE2Aand [97,102]
limitsthecAMP-dependenttranslocationandationvactiofAHR
Phosphodiesterase4A5 AssociationwithXAP2diminishesthephosphodiesterasevityactiofPDE4A5 [98,102]
(PDE4A5) andthusmayateveleintracellularvelsleofcAMP
vinSurvi(BRIC5) BindingtoXAP2stabilizesvinsurvitoatevelethecellularantiapoptotic [88,102]
threshold
Epstein(cid:133)Barrvirus-encoded AssociationbetweenA-3EBNandXAP2in(cid:148)uencesligand-mediated [82]
nuclearantigen-3 ationvactiofAHR
AHR xcomple results in a marked retention of AHR in are not matched by their Ahr -null counterparts [75]. These
the oplasmiccyt compartment, probably through disruption non-AHRXAP2escomplexandtheirbiologicalimplications
of the association of AHR with the nuclear transport ma- are outlinedinableT3.1.
.chinery Such retention obviously sequesters AHR away TheerydiscovofXAP2wastheresultofyeasttwo-hybrid
fromthesiteofitsprimaryfunctionasatranscription,factor assays performed to identify proteins that interact with the
that is, the nucleus. wHo these two opposing phenomena HBV X protein [76]. HBx is a promiscuous indirect tran-
canbereconciledremainstobeestablished,anditsuggests scriptional atorvacti that appears to be required for viral
that XAP2 regulation of AHR function is a highly ordered replication and thus contributes to the pathology of HBV
and xcompleprocess. infection[77(cid:133)80].HBxisthoughttopromotecirrhosisofthe
verli and hepatocarcinoma in -infectedHBV vidualsindi
through multiple mechanisms [81]. Despite the yvelrelati
3.2.4 Insights from XAP2 Activity in Other
wloexpression ofXAP2 inhepatocytes, studiesvealrethat
Protein Complexes
XAP2 can abolish HBx-mediated ationvtransacti [76] and
Since the initial cloning of XAP2, its role in the cell has through this mechanism may provide some degree of pro-
predominantlybeenstudiedandcharacterizedinthecontext tection against HBV replication and associated verli dys-
ofitsassociationwithAHR;,verwehoanincreasingnumber function. XAP2 is also reported to interact with another
ofadditionalinteractionsehavrecentlybeendemonstrated, virally encoded protein, Epstein(cid:133)Barr virus nuclear anti-
whichsuggeststhatXAP2hasphysiologicalrolesindepen- gen-3A-3)(EBN[82].Althoughthisassociationisreported
dentofAHR.Evidenceforsuchrolesisprovidedby-velde toin(cid:148)uenceligand-dependentationvactiofAHR,thefectef
opmental expression studies, which illustrate that XAP2 of XAP2 on-mediatedEBVlymphocyte pathology has not
expressionprecedesthatofAHRandthatXAP2expression been established. XAP2 has been demonstrated to be a
exhibits a more ubiquitous tissue expression pro(cid:147)le than componentofnuclearreceptorxes,compleforexample,GR,
AHR [74]. Furthermore, ablation of XAP2 in homozygous peroxisomeatedvproliferator-actireceptor a AR(PP a ),and
Xap2 nullmiceresultsinembryoniclethalphenotypesthat thyroidhormonereceptors(TR b ).Theseinteractionsrequire
54 OLEROFONECHAPERTEINSOPRINAHRFUNCTION
FIGURE3.3 Homologybetweentheco-chaperoneXAP2andtheimmunophilins.XAP2sharesa
common domain structure with many of the FK506 binding immunophilin family of proteins
consistingofanamino-terminalFKpeptidylprolylisomerasedomainthatdoesnottexhibiFK506
bindingorisomerasevityacticommontotheimmunophilinssuchasFKBP52.Thecarboxyl-terminal
half of XAP2 encompasses 3 (cid:3) TPR protein(cid:133)protein interaction domains, the most C-terminal of
whichfacilitatesitsinteractionwithAHRandHSP90.
binding of XAP2 to HSP90 through the TPR domains and which then diminishes the stabilizing fect.ef,Clearlythese
appeartolimitvenessresponsitoligandinthecaseofGRand interactionsimplicateXAP2asfactorthatmayimpacttumor
ARPP[83,84].ItispossiblethatinthiscontextXAP2may wthgroandprogressionandthusrequiresfurthervestiga-in
mimictheroleofFKBP51,whichiserexpressedovinwNe tion (Fig. 3.3). While no data are currently ailable,av the
orldW primates and in part contributes to their verelati actionofXAP2indeterminingapoptoticsignalingmayehav
vityinsensititoglucocorticoids[85].,verselyConTR b tran- particular ancevrele in the context of AHR. AHR is well
scriptional vityacti is diminished in an isoform-speci(cid:147)c characterized as a mediator of xenobiotic-induced carcino-
fashion wingfollo A-mediatedsiRN wnknockdo of XAP2, genesis through the induction of CYP1A1 and subsequent
with TR b 1 being in(cid:148)uenced while TR b 2 vityacti is unaf- biotransformationofprocarcinogens(e.g.,benzo[ a ]pyrene)
fected. This implies that XAP2 speci(cid:147)cally stabilizes intogenotoxicintermediates.ationvActiofAHRultimately
TR b 1,thusfacilitatingthyroidhormone-mediatedtranscrip- leadstodissociationofXAP2fromtheAHRxcompleandthe
tion[86, 87]. fateofliberatedXAP2remainstobeestablished.Itmaybe
In addition to in(cid:148)uencing immediate ligand-induced speculatedthatonceXAP2isliberated,itmayassociatewith
transcriptional responses, videncee exists to suggest that proteinssuchasRET51orvinsurviandthusmaycontribute
XAP2mayalsofunctionasadeterminateofcellfatethrough wardtothetumorigenicpropertiesofcarcinogenicxenobio-
nontranscriptionalmodesofaction.Forexample,XAP2has tics that bind to the AHR.
beendemonstratedtointeractwithvinsurvi( BRIC5 )through ,RecentlyageneticlinkagehasassociatedXAP2withthe
itsTPR-containingC-terminalend[88,89].Asamemberof occurrence of familial and sporadic isolated pituitary ade-
the inhibitor of the apoptosis family of proteins, vinsurvi nomawithXAP2mutations(49mutationscharacterizedto
serves as a alvsurvi factor by inhibiting the proapoptotic date)foundin15(cid:133)40%ofcases[90(cid:133)93].Inthemajorityof
vityactiofedcleavcaspases3and7.,Interestinglyvinsurviis cases, the mutations resulted in truncated or nonsense ex-
highlyessedexprintumorsandthuslikelycontributestothe pressionofXAP2,withthelossoftheTPRdomain.Sincethe
malignant phenotype. The interaction between XAP2 and TPR domain is the primary site of protein(cid:133)protein interac-
vinsurviappearstoenhancethestabilityofvin,survirender- tionforXAP2,itislikelythatthesefunctionsofXAP2are
ingcellslessvesensititoprogrammedcelldeath.,Recently lostordiminished.Itisunclearbywhichmechanismtheloss
XAP2hasbeenwnshotoassociate invivo withthetyrosine of XAP2 function contributes to the speci(cid:147)c velopdmenet
receptorkinaseRET51[89].RET51servesasthereceptorfor and progression of pituitary adenomas. In vitro studies
glialcellvedline-derineurotrophicfamilyofwthgrofactors examiningthefectefofpathogenicXAP2mutationsvealre
and has the capacity to transduce proapoptotic stimuli, no disruption in the context of RET51 binding. Similar
resulting in cell death. The XAP2/RET51 association has studies examining the vsuinrvi protein ehav yet to be per-
been suggested to promote apoptosis by a mechanism in- formed; therefore, the role of mutated XAP2 and its inter-
vingvol the sequestration of XAP2 ayaw from vin,survi action with these cell fate factors with regard to pituitary
NCOMPOSITIOOFTHEDEDUNLIGANAHRCORECOMPLEX 55
pathogenesisremaintobeestablished.Otherpotentialme- physiological signi(cid:147)cance of its interaction with XAP2.
chanismsmayvolveintheendocrinenatureofthepituitary ,Intriguinglyationveleofintracellular,cAMPaswouldoccur
gland. As highlighted ,viopusrely XAP2 can speci(cid:147)cally through inhibition of PDE4A5, such as with rolipram or
enhance TR b 1-mediated gene expression, with increased reduced phosphodiesterase ,vityactihas been wnsho to at-
expressionofhypothalamicTSHbeingtheprimeexample. tenuate in(cid:148)ammatory responses and is currently an veacti
Also,thefectefofXAP2ondiminishedGRsenvitsityimay areaofresearch.Itcouldbespeculatedthatsomeoftheanti-
together promote an endocrine disrupting fectef on the in(cid:148)ammatory fectsef may arise from liberation of XAP2
hypothalamic(cid:133)pituitary(cid:133)adrenalaxis,thusprovidingapro- from the AHR xcomple (which has also been wnsho to
tumorigenichormonalvironmenent.,ClearlyXAP2mustbe exhibitanti-in(cid:148)ammatoryproperties)wingfolloligandacti-
providing a tumor suppressor function, which is lost upon ation.v This free XAP2 might then subsequently bind to
mutation,and thusadditionalstudiesarerequired to delin- PDE4A5 and attenuate its catalytic,vityactithus atingvele
eate the mechanisms by which wild-type XAP2 ertsex its .cAMP,verweHo such a mechanism remains to be estab-
suppressor.vaitctiy,Currentlynolinkageanalysishasbeen lished.Thesecondisotypeofphosphodiesterasedemonstrat-
performed or detected with other nonpituitary tumors, but ed to bind XAP2 is PDEA2. Similar to PDE4A5, PDEA2
vengithealencvpereofhepatitisB-associatedhepaticcancer binding is mediated through the XAP2 TPR domain; w-ho
andtheinteractionbetweenHBxandXAP2,itislikelythat ,vereunlikePDE4A5,PDEA2utilizesaferentdifbindingsite
this interaction hassome impacton hepatic tumorigenesis. thatesvolvintheGAF-Bcyclicnucleotidebindingsite[97].
,UnfortunatelyXAP2nullanimalmodelsarenotailablaveto ,InterestinglydespiteutilizingtheGAF-Bdomain,bindingof
exploretheroleofXAP2incarcinogenesisduetoembryonic XAP2 does not attenuate the catalytic vityacti of PDEA2,
lethality [75].,verweHothe velopmedent of tissue-speci(cid:147)c which is in contrast to PDE4A5, suggesting that XAP2
conditionalknockout,mutant,orhypomorphicmodelsmay interaction serves two distinct roles with gardre to these
proveinformveati[94]. phosphodiesteraseisotypes.UnlikethePDE4A5interaction,
AnothermechanismbywhichXAP2maymodulatecel- the binding of XAP2 to PDEA2 has been demonstrated to
lular function is through its association with phosphodies- ertexafunctionalfectefonAHRsignaling[97].Anumberof
terases, enzymes that degrade the second messenger cyclic groupsehavhighlightedthephenomenonofligand-indepen-
nucleotidescAMPand.cGMPPhosphodiesterasesareubiq- dent AHR ationvacti wingfollo exposure to cAMP or the
uitouslyexpressedandtherearenumeroussubtypesencoded adenylatecyclaseatorvactiforskolin,oftenwithcon(cid:148)icting
by ferentdif genes with ferentdif modes of regulation and ationsobserv that are attributed to species, cell, or tissue
nucleotide speci(cid:147)city[95,96].wo-hybridTassays andsub- ferencesdif[99(cid:133)101].DataindicatethatcAMPoritsanalogs
sequentGSTwnpull-dostudiesehavidenti(cid:147)edandalidatedv promotenucleartranslocationofAHRandsubsequenttran-
twophosphodiesterases,,namelyPDE2AandPDE4A5,that scriptionofAHRtargetgenes;,verwehoforcedexpressionof
interactwithXAP2[97,98].BothPDE2AandPDE4A5were PDEA2 through transient transfection results in increased
wnshotoassociatewithXAP2throughtheC-terminalTPR oplasmiccyt retention of AHR under basal, TCDD-, and
domain.,Interestinglydespitesharingthesamebindingsite cAMP-inducedstates[97,99,102].Undertheseconditions,
onXAP2,thedomainsontheverespectiphosphodiesterase the reduction in AHR vityacti is attributed to increased
isotypesthatfacilitatetheinteractionareferent,difsuggest- binding of XAP2 to PDEA2. Such data may indicate that
ingtwoindependentmodesofXAP2binding[97,98].Inthe PDEA2 forms a xcomple with AHR mediated through a
caseofPDE4A5,theinteractionishighlyspeci(cid:147)cforXAP2 mutual interaction with XAP2, an interaction that may be
since there is no videncee for any association with the further stabilized through the ementvolvin of HSP90. The
immunophilins, for example, FKBP52 [98]. ,Similarly presenceofPDEA2asacomponentoftheAHRxcomplehas
amongthe PDE4 isotypes,PDE4A5appearstobetheonly beenspeculatedtodiminishthelocalcAMPconcentrationin
onethatxescomplewithXAP2.Thefunctionalancesigni(cid:147)c thevicinityofAHR,thusdecreasingitscapacitytoundergo
oftheXAP2(cid:133)PDE4A5interactionhasyettobeelucidatedin nucleartranslocation.Suchamechanismmayprovidescope
thecontextofAHRsignaling,butformationofthiscomplex forthesubtleregulationofAHRvityactiorwallocontextual
hasprofoundfectsefonPDE4A5function.Thephosphodi- ationvacti or inhibition of AHR when multiple signaling
esterasecatalyticvityactiofPDE4A5isgreatlydiminished stimuli areertedex(Fig. 3.4).
by60%anditsvitysensititothephosphodiesteraseinhibitor Thus,inconclusion,XAP2appearstoehavedvolvetobea
rolipramisenhancedwhenboundtoXAP2[98].Inaddition, satilever pleiotropic factor with the capacity to modulate
XAP2 attenuates the capacity of protein kinase A to phos- severdibiologicalvitiesactithroughnumerousproteininter-
phorylate PDE4A5. Data are limited regarding the physio- actions. Evidence suggests that some of these interactions
logicalroleofPDE4A5,butithasbeenimplicatedinsperm may in(cid:148)uence the vityacti and biological functions of the
,motility apoptosis, in(cid:148)ammation, and adipocyte metabo- mostwidelyexaminedXAP2bindingpartnertheAHR.Itis
lism. venGi the lack of information at this time regarding likelythatfurthervestigationinwillidentifyadditionalelnov
the role of PDE4A5, it is (cid:147)cultdif to ascertain the precise XAP2-interactingproteinsandprovidealuablevinformation