BackgroundInformation | TheUPSTATEcolorimetricSTAR(SignalTransductionAssayReaction)ELISAkitisasolidphasesandwichenzymelinkedimmunosorbentassaythatprovidesafast,sensitivemethodtodetectspecificlevelsofsignalingtargetsinwholecellextracts.TheAktplateiscoatedwithaspecificmousemonoclonalAktcaptureantibodyonthemicrowellsofthe96-wellclearplate.SamplelysateorthestandardincludedinthekitareincubatedinthemicrowellsallowingAktantigentobecapturedintheplatewells.Theplateisthenwashedtoremoveanynon-boundunspecificmaterial.Thewellsarethenincubatedwithaspecificrabbitanti-phospho-Akt(Ser473)antibodytodetectthecapturedAktontheplatewellthatisphosphorylatedonSer473.TheunbounddetectionantibodyiswashedawayfollowedbyincubationwithanHRP-conjugatedanti-rabbitantibody.Thisallowsforasensitiveenzymaticdetectionofthesample.AftertheadditionofTMBsubstrateandstopsolutiontheabsorbanceismeasuredat450nmusingaplatereader.Theentireassaytakeslessthan5hourstocompletewithminimalhands-ontime.Manyofthereagentsaresuppliedinready-touseformulationsforeaseofuse.Thekitalsoincludesastandardthatisrunasbothapositivecontrolandtodevelopastandardlineofdetection.
II.AktBACKGROUND
Akt(ProteinKinaseB),aSer/Thrkinase,isamajorknowneffecterofthePI3KinasepathwayandisinvolvedinmultiplesignalingpathwaysthatrelatetomanyBIOLOGicalprocessesincludingglucosemetabolism,cellsurvival/apoptosis,cellcyclecontrol,angiogenesis,differentiation,andcellgrowthandproliferation.Aktisactivatedbyligand-stimulatedgrowthfactorreceptorsignalingthatactivatesthePhosphatidylinositol3-kinase(PI3Kinase,PI3K)dependentmanner.PKBisoneofthemostfrequentlyhyperactivatedproteinkinasesinhumancancers.InmammalsthreeisoformsofAkt(Akt1/PKBα,Akt2/PKBβ,andAkt3/PKBγ)exists.Theyexhibitahighdegreeofhomology,butdifferslightlyinthelocalizationoftheirregulatoryphosphorylationsites.Akt1isthepredominantisoformthatisinmosttissuesandisthoughttohaveadominantroleingrowth,survival,embryonicdevelopment,andpost-natalsurvival.Additionally,Akt1/PKBαisrequiredforADIpocytedifferentiation,whereasAkt2/PKBβandAkt3/PKBγarenot.Akt2isstronglycorrelatedwiththeregulationofglucosehomeostasisandisthepredominantPKBisoformexpressedininsulin-responsivetissueswheredefectiveAkt2resultsinimpairedinsulin-stimulatedglucoseuptakeinmuscleandadipocytes.Akt3isabundantinbraintissue.EachAktisoformiscomposedofthreefunctionallydistinctregions:anN-terminalPleckstrinHomology(PH)domainthatprovidesalipid-bindingmoduletodirectAkttoPIP3atthecellmembraneasaresultofPI3Kinase(PI3K)activitythatisnecessaryforitsactivation,acentralcatalyticdomain,andaC-terminalhydrophobicmotif.TheactivationandregulationofAKTisdependentonadualregulatorymechanismthatrequiresbothitstranslocationtotheplasmamembraneanddualphosphorylationonThr308andSer473byPDK1andtheTORC2complex,respectively.Thisisaccomplishedbythegenerationandbuild-upofPIP3byPI3KinconjunctionwithreducedPTENfunctionthatresultsintheactivationofPDK1(3-phosphoinositide-dependentproteinkinase-1)andtherecruitmentofAKTtotheplasmamembranebydirectinteractionwithitsPHdomain.TheactivatedPDK1theninturnphosphorylatesAktonThr308initsactivationloop.ThisphosphorylationisnecessaryandsufficientforAKTactivation;howevermaximalactivationrequirestheadditionalphosphorylationatSer473.Anotherkinasecomplex,recentlyidentifiedasTORC2,whichiscomposedofthemTOR,Rictor,GL,Sin1,andProtor1and2(previouslyreferredtoastheunidentifiedkinasePDK2),phosphorylatesAKTonSer473initshydrophobicmotif.AfterAktisactivated,itisliberatedfromtheplasmamembraneandreleasedintothecytosolandnucleuswhereitinteractswithandphosphorylatesmultiplebindingpartners.Ithasbeenshowntophosphorylateover40substrates,someofwhichareactivatedbyphosphorylationsuchasmTOR,AS160,PRAS40(Thr246),IKK,MDM2,NFκB,andTSC1&2andsomethatareinhibitedbyitsphosphorylationthatincludeBad(Ser136),GSK3(Ser9),FKHR(Ser256),andCaspase9(Ser196). JustasthesetwoAKTphosphorylationsitesarephosphorylatedbytwoseparatemechanisms,theyarebothregulatedbytwodifferentphosphatases.ThedephosphorylationandsubsequentinactivationofAKTismuchlessunderstoodthanitsactivation.Itwasnotuntiltherecentdiscoveryoftwonewphosphatases,PHLPP1andPHLPP2(PHdomainleucine-richrepeatproteinphosphatase)thattheprocesswasbetterelucidated.DephosphorylationofAKTatSer473,butnotatThr308,wasfoundtobemediatedbyoneorbothofthePHLPPfamilyofphosphatases.Anothermorepromiscuousphosphatase,PP2A,isnowbelievedtodephosphorylateAKTonthePDK1phosphorylationsiteatThr308.TogetherthesephosphataseshelpregulatetheactivityofAKT.WithAKThavingsomanysignalingpartnersanditsinvolvementinmultiplesignalingpathwaysandcellularmechanisms,itisnowonderwhyAKTissowellstudiedandahighlysoughtafterdrugtarget. |