ProjectSummary: Anti-steroidtherapyisstandardofcaretherapyforbothER+breastcancerandforprostatecancer.Both cancersmetastasizepredominantlytobone.Estrogensandandrogensarealsokeymediatorsofnormalbone growth,homeostasis,andmaintenanceoftheosteogenicnicheinbothfemalesandmales.Bothcancers metastasizetotheosteogenic/hematopoieticnicheintrabecularbone.Accordingly,theearlystagebone colonizationofprostateandER+breastcancermayinvolvesimilarosteogeniccell-dependentmechanisms. TheosteoprogenitorandosteoblastsintheosteogenicnicheareregulatedprimarilybyER?andARactionsin bothgenders.Deficienciesthesereceptorsoftentranslateintoseverepathologicalboneconditions.Thus,any anti-steroidtherapiestargetingprostateandER+breastcancerswillalsoinevitablyaffectthe microenvironment,i.e.,theosteogenicnichecells.However,thereisalackofunderstandingofhowanti- steroidtherapiesaffectthebiologyofosteogenicnichecells,andhowthisaffectscancerprogressionand evolutiontotherapeuticresistance.TheZhangandRowleylaboratorieshavebothdevelopednovel3D osteogenicheterotypicalorganoidmodelsthataddresseshumanbreastandprostatecancercellsinteraction withhumanosteogeniccellsrespectively.Moreover,novelintra-iliacarteryinjectionandmousecalvaria intravitalimagingmodelshavepermittedthestudyofdirectinteractionswiththeosteogenicnicheintrabecular boneinvivo.Preliminarydatasuggestsanti-steroidtherapymayresultinarepairphenotypeintheosteogenic nichethatmaypromotecancerprogressionandtherapeuticresistance.Hence,itisourhypothesisthatanti- steroidtherapyaffectstheosteogenicnichetoamorehomeostasis-repairphenotypethatiscancer-promoting. Toaddressthis,weproposetwoSpecificAims.SpecificAim1.Toaddressboneosteogenicniche-cancer interactionsindifferentialsteroidandanti-steroidactionconditionsusingnovel3Dosteogenicorganoid approaches.ThisAimwilladdresstherelativeimportanceofER?,ER?,andARinmediatingestrogen, androgen,andanti-steroidactionsinthegenesisofareactiveosteogenicniche,howitaffectsbreastand prostatecancerbiology,andhowitaltersanti-steroidtherapeuticefficacies.SpecificAim2.Toaddress mechanismsofanti-steroid(estrogenandandrogen)biologyintheosteogenicnicheandhowthisaffects colonyinitiationandprogressionofbreastandprostatecancerinvivo.Usinggeneticallyengineeredmouse models,noveltumortransplantationapproachesandcutting-edgeintravitalmicroscopy,wewillexaminethe impactofanti-steroidtreatmentsontheosteogenicnicheinvivoandhowitleadstoendocrineresistance.The overallgoalofthismulti-PIproposalistoidentifycommonmechanismsofosteogenicnichebiologythataffects theevolutionofbreastandprostatemetastaticprogressionduringanti-steroidtherapy.Thesepathwaysmay representtargetsfornoveltherapeuticapproaches.