title: Ligands specify estrogen receptor alpha nuclear localization and degradation.
abstract: BACKGROUND
  The estrogen receptor alpha (ERα) is found predominately in the nucleus, both in hormone stimulated and untreated cells. Intracellular distribution of the ERα changes in the presence of agonists but the impact of different antiestrogens on the fate of ERα is a matter of debate.
  
  
  RESULTS
  A MCF-7 cell line stably expressing GFP-tagged human ERα (SK19 cell line) was created to examine the localization of ligand-bound GFP-ERα. We combined digitonin-based cell fractionation analyses with fluorescence and immuno-electron microscopy to determine the intracellular distribution of ligand-bound ERα and/or GFP-ERα.Using fluorescence- and electron microscopy we demonstrate that both endogenous ERα and GFP-ERα form numerous nuclear focal accumulations upon addition of agonist, 17β-estradiol (E2), and pure antagonists (selective estrogen regulator disruptor; SERD), ICI 182,780 or RU58,668, while in the presence of partial antagonists (selective estrogen regulator modulator; SERM), 4-hydroxytamoxifen (OHT) or RU39,411, diffuse nuclear staining persisted.Digitonin based cell fractionation analyses confirmed that endogenous ERα and GFP-ERα predominantly reside in the nuclear fraction. Overall ERα protein levels were reduced after estradiol treatment. In the presence of SERMs ERα was stabilized in the nuclear soluble fraction, while in the presence of SERDs protein levels decreased drastically and the remaining ERα was largely found in a nuclear insoluble fraction. mRNA levels of ESR1 were reduced compared to untreated cells in the presence of all ligands tested, including E2. E2 and SERDs induced ERα degradation occurred in distinct nuclear foci composed of ERα and the proteasome providing a simple explanation for ERα sequestration in the nucleus.
  
  
  CONCLUSIONS
  Our results indicate that chemical structure of ligands directly affect the nuclear fate and protein turnover of the estrogen receptor alpha independently of their impact on transcription. These findings provide a molecular basis for the selection of antiestrogen compounds issue from pharmacological studies aimed at improving treatment of breast cancer.
authors.affiliation: Université de Toulouse; UPS; Laboratoire de Biologie Moléculaire Eucaryote; F-31062 Toulouse, France.
authors.forename: Silvia
authors.initials: S
authors.lastname: Kocanova
authors.forename: Mahta
authors.initials: M
authors.lastname: Mazaheri
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Caze-Subra
authors.forename: Kerstin
authors.initials: K
authors.lastname: Bystricky
mesh_terms: D045744:Cell Line, Tumor; D004072:Digitonin / Q000737:chemistry; D004958:Estradiol / Q000031:analogs & derivatives / Q000494:pharmacology; D047628:Estrogen Receptor alpha / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D000077267:Fulvestrant; D049452:Green Fluorescent Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008024:Ligands; D016253:Microscopy, Immunoelectron; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D013629:Tamoxifen / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: C506487:ESR1 protein, human; D047628:Estrogen Receptor alpha; D008024:Ligands; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D013629:Tamoxifen; D049452:Green Fluorescent Proteins; C016601:afimoxifene; D000077267:Fulvestrant; D004958:Estradiol; D046988:Proteasome Endopeptidase Complex; D004072:Digitonin