title: Estrogen down-regulation of the corepressor N-CoR: mechanism and implications for estrogen derepression of N-CoR-regulated genes.
abstract: The nuclear receptor corepressor N-CoR plays a crucial role in the repressive activity of diverse transcription factors, yet little is known about what regulates its cellular level. We have found that estrogen markedly down-regulates N-CoR protein levels in estrogen receptor (ER)-positive breast cancer cells without affecting N-CoR mRNA levels, whereas levels of the related corepressor SMRT are unaffected. This effect is attributable to estrogen up-regulation of the ubiquitin ligase Siah2, which is a rapid and primary transcriptional response mediated by the ER, and precedes the loss of N-CoR. Treatment with proteasomal inhibitor or with small interfering RNA against Siah2 prevented the down-regulation of N-CoR by estrogen. Furthermore, the expression of 24-hydroxylase, a gene repressed by unliganded vitamin D receptor through its interaction with N-CoR, was up-regulated by estrogen and required Siah2. Our results illustrate a mechanism by which the estrogen-ER complex markedly reduces the level of N-CoR through a process involving the up-regulation of Siah2 and the subsequent targeting of N-CoR for proteasomal degradation. These findings reveal that, although estrogen directly regulates the transcription of many genes, by regulating a gene such as Siah2 it can exert profound "secondary" effects on cellular activity through mechanisms such as targeting regulatory proteins for degradation. This estrogen-evoked down-regulation of N-CoR could have a global derepressive effect on genes whose repression depends on N-CoR and thereby have broad impact on the activity of transcription factors and nuclear receptors whose actions involve N-CoR.
authors.affiliation: Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
authors.forename: Jonna
authors.initials: J
authors.lastname: Frasor
authors.forename: Jeanne M
authors.initials: JM
authors.lastname: Danes
authors.forename: Cory C
authors.initials: CC
authors.lastname: Funk
authors.forename: Benita S
authors.initials: BS
authors.lastname: Katzenellenbogen
mesh_terms: D045744:Cell Line, Tumor; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015536:Down-Regulation; D004967:Estrogens / Q000494:pharmacology / Q000502:physiology*; D005260:Female; D006801:Humans; D009687:Nuclear Proteins / Q000032:analysis / Q000235:genetics* / Q000378:metabolism; D056971:Nuclear Receptor Co-Repressor 1; D012333:RNA, Messenger / Q000032:analysis; D011960:Receptors, Estrogen / Q000378:metabolism; D012097:Repressor Proteins / Q000032:analysis / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics* / Q000502:physiology; D044767:Ubiquitin-Protein Ligases; D015854:Up-Regulation; D000098563:Seven in Absentia Proteins
chemical_list: D004967:Estrogens; C539127:NCOR1 protein, human; D009687:Nuclear Proteins; D056971:Nuclear Receptor Co-Repressor 1; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D012097:Repressor Proteins; D014157:Transcription Factors; D044767:Ubiquitin-Protein Ligases; D000098563:Seven in Absentia Proteins; D003546:Cysteine Endopeptidases