title: OCT-4: a novel estrogen receptor-α collaborator that promotes tamoxifen resistance in breast cancer cells.
abstract: Tamoxifen has shown great success in the treatment of breast cancer; however, long-term treatment can lead to acquired tamoxifen (TOT) resistance and relapse. TOT classically antagonizes estradiol (E2) -dependent breast cancer cell growth, but exerts partial agonist/antagonist behavior on gene expression. Although both E2 and TOT treatment of breast cancer cells results in recruitment of the estrogen receptor (ER) to common and distinct genomic sites, the mechanisms and proteins underlying TOT preferential recruitment of the ER remains poorly defined. To this end, we performed in silico motif-enrichment analyses within the ER-binding peaks in response to E2 or TOT, to identify factors that would specifically recruit ER to genomic binding sites in the presence of TOT as compared to E2. Intriguingly, we found Nkx3-1 and Oct-transcription factor homodimer motifs to be enriched in TOT preferential binding sites and confirmed the critical role of Oct-3/4 (aka Oct-4) in directing ER recruitment to TOT preferential genomic binding sites, by chromatin immunoprecipitation (ChIP) analyses. Further investigation revealed Oct-4 expression to be basally repressed by Nkx3-1 in MCF-7 cells and TOT treatment appeared to elevate Nkx3-1 degradation through a p38MAPK-dependent phosphorylation of the E3 ligase, Skp2 at serine-64 residue, as observed by quantitative mass-spectrometry analyses. Consistently, Oct-4 upon induction by phospho-Ser64-Skp2-mediated proteasomal degradation of Nkx3-1, participated in ER transcriptional complexes along with p38MAPK and Skp2 in a tamoxifen-dependent manner leading to TOT-dependent gene activation and cell proliferation of the TOT-resistant MCF-7-tamrbreast cancer cells. Notably, Oct-4 levels were highly elevated in MCF-7-tamrcells, and appeared critical for their TOT sensitivity in cell proliferation assays. Furthermore, overexpression of Oct-4 enhanced tumor growth in the presence of tamoxifen in mice in vivo. Collectively, our work presents a novel mechanism for tamoxifen-specific gene activation by ER, secondary to its TOT preferential recruitment to genomic sites by specific activation of Oct-4, a phenomenon that appears to underlie tamoxifen resistance in breast cancer cells and in xenograft tumor models, and could be useful in designing therapeutic interventions to improve treatment outcome.
authors.affiliation: Departments of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL, USA.
authors.forename: S
authors.initials: S
authors.lastname: Bhatt
authors.affiliation: Departments of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Stender
authors.affiliation: Agilent Technologies, Cedar Creek, TX, USA.
authors.forename: S
authors.initials: S
authors.lastname: Joshi
authors.affiliation: Research Division, Joslin Diabetes Center, Harvard Medical School, Boston, MA, USA.
authors.forename: G
authors.initials: G
authors.lastname: Wu
authors.affiliation: Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, IL, USA.
authors.forename: B S
authors.initials: BS
authors.lastname: Katzenellenbogen
mesh_terms: D000818:Animals; D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology*; D001665:Binding Sites; D001943:Breast Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000378:metabolism; D004195:Disease Models, Animal; D019008:Drug Resistance, Neoplasm / Q000235:genetics; D047628:Estrogen Receptor alpha / Q000378:metabolism*; D005260:Female; D015870:Gene Expression; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D055785:Gene Knockdown Techniques; D018398:Homeodomain Proteins / Q000378:metabolism; D006801:Humans; D051379:Mice; D008954:Models, Biological; D059372:Nucleotide Motifs; D050814:Octamer Transcription Factor-3 / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding; D020218:Response Elements; D044786:S-Phase Kinase-Associated Proteins / Q000378:metabolism; D015398:Signal Transduction; D013629:Tamoxifen / Q000494:pharmacology*; D014157:Transcription Factors / Q000378:metabolism; D023041:Xenograft Model Antitumor Assays; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
chemical_list: D018931:Antineoplastic Agents, Hormonal; D047628:Estrogen Receptor alpha; D018398:Homeodomain Proteins; C491178:NKX3-1 protein, human; D050814:Octamer Transcription Factor-3; D044786:S-Phase Kinase-Associated Proteins; D014157:Transcription Factors; D013629:Tamoxifen; D048051:p38 Mitogen-Activated Protein Kinases