Original Article | Published:

OCT-4: a novel estrogen receptor-α collaborator that promotes tamoxifen resistance in breast cancer cells

Oncogene volume 35, pages 57225734 (03 November 2016) | Download Citation

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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-tamr breast cancer cells. Notably, Oct-4 levels were highly elevated in MCF-7-tamr cells, 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.

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Acknowledgements

We thank Dr Peter Yau, Biotechnology Core, UIUC, for contributions to the mass-spectrometry analyses. This work was supported by grants from the NIH 5RO1 CA18119 (BSK), The Breast Cancer Research Foundation (BSK) and Juvenile Diabetes Research Foundation (GW). SB conceived of, designed and executed the study, performed experiments, data analyses and interpretation and wrote the manuscript. JDS conceived the study, performed computational analysis and contributed to experimental data. GW and SJ contributed to experimental data. BSK conceived and designed the study, performed data analyses and interpretation and wrote the manuscript.

Author information

Affiliations

  1. Departments of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL, USA

    • S Bhatt
    •  & J D Stender
  2. Agilent Technologies, Cedar Creek, TX, USA

    • S Joshi
  3. Research Division, Joslin Diabetes Center, Harvard Medical School, Boston, MA, USA

    • G Wu
  4. Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, IL, USA

    • B S Katzenellenbogen

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Competing interests

The authors declare no conflict of interest.

Corresponding author

Correspondence to B S Katzenellenbogen.

Supplementary information

Glossary

CoIP

Co-immunoprecipitation

Ub

ubiquitin

E2

estradiol

TOT

tamoxifen

ER

estrogen receptor

Skp2

S-phase Kinase-associated Protein-2

p38MAPK

p38-Mitogen Activated Protein Kinase

NFATc4

Nuclear Factor Activated T-cell 4

siRNA

short interfering RNA

Ad

Adenovirus

ERE

estrogen response element.

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DOI

https://doi.org/10.1038/onc.2016.105

Supplementary Information accompanies this paper on the Oncogene website (http://www.nature.com/onc)

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