Estrogen receptor-α36 is involved in development of acquired tamoxifen resistance via regulating the growth status switch in breast cancer cells

Mol Oncol. 2013 Jun;7(3):611-24. doi: 10.1016/j.molonc.2013.02.001. Epub 2013 Feb 26.

Abstract

Acquired tamoxifen (TAM) resistance limits the therapeutic benefit of TAM in patients with hormone-dependent breast cancer. The switch from estrogen-dependent to growth factor-dependent growth is a critical step in this process. However, the molecular mechanisms underlying this switch remain poorly understood. In this study, we established a TAM resistant cell sub line (MCF-7/TAM) from estrogen receptor-α (ER-α66) positive breast cancer MCF-7 cells by culturing ER-α66-positive MCF-7 cells in medium plus 1 μM TAM over 6 months. MCF-7/TAM cells were then found to exhibit accelerated proliferation rate together with enhanced in vitro migratory and invasive ability. And the estrogen receptor-α36 (ER-α36), a novel 36-kDa variant of ER-α66, was dramatically overexpressed in this in vitro model, compared to the parental MCF-7 cells. Meanwhile, the expression of epidermal growth factor receptor (EGFR) in MCF-7/TAM cells was significantly up-regulated both in mRNA level and protein level, and the expression of ER-α66 was greatly down-regulated oppositely. In the subsequent studies, we overexpressed ER-α36 in MCF-7 cells by stable transfection and found that ER-α36 transfected MCF-7 cells (MCF-7/ER-α36) similarly exhibited decreased sensitivity to TAM, accelerated proliferative rate and enhanced in vitro migratory and invasive ability, compared to empty vector transfected MCF-7 cells (MCF-7/V). Real-time qPCR and Western blotting analysis revealed that MCF-7/ER-α36 cells possessed increased EGFR expression but decreased ER-α66 expression both in mRNA level and protein level, compared to MCF-7/V cells. This change in MCF-7/ER-α36 cells could be reversed by neutralizing anti-ER-α36 antibody treatment. Furthermore, knock-down of ER-α36 expression in MCF-7/TAM cells resulted in reduced proliferation rate together with decreased in vitro migratory and invasive ability. Decreased EGFR mRNA and protein expression as well as increased ER-α66 mRNA expression were also observed in MCF-7/TAM cells with down-regulated ER-α36 expression. In addition, blocking EGFR/ERK signaling in MCF-7/ER-α36 cells could restore the expression of ER-α66 partly, suggesting a regulatory function of EGFR/ERK signaling in down-regulation of ER-α66 expression. In conclusion, our results indicated for the first time a regulatory role of ER-α36 in up-regulation of EGFR expression and down-regulation of ER-α66 expression, which could be an underlying mechanism for the growth status switch in breast tumors that contribute to the generation of acquired TAM resistance. And ER-α36 could be considered a potential new therapeutic target in breast tumors which have acquired resistance to TAM.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antineoplastic Agents, Hormonal / pharmacology*
  • Breast / drug effects*
  • Breast / metabolism
  • Breast / pathology
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / genetics
  • Breast Neoplasms / pathology
  • Cell Movement
  • Cell Proliferation / drug effects
  • Down-Regulation / drug effects
  • Drug Resistance, Neoplasm*
  • ErbB Receptors / genetics
  • Estrogen Receptor alpha / genetics*
  • Female
  • Gene Expression Regulation, Neoplastic / drug effects
  • Humans
  • MCF-7 Cells
  • Tamoxifen / pharmacology*
  • Up-Regulation / drug effects

Substances

  • Antineoplastic Agents, Hormonal
  • Estrogen Receptor alpha
  • estrogen receptor alpha, human
  • Tamoxifen
  • ErbB Receptors