Inhibition of STAT3 enhances sensitivity to tamoxifen in tamoxifen-resistant breast cancer cells

BMC Cancer. 2021 Aug 18;21(1):931. doi: 10.1186/s12885-021-08641-7.

Abstract

Background: The mechanisms of endocrine resistance are complex, and deregulation of several oncogenic signalling pathways has been proposed. We aimed to investigate the role of the EGFR and Src-mediated STAT3 signalling pathway in tamoxifen-resistant breast cancer cells.

Methods: The ER-positive luminal breast cancer cell lines, MCF-7 and T47D, were used. We have established an MCF-7-derived tamoxifen-resistant cell line (TamR) by long-term culture of MCF-7 cells with 4-hydroxytamoxifen. Cell viability was determined using an MTT assay, and protein expression levels were determined using western blot. Cell cycle and annexin V staining were analysed using flow cytometry.

Results: TamR cells showed decreased expression of estrogen receptor and increased expression of EGFR. TamR cells showed an acceleration of the G1 to S phase transition. The protein expression levels of phosphorylated Src, EGFR (Y845), and STAT3 was increased in TamR cells, while phosphorylated Akt was decreased. The expression of p-STAT3 was enhanced according to exposure time of tamoxifen in T47D cells, suggesting that activation of STAT3 can cause tamoxifen resistance in ER-positive breast cancer cells. Both dasatinib (Src inhibitor) and stattic (STAT3 inhibitor) inhibited cell proliferation and induced apoptosis in TamR cells. However, stattic showed a much stronger effect than dasatinib. Knockdown of STAT3 expression by siRNA had no effect on sensitivity to tamoxifen in MCF-7 cells, while that enhanced sensitivity to tamoxifen in TamR cells. There was not a significant synergistic effect of dasatinib and stattic on cell survival. TamR cells have low nuclear p21(Cip1) expression compared to MCF-7 cells and inhibition of STAT3 increased the expression of nuclear p21(Cip1) in TamR cells.

Conclusions: The EGFR and Src-mediated STAT3 signalling pathway is activated in TamR cells, and inhibition of STAT3 may be a potential target in tamoxifen-resistant breast cancer. An increase in nuclear p21(Cip1) may be a key step in STAT3 inhibitor-induced cell death in TamR cells.

Keywords: Breast cancer; Endocrine resistance; Epidermal growth factor receptor; Signal transducer and activator of transcription protein; Src.

MeSH terms

  • Antineoplastic Agents, Hormonal / pharmacology
  • Apoptosis
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / genetics
  • Breast Neoplasms / metabolism
  • Breast Neoplasms / pathology
  • Cell Cycle
  • Cell Movement
  • Cell Proliferation
  • Cyclic S-Oxides / pharmacology*
  • Drug Resistance, Neoplasm / drug effects*
  • Female
  • Gene Expression Regulation, Neoplastic / drug effects*
  • Humans
  • STAT3 Transcription Factor / antagonists & inhibitors*
  • Tamoxifen / pharmacology*
  • Tumor Cells, Cultured

Substances

  • Antineoplastic Agents, Hormonal
  • Cyclic S-Oxides
  • STAT3 Transcription Factor
  • STAT3 protein, human
  • stattic
  • Tamoxifen