Background
Approximately 80 % of primary breast cancers express estrogen receptor α (ER), a nuclear transcription factor that is the product of the
ESR1 gene [
1‐
3]. Aromatase inhibitors and anti-estrogens such as tamoxifen are used to treat these estrogen receptor positive (ER+) cancers, and act to reduce the growth-promoting effects of estrogen. However, 33 % of women treated with tamoxifen see a recurrence in their breast cancer within 5 years, and resistance to all endocrine therapies is common [
4]. Many potential molecular changes could allow breast cancer cells to continue growth in the absence of estrogen, and the acquired drug resistance is incompletely understood. Mutations to the
ESR1 gene [
1,
5], chromatin restructuring [
6], and disruption of many cellular pathways [
7,
8] have been described in endocrine resistant breast cancers, as reviewed [
9‐
11]. We have previously shown that tamoxifen treatment alters DNA methylation of various genes involved in cell growth in breast cancer cell lines [
12].
Despite their insensitivity to tamoxifen, most resistant breast cancers still express ER [
9]. Resistant cell lines developed in previous experiments were primarily ER+, with exceptions such as the triple negative, TMX 2-28 [
13,
14]. The characterization of our novel cell lines began with measuring the intracellular protein levels of three receptors: ER, progesterone receptor (PR), and human epidermal growth factor 2 (HER2). The intracellular protein levels of ER, PR, and HER2 in breast cancers often advise the treatment choices for the patient. Cancers with decreased levels of PR are more likely to become resistant to endocrine therapy [
15], and HER2 overexpression is known to promote tamoxifen resistance via cross-talk with ER [
16].
Due to the frequency of acquired endocrine resistance and the lack of treatment for this type of breast cancer, many researchers have studied biological mechanisms that predict acquired endocrine resistance. One recently recognized marker of acquired endocrine resistance is E74-liked factor 5 (ELF5) [
8]. ELF5 is a transcription factor involved in keratinocyte and mammary gland differentiation, especially alveolar differentiation, milk secretion and ductal morphogenesis [
17]. ELF5 is frequently down-regulated in breast cancer [
18]. A recent study identified ELF5 as playing a potential role in the development of tamoxifen resistance that is positively correlated to ER expression [
8]. Progesterone has been shown to induce ELF5 levels, and the action of progesterone on ELF5 expression in alveolar development is thought to be via a paracrine mechanism [
19]. For the above reasons, ELF5 expression in each of our tamoxifen-selected novel cell lines was compared to that in MCF-7, as well as that in the previously established tamoxifen-resistant cell lines: TMX2-4, TMX2-11 and TMX2-28.
Additionally, expression and methylation of docking protein 7 (DOK7) were measured due to the protein’s potential role as a tumor suppressor. A recent study of identical twins with differential breast cancer statuses identified DOK7 gene methylation as an indicator of breast cancer risk [
20]. Increased expression of DOK7 has been correlated with longer patient survival time, and decreased expression of the gene has been seen in patients with recurrent cancers [
21]. To our knowledge, a role of DOK7 in tamoxifen resistance has not been previously studied.
In this study, we sought to characterize tamoxifen-selected MCF-7 derivative cell lines based on changes in the expression and methylation of DOK7, ELF5, and ERα, and the protein levels of ERα, PR, and HER2. To extend these findings clinically, we examined the relationship between hormone receptor status, anti-estrogen treatment and promoter methylation of DOK7 and ELF5 in a set of matched primary and recurrent breast tumors from 24 women.
Authors’ contributions
KFA, KEW, BTP, LMF and EPB designed the study. KEW and KDC performed the 6-month tamoxifen treatment. LMF generated the clonal cell lines. LMF, EPB, ECP and LOS cultured the cell lines, isolated DNA and RNA, and performed RT qPCR. EPB performed the pyrosequencing assays. RMJ and CNO performed the IHC and scored the protein expression. KEW provided the HM450 data for cell lines and human tumors. KFA, EPB and LMF drafted the manuscript. All authors reviewed and edited the manuscript. All authors read and approved the final manuscript.