Estrogen Receptor α Roles in Breast Cancer Chemoresistance

The estrogen receptor α (ERα) plays an important role in the progression of breast cancer. About 65% of the human breast cancers were estrogen dependent and express ER α (Puhalla et al., 2012; Xu et al., 2012). Chemotherapy is a critically important treatment choice for most breast cancer patients, which has been shown to substantially improve disease-free survival and overall survival, but chemotherapy often fails to cure the disease due to unexpected drug resistance (Karroum et al., 2012; Natarajan et al., 2012). In recent years, accumulative data from clinical trials and laboratory trials suggested that ERα status might also affect the efficacy of chemotherapy. Specifically, it has been observed that some chemotherapeutic agents may be less effective in patients with ERα+ tumors than those with ERαtumors (Precht et al., 2010; Bailey et al., 2012; Lips et al., 2012). These findings indicate that ERα status may play an important role in determining the sensitivity of breast tumors to chemotherapy. Drug resistance is one of the major obstacles limiting the success of breast cancer chemotherapy. In this field, inherent or acquired chemotherapy resistance, which can include development of resistance to multiple drugs, is a frequent phenomenon in breast cancer cells. Forty percent of breast cancer patients with surgery and 80% of breast cancer patients with unresectable disease have poor response to chemotherapy (Petit et al., 2010; Osako et al., 2012). Several mechanisms of drug resistance have been examined. Overexpression of a membrane efflux transporter, P-glycoprotein (P-gp) (Shi et al., 2011),


Introduction
The estrogen receptor α (ERα) plays an important role in the progression of breast cancer. About 65% of the human breast cancers were estrogen dependent and express ER α (Puhalla et al., 2012;Xu et al., 2012). Chemotherapy is a critically important treatment choice for most breast cancer patients, which has been shown to substantially improve disease-free survival and overall survival, but chemotherapy often fails to cure the disease due to unexpected drug resistance (Karroum et al., 2012;Natarajan et al., 2012). In recent years, accumulative data from clinical trials and laboratory trials suggested that ERα status might also affect the efficacy of chemotherapy. Specifically, it has been observed that some chemotherapeutic agents may be less effective in patients with ERα+ tumors than those with ERα-tumors (Precht et al., 2010;Lips et al., 2012). These findings indicate that ERα status may play an important role in determining the sensitivity of breast tumors to chemotherapy.
Drug resistance is one of the major obstacles limiting the success of breast cancer chemotherapy. In this field, inherent or acquired chemotherapy resistance, which can include development of resistance to multiple drugs, is a frequent phenomenon in breast cancer cells. Forty percent of breast cancer patients with surgery and 80% of breast cancer patients with unresectable disease have poor response to chemotherapy (Petit et al., 2010;Osako et al., 2012). Several mechanisms of drug resistance have been examined. Overexpression of a membrane efflux transporter, P-glycoprotein (P-gp) (Shi et al., 2011),

Estrogen receptor and its isoforms
Estrogen receptors are ligand-regulated transcription factors; its biological action is mediated by binding to estrogen. Ligand-binding induces conformation changes in the receptor leading to dimerization, protein-DNA interaction, recruitment of co-regulator proteins and other transcription factors and ultimately the formation of the pre-initiation complex. The Estrogen receptors including isoforms ERα and ERβ, both receptors are expressed in breast cancer. ERα and ERβ have a common structural architecture composed of three functional domains including the N-terminal A/B domain, the C or DNAbinding domain and the D/E/F or ligand-binding domain. ERα isoform has attracted a great deal of attention as a key molecule in the progression of breast cancer.
The ERα consists of at least three different variants. ERα66, which was a full-length counterpart divided into six distinct regions (A-F), was the first identified estrogen receptor (Figure 1). The N-terminal domain A/B is the ligand-independent transactivation function (AF-1). Domain C is the receptor dimerization and the binding to specific DNA sequences. The C-terminal domain E/F is the ligand-dependent transactivation (AF-2). In contrast to its full-length counterpart, the truncated ERα46 lacks the transactivation domain AF-1 and encodes a protein with a predicted molecular weight of 46kDa. This ERα36 variant lacks both transactivation domains AF-1 and AF-2 compared to ERα66 and encodes a 36kDa protein.

Estrogen receptor α function
Estrogen receptor, ERα, for 17β-oestradiol, was first identified from rat uteri in 1966. Human ERα is located on human chromosome 6q25, first cloned in 1986. Estrogens are steroid hormones, which is associated with the human female reproductive cycle, interaction with estrogen receptors affect the growth, differentiation, and function of target tissues. Binding of ERα causes a conformational change allowing disassociation from co-repressors and recruitment of co-activator molecules. Estrogen-bound ERα translocation to the nucleus, where they increased transcription of target genes by two different mechanisms, classical pathway where ERα binds to estrogen-response element (ERE) and modulates target genes ( Figure 2) and non-classical pathway where ERα binds to co-activator molecules, such as specificity protein 1 (Sp1), activating protein 1 (AP-1), or nuclear factor kappa b (NF-kB), associated with their recognition sites in enhancer elements and modifies their function.

Significance of ERα in breast cancer diseases
The attempt to define the prognostic and treatment value of ERα expression concerning mainly breast cancer patients treated has already been undertaken by many researchers.
The breast cancer patients with high ERα expression were benefit from endocrine therapy (selective estrogenreceptor modulators or aromatase inhibitors) (Kemp et al., 2011;Geisler et al., 2012;Goncalves et al., 2012;Walker et al., 2012;Walker et al., 2012). Tamoxifen (selective estrogenreceptor modulators) is currently used in both premenopausal and postmenopausal breast cancer patients with ERα positive expression (Fisher et al., 1996;Fleeman et al., 2011). The Early Breast Cancer Trialists Collaborative Group meta-analysis of 194 randomized trials employing adjuvant chemotherapy and endocrine therapy demonstrated the 15-year breast cancer recurrence rate was reduced from 45% to 33% with use of tamoxifen, and reduction of breast cancer mortality by 35% (EBCTCG et al., 2008). This meta-analysis determined that the risk of the breast cancer patients with ERα positive expression was reduction with tamoxifen both in premenopausal and postmenopausal women.
Tamoxifen has shown benefit to women with ERα positive expression breast cancers irrespective of menopausal status. However, a number of studies evaluating the use of aromatase inhibitors as an alternative to tamoxifen in postmenopausal women with ERα positive expression breast cancers, in the past 10 years (Bachelot et al., 2012;Gilani et al., 2012). In contrast to tamoxifen, aromatase inhibitors act by preventing the conversion of androgens to estrogens by blocking the aromatase enzyme and result in subsequent decline in estrogens levels (Saylam et al., 2011). In postmenopausal women aromatase inhibitors agents offer the advantage of modestly improved activity over tamoxifen, and aromatase inhibitors agents do not possess the thrombotic or endometrial cancer risks (Cohen et al., 2008;Coleman et al., 2008).

Resistance to chemotherapy in ERα positive breast cancer
The potential association between expression of ERα in breast tumors and resistance to chemotherapy treatment was first reported about two decades age (Tokuda et al., 2012). In a retrospective study, thirty-four of 45 patients with low or absent ERα expression level had objective responses to chemotherapy, whereas only three of 25 patients with higher ERα expression responses to chemotherapy. Since then, many papers have indicated that expression level of ERα may interfere with the therapeutic efficacy of chemotherapy in breast tumors (Dziadyk et al., 2004;Mutoh et al., 2006;Tabuchi et al., 2009). To identification the role of ERα in breast cancer threatment, the 97 breast cancer patients were studied, including the expression level of Bcl-2, ERα, P53 HER2 and Ki-67 associated with neoadjuvant chemotherapy response. The results showed that ERα negativity is associated with better chemotherapy response (Wang et al., 2009). The similar results were showed in others (Lv et al., 2011;Jeong et al., 2012).
The neoadjuvant chemotherapy permits the assessment of pathologic response of the primary cancer tissues to chemotherapy treatment and provides an early opportunity to alter the agents if the cancer tissues appear chemotherapy resistant. In recently our retrospective study, 118 primary breast tumor patients with neoadjuvant chemotherapy were investigated (Wang et al., 2009). The result showed that ERα is an independent predictive factor for pathologic response to neoadjuvant chemotherapy in primary breast tumors. Another study showed the similar results, preoperational biopsies from 517 patients with locally advanced breast cancer were analyzed for expression of ERα, and these data were compared to the pathological response after preoperative chemotherapy (+/-radiotherapy). The result showed that patients with locally advanced breast cancer, lower ERα had a significantly greater benefit from chemotherapy.
In laboratory study showed that the combination of paclitaxel with exemestane, a non-steroidal aromatase inhibitor that shuts down estrogen synthesis, produced additive antitumor effect in human breast cancer cell lines (Chen et al., 2004). Another study conducted by Wu et al. showed that arzoxifene, a short-acting selective estrogen receptor modulator (SERMs), can inhibit repopulation of ERα MCF-7 breast cancer xenograft tumors when given between courses of chemotherapy (5-fluorouracil or paclitaxel) (Wu et al., 2005). These findings suggest that the combined use of endocrine therapy agents with chemotherapeutic drug might benefit the clinical treatment for ERα breast cancer patients.
ERα may play an important role in determining the sensitivity of breast tumors to chemotherapy treatment, but the mechanisms ERα−mediated drug resistance is not entirely clear. In a recently study, sui et al. stable transfection and expression of ERα result in reduced sensitivity of BCap37 cells to paclitaxel, particularly to paclitaxel-induced apoptotic cell death. The potential mechanisms indicate that several apoptosis associated proteins, including bcl-2 and IκB exhibit differential responses to paclitaxel in BCap37 cells with ER positive expression (Sui et al., 2007). However, there are different opinions in this study. The ERαtransfected Bcap37 cells and natural ERαpositive T47D breast cancer cells were treated using chemotherapeutic agents with or without 17-beta estradiol (E2) pretreatment. The breast cancer cells with ERα positive were decreased in response to chemotherapeutic agents due to the influence of ERαon the growth of breast cancer cells (Jiang et al., 2012). It is important to note that the exact molecular mechanisms by which ERαresistance to chemotherapy treatment remain unclear, even though the known ERαis well documented for their oncogenic roles in cancer development and progression.

ICI 182,780 sensitizes ERα positive breast cancer cells to chemotherapy treatment
The next obvious question is the if down-regulates the expression of ERαin breast cancer cells could sensitizes cancer cells to the treatment of chemotherapy. ICI 182,780(Fulvestrant) is a potent steroidal antiestrogen, which can bind to both estrogen receptor subtypes, as well as for estradiol, are distinct on both ERαand ERβ. ICI 182,780 caused a gradual but dramatic decrease in ERαexpression in the human tissues, but no change ERβexpression (Oliveira et al., 2003) (Sui et al., 2007). Moreover, ICI 182,780 was also reported to changes in Bax and bcl-2 in ERα breast cancer cells. It is thus possible that ICI 182,780 could reduce the expression of ERα in breast cancer cell, which can sensitize ERα breast cancer cells to chemotherapy treatment.

Summary
The past several years have seen significant strides in elucidating the role of ERα in breast cancer drug resistance. It is clear that ERα exerts a negative effect on cancer treatment outcomes by virtue of its effects on anticancer drug.
Data are starting to accumulate defining the role that ERα play in the chemotherapy treatment. Although some studies found enhanced drug toxicity or more favorable tumor response with ICI 182,780 treatment in breast cancer cells with ERα expression, other studies did not substantiate these findings. Further work in this area is clearly needed.
Given this, in the foreseeable future, drug treatment may be guided by individualized genotype databases that can enable customized drug dosing to minimize toxicity and to enhance therapeutic effect. The past several years have provided mounting evidence for expression of ERα in breast cancer and ERα expression frequently correlates with chemotherapy-resistant disease.