Oestrogen- and anti-oestrogen-regulated genes in human breast cancer

Alterations in estrogen signalling pathways upon acquisition of anthracycline resistance in breast tumor cells

Intrinsic or acquired drug resistance is a major impediment to the successful treatment of women with breast cancer using chemotherapy. We have observed that MCF-7 breast tumor cells selected for resistance to doxorubicin or epirubicin (MCF-7_DOX2 and MCF-7_EPI cells, respectively) exhibited increased expression of several members of the aldo-keto reductase (AKR) gene family (in particular AKR1C3 and AKR1B10) relative to control MCF-7_CC cells selected by propagation in the absence of drug. Normal cellular roles for the AKRs include the promotion of estrogen (E2) synthesis from estrone (E1) and the hydroxylation and detoxification of exogenous xenobiotics such as anthracycline chemotherapy drugs. While hydroxylation of anthracyclines strongly attenuates their cytotoxicity, it is unclear whether the enhanced AKR expression in the above anthracycline-resistant cells promotes E2 synthesis and/or alterations in E2 signalling pathways and whether such changes contribute to enhanced survival and anthracycline resistance. To determine the role of AKRs and E2 pathways in doxorubicin resistance, we examined changes in the expression of E2-related genes and proteins upon acquisition of doxorubicin resistance. We also assessed the effects of AKR overexpression or downregulation or the effects of activators or inhibitors of E2-dependent pathways on previously acquired resistance to doxorubicin. In this study we observed that the enhanced AKR expression upon acquisition of anthracycline resistance was, in fact, associated with enhanced E2 production. However, the expression of estrogen receptor α (ERα) was reduced by 2- to 5-fold at the gene transcript level and 2- to 20-fold at the protein level upon acquisition of anthracycline resistance. This was accompanied by an even stronger reduction in ERα phosphorylation and activity, including highly suppressed expression of two proteins under E2-dependent control (Bcl-2 and cyclin D1). The diminished Bcl-2 and cyclin D1 expression would be expected to reduce the growth rate of the cells, a hypothesis which was confirmed in subsequent cell proliferation experiments. AKR1C3 or AKR1B10 overexpression alone had no effect on doxorubicin sensitivity in MCF-7_CC cells, while siRNA-mediated knockdown of AKR1C3 and/or AKR1B10 expression had no significant effect on sensitivity to doxorubicin in MCF-7_DOX2 or MCF-7_EPI cells. This suggested that enhanced or reduced AKR expression/activity is insufficient to confer anthracycline resistance or sensitivity to breast tumor cells, respectively. Rather, it would appear that AKR overexpression acts in concert with other proteins to confer anthracycline resistance, including reduced E2-dependent expression of both an important apoptosis inhibitor (Bcl-2) and a key protein associated with activation of cell cycle-dependent kinases (cyclin D1).

The Wilms' tumor suppressor WT1 induces estrogen-independent growth and anti-estrogen insensitivity in ER-positive breast cancer MCF7 cells

A switch from estrogen-dependent to estrogen-independent growth is a critical step in malignant progression of breast cancer and is a major problem in endocrine therapy. However, the molecular mechanisms underlying this switch remain poorly understood. The Wilms' tumor suppressor gene, wt1, encodes a zinc finger protein WT1 that functions as a transcription regulator. High levels of the WT1 expression have been associated with malignancy of breast cancer. The goal of this study was to investigate the function of WT1 in malignant progression of breast cancer. We found that the high passage ER-positive breast cancer MCF7H cells expressed EGFR, HER2 and WT1 at higher levels compared to the low passage MCF7L cells. MCF7H cells responded weakly to estrogen stimulation, grew rapidly in the absence of estrogen and were insensitive to anti-estrogens such as ICI 182,780 and 4-hydroxy-tamoxifen (4OH-TAM). We also established stable cell lines from the low passage MCF7L cells to constitutively express exogenous WT1 and found elevated levels of EGFR and HER2 expression, estrogen-independent growth and anti-estrogen insensitivity in WT1-transfected MCF7L cells. These results suggested WT1 promotes estrogen-independent growth and anti-estrogen resistance in ER-positive breast cancer cells presumably through activation of the signaling pathways mediated by the members of EGFR family.

Induction of Hsp22 (HspB8) by estrogen and the metalloestrogen cadmium in estrogen receptor-positive breast cancer cells

Estrogen (E2) plays a critical role in the etiology and progression of human breast cancer. The estrogenic response is complex and not completely understood, including in terms of the involved responsive genes. Here we show that Hsp22 (synonyms: HspB8, E2lG1, H11), a member of the small heat shock protein (sHSP) superfamily, was induced by E2 in estrogen receptor-positive MCF-7 breast cancer cells, resulting in an elevated Hsp22 protein level, whereas it was not induced in estrogen receptor-negative MDA-MB-231 cells. This induction was prevented by the pure anti-estrogen ICI182780 (faslodex, fulvestrant), whereas tamoxifen, a substance with mixed estrogenic and antiestrogenic properties, had no major inhibitory effect on this induction, nor did it induce Hsp22 on its own. Cadmium (Cd) is an environmental pollutant with estrogenic properties (metalloestrogen) that has been implicated in breast cancer. Treatment of MCF-7 cells with Cd also resulted in induction of Hsp22, and this induction was also inhibited by ICI182780. In live MCF-7 cells, Hsp22 interacted at the level of dimers with Hsp27, a related sHSP, as was shown by quantitative fluorescence resonance energy transfer measurements. In cytosolic extracts of MCF-7 cells, most of the E2- and Cd-induced Hsp22 was incorporated into high-molecular mass complexes. In part, Hsp22 and Hsp27 were components of distinct populations of these complexes. Finally, candidate elements in the Hsp22 promoter were identified by sequence analysis that could account for the induction of Hsp22 by E2 and Cd. Taken together, Hsp22 induction represents a new aspect of the estrogenic response with potential significance for the biology of estrogen receptor-positive breast cancer cells.

The role of estrogen receptors in breast cancer metastasis

It has long been appreciated that the estrogen receptor (ER) plays an important role in the biology of breast cancer. It is an accepted factor predicting favorable disease outcome and treatment response, and as such is generally considered to represent a "good" prognostic marker in breast cancer. In this review we present data suggesting that the ER may also play a pivotal role in the metastatic behavior of breast cancer, and present an argument that the up-regulation of ER and/or the selection of specific ER mutations are early events important for facilitating tumor progression. Thus, ER could serve dual roles in breast cancer, acting as a "bad" prognostic marker later in the disease process.

Estrogen receptor mediated inhibition of cancer cell invasion and motility: an overview

In this overview of results from our laboratory, we address the question of the role of estrogens during early steps of metastasis, involving cell invasion through the basement membrane and cell motility. The motility of several estrogen receptor (ER) positive breast (MCF7, T47D) and ovarian (BG-1, SKOV3, PEO4) cancer cell lines was studied using a modified Boyden chamber assay. We observed, in all cases, estradiol induced inhibition of cancer cell invasion and motility. A similar inhibitory effect of estradiol was found when the wild-type ER alpha was stably transfected in the ER-negative MDA-MB231 cells and 3Y1-Ad12 cancer cells. The mechanism of this inhibitory effect is unknown. In ovarian cancer, however, it may involve intermediary proteins such as fibulin-1, an extracellular matrix protein that strongly interacts with fibronectin and which is induced by estrogen and secreted by ovarian cancer cells. We conclude that estrogens in ER-positive breast and ovarian cancers have a dual effect, since they stimulate tumor growth but inhibit invasion and motility. This may be consistent with the good initial prognostic value of ER-positive breast cancers compared to ER negative breast cancers noted in several clinical studies.