Translational cell study exploring the role of estrogen receptor β expression as a predictive and/or prognostic factor in breast cancer patients

e22185 Background: We have reported that positive expression of Estrogen receptor β (ERβ) was associated with better prognosis in the early breast cancer patients treated with adjuvant tamoxifen monotherapy (J Clin Oncol. 2008). In addition, this was also true in the ERα-negative/PR-negative/Her-2 negative patients. We explored the biological impact of ERβ in breast cancer cell lines to determine whether these observations were due to its prognostic power or predictive power of response to the therapy. Methods: Since MCF-7 cell was ERβ-negative ERα-positive cell line, we established two stable clones of MCF-7 by introducing ERβ expression vector (β-clone 1, β-clone 2) as the model of ERβ-positive ERα-positive breast cancer. MDA-MB 231 cell was used as ERβ-positive triple-negative cell line. These cells were subjected to proliferation, expression and functional analysis. Results: In western blotting, both β-clone 1 and clone 2 showed decreased expression of PR and Her-2 than parent MCF-7, although there were no differences in ERα expression. Expression of ERβ decreased estradiol (E2) induced proliferation ability and rate of cells in S-phase cycle. PPT (ERα-specific agonist) and DPN (ERβ-specific agonist) did not show any difference in response, and IC 50 for 4 OH-tamoxifen and fulvestrant did not differ among MCF-7, β-clone 1 and clone 2 (0.05–0.1 μM). Whereas, cell death due to deprivation of E2 from 1nM to 1pM was more frequently observed in ERβ-expressing clones than in parent MCF-7 cell. These cell deaths did not involve standard apoptosis pathway with caspase-3/7 activation and PARP cleavage. E2, DPN and PPT did not affect the proliferation of ERβ-positive triple negative MDA-MB 231 cell, and IC 50 for 4-OH tamoxifen was too high (8 μM) to be achieved in clinical pharmacological dose. Conclusions: From our cell study, better prognosis of ERβ-positive breast cancer patient who treated with adjuvant tamoxifen is mainly due to its own favorable biological behavior. However, this prognostic impact may include the favorable response to the treatment, when we use estrogen-deprivation therapy such as aromatase inhibitors (AIs). Additional clinical study in AI users would be required to address this issue. No significant financial relationships to disclose.