Endocrine Abstracts

Epidemiological data suggests a correlation between phytoestrogen consumption and a reduced incidence of breast cancer. Current in vitro data is ambiguous, suggesting differential proliferative effects of phytoestrogens depending on ER expression status and exogenous estradiol levels. This study aims to characterise the growth inhibitory effects of the isoflavones genistein and daidzein using a panel of human breast cancer cell lines.
Flow cytometry revealed low levels of ER-alpha and ER-beta expression in MDA-MB-231 cells. In contrast, comparatively high ER expression was detected in MCF-7 and T47-D cultures. ER-alpha was the predominant receptor subtype in MCF-7 cells, whilst T47-D expressed equal protein levels of ER-alpha and ER-beta. The cell lines were treated with 0.1-50 micromolar isoflavone in the absence of exogenous estrogens. Despite low ER expression, genistein and daidzein inhibited MDA-MB-231 proliferation in a dose-dependent manner, suggesting the existence of ER-independent signalling mechanisms. Growth inhibition of MDA-MB-231 correlated with G₂/M phase accumulation. Both genistein and daidzein stimulated the proliferation of T47-D cultures, with maximal effect at 0.1-10 micromolar.
MCF-7 cell proliferation was stimulated by daidzein in a dose-dependent manner, whilst exhibiting a biphasic response to genistein. Transient transfection of MCF-7 with ERE and AP-1 response elements revealed phytoestrogen signalling via ER-dependent pathways. ERE and AP-1 transcriptional activity was increased in excess of three-fold in isoflavone-treated MCF-7 cells, as determined by luciferase reporter gene assays. Furthermore, expression of the downstream ER proteins cyclin D1, cathepsin D and progesterone receptor was up-regulated. Our data suggests that dietary phytoestrogens may facilitate the growth of breast cancer cells containing functional ER protein, via the activation of ERE and AP-1 transcription.