Oestrogen receptors pathways to oestrogen responsive elements:: The transactivation function-1 acts as the keystone of oestrogen receptor (ER)β-mediated transcriptional repression of ERα

Oestrogen receptors (ER)alpha and beta modify the expression of genes involved in cell growth, proliferation and differentiation through binding to oestrogen response elements (EREs) located in a number of gene promoters. Transient transfection of different luciferase reporter vectors 3xEREs-Vit, 2xEREs-tk and ERE-C3 showed that the transactivation capacity of both ER subtypes was influenced by 1) the nature of the inducer (oestradiol (EA phyto- and anti-oestrogen (AE)), 2) the structure of the promoter (nucleotidic sequence, number of ERE, length of the promoter sequence) and 3) the cell line (containing endogenous ER (MCF-7) or in which ER was stably expressed (MDA-MB-23 1 -HE-5 (ER alpha+) or MDA-MB-23 1 -HERB (ERP+)). ER subtype did not display the same efficacy on the different constructions in the presence of E-2 and of AE according to the cell (e.g. in MCF-7 cells: tk>>Vit>>C3 similar to 0 while in MDA-MB-231 cells: Vit>>tk similar to C3). E2 response was higher in MCF-7 cells, probably due to higher ER expression level (maximal at 10(-10) M instead of 10(-8) M for E-2 in HE-5 cells). Finally, the same ligand could exert opposite activities on the same promoter according to the ER isoform expressed: in the MDA-MB-231 cells, AE acted as inducers of the C3 promoter via ERP whereas ER alpha/AE complexes down-regulated this promoter. Approximately 70% of breast tumours express ER and most turnour cells coexpress both ER isotypes. Thus, different types of ER dimers can be formed in such turnours (ERP or ER alpha homodimers or ER alpha-/ER beta heterodimers). We therefore studied the influence of the coexistence of the two ERs on the ligand-induced transcriptional process following transient transfection of ER alpha in ER beta(+) cells, and inversely ERP in ER alpha+ cells. ERP-transfection inhibited the E-2- and genistein-induced ER(x-dependent transcription on all promoters in all cell lines except C3 in MCF-7; this inhibitory effect was lost following transfection of ERP deleted of its AF-1 (ERP-AF-2). These results suggest that the dominant negative properties of ERP are mainly due to its AF-I function. Interestingly, transfection of an ERP-AF-2 construct into MCF-7 cells potentiated the transcription inhibitory capacity of 4-OH-tamoxifen (OHT) on the Vit and tk promoters. Thus, (1) OHT exerts an agonistic activity through the AF-I function of ER and (2) expression of ERP in breast cancer cells seems to favour the AE treatment. Contrary to ERP, ER alpha-transfection had little effect on ERP transactivation capacity in HERB cells. Finally, the ratio ER alpha/ER beta constitutes one decisive parameters to orientate the transcriptional mechanism of a target gene in the presence of agonist as well as of antagonist ligands. (C) 2007 Elsevier Ltd. All rights reserved.