Oestrogens and selective oestrogen receptor (ER) modulators regulate EGF receptor gene expression through human ER alpha and beta subtypes via an Sp1 site

Through the analysis of the transient expression of the luciferase reporter gene in HeLa cells, an evaluation has been made of the transcriptional activity of oestrogens and of selective oestrogen receptor (ER) modulators (SERMs), mediated by the alpha and beta isoforms of the ER, on the epidermal growth factor receptor gene promoter. Oestrogen-activated ERbeta presents a lower transcriptional activity compared with ERalpha, probably due to structural differences in the AF-1 regions of the receptors. Also SERMs induce different responses depending on the receptor isoform bound. Indeed, the phyto-oestrogens, genistein and daidzein, act as weak agonists of the oestrogenic activity via ERalpha, but as full agonists when bound to ERbeta. The synthetic SERM 4OH-tamoxifen, on the other hand, displays an opposite behaviour since it exerts a full agonist action through ERalpha, but acts as a full antagonist via ERbeta. As we have previously shown for ERalpha, an ERbeta/Sp1 functional synergism has also been highlighted, by means of gel mobility shift assays. Moreover, our results show that the sensitivity of target tissues to oestrogens and SERMs can be affected by coexpression of ERs, depending on the formation of appropriate levels of homo- and heterodimers, thus providing a useful approach to predict the effects of hormonal treatment.