Cofactor competition between the ligand-bound oestrogen receptor and an intron 1 enhancer leads to oestrogen repression of ERBB2 expression in breastcancer

Overexpression of the ERBB2 proto oncogene in breast tumours, which occurs in 25-30% of patients, correlates with poor prognosis, In oestrogen recepto r (ER) positive breast epithelial cells oestrogens reduce ERBB2 mRNA and pr otein levels, an effect that is reversed in the presence of anti-oestrogens such as tamoxifen and ICI 182780. Our previous studies have shown that the major effect of oestrogen on ERBB2 expression is at the level of transcrip tion and that this is mediated through a region within the EXBB2 first intr on which can act as an oestrogen-suppressible enhancer in ER positive breas t cells. in vitro footprinting of the smallest DNA fragment that retained f ull activity revealed four transcription factor binding sites. We report he re that two of these sites are recognized by AP-2 proteins and the other tw o are bound by a variety of bZIP factors, including CREB and ATF1, with a m ajor complex containing ATFa/JunD. However, by using ER mutants it is clear that repression occurs essentially off the DNA. Indeed, the essential doma in of the ER responsible for repression of the ERBB2 enhancer is a region t ermed AF2 which is required for the ligand-dependent association of non-DNA binding cofactors, We further demonstrate that one of these ER cofactors, SRC-1, can relieve oestrogen repression of the ERBB2 enhancer and conclude that these data fit with a model whereby the ER and the ERBB2 enhancer comp ete for this limiting, non-DNA binding cofactor, Thus, in oestrogenic condi tions SRC-1 preferentially binds to the ER which effectively sequesters it thereby reducing enhancer activity, but in antioestrogenic media the cofact or is released from the ER and is therefore available to activate the EXBB2 enhancer.