FRA-1 expression level modulates regulation of activator protein-1 activity by estradiol in breast cancer cells.

We compared the effect of estradiol on activator protein-1 (AP-1) activity in estrogen receptor positive (ER alpha+) and estrogen receptor negative (ER alpha-) human breast cancer cell lines transiently transfected with the AP-1-responsive reporter plasmid AP-1-TK-CAT and an ER alpha expression vector. While estradiol increased AP-1 activity in the ER alpha+ cell lines MCF7, ZR75.1, and T47D, it decreased (MDA-MB231 and BT20 cells) or had no significant effect (MDA-MB435 cells) on AP-1-mediated transcription in ER alpha- cells. Estradiol also inhibited AP-1 activity in ER alpha-MDA-MB231 cells stably transfected with ER alpha and in which ER alpha levels are close to those found in MCF7. Use of ER alpha mutant expression vectors demonstrated that the DNA-binding domain of ER alpha was needed for stimulation or inhibition of AP-1 activity by estradiol but suggested that ER alpha binding to estrogen-responsive elements was not required for these effects. Changes in regulation paralleled quantitative and qualitative changes in protein binding to AP-1 sites, as demonstrated by gel shift assay: protein binding was greater and DNA/protein complexes migrated faster for ER alpha--than for ER alpha+ cells. In fact, by Northern blot, a high level of Fra-1 mRNA was found in BT20 and MDA-MB231 cells as compared with ER alpha+ cells, and MDA-MB435 cells showed an intermediary level of expression. The differential expression of Fra-1 in MCF7 and MDA-MB231 cells was confirmed at the protein level by supershift experiments. In addition, overexpression of Fra-1 in MCF7 cells decreased the positive effect of estradiol while inhibition of Fra-1 expression in MDA-MB231 cells, by transient transfection of the Fra-1 antisense expression vector, abolished the negative effect of the hormone. In conclusion, we demonstrated that ER alpha- breast cancer cell lines differ from ER+ cells by a high level of AP-1 DNA-binding activity due, at least in part, to high Fra-1 constitutive expression. High Fra-1 concentration is crucial for the negative regulation of AP-1 activity by estradiol and thus may take part in estradiol-induced inhibition of cell proliferation in ER alpha- breast cancer cells transfected with ER alpha expression construct.