The expression of two human estrogen receptor-α (hERα) isoforms has been characterized within estrogen receptor-α-positive breast cancer cell lines such as MCF7: the full-length hERα66 and the N terminally deleted hERα46, which is devoid of activation function (AF)-1. Although hERα66 is known to mediate the mitogenic effects that estrogens have on MCF7 cells, the exact function of hERα46 in these cells remains undefined. Here we show that, during MCF7 cell growth, hERα46 is mainly expressed in the nucleus at relatively low levels, whereas hERα66 accumulates in the nucleus. When cells reach confluence, the situation reverses, with hERα46 accumulating within the nucleus. Although hERα46 expression remains rather stable during an estrogen-induced cell cycle, its overexpression in proliferating MCF7 cells provokes a cell-cycle arrest in G0/G1 phases. To gain further details on the influence of hERα46 on cell growth, we used PC12 estrogen receptor-α-negative cell line, in which stable transfection of hERα66 but not hERα46 allows estrogens to behave as mitogens. We next demonstrate that, in MCF7 cells, overexpression of hERα46 inhibits the hERα66-mediated estrogenic induction of all AF-1-sensitive reporters: c-fos and cyclin D1 as well as estrogen-responsive element-driven reporters. Our data indicate that this inhibition occurs likely through functional competitions between both isoforms. In summary, hERα46 antagonizes the proliferative action of hERα66 in MCF7 cells in part by inhibiting hERα66 AF-1 activity.
RNF8 identified as a co-activator of estrogen receptor α promotes cell growth in breast cancer
