The Human Estrogen Receptor-α Isoform hERα46 Antagonizes the Proliferative Influence of hERα66 in MCF7 Breast Cancer Cells

Abstract

The expression of two human estrogen receptor-alpha (hERalpha) isoforms has been characterized within estrogen receptor-alpha-positive breast cancer cell lines such as MCF7: the full-length hERalpha66 and the N terminally deleted hERalpha46, which is devoid of activation function (AF)-1. Although hERalpha66 is known to mediate the mitogenic effects that estrogens have on MCF7 cells, the exact function of hERalpha46 in these cells remains undefined. Here we show that, during MCF7 cell growth, hERalpha46 is mainly expressed in the nucleus at relatively low levels, whereas hERalpha66 accumulates in the nucleus. When cells reach confluence, the situation reverses, with hERalpha46 accumulating within the nucleus. Although hERalpha46 expression remains rather stable during an estrogen-induced cell cycle, its overexpression in proliferating MCF7 cells provokes a cell-cycle arrest in G(0)/G(1) phases. To gain further details on the influence of hERalpha46 on cell growth, we used PC12 estrogen receptor-alpha-negative cell line, in which stable transfection of hERalpha66 but not hERalpha46 allows estrogens to behave as mitogens. We next demonstrate that, in MCF7 cells, overexpression of hERalpha46 inhibits the hERalpha66-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, hERalpha46 antagonizes the proliferative action of hERalpha66 in MCF7 cells in part by inhibiting hERalpha66 AF-1 activity.

BARX2 and estrogen receptor-α (ESR1) coordinately regulate the production of alternatively spliced ESR1 isoforms and control breast cancer cell growth and invasion

“…In contrast, increased ESR1-46 expression did not induce AIG. Moreover, ESR1-46 can compete with ESR1-66 in a variety of contexts and inhibit monolayer growth in response to estrogen (Penot et al, 2005). Thus, the increased ESR1-46/ESR1-66 ratio in BARX2-overexpressing cells may seem at odds with increased AIG.…”

“…ESR1-46 is expressed in breast cancer cell lines, mammary gland and osteoblasts (Fasco et al, 2000;Flouriot et al, 2000). In osteoblasts, competition between ESR1-46 and ESR1-66 for DNA binding leads to reduced cell proliferation (Denger et al, 2001), and in other cell types, expression of ESR1-46 can inhibit the growth stimulatory effect of ESR1-66 (Penot et al, 2005).…”

Estrogen receptor (ER) expression and function in the pregnant human myometrium: estradiol via ERα activates ERK1/2 signaling in term myometrium

“…We performed further testing of the specificity of ERa immunodetection, since truncated ERa isoforms of various sizes have been reported previously in a number of human cells and tissues following the use of the ERa F10 and HC-20 antibodies (Flouriot et al 2000, Russell et al 2000, Horvath et al 2002, Figtree et al 2003, Penot et al 2005. By RNA interference, we reduced the expression of endogenous or overexpressed ERa protein in hTERT-HM myometrial smooth muscle cells, followed by ERa immunoblotting (Fig.…”

Estrogen Receptor Isoform-specific Regulation of the Retinoblastoma-binding Protein 1 (RBBP1) Gene

“…hER␣46 is found to be an effective E2-inducible transcription factor and exhibits a strong inhibitory effect on wild type ER␣ (hER␣66) transactivation, possibly because of interference with coregulator binding to the hER␣46/hER␣66 heterodimer (36,53). Furthermore, hER␣46 form is demonstrated to be naturally expressed in human primary trabecular osteoblasts (37).…”