Estrogen Up-regulates ATBF1 Transcription but Causes Its Protein Degradation in Estrogen Receptor-α-positive Breast Cancer Cells

Abstract

The proper level of estrogen-estrogen receptor (ER) signaling is important for the maintenance of epithelial homeostasis in the breast. In a previous study we demonstrated that ATBF1, which has been suggested as a tumor suppressor in breast cancer, inhibited estrogen-mediated cell proliferation by selectively competing with AIB1 for binding to the ER. However, the expression of ATBF1 mRNA was shown to positively correlate with ER in breast cancer specimens. We, therefore, examined whether estrogen regulates ATBF1. We demonstrated that estrogen up-regulated the transcription of ATBF1, which was mediated by the direct binding of the ER onto the ATBF1 promoter, and that a half-estrogen-responsive element in the ATBF1 promoter was essential for ER direct binding. Furthermore, we found that estrogen at lower levels increased, but at higher levels decreased the expression of ATBF1 protein, which involved the degradation of ATBF1 protein by the estrogen-responsive proteasome system. ATBF1 protein levels fluctuate with estrogen levels. Although lower levels of estrogen increased ATBF1 protein expression, ATBF1 still inhibited cell proliferation caused by lower levels of estrogen. These findings not only reveal an autoregulatory feedback loop between ATBF1 and estrogen-ER signaling but also suggest that ATBF1 plays a role in both the maintenance of breast epithelial homeostasis and breast tumorigenesis caused by elevated estrogen levels.Extensive studies have established that estrogen is important for the development of the mammary gland and is positively associated with the initiation and development of breast cancer (1). Estrogen exerts its effect through the activation of estrogen receptors, mainly the ␣ subtype (ER␣, 2 or ER hereafter), a member of the nuclear receptor superfamily of transcription factors, which in turn regulates the expression of a variety of genes involved in multiple physiopathological processes including proliferation, differentiation, and apoptosis (2). Tight regulation of estrogen-ER signaling is important for the maintenance of epithelial homeostasis in the breast. In normal mammary epithelial cells, the level and function of ER fluctuates during the menstrual cycle in response to cyclical changes in estrogen (3, 4). ER regulates the expression of a variety of genes through the recruitment of coactivators and corepressors, which are often oncoproteins and tumor suppressors, respectively (5). Estrogen, ER, and coregulators are present in limiting amounts in normal cells. In breast cancer, however, their levels and/or structure are often interrupted, which contributes to the initiation and development of breast cancer (5).The AT-motif binding factor 1 (ATBF1) is a transcription factor composed of 3703 amino acid residues. It was originally identified for its direct binding to and negative regulation of the ␣-fetoprotein gene in a hepatoma cell line (6). In recent studies we identified ATBF1 as a strong candidate for the 16q22 tumor suppressor gene in human cancers, including prostate and breast ...

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Upregulation of ATBF1 by progesterone-PR signaling and its functional implication in mammary epithelial cells

“…T-47D cells were seeded in 24-well plates at 1 Â 10 5 cells per well. On the next day, cells were transfected with the ATBF1 promoter reporter plasmids constructed in our previous study [16] in regular medium. One day after transfection, cells were treated with 100 nM Pg for 12 h or 36 h, and luciferase activities were measured using the dual luciferase reporter assay system (Promega, Madison, WI).…”

“…Promoter-luciferase reporter assay was performed as previously described [16,20]. T-47D cells were seeded in 24-well plates at 1 Â 10 5 cells per well.…”

“…In addition, there appears to be an autoregulatory negative feedback loop between ATBF1 and estrogen-ER signaling, as the transcription of ATBF1 is increased by estrogen-ER while ATBF1 protein is degraded by the estrogen-responsive ubiquitin E3 ligase EFP [15,16]. PR is a well known target gene of estrogen-ER signaling [17], and ERa mediates progestin-induced tumor growth by interacting with PR [18].…”