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Key words: breast cancer, estrogen receptor, Sp1, transcription, USF-1
Summary
The exact molecular mechanisms regulating estrogen receptor [alpha] (ER[alpha]) expression in breast tumors are unclear, but studies suggest that they are partly at the level of transcription. We have focused on the transcription factors that regulate the ER[alpha] minimal promoter, which we have previously shown to reside within the first 245 bp of the 5'flanking region of the gene.Within this region are several elements essential for full ER[alpha] promoter transcriptional activity, including a GC box and an imperfect E box. In earlier studies we demonstrated an essential function for the Sp1 family of transcription factors in the regulation of ER[alpha] expression. We have now identified both USF-1 and ER[alpha] itself as components of a multi-protein complex of transcription factors that interacts at the ER[alpha] minimal promoter and is essential for its full transcriptional activity. Electrophoretic mobility shift assays demonstrated that Sp1 and USF-1, but not ER[alpha], bind directly to the ER[alpha] minimal promoter. We showed by GST pull-down assays that ER[alpha] is able to interact in vitro with USF-1, suggesting, in addition to a possible interaction between ER[alpha] and Sp1, a mechanism whereby ER[alpha] is able to interact with the protein complex. Combined exogenous expression of the components of the complex inMCF-7 breast cancer cells resulted in a synergistic effect on transactivation of the ER[alpha] minimal promoter, suggesting that the importance of the protein complex is in the interactions among the components. Based upon these findings, we propose a possiblemodel for transcription from the ER[alpha] minimal promoter.
Introduction
The levels of estrogen receptor [alpha] (ER[alpha]) observed at any one time in a breast tumor are the result of a combination of many regulatory mechanisms, both at the gene and protein levels. The clinical significance of this expression has long been established [1]. A strong correlation between an ER-negative tumor status and a poorer prognosis has long existed [2]. However, studies have also shown that elevated levels of ER[alpha] in benign breast epithelium is itself a risk factor for progression to invasive breast disease [3, 4]. This is not surprising, since the ER[alpha] is a well known mitogen for breast cancer cells [5], and the ER[alpha] level observed in both hyperplastic lesions and ER-positive tumors...