Sp1 is essential for estrogen receptor alpha gene transcription.

The exact molecular mechanisms regulating estrogen receptor (ER)alpha expression in breast tumors are unclear, but studies suggest that the regulation is at least partly transcriptional. We therefore undertook a detailed analysis of ERalpha promoter activity in a number of breast cancer cell lines. We find that the majority of ERalpha promoter activity lies within the first 245bp of the 5'-flanking region of the gene. Three elements essential for full ERalpha promoter transcriptional activity were identified within the -245 to -192bp region in transient transactivation assays using linker-scanner mutation analysis. These three elements include two binding sites for the Sp1 family of transcription factors as well as a non-consensus E box. We show that both Sp1 and Sp3 bind to this region using electrophoretic mobility shift assays. Exogenous expression of Sp1 or Sp3 in Sp1/3-negative Drosophila Schneider SL2 cells results in transactivation of the -245 to +212bp fragment of the ERalpha promoter. These data demonstrate that transcription of ERalpha is dependent upon the expression of members of the Sp1 family.