Ligand-, cell-, and estrogen receptor subtype (α/β)-dependent activation at GC-rich (Sp1) promoter elements

17β-Estradiol (E2) induces expression of several genes via estrogen receptor (ER)-Sp1 protein interactions with GC-rich promoter elements in which Sp1 but not ER binds DNA. This study reports the ligand- and cell context-dependent ER(α)/Sp1 and ER(β)/Sp1 action using an E2-responsive construct (pSp1) containing a GC-rich promoter. Both ER(α) and ER(β) proteins physically interact with Sp1 (coimmunoprecipitation) and preferentially bind to the C-terminal region of this protein in pull-down assays. E2- and antiestrogen-dependent transcriptional activation of ER(α)/Sp1 was observed in MCF-7, MDA-MB-231, and LnCaP cells, but not in HeLa cells. E2 did not affect or significantly decrease ER(β)/Sp1 action, and antiestrogens had minimal effects in the same 4 cell lines. Exchange of activation function-1 (AF-1) domains of ER subtypes gave chimeric ER(α/β) (AF1α/AF-2β) and ERβ/α (AF-1β/AF-2α) proteins that resembled wild-type ER (α or β) in terms of physical association with Sp1 protein. Transcriptional activation studies with chimeric ER(β/α) and ER(α/β) showed that only ER(α/β), can activate transcription from an Sp1 element, not ER(β/α). This indicates that the AF-1 domain from ER(α) is responsible for activation at an Sp1 element, independent of ER subtype context. In order to further characterize this observation, deletion constructs in the AF-1 domain of both ER(α) and ER(α/β) were made, and transactivation studies indicated that the region between amino acids 79 and 117 of this domain is important for activation at an Sp1 element.