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 ERα/Sp1 and ERβ/Sp1 action using an E2-responsive construct (pSp1) containing a GC-rich promoter. Both ERα and ERβ 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 ERα/Sp1 was observed in MCF-7, MDA-MB-231, and LnCaP cells, but not in HeLa cells. E2 did not affect or significantly decrease ERβ/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 ERα/β(AF-1α/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 ERβ/α and ERα/β showed that only ERα/β can activate transcription from an Sp1 element, not ERβ/α. This indicates that the AF-1 domain from ERα 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 ERα and ERα/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.

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