title: Stimulation of steroid receptor coactivator-3 (SRC-3) gene overexpression by a positive regulatory loop of E2F1 and SRC-3.
abstract: Steroid receptor coactivator 3 (SRC-3, amplified in breast cancer 1, or ACTR) is a transcriptional coactivator for nuclear receptors and certain other transcription factors such as E2F1. SRC-3 is overexpressed in breast cancers, and its overexpression is sufficient to cause mammary carcinomas in vivo. However, the mechanisms controlling endogenous SRC-3 overexpression are unknown. In this study, we identified the first exon and analyzed the 5' regulatory sequence of the SRC-3 gene. We found three evolutionarily conserved regions (ECRs) in the 5' SRC-3 regulatory sequence, and ECR2 makes a major contribution to the SRC-3 promoter activity. The ECR2 region (bp -250/+350) contains several specificity protein 1 (Sp1) binding sites and two E2F1 binding sites. We show that E2F1 can significantly activate the ECR2 promoter activity in a dose-dependent manner. Furthermore, overexpression of E2F1 significantly increases the promoter activity of the endogenous SRC-3 gene and boosts SRC-3 expression in vivo. Conversely, knockdown of E2F1 reduces SRC-3 expression. We demonstrate that the mechanism of E2F1 activity on SRC-3 promoter is independent of the E2F binding sites but relies on the Sp1 element located at bp +150/+160. Sp1, E2F1, and SRC-3 are specifically recruited to this Sp1 site and the interaction between E2F1 and Sp1 is essential to modulate SRC-3 expression. Moreover, SRC-3 coactivates E2F1 activity and thereby additively stimulates a further increase in SRC-3 expression in vivo. These results suggest that in cells with hyperactive E2F1, such as the case encountered in breast cancer cells, there is a positive feedback regulatory loop consisting of E2F1 and SRC-3 to maintain high levels of SRC-3 and E2F1 activity, which may partially interpret the oncogenic role of SRC-3 overexpression.
authors.affiliation: Department of Molecular and Cellular Biology, Baylor College of Medicine, 1 Baylor Plaza, Houston, Texas 77030, USA.
authors.forename: Paola
authors.initials: P
authors.lastname: Mussi
authors.forename: Chundong
authors.initials: C
authors.lastname: Yu
authors.forename: Bert W
authors.initials: BW
authors.lastname: O'Malley
authors.forename: Jianming
authors.initials: J
authors.lastname: Xu
mesh_terms: D020816:Amino Acid Motifs; D001665:Binding Sites / Q000235:genetics; D002478:Cells, Cultured; D050687:E2F1 Transcription Factor / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D005091:Exons; D018628:Gene Dosage; D005786:Gene Expression Regulation; D051548:Histone Acetyltransferases / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D009154:Mutation; D056921:Nuclear Receptor Coactivator 3; D011401:Promoter Regions, Genetic / Q000187:drug effects; D034741:RNA, Small Interfering / Q000494:pharmacology; D016329:Sp1 Transcription Factor / Q000378:metabolism; D015534:Trans-Activators / Q000235:genetics* / Q000378:metabolism*
chemical_list: D050687:E2F1 Transcription Factor; D034741:RNA, Small Interfering; D016329:Sp1 Transcription Factor; D015534:Trans-Activators; D051548:Histone Acetyltransferases; C538866:NCOA3 protein, human; D056921:Nuclear Receptor Coactivator 3