Transcription factor E2F1 is a potent transactivator of the insulin-like growth factor-I receptor (IGF-IR) gene

Objectives: The insulin-like growth factor-I receptor (IGF-IR) plays an important role in cancer development. The E2F1 transcription factor activates S-phase promoting genes and mediates apoptosis. Microarray analyses of E2F1-induced genes revealed that genes associated with proliferation as well as apoptosis are upregulated by E2F1. Among other candidate genes, DNA microarrays identified the IGF-IR gene as a putative E2F1 target. The aim of this study was to investigate the involvement of E2F1 in regulation of IGF-IR gene transcription. Methods: To examine the potential regulation of IGF-IR gene expression by E2F1, an E2F1 expression vector was transfected into P69 and M12 prostate cancer cell lines, after which IGF-IR levels were measured by Western blots. Transient transfections were used to evaluate IGF-IR promoter activity and chromatin immunoprecipitation (ChIP) assays were employed to assess E2F1-binding to the IGF-IR promoter. Results: Results obtained showed that E2F1 expression induced a significant increment in endogenous IGF-IR levels. ChIP assays showed enhanced E2F1-binding to the IGF-IR promoter in E2F1-expressing cells. Transient coexpression of an E2F1 vector along with an IGF-IR promoter-luciferase reporter resulted in a ∼140-fold increase in IGF-IR promoter activity. Furthermore, deletion and bioinformatic analyses indicate that the ability of E2F1 to stimulate IGF-IR promoter activity was correlated with the number of E2F1 sites in the promoter region. Conclusions: In summary, we provide evidence that E2F1 regulates IGF-IR gene transcription in prostate cancer cells via a mechanism that involves direct binding to specific elements in the proximal IGF-IR promoter.