The Transcriptional Coactivator DEAD/H Box 5 (DDX5) Gene Is a Target of the Transcription Factor E2F1 Deregulated from the Tumor Suppressor pRB

DEAD/H box 5 (DDX5) serves as a transcriptional coactivator for several transcription factors including E2F1, the primary target of the tumor suppressor pRB. E2F1 physiologically activated by growth stimulation activates growth-related genes and promotes cell proliferation. In contrast, upon loss of pRB function due to oncogenic changes, E2F1 is activated out of restraint by pRB (deregulated E2F1) and stimulates tumor suppressor genes such as ARF, which activates the tumor suppressor p53, to suppress tumorigenesis. We have recently reported that DDX5 augments deregulated E2F1 activity to induce tumor suppressor gene expression and apoptosis. During the analyses, we noted that over-expression of E2F1 increased DDX5 expression, suggesting a feed forward loop in E2F1 activation through DDX5. We thus examined whether the DDX5 gene is a target of deregulated E2F1. For this purpose, we performed promoter analysis and ChIP assay. The DDX5 promoter did not possess typical E2F binding consensus but contained several GC repeats observed in deregulated E2F1 targets. Insertion of point mutations in these GC repeats decreased responsiveness to deregulated E2F1 induced by over-expression of E2F1, but scarcely affected responsiveness to growth stimulation. ChIP assays showed that deregulated E2F1 induced by over-expression of E2F1 or expression of E1a, which binds pRB and releases E2F1, bound to the DDX5 gene, while physiological E2F1 induced by growth stimulation did not. These results suggest that the DDX5 gene is a target of deregulated E2F1, generating a feed forward loop mediating tumor suppressive E2F1 activity.