The molecular basis of E2F-1/DP-1-induced S-phase entry and apoptosis

The transcription factor E2F plays a critical role in the G1 to S transition. E2F is a heterodimer formed by members of the E2F and DP families of DNA-binding proteins. Ectopic expression of E2F-1, the first member of the E2F family identified, is sufficient to cause quiescent cells to enter S phase. Thus, the biological significance of the interaction of E2F-1 with its DP protein partner, DP-1, was unclear. Here, we report on the role of DP-1 in the mediation of E2F-induced S-phase entry and apoptosis. Cells inducible for DP-1, E2F-1, or both were established and characterized. Ectopic expression of DP-1 alone fails to promote cell cycle entry, even when the potent transactivation domain of human papillomavirus-VP16 is fused to the DNA-binding domain of DP-1. In contrast, coexpression of DP-1 and E2F-1 results in greater loss of G1 regulation and significantly more apoptosis than does E2F-1 alone. Using clones co-inducible for DP-1 and E2F-1, expression of potential target genes of E2F activity that may account for its ability to induce S-phase entry was also examined. Induction of E2F-1/DP-1 resulted in increased expression and activity of cyclins A and E, as well as CDK2, prior to S-phase entry. Cyclin D and CDK4, however, were not induced. Phosphorylation of the retinoblastoma protein is also increased following induction of E2F-1/DP-1, suggesting that E2F can feed-back on the retinoblastoma protein, presumably through activation of cyclin A- and/or E-associated kinase activity.