THE MOLECULAR-BASIS OF E2F-1 DP-1-INDUCED S-PHASE ENTRY AND APOPTOSIS/

The transcription factor E2F plays a critical role in the G(1) to S tr ansition. E2F is a heterodimer formed by members of the E2F and Up fam ilies 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 inter action 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 est ablished 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 G(1) regulation and significantly more apoptosis than does E2F-1 a lone, Using clones co-inducible for DP-1 and E2F-1, expression of pote ntial 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 resul ted in increased expression and activity of cyclins A and E, as well a s CDK2, prior to S-phase entry, Cyclin D and CDK4, however, were not i nduced, Phosphorylation of the retinoblastoma protein is also increase d following induction of E2F-1/DP-1, suggesting that E2F can feed-back on the retinoblastoma protein, presumably through activation of cycli n A- and/or E-associated kinase activity.