No abstract

Although a number of transfection experiments have suggested potential targets for the action of the E2F1 transcription factor, as is the case for many transcriptional regulatory proteins, the actual targets in their normal chromosomal environment have not been demonstrated. We have made use of a recombinant adenovirus containing the E2F1 cDNA to infect quiescent cells and then measure the activation of endogenous cellular genes as a consequence of E2F1 production. We find that many of the genes encoding S-phase-acting proteins previously suspected to be E2F targets, including DNA polymerase alpha, thymidylate synthase, proliferating cell nuclear antigen, and ribonucleotide reductase, are indeed induced by E2F1. Several other candidates, including the dihydrofolate reductase and thymidine kinase genes, were only minimally induced by E2F1. In addition to the S-phase genes, we also find that several genes believed to play regulatory roles in cell cycle progression, such as the cdc2, cyclin A, and B-myb genes, are also induced by E2F1. Moreover, the cyclin E gene is strongly induced by E2F1, thus defining an autoregulatory circuit since cyclin E-dependent kinase activity can stimulate E2F1 transcription, likely through the phosphorylation and inactivation of Rb and Rb family members. Finally, we also demonstrate that a G1 arrest brought about by gamma irradiation is overcome by the overexpression of E2F1 and that this coincides with the enhanced activation of key target genes, including the cyclin A and cyclin E genes.

E2F transcription activity is composed of a family of heterodimers encoded by distinct genes. Through the overproduction of each of the five known E2F proteins in mammalian cells, we demonstrate that a large number of genes encoding proteins important for cell cycle regulation and DNA replication can be activated by the E2F proteins and that there are distinct specificities in the activation of these genes by individual E2F family members. Coexpression of each E2F protein with the DP1 heterodimeric partner does not significantly alter this specificity. We also find that only E2F1 overexpression induces cells to undergo apoptosis, despite the fact that at least two other E2F family members, E2F2 and E2F3, are equally capable of inducing S phase. The ability of E2F1 to induce apoptosis appears to result from the specific induction of an apoptosis-promoting activity rather than the lack of induction of a survival activity, because co-expression of E2F2 and E2F3 does not rescue cells from E2F1-mediated apoptosis. We conclude that E2F family members play distinct roles in cell cycle control and that E2F1 may function as a specific signal for the initiation of an apoptosis pathway that must normally be blocked for a productive proliferation event.Various studies have led to the delineation of a pathway controlling the progression of cells from quiescence, through G 1 , and into S phase that involves the activation of G 1 cyclin-dependent kinases (cdk), inactivation of Rb and related proteins, and accumulation of E2F transcription factor activity (for reviews see refs. 1-7). It is also now clear that the disruption of components of this control pathway, either the activation of positive acting components such as the G 1 cyclins or the inactivation of negative components such as p53, Rb, and the cyclin-dependent kinase inhibitors (CKI), can lead to the loss of cell growth control underlying the development of various forms of human cancer (7,8).Like many other signal transduction activities, E2F consists of a family of related proteins that include five distinct E2F members and at least two heterodimer partners, DP1 and DP2. The complexity of the E2F activity, as generated by the formation of a variety of heterodimeric protein complexes, suggests a complexity of function whereby the individual family members might play distinct roles in cellular growth control. For instance, the individual E2F family members might integrate distinct signaling pathways within the cell to facilitate the orderly progression through the growth cycle. That is, individual E2F genes might respond to different components of a growth signaling process, either distinct extracellular growth factors or simply distinct signal transduction pathways that integrate a complex growth response. Additionally, but not exclusive of the first instance, the individual E2F proteins could activate distinct target genes, the total of which constitutes the range of activities necessary for cells to progress into and through S phase. Finally, the complexity...