Constitutive expression of the E2F1 transcription factor in fibroblasts alters G0 and S phase transit following serum stimulation

The E2F-1 transcription factor is negatively regulated by its interaction with the MDMX protein

Several proteins with important roles in oncogenesis have been shown to regulate the function of the E2F-1 transcription factor, which is known to activate the expression of genes required for proliferation and apoptosis. Here we identify the MDMX oncoprotein as an E2F-1-binding factor, from a yeast-two hybrid screen using a portion of the E2F-1 protein as "bait." We demonstrate that the region within MDMX needed for the E2F-1:MDMX interaction is located in the central part of the protein, C-terminal of the p53-binding domain. The region within E2F-1 needed for this association is adjacent to the DNA binding domain. Further, when expressed in vivo or in vitro the MDMX protein migrates as two isoforms on SDS-PAGE, the faster migrating isoform having the stronger affinity for the E2F-1 proteins. It appears that this interaction reduces the ability of E2F-1 to bind DNA. Expression of MDMX along with E2F-1 and Dp-1 in Saos2 cells reduces the ability of E2F-1 to bind to its consensus DNA sequence, without altering E2F-1 protein levels. These data indicate that the MDMX protein is capable of associating with E2F-1 and negatively regulating its DNA binding ability.

An early S phase checkpoint is regulated by the E2F1 transcription factor

The E2F1 transcription factor regulates transit of cells through the S phase checkpoint, dependent on its association with cyclin A/cdk2. Expression in cells of a mutant E2F1 lacking the cyclin A/cdk2 binding domain leads to partial arrest of cells at the S phase checkpoint. When subconfluent growing cells expressing this mutant E2F1 are analyzed in detail, it is shown here that they display a significantly reduced incorporation of 3H-thymidine into the DNA of each S phase cell, compared to control cells or to cells overexpressing full-length E2F1. Further, when cells are blocked at the G1/S phase border and released, there is a clear reduction in the amount of 3H-thymidine incorporated into the DNA of S phase cells by 1.5 hours post release. Considering a normal 6 hour S phase duration, the results show that the S phase checkpoint mediated by E2F1 is not a late S phase event but an early one.