title: Transcriptional control during the cell cycle and apoptosis
Creators: name: Lee, Chang-Woo
Publisher: name: University of Glasgow
Year: 1998
Subjects: subject: Cellular biology.

description: Both E2F and p53 are sequence specific transcription factors that play important roles in controlling early cell cycle progression. The pathway of control mediated through E2F governs the transition from G1 into S phase whereas p53 in response to genotoxic stress can facilitate cell cycle arrest or apoptosis. Here, I show that there is a physical and functional interaction between p53 and DP-1. p53, which interacts with a distinct form of DP-1, competes with E2F-1 for DP-1 leading to a reduction in the level of E2F-1/DP-1 heterodimer transcriptional activity. The results establish DP-1 as a common cellular target in two distinct pathways of growth control mediated through the activities of pRb and p53 tumour suppressor proteins. From previous studies it is known that cells expressing aberrant levels of E2F-1 can undergo p53-dependent apoptosis. However, I demonstrate that E2F-1 can induce apoptosis in p53-/- tumour cells and that DP-1 is able to augment the E2F-1-induced apoptosis, yet by itself it has no ability to induce apoptosis, consistent with the ability of DP-1 to enhance E2F-1-mediated transcriptional activation. Surprisingly, E2F-1-induced apoptosis requires DNA-binding but not trans activation, suggesting that this apoptosis is not simply the consequence of the direct activation of genes required for apoptosis. The mechanisms which influence the outcome of p53 induction are not clear, although transcription of the p53 target gene, encoding the cdk-inhibitor p21Waf1/Cip1 correlates with p53-mediated cell cycle arrest. Using a combination of biochemical and functional assays, I identify that p300 as a co-activator required for p5 3-dependent transcriptional activation of Waf1/Cip1. Furthermore, I show that the cdk-inhibitor p2lWaf1/Cip1 autoregulates in a positive fashion transcription through modulating the activity of the p53/p300 complex, whilst negatively regulating the activity of E2F by preventing cdk-dependent phosphorylation of pRb. Consistent with a role for p2lWaf1/Cip1 in the autoregulation of p53-dependent transcription, p300 augments the ability of p53 to cause G1 arrest. In addition, I find that p300, which also functions as a co-activator for E2F/DP heterodimer, enhances the E2F-1- induced apoptotic activity. Thus, a functional interaction between p300 and either p53 or E2F-1 has a profound impact on early cell cycle progression, specifically in regulating the contrasting outcomes of cell cycle arrest and apoptosis. p300/CBP proteins have been implicated as critical regulators of distinct cellular pathways, such as those leading to differentiation, cell cycle arrest and apoptosis. To elucidate the mechanisms of transcriptional activation by p300, it was considered possible that additional control may be exerted through proteins that physically interact to regulate the activity of p300. I demonstrate that a newly identified protein, termed JMY, makes a physiological complex with p300 in mammalian cells. Also, this new protein functionally co-operates with p300 in the transcriptional activation of p53, and possesses the properties of a co-activator for p53. I find that JMY has potent effects on certain p53-dependent genes, such as bax, and physiologically JMY co-operates with p300 in promoting apoptosis. These results reveal a new level of control that is important in dictating the cellular response to p53.