Transcription factors p53 and E2F-1 coordinate and mediate apoptosis in response to genotoxic stress. However, the mechanism whereby how p53 and E2F-1 mediate apoptosis is unclear. We found that p53 and E2F-1 co-regulate the expression of ECK, an epithelial cell receptor protein-tyrosine kinase and that ECK mediates apoptosis following oxidative stress. In p53 inducible systems, ECK is greatly upregulated upon the activation of p53. The expression of ECK is increased following oxidative stress in a p53-dependent manner. We have cloned the promoter region of human ECK gene and identified two potential p53-binding sites within a 700 bp from the transcriptional initiation site of ECK. We show that p53 greatly induces luciferase activity of the luciferase reporter containing this sequence upstream of the luciferase gene. Interestingly, there is also an E2F-binding site in the ECK promoter. Cotransfection of E2F-1 with the luciferase reporter containing the potential E2F-1 binding site leads to the activation of this promoter reporter. We further demonstrate that the ectopic expression of E2F-1 elevates the levels of ECK protein. Finally, overexpression of ECK greatly increases cellular susceptibility to apoptosis and suppresses colony formation in soft agar. Our findings indicate that p53 and E2F-1 are transcriptional regulators of ECK and that ECK is a cell death receptor in the p53 and E2F-1 pathways. This is the first evidence that p53 and E2F-1 share a common target gene in signaling apoptosis. In this grant application, we propose a series of experimental approaches to classify ECK as a new member of the p53-downstream gene family and a dual target for p53 and E2F-1. We will determine whether and how ECK is regulated by these two fundamental tumor suppressors. We will demonstrate the importance of ECK in signaling oxidative cell death by apoptosis and its potential role in tumorigenesis. Successful achievement of our specific aims will provide evidence for how p53 and E2F-1 cooperatively regulate a protein receptor, which receives damage signals and mediates cell death. Characterization of ECK as a cell death receptor may reveal a new target for gene therapy or lead to the development of effective chemotherapeutical strategies in cancer treatment.
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