Abstract LB-148: DDX5 promotes ARF gene expression and apoptosis induced by deregulated E2F1

Abstract

Abstract The transcription factor E2F, the main target of the tumor suppressor pRB, plays central roles not only in cell proliferation but also in tumor suppression. E2F promotes cell proliferation by activating growth-related genes in response to normal growth stimulation. On the other hand, E2F contributes to tumor suppression by activating pro-apoptotic and growth-suppressive genes such as ARF and TAp73 when deregulated from pRB upon dysfunction of pRB, a major oncogenic change. ARF is an upstream activator of the tumor suppressor p53, one of the two major tumor suppressors in addition to pRB. TAp73 is a member of the p53 family and activates p53-responsive genes. Importantly, physiological E2F activity induced by growth stimulation does not activate the latter genes, underscoring importance of deregulated E2F activity in tumor suppression upon oncogenic changes. However, the regulatory mechanism of deregulated E2F activity is not known in detail. We found that deletion of N-terminal domain of E2F1 dramatically reduced its ability to activate ARF and TAp73 promoter, suggesting that E2F1 interacts with a factor(s) through its N-terminal domain to activate the tumor suppressor genes. Hence we explored novel E2F1-interacting factors using co-immunoprecipitation and LC/MS. Among the candidates, we found DEAD box 5 (DDX5) that functions not only RNA helicase but also transcriptional coactivator. DDX5 is reported to function as a coactivator for p53. Furthermore, E2F1 activity is regulated in similar manners as p53 such as by Chk1 and PRMT5. These observations suggest that DDX5 enhances not only p53 activity but also deregulated E2F1 activity. We thus examined effect of DDX5 on deregulated E2F1 activity induced by overexpression of E2F1. DDX5 enhanced E2F1 activation of ARF promoter depending on the integrity of N-terminal domain of E2F1. DDX5 mutant lacking RNA helicase activity retained the ability to enhance E2F1 activation of ARF promoter, indicating that the enhancement is independent of helicase activity. Furthermore, DDX5 enhanced endogenous ARF gene expression and apoptosis induced by E2F1 without affecting E2F1 expression level. Finally, knock down of DDX5 reduced E2F1-mediated activation of ARF promoter and ARF gene expression. These results suggest that DDX5 upregulates deregulated E2F1 activity and contributes to E2F1-induced apoptosis to protect cells from tumorigenesis. Citation Format: Kenta Kurayoshi, Kiyoshi Ohtani. DDX5 promotes ARF gene expression and apoptosis induced by deregulated E2F1. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-148.