title: E2F1 mediated apoptosis induced by the DNA damage response is blocked by EBV nuclear antigen 3C in lymphoblastoid cells.
abstract: EBV latent antigen EBNA3C is indispensible for in vitro B-cell immortalization resulting in continuously proliferating lymphoblastoid cell lines (LCLs). EBNA3C was previously shown to target pRb for ubiquitin-proteasome mediated degradation, which facilitates G1 to S transition controlled by the major transcriptional activator E2F1. E2F1 also plays a pivotal role in regulating DNA damage induced apoptosis through both p53-dependent and -independent pathways. In this study, we demonstrate that in response to DNA damage LCLs knocked down for EBNA3C undergo a drastic induction of apoptosis, as a possible consequence of both p53- and E2F1-mediated activities. Importantly, EBNA3C was previously shown to suppress p53-induced apoptosis. Now, we also show that EBNA3C efficiently blocks E2F1-mediated apoptosis, as well as its anti-proliferative effects in a p53-independent manner, in response to DNA damage. The N- and C-terminal domains of EBNA3C form a stable pRb independent complex with the N-terminal DNA-binding region of E2F1 responsible for inducing apoptosis. Mechanistically, we show that EBNA3C represses E2F1 transcriptional activity via blocking its DNA-binding activity at the responsive promoters of p73 and Apaf-1 apoptosis induced genes, and also facilitates E2F1 degradation in an ubiquitin-proteasome dependent fashion. Moreover, in response to DNA damage, E2F1 knockdown LCLs exhibited a significant reduction in apoptosis with higher cell-viability. In the presence of normal mitogenic stimuli the growth rate of LCLs knockdown for E2F1 was markedly impaired; indicating that E2F1 plays a dual role in EBV positive cells and that active engagement of the EBNA3C-E2F1 complex is crucial for inhibition of DNA damage induced E2F1-mediated apoptosis. This study offers novel insights into our current understanding of EBV biology and enhances the potential for development of effective therapies against EBV associated B-cell lymphomas.
authors.affiliation: Department of Microbiology and Tumor Virology Program, Abramson Comprehensive Cancer Center, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
authors.forename: Abhik
authors.initials: A
authors.lastname: Saha
authors.forename: Jie
authors.initials: J
authors.lastname: Lu
authors.forename: Lise
authors.initials: L
authors.lastname: Morizur
authors.forename: Santosh K
authors.initials: SK
authors.lastname: Upadhyay
authors.forename: Mahadesh Prasad
authors.initials: MP
authors.lastname: Aj
authors.forename: Erle S
authors.initials: ES
authors.lastname: Robertson
mesh_terms: D000956:Antigens, Viral / Q000235:genetics / Q000378:metabolism*; D017209:Apoptosis / Q000235:genetics*; D002460:Cell Line; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D004249:DNA Damage; D050687:E2F1 Transcription Factor / Q000037:antagonists & inhibitors / Q000378:metabolism*; D020031:Epstein-Barr Virus Infections / Q000235:genetics / Q000378:metabolism* / Q000821:virology; D019309:Epstein-Barr Virus Nuclear Antigens; D015967:Gene Expression Regulation, Viral; D057809:HEK293 Cells; D006801:Humans; D008214:Lymphocytes / Q000378:metabolism / Q000473:pathology / Q000821:virology*; D016393:Lymphoma, B-Cell / Q000276:immunology / Q000821:virology; D010006:Osteoblasts / Q000276:immunology / Q000821:virology; D014162:Transfection
chemical_list: D000956:Antigens, Viral; D050687:E2F1 Transcription Factor; C493753:E2F1 protein, human; C498774:EBNA-3C, epstein-barr virus; D019309:Epstein-Barr Virus Nuclear Antigens