Date Published: May 2, 2013
Publisher: Public Library of Science
Author(s): Shuvomoy Banerjee, Jie Lu, Qiliang Cai, Abhik Saha, Hem Chandra Jha, Richard Kuo Dzeng, Erle S. Robertson, Nancy Raab-Traub.
Epstein-Barr virus (EBV) is linked to a broad spectrum of B-cell malignancies. EBV nuclear antigen 3C (EBNA3C) is an encoded latent antigen required for growth transformation of primary human B-lymphocytes. Interferon regulatory factor 4 (IRF4) and 8 (IRF8) are transcription factors of the IRF family that regulate diverse functions in B cell development. IRF4 is an oncoprotein with anti-apoptotic properties and IRF8 functions as a regulator of apoptosis and tumor suppressor in many hematopoietic malignancies. We now demonstrate that EBNA3C can contribute to B-cell transformation by modulating the molecular interplay between cellular IRF4 and IRF8. We show that EBNA3C physically interacts with IRF4 and IRF8 with its N-terminal domain in vitro and forms a molecular complex in cells. We identified the Spi-1/B motif of IRF4 as critical for EBNA3C interaction. We also demonstrated that EBNA3C can stabilize IRF4, which leads to downregulation of IRF8 by enhancing its proteasome-mediated degradation. Further, si-RNA mediated knock-down of endogenous IRF4 results in a substantial reduction in proliferation of EBV-transformed lymphoblastoid cell lines (LCLs), as well as augmentation of DNA damage-induced apoptosis. IRF4 knockdown also showed reduced expression of its targeted downstream signalling proteins which include CDK6, Cyclin B1 and c-Myc all critical for cell proliferation. These studies provide novel insights into the contribution of EBNA3C to EBV-mediated B-cell transformation through regulation of IRF4 and IRF8 and add another molecular link to the mechanisms by which EBV dysregulates cellular activities, increasing the potential for therapeutic intervention against EBV-associated cancers.
Tumor viruses have evolved multiple strategies for modulating the expression of an array of cellular genes to enhance persistence, latency and survival of infected cells. Studies into these strategies have provided several lines of evidence as to the mechanisms of differential gene expression and their deregulation during oncogenesis. Particularly, EBV is responsible for the development of lympho-proliferative diseases manifested in immuno-compromised AIDS patients , and is also linked to Burkitt’s lymphoma, Hodgkin’s lymphoma, B and T cell lymphomas, anaplastic nasopharyngeal carcinoma, and also some forms of gastric carcinomas . Human primary B lymphocytes are the principal target for EBV infection, although the virus has the potential to infect other lymphocytes and epithelial cells .
EBNA3C, one of the EBV essential latent proteins, regulates transcription of several viral and cellular genes. Previous studies showed that EBNA3C also co-operates with EBNA2 to activate the viral LMP1 promoter via interaction with cellular transcription factors, including Spi-1/Spi-B . Additionally, EBNA3C associates with the metastasis suppressor protein Nm23-H1 to regulate the transcription of cellular genes which are critically involved in cell migration and invasion . Recent studies also demonstrated that EBNA3C can interact and stabilize cellular oncoproteins, including c-Myc , and the major cell cycle regulatory protein Cyclin D1  to drive B-cell transformation.