Interferon Regulatory Factor 4 Is Involved in Epstein-Barr Virus-Mediated Transformation of Human B Lymphocytes

Epstein-Barr virus (EBV) infection has been associated with the development of several human malignancies, including nasopharyngeal carcinoma, Burkitt's lymphoma (BL), Hodgkin's lymphoma, T-cell lymphoma, and gastric carcinoma (32, 48). In immunocompromised individuals such as organ transplant recipients and AIDS patients, EBV almost certainly triggers two fatal cancers without the necessity for cofactors: AIDS-associated central nervous system (CNS) lymphoma and posttransplantation lymphoproliferative disorder (46). EBV establishes several types of latencies in host cells. In type I latency, EBV nuclear antigen 1 (EBNA-1) and small EBV-encoded, nonpolyadenylated nuclear RNAs (EBER-1 and -2) are expressed in host cells. In contrast, six nuclear proteins (EBNA-1, EBNA-2, EBNA-3A, EBNA-3B, EBNA-3C, and EBNA-LP), three membrane proteins (latent membrane protein 1 [LMP-1], LMP-2A, and LMP-2B), plus EBERs are expressed in type III latency (32, 48). EBV transforms adult primary B cells into continually growing lymphoblastoid cell lines (LCLs) and concomitantly establishes type III latency in vitro. LMP-1 is required for the transformation process: the deletion of LMP-1 prevents the transformation of primary B cells (27, 31), and the inhibition of LMP-1 expression in EBV-transformed cells reverts the transformed phenotypes (33). LMP-1 is an integral membrane protein with transmembrane domains and a C-terminal domain located in the cytoplasm (32, 35). LMP-1 acts as a constitutively active, receptor-like molecule that activates signaling pathways without the binding of a ligand (19). In addition, LMP-1 appears to be a central effector of altered cell growth, survival, adhesive, invasive, and antiviral potential (15, 41, 62, 63, 65, 69, 71). Interferon (IFN) regulatory factors (IRFs) are a small family of transcription factors with multiple functions. IRFs are apparently associated with viral transformation. IRF-7 is associated with EBV-transformed CNS lymphomas and has oncogenic properties (78). Oncogenic human herpesvirus 8 (HHV-8), also called Kaposi's sarcoma-associated herpesvirus, encodes four IRF-like molecules (viral IRFs [vIRFs]). Rhesus rhadinovirus, another oncogenic herpesvirus, has eight vIRFs in the genome (3, 47). At least human herpesvirus 8 vIRF-1 causes oncogenic transformation (17). IRF-4, also known as LSIRF, ICSAT, Pip, and Mum1, was cloned independently as a homologous member of the IRF gene family (67) and as an interacting partner of PU.1 (14). IRF-4 is expressed at all stages of B-cell development, in mature T cells, and in macrophages. The analysis of mice lacking IRF-4 (IRF-4−/−) revealed that IRF-4 is essential for the function and homeostasis of both mature B and T lymphocytes (42). IRF-4 is a critical factor for pre-B-to-B transition and the development of certain dendritic cells (37, 58). In addition, IRF-4 is closely associated with the human T-cell leukemia virus transformation process (67), has oncogenic potential in vitro, and may prevent apoptosis (26, 36). IRF-4 is also implicated in the pathogenesis of multiple myeloma: some myeloma cells express high levels of IRF-4 resulting from the chromosome translocation of the IRF-4 gene (26). In this report, we have examined the role of IRF-4 in the EBV transformation process. The expression of IRF-4 is associated with the EBV transformation of primary B lymphocytes in vitro and with primary CNS lymphomas in vivo. The reduced expression of IRF-4 in EBV-transformed cells decreases the cell proliferation rate and enhances apoptosis. These data suggest that IRF-4 may be a critical factor in the EBV-mediated transformation process.