Interferon Regulatory Factor 4 Is Activated through c-Src-Mediated Tyrosine Phosphorylation in Virus-Transformed Cells

Abstract

Interferon regulatory factors (IRFs) are a small but important family of transcription factors in multiple facets of host defense systems and are also involved in the regulation of tumorigenesis, cell growth, differentiation, and myeloid cell development (1). Among IRFs, IRF2 (2), -4 (3), and -7 (4) have oncogenic and transforming potentials and antiapoptotic activity (3,5,6). These oncogenic IRFs all intimately interact with Epstein-Barr virus (EBV) latency programs (3, 7-10), which are associated with a variety of hematological and epithelial malignancies.IRF4, also known as multiple myeloma (MM) oncoprotein 1 (MUM1), is lymphocyte specific and is overexpressed in EBVtransformed cells (3,7,11,12), MM (13,14), and human T cell leukemia virus 1 (HTLV1)-infected cell lines and associated adult T cell lymphoma/leukemia (ATLL) (15)(16)(17)(18)(19). IRF4 overexpression is a hallmark of the ABC type of DLBCL and MM (20,21) and is frequently used as a diagnostic and prognostic marker for these and other proliferative disorders (21-23). Chromosomal translocation and genetic mutation of IRF4 have been found in MM, peripheral T cell lymphomas (24), and chronic lymphocytic leukemia (CLL) (13, 25). More recently, IRF4 was shown to be expressed in all LMP1-driven tumors in mice (26). These lines of evidence underscore the importance of IRF4 in these malignancies. However, the role of IRF4 in tumorigenesis remains to be elucidated.Importantly, we have recently identified B cell integration cluster (BIC), which encodes the oncogenic microRNA (miRNA) miR-155, as the first miRNA-encoding gene induced by IRF4 in virus-transformed cells (6). miR-155 plays important roles in innate immunity (27,28) and is the first identified oncogenic miRNA (oncomiR) that is implicated in various types of cancers, including lymphomas (29-31), breast cancer, leukemia, pancreatic cancer, and lung cancer (32,33). Like oncogenic IRFs, miR-155 is also associated with EBV latency (29,(34)(35)(36). Our findings therefore made a connection between these two pivotal players of cancer and immunity and have provided valuable insights into the interaction between viral oncogenesis and immune mechanisms governed by them. For example, both factors are crucial regulators of germinal center reaction (37, 38), which is implicated in lymphoma development and EBV latent infection (39). Furthermore, our microarray analysis shows that IRF4 regulates a pool of interesting genes in EBV-transformed cells (our unpublished data). Future pursuits on selected targets may disclose novel roles for IRF4 and broaden our knowledge in its interaction with viral oncogenesis and other associated cancers.Activation of IRFs by phosphorylation is prerequisite for their functions. Serine phosphorylation of IRF4 by the kinase ROCK2 activates IRF4, leading to interleukin 17/21 (IL-17/21) production in the autoimmune response in mice (40). However, how IRF4 is activated in cancer is an open question. Many proteins involved in signal transduction are tyrosine phosphorylated. Interestingly,...

LMP1 signaling pathway activates IRF4 in latent EBV infection and a positive circuit between PI3K and Src is required

“…37 We have previously provided strong evidence that c-Src activates IRF4, and Src(Y418) is phosphorylated and therefore Src is active, in EBV-transformed cells. 36 We have also shown that the total endogenous Src protein levels and activity correlate with the LMP1 protein levels in EBV latency. 36 These observations suggest that, in EBV latency, LMP1 can stimulate Src activation, which then leads to IRF4 phosphorylation.…”

“…We also used the IRF4 mutant, IRF4(Y121/124F) that is deficient in response to Src, 36 to test its ability to respond to LMP1. As shown in Figure 1d, activation of IRF4(Y121/124F) by LMP1 is significantly impaired, compared with wild-type IRF4.…”

“…35 Furthermore, we have recently demonstrated that IRF4 is activated through Src-mediated tyrosine phosphorylation in EBV-transformed cells. 36 Correspondingly, Src is functional in these cells. 36 These findings have prompted us to further identify the molecular mechanism underlying IRF4 activation in the context of EBV infection.…”

“…36 Correspondingly, Src is functional in these cells. 36 These findings have prompted us to further identify the molecular mechanism underlying IRF4 activation in the context of EBV infection.…”

IRF4 and IRF8: governing the virtues of B lymphocytes

“…Furthermore, EBV transformed lymphoblastoid cell lines showed reduced proliferation and enhanced apoptosis upon IRF4 knock-down (Banerjee et al, 2013). IRF4 was further shown to be phosphorylated in a c-src dependent manner which interfered with the DNA binding ability of IRF4 in EBV transformed cells (Wang and Ning, 2013). Furthermore, EBNA3C was shown to bind AICE and EICE sites in p14 (ARF)/CDKN2A locus, to repress its expression (Jiang et al, 2014b; Portal et al, 2013).…”