Interferon regulatory factor 4 is activated through c-Src-mediated tyrosine phosphorylation in virus-transformed cells.

The importance of the oncogenic transcription factor interferon regulatory factor 4 (IRF4) in hematological malignancies has been increasingly recognized. We have previously identified the B cell integration cluster (BIC), the gene encoding miR-155, as the first microRNA (miRNA)-encoding gene transcriptionally targeted by IRF4 in virus-transformed cancer cells. Activation of IRFs is prerequisite for their functions. However, how IRF4 is activated in cancer is an open question. Our phosphoproteome profiling has identified several tyrosine phosphorylation sites on IRF4 in Epstein-Barr virus (EBV)-transformed cells. Further, we show here that c-Src dramatically stimulates IRF4 phosphorylation and activity and that Y61 and Y124 are two key sites responding to c-Src-mediated activation. Consistently, c-Src is constitutively expressed and active in EBV-transformed cells. However, c-Src is unlikely to be a direct kinase for IRF4. Furthermore, we have a polyclonal antibody specific to phospho-IRF4(Y121/124) developed in rabbit. We have further shown that inhibition of c-Src activity reduces p-IRF4(Y121/124) and significantly represses transcription of the IRF4 target BIC in EBV-transformed cells. Our results therefore, for the first time, demonstrate that IRF4 is phosphorylated and activated through a c-Src-mediated pathway in virus-transformed cells. These findings will improve our understanding of IRF4 in neoplasia and will provide profound insights into the interaction of oncogenic viruses with IRF4 in the development of hematological malignancies.