IRF6 controls Epstein-Barr virus (EBV) lytic reactivation and differentiation in EBV-infected epithelial cells

Latent Epstein-Barr virus (EBV) infection promotes undifferentiated nasopharyngeal carcinoma (NPC) and gastric carcinoma (GC), while EBV infection of normal differentiated oropharyngeal epithelial cells is lytic and kills the cell. Establishment of viral latency within epithelial cells is likely essential for the development of EBV-induced NPCs and GCs, but the mechanism(s) by which EBV latency is maintained in epithelial cells are not fully understood. Here we demonstrate that the cellular tumor suppressor protein IRF6, a master regulator of squamous cell epithelial cell differentiation, plays a critical role in promoting TPA-induced lytic EBV reactivation in vitro in both EBV-infected NPC cells and EBV-infected GC cells. Using a telomerase-immortalized normal oral keratinocyte cell line (NOKs) model which retains the ability to differentiate in response to TPA treatment, we show that TPA-induced lytic EBV reactivation requires the PKCδ-RIPK4-IRF6 signaling pathway. RIPK4 is a PKCδ (PRKCD)-activated cellular S/T kinase that phosphorylates and activates the IRF6 transcription factor. We demonstrate that inhibition of PKCδ, RIPK4 or IRF6 expression is sufficient to suppress TPA-induced epithelial cell differentiation, as well as lytic EBV reactivation, in NOKs. Furthermore, we find that latent EBV infection in NOKs inhibits the expression of IRF6. Importantly, we show that inducible expression of a constitutively active (phospho-mimetic) IRF6 mutant is sufficient to activate the lytic form of EBV infection in both EBV-infected NOKs and EBV-infected SNU719 GC cells. Finally, we demonstrate that the ability of constitutively active IRF6 to promote lytic EBV infection in NOKs is at least partially mediated by IRF6-induced expression of the BLIMP1 transcription factor, which we previously showed synergistically activates expression of the two EBV immediate-early proteins, BZLF1 and BRLF1, in conjunction with KLF4. Thus, suppression of IRF6 expression may promote NPC and GC tumors by blocking lytic EBV reactivation and differentiation.