KSHV vIRF4 enhances BCL6 transcription via downregulation of IRF4 expression.

Primary effusion lymphoma (PEL), strongly linked with latent infection of Kaposi's sarcoma-associated herpesvirus (KSHV), constitutively expresses cellular interferon regulatory factor 4 (IRF4) while suppressing the expression of B cell lymphoma 6 (BCL6). Recently, it was shown that IRF4, a key transcriptional repressor of BCL6, might be a pivotal regulator of KSHV for balancing between latency and its reactivation in PEL cells. However, the action of the BCL6-IRF4 transcription factor axis during KSHV's life cycle is not clear. Herein we found that the KSHV lytic protein viral interferon regulatory factor 4 (vIRF4) dramatically enhanced the transcriptional activity of the BCL6 through the inhibition of its negative regulator IRF4. Using a chromatin immunoprecipitation (ChIP) assay, we further showed that vIRF4 bound to the specific promoter region of IRF4, contributing to a dramatic suppression of IRF4 gene expression. Correspondingly, we also found BCL6 expression to be positively and inversely correlated with vIRF4 and IRF4 expression, respectively, during KSHV reactivation. Finally, we observed that these processes require efficient KSHV lytic replication. Thus, our findings suggest a crucial role of the BCL6-IRF4 axis in triggering the transition between KSHV latency and lytic reactivation.

Deregulation of HDAC5 by Viral Interferon Regulatory Factor 3 Plays an Essential Role in Kaposi's Sarcoma-Associated Herpesvirus-Induced Lymphangiogenesis.

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent for Kaposi's sarcoma (KS), which is one of the most common HIV-associated neoplasms. The endothelium is the thin layer of squamous cells where vascular blood endothelial cells (BECs) line the interior surface of blood vessels and lymphatic endothelial cells (LECs) are in direct contact with lymphatic vessels. The KS lesions contain a prominent compartment of neoplastic spindle morphology cells that are closely related to LECs. Furthermore, while KSHV can infect both LECs and BECs in vitro, its infection activates genetic programming related to lymphatic endothelial cell fate, suggesting that lymphangiogenic pathways are involved in KSHV infection and malignancy. Here, we report for the first time that viral interferon regulatory factor 3 (vIRF3) is readily detected in over 40% of KS lesions and that vIRF3 functions as a proangiogenic factor, inducing hypersprouting formation and abnormal growth in a LEC-specific manner. Mass spectrometry analysis revealed that vIRF3 interacted with histone deacetylase 5 (HDAC5), which is a signal-responsive regulator for vascular homeostasis. This interaction blocked the phosphorylation-dependent cytosolic translocation of HDAC5 and ultimately altered global gene expression in LECs but not in BECs. Consequently, vIRF3 robustly induced spindle morphology and hypersprouting formation of LECs but not BECs. Finally, KSHV infection led to the hypersprouting formation of LECs, whereas infection with a ΔvIRF3 mutant did not do so. Collectively, our data indicate that vIRF3 alters global gene expression and induces a hypersprouting formation in an HDAC5-binding-dependent and LEC-specific manner, ultimately contributing to KSHV-associated pathogenesis.IMPORTANCE Several lines of evidences indicate that KSHV infection of LECs induces pathological lymphangiogenesis and that the results resemble KS-like spindle morphology. However, the underlying molecular mechanism remains unclear. Here, we demonstrated that KSHV vIRF3 is readily detected in over 40% of various KS lesions and functions as a potent prolymphangiogenic factor by blocking the phosphorylation-dependent cytosolic translocation of HDAC5, which in turn modulates global gene expression in LECs. Consequently, vIRF3-HDAC5 interaction contributes to virus-induced lymphangiogenesis. The results of this study suggest that KSHV vIRF3 plays a crucial role in KSHV-induced malignancy.