Date Published: January 18, 2017
Publisher: Public Library of Science
Author(s): Chong Wang, Caixia Zhu, Fang Wei, Shujun Gao, Liming Zhang, Yuhong Li, Yanling Feng, Yin Tong, Jianqing Xu, Bin Wang, Zhenghong Yuan, Erle S. Robertson, Qiliang Cai, Paul D. Ling.
Emerging evidence implies that STAT6 plays an important role in both the adaptive and innate immune responses to virus infection. Kaposi’s sarcoma-associated herpesvirus (KSHV) is an oncogenic γ-herpesvirus agent associated with several human malignancies, including Kaposi’s sarcoma (KS) and primary effusion lymphomas (PELs). Previously, we demonstrated that KSHV blocks IL-4-induced STAT6 phosphorylation and retains a basal IL-13/STAT6 constitutive activation for cell survival and proliferation. However, the mechanism by which KSHV regulates STAT6 remains largely unknown. Here, we found that KSHV-encoded LANA interacts with STAT6 and promotes nuclear localization of STAT6 independent of the tyrosine 641-phosphorylation state. Moreover, nuclear localization of STAT6 is also dramatically increased in KS tissue. The latent antigen LANA induces serine protease-mediated cleavage of STAT6 in the nucleus, where the cleaved STAT6 lacking transactivation domain functions as a dominant-negative regulator to repress transcription of Replication and Transcription Activator (RTA) and in turn shut off viral lytic replication. Blockade of STAT6 by small interference RNA dramatically enhances expression of RTA, and in turn reduces KSHV-infected endothelial cell growth and colony formation. Taken together, these results suggest that nuclear localization and cleavage of STAT6 is important for modulating the viral latency and pathogenesis of KSHV.
Signal transducer and activator of transcription (STAT) is a family of latent cytoplasmic transcription factors activated by specific cytokine receptor-mediated signal transducers. Seven members of the STAT family, including STAT1, 2, 3, 4, 5a, 5b, and 6, have been described so far . STAT6 is activated by cytokines like IL-4 and IL-13 that interact with a receptor complex containing IL-4Rα chain . Selective activation of STAT6 by IL-4 or IL-13 involves phosphorylation, dimerization and then translocation into the nucleus, where it binds to specific DNA elements TTC(N3/4)GAA within the promoter region, activating gene transcription . It has been demonstrated that STAT6 is required to induce the expression of CD23 and MHC class II, IgE isotype switching in B cells , as well as differentiation of Th2 type T cells . However, STAT6 blocks IL-4-dependent inhibition of IFN-γ-induced gene expression in macrophages or Th1 type T-cell differentiation, indicating that STAT6 plays a key role in negative regulation of gene expression [5,6]. Although little is known regarding the mechanisms of down-regulation of STAT6-dependent signaling, recent reports of STAT6 isoform with carboxyl-truncation in both bone marrow-derived mast cells and mast cell lines suggest that STAT6 could function as a dominant-negative regulator in gene expression, which, due to lack of the carboxyl-terminus, interferes with the normal ability of STAT6 to induce transcription of target genes [7,8]. For instance, a 70kDa carboxyl-truncated isoform of STAT6 was detected in IL-4-stimulated mast cells , and this cleavage of STAT6 is induced by serine proteases in the nucleus. Interestingly, full length STAT6 (94kDa) can also be cleaved at different sites to yield short STAT6 (60kDa and 55kDa) in the cytoplasm of mast cells by neutrophil elastase and proteinase, respectively –a phenomenon not observed in B cells.
In the present study, we demonstrated a novel regulatory mechanism of KSHV-mediated STAT6 signaling in KSHV-infected cells. We found that STAT6 was induced by KSHV to translocate into the nucleus independent of tyrosine 641-phosphorylation. The nuclear localization of STAT6 was due to the interaction with the latent antigen LANA, and in turn led to STAT6 cleavage in the nucleus of KSHV-infected cells. The nuclear-localized and cleaved STAT6, induced by LANA, inhibited the transcription of RTA, and blocked lytic replication and viral progeny production. Conversely, full length nuclear-localized STAT6 enhanced the protein stability of LANA for viral latency (Fig 10). Consistent with our previous studies [23,24], these findings explain why LANA could block the IL-4-induced phosphorylation of STAT6, and why IL-13-mediated constitutively phosphorylation of STAT6 was dramatically enhanced at the early stage (< 3 days), but reduced later (>5 days) along with the increased expression of LANA during KSHV primary infection (which could be due to the effect of nuclear localization of STAT6 induced by LANA).