Human herpesvirus 8 (HHV-8) encodes four viral interferon regulatory factors (vIRF-1 to -4) that likely function to suppress innate immune and cellular stress responses through inhibitory interactions with various cellular proteins involved in these activities. It is notable that vIRF-1 and -4 have been reported to interact with the deubiquitinase ubiquitin-specific protease 7 (USP7), substrates of which include p53 and the p53-targeting and -destabilizing ubiquitin E3 ligase MDM2. Structural studies of vIRF-1 and vIRF-4 USP7 binding sequences in association with USP7 have been reported; both involve interactions with N-terminal-domain residues of USP7 via EGPS and ASTS motifs in vIRF-1 and vIRF-4, respectively, but vIRF-4 residues also contact the catalytic site. However, the biological activities of vIRF-1 and vIRF-4 via USP7 interactions are unknown. Here, we report that vIRF-3, which is latently, as well as lytically, expressed in HHV-8-infected primary effusion lymphoma (PEL) cells, also interacts with USP7-via duplicated EGPS motifs-and that this interaction is important for PEL cell growth and viability. The interaction also contributes to suppression of productive virus replication by vIRF-3, which we identify here. We further show that vIRF-1, which is expressed at low levels in PEL latency, promotes latent PEL cell viability and that this activity and vIRF-1-promoted productive replication (reported previously) involve EGPS motif-mediated USP7 targeting by vIRF-1. This study is the first to identify latent and lytic functions of vIRF-1 and vIRF-3, respectively, and to address the biological activities of these vIRFs through their interactions with USP7. HHV-8 is associated with Kaposi's sarcoma, primary effusion lymphoma (PEL), and multicentric Castleman's disease; both latent and lytic viral functions are believed to contribute. Viral interferon regulatory factors specified by HHV-8 are thought to be critically important for successful productive replication through suppression of innate immune and stress responses triggered by the lytic cycle. Latently expressed vIRF-3 contributes significantly to PEL cell survival. Here, we identify ubiquitin-specific protease 7 (USP7) deubiquitinase targeting by vIRF-3 (in addition to previously reported USP7 binding by vIRF-1 and vIRF-4); the importance of vIRF-1 and vIRF-3 interactions with USP7 for latent PEL cell growth and viability; and the positive and negative contributions, respectively, of USP7 targeting by vIRF-1 and vIRF-3 to HHV-8 productive replication. This is the first report of the biological importance of vIRF-1 in PEL cell latency, the modulation of productive replication by vIRF-3, and the contributions of vIRF-USP7 interactions to HHV-8 biology.

Viral interleukin-6 (vIL-6) encoded by human herpesvirus 8 (HHV-8) is believed to contribute via mitogenic, survival, and angiogenic activities to HHV-8-associated Kaposi's sarcoma, primary effusion lymphoma (PEL), and multicentric Castleman's disease through autocrine or paracrine mechanisms during latency or productive replication. There is direct evidence that vIL-6 promotes latently infected PEL cell viability and proliferation and also viral productive replication in PEL and endothelial cells. These activities are mediated largely through endoplasmic reticulum (ER)-localized vIL-6, which can induce signal transduction via the gp130 signaling receptor, activating mitogen-activated protein kinase and signal transducer and activator of transcription signaling, and interactions of vIL-6 with the ER membrane protein vitamin K epoxide reductase complex subunit 1 variant 2 (VKORC1v2). The latter functional axis involves suppression of proapoptotic lysosomal protein cathepsin D by promotion of the ER-associated degradation of ER-transiting, preproteolytically processed procathepsin D. Other interactions of VKORC1v2 and activities of vIL-6 via the receptor have not been reported. We show here that both vIL-6 and VKORC1v2 interact with calnexin cycle proteins UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT1), which catalyzes monoglucosylation of N-glycans, and oppositely acting glucosidase II (GlucII), and that vIL-6 can promote protein folding. This activity was found to require VKORC1v2 and UGGT1, to involve vIL-6 associations with VKORC1v2, UGGT1, and GlucII, and to operate in the context of productively infected cells. These findings document new VKORC1v2-associated interactions and activities of vIL-6, revealing novel mechanisms of vIL-6 function within the ER compartment. HHV-8 vIL-6 prosurvival (latent) and proreplication functions are mediated from the ER compartment through both gp130 receptor-mediated signal transduction and interaction of vIL-6 with the ER membrane protein VKORC1v2. This report identifies interactions of vIL-6 and VKORC1v2 with calnexin cycle enzymes GlucII and UGGT1, which are involved in glycan processing and nascent protein folding. The presented data show that vIL-6 and VKORC1v2 can cocomplex with GlucII and UGGT1, that vIL-6 promotes protein folding, and that VKORC1v2, UGGT1, and vIL-6 interactions with GlucII and UGGT1 are important for the profolding activity of vIL-6, which can be detected in the context of infected cells. This newly identified ER activity of vIL-6 involving VKORC1v2 may promote viral latency (in PEL cells) and productive replication by limiting the damaging effects of unfolded protein response signaling in addition to enhancing viral protein folding. This is the first report of such a function for a cytokine.

Human herpesvirus 8 (HHV-8) encodes four viral interferon regulatory factors (vIRFs 1 to 4), all of which are expressed during lytic replication and inhibit a variety of antiviral signaling pathways. Viral IRFs 1, 2, and 3 are also expressed during latency in primary effusion lymphoma (PEL) cells, and vIRF-1 and vIRF-3 have been reported to promote PEL cell viability. Viral IRFs 1, 3, and 4 are known to interact with ubiquitin-specific protease 7 (USP7); interactions of vIRF-1 and vIRF-3 with USP7 promote PEL cell viability and regulate productive replication. Here, we report that vIRF-2 also targets USP7, utilizing a PSTS motif matching the USP7 N-terminal domain-binding A/PxxS consensus, but uniquely requires catalytic domain residues for intracellular interaction. In functional and mechanistic analyses, tumor necrosis factor receptor-associated factor (TRAF)-mediated signaling and associated polyubiquitination of TRAFs 3 and 6, specifically, were regulated negatively by USP7 and positively by vIRF-2-USP7 interaction, the latter competing for USP7-TRAF association. Using depletion, depletion-complementation, and targeted mutagenesis approaches, vIRF-2 was determined to promote latent PEL cell viability, likely independently of USP7 interaction, while lytic replication was inhibited by vIRF-2, in part or in whole via USP7 interaction. Together, our data identify a new molecular determinant of USP7 recognition, TRAF3/6-specific targeting by the deubiquitinase, associated activation of these TRAFs by vIRF-2, and activities of vIRF-2 and vIRF-2-USP7 interaction in HHV-8 latent and lytic biology. IMPORTANCE Human herpesvirus 8-encoded IRF homologues were the first to be identified in a virus. Through inhibitory interactions with cellular IRFs and other mediators of antiviral signaling, the vIRFs are believed to be essential for productive replication and also for latency in particular cell types. The deubiquitinase USP7 is a regulator of key cellular pathways, modulates HHV-8 latent and lytic infection, and is targeted by vIRFs 1, 3, and 4. Here, we report that vIRF-2 also interacts with USP7, via a means distinguishable from USP7 interactions with other vIRFs and other proteins, that this interaction modulates antiviral signaling via disruption of USP7 interactions with innate immune signaling proteins TRAF3 and TRAF6, and that vIRF-2 targeting of USP7 regulates HHV-8 productive replication. The presented data are the first to identify vIRF-2 targeting of USP7 and its role in HHV-8 biology, expanding our understanding of the repertoire and importance of virus-host interactions.