IFNs orchestrate immune defense through induction of hundreds of genes. Small ubiquitin-like modifier (SUMO) is involved in various cellular functions, but little is known about its role in IFN responses. Prior work identified STAT1 SUMOylation as an important mode of regulation of IFN-γ signaling. In this study, we investigated the roles of SUMO in IFN signaling, gene expression, protein stability, and IFN-induced biological responses. We first show that SUMO overexpression leads to STAT1 SUMOylation and to a decrease in IFN-induced STAT1 phosphorylation. Interestingly, IFNs exert a negative retrocontrol on their own signaling by enhancing STAT1 SUMOylation. Furthermore, we show that expression of each SUMO paralog inhibits IFN-γ–induced transcription without affecting that of IFN-α. Further, we focused on IFN-induced gene products associated to promyelocytic leukemia (PML) nuclear bodies, and we show that neither IFN-α nor IFN-γ could increase PML and Sp100 protein expression because they enhanced their SUMO3 conjugation and subsequent proteasomal degradation. Because it is known that SUMO3 is important for the recruitment of RING finger protein 4, a poly–SUMO-dependent E3 ubiquitin ligase, and that PML acts as a positive regulator of IFN-induced STAT1 phosphorylation, we went on to show that RING finger protein 4 depletion stabilizes PML and is correlated with a positive regulation of IFN signaling. Importantly, inhibition of IFN signaling by SUMO is associated with a reduction of IFN-induced apoptosis, cell growth inhibition, antiviral defense, and chemotaxis. Conversely, inhibition of SUMOylation results in higher IFN-γ–induced STAT1 phosphorylation and biological responses. Altogether, our results uncover a new role for SUMO in the modulation of IFN response.

The promyelocytic leukemia (PML) protein is expressed in the diffuse nuclear fraction of the nucleoplasm and in matrix-associated structures, known as nuclear bodies (NBs). PML NB formation requires the covalent modification of PML to SUMO. The noncovalent interactions of SUMO with PML based on the identification of a SUMO-interacting motif within PML seem to be required for further recruitment within PML NBs of SUMOylated proteins. RNA viruses whose replication takes place in the cytoplasm and is inhibited by PML have developed various strategies to counteract the antiviral defense mediated by PML NBs. We show here that primary fibroblasts derived from PML knockout mice are more sensitive to infection with encephalomyocarditis virus (EMCV), suggesting that the absence of PML results in an increase in EMCV replication. Also, we found that EMCV induces a decrease in PML protein levels both in interferon-treated cells and in PMLIIIexpressing cells. Reduction of PML was carried out by the EMCV 3C protease. Indeed, at early times postinfection, EMCV induced PML transfer from the nucleoplasm to the nuclear matrix and PML conjugation to SUMO-1, SUMO-2, and SUMO-3, leading to an increase in PML body size where the viral protease 3C and the proteasome component were found colocalizing with PML within the NBs. This process was followed by PML degradation occurring in a proteasome-and SUMO-dependent manner and did not involve the SUMOinteracting motif of PML. Together, these findings reveal a new mechanism evolved by EMCV to antagonize the PML pathway in the interferon-induced antiviral defense.The PML (promyelocytic leukemia) gene was originally identified through its fusion with the RAR␣ gene in the t(15;17) translocation found in acute promyelocytic leukemia (APL) (14). PML, also known as TRIM19, is expressed in the diffuse nuclear fraction of the nucleoplasm and in matrixassociated structures, known as nuclear bodies (NBs) (17, 30). PML functions as the organizer of PML NBs, which are dynamic structures harboring numerous transiently and permanently localized proteins (35). The RBCC/TRIM motif, which contains a C 3 HC 4 (RING finger) zinc-binding domain, two cysteine/histidine-rich motifs (the B boxes B1 and B2), an a helical coiled-coil region (RBCC), is embedded within the PML protein and is required for PML NB formation (28). Due to alternative splicing, many PML isoforms are synthesized, and they are classified into seven groups, designated PML I to PML VII (reviewed in reference 28). They share the N-terminal region (exons 1 to 3), which encodes the RBCC motif, whereas they differ in their C-terminal regions. Posttranslational modification of PML by SUMO (small ubiquitin-like modifier), a ubiquitin-like protein of 11 kDa, is another requirement for PML NB formation. SUMO is covalently coupled to PML through its lysines 65, 160, and 490 via a process called SUMOylation (29). The noncovalent interaction of SUMO with PML through a SUMO-interacting motif (SIM; also named SBD for SUMO binding domain) (46) has been sug...

Human T-cell lymphotropic virus type I (HTLV-I) is the causative agent of adult T-cell leukemia/lymphoma (ATL). ATL is an aggressive proliferation of mature activated T cells associated with a poor prognosis. The combination of the antiviral agents, zidovudine (AZT) and  interferon (IFN), is a potent treatment of ATL. Recently, arsenic trioxide (As) was shown to be an effective treatment of acute promyelocytic leukemia (APL). We have tested the effects of the combination of As and IFN on cell proliferation, cell cycle phases distribution, and apoptosis in ATL-derived or control T-cell lines. A high synergistic effect between IFN and As was observed in ATL-derived cell lines in comparison to the control cell lines, with a dramatic inhibition of cell proliferation, G1 arrest, and induction of apoptosis. Similar results were obtained with fresh leukemia cells derived from an ATL patient. Although the mechanisms involved are unclear, these results could provide a rational basis for combined As and IFN treatments in ATL.