SUMO-Modification of Human Nrf2 at K110 and K533 Regulates Its Nucleocytoplasmic Localization, Stability and Transcriptional Activity.

BACKGROUND/AIMS: Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that binds to the antioxidant response element(s) (ARE) in target gene promoters, enabling oxidatively stressed cells to respond in order to restore redox homeostasis. Post-translational modifications (PTMs) that mediate activation of Nrf2, in the cytosol and its release from Keap1, have been extensively studied but PTMs that impact its biology after activation are beginning to emerge. In this regard, PTMs like acetylation, phosphorylation, ubiquitination and sumoylation contribute towards the Nrf2 subcellular localization, and its transactivation function. We previously demonstrated that Nrf2 traffics to the promyelocytic leukemia-nuclear bodies (PML-NB), where it is a target for modification by small ubiquitin-like modifier (SUMO) proteins (sumoylation), but the site(s) for SUMO conjugation have not been determined. In this study, we aim to identify SUMO-2 conjugation site(s) and explore the impact, sumoylation of the site(s) have on Nrf2 stability, nuclear localization and transcriptional activation of its target gene expression upon oxidative stress. METHODS: The putative SUMO-binding sites in Nrf2 for human isoform1 (NP_006155.2) and mouse homolog (NP_035032.1) were identified using a computer-based SUMO-predictive software (SUMOplot™). Site-directed mutagenesis, immunoblot analysis, and ARE-mediated reporter gene assays were used to assess the impact of sumoylation on these site(s) in vitro. Effect of mutation of these sumoylation sites of Nrf2 on expression of Heme Oxygenase1 (HO-1) was determined in HEK293T cell. RESULTS: Eight putative sumoylation sites were identified by SUMOplot™ analysis. Out of the eight predicted sites only one 532LKDE535 of human (h) and its homologous 524LKDE527 of mouse (m) Nrf2, exactly matches the SUMO-binding consensus motif. The other high probability SUMO-acceptor site identified was residue K110, in the motifs 109PKSD112 and 109PKQD112 of human and mouse Nrf2, respectively. Mutational analysis of putative sumoylation sites (human (h)/mouse (m)K110, hK533 and mK525) showed that these residues are needed for SUMO-2 conjugation, nuclear localization and ARE driven transcription of reporter genes and the endogenous HO-1 expression by Nrf2. These residues also stabilized Nrf2, as evident from shorter half-lives of the mutant protein compared to wild-type Nrf2. CONCLUSION: Our findings indicate that SUMO-2mediated sumoylation of K110 and K533 in human Nrf2 regulates in part its transcriptional activity by enhancing its stabilization and nuclear localization.

Arkadia (RING Finger Protein 111) Mediates Sumoylation-Dependent Stabilization of Nrf2 Through K48-Linked Ubiquitination.

BACKGROUND/AIMS: The transcription factor Nrf2 is a master regulator of the antioxidant defense system, protecting cells from oxidative damage. We previously reported that the SUMO-targeted E3 ubiquitin ligase (STUbL), RING finger protein 4 (RNF4) accelerated the degradation rate of Nrf2 in promyelocytic leukemia-nuclear body (PML-NB)-enriched fractions and decreased Nrf2-mediated gene transcription. The mechanisms that regulate Nrf2 nuclear levels are poorly understood. In this study, we aim to explore the role of the second mammalian STUbL, Arkadia/RNF111 on Nrf2. METHODS: Arkadia mediated ubiquitination was detected using co-immunoprecipitation assays in which whole cell lysates were immunoprecipated with anti-Nrf2 antibody and Western blotted with anti-hemagglutinin (HA) antibody or anti-Lys-48 ubiquitin-specific antibody. The half-life of Nrf2 was detected in whole cell lysates and promyelocytic leukemia-nuclear body enriched fractions by cycloheximide-chase. Reporter gene assays were performed using the antioxidant response element (ARE)-containing promoter Heme oxygenase-1 (HO-1). RESULTS: We show that Arkadia/RNF111 is able to ubiquitinate Nrf2 resulting in the stabilization of Nrf2. This stabilization was mediated through Lys-48 ubiquitin chains, contrary to traditionally degradative role of Lys-48 ubiquitination, suggesting that Lys-48 ubiquitination of Nrf2 protects Nrf2 from degradation thereby allowing Nrf2-dependent gene transcription. CONCLUSION: Collectively, these findings highlight a novel mechanism to positively regulate nuclear Nrf2 levels in response to oxidative stress through Arkadia-mediated K48-linked ubiquitination of Nrf2.

Trafficking of the transcription factor Nrf2 to promyelocytic leukemia-nuclear bodies: implications for degradation of NRF2 in the nucleus.

Ubiquitylation of Nrf2 by the Keap1-Cullin3/RING box1 (Cul3-Rbx1) E3 ubiquitin ligase complex targets Nrf2 for proteasomal degradation in the cytoplasm and is an extensively studied mechanism for regulating the cellular level of Nrf2. Although mechanistic details are lacking, reports abound that Nrf2 can also be degraded in the nucleus. Here, we demonstrate that Nrf2 is a target for sumoylation by both SUMO-1 and SUMO-2. HepG2 cells treated with As2O3, which enhances attachment of SUMO-2/3 to target proteins, increased SUMO-2/3-modification (polysumoylation) of Nrf2. We show that Nrf2 traffics, in part, to promyelocytic leukemia-nuclear bodies (PML-NBs). Cell fractions harboring key components of PML-NBs did not contain biologically active Keap1 but contained modified Nrf2 as well as RING finger protein 4 (RNF4), a poly-SUMO-specific E3 ubiquitin ligase. Overexpression of wild-type RNF4, but not the catalytically inactive mutant, decreased the steady-state levels of Nrf2, measured in the PML-NB-enriched cell fraction. The proteasome inhibitor MG-132 interfered with this decrease, resulting in elevated levels of polysumoylated Nrf2 that was also ubiquitylated. Wild-type RNF4 accelerated the half-life (t½) of Nrf2, measured in PML-NB-enriched cell fractions. These results suggest that RNF4 mediates polyubiquitylation of polysumoylated Nrf2, leading to its subsequent degradation in PML-NBs. Overall, this work identifies Nrf2 as a target for sumoylation and provides a novel mechanism for its degradation in the nucleus, independent of Keap1.