Chordate PIAS proteins act as conserved repressors of the TRAF6 self-polyubiquitination

TRIM28 functions as SUMO ligase to SUMOylate TRAF6 and regulate NF-κB activation in HBV-replicating cells

BACKGROUND

Hepatitis B virus (HBV) is a pathogen that poses a serious threat to human health. The interaction between HBV and host has made great progress in recent years. SUMOylation is involved in virus-related cancer progression, but there are fewer studies on the mechanism of SUMOylation on HBV replication and antiviral defense. Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a critical adaptor of the NF-κB pathways. Here, we focus on the roles of TRIM28 in regulating TRAF6 SUMOylation in HBV-replicating cells.

METHODS

The SUMO1-modified TRAF6 proteins were enriched from total cellular proteins by immunoprecipitation with anti-SUMO1 antibody, then the SUMOylated TRAF6 was detected by western blot using an anti-TRAF6 antibody. The interaction between TRAF6 and TRIM28 was identified by immunoprecipitation and LC-MS/MS. The modification sites of TRAF6 SUMOylation were identified by amino acid site mutation. Expression and localization of TRAF6 and TRIM28 were assessed by immunohistochemistry and immunofluorescence. The hydrodynamic injection HBV mouse model was used to determine the function of TRIM28-mediated TRAF6 SUMOylation in vivo.

RESULTS

The results show that the levels of SUMO1-modified TRAF6 are elevated in HBV-replicating cells. Lys453 is a major SUMO1 modification site of TRAF6. There is an antagonistic interaction between SUMOylation and ubiquitination of TRAF6 protein. The SUMO ligase TRIM28 is responsible for catalyzing TRAF6 SUMOylation. Compared to the wild-type TRAF6, its SUMO site mutant TRAF6K453R promotes NF-κB activation. Moreover, TRIM28 overexpression attenuates TRAF6-mediated NF-κB activation, thereby inhibiting HBV replication in vivo.

CONCLUSIONS

Our findings demonstrate that SUMO ligase TRIM28 affects the ability of TRAF6 on NF-κB activation, nucleocytoplasmic shuttling and HBV replication-related indicators. Our data reveal that TRIM28-mediated SUMOylation of TRAF6 is a novel mechanism to regulate the inflammatory response, which may pave the way for new strategies to control anti-HBV.

SUMOylation and DeSUMOylation: Prospective therapeutic targets in cancer

The SUMO family is a type of ubiquitin-like protein modification molecule. Its protein modification mechanism is similar to that of ubiquitination: both involve modifier-activating enzyme E1, conjugating enzyme E2 and substrate-specific ligase E3. However, polyubiquitination can lead to the degradation of substrate proteins, while poly-SUMOylation only leads to the degradation of substrate proteins through the proteasome pathway after being recognized by ubiquitin as a signal factor. There are currently five reported subtypes in the SUMO family, namely SUMO1-5. As a reversible dynamic modification, intracellular sentrin/SUMO-specific proteases (SENPs) mainly regulate the reverse reaction pathway of SUMOylation. The SUMOylation modification system affects the localization, activation and turnover of proteins in cells and participates in regulating most nuclear and extranuclear molecular reactions. Abnormal expression of proteins related to the SUMOylation pathway is commonly observed in tumors, indicating that this pathway is closely related to tumor occurrence, metastasis and invasion. This review mainly discusses the composition of members in the protein family related to SUMOylation pathways, mutual connections between SUMOylation and other post-translational modifications on proteins as well as therapeutic drugs developed based on these pathways.

SUMO and PIAS repress NF-κB activation in a basal chordate

Small ubiquitin-like modifier (SUMO) regulates various biological processes, including the MyD88/TICAMs-IRAKs-TRAF6-NF-κB pathway, one of the core immune pathways. However, its functions are inconsistent between invertebrates and vertebrates and have rarely been investigated in lower chordates, including amphioxus and fishes. Here, we investigated the SUMOylation gene system in the amphioxus, a living basal chordate. We found that amphioxus has a SUMOylation system that has a complete set of genes and preserves several ancestral traits. We proceeded to study their molecular functions using the mammal cell lines. Both amphioxus SUMO1 and SUMO2 were shown to be able to attach to NF-κB Rel and to inhibit NF-κB activation by 50-75% in a dose-dependent fashion. The inhibition by SUMO2 could be further enhanced by the addition of the SUMO E2 ligase UBC9. In comparison, while human SUMO2 inhibited RelA, human SUMO1 slightly activated RelA. We also showed that, similar to human PIAS1-4, amphioxus PIAS could serve as a SUMO E3 ligase and promote its self-SUMOylation. This suggests that amphioxus PIAS is functionally compatible in human cells. Moreover, we showed that amphioxus PIAS is not only able to inhibit NF-κB activation induced by MyD88, TICAM-like, TRAF6 and IRAK4 but also able to suppress NF-κB Rel completely in the presence of SUMO1/2 in a dose-insensitive manner. This suggests that PIAS could effectively block Rel by promoting Rel SUMOylation. In comparison, in humans, only PIAS3, but not PIAS1/2/4, has been reported to promote NF-κB SUMOylation. Taken together, the findings from amphioxus, together with those from mammals and other species, not only offer insights into the functional volatility of the animal SUMO system, but also shed light on its evolutionary transitions from amphioxus to fish, and ultimately to humans.

Identification and Allelic Variants Associated With Cold Tolerance of PmPIAS in Pinctada fucata martensii

The protein inhibitor of activated STAT (PIAS) functions in diverse aspects, including immune response, cell apoptosis, cell differentiation, and proliferation. In the present study, the PIAS in the pearl oyster Pinctada fucata martensii was characterized. The sequence features of PmPIAS were similar to that of other PIAS sequences with PIAS typical domains, including SAP, Pro-Ile-Asn-Ile-Thr (PINIT), RLD domain, AD, and S/T-rich region. Homologous analysis showed that PmPIAS protein sequence showed the conserved primary structure compared with other species. Ribbon representation of PIAS protein sequences also showed a conserved structure among species, and the PINIT domain and RLD domain showed the conserved structure compared with the sequence of Homo sapiens. The expression pattern of PmPIAS in different tissues showed significant high expression in the gonad. PmPIAS also exhibited a significantly higher expression in the 1 and 2 days after cold tolerance stress (17°C) and showed its potential in the cold tolerance. The SNP analysis of the exon region of PmPIAS obtained 18 SNPs, and among them, 11 SNPs showed significance among different genotypes and alleles between cold tolerance selection line and base stock, which showed their potential in the breeding for cold tolerance traits.

Duck PIAS2 negatively regulates RIG-I mediated IFN-β production by interacting with IRF7

The protein inhibitor of activated STAT (PIAS) proteins are important signal transduction modulator family and regulate the innate immune signaling pathway induced by certain transcription factors, including NF-κB, IRF3, and JAK/STAT. The PIAS protein mechanism that regulates innate immune response in mammals has been well described in the literature; however, whether the PIAS gene exists in ducks as well as the role of PIAS in duck IFN-β expression is still unclear. Here, we cloned duck PIAS (duPIAS), finding PIAS2 could repress IFN-β production. DuPIAS2 contains SAP-PINIT-RLD-S/T characteristic domains, and its overexpression could inhibit virus-induced IFN-β promoter activation. Moreover, duPIAS2 interacts with duck interferon regulatory factor 7 (IRF7) and inhibits IFN-β promoter activation induced by duck IRF7. Additionally, its inhibitory function does not rely on its SUMO E3 ligase activity but rather its C-terminal portion. The above results demonstrate that duPIAS2 is a repressor of IFN-β production induced by duck IRF7.