Ubc9‐mediated SUMOylation of Ninj1 alleviates inflammatory responses in hepatic ischaemia/reperfusion injury

BACKGROUND: Hepatic ischaemia/reperfusion (I/R) injury poses a common clinical dilemma encountered during liver transplantation (LT), characterised by substantial cellular death and inflammation reactions. Ubc9, the sole E2 conjugating enzyme of SUMOylation, has long been recognised to regulate diverse biological and pathological processes. However, its impact on I/R-induced liver damage is yet to be elucidated. METHODS: The expression levels of UBC9 in patients undergoing LT were analysed. Hepatocyte-specific Ubc9-deficient or transgenic mice were utilised in an in vivo model of hepatic I/R, alongside in vitro experiments that employed hypoxia/reoxygenation stimulation. The investigation focused on Ubc9's role in liver damage due to I/R and the underlying mechanisms through a range of phenotypic analyses and biological techniques. RESULTS: Herein, we found that hepatic tissues from patients with LT are featured by a significant downregulation of UBC9 expression. Studies in 68 donor hepatic biopsies further demonstrated a negative correlation between UBC9 expression and liver injury in patients with LT. Similarly, murine liver I/R was coupled with an obvious decrease in Ubc9 expression. Hepatocyte deficient in Ubc9 exacerbated liver injury in liver I/R, while Ubc9-overexpression showed the opposite phenotype. Mechanistically, Ubc9-mediated SUMOylation of Ninj1 at lysine K103 inhibited its membrane localisation and damage-associated molecular patterns (DAMPs) release in hepatocytes, subsequently inhibited nuclear factor-kappa B (NF-κB) signalling in macrophages and curtailing inflammatory cytokines production. CONCLUSIONS: These findings further suggest that Ubc9-mediated SUMOylation of Ninj1 at lysine K103 may represent a potential therapeutic strategy for safeguarding the liver against I/R injury in clinical settings. KEY POINTS: Ubc9 expression is downregulated in hepatocytes during hepatic ischaemia/reperfusion (I/R) injury. Higher UBC9 expression is associated with improved post-liver transplantation (LT) liver function. Ubc9 ameliorates liver damage and inflammation responses in hepatic I/R injury. Ubc9-mediated Ninj1 SUMOylation at K103 is essential for regulating the subcellular distribution of Ninj1. Ubc9 inhibits the release of hepatocyte-derived damage-associated molecular patterns (DAMPs) in a Ninj1 K103 SUMOylation-dependent manner.