Hepatocyte-specific role of the deubiquitinating enzyme OTUB1 during inflammatory liver diseases

Abstract

Both infectious and sterile liver inflammation may trigger death of the hepatocytes. In particular, in acute hepatitis, liver failure may develop due to an excessive hepatocellular death as a consequence of an imbalanced survival and death signaling. Both survival and death signaling are tightly regulated by post-translational modifications, including ubiquitination and deubiquitination. Here, we investigated the hepatocyte-specific function of the deubiquitinating enzyme OTUB1 during infection with the intracellular bacterium Listeria monocytogenes and acute liver injury induced by DGal/TNF using conditional OTUB1 knockout mice (OTUB1LPC - KO). Upon both Listeria infection and DGal/TNF challenge, the OTUB1LPC-KO mice developed severe liver damage and synthetized increased TNF levels in the liver as compared to the OTUB1FL control mice. The aggravated liver damage resulted in liver failure and ultimately in death of OTUB1LPC-KO mice. The enhanced liver damage in the absence of OTUB1 expression in hepatocytes was a consequence of an augmented TNF-driven necroptotic hepatocellular death which was further amplified by the enhanced hepatocyte-intrinsic production of TNF. Death of OTUB1-deficient hepatocytes could be prevented by Necrostatin-1s treatment and genetic ablation of MLKL, the executioner of necroptosis, respectively. Consequently, Listeria-infected OTUB1LPC-KO mice treated with Necrostatin-1s or deficient for MLKL survived listeriosis with minimal liver damage equivalent to OTUB1FL control mice. Mechanistically, hepatocyte-specific OTUB1 prevented cell death by protecting c-IAP1 from proteasomal degradation by reducing its K48-linked polyubiquitination. This stabilized c-IAP1 in turn conjugated K63-linked polyubiquitin chains RIPK1, thereby acting as scaffold for survival NF-kappaB activation while inhibiting its kinase activity required for the induction of necroptosis. In the absence of OTUB1, the kinase active RIPK1 further amplified the hepatocyte-intrinsic TNF production in a RIPK1/ERK-dependent manner which augmented the hepatocellular death in these mice. Taken together, our studies identified OTUB1 as a hepatocyte-intrinsic inhibitor of necroptosis and TNF-production in both bacterial- and TNF-induced inflammation.