ABRO1 Deficiency Selectively Inhibits The Activation Of LPS/TLR4 Signaling Pathway In Mouse Kupffer Cells

ABRO1(Abraxas brother 1),also known as KIAA0157 or FAM175 B,together with BRCC36/BRCC3,MERIT40/NBA1,and BRCC45/BRE,assembles into the BRCC36 isopeptidase complex(BRISC)that are specific for cleaving K63-linked ubiquitin chains.Among these components,BRCC3 is the catalytic subunit,while ABRO1 served as a scaffold for assembly of the complex.In murine bone marrow-derived macrophages(BMDMs),BRISC plays a critical role in regulating inflammation by promoting the activation of type 1interferon(IFN)receptor chain 1(IFNAR1)and NLRP3 inflammasome.However,the function of BRISC in Kupffer cells and its role in Toll like receptor 4(TLR4)signaling pathway are still unclear.Previous studies in our lab have shown that ABRO1-knockout mice protected against LPS/D-Gal N-induced acute liver injury,and this hepatoprotective effect was dependent on Kupffer cells.ABRO1 deficiency inhibited LPS-induced activation of NF-κB pathway,thereby reducing the expression and release of proinflammatory cytokines,such as TNF-α and IL-6.Based on these findings,this study further revealed that lack of ABRO1 does not affect LPS/TLR4 pathway activation in mouse embryonic fibroblasts(MEFs),BMDMs,resident peritoneal macrophages,and neutrophils,which indicates that ABRO1 may regulate TLR4 pathway in a cell-specific manner.Mechanistically,we demonstrated that ABRO1 binds to the E3 ubiquitin ligase HOIP(HOIL-1-interacting protein)and enhances the linear ubiquitin chain assembly complex(LUBAC)activity.LUBAC is a multisubunit complex containing HOIP,HOIL-1L,and SHARPIN,which plays a critical role in TLR4/ NF-κB pathway.The activation of LUBAC is self-restricted by the ubiquitination of HOIP.Our results showed that there are interactions between ABRO1 and HOIP,and LPS enhances the binding of the two in Kupffer cells.ABRO1 reduces the K63-linked ubiquitination and linear ubiquitination of HOIP,and promotes the linear ubiquitination of IRAK1 and NEMO,two substrates of LUBAC.Taken together,we found that HOIP is a new substrate of BRISC and revealed the possible regulatory mechanism of BRISC in the activation of LPS/TLR4 pathway.Moreover,our findings may bring new insights into the understanding of the heterogeneity in response to LPS.