Hepatic RACK1 deficiency protects against fulminant hepatitis through myeloid-derived suppressor cells.

Fulminant hepatitis (FH) is a life-threatening disease with partially understood pathogenesis. It has been demonstrated that myeloid-derived suppressor cells (MDSCs) are recruited into the liver during this process, and their augmented accumulation by various strategies protects against liver injury. However, the underlying mechanism(s) remain elusive. Receptor for activated C kinase 1 (RACK1), a multi-functional scaffold protein, is highly expressed in normal liver and has been implicated in liver physiology and diseases, but the in vivo role of hepatic RACK1 in FH remains unknown. Methods: Survival curves and liver damage were monitored to investigate the in vivo role of hepatic RACK1 in FH. The liver microenvironment was explored by microarray-based transcriptome analysis, flow cytometry, immunoblotting, and immunohistochemistry. MDSCs were identified with phenotypic and functional characteristics. Functional antibodies were used to target MDSCs. Co-culture techniques were used to study the underlying mechanism(s) of protection. The interaction of RACK1 with histone deacetylase 1 (HDAC1) and the consequent effects on HDAC1 ubiquitination were analyzed. Ectopic expression of HDAC1 with recombinant adeno-associated virus serotype 8 was conducted to confirm the role of HDAC1 in the protective effects of hepatic RACK1 deficiency against FH. Post-translational modifications of RACK1 were also investigated during the induction of FH. Results: Liver-specific RACK1 deficiency rendered mice resistant to FH. RACK1-deficient livers exhibited high basal levels of chemokine (C-X-C motif) ligand 1 (CXCL1) and S100 calcium-binding protein A9 (S100A9), associated with MDSC accumulation under steady-state conditions. Targeting MDSCs with an antibody against either Gr1 or DR5 abrogated the protective effects of liver-specific RACK1 deficiency. Accumulated MDSCs inhibited inflammatory cytokine production from macrophages and enhanced IκB kinase (IKK)/NF-κB pathway activation in hepatocytes. Further investigation revealed that RACK1 maintained HDAC1 protein level in hepatocytes by direct binding, thereby controlling histone H3K9 and H3K27 acetylation at the Cxcl1 and S100a9 promoters. Ectopic expression of HDAC1 in livers with RACK1 deficiency partially reversed the augmented Cxcl1/S100a9 → MDSCs → IKK/NF-κB axis. During FH induction, RACK1 was phosphorylated at serine 110, enhancing its binding to ubiquitin-conjugating enzyme E2T and promoting its ubiquitination and degradation. Conclusion: Liver-specific RACK1 deficiency protects against FH through accelerated HDAC1 degradation and the consequent CXCL1/S100A9 upregulation and MDSC accumulation.

RACK1 T50 Phosphorylation by AMPK Potentiates Its Binding with IRF3/7 and Inhibition of Type 1 IFN Production.

The receptor for activated C kinase 1 (RACK1) adaptor protein has been implicated in viral infection. However, whether RACK1 promotes in vivo viral infection in mammals remains unknown. Moreover, it remains elusive how RACK1 is engaged in antiviral innate immune signaling. In this study, we report that myeloid RACK1 deficiency does not affect the development and survival of myeloid cells under resting conditions but renders mice less susceptible to viral infection. RACK1-deficient macrophages produce more IFN-α and IFN-ß in response to both RNA and DNA virus infection. In line with this, RACK1 suppresses transcriptional activation of type 1 IFN gene promoters in response to virus infection. Analysis of virus-mediated signaling indicates that RACK1 inhibits the phosphorylation of IRF3/7. Indeed, RACK1 interacts with IRF3/7, which is enhanced after virus infection. Further exploration indicates that virus infection triggers AMPK activation, which in turn phosphorylates RACK1 at Thr50 RACK1 phosphorylation at Thr50 enhances its interaction with IRF3/7 and thereby limits IRF3/7 phosphorylation. Thus, our results confirm that myeloid RACK1 promotes in vivo viral infection and provide insight into the control of type 1 IFN production in response to virus infection.

[Establishment and evaluation of hepatocyte injury model induced by LPS/D-galactosamine in vitro].

Objective To establish a novel hepatocyte injury model induced by lipopolysaccharide/D-galactosamine (LPS/D-GalN) in vitro. Methods Freshly isolated mouse primary hepatocytes were cultured in vitro and treated with different doses of tumor necrosis factor-α (TNF-α) and 5 mg/mL of D-GalN. The supernatants from hepatocyte culture were detected for alanine aminotransferase (ALT) activity by chemiluminescence assay. Bone marrow-derived macrophages (BMDMs) were stimulated with 1 µg/mL of LPS and the level of TNF-α in supernatants were detected by ELISA. Primary hepatocytes were treated with the BMDM supernatants combined with 5 mg/mL D-GalN or 50 ng/mL actinomycin D (ActD) for 24 hours. The level of ALT from hepatocyte supernatant was detected and morphology of hepatocytes was observed with microscopy. BMDMs and hepatocytes were co-cultured and treated with 1 µg/mL of LPS combined with D-GalN or ActD for 24 hours. Hepatocyte injury was reflected by the ALT activity and hepatocyte morphology. Results The ALT activity was significantly increased in the supernatants of hepatocytes treated with TNF-α and D-GalN, indicating the obvious hepatocyte injury. Co-treatment with LPS-primed BMDM supernatants and D-GalN or ActD could cause hepatocyte injury, as reflected by markedly increased ALT activity and the deformed and cracked hepatocytes. In the context of co-culture of BMDM and hepatocytes, treatment with LPS and D-GalN led to obvious hepatocyte injury as expected. LPS combined with ActD could not cause hepatocyte injury, since the BMDMs started to die earlier than they could secret TNF-α to destruct hepatocytes. Hepatocytes with normal morphology and deformed BMDMs were observed. Conclusion LPS/D-GalN can be used to induce hepatocyte injury in vitro. D-GalN, rather than ActD, should be used as a transcriptional inhibitor when the TNF-α -induced hepatocyte injury is evaluated in a co-culture system of BMDMs and hepatocytes.