Increased hepatic receptor interacting protein kinase 3 expression due to impaired proteasomal functions contributes to alcohol-induced steatosis and liver injury

Abstract

Chronic alcohol exposure increased hepatic receptor-interacting protein kinase (RIP) 3 expression and necroptosis in the liver but its mechanisms are unclear. In the present study, we demonstrated that chronic alcohol feeding plus binge (Gao-binge) increased RIP3 but not RIP1 protein levels in mouse livers. RIP3 knockout mice had decreased serum alanine amino transferase activity and hepatic steatosis but had no effect on hepatic neutrophil infiltration compared with wild type mice after Gao-binge alcohol treatment. The hepatic mRNA levels of RIP3 did not change between Gao-binge and control mice, suggesting that alcohol-induced hepatic RIP3 proteins are regulated at the posttranslational level. We found that Gao-binge treatment decreased the levels of proteasome subunit alpha type-2 (PSMA2) and proteasome 26S subunit, ATPase 1 (PSMC1) and impaired hepatic proteasome function. Pharmacological or genetic inhibition of proteasome resulted in the accumulation of RIP3 in mouse livers. More importantly, human alcoholics had decreased expression of PSMA2 and PSMC1 but increased protein levels of RIP3 compared with healthy human livers. Moreover, pharmacological inhibition of RIP1 decreased Gao-binge-induced hepatic inflammation, neutrophil infiltration and NF-κB subunit (p65) nuclear translocation but failed to protect against steatosis and liver injury induced by Gao-binge alcohol. In conclusion, results from this study suggest that impaired hepatic proteasome function by alcohol exposure may contribute to hepatic accumulation of RIP3 resulting in necroptosis and steatosis while RIP1 kinase activity is important for alcohol-induced inflammation.

Differential contribution of complement receptor C5aR in myeloid and non-myeloid cells in chronic ethanol-induced liver injury in mice

“…Necroptosis, a caspase-independent form of programmed cell death, also contributes to ethanol-induced liver injury [22, 33]. Because TNFα is an important activator of the necroptotic pathway [22,33], we also expected for necroptosis to be reduced in C5aR−/− mice compared to WT, since they expressed less TNFα in liver after chronic ethanol exposure.…”

“…Because TNFα is an important activator of the necroptotic pathway [22,33], we also expected for necroptosis to be reduced in C5aR−/− mice compared to WT, since they expressed less TNFα in liver after chronic ethanol exposure. Ethanol feeding increased the expression of receptor interacting protein kinase 3 (RIP3), an important effector protein in necroptosis, in the centralobular zone in WT mice; this induction was similar in C5aR−/− mice (Figure 3B/D).…”

Caspase Inhibition Prevents Tumor Necrosis Factor-α–Induced Apoptosis and Promotes Necrotic Cell Death in Mouse Hepatocytes in Vivo and in Vitro

“…43 It should be noted that we and others recently demonstrated that the expression levels of RIP1 and RIP3 are remarkably higher in immune cells than in hepatocytes. 39,44 Therefore, it is highly likely that the functions of RIP1 and RIP3 in nonparenchymal cells may play important roles in LPS/GalN/ZVD-induced liver injury, which could be different from their direct role in cultured hepatocytes. Indeed, we found the mRNA levels of TNF-a and IL-1b were significantly higher in the livers of mice treated with RIP1 inhibitor and in RIP3 KO mouse livers after LPS/ GalN/ZVD administration.…”

Necroptosis: Mechanisms and Relevance to Disease

“…Thus, the Ripk3 −/− genotype, the intraperitoneal delivery of Ripk3 -targeting antisense oligonucleotides, or the administration of dabrafenib (an FDA-approved inhibitor of oncogenic BRAF that also suppresses RIPK3 kinase activity) provides considerable hepatoprotection in mouse models of acetaminophen toxicity (134, 170, 171), concanavalin A–driven hepatitis (134), ethanol and dietary intoxication (172, 173), and nonalcoholic steatohepatitis (NASH) (174, 175). Along similar lines, mice treated with Nec-1 exhibit an increased resistance to the hepatotoxic effects of acetaminophen as compared with control animals (134).…”