The sensitization of hepatocytes to cell death from tumor necrosis factor α (TNFα) underlies many forms of hepatic injury, including that from toxins. Critical for hepatocyte resistance to TNFα toxicity is activation of nuclear factor κB (NF-κB) signaling, which prevents TNFα-induced death by the up-regulation of protective proteins. To further define the mechanisms of hepatocyte sensitization to TNFα killing, immunoblot analysis comparing livers from mice treated with lipopolysaccharide (LPS) alone or LPS together with the hepatotoxin galactosamine (GalN) was performed to identify TNFα-induced protective proteins blocked by GalN. Levels of CCAAT/enhancer-binding protein β (C/EBPβ) were increased after LPS treatment but not GalN/LPS treatment. In a nontransformed rat hepatocyte cell line, TNFα-induced increases in C/EBPβ protein levels were dependent on NF-κB-mediated inhibition of proteasomal degradation. Pharmacological inhibition of c-Jun N-terminal kinase (JNK) did not affect C/EBPβ degradation, indicating that the process was JNK-independent. C/EBPβ functioned to prevent cell death as adenoviral C/EBPβ overexpression blocked TNFα-induced apoptosis in cells sensitized to TNFα toxicity by NF-κB inhibition. C/EBPβ inhibited TNFα-induced caspase 8 activation and downstream mitochondrial cytochrome c release and caspase 3 and caspase 7 activation. Studies in primary hepatocytes from c/ebpβ(-/-) mice confirmed that loss of C/EBPβ increased death from TNFα. c/ebpβ(-/-) mice were also sensitized to liver injury from a nontoxic dose of LPS or TNFα. The absence of jnk2 failed to reverse the GalN-induced block in C/EBPβ induction by LPS, again demonstrating that C/EBPβ degradation was JNK-independent.
Conclusion: C/EBPβ is up-regulated by TNFα and mediates hepatocyte resistance to TNFα toxicity by inhibiting caspase-dependent apoptosis. In the absence of NF-κB signaling, proteasomal degradation of C/EBPβ is increased by a JNK-independent mechanism and promotes death from TNFα.
Copyright © 2010 American Association for the Study of Liver Diseases.