Nuclear factor [kappa]B up-regulation of CCAAT/enhancer-binding protein [beta] mediates hepatocyte resistance to tumor necrosis factor [alpha] toxicity

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