Nuclear factor κB up-regulation of CCAAT/enhancer-binding protein ß mediates hepatocyte resistance to tumor necrosis factor α toxicity.

ABSTRACT

UNLABELLED 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 ß (C/EBPß) were increased after LPS treatment but not GalN/LPS treatment. In a nontransformed rat hepatocyte cell line, TNFα-induced increases in C/EBPß 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/EBPß degradation, indicating that the process was JNK-independent. C/EBPß functioned to prevent cell death as adenoviral C/EBPß overexpression blocked TNFα-induced apoptosis in cells sensitized to TNFα toxicity by NF-κB inhibition. C/EBPß 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/ebpß(-/-) mice confirmed that loss of C/EBPß increased death from TNFα. c/ebpß(-/-) 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/EBPß induction by LPS, again demonstrating that C/EBPß degradation was JNK-independent. CONCLUSION: C/EBPß 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/EBPß is increased by a JNK-independent mechanism and promotes death from TNFα.