12 Hepatocytes are normally Raf inhibitor resistant to TNFα cytotoxicity through TNFα-induced activation of the transcription factor nuclear factor κB (NF-κB).13, 14 Loss of NF-κB activity in hepatocytes in culture,14 or in vivo,15 sensitizes the cells to death through this apoptotic pathway.10, 13, 14 TNFα-dependent liver injury from hepatotoxins such as carbon tetrachloride, galactosamine, and alcohol may result from a block in protective NF-κB signaling. A mechanism of NF-κB–mediated resistance to TNFα toxicity is down-regulation

of the mitogen-activated protein kinase c-Jun N-terminal kinase (JNK).16–18 In the absence of NF-κB signaling, TNFα-induced JNK activation is converted from a transient to a prolonged response that triggers cell death. Although the central function of JNK is to increase gene transcription through the phosphorylation and activation of the activator protein-1 component

c-Jun, JNK regulates TNFα toxicity through nontranscriptional effects on protein degradation. The induction of cell death by JNK overactivation occurs in part from alterations in the half-lives of two proteins that mediate hepatocyte TNFα resistance: cFLIP and Mcl-1.19, 20 Loss of the transcription factor NF-κB therefore sensitizes hepatocytes to TNFα cytotoxicity in part through JNK-dependent effects on protein degradation. CCAAT/enhancer-binding protein β (C/EBPβ) is a member of a family of transcription factors that regulate several critical cellular functions, including apoptosis.21 C/EBPβ has three protein forms that in EPZ-6438 rodents are termed LAP1 (38 kDa),

LAP2 (34 kDa), and LIP (20 kDa).21 LAP1 and LAP2 act as transcriptional activators and LIP as a MCE公司 dominant negative. C/EBPβ promotes cell survival in several nonhepatic cell types after DNA damage.22–24 In addition, a novel nontranscriptional function of C/EBPβ as a caspase inhibitor has been described in hepatic stellate cells.22 Whether C/EBPβ performs this function in other cell types is unknown. In hepatocytes, C/EBPβ promotes apoptosis from the death receptor Fas.25 C/EBPβ regulation by TNFα has been shown to occur in hepatocytes at the level of subcellular localization as TNFα induces C/EBPβ translocation to the nucleus, where it regulates hepatocyte differentiation and proliferation through effects on gene transcription.26–28 It is unknown whether C/EBPβ functions in hepatocyte death from TNFα. Galactosamine/lipopolysaccharide (GalN/LPS)-induced liver injury is a well-established experimental model of TNFα-dependent hepatic injury.29, 30 GalN is a hepatocyte-specific transcriptional inhibitor that at subtoxic doses sensitizes hepatocytes to death from LPS-induced TNFα. A feature of this model is that protein changes induced by LPS alone can be contrasted with those from combined GalN/LPS treatment to identify protective proteins whose induction by TNFα is blocked by GalN.