The Role of TNF in Hepatotoxicity

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Tumor necrosis factor alpha (TNFalpha) has been demonstrated to be a primary mediator of endotoxin shock. One mechanism of TNF-induced injury is via the activation of NF-kB and the generation of reactive oxygen species. We have hypothesized that 2,3,7,8-tetrachlorodibenzodioxin (TCDD) -induced endotoxin hypersensitivity and subsequent induction of apoptosis may occur through modulation of TNFalpha signaling pathways. Recent efforts in our laboratory have focused on in vivo models of endotoxin hypersensitivity and the relationship between TNFalpha signaling and oxidative stress. We have characterized the kinetics of TCDD-induced damage in the liver by evaluating serum enzyme levels and quantitating apoptotic cells, and have evaluated alterations in gene expression at critical time points. We have shown that when rodents are treated with TCDD prior to endotoxin exposure a significant increase in toxicity occurs, and that inhibition of protein synthesis with cycloheximide blocks the TCDD-induced sensitivity to TNFalpha in this model. Expression of mRNAs for genes associated with the TNF and Fas apoptotic pathways have been quantitated in liver tissue from control and treated animals using RT-PCR. TCDD modulated endotoxin-regulated early expression of Fas and altered the expression of TNF receptor 1 and nuclear factor kappa B (NFKB). We are currently examining the effects of TCDD on NFKB expression. Preliminary studies suggest that combined TCDD/endotoxin treatment alters nuclear translocation of the p65 subunit or NFKB. While high levels of TNFalpha in the liver are usually associated with septic shock, cytotoxicity, inflammation and/or apoptosis, there is an increasing awareness that TNFalpha also plays a pivotal role in regulation of hepatocyte growth. Previous studies have suggested that peroxisome proliferator-induced alterations in hepatocyte proliferation may be due to the mitogenic activity of TNFalpha released from activated Kupffer cells. Modulation of Kupffer cell activity with agents that limit or prevent cytokine production could be an important therapeutic tool against hepatic toxicity and neoplasia. In collaboration with Dr. Ronald Thurman we have determined that treatment with the peroxisome proliferator Wyeth-14,643 activates Kupffer cells in the liver, stimulating phagocytosis and increasing TNFalpha gene expression and secretion. This increase can be modulated by a number of factors which prevent Kupffer cell activation, such as methyl palmitate, gadolinium chloride and nimodipine. We have also demonstrated that dietary glycine inhibits Kupffer cells activation and prevents TNFalpha production in vivo following treatment with LPS or Wyeth-14,643. This project was previously reported as part of Z01 ES 30106 23 LT