Role of FAN in tumor necrosis factor-alpha and lipopolysaccharide-induced interleukin-6 secretion and lethality in D-galactosamine-sensitized mice.

Tumor necrosis factor (TNF) alpha-induced neutral sphingomyelinase-mediated generation of ceramide, a bioactive lipid molecule, is transduced by the adaptor protein FAN, which binds to the intracellular region of the CD120a TNFalpha receptor. FAN-deficient mice do not exhibit any gross abnormality. To further explore the functions of FAN in vivo and because CD120a-deficient mice are resistant to endotoxin-induced liver failure and lethality, we investigated the susceptibility of FAN-deficient animals to lipopolysaccharide (LPS). We show that after d-galactosamine sensitization, FAN-deficient mice were partially resistant to LPS- and TNFalpha-induced lethality. Although LPS challenge resulted in a hepatic ceramide content lower in mutant mice than in control animals, it triggered similar histological alterations, caspase activation, and DNA fragmentation in the liver. Interestingly, LPS-induced elevation of IL-6 (but not TNFalpha) serum concentrations was attenuated in FAN-deficient mice. A less pronounced secretion of IL-6 was also observed after LPS or TNFalpha treatment of cultured peritoneal macrophages and embryonic fibroblasts isolated from FAN-deficient mice, as well as in human fibroblasts expressing a mutated FAN. Finally, we show that d-galactosamine-sensitized IL-6-deficient mice were partially resistant to endotoxin-induced liver apoptosis and lethality. These findings highlight the role of FAN and IL-6 in the inflammatory response initiated by endotoxin, implicating TNFalpha.

Resistance to TNF-alpha cytotoxicity can be achieved through different signaling pathways in rat mesangial cells.

We reported previously that Ro-318220 blocked expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) induced by tumor necrosis factor-alpha (TNF-alpha) and subsequently caused apopotosis in mesangial cells (Y.-L. Guo, B. Kang, and J. R. Williamson. J. Biol. Chem. 273: 10362-10366, 1998). These data support our hypothesis that a TNF-alpha-inducible phosphatase may be responsible for preventing sustained activation of c-Jun NH2-terminal protein kinase (JNK) and consequent cell death in these cells (Y.-L. Guo, K. Baysal, B. Kang, L.-J. Yang, and J. R. Williamson. J. Biol. Chem. 273: 4027-4034, 1998). In this study, we investigated the involvement of protein kinase C (PKC) in regulation of MKP-1 expression in mesangial cells together with effects on viability. Although originally characterized as a PKC inhibitor, Ro-318220 inhibited TNF-alpha-induced MKP-1 expression through a mechanism other than blocking the PKC pathway. Furthermore, inhibition of the PKC pathway neither significantly affected TNF-alpha-induced MKP-1 expression nor made cells susceptible to toxic effect of TNF-alpha. Thus PKC activation is not essential for cells to achieve the resistance to TNF-alpha cytotoxicity displayed by normal mesangial cells. However, activation of PKC by phorbol 12-myristate 13-acetate (PMA) dramatically increased cellular resistance to the apoptotic effect of TNF-alpha. Coincidentally, PMA stimulated MKP-1 expression and suppressed JNK activation. Therefore, PMA-induced MKP-1 expression may contribute to the protective effect of PMA. These results provide a mechanistic explanation for previous documentation that PKC activation can rescue some cells from apopotosis.

Glutathione modulates rat and mouse hepatocyte sensitivity to tumor necrosis factor toxicity.

BACKGROUND & AIMS: Tumor necrosis factor (TNF)-alpha causes much of the hepatocellular injury and cell death that follows toxin-induced liver damage. The mechanism by which toxic liver injury sensitizes hepatocytes to TNF-alpha cytotoxicity is unknown. The aim of this study was to determine the role of the antioxidant glutathione in this process. METHODS: A rat hepatocyte cell line and primary hepatocytes sensitized to TNF-alpha toxicity by the addition of actinomycin D were examined for changes in glutathione levels and for the effects of glutathione depletion or supplementation on cell death. The in vivo effects of glutathione depletion were determined in mice treated with galactosamine plus lipopolysaccharide. RESULTS: Treatment of hepatocytes with actinomycin D and TNF-alpha induced apoptotic cell death without affecting cellular glutathione levels or production of the reactive oxygen intermediate H2O2. Glutathione depletion induced by diethyl maleic acid significantly increased TNF-alpha-induced cell death even when this agent was administered 2 hours after TNF-alpha treatment. Hepatocyte cell death was not affected by glutathione supplementation. In mice treated with galactosamine plus lipopolysaccharide, glutathione depletion increased mortality from liver injury from 32% to 72%. CONCLUSIONS: TNF-alpha-induced cytotoxicity in hepatocytes occurs in the absence of glutathione depletion. However, a preexisting reduction in glutathione levels can significantly increase cell death from TNF-alpha.