Background/aims: Tumor necrosis factor (TNF)-α itself does not induce liver injury in normal mice or hepatocytes. Rather, this event, especially in vitro, is explained by the fact that the TNF-α/TNF receptor system not only triggers downstream signals leading to apoptosis but also induces an antiapoptotic pathway through the activation of nuclear factor (NF)-κB. The aim of this study was to determine whether inhibition of antiapoptotic pathways influences the susceptibility of mice to TNF-α. Here, we focused on the roles of NF-κB and phosphatidylinositol 3-kinase (PI3K)-regulated serine/threonine kinase Akt.
Methods: TNF-α was administered to BALB/c mice after treatment with an adenovirus expressing a mutant form IκBα (Ad5IκB), the PI3K inhibitor wortmannin, or both. Liver injury was assessed biochemically and histologically. The expression of Bcl-2 family members and caspase activity were examined.
Results: In the mice livers, treatment with Ad5IκB or the wortmannin suppressed the activation of NF-κB or Akt, respectively. Suppression of either NF-κB or Akt showed a slight increase in transaminase levels and focal liver cell death after TNF-α administration. However, in mice treated with both Ad5IκB and wortmannin, TNF-α administration resulted in massive hepatocyte apoptosis and hemorrhagic liver destruction in mice. The combination of Ad5IκB, wortmannin, and TNF-α markedly increased the activation of caspase-3 and -9, and activated caspase-8 to a lesser degree, suggesting that TNF-α-induced hepatocyte apoptosis is dependent on type II cell death signaling pathway, probably through the mitochondria. Inhibition of the NF-κB and PI3K/Akt pathways had no effect on expression of Bcl-2 families.
Conclusion: The inducible activation of NF-κB and constitutive activation of Akt regulate hepatocyte survival against TNF-α, which occurs independent of Bcl-2 families.