Tissue transglutaminase (TG2) acting as G protein protects hepatocytes against Fas-mediated cell death in mice

Tissue transglutaminase (TG2) is a protein cross-linking enzyme known to be expressed by hepatocytes and to be induced during the in vivo hepatic apoptosis program. TG2 is also a G protein that mediates intracellular signaling by the alpha-1b-adrenergic receptor (AR) in liver cells. Fas/Fas ligand interaction plays a crucial role in various liver diseases, and administration of agonistic anti-Fas antibodies to mice causes both disseminated endothelial cell apoptosis and fulminant hepatic failure. Here we report that an intraperitoneal dose of anti-Fas antibodies, which is sublethal for wild-type mice, kills all the TG2 knock-out mice within 20 hours. Although TG2^-/- thymocytes exposed to anti-Fas antibodies the at the same rate as wild-type mice, TG2^-/- hepatocytes show increased sensitivity toward anti-Fas treatment both in vivo and in vitro, with no change in their cell surface expression of Fas, levels of FLIP_L (FLICE-inhibitory protein), or the rate of I-κBα degradation, but a decrease in the Bcl-x_L expression. We provide evidence that this is the consequence of the impaired AR signaling that normally regulates the levels of Bcl-x_L in the liver. In conclusion, our data suggest the involvement of adrenergic signaling pathways in the hepatic regeneration program, in which Fas ligand-induced hepatocyte proliferation with a simultaneous inhibition of the Fas-death pathway plays a determinant role.