Cytokine-Mediated Hepatic Apoptosis

Abstract

Thirty years ago liver pathology defined apoptosis as a novel mode of cell death. Recently, experimental models of liver injury have been made available for examining the signaling molecules and receptors of apoptotic mechanisms as well as their pathological relevance. Experimental evidence suggests the involvement of apoptosis not only in various inflammatory liver disorders, but also in conditions of poisoning with xenobiotic hepatotoxins. The presence of several differentially regulated apoptosismediating receptors and their ligands on hepatocytes may explain the liver's susceptibility to autoimmune reactions, toxins, and viruses causing chronic liver disease, as well as the differential sensitivity of this system in various metabolic and pathologic conditions. Tumor necrosis factor (TNF) and its receptors (TNF-R), as well as CD95L and its receptor (CD95), are well-known cytokine/cytokine receptor systems relevant to hepatic disease and to apoptosis. Neutralization of endogenously released TNF prevents hepatocyte apoptosis associated with inflammatory liver damage. Direct injection of TNF in sensitized mice results in large scale hepatocyte apoptosis which is exclusively and selectively mediated by the 55-kDa TNF-R. Fulminant apoptotic liver damage is also triggered upon stimulation of CD95. Possible triggering cells include hepatocytes that express CD95L under pathological conditions. Despite the lack of interaction between TNF-R and CD95 on the receptor level, their signal transduction inside the cell seems to involve common proteolytic steps since inhibition of proteases of the caspase family blocks hepatocyte death, liver damage, or lethality in mice signaled by either receptor.