Hepatocytes express keratins 8 and 18 (K8/18) as their only cytoskeletal intermediate filament (IF) proteins, and K8/18 mutations predispose their carriers to liver cirrhosis. Transgenic mice that overexpress mutant human K18 (Arg89-->Cys [R89C]) develop mild chronic hepatitis, hepatocyte fragility, keratin filament disruption, and increased susceptibility to drug-induced liver injury. K18 is a major caspase substrate during apoptosis, and K8- or K18-null mice are significantly predisposed to Fas- and possibly tumor necrosis factor (TNF)-mediated apoptosis in the liver. Here we tested the potential role of the K18 R89C mutation on Fas- or TNF-mediated apoptotic liver injury by injecting Fas antibody (Ab) or TNF-alpha plus actinomycin D into mice that overexpress wild-type (WT) human K18 (with intact filament network, termed TG2 mice) or into K18 R89C mice (with disrupted filament network). K18 R89C mice are significantly more susceptible to Fas-mediated liver injury compared with nontransgenic and TG2 mice. This included differences in lethality, histology, apoptosis, and serum transaminase levels. In contrast, K18 WT and R89C mice manifest similar sensitivity to TNF-induced injury. Both Fas- and TNF-induced apoptosis in liver tissues are associated with caspase-mediated K18 degradation and increased keratin phosphorylation on several but not all sites. In conclusion, transgenic mouse K18 mutation and its consequent keratin filament disruption predispose hepatocytes to Fas- but not TNF-mediated apoptotic injury. This supports the association of keratin mutations with cirrhosis in patients with liver disease and suggests that keratins modulate apoptosis induced by Fas but not TNF.