Inhibition of Fas receptor (CD95)-induced hepatic caspase activation and apoptosis by acetaminophen in mice

The mechanism of liver cell injury induced by an overdose of the analgesic acetaminophen (AAP) remains controversial. Recently, it was hypothesized th at a significant number of hepatocytes die by apoptosis. Since caspases hav e been implicated as critical signal and effector proteases in apoptosis, w e investigated their potential role in the pathophysiology of AAP-induced l iver injury. Male C3Heb/FeJ mice were fasted overnight and then treated wit h 500 mg/kg AAP. Liver injury became apparent at 4 h and was more severe at 6 h (plasma ALT activities: 4110 +/- 320 U/liter; centrilobular necrosis). DNA fragmentation increased parallel to the increase of plasma ALT values. At 6 h there was a 420% increase of DNA fragmentation and a 74-fold increa se of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive cells located predominantly around central veins. However , the activity of the proapoptotic caspase-3 was not increased at any time after AAP. In contrast, injection of the anti-Fas antibody Jo-2 (positive c ontrol) caused a 28-fold increase of caspase-3 activity and severe DNA frag mentation before significant ALT release. Treatment with the caspase inhibi tor ZVAD-CHF2 had no effect on AAP toxicity but completely prevented Jo-med iated apoptosis. In contrast, Jo-induced caspase activation and apoptosis c ould be inhibited by AAP treatment in a time- and dose-dependent manner. We conclude that AAP-induced DNA fragmentation does not involve caspases, sug gesting a direct activation of endonucleases through elevated Ca2+ levels. In addition, electrophilic metabolites of AAP may inactivate caspases or th eir activation pathway. This indicates that AAP metabolism has the potentia l to inhibit signal transduction mechanisms of receptor-mediated apoptosis. (C) 1999 Academic Press.