Liver injury is a clinical prognostic indicator in acute pancreatitis (AP). We have demonstrated that Kupffer cell-derived FasL mediates liver injury during AP and sought to determine its role in AP-induced hepatocyte apoptosis. AP was induced in National Institutes of Health (NIH) Swiss mice, C57/C57, and Fas-/-, FasL-/- mice by a choline-deficient ethionine-supplement diet. Liver Fas, FasL, p38-mitogen activated phosphokinase (p38-MAPK), poly-ADP ribose polymerase (PARP), and cytochrome C were measured by immunoblotting. Apoptosis was assessed by terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) and DNA fragmentation (ELISA). AP upregulated liver FasL (4280 +/- 580 vs. 733 +/- 336), Fas (2866 +/- 595 vs. 649 +/- 111), cytochrome C (6980 +/- 237 vs. 903 +/- 156), and PARP (6393 +/- 591 vs. 466 +/- 261) as well as increased TUNEL staining (40 +/- 2 vs. 14 +/- 1) and DNA fragmentation (all P < 0.03 vs. control). In FasL-/- and Fas-/- mice, AP-induced upregulation of p38-MAPK, PARP, and cytochrome C was significantly attenuated (all P < 0.01 compared to C57/C57 control). In addition, AP-induced DNA fragmentation was reduced 60% in Fas-/- and FasL-/- mice (P < 0.01 vs. C57/C57). AP induces apoptosis by transcriptional activation of Fas/FasL. AP-induced apoptosis was significantly reduced in Fas and FasL knockout mice along with downregulation of p38-MAPK, PARP, and cytochrome C, thereby suggesting a central role for Fas/FasL in hepatocyte apoptosis. The manipulation of interactions between Kupffer cell-derived FasL and hepatocytes may have important therapeutic implications.