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Background & Aims: Free radicals are involved in the pathogenesis of acute pancreatitis, during which pancreatitis-associated protein (PAP)-I is overexpressed. We explored whether PAP-I expression could be induced by oxidative stress and whether it could affect apoptosis. Methods: AR4-2J cells were exposed to H"2O"2 or menadione, and PAP-I messenger RNA (mRNA) expression was analyzed by Northern blotting. Results: Maximal expression was observed with 0.1 mmol/L H"2O"2 or with 0.05 mmol/L menadione. Induction was detectable after 12 hours, reached a climax at 18 hours, and then decreased. Pretreatment of the cells with pyrrolidine dithiocarbamate completely abolished PAP-I mRNA induction, suggesting involvement of NF@kB in the signaling pathway. These findings were confirmed in transient transfection assays using a plasmid containing the PAP-I promoter linked to the chloramphenicol acetyltransferase reporter gene. Then the relationship between PAP-I induction and protection against cell damage during oxidative stress was considered. Constitutive PAP-I expression in AR4-2J cells after transfection with PAP-I complementary DNA conferred significant resistance to apoptosis induced by low doses of H"2O"2 but not to necrosis induced by high doses of H"2O"2. Conclusions: These results suggest that during oxidative stress, PAP-I might be part of a mechanism of pancreatic cell protection against apoptosis. GASTROENTEROLOGY 1998;114:808-816