CLASS I PI3K ISOFORMS ?? AND ?? REGULATE KEY RESPONSES OF CERULEIN-INDUCED PANCREATITIS. PI3K?? IS NECESSARY FOR TRYPSIN ACTIVATION

Abstract

Background & Aims: Phosphoinositide 3-kinase (PI3K) is an important signaling molecule; however, its function in pancreatitis is poorly understood. We recently showed that PI3Kγ isoform regulates key pathologic responses of the pancreatic acinar cell. In this study, using genetic approaches, we sought to determine the roles of class I PI3K isoforms δ and γ in the cerulein (CR) model of acute pancreatitis. Methods: To inactivate PI3K we used mice deficient in PI3Kγ catalytic subunit, p110γ, or homozygous for the loss-of-function p110δD910A mutant of PI3Kδ. Pancreatitis was induced by hourly i.p. injections of 50 μg/kg CR. We measured both early and late responses of pancreatitis at, respectively, 30 min and 7 h after the first CR injection. Results: Genetic inactivation of either PI3Kδ or γ reduced necrosis and neutrophil infiltration in CR pancreatitis by ∼2-fold. However, serum amylase and lipase, and acinar cell vacuolization were unaffected in PI3K mutant mice. PI3K inactivation markedly stimulated apoptosis. Caspase-3 activity did not increase, suggesting that PI3K protects from apoptosis in pancreatitis via caspase-independent mechanism. In both mutants, trypsin activation in pancreatitis was markedly reduced at both early (30 min) and late (7 h) time points. Of note, the late phase of trypsin activation was completely inhibited in PI3Kδ mutant mice. In pancreatic acini isolated from PI3Kδ mutant mice, trypsin activation by CR was only partly reduced (compared with wild type), suggesting that in pancreatitis non-acinar cells may regulate trypsin activation via PI3K. Previously we showed that neutrophils regulate trypsin activation; and indeed, blood neutrophils isolated from PI3K mutant mice displayed impaired oxidative burst. Conclusions: PI3Kδ and γ isoforms regulate key responses of CR pancreatitis, i.e., inflammatory infiltration, trypsin activation, and parenchymal cell death, but are not involved in other responses, i.e., hyperamylasemia. The results further elucidate the pathways of trypsinogen activation and the role of the inflammatory response in pancreatic injury.