THE ROLE OF PANCREATIC PHOSPHOLIPASE A2 IN EXPERIMENTAL PANCREATITIS

Abstract

Premature intracellular activation of digestive zymogens is regarded as an initial and initiating event for acute pancreatitis. While the activation of proteolytic zymogens was shown in numerous studies, the role of other digestive enzymes is much less clear. Several authors have suggested that Phosholipase A2 is responsible for proteolytic injury of pancreatic acini. However, earlier studies have never investigated the role of the ancreatic secretory phospholipase A2 (PLA2, type 1b) in contrast to the well known and studied leukocyte PLA2, (PLA2, type 2a). Pancreatic secretory-PLA2-deficient mice were generated by targeted disruption of the PLA2 gene (Gastroenterology 2001; 120:1193-1202). Acute pancreatitis was induced by 7 hourly injections of caerulein (50μg/h, i.p.). Control mice received i.p. saline. Activities of amylase, lipase, myeloperoxidase, trypsin and PLA2 were enzymatically measured with specific substrates. Histological evaluation of pancreatic injury was performed after H&E staining, and apoptosis was analysed by 3-OH nick-end labelling. Pancreatic edema was calculated as ratio of pancreatic wet weight to body mass. CCK-induced necrosis in isolated acinar cells was measured by propidium iodide inclusion. PLA2 expression and PLA2 activity was absent in PLA2-KO animals. In wild type animals we detected a high PLA2 basal activity with a 3-fold increase after supramaximal caerulein stimulation. As an endogenous activator of PLA2 we identified Cathepsin L. Supramaximal caerulein stimulation induced early pancreatic injury as indicated by high levels of serum amylase and lipase in wild-type mice but there was no significant difference in comparison to knock-out animals. The pancreatic inflammatory reaction indicated by myeloperoxidase levels and CD45 staining was reduced; in parallel we detected a 7 fold increase in the rate of apoptosis in PLA2 KO animals. Isolated acinar cells from both animal groups showed no difference in CCK-induced necrosis or trypsin activity. Our data clearly indicate that activation of the secretory pancreatic pro-PLA2 plays a role in mediating the systemic inflammatory response, but has little or no impact on the initiation or propagation of the digestive enzyme activation cascade in pancreatic acini eventually leading to cell necrosis. The deletion of PLA2 leads to a shift of cell death from predominantly necrosis to apoptosis.