J Biol Chem, Vol. 274, Issue 16, 10758-10764, April 16, 1999

Porcine Pancreatic Phospholipase A₂ Stimulates Secretin Release from Secretin-producing Cells

From the Konar Center for Digestive and Liver Diseases, Department of Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642

We have isolated, from canine pancreatic juice, two 14-kDa proteins with secretin-releasing activity that had N-terminal sequence homology with canine pancreatic phospholipase A₂ (PLA₂). In this study we have obtained evidence that secretin-releasing activity is an intrinsic property of pancreatic PLA₂. Porcine pancreatic PLA₂ from Sigma or Boehringer Mannheim was fractionated into several peaks by reverse phase high performance liquid chromatography. They were tested for stimulation of secretin release from murine neuroendocrine intestinal tumor cell line STC-1 and secretin cells enriched mucosal cell preparations isolated from rat upper small intestine. Each enzyme preparation was found to contain several components of secretin-releasing activity. Each bioactive fraction was purified to homogeneity by rechromatography and then subjected to mass spectral analysis and assays of PLA₂ and secretin-releasing activities. It was found that the fraction with highest enzymatic activity also had the highest secretin-releasing activity and the same M_r as porcine pancreatic PLA₂. Moreover, it also had the same N-terminal amino acid sequence (up to 30 residues determined) as that of porcine pancreatic PLA₂, suggesting that it was identical to the enzyme. Purified porcine pancreatic PLA₂ also stimulated secretin release concentration-dependently from both STC-1 cells and a mucosal cell preparation enriched in secretin-containing endocrine cells isolated from rat duodenum. Abolishment of the enzymatic activity by pretreatment with bromophenacyl bromide did not affect its secretin-releasing activity. The stimulatory effect of purified pancreatic PLA₂ on secretin secretion from STC-1 cells was inhibited by an L-type Ca²⁺ channel blocker, by down-regulation of protein kinase C or by pretreatment of the cell with pertussis toxin. It is concluded that porcine pancreatic PLA₂ possesses an intrinsic secretin-releasing activity that was independent of its enzymatic activity. This action is pertussis toxin-sensitive and is in part dependent on Ca²⁺ influx through the L-type channel and activation of protein kinase C.