Alcohol/Cholecystokinin-evoked Pancreatic Acinar Basolateral Exocytosis Is Mediated by Protein Kinase Cα Phosphorylation of Munc18c

• Published in: The Journal of biological chemistry Vol. 282; no. 17; p. 13047
• Main Authors: Laura I. Cosen-Binker, Patrick P. L. Lam, Marcelo G. Binker, Joseph Reeve, Stephen Pandol, Herbert Y. Gaisano
• Format: Journal Article
• Language: English
• Published: American Society for Biochemistry and Molecular Biology 27.04.2007
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M611132200

The pancreatic acinus is the functional unit of the exocrine pancreas whose role is to secrete zymogens into the gut lumen for food digestion via apical exocytosis. We previously reported that supramaximal CCK induced apical blockade and redirected exocytosis to ectopic sites on the basolateral plasma membrane (BPM) of this polarized cell, leading to pancreatitis. Basolateral exocytosis was mediated by protein kinase C phosphorylation of BPM Munc18c, causing its displacement into the cytosol and activation of BPM-bound Syntaxin-4 to form a SNARE complex. To mimic the conditions of alcoholic pancreatitis, we now examined whether 20 m m alcohol followed by submaximal CCK might mimic supramaximal CCK in inducing these pathologic exocytotic events. We show that a non-secretory but clinically relevant alcohol concentration (20 m m ) inhibited submaximal CCK (50 p m )-stimulated amylase secretion by blocking apical exocytosis and redirecting exocytosis to less efficient BPM, indeed mimicking supramaximal CCK (10 n m ) stimulation. We further demonstrate that basolateral exocytosis caused by both stimulation protocols is mediated by PKCα-induced phosphorylation of Munc18c: 1) PKCα is activated, which binds and induces phosphorylation of PM-Munc18c at a Thr site, and these events can be inhibited by PKCα blockade; 2) PKCα inhibition blocks Munc18c displacement from the BPM; 3) PKCα inhibition prevents basolateral exocytosis but does not rescue apical exocytosis. We conclude that 20 m m alcohol/submaximal CCK as well supramaximal CCK stimulation can trigger pathologic basolateral exocytosis in pancreatic acinar cells via PKCα-mediated activation of Munc18c, which enables Syntaxin-4 to become receptive in forming a SNARE complex in the BPM; and we further postulate this to be an underlying mechanism contributing to alcoholic pancreatitis.