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Acinar cells require a functional apical actin web for secretion. During stimulation with supraphysiological concentrations of cholecystokinin (CCK), a condition that mimics acute pancreatitis, the actin filaments disintegrate. This leads to retention of secretory enzymes and, together with their premature activation, results in cell injury. Actin filaments are anchored through membrane-associated protein complexes that can be regulated through Src-family kinases in some model systems. Here we show that the Src-family kinases Yes and Lyn, but not Src and Fyn, are expressed in isolated pancreatic acini of Wistar rats. Upon stimulation with supramaximal secretory CCK (10(-8) M), Yes became reversibly tyrosine-phosphorylated and activated within 2 min. Immunocytochemical and subcellular fractionation studies showed reversible redistribution of Yes to the apical actin web and to the membrane fraction within 5 min. Coimmunoprecipitation demonstrated that Yes forms a complex with the focal adhesion protein Pyk2, which increased with CCK stimulation. In functional studies, inhibition of Src-kinase activity with PP2 partially reversed actin disintegration and also restored amylase secretion. We conclude that Yes participates in the regulation of the acinar cell actin, probably by interaction with Pyk2.

Pancreatic acinar cells depend on the integrity of the cytoskeleton for regulated secretion. Stimulation of isolated rat pancreatic acini with the secretagogue CCK serves as a model for human acute edematous pancreatitis. It induces the breakdown of the actin filament system (F-actin) with the consecutive inhibition of secretion and premature activation of digestive enzymes. However, the mechanisms that regulate F-actin breakdown are largely unknown. Plectin is a versatile cytolinker protein regulating F-actin dynamics in fibroblasts. It was recently demonstrated that plectin is a substrate of caspase 8. In pancreatic acinar cells, plectin strongly colocalizes with apical and basolateral F-actin. Supramaximal secretory stimulation of acini with CCK leads to a rapid redistribution and activation of caspase 8, followed by degradation of plectin that in turn precedes the F-actin breakdown. Inhibition of caspase 8 before CCK hyperstimulation prevents plectin cleavage, stabilizes F-actin morphology, and reverses the inhibition of secretion. Thus we propose that the caspase 8-mediated degradation of plectin represents a critical biochemical event during CCK-induced secretory blockade and cell injury.

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