Phosphatidylinositol 3-kinase regulates Ca2+signaling in pancreatic acinar cells through inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase

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Abstract
Calcium is a key mediator of hormone-induced enzyme secretion in pancreatic acinar cells. At the same time, abnormal Ca2+responses are associated with pancreatitis. We have recently shown that inhibition of phosphatidylinositol 3-kinase (PI3-kinase) by LY-294002 and wortmannin, as well as genetic deletion of PI3-kinase-γ, regulates Ca2+responses and the Ca2+-sensitive trypsinogen activation in pancreatic acinar cells. The present study sought to determine the mechanisms of PI3-kinase involvement in Ca2+responses induced in these cells by CCK and carbachol. The PI3-kinase inhibitors inhibited both Ca2+influx and mobilization from intracellular stores induced by stimulation of acini with physiological and pathological concentrations of CCK, as well as with carbachol. PI3-kinase inhibition facilitated the decay of cytosolic free Ca2+concentration ([Ca2+]i) oscillations observed in individual acinar cells. The PI3-kinase inhibitors decreased neither CCK-induced inositol 1,4,5-trisphosphate [Ins( 1 , 4 , 5 )P3] production nor Ins( 1 , 4 , 5 )P3-induced Ca2+mobilization, suggesting that the effect of PI3-kinase inhibition is not through Ins( 1 , 4 , 5 )P3or Ins( 1 , 4 , 5 )P3receptors. PI3-kinase inhibition did not affect Ca2+mobilization induced by thapsigargin, a specific inhibitor of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA). Moreover, SERCA blockade with thapsigargin abolished the effects of pharmacological and genetic PI3-kinase inhibition on [Ca2+]isignals, suggesting SERCA as a downstream target of PI3-kinase. Both pharmacological PI3-kinase inhibition and genetic deletion of PI3-kinase-γ increased the amount of Ca2+in intracellular stores during CCK stimulation. Finally, addition of the PI3-kinase product phosphatidylinositol 3,4,5-trisphosphate to permeabilized acini significantly attenuated Ca2+reloading into the endoplasmic reticulum. The results indicate that PI3-kinase regulates Ca2+signaling in pancreatic acinar cells through its inhibitory effect on SERCA.