Phosphatidylinositol 3-kinase facilitates bile acid-induced Ca²⁺ responses in pancreatic acinar cells

Bile acids are known to induce Ca²⁺ signals in pancreatic acinar cells. We have recently shown that phosphatidylinositol 3-kinase (PI3K) regulates changes in free cytosolic Ca²⁺ concentration ([Ca ²⁺]_i) elicited by CCK by inhibiting sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA). The present study sought to determine whether PI3K regulates bile acid-induced [Ca²⁺]_i responses. In pancreatic acinar cells, pharmacological inhibition of PI3K with LY-294002 or wortmannin inhibited [Ca²⁺]_i responses to taurolithocholic acid 3-sulfate (TLC-S) and taurochenodeoxycholate (TCDC). Furthermore, genetic deletion of the PI3K γ-isoform also decreased [Ca²⁺]_i responses to bile acids. Depletion of CCK-sensitive intracellular Ca²⁺ pools or application of caffeine inhibited bile acid-induced [Ca²⁺]_i signals, indicating that bile acids release Ca²⁺ from agonist-sensitive endoplasmic reticulum (ER) stores via an inositol (1,4,5)-trisphosphate-dependent mechanism. PI3K inhibitors increased the amount of Ca²⁺ in intracellular stores during the exposure of acinar cells to bile acids, suggesting that PI3K negatively regulates SERCA-dependent Ca²⁺ reloading into the ER. Bile acids inhibited Ca²⁺ reloading into ER in permeabilized acinar cells. This effect was augmented by phosphatidylinositol (3,4,5)-trisphosphate (PIP₃), suggesting that both bile acids and PI3K act synergistically to inhibit SERCA. Furthermore, inhibition of PI3K by LY-294002 completely inhibited trypsinogen activation caused by the bile acid TLC-S. Our results indicate that PI3K and its product, PIP₃, facilitate bile acid-induced [Ca²⁺]_i responses in pancreatic acinar cells through inhibition of SERCA-dependent Ca²⁺ reloading into the ER and that bile acid-induced trypsinogen activation is mediated by PI3K. The findings have important implications for the mechanism of acute pancreatitis since [Ca²⁺]_i increases and trypsinogen activation mediate key pathological processes in this disorder.