BH4 domain peptides derived from Bcl-2/Bcl-XL as novel tools against acute pancreatitis

Biliary acute pancreatitis (AP) is a serious condition, which currently has no specific treatment. Taurolithocholic acid 3-sulfate (TLC-S) is one of the most potent bile acids causing cytosolic Ca²⁺ overload in pancreatic acinar cells (PACs), which results in premature activation of digestive enzymes and necrosis, hallmarks of AP. The inositol 1,4,5-trisphosphate receptor (IP₃R) and the ryanodine receptor (RyR) play major roles in intracellular Ca²⁺ signaling. Inhibition of these endoplasmic reticulum-located channels suppresses TLC-S-induced Ca²⁺ release and necrosis, decreasing the severity of AP. Anti-apoptotic B-cell lymphoma (Bcl)-2-family members, such as Bcl-2 and Bcl-X_L, have emerged as important modulators of IP₃Rs and RyRs. These proteins contain four Bcl-2 homology (BH) domains of which the N-terminal BH4 domain exerts critical roles in regulating intracellular Ca²⁺ release channels. The BH4 domain of Bcl-2, but not of Bcl-X_L, binds to and inhibits IP₃Rs, whereas both BH4 domains inhibit RyRs. Although clear cytoprotective effects have been reported for these BH4 domains, it remains unclear whether they are capable of inhibiting pathological Ca²⁺-overload, associated with AP. Here we demonstrate in PACs that the BH4 domains of Bcl-2 and Bcl-X_L inhibit RyR activity in response to the physiological agonist cholecystokinin. In addition, these BH4 domains inhibit pathophysiological TLC-S-induced Ca²⁺ overload in PACs via RyR inhibition, which in turn protects these cells from TLC-S-induced necrosis. This study shows for the first time the therapeutic potential of BH4 domain function by inhibiting pathological RyR-mediated Ca²⁺ release and necrosis, events that trigger AP.