Intracellular Ca2+ Signalling in the Pathogenesis of Acute Pancreatitis: Recent Advances and Translational Perspectives

Abstract

Intracellular Ca²⁺ signalling is a major signal transductional pathway in non-excitable cells, responsible for the regulation of a variety of physiological functions. In the secretory epithelial cells of the exocrine pancreas, such as acinar and ductal cells, intracellular Ca²⁺ elevation regulates digestive enzyme secretion in acini or fluid and ion secretion in ductal cells. Although Ca²⁺ is a uniquely versatile orchestrator of epithelial physiology, unregulated global elevation of the intracellular Ca²⁺ concentration is an early trigger for the development of acute pancreatitis (AP). Regardless of the aetiology, different forms of AP all exhibit sustained intracellular Ca²⁺ elevation as a common hallmark. The release of endoplasmic reticulum (ER) Ca²⁺ stores by toxins (such as bile acids or fatty acid ethyl esters (FAEEs)) or increased intrapancreatic pressure activates the influx of extracellular Ca²⁺ via the Orai1 Ca²⁺ channel, a process known as store-operated Ca²⁺ entry (SOCE). Intracellular Ca²⁺ overload can lead to premature activation of trypsinogen in pancreatic acinar cells and impaired fluid and HCO₃^- secretion in ductal cells. Increased and unbalanced reactive oxygen species (ROS) production caused by sustained Ca²⁺ elevation further contributes to cell dysfunction, leading to mitochondrial damage and cell death. Translational studies of AP identified several potential target molecules that can be modified to prevent intracellular Ca²⁺ overload. One of the most promising drugs, a selective inhibitor of the Orai1 channel that has been shown to inhibit extracellular Ca²⁺ influx and protect cells from injury, is currently being tested in clinical trials. In this review, we will summarise the recent advances in the field, with a special focus on the translational aspects of the basic findings.

Keywords: Ca2+ signalling; acinar cell necrosis; acute pancreatitis; bile acid; epithelial ion transport.

Figure 1

Intracellular Ca²⁺ signalling in biliary acute pancreatitis. Bile acids dose-dependently release Ca²⁺ from intracellular stores via activation of IP₃ and ryanodine receptors (RyR). The inhibition of the sarco-endoplasmic reticulum Ca²⁺ pump (SERCA) and activation of Orai1-mediated extracellular Ca²⁺ influx contributes to the sustained global Ca²⁺ signals. Bile acids can activate the G-protein-coupled cell surface bile acid receptor (Gpbar) at the apical membrane of pancreatic acinar cells that also release Ca²⁺ from the endoplasmic reticulum. Mitochondrial Ca²⁺ overload can lead to mitochondrial damage by opening the mitochondrial permeability transition pore and dissipating the mitochondrial membrane potential. In addition, bile acids have been demonstrated to activate calcineurin in a Ca²⁺-dependent manner in pancreatic cells, leading to premature digestive enzyme and NF-κB activation.

Figure 2

Intracellular Ca²⁺ signalling in alcoholic acute pancreatitis. In the pancreas, non-oxidative ethanol metabolism is the dominant metabolic pathway mediated by enzymes with fatty acid ethyl ester (FAEE) synthase activity, which combine ethanol and fatty acids to generate FAEE. In pancreatic acinar cells, FAEEs are accumulated in the mitochondria and their local breakdown leads to localised high concentrations of fatty acids. Similar to bile acids, FAEEs induce sustained [Ca²⁺]_i elevation and drop of cellular ATP leading to necrosis. In addition, alcohol and fatty acids inhibit fluid and HCO₃^- secretion in the pancreatic ductal epithelia, mainly due to the impaired expression and function of the cystic fibrosis transmembrane conductance regulator (CFTR).

Figure 3

The role of Transient Potential Melastatin-like 2 (TRPM2) in biliary pancreatitis. Increased, unbalanced production of ROS, which are generated during physiological mitochondrial respiration, are mainly derived from complexes I and III of the mitochondrial electron transport chain, a crucial event in the pathogenesis of biliary acute pancreatitis. In our recent study, we described the expression of a redox sensitive cation channel, TRPM2, in the exocrine pancreas with basolateral localisation in acinar cells and apical localisation in ductal cells. H₂O₂-induced oxidative stress activated TRPM2, whereas TRPM2 knockout decreased the bile acid-induced Ca²⁺ elevation in acinar cells and prevented acinar cells from bile-acid-induced necrosis. Genetic deletion of TRPM2 reduced the severity of bile-acid-induced experimental pancreatitis.

Figure 4

Store-operated Ca²⁺ entry and acute pancreatitis. In pancreatic acinar cells physiological agonist stimulation releases Ca²⁺ from the endoplasmic reticulum (ER) stores at the apical granular region of the cell. The spatiotemporal localisation of IP₃-evoked apical signals is maintained by the mitochondria surrounding the apical region of the acinar cells, whereas the plasma membrane Ca²⁺ ATPase (PMCA) pumps extrude the Ca²⁺ at the apical membrane. The influx of extracellular Ca²⁺ in non-excitable cells is mediated by a process called store-operated Ca²⁺ entry (SOCE), which is determined by the ER Ca²⁺ sensor stromal interaction molecule 1 (Stim1) and the plasma membrane Ca²⁺ channel Orai1. In unstimulated cells, Stim1 is distributed in the ER membrane, whereas mobilisation of the ER Ca²⁺ stores induces a conformational change and translocation with puncta formation, of Stim1 to the ER–PM junctions, which activates Orai1. The basal Ca²⁺ uptake can refill the apical Ca²⁺ stores by a mechanism called Ca²⁺ tunnelling. Under pathological conditions, the spatiotemporal regulation of the Ca²⁺ signalling fails and extracellular Ca²⁺ entry is an important contributor to the Ca²⁺ toxicity. Recently, an Orai1 channel regulator protein called store-operated calcium entry-associated regulatory factor (Saraf) was described as a crucial component in pathological Ca²⁺ signal development. Saraf knockout mice developed more severe acute pancreatitis (AP) compared to controls accompanied by increased Ca²⁺ influx in acinar cells, whereas Saraf overexpression reduced acinar Ca²⁺ influx and decreased the severity of AP.

Figure 5

Ca²⁺ signalling in pressure-related acute pancreatitis. Increased intrapancreatic pressure during endoscopic retrograde cholangiopancreatography (ERCP) can damage the pancreas and cause post-ERCP acute pancreatitis (AP). Similar to other forms of AP, pancreatic acinar cells represented aberrant intracellular Ca²⁺ signalling and impaired mitochondrial function and calcineurin downstream activation. Recently, Piezo1 expression, a mechanoreceptor directly gated by mechanical forces, was described in pancreatic acinar cells. Piezo1 activation triggered intracellular Ca²⁺ elevation and acinar cell injury. Genetic deletion of Piezo1 remarkably decreased the severity of pressure-induced AP, and it’s activation by Yoda1 triggered AP without the application of pressure. In addition, Piezo1 activation induces phospholipase A2 activation, which activates TRPV4, which is necessary for the development of sustained toxic Ca²⁺ signals in acinar cells upon pressure.