Structure-Function Relationships in the Pancreatic Acinar Cell

Abstract The pancreatic Acinar Cell synthesizes, stores, and secretes the enzymes and enzyme precursors required for the digestion of dietary proteins, carbohydrates, and lipids. To meet the daily needs for digestion, the Acinar Cell has the highest average rates of protein synthesis in the body. Enzymes are stored in zymogen granules that are localized to the apical region of the Cell. Signals generated by food in the intestine reach the Acinar Cell by neural and hormonal routes stimulate the secretion of enzymes from zymogen granules by exocytosis into the pancreatic duct. The Acinar Cell has been a model system for examining mechanisms of protein synthesis and export. Thus, nascent digestive enzyme proteins are transported from the endoplasmic reticulum to the Golgi complex, segregated from lysosomal enzymes, and then directed to zymogen granules by vesicular transport in a time-dependent and vectorial manner. The exocrine pancreas has two major physiologic functions: it supplies the enzymes and enzyme precursors (zymogens) that are needed for digesting dietary lipids, carbohydrates, and proteins and secretes a bicarbonate-rich fluid that neutralizes acidic gastric secretions and thus provides the correct pH for intestinal digestion by pancreatic enzymes. Two major Cell types form the exocrine pancreas; Acinar Cells and duct Cells. The chapter focuses on the Acinar Cell which synthesizes, stores, and secretes digestive enzymes while duct Cells secrete chloride and bicarbonate. Reasons to focus on the Acinar Cell include its critical physiologic function and its history as the model for the scientific studies that described the steps responsible for regulating protein synthesis and export and Cell signaling. Thus, after electron microscopy was developed, Cell biologists first visualized Acinar Cell organelles, they often then determined their Cellular function by studying this Cell. Although this review is largely based on the data from rodent Acinar Cells, limited data from human Acinar Cells suggest they will exhibit the same fundamental responses.

tlr9 and the nlrp3 inflammasome link Acinar Cell death with inflammation in acute pancreatitis

Acute pancreatitis is characterized by a sequence of events that begins with intraCellular activation of stored zymogens within the Acinar Cell. This can lead to Acinar Cell death and subsequent pancreatic inflammation.1 This model has received support from experimental systems such as caerulein-induced pancreatitis in which each of these pathologic responses can be demonstrated.2 Clinical and experimental studies have documented that the extent of the subsequent inflammatory response is directly related to disease severity and survival.3 An important role for inflammation in the pathogenesis of pancreatitis is supported by systems which show a requirement for inflammatory components in the development of pancreatitis. For example, neutrophils contribute to injury through the generation of reactive oxygen species.4 Other inflammatory components which are required for maximal pancreatitis include the IL-1 receptor, caspase -1, and TNF-α.5

Collectively, these studies suggest that a complex cytokine and Cellular inflammatory response mediates injury in acute pancreatitis. However, whether Acinar Cell death could promote an inflammatory response has not been explored. A central feature of necrotic Cell death is the release of intraCellular contents. Some intraCellular molecules on release into the extraCellular compartment induce an immune response, and are known as damage associated molecular patterns (DAMPs).6 Over twenty DAMPs have been identified; they bind to a wide range of DAMP-receptors including members of the TOLL, and purinergic receptor families. Activation of DAMP-receptors results in up-regulation of pro-cytokines such as pro-IL-1β, and activation of Cellular proteases required for their cleavage to functional cytokines.7 Recently caspase-1 has been identified as a central component of a cytosolic complex termed the inflammasome which is required for the initiation of many types of sterile inflammatory responses. We demonstrate that the inflammasome is activated during acute pancreatitis, and components of the inflammasome are required for maximal pancreatitis. Furthermore, we identify the DAMP-receptors TLR9 and P2X7 as upstream of the inflammasome and vital for the development of pancreatic injury.

low extraCellular ph induces damage in the pancreatic Acinar Cell by enhancing calcium signaling

Low extraCellular pH (pHe) occurs in a number of clinical conditions and sensitizes to the development of pancreatitis. The mechanisms responsible for this sensitization are unknown. Because abnormal Ca2+ signaling underlies many of the early steps in the pathogenesis of pancreatitis, we evaluated the effect of decreasing pHe from 7.4 to 7.0 on Ca2+ signals in the Acinar Cell. Low pHe significantly increased the amplitude of cerulein-induced Ca2+ signals. The enhancement in amplitude was localized to the basolateral region of the Acinar Cell and was reduced by pretreatment with ryanodine receptor (RYR) inhibitors. Because basolateral RYRs also have been implicated in the pathogenesis of pancreatitis, we evaluated the effects of RYR inhibitors on pancreatitis responses in acidic conditions. RYR inhibitors significantly reduced the sensitizing effects of low pHe on zymogen activation and Cellular injury. These findings suggest that enhanced RYR-mediated Ca2+ signaling in the basolateral region of the Acinar Cell is responsible for the injurious effects of low pHe on the exocrine pancreas.

low extraCellular ph induces damage in the pancreatic Acinar Cell by enhancing calcium signaling

Low extraCellular pH (pHe) occurs in a number of clinical conditions and sensitizes to the development of pancreatitis. The mechanisms responsible for this sensitization are unknown. Because abnormal Ca2+ signaling underlies many of the early steps in the pathogenesis of pancreatitis, we evaluated the effect of decreasing pHe from 7.4 to 7.0 on Ca2+ signals in the Acinar Cell. Low pHe significantly increased the amplitude of cerulein-induced Ca2+ signals. The enhancement in amplitude was localized to the basolateral region of the Acinar Cell and was reduced by pretreatment with ryanodine receptor (RYR) inhibitors. Because basolateral RYRs also have been implicated in the pathogenesis of pancreatitis, we evaluated the effects of RYR inhibitors on pancreatitis responses in acidic conditions. RYR inhibitors significantly reduced the sensitizing effects of low pHe on zymogen activation and Cellular injury. These findings suggest that enhanced RYR-mediated Ca2+ signaling in the basolateral region of the Acinar Cell is responsible for the injurious effects of low pHe on the exocrine pancreas.

the Acinar Cell and early pancreatitis responses

Pathologic responses arising from the pancreatic Acinar Cell appear to have a central role in initiating acute pancreatitis. Environmental factors that sensitize the Acinar Cell to harmful stimuli likely have a critical role in many forms of pancreatitis, including that induced by alcohol abuse. Activation of zymogens within the Acinar Cell and an inhibition of secretion are critical, but poorly understood, early pancreatitis events. While there is firm evidence relating trypsinogen activation to pancreatitis, the importance of other zymogens has been less studied. Preliminary studies suggest that trypsin may be activated by mechanisms that are distinct from other zymogens. Further, unlike the small intestine, it may not catalyze the activation of other zymogens. These features could affect strategies aimed at inhibiting proteases to treat pancreatitis. Specific intraCellular signals are required to activate pancreatitis pathways in the Acinar Cell. The most important is calcium. Recent studies have suggested that calcium release through specific calcium channels in the endoplasmic reticulum is the means by which pathological elevations in cytosolic calcium occur. Although the targets of abnormal calcium signaling are unknown, calcineurin, a calcium-dependent phosphatase, may serve such a role. Finally, recent work suggests that an acute acid load might sensitize the Acinar Cell to pancreatitis responses. Therapies aimed at preventing or reversing the effects of an acid load on the pancreas may be important for treatment.

reducing extraCellular ph sensitizes the Acinar Cell to secretagogue induced pancreatitis responses in rats

Background & Aims Protease activation within the pancreatic Acinar Cell is a key early event in acute pancreatitis and may require low pH intraCellular compartments. Clinical studies suggest that acidosis may affect the risk for developing pancreatitis. We hypothesized that exposure to an acid load might sensitize the Acinar Cell to secretagogue-induced pancreatitis. Methods Secretagogues (cerulein, carbachol, and bombesin) can induce protease activation in Acinar Cells at high (100 nmol/L, 1 mmol/L, and 10 μmol/L, respectively) but not at physiologically relevant concentrations. The effects of decreasing extraCellular pH (pHe) in early secretagogue-induced pancreatitis (zymogen activation and injury) were examined in rats (1) in vitro with isolated acini and (2) in vivo with an acid challenge. Results In acini, lowering pHe from 7.6 to 6.8 enhanced secretagogue-induced zymogen activation and injury, but did not affect secretion. For cerulein, this sensitization was seen over a range of concentrations (0.01–100.00 nmol/L). However, reduced pHe alone had no effect on zymogen activation, amylase secretion, or Cell injury. We have reported that zymogen activation is mediated by the vacuolar ATPase (vATPase), a proton transporter. vATPase inhibition, using concanamycin (100 nmol/L), blocked the low pHe effects on zymogen activation. An acute acid load given in vivo enhanced cerulein-induced (50 μg/kg) trypsinogen activation and pancreatic edema. Conclusion These studies suggest that acid challenge sensitizes the pancreatic Acinar Cell to secretagogue-induced zymogen activation and injury and may increase the risk for the development and severity of acute pancreatitis.

basal autophagy maintains pancreatic Acinar Cell homeostasis and protein synthesis and prevents er stress

Pancreatic Acinar Cells possess very high protein synthetic rates as they need to produce and secrete large amounts of digestive enzymes. Acinar Cell damage and dysfunction cause malnutrition and pancreatitis, and inflammation of the exocrine pancreas that promotes development of pancreatic ductal adenocarcinoma (PDAC), a deadly pancreatic neoplasm. The Cellular and molecular mechanisms that maintain Acinar Cell function and whose dysregulation can lead to tissue damage and chronic pancreatitis are poorly understood. It was suggested that autophagy, the principal Cellular degradative pathway, is impaired in pancreatitis, but it is unknown whether impaired autophagy is a cause or a consequence of pancreatitis. To address this question, we generated Atg7Δpan mice that lack the essential autophagy-related protein 7 (ATG7) in pancreatic epithelial Cells. Atg7Δpan mice exhibit severe Acinar Cell degeneration, leading to pancreatic inflammation and extensive fibrosis. Whereas ATG7 loss leads to the expected decrease in autophagic flux, it also results in endoplasmic reticulum (ER) stress, accumulation of dysfunctional mitochondria, oxidative stress, activation of AMPK, and a marked decrease in protein synthetic capacity that is accompanied by loss of rough ER. Atg7Δpan mice also exhibit spontaneous activation of regenerative mechanisms that initiate Acinar-to-ductal metaplasia (ADM), a process that replaces damaged Acinar Cells with duct-like structures.

loss of Acinar Cell ikkα triggers spontaneous pancreatitis in mice

Chronic pancreatitis is an inflammatory disease that causes progressive destruction of pancreatic Acinar Cells and, ultimately, loss of pancreatic function. We investigated the role of I?B kinase ? (IKK?) in pancreatic homeostasis. Pancreas-specific ablation of IKK? (Ikk?(?pan)) caused spontaneous and progressive Acinar Cell vacuolization and death, interstitial fibrosis, inflammation, and circulatory release of pancreatic enzymes, clinical signs resembling those of human chronic pancreatitis. Loss of pancreatic IKK? causes defective autophagic protein degradation, leading to accumulation of p62-mediated protein aggregates and enhanced oxidative and ER stress in Acinar Cells, but none of these effects is related to NF-?B. Pancreas-specific p62 ablation prevented ER and oxidative stresses and attenuated pancreatitis in Ikk?(?pan) mice, suggesting that Cellular stress induced by p62 aggregates promotes development of pancreatitis. Importantly, downregulation of IKK? and accumulation of p62 aggregates were alsoobserved in chronic human pancreatitis. Our studies demonstrate that IKK?, which may control autophagic protein degradation through its interaction with ATG16L2, plays a critical role in maintaining pancreatic Acinar Cell homeostasis, whose dysregulation promotes pancreatitis through p62 aggregate accumulation.