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 Atg7^Δpan mice that lack the essential autophagy-related protein 7 (ATG7) in pancreatic epithelial cells. Atg7^Δpan 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. Atg7^Δpan 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.The pancreatic acinar cell is responsible for production and secretion of numerous digestive enzymes, including amylase, lipase, and various proteases. To cope with the high daily demand for these enzymes, the acinar cell possesses one of the highest protein biosynthetic rates of all cells, together with an extensive rough endoplasmic reticulum (RER) network (1). Due to its high protein synthetic rates, the acinar cell is prone to the accumulation of misfolded proteins and subsequent induction of ER stress (2, 3). ER stress was suggested to be involved in the pathogenesis of pancreatitis, a potentially fatal inflammatory disease of the exocrine pancreas (2, 4). By progressing from acute (sudden onset; duration <6 mo), to recurrent acute (>1 episode of acute pancreatitis), and chronic (duration >6 mo) disease (5), pancreatitis increases the risk of pancreatic ductal adenocarcinoma (PDAC), the fourth deadliest cancer worldwide, with a median survival of 6 mo (6). The molecular mechanisms mediating the progression of pancreatitis from acinar cell damage and inflammation to formation of pancreatic intraepithelial neoplasia (PanIN) and PDAC are not fully understood. Recent studies suggest that in addition to ER stress, insufficient autophagy also contributes to development of pancreatitis (7).Autophagy is an evolutionarily conserved, catabolic quality control process that maintains cellular homeostasis by degrading damaged organelles, misfolded protein aggregates, and foreign organisms (8). Autophagy is also important for generation of amino acids and other building blocks during starvation (9). There are three classes of autophagy: macroautophagy, microautophagy, and chaperone-mediated autophagy (9). Macroautophagy, the major type of autophagy (hereafter referred to as autophagy), entails formation of double-membrane vesicles (autophagosomes) that sequester damaged organelles and biomolecules and recycle them after transport into lysosomes, where they are degraded. The rate of autophagy is increased in response to diverse stress conditions, including nutrient deprivation, viral infection, and genotoxic stress. In this way, autophagy controls the cross-talk between the intracellular demand for energy, building blocks, and external stimuli (9). Autophagy is critically involved in mammalian development, cell survival, and longevity (10), and its impairment correlates with many pathological conditions (11), including pancreatitis (7). Notably, the mammalian exocrine pancreas exhibits a higher autophagy rate (or autophagic flux) than the liver, kidney, heart, or endocrine pancreas (12), underscoring the likely importance of autophagy in maintaining acinar cell homeostasis and function. Until recently, however, the role of autophagy in pancreatitis has been controversial. On one hand, the genetic inhibition of autophagy [Autophagy Related 5 (Atg5) gene ablation] was found to reduce trypsinogen activation and attenuate pancreatic damage in mice challenged with the pancreatic enzyme secretagogue cerulein (13); but selective and protective autophagy can sequester and degrade potentially deleterious activated zymogens during early pancreatitis (14). More recent findings from experimental models (cerulein-induced pancreatitis), and genetically altered mice (Pdx-Cre; Ikkα^F/F, also known as Ikkα^Δpan, and Ptf1a-Cre; Atg5^F/F) have demonstrated that insufficient autophagy, at least in mice, can lead to the onset of pancreatitis (15–17). The mechanisms by which disruption of autophagy triggers pancreatitis are poorly understood.Degradation of long-lived proteins, a major function of autophagy, is impaired during experimental pancreatitis, especially after administration of cerulein, which causes acinar cell vacuolization and excessive trypsinogen activation (18). Moreover, autophagic flux is reduced during pancreatitis due to defective cathepsin-mediated processing of lysosomal proteases (18). Similarly, pancreas-specific ablation of inhibitor of IκB kinase (IKK)α results in acinar damage ranging from vacuole accumulation to chronic pancreatitis (15). IKKα deficiency impairs the completion of autophagy in acinar cells, with accumulation of the chaperon and autophagy substrate ubiquitin-binding protein p62/SQSTM1 as the key pathogenic mechanism (15). However, ablation of ATG5 was reported to either inhibit (13) or promote (16) pancreatitis. In addition, inhibition of autophagy can either accelerate the development of early malignant lesions in mice lacking the transformation-related protein 53 (p53) (19) or cause the death of established pancreatic cancer (20). The latter results gave rise to several ongoing clinical trials (clinicaltrials.gov numbers {"type":"clinical-trial","attrs":{"text":"NCT01494155","term_id":"NCT01494155"}}NCT01494155, {"type":"clinical-trial","attrs":{"text":"NCT01978184","term_id":"NCT01978184"}}NCT01978184, {"type":"clinical-trial","attrs":{"text":"NCT01506973","term_id":"NCT01506973"}}NCT01506973, {"type":"clinical-trial","attrs":{"text":"NCT01128296","term_id":"NCT01128296"}}NCT01128296, and {"type":"clinical-trial","attrs":{"text":"NCT01273805","term_id":"NCT01273805"}}NCT01273805) that intend to evaluate the impact of autophagy inhibitors on human pancreatic cancer. However, a recent commentary has raised concerns about the safety of this therapeutic approach (21). Given all of these questions, we decided to take a closer look at the impact of autophagy inhibition on pancreatic health and function by generating mice that lack autophagy-related protein 7 (ATG7) in pancreatic epithelial cells. These mice, termed Atg7^Δpan, exhibit striking acinar cell degeneration, which is followed by pronounced pancreatic inflammation and fibrosis. Whereas loss of ATG7 leads to the expected decrease in autophagic flux, it also results in ER stress, accumulation of dysfunctional mitochondria, oxidative stress, and a marked reduction in protein synthetic ability.