Environmental and genetic stressors and the unfolded protein response in exocrine pancreatic function - a hypothesis

Abstract

The exocrine pancreas has the greatest protein synthetic capacity of any mammalian organ and is challenged with the synthesis, processing and transporting a large load of digestive enzymes. Based on recent findings we present a hypothesis proposing that mutations in the digestive enzymes and environmental risks impacting the pancreas (i.e., alcohol abuse, smoking, metabolic disorders, and drugs) cause endoplasmic reticulum (ER) stress. We review recent findings showing that in normal pancreas the ER stress resulting from alcohol abuse leads to an adaptive unfolded protein response (UPR) allowing for maintenance of protein synthesis, processing, and transport. However, when key pathways necessary for the adaptive UPR are altered, the exocrine cell of the pancreas is unable to maintain these processes and cellular pathology results. These findings may explain why some individuals with alcohol abuse disorders develop organ injury and disease while most do not. Further, the findings allow us to hypothesize that the UPR in the exocrine pancreas adapts the protein synthetic machinery of the ER stress resulting from mutational and environmental stressors. When the ability of the UPR to adapt to the stressors is exceeded, pathologic pathways and disease develop.

Keywords: UPR; exocrine; gastrointestinal; pancreas; pancreatic function; research.

Figure 1

Pancreatic ER stress. This figure depicts factors we hypothesize cause pancreatic exocrine cell ER stress. For example, an increased folding demand will result when there is a need for increased digestive enzyme synthesis. Increased expression of chaperones and foldases are needed for processing these proteins. Altered ER calcium levels as occurs during forms of pancreatitis can also cause ER stress. On the left hand side of the figure are situations in which pancreatic ER stress is further amplified. These include genetic mutations in digestive enzymes; alcohol abuse; smoking; metabolic disorders such as diabetes and hyperlipidemia; xenobiotics such as drugs and intestinal bacterial metabolites; and reactive oxygen species that are generated in many of these situations as well as during acute and chronic pancreatitis. These pancreatic ER stressors lead to further accumulation of unfolded and misfolded proteins which, in turn, lead to further activation of the Unfolded Proteins Response in an attempt to adapt the pancreas to function in the face of the ER stressors.

Figure 2

Sensor–transducers of the mammalian UPR. The three sensor–transducers of the UPR are inositol-requiring protein-1 (IRE1), activating transcription factor-6 (ATF6), and protein kinase RNA (PKR)-like ER kinase (PERK). These sensor–transducers determine the state of unfolded proteins in the ER lumen. Activation of each of the sensor–transducers is followed specific pathways resulting in transcriptional regulation of chaperones, foldases, and components of ER-associated protein degradation (ERAD) system and lipid synthesis for expansion of the ER mediated by the combined effects of IRE1 and ATF6; or translational attenuation and transcriptional upregulation pathways involved in antioxidant synthesis and cell death through the transcription factor C/EBP homologous protein (CHOP) as in the case of PERK. The participants in the pathways involved in the downstream effects of IRE1, ATF6, and PERK activation include X-box binding protein1 (XBP1), site-1 and site-2 proteases (Sp1/Sp2), and eukaryotic translation initiation factor-2a (eIF-2a), and activating transcription factor-4 (ATF4) as discussed in the text.

Figure 3

Alcohol, ER stress, and the IRE1 pathway of the UPR. This figure illustrates the results of our experiments in alcohol fed animals showing ER stress from ethanol and its metabolites, acetaldehyde, and fatty acid ethanol ester (FAEE), by inducing an oxidative state in the ER mediating misfolding and unfolding of proteins and oxidation of lipids. The ER stress leads to upregulation of the IRE1 pathway leading to splicing of XBP1 mRNA resulting in the translation of the transcription factor XBP1-S which is transported to the nucleus where it upregulates the expression of chaperones, oxido-reductases/foldases such as PDI, ERAD proteins, and lipid synthetic enzymes through DNA binding elements, ER stress element (ERSE) and the UPR element (UPRE). The activation of this pathway leads to adaptation of the pancreas to the stress of ethanol abuse and prevents pancreatic injury and disease.

Figure 4

Oxidative folding in the ER. This figure illustrates the ER oxido-reductases involved in oxidative folding of nascent proteins in the ER. The oxido-reductases PDI and ERO1 are coupled and mediate the disulfide bridge formation and protein folding of proteins such as digestive enzymes synthesized in the ER. Another product of these coupled reactions is ROS. Reduced glutathione is necessary to reduce incorrectly placed disulfide bonds so that the protein can be recycled for correct disulfide bridge formation and folding.