Gastrointestinal , Hepatobiliary and Pancreatic Pathology Grp 78 Heterozygosity Regulates Chaperone Balance in Exocrine Pancreas with Differential Response to Cerulein-Induced Acute Pancreatitis

The endoplasmic reticulum (ER) is abundant in the acinar cells of the exocrine pancreas. To test the role of ER homeostasis in acute pancreatitis, we manipulated GRP78 levels, a major ER chaperone, in mice. Grp78 / and / littermates were fed either a regular diet (RD) or a high-fat diet. Acinar cells were examined for ER structure by electron microscopy, and ER chaperone levels were assessed by immunoblotting. Pancreatitis was induced by cerulein injection, and multiple pathological parameters were analyzed. Grp78 / mice showed decreased GRP78 expression in acinar cells. Exocrine pancreata of RD-fed Grp78 / mice in an outbred C57BL/6 129/sv genetic background exhibited ER lumen dilation, a reduction in chaperones calnexin (CNX) and calreticulin (CRT), and exacerbated pancreatitis associated with high CHOP induction. With the high-fat diet regimen, Grp78 heterozygosity triggered GRP94 up-regulation and restoration of GRP78, CNX, and CRT to wild-type levels, corresponding with mitigated pancreatitis on cerulein insult. Interestingly, after backcrossing into the C57BL/6 background, RD-fed Grp78 / mice exhibited an increase in GRP94 and levels of CNX and CRT equivalent to wild type, associated with decreased experimental pancreatitis severity. Administration of a chemical chaperone, 4-phenolbutyrate, was protective against cerulein-induced death. Thus, in exocrine pancreata, Grp78 heterozygosity regulates ER chaperone balance, in dietaryand genetic background–dependent manners, and improved ER protein folding capacity might be protective against pancreatitis. (Am J Pathol 2010, 177:2827–2836; DOI: 10.2353/ajpath.2010.100368) The exocrine pancreas is highly specialized for production and secretion of digestive enzymes. In response to meal stimulation, acinar cells of the exocrine pancreas exhibit the highest protein synthesis rate among human tissues. Consistent with this cellular function, acinar cells are morphologically characterized by abundant endoplasmic reticulum (ER). The normal function of exocrine pancreas depends on balance between protein load and folding capacity of the ER. Protein folding in ER is mediated by multiple chaperoning systems. The 78-kilodalton glucose regulated protein (GRP78), also known as BiP (immunoglobulin heavy chain binding protein), is a general chaperone recognizing the unfolded proteins by its hydrophobic residues. GRP94, another major ER chaperone, forms a large complex with GRP78 and other chaperones to process nascent peptides. Calnexin (CNX) and calreticulin (CRT) are lectin chaperones, composing a system specifically targeting proteins with monoglucosylated N-glycan for subsequent assembly. Acute pancreatitis is pathologically characterized by inflammation, edema, and cell necrosis of exocrine pancreas. ATP depletion and premature activation of digestive enzymes contribute to necrosis of pancreatic acinar cells. To study the pathogenesis of pancreatitis, multiple experimental models for pancreatitis have been established in rodent. Administration of cerulein, a cholecystokinin analogue, leads to activation of digestive proenzymes, severe acinar cell necrosis, and inflammation in mouse. Recently, activation of ER stress signaling was observed in pancreas of rats treated with secretagogues or following arginine-induced acute pancreatitis. ER