Digestive Diseases News Summer 2011
NIDDK-funded Scientists Develop Acute Pancreatitis Mouse Model
Scientists at the University of Iowa have developed a mouse model to study acute pancreatitis. The model, which involves pancreatic duct ligation, is the first to closely mimic human gallstone-induced pancreatitis, which is the most common cause of acute pancreatitis in the United States after alcoholism.
“Acute pancreatitis lacks effective treatment options,” said Isaac Samuel, M.D., associate professor at the University of Iowa’s Carver College of Medicine, who guided development of the model. “To find a drug, you need a target. And to find a target, you need a model,” he said. The model is described in the October 26, 2010, issue of Pancreatology.
Gallstones trigger almost half of the approximately 200,000 acute pancreatitis cases that occur annually in the United States. Gallstones form in the gallbladder, a saclike structure that stores fat-digesting bile made by the liver.
Most gallstones pass out of the body unnoticed, but some become lodged in the common bile duct, causing jaundice. The common bile duct is a tubelike structure that carries bile from the gallbladder to the small intestine. The common bile duct and the pancreatic duct, which carries pancreatic juice made in the pancreas, converge to form a common channel before meeting the small intestine. A frequent site of gallstone impaction is the ampulla of Vater, where the common channel meets the small intestine. Blockage of the common channel by a gallstone can induce acute pancreatitis. Bile and pancreatic juice backing up in the ducts affects not only the pancreas and liver but also causes a systemic inflammatory response, affecting multiple organs including the kidneys and lungs.
Most patients with acute pancreatitis arrive at the hospital within 48 hours of symptom onset with severe upper abdominal pain, nausea, and vomiting. “Often by this time, the gallstone has already passed,” said Samuel. Systemic inflammation may persist and progress, however, with risk of serious complications including organ failure and even death.
“Unfortunately, modern-day treatment is just supportive. There is no specific treatment because the pathogenesis is unknown,” said Samuel. “Until we define the pathogenesis, we cannot find a therapeutic target.”
Average hospital stays for acute pancreatitis last 5 to 14 days, according to The Society for Surgery of the Alimentary Tract. Gallstones that fail to pass on their own must be surgically removed by gallbladder removal once the patient’s condition has stabilized.
Samuel and colleagues produced a mouse version of acute pancreatitis by ligating, or tying, the small and difficult-to-find pancreatic duct in mice. That Samuel is a gastric surgeon helped, as did use of larger-than-average mice.
Signs of acute pancreatitis were evident in samples taken an hour after ligation. Compared with shams—mice that underwent similar surgeries but whose ducts were not ligated—pancreatic duct-ligated (PD) mice had higher blood levels of aspartate transaminase, a sign of liver inflammation. Tumor necrosis factor-α and interleukin-1β, both signs of systemic inflammation, were higher in blood samples taken 24 hours after ligation. And elevated creatinine levels, a sign of kidney injury, were observed in blood samples taken 48 hours after ligation.
Within 4 days of surgery, all PD mice had died. Examination of lung tissue showed an infiltration of neutrophils—immune cells whose presence indicates an inflammatory response. Analysis of liver and kidney tissue also showed neutrophilic infiltration and signs of hemorrhage and necrosis.
For comparison, the researchers included a group of bile duct-ligated (BD) mice. Although mice in this group had their bile ducts ligated, pancreatic juice flowed normally through the common channel to the small intestine. Compared with BD mice, PD mice experienced greater lung dysfunction and mortality, a phenomenon of particular clinical relevance, according to Samuel and co-authors, who stated in their report that “acute lung injury is the major determinant of morbidity and mortality in human acute pancreatitis of any cause.”
Existing acute pancreatitis animal models of pancreatic duct obstruction have failed to parallel human disease, while models that resemble severe human pancreatitis—such as special diets—are induced by methods that do not cause pancreatitis in humans.
“The finding of multiple organ dysfunction and mortality in the mouse was a big surprise,” said Samuel. Most models use rats because the ducts are much easier to find. “The fact that the pancreas community overlooked the mouse pancreatic duct ligation model as a suitable acute pancreatitis model really astonished me. Everyone assumed that mice would behave like the rat: you get mild inflammation and that’s all.”
The model has already yielded new information about the pathogenesis of acute pancreatitis. Using their model, Samuel and colleagues observed activation of the extracellular regulated kinase (ERK) within 1 hour of pancreatic duct ligation. ERK is a mitogen-activated protein kinase that stimulates proinflammatory cytokines—biochemical messengers that stimulate the immune system. Proinflammatory cytokines help fend off infection but when expressed at high levels can lead to organ dysfunction.
Data presented by Zuobiao Yuan, M.D., Ph.D., a postdoctoral investigator in Samuel’s laboratory, at the May 18–21, 2011, American Society of Gene & Cell Therapy’s Annual Meeting, showed that modulation of ERK through an adeno-associated virus in PD mice significantly reduced mortality. The finding, according to Samuel, is strong experimental evidence of a distinct role for ERK in the early phase of acute gallstone pancreatitis.
These discoveries, said Samuel, highlight the important role animal models play in facilitating the testing of diagnostic, preventive, and therapeutic interventions.
The research was funded by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Additional research support was provided by the Veterans Health Administration and the American Recovery and Reinvestment Act of 2009.
The National Digestive Diseases Information Clearinghouse, an information service of the NIDDK, has free fact sheets and easy-to-read booklets about pancreatitis. For more information or to obtain copies, visit www.digestive.niddk.nih.gov.
NIH Publication No. 11–4552