RA. Transgenic expression of pancreatic secretory trypsin inhibitor-1 rescues SPINK3-deficient mice and restores a normal pancreatic phenotype. Am J Physiol Gastrointest Liver Physiol 298: G518 -G524, 2010. First published January 28, 2010; doi:10.1152/ajpgi.00431.2009.-Endogenous trypsin inhibitors are synthesized, stored, and secreted by pancreatic acinar cells. It is believed that they play a protective role in the pancreas by inhibiting trypsin within the cell should trypsinogen become prematurely activated. Rodent trypsin inhibitors are highly homologous to human serine protease inhibitor Kazal-type 1 (SPINK1). The mouse has one pancreatic trypsin inhibitor known as SPINK3, and the rat has two trypsin inhibitors commonly known as pancreatic secretory trypsin inhibitors I and II (PSTI-I and -II). Rat PSTI-I is a 61-amino acid protein that shares 65% sequence identity with mouse SPINK3. It was recently demonstrated that mice with genetic deletion of the Spink3 gene (Spink3 Ϫ/Ϫ ) do not survive beyond 15 days and lack normal pancreata because of pancreatic autophagy. We have shown that targeted transgenic expression of the rat Psti1 gene to acinar cells in mice [TgN(Psti1)] protects mice against caerulein-induced pancreatitis. To determine whether the autophagic phenotype and lethality in Spink3 Ϫ/Ϫ mice were due to lack of pancreatic trypsin inhibitor, we conducted breeding studies with Spink3 ϩ/Ϫ heterozygous mice and TgN(Psti1) mice. We observed that, whereas Spink3 ϩ/ϩ , Spink3 ϩ/Ϫ , and Spink3 Ϫ/Ϫ /TgN(Psti1) mice had similar survival rates, no Spink3 Ϫ/Ϫ mice survived longer than 1 wk. The level of expression of SPINK3 protein in acini was reduced in heterozygote mice compared with wild-type mice. Furthermore, endogenous trypsin inhibitor capacity was reduced in the pancreas of heterozygote mice compared with wild-type or knockout mice rescued with the rat Psti1 gene. Surprisingly, the lesser amount of SPINK3 present in the pancreata of heterozygote mice did not predispose animals to increased susceptibility to caerulein-induced acute pancreatitis. We propose that a threshold level of expression is sufficient to protect against pancreatitis.PSTI-I; pancreatitis; autophagy TRYPSIN IS SYNTHESIZED in the pancreas in the form of inactive trypsinogen and stored in zymogen granules within acinar cells. Pancreatic secretory trypsin inhibitor (PSTI) is an endogenous protein inhibitor of trypsin that is also present in zymogen granules and inhibits prematurely activated trypsin to protect the pancreas from possible damage (11). Serine protease inhibitor Kazal-type 1 (SPINK1) is the human homolog of PSTI and is expressed at high levels in the pancreas (7) representing 0.1-0.6% of total pancreatic protein (8) and 0.1-0.8% of the total protein in pancreatic juice (28). It has been estimated that the amount of trypsinogen exceeds the amount of trypsin inhibitor in the pancreas, and in humans it has been shown that SPINK1 mRNA levels are 1,000-fold lower than cationic trypsinogen (PRSS1) levels (14) raising the possib...

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