Premature intrapancreatic
trypsinogen activation is widely regarded as an initiating event for
acute pancreatitis. Previous studies have alternatively implicated secretory vesicles, endosomes, lysosomes, or autophagosomes/autophagolysosomes as the primary site of
trypsinogen activation, from which a cell-damaging proteolytic cascade originates. To identify the subcellular compartment of initial
trypsinogen activation we performed a time-resolution analysis of the first 12 h of
caerulein-induced
pancreatitis in transgenic light chain 3 (LC3)-GFP autophagy reporter mice. Intrapancreatic
trypsin activity increased within 60 min and serum
amylase within 2 h, but fluorescent autophagosome formation only by 4 h of
pancreatitis in parallel with a shift from cytosolic LC3-I to membranous LC3-II on Western blots. At 60 min, activated
trypsin in heavier subcellular fractions was co-distributed with
cathepsin B, but not with the autophagy markers LC3 or autophagy
protein 16 (ATG16). Supramaximal
caerulein stimulation of primary pancreatic acini derived from LC3-GFP mice revealed that
trypsinogen activation is independent of autophagolysosome formation already during the first 15 min of exposure to
caerulein. Co-localization studies (with GFP-LC3 autophagosomes versus Ile-
Pro-Arg-AMC
trypsin activity and immunogold-labelling of
lysosomal-associated membrane protein 2 [LAMP-2] versus
trypsinogen activation peptide [TAP]) indicated active
trypsin in autophagolysosomes only at the later timepoints. In conclusion, during the initiating phase of
caerulein-induced
pancreatitis, premature
protease activation develops independently of autophagolysosome formation and in vesicles arising from the secretory pathway. However, autophagy is likely to regulate overall intracellular
trypsin activity during the later stages of this disease.