Acute pancreatitis is an auto-digestive disease resulting in
inflammation. At the cellular level,
acute pancreatitis disrupts posttranslational protein processing and traffic in the secretory pathway, and
zymogens become activated in the acinar cell. To better understand the disruption of the secretory pathway in
pancreatitis, pulse-chase [(35)S]met/cys analysis was used to study the effects of supramaximal
cerulein stimulation on posttranslational modification in the secretory pathway of the major sulfated
glycoprotein of the mouse pancreas, pro-Muclin, and the
lysosomal membrane protein LAMP1. Maximal
cerulein or high concentration
bombesin stimulation had little effect on
glycoprotein processing. By contrast, supramaximal
cerulein stimulation strongly inhibited pro-Muclin processing as measured by the failure of Muclin to attain its normal mature size of 300 kDa and to become highly sulfated and decreased proteolytic cleavage of pro-Muclin to produce apactin. Digestion of immunoprecipitated [35S]met/cys-labeled Muclin and LAMP1 with
endoglycosidase H demonstrated that the supramaximal
cerulein-induced block in processing occurred before the medial Golgi compartment. With supramaximal
cerulein stimulation, vacuoles formed which contained Muclin,
amylase, and LAMP1. Earlier autoradiographic studies showed that newly synthesized
proteins end up in
pancreatitis-associated vacuoles, so it is likely that
glycoproteins with incomplete posttranslational processing are also present in vacuoles. Because
glycoproteins are believed to protect the membranes of lysosomes and zymogen granules, when they are not correctly processed, their defensive mechanisms may be impaired, and this could contribute to vacuole fragility in
pancreatitis.