Brefeldin A affects early events but does not affect late events along the exocytic pathway in pancreatic acinar cells

Abstract

Brefeldin A (BFA) blocks protein export from the endoplasmic reticulum (ER) to Golgi complex and causes dismantling of the Golgi complex with relocation of resident Golgi proteins to the ER in some cultured cells. It is not known whether later steps in the secretory process are affected. We previously have shown that in BFA-treated rat pancreatic lobules, there is no detectable relocation of Golgi proteins to the ER and, although Golgi cisternae are rapidly dismantled, clusters of small smooth vesicles consisting of both bona fide Golgi remnants and associated vesicular carriers persist even with prolonged BFA exposure. We now report the effects of BFA on transport of proteins through the secretory pathway in exocrine pancreatic cells; we pulse-labeled pancreatic lobules with [35S]methionine and then chased for various times before adding BFA. When BFA was added at pulse, treated lobules released less than 10% of radioactive protein in comparison with controls, regardless of whether or not the lobule cultures were stimulated with carbamoylcholine. However, when lobules were pulsed and then chased for 30, 45, or 60 min before BFA addition, the amount of labeled protein released was comparable in both BFA-treated and untreated cultures. Furthermore, the kinetics and amounts of basal and carbamoylcholine-stimulated release of unlabeled alpha-amylase from storage in zymogen granules were similar in both control and BFA-treated lobules. Therefore, in the rat pancreas, BFA blocks ER to Golgi transport but does not affect later stages along the secretory pathway, including intra-Golgi transport, exit from the Golgi complex, formation and concentration of secretory granules, and exocytosis.