Brefeldin A induced dose-dependent changes to Golgi structure and function in the rat exocrine pancreas

We have studied the effect of increasing concentrations of Brefeldin A (BfA) on the rate of secretion in vitro of pulse-labeled proteins and individual enzymes and on the fine structure of the Golgi apparatus in pancreatic acinar cells derived from control rats and from animals stimulated in vivo by feeding a synthetic proteinase inhibitor (FOY 305). A half-maximal inhibition of intracellular transport of newly synthesized proteins was observed at 0.125 microgram/ml BfA which in FOY-stimulated pancreatic lobules was more pronounced. The Golgi apparatus at this low dose of BfA was still preserved and consisted of stacks of narrow cisternae devoid of secretory material. Granule formation from the trans-Golgi network (TGN) was greatly reduced. A nearly complete inhibition of both total protein and individual enzyme transport was observed at 0.5 microgram/ml BfA. This inhibitory effect was more pronounced with enzyme proteins which are transported slowly through the cellular compartments (e.g. lipase) as compared to faster moving proteins (e.g. chymotrypsinogen). The Golgi apparatus at 0.5 microgram/ml BfA was fragmented into clouds of small uniform vesicles which were stained with an antibody directed against TGN 38 and which were surrounded by a network of tubular membranes reacting with an antibody against p58, a marker protein of the intermediate compartment between rough endoplasmic reticulum (RER) and Golgi. Incubation of pancreatic lobules in 10 micrograms/ml BfA led to a disappearance of the small vesicles and a relocation of TGN 38 into the RER, while the tubuloreticular membranes of the intermediate compartment remained unaffected. Aggregates of clathrin cages devoid of membranes accumulated in the area of the previous Golgi vesicles. The inhibitory effect of 0.5 microgram/ml BfA on both intracellular transport and Golgi fine structure was readily reversible within 15 to 30 min upon removal of the drug, but took 1 h or longer after incubation in 5 or 10 micrograms/ml BfA.