Differential effects of brefeldin A on transport of secretory and lysosomal proteins

Abstract

Brefeldin A (BFA) rapidly blocks anterograde exocytotic transport through the Golgi complex. Sustained retrograde traffic induced by brefeldin A causes redistribution of constituents of the Golgi, but not the trans-Golgi network (TGN), to the endoplasmic reticulum (ER). In the present study on HepG2 cells, we have observed a differential effect of BFA on transport from the TGN of two soluble proteins: α1-antitrypsin as a representative of secretory proteins and cathepsin D as a prototype of lysosomal enzymes. The Golgi complex of HepG2 cells is sensitive to BFA, as within minutes after its addition nearly all activity of three resident Golgi enzymes was recovered in the ER as monitored by cell fractionation on sucrose density gradients. In accordance with this, 'high mannose'-glycosylated α1-antitrypsin was retained in or transported back to the ER. 'Complex'-glycosylated α1- antitrypsin was neither secreted into the medium nor transported back to the ER. Most of it was retained in vesicles with the buoyant density of Golgi. These vesicles contained the fluid phase endocytotic marker horseradish peroxidase when this was added to the culture medium prior to the BFA, suggesting that the vesicles derived from the TGN. After BFA addition, the compartment became inaccessible to endocytosed horseradish peroxidase. In contrast to blocking transport of complex α1-antitrypsin, BFA did not affect processing of newly synthesized complex-glycosylated procathepsin D (53 kDa) to the mature 31-kDa form. Neither did it interfere with processing of endocytosed procathepsin D. That the mature cathepsin D had indeed reached the lysosomes was verified by Percoll density gradient fractionation. In conclusion, in HepG2 cells, BFA induces two blocks in the secretory pathway: one at the level of the ER-Golgi juncture and the other in the TGN. In contrast, transport from the Golgi complex to the lysosomes and from the plasma membrane to the lysosomes continued.