BREFELDIN-A PROTECTS RICIN-INDUCED CYTOTOXICITY IN HUMAN CANCER KB CELL-LINE, BUT NOT IN ITS RESISTANT COUNTERPART WITH ALTERED GOLGI STRUCTURES

Brefeldin A (BFA), an isoprenoid fungal metabolite, dramatically disru pts intracellular protein transport and protein secretion. BFA protect s cells from the cytotoxicity of a plant toxin, ricin or pseudomonas t oxin, but not that of diphtheria toxin (YOSHIDA et al., 1991. Expt. Ce ll Res., 192: 389-395.). In this study, we examined whether BFA could differentially change the cytotoxicity of ricin between BFA-sensitive cells and BFA-resistant cells. As a BFA-resistant cell line, we used a resistant cell line, KB/BF2-2, derived from BFA-sensitive human cance r KB cells. BFA treatment caused the disappearance of typical Golgi ci sternae and the concomitant appearance of dilated vesicles in the cyto plasm in KB cells. By contrast, KB/BF2-2 cells had already altered Gol gi structures with poor development of cisternae and also many vesicle s in the absence of BFA, and BFA treatment did not further induce the morphological changes. Although a plasma membrane-specific marker prot ein, alpha-adaptin, was localized similarly in KB/BF2-2 as KB, Golgi s pecific markers such as beta-cop and gamma-adaptin were distributed in the cytoplasmic small vesicles as well as Golgi compartments in KB/BF 2-2 cells in the absence of BFA, and the mutant cells showed no appare nt changes in the distribution even when exposed to BFA. Ricin inhibit ed protein synthesis in KB and KB/BF2-2 to similar levels while pretre atment of KB cells with BFA at 0.1 mu g/ml almost completely reversed the inhibitory effect of ricin. By contrast, the pre-exposure of KB/BF 2-2 cells to 1.0 mu g/ml BFA only partially rescued the ricin-induced inhibition of protein synthesis. Exposure to BFA at 30 min before rici n addition or at 0 min with ricin rescued the protein synthesis inhibi tion, but no rescue occured when BFA was added 30 min after ricin addi tion. BFA could not rescue the protein synthesis inhibition by another toxin, diphtheria toxin. Our results suggest that BFA-resistant mutat ion causes a specific change in the endocytic membrane traffic of rici n in human cells, and also that cytotoxicity of diphtheria toxin does not share a common pathway of the intracellular transport with that of ricin.