Protein synthesis is required for expression of anthrax lethal toxin cytotoxicity

Abstract

Anthrax lethal toxin, which is composed of two proteins, i.e., protective antigen and lethal factor, is cytolytic to mouse peritoneal macrophages and the macrophage-like cell line J774A.1. After exposure of cells to lethal toxin, inhibition of protein synthesis occurred only slightly before the onset of cytolysis. Thus, cell death did not appear to be due to inhibition of protein synthesis. However, prior treatment of J774A.1 cells with cycloheximide or puromycin, which inhibited protein synthesis, protected them completely against lethal toxin-induced cytolysis, which suggested that continuous protein synthesis is required for the expression of lethal toxin activity. Inhibition of protein synthesis had no appreciable effect on the binding of protective antigen to the cell surface receptor or on proteolytic cleavage of surface-bound protective antigen. Furthermore, inhibition of protein synthesis did not alter the uptake of toxin, which suggested that protein synthesis is required at a later stage of the intoxication process. The protection provided by inhibition of protein synthesis was effective, even up to 1 h after exposure to anthrax lethal toxin. The increased uptake of calcium observed in cells exposed to lethal toxin did not occur when they were protected by blocking protein synthesis. Identifying the protein(s) synthesized during the intoxication process may help to understand the mechanism of cell death produced by anthrax lethal toxin.