Trafficking of Pseudomonas exotoxin A in mammalian cells

Intoxication of epithelial cells by plasmid-encoded toxin requires clathrin-mediated endocytosis

It has been shown that the autotransporter plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli (EAEC) produces cytotoxic and enterotoxic effects. Both effects can be explained by the proteolytic activity of Pet on its intracellular target alpha-fodrin (alphaII spectrin). In addition, Pet cytotoxicity and enterotoxicity depend on Pet serine protease activity, and on its internalization into epithelial cells. However, the mechanisms of Pet uptake by epithelial cells are unknown. Here, we show that Pet interacts with the plasma membrane of epithelial cells, and afterwards is detected inside the cells. Furthermore, Pet was internalized via clathrin-mediated endocytosis, since its internalization was inhibited by monodansylcadaverine and sucrose, but not by filipin or methyl-beta-cyclodextrin, which are drugs that interfere with protein entry via a clathrin-independent pathway. Additionally, Pet was immunoprecipitated by anti-clathrin antibodies, but not by anti-caveolin antibodies. Moreover, small interfering RNA (siRNA), designed to knock out clathrin gene expression in HEp-2 cells, prevented Pet internalization, and thereby the Pet-induced cytotoxic effect. However, the use of siRNA to knock out caveolin expression had no effect on Pet internalization, and the cytotoxic effect was clearly observed. Together, these data indicate that Pet secreted by EAEC binds to the cell surface via an unknown receptor, to be taken up by clathrin-mediated endocytosis, and exert its toxic effect in the cytoplasm.

Pseudomonas exotoxin-mediated delivery of exogenous antigens to MHC class I and class II processing pathways

Peptides associated with class II MHC molecules are normally derived from exogenous proteins, whereas class I MHC molecules normally associate with peptides from endogenous proteins. We have studied the ability of Pseudomonas exotoxin A (PE) fusion proteins to deliver exogenously added antigen for presentation by both MHC class I and class II molecules. A MHC class II-restricted antigen was fused to PE; this molecule was processed in a manner typical for class II-associated antigens. However, a MHC class I-restricted peptide fused to PE was processed by a mechanism independent of proteasomes. Furthermore, we also found that the PE fusion protein was much more stable in normal human plasma than the corresponding synthetic peptide. We believe that effective delivery of an antigen to both the MHC class I and class II pathways, in addition to the increased resistance to proteolysis in plasma, will be important for immunization.

Pseudomonas exotoxin-mediated selection yields cells with altered expression of low-density lipoprotein receptor-related protein

The alpha 2-macroglobulin (alpha 2M) receptor/low-density lipoprotein receptor-related protein (LRP) is important for the clearance of proteases, protease-inhibitor complexes, and various ligands associated with lipid metabolism. While the regulation of receptor function is poorly understood, the addition of high concentrations of the 39-kD receptor-associated protein (RAP) to cells inhibits the binding and/or uptake of many of these ligands. Previously, we (Kounnas, M.Z., R.E. Morris, M.R. Thompson, D.J. FitzGerald, D.K. Strickland, and C.B. Saelinger. 1992. J. Biol. Chem. 267:12420-12423) [corrected] showed that Pseudomonas exotoxin (PE) could bind immobilized LRP. Also, the addition of RAP blocked toxin-mediated cell killing. These findings suggested that PE might use LRP to gain entry into toxin-sensitive cells. Here we report on a strategy to select PE-resistant lines of Chinese hamster ovary cells that express altered amounts of LRP. An important part of this strategy is to screen PE-resistant clones for those that retain sensitivity to both diphtheria toxin and to a fusion protein composed of lethal factor (from anthrax toxin) fused to the adenosine diphosphate-ribosylating domain of PE. Two lines, with obvious changes in their expression of LRP, were characterized in detail. The 14-2-1 line had significant amounts of LRP, but in contrast to wild-type cells, little or no receptor was displayed on the cell surface. Instead, receptor protein was found primarily within cells, much of it apparently in an unprocessed state. The 14-2-1 line showed no uptake of chymotrypsin-alpha 2M and was 10-fold resistant to PE compared with wild-type cells. A second line, 13-5-1, had no detectable LRP mRNA or protein, did not internalize alpha 2M-chymotrypsin, and exhibited a 100-fold resistance to PE. Resistance to PE appeared to be due to receptor-specific defects, since these mutant lines showed no resistance to a PE chimeric toxin that was internalized via the transferrin receptor. The results of this investigation confirm that LRP mediates the internalization of PE.

Reduced temperature alters Pseudomonas exotoxin A entry into the mouse LM cell

The movement of Pseudomonas exotoxin A (PE) into the cytoplasm of mouse LM fibroblasts was followed by using inhibition of protein synthesis as a biochemical index of toxin activity; biotinyl-PE and avidin-gold colloids were used for electron microscopy. At 37 degrees C both specific antitoxin and pronase-trypsin protected cells against PE toxicity when added within seconds of warming cells, whereas methylamine was protective when added during the first 7 min of endocytosis. Lowering the temperature to 19 degrees C afforded protection when the temperature transition was accomplished within 15 min of the original endocytic event. These data suggest that PE enters an acidic compartment before reaching a step blocked by shifting cells from 37 to 19 degrees C. PE expressed toxicity for LM cells at 19 degrees C, but at a concentration 1 order of magnitude higher than that required at 37 degrees C. At 19 degrees C, antitoxin or trypsin-pronase protection was rapidly ablated. In contrast cells were fully protected by methylamine for 90 min. Using electron microscopy we demonstrated that toxin moved normally (30 s) to coated areas at 19 degrees C, but remained at this site for up to 20 min before being internalized. The majority of the toxin internalized at 19 degrees C remained in endosomes or in Golgi-associated vesicles and was not delivered to lysosomes. The results suggest that, under physiological conditions (37 degrees C), PE rapidly enters cells through coated areas, moves to an acidic compartment (i.e., the endosome), and then probably to the Golgi region en route to lysosomes. The evidence suggests that movement of toxin from endosomes or Golgi vesicles to lysosomes is blocked at 19 degrees C. We hypothesize that the active form of PE enters the cytosol, where it expresses its toxicity during fusion of Golgi-derived, toxin-laden vesicles with lysosomes.