SHIGA TOXIN MODE OF ACTION IN E. COLI O157:H7 DISEASE

Tom G. Obrig

Department of Microbiology and Immunology, University of Rochester, Box 672, 601 Elmwood Avenue, Rochester, NY 14642

Received 12/2/97 Accepted 12/8/97

6. INTERNALIZATION AND PROCESSING OF STX BY EUKARYOTIC CELLS

6.1 Stx enters by receptor-mediated endocytosis

Toxin internalization and processing is fundamental in dictating the effect of Stxs' on individual cell types. Stx enters and is processed in eukaryotic cells by a series of steps which are collectively referred to as receptor mediated endocytosis (33). This process involves trafficking of Stx between intracellular vesicles, endosomes and lysosomes to the golgi and finally to the endoplasmic reticulum where it is released into the cytoplasm. It is in the cytoplasm where Stx comes into contact with and enzymatically inactivates the ribosomes. The Gb3 receptor is required, but not sufficient for Stx action. Examples have been described of cells being resistant to Stx action because Stx could bind to, but not be internalized by the cells (34). In contrast, part of the reason that human renal microvascular endothelial cells are 1,000-times more sensitive to Stx2 vs. Stx1 is likely due to the efficiency of toxin internalization and processing by this cell type (23). Routing of Stx within cells is also dictated by the chemical structure of the Gb3 receptor itself. For example, it has recently been demonstrated that shorter fatty acids (i.e. C16) in the ceramide portion of Gb3 direct the internalized Stx-Gb3 complexes directly to the nucleus in certain cell types (29). This is important because in the latter case, the cells succumb to apoptotic cell death rather than to necrosis through inhibition of cytoplasmic protein synthesis.

6.2 Processing and activation of Stx in target cells

Processing of Stx within the target cell by trypsin-like action releases the Stx-A1 peptide fragment (the enzymatic portion) from a shorter C-terminal Stx-A2 fragment. It is thought that this action, along with reduction of a disulfide bond within the same region of the Stx-A subunit is required for "activation" of Stx biological activity. However, changes in the trypsin cleavage site or in the cysteines of the disulfide bond yield Stx that is toxic to cells because the cells remain capable of cleaving and releasing the Stx-A1 subunit (35,36).