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Diarrhea-associated hemolytic uremic syndrome (D(+)HUS) is caused by the ingestion of Escherichia coli that produce Shiga toxin (Stx), which is composed of a cytotoxic A subunit and pentameric B subunits that bind globotriaosylceramide on susceptible cells. Stx occurs in 2 types, Stx1 and Stx2. B subunits of either type stimulate von Willebrand factor (VWF) secretion from human umbilical vein endothelial cells (HUVECs), and Stx2B can cause thrombotic microangiopathy in Adamts13(-/-) mice. We have now determined that Stx1B and Stx2B activate different signaling pathways in HUVECs. VWF secretion induced by Stx1B is associated with a transient rise in intracellular Ca(2+) level that is blocked by chelation with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester, removal of extracellular Ca(2+), the phospholipase C inhibitor U73122, the protein kinase inhibitor staurosporine, or small interfering RNA knockdown of protein kinase Cα. In contrast, Stx2B-induced VWF secretion is associated with activation of protein kinase A (PKA) and is blocked by the PKA inhibitor H89 or small interfering RNA knockdown of PKA. Stx2B does not increase cAMP levels and may activate PKA by a cAMP-independent mechanism. The activation of distinct signaling pathways may be relevant to understanding why E coli that express Stx2 are more likely to cause D(+)HUS than are E coli expressing only Stx1.

A growing body of evidence suggests that ribosome-inactivating proteins (RIPs) remove adenine moieties not only from rRNA, but also from DNA--an effect leading to DNA damage in cultured cells. We herein report that two distinct RIPs of bacterial (shiga toxin 1, Stx1) and plant (ricin) origin, inhibit the repair of the DNA lesions generated by hydrogen peroxide in cultured human cells. This effect is unrelated either to inhibition of protein synthesis or to depletion of cellular antioxidant defenses and is likely to derive from direct interactions with cellular DNA repair machinery. Therefore, the genotoxicity of these toxins on mammalian cells seems to be a complex phenomenon resulting from the balance between direct (DNA damaging activity), indirect (DNA repair inhibition) effects and the eventual presence of other DNA damaging species. In particular, with regard to Stx1, it could be hypothesized that Stx-producing bacteria increase the risk of transformation of surrounding, inflamed tissues in the course of human infections.