Treatment of human endothelial cells with
Shiga toxin 1 and 2 leads to the upregulation of genes encoding proinflammatory molecules involved in the pathogenesis of
hemolytic-uremic syndrome. The paradoxical effect of inhibitors of mRNA translation, such as
Shiga toxins, that at the same time induce
protein expression was investigated by studying the relationship between their enzymatic activity (abstraction of
adenine from
nucleic acids) and the induction of
interleukin-8 and
granulocyte-macrophage colony-stimulating factor in human endothelial cells. As a positive control, the fungal toxin
alpha-sarcin, acting on the same rRNA sequence targeted by
Shiga toxins with a different mechanism (
RNase activity), was used. The three toxins caused ribosomal lesions that, in turn, induced the activation of p38 stress
kinase with kinetics that paralleled the inhibition of translation.
Alpha-sarcin was devoid of activity on
DNA.
Shiga toxin 2 targeted nuclear
DNA with more rapid kinetics than did
Shiga toxin 1. Since the fungal ribotoxin was fully effective in the induction of proinflammatory
proteins, we conclude that damage to ribosomes is indispensable and sufficient to activate
protein expression via induction of the stress-
kinase cascade. However, gene upregulation events induced by
Shiga toxin 2 were much more efficient than those triggered by
Shiga toxin 1, although the two toxins impaired translation to the same extent and had overlapping time courses of stress
kinase activation. Regulations independent of the ribotoxic stress were assumed to operate in intoxicated cells. We hypothesized that the two
bacterial toxins recognize different DNA sequences inducing different regulating effects on gene expression.