Infection with
Shiga toxin (Stx)-producing bacteria and the subsequent release of Stxs and
endotoxins into the bloodstream may damage blood vessels in the colon, kidneys, and central nervous system, leading to bloody
diarrhea,
acute renal failure, and neurological complications. The proinflammatory
cytokines tumor necrosis factor alpha (
TNF-alpha) and
interleukin-1beta (IL-1beta) may contribute to the pathogenesis of Stx-induced vascular lesions by up-regulating toxin receptor expression on endothelial cells. We previously showed that macrophages treated with purified
Shiga toxin 1 (Stx1) or
lipopolysaccharides (LPS) secrete
TNF-alpha and IL-1beta. Northern blot analysis revealed that treatment of the human monocytic cell line THP-1 with LPS induced a rapid and transient increase in steady-state
TNF-alpha and IL-1beta transcripts. In contrast, Stx1 induced slower but prolonged elevations in
cytokine transcripts. The presence of both stimulants resulted in optimal
cytokine mRNA induction in terms of kinetics and prolonged expression. Compared to LPS, Stx1 was a poor inducer of IL-1beta
protein expression, although levels of soluble IL-1beta induced by all treatments continually increased over 72 h. IL-1beta transcripts were not induced by Stx1 B-subunits. Using the transcriptional inhibitor
actinomycin D, we determined that treatment with Stx1 or Stx1 plus LPS induced
cytokine transcripts with increased stability compared to transcripts induced by LPS alone. For all treatments, IL-1beta mRNA decay was slower than
TNF-alpha. Collectively, our data suggest that Stxs affect
cytokine expression, in part, at the posttranscriptional level by stabilizing mRNAs. Optimal
TNF-alpha expression occurs when both Stxs and LPS are present.