Roles of Intracellular Calcium and NF-κB in the Clostridium difficile Toxin A-Induced Up-Regulation and Secretion of IL-8 from Human Monocytes

Clostridium difficile causes an intense inflammatory colitis through the actions of two large exotoxins, toxin A and toxin B. IL-8 is believed to play an important role in the pathophysiology of C. difficile-mediated colitis, although the mechanism whereby the toxins up-regulate the release of IL-8 from target cells is not well understood. In this study, we investigated the mechanisms through which toxin A induces IL-8 secretion in human monocytes. We found that cellular uptake of toxin A is required for the up-regulation of IL-8, an effect that is not duplicated by a recombinant toxin fragment comprising the cell-binding domain alone. Toxin A induced IL-8 expression at the level of gene transcription and this effect occurred through a mechanism requiring intracellular calcium and calmodulin activation. Additionally, the effects of toxin A were inhibited by the protein tyrosine kinase inhibitor genistein, but were unaffected by inhibitors of protein kinase C and phosphatidylinositol-3 kinase. We determined that toxin A activates nuclear translocation of the transcription factors NF-κB and AP-1, but not NF-IL-6. NF-κB inhibitors blocked the ability of toxin A to induce IL-8 secretion, and supershift analysis indicated that the major isoform of NF-κB activated by the toxin is a p50-p65 heterodimer. This study is the first to identify intracellular signaling pathways and transcription factors involved in the C. difficile toxin-mediated up-regulation of IL-8 synthesis and release by target cells. This information should increase our understanding of the pathogenesis of C. difficile colitis and the nature of IL-8 gene regulation as well.