Listeriolysin O secreted by Listeria monocytogenes induces NF‐ κ B signalling by activating the I κ B kinase complex

Summary Listeriolysin O (LLO) is a pore‐forming cytolysin secreted by the pathogen Listeria monocytogenes and is required for its intracellular survival. We recently demonstrated that in endothelial cells, LLO activates the NF‐κB signalling pathway. In this work, we studied the LLO‐induced molecular cascade of NF‐κB activation with a cellular model extensively used to analyse the signalling pathway of NF‐κB activation, i.e. the human embryonic kidney HEK‐293 cell line and its derivatives (transfectants or mutants). When the stably transfected derivative HEK‐293 cells expressing IL‐1RI were exposed to LLO, a strong NF‐κB activation was detected, contrasting with other cell lines (HEK‐293 wild type, HEK‐293.T and COS) expressing a very low level of IL‐1RI. Although a delayed kinetics of LLO‐dependent NF‐κB activation suggests an autocrine or paracrine IL‐1‐dependent pathway, we found that LLO‐dependent NF‐κB activation did not require the IL‐1 protein synthesis nor the interaction with the IL‐1RI specific receptor. Herein, we demonstrated that LLO‐dependent NF‐κB activation requires the activation of the IκB kinase β (IKKβ) subunit of IKK complex to phosphorylate and degrade cytoplasmic IκBα, a natural inhibitor of NF‐κB. The activation induced by LLO does not require the adapters MyD88 and IL‐1R‐associated kinase (IRAK). We suggested that LLO induces a distinct signalling pathway from that of IL‐1 and its receptor.