CARD4/Nod1 mediates NF-kappa B and JNK activation by invasive Shigella flexneri

Epithelial cells are refractory to extracellular lipopolysaccharide (LPS), yet when presented inside the cell, it is capable of initiating an inflamma tory response. Using invasive Shigella flexneri to deliver LPS into the cyt osol, we examined how this factor, once intracellular, activates both NF-KB and c-jun N-terminal kinase (JNK). Surprisingly, the mode of activation is distinct from that induced by toll-like receptors (TLRs), which mediate LP S responsiveness from the outside-in. Instead, our findings demonstrate tha t this response is mediated by a cytosolic, plant disease resistance-like p rotein called CARD4/Nod1. Biochemical studies reveal enhanced oligomerizati on of CARD4 upon S. flexneri infection, an event necessary for NF-KB induct ion. Dominant-negative versions of CARD4 block activation of NF-KB and JNK by S. flexneri as well as microinjected LPS. Finally, we showed that invasi ve S. flexneri triggers the formation of a transient complex involving CARD 4, RICK and the IKK complex. This study demonstrates that in addition to th e extracellular LPS sensing system mediated by TLRs, mammalian cells also p ossess a cytoplasmic means of LPS detection via a molecule that is related to plant disease-resistance proteins.