CARD4/Nod1 mediates NF‐κB and JNK activation by invasive Shigella flexneri

Abstract

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

TIR, CARD and PYRIN: three domains for an antimicrobial triad

“…56,77 . Nod1 and Nod2 stimulation leads also to the production of proinflammatory cytokines as well as JNK activation.…”

“…Nod1 and Nod2 stimulation leads also to the production of proinflammatory cytokines as well as JNK activation. 56,78 Virus infection can activate NF-kB but the prominent cell response is a massive type I IFN release, mediated by the activation of transcription factors, IRF3 and IRF7 (see Part I).…”

“…RIP2 mediates Nod1-and Nod2-dependent NF-kB activation. 56,80 Upon Nod1 activation, a transient complex between Nod1/RIP2 and IKKs is formed, 56,80 suggesting a mechanism of downregulation. Recently, it has been shown that activation of Nod2 induces a RIP2-dependent ubiquitinylation of NF-kB essential modulator (IKKg), at a novel site, thereby enhancing the NF-kB activation.…”

Synergistic enhancement of Toll-like receptor responses by NOD1 activation

“…NOD1 mediates epithelial cell signal transduction following infection with Shigella flexneri and enteroinvasive Escherichia coli [5,6], and in vivo NOD1-deficient mice show increased susceptibility to cag pathogenicity island positive Helicobacter pylori infection [7]. Genetic variation in the NOD1 gene is associated with suscept-ibility to asthma and inflammatory bowel disease [8,9].…”

RICK/RIP2 Mediates Innate Immune Responses Induced through Nod1 and Nod2 but Not TLRs

“…However, the mechanism by which RICK regulates Listeria infection remains poorly understood. Stimulation of Nod1 or Nod2 by their specific bacterial activators causes the recruitment of RICK and the association of RICK with Nod1 or Nod2 via CARD-CARD interactions (11,12). RICK directly binds to I B kinase (IKK) ␥ and activates the IKK complex by promoting the ubiquitinylation of IKK␥ and stimulating the kinase activity of the two other components of the IKK complex, IKK␣ and IKK␤ (11,13).…”