IKKα Contributes to Canonical NF-κB Activation Downstream of Nod1-Mediated Peptidoglycan Recognition

Background: During pathogen infection, innate immunity is initiated via the recognition of microbial products by pattern recognition receptors and the subsequent activation of transcription factors that upregulate proinflammatory genes. By controlling the expression of cytokines, chemokines, anti-bacterial peptides and adhesion molecules, the transcription factor nuclear factor-kappa B (NF-kappa B) has a central function in this process. In a typical model of NF-kappa B activation, the recognition of pathogen associated molecules triggers the canonical NF-kappa B pathway that depends on the phosphorylation of Inhibitor of NF-kappa B (I kappa B) by the catalytic subunit IkB kinase beta (IKK beta), its degradation and the nuclear translocation of NF-kappa B dimers. Methodology: Here, we performed an RNA interference (RNAi) screen on Shigella flexneri-induced NF-kappa B activation to identify new factors involved in the regulation of NF-kappa B following infection of epithelial cells by invasive bacteria. By targeting a subset of the human signaling proteome, we found that the catalytic subunit IKK alpha is also required for complete NF-kappa B activation during infection. Depletion of IKK alpha by RNAi strongly reduces the nuclear translocation of NF-kappa B p65 during S. flexneri infection as well as the expression of the proinflammatory chemokine interleukin-8. Similar to IKK beta, IKK alpha contributes to the phosphorylation of IkBa on serines 32 and 36, and to its degradation. Experiments performed with the synthetic Nod1 ligand L-Ala-D-gamma-Glu-meso-diaminopimelic acid confirmed that IKK alpha is involved in NF-kappa B activation triggered downstream of Nod1-mediated peptidoglycan recognition. Conclusions: Taken together, these results demonstrate the unexpected role of IKK alpha in the canonical NF-kappa B pathway triggered by peptidoglycan recognition during bacterial infection. In addition, they suggest that IKK alpha may be an important drug target for the development of treatments that aim at limiting inflammation in bacterial infection.