IKKalpha Contributes to Canonical NF-kappaB 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-kappaB) has a central function in this process. In a typical model of NF-kappaB activation, the recognition of pathogen associated molecules triggers the canonical NF-kappaB pathway that depends on the phosphorylation of Inhibitor of NF-kappaB (IkappaB) by the catalytic subunit IkappaB kinase beta (IKKbeta), its degradation and the nuclear translocation of NF-kappaB dimers. METHODOLOGY: Here, we performed an RNA interference (RNAi) screen on Shigella flexneri-induced NF-kappaB activation to identify new factors involved in the regulation of NF-kappaB following infection of epithelial cells by invasive bacteria. By targeting a subset of the human signaling proteome, we found that the catalytic subunit IKKalpha is also required for complete NF-kappaB activation during infection. Depletion of IKKalpha by RNAi strongly reduces the nuclear translocation of NF-kappaB p65 during S. flexneri infection as well as the expression of the proinflammatory chemokine interleukin-8. Similar to IKKbeta, IKKalpha contributes to the phosphorylation of IkappaBalpha 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 IKKalpha is involved in NF-kappaB activation triggered downstream of Nod1-mediated peptidoglycan recognition. CONCLUSIONS: Taken together, these results demonstrate the unexpected role of IKKalpha in the canonical NF-kappaB pathway triggered by peptidoglycan recognition during bacterial infection. In addition, they suggest that IKKalpha may be an important drug target for the development of treatments that aim at limiting inflammation in bacterial infection.