Variant IL-1 receptor-associated kinase-1 mediates increased NF-κB activity

IL-1R-associated kinase (IRAK)-1 is a critical mediator of TLR/IL-1R-induced activation of the transcription factor NF-kappa B. We previously described that a commonly occurring IRAK-1 variant haplotype, containing amino acid changes from serine to phenylalanine at position 196 and from leucine to serine at position 532, is associated with increased activation of NF-kappa B in LPS-stimulated neutrophils from patients with sepsis-induced acute lung injury and also higher mortality and more severe clinical outcomes in such patients. To investigate the underlying molecular mechanisms, we examined the ability of wild-type and variant IRAK-1 to modulate NF-kappa B activation. We found increased NF-kappa B transcriptional activity and expression of NF-kappa B-dependent proinflammatory cytokines in IL-1 beta stimulated IRAK-1-deficient cells transfected with variant IRAK-1 as compared with IRAK-1 wild type. I kappa B-alpha degradation was faster and p65 phosphorylation more prolonged after IL-1 beta stimulation in cells expressing the IRAK-1 variant. However, IL-1-induced activation of MAPKs and nuclear translocation of NF-kappa B are comparable in both IRAK-1 variant- and IRAK-1 wild-type-expressing cells. Autophosphorylation of the IRAK-1 variant is greater than that found with wild-type IRAK-1. Additionally, variant IRAK-1 has greater interaction with TNFR-associated factor 6 than does wild-type IRAK-1. The enhanced activity of variant IRAK-1 appeared to be due to the alteration at as 532, with only minimal effects being associated with change at as 196. These results demonstrate that variant IRAK-1 is associated with alterations in multiple intracellular events that are likely to contribute to increased NF-kappa B activation and inflammatory responses in individuals with this IRAK-1 haplotype.