An IκB Kinase-Regulated Feedforward Circuit Prolongs Inflammation

• IKKs phosphorylate ITCH in a highly conserved region of the HECT domain
• E3 ubiquitin ligase activity is impaired in IKK-phosphorylated ITCH
• Impaired ITCH results in heightened TNF signaling
• Activated IKKs can affect multiple pathways through inhibitory ITCH phosphorylation

SummaryLoss of NF-κB signaling causes immunodeficiency, whereas inhibition of NF-κB can be efficacious in treating chronic inflammatory disease. Inflammatory NF-κB signaling must therefore be tightly regulated, and although many mechanisms to downregulate NF-κB have been elucidated, there have only been limited studies demonstrating positive feedforward regulation of NF-κB signaling. In this work, we use a bioinformatic and proteomic approach to discover that the IKK family of proteins can phosphorylate the E3 ubiquitin ligase ITCH, a critical downregulator of TNF-mediated NF-κB activation. Phosphorylation of ITCH by IKKs leads to impaired ITCH E3 ubiquitin ligase activity and prolongs NF-κB signaling and pro-inflammatory cytokine release. Since genetic loss of ITCH mirrors IKK-induced ITCH phosphorylation, we further show that the ITCH−/− mouse’s spontaneous lung inflammation and subsequent death can be delayed when TNF signaling is genetically deleted. This work identifies a new positive feedforward regulation of NF-κB activation that drives inflammatory disease.

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