Interleukin-1-induced NF-κB activation is NEMO-dependent but does not require IKKβ

Activation of NF-kappa B by the pro-inflammatory cytokines tumor necrosis factor (TNF) and interleukin-1 (IL-1) requires the I kappa B kinase (IKK) complex, which contains two kinases named IKK alpha and IKK beta and a critical regulatory subunit named NEMO. Although we have previously demonstrated that NEMO associates with both IKKs, genetic studies reveal that only its interaction with IKK beta is required for TNF-induced NF-kappa B activation. To determine whether NEMO and IKK alpha can form a functional IKK complex capable of activating the classical NF-kappa B pathway in the absence of IKK beta, we utilized a panel of mouse embryonic fibroblasts (MEFs) lacking each of the IKK complex subunits. This confirmed that TNF-induced I kappa B alpha degradation absolutely requires NEMO and IKK beta. In contrast, we consistently observed intact I kappa B alpha degradation and NF-kappa B activation in response to IL-1 in two separate cell lines lacking IKK beta. Furthermore, exogenously expressed, catalytically inactive IKK beta blocked TNF- but not IL-1-induced I kappa B alpha degradation in wild-type MEFs, and reconstitution of IKK alpha/beta double knockout cells with IKK alpha rescued IL-1- but not TNF-induced NF-kappa B activation. Finally, we have shown that incubation of IKK beta-deflcient MEFs with a cell-permeable peptide that blocks the interaction of NEMO with the IKKs inhibits IL-1-induced NF-kappa B activation. Our results therefore demonstrate that NEMO and IKK alpha can form a functional IKK complex that activates the classical NF-kappa B pathway in response to IL-1 but not TNF. These findings further suggest NEMO differentially regulates the fidelity of the IKK subunits activated by distinct upstream signaling pathways.