IKKα and IKKβ Each Function to Regulate NF-κB Activation in the TNF-Induced/Canonical Pathway

Background: Activation of the transcription factor NF-kappa B by cytokines is rapid, mediated through the activation of the IKK complex with subsequent phosphorylation and degradation of the inhibitory I kappa B proteins. The IKK complex is comprised of two catalytic subunits, IKK alpha and IKK beta, and a regulatory protein known as NEMO. Using cells from mice that are genetically deficient in IKK beta or IKK alpha, or using a kinase inactive mutant of IKK beta, it has been proposed that IKK beta is critical for TNF-induced I kappa B phosphorylation/degradation through the canonical pathway while IKK alpha has been shown to be involved in the non-canonical pathway for NF-kappa B activation. These conclusions have led to a focus on development of IKK beta inhibitors for potential use in inflammatory disorders and cancer. Methodology: Analysis of NF-kappa B activation in response to TNF in MEFs reveals that IKK beta is essential for efficient phosphorylation and subsequent degradation of I kappa B alpha, yet IKK alpha contributes to the NF-kappa B activation response in these cells as measured via DNA binding assays. In HeLa cells, both IKK alpha and IKK beta contribute to I kappa B alpha phosphorylation and NF-kappa B activation. A kinase inactive mutant of IKK beta, which has been used as evidence for the critical importance of IKK beta in TNF-induced signaling, blocks activation of NF-kappa B induced by IKK alpha, even in cells that are deficient in IKK beta. Conclusions: These results demonstrate the importance of IKK alpha in canonical NF-kappa B activation, downstream of cytokine treatment of cells. The experiments suggest that IKK alpha will be a therapeutic target in inflammatory disorders.