IKKalpha and IKKbeta each function to regulate NF-kappaB activation in the TNF-induced/canonical pathway

Activation of the transcription factor NF-kappaB by cytokines is rapid, mediated through the activation of the IKK complex with subsequent phosphorylation and degradation of the inhibitory IkappaB proteins. The IKK complex is comprised of two catalytic subunits, IKKalpha and IKKbeta, and a regulatory protein known as NEMO. Using cells from mice that are genetically deficient in IKKbeta or IKKalpha, or using a kinase inactive mutant of IKKbeta, it has been proposed that IKKbeta is critical for TNF-induced IkappaB phosphorylation/degradation through the canonical pathway while IKKalpha has been shown to be involved in the non-canonical pathway for NF-kappaB activation. These conclusions have led to a focus on development of IKKbeta inhibitors for potential use in inflammatory disorders and cancer.

Analysis of NF-kappaB activation in response to TNF in MEFs reveals that IKKbeta is essential for efficient phosphorylation and subsequent degradation of IkappaB alpha, yet IKKalpha contributes to the NF-kappaB activation response in these cells as measured via DNA binding assays. In HeLa cells, both IKKalpha and IKKbeta contribute to IkappaB alpha phosphorylation and NF-kappaB activation. A kinase inactive mutant of IKKbeta, which has been used as evidence for the critical importance of IKKbeta in TNF-induced signaling, blocks activation of NF-kappaB induced by IKKalpha, even in cells that are deficient in IKKbeta.

These results demonstrate the importance of IKKalpha in canonical NF-kappaB activation, downstream of cytokine treatment of cells. The experiments suggest that IKKalpha will be a therapeutic target in inflammatory disorders.