Distinct roles of the IκB kinase α and β subunits in liberating nuclear factor κB (NFκB) from IκB and in phosphorylating the p65 subunit of NF-κB

Phosphatidylinositol 3'-kinase (PI3K) and the serine/threonine kinase AKT have critical roles in phosphorylating and transactivating the p65 subunit of nuclear factor kappaB (NF-kappaB) in response to the pro-inflammatory cytokines interleukin-1 (IL-1) and tumor necrosis factor (TNF). Mouse embryo fibroblasts (MEFs) lacking either the alpha or beta subunit of IkappaB kinase (IKK) were deficient in NF-kappaB-dependent transcription following treatment with IL-1 or TNF. However, in contrast to IKKbeta-null MEFs, IKKalpha-null MEFs were not substantially defective in the cytokine-stimulated degradation of Ikappabetaalpha or in the nuclear translocation of NF-kappaB The IKK complexes from IKKalpha- or IKKbeta-null MEFs were both deficient in PI3K-mediated phosphorylation of the transactivation domain of the p65 subunit of NF-kappaB in response to IL-1 and TNF, and constitutively activated forms of PI3K or AKT did not potentiate cytokine-stimulated activation of NF-kappaB in either IKKalpha- or IKKbeta-null MEFs. Collectively, these data indicate that, in contrast to IKKbeta, which is required for both NF-kappaB liberation and p65 phosphorylation, IKKalpha is required solely for the cytokine-induced phosphorylation and activation of the p65 subunit of NF-kappaB that are mediated by the PI3K/AKT pathway.