Epidermal growth factor activation of NF-κB is mediated through IκBα degradation and intracellular free calcium

The transcription factor NF-kappa B is normally sequestered in the cytoplasm by its inhibitory subunit I kappa B. Most extracellular signals activate NF-kappa B through a mechanism involving the phosphorylation and proteasome-dependent degradation of I kappa B, EGF activates NF-kappa B in A-431 carcinoma cells, which overexpress EGF receptors and in mouse embryo fibroblasts, which have a normal complement of receptors, Supershift experiments indicate that the NF-kappa B complexes induced by EGF are composed of p50/p50 homodimers and p65/p50 heterodimers, but not c-rel, EGF stimulation enhances the degradation of I kappa B alpha, but not I kappa-B beta nor an N-terminal deletion mutant of II;Ba. Treatment of cells with a proteasome inhibitor, such as ALLN or MG132, blocks EGF-mediated NF-kappa B activation, indicating that EGF-induced NF-kappa B activation requires proteasome-dependent I kappa B degradation, Also, Bapta A/M (a cell-permeable chelator of intracellular calcium) blocks EGF-induced NF-kappa B activation and I kappa B alpha degradation, suggesting a requirement of intracellular free Ca2+ for this growth factor response, Protein kinase C inhibition, in contrast, did not influence EGF activation of NF-kappa B.