Regulation of NF-κB by Calmodulin

Abstract: Cells experience numerous external signals which they must respond to. Such signals arriving at the cell surface are transduced via various signal transduction pathways and often ultimately result in regulation of transcription. NF-κB is a family of transcription factors involved in the regulation of genes important for processes such as immune and inflammatory responses, cell growth, development and cell survival. NF-κB proteins are normally kept inactive in the cytoplasm due to masking of their nuclear localisation signal (NLS) by inhibitory IκB proteins. A large number of stimuli lead to the activation of IκB-kinase (IKK). Active IKK phosphorylates IκB and thereby labels it for ubiquitination and, subsequently, degradation by the proteasome. Liberated NF-κB enters the nucleus, where it takes part in the regulation of its target genes. Calmodulin (CaM) is a ubiquitous Ca2+-binding protein which is considered to be the predominant intracellular Ca2+ sensor. CaM plays a major role in the Ca2+-dependent regulation of a wide variety of cellular processes, including transcription. CaM regulates transcription both indirectly through CaM-dependent kinases and phosphatases and directly through interaction with transcription factors.CaM was found to bind directly and in a Ca2+-dependent fashion to the two NF-κB family members c-Rel and RelA. The CaM-NF-κB interactions were strongly enhanced by NF-κB activating stimuli and this enhancement was blocked by the addition of IκB, suggesting that c-Rel and RelA can bind CaM after their signal-induced release from IκB. Compared to wild-type c-Rel, CaM binding-deficient mutants were shown to exhibit an increased nuclear accumulation and transcriptional activity on Ca2+-regulated cytokine promoters. The results suggest that CaM can inhibit transport of c-Rel, but not of RelA, to the nucleus and thereby differentially regulate the activation of NF-κB proteins following cell stimulation. CaM was also found to affect NF-κB activity indirectly through the action of a CaM-dependent kinase (CaMK). Studies of the events leading to IκBα phosphorylation revealed that CaM and CaMKII inhibitors blocked phorbol ester induced activation of IKK. Furthermore, CaM and CaMKII inhibitors also blocked T cell receptor/CD3 induced IκBα degradation, and expression of an inhibitor-resistant derivative of the γ isoform of CaMKII caused the inhibitors lose their effect on phorbol ester induced IκBα degradation. Finally, expression of a constitutively active CaMKII resulted in the activation of NF-κB. These results identify CaMKII as a mediator of IKK activation, specifically in response to T cell receptor/CD3 and phorbol ester stimulation.In conclusion, this thesis describes the identification of CaM as a dual regulator of NF-κB proteins, acting both directly and indirectly to affect the activity of this family of transcription factors.