Calmodulin-dependent kinase II mediates T cell receptor/CD3-and phorbol ester-induced activation of IκB kinase

Numerous fundamental biological processes involve the NF kappaB family of transcription factors. The mechanisms by which this family of proteins is regulated are therefore of widespread importance. In most cells, NF kappaB is bound to inhibitory I kappaB proteins and sequestered in the cytoplasm. NF kappaB-inducing signals result in activation of a large multisubunit kinase complex, IKK, which phosphorylates I kappaB. I kappaB is subsequently degraded, releasing NF kappaB, which translocates to the nucleus. We previously reported that inhibitors of the calcium-binding protein calmodulin (CaM) prevent phorbol ester-induced phosphorylation of I kappaB. Here we show that KN93, an inhibitor of CaM-dependent kinases (CaMKs), also inhibits the phosphorylation of I kappaB. The effect of both CaM and CaMK inhibitors on I kappaB phosphorylation is due to the inhibition of the activity of CaMK II because neither drug has any effect when a derivative of CaMK II that is insensitive to these inhibitors is expressed. When CaMK II is inhibited, phorbol ester is no longer able to activate IKK, placing CaMK II in the signaling pathway that leads to IKK activation. CaM and CaMK inhibitors also block T cell receptor/CD3-induced activation but have no effect on the ability of the cytokine tumor necrosis factor alpha or the phosphatase inhibitor calyculin A to induce degradation of I kappaB. Finally we show that expression of a constitutively active CaMK II results in the activation of NF kappaB. The results identify CaMK Il as a mediator of IKK activation specifically in response to T cell receptor/CD3 and phorbol ester stimulation.