Inducible IκB kinase/IκB kinase ε expression is induced by CK2 and promotes aberrant nuclear factor-κB activation in breast cancer cells

Aberrant activation of nuclear factor-kappa B (NF-kappa B) transcription factors has been implicated in the pathogenesis of breast cancer. We previously showed elevated activity of I kappa B kinase alpha (IKK alpha), IKK beta, and protein kinase CK2 in primary human breast cancer specimens and cultured cells. A novel inducible IKK protein termed IKK-i/IKK epsilon has been characterized as a potential TNF-kappa B activator. Here, we provide evidence that implicates IKK-i/IKK epsilon: in the pathogenesis of breast cancer. We show IKK-i/IKK epsilon expression in primary human breast cancer specimens and carcinogen-induced mouse mammary tumors. Multiple breast cancer cell lines showed higher levels of IKK-i/IKK epsilon and kinase activity compared with untransformed MCF-10F breast epithelial cells. Interestingly, IKK-i/ IKK epsilon expression correlated with CK2 alpha. expression in mammary glands and breast tumors derived from TMMTV-CK2 alpha transgenic mice. Ectopic CK2 expression in untransformed cells led to increased IKK-i/IKK epsilon mRNA and protein levels. Inhibition of CK2 alpha via the pharmacologic inhibitor apigenin or upon transfection of a CK2 kinase-inactive subunit reduced IKK-i/IKK epsilon levels. Expression of a kinase-inactive IKK-i/IKK epsilon mutant in breast cancer cells reduced NI-KII activity as judged by transfection assays of reporters driven either by NF-kappa B elements or the promoters of two NT-kappa B target genes, cyclin D1 and relB. Importantly, the kinase-inactive IKK-i/IKK epsilon mutant reduced the endogenous levels of these genes as well as the ability of breast cancer cells to grow in soft agar or form invasive colonies in Matrigel. Thus, CK2 induces functional IKK-i/IKK epsilon, which is an important mediator of the activation of NF-kappa B that plays a critical role in the pathogenesis of breast cancer.