Abstract. Casein kinase II contributes to the growth and survival of malignant gliomas and attracts increasing attention as a therapeutic target in these tumors. Several reports have suggested that this strategy might be most relevant for specific subgroups of patients, namely Verhaak's classical and TP53 wild-type tumors. Using kinase assays and microarray genetic profiling in a series of 27 proprietary fresh frozen surgical glioma samples, we showed that constitutive CK2 kinase activation is not restricted to tumors that present increased copy numbers or mRNA expression of its catalytic or regulatory subunits, and can result from a functional activation by various cytokines from the glioma microenvironment. Using corresponding primary tumor and human astrocyte cell cultures as well as glioma cell lines, we confirmed that CK2 inhibition is selectively toxic to malignant glial tumors, without any restriction to tumor class or to TP53 status. We finally showed that while the contribution of CK2 to the constitutive NF-κB hyperactivation in malignant gliomas is at best moderate, a delayed activation of NF-κB may associate with the therapeutic resistance of glioma cells to CK2 inhibition.
IntroductionGlioblastomas (GBMs) are the most aggressive and prevalent type of primary brain tumors and present a dismal prognosis with a median survival of less than two years (1).Casein kinase 2 is a ubiquitous serine/threonine tetrameric kinase that consists of two catalytic subunits (α and α') and two β regulatory subunits. CK2 is frequently overexpressed or overactive in aggressive forms of solid and hematological malignancies (2,3), and contributes to the dysregulation of cell growth, invasion, survival and senescence via the β-catenin, JAK/STAT, mTOR, and NF-κB pathways (4-9). As a result, CK2 inhibitors have recently entered preliminary clinical trials for advanced solid and hematological tumors (source: ClinicalTrial.gov, accessed in December 2015).In GBMs, CSNK2A1, the gene encoding the α catalytic subunit of the kinase, is amplified in one third of the tumors, and especially in those that belong to the Verhaak's 'classical' gene expression phenotype (10). CK2 has been reported to regulate tumor-initiating cell growth, tumor cell survival, DNA repair following ionizing radiation, and apoptosis in these tumors (4,5,7,8,(11)(12)(13). As a result, CK2 inhibition with drug inhibitors or siRNA technology was shown to alter the growth, survival and migration of glioma cell lines and slow down the growth of GBMs xenografts in immunodeficient mice (9,14).Prior to translating these findings to the bedside however, it remains unclear whether CK2 hyperactivity is restricted to classical GBMs presenting a CSNK2A1 amplification or not and/or whether all malignant glial tumors are evenly likely to benefit from CK2 inhibitory strategies.This report provides evidence that CK2 kinase hyperactivity occurs in vivo in all classes of GBMs as well as in glial tumors of lower grades and histology, and can be a target independently of Verhaak's ...