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Regulation and function of protein Kinase CK2 in cancer cells Sayed, Mohamed Rabi


Protein kinase CK2 is a ubiquitous eukaryotic protein serine/threonine protein kinase present in both the nucleus and cytoplasm. In most cells, the holoenzyme form of CK2 is a constitutively active heterotetramer of two catalytic (aand a') and two noncatalytic P-subunits, but CK2(3-free pools of CK2a also exist. The activity of CK2 is increased in rapidly growing tissues, tumor cells and human cancers. Despite the identification of more than 160 substrates, many of which are important in cell division and differentiation, the signaling pathways that regulate this enzyme have been enigmatic. Here I provide evidence that protein kinase CK2 is regulated by the stressactivated p38 MAP kinase signaling pathway. I found that p38 MAP kinase, in a phosphorylation-dependent manner, can directly interact with and activate CK2 apparently through an allosteric mechanism. The tumor suppressor protein p53 is one of the key molecules involved in maintaining the stability of the genome, through both cell cycle arrest and apoptosis, especially under circumstances of genotoxic stress. p53 has previously been shown to be associated with protein kinase CK2, and undergo phosphorylation on Ser-392. However, the physiological regulation and the functional consequences of this event remain unknown. I found that this phosphorylation is dependent upon the stepwise activation of p38 MAP kinase and CK2 in response to cellular stress. Furthermore, this activation was demonstrated to be responsible for the biochemical regulation of the cyclin-dependent kinase-1 (CDK-1), as both 5,6-dichloro-l-((3-D-ribofuranosyl)-benzimidazole (DRB; a semi-specific inhibitor of CK2)) and antisense depletion of CK2, as well as SB203580 (a specific inhibitor of p38 MAP kinase) were each associated with an inhibition of its activation in response to nocodazole (microtubule inhibitor). I observed that depletion or inhibition of the catalytic subunits of CK2, in the presence of microtubule inhibitor (nocodazole), resulted in a compromise of the spindle assembly checkpoint. Furthermore, the CK2-depleted cells exposed to nocodazole underwent reduced apoptosis as well as abnormal duplication and alignment of centrosomes. These oncogenic properties were observed in both human cervical carcinoma cells (i.e. HeLa cells) and human colon tumor cells (HCT116). I also showed that this effect was dependent on the presence of functional wild-type p53, as this phenomenon was not apparent in HCT116 p53"/_ cells. My results support a novel role for CK2 in concert with p53, to maintain the stability of the genome. These findings may provide for an improved understanding of abnormalities of the spindle checkpoint in human cancers, which is a prerequisite for defining future therapies.

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