Expression of the Casein Kinase 2 Subunits in Chinese Hamster Ovary and 3T3 L1 Cells Provides Information on the Role of the Enzyme in Cell Proliferation and the Cell Cycle

Abstract

In order to investigate the in vivo functions of protein kinase CK2 (CK2), the expression of Myc-tagged versions of the subunits, Myc-CK2␣ and Myc-CK2␤, was carried out in Chinese hamster ovary cells (CHO cells) and in 3T3 L1 fibroblasts. Cell proliferation in these cells was examined. CHO 3 H]thymidine incorporation, it was found that expression of Myc-CK2␤ prolonged the G 1 phase and inhibited up-regulation of cyclin D1 expression during G 1 . In addition, a lower mitotic index and lower mitotic cyclin-dependent kinase activities were detected in Myc-CK2␤-expressing cells. Detailed analysis of stable cells that were synchronously released into the cell cycle revealed that the expression of Myc-CK2␤ inhibited cells entering into mitosis and prevented the activation of mitotic cyclin-dependent kinases. Taken together, results from both transient and stable expression of CK2 subunits strongly suggest that CK2 may be involved in the control of cell growth and progression of the cell cycle. Casein kinase 2 (CK2)1 is a ubiquitous, multifunctional eukaryotic serine/threonine protein kinase that phosphorylates many different substrates including metabolic enzymes, structural proteins, transcription factors, and proto-oncoproteins (1). The holoenzyme form of CK2 is a heterotetramer, composed of ␣, ␣Ј, and ␤ subunits combined to form ␣ 2 ␤ 2 , ␣␣Ј␤ 2 , and ␣Ј 2 ␤ 2 . The ␣ and ␣Ј subunits are catalytically active, whereas the ␤ subunit is thought to be a regulatory subunit that stimulates the catalytic activity of ␣ or ␣Ј subunits and may also influence substrate specificity (for reviews, see Refs. 1-4). CK2 exhibits remarkable evolutionary conservation of primary structure in all eukaryotes from yeast to human, e.g. the identity of amino acid sequences of ␣ and ␤ subunits between human and Drosophila melanogaster is 90 and 88%, respectively. The amino acid sequences of the ␤ subunits of human, pig, and chicken are even identical, underscoring this point (5-6).The physiological role of CK2 has been explored in yeast and in a number of mammalian cell types, and these studies suggest that the enzyme is involved in cell growth and progression of the cell cycle. For example, genetic studies in Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Dictyostelium discoideum (7-9) showed that CK2 activity is essential for cell viability, e.g. the simultaneous disruption of the genes encoding the catalytic subunits, cka1 and cka2, in S. cerevisiae is lethal (7). An essential role of CK2 in control of cell cycle progression has also been demonstrated in the yeast S. cerevisiae (10). Through the use of mutant strains temperaturesensitive for the CK2 gene, the function of CK2 during the cell cycle was analyzed. It was shown that following a shift to the nonpermissive temperature, the mutant strains arrested within a single cell cycle and showed a dual arrest phenotype consisting of 50% of cells in G 1 and 50% cells in G 2 /M. Further analysis by flow cytometry of pheromone-synchronized cells confirmed that CK2 is required at a...

Phosphorylation regulates the stability of the regulatory CK2β subunit

“…With this conventional view, it might be therefore predicted that autophosphorylated CK2b only exists within tetrameric CK2 complexes. However, a mounting body of evidence indicates that CK2b has functions that are independent of its role in CK2 complexes (Boldyre and Issinger, 1997;Li et al, 1999;Toczyski et al, 1997;Teitz et al, 1990). The three-dimensional structure of tetrameric CK2 that has emerged from X-ray crystallography also challenges the traditional view that CK2 complexes are stable tetramers (Nie®nd et al, 2001).…”

Protein kinase CK2 regulates CDC25B phosphatase activity

“…In mammalian cells, a possible role for CK2 at G2/M has been suggested from results of various experimental approaches. In 3T3 L1 fibroblast cells, the increased expression of CK2b was shown to cause accumulation in G2/M, suggesting a role of CK2 at this point of the cell cycle (Li et al, 1999). Furthermore, both types of CK2 subunits can be phosphorylated by CDK1 in vitro and in intact cells during mitosis (Litchfield et al, 1992;Bosc et al, 1995).…”

Modulation of human checkpoint kinase Chk1 by the regulatory β-subunit of protein kinase CK2

“…It has been shown that CK2b can inhibit the activity of c-Mos preventing Xenopus oocytes maturation (Chen et al, 1997). Moreover, the transient or the stable ectopic expression of Myc-tagged CK2b inhibits cell proliferation and reduces mitotic cyclin-dependent kinases activity (Li et al, 1999). In S. cerevisiae, CK2b has been implicated in checkpoint regulation (Toczyski et al, 1997).…”