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Casein Kinase 2 Regulates Both Apoptosis and the Cell Cycle Following DNA Damage Induced by 6-Thioguanine

¹ Department of Radiation Oncology, Case Western Reserve University and ² University Hospitals of Cleveland, Case Comprehensive Cancer Center, Cleveland, Ohio

Requests for reprints: Timothy J. Kinsella, Department of Radiation Oncology, LTR6068, University Hospitals of Cleveland/Case Comprehensive Cancer Center, 11100 Euclid Avenue, Cleveland, OH 44106-6068. Phone: 216-844-2530; Fax: 216-844-4799; E-mail: timothy.kinsella{at}uhhs.com.

Purpose: The purine antimetabolite, 6-thioguanine (6-TG), is an effective drug in the management of acute leukemias. In this study, we analyze the mechanisms of apoptosis associated with 6-TG treatment and casein kinase 2 (CK2 or CKII) in human tumor cells.

Experimental Design: Small interfering RNA and chemical CK2 inhibitors were used to reduce CK2 activity. Control and CK2 activity–reduced cells were cultured with 6-TG and assessed by flow cytometry to measure apoptosis and cell cycle profiles. Additionally, confocal microscopy was used to assess localization of CK2 catalytic units following 6-TG treatment.

Results: Transfection of small interfering RNA against the CK2 and/or ' catalytic subunits results in marked apoptosis of HeLa cells following treatment with 6-TG. Chemical inhibitors of CK2 also induce apoptosis following 6-TG treatment. Apoptosis induced by 6-TG is similarly observed in both mismatch repair-proficient and -deficient HCT116 and HeLa cells. Concomitant treatment with a pan-caspase inhibitor or transfection of apoptosis repressor with caspase recruitment domain markedly suppresses the apoptotic response to DNA damage by 6-TG in the CK2-reduced cells, indicating caspase regulation by CK2. CK2 relocalizes to the endoplasmic reticulum after 6-TG treatment. Additionally, transfection of Cdc2 with a mutation at Ser³⁹ to Ala, which is the CK2 phosphorylation site, partially inhibits cell cycle progression in G₁ to G₂ phase following 6-TG treatment.

Conclusion: CK2 is essential for apoptosis inhibition following DNA damage induced by 6-TG, controlling caspase activity.

Key Words: 6-TG • caspase • Casein kinase II • emodin • apigenin • DRB

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