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 alpha and/or alpha' 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 alpha relocalizes to the endoplasmic reticulum after 6-TG treatment. Additionally, transfection of Cdc2 with a mutation at Ser(39) to Ala, which is the CK2 phosphorylation site, partially inhibits cell cycle progression in G(1) to G(2) phase following 6-TG treatment.
Conclusion: CK2 is essential for apoptosis inhibition following DNA damage induced by 6-TG, controlling caspase activity.