We previously showed that protein kinase CK2 downregulation mediates senescence through the reactive oxygen species (ROS)-p53-p21(Cip1/WAF1) pathway in various human cells. In the present study, we investigated whether the FoxO3a transcription factor is associated with ROS production during CK2 downregulation-induced senescence in human colon cancer HCT116 and breast cancer MCF-7 cells. FoxO3a overexpression suppressed ROS production and p53 stabilization induced by a CK2 alpha knockdown. CK2 alpha downregulation induced nuclear export of FoxO3a through stimulation of ART-mediated phosphorylation of FoxO3a and decreased transcription of its target genes (Cu/ZnSOD, MnSOD, and catalase). In contrast, CK2 alpha overexpression inhibited AICT-mediated FoxO3a phosphorylation. This resulted in nuclear accumulation of FoxO3a, and elevated expression of its target genes. Therefore, these data indicate for the first time that CK2 downregulation stimulates ROS generation by inhibiting FoxO3a during premature senescence in human colon and breast cancer cells. (C) 2016 Elsevier Inc. All rights reserved.