Casein kinase I δ/ε phosphorylates topoisomerase II at serine-1106 and modulates DNA cleavage activity

We previously reported that phosphorylation of topoisomerase (topo) II at serine-1106 (Ser-1106) regulates enzyme activity and sensitivity to topo II-targeted drugs. In this study we demonstrate that phosphorylation of Ser-1106, which is flanked by acidic amino acids, is regulated in vivo by casein kinase (CK) I and/or CKI, but not by CKII. The CKI inhibitors, CKI-7 and IC261, reduced Ser-1106 phosphorylation and decreased formation of etoposide-stabilized topo IIDNA cleavable complex. In contrast, the CKII inhibitor, 5,6-dichlorobenzimidazole riboside, did not affect etoposide-stabilized topo IIDNA cleavable complex formation. Since, IC261 specifically targets the Ca-2-regulated isozymes, CKI and CKI, we examined the effect of down-regulating these enzymes on Ser-1106 phosphorylation. Down-regulation of these isozymes with targeted si-RNAs led to hypophosphorylation of the Ser-1106 containing peptide. However, si-RNA-mediated down-regulation of CKII and did not alter Ser-1106 phosphorylation. Furthermore, reduced phosphorylation of Ser-1106, observed in HRR25 (CKI/ homologous gene)-deleted Saccharomyces cerevisiae cells transformed with human topo II, was enhanced following expression of human CKI. Down-regulation of CKI and CKI also led to reduced formation of etoposide stabilized topo IIDNA cleavable complex. These results provide strong support for an essential role of CKI/ in phosphorylating Ser-1106 in human topo II and in regulating enzyme function.