Background: Kinases are under extensive investigation as targets for drug development. Discovering novel kinases whose inhibition induces cancer-cell-selective lethality would be of value. Recent advances in RNA interference have enabled the realization of this goal.
Results: We screened 5,760 short hairpin RNA clones targeting the human kinome to detect human kinases on which cancer cells are more dependent than normal cells. We employed a two-step screening strategy using human sarcoma cell lines and human fibroblast-derived isogenic cell lines, and found that short hairpin RNAs targeting CSNK1E, a clock gene that regulates circadian rhythms, can induce selective growth inhibition in engineered tumor cells. Analysis of gene-expression data revealed that CSNK1E is overexpressed in several cancer tissue samples examined compared to non-tumorigenic normal tissue, suggesting a positive role of CSNK1E in neogenesis or maintenance. Treatment with IC261, a kinase domain inhibitor of casein kinase 1-epsilon (CK1epsilon), a protein product of CSNK1E, showed a similar degree of cancer-cell-selective growth inhibition. In a search for substrates of CK1epsilon that mediate IC261-induced growth inhibition, we discovered that knocking down PER2, another clock gene involved in circadian rhythm control, rescues IC261-induced growth inhibition.
Conclusion: We identified CK1epsilon as a potential target for developing anticancer reagents with a high therapeutic index. These data support the hypothesis that circadian clock genes can control the cell cycle and cell survival signaling, and emphasize a central role of CK1epsilon and PERIOD2 in linking these systems.