Circadian Rhythms in Doxorubicin Nuclear Uptake and Clock Control of C6 Glioma Cells

Alterations of drug efficacy by the circadian clock are a concern when assessing drug therapies. Circadian rhythms persist in some cancer cells and are repressed in others. A better understanding of circadian activities generated within cancer cells could indicate therapeutic approaches that selectively disrupt rhythms and deprive cells of any benefits provided by circadian timing. Another option is to induce expression of the core clock gene Per2 to suppress cancer cell proliferation. We used the C6 rat glioblastoma cell line to identify rhythmic cancer cell properties that could provide improved therapeutic targets. Nuclear uptake of the anti-cancer agent doxorubicin by C6 cells showed a circadian rhythm that was shifted six hours from the rhythm in Per2 expression. We also observed circadian expression of the Crm1 (Xpo1) gene that is responsible for a key component of molecular transport through nuclear pores. C6 cultures include glioma stem cells (GSCs) that have elevated resistance to chemotherapeutic agents. We examined C6 tumorsphere cultures formed from GSCs to determine whether Hes1 and Bmi1 genes that maintain GSCs are under circadian clock control. Unlike Per2 gene expression in tumorspheres, Hes1 and Bmi1 expression did not oscillate in a circadian rhythm. These results highlight the importance of the nuclear pore complex in cancer treatments and suggest that the nuclear export mechanism and genes maintaining the cancer stem cell state could be inhibited therapeutically at a particular phase of the circadian cycle while preserving the tumor-suppressing abilities of Per2 gene expression.