Circadian oscillations persist in cervical and esophageal cancer cells displaying decreased expression of tumor-suppressing circadian clock genes

There is accumulating evidence for a link between circadian clock disruption and cancer progression. In this study, the circadian clock was investigated in cervical and esophageal cancers, to determine whether it is disrupted in these cancer types. Oncomine datamining revealed downregulation of multiple members of the circadian clock gene family in cancer patient tissue compared with matched normal epithelium. Real-time RT-PCR analysis confirmed significant downregulation of CLOCK, PER1, PER2, PER3, CRY1, CRY2, REV-ERBa, and RORa in esophageal tumor tissue. In cell line models, expression of several circadian clock genes was significantly decreased in transformed and cancer cells compared with noncancer controls, and protein levels were dysregulated. These effects were mediated, at least in part, by methylation, where CLOCK, CRY1, and RORa gene promoter regions were found to be methylated in cancer cells. Overexpression of CLOCK and PER2 in cancer cell lines inhibited cell proliferation and activation of RORa and REV-ERBa using agonists resulted in cancer cell death, while having a lesser effect on normal epithelial cells. Despite dysregulated circadian clock gene expression, cervical and esophageal cancer cells maintain functional circadian oscillations after Dexamethasone synchronization, as revealed using real-time bioluminescence imaging, suggesting that their circadian clock mechanisms are intact. Implications: This study is a first to describe dysregulated, yet oscillating, circadian clock gene expression in cervical and esophageal cancer cells, and knowledge of circadian clock functioning in these cancer types has the potential to inform chronotherapy approaches, where the timing of administration of chemotherapy is optimized on the basis of the circadian clock.