The Per1 gene is a core clock factor that plays an essential role in generating circadian rhythms. Recent data reveal that major biological pathways, including those critical to cell division, are under circadian control. We report here that Per1 provides an important link between the circadian system and the cell cycle system. Overexpression of Per1 sensitized human cancer cells to DNA damage-induced apoptosis; in contrast, inhibition of Per1 in similarly treated cells blunted apoptosis. The apoptotic phenotype was associated with altered expression of key cell cycle regulators. In addition, Per1 interacted with the checkpoint proteins ATM and Chk2. Ectopic expression of Per1 in human cancer cell lines led to significant growth reduction. Finally, Per1 levels were reduced in human cancer patient samples. Our results highlight the importance of circadian regulation to fundamental cellular functions and support the hypothesis that disruption of core clock genes may lead to cancer development.

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Circadian rhythms regulate diverse physiologic processes, including homeostatic functions of steroid hormones and their receptors. Perturbations of these rhythms are associated with pathogenic conditions, such as depression, diabetes, and cancer. Androgens play an important role in both normal development and carcinogenesis of the prostate. In the present study, we investigated a potential role for the core clock factor Per1 in the pathogenesis of prostate cancer. Serum-shocked synchronized prostate cancer cells displayed disrupted circadian rhythms compared with the normal prostate tissue. Using Oncomine to perform a meta-analysis of microarray expression studies, we found that Per1 is downregulated in human prostate cancer samples compared with normal prostates. Reporter assays showed that Per1 inhibited transactivation of the androgen receptor (AR) both in 293T cells overexpressing the AR and in the prostate cancer cell line LNCaP. Forced expression of Per1 in LNCaP cells diminished the expression of known androgen-sensitive genes following stimulation with dihydrotestosterone. We showed that Per1 physically interacted with AR; in addition, we found that Per1 itself is regulated by androgens in prostate cancer cells. Overexpression of Per1 in prostate cancer cells resulted in significant growth inhibition and apoptosis. Our results support the emerging role of circadian genes as key players in malignant transformation. Further elucidating the connections between clock genes and the AR pathway could benefit the development of new therapeutic strategies for prostate cancer as well as provide insights into chronotherapy as a way to optimize current therapies.