Previous studies have shown that the aberrant expression of period circadian clock 2 (Per2) is closely related to the occurrence and development of cancers, but the specific mechanism remains unclear. In the present study, we used shRNA to downregulate Per2 in oral squamous cell carcinoma (OSCC) Tca8113 cells, and then detected the alterations in cell cycle, cell proliferation and apoptosis by flow cytometric analysis and mRNA expression alterations in all the important genes in the cyclin/cyclin-dependent protein kinase (CDK)/cyclin-dependent kinase inhibitor (CKI) cell cycle network by RT-qPCR. We found that in the Tca8113 cells, after Per2 downregulation, the mRNA expression levels of cyclin A2, B1 and D1, CDK4, CDK6 and E2F1 were significantly increased (P<0.05), the mRNA expression levels of p53, p16 and p21 were significantly decreased (P<0.05), cell proliferation was significantly higher (P<0.05), apoptosis was significantly lower (P<0.05) and the number of cells in the G1/G0 phase was significantly decreased (P<0.05). The present study proves that in OSCC, clock gene Per2 plays an important role in cell cycle progression and the balance of cell proliferation and apoptosis by regulation of the cyclin/CDK/CKI cell cycle network. Further research on Per2 may provide a new effective molecular target for cancer treatments.

Abstract. Low expression of the clock gene PER2 is closely related to carcinogenesis and the development of cancer; however, the mechanism of the low expression of PER2 that led to cell malignant transformation remains unclear. This study used RNA interference (RNAi) technology to silence PER2 in SCC15 human oral squamous cell carcinoma (OSCC) cells. Then it was found that the ability of cancer cell proliferation, migration, and invasion were markedly increased (P<0.05), and the ability of cancer cell apoptosis and the number of cells in G1/G0 phase were markedly reduced (P<0.05) after PER2 knockdown. PER2 knockdown increased the expression of MDM2, MMP2, and VEGF mRNA (P<0.05), and decreased the expression of p53, Bax, and TIMP-2 mRNA (P<0.05). The in vivo experiments also proved that the tumorigenicity of SCC15 cells was significantly enhanced after PER2 silence (P<0.05). Overall, these results show that PER2 through the regulation of the numerous important downstream tumor-related genes, plays a major role in tumor suppression, and it may be a novel molecular target for cancer treatment.

The various clock genes in normal cells, through their interaction, establish a number of positive and negative feedback loops that compose a network structure. These genes play an important role in regulating normal physiological activities. The expression of clock gene PER1 is decreased in many types of cancer. PER1 is highly correlated with the initiation and progression of cancer by regulating numerous downstream genes. However, it is still unclear whether the lower expression of PER1 in cancer can influence the expression of other clock genes in the clock gene network. In this study, we used short hairpin RNA interference to knock down PER1 effectively in SCC15 human oral squamous cell carcinoma cells. These cancer cells later were subcutaneously injected into the back of nude mice. We discovered that after PER1 knockdown, apoptosis was decreased and cell proliferation and in vivo tumor formation were enhanced. Quantitative real-time PCR result indicated that in vitro and in vivo cancer cells after PER1 knockdown, PER2, DEC1, DEC2, CRY1, CRY2 and NPAS2 were significantly down-regulated at the mRNA level, while PER3, TIM, RORα and REV-ERBα were significantly up-regulated. It prompts that the role of PER1 in carcinogenesis is exerted not only by regulating downstream genes, but also through the synergistic dysregulation of many other clock genes in the clock gene network.

Clock genes are the core of the circadian rhythms in the human body and are important in regulating normal physiological functions. To date, research has indicated that the clock gene, period circadian clock 2 (PER2), is downregulated in numerous types of cancer, and that it is associated with cancer occurrence and progression via the regulation of various downstream cell cycle genes. However, it remains unclear whether the decreased expression of PER2 influences the expression of other clock genes in cancer cells. In the present study, short hairpin RNA interference was used to knockdown PER2 effectively in human oral squamous cell carcinoma SCC15 cells. Quantitative polymerase chain reaction was used to assess the mRNA expression levels of various clock genes and revealed that, following the knockdown of PER2 in SCC15 cells, the mRNA expression levels of PER3, brain and muscle ARNT-like 1, deleted in esophageal cancer (DEC)1, DEC2, cryptochrome circadian clock (CRY)2, timeless circadian clock, retinoic acid receptor-related orphan receptor-alpha and neuronal PAS domain protein 2 were significantly downregulated, while the mRNA expression levels of PER1 and nuclear receptor subfamily 1 group D member 1 were significantly upregulated. In addition, flow cytometric analysis demonstrated that proliferation was enhanced and apoptosis was reduced following PER2 knockdown in SCC15 cells (P<0.05). To the best of our knowledge, the present study is the first to report that PER2 is important for the regulation of other clock genes of the clock gene network in cancer cells. This is of great significance in elucidating the molecular function and tumor suppression mechanism of PER2.