Clock genes and the role of melatonin in cancer cells: an overview

     Circadian rhythms control most biological processes in every organism and their disruption or an aberrant function in the expression of clock genes are associated with a number of cancers including some hormone-dependent and independent cancers. The processes involved in carcinogenesis and tumor progression are complex, but understanding the daily profiles of the core clock genes and their clock-controlled genes is essential to evaluate specifically the molecular program of the cancer phenotype; this may be helpful in providing a more realistic strategy for both diagnosis and treatment during the course of the disease. Because melatonin production and secretion oscillates rhythmically through the light:dark cycle and is related to the circadian machinery genes (Clock, Bmal1, Periods, and Cryptochromes), its regulatory role on clock genes in cancer cells may bring additional evidence regarding the mechanism(s) by which melatonin is involved. Mechanistically, melatonin acts via proteasome inhibition and sirtuins to indirectly modulate clock genes in cancer; however, melatonin seems to be capable of directly altering the expression of clock genes to affect cancer development. Depending on cancer cell type, melatonin might up or downregulate specific clock genes to control cell cycle, survival, repair mechanisms, etc. In parallel, melatonin exerts pro-apoptotic, anti-proliferative and pro-oxidative effects, metabolic shifting, reduction in neovasculogenesis and inflammation, and restores chemosensitivity of cancer cells. Finally, melatonin improves the life quality of patients. This review focuses on the main functions of melatonin on clock genes, and reviews, from a clinical and experimental standpoint, how melatonin regulates the expression of clock genes in some prevalent cancer types such as breast, prostate, liver, and colon cancers, leukemia and melanoma. We further emphasized possible signaling mechanisms whereby melatonin interferes with clockwork genes and circadian-controlled genes within cancer cells.