Risk of Breast Cancer After Night- and Shift Work: Current Evidence and Ongoing Studies in Denmark

Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, DK-2100, Copenhagen, Denmark.
Cancer Causes and Control (Impact Factor: 2.74). 06/2006; 17(4):531-7. DOI: 10.1007/s10552-005-9006-5
Source: PubMed


Breast cancer is the most frequent cancer among women, and the number is increasing worldwide. This tumour is strongly associated with Western lifestyle, but the specific risk factors behind this observation are not well known. Exposure to light-at-night, including disturbance of the circadian rhythm, possibly mediated via the melatonin synthesis and clock genes, has been suggested as a contributing cause of breast cancer. Since shift- and night-time work is prevalent and increasing in modern societies, this exposure may be of public health concern, and contribute to the continuing elevation in breast cancer risk. Until now only few epidemiological studies have evaluated breast cancer risk after shift and night work. Although these studies are all suffering from methodological problems, especially concerning assessment of light exposure, results have consistently shown an increase in risk associated with night and shift work. Good opportunities for epidemiological cancer research exist in Denmark, and several studies on different aspects of breast cancer, work schedules, light exposure and melatonin levels are ongoing in order to further examine different aspects of this issue.

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Available from: Johnni Hansen, Mar 28, 2014
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    • "In laboratory studies that experimentally imposed severe acute circadian misalignment, healthy participants showed adverse metabolic responses that are risk factors for cardiovascular disease and type 2 diabetes [1] [8] [9]. When experienced chronically like in night-shift work, circadian misalignment increases the risk of a number of diseases , including cancer [13] [14] [15] [16] [17] [18]. "
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    ABSTRACT: Objective: Efficient treatments to phase-advance human circadian rhythms are needed to attenuate circadian misalignment and the associated negative health outcomes that accompany early-morning shift work, early school start times, jet lag, and delayed sleep phase disorder. This study compared three morning bright-light exposure patterns from a single light box (to mimic home treatment) in combination with afternoon melatonin. Methods: Fifty adults (27 males) aged 25.9 ± 5.1 years participated. Sleep/dark was advanced 1 h/day for three treatment days. Participants took 0.5 mg of melatonin 5 h before the baseline bedtime on treatment day 1, and an hour earlier each treatment day. They were exposed to one of three bright-light (~5000 lux) patterns upon waking each morning: four 30-min exposures separated by 30 min of room light (2-h group), four 15-min exposures separated by 45 min of room light (1-h group), and one 30-min exposure (0.5-h group). Dim-light melatonin onsets (DLMOs) before and after treatment determined the phase advance. Results: Compared to the 2-h group (phase shift = 2.4 ± 0.8 h), smaller phase-advance shifts were seen in the 1-h (1.7 ± 0.7 h) and 0.5-h (1.8 ± 0.8 h) groups. The 2-h pattern produced the largest phase advance; however, the single 30-min bright-light exposure was as effective as 1 h of bright light spread over 3.25 h, and it produced 75% of the phase shift observed with 2 h of bright light. Conclusions: A 30-min morning bright-light exposure with afternoon melatonin is an efficient treatment to phase-advance human circadian rhythms.
    Sleep Medicine 12/2014; 16(2). DOI:10.1016/j.sleep.2014.12.004 · 3.15 Impact Factor
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    • "Apparently, women who have been exposed to artificial light at night for long term, or have been working with jobs that can alter their circadian rhythm are predisposed to breast cancer [54]–[56]. Growing evidence suggests that breast tumorigenesis is associated with the disruption of circadian clocks [23], [56], [57]. Thus it is important to investigate how estrogen signaling, which is vital to a number of cellular processes and the onset of breast cancer, is integrated with the circadian clock. "
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    ABSTRACT: Growing genetic and epidemiological evidence suggests a direct connection between the disruption of circadian rhythm and breast cancer. Moreover, the expression of several molecular components constituting the circadian clock machinery has been found to be modulated by estrogen-estrogen receptor α (E2-ERα) signaling in ERα-positive breast cancer cells. In this study, we investigated the regulation of CLOCK expression by ERα and its roles in cell proliferation. Immunohistochemical analysis of human breast tumor samples revealed high expression of CLOCK in ERα-positive breast tumor samples. Subsequent experiments using ERα-positive human breast cancer cell lines showed that both protein and mRNA levels of CLOCK were up-regulated by E2 and ERα. In these cells, E2 promoted the binding of ERα to the EREs (estrogen-response elements) of CLOCK promoter, thereby up-regulating the transcription of CLOCK. Knockdown of CLOCK attenuated cell proliferation in ERα-positive breast cancer cells. Taken together, these results demonstrated that CLOCK could be an important gene that mediates cell proliferation in breast cancer cells.
    PLoS ONE 05/2014; 9(5):e95878. DOI:10.1371/journal.pone.0095878 · 3.23 Impact Factor
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    • "Some biological and clinical experiments have shown that disrupted circadian clocks may enhance tumour growth [15] [17] [18] [21]. Although it has clearly been observed that circadian rhythms play a role in cell cycle regulation, this role has still to be investigated to understand their mechanisms of action, and how circadian control could be used to prevent or treat cancer. "
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