Light at night, melatonin and breast cancer

A recent paper published in Chronobiology International hypothesized that exposure to light at night (LAN) is coincident with decreased melatonin secretion, epigenetic modifications and results in a higher breast cancer tumor growth rate (Schwimmer et al., 2013). The study involved the inoculation of Balb/ c mice with 4T1 mouse breast cancer cells and their followup over the subsequent 21 days. Three groups of mice were studied: (1) mice maintained on a 8L:16D photoperiod, (2) mice subjected to 30 minutes of light, starting 7 h after the onset of darkness (LAN; presumably commencing the day of inoculation) and (3) mice exposed to the LAN, but given melatonin in their drinking water. The primary outcomes of the experiment were that mice subjected to LAN had the greatest tumor burden while those mice subjected to LAN and given melatonin had the lowest tumor burden. In addition, LAN decreased global DNA methylation in the tumors compared to the control group, while treatment with melatonin partially reversed this effect. The authors commented in the introduction that ‘‘a possible mediator of LAN effects is the pineal neurohormone melatonin’’ and that ‘‘the exposure to LAN, through the disruption of the melatonin daily rhythm, is a temporal environmental misleading signal that may result in cellular malfunction. This may promote abnormal proliferation of modified cells’’. Urinary 6-sulphatoxy melatonin urinary levels were measured to monitor changes in melatonin production levels caused by LAN. They ‘‘verified that relatively high melatonin levels were secreted, but were reduced after LAN exposure in the LAN group. As we hypothesized, this may imply an involvement of pineal melatonin in the effect of LAN, or in the regulation of breast cancer growth rates’’. The paper provides some interesting new results in the area, especially the effects on global DNA methylation, although it would have been a better design to have included a melatonin treated control group. What makes this of more interest is the fact that it inadvertently strengthens the case against melatonin being responsible for the LAN effects on tumor growth! It has been known for nearly 30 years that Balb/c mice and the vast majority of other commonly used mouse strains have an extremely low pineal melatonin synthesizing enzyme capacity (both AANAT and HIOMT) and extremely low melatonin content (Ebihara et al., 1986, 1987; Goto et al., 1989). While the actual mutation in Aanat of C57Bl/6 mice that results in the loss of protein has been identified (Roseboom et al., 1998), the defect in the Balb/c Aanat gene is currently not known. Schwimmer et al. conceded that Balb/c mice had been previously reported to have low endogenous melatonin levels by referencing the study by Conti et al. (2000). That study reported the extraordinary result of a one hour pulse of pineal melatonin content and no significant increase in serum melatonin levels at night. Nevertheless, Schwimmer et al. used a 6-sulphatoxymelatonin ELISA (IBL International, Hamburg, Germany) to monitor the effects of LAN on melatonin production by their mice. They reported that 6-sulphatoxymelatonin could be detected in urine and moreover that it was significantly decreased at the end of the 30-minute light pulse. Therefore, it would not be unreasonable to conclude that the LAN did in fact decrease melatonin production. The assay used is, according to the manufacturer for human use and the authors have not reported any validation studies on its use in mice. This is extremely important since we showed that mice