Melatonin Depletion Spurs Tumor Growth

David E. Blask, Ph.D., MD and George C. Brainard, Ph.D.
Thomas Jefferson University
R21ES11659

Background: The risk of developing breast cancer is up to five times higher in industrialized countries than in underdeveloped nations and approximately 50% of breast cancers cannot be accounted for by conventional risk factors. Western nations have become "24-hour societies" meaning that greater numbers of people are exposed to more artificial light at night both at home but perhaps more importantly in the workplace.

In the early 1990's, epidemiologists noted that women who frequently work night shifts seem predisposed to the development of breast cancer. These researchers postulated that nighttime exposure to bright lights interrupted the body's mainly nocturnal production of melatonin, a hormone produced by the pineal gland, and that this disruption somehow increased the risk of breast cancer. Later, melatonin was shown to inhibit the growth of mammary tumors in experimental models and either removal of the pineal gland or constant exposure to light stimulates mammary tumor development in laboratory animal studies.

Advance: In a new study using a unique laboratory model, NIH-supported researchers have shown that a woman's blood provides better sustenance for breast tumors just after exposure to a bright light source than when she has been in steady darkness. The model is based on the implantation of human breast tumors into rats such that a single artery supplied blood to the tumor and a single vein drained the tumor. Results indicated that tumor cells divided most rapidly when supplied by blood taken from women either in daylight or at night after exposure to bright artificial lighting; this blood contained low levels of melatonin. Spiking the samples with synthetic melatonin removed the capacity to promote the tumor cell growth.

Implications: This study could have far reaching implications for the prevention and treatment of breast cancer. It could encourage trials that test whether breast cancer can be slowed down by altering a woman's light environment or by using melatonin supplements. These researchers state that women should avoid even short exposures to bright light at night. They also point out that blue or white lights suppress melatonin more effectively than red or yellow; therefore, lights could be designed to filter out the more offending wavelengths. Similar studies could show whether nocturnal light exposure is associated with increased risk for prostate cancer in men as some researchers suspect.

Citation: Blask DE, Brainard GC, Dauchy RT, Hanifin JP, Davidson LK, Krause JA, Sauer LA, Rivera-Bermudez MA, Dubocovich ML, Jasser SA, Lynch DT, Rollag MD, Zalatan F. Melatonin-depleted blood from premenopausal women exposed to light at night stimulates growth of human breast cancer xenografts in nude rats. Cancer Res. 2005 Dec 1;65(23):11174-84.