OBJECTIVES AND DESIGN: The effect and the mechanism of light regimen and melatonin on the development of mammary tumors in HER2/neu transgenic mice were investigated. Female HER-2/neu mice starting from the age of 2 months were kept under standard light/dark regimen (LD) or constant light illumination (LL) and a part of each group was given melatonin (20 mg/l) during the night time.
RESULTS: The exposure to LL failed to change the incidence of spontaneous mammary adenocarcinoma development, the size of mammary tumors, as well as the incidence and size of lung metastases. However, the number of tumors per mouse was significantly increased in the LL group as compared to the LD group. The number of mice bearing 4 and more tumors was higher in the LL group than in the LD group, whereas the number of mice bearing 1 to 3 tumors was lower in the LL group in comparison with the LD group. Melatonin decreased the incidence and size of mammary adenocarcinomas, and the incidence of lung metastases in the LD group but not in the LL group. The mean number of tumors per mouse was not changed by melatonin treatment in both light regimens. The number of mice bearing 4 and more tumors was reduced by melatonin more significantly in the LL group than in LD group. Melatonin treatment resulted in a 2.5-fold reduction in the expression of HER-2/neu mRNA in mammary tumors from HER-2 /neu transgenic mice.
CONCLUSION: The data demonstrate the influence of the LD light regiment and melatonin treatment in the development of spontaneous mammary tumors in HER-2/neu mice suggesting a melatonin-dependent modulation of HER-2/neu gene expression in mammary adenocarcinoma.