Physiological Melatonin Inhibition of Human Breast Cancer Cell Growth in Vitro: Evidence for a Glutathione-mediated Pathway¹

Melatonin, the chief hormone secreted by the pineal gland, has been previously shown to inhibit human breast cancer cell growth at the physiological concentration of 1 nm in vitro. In this study, using the estrogen receptor (ER)-positive human breast tumor cell line MCF-7, we have shown that 10 µm l-buthionine-[S,R]-sulfoximine (L-BSO), an inhibitor of γ-glutamylcysteine synthetase (the rate-limiting enzyme in glutathione synthesis), blocks the oncostatic action of 1 nm melatonin over a 5-day incubation, indicating that glutathione is required for melatonin action. The result was repeated with ZR75-1 cells, suggesting that the glutathione requirement is a general phenomenon among ER⁺ breast cancer cells. Addition of exogenous glutathione (1 µm) to L-BSO-treated groups restored the melatonin response in both cell lines. Further demonstration of the importance of glutathione was shown using the ER^- breast tumor cell line HS578T, which is normally unresponsive to melatonin. Growth in this cell line was inhibited in the presence of 1 µm ethacrynic acid (an inhibitor of glutathione S-transferase) plus 1 nm melatonin, and this effect was blocked with 10 µm L-BSO. We also observed a steady decrease of intracellular glutathione in MCF-7 cells over a 5-day incubation, suggesting that these cells metabolize glutathione differently than do normal cells.