Physiological melatonin inhibition of human breast cancer cell growth in vitro: Evidence for a glutathione-mediated pathway

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 CER)-positive human breast tumor cell Line MCF-7, we have shown that 10 mu M L-buthionine-[S,R]-sulfoximine (L-BSO), an inhibitor of gamma-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 mu M) 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 mu M ethacrynic acid (an inhibitor of glutathione S-transferase) plus 1 nM melatonin, and this effect was blocked with 10 mu M 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.