Disruption of mitochondrial respiration by melatonin in MCF-7 cells

Clinical and laboratory studies have provided evidence of oncostatic activi ty by the pineal neurohormone melatonin. However, these studies have not el ucidated its mechanism of action. The following series of MCF-7 breast tumo r cell studies conducted in the absence of exogenous steroid hormones provi de evidence for a novel mechanism of oncostatic activity by this endogenous hormone. We observed a 40-60% loss of MCF-7 cells after 20-h treatment wit h 100 nM melatonin, which confirmed and extended previous reports of its on costatic potency. Interestingly, there were no observed changes in tritiate d thymidine uptake, suggesting a lack of effect on cell cycle/nascent DNA s ynthesis. Further evidence of a cytocidal effect came from morphologic obse rvations of acute cell death and autophagocytosis accompanied by degenerati ve changes in mitochondria. Studies of mitochondrial function via standard polarography revealed a significant increase in oxygen consumption in melat onin-treated MCF-7 cells. Enzyme-substrate studies of electron transport ch ain (complex IV) activity in detergent permeabilized cells demonstrated a c oncomitant 53% increase (p < 0.01) in cytochrome c oxidase activity. Additi onal studies of succinate dehydrogenase activity (complex II) as determined by reduction of (3-4,5-dimethylthiazol-2-yl)2,5-diphenyl razolium bromide demonstrated a significant increase (p < 0.05) in melatonin-treated cells a nd further confirmed the accelerated ET activity. Finally, there was a 64% decrease (p < 0.05) in cellular ATP levels in melatonin-treated cells. The G-protein-coupled melatonin receptor antagonist luzindole abrogated the cyt otoxic and mitochondrial effects. These studies suggest a receptor-modulate d pathway of cytotoxicity in melatonin-treated MCF-7 tumor cells with appar ent uncoupling of oxidative phosphorylation. (C) 2001 Academic Press.