Effect of melatonin on cell growth, metabolic activity, and cell cycle distribution

We have recently demonstrated that the pineal secretory product melatonin inhibits the key transcriptional regulator nuclear factor-kappaB (NF-kappaB). As the activation of NF-kappaB is known to regulate the expression of cellular genes associated with cell cycle progression, cell growth, and differentiation, we investigated the effect of melatonin treatment on several cellular processes. These include cell viability, metabolic activity, and cell cycle phase distribution. Human embryonic kidney (293S) cells were treated with melatonin at concentrations of 0.02, 0.2, or 2 mM. When cell viability was measured 24, 48, and 72 hr after continuous exposure to melatonin using the trypan blue dye exclusion method, no significant cell death was observed. Even after exposure to 2 mM melatonin for 72 hr, cell viability remained at 98%. In contrast, another antioxidant compound, pyrrolidine dithiocarbomate (PDTC), at a 2 mM concentration reduced cell viability to 80.7 +/- 2.1% as early as 24 hr compared with untreated controls (P < 0.05). When the metabolic activity was determined at 24, 48, and 72 hr using the colorimetric MTT assay, no significant changes in metabolic activity were observed. Even if the cells were treated with 10 mM melatonin for 72 hr, the metabolic activity was similar to that of the control cells. When cell cycle analysis was performed by flow cytometry, no marked difference in cell cycle distribution was observed. Melatonin at a concentration of 2 mM, however, did slightly alter the cell cycle (percentage of S phase cells) at 48 hr. This study revealed that when 293S cells are treated with concentrations of melatonin up to 2 mM, no significant alterations in three important cellular functions occurred. Exogenously added melatonin appeared to have a limited influence on the normal functioning of the cells even when the exposure continued for 72 hr.