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

We have recently demonstrated that the pineal secretory product melatonin i nhibits the key transcriptional regulator nuclear factor-kappaB (NF-kappaB) . As the activation of NF-kappaB is known to regulate the expression of cel lular genes associated with cell cycle progression, cell growth, and differ entiation, we investigated the effect of melatonin treatment on several cel lular 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 viabilit y was measured 24, 48, and 72 hr after continuous exposure to melatonin usi ng the trypan blue dye exclusion method, no significant cell death was obse rved. Even after exposure to 2 mM melatonin for 72 hr, cell viability remai ned at 98%. In contrast, another antioxidant compound, pyrrolidine dithioca rbomate (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 th e metabolic activity was determined at 24, 48, and 72 hr using the colorime tric MTT assay, no significant changes in metabolic activity were observed. Even if the cells were treated with 10 mM melatonin for 72 hr, the metabol ic activity was similar to that of the control cells. When cell cycle analy sis was performed by flow cytometry, no marked difference in cell cycle dis tribution 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 mela tonin up to 2 mM, no significant alterations in three important cellular fu nctions occurred. Exogenously added melatonin appeared to have a limited in fluence on the normal functioning of the cells even when the exposure conti nued for 72 hr.