Diquat-induced cytotoxicity on Vero and HeLa cell lines: effect of melatonin and dihydromelatonin

Diquat dibromide is a moderately toxic contact herbicide belonging to the bipyridyl group of redox-active compounds that induce a strong oxidative damage. Melatonin (MEL) can protect against oxidative damage under in vivo conditions, probably through its anti-oxidative capacity and ability to induce expression of anti-oxidative enzymes. The objective of this study was to investigate effects of diquat on viability of Vero and HeLa cells and possible protective effects of MEL and its analogue 2,3-dihydromelatonin (DMEL). Cell viability was evaluated with the MTT test. First, we analyzed dose-dependent effects of diquat on cell viability using the concentration range of 0.1-100 μM. Second, we used the diquat dose which reduced cell viability by 50% and treated cells with either MEL or DMEL (both in the concentration range of 1-100 μM) in the presence or absence of diquat. In addition, effects of both diquat and MEL on oxidative stress in HeLa cells were measured by flow cytometry using 2',7'-dichlorofluorescin diacetate. We confirmed the expected negative effects of diquat on viability of Vero and HeLa cells. Melatonin and DMEL were able to prevent diquat reduced viability of Vero cells in rather low concentrations (1 μM) and DMEL exerted substantially stronger protective effects than MEL. However in HeLa cells, we did not find the same effects and MEL even reduced their viability. Moreover, treatment of HeLa cells with high concentrations of MEL (100 μM) exaggerated the pro-oxidative effects of diquat. The results suggest that in addition to the expected anti-oxidative effects, MEL exerts a pro-oxidative action which is cell type and dose dependent.