| Objective: Hydroquinone (1, 4-dihydroxybenzene, HQ) is widely used in various industrial fields. HQ and products containing HQ are used in over-the-counter (OTC) drugs as an ingredient in skin lighteners and also a natural ingredient in many plant-derived products. Human exposure to HQ can occur by environmental, occupational, dietary and cigarette smoke exposure and from exposure to benzene, which can be metabolized to HQ. In 2001, HQ was banned in cosmetic skin lightening formulations in European Union countries although products containing arbutin being an analogue of HQ and botanicals including plants that naturally contain HQ and arbutin, continued to remain available in European countries.In 1999 the International Agency for Research on Cancer (IARC), assessed Sudan I as a Group 3 carcinogen. HQ was mutagenic in many mammalian cells in vitro using a variety of end- points. HQ is metabolized by several routes, reactive oxygen species (ROS) are produced. The liver is the initial site of HQ metabolism. The aim of this study was to assess the genotoxic effects of HQ in vitro and to elucidate the mechanism of oxidative DNA damage. In this study, we selected a metabolically competent human hepatoma line (HepG2), which retains many of the functions of normal liver cells and expresses the activities of several phase I and phase II xenobiotic metabolising enzymes. HepG2 cells have been shown to be a suitable system for genotoxicity testing.The purpose of this study was to assess the genotoxic effects of HQ in vitro and to illuminate the mechanisms in HepG2 cells. Thus it may provide some information for occupational hazard assessment to humans on HQ.Methods: HepG2 cells were selected as test system. Genotoxicity of HQ was assessed by standard and proteinase K-modified alkaline single cell gel electrophoresis (SCGE) and micronucleus test (MNT). To further investigate the mechanism of genotoxicity of HQ in HepG2 cells, we used the 2, 7-dichlorofluorescein diacetate (DCFH-DA) and o-phthalaldehyde (OPT) to monitor the levels of reactive oxygen species (ROS) and glutathione (GSH). We analyzed the oxidative DNA damage in HQ-treated cells by immunocytochemistry staining of 8-hydroxydeoxyguanosine (8-OHdG).Results: The damage to DNA-strand of HepG2 cells significantly increased after exposure to 6.25, 12.5, 25, 50μM HQ for 1h. A significant dose-dependent increment in DNA migration was detected at lower concentrations of HQ (6.25 - 25μM); but at the higher tested concentrations (50μM), a reduction in the migration compared to the maximum migration at 25μM was observed. Post-incubation with proteinase K significantly increased DNA migration in cells exposed to higher concentrations of HQ (50μM). These results indicated that HQ caused DNA strand breaks at lower concentrations of HQ (6.25 - 25μM) and DNA-protein crosslinks (DPC) formation at higher concentrations. Frequencies of micronuclei significantly increased in HepG2 cells after treatment with 12.5-50μM ?HQ for 24h. The present study showed that ?HQ induced the increased levels of ROS and depletion of GSH in HepG2 cells, the doses being 25-50μM and 6.25-50μM, respectively. Moreover, HQ significantly caused 8-OHdG formation in HepG2 cells at concentrations from 12.5 to 50μM.Conclusion: The data suggest that HQ caused DNA strand breaks and chromosome breaks, which indicate that HQ induced genotoxic effects in HepG2 cells. HQ exerts genotoxic effects in HepG2 cells, probably through the formation of ROS, depletion of GSH and increase of 8-OHdG formation, which cause oxidative DNA damage and DPC.
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