Investigation of the differential transport mechanism of cinnabar and mercury containing compounds

Abstract

Background: Cinnabar has a long history of uses in Chinese traditional medicines as an ingredient in various remedies. However, the detailed mechanism of cinnabar in medication remains unclear, and the toxicity of cinnabar has been a debate due to its containing mercury sulfide. This study was designed to investigate the differential transport mechanism of cinnabar and other Hg-containing compounds HgCl₂, MeHg and HgS, and to determine if organic anion transporters OAT1 and OAT3 were involved in the differential transport mechanism.

Materials and methods: The 293T cells were employed to investigate and compare the differential transport mechanism of cinnabar and HgCl₂, MeHg and HgS. Cells were incubated with a low dose (5 μM HgCl₂ and MeHg, 200 μM HgS and cinnabar), medium dose (10 μM HgCl₂ and MeHg, 400 μM HgS and cinnabar), and high dose (20 μM HgCl₂ and MeHg, 800 μM HgS and cinnabar) of HgCl₂, MeHg, HgS and cinnabar for 24 h. Following treatment, the cells were collected and the cell viability was determined by MTT assay. The intracellular mercury content was measured at 1, 4, and 24 h after treatment with 10 μM of the tested agents by an atomic fluorescence spectrophotometer. The effect of these tested agents on mitochondrial respiration was determined in a high-resolution oxygraphyat 24 h following treatment. Furthermore, the effect of modulation of expression of transporters OAT1 and OAT3 on the transport and cytotoxicity of the tested agents was evaluated. The up and down regulation of OAT1 and OAT3 were achieved by overexpression and siRNA transfection, respectively.

Results: Compared with HgCl₂ and MeHg, the cytotoxicity of cinnabar and HgS was lower, with cell viability at the high dose cinnabar and HgS being about 65%, while MeHg and HgCl₂ were 40% and 20%, respectively. The intracellular mercury accumulation was time-dependent. At 24 h the intracellular concentrations of HgCl₂ and MeHg were about 7 and 5 times higher, respectively, than that of cinnabar. No significant difference was found in the intracellular mercury content in cells treated with cinnabar compared to HgS. The knockdown and overexpression of the transporter OAT1 resulted in significant reduction and increase, respectively, in mercury accumulation in HgCl₂ -treated cells in relative to control cells, while no significant changes were observed in cells treated with cinnabar, MeHg, and HgS. In addition, the knockdown and overexpression of the transporter OAT3 caused significant reduction and increase, respectively, in mercury accumulation in both HgCl₂ and MeHg-treated cells in relative to control cells, while no significant changes were observed in cells treated with cinnabar and HgS. Furthermore, it was found that cells transfected with siOAT1 caused significant resistance to the cytotoxicity induced by HgCl₂, while no noticeable changes in cell viability were observed in cells treated with other tested agents. Additionally, cells transfected with OAT3 did not change cell sensitivity to cytotoxicity induced by all of the four tested agents.

Conclusion: This study demonstrates that differential transport and accumulation of mercury in 293T cells exists among cinnabar and the three mercury-containing compounds HgCl₂, MeHg and HgS, leading to distinct sensitivity to mercury induced cytotoxicity. The kidney organic anion transporters OAT1 and OAT3 are partially involved in the regulation of the transport of HgCl₂ and MeHg, but not in the regulation of the transport of cinnabar.

Keywords: Cinnabar; Cytotoxicity; Hg-containing compounds; Intracellular mercury content; Organic anion transporter.