Mutual synergistic toxicity between environmental toxicants: A study of mercury chloride and 4-nonylphenol

Mercury chloride (HgCl2) and 4-nonylphenol (NP) are widespread environmental and industrial pollutants that are known to have toxic effects as well as endocrine disrupting activities. Although the individual effects of HgCl2 and NP in liver have been relatively well recognized, little is known about the interaction of NP and HgCl2 during the induction of their toxicity. In the current study, we investigated the synergism between HgCl2 and NP using HepG2 cells. Surprisingly, the concurrent treatment of HepG2 with HgCl2 and NP induced a significant cytotoxicity at concentrations where neither of them have any cytotoxic effect when treated alone. The cytotoxicity of NP is enhanced in the presence of HgCl2 (a shift from 74.9 to 47.4 μM in LC50) and vice versa (a shift from 94.9 to 66.3 μM in LC50). Estrogen receptor antagonists such as ICI 182,780 did not protect HepG2 cells from these cytotoxic insults. Whereas the intracellular level of reduced form glutathione (GSH) was considerably decreased upon the co-treatment with NP and HgCl2. Furthermore, the synergistic cytotoxicity was significantly inhibited by 20-mM N-acetylcysteine (NAC). These results indicate that the mutual synergistic cytotoxicity of HgCl2 and NP on HepG2 cell is not associated with estrogen receptor signaling but mediated by reactive oxygen species (ROS) generation. In our real life, we are continuously and often simultaneously exposed to many different kinds of environmental pollutants. The present study suggests a mechanism of potential synergistic adverse effects of these toxic pollutants.