Combined exposure to 3-chloro-4-dichloromethyl-5-hydroxy-2(5H)-furanone and microsytin-LR increases genotoxicity in Chinese hamster ovary cells through oxidative stress

Assessing Additivity of Cytotoxicity Associated with Disinfection Byproducts in Potable Reuse and Conventional Drinking Waters

Recent studies used the sum of the measured concentrations of individual disinfection byproducts (DBPs) weighted by their Chinese hamster ovary (CHO) cell cytotoxicity LC₅₀ values to estimate the DBP-associated cytotoxicity of disinfected waters. This approach assumed that cytotoxicity was additive rather than synergistic or antagonistic. In this study, we evaluated whether this assumption was valid for mixtures containing DBPs at the concentration ratios measured in authentic disinfected waters. We examined the CHO cell cytotoxicity of defined DBP mixtures based on the concentrations of 43 regulated and unregulated DBPs measured in eight drinking and potable reuse waters. The hypothesis for additivity was supported using three experimental approaches. First, we demonstrated that the calculated additive toxicity (CAT) and bioassay-based calculated additive toxicity (BCAT) of the DBP mixtures agree within 12% on a median basis. We also found an additive toxicity response (CAT ≈ BCAT) between the regulated and unregulated DBP classes. Finally, the empirical biological cytotoxicity of the DBP subset mixtures, independent of the calculated toxicity, was additive. These results support the validity of using the sum of cytotoxic potency-weighted DBP concentrations as an estimate of the CHO cell cytotoxicity associated with known DBPs in real disinfected waters.

Genotoxicity of a Low-Dose Nitrosamine Mixture as Drinking Water Disinfection Byproducts in NIH3T3 Cells

N-nitrosamines (NAms), which can arise as byproducts of disinfection agents, are reportedly found in drinking water, and their potential carcinogenicity is a concern; however, little research exists regarding the genotoxicity or carcinogenicity of NAms exposure as a low-dose mixture. The three most common NAms components in China's drinking water are N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA) and N-nitrosomethylethylamine (NMEA). Thus, we measured the genotoxic and carcinogenic potential of these compounds and measured the cell cycle and gene expression. The data show that exposure to the NAms-mixture doubled the revertants in the TA98 and TA100 S. typhimurium strains and increased the DNA double-strand breaks and the micronuclear frequency in the NIH3T3 cells compared to a single exposure. After long-term NAms mixture exposure, a malignant transformation of NIH3T3 and a significantly increased G2/M distribution were observed. Furthermore, P53, CDK1, P38, CDC25A and CyclinB expressions were down-regulated in the NAms-mixture exposure group; however, P21 and GADD45A genes were up-regulated. Interestingly, the CHK1/CHK2 and CDC25A genes had two responses, depending on the NAms concentrations. Thus, we observed mutagenic, genotoxic and carcinogenic effects after a low-dose NAms-mixture exposure in drinking water, and DNA repair and apoptosis pathways may contribute to these adverse effects.