Toxicity of mixtures of trichloroethylene, tetrachloroethylene and 1,1,1-trichloroethane: similarity of in vitro to in vivo responses

Mixed organic solvents induce renal injury in rats

To investigate the injury effects of organic solvents on kidney, an animal model of Sprague-Dawley (SD) rats treated with mixed organic solvents via inhalation was generated and characterized. The mixed organic solvents consisted of gasoline, dimethylbenzene and formaldehyde (GDF) in the ratio of 2∶2:1, and were used at 12,000 PPM to treat the rats twice a day, each for 3 hours. Proteinuria appeared in the rats after exposure for 5–6 weeks. The incidences of proteinuria in male and female rats after exposure for 12 weeks were 43.8% (7/16) and 25% (4/16), respectively. Urinary N-Acetyl-β-(D)-Glucosaminidase (NAG) activity was increased significantly after exposure for 4 weeks. Histological examination revealed remarkable injuries in the proximal renal tubules, including tubular epithelial cell detachment, cloud swelling and vacuole formation in the proximal tubular cells, as well as proliferation of parietal epithelium and tubular reflux in glomeruli. Ultrastructural examination found that brush border and cytoplasm of tubular epithelial cell were dropped, that tubular epithelial cells were partially disintegrated, and that the mitochondria of tubular epithelial cells were degenerated and lost. In addition to tubular lesions, glomerular damages were also observed, including segmental foot process fusion and loss of foot process covering on glomerular basement membrane (GBM). Immunofluorescence staining indicated that the expression of nephrin and podocin were both decreased after exposure of GDF. In contrast, increased expression of desmin, a marker of podocyte injury, was found in some areas of a glomerulus. TUNEL staining showed that GDF induced apoptosis in tubular cells and glomerular cells. These studies demonstrate that GDF can induce both severe proximal tubular damage and podocyte injury in rats, and the tubular lesions appear earlier than that of glomeruli.

The impact of CYP2E1 genetic variability on risk assessment of VOC mixtures

Humans are simultaneously exposed to multiple chemicals in the environment. Many of the chemicals use the same enzymes in their metabolic pathways. Competitive inhibition may occur as one of the possible interactions between the xenobiotics in human body. For example, many volatile organic compounds (VOCs) are metabolized using P450 enzymes, specifically CYP2E1. Inheritable gene alterations may result in changes of function of the enzymes in different human subpopulations. Variations in quantity and/or quality of particular isoenzymes may cause differences in the metabolism of VOCs. These variations may cause higher sensitivity in certain populations. Using examples of three different mixtures, this review paper outlines the variances in CYP2E1 isoenzymes, effect of exposure to such mixtures on sensitive populations, and approaches to mixtures risk assessment.

Comparative phylogenetic microarray analysis of microbial communities in TCE-contaminated soils

The arrival of chemicals in a soil or groundwater ecosystem could upset the natural balance of the microbial community. Since soil microorganisms are the first to be exposed to the chemicals released into the soil environment, we evaluated the use of a phylogenetic microarray as a bio-indicator of community perturbations due to the exposure to trichloroethylene (TCE). The phylogenetic microarray, which measures the presence of different members of the soil community, was used to evaluate unpolluted soils exposed to TCE as well as to samples from historically TCE polluted sites. We were able to determine an apparent threshold at which the microbial community structure was significantly affected (about 1ppm). In addition, the members of the microbial community most affected were identified. This approach could be useful for assessing environmental impact of chemicals on the biosphere as well as important members of the microbial community involved in TCE degradation.

Binary weight-of-evidence evaluations of chemical interactions--15 years of experience

The paper reflects on the last 15years of experience in the field of mixtures risk assessment. It summarizes results found in various documents developed by the Agency for Toxic Substances and Disease Registry (ATSDR) of the weight-of-evidence (WOE) approach applied to 380 binary combinations of chemicals. Of these evaluations, 156 assessments indicated possible additivity of effects [=], 76 indicated synergism (greater-than-additive effects [>]), and 57 indicated antagonism (less-than-additive effects [<]). However, 91 combinations lacked the minimum information needed for making any assessments and, hence, were undetermined. The paper provides examples of the rationale behind some of the WOE decisions and discusses the importance of expert judgments in risk assessment evaluations. Examples are given regarding the importance of human variability in mixtures' ability to affect human health and regarding the dose versus effect relationships.

In vitro exposure of jurkat T-cells to industrially important organic solvents in binary combination: interaction analysis

Humans are frequently exposed to mixtures of environmental pollutants at low levels over prolonged periods of time yet most toxicity studies deal with acute exposure to high concentrations of single chemicals. Investigation of the biological effects and possible toxic interactions during long-term exposure to such mixtures is warranted. Here Jurkat T-cells were exposed to toluene, n-hexane and methyl ethyl ketone in binary combination. Concentration ranges were centered on thresholds at which the individual agents caused cell toxicity under otherwise similar conditions, and concentrations were confirmed by headspace gas chromatography. After 5 days cells were harvested and toxicity measured in terms of membrane damage (lactate dehydrogenase [LDH] leakage), perturbations in [Ca(2+)](i) and changes in glutathione redox status. Data for all three endpoints were subjected to isobolographic analysis to test for interaction between components of the solvent mixture. Almost all combinations of toluene and n-hexane elicited greater than additive toxicity in terms of each of the three endpoints, as did methyl ethyl ketone (MEK)/n-hexane and MEK/toluene combinations for the LDH and glutathione endpoints. The main exceptions were the two combinations involving MEK, which caused less than additive effects on perturbations of [Ca(2+)](i). It is concluded that toxicity in immune-derived T cells may exhibit greater than additive effects when there is coexposure to organic solvents. This may have implications for risk assessment of environmental exposure to these agents.

Six interaction profiles for simple mixtures

The Agency for Toxic Substances and Disease Registry (ATSDR) has a program for chemical mixtures that encompasses research on chemical mixtures toxicity, health risk assessment, and development of innovative computational methods. ATSDR prepared a guidance document that instructs users on how to conduct health risk assessment on chemical mixtures (Guidance Manual for the Assessment of Joint Toxic Action of Chemical Mixtures). ATSDR also developed six interaction profiles for chemical mixtures. Two profiles were developed for persistent environmental chemicals that are often found in contaminated fish and also can be detected in human breast milk. The mixture included chlorinated dibenzo-p-dioxins, hexachlorobenzene, dichlorodiphenyl dichloroethane, methyl mercury, and polychlorinated biphenyls. Two profiles each were developed for mixtures of metals and mixtures of volatile organic chemicals (VOCs) that are frequently found at hazardous waste sites. The two metal profiles dealt with (a) lead, manganese, zinc, and copper; and (b) arsenic, cadmium, chromium, and lead; the two VOCs mixtures dealt with (a) 1,1,1-trichloroethane, 1,1-dichloroethane, trichloroethylene, and tetrachloroethylene; and (b) benzene, ethylbenzene, toluene, and xylenes (BTEX). Weight-of-evidence methodology was used to assess the joint toxic action for most of the mixtures. Physiologically based pharmacokinetic modeling was used for BTEX. In most cases, a target-organ toxicity dose modification of the hazard index approach is recommended for conducting exposure-based assessments of noncancer health hazards.