Induction of xenobiotic-metabolizing enzymes and peroxisome proliferation in rat liver caused by dietary exposure to di(2-ethylhexyl)phosphate

Comparison of tris(2-ethylhexyl) phosphate and di(2-ethylhexyl) phosphoric acid toxicities in a rat 28-day oral exposure study

Tris(2-ethylhexyl) phosphate (TEHP, CAS no. 78-42-2) is a plasticizer and a flame retardant, while di(2-ethylhexyl) phosphoric acid (DEHPA, CAS no. 298-07-7) is an oil additive and extraction solvent. Publicly-available information on repeated exposure to these two related organophosphate compounds is fragmentary. Hence, adult male and female Fischer rats were exposed to TEHP (300, 1000 and 3000 mg/kg body weight [BW]/day) or DEHPA (20, 60 and 180 mg/kg BW/day) by gavage for 28 consecutive days, to assess and compare their toxicities. Although significantly impaired BW gains and evidence of TEHP enzymatic hydrolysis to DEHPA were observed only in males, exposures to the highest TEHP and DEHPA doses often resulted in similar alterations of hematology, serum clinical chemistry and liver enzymatic activities in both males and females. The squamous epithelial hyperplasia and hyperkeratosis observed in the non-glandular forestomach of rats exposed to the middle and high DEHPA doses were most likely caused by the slightly corrosive nature of this chemical. Although tubular degeneration and spermatid retention were observed only in the testes of males exposed to the highest TEHP dose, numerous periodic acid-Schiff stained crystalline inclusions were observed in testis interstitial cells at all TEHP dose levels. No-observed-adverse-effect levels for TEHP and DEHPA are proposed, but the lower serum pituitary hormone levels resulting from TEHP and DEHPA exposures and the perturbations of testicular histology observed in TEHP-treated males deserve further investigation. Improved characterization of the toxicity of flame retardants will contribute to better informed substitution choices for legacy flame retardants phased-out over health concerns.

Classification and toxicity mechanisms of novel flame retardants (NFRs) based on whole genome expression profiling

Recently some novel alternative flame retardants (NFRs), which have been widely applied to meet demands for mandated flame retardation of products, have been detected in various matrices of the environment. However, knowledge on toxic effects and associated molecular mechanisms of these chemicals was limited. Here, toxic mechanisms of action of six NFRs, bis (2-ethylhexyl) phosphate (BEHP), chlorendic acid (Het acid), 2,2-bis (bromomethyl)-1,3-propanediol (BMP), tris (2-butoxyethyl) phosphate (TBEP), triethyl phosphate (TEP), tributyl phosphate (TBP) were investigated by use of a library containing ∼1820 modified green fluorescent protein (GFP) expressing promoter reporter vectors constructed from Escherichia coli K12(E.coli). BEHP, Het acid, BMP, TBEP, TEP, TBP inhibited growth of E. coli with 4 h 10%-inhibition concentrations of 53.0-3102.3 μM. A total of 119, 44, 26, 131, 62, 103 genes out of 336 genes selected during preliminary screening were significantly altered with fold-changes greater than 1.5 by BEHP, Het acid, BMP, TBEP, TEP and TBP, respectively. GO analyses of responsive genes suggested that RNA and primary metabolism process were involved in molecular mechanisms of toxicity. Chemical clustering based on expression of 62 multi-responsive genes showed that BEHP, TBP and TBEP were grouped together, which is consistent with similarity of their chemical structures, especially for BEHP and TBP. Clustering by molecular descriptors and molecular activity by use of the multivariate classification system ToxCast was consistent with that by profiles of multi-responsive genes. The results of this study demonstrated the utility of the E. coli, whole-cell assay for determining mechanisms of toxic action of chemicals.

Characterization of the adipose tissue atrophy induced by peroxisome proliferators in mice

In the present study, we characterized the effects of peroxisome proliferators (PP) on adipose tissue in mice. Treatment with potent PP, such as perfluorooctanoic acid (PFOA), 2-methyl-2-(p(1,2,3,4-tetrahydroxy-naphthyl)-phenoxy)propionic acid, (4-chloro-6-(2,3-xylidino)2-pyrimidinylthio) acetic acid, and di(2-ethylhexyl)phthalate, caused dramatic decreases in adipose tissue weight, whereas the moderately potent PP, acetylsalicylic acid, had a relatively weak effect. This decrease in weight reflects a loss of fat from adipocytes rather than a loss of cells, as demonstrated by constant DNA content. The dose-dependency and time-course experiments indicate that peroxisome proliferation occurs simultaneously with or prior to adipose tissue atrophy. Thus, hepatic peroxisome proliferation might result in the increased mobilization of lipids and lipid utilization in liver. The enhanced adipose tissue hormone-sensitive lipase (HSL) activity and down-regulated lipoprotein lipase (LPL) activity observed upon PP treatment might, at least in part, explain the loss of fat via increased FA release from adipocytes and/or decreased FA uptake from the circulation, respectively. In addition, the possible involvement of the increased tumor necrosis factor alpha expression found upon PFOA treatment in reducing the insulin sensitivity of adipose tissue and thereby altering LPL and HSL activities is discussed.

Peroxisome proliferator nafenopin potentiated cytotoxicity and genotoxicity of cyclophosphamide in the liver and bone marrow cells

Peroxisome proliferators are ubiquitous rodent hepatocarcinogens, known to modulate the activities of xenobiotic-metabolising enzymes such as glutathione S-transferases (GST) and mixed-function oxidase (cytochrome P-450). In addition these compounds induce pleiotropic changes in the liver of rodents even after a short-term treatment. It has been hypothesised that the enzymatic and cellular changes induced by peroxisome proliferators may alter the toxicity of other compounds activated by cytochrome P-450 and detoxified by GST isoenzymes. The effect of nafenopin-induced changes in the liver of rats on the toxicity of an anti-cancer drug cyclophosphamide was studied using cyto- and geno-toxicity parameters in the liver and bone marrow cells. The administration of cyclophosphamide (10 or 20 mg/kg bw) to the rats pre-treated with 80 mg/kg bw of nafenopin for 2 days resulted in significantly increased cytotoxic response in bone marrow cells. However, genotoxicity of cyclophosphamide was increased only in the liver of nafenopin pre-treated rats. Low level of genotoxicity in bone marrow could be accounted for potentiated cytotoxicity of cyclophosphamide. These events coincided with a significant, up to 5-fold, increase in indirect activation-detoxication index for cyclophosphamide, determined as a ratio of ECOD and GST activities, in nafenopin treated rats. This resulted from the induction of ECOD responsible for the formation of reactive metabolites of cyclophosphamide and reduced activity of GST responsible for their detoxication. In addition, mitotic activity of hepatocytes was increased in nafenopin treated rats that might also have an impact on the genotoxicity of cyclophosphamide in this organ.

Effect of peroxisome proliferator nafenopin on the cytotoxicity of dihaloalkanes in isolated rat hepatocytes

Hepatocytes were isolated from nafenopin-treated animals (80 mg/kg body weight in 1.2 ml/kg body weight olive oil for 2 consecutive days) and exposed to various doses of 1,2 dichloroethane (DCE) (64-159 mumol) and 1,2-dibromoethane (DBE) (5.5-27.5 mumol) for up to 3 hr to assess the effect of nafenopin on the toxicity of dihaloalkanes. The activity of biotransformation enzymes involved in the activation and detoxication of these solvents was measured. Although cytochrome P450IIE1 activity was apparently unaltered, glutathione S-transferase activity was significantly reduced; the reduction was 20% for 1-chloro-2,4-dinitrobenzene as substrate but 40% and 80%, respectively for DBE and DCE. DBE was more than 10 times more cytotoxic to nafenopin-treated hepatocytes than DCE, and while very little change in DCE cytotoxicity was observed in hepatocytes isolated from nafenopin pretreated rats compared with control animals, DBE cytotoxicity was significantly potentiated in cells isolated from nafenopin-pretreated rats compared with cells from controls. It is believed that enhanced toxicity of DBE in isolated cells from nafenopin-treated rats is the result of modulation of dihaloalkane metabolism (glutathione conjugation).

Comparison of the potencies of (+)- and (-)-2-ethylhexanoic acid in causing peroxisome proliferation and related biological effects in mouse liver

Male C57BL/6 mice were exposed to 1% (w/w) (+)- or (-)-2-ethylhexanoic acid or an equimolar mixture of these enantiomers in their diet for 4 or 10 days. A significant increase in liver weight and a 2- to 3-fold increase in the protein content of the mitochondrial fraction were seen in all cases. Peroxisomal palmitoyl-CoA oxidation was increased 2- to 3.5-fold after 4 days of treatment and 4- to 5-fold after 10 days, while the corresponding increases in peroxisomal lauroyl-CoA oxidase activity were 2- to 3-fold and 9- to 12-fold, respectively. Peroxisomal catalase activity was unchanged, whereas the microsomal and cytosolic activities were increased 2- to 3-fold and 6- to 16-fold, respectively. These treatments also induced microsomal omega-hydroxylation of lauric acid 7-fold and soluble epoxide hydrolase activity in the mitochondrial and cytosolic fractions, as well as microsomal epoxide hydrolase activity about 50-100%. The only significant differences observed between the effects of (+)-2-ethylhexanoic acid and its (-)-enantiomer were on peroxisomal palmitoyl-CoA oxidation and lauroyl-CoA oxidase activity after 4 days of treatment. In both these cases the (+)-enantiomer resulted in increases which were 50-75% greater than those seen with the (-)-form.

Effect of hypolipidemic compounds on lauric acid hydroxylation and phase II enzymes

Treatment of male Fischer 344 rats with various hypolipidemic drugs of different peroxisome proliferating potency (1-benzylimidazole, acetylsalicylic acid, clofibrate, tiadenol) led to an induction of liver lauric acid hydroxylase, whereas probucol, which is not a peroxisome proliferator, did not induce this enzyme. Activity of bilirubin UDP-glucuronosyltransferase was increased by all the compounds tested. The highest increase was observed after treatment with acetylsalicylic acid (2.3-fold). High correlation (r = 0.953) was observed between the activities of lauric acid hydroxylase and the corresponding activities of cytosolic epoxide hydrolase reported previously. The amount of microsomal epoxide hydrolase was not changed by any of the compounds. Whereas clofibrate and tiadenol decreased glutathione S-transferase activity with 1-chloro-2,4-dinitrobenzene as substrate, 1-benzylimidazole and probucol increased this activity. With 4-hydroxynonenal as a substrate qualitatively the same results were obtained with the exception that probucol did not affect the enzyme activity. When glutathione S-transferase activity was measured with cis-stilbene oxide as substrate only the more than five-fold increase after treatment with 1-benzylimidazole was significantly different from control values. Activity of dihydrodiol dehydrogenase was increased after treatment of rats with 1-benzylimidazole (1.5-fold), whereas application of tiadenol led to a decrease of enzyme activity. Feeding of male guinea pigs with clofibrate did not change the activity of peroxisomal beta-oxidation, cytosolic epoxide hydrolase or lauric acid hydroxylase. However, treatment with tiadenol caused an increase of these activities.

Cytosolic epoxide hydrolase

Epoxide hydrolase activity is recovered in the high-speed supernatant fraction from the liver of all mammals so far examined, including man. For some as yet unexplained reason, the rat has a very low level of this activity, so that cytosolic epoxide hydrolase is generally studied in mice. This enzyme selectively hydrolyzes trans epoxides, thereby complementing the activity of microsomal epoxide hydrolase, for which cis epoxides are better substrates. Cytosolic epoxide hydrolase has been purified to homogeneity from the livers of mice, rabbits and humans. Certain of the physicochemical and enzymatic properties of the mouse enzyme have been thoroughly characterized. Neither the primary amino acid, cDNA nor gene sequences for this protein are yet known, but such characterization is presently in progress. Unlike microsomal epoxide hydrolase and most other enzymes involved in xenobiotic metabolism, cytosolic epoxide hydrolase is not induced by treatment of rodents with substances such as phenobarbital, 2-acetylaminofluorene, trans-stilbene oxide, or butylated hydroxyanisole. The only xenobiotics presently known to induce cytosolic epoxide hydrolase are substances which also cause peroxisome proliferation, e.g., clofibrate, nafenopin and phthalate esters. These and other observations indicate that this enzyme may actually be localized in peroxisomes in vivo and is recovered in the high-speed supernatant because of fragmentation of these fragile organelles during homogenization, i.e., recovery of this enzyme in the cytosolic fraction is an artefact. The functional significance of cytosolic epoxide hydrolase is still largely unknown. In addition to deactivating xenobiotic epoxides to which the organism is exposed directly or which are produced during xenobiotic metabolism, primarily by the cytochrome P-450 system, this enzyme may be involved in cellular defenses against oxidative stress.