Metallothionein's role in PCB126 induced hepatotoxicity and hepatic micronutrient disruption.

Polychlorinated biphenyls (PCBs), industrial chemicals and persistent environmental pollutants, are found in rural and urban settings. Rodent studies have shown that exposure to PCB126, a dioxin-like PCB, causes a significant disruption of hepatic micronutrient homeostasis and an increase in metallothionein (MT), an antioxidant protein and metal carrier. A MT knockout mouse strain was used to assess metallothionein's role in micronutrient disruption and overall hepatotoxicity. Twenty four 129S male mice (12 wild type (WT) and 12 MT knockout (MTKO)) were placed on a purified diet (AIN-93G) for 3 weeks to achieve hepatic metal equilibrium. Mice were then given a single IP injection, of either vehicle or 150 umol/kg PCB126 in vehicle. The animals were sacrificed 2 weeks later and organs processed for analysis. Liver histology, hepatic lipids, gene expression, micronutrient and ROS status were investigated. Liver weights, liver lipids, ROS, and hepatocyte vacuolation were increased with PCB126 exposure along with AhR responsive genes. The MTKO animals had more severe histological changes in the liver and elevated liver lipids than their wild type counterparts. Hepatic and renal metals levels (Cu, Zn, Se and Mn) were mostly reduced by PCB126 treatment. Renal micronutrients were more affected by PCB126 treatment in the MTKO animals. This research suggests that MT may not be the sole/primary cause of the metal disruption caused by PCB126 exposure in mice, but may provide protection against overall hepatotoxicity.

PCB153 reduces telomerase activity and telomere length in immortalized human skin keratinocytes (HaCaT) but not in human foreskin keratinocytes (NFK).

Polychlorinated biphenyls (PCBs), ubiquitous environmental pollutants, are characterized by long term-persistence in the environment, bioaccumulation, and biomagnification in the food chain. Exposure to PCBs may cause various diseases, affecting many cellular processes. Deregulation of the telomerase and the telomere complex leads to several biological disorders. We investigated the hypothesis that PCB153 modulates telomerase activity, telomeres and reactive oxygen species resulting in the deregulation of cell growth. Exponentially growing immortal human skin keratinocytes (HaCaT) and normal human foreskin keratinocytes (NFK) were incubated with PCB153 for 48 and 24days, respectively, and telomerase activity, telomere length, superoxide level, cell growth, and cell cycle distribution were determined. In HaCaT cells exposure to PCB153 significantly reduced telomerase activity, telomere length, cell growth and increased intracellular superoxide levels from day 6 to day 48, suggesting that superoxide may be one of the factors regulating telomerase activity, telomere length and cell growth compared to untreated control cells. Results with NFK cells showed no shortening of telomere length but reduced cell growth and increased superoxide levels in PCB153-treated cells compared to untreated controls. As expected, basal levels of telomerase activity were almost undetectable, which made a quantitative comparison of treated and control groups impossible. The significant down regulation of telomerase activity and reduction of telomere length by PCB153 in HaCaT cells suggest that any cell type with significant telomerase activity, like stem cells, may be at risk of premature telomere shortening with potential adverse health effects for the affected organism.

Airborne polychlorinated biphenyls (PCBs) reduce telomerase activity and shorten telomere length in immortal human skin keratinocytes (HaCat).

PCBs, a group of 209 individual congeners, are ubiquitous environmental pollutants and classified as probable human carcinogens. One major route of exposure is by inhalation of these industrial compounds, possibly daily from inner city air and/or indoor air in contaminated buildings. Hallmarks of aging and carcinogenesis are changes in telomere length and telomerase activity. We hypothesize that semi-volatile PCBs, like those found in inner city air, are capable of disrupting telomerase activity and altering telomere length. To explore this possibility, we exposed human skin keratinocytes to a synthetic Chicago Airborne Mixture (CAM) of PCBs, or the prominent airborne PCB congeners, PCB28 or PCB52 for up to 48 days and determined telomerase activity, telomere length, cell proliferation, and cell cycle distribution. PCBs 28, 52 and CAM significantly reduced telomerase activity from days 18-48. Telomere length was shortened by PCB 52 from day 18 and PCB 28 and CAM from days 30 on. All PCBs decreased cell proliferation from day 18; only PCB 52 produced a small increase of cells in G0/G1 of the cell cycle. This significant inhibition of telomerase activity and reduction of telomere length by PCB congeners suggest a potential mechanism by which these compounds could lead to accelerated aging and cancer.

3-Methylcholanthrene (3-MC) and 4-chlorobiphenyl (PCB3) genotoxicity is gender-related in Fischer 344 transgenic rats.

Polychlorinated biphenyls (PCBs) are a class of persistent organic pollutants with myriad biological effects, including carcinogenicity. We present data showing gender-specific genotoxicity in Fischer 344 transgenic BigBlue rodents exposed to 4-chlorobiphenyl (PCB3), a hydroxylated metabolite, and the positive control 3-methylcholanthrene (3-MC) where female rats are more resistant to the genotoxic effects of the test compounds compared to their male counterparts. This difference is further highlighted through our examination of gene expression, organ-specific weight changes, and tissue morphology. The purpose of the present study was to explore the complex and multifaceted issues of lower molecular weight PCBs as initiators of carcinogenesis, by examining the mutagenicity of PCB3, a hydroxylated metabolite (4'-OH-PCB3), and 3-methylcholanthrene (3-MC, positive control) in a transgenic rodent model. Previous findings indicated that PCB3 is mutagenic in the liver of male BigBlue transgenic rats under identical exposure conditions. We expected that female rats would be equally, if not more sensitive than male rats, since a 2-year carcinogenesis bioassay with Sprague-Dawley rats and commercial PCB mixtures reported much higher liver cancer rates in female than in male rats. The current study, however, revealed a similar trend in the mutation frequencies across all four treatment groups in females as reported previously in males, but increased variability among animals within each group and a lower overall effect, led to non significant differences in mutation frequencies. A closer analysis of the possible reasons for this negative result using microarray, organ weight and histology data comparisons shows that female Fischer 344 rats 1) had a higher baseline mutation frequency in the corn oil control group and greater variability than male rats; 2) responded with robust gene expression changes, which may also play a role in our observation of 3) highly increased liver, spleen, and lung weight in 3-MC and PCB3-treated female rats and thus changed distribution and kinetics of the test compounds. Our analysis indicates that female transgenic BigBlue Fischer 344 rats are more resistant to PCB3 and 3-MC genotoxicity compared to their male counterparts.

A low halogenated biphenyl (PCB3) increases CYP1A1 expression and activity via the estrogen receptor beta in the porcine ovary.

Polychlorinated biphenyls (PCBs) have been detected at high levels, up to hundreds of pg/ml, in human ovarian follicle fluid. The effect of PCBs on the ovary and the consequences of exposure are largely unknown. We have previously shown that PCB3 (4-chlorobiphenyl) increases the secretion of estradiol and the activity of cytochrome P450s (CYPs) in ovarian follicle cells. Our goal here is to elucidate the mechanism of CYP induction by this congener. Exposure of porcine follicle cells, a co-culture of theca and granulosa cells, to 6 ng/ml of PCB3 caused an increase in CYP1A1 protein and enzymatic activity, in the same manner as exposure to exogenous 17beta-estradiol. No changes were seen in the protein level of the aryl hydrocarbon receptor (AhR), which mediates the first step in the signaling pathway of CYP1A1 induction. However, a strong reduction was seen in the protein level of estrogen receptor beta (ERbeta), while no effect was seen on ERalpha protein levels. These result suggest that: 1) PCB3 acts as an agonist of ERbeta but not the Ah receptor in the ovarian follicles, 2) PCB3 is not only an efficacious inducer of CYP1A1 expression and activity, but also a substrate for this enzyme. Changes in the expression level of CYP1A1 not only alter the intensity of the activity of PCB3, but also the activity of estrogen in the ovary.

2-(4'-CHLOROPHENYL)-1,4-BENZOQUINONE INCREASES THE FREQUENCY OF MICRONUCLEI AND SHORTENS TELOMERES.

The toxicity of polychlorinated biphenyls (PCBs) has been attributed widely to receptor-mediated effects, buttressed by the popularity of the Toxic Equivalency Factor. We propose that a crucial toxic mechanism of lower-chlorinated PCBs is their enzymatic biotransformation to electrophiles, including quinoid metabolites, that bind intracellular sulfhydryl groups, such as those found in microtubulin and enzymes like telomerase. To test this hypothesis, we have examined micronuclei induction, cell cycle, and telomere shortening in cells in culture. Our findings show a large increase in micronuclei frequency and cell cycle perturbation in V79 cells, and a marked decrease in telomere length in HaCaT cells exposed to 2-(4'-chlorophenyl)-1,4-benzoquinone (PCB3pQ).

Sulfhydryl binding and topoisomerase inhibition by PCB metabolites.

Polychlorinated biphenyls (PCBs) are highly persistent contaminants in our environment. Their persistence is due to a general resistance to metabolic attack. Lower halogenated PCBs, however, are metabolized to mono- and dihydroxy compounds, and the latter may be further oxidized to quinones with the formation of reactive oxygen species (ROS). We have shown that PCB metabolism generates ROS in vitro and in cells in culture and this leads to oxidative DNA damage, like DNA strand breaks and 8-oxo-dG formation. In the present study, we have evaluated the reactivity of PCB metabolites with other nucleophiles, like glutathione (GSH), by assessing (1) quantitative GSH binding in vitro, (2) GSH and thiol (sulfhydryl) depletion in HL-60 cells, (3) the associated cytotoxicity, and (4) the inhibition of topoisomerase II activity in vitro. PCB quinones were found to bind GSH in vitro at a ratio of 1:1.5 and to deplete GSH in HL-60 cells as measured by both spectrophotometric and spectrofluorometric methods. By flow cytometry analysis, we confirmed that there was intracellular GSH depletion in HL-60 cells by PCB quinones and this is associated with cytotoxicity. On the other hand, the PCB hydroquinone metabolites did not bind GSH or other thiols within 1 h of exposure. However, by spectral analyses we found that the PCB hydroquinones could be oxidized enzymatically to the quinones, which could then bind GSH. The resulting hydroquinone-glutathione addition product(s) could undergo a second and third cycle of oxidation and GSH addition with the formation of di- and tri-GSH-PCB adducts. The effect of the PCB metabolites was also tested on a sulfhydryl-containing enzyme, topoisomerase II. PCB quinones inhibited topoisomerase II activity while the PCB hydroquinone metabolites did not. Hence, the oxidation of PCB hydroquinone metabolites to quinones in cells followed by the binding of quinones to GSH and to protein sulfhydryl groups and the resulting oxidative stress may be important aspects of the toxicity of these compounds.

Production of DNA strand breaks in vitro and reactive oxygen species in vitro and in HL-60 cells by PCB metabolites.

PCBs are industrial chemicals that continue to contaminate our environment. They cause various toxic effects in animals and in exposed human populations. The mechanisms of toxicity, however, are not completely understood. PCBs are metabolized by cytochromes P450 to mono- and dihydroxylated compounds. Dihydroxy-PCBs can potentially be oxidized to the corresponding quinones. We hypothesized that reactive oxygen species (ROS) are produced by redox reactions of PCB metabolites. We tested several synthetic dihydroxy- and quinoid-PCBs with 1-3 chlorines for their potential to produce ROS in vitro and in HL-60 human leukemia cells, and DNA strand breaks in vitro. All dihydroxy-PCBs tested produced superoxide. The quinones generated superoxide only in the presence of GSH, probably during the autoxidation of the glutathione conjugates. We observed increased superoxide production with decreasing halogenation. Incubation of dihydroxy-PCBs or PCB quinones + GSH with plasmid DNA resulted in DNA strand break induction in the presence of Cu(II). Tests with various ROS scavengers indicated that hydroxyl radicals and singlet oxygen are likely involved in this strand break induction. Finally, dihydroxy- and quinoid PCBs also produced ROS in HL-60 cells in a dose- and time-dependent manner. We conclude that dihydroxylated PCBs, and PCB quinones after reaction with GSH, produce superoxide and other ROS both in vitro and in HL-60 cells, and oxidative DNA damage in the form of DNA strand breaks in vitro. The reactions seen in vitro and in cells may well be a predictor of the toxicity of PCBs in animals.

Mutations in the promoter reveal a cause for the reduced expression of the human manganese superoxide dismutase gene in cancer cells.

Manganese superoxide dismutase (MnSOD) has been shown to play an important role in preventing the development of cancer. MnSOD activity is reduced in many transformed cells and tumor tissues. We previously showed that the reduced level of MnSOD activity in cancer cells was not due to a defect in the primary structure of MnSOD protein, but rather was due to defects in gene expression. To elucidate the cause for the reduced expression of human MnSOD in cancer, we investigated the nucleotide sequence in the regulatory region of the MnSOD gene in a normal human cell line and various human tumor cell lines. A DNA fragment spanning 3.4 kb 5' flanking region of the MnSOD gene isolated from a normal human genomic DNA library was used to determine the DNA sequence of MnSOD promoter. PCR primers were used for amplification of the 3.4 kb 5' flanking region of the human MnSOD gene in cancer cells. Sequence analysis identified three heterozygous mutations in the proximal region of the promoter in five human tumor cell lines. These mutations, clustered around the GC-rich region of the human MnSOD promoter, change the binding pattern of AP-2 and lead to a reduction in transcription activity using a luciferase reporter assay system. These results suggest that the reduced level of MnSOD expression in some tumor cells is, at least in part, due to a defect in the DNA sequence of the promoter region.

Activation of hepatic NF-kappaB by the herbicide Dicamba (2-methoxy-3,6-dichlorobenzoic acid) in female and male rats.

Nuclear factor-kappaB is a transcription factor that is activated in many different cell types by pathologic stimuli, such as reactive oxygen intermediates. One class of hepatocarcinogens, the peroxisome proliferators, may produce reactive oxygen intermediates, and one potent peroxisome proliferator, ciprofibrate, was recently reported to activate nuclear factor-kappaB. In this study, we investigated whether Dicamba, a broad leaf herbicide and peroxisome proliferator, could activate nuclear factor-KB in the livers of rats. Female and male Sprague Dawley rats (n = 4) were fed diets containing either 0, 1, or 3% Dicamba or 0.01% ciprofibrate for 7 days. As expected, the potent peroxisome proliferator, ciprofibrate, significantly increased fatty acyl CoA oxidase, peroxisomal beta-oxidation, and catalase activities in male rats and, except for catalase, also in female rats. Dicamba significantly increased peroxisomal fatty acyl CoA oxidase, peroxisomal beta-oxidation, and catalase activities, but decreased the activity of the cytosolic antioxidant enzyme, Se-dependent glutathione peroxidase, in both female and male rats. Dicamba increased nuclear factor-kappaB binding in the nuclear protein of livers from male rats fed both the 1 and 3% Dicamba diets. However, the highest binding was seen in nuclear protein from female rats fed 3% Dicamba. Both supershift and cold competition assays confirmed that this DNA binding activity was specific for nuclear factor-kappaB. Our results in this study suggest that the herbicide and peroxisome proliferator Dicamba has the ability to activate nuclear factor-kappaB.

Characterization of the PNT1 pentamidine resistance gene of Saccharomyces cerevisiae.

The Saccharomyces cerevisiae PNT1 gene was isolated and characterized. When present in high copy number in S. cerevisiae, PNT1 confers resistance to the anti-Pneumocystis carinii drug pentamidine. The PNT1 gene encodes a previously uncharacterized polypeptide of 409 amino acids. The predicted gene product is a very basic (pI 9.9) polypeptide with one potential membrane-associated region. PNT1 is located on chromosome XVR of S. cerevisiae. It is transcribed at a very low level. Overexpression of the gene increases resistance to the cytostatic and mitochondrial DNA-damaging effects of pentamidine and related cationic compounds. Disruption of the gene leads to slightly increased levels of susceptibility to pentamidine and some related compounds.

Effects of pentamidine isethionate on Saccharomyces cerevisiae.

We used Saccharomyces cerevisiae as a model system in which to examine the mechanism of action of the anti-Pneumocystis drug pentamidine. Pentamidine at low concentrations inhibited S. cerevisiae growth on nonfermentable carbon sources (50% inhibitory concentration [IC50] of 1.25 micrograms/ml in glycerol). Pentamidine inhibited growth on fermentable energy sources only at much higher concentrations (IC50 of 250 micrograms/ml in glucose). Inhibition at low pentamidine concentrations in glycerol was due to cytostatic activity rather than cytotoxic or mutagenic activity. Pentamidine also rapidly inhibited respiration by intact yeast cells, although inhibitory concentrations were much higher than those inhibitory to growth (IC50 of 100 micrograms/ml for respiration). Pentamidine also induced petite mutations, although only at concentrations much higher than those required for growth inhibition. These results suggest that a function essential for respiratory growth is inhibited by pentamidine and that pentamidine affects mitochondrial processes. We propose the hypothesis that the primary cellular target of pentamidine in S. cerevisiae is the mitochondrion.

Multiple activation pathways of benzene leading to products with varying genotoxic characteristics.

Benzene and 13 potential metabolites were investigated for genotoxicity in Salmonella typhimurium and V79 Chinese hamster cells. In the presence of NADPH-fortified hepatic postmitochondrial fraction (S9 mix), benzene reverted his- S. typhimurium strains. The effect was strongest in strain TA1535. Among the potential metabolites, only the trans-1,2-dihydrodiol, in the presence of S9 mix, and the diol epoxides, in the presence and absence of S9 mix, proved mutagenic in this strain. The anti-diol epoxide was more potent than the syn-diastereomer. Both enantiomers of the anti-diastereomer showed similar activities. S9 mix did not appreciably affect the mutagenicity of the anti-diol epoxide. However, detoxification was observed when purified rat liver dihydrodiol dehydrogenase (EC 1.3.1.20) was used at concentrations comparable to that present in the liver. The (1S)-anti-diol epoxide was a much better substrate than the (1R)-enantiomer, as was true also for (1S)-versus (1R)-trans-1,2-dihydrodiol. The anti-diol epoxide reverted all six strains of S. typhimurium used and induced all four genotoxic effects studied in V79 cells (sister chromatid exchange greater than acquisition of 6-thioguanine resistance, acquisition of ouabain resistance, micronuclei). However, other potential benzene metabolites showed genotoxic effects in V79 cells, as well: sister chromatid exchange was induced by the syn-diol epoxide, 1,2,4-trihydroxybenzene, hydroquinone, catechol, and 1,2,3-trihydroxybenzene. Elevated frequencies of micronucleated cells were observed after treatment with hydroquinone, 1,2,4-trihydroxybenzene, catechol, phenol, 1,2,3-trihydroxybenzene, and quinone. Mutations to 6-thioguanine resistance were induced by quinone, hydroquinone, 1,2,4-trihydroxybenzene, catechol, and the trans-1,2-dihydrodiol.(ABSTRACT TRUNCATED AT 250 WORDS)

Genotoxicity of 1,4-benzoquinone and 1,4-naphthoquinone in relation to effects on glutathione and NAD(P)H levels in V79 cells.

1,4-Benzoquinone is cytotoxic in V79 Chinese hamster cells and induces gene mutations and micronuclei. The cell-damaging effects of quinones are usually attributed to thiol depletion, oxidation of NAD(P)H, and redox-cycling involving the formation of semiquinone radicals and reactive oxygen species. To elucidate the role of these mechanisms in the genotoxicity of 1,4-benzoquinone, we measured various genotoxic effects, cytotoxicity, and the levels of glutathione, NADPH, NADH, and their oxidized forms all in the same experiment. 1,4-Naphthoquinone, which does not induce gene mutations in V79 cells, was investigated for comparative reasons. The quinones had a similar effect on the levels of cofactors. Total glutathione was depleted, but levels of oxidized glutathione were slightly increased. The levels of NADPH and NADH were reduced at high concentrations of the quinones with a simultaneous increase in the levels of NADP+ and NAD+. Both compounds induced micronuclei, but neither increased the frequency of sister chromatid exchange. Only 1,4-benzoquinone induced gene mutations. This effect was observed at low concentrations, where none of the other parameters studied was affected. When the cells were depleted of glutathione prior to treatment with the quinones, the induction of gene mutations and micronuclei remained virtually unchanged. We conclude that a) induction of micronuclei and glutathione depletion by the two quinones are not linked causally, b) 1,4-benzoquinone induces gene mutations by a mechanism different from oxidative stress and glutathione depletion, and c) glutathione does not fully protect the cells against the genotoxicity of quinones.

Glutaraldehyde-fixed transformed and non-transformed cells induce contact-dependent inhibition of growth in non-transformed C3H/10T1/2 mouse fibroblasts, but not in 3-methylcholanthrene-transformed cells.

C3H/10T1/2 mouse fibroblasts showed a pronounced inhibition of growth when reaching a critical cell density. The situation of high cell density could be mimicked by the addition of glutaraldehyde-fixed cells to sparsely seeded proliferating cells. Treatment of the C3H/10T1/2 cells with 3-methylcholanthrene led to a high frequency of piled up foci (118 type II and type III foci in 78 cultures). Cells of a type III focus of a treated culture were cloned. These cells grew in soft-agar and reached 10 times higher cell densities when grown in culture dishes, than did their non-transformed counterparts. Glutaraldehyde-fixed transformed cells did not differ from fixed non-transformed cells in the ability to inhibit the growth of sparsely seeded non-transformed cells. On the other hand, both the addition of fixed normal or transformed C3H/10T1/2 cells did not affect the growth rate of transformed cells. In a concept explaining the density-dependent inhibition of growth of non-transformed cells by a specific interaction of plasma membrane-localized effectors with plasma membrane-localized receptors, the present findings would indicate that the transformed cells used express active effectors but are functionally defective in the receptors or in the signal transmission.

Arene imines, a new class of exceptionally potent mutagens in bacterial and mammalian cells.

K-region aziridines of polycyclic aromatic hydrocarbons reverted Salmonella typhimurium his- (TA100, TA98) and Escherichia coli trp- strains (WP2 uvrA), without requiring activation by mammalian enzymes. The number of revertants induced per nmol in S. typhimurium TA 100, the most responsive strain, variea from 6 to 10,000 for the seven monoaziridines and the two bisaziridines tested. Interestingly, the mutagenic potencies (y) of the monoaziridines were closely related (r = 0.984) with those of the corresponding epoxide analogues (x) by the equation y = 19.6 X0.97, i.e., the aziridines were about 20-fold stronger mutagens than were the epoxides. One of the aziridines, benzo(a)pyrene (BP)-4,5-imine, was investigated in several additional mutagenicity test systems: toxicity in DNA repair-deficient (rec-) and -proficient (rec+) Bacillus subtilis strains; induction of 6-thioguanine resistance in V79 Chinese hamster cells; and induction of sister chromatid exchanges in cultured human fibroblasts. In all systems, BP-4,5-imine was much more active than the epoxide analogue, BP-4,5-oxide. The difference in activity was particularly large in the two test systems with mammalian target cells in which several hundredfold higher concentrations of the epoxide had to be used in order to elicit equipotent effects. Even r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydro-BP, which is one of the most potent mutagens known for V79 cells, was less active in the mammalian cells than was BP-4,5-imine. One reason that arene imines are such potent mutagens may be that they are poorly detoxified. Addition of highly purified microsomal epoxide hydrolase, which strongly reduced the mutagenicity of BP-4,5-oxide and benz(a)anthracene-5,6-oxide in S. typhimurium, had no effect on the mutagenicity of the corresponding aziridines. Furthermore, while benz(a)anthracene-5,6-oxide was inactivated by highly purified cytosolic epoxide hydrolase, benz(a)anthracene-5,6-imine was not inactivated. It is noteworthy that the arene imines are isomeric with and structurally closely related to aromatic amines. Some aziridines derived from nonaromatic structures (ethylene imines) have been reported as metabolites of xenobiotics; others are used as chemotherapeutics. At present, however, the results are mainly of theoretical interest in that a new type of arene derivatives with exceptionally potent, probably ultimate, mutagenicity was discovered and may be exploited for the study of mechanisms of chemical carcinogenesis.