Dietary selenium as a modulator of PCB 126-induced hepatotoxicity in male Sprague-Dawley rats

Chronic Aroclor 1260 exposure alters the mouse liver proteome, selenoproteins, and metals in steatotic liver disease

The prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD) continues to increase due in part to the obesity epidemic and to environmental exposures to metabolism disrupting chemicals. A single gavage exposure of male mice to Aroclor 1260 (Ar1260), an environmentally relevant mixture of non-dioxin-like polychlorinated biphenyls (PCBs), resulted in steatohepatitis and altered RNA modifications in selenocysteine tRNA 34 weeks post-exposure. Unbiased approaches identified the liver proteome, selenoproteins, and levels of 25 metals. Ar1260 altered the abundance of 128 proteins. Enrichment analysis of the liver Ar1260 proteome included glutathione metabolism and translation of selenoproteins. Hepatic glutathione peroxidase 4 (GPX4) and Selenoprotein O (SELENOO) were increased and Selenoprotein F (SELENOF), Selenoprotein S (SELENOS), Selenium binding protein 2 (SELENBP2) were decreased with Ar1260 exposure. Increased copper, selenium (Se), and zinc and reduced iron levels were detected. These data demonstrate that Ar1260 exposure alters the (seleno)proteome, Se, and metals in MASLD-associated pathways.

Insight into the mechanism of melatonin in attenuating PCB126-induced liver injury: Resistance to ROS-dependent NETs formation to alleviate inflammation and lipid metabolism dysfunction

3,3',4',4',5-Polychlorinated biphenyls (PCB126) is classified as a persistent organic environmental pollutant that can cause liver damage by producing excessive reactive oxygen species (ROS). ROS also can stimulate neutrophil extracellular traps (NETs) formation, which cause damage to organism if NETs are produced in excess. Melatonin is generally considered to possess strong antioxidant and anti-inflammation prosperities, but it is unclear whether it can alleviate PCB126-induced injury. To explore whether PCB126-induced liver injury is related to the formation of NETs and whether melatonin has a potent protective effect, we established PCB126 exposure/ PCB126 and melatonin co-treatment mouse models by gavage. To further clarify the specific mechanism, we also cultured neutrophils and AML12 cells to replicate in vivo model. Here, we found PCB126 exposure resulted in an elevation in the activities of MDA, LPO, PCO, and 8-OHdG, and a reduction in the activities of CAT, GSH-PX and SOD. We found that PCB126 exposure led to an elevation in the expression levels of chemokines (CCL2, CCL3, CCL4, CXCL12, and CXCL8) and marker factors for NETs formation (MPO, NE, NOX2, PKCα, and PKCζ) in the PCB126 group. IF, SYTOX staining, and SEM results also revealed that PCB126 could stimulate NETs formation. In addition, results of a co-culture system of PBNs and AML12 cells revealed that the expression levels of inflammatory cytokines (IL-1β, IL-6, and TNF-α) significantly decreased and the expression levels of metabolism factors (Fas, Acc, and Srebp) slightly decreased for scavenging NETs, indicating NETs formation aggravated PCB126-induced hepatic damages. Noteworthy, treatment with melatonin reversed these results. In summary, our findings revealed that melatonin alleviated hepatic damage aggravated by PCB126-induced ROS-dependent NETs formation through suppressing excessive ROS production. This finding not only enriches toxicological mechanism of PCB126, but more importantly extends biological effects of melatonin and its potential application values.

Selenium, Stroke, and Infection: A Threefold Relationship; Where Do We Stand and Where Do We Go?

Stroke is currently the second most common cause of death worldwide and a major cause of serious long-term morbidity. Selenium is a trace element with pleotropic effects on human health. Selenium deficiency has been associated with a prothrombotic state and poor immune response, particularly during infection. Our aim was to synthesize current evidence on the tripartite interrelationship between selenium levels, stroke, and infection. Although evidence is contradictory, most studies support the association between lower serum selenium levels and stroke risk and outcomes. Conversely, limited evidence on the role of selenium supplementation in stroke indicates a potentially beneficial effect of selenium. Notably, the relationship between stroke risk and selenium levels is bimodal rather than linear, with higher levels of serum selenium linked to disturbances of glucose metabolism and high blood pressure, morbidities which are, in turn, substrates for stroke. Another such substrate is an infection, albeit forming a bidirectional relationship with both stroke and the consequences of impaired selenium metabolism. Perturbed selenium homeostasis leads to impaired immune fitness and antioxidant capacity, which both favor infection and inflammation; specific pathogens may also contend with the host for transcriptional control of the selenoproteome, adding a feed-forward loop to this described process. Broader consequences of infection such as endothelial dysfunction, hypercoagulation, and emergent cardiac dysfunction both provide stroke substrates and further feed-forward feedback to the consequences of deficient selenium metabolism. In this review, we provide a synthesis and interpretation of these outlined complex interrelationships that link selenium, stroke, and infection and attempt to decipher their potential impact on human health and disease. Selenium and the unique properties of its proteome could provide both biomarkers and treatment options in patients with stroke, infection, or both.

PCB126 induced toxic actions on liver energy metabolism is mediated by AhR in rats

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor involved in the regulation of biological responses to more planar aromatic hydrocarbons, like TCDD. We previously described the sequence of events following exposure of male rats to a dioxin-like polychlorinated biphenyl (PCB) congener, 3,3',4,4',5-pentachlorobiphenyl (PCB126), that binds avidly to the AhR and causes various types of toxicity including metabolic syndrome, fatty liver, and disruption of energy homeostasis. The purpose of this study was, to investigate the role of AhR to mediate those toxic manifestations following sub-acute exposure to PCB126 and to examine possible sex differences in effects. For this goal, we created an AhR knockout (AhR-KO) model using CRISPR/Cas9. Comparison was made to the wild type (WT) male and female Holtzman Sprague Dawley rats. Rats were injected with a single IP dose of corn oil vehicle or 5 μmol/kg PCB126 in corn oil and necropsied after 28 days. PCB126 caused significant weight loss, reduced relative thymus weights, and increased relative liver weights in WT male and female rats, but not in AhR-KO rats. Similarly, significant pathologic changes were visible which included necrosis and regeneration in female rats, micro- and macro-vesicular hepatocellular vacuolation in males, and a paucity of glycogen in livers of both sexes in WT rats only. Hypoglycemia and lower IGF1, and reduced serum non-esterified fatty acids (NEFAs) were found in serum of both sexes of WT rats, low serum cholesterol levels only in the females, and no changes in AhR-KO rats. The expression of genes encoding enzymes related to xenobiotic metabolism (e.g. CYP1A1), gluconeogenesis, glycogenolysis, and fatty acid oxidation were unaffected in the AhR-KO rats following PCB126 exposure as opposed to WT rats where expression was significantly upregulated (PPARα, females only) or downregulated suggesting a disrupted energy homeostasis. Interestingly, Acox2, Hmgcs, G6Pase and Pc were affected in both sexes, the gluconeogenesis and glucose transporter genes Pck1, Glut2, Sds, and Crem only in male WT-PCB rats. These results show the essential role of the AhR in glycogenolysis, gluconeogenesis, and fatty acid oxidation, i.e. in the regulation of energy production and homeostasis, but also demonstrate a significant difference in the effects of PCB126 in males verses females, suggesting higher vulnerability of glucose homeostasis in males and more changes in fatty acid/lipid homeostasis in females. These differences in effects, which may apply to more/all AhR agonists, should be further analyzed to identify health risks to specific groups of highly exposed human populations.

The Aryl hydrocarbon receptor mediates reproductive toxicity of polychlorinated biphenyl congener 126 in rats

Polychlorinated biphenyls (PCBs) are endocrine disrupting chemicals with documented, though mechanistically ill-defined, reproductive toxicity. The toxicity of dioxin-like PCBs, such as PCB126, is mediated via the aryl hydrocarbon receptor (AHR) in non-ovarian tissues. The goal of this study was to examine the uterine and ovarian effects of PCB126 and test the hypothesis that the AHR is required for PCB126-induced reproductive toxicity. Female Holzman-Sprague Dawley wild type (n = 14; WT) and Ahr knock out (n = 11; AHR-/-) rats received a single intraperitoneal injection of either corn oil vehicle (5 ml/kg: WT_O and AHR-/-_O) or PCB126 (1.63 mg/kg in corn oil: WT_PCB and AHR-/-_PCB) at four weeks of age. The estrous cycle was synchronized and ovary and uterus were collected 28 days after exposure. In WT rats, PCB126 exposure reduced (P < 0.05) body and ovary weight, uterine gland number, uterine area, progesterone, 17β-estradiol and anti-Müllerian hormone level, secondary and antral follicle and corpora lutea number but follicle stimulating hormone level increased (P < 0.05). In AHR-/- rats, PCB126 exposure increased (P ≤ 0.05) circulating luteinizing hormone level. Ovarian or uterine mRNA abundance of biotransformation, and inflammation genes were altered (P < 0.05) in WT rats due to PCB126 exposure. In AHR-/- rats, the transcriptional effects of PCB126 were restricted to reductions (P < 0.05) in three inflammatory genes. These findings support a functional role for AHR in the female reproductive tract, illustrate AHR's requirement in PCB126-induced reprotoxicity, and highlight the potential risk of dioxin-like compounds on female reproduction.

Selenium-Rich-Yeast Protects Against Aluminum-Induced Activating Nuclear Xenobiotic Receptors and Triggering Inflammation and Cytochromes P450 Systems in Mice Heart

Aluminum (Al) poisoning is linked to the development of cardiovascular diseases, and dietary supplementation with selenium-rich-yeast (SeY) has been shown to prevent inflammatory conditions. We evaluated the preventive effect of SeY on Al-induced cardiotoxicity, and the possible underlying mechanisms. Mice were treated with SeY (0.1 mg/kg) and/or Al (10 mg/kg) by oral gavage for 4 weeks. Histopathological damage was observed in the heart of Al-treated mice, in addition to the transcriptional up/downregulation of nuclear xenobiotic receptors (NXRs), inflammatory cytokines and 15 CYP450s genes. SeY significantly inhibited these Al-induced histopathological and molecular changes, and restored these indicators to the control levels. These results suggest that SeY exerts a cardio-protective effect against Al-induced toxicity through the NXR system, inflammatory cytokines, and CYP450s genes.

Metabolic Impairments Caused by a "Chemical Cocktail" of DDE and Selenium in Mice Using Direct Infusion Triple Quadrupole Time-of-Flight and Gas Chromatography-Mass Spectrometry

Among organic contaminants, pesticides are one of the most important groups of chemicals due to their persistent character and toxicity. However, the biological systems are exposed to a complex environment in which the contaminants can interact in a synergistic/antagonistic fashion, and for this reason, the study of "chemical cocktails" is of great interest to fully understand the final biological effect. In this way, selenium is known for its antagonistic action against several toxicants. In this paper, metabolic impairments caused by the joint exposure of p,p'-dichloro diphenyl trichloroethane (DDE) and selenium (Se) have been issued for the first time. A metabolomic workflow was applied to mice fed DDE and DDE with Se diet, on the basis of the complementary use of two organic mass spectrometric techniques, combining direct infusion mass spectrometry (DI-ESI-QqQ-TOF MS) and gas chromatography-mass spectrometry (GC-MS). The results show a good classification between the studied groups caused by about 70 altered metabolites in the liver, kidney, or brain, including the pathways of energy metabolism, degradation of phospholipidic membrane, β-oxidation, and oxidative stress, which confirm the potential of combined metabolomic platforms in environmental studies.

Spatial distribution of metals within the liver acinus and their perturbation by PCB126

Animal studies show that exposure to the environmental pollutant 3,3',4,4',5-pentachlorobiphenyl (PCB126) causes alterations in hepatic metals as measured in acid-digested volume-adjusted tissue. These studies lack the detail of the spatial distribution within the liver. Here we use X-ray fluorescence microscopy (XFM) to assess the spatial distribution of trace elements within liver tissue. Liver samples from male Sprague Dawley rats, treated either with vehicle or PCB126, were formalin fixed and paraffin embedded. Serial sections were prepared for traditional H&E staining or placed on silicon nitride windows for XFM. With XFM, metal gradients between the portal triad and the central vein were seen, especially with copper and iron. These gradients change with exposure to PCB126, even reverse. This is the first report of how micronutrients vary spatially within the liver and how they change in response to toxicant exposure. In addition, high concentrations of zinc clusters were discovered in the extracellular space. PCB126 treatment did not affect their presence, but did alter their elemental makeup suggesting a more general biological function. Further work is needed to properly evaluate the gradients and their alterations as well as classify the zinc clusters to determine their role in liver function and zinc homeostasis.

PCB126 Inhibits the Activation of AMPK-CREB Signal Transduction Required for Energy Sensing in Liver

3,3',4,4',5-pentachlorobiphenyl (PCB126), a dioxin-like PCB, elicits toxicity through a wide array of noncarcinogenic effects, including metabolic syndrome, wasting, and nonalcoholic fatty-liver disease. Previously, we reported decreases in the transcription of several enzymes involved in gluconeogenesis, before the early onset of lipid accumulation. Hence, this study was aimed at understanding the impact of resultant decreases gluconeogenic enzymes on growth, weight, and metabolism in the liver, upon extended exposure. Male Sprague Dawley rats (75-100 g), fed a defined AIN-93G diet, were injected (ip) with single dose of soy oil (5 ml/kg body weight; n = 14) or PCB126 (5 µmol/kg; n = 15), 28 days, prior euthanasia. A subset of rats from each group were fasted for 12 h (vehicle [n = 6] and PCB126 [n = 4]). Rats only showed significant weight loss between days 14 and 28 (p < .05) and some mortality (p = .0413). As in our previous studies, the expression levels of enzymes involved in gluconeogenesis (Pepck-c, G6Pase, Sds, Pc, and Ldh-A) and glycogenolysis (Pygl) were strongly downregulated. The decreased expression of these enzymes in PCB126-treated rats after a 12 h fast decreased hepatic glucose production from glycogen and gluconeogenic substrates, exacerbating the hypoglycemia. Additionally, PCB126 caused hepatic steatosis and decreased the expression of the transcription factor Pparα and its targets, necessary for fatty-acid oxidation. The observed metabolic disruption across multiple branches of fasting metabolism resulted from inhibition in the activation of enzyme AMPK and transcription factor CREB signaling, necessary for "sensing" energy-deprivation and the induction of enzymes that respond to the PCB126 triggered fuel crisis in liver.

Sources and toxicities of phenolic polychlorinated biphenyls (OH-PCBs)

Polychlorinated biphenyls (PCBs), a group of 209 congeners that differ in the number and position of chlorines on the biphenyl ring, are anthropogenic chemicals that belong to the persistent organic pollutants (POPs). For many years, PCBs have been a topic of interest because of their biomagnification in the food chain and their environmental persistence. PCBs with fewer chlorine atoms, however, are less persistent and more susceptible to metabolic attack, giving rise to chemicals characterized by the addition of one or more hydroxyl groups to the chlorinated biphenyl skeleton, collectively known as hydroxylated PCBs (OH-PCBs). In animals and plants, this biotransformation of PCBs to OH-PCBs is primarily carried out by cytochrome P-450-dependent monooxygenases. One of the reasons for infrequent detection of lower chlorinated PCBs in serum and other biological matrices is their shorter half-lives, and their metabolic transformation, resulting in OH-PCBs or their conjugates, such as sulfates and glucuronides, or macromolecule adducts. Recent biomonitoring studies have reported the presence of OH-PCBs in human serum. The occurrence of OH-PCBs, the size of this group (there are 837 mono-hydroxyl PCBs alone), and their wide spectra of physical characteristics (pKa's and log P's ranging over 5 to 6 orders of magnitude) give rise to a multiplicity of biological effects. Among those are bioactivation to electrophilic metabolites that can form covalent adducts with DNA and other macromolecules, interference with hormonal signaling, inhibition of enzymes that regulate cellular concentrations of active hormones, and interference with the transport of hormones. This new information creates an urgent need for a new perspective on these often overlooked metabolites.

Identification of lipidomic markers of chronic 3,3',4,4',5-pentachlorobiphenyl (PCB 126) exposure in the male rat liver

Exposure to PCB 126, an environmentally relevant aryl hydrocarbon receptor agonist, is an environmental factor causing hepatic steatosis in rodent models; however, the lipidome of PCB 126-exposed rats has not been investigated in-depth. The objective of the present study was therefore to characterize dose-dependent changes in the lipid profile in the liver of male Sprague-Dawley rats exposed to PCB 126. Rats were exposed for three month to intraperitoneal injections of 0.01, 0.05 and 0.2μmol/kg bw PCB 126 in corn oil. Control animals were exposed in parallel and received corn oil alone. Lipids were extracted from whole liver homogenate and levels of polar lipids and fatty acids incorporated into triglycerides (FATAGs) were determined with tandem mass spectrometry using electrospray ionization. PCB 126 exposure increased the hepatic content of polar lipids and FATAGs. Protein adjusted levels of several polar lipid classes, in particular phosphatidylserine levels, decreased, whereas FATAGs levels typically increased with increasing PCB 126 dose. Sensitive, dose-dependent endpoints of PCB 126 exposure included an increase in levels of adrenic acid incorporated into triglycerides and changes in levels of certain ether-linked phospholipid and 1-alkyl/1-alkenyldiacylglycerol species, as determined using partial least square discriminant analysis (PLS-DA) and ANOVA. These changes in the composition of polar lipids and fatty acid in the liver of PCB 126 exposed rats identified several novel markers of PCB 126-mediated fatty liver disease that need to be validated in further studies.

Selenoproteins are involved in antioxidant defense systems in thalassemia

Thalassemia major (TM) is a hereditary blood disease that affects the production of hemoglobin, resulting in severe anemia.

3,3',4,4',5-Pentachlorobiphenyl (PCB 126) Decreases Hepatic and Systemic Ratios of Epoxide to Diol Metabolites of Unsaturated Fatty Acids in Male Rats

Disruption of the homeostasis of oxygenated regulatory lipid mediators (oxylipins), potential markers of exposure to aryl hydrocarbon receptor (AhR) agonists, such as 3,3',4,4',5-pentachlorobiphenyl (PCB 126), is associated with a range of diseases, including nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Here we test the hypothesis that PCB 126 exposure alters the levels of oxylipins in rats. Male Sprague-Dawley rats (5-weeks old) were treated over a 3-month period every 2 weeks with intraperitoneal injections of PCB 126 in corn oil (cumulative doses of 0, 19.8, 97.8, and 390 µg/kg b.w.; 6 injections total). PCB 126 treatment caused a reduction in growth rates at the highest dose investigated, a dose-dependent decrease in thymus weights, and a dose-dependent increase in liver weights. Liver PCB 126 levels increased in a dose-dependent manner, while levels in plasma were below or close to the detection limit. The ratios of several epoxides to diol metabolites formed via the cytochrome P450 (P450) monooxygenase/soluble epoxide hydrolase (sEH) pathway from polyunsaturated fatty acids displayed a dose-dependent decrease in the liver and plasma, whereas levels of oxylipins formed by other metabolic pathways were generally not altered by PCB 126 treatment. The effects of PCB 126 on epoxide-to-diol ratios were associated with an increased CYP1A activity in liver microsomes and an increased sEH activity in liver cytosol and peroxisomes. These results suggest that oxylipins are potential biomarkers of exposure to PCB 126 and that the P450/sEH pathway is a therapeutic target for PCB 126-mediated hepatotoxicity that warrants further attention.

Assessment of the Mitigative Capacity of Dietary Zinc on PCB126 Hepatotoxicity and the Contribution of Zinc to Toxicity

Hepatic levels of the essential micronutrient, zinc, are diminished by several hepatotoxicants, and the dietary supplementation of zinc has proven protective in those cases. 3,3',4,4',5-Pentachlorobiphenyl (PCB126), a liver toxicant, alters hepatic nutrient homeostasis and lowers hepatic zinc levels. The current study was designed to determine the mitigative potential of dietary zinc in the toxicity associated with PCB126 and the role of zinc in that toxicity. Male Sprague-Dawley rats were divided into three dietary groups and fed diets deficient in zinc (7 ppm Zn), adequate in zinc (30 ppm Zn), and supplemented in zinc (300 ppm). The animals were maintained for 3 weeks on these diets, then given a single IP injection of vehicle or 1 or 5 μmol/kg PCB126. After 2 weeks, the animals were euthanized. Dietary zinc increased the level of ROS, the activity of CuZnSOD, and the expression of metallothionein but decreased the levels of hepatic manganese. PCB126 exposed rats exhibited classic signs of exposure, including hepatomegaly, increased hepatic lipids, increased ROS and CYP induction. Liver histology suggests some mild ameliorative properties of both zinc deficiency and zinc supplementation. Other metrics of toxicity (relative liver and thymus weights, hepatic lipids, and hepatic ROS) did not support this trend. Interestingly, the zinc supplemented high dose PCB126 group had mildly improved histology and less efficacious induction of investigated genes than did the low dose PCB126 group. Overall, decreases in zinc caused by PCB126 likely contribute little to the ongoing toxicity, and the mitigative/preventive capacity of zinc against PCB126 exposure seems limited.

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 μmol/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.

Dietary Manganese Modulates PCB126 Toxicity, Metal Status, and MnSOD in the Rat

PCB126 (3,3',4,4',5-pentachlorobiphenyl) is a potent aryl hydrocarbon receptor agonist and induces oxidative stress. Because liver manganese (Mn) levels decrease in response to PCB126, a Mn dietary study was designed to investigate the role of Mn in PCB126 toxicity. Male Sprague Dawley rats received diets containing 0, 10, or 150 ppm added Mn for 3 weeks, followed by a single ip injection of corn oil or PCB126 (5 µmol/kg body weight). After 2 weeks, Mn, Cu, Zn, and Fe levels in the heart, liver, and liver mitochondria, and Mn-containing superoxide dismutase (MnSOD) and metallothionein mRNA, MnSOD protein, and MnSOD activity were determined. Mn levels in liver, heart, and liver mitochondria were strongly decreased by the Mn-deficient diet. Small effects on Fe levels and a stepwise increase in MnSOD activity with dietary Mn were also visible. PCB126 caused profound changes in Cu (up), Zn, Fe, and Mn (down) in liver, but not in heart, and differing effects (Cu, Zn, and Fe up, Mn down) in liver mitochondria. Liver MnSOD and metallothionein mRNA levels and MnSOD protein were increased but MnSOD activity was decreased by PCB126. PCB126-induced liver enlargement was dose-dependently reduced with increasing dietary Mn. These changes in metals homeostasis and MnSOD activity in liver but not heart may be a/the mechanism of PCB126 liver-specific toxicity. Specifically, transport of Fenton metals (Cu, Fe) into and Mn out of the mitochondria, a probable mechanism for lower MnSOD activity, may be a/the cause of PCB126-induced oxidative stress. The role of metallothioneins needs further evaluation. Dietary Mn slightly alleviated PCB126-induced toxicities.

Progression of micronutrient alteration and hepatotoxicity following acute PCB126 exposure

Polychlorinated Biphenyls (PCBs) are industrial chemicals that have become a persistent threat to human health due to ongoing exposure. A subset of PCBs, known as dioxin-like PCBs, pose a special threat given their potent hepatic effects. Micronutrients, especially Cu, Zn and Se, homeostatic dysfunction is commonly seen after exposure to dioxin-like PCBs. This study investigates whether micronutrient alteration is the byproduct of the ongoing hepatotoxicity, marked by lipid accumulation, or a concurrent, yet independent event of hepatic damage. A time course study was carried out using male Sprague-Dawley rats with treatments of PCB126, the prototypical dioxin-like PCB, resulting in 6 different time points. Animals were fed a purified diet, based on AIN-93G, for three weeks to ensure micronutrient equilibration. A single IP injection of either tocopherol-stripped soy oil vehicle (5 mL/kg) or 5 μmol/kg PCB126 dose in vehicle was given at various time points resulting in exposures of 9h, 18 h, 36 h, 3 days, 6 days, and 12 days. Mild hepatic vacuolar change was seen as early as 36 h with drastic changes at the later time points, 6 and 12 days. Micronutrient alterations, specifically Cu, Zn, and Se, were not seen until after day 3 and only observed in the liver. No alterations were seen in the duodenum, suggesting that absorption and excretion may not be involved. Micronutrient alterations occur with ROS formation, lipid accumulation, and hepatomegaly. To probe the mechanistic underpinnings, alteration of gene expression of several copper chaperones was investigated; only metallothionein appeared elevated. These data suggest that the disruption in micronutrient status is a result of the hepatic injury elicited by PCB126 and is mediated in part by metallothionein.

Effect of caffeic acid derivatives on polychlorinated biphenyls induced hepatotoxicity in male mice

Chronic exposure to coplanar polychlorinated biphenyls (PCBs), a potent inducer of toxic reactive oxygen species (ROS), in the environment and food can cause liver diseases. It remains unknown whether caffeic acid derivatives (CADs) exerted protective effect on PCB-induced hepatotoxicity. We sought to evaluate the activities of 3 CADs on PCB169-induced oxidative stress and DNA damage in the liver. Male ICR mice were administered with 1 μmol/mL PCB169 at 5 mL/kg body weight for 2 weeks. The mice were given CADs by gastric gavage for 3 weeks. We found that PCB169 decreased the growth rate and reduced the levels of superoxide dismutase (SOD), glutathione (GSH) and GSH peroxidase (GPx). It increased the liver weight, malondialdehyde (MDA) and 8-hydroxy-2′-deoxyguanosine (8-OHdG) levels and CYP1A1 activity in the liver tissues and plasma of mice (P<0.05). Pretreatment of mice with CADs restored the above parameters to normal levels. There was a synergistic protective effect between CADs in preventing MDA and 8-OHdG formation and inducing CYP1A1 and phase II metabolism enzyme (SOD, GPx) activities (P<0.05). In conclusion, PCB169 induced hepatotoxicity and pretreatment with CADs had synergistic protective effects on liver damage.

Dietary antioxidants (selenium and N-acetylcysteine) modulate paraoxonase 1 (PON1) in PCB 126-exposed rats

Environmental pollutants polychlorinated biphenyls (PCBs), especially dioxin-like PCBs, cause oxidative stress and associated toxic effects, including cancer and possibly atherosclerosis. We previously reported that PCB 126, the most potent dioxin-like PCB congener, not only decreases antioxidants such as hepatic selenium (Se), Se-dependent glutathione peroxidase, and glutathione (GSH) but also increases levels of the antiatherosclerosis enzyme paraoxonase 1 (PON1) in liver and serum. To probe the interconnection of these three antioxidant systems, Se, GSH, and PON1, we examined the influence of varying levels of dietary Se and N-acetylcysteine (NAC), a scavenger of reactive oxygen species (ROS) and precursor for GSH synthesis, on PON1 in the absence and presence of PCB 126 exposure. Male Sprague-Dawley rats, fed diets with differing Se levels (0.02, 0.2, or 2 ppm) or NAC (1%), were treated with a single intraperitoneal injection of corn oil or various doses of PCB 126 and euthanized 2 weeks later. PCB 126 significantly increased liver PON1 mRNA, protein level and activity, and serum PON1 activity in all dietary groups but did not consistently increase thiobarbituric acid levels (thiobarbituric acid reactive substances, TBARS), an indicator of lipid oxidation and oxidative stress, in liver or serum. Inadequate (high or low) dietary Se decreased baseline and PCB 126-induced aryl hydrocarbon receptor (AhR) expression but further increased PCB 126-induced cytochrome P450 1A1 (CYP1A1) expression, the enzyme believed to be the cause for PCB 126-induced oxidative stress. In addition, a significant inverse relationship was observed not only between dietary Se levels and PON1 mRNA and PON1 activity but also with TBARS levels in the liver, suggesting significant antioxidant protection from dietary Se. NAC lowered serum baseline TBARS levels in controls and increased serum PON1 activity but lowered liver PON1 activities in animals treated with 1 μmol/kg PCB 126, suggesting antioxidant activity by NAC primarily in serum. These results also show an unexpected predominantly inverse relationship between Se or NAC and PON1 during control and PCB 126 exposure conditions. These interactions should be further explored in the development of dietary protection regimens.

The effects of 3,3',4,4'-tetrabromobiphenyl on rats fed diets containing a constant level of copper and varying levels of molybdenum

Copper (Cu) metabolism is altered in rats fed diets high in molybdenum (Mo) and low in Cu. This 10-week study was carried out to examine the effects of supplemental Mo (7.5-240 μg/g diet) on male Sprague-Dawley rats fed diets adequate in Cu (5 μg/g diet) and to determine the susceptibility of Mo-treated animals to the environmental pollutant 3,3',4,4'-tetrabromobiphenyl (TBB). After 7 weeks of dietary treatment, half of the rats in each group received a single IP injection of TBB (150 μM/kg bw), while the other half received the corn oil vehicle. Rats sacrificed at 10 weeks showed no effects of Mo on growth, feed efficiency, or selected organ or tissue weights. Dose-dependent effects on plasma Mo (0-5.1 μg/mL), plasma Cu (0.95-0.20 μg/mL), and bone Cu (3.4-10 μg/g) in control through the high dose were found. Cu sequestration in the bone of Mo-treated rats is a new finding. TBB treatment resulted in dramatic weight loss and loss of absolute organ mass. Relative organ weights were increased, except for the thymus. TBB altered the concentrations of certain amino acids. Compared to control rats, this polybrominated biphenyl congener significantly decreased plasma Cu and ceruloplasmin at higher concentrations of dietary Mo and promoted the process of plasma Cu decrease by Mo, suggesting a combined effect.

Sustained expression of CYPs and DNA adduct accumulation with continuous exposure to PCB126 and PCB153 through a new delivery method: Polymeric implants

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Polymeric implants successfully achieved continuous exposure to PCBs in rats.

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PCB126 resulted in significant oxidative DNA damage (8-oxodG) in rat liver and lung.

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PCB126 or in combination with PCB153 induced PON1 and its activity in the liver.

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The induction was even greater for PON3 and AhR gene transcription.

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Co-treatment reduced mammary PCB153 and increased liver PCB126 and PCB153 levels.

A new delivery method via polymeric implants was used for continuous exposure to PCBs. Female Sprague-Dawley rats received subcutaneous polymeric implants containing PCB126 (0.15% load), PCB153 (5% load), or both, for up to 45 d and release kinetics and tissue distribution were measured. PCB153 tissue levels on day 15 were readily detected in lung, liver, mammary and serum, with highest levels in the mammary tissue. PCB126 was detected only in liver and mammary tissues. However, a completely different pharmacokinetics was observed on co-exposure of PCB153 and PCB126, with a 1.8-fold higher levels of PCB153 in the liver whereas a 1.7-fold lower levels in the mammary tissue. PCB126 and PCB153 caused an increase in expression of key PCB-inducible enzymes, CYP 1A1/2 and 2B1/2, respectively. Serum and liver activities of the antioxidant enzymes, PON1 and PON3, and AhR transcription were also significantly increased by PCB126. ³²P-postlabeling for polar and lipophilic DNA-adducts showed significant quantitative differences: PCB126 increased 8-oxodG, an oxidative DNA lesion, in liver and lung tissues. Adduct levels in the liver remained upregulated up to 45 d, while some lung DNA adducts declined. This is the first demonstration that continuous low-dose exposure to PCBs via implants can produce sustained tissue levels leading to the accumulation of DNA-adducts in target tissue and induction of indicator enzymes. Collectively, these data demonstrate that this exposure model is a promising tool for long-term exposure studies.

Biological and tumor-promoting effects of dioxin-like and non-dioxin-like polychlorinated biphenyls in mouse liver after single or combined treatment

To assess the impact of a mixture containing dioxin-like and non-dioxin-like polychlorinated biphenyls (PCBs), male mice were initiated with N-nitroso-diethylamine and subsequently treated with PCB126, an Ah-Receptor agonist, and PCB153, acting via activation of the constitutive androstane receptor. The two congeners were given at two dose levels: the low dose was adjusted to induce ~150-fold increases in cytochrome P450 (Cyp)1a1 (PCB126) and Cyp2b10 mRNAs (PCB153), and the high dose was chosen as twice the low dose. To keep the liver PCB levels constant, mice were given initial loading doses followed by weekly maintenance doses calculated on the basis of the PCBs' half-lives. Mice were treated with the individual congeners (low and high dose) or with a mixture consisting of the low doses of the 2 PCBs. The following results were obtained: (1) the 2 PCBs produced dose-dependent increases in Cyp1a1 and Cyp2b10 mRNA, protein, and activity when given individually; (2) combined treatment caused more than additive effects on Cyp1a1 mRNA expression, protein level, and ethoxyresurofin activity; (3) changes in the levels of several proteins were detected by proteome analysis in livers of PCB-treated mice; (4) besides these biological responses, the individual PCBs caused no significant increase in the number of glucose-6-phospatase (G6Pase)-deficient neoplastic lesions in liver, whereas a moderate significant effect occurred in the combination group. These results suggest weak but significant response-additive effects of the 2 PCBs when given in combination. They also suggest that the Cyp biomarkers tend to overestimate the carcinogenic response produced by the PCBs in mouse liver.

Does dietary copper supplementation enhance or diminish PCB126 toxicity in the rodent liver?

Copper is essential for the function of the mitochondrial electron transport chain and several antioxidant proteins. However, in its free form copper can participate in Fenton-like reactions that produce reactive hydroxyl radicals. Aryl-hydrocarbon receptor (AhR) agonists, including the most potent polychlorinated biphenyl (PCB) congener, 3,3',4,4',5-pentachlorobiphenyl (PCB126), increase copper levels in rodent livers. This is accompanied by biochemical and toxic changes. To assess the involvement of copper in PCB toxicity, male Sprague-Dawley rats were fed an AIN-93G diet with differing dietary copper levels: low (2 ppm), adequate (6 ppm), and high (10 ppm). After three weeks, rats from each group were given a single ip injection of corn oil (control), 1, or 5 μmol/kg body weight PCB126. Two weeks following injections, biochemical and morphological markers of hepatic toxicity, trace metal status, and hepatic gene expression of metalloproteins were evaluated. Increasing dietary copper was associated with elevated tissue levels of copper and ceruloplasmin. In the livers of PCB126-treated rats, the hallmark signs of AhR activation were present, including increased cytochrome P450 and lipid levels and decreased glutathione. In addition, a doubling of hepatic copper levels was seen, and overall metal homeostasis was disturbed, resulting in decreased hepatic selenium, manganese, zinc, and iron. Expression of key metalloproteins was either decreased (cytochrome c oxidase), unchanged (ceruloplasmin and CuZnSOD), or increased (tyrosinase and metallothioneins 1 and 2) with exposure to PCB126. Increases in metallothionein may contribute/reflect the increased copper seen. Alterations in dietary copper did not amplify or abrogate the hepatic toxicity of PCB126. PCB126 toxicity, i.e., oxidative stress and steatosis, is clearly associated with disturbed metal homeostasis. Understanding the mechanisms of this disturbance may provide tools to prevent liver toxicity by other AhR agonists.

2,2',3,5',6-Pentachlorobiphenyl (PCB 95) and its hydroxylated metabolites are enantiomerically enriched in female mice

Epidemiological and laboratory studies link polychlorinated biphenyls and their metabolites to adverse neurodevelopmental outcomes. Several neurotoxic PCB congeners are chiral and undergo enantiomeric enrichment in mammalian species, which may modulate PCB developmental neurotoxicity. This study measures levels and enantiomeric enrichment of PCB 95 and its hydroxylated metabolites (OH-PCBs) in adult female C57Bl/6 mice following subchronic exposure to racemic PCB 95. Tissue levels of PCB 95 and OH-PCBs increased with increasing dose. Dose-dependent enantiomeric enrichment of PCB 95 was observed in brain and other tissues. OH-PCBs also displayed enantiomeric enrichment in blood and liver, but were not detected in adipose and brain. In light of data suggesting enantioselective effects of chiral PCBs on molecular targets linked to PCB developmental neurotoxicity, our observations highlight the importance of accounting for PCB and OH-PCB enantiomeric enrichment in the assessment of PCB developmental neurotoxicity.

N-acetylcysteine (NAC) diminishes the severity of PCB 126-induced fatty liver in male rodents

Potent aryl hydrocarbon receptor agonists like PCB 126 (3,3',4,4',5-pentachlorobiphenyl) cause oxidative stress and liver pathology, including fatty liver. Our question was whether dietary supplementation with N-acetylcysteine (NAC), an antioxidant, can prevent these adverse changes. Male Sprague-Dawley rats were fed a standard AIN-93G diet (sufficient in cysteine) or a modified diet supplemented with 1.0% NAC. After one week, rats on each diet were exposed to 0, 1, or 5μmol/kg body weight PCB 126 by i.p. injection (6 rats per group) and euthanized two weeks later. PCB-treatment caused a dose-dependent reduction in growth, feed consumption, relative thymus weight, total glutathione and glutathione disulfide (GSSG), while relative liver weight, glutathione transferase activity and hepatic lipid content were dose-dependently increased with PCB dose. Histologic examination of liver tissue showed PCB 126-induced hepatocellular steatosis with dose dependent increase in lipid deposition and distribution. Dietary NAC resulted in a reduction in hepatocellular lipid in both PCB groups. This effect was confirmed by gravimetric analysis of extracted lipids. Expression of CD36, a scavenger receptor involved in regulating hepatic fatty acid uptake, was reduced with high dose PCB treatment but unaltered in PCB-treated rats on NAC-supplemented diet. These results demonstrate that NAC has a protective effect against hepatic lipid accumulation in rats exposed to PCB 126. The mechanism of this protective effect appears to be independent of NAC as a source of cysteine/precursor of glutathione.

Glutathione transferase pi class 2 (GSTp2) protects against the cardiac deformities caused by exposure to PAHs but not PCB-126 in zebrafish embryos

Glutathione transferases (GSTs) are phase II enzymes that detoxify a wide range of toxicants and reactive intermediates. One such class of toxicants is the ubiquitous polycyclic aromatic hydrocarbons (PAHs). Certain PAHs are known to cause developmental cardiac toxicity in fish. Herein, we explored the role of GST pi class 2 (GSTp2) in PAH- and PCB-induced cardiac toxicity in zebrafish (Danio rerio) embryos. We measured expression of GSTp2 in embryos exposed to individual and co-exposures of the PAHs benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), and fluoranthene (FL) as well as 3,3',4,4',5-pentachlorobiphenyl (PCB-126). GSTp2 mRNA expression was induced by exposure to BkF, BaP, PCB-126, and BaP+FL and BkF+FL co-exposure. A splice junction morpholino was then used to knockdown GSTp2 in developing zebrafish. GSTp2 knockdown exacerbated the toxicity caused by co-exposures to BkF+FL and BaP+FL. However, GSTp2 knockdown did not affect PCB-126 toxicity. These results further suggest that pi class GSTs serve a protective function against the synergistic toxicity caused by PAHs in developing zebrafish.