Concentration and distribution of dioxins and related compounds in human tissues

Persistent organic pollutants disrupt the oxidant/antioxidant balance of INS-1E pancreatic β-cells causing their physiological dysfunctions

BACKGROUND

Persistent organic pollutants (POPs) have emerged as potent diabetogenic agents, but their mechanisms of action remain poorly identified.

OBJECTIVES

In this study, we aim to determine the mechanisms regulating the damaging effects of POPs in pancreatic β-cells, which have a central role in the development of diabetes.

METHODS

We treated INS-1E pancreatic β-cells with PCB-153, p,p'-DDE, PCB-126, or TCDD at doses ranging from 1 × 10-15to 5 × 10-6M. We measured insulin content and secretion, cell viability and assessed the mRNA expression of the xenobiotic nuclear receptors Nr1i2 and Nr1i3, and the aryl hydrocarbon receptor (Ahr). In addition, we evaluated the antioxidant defense and production of reactive oxygen species (ROS). Finally, we studied the ability of the antioxidant N-acetyl-L-cysteine (NAC) to counteract the effects of POPs in INS-1E cells.

RESULTS

When exposed to environmental POP levels, INS-1E cells had impaired production and secretion of insulin. These defects were observed for all tested POPs and were paralleled by reduced Ins1 and Ins2 mRNA expression. While POP treatment for 3 days did not affect INS-1E cell viability, longer treatment progressively killed the cells. Furthermore, we found that the xenobiotic detoxification machinery is poorly expressed in the INS-1E cells, as characterized by the absence of Nr1i2 and Nr1i3 and their respective downstream targets Cyp3a1/Cyp3a2 and Cyp2b1/Cyp2b3, and the weak functionality of the Ahr/Cyp1a1 signaling. Interestingly, POPs dysregulated key antioxidant enzymes such as glutathione peroxidases, peroxiredoxins, thioredoxins, and catalases. In parallel, the production of intracellular ROS, including superoxide anion (O2•-) and hydrogen peroxide (H2O2), was increased by POP exposure. Improving the oxidant scavenging capacity of INS-1E cells by NAC treatment restored the production and secretion of insulin.

CONCLUSION

By promoting oxidative stress and impairing the ability of INS-1E cells to produce and secrete insulin, this study reveals how POPs can mechanistically act as diabetogenic agents, and provides new scientific evidence supporting the concept that POPs are fueling the diabetes epidemics.

A link between energy metabolism and plant host adaptation states in the two-spotted spider mite, Tetranychus urticae (Koch)

Energy metabolism is a highly conserved process that balances generation of cellular energy and maintenance of redox homeostasis. It consists of five interconnected pathways: glycolysis, tricarboxylic acid cycle, pentose phosphate, trans-sulfuration, and NAD+ biosynthesis pathways. Environmental stress rewires cellular energy metabolism. Type-2 diabetes is a well-studied energy metabolism rewiring state in human pancreatic β-cells where glucose metabolism is uncoupled from insulin secretion. The two-spotted spider mite, Tetranychus urticae (Koch), exhibits a remarkable ability to adapt to environmental stress. Upon transfer to unfavourable plant hosts, mites experience extreme xenobiotic stress that dramatically affects their survivorship and fecundity. However, within 25 generations, mites adapt to the xenobiotic stress and restore their fitness. Mites' ability to withstand long-term xenobiotic stress raises a question of their energy metabolism states during host adaptation. Here, we compared the transcriptional responses of five energy metabolism pathways between host-adapted and non-adapted mites while using responses in human pancreatic islet donors to model these pathways under stress. We found that non-adapted mites and human pancreatic β-cells responded in a similar manner to host plant transfer and diabetogenic stress respectively, where redox homeostasis maintenance was favoured over energy generation. Remarkably, we found that upon host-adaptation, mite energy metabolic states were restored to normal. These findings suggest that genes involved in energy metabolism can serve as molecular markers for mite host-adaptation.

Persistent organic pollutants and β-cell toxicity: a comprehensive review

Persistent organic pollutants (POPs) are a diverse family of contaminants that show widespread global dispersion and bioaccumulation. Humans are continuously exposed to POPs through diet, air particles, and household and commercial products; POPs are consistently detected in human tissues, including the pancreas. Epidemiological studies show a modest but consistent correlation between exposure to POPs and increased diabetes risk. The goal of this review is to provide an overview of epidemiological evidence and an in-depth evaluation of the in vivo and in vitro evidence that POPs cause β-cell toxicity. We review evidence for six classes of POPs: dioxins, polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), organophosphate pesticides (OPPs), flame retardants, and per- and polyfluoroalkyl substances (PFAS). The available data provide convincing evidence implicating POPs as a contributing factor driving impaired glucose homeostasis, β-cell dysfunction, and altered metabolic and oxidative stress pathways in islets. These findings support epidemiological data showing that POPs increase diabetes risk and emphasize the need to consider the endocrine pancreas in toxicity assessments. Our review also highlights significant gaps in the literature assessing islet-specific endpoints after both in vivo and in vitro POP exposure. In addition, most rodent studies do not consider the impact of biological sex or secondary metabolic stressors in mediating the effects of POPs on glucose homeostasis and β-cell function. We discuss key gaps and limitations that should be assessed in future studies.

Polychlorinated dibenzo-dioxins and polychlorinated dibenzo-furans exposure and altered lung function: The mediating role of oxidative stress

The lung has been reported to be one of the target organs of polychlorinated dibenzo-dioxins and polychlorinated dibenzo-furans (PCDD/Fs) in many toxicological studies. While the associations between PCDD/Fs exposure and lung function levels have not been investigated thoroughly. This study aimed to explore these associations and the potential mediating role of oxidative stress. In this study, 201 foundry workers and 222 non-exposed general residents were recruited from central China, and their lung function parameters were measured. Air and food samples were collected to determine the PCDD/Fs levels for individual PCDD/Fs exposure estimation. Serum PCDD/Fs levels were determined in a subgroup of individuals randomly selected from the study population to reflect the body burden. It was found that each 1-unit increase in ln-transformed concentration of PCDD/Fs exposure (fg TEQ/bw/day) was associated with a 0.47 L decrease in FVC and a 0.25 L decrease in FEV₁. Each 1-unit increase in ln-transformed concentration of serum PCDD/Fs (fg TEQ/g lipid) was associated with a 0.36 L decrease in FVC and a 0.24 L decrease in FEV₁. Urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) was not only positively related to PCDD/Fs exposure, but also inversely associated with FVC and FEV₁ are FVC (β = -0.15, 95% CI: -0.22 to -0.08) and FEV₁ (β = -0.07, 95% CI: -0.13 to -0.02). Mediation analysis revealed that urinary 8-OHdG mediated 12.22% of the associations of external PCDD/Fs exposure with FVC levels, 28.61% and 27.87% of the associations of serum PCDD/Fs with FVC and FEV₁ levels respectively. Our findings suggested that PCDD/Fs exposure was associated with decreased lung function levels by a mechanism partly involving oxidatively generated damage to DNA.

Concentrations of dioxins and furans in breast milk of women living near a hazardous waste incinerator in Catalonia, Spain

Since 1999, a hazardous waste incinerator (HWI) is operating in Constantí (Tarragona County, Catalonia, Spain). In 1996-1998, when the facility was being built, we started a monitoring program aimed at evaluating the impact of the emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) by the HWI on the environment and the human health. This study aimed at determining the current concentrations of PCDD/Fs in breast milk samples of women living nearby the HWI and at comparing these levels and profiles with those obtained in the baseline (1996-1998) and previous surveys (2002, 2007 and 2012). Furthermore, the association between the dietary intake of PCDD/Fs and the congener profiles in breast milk was also explored. Twenty milk samples were collected from women living in Tarragona downtown and near the industrial area where the HWI is placed. The content of PCDD/Fs was analyzed by following a procedure derived from the US EPA methods 1613 and 8290A. The mean concentration of PCDD/Fs was 2.26 pg WHO-TEQ/g fat. No significant differences were found between women living in industrial and urban areas (1.67 pg vs. 2.48 pg WHO-TEQ/g fat). Interestingly, a notable significant reduction (81%) was observed with respect to the concentrations found in the baseline study. The profiles of PCDD/Fs in breast milk were similar independently on the area and period of collection, being mainly influenced by the intake of fish, meat, oils and fats. The decreasing trend of PCDD/Fs in human milk agrees with the reduction observed in the dietary intake of these pollutants. It was also concluded that the current levels of PCDD/Fs in human milk levels are not influenced by the HWI stack emissions.

Retinoid-xenobiotic interactions: the Ying and the Yang

The literature provides compelling evidence pointing to tight metabolic interactions between retinoids and xenobiotics. These are extensive and important for understanding xenobiotic actions in the body. Within the body, retinoids affect xenobiotic metabolism and actions and conversely, xenobiotics affect retinoid metabolism and actions. This article summarizes data that establish the importance of retinoid-dependent metabolic pathways for sustaining the body's responses to xenobiotic exposure, including the roles of all-trans- and 9-cis-retinoic acid for protecting mammals from harmful xenobiotic effects and for ensuring xenobiotic elimination from the body. This review will also consider molecular mechanisms underlying xenobiotic toxicity focusing on how this may contribute to retinoid deficiency and disruption of normal retinoid homeostasis. Special attention is paid to xenobiotic molecular targets (nuclear receptors, regulatory proteins, enzymes, and transporters) which affect retinoid metabolism and signaling.

Polychlorinated biphenyls in serum, liver and adipose tissue of subjects with hepatocellular carcinoma living in a highly polluted area

Brescia (North Italy) is located in a highly industrialized area with diffuse environmental pollution by polychlorinated biphenyls (PCBs) and with a high incidence of hepatocellular carcinoma (HCC). We aimed to investigate the concentration of PCBs congeners in serum, liver and adipose tissue of patients with HCC living in this area. We enrolled prospectively 101 HCC incident cases. Lipid-adjusted PCBs concentration of single congeners and total PCBs concentration, computed as the sum of 24 PCBs congeners, were measured in the serum of all subjects and in the liver and abdominal fat of 59 HCC cases undergoing surgery. A high correlation was found between PCBs serum and liver (Spearman r=0.79), serum and fat (r=0.91), and liver and fat (r=0.75) concentrations. Serum PCBs concentration in HCC patients did not vary according to demographical, clinical or epidemiological variables, apart from age, as PCBs concentration increased linearly with the subject's age. This study confirms that serum lipid-adjusted PCBs concentration is a valid measure of PCBs body storage and is highly correlated with PCBs concentration detected in other biological samples.

Toxicokinetics of dioxins and other organochlorine compounds in Japanese people: association with hepatic CYP1A2 expression levels

Concentrations of persistent organochlorine compounds (OCs) including polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs) in the liver and adipose tissue of Japanese cadavers were measured, and their toxicokinetics were examined in association with hepatic cytochrome P450 (CYP) 1A protein expression levels. Total 2,3,7,8-tetrachlorodibenzo-p-dioxin toxic equivalents (TEQs) were 66±74 and 65±57 pg/g lipid weight (mean±S.D.) in the liver and adipose tissue, respectively. Total PCBs (sum of 62 congeners targeted), p,p'-dichlorodiphenyl-dichloroethylene (p,p'-DDE) and β-hexachlorocyclohexane (β-HCH) were detected at concentrations over 1 μg/g lipid in both tissues of some specimens. For most of the dioxin-like congeners, total PCBs, p,p'-DDE, oxychlordane, α- and β-HCH, hexachlorobenzene (HCB), and tris(4-chlorophenyl)methane (TCPMe), age-dependent increases in concentrations were found in the adipose tissue of males. No such age-dependent trend was observed in the liver, suggesting that there are different mechanisms underlying the hepatic concentrations of OCs. Immunoblot analyses indicated detectable expression of hepatic CYP1A2 protein, whereas no CYP1A1 protein was detected. The CYP1A2 expression levels were positively correlated with concentrations (on wet weight basis) of 2,3,4,7,8-P₅CDF, the dominant TEQ-contributed congeners in the liver, indicating the induction of this CYP. Hepatic CYP1A2 protein levels were strongly correlated with the liver to adipose concentration (L/A) ratios of PCDD/F congeners with more than 5 chlorine atoms. Together with higher concentrations of the congeners in the liver than in the adipose tissue, the observation on L/A ratios of highly chlorinated PCDD/Fs suggests that induced hepatic CYP1A2 protein is involved in their sequestration in this human population, as observed in model animals (rodents). Nonetheless, the magnitude of hepatic sequestration (L/A ratio) of PCDD/Fs in this human population was lower than in other mammals and birds, reported previously. This study emphasizes the fact that toxicokinetics of some OCs can be affected at least partly by CYP1A2 protein levels in humans. For the extrapolation of their toxicokinetics from model animals to humans, knowledge on the induction and sequestration potencies of CYP1A is necessary.

PCDD/F and PCB concentrations in sera from the Canadian Health Measures Survey (CHMS) from 2007 to 2009

In order to establish a national baseline estimate of the concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs) in Canadians, pooling of individual human sera was performed to ensure that a high frequency of detectable concentrations of analytes would be achieved. Nearly 5000 (4583) sera samples from Cycle 1 of the Canadian Health Measures Survey (CHMS) collected between 2007 and 2009 were used to form 59 composite pools of approximately 25 mL each. Pools were categorized by sex and age with participants ranging from 6 to 79 years. The pooled samples were analysed for 17 PCDD/Fs and 36 PCB congeners, and from these data, total toxic equivalent concentrations (TEQ(2005 PCDD/F+Dioxin-like [DL]-PCB)) were estimated. The average 2,3,7,8 tetrachlorodibenzodioxin (TCDD) concentration was <1 pg g⁻¹ on a lipid extractable basis. The average total TEQ(2005 PCDD/F+DL-PCB) was 11 pg TEQ g⁻¹ lipid and average ΣPCB concentrations were about 100 ng g⁻¹ lipid. Sex did not affect the concentrations, while PCB and PCDD/F concentrations were positively correlated with age (p<0.001). It appeared in some cases that the age group 6-11 years had higher concentrations of these persistent organic pollutants (POPs) than the concentrations observed in 12-19 year olds, however, the results were not statistically significant based on pair-wise comparisons. Concentration levels and patterns observed in this study of Canadians were similar to those reported in the US and European populations.

Polychlorinated biphenyls in bile of patients with biliary tract cancer

BACKGROUND

Polychlorinated biphenyls (PCBs) are anthropogenic, organic compounds. Although banned in the 1970s, PCBs are poorly biodegradable and hence ubiquitous in the environment. They accumulate in adipose tissue and are implicated various malignancies, including breast and pancreatic cancer. The hepatobiliary system is the main excretory route for such xenobiotic toxins. Incidence rates of intrahepatic biliary tract cancer are increasing worldwide. Measurement and comparison of PCB levels in bile from human patients with benign and malignant bile duct disease has not previously been done.

OBJECTIVES

To compare PCB concentrations in bile from patients with malignant (n=8) and non-malignant (n=7) biliary disease.

METHODS AND RESULTS

Fifteen human bile samples, collected endoscopically, were analysed using gas chromatography mass spectrometry for seven target PCB congeners (28, 52, 101, 118, 153, 138, and 180), known to occur in the environment and food. Amongst males, total PCB concentrations in bile ranged from 6 ng mL(-1) (aged 73 years) to 49 ng mL(-1) (aged 90 years); and in females between 8 ng mL(-1) (aged 33 years) to 43 ng mL(-1) (aged 67 years) bile. Although there was no overall difference in mean PCB levels between non-cancer and cancer patients, levels of congener 28 were significantly higher in patients with biliary tract cancer (p<0.05).

CONCLUSIONS

Despite the banning of PCBs over 30 years ago, these xenobiotics are present in the bile of patients with biliary disease. PCB levels tend to increase with age, suggesting chronic bioaccumulation. Further research is necessary to investigate the relevance of increased levels of congener 28 in bile in biliary tract cancer.

Apparent half-lives of dioxins, furans, and polychlorinated biphenyls as a function of age, body fat, smoking status, and breast-feeding

OBJECTIVE

In this study we reviewed the half-life data in the literature for the 29 dioxin, furan, and polychlorinated biphenyl congeners named in the World Health Organization toxic equivalency factor scheme, with the aim of providing a reference value for the half-life of each congener in the human body and a method of half-life estimation that accounts for an individual's personal characteristics.

DATA SOURCES AND EXTRACTION

We compared data from >30 studies containing congener-specific elimination rates. Half-life data were extracted and compiled into a summary table. We then created a subset of these data based on defined exclusionary criteria.

DATA SYNTHESIS

We defined values for each congener that approximate the half-life in an infant and in an adult. A linear interpolation of these values was used to examine the relationship between half-life and age, percent body fat, and absolute body fat. We developed predictive equations based on these relationships and adjustments for individual characteristics.

CONCLUSIONS

The half-life of dioxins in the body can be predicted using a linear relationship with age adjusted for body fat, smoking, and breast-feeding. Data suggest an alternative method based on a linear relationship between half-life and total body fat, but this approach requires further testing and validation with individual measurements.