The Influence of Obesity on the Pharmacokinetics of Dioxin in Mice: An Assessment Using Classical and PBPK Modeling

A Physiologically Based Pharmacokinetic (PBPK) Modeling Framework for Mixtures of Dioxin-like Compounds

Humans are exposed to persistent organic pollutants, such as dioxin-like compounds (DLCs), as mixtures. Understanding and predicting the toxicokinetics and thus internal burden of major constituents of a DLC mixture is important for assessing their contributions to health risks. PBPK models, including dioxin models, traditionally focus on one or a small number of compounds; developing new or extending existing models for mixtures often requires tedious, error-prone coding work. This lack of efficiency to scale up for multi-compound exposures is a major technical barrier toward large-scale mixture PBPK simulations. Congeners in the DLC family, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), share similar albeit quantitatively different toxicokinetic and toxicodynamic properties. Taking advantage of these similarities, here we reported the development of a human PBPK modeling framework for DLC mixtures that can flexibly accommodate an arbitrary number of congeners. Adapted from existing TCDD models, our mixture model contains the blood and three diffusion-limited compartments-liver, fat, and rest of the body. Depending on the number of congeners in a mixture, varying-length vectors of ordinary differential equations (ODEs) are automatically generated to track the tissue concentrations of the congeners. Shared ODEs are used to account for common variables, including the aryl hydrocarbon receptor (AHR) and CYP1A2, to which the congeners compete for binding. Binary and multi-congener mixture simulations showed that the AHR-mediated cross-induction of CYP1A2 accelerates the sequestration and metabolism of DLC congeners, resulting in consistently lower tissue burdens than in single exposure, except for the liver. Using dietary intake data to simulate lifetime exposures to DLC mixtures, the model demonstrated that the relative contributions of individual congeners to blood or tissue toxic equivalency (TEQ) values are markedly different than those to intake TEQ. In summary, we developed a mixture PBPK modeling framework for DLCs that may be utilized upon further improvement as a quantitative tool to estimate tissue dosimetry and health risks of DLC mixtures.

Seasonal peculiarities of PCDD/Fs levels in bank voles inhabiting sites in the vicinity of the landfill with municipal wastes (Moscow, Russia)

Differences in PCDD/Fs concentrations were registered among adult bank voles (Myodes glareolus) inhabiting forest in the vicinity of the Landfill Salariyevo (Moscow, Russia) and caught in early spring (over-winter survivors) and late summer. The levels of highly toxic congeners and WHO-TEQ₀₅ in samples of wintering voles were much lower than those in samples got in summer. This difference was investigated analyzing PCDD/Fs in sexually matured animals and offspring obtained in vivarium. Practically, no significant differences of PCDD/Fs concentrations and general toxicity were revealed among sexes. The main impact to these seasonal differences in PCDD/Fs accumulation was assigned to the types of functional development and activity, diet variation as natural growth of all voles occurs on the same territory but in seasonally diverse conditions. So, the species functional ecology (lifestyle and seasonal features) should be considered in practical application of animal models from natural populations for local monitoring of PCDD/Fs body burden, and individual functional type of ontogenesis among cyclomorphic mammalian species, especially. Thus, the results obtained during monitoring should be carefully interpreted for a proper environmental management.

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.

Impact of obese levels on the hepatic expression of nuclear receptors and drug-metabolizing enzymes in adult and offspring mice

The prevalence of obesity-associated conditions raises new challenges in clinical medication. Although altered expression of drug-metabolizing enzymes (DMEs) has been shown in obesity, the impacts of obese levels (overweight, obesity, and severe obesity) on the expression of DMEs have not been elucidated. Especially, limited information is available on whether parental obese levels affect ontogenic expression of DMEs in children. Here, a high-fat diet (HFD) and three feeding durations were used to mimic different obese levels in C57BL/6 mice. The hepatic expression of five nuclear receptors (NRs) and nine DMEs was examined. In general, a trend of induced expression of NRs and DMEs (except for Cyp2c29 and 3a11) was observed in HFD groups compared to low-fat diet (LFD) groups. Differential effects of HFD on the hepatic expression of DMEs were found in adult mice at different obese levels. Family-based dietary style of an HFD altered the ontogenic expression of DMEs in the offspring older than 15 days. Furthermore, obese levels of parental mice affected the hepatic expression of DMEs in offspring. Overall, the results indicate that obese levels affected expression of the DMEs in adult individuals and that of their children. Drug dosage might need to be optimized based on the obese levels.

Release and toxicity of adipose tissue-stored TCDD: Direct evidence from a xenografted fat model

BACKGROUND

Persistent organic pollutants (POPs) are known to accumulate in adipose tissues (AT). This storage may be beneficial by diverting POPs from other sensitive tissues or detrimental because of chronic release of pollutants as indirectly suggested during weight loss. The aim is to study the biological and/or toxic effects that chronic POP release from previously contaminated grafted AT could exert in a naïve mouse.

METHODS

C57BL/6J male mice were exposed intraperitoneally to 2,3,7,8-tetrachlorodibenzo-p-doxin (TCDD); their epididymal fat pads were collected and grafted on the back skin of uncontaminated recipient mice whose brain, liver, and epididymal ATs were analyzed (TCDD concentration, relevant gene expression). Kinetics of release and redistribution were modeled using Physiologically Based PharmacoKinetics (PBPK).

RESULTS

The grafts released TCDD over a period of 10 weeks with different kinetics of distribution in the three organs studied. A PBPK model was used to simulate the AT releasing process and the incorporation of TCDD into the major organs. At three weeks post-graft, we observed significant changes in gene expression in the liver and the host AT with signatures reminiscent of inflammation, gluconeogenesis and fibrosis as compared to the control.

CONCLUSIONS

This study confirms that AT-stored TCDD can be released and distributed to the organs of the recipient hence leading to distinct changes in gene expression. This original model provides direct evidence of the potential toxic-relevant effects when endogenous sources of contamination are present.