Effects on thyroid hormone metabolism and depletion of lung vitamin A in rats by airborne particulate matter

Metribuzin-induced non-adverse liver changes result in rodent-specific non-adverse thyroid effects via uridine 5'-diphospho-glucuronosyltransferase (UDPGT, UGT) modulation

Metribuzin is a herbicide that inhibits photosynthesis and has been used for over 40 years. Its main target organ is the liver and to some extent the kidney in rats, dogs, and rabbits. Metribuzin shows a specific thyroxine (T4) profile in rat studies with T4 increases at low doses and T4 decreases at higher doses. Only the T4 decreases occur together with histopathological changes in the thyroid and weight changes of liver and thyroid. A set of experiments was conducted to investigate metribuzin's endocrine disruptor potential according to European guidance and regulations. The results indicate that a liver enzyme modulation, i.e. of the uridine 5'-diphospho-glucuronosyltransferase (UDPGT, UGT), is most likely responsible for both increased and decreased plasma thyroxine level and for thyroid histopathological observations. Animals with high T4 levels show low UGT activity, while animals with low T4 levels show high UGT activity. A causal relationship was inferred, since other potentially human-relevant mode of action (MOA) pathways were excluded in dedicated studies, i.e. inhibition of deiodinases (DIO), inhibition of thyroid peroxidase (TPO) or of the sodium importer system (NIS). This liver metabolism-associated MOA is considered not relevant for human hazard assessment, due to species differences in thyroid homeostasis between humans and rats and, more importantly, based on experimental data showing that metribuzin affects UGT activity in rat but not in human hepatocytes. Further, we discuss whether or not increased T4 levels in the rat, in the absence of histopathological changes, should be considered as adverse and therefore used as an appropriate hazard model for humans. Based on a weight of evidence approach, metribuzin should not be classified as an endocrine disruptor with regard to the thyroid modality.

Effects of cigarette smoke exposure during suckling on food intake, fat mass, hormones, and biochemical profile of young and adult female rats

PURPOSE

Children from smoking mothers have a higher risk of developing obesity and associated comorbidities later in life. Different experimental models have been used to assess the mechanisms involved with this increased risk. Using a rat model of neonatal nicotine exposure via implantation of osmotic minipumps in lactating dams, we have previously shown marked sexual dimorphisms regarding metabolic and endocrine outcomes in the adult progeny. Considering that more than four thousand substances are found in tobacco smoke besides nicotine, we then studied a rat model of neonatal tobacco smoke exposure: adult male offspring had hyperphagia, obesity, hyperglycemia, hypertriglyceridemia, secondary hyperthyroidism and lower adrenal hormones. Since litters were culled to include only males and since sexual dimorphisms had already been identified in the nicotine exposure model, here we also evaluated the effects of tobacco smoke exposure during lactation on females.

METHODS

Wistar rat dams and their pups were separated into two groups of 8 litters each: SMOKE (4 cigarettes per day, from postnatal day 3 to 21) and CONTROL (filtered air). Offspring of both sexes were euthanized at PN21 and PN180.

RESULTS

Changes in male offspring corroborated previous data. At weaning, females showed lower body mass gain and serum triglycerides, but no alterations in visceral fat and hormones. At adulthood, females had higher body mass, hyperphagia, central obesity, hyperleptinemia, hypercholesterolemia, hypercorticosteronemia, but no change in serum TSH and T3, and adrenal catecholamine CONCLUSIONS: Sexual dimorphisms were observed in several parameters, thus indicating that metabolic and hormonal changes due to smoke exposure during development are sex-dependent.

Vitamin A depletion induced by cigarette smoke is associated with an increase in lung cancer-related markers in rats

BACKGROUND

We have previously demonstrated that cigarette smoke is associated with a significant reduction of retinoic acid in rat lungs and the formation of tracheal precancerous lesions. However, the underlying mechanism of cancer risk induced by vitamin A deficiency is unclear. The purpose of this study was to determine whether the cigarette smoke-induced depletion of vitamin A is related to changes in lung cancer risk-related molecular markers.

RESULTS

We investigated the roles of the retinoic acid receptors (RARs) as well as other biomarkers for potential cancer risk in the lungs of rats exposed to cigarette smoke. Twenty-four male weanling rats were fed a purified diet and divided equally into four groups. Three experimental groups were exposed to increasing doses of cigarette smoke from 20, 40 or 60 commercial cigarettes/day for 5 days/week. After 6 weeks, the retinoic acid concentrations in the lung tissue as measured via high performance liquid chromatography (HPLC) significantly decreased (P < 0.01) in cigarette smoke exposed groups. Western Blot analysis revealed that cigarette smoke exposure increased lung protein expression of RAR α in a threshold manner and decreased RAR β and RAR γ expression in a dose-dependent fashion. Protein expressions of cyclin E and proliferating cell nuclear antigen (PCNA) were increased significantly in a dose-dependent manner in cigarette smoke exposed-groups. Additionally, there was a significant increase in protein expression of cJun and cyclin D1 demonstrating a threshold effect similar to that exhibited by RARα, suggesting a potential independent signaling pathway for RARα in lung carcinogenesis.

CONCLUSIONS

Findings from this study suggest that cigarette smoke-induced lung retinoic acid depletion may involve two independent pathways, RARα- and RARβ-mediated, responsible for the increased cancer risk associated with cigarette smoke-induced vitamin A deficiency.

Effects of outdoor and indoor airborne particulate matter on thyroid hormone and vitamin A metabolism

Vitamin A is an important regulator of normal epithelial differentiation and proliferation and might act in the promotion phase of carcinogenesis. Vitamin A and thyroid hormone metabolism are linked by a common plasma carrier protein transthyretin (TTR). Results indicated that extracts of outdoor and indoor airborne particulate matter (APM), originating from different pollution sources, significantly interfered with thyroxine (T4) binding to TTR. The neutral fraction accounted for most of the inhibitory activity. Polycyclic aromatic hydrocarbons and nitrated derivatives were not responsible for the activity of the neural fraction. A single treatment of rats with an outdoor and cigarette smoke APM extract depleted plasma T4 and triiodothyronine levels and increased plasma retinol levels, while liver and lung retinol levels were depleted. The studies show that APM extracts have the potency to interfere with thyroid hormone metabolism both in vitro and in vivo and to deplete lung vitamin A in vivo.

Inhibition of gap-junctional intercellular communication by outdoor and indoor airborne particulate matter

The inhibition of gap-junctional intercellular communication (GJIC) by different airborne particulate matter (APM) extracts was tested in V79 cells and in primary cultures of alveolar type II cells. The results show that extracts of APM were able to inhibit GJIC in V79 and in alveolar type II cells at non-cytotoxic doses. Chemical fractionation of extracts showed that the neutral fractions accounted for most of the inhibitory activity on GJIC. The activities of basic and acid fractions was less than 5% of the total activity. Polycyclic aromatic hydrocarbons and nitrated derivatives are unlikely to be responsible for the activity of the neutral fractions, because several representatives of these compounds failed to inhibit GJIC. Taken together, these results suggest that extracts of APM, in addition to the genotoxic activity that has been known for many years, also have a tumor promoting activity.