Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in cold-adapted rats

Endocrine disrupting chemicals: Friend or foe to brown and beige adipose tissue?

The effects of Endocrine Disrupting Chemicals (EDCs) on the current obesity epidemic is a growing field of interest. Numerous EDCs have shown the potential to alter energy metabolism, which may increase the risk of obesity, in part, through direct actions on adipose tissue. While white adipose tissue has historically been the primary focus of this work, evidence of the EDC-induced disruption of brown and beige adipose tissues continues to build. Both brown and beige fat are thermogenic adipose depots rich in mitochondria that dispense heat when activated. Due to these properties, brown and beige fat are implicated in metabolic diseases such as obesity, diabetes, and cachexia. This review delves into the current literature of different EDCs, including bisphenols, dioxins, air pollutants, phthalates, and phytochemicals. The possible implications that these EDCs have on thermogenic adipose tissues are covered. This review also introduces the possibility of using brown and beige fat as a therapeutic target organ by taking advantage of some of the properties of EDCs. Collectively, we provide a comprehensive discussion of the evidence of EDC disruption in white, brown, and beige fat and highlight gaps worthy of further exploration.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Toxic equivalency factors of polychlorinated dibenzo-p-dioxins in an ovulation model: validation of the toxic equivalency concept for one aspect of endocrine disruption

Polychlorinated dibenzo-p-dioxins (PCDDs) are structural analogues, which produce a similar spectrum of biological and toxicological responses in animals, albeit with differential potencies. Very consistent structure-activity relationships have been found for acute toxicity and some biochemical effects among these compounds. For the current experiments, the gonadotropin-primed immature female rat model was used to study the effect of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7, 8-pentachlorodibenzo-p-dioxin (PeCDD), and 1,2,3,4,7, 8-hexachlorodibenzo-p-dioxin (HxCDD) on ovulation. Single doses of different PCDDs and their mixture were given orally to 23-day-old rats. Gonadotropin from pregnant mare's serum (PMSG) was injected (5 IU) 24 h later to induce follicular maturation. Rats were decapitated at various times after PMSG, blood was collected, and ovarian weight was measured. Serum concentrations of 17beta-estradiol (E2), progesterone (P4), luteinizing hormone (LH), follicle stimulating hormone (FSH), and prolactin (PrL) were determined by radioimmunoassay. Ovulation was measured at 72 h after injection of PMSG by counting ova flushed from oviducts. PCDDs dose dependently decreased the number of ova per ovary and reduced ovarian weight gain induced by PMSG. The slopes of the dose-response curves generated by individual PCDDs and/or their mixture were similar. PMSG-induced increase in serum E2 was enhanced on the day of expected ovulation by PCDDs; in contrast, serum P4 and FSH were decreased at that same time point. PCDDs also altered the temporal pattern of serum E2, FSH, and LH but not that of PrL. Histologically the effect of all three PCDDs consisted of ova trapped in preovulatory follicles and a lack of or reduced number of corpora lutea. The results indicate that the PCDDs, tested in the present model, have the same mode of action on ovulation and the reproductive hormones, e.g., LH, FSH, P4 and E2. Furthermore, the dose responses of the individual congeners are parallel to each other and also to that of their equipotent mixture, which represent a validation of the TEQ concept for one aspect of endocrine disruption, that is for inhibition of ovulation.

Alterations of hepatic acetyl-CoA carboxylase by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on acetyl-CoA carboxylase (ACC) activity and synthesis was examined. Male Wistar rats received a single i.p. injection of TCDD (53 micrograms/kg), and nine days later body weight, liver weight, hepatic lipid, ACC activity and mass were determined and compared to pair-fed controls. Body weights of TCDD-treated animals decreased, while liver weights increased resulting in an increase in liver to body weight ratios. ACC activity was decreased by 65%, however sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western analysis using a biotin specific probe revealed that ACC protein levels were not appreciably changed. In addition, there was a large increase in exogenous lipid material in TCDD-treated livers as determined by osmium tetroxide staining. These data suggest that the decrease in ACC activity may be due to direct inhibition of the enzyme by negative allosteric interactions with free fatty acids released from adipose tissue that subsequently accumulate in liver.

Increased copper concentrations in rat tissues after acute intoxication with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Recently, acutely toxic doses of the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been reported to affect the hepatic distribution of essential metals in the rat. However, the reduced food intake by TCDD was not taken into account. Therefore, metal concentrations were determined in different rat tissues at the end of a toxicity study with TCDD in which a pair-fed control group was introduced. Male Sprague-Dawley rats received a single i.p. injection of corn oil/acetone with or without TCDD at 125 micrograms/kg. Controls and TCDD-treated rats were fed ad libitum; additionally, pair-fed controls received the amount of food consumed by their TCDD-treated partners 1 day before. Twenty-one days after dosing rats were killed and samples of liver, kidney and jejunum were taken for the analysis of Ca, Cu, Fe, Mg, Mn, and Zn. After acid digestion of the tissues metals were determined by atomic emission spectrometry (AES). The most outstanding effect of TCDD treatment was an increase of the copper levels in the kidney (4-fold, versus pair-fed controls) and in the liver (greater than 2-fold, versus pair-fed controls). Other metals were mainly affected by the reduced food intake only. Since Cu represents a trace metal the homeostasis of which depends on its biliary excretion and since TCDD is known to impair biliary flow and excretion, an impaired biliary excretion of Cu by TCDD is suggested as the causal mechanism.

Correlation between acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and total body fat content in mammals

Single oral 30-day LD50s of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were correlated with total body fat (TBF) content in various species and strains of laboratory mammals. LD50 values and TBF contents were either obtained from the literature or determined by experiments. A log (LD50) vs. log (TBF) plot yielded a highly significant linear regression equation (r2 = 0.834, P less than 0.001, n = 20). It is suggested that this correlation exists for at least two reasons: (1) increasing TBF content in organisms represents an enhanced capacity to remove TCDD from the systemic circulation and (2) different TBF content reflects a differential role and regulation of fat metabolism for various organisms. Extrapolation of this correlation to man suggests that adult humans are among the less sensitive species to the acute toxicity of TCDD.

Mode of metabolism is altered in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

Male Sprague-Dawley rats were fed either a high-fat (HF) or a high-carbohydrate (HC) diet and subsequently injected with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (125 micrograms/kg) or vehicle (pair-fed controls). In all TCDD-treated animals, a reduction in caloric intake was evident as early as 1 day after dosage. Respiratory quotients (RQ) were determined at 5-day intervals. Their pattern for the HC-fed but not for the HF-fed TCDD-treated rats was different from that of the corresponding pair-fed controls. After an initial parallel decrease the RQ values remained low for TCDD-treated rats whereas they increased again for pair-fed controls. Serum total thyroxine (T4) was significantly lower in TCDD-treated animals and this reduction was not influenced by the composition of the diet. Serum triiodothyronine (T3) was neither altered by diet nor by TCDD. Thymic atrophy was as severe in pair-fed as in TCDD-treated rats fed the HC diet but not in rats fed the HF diet. Our results suggest that TCDD-treated rats are in a different mode of metabolism from pair-fed rats and that this difference is related to gluconeogenesis.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on thermogenesis in brown adipose tissue of rats

Male Sprague-Dawley rats were treated with a usually lethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 125 micrograms/kg i.p. in corn oil) or with vehicle alone. Two, 4, and 8 days after treatment the temperature of interscapular brown adipose tissue (IBAT) was monitored during venous infusion of norepinephrine (480 ng/min) for 60 min. The temperature response was about 1.0-1.5 degrees C within 1 h in vehicle-treated, pair-fed and ad libitum-fed controls. In TCDD-treated animals, the response of IBAT decreased with time after TCDD dosage, amounting to only 0.3 +/- 0.1 degree C at 8 days after dosing (differences significant with respect to both controls, P less than 0.05). GDP binding to IBAT mitochondria (a measure of thermogenic capacity) was unchanged in all groups, indicating that the reduced thermogenic response was probably not caused by an impairment of the mitochondrial uncoupling process by TCDD.

Composition of diet modifies toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in cold-adapted rats

The effect of a high carbohydrate, high fat or high protein diet was studied on the acute toxicity of TCDD (125 micrograms/kg) in cold-adapted (4 +/- 1 degrees C) rats. Within 10 days after dosing, TCDD-treated rats fed a high carbohydrate or a high protein diet reduced their caloric intake by 25% whereas those fed a high fat diet consumed only 15% fewer kcal/MBS (metabolic body size). TCDD-treated rats fed a high protein diet lost body weight at the same rate as their pair-fed controls, whereas body weight loss in high fat-fed rats was significantly higher than in their pair-fed controls. In contrast, TCDD-treated rats fed a high carbohydrate diet effectively maintained their body weight in the 4 days immediately after TCDD dosage, whereas their pair-fed controls lost weight. Mortality in TCDD-treated animals was 100% irrespective of the diet; all pair-fed control rats (except one fed a high protein diet) were terminated on days corresponding to the spontaneous death of their TCDD-treated pairs. Mean time to 50% mortality and mean time to death were significantly longer in TCDD-treated rats fed a high carbohydrate diet in comparison with the other two TCDD-treated groups (p less than 0.05), although caloric intake was comparable. Serum triiodothyronine (T3) was reduced in TCDD-treated animals fed a high fat or a high carbohydrate diet but not in those fed a high protein diet; serum thyroxine (T4) was reduced in all the treated groups, irrespective of diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a sublethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin on interscapular brown adipose tissue of rats

The effect of a sublethal dose (15 micrograms/kg) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied in selected tissues of male Sprague-Dawley rats by histological techniques 1, 3, 7 and 14 days after TCDD dosage. Histology of the heart, muscle, white adipose tissue, pancreas and the thyroid was unremarkable and that of the liver was found in agreement with previous reports. However, considerable changes were seen in interscapular brown adipose tissue (IBAT) of TCDD-treated rats. Initial accumulation followed by depletion of lipids, appearance of glycogen, cellular, mitochondrial and nuclear transformations were observed. In conjunction with other experiments it is concluded that a sublethal dose of TCDD alters fat and glucose metabolism in IBAT.

Brown adipose tissue is a target tissue in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced toxicity

This paper demonstrates by electron microscopy that interscapular brown adipose tissue (IBAT) is a target tissue of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rats. Because of the known importance of IBAT in energy metabolism, it is suggested that brown adipose tissue may be a major target tissue in the toxicity of TCDD.