Tissue-specific alterations of de novo fatty acid synthesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

AhR, PXR and CAR: From Xenobiotic Receptors to Metabolic Sensors

Traditionally, xenobiotic receptors are known for their role in chemical sensing and detoxification, as receptor activation regulates the expression of various key enzymes and receptors. However, recent studies have highlighted that xenobiotic receptors also play a key role in the regulation of lipid metabolism and therefore function also as metabolic sensors. Since dyslipidemia is a major risk factor for various cardiometabolic diseases, like atherosclerosis and non-alcoholic fatty liver disease, it is of major importance to understand the molecular mechanisms that are regulated by xenobiotic receptors. In this review, three major xenobiotic receptors will be discussed, being the aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Specifically, this review will focus on recent insights into the metabolic functions of these receptors, especially in the field of lipid metabolism and the associated dyslipidemia.

Atypical functions of xenobiotic receptors in lipid and glucose metabolism

Xenobiotic receptors are traditionally defined as xenobiotic chemical-sensing receptors, the activation of which transcriptionally regulates the expression of enzymes and transporters involved in the metabolism and disposition of xenobiotics. Emerging evidence suggests that "xenobiotic receptors" also have diverse endobiotic functions, including their effects on lipid metabolism and energy metabolism. Dyslipidemia is a major risk factor for cardiovascular disease, diabetes, obesity, metabolic syndrome, stroke, nonalcoholic fatty liver disease (NAFLD), and nonalcoholic steatohepatitis (NASH). Understanding the molecular mechanism by which transcriptional factors, including the xenobiotic receptors, regulate lipid homeostasis will help to develop preventive and therapeutic approaches. This review describes recent advances in our understanding the atypical roles of three xenobiotic receptors: aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), and constitutive androstane receptor (CAR), in metabolic disorders, with a particular focus on their effects on lipid and glucose metabolism. Collectively, the literatures suggest the potential values of AhR, PXR and CAR as therapeutic targets for the treatment of NAFLD, NASH, obesity and diabetes, and cardiovascular diseases.

Environmental toxicants, brown adipose tissue, and potential links to obesity and metabolic disease

Rates of human obesity, type 2 diabetes, and non-alcoholic fatty liver disease (NAFLD) have risen faster than anticipated and cannot solely be explained by excessive caloric intake or physical inactivity. Importantly, this effect is also observed in many other domesticated and non-domesticated mammals, which has led to the hypothesis that synthetic environmental pollutants may be contributing to disease development. While the impact of these chemicals on appetite and adipogenesis has been extensively studied, their potential role in reducing energy expenditure is less studied. An important component of whole-body energy expenditure is adaptive and diet-induced thermogenesis in human brown adipose tissue (BAT). This review summarizes recent evidence that environmental pollutants such as the pesticide chlorpyrifos inhibit BAT function, diet-induced thermogenesis and the potential signaling pathways mediating these effects. Lastly, we discuss the importance of housing experimental mice at thermoneutrality, rather than room temperature, to maximize the translation of findings to humans.

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.

Gut Microbiota-Derived Tryptophan Metabolites Modulate Inflammatory Response in Hepatocytes and Macrophages

The gut microbiota plays a significant role in the progression of fatty liver disease; however, the mediators and their mechanisms remain to be elucidated. Comparing metabolite profile differences between germ-free and conventionally raised mice against differences between mice fed a low- and high-fat diet (HFD), we identified tryptamine and indole-3-acetate (I3A) as metabolites that depend on the microbiota and are depleted under a HFD. Both metabolites reduced fatty-acid- and LPS-stimulated production of pro-inflammatory cytokines in macrophages and inhibited the migration of cells toward a chemokine, with I3A exhibiting greater potency. In hepatocytes, I3A attenuated inflammatory responses under lipid loading and reduced the expression of fatty acid synthase and sterol regulatory element-binding protein-1c. These effects were abrogated in the presence of an aryl-hydrocarbon receptor (AhR) antagonist, indicating that the effects are AhR dependent. Our results suggest that gut microbiota could influence inflammatory responses in the liver through metabolites engaging host receptors.

Association of dioxin and other persistent organic pollutants (POPs) with diabetes: epidemiological evidence and new mechanisms of beta cell dysfunction

The worldwide explosion of the rates of diabetes and other metabolic diseases in the last few decades cannot be fully explained only by changes in the prevalence of classical lifestyle-related risk factors, such as physical inactivity and poor diet. For this reason, it has been recently proposed that other "nontraditional" risk factors could contribute to the diabetes epidemics. In particular, an increasing number of reports indicate that chronic exposure to and accumulation of a low concentration of environmental pollutants (especially the so-called persistent organic pollutants (POPs)) within the body might be associated with diabetogenesis. In this review, the epidemiological evidence suggesting a relationship between dioxin and other POPs exposure and diabetes incidence will be summarized, and some recent developments on the possible underlying mechanisms, with particular reference to dioxin, will be presented and discussed.

Biochemical study of retired pentachlorophenol workers with and without following dietary exposure to PCDD/Fs

PCDD/Fs are found as impurities in commercial pesticide sodium pentachlorophenol (Na-PCP) salt. We compared, using multivariate logistic regression analysis adjusted for confounding factors, serum PCDD/F levels and biochemical examinations of retired Na-PCP workers and other inhabitants living near a closed Na-PCP plant that discharged PCP-contaminated wastewater into a nearby pond in Tainan, Taiwan. In this cross-sectional study from October 2006 through May 2009, 1167 participants were divided into groups according to their occupational (retired Na-PCP plant workers versus other residents) and dietary exposure (eating polluted fish versus not eating polluted fish) to PCDD/Fs, and a general population from a large-scale survey. Serum PCDD/F levels were significantly different between these groups (range: from 22.9±10.0pg WHO(98)-TEQ(DF) g(-1) lipid in the general population to 109.6±94.5pg WHO(98)-TEQ(DF) g(-1) lipid in retired Na-PCP workers eating polluted seafood; P(trend)<0.001). Distinct patterns of PCDD/Fs congener profiles, showing a significantly higher proportion of 1,2,3,7,8-PeCDD, 1,2,3,6,7,8-HxDD, and less 2,3,4,6,7,8-HpCDF, OCDF, were also found among workers and residents with different serum PCDD/F levels versus the general population. After adjusting for confounding factors, glucose (adjusted odds ratio [AOR] 7.22 [95% CI: 4.04-12.90]), triglycerides (AOR 4.31 [95% CI: 2.57-7.22]), blood urea nitrogen (AOR 2.90 [95% CI: 1.58-5.33]), creatine (AOR 5.83 [95% CI: 1.12-30.30]) and total protein (AOR 3.74 [95% CI: 1.91-7.31]) levels in retired workers were significantly higher than in the reference group. Occupational exposure to PCDD/Fs is associated with biochemical abnormalities that may persist for years after serum PCDD/F levels have declined.

PCB 126 and other dioxin-like PCBs specifically suppress hepatic PEPCK expression via the aryl hydrocarbon receptor

Dioxins and dioxin-like compounds encompass a group of structurally related heterocyclic compounds that bind to and activate the aryl hydrocarbon receptor (AhR). The prototypical dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a highly toxic industrial byproduct that incites numerous adverse physiological effects. Global commercial production of the structurally similar polychlorinated biphenyls (PCBs), however, commenced early in the 20(th) century and continued for decades; dioxin-like PCBs therefore contribute significantly to total dioxin-associated toxicity. In this study, PCB 126, the most potent dioxin-like PCB, was evaluated with respect to its direct effects on hepatic glucose metabolism using primary mouse hepatocytes. Overnight treatment with PCB 126 reduced hepatic glycogen stores in a dose-dependent manner. Additionally, PCB 126 suppressed forskolin-stimulated gluconeogenesis from lactate. These effects were independent of acute toxicity, as PCB 126 did not increase lactate dehydrogenase release nor affect lipid metabolism or total intracellular ATP. Interestingly, provision of cells with glycerol instead of lactate as the carbon source completely restored hepatic glucose production, indicating specific impairment in the distal arm of gluconeogenesis. In concordance with this finding, PCB 126 blunted the forskolin-stimulated increase in phosphoenolpyruvate carboxykinase (PEPCK) mRNA levels without affecting glucose-6-phosphatase expression. Myricetin, a putative competitive AhR antagonist, reversed the suppression of PEPCK induction by PCB 126. Furthermore, other dioxin-like PCBs demonstrated similar effects on PEPCK expression in parallel with their ability to activate AhR. It therefore appears that AhR activation mediates the suppression of PEPCK expression by dioxin-like PCBs, suggesting a role for these pollutants as disruptors of energy metabolism.

Some clinico and histopathological changes in female goats experimentally exposed to dioxin

Female Baladi goats were used for investigating the toxicological effects of dioxin. Each animal in the treated group was given an oral dose of 4 mL of stock standard solution of dioxin (labelled and native congeners) diluted in 5 mL distilled water (1/3 of LD50) for three times with 2 days interval and slaughtered 16 days post treatment. Blood and tissue samples were taken and subjected for haemogram, biochemical and pathological studies as well as for determination of dioxin residues. Results revealed that exposure of female goats to dioxin induced anemia, leucocytopenia, neutropenia and eosinophilia with non significant increases in activities of serum ALT and AST as compared with untreated group. Meanwhile, activity of ALP and BUN concentration were significantly increased. Histopathological examination showed degenerative and necrotic changes associated with inflammatory reaction in liver and kidney, in addition to cystic glandular hyperplasia and adenomyosis in uterus. In ovarian tissue, marked decrease ofpreantral follicles together with cystic atretic follicle were noticed. The average percentage residues ofpg WHO-TEQ values for dioxins (PCDDs and PCDFs) in liver, kidney, mammary gland, uterus and milk after oral dose were 0.013, 0.0012, 0.0012, 0.009 and 0.0012%, respectively. It was concluded that oral exposure to dioxin in female goats induced adverse effects on liver and kidney. Dioxins had estrogenic like effect as indicated by uterine and ovarian histopathological changes.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced impairment of glucose-stimulated insulin secretion in isolated rat pancreatic islets

We have explored the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administration on the secretory function of isolated rat pancreatic islets. Twenty-four hours after TCDD administration (1 microg/kg b.w., i.p.), rats showed no significant differences in plasma glucose, insulin, triglycerides and leptin levels whereas plasma-free fatty acids were significantly increased with respect to untreated controls. In isolated islets, DNA and protein content were unchanged, whereas insulin content was significantly decreased in TCDD-treated rats. Incubation with different concentrations of glucose demonstrated a significant impairment of glucose-stimulated insulin secretion in islets isolated from TCDD-treated rats, whereas insulin release was better preserved upon alpha-ketoisocaproate stimulation. A significant reduction of [3H]-2-deoxy-glucose uptake was observed in pancreatic tissue of TCDD-treated rats, whereas no significant reduction in GLUT-2 protein levels was detectable by immunoblotting in islets from TCDD-treated rats. We concluded that low-dose TCDD could rapidly induce significant alterations of the pancreatic endocrine function in the rat.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) alter body weight by decreasing insulin-like growth factor I (IGF-I) signaling

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) affects glycemia due to reduced gluconeogenesis; when combined with a reduction in feed intake, this culminates in decreased body weight. We investigated the effects of steady-state levels of TCDD (loading dose rates of 0.0125, 0.05, 0.2, 0.8, and 3.2 microg/kg) or approximately isoeffective dose rates of 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) (loading dose rates of 0.3125, 1.25, 5, 20, and 80 microg/kg) on body weight, phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression and activity, and circulating concentrations of insulin, glucose, and insulin-like growth factor-I (IGF-I), and expression of hepatic phosphorylated AMP kinase-alpha (p-AMPK) protein in female Sprague-Dawley rats (approximately 250 gm) at 2, 4, 8, 16, 32, 64, and 128 days after commencement of treatment. At the 0.05 and 1.25 microg/kg loading dose rates of TCDD and HxCDD, respectively, there was a slight increase in body weight as compared to controls, whereas at the 3.2 and 80 microg/kg loading dose rates of TCDD and HxCDD, respectively, body weight of the rats was significantly decreased. TCDD and HxCDD also inhibited PEPCK activity in a dose-dependent fashion, as demonstrated by reductions in PEPCK mRNA and protein. Serum IGF-I levels of rats treated initially with 3.2 microg/kg TCDD or 80 microg/kg HxCDD started to decline at day 4 and decreased to about 40% of levels seen in controls after day 16, remaining low for the duration of the study. Eight days after initial dosing, hepatic p-AMPK protein was increased in a dose-dependent manner with higher doses of TCDD and HxCDD. There was no effect with any dose of TCDD or HxCDD on circulating insulin or glucose levels. In conclusion, doses of TCDD or HxCDD that began to inhibit body weight in female rats also started to inhibit PEPCK, inhibited IGF-I, while at the same time inducing p-AMPK.

Glycogen metabolism and glucose transport in experimental porphyria

Hexachlorobenzene (HCB) is a fungicide of well-known porphyrinogenic ability, which induces an experimental porphyria that resembles human porphyria cutanea tarda (PCT) in several animal species. It has been demonstrated that high glucose ingestion prevents porphyria development, and high-fat/high-protein diets enhance HCB porphyrinogenic ability. On the contrary, a diet rich in carbohydrates reduces HCB effects. The aim of this work was to study HCB effects on glycogen synthesis and degradation, as well as on glucose synthesis and transport, in order to elucidate whether would justify the beneficial use of carbohydrates in this porphyria. Rats were treated with HCB dissolved in corn oil (five daily doses 100mg/kg body weight). Results showed that: (1) HCB caused an increase in glycogen content; (2) glycogen synthase activity increased three times, and phosphorylase activity decreased about 40% due to fungicide intoxication. The effect of HCB on these two activities accounted for the higher glycogen content observed in treated animals; (3) three gluconeogenic enzymes were reduced 30-50%; (4) glucose uptake in the liver decreased in all weeks studied. The alterations found in glucose synthesis, its uptake in liver and other tissues, and its release from glycogen might contribute to the biochemical porphyria picture and would account for the effect of glucose above mentioned.

Effect of estrogen and 2,3,7,8-tetrachlorodibenzo-rho-dioxin (TCDD) on plasma fatty acids of immature male chickens (Gallus domesticus)

This study investigated the effects of 2,3,7,8-tetrachlorodibenzo-rho-dioxin (TCDD) and estrogen on plasma lipids in immature male chickens. Fatty acids were quantified in plasma collected on day 14 from chickens injected with either: Estrogen plus TCDD-1 mg estradiol cypionate /kg body wt. daily for 3 days and 50 microg TCDD/kg body wt. on day 4; Estrogen--1 mg estradiol cypionate/kg body wt. daily for 3 days and vehicle only on day 4; TCDD-vehicle only for 3 days and 50 microg TCDD/kg body wt. on day 4; or Vehicle--same volume of appropriate vehicle for 4 days. TCDD treatment alone increased the plasma concentrations of total triacylglycerides and of the specific fatty acids 14:0, 15:0, 18:0, 18:2n6, 18:3n3, 20:0, 20:1n9, 20:2n6, 20:3n6, 20:5n3 and 22:1n9, compared with vehicle treatment. The concentration of 22:6n3 was increased in all plasma lipid classes of the estrogen group compared with the vehicle group, but was not increased in the estrogen plus TCDD group. Overall, TCDD treatment alone increased plasma lipids, possibly as a result of decreased clearance or utilization; whereas estrogen plus TCDD treatment antagonized estrogen-induced increases in 22:6n3 but did not cause hyperlipidemia.

Effect of hexachlorobenzene on NADPH-generating lipogenic enzymes and L-glycerol-3-phosphate dehydrogenase in brown adipose tissue

The effect of the in vivo administration of hexachlorobenzene (HCB) (100 mg/100 g bw) for 30 days, on the activities of brown adipose tissue (BAT) lipogenic enzymes, i.e. malic enzyme (ME), and glucose-6-phosphate dehydrogenase (G6PD) and the mitochondrial non lipogenic enzyme, L-glycerol-3-phosphate dehydrogenase (LG3PD), was studied in male Wistar rats, submitted to various neurohormonal manipulations. BAT ME, G6PD and LG3PD activities showed significant reductions in response to HCB treatment both in euthyroid and surgically thyroidectomized rats, showing that the effect does not depend on the presence of thyroid hormones. These results differ from those obtained for hepatic ME and G6PD activities, which increased in HCB intoxicated rats without alteration in LG3PD. HCB decreased BAT ME activity under BAT denervation. Administration of HCB resulted in time and dose-dependent decreases in the activity of BAT malic enzyme. The basal activity of ME was increased in hypothyroid rats, while that of LG3PD was reduced. A stimulatory effect of receptor-saturating doses of triiodothyronine (T3) (50 microg/100 g body weight) was observed on BAT ME and LG3PD activities in thyroidectomized rats, showing that the enzymes responded to thyroid hormone stimulation in a normal manner. The stimulatory effect of saturating doses of T3 on ME and LG3PD was reduced by HCB. The results presented herein unequivocally show that brown adipose tissue is a specific target in HCB-induced toxicity, which in turn involves severe alterations in the regulation of BAT lipogenesis.

Subchronic/chronic toxicity of a mixture of four chlorinated dibenzo-p-dioxins in rats. II. Biochemical effects

Groups of 20 male and 20 female rats were given five different oral doses of a mixture of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1, 2,3,7,8-pentaCDD, (PCDD) 1,2,3,4,7,8-hexaCDD (HxCDD), and 1,2,3,4,6, 7,8-heptaCDD (HpCDD) divided into four daily loading doses and six biweekly maintenance doses. PCDD and HxCDD were used as positive controls. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest provided with an off-dose period of another 13 weeks. Liver ethoxyresorufin O-deethylase (EROD) activity was dose-dependently increased in rats dosed with the mixture starting at the lowest dose (13- to 16-fold increase), with the effect reaching maximum at the middle dosage (74- to 112-fold increase), as well as in the positive control groups. There was some indication of reversibility at the lower doses and in positive controls during the off-dose period. The activity of phosphoenolpyruvate carboxykinase (PEPCK) in liver was dose-dependently decreased (maximally by 51%). This effect was more distinct in males than in females. Liver tryptophan 2,3-dioxygenase (TdO) activity decreased maximally by 53% at the two highest doses. This effect was more distinct in females than in males. Serum tryptophan concentrations were increased in rats moribund due to wasting. Some reversibility was apparent by the end of the off-dose period regarding all three biochemical markers of CDD toxicity. Serum glucose concentrations were decreased at the three highest doses of the mixture and in positive controls, maximally by 30%, with some reversibility during the off-dose period. There was a dose-dependent decrease of serum thyroxine (T4) concentrations in rats given the mixture and in the PCDD and HxCDD dosage groups (maximally by 69%), with some reversibility in males during the off-dose period. Serum triiodothyronine (T3) levels were not much affected, except that they tended to be decreased in rats moribund with hemorrhage or anemia. The results demonstrate that comparable biochemical changes occur after multiple as after single dosing with CDDs and that TEFs derived from acute studies can be used to predict the toxicity of mixtures of CDDs regardless whether they are administered as single compounds or as a mixture. This study supports the validity of the toxic equivalency factor (TEF) method and the notion of additive toxicity for CDDs as currently used in the risk assessment of these compounds.

Subchronic/chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) in rats. Part II. Biochemical effects

Groups of 20 male and 20 female rats were given five different oral doses of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) or one dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) divided into four daily loading doses and six biweekly maintenance doses. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest assigned to an off-dose period of another 13 weeks. At the end of the dosing period, liver ethoxyresorufin O-deethylase (EROD) activity was dose-dependently increased starting at the lowest dose (7- to 10-fold) with maximum induction (50- to 100-fold) at the middle or second highest dose. There was a slight reversibility of this effect in HpCDD-treated rats, particularly at lower doses, and a pronounced reversibility in TCDD-dosed rats, both in accordance with respective toxicokinetics. The activity of phosphoenolpyruvate carboxykinase in liver was dose-dependently decreased (up to 60%) at the two or three highest doses of HpCDD and also in the TCDD dosage group. Liver tryptophan 2,3-dioxygenase activity was decreased at the two highest doses of HpCDD (up to 41%), particularly in females. Serum tryptophan concentrations were elevated in rats found moribund due to wasting. There was a dose-dependent decrease in serum glucose concentrations (up to 30%) at the end of the dosing period. Serum thyroxin (T4) concentrations showed a dose-dependent decrease (78% at the highest dose) beginning in the middle dose for HpCDD and in the TCDD dosage group. Serum triiodothyronine (T3) concentrations were only slightly affected, except that they were somewhat decreased in moribund animals. The results demonstrate that similar biochemical changes occur in rats after single as after multiple dosing with HpCDD and TCDD. Based on these endpoints, the relative potency of HpCDD after subchronic exposure is in agreement with the international toxic equivalency factor (I-TEF) of 0.01 and, more specifically, with a TEF of 0.007 based on LD50 values in the same strain of rats.

Identification of dioxin-responsive elements (DREs) in the 5' regions of putative dioxin-inducible genes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an exogenous ligand for the cytosolic aryl hydrocarbon receptor (AhR), a ligand-inducible transcription factor whose exact physiological role remains elusive. TCDD has been shown to modulate the expression of a large array of genes, albeit often indirectly, by demonstration of protein or mRNA upregulation. Here, by computer analysis of available promoter sequences, we identify dioxin-responsive elements in the promoter regions of many putative AhR regulated and therefore dioxin-inducible genes.

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.

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.

Reduced gluconeogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

The effect of a usually lethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 125 micrograms/kg) was studied on the conversion of 14C-alanine into 14C-glucose in male Sprague-Dawley rats by established procedures (determination of plasma alanine and blood glucose by enzymatic assays and isolation of 14C-alanine and 14C-glucose from whole blood by column chromatography). TCDD-treated rats converted significantly (p less than 0.05) less 14C-alanine into 14C-glucose than did their pair-fed or ad libitum-fed counterparts, indicating reduced gluconeogenesis as a result of TCDD treatment. This finding suggests that reduced gluconeogenesis in TCDD-treated rats contributed to the progressively developing, severe hypoglycemia observed in these animals. Corticosterone, a key hormone in gluconeogenesis, provides partial protection from TCDD-induced toxicity in hypophysectomized rats. Therefore, the conversion of 14C-alanine into 14C-glucose was also determined in hypophysectomized rats dosed with TCDD (125 micrograms/kg) and given corticosterone (25 micrograms/ml in drinking water). These rats also converted significantly (p less than 0.05) less 14C-alanine into 14C-glucose than did their pair-fed counterparts. However, in contrast to non-hypophysectomized TCDD-treated rats, these rats maintained marginal normoglycemia even at 64 days after dosing with TCDD, which suggests that the partial protective effect of corticosterone in hypophysectomized, TCDD-treated rats is unrelated to its effect on gluconeogenesis. The protection provided by corticosterone supplementation in TCDD toxicity is more likely due to reduced peripheral utilization of glucose enabling the animals to maintain marginal normoglycemia.

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most potent toxins and tumor promoters known to man. It is prototypical of many halogenated polycyclic hydrocarbons that occur as environmental contaminants. Pathologic lesions produced by these compounds are mediated by an intracellular receptor protein called the TCDD (Ah) receptor which functions as a trans-acting effector of gene expression. However, the ultimate posttranslational pathways and mechanisms involved in the expression of the toxic manifestations of TCDD have received little attention and remain unclear, yet constitute an important segment in our understanding of the overall mechanism of action of TCDD. Recent studies have demonstrated that an oxidative stress occurs in various tissues of TCDD-treated animals. Evidence indicating production of an oxidative stress by TCDD in rodents is reviewed and includes:enhanced in vitro and in vivo hepatic and extrahepatic lipid peroxidation; increased hepatic and macrophage DNA damage; increased urinary excretion of malondialdehyde; decreased hepatic membrane fluidity; increased production of superoxide anion by peritoneal macrophage; and decreased glutathione, nonprotein sulfhydryl, and NADPH contents in liver. The potential role of reactive oxygen species in tumor promotion by TCDD is discussed. Possible sources and mechanisms of production of reactive oxygen species in response to TCDD are considered in light of current information. Evidence demonstrating the involvement of iron in TCDD-induced formation of reactive oxygen species and DNA damage is reviewed. Oxidative damage may contribute to many of the toxic responses produced by TCDD and its bioisosteres, and may be common to most of the tissue-damaging effects.

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.