Key enzymes of gluconeogenesis are dose-dependently reduced in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

Association of PKLR gene copy number, expression levels and enzyme activity with 2,3,7,8-TCDD exposure in individuals exposed to Agent Orange/Dioxin in Vietnam

2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) is the most toxic congener of dioxin and has serious long-term effects on the environment and human health. Pyruvate Kinase L/R (PKLR) gene expression levels and gene variants are associated with pyruvate kinase enzyme deficiency, which has been identified as the cause of several diseases linked to dioxin exposure. In this study, we estimated PKLR gene copy number and gene expression levels using real-time quantitative PCR (RT-qPCR) assays, genotyped PKLR SNP rs3020781 by Sanger sequencing, and quantified plasma pyruvate kinase enzyme activity in 100 individuals exposed to Agent Orange/Dioxin near Bien Hoa and Da Nang airfields in Vietnam and 100 healthy controls. The means of PKLR copy numbers and PKLR gene expression levels were significantly higher, while pyruvate kinase enzyme activity was significantly decreased in Agent Orange/Dioxin-exposed individuals compared to healthy controls (P < 0.0001). Positive correlations of PKLR gene copy number and gene expression with 2,3,7,8-TCDD concentrations were observed (r = 0.2, P = 0.045 and r = 0.54, P < 0.0001, respectively). In contrast, pyruvate kinase enzyme activity was inversely correlated with 2,3,7,8-TCDD concentrations (r = -0.52, P < 0.0001). PKLR gene copy number and gene expression levels were also inversely correlated with pyruvate kinase enzyme activity. Additionally, PKLR SNP rs3020781 was found to be associated with 2,3,7,8-TCDD concentrations and PKLR gene expression. In conclusion, PKLR copy number, gene expression levels, and pyruvate kinase enzyme activity are associated with 2,3,7,8-TCDD exposure in individuals living in Agent Orange/Dioxin-contaminated areas.

Aryl hydrocarbon receptor (AHR) functions in infectious and sterile inflammation and NAD+-dependent metabolic adaptation

Aryl hydrocarbon receptor (AHR) research has shifted from exploring dioxin toxicity to elucidation of various physiologic AHR functions. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to exert cellular stress-mediated sterile inflammatory responses in exposed human tissues but may be lethal in sensitive species. Inflammation can be thought of as the extreme end of a spectrum ranging from homeostasis to stress responses (sterile inflammation) and to defense against infection (infectious inflammation). Defense against bacterial infection by generation of reactive oxygen species has to be strictly controlled and may use up a considerable amount of energy. NAD⁺-mediated energy metabolism adapts to various inflammatory responses. As examples, the present commentary tries to integrate responses of AHR and NAD⁺-consuming enzymes (PARP7/TiPARP, CD38 and sirtuins) into infectious and stress-induced inflammatory responses, the latter exemplified by nonalcoholic fatty liver disease (NAFLD). TCDD toxicity models in sensitive species provide hints to molecular AHR targets of energy metabolism including gluconeogenesis and glycolysis. AHR research remains challenging and promising.

Aryl hydrocarbon receptor (AHR) functions in NAD+ metabolism, myelopoiesis and obesity

Diverse physiologic functions of AHR, a transcription factor discovered in studies of dioxin toxicity, are currently elucidated in many laboratories including chemical and microbial defense, immunity and myelopoiesis. Accumulating evidence suggests that AHR may also be involved in obesity and TCDD-mediated lethality in sensitive species. Underlying mechanisms include NAD⁺- and sirtuin-mediated deregulation of lipid, glucose and NAD⁺ homeostasis. Progress in NAD metabolome research suggests large consumption of NAD⁺ by NAD glycohydrolases (NADases) and NAD-dependent sirtuins. In focus are two NADases: (i) TiPARP (TCDD-induced poly(ADP-ribose) polymerase), one of several nuclear NADases, and (ii) plasma membrane-bound ectoNADase/CD38, a multifunctional enzyme and receptor. CD38 is involved in extra- and intracellular NAD degradation but acts also as differentiation marker. Both CD38 and AHR are components of a complex signalsome that enhances retinoic acid-induced differentiation of myeloid progenitor cells to granulocytes. Further advances of NAD metabolome research may lead to therapeutic options in the control of obesity and to improved risk assessment of TCDD toxicity.

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.

Aryl hydrocarbon receptor activation by dioxin targets phosphoenolpyruvate carboxykinase (PEPCK) for ADP-ribosylation via 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly(ADP-ribose) polymerase (TiPARP)

Effects of the environmental toxin and carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) include a wasting syndrome associated with decreased gluconeogenesis. TCDD is a potent activator of the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. The relationship between gene activation by the AHR and TCDD toxicities is not well understood. We recently identified a pathway by which the AHR target gene TiPARP (TCDD-inducible poly(ADP-ribose) polymerase) contributes to TCDD suppression of transcription of phosphoenolpyruvate carboxykinase (PEPCK), a key regulator of gluconeogenesis, by consuming NAD(+) and decreasing Sirtuin 1 activation of the peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), a transcriptional activator of PEPCK. We report here that TCDD-induced TiPARP also targets PEPCK for ADP-ribosylation. Both cytosolic and mitochondrial forms of PEPCK were found to undergo ADP-ribosylation. Unexpectedly, AHR suppression also enhanced ADP-ribosylation and did so by a poly(ADP-ribose) polymerase-independent mechanism. This report 1) identifies ADP-ribosylation as a new posttranslational modification for PEPCK, 2) describes a pathway by which transcriptional induction of TiPARP by the AHR can lead to a downstream posttranslational change in a TCDD target protein (PEPCK), and 3) reveals that the AHR exerts complex, previously unidentified modulatory effects on ADP-ribosylation.

Polybrominated diphenyl ethers alter hepatic phosphoenolpyruvate carboxykinase enzyme kinetics in male Wistar rats: implications for lipid and glucose metabolism

Xenobiotics such as phenobarbital, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and Aroclor 1254 significantly suppress the activity of a key gluconeogenic and glyceroneogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK), suggesting that xenobiotics disrupt hepatic glucose and fat metabolism. The effects of polybrominated diphenyl ethers (PBDE), a family of synthetic flame-retardant chemicals, on PEPCK activity is unknown. This study investigated the effect of DE-71, a commercial PBDE mixture, on PEPCK enzyme kinetics. Forty-eight 1-mo-old male Wistar rats were gavaged daily with either corn oil or corn oil containing 14 mg/kg DE-71 for 3, 14, or 28 d (n = 8/group). At each time point, fasting plasma glucose, insulin, and C-peptide were measured and hepatic PEPCK activity, lipid content, and three cytochrome P-450 enzymes (CYP1A, -2B, and -3A) were assayed. PBDE treatment for 28 d significantly decreased PEPCK Vmax ( μ mol/min/g liver weight) by 43% and increased liver lipid by 20%, compared to control. CYP1A, -2B, and -3A Vmax values were enhanced by 5-, 6-, and 39-fold, respectively, at both 14 and 28 d in treated rats compared to control. There was a significant inverse and temporal correlation between CYP3A and PEPCK Vmax for the treatment group. Fasting plasma glucose, insulin, and C-peptide levels were not markedly affected by treatment, but the glucose:insulin ratio was significantly higher in treated compared to control rats. Data suggest that in vivo PBDE treatment compromises liver glucose and lipid metabolism, and may influence whole-body insulin sensitivity.

Persistent effects on adult swim performance and energetics in zebrafish developmentally exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) remains a potent and persistent toxicant in aquatic environments, causing lethal developmental deformities in fish. However, few studies have examined sublethal or persistent effects of developmental TCDD exposure and none have examined its effects on swimming capabilities in sub-adult fish. The objective of the current study was to examine whether effects of TCDD exposure during the critical period of cardiovascular development (2-4 days post fertilization) on swim performance, triglyceride stores and cardiovascular deformities would persist until adulthood in zebrafish. Zebrafish larvae were exposed between 48 and 96 h post fertilization to 1, 0.1, 0.01 ng/L TCDD or DMSO control (0.005%), then raised in clean water for 90 days. Despite having equal survivability, no significant increase in gross deformities and no change in cytochrome P450 1A (CYP1A) activity was observed, while critical swimming speed and dorsal aorta diameter were significantly decreased in TCDD-exposed fish at 90 days. Furthermore, whole body triglycerides were significantly elevated in TCDD-exposed fish both before and after swim testing. Therefore sublethal TCDD exposure during zebrafish development caused a persistent decrease in swim endurance. The cause of this persistent decrease in swim endurance is not known, but may be related to behavioral adaptations limiting swimming capabilities, failure to mobilize triglyceride stores, vascular deformities limiting blood flow to the periphery, or a combination of these factors.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) reverses hyperglycemia in a type II diabetes mellitus rat model by a mechanism unrelated to PPAR gamma

It has been asserted that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases the risk for diabetes mellitus in humans, observable as hyperglycemia resulting from insulin resistance. There is no animal model for the induction of diabetes by TCDD. On the contrary, TCDD has been shown to increase insulin sensitivity in rats. Therefore, a diabetic rat model was used to study the effects of TCDD on preexisting diabetes. Type II diabetes was induced in male rats by a high-fat diet and streptozotocin. After manifestation of the disease, these rats received loading dose rates (LDRs) of 3.2, 6.4, and 12.8 microg/kg of TCDD p.o., followed by weekly maintenance dose rates. Rats fed a high-fat diet and not dosed with streptozotocin nor with TCDD served as nondiabetic controls. By day 2, serum-glucose levels in diabetic rats treated with the high LDR of 12.8 microg/kg TCDD were already significantly reduced. By day 8, serum-glucose levels had decreased to control levels and were maintained for the duration of the study (32 days). Thus, TCDD effectively counteracted hyperglycemia in this diabetic rat model. In healthy animals, TCDD induced PPAR gamma transcription and activity in a different dose range than that observed for the hypoglycemic effect.

Cell death and impairment of glucose-stimulated insulin secretion induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the beta-cell line INS-1E

The aim of this research was to characterize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity on the insulin-secreting beta-cell line INS-1E. A sharp decline of cell survival (below 20%) was observed after 1 h exposure to TCDD concentrations between 12.5 and 25 nM. Ultrastructurally, beta-cell death was characterized by extensive degranulation, appearance of autophagic vacuoles, and peripheral nuclear condensation. Cytotoxic concentrations of TCDD rapidly induced a dose-dependent increase in intracellular calcium concentration. Blocking calcium entry by EGTA significantly decreased TCDD cytotoxicity. TCDD was also able to rapidly induce mitochondrial depolarization. Interestingly, 1 h exposition of INS-1E cells to very low TCDD concentrations (0.05-1 nM) dramatically impaired glucose-stimulated but not KCl-stimulated insulin secretion. In conclusion, our results clearly show that TCDD exerts a direct beta-cell cytotoxic effect at concentrations of 15-25 nM, but also markedly impairs glucose-stimulated insulin secretion at concentrations 20 times lower than these. On the basis of this latter observation we suggest that pancreatic beta-cells could be considered a specific and sensitive target for dioxin toxicity.

CYP1A in TCDD toxicity and in physiology-with particular reference to CYP dependent arachidonic acid metabolism and other endogenous substrates

Toxicologic and physiologic roles of CYP1A enzyme induction, the major biochemical effect of aryl hydrocarbon receptor activation by TCDD and other receptor ligands, are unknown. Evidence is presented that CYP1A exerts biologic effects via metabolism of endogenous substrates (i.e., arachidonic acid, other eicosanoids, estrogens, bilirubin, and melatonin), production of reactive oxygen, and effects on K(+) and Ca(2+) channels. These interrelated pathways may connect CYP1A induction to TCDD toxicities, including cardiotoxicity, vascular dysfunction, and wasting. They may also underlie homeostatic roles for CYP1A, especially when transiently induced by common chemical exposures and environmental conditions (i.e., tryptophan photoproducts, dietary indoles, and changes in oxygen tension).

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the mRNA expression of critical genes associated with cholesterol metabolism, bile acid biosynthesis, and bile transport in rat liver: a microarray study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatotoxin that exerts its toxicity through binding to the aryl hydrocarbon receptor (AhR) and the subsequent induction or repression of gene transcription. In order to further identify novel genes and pathways that may be associated with TCDD-induced hepatotoxicity, we investigated gene changes in rat liver following exposure to single oral doses of TCDD. Male Sprague-Dawley rats were administered single doses of 0.4 microg/kg bw or 40 microg/kg bw TCDD and killed at 6 h, 24 h, or 7 days, for global analyses of gene expression. In general, low-dose TCDD exposure resulted in greater than 2-fold induction of genes coding for a battery of phase I and phase II metabolizing enzymes including CYP1A1, CYP1A2, NADPH quinone oxidoreductase, UGT1A6/7, and metallothionein 1. However, 0.4 microg/kg bw TCDD also altered the expression of Gadd45a and Cyclin D1, suggesting that even low-dose TCDD exposure can alter the expression of genes indicative of cellular stress or DNA damage and associated with cell cycle control. At the high-dose, widespread changes were observed for genes encoding cellular signaling proteins, cellular adhesion, cytoskeletal and membrane transport proteins as well as transcripts coding for lipid, carbohydrate and nitrogen metabolism. In addition, decreased expression of cytochrome P450 7A1, short heterodimer partner (SHP; gene designation nr0b2), farnesyl X receptor (FXR), Ntcp, and Slc21a5 (oatp2) were observed and confirmed by RT-PCR analyses in independent rat liver samples. Altered expression of these genes implies major deregulation of cholesterol metabolism and bile acid synthesis and transport. We suggest that these early and novel changes have the potential to contribute significantly to TCDD induced hepatotoxicity and hypercholesterolemia.

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.

Basis for a proposed reference dose (RfD) for dioxin of 1-10 pg/kg-day: a weight of evidence evaluation of the human and animal studies

The dioxins have been perhaps the most studied of all chemicals to which humans are routinely exposed. It has been reported that more than 5,000 scientific papers have been published that have evaluated the toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Although the cancer hazard posed by this chemical has probably received the bulk of attention over the past 20 years, the U.S. Environmental Protection Agency (EPA) and the recent U.S. EPA Science Advisory Board (SAB) that reviewed the "Reassessment" have suggested that the noncancer hazard may well be more important than the cancer hazard at current background doses to the general public. The World Health Organization (WHO) and U.K. Food Standards Agency (FAO) committee (JECFA) on dioxins has reached similar conclusions. This article reviews the published studies involving laboratory animals and humans that address the noncancer effects. Based on our review, developmental toxicity is the most sensitive effect of TCDD consistently seen in mice and rats. Specifically, of the various studies, a no-observed-adverse-effects level (NOAEL) of 13 ng/kg (maternal body burden) was identified as the most pertinent for deriving a reference dose (RfD) for humans. Although more than a dozen different adverse effects have been reported in various studies of humans over the past 25 years, the most consistent clinically important adverse effect of human exposure appears to be chloracne. Following a review of all published studies, we concluded that the best estimate of a LOAEL for production of chloracne is approximately 160 ng/kg (body burden). Based on our analysis, an RfD of between 1 and 10 pg/kg-d (TCDD TEQ) is consistent with the objectives of this risk criterion. Maintaining a lifetime average daily dose below this concentration, based on what is known today, should prevent noncancer effects in virtually all persons. This value is consistent with the JECFA recommendation of 70 pg/kg-mo.

Hepatic vitamin a depletion is a sensitive marker of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in four rodent species

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-treated animals show altered retinoid homeostasis and exhibit signs of toxicity similar to those of vitamin A-deficient animals. In this study we established dose-response curves for sublethal oral doses of TCDD and hepatic vitamin A gain in four rodent species. This was done to evaluate any potential correlation between decreased hepatic vitamin A gain and other TCDD-induced effects, particularly depressed body weight gain and hepatic CYP1A induction. Young Hartley guinea pigs, Sprague-Dawley rats, C57BL/6 mice, and Golden Syrian hamsters were given single oral doses of TCDD at up to 2.5, 100, 1000, and 1000 microg/kg bw, respectively, and killed 28 days after treatment. Hepatic vitamin A gain was decreased 25% compared to controls at estimated doses of 0.1, 0.9, 1.1 and 3.6 microg/kg bw in guinea pigs, hamsters, rats, and mice, respectively. CYP1A induction and hepatic vitamin A gain were affected at similar dose levels and showed similar, but inverse dose-response curves in each of the four species, consistent with the hypothesis that altered vitamin A homeostasis is Ah-receptor mediated. In addition, there was an apparent correlation between the dose-response curves for decreased hepatic vitamin A gain and decreased body weight gain in all species. Taken together with the known importance of vitamin A in body weight regulation, this result was consistent with a contributing role for altered retinoid homeostasis in the wasting syndrome induced by TCDD.

Role of the aromatic hydrocarbon receptor and [Ah] gene battery in the oxidative stress response, cell cycle control, and apoptosis

The chronology and history of characterizing the aromatic hydrocarbon [Ah] battery is reviewed. This battery represents the Ah receptor (AHR)-mediated control of at least six, and probably many more, dioxin-inducible genes; two cytochrome P450 genes-P450 1A1 and 1A2 (Cypla1, Cypla2-and four non-P450 genes, have experimentally been documented to be members of this battery. Metabolism of endogenous and exogenous substrates by perhaps every P450 enzyme, but certainly CYP1A1 and CYP1A2 (which are located, in part, in the mitochondrion), have been shown to cause reactive oxygenated metabolite (ROM)-mediated oxidative stress. Oxidative stress activates genes via the electrophile response element (EPRE) DNA motif, whereas dioxin (acutely) activates genes via the AHR-mediated aromatic hydrocarbon response element (AHRE) DNA motif. In contrast to dioxin, AHR ligands that are readily metabolized to ROMs (e.g. benzo[a]pyrene, beta-naphthoflavone) activate genes via both AHREs and the EPRE. The importance of the AHR in cell cycle regulation and apoptosis has just begun to be realized. Current evidence suggests that the CYP1A1 and CYP1A2 enzymes might control the level of the putative endogenous ligand of the AHR, but that CYPA1/1A2 metabolism generates ROM-mediated oxidative stress which can be ameliorated by the four non-P450 EPRE-driven genes in the [Ah] battery. Oxidative stress is a major signal in precipitating apoptosis; however, the precise mechanism, or molecule, which determines the cell's decision between apoptosis and continuation with the cell cycle, remains to be elucidated. The total action of AHR and the [Ah] battery genes therefore represents a pivotal upstream event in the apoptosis cascade, providing an intricate balance between promoting and preventing ROM-mediated oxidative stress. These proposed endogenous functions of the AHR and [Ah] enzymes are, of course, in addition to the frequently described functions of "metabolic potentiation" and "detoxification" of various foreign chemicals.

Suppression of C/EBPalpha and induction of C/EBPbeta by 2,3,7,8-tetrachlorodibenzo-p-dioxin in mouse adipose tissue and liver

We examined the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on two transcription factors, CAAT/enhancer binding protein-alpha (C/EBPalpha) and beta (C/EBPbeta), involved in the coordination of gene expression in adipose and liver. A single dose of TCDD (100 microg/kg) to male C57BL mice resulted in a time- and dose-dependent decrease in the level of C/EBPalpha mRNA in adipose tissue and liver, and a reciprocal increase in C/EBPbeta mRNA. Gel shift analysis using hepatic nuclear extracts from control and TCDD-treated mice and an oligonucleotide containing a C/EBP recognition element revealed a time-dependent change in DNA-protein complexes formed. Bands corresponding to C/EBPalpha, as determined by supershift analysis, diminished in TCDD-treated animals over a 7-day time period, whereas two new bands corresponding to C/EBPbeta, not present in control extracts, were increased significantly in treated samples. TCDD induced C/EBPbeta mRNA in wild-type mouse hepatoma cells, but not in aryl hydrocarbon receptor (AhR) nuclear translocator-deficient hepatoma cells. Induction in wild-type hepatoma cells was antagonized effectively by a molar excess of alpha-naphthoflavone. These results showed that TCDD caused rapid, reciprocal changes in C/EBPalpha and C/EBPbeta mRNAs and DNA binding in the adipose and liver of male C57BL mice and induced C/EBPbeta in hepatoma cells in an AhR-dependent manner. C/EBPs play vital roles in the coordination of energy homeostasis, and their alteration by TCDD may provide insight into the mechanism by which TCDD perturbs energy storage and utilization in vivo.

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.

Cytokines (IL-1beta and TNFalpha) in relation to biochemical and immunological effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rats

Previous studies in different strains of rats and mice have shown that the inhibition of gluconeogenesis as a result of reduced liver phosphoenolpyruvate carboxykinase (PEPCK) activity together with appetite suppression play critical roles in the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Recent immunological studies in rats demonstrated that exposure to low doses of TCDD resulted in an early and enhanced IgG response to immunization with sheep red blood cells (SRBC) and an enhanced delayed-type hypersensitivity (DTH) reaction as well as a positive popliteal lymph node (PLN) response. However, high doses of TCDD suppressed the DTH reaction. This study aimed at examining the involvement of cytokines (IL-1 and TNF) in mediating the above effects. Liver samples from a previous dose-response study on DTH reaction were investigated, in which rats were treated with TCDD (1, 3, 10, 30 and 90 microg/kg) and immunized with an antigen. mRNA levels of IL-1beta were elevated begining at the 1 microg/kg (non-lethal) dosage group with a maximum increase of about 5-fold above controls in the 90 microg/kg (lethal) dosage group. mRNA levels of TNFalpha were also significantly elevated begining at the 30 microg/kg dosage group. These results suggest that at low doses of TCDD, increased IL-1beta could be responsible for immune function stimulation, whereas at high doses of TCDD, greatly elevated TNFalpha and IL-1beta levles may exacerbate or mediate acute toxicity including immune suppression and related biochemical effects. A time course study (60 microg TCDD/kg without immunization) revealed that liver mRNA levels of TNFalpha were significantly elevated starting 24 h, and reaching a maximum 48 h after dosing with TCDD. This change was accompanied by a transient increase of mRNA levels of IL-1beta at day 4 after TCDD dosage. Thus, these data demonstrated that TCDD alone (without immunization) can cause transient increases of mRNA levels of TNFalpha and IL-1beta in liver. Results from these experiments suggest that TCDD-induced cytokine changes may play important roles in various effects of TCDD.

Differences in binding of epidermal growth factor to liver membranes of TCDD-resistant and TCDD-sensitive rats after a single dose of TCDD

Epidermal growth factor (EGF) receptor has been implied as having a role in certain actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). After a single dose of TCDD, the receptor has been shown to be downregulated in several tissues including the liver. Two rat substrains, the Han/Wistar (Kuopio; H/W) rat and the Long-Evans (Turku AB; L-E) rat exhibit over a 1000-fold difference in their sensitivity to the lethal effect of TCDD. This large sensitivity difference was utilized in the current study to investigate whether or not a correlation exists between TCDD lethality and biochemical endpoints related to the hepatic EGF receptor. In the TCDD-sensitive L-E strain both the B(max) of the EGF receptor and the receptor protein as measured by Western blots, decreased dose and time dependently. Ten days after a lethal dose of TCDD (50 μg/kg), the downregulation was 80%. In the resistant H/W strain, two non-lethal doses were used (50 and 500 μg/kg), since the lethal dose is not known. These doses caused a downregulation already at 4 days after dosing, but no further decrease by day 10. The activity of phosphoenolpyruvate carboxykinase (PEPCK, the main gluconeogenetic enzyme in the liver and a proposed target of TCDD) decreased in H/W rats at least to the same extent as in L-E rats at both 4 and 10 days. It is concluded that EGF receptor downregulation is different in the two rat strains studied, despite the fact that a classical Ah receptor-regulated response (CYP1A1 induction) is similar. The results demonstrate that downregulation of the EGF receptor by TCDD is strain-dependent as well as dose- and time-dependent.

2,3,7,8-Tetrachlorodibenzo-p-dioxin blocks the physiological regulation of hepatic phosphoenolpyruvate carboxykinase activity in primary rat hepatocytes

It has been previously reported that TCDD dose-dependently reduces the activity of PEPCK, the rate-limiting enzyme of hepatic gluconeogenesis. To further investigate the mechanism, whereby TCDD decreases PEPCK activity, we studied the effect of TCDD on PEPCK activity in primary rat hepatocytes (PRH). PRH were isolated from male Sprague-Dawley rats by collagenase perfusion and incubated on collagen-coated culture dishes in medium M199 containing 1 nM insulin. Cells were pretreated with dexamethasone (100 nM) 8 h before PEPCk induction was initiated by addition of glucagon (10 nM) and concurrent withdrawal of insulin. This hormonal treatment induced the enzymatic activity of PEPCK in control cells about 2-fold within 8 h. This PEPCK induction regimen was used to perform two sets of experiments. In the first set of experiments, rats were pretreated with TCDD (125 micrograms/kg p.o. in corn oil, 4 ml/kg) 4 days prior to isolation of PRH. This resulted in a complete block of the glucagon-dependent induction of PEPCK in PRH from TCDD-pretreated animals. In the second set of experiments, TCDD (100 nM) was added directly to the PRH either 24 or 48 h prior to the induction regimen. Incubation of PRH with TCDD 24 h prior to initiation of the induction regimen resulted in a slight decrease in the degree of PEPCK induction when compared to controls. However, treatment of PRH with TCDD 48 h prior to initiation of the induction regimen almost completely blocked PEPCK induction. It is, therefore, suggested that the effect of TCDD on liver PEPCK activity is due to a direct effect on liver cells and is not mediated by factors from outside the liver.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on tryptophan and glucose homeostasis in the most TCDD-susceptible and the most TCDD-resistant species, guinea pigs and hamsters

We have previously reported that in rats 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) lethality is associated (although not necessarily causally) with changes in brain serotonin (5-HT) metabolism. In the present study, we have examined whether this holds for other species by comparing the effect of TCDD in the most TCDD-susceptible and the most TCDD-resistant species, guinea pigs and hamsters, respectively. Body weight gain of guinea pigs exposed to TCDD (0.3-2.7 micrograms/kg) diminished dose dependently, while the effect was marginal in hamsters (900-4600 micrograms/kg). Brain 5-hydroxyindoleacetic acid (the main metabolite of brain 5-HT), brain tryptophan (the precursor amino acid of 5-HT), and plasma free and total tryptophan were not affected at any dose in guinea pigs. In contrast, 4 days after exposure, the levels of plasma free and total tryptophan were consistently increased in hamsters. These, as well as brain tryptophan, were still elevated 10 days after exposure. TCDD did not affect plasma glucose level in either species. Liver glycogen was decreased in a dose-dependent manner in TCDD-treated guinea pigs as well as in their pair-fed controls on day 10. There was no change in liver glycogen in hamsters. The activity of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase was only depressed in hamsters by all doses of TCDD. We conclude that changes in tryptophan metabolism or in carbohydrate homeostasis cannot explain the wide interspecies differences in susceptibility to the acute lethality of TCDD, although they may correlate with some aspects of its toxicity in certain species.

Ah receptor involvement in mediation of pyruvate carboxylase levels and activity in mice given 2,3,7,8-tetrachlorodibenzo-p-dioxin

The arylhydrocarbon receptor (AhR) plays a central role in mediating 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity in animals. The investigations described here provide evidence that support a role for the AhR in TCDD-mediated pyruvate carboxylase (PC) level/activity reductions in mice. Pyruvate carboxylase plays a pivotal role in gluconeogenesis and in supplying carbon units for the citric acid cycle. Delivered ip in a corn oil carrier, TCDD suppresses PC activity/amount at doses as low as 1 microgram/kg in responsive C57BL/6J(Ahb/b) mice. Corn oil alone injected ip into mice at 4 mL/kg appears to be an inducer that increases the amount and activity of PC. However, TCDD suppresses this induction. In the Ahb/b mouse, PC levels and activity are reduced to 10% of control values at a dose of 75 micrograms/kg. A time-course experiment shows that the PC reductions are apparent within 16 hours post-TCDD exposure. Here we report investigations on the PC/TCDD response using a congenic C57BL/6J(Ahd/d) mouse strain having an AhR with a low affinity for TCDD. If the PC/TCDD response is AhR mediated, the congenic mouse strain (Ahd/d) would require much higher doses of TCDD to suppress PC. In the Ahd/d mice, we observe that an approximately 60-fold increase in TCDD dose is necessary to produce a PC/TCDD effect. We also find that in Ahd/d mice, corn oil does not induce an increase in PC activity/amounts, as reported for Ahb/b mice.

Subchronic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and their reversibility in male Sprague-Dawley rats

The hypothesis tested in this experiment is that effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) show identical dose-responses after subchronic as after acute exposure when the dose is corrected for toxicokinetics. Groups of male Sprague-Dawley (S-D) rats were administered orally a total dose of 0, 0.2, 2.3, 11.5, 35, 70 or 115 micrograms/kg of TCDD over a period of 10 weeks at 4 ml/kg of vehicle. Body weight was recorded weekly. One week after the last dose of TCDD one half of the rats was killed and tryptophan 2,3-dioxygenase (TdO), 7-ethoxyresorufin-O-deethylase (EROD) and phosphoenolpyruvate carboxykinase (PEPCK) activities were measured in livers, whereas tryptophan and total T4 (TT4) were determined in serum. The results show that the dose-response for decreased TdO and PEPCK activity and elevated serum tryptophan levels are similar if not the same as the dose-response for subchronic retardation of body weight increase. They also demonstrate that the dose-responses for the induction of EROD activity and the reduction of serum TT4 occurred at much lower doses than those for decreased TdO and PEPCK activities or elevated tryptophan levels and mortality. After a 6-week recovery period, PEPCK and TdO activities in liver as well as tryptophan in serum returned to near control values, whereas EROD activity and serum TT4 still displayed a dose-dependent induction and reduction, respectively, albeit both shifted to the right in accordance with toxicokinetics. These data support the notion that subchronic dose-responses of TCDD are similar to acute dose-responses when corrected for toxicokinetics and that at least some TCDD-induced effects are reversible also in accordance with toxicokinetics.

Modulation of TCDD-induced wasting syndrome by portocaval anastomosis and vagotomy in Long-Evans and Han/Wistar rats

Portocaval anastomosis and vagotomy operations were performed in Long-Evans (L-E) and Han/Wistar (H/W) rats to elucidate the mechanism of anorexia induced by TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin). TCDD-sensitive L-E rats were given a sublethal (5 micrograms/kg) or a lethal dose (20 micrograms/kg) by gavage 5-8 weeks after portocaval anastomosis. TCDD-resistant H/W rats were given a nonlethal dose (500 or 7200 micrograms/kg). The shunt operation did not reduce lethality from TCDD. The effect on wasting of the marginally toxic dose of 5 micrograms/kg in L-E rats was potentiated by the portocaval operation, and the lethal dose was effective in both shunted and sham-operated L-E rats. TCDD failed to decrease food intake and body weight in shunted rats of H/W strain at either dose level though it did so in sham-operated controls. The lack of effect may be due to the already reduced weight of shunted rats at the time of TCDD dosing. TCDD anorexia was not explained by changes in histamine or serotonin (5-HT) turnover in the brain. Vagotomy did not influence lethality after TCDD, although reduction in food intake was somewhat blunted in H/W rats. The results seem to indicate that the anorectic effect of TCDD is modified when portal blood bypasses the liver. The mechanisms remain to be elucidated in detail, but the results do not favor the role of liver as the only or the major initiator of TCDD anorexia. Little evidence was found to support a crucial role of vagal afferent input.

Short- and long-term biochemical effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin in female Long-Evans rats

The aim of this study was to examine short- and long-term biochemical effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in female Long-Evans (L-E) rats. In the short-term study, female rats were dosed orally with 5.3, 12, 18 and 60 micrograms TCDD/kg and sacrificed 4 days after dosing. In the long-term study, rats were dosed with 27, 40 and 60 micrograms TCDD/kg and sacrificed 90 days after dosing. Four days after dosing, ethoxyresorufin O-deethylase (EROD) activity was fully induced at all doses studied, hepatic phosphoenolpyruvate carboxykinase (PEPCK) and gamma-glutamyl transpeptidase (gamma-GT) activities were dose-dependently reduced, whereas hepatic tryptophan 2,3-dioxygenase (TdO) activity was stimulated at low doses but decreased at high doses. Serum total T4 (TT4) levels were dose-dependently decreased, whereas serum total T3 (TT3) and tryptophan levels were unaffected. The short-term effects of TCDD examined in this study indicate only small differences in the response of female L-E rats to TCDD as compared to males. Ninety days after dosing, liver EROD activity revealed considerable reversibility although it was still elevated compared to controls. Hepatic PEPCK activity at this time point was no more different from controls. In contrast to 4 days after dosing, serum TT3, TT4 and hepatic gamma-GT activity were dose-dependently elevated at the 90-day time point. These findings have significant implications for the interpretations of subchronic and chronic effects of TCDD on thyroid homeostasis and on the formation of preneoplastic liver foci.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induced alterations of pyruvate carboxylase levels and lactate dehydrogenase isozyme shifts in C57BL/6J male mice

A dose-dependent reduction of hepatic pyruvate carboxylase levels and activity occurs in C57BL/6J male mice given 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) i.p. in a corn oil carrier. The dose range was from 1 to 75 micrograms/kg body weight and the analysis was done 8 days postinjection. At the maximum TCDD level investigated, we found a 10-fold reduction in pyruvate carboxylase activity. Furthermore, TCDD at a dose of 1 microgram/kg body weight blocks corn oil induction of an increase in the amount of pyruvate carboxylase in liver protein extracts. At doses beyond those required to initiate a reduction in pyruvate carboxylase, lactate dehydrogenase isozyme patterns shift. This is accompanied by an increase in blood lactic acid levels. We propose that TCDD-mediated reduction in pyruvate carboxylase and lactate dehydrogenase isozyme shifts may represent a major component in TCDD toxicity.

Relationship between acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and disturbance of intermediary metabolism in the Long-Evans rat

The aim of this study was to examine the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin, (TCDD) in a rat strain other than the Sprague-Dawley (S-D) rat, for which most of our data have been generated thus far. Doses for the biochemical study were selected based on an acute range-finding study, which indicated that Long-Evans (L-E) rats are somewhat less susceptible to TCDD toxicity than are S-D rats. Male L-E rats were dosed orally with 10, 20, 45, 67, 100 and 150 micrograms/kg TCDD. Body weight and feed intake were dose-dependently decreased prior to killing of the animals. Eight days after dosing, animals were killed and tryptophan, total T4 (TT4) and total T3 (TT3) levels were determined in serum, whereas the activities of ethoxy-resorufin-O-deethylase (EROD), phosphoenolpyruvate carboxykinase (PEPCK), gamma-glutamyl transpeptidase (gamma-GT) and tryptophan 2,3-dioxygenase (TdO) were measured in liver. EROD activity was fully induced at all doses studied, indicating that as in S-D rats, Ah-receptor-mediated effects do not seem to play any major role in the acute toxicity of TCDD in this rat strain either. Hepatic PEPCK activity was dose-dependently decreased in a similar dose range as in S-D rats, indicating inhibition of gluconeogenesis. Feed intake was dose-dependently decreased as a result of a dose-dependent elevation in serum tryptophan levels, which in turn were related to reduced liver TdO activity. Hepatic gamma-GT activity was also dose-dependently reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of hepatic phosphoenolpyruvate carboxykinase (PEPCK) activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is due to decreased mRNA levels

We have previously shown that the rate of hepatic gluconeogenesis is reduced in TCDD-treated rats and that this decrease in carbohydrate production is associated with a dose-dependent reduction of the activity of PEPCK, the rate limiting enzyme of gluconeogenesis. This derailment of glucose metabolism has been suggested to be the critical lesion in acute TCDD toxicity. To further elucidate the mechanism of decreased PEPCK activity we performed Northern blot analyses using a cDNA probe complementary to a portion of the mRNA coding for PEPCK. We have demonstrated that 4 and 8 days after TCDD treatment (125 micrograms/kg, p.o.) liver PEPCK mRNA in Sprague-Dawley rats was decreased to very low levels as compared to vehicle-treated and pair-fed control animals. This decline of PEPCK mRNA was paralleled by decreased levels of PEPCK protein, as revealed by Western blot analyses and was accompanied by a reduction in the enzymatic activity of PEPCK. These results indicate that the decrease of PEPCK activity by TCDD is most likely the result of decreased expression of the PEPCK gene. These together with previous results also suggest that many of the physiological responses occurring in TCDD-treated animals (reduced feed intake, decreased insulin, increased corticosterone, increased glucagon and cAMP levels) which would normally stimulate PEPCK gene expression, are ineffective. Furthermore tryptophan 2,3-dioxygenase (TdO) activity, which is regulated in a very similar fashion to PEPCK activity, is also reduced after TCDD treatment, suggesting a common mechanism by which TCDD alters the regulation of these enzymes. P-450 1A1 mRNA and related EROD activity were maximally induced under the conditions of these experiments and represent a positive control for TCDD-related alterations of gene expression. However, because of differences in the dose-response characteristics of TCDD-induced reduction of PEPCK activity and induction of EROD activity an involvement of the Ah receptor in the reduction of PEPCK activity cannot be postulated.

Comparative toxicity of four chlorinated dibenzo-p-dioxins (CDDs) and their mixture. Part III: Structure-activity relationship with increased plasma tryptophan levels, but no relationship to hepatic ethoxyresorufin o-deethylase activity

Male Sprague-Dawley rats were treated with an LD20, an LD50, and an LD80 of 2,3,7,8-tetrachlorodibenzo-p-dioxin (tetra-CDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (penta-CDD), 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (hexa-CDD), 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (hepta-CDD), respectively, and a mixture of the four homologues where each CDD was represented at one-fourth its previously established LD20, LD50, and LD80, respectively. Plasma tryptophan levels, liver ethoxyresorufin O-deethylase (EROD) activities, and liver weights were determined at 2 and 8 days after treatment. Plasma tryptophan levels were dose-dependently elevated, particularly at 8 days after treatment, by as much as 75% over control levels. EROD activity in CDD-treated animals was induced 27- to 28-fold, as compared with vehicle-treated controls, but did not show any dose-response. Liver weights were also significantly increased by the CDD treatments, but the increase was not dose related. There was no correlation between plasma tryptophan levels, a biomarker of acute toxicity of CDDs, and EROD activity, a biomarker of arylhydrocarbon (Ah) receptor-mediated enzyme induction. It is concluded that the acute toxicity of CDDs, which correlates and shows perfect structure-activity relationship with reduced activities of key enzymes of intermediary metabolism, and the induction of enzymes by much lower doses of CDDs in the liver, have different mechanisms of action.

Comparative toxicity of four chlorinated dibenzo-p-dioxins (CDDs) and their mixture

Male Sprague-Dawley rats were treated with an LD20, LD50 and LD80 respectively, of tetra-, penta-, hexa-, hepta-CDD and a mixture of the four CDDs, all carrying chlorine substituents in the biologically crucial 2, 3, 7, and 8 positions. Specific activities of two key enzymes of gluconeogenesis, viz, phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC), as well as the activity of the preneoplastic marker enzyme gamma-glutamyl transpeptidase (gamma-GT), were determined in livers of CDD-treated and ad libitum-fed control animals. PEPCK activity showed evidence for dose-related inhibition on the second day after dosing; PC activity was slightly reduced, whereas gamma-GT activity was dose-dependently inhibited. By 8 days after dosing PEPCK activities were dose-dependently decreased after administration of all four CDDs and their mixture. PC activities were significantly reduced, but no dose-response was evident. The activity of gamma-GT was dose-dependently inhibited, but only to a value of 25% below control activities. It is concluded that CDDs share a common mechanism of acute toxicity, viz, inhibition of glucocorticoid-dependent enzymes which results in a derailment of intermediary metabolism not compatible with survival of the animals.

Tissue-specific, developmental, hormonal, and dietary regulation of rat phosphoenolpyruvate carboxykinase-human growth hormone fusion genes in transgenic mice

The cytosolic phosphoenolpyruvate carboxykinase (PEPCK) gene is expressed in multiple tissues and is regulated in a complex tissue-specific manner. To map the cis-acting DNA elements that direct this tissue-specific expression, we made transgenic mice containing truncated PEPCK-human growth hormone (hGH) fusion genes. The transgenes contained PEPCK promoter fragments with 5' endpoints at -2088, -888, -600, -402, and -207 bp, while the 3' endpoint was at +69 bp. Immunohistochemical analysis showed that the -2088 transgene was expressed in the correct cell types (hepatocytes, proximal tubular epithelium of the kidney, villar epithelium of the small intestine, epithelium of the colon, smooth muscle of the vagina and lungs, ductal epithelium of the sublingual gland, and white and brown adipocytes). Solution hybridization of hGH mRNA expressed from the transgenes indicated that white and brown fat-specific elements are located distally (-2088 to -888 bp) and that liver-, gut-, and kidney-specific elements are located proximally (-600 to +69 bp). However, elements outside of the region tested are necessary for the correct developmental pattern and level of PEPCK expression in kidney. Both the -2088 and -402 transgenes responded in a tissue-specific manner to dietary stimuli, and the -2088 transgene responded to glucocorticoid stimuli. Thus, different tissues utilize distinct cell-specific cis-acting elements to direct and regulate the PEPCK gene.

Tissue-specific, developmental, hormonal, and dietary regulation of rat phosphoenolpyruvate carboxykinase-human growth hormone fusion genes in transgenic mice

The cytosolic phosphoenolpyruvate carboxykinase (PEPCK) gene is expressed in multiple tissues and is regulated in a complex tissue-specific manner. To map the cis-acting DNA elements that direct this tissue-specific expression, we made transgenic mice containing truncated PEPCK-human growth hormone (hGH) fusion genes. The transgenes contained PEPCK promoter fragments with 5' endpoints at -2088, -888, -600, -402, and -207 bp, while the 3' endpoint was at +69 bp. Immunohistochemical analysis showed that the -2088 transgene was expressed in the correct cell types (hepatocytes, proximal tubular epithelium of the kidney, villar epithelium of the small intestine, epithelium of the colon, smooth muscle of the vagina and lungs, ductal epithelium of the sublingual gland, and white and brown adipocytes). Solution hybridization of hGH mRNA expressed from the transgenes indicated that white and brown fat-specific elements are located distally (-2088 to -888 bp) and that liver-, gut-, and kidney-specific elements are located proximally (-600 to +69 bp). However, elements outside of the region tested are necessary for the correct developmental pattern and level of PEPCK expression in kidney. Both the -2088 and -402 transgenes responded in a tissue-specific manner to dietary stimuli, and the -2088 transgene responded to glucocorticoid stimuli. Thus, different tissues utilize distinct cell-specific cis-acting elements to direct and regulate the PEPCK gene.

Decreased hepatic phosphoenolpyruvate carboxykinase gene expression after 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment: implications for the acute toxicity of chlorinated dibenzo-p-dioxins in the rat

Decreased activity of the rate limiting gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK), has been recently suggested to be the critical lesion in the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We now show that other toxicologically relevant chlorinated dibenzo-p-dioxins (CDDs), with chlorine substituents in the crucial 2-,3-,7-, and 8-positions, exert the same effect on PEPCK activity. The doses required to cause this enzyme inhibition are within the acutely toxic range for each homologue, suggesting the same mechanism of action for these compounds. To further investigate the mechanism whereby dioxins decrease PEPCK activity, Northern analysis was performed using a cDNA probe complementary to a portion of the PEPCK mRNA. We could demonstrate that after TCDD treatment hepatic PEPCK mRNA was decreased by as much as 90% compared to pair-fed control animals (day 8 after dosing). This decrease in PEPCK mRNA was paralleled by a decrease of the amount of PEPCK protein and enzymatic activity. These results indicate that the physiological changes which occur in TCDD-treated animals (decreased feed consumption, low plasma insulin and elevated plasma corticosterone levels) which under normal conditions increase PEPCK gene expression and enzyme activity, are not effective in stimulating PEPCK synthesis in TCDD-treated animals.

Depletion of brain serotonin does not alter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced starvation syndrome in the rat

We have previously reported a series of biological events in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-intoxicated rats which resulted in elevated brain serotonin (5-HT) levels, offering a possible explanation of the acute toxicity (reduced feed intake and death) in these animals. It was thus hypothesized that depletion of central 5-HT stores should alter the TCDD-induced starvation syndrome. Brain 5-HT was selectively depleted by intracerebroventricular infusions of the neurotoxin 5,7-dihydroxytrytamine (5,7-DHT). Subsequently the animals were given a lethal dose of TCDD. In rats treated with 5,7-DHT hypothalamic 5-HT was depleted up to 90% compared to control animals, yet TCDD induced the expected reduction of bodyweight and feed intake. These results suggest that although TCDD increases central 5-HT levels as a result of increased plasma tryptophan, this may not be the main cause for reduced feed intake and lethality in these animals.

Is a serotonergic mechanism involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced appetite suppression in the Sprague-Dawley rat?

The major cause of TCDD-induced death in rats is a progressive voluntary feed refusal which has been correlated with reduced gluconeogenesis. Since centrally administered TCDD does not cause death or decreased feed intake in rats, the ability of TCDD to suppress appetite via peripheral mechanisms acting on the central nervous system was examined in two experimental models. First, it was found that the feed intake of rats on scheduled feeding cycles was not decreased by blood transfused from rats with TCDD-induced appetite suppression (8 days after a lethal dose of TCDD, i.p.). In contrast, a similar transfusion from normal, satiated rats did reduce feed intake of recipient rats by approximately 40%, suggesting that TCDD-treated rats are not satiated but rather that they are not hungry. In the second study tryptophan (the amino acid precursor of the neurotransmitter serotonin) was measured in the plasma and tryptophan, serotonin, norepinephrine and dopamine in the hypothalamus as well as dopamine and its metabolites in the striatum 4, 8, and 16 days after TCDD dosage (125 micrograms/kg, i.p.). Progressive time-dependent increases in tryptophan levels in plasma and brain were paralleled by increases in brain serotonin and 5-hydroxyindoleacetic acid (the primary metabolite of serotonin) in TCDD-treated rats. No changes were observed regarding the other biogenic amines. It is suggested based on these data and on substantial evidence from the published literature that a serotonergic mechanism may be involved in TCDD-induced feed intake reduction.