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

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.

Patterns of dioxin-altered mRNA expression in livers of dioxin-sensitive versus dioxin-resistant rats

Dioxins exert their major toxicologic effects by binding to the aryl hydrocarbon receptor (AHR) and altering gene transcription. Numerous dioxin-responsive genes previously were identified both by conventional biochemical and molecular techniques and by recent mRNA expression microarray studies. However, of the large set of dioxin-responsive genes the specific genes whose dysregulation leads to death remain unknown. To identify specific genes that may be involved in dioxin lethality we compared changes in liver mRNA levels following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in three strains/lines of dioxin-sensitive rats with changes in three dioxin-resistant rat strains/lines. The three dioxin-resistant strains/lines all harbor a large deletion in the transactivation domain of the aryl hydrocarbon receptor (AHR). Despite this deletion, many genes exhibited a "Type-I" response-that is, their responses were similar in dioxin-sensitive and dioxin-resistant rats. Several genes that previously were well established as being dioxin-responsive or under AHR regulation emerged as Type-I responses (e.g. CYP1A1, CYP1A2, CYP1B1 and Gsta3). In contrast, a relatively small number of genes exhibited a Type-II response-defined as a difference in responsiveness between dioxin-sensitive and dioxin-resistant rat strains. Type-II genes include: malic enzyme 1, ubiquitin C, cathepsin L, S-adenosylhomocysteine hydrolase and ferritin light chain 1. In silico searches revealed that AH response elements are conserved in the 5'-flanking regions of several genes that respond to TCDD in both the Type-I and Type-II categories. The vast majority of changes in mRNA levels in response to 100 microg/kg TCDD were strain-specific; over 75% of the dioxin-responsive clones were affected in only one of the six strains/lines. Selected genes were assessed by quantitative RT-PCR in dose-response and time-course experiments and responses of some genes were assessed in Ahr-null mice to determine if their response was AHR-dependent. Type-II genes may lie in pathways that are central to the difference in susceptibility to TCDD lethality in this animal model.

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.

Dioxin-induced perturbations in tryptophan homeostasis in laboratory animals

Polychlorinated dioxins (PCDD) are widespread environmental contaminants. The most potent and the general model compound for dioxins is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Our laboratory has developed a new model for studies of dioxin toxicity based on totally disparate sensitivity to the lethal action of TCDD between Long-Evans (L-E, Turku AB; LD50 ca. 10 micrograms/kg) and Han/Wistar (H/W, Kuopio; LD50 over 10,000 micrograms/kg) rat strains. We have shown that body weight regulation is differentially regulated by TCDD in these rat strains: body weight gain is permanently reduced in the sensitive L-E but not in the resistant H/W strain. In concert with reduced body weight, TCDD increased brain TRP concentration, 5-HT synthesis and its metabolism to 5-HIAA at lethal doses in TCDD-susceptible L-E rats, and almost not at all in resistant H/W rats in which lethal dose levels were not reached. Further studies showed that TCDD indirectly increases free TRP concentration in the circulation in TCDD-susceptible L-E rats. Blood free fatty acids seem to be involved in the latter phenomenon. It is not likely that the enhanced serotonergic tone in the CNS is a causative factor in TCDD-induced anorexia. However, the present results may open up an interesting avenue to better understand physiology of TRP and the complex regulation of energy balance.

Effects of polychlorinated dibenzofurans, biphenyls, and their mixture with dibenzo-p-dioxins on ovulation in the gonadotropin-primed immature rat: support for the toxic equivalency concept

Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), and 1,2,3,4,7, 8-hexachlorodibenzo-p-dioxin (HxCDD), and their equipotent mixture block ovulation, reduce ovarian weight gain and alter preovulatory hormone levels in a similar manner. The objective of the current experiment was to investigate the effect of other structurally related compounds such as chlorinated furans and biphenyls on ovulation and related hormonal endpoints. The gonadotropin-primed immature female rat model was used to study the effect of 2,3,4,7, 8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PeCB), and 2,2',5,5' tetrachlorobiphenyl (TCB) and their mixture with polychlorinated dibenzo-p-dioxins (PCDDs) on ovulation. Rats were dosed on Day 23 of age at 0900 h with individual congeners (PeCDF, PeCB, TCB) or a mixture of five compounds, which included TCDD, PeCDD, HxCDD, in addition to PeCDF and PeCB. Equine choronic gonadotropin (eCG; 5 IU) was injected 24 h later to induce follicular development. Blood and ovaries were harvested, and ovarian weights determined at various times after eCG. Serum concentrations of 17beta-estradiol (E(2)), progesterone (P(4)), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. At 72 h after injection of eCG, the number of ova shed was measured by irrigating the ova from oviducts. The slopes of the dose-responses for inhibition of ovulation generated by the individual PeCDF, PeCB, and/or their mixture with PCDDs were similar. PeCDF, PeCB, and the mixture increased serum concentrations of E(2) at 72 h after eCG injection, the day of expected ovulation; in contrast, serum P(4) and FSH were decreased at that same time point. Only the high doses of TCDD, PeCDF, and PeCB blocked LH and FSH surges at 58 h after eCG. The ovarian histology revealed that the effects of PeCDF, PeCB, and the mixture were very similar to those of PCDDs, consisting of ova in large preovulatory follicles and a lack of or reduced number of corpora lutea. Parallel dose-responses of the individual congeners (PeCDF and PeCB) and their equipotent mixture with PCDDs support the toxic equivalency (TEQ) concept for the blockage of ovulation. Thus, PCDDs, PCDFs, and PeCBs appear to block ovulation by the same or a very similar mechanism of action.

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

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

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 a mixture of four chlorinated dibenzo-p-dioxins in rats. I. Design, general observations, hematology,and liver concentrations

Groups of 20 male and 20 female rats were administered five different 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). Doses were selected based on relative potency factors derived from acute toxicity data and a previous subchronic study with HpCDD. The mixture was constituted such that each of the congeners contributed one fourth to total toxic equivalency. Total doses were divided into four daily loading doses and six biweekly maintenance doses. The highest total dose for males was 17.5 microg/kg of TCDD, 87.5 microg/kg of PCDD, 350 microg/kg of HxCDD, and 2500 microg/kg of HpCDD. Positive controls were administered PCDD (350 microg/kg) or HxCDD (1400 microg/kg). Females were given 1.5 times lower doses than males. 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 concentrations as determined by GC-MS reflected the doses administered. Body weight gain was dose-dependently reduced throughout the study. Mortality rates at the end of the off-dose period were 80 and 60% for the two highest dosages (mixture) in males and 70 and 10% for females. Clinical signs and necropsy findings suggested that the cause of death was related to wasting, hemorrhage, and anemia. Prothrombin times were prolonged and platelet counts were decreased in some rats receiving high doses. This study provides in vivo support for the validity of the assumption of additive toxicity of CDDs as currently used in the toxicity equivalency factor approach to assess the toxicity of mixtures of CDDs.

The enzyme inducers 3-methylcholanthrene and phenobarbital affect the activities of glucocorticoid hormone-regulated enzymes in rat liver and kidney

3-Methylcholanthrene, an inducer of P448-type cytochromes (mostly 1A1 and 1A2), and phenobarbital, an inducer of P450-type cytochromes (mostly 2B1 and 2B2), are prototypical for the actions of many xenobiotics. They cause endocrine disruption by affecting, among others, steroid hormone levels. Rats were treated with single bolus doses of 3-methylcholanthrene or phenobarbital, and enzyme activities that are controlled by glucocorticoids were measured in liver and kidney. The activities of the cytosolic enzymes L-alanine aminotransferase, indoleamine 2,3-dioxygenase (L-tryptophan pyrrolase), phosphoenolpyruvate carboxykinase, L-serine dehydratase and L-tyrosine aminotransferase were affected in a similar fashion: an initial activity reduction followed by two overshoots of activity 1 and 2 days after dosing. 3-Hydroxy-3-methylglutaryl coenzyme A reductase, the microsomal key enzyme of sterol synthesis, responded with a temporary reduction of activity only and evidently lost its diurnal rhythm. The time course of these changes is most likely caused by a combination of sub-physiological levels of glucocorticoids plus changes of other regulatory hormones elicited by feed intake, postprandial state, etc. A possible role for a combined action of the arylhydrocarbon (Ah) and glucocorticoid receptors in the effects of 3-methylcholanthrene is also suggested.

Subchronic/chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) in rats. Part I. Design, general observations, hematology, and liver concentrations

Groups of 20 male and 20 female adult Sprague-Dawley rats were given five different doses of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD). Total doses for males and females (30.9/18.5, 370/222, 2222/1333, 6667/4000, and 10000/6000 microg/kg) were divided into four daily loading doses and six biweekly maintenance doses. Positive controls were administered 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD, total dose 70/41.9 microg/kg). Liver concentrations, as determined by GC/MS, reflected quite accurately the calculated dose ratios. 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. Body weight gain was dose-dependently reduced throughout the study. Mortality occurred dose-dependently, starting on Day 22 and continuing until the end of the off-dose period. Mortality rates at the end of the off-dose period were 90 and 40% for males and 60 and 10% for females in the two highest dose groups. Clinical signs and necropsy findings suggested that the cause of death was related to wasting (early deaths), gastrointestinal and nasal hemorrhage (between Days 64 and 126), or anemia (late deaths, after Day 111). Prothrombin times were prolonged intermittently, mainly at the highest dose of HpCDD. Platelet counts were dose-dependently decreased at the two highest doses of HpCDD and in the TCDD-treated group. This study demonstrates that the relative potency derived from acute toxicity studies is the same as that observed in this subchronic/chronic toxicity study of HpCDD and TCDD, confirming the validity of 0.007 as the toxic equivalency factor (TEF) for HpCDD, which is in good agreement with the international TEF of 0.01.

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.

Comparative toxicity of four chlorinated dibenzo-p-dioxins (CDDs) and their mixture. IV. Determination of liver concentrations

Groups of male Sprague-Dawley rats were administered orally the following chlorinated dibenzo-p-dioxins (CDDs) in corn oil/acetone (95/5; v/v): 30-60 micrograms/kg 2,3,7,8-tetrachlorodibenzo-dioxin (tetra-CDD), 160-270 micrograms/kg1,2,3,7,8-pentachlorodibenzo-p-dioxins (penta-CDD), 630-1249 micrograms/kg 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (hexa-CDD) and 5000-8000 micrograms/kg 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (hepta-CDD) or a mixture of the four homologues such that each was present in the mixture at one quarter of its dose as a single compound. Animals were killed at 2 and 8 days after dosing. Livers were immediately removed, and aliquots frozen in liquid nitrogen. Storage occurred at -80 degrees C until further use. About 0.2 g of each lyophilized rat liver was extracted, the extract purified by column chromatography and analyzed by GC/MS for CDD content. Results obtained suggest that the absorption of CDDs after oral administration decreases in the order of tetra-CDD > or = penta-CDD > hexa-CDD > hepta-CDD, indicating that the dose was an incomplete surrogate of exposure in parts I-III of this publication series (Stahl et al. 1992; Weber et al. 1992a,b). Moreover, data also support the notion that the pharmacokinetics of CDD mixtures at high doses are somewhat different from those expected based on single compound exposures. Our findings suggest that the intrinsic relative potency in terms of toxic equivalents (TEQ) of the higher chlorinated homologues is slightly greater (about a factor of 2) than suggested by Stahl et al.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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)

Subchronic (13-week) toxicity of heptachlorodibenzo-p-dioxin in male Sprague-Dawley rats

A 13-week oral toxicity study with 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) was performed in Sprague-Dawley rats. Rats received HpCDD at five different dose levels or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at one dose level. The doses were divided into 4 daily loading doses and 6 biweekly maintenance doses. At the end of the 13-week dosing period half of the rats were scheduled for necropsy and the other half after another 13-week off-dose period. This preliminary report contains only data from male rats during the 13-week main study period. At the two highest doses of HpCDD and in the TCDD dosage group the body weight or body weight gain was reduced. Mortality was 15, 50 and 5%, respectively. Wasting syndrome was the primary cause of death, but some rats died of hemorrhage without wasting, which may be related to the dose-dependent decrease in platelet counts. Phosphoenolpyruvate carboxykinase (PEPCK), the rate limiting enzyme of gluconeogenesis, was decreased only at the two highest dose levels of HpCDD and in the TCDD group, all of which also showed mortality. Ethoxyresorufin O-deetylase (EROD) was induced dose-dependently in all treated groups. Serum total thyroxine (T4) concentrations were decreased beginning at the middle dose of HpCDD. The study demonstrates that the toxicity observed after subchronic exposure to HpCDD is very similar to that of TCDD. Most importantly, most of the effects after subchronic and acute dose exposure are identical, confirming the validity of 0.007 as the toxic equivalency factor for HpCDD.

Endocrine considerations in toxicologic pathology

Detection of xenobiotic-induced toxicity on the endocrine system is a very difficult task because of the close relationship that the endocrine system has with the neural and immune systems. This is further complicated when one is asked to extrapolate from lab animals to man. Knowledge across species of hormonal action, solubility, transportation, plasma half life, receptor location, type of mediator, rhythmicity and pattern of secretion, is essential. One hormone can exert various effects in different tissues, or one function can be regulated by several hormones or even many functions of one endocrine target tissue can be regulated by several hormones acting in concert. The endocrine toxic response is determined by the state of differentiation of the target site. Feedback mechanisms both positive and negative, should also be taken into consideration initially. Because the effects of hormones have wide-ranged ramifications, the toxic responses likewise encompass broad areas such as the regulation of energy availability, maintenance of the internal environmental, development, growth and reproduction. The initial step involves the ascertaining of interference with the general trophic and target gland function and the characterization of the primary toxic effect. Equally important is to calculate the dose which elicited this primary effect, taking into consideration the area under the curve of the target endocrine site. Adapting this step-by-step approach, the causality between a specific toxic dose and a specific toxic effect can be readily and reliably established across all lab animal species and man.

Reduced activity of tryptophan 2,3-dioxygenase in the liver of rats treated with chlorinated dibenzo-p-dioxins (CDDs): dose-responses and structure-activity relationship

The activity of tryptophan 2,3-dioxygenase (TdO) was measured in the livers of male Sprague-Dawley rats after acutely toxic doses (LD20-LD80) of chlorinated dibenzo-p-dioxins (CDDs) with 4 of the up to 7 chlorine substituents occupying the 2,3,7,8-positions. Treatment with toxic doses of CDDs results in voluntary feed refusal of rats. A corresponding involuntary reduction of feed intake in naive animals (pair-feeding) causes elevated levels of TdO activity. In the CDD treated rats, however, TdO activities were dose-dependently reduced. An LD80 reduced TdO activity to about 50% of the level found in the corresponding pair-fed animals. This decrease of TdO activity explains the dose-dependent increase of serum tryptophan, which in turn is the likely cause of voluntary feed refusal observed in CDD-treated rats. The activity of another enzyme which is regulated in a fashion very similar to that of TdO, viz., tyrosine aminotransferase (TAT), was consistently, but not dose-dependently, affected by treatment with CDDs.

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.

Relative potency of chlorinated dibenzo-p-dioxins (CDDs) in acute, subchronic and chronic (carcinogenicity) toxicity studies: implications for risk assessment of chemical mixtures

This paper shows that the relative toxic potency of four chlorinated dibenzo-p-dioxins (CDDs) is similar in two species with different sensitivities (guinea pig, Sprague-Dawley rat). More importantly, it also demonstrates that the relative toxic potencies of these homologues are very similar for acute, subchronic and chronic dosing in the same species (rat). Furthermore, examination of different endpoints of toxicity (mortality, porphyria, carcinogenicity) suggests that the dose-responses for these diverse end-points after acute, subchronic, and chronic administration are very similar if not identical for tetra-CDD. Based on toxicokinetic and toxicodynamic considerations, a new, possibly generalizable rule (average tissue concentration x time = toxicity) is derived for CDDs. Implicit in the relative potency arguments of CDDs is the requirement of a practical threshold dose for all endpoints of toxicity including cancer.