Induction of DT-diaphorase by 2,3,7,8-tetrachlorodibenzo-p-dioxin (ICDD).

The compounds 3-methylcholanthrene (3-MC) and 2,3,7,8-tetrachlorodibenzo-$\text{p}$-dioxin (TCDD) are both inducers of the enzyme system aryl hydrocarbon hydroxylase. It has recently been reported that 3-MC is also an inducer of DT-diaphorase activity in rat liver. In this report the ability of TCDD to induce hepatic DT-diaphorase activity was examined. The results indicate that TCDD is approximately 17,000 times more potent as an inducer of DT-diaphorase activity than 3-MC.     

Growth stimulation of primary rat hepatocytes by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The modulation of liver growth control by the tumor promoter, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was investigated in primary hepatocytes of adult rats. Under defined conditions in serum-free cultures, the interaction of TCDD with growth-related hormones was studied. TCDD-treatment of the cultured hepatocytes for two days caused a transient stimulation of both DNA synthesis and mitotic activity. This effect was maximal at the very low nontoxic concentration of 10^–12 M TCDD, i.e., two orders of magnitude below the optinzal concentrations for induction of drug metabolizing enzymes. Growth stimuladon by TCDD was dependent on the presence of growth-related hormones; in primary rat hepatocytes, TCDD acted synergistically with insulin and epidermal growth factor (EGF) and antagonized the growth inhibition by dexamethasone. Under culture conditions allowing high rates of DNA synthesis, e.g., at low concentrations of dexamethasone, in the presence of EGF plus alphal-adrenergic agonists or rat serum, no significant effect of TCDD on cellular growth was observed. Furthermore, TCDD failed to stimulate DNA synthesis in a rat hepatoma cell line, H4IIE, which is less sensitive to growth controlling factors than normal hepatocytes. Therefore, the results suggest that the growth modulation of primary rat hepatocytes by TCDD is the most sensitive parameter of the agent thus far observed. This effect may involve both a release from the growth inhibition caused, for instance, by glucocorticoids, as well as a direct growth-stimulating effect, synergistic to the one induced by insulin.Abbreviations Ah aryl hydrocarbon - EGF epidermal growth factor - EROD 7-ethoxyresorufin-0-deethylase - ³HdT [³H]thymidine - TCB 3,4,3,4-tetrachlorobiphenyl - TCDD 2,3,7,8-tetrachlorodibenzo-p-dioxin     

Cell-type specificity of ectonucleotidase expression and upregulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

We report here that induction of ectoATPase by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is cell-type specific and not a generalized response to aryl hydrocarbon (Ah) receptor activation. TCDD increased [14C]-ATP and -ADP metabolism in two mouse hepatoma lines, Hepa1c1c7 and Hepa1-6 cells, but not in human hepatoma HepG2 or HuH-7 cells, human umbilical vein endothelial cells (HUVEC), chick hepatoma (LMH) cells, or chick primary hepatocytes or cardiac myocytes, even though all of those cell types were Ah receptor-responsive, as evidenced by cytochrome P4501A induction. To determine whether the differences in ectonucleotidase responsiveness to TCDD might be related to differences in cell-type ectonucleotidase expression, ATP and ADP metabolite patterns, the products of several classes of ectonucleotidases including ectonucleoside triphosphate diphosphohydrolases (E-NTPDases), ectophosphodiesterase/pyrophosphatases (E-NPP enzymes) and ectoalkaline phosphatase activities were examined. Those patterns, together with results of enzyme assays, Western blotting, or semiquantitative RT-PCR show that NTPDase2 is the main ectonucleotidase for murine and human hepatoma cells, NTPDase3 for chick hepatocytes and LMH cells, and an E-NPP enzyme for chick cardiac myocytes. Evidence for NTPDase2 expression was lacking in all cells except the mouse and human hepatoma cells. TCDD increased expression of the NTPDase2 gene but only in the mouse and not in the human hepatoma cells. TCDD did not increase NTPDase3, NTPDase1, E-NPP, or alkaline phosphatase in any of the cell types examined. The failure of TCDD to increase ATP metabolism in HUVEC, chick LMH cells, hepatocytes, and cardiac myocytes can be attributed to their lack of NTPDase2 expression, while the increase in ATP metabolism by TCDD in the mouse but not the human hepatoma cells can be explained by differences in TCDD effects on mouse and human hepatoma NTPDase2 gene expression. In addition to characterizing effects of TCDD on ectonucleotidases, these studies reveal major differences in the complements of ectonucleotidases present in different cell types. It is likely that such differences are important for cell-specific susceptibility to extracellular nucleotide toxicity and responses to purinergic signaling.     

Uptake of 2,3,7,8-tetrachlorodibenzo-p-dioxin from plasma lipoproteins by cultured human fibroblasts

Tritiated 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) added to human plasma in vitro associated with the plasma lipoproteins. The effects of plasma and lipoproteins on cellular uptake of dioxin were studied using normal human skin fibroblasts and mutant fibroblasts from a patient with homozygous familial hypercholesterolemia. The latter cells lack the normal cell membrane receptor for low density lipoprotein (LDL). The time- and temperature-dependent cellular uptake of [3H]dioxin was greatest from LDL, intermediate from high density lipoprotein (HDL) and least from serum. A significantly greater uptake from LDL by the normal cells compared to the mutant cells indicated the involvement of the LDL receptor-mediated pathway. Concentration-dependent studies indicated that the cellular uptake at 37 degrees C of [3H]dioxin varied linearly with dioxin concentration at constant LDL concentration. Thin-layer chromatography (TLC) showed that conversion to more polar compounds may have occurred after 24-h incubation with cells. [3H]Dioxin could be removed from cells efficiently by incubation with 20% serum greater than HDL greater than LDL. Since the vehicle of delivery may influence subsequent location and metabolism of this compound in cells, it is concluded that the physiologic vehicles (either serum- or LDL-associated dioxin), rather than organic solvents, should be used in experiments with cultured cells or perfused organs.     

Endogenous production of prothymocyte differentiating activity by phytohemagglutinin-stimulated T-cell-depleted human marrow

The PHA responsiveness of marrow T-cell precursors remains a matter of controversy. We have investigated the capacity of human marrow to proliferate under phytohemagglutinin (PHA) stimulation following extensive removal of mature T cells by complement-dependent cytotoxicity with MBG6 and RFT8 monoclonal antibodies. PHA-induced thymidine uptake by marrow cells occurred with a peak on Days 6-8 of incubation instead of Day 3 for PBL. This peak was observed 48 hr earlier in the presence of PHA-stimulated T-depleted marrow cell supernatants. These supernatants can also promote the growth of mature T-cell colonies from MBG6-, RFT8-, T11-, T3- marrow. However, full colony development requires exogenous interleukin 2 (IL-2). IL-2 could be detected in marrow supernatants but only at very low levels and beyond Days 3 and 4. In contrast Days 1-6 marrow supernatants were equally effective in promoting MBG6-RFT8- marrow cell responsiveness to PHA. We conclude that marrow T-cell precursors are not PHA responsive and that PHA induces the production by marrow non-T cells of a prothymocyte-differentiating activity (PTDA); PTDA can differentiate marrow T-cell progenitors into PHA-responsive T cells; following activation by PHA, these cells undergo limited proliferation induced by IL-2 endogenously released from de novo differentiated T cells. It is suggested that this mechanism may account for extrathymic differentiation of the T-cell lineage in heavily irradiated marrow transplantation recipients.     

Hepatic Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin. Partial stabilization by molybdate

In structure and general mode of action, the Ah receptor is very similar to the receptors for steroid hormones. Molybdate previously has been shown to be highly effective at preserving ligand-binding function in steroid receptors during their exposure to elevated temperature or high ionic strength and at stabilizing steroid receptors as high molecular weight oligomeric complexes. Since such stabilization by molybdate can be very useful during characterization and purification of receptors, we tested the effects of molybdate on the Ah receptor to determine if the Ah receptor, like the receptors for steroid hormones, might be stabilized. In hepatic cytosols from C57BL/6N mice and Sprague-Dawley rats, molybdate concentrations up to 30 mM in homogenizing and analysis buffers did not alter the concentration of specific Ah receptor sites detected by binding of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin. However, inclusion of 20 mM molybdate in the homogenizing buffer did significantly protect unliganded Ah receptor from thermal inactivation at 20 degrees C and from KCl-induced loss of ligand-binding ability. In accord with previous reports, 20 mM molybdate in homogenizing and analysis buffers greatly increased the concentration of detectable glucocorticoid receptor in rat hepatic cytosol and estrogen receptor in rat uterine cytosol. Exposure to 0.4 M KC1 caused the glucocorticoid receptor from rat liver to shift sedimentation from approximately equal to 8 S to approximately equal to 4 S and caused a severe loss of specific glucocorticoid binding. Presence of 20 mM molybdate stabilized the glucocorticoid receptor as a single discrete peak sedimenting at approximately equal to 8 S. In contrast, the Ah receptor from rat liver exposed to 0.4 M KC1 in the presence of molybdate sedimented as biphasic peaks; one peak (approximately equal to 9.5 S) corresponded to the form of Ah receptor observed at low ionic strength, while the other peak (approximately equal to 5.5 S) corresponded to the form of Ah receptor seen in cytosol treated with 0.4 M KC1 in the absence of molybdate. Addition of heparin to hepatic cytosols from mice or rats shifted sedimentation of Ah receptor from approximately equal to 9.5 S to approximately equal to 5.5 S. Molybdate, again, provided stabilization in the approximately equal to 9.5 S form, but only for about one-half the total Ah receptor content in both rat and mouse hepatic cytosols. In sum, molybdate is far less effective at stabilizing rodent Ah receptors than it is at stabilizing steroid receptors in the same species.     

Induction of olfactory mucosal and liver metabolism of lidocaine by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Formulation of drugs for administration via the nasal cavity is becoming increasingly common. It is of potential clinical relevance to determine whether intranasal drug administration itself, or exposure to other xenobiotics, can modulate the levels and/or activity of nasal mucosal metabolic enzymes, thereby affecting the metabolism and disposition of the drug. In these studies, we examined changes in several of the major metabolic enzymes in nasal epithelial tissues upon exposure to the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), as well as the impact of these changes on the metabolism of a model intranasally administered drug, lidocaine. Results of these studies show that TCDD can induce multiple metabolic enzymes in the olfactory mucosa and that the pattern of induction in the olfactory mucosa does not necessarily parallel that which occurs in the liver. Further, increases in enzyme levels noted by Western blot analysis were associated with increased activities of several nasal mucosal enzymes as well as with enhanced conversion of lidocaine to its major metabolite, monoethyl glycine xylidide (MEGX). These results demonstrate that environmental exposures can influence the levels and activity of nasal mucosal enzymes and impact the pharmacology of drugs administered via the nasal route.     

Aryl hydrocarbon hydroxylase induction in human lymphocyte cultures by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity is induced in cultured human lymphocytes by 2,3,7,8-tetrachlorodibenzo-$\text{p}$-dioxin (TCDD) at a concentration in the growth medium 40 to 60 times less than the concentration of 3-methylcholanthrene (MC) necessary for maximal hydroxylase induction. In cultured lymphocytes from 19 individuals, the extent of hydroxylase induction by TCDD or MC ranged between 1.7- and 2.9-fold. Those individuals having (presumably under genetic control) lower basal and MC-inducible hydroxylase activities in their lymphocytes also have lower TCDD-inducible hydroxylase activity. Because of the day-to-day experimental variability, the variations within each assay, and for several other reasons discussed, we suggest that the observed variance of expression of hydroxylase induction more closely fits a unimodal, polygenic ($\text{i.e.}$ 2 or more genes) pattern rather than the trimodal (single gene) form of inheritance proposed recently by Kellermann and coworkers.     

Aryl hydrocarbon receptor (AhR)-mediated induction of xanthine oxidase/xanthine dehydrogenase activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an environmental contaminant, induced xanthine oxidase and xanthine dehydrogenase (XO/XDH) activities, in addition to ethoxyresorufin-O-dealkylase and methoxyresorufin-O-dealkylase activities in liver of mice. When TCDD was given to mice as a single oral dose of 40 microg/kg, the activities of XO and XDH increased about threefold within 3 days and the increased levels were maintained for 4 weeks. The treatment of mice with 3-methylcholanthrene also induced XO/XDH activities, but phenobarbital and dexamethasone had no effect. The level of aldehyde oxidase, a molybdenum flavoenzyme related to XO/XDH, in mouse liver was also enhanced about 1.5-fold by TCDD treatment. The inducing effect of TCDD and 3-methylcholanthrene was not observed in null mice (AhR(-/-)), which lack the AhR gene. XO and XDH activities were induced by TCDD in heterozygous mice (AhR(+/-)). The lipid peroxidation in liver was stimulated by TCDD. The induction of XO and XDH, which produces reactive oxygen species, may contribute to the various toxicities of TCDD.     

Long-term carcinogenesis studies on 2,3,7,8-tetrachlorodibenzo-p-dioxin and hexachlorodibenzo-p-dioxins

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,6,7,8- and 1,2,3,7,8,9-hexachlorodibenzo-p-dioxins (HCDDs) are among the most toxic and carcinogenic of man-made chemicals. These dioxins, as well as many of the other polychlorinated dibenzodioxins (PCDDs) and dibenzofuran (PCDFs) derivatives, are chlorinated aromatic compounds which are chemically stable, insoluble in water, and highly soluble in fats and oils.TCDD acts as a complete carcinogen in several species, causing both common and uncommon tumors at multiple sites. It is a highly potent chemical carcinogen in chronic animal studies, producing carcinogenic effects in laboratory animals with doses as low as 0.001 µg/kg/day. In rats, TCDD induces neoplasms in the lung, oral/nasal cavities, thyroid and adrenal glands, and liver. In mice, TCDD induces neoplasms in the liver and subcutaneous tissue, thyroid gland, and thymic lymphomas. In hamsters, it induces squamous cell carcinomas of the facial skin. Tumors of the integumentary system are reported after oral (mice and rats), intraperitoneal (hamsters), and dermal (mice) administration. A mixture of HCDDS (defined as the mixture of the 1,2,3,6,7,8- and 1,2,3,7,8,9 isomers used in the NTP experiments) are potent liver carcinogens in mice and rats.Pharmacokinetic studies in laboratory animals indicate that 50–90% of dietary TCDD is absorbed. It concentrates in adipose tissue and the liver. In mammals, the TCDD present in the liver is slowly redistributed and stored in fatty tissue. Elimination of TCDD occurs via excretion of metabolites in the bile and urine and passively through the gut wall. Metabolism is slow: the biological half-life of TCDD varies from weeks (rodents) to years (humans), and is strongly dependent upon the rate of TCDD metabolism.Many of the toxic effects of TCDD, including teratogenicity, may arise by receptor-mediated mechanisms. The induction of cytochrome P-448 and related enzymes by TCDD occurs by such a mechanism, and is related to the binding of TCDD to the Ah receptor. The specific mechanism(s) by which TCDD exerts its carcinogenic effects is unclear: receptor-binding may be part of the story. The role of the Ah receptor has been indicated in a skin promotion assay. The evidence for mutagenicity is inconclusive. TCDD did not induce lethal mutations, chromosomal aberrations, micronuclei or sister chromotid exchanges in rodents treated in vivo, nor was it mutagenic to bacteria, but it did enhance transformation of mouse C3H 10T1/2 cells by N-methyl-N-nitro-N-nitrosoguanidine and was mutagenic to mouse lymphoma cells.Under the State of California's Safe Drinking Water and Toxic Enforcement Act of 1986 (Proposition 65), TCDD and HCDDs were listed as chemicals known to the state to cause cancer. The International Agency for Research on Cancer (IARC) has classified TCDD as a Group 2B carcinogen (possibly carcinogenic to humans). Similarly, the EPA classified TCDD and HCDDs as Group B2 carcinogens (probable human carcinogens); and the DHHS' National Toxicology Program placed TCDD into the category of substances which may reasonably be anticipated to be carcinogens for humans.     

Thymoquinone is protective against 2,3,7,8-tetrachlorodibenzo-p-dioxin induced hepatotoxicity

We investigated changes in rat liver tissues following administration of thymoquinone (TQ) against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced hepatotoxicity. Fifty rats were assigned randomly to five groups of 10 as follows: control, corn oil, TCDD, TQ and TCDD + TQ. Biochemical and histopathological analyses were conducted on liver tissue. We found that 30 day TCDD administration caused histopathological changes in liver including thickening of Glisson’s capsule, intracytoplasmic vacuolization in hepatocytes, sinusoidal dilation, vascular and sinusoidal congestion and inflammatory cell infiltration. TCDD administration increased malondialdehyde (MDA), total oxidant status (TOS), alanine aminotransferase (ALT), aspartate aminotransferase (AST) and alkaline phosphatase (ALP) levels in rat liver tissue and reduced glutathione (GSH), superoxide dismutase (SOD), catalase (CAT) and total antioxidant status (TAS) levels compared to all other groups. In the TQ treated group, GSH, SOD, CAT and TAS levels increased compared to all other groups. MDA, TOS, ALT, AST, ALP levels decreased compared to all other groups. Our histological findings were consistent with the biochemical findings. The oxidative and histologic effects of TCDD were eliminated by TQ treatment. TCDD administration caused oxidative stress in rat liver and TQ administered with TCDD prevented TCDD induced hepatotoxicity. TQ could be considered an alternative anti-TCDD toxicity agent.     

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure on bone material properties

Dioxins are known to decrease bone strength, architecture and density. However, their detailed effects on bone material properties are unknown. Here we used nanoindentation methods to characterize the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on nanomechanical behaviour of bone matrix.Pregnant rats were treated with a single intragastric dose of TCDD (1 μg/kg) or vehicle on gestational day 11. Tibias of female offspring were sampled on postnatal day (PND) 35 or 70, scanned at mid-diaphysis with pQCT, and evaluated by three-point bending and nanoindentation.TCDD treatment decreased bone mineralization (p<0.05), tibial length (p<0.01), cross-sectional geometry (p<0.05) and bending strength (p<0.05). Controls showed normal maturation pattern between PND 35 and 70 with decreased plasticity by 5.3% and increased dynamic hardness, storage and complex moduli by 26%, 13% and 12% respectively (p<0.05), while similar maturation was not observed in TCDD-exposed pups.In conclusion, for the first time, we demonstrate retardation of bone matrix maturation process in TCDD-exposed animals. In addition, the study confirms that developmental TCDD exposure has adverse effects on bone size, strength and mineralization. The current results in conjunction with macromechanical behaviour suggest that reduced bone strength caused by TCDD is more associated with the mineralization and altered geometry of bones than with changes at the bone matrix level.     

The determination of 2,3,7,8-tetrachlorodibenzo-p-dioxin in biological extracts by gas chromatography mass spectrometry

Using both gas chromatography low resolution mass spectrometry and gas chromatography high resolution mass spectrometry, 2,3,7,8-tetrachlorodibenzo-p-dioxin at the part per trillion level may be determined in pre-concentrated extracts of bovine fat, liver and milk; human milk; rats; rice; grass; soil and water. Criteria are set forth for the objective determination of detection limit, signal and noise as applied to these determinations.     

Depressed guinea pig testicular microsomal cytochrome P-450 content by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Human exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can result in hirsutism and chloracne, symptoms that suggest an alteration in endocrine regulation. Consequently, the effect of TCDD on guinea pig testicular cytochrome P-450 has been investigated. Twelve hours after a single, oral dose of TCDD (1 μg/kg), testicular microsomal cytochrome P-450 content was depressed by approximately 36%. Microsomal cytochrome P-450 content reached a maximal depression at approximately 1 day (52% of control) and remained at this level for 9 days. No appreciable alterations of testicular microsomal heme levels or activity of testicular δ-aminolevulinic acid (ALA) synthetase were observed.     

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced reduction of adenosine deaminase activity in vivo and in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressant in several animal species. The purpose of this study was to determine if TCDD affected the activity of adenosine deaminase (ADA), a purine metabolizing enzyme that is vital to the proper functioning of the immune system. The effect of TCDD on ADA ctivity was studied in various tissues of male Balb/c mice (a TCDD-responsive strain) and DBA/2 mice (a less-responsive strain). Of the tissues examined after administration of TCDD in vivo (115 μ/kg, i.p.), ADA activity was found to be significantly reduced in thymic and splenic tissues of Balb/c mice at 24 hours postadministration. The enzyme activity in these affected tissues remained consistently low through 10 days postadministration. Such an effect of TCDD was both dose and time related in the thymic tissue of Balb/c mice. In contrast, no appreciable alterations in ADA activity were evident in any of the tissues of DBA/2 mice at any of the sampling intervals, indicating that such an effect of TCDD is likely to be mediated through the Ah receptor. This in vivo effect of TCDD on thymic ADA activity was also reproducible in situ where isolated whole thymuses were directly incubated with 10 nM TCDD. In this model, TCDD's effects on ADA activity were antagonized by known protein kinase or phosphorylation inhibitors such as quercetin, genistein, tyrphostin, and neomycin. These results indicate that the effect of TCDD on ADA activity in the thymus may be related to its property to elevate protein kinase activities in this tissue. ADA activity was also reduced in 3T3 cells that were treated with 10 nM TCDD in a low (1%) serum media. In contrast, 25 ng/mL epidermal growth factor (EGF) under such conditions consistently stimulated ADA activity. Interestingly, EGF at a similar concentration failed to elicit a stimulatory effect on ADA activity when cells were pretreated with TCDD. The property of TCDD to lower ADA activity under in vivo, in situ, as well as in vitro conditions appears to be largely related to its action to modulate protein phosphorylation activities.     

Mutagenic and genotoxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, a review

2,3,7,8-Tetrachlorodibenzo-p-dioxin is one of the most toxic and highly stable compounds known in the environment. Due to uncontrolled exothermic reactions during manufacture of 2,4,5-T and other chlorophenoxy compounds, TCDD is released in the environment. The importance of determining the hazards to human health from dioxin became apparent because of its severe adverse health effects. 'Agent Orange' which is a mixture of 2,4,5-T and 2,4-D has been extensively used during the late Vietnam War. In this review an attempt has been made to evaluate the mutagenic and genotoxic effects of 2,3,7,8-TCDD as known at present.     

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on vitamin A metabolism in mice

This study aimed to clarify the effects of single and repeated administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the activities or expression of some metabolic enzymes of retinoids and the influence of supplemental vitamin A on changed vitamin A homeostasis by TCDD. In Experiment I, the mice were given a single oral dose of 40 mug TCDD/kg body weight, with or without continuous administration of 2,500 IU vitamin A/kg body weight/day, and were killed on day 1, 3, 7, 14, and 28. In Experiment II, the mice were daily given 0.1 microg TCDD/kg body weight, with or without supplemental 2,000 IU vitamin A/kg body weight, and were killed on day 14, 28, and 42. In both experiments, TCDD significantly decreased the hepatic all-trans-retinol level and increased the hepatic all-trans-retinoic acid (RA) content, increased the mRNA and enzymatic activities of retinal oxidase. In TCDD + vitamin A mice, the all-trans retinol content was significantly higher, and the retinal oxidase mRNA was significantly lower on day 3 or 7 in Experiment I and on day 14 in Experiment II, compared to TCDD-treated mice. The induction of the retinal oxidase may contribute to the decrease in hepatic all-trans-retinol level and the increase in hepatic all-trans-RA caused by TCDD. Supplemental vitamin A might decelerate the effect of TCDD on retinal oxidase mRNA.     

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 μg/kg in responsive C57BL/6J(Ah^b/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 Ah^b/b mouse, PC levels and activity are reduced to 10% of control values at a dose of 75 μg/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(Ah^d/d) mouse strain having an AhR with a low affinity for TCDD. If the PC/TCDD response is AhR mediated, the congenic mouse strain (Ah^d/d) would require much higher doses of TCDD to suppress PC. In the Ah^d/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 Ah^d/d mice, corn oil does not induce an increase in PC activity/amounts, as reported for Ah^b/b mice.     

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on hepatic and renal prostaglandin synthetase

This study examines the possibility that the very toxic compound, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), produces its toxic effects through induction or repression of microsomal prostaglandin synthetase (cyclooxygenase). The effects of TCDD on microsomal synthesis of prostaglandin from [¹⁴C]arachidonic acid in rabbit liver and kidney medulla were examined 24 and 72 hr after TCDD administration. A hepatotoxic dose of TCDD (30 μg/kg) did not affect prostaglandin synthetase activity of rabbit liver or kidney medulla microsomes at either time point, although other microsomal enzymes (cytochrome P-488) were altered in both tissues.