Early and differential decrease in natural retinoid levels in C57BL/Rij and DBA/2 mice by 3,4,3',4'-tetrachlorobiphenyl

Constitutive androstane receptor mediates PCB-induced disruption of retinoid homeostasis

Environmental exposure to polychlorinated biphenyls (PCBs) is associated with an increased risk of incidence of metabolic disease, however the molecular mechanisms underlying this phenomenon are not fully understood. Our study provides new insights into molecular interactions between PCBs and retinoids (vitamin A and its metabolites) by defining a role for constitutive androstane receptor (CAR) in the disruption of retinoid homeostasis by non-coplanar 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153). Administration of four weekly 50 mg/kg doses of PCB153 to C57BL/6 male mice resulted in a significant decline in the tissue concentrations of retinyl esters, retinol and all-trans-retinoic acid (atRA), while no decline in hepatic and adipose tissue retinoid levels were detected in Car-null littermates. Our data imply that disrupted retinoid homeostasis occurs as a consequence of PCB153-induced activation of CAR, and raise the possibility that CAR signaling can affect atRA homeostasis in vivo. A strong correlation between the changes in retinoid metabolism and extensive upregulation of hepatic CAR-driven Cyp2b10 expression implicates this CYP isoform as contributing to retinoid homeostasis disruption via atRA oxidation during PCB153 exposure. In response to PCB153-induced CAR activation and disruption of retinoid homeostasis, expression of hepatic Pepck, Cd36 and adipose tissue Pparγ, Cd36, Adipoq, and Rbp4 were altered; however, this was reversed by administration of exogenous dietary retinoids (300 IU daily for 4 weeks). Our study establishes that PCB153 exposure enables a significant disruption of retinoid homeostasis in a CAR-dependent manner. We propose that this contributes to the obesogenic properties of PCB153 and may contribute to the predisposition to the metabolic disease.

Retinoid-xenobiotic interactions: the Ying and the Yang

The literature provides compelling evidence pointing to tight metabolic interactions between retinoids and xenobiotics. These are extensive and important for understanding xenobiotic actions in the body. Within the body, retinoids affect xenobiotic metabolism and actions and conversely, xenobiotics affect retinoid metabolism and actions. This article summarizes data that establish the importance of retinoid-dependent metabolic pathways for sustaining the body's responses to xenobiotic exposure, including the roles of all-trans- and 9-cis-retinoic acid for protecting mammals from harmful xenobiotic effects and for ensuring xenobiotic elimination from the body. This review will also consider molecular mechanisms underlying xenobiotic toxicity focusing on how this may contribute to retinoid deficiency and disruption of normal retinoid homeostasis. Special attention is paid to xenobiotic molecular targets (nuclear receptors, regulatory proteins, enzymes, and transporters) which affect retinoid metabolism and signaling.

Vitamin A physiology and its application as a biomarker of contaminant-related toxicity in marine mammals: a review

In recent decades, marine mammal populations living in highly polluted areas have experienced incidences of low reproductive success, developmental abnormalities and disease outbreaks. In many of these cases, environmental contaminants were suspected as causal or contributing factors. However, demonstrating a mechanistic link between contaminant exposure and effect in marine mammal populations has proven challenging. Consequently, the development and application of relatively noninvasive biomarkers represents a potentially valuable means of monitoring wildlife populations exposed to elevated levels of contaminants. One touted biomarker is vitamin A (retinol), a "dietary hormone" whose metabolites are required for reproduction, growth, development, immune function, vision and epithelial maintenance. Laboratory studies have shown that many contaminants, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-para-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), can disrupt vitamin A physiology and alter the distribution of its essential metabolites. Field studies suggest that complex environmental mixtures of these chemicals can also interfere with vitamin A dynamics in free-ranging marine mammals and other fish-eating wildlife. However, circulatory retinol, which is the least invasive measurement of vitamin A status, appears to have variable responses to contaminant exposure. In addition, "normal" circulatory retinol levels have not yet been described for most wildlife species, and not enough is known about the natural physiological events that can alter these concentrations. Confounding factors must therefore be characterized before retinoids can be used as an effective indicator of adverse health effects in marine mammals exposed to elevated levels of environmental contaminants.

Low hepatic 7-ethoxyresorufin-O-deethylase (EROD) activity and minor alterations in retinoid and thyroid hormone levels in flounder (Platichthys flesus) exposed to the polychlorinated biphenyl (PCB) mixture, Clophen A50

The effect of the polychlorinated biphenyl (PCB) mixture Clophen A50 on hepatic cytochrome P4501A1 dependent EROD (7-ethoxyresorufin-O-deethylase) activity, plasma thyroid hormone levels and plasma, kidney and liver retinoid concentrations of the euryhaline flatfish flounder (Platichthys flesus) was determined 2 and 10 days after i.p. (intraperitoneal) injection with 20, 100 and 500 mg Clophen A50/kg body weight. No effect of Clophen A50 on total cytochrome P450 content in flounder liver was observed at both time points. A six-fold, dose-dependent, significant increase in EROD activity was found at exposure day 10 in flounder receiving 100 or 500 mg Clophen A50/kg body weight. Plasma retinol concentrations were not altered at both time points after Clophen A50 administration, whereas renal retinol levels showed a minor dose-related increase at day 2 and day 10 of exposure. Significant alterations in hepatic retinoid concentrations were observed, which were not dependent on the dose of PCB administered. In addition Clophen A50 administration did not result in a dose-related alteration of total T4 concentrations in plasma. Total T3 concentrations in plasma were only significantly increased at day 2 after exposure, whereas free T4 concentrations were increased at both time points after Clophen A50 administration. These data indicate that with regard to the parameters investigated and in contrast to other fish species studied, the flounder is not a sensitive species to PCB exposure.

Biotransformation and toxicity of halogenated benzenes

1. Multiple potentially harmful metabolites can be distinguished in the metabolic activation of halogenated benzenes: epoxides, phenols, benzoquinones and benzoquinone-derived glutathione conjugates. 2. The role of these (re-) active metabolites in the toxic effects induced by halogenated benzenes such as hepatotoxicity, nephrotoxicity, porphyria and thyroid toxicity is discussed. 3. Evidence is presented suggesting that the formation of reactive benzoquinone metabolites rather than the traditional epoxides is linked to halogenated benzene-induced hepatotoxicity. 4. A crucial role for the benzoquinone-derived glutathione adducts in halogenated benzene-induced nephrotoxicity is clearly established. 5. Although metabolic activation appears to be involved in porphyria, the nature of the ultimate porphyrinogenic metabolite has not been elucidated yet. 6. Disturbances in thyroid hormone (and retinoid) homeostasis can be (at least partially) explained by the formation of halogenated phenol metabolites. 7. In conclusion, for a relevant prediction of the ultimate fate of a compound in a living organism, one should know the chemical characteristics and reactivity of the parent compound and its metabolites, together with insight into the formation mechanism of each of the suspected metabolites, and an understanding of the interaction between a specific chemical (reactive) structure and its target molecule.

Effects of outdoor and indoor airborne particulate matter on thyroid hormone and vitamin A metabolism

Vitamin A is an important regulator of normal epithelial differentiation and proliferation and might act in the promotion phase of carcinogenesis. Vitamin A and thyroid hormone metabolism are linked by a common plasma carrier protein transthyretin (TTR). Results indicated that extracts of outdoor and indoor airborne particulate matter (APM), originating from different pollution sources, significantly interfered with thyroxine (T4) binding to TTR. The neutral fraction accounted for most of the inhibitory activity. Polycyclic aromatic hydrocarbons and nitrated derivatives were not responsible for the activity of the neural fraction. A single treatment of rats with an outdoor and cigarette smoke APM extract depleted plasma T4 and triiodothyronine levels and increased plasma retinol levels, while liver and lung retinol levels were depleted. The studies show that APM extracts have the potency to interfere with thyroid hormone metabolism both in vitro and in vivo and to deplete lung vitamin A in vivo.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.

Effects on thyroid hormone metabolism and depletion of lung vitamin A in rats by airborne particulate matter

Thyroxine (T4) and vitamin A are important regulators of normal epithelial differentiation and proliferation and might act in the promotion phase of carcinogenesis. Thyroid hormone and vitamin A metabolism are linked by a common plasma carrier protein, transthyretin (TTR). Polychlorinated biphenyls (PCBs) and related organochlorine compounds deplete vitamin A and thyroxine by interaction with TTR and alteration of their metabolism in hepatic and other organs. In the present report an outdoor airborne particulate matter (APM) extract was tested for both interaction with thyroid hormone and vitamin A metabolism, in order to address the question of whether APM has the potency to deplete vitamin A and thyroid hormones. Furthermore, studies were performed to characterize compounds present in APM that interact with TTR. A third aim was to compare the interaction of APM extracts with TTR and thyroxine-binding globulin (TBG), the major carrier protein for thyroxine in humans. Results showed that a single treatment of rats with an outdoor APM extract depleted plasma thyroxine and triiodothyronine levels and increased plasma retinol levels gradually over the time period studied, while liver retinol, lung retinol, and retinyl palmitate levels were depleted by 30-50%. As outdoor APM was able to inhibit T4-TTR binding in vitro, this suggests that the reduction in thyroxine levels in vivo is caused by the same phenomenon. Experiments showed that the neutral fraction of the APM extract accounted for most of the inhibitory activity on T4-TTR binding. Polycyclic aromatic hydrocarbons and nitrated derivatives are not likely to be responsible for the activity of the neutral fraction, because several representatives of these compounds showed no or very little interaction with TTR. Pentachlorophenol, a compound with known inhibitory activity on T4-TTR binding, was detected in the organic acid fraction of both a cigarette smoke sample and an outdoor APM sample. Finally, it was shown that several indoor and outdoor APM extracts only interact with TTR, but not with TBG. As APM has the potency to deplete lung vitamin A in vivo and vitamin A might have a protective effect in the process of lung carcinogenesis, APM might increase the susceptibility for the development of lung cancer.

Effect of prototypic polychlorinated biphenyls on hepatic and renal vitamin contents and on drug-metabolizing enzymes in rats fed diets containing low or high levels of retinyl palmitate

Two groups of weanling male Sprague-Dawley rats fed a diet supplemented with either 0.6 or 6 retinol equivalents/g diet were each separated into three further groups receiving 300 mumol 2,2',4,4',5,5'-hexachlorobiphenyl/kg body weight, 300 mumol 3,3',4,4'-tetrachlorobiphenyl kg/body weight or vehicle only (corn oil). Only the coplanar (3,4)2Cl congener caused a slight reduction in food intake, thymic atrophy and led to a significant decrease in the liver vitamin A storage. The vitamin A lost by the liver was approximately the same in both dietary groups; however an increased renal accumulation of vitamin A was observed in the high vitamin A group. Serum retinol was reduced by (3,4)2Cl treatment but remained unchanged by (2,4,5)2Cl exposure. Total amounts of ascorbic acid and its oxidation products were increased in the liver and in the kidney by both xenobiotics while niacin and thiamine concentrations were lowered by (3,4)2Cl only. Microsomes from vitamin A-deficient rats exhibited a marked decrease in the anisotropy parameter. After (2,4,5)2Cl exposure, an increase in membrane fluidity was observed linked to a decrease in cholesterol/phospholipid (C/P) ratio. Treatment with (3,4)2Cl caused a significant decrease in the index of fluorescence polarization only in the low vitamin A group even if the C/P ratio was enhanced in both dietary groups. This study shows that the polychlorinated biphenyl with the 3-methylcholanthrene-type pattern of induction of cytochrome P-450 has more profound effects on B group vitamins and particularly vitamin A homeostasis than does the phenobarbital-type inducer. Moreover, this situation, which has been found to be similar to that in vitamin A deficiency, is not ameliorated by a high dietary vitamin A intake.

Retinyl ester hydrolase and vitamin A status in rats treated with 3,3',4, 4'-tetrachlorobiphenyl

Previous studies have shown that rats exposed to 3,3',4,4'-tetrachlorobiphenyl (TCB) exhibit decreased liver vitamin A stores. The activity of retinyl ester hydrolase (REH), the enzyme responsible for the hydrolysis of the storage form of vitamin A (retinyl esters) into free retinol, may therefore be altered by TCB. This study was carried out to investigate the effect of TCB on vitamin A distribution and on REH activity in the rat. REH activity was measured in liver homogenates and microsomes (650 micrograms protein), in Tris-maleate buffer 0.1 M at pH 7.2 in the presence of 150 mM CHAPS and 1.5 mM retinyl palmitate dispersed in Triton X-100 0.2%. Using these conditions, the kinetic parameters of the enzyme were determined and the inter-animal variation coefficient (10%) allowed statistical comparisons between experimental groups. Male Wistar rats of sufficient or deficient vitamin A status were treated IP with 340 mumol of TCB/kg. Vitamin A levels were significantly depressed in liver. REH activity was decreased about 20%, and serum retinol was decreased about 50%, independent of the initial vitamin A status of the animals. Vitamin A levels in lungs and testes were also decreased, suggesting that TCB could interfere with vitamin A delivery to target organs. The negative effect of TCB on REH activity in vivo was also observed when TCB was added in vitro to the incubation medium at concentrations near to those expected after in vivo treatment. TCB is a non-competitive inhibitor of retinyl palmitate hydrolase.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)

Halogenated aromatic compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and diphenylethers (PCDEs), are industrial compounds or byproducts which have been widely identified in the environment and in chemical-waste dumpsites. Halogenated aromatics are invariably present in diverse analytes as highly complex mixtures of isomers and congeners and this complicates the hazard and risk assessment of these compounds. Several studies have confirmed the common receptor-mediated mechanism of action of toxic halogenated aromatics and this has resulted in the development of structure-activity relationships for this class of chemicals. The most toxic halogenated aromatic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and based on in vivo and in vitro studies the relative toxicities of individual halogenated aromatics have been determined relative to TCDD (i.e., toxic equivalents). The derived toxic equivalents can be used for hazard and risk assessment of halogenated aromatic mixtures; moreover, for more complex mixtures containing congeners for which no standards are available (e.g., bromo/chloro mixtures), several in vitro or in vivo assays can be utilized for hazard or risk assessment.

Marked alterations in retinoid homeostasis of Sprague-Dawley rats induced by a single i.p. dose of 10 micrograms/kg of 2,3,7,8-tetrachlorodibenzo-p-dioxin

Interference of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in retinoid homeostasis was investigated in Sprague-Dawley rats with a low (dietary induced) retinoid status, that were fed a [3H]retinol-containing diet (37 MBq, 10,000 IU/kg diet) for 21 days to facilitate determination of retinoid concentrations in various tissues. The rats were exposed to a single i.p. dose of 10 micrograms TCDD/kg body weight in corn oil, or to corn oil at day 7 of [3H]retinol supplementation. TCDD induced significant reductions in retinol and retinyl ester concentrations and [3H] retinol-derived radioactivity in the liver, the lung, the intestine and the adrenals to 3-5%, 40-45%, 37%, and 56% of control values, respectively, at 14 days after exposure. In contrast, the retinoid concentrations and the amount of [3H]retinol-derived radioactivity in the kidney and serum of TCDD-treated rats was increased to 440% and 140% of corn oil-treated controls, respectively, at the termination time of the experiment. Analysis of the amount of serum retinol binding protein (RBP) by gel-permeation chromatography revealed an 150% increase in the free fraction of retinol-RBP, i.e., uncoupled to transthyretin (TTR), in serum of TCDD-treated rats. In addition, urinary excretion of [3H]retinol-derived radioactivity was significantly enhanced (to 140% of controls) by TCDD. These data indicate that TCDD induces an increased mobilization of retinoids from hepatic and extrahepatic storage sites into serum accompanied by an enhanced elimination via the kidney into the urine of rats.

Effects of dietary retinyl acetate on the promotion of hepatic enzyme-altered foci by polybrominated biphenyls in initiated rats

Vitamin A inhibits the development of some chemically-induced tumours. Since polybrominated biphenyls (PBBs) are hepatic tumour promoters and they affect vitamin A homeostasis in rats, we put forward the hypothesis that dietary levels of vitamin A would influence tumour promotion by PBBs. In the study described here, female Sprague-Dawley rats were initiated on day 1 by ip administration of diethylnitrosamine. On day 7 after initiation, the rats were fed a vitamin A-deficient basal diet that was supplemented with either 2000 IU (low-vitamin A) or 200,000 IU (high-vitamin A) retinyl acetate/kg feed. From day 30 after initiation until the end of the study the following PBBs were added to the diets: Firemaster BP-6 (10 ppm), 2,4,5,2',4',5'-hexabromobiphenyl (10 ppm) or 3,4,5,3',4',5'-hexabromobiphenyl (1 ppm). The control animals received low- or high-vitamin A diets containing no PBBs. On day 180, the rats were necropsied, sections of various tissues were stained for histopathological examination and an evaluation of hepatic enzyme-altered foci was performed. Numbers of gamma-glutamyl transpeptidase-positive foci/cm3 liver and the mean volumes of these foci were lower in the high-vitamin A groups than those in the corresponding low-vitamin A groups, but these differences were not significant. The percentage of the liver volume occupied by foci was significantly greater in the low-vitamin A with 345-HBB group than in the corresponding high-vitamin A group. Thus, high dietary levels of vitamin A had some inhibitory effect on the promotion of hepatic-altered foci by 345-HBB in initiated rats.

Alterations in retinoid concentrations in several extrahepatic organs of rats by 3,4,3',4'-tetrachlorobiphenyl

In this study, the effect of polychlorinated biphenyls on retinoid homeostasis was investigated in Sprague-Dawley rats, by analysing [3H]retinoid concentrations in peripheral organs, following exposure to 3,4,3',4'-tetrachlorobiphenyl (TCB). The rats were rendered retinoid-deficient through dietary restriction, followed by dietary supplementation with [3H]retinol for 14 days, in order to facilitate determination of retinoid concentrations in various tissues. At day 7 of [3H]retinol supplementation the rats were exposed to a single i.p. dose of 15 mg TCB dissolved in corn oil/kg body weight. In corn oil-treated control rats, the highest concentrations of [3H]retinoid radioactivity, consisting mainly of retinol and several retinylesters, were obtained in the liver (greater than 10(6) cpm/g,), followed by the kidney and the lung, while only minor concentrations were found in skin and heart. Exposure to TCB resulted in a significant reduction of both retinol and retinylester concentrations in the liver (to 25% of controls) and the lung (to 44% of controls), while in the heart a reduction of retinol to 35% of controls was observed. No significant alterations in retinoid concentrations were observed in the skin and kidney. It is suggested that the reductions in retinoid concentrations might contribute to the toxicological alterations reported in these organs upon exposure to TCB.

Chronic toxicity of 3,4,3',4'-tetrachlorobiphenyl in the marmoset monkey (Callithrix jacchus)

Cotton top marmoset monkeys (Callithrix jacchus) were orally dosed with 3, 1, 0.1 or 0 mg 3,4,3',4'-tetrachlorobiphenyl (TCB)/kg body weight twice per week for 18-23 weeks. Severe toxicity occurred in the highest dose group. Clinical signs of toxicity were a rapid decrease in body weight, alopecia, abnormal nail growth, nodular enlargement of the nipple area and scaly skin. Haematological analysis of peripheral blood revealed mild leukocytosis and anemia. Biochemical alterations observed were elevated triglyceride levels and cholesterol levels. Histopathology revealed dose dependent changes in a variety of tissues. Squamous metaplasia was found in skin and adnexa as well as in salivary glands. In the stomach, parietal cells were decreased and mucus producing cells were increased. The duodenal mucosa was hyperplastic. Ovaries showed an absence of corpora lutea. In the thyroid follicular cell hyperplasia and hypertrophy were noted. Toxicity was less severe in marmoset monkeys dosed with 1 mg TCB/kg, while minor toxic effects were observed in the animals dosed with 0.1 mg TCB/kg. The marmoset monkey appears to be less sensitive to the toxic action of TCB than the rhesus monkey. The pattern of histological and biochemical changes induced by TCB in marmoset monkeys is comparable to that described in humans and in other primate species exposed to PCBs. The marmoset monkey model may be valuable for investigations on human-related toxicity of PCBs.

Study on the mechanism of interference of 3,4,3',4'-tetrachlorobiphenyl with the plasma retinol-binding proteins in rodents

The mechanism of plasma retinol reduction in rodents by 3,4,3',4'-tetrachlorobiphenyl (TCB) was investigated by radioimmunochemical analysis of the amounts of circulating and hepatic retinol-binding protein (RBP) and transthyretin (TTR) in exposed and control animals. Plasma RBP concentrations were markedly reduced in C57BL/Rij mice (50%) at 4 days, in DBA/2 mice (37-41%) at 4 and 8 days, and in Sprague-Dawley rats (58%) at 2 days after exposure to TCB. These reductions paralleled the time course of reduction of plasma retinol after exposure to TCB. Hepatic RBP concentrations were somewhat increased in TCB-treated animals, especially in the C57BL/Rij mouse and Sprague-Dawley rat. However, the release of hepatic RBP into the circulation was not blocked by TCB treatment, as analysed in vitamin A deficient rats. In addition, the amount of plasma TTR was in the normal range in TCB-treated rats. The dissociation constants of the RBP-TTR complex as analysed by polarization of fluorescence appeared to be significantly increased (from 0.5 x 10(-7) M-1 to 2.4 x 10(-7) M-1) in the presence of a TCB metabolite, isolated from plasma of TCB-treated rats. In addition, the estimated number of binding sites for RBP on the TTR molecule was reduced (from 2.8 to 1.7 sites) upon treatment of TTR with the TCB metabolite. These data support the hypothesis that plasma retinol reduction by TCB might result from a weakening of the RBP-TTR complex, in the presence of the TCB metabolite bound to the TTR.

Vitamin A metabolism in rats chronically treated with 3,3',4,4',5,5'-hexabromobiphenyl

Chronic dietary administration of 3,3',4,4',5,5'-hexabromobiphenyl (HBB), 1 mg/kg diet, caused a decrease in retinol (20-fold) and retinyl esters (23-fold) in the livers of female rats, but resulted in a 6.4-fold increase in retinol and 7.4-fold increase in retinyl esters in the kidneys. Liver acyl-CoA:retinol acyltransferase and retinyl palmitate hydrolase activities were reduced while serum concentration of retinol was unaffected by HBB feeding. Metabolism of a physiological dose of [11-3H]retinyl acetate (10 micrograms), was examined in rats fed either vitamin A-adequate diet, or marginal amounts of vitamin A, or vitamin A-adequate diet containing HBB. A 13-fold greater amount of the administered vitamin A was found in kidneys of HBB-treated rats. In rats fed adequate or low amounts of vitamin A, kidney radioactivity was primarily in the retinol fraction, while in HBB-fed rats the radioactivity was associated mostly with retinyl esters. Fecal and urinary excretion of radioactivity was greatly increased in HBB-treated rats. Chronic HBB feeding results in a loss of ability of liver to store vitamin A, and severely alters the uptake and metabolism of vitamin A in the kidneys. We conclude that HBB causes major disturbances in the regulation of vitamin A metabolism.

Rapid oxidative stress induced by N-nitrosamines

We have investigated the generation of prooxidant state shortly after administration of N-nitrosamines (NA) to rats. N-Nitrosodimethylamine (NDMA) was found to increase ethane exhalation (EE) rapidly in a dose-related manner. EE remained elevated for several days after single doses of NDMA. Similarly, lipid peroxidation (LP) in the liver (measured by four methods) increased rapidly showing a peak 20 min after NDMA dose. The increase of LP was preceded by a decrease in retinol concentration in the liver. N-Nitrosodiethanolamine, too, increased EE and LP in the liver, whereas N-nitrosomethylbenzylamine had no effect. Thus, hepatocarcinogenic NA induced LP in their target tissue, and the LP enhancing effects of NA were not related to their acute toxic effects.

The effect of 3,4,3',4'-tetrachlorobiphenyl on plasma retinol and hepatic retinyl palmitate hydrolase activity in female Sprague-Dawley rats

The Effect of 3,4,3',4'-Tetrachlorobiphenyl on Plasma Retinol and Hepatic Retinyl Palmitate Hydrolase Activity in Female Sprague-Dawley Rats. Powers, R.H., Gilbert, L.C., and Aust, S.D. (1987). Toxicol Appl. Pharmacol. 89, 370-377. A single ip dose of 1, 5, or 15 mg/kg 3,4,3',4'-tetrachlorobiphenyl (TCB) caused a dose-dependent depression of plasma retinol levels 24 hr after treatment of female Sprague-Dawley rats. The loss of plasma retinol appeared to be a function of depressed levels of the retinol-retinol-binding protein (RBP)-transthyretin ternary complex. No free retinol-RBP was observed in plasma from treated animals. Hepatic retinyl palmitate hydrolase (RPH) activity was also depressed and highly and positively correlated to the plasma retinol levels. TCB was determined to be a noncompetitive inhibitor of partially purified RPH with a KI of 91 microM. Incubation of TCB with liver microsomes and NADPH decreased the inhibition of RPH. Doses of either 2,4,5,2',4',5'-hexachlorobiphenyl (HCB) or 3,4,5,3',4',5'-HCB equimolar to the 15 mg/kg TCB dose failed to cause a similar depression of plasma retinol in treated female rats. We conclude that, unlike other polychlorinated biphenyl congeners, TCB causes a depression of plasma retinol by inhibition of hepatic RPH.

A time-course investigation of vitamin A levels and drug metabolizing enzyme activities in rats following a single treatment with prototypic polychlorinated biphenyls and DDT

Xenobiotics previously characterized as selective inducers of drug-metabolizing enzymes were chosen to probe possible relationships between enzyme induction and vitamin A metabolism. Liver, kidney and serum retinol and retinyl palmitate levels were investigated in male Sprague--Dawley rats receiving a single i.p. injection of the polychlorinated biphenyls (PCBs), 2,2',5,5'-tetrachlorobiphenyl, 3,3',4,4'-tetrachlorobiphenyl or 2,2',4,4',5,5'-hexachlorobiphenyl (300 mumol/kg) or 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (DDT) (150 mumol/kg). While 2,2',5,5'-tetrachlorobiphenyl, a weak or non-inducer, and 2,2',4,4',5,5'-hexaclorobiphenyl and DDT, phenobarbital-type inducers of cytochrome P-450, led to no reduction in total vitamin A content of liver or kidney during the 7 day time-course, administration of 3,3',4,4'-tetrachlorobiphenyl, a toxic PCB and a potent 3-methylcholanthrene-type inducer of cytochrome P-450, resulted in progressively lowered liver vitamin A levels (to 40% of control values by day 7). During this time, kidney total vitamin A content increased 3-fold. The increase in kidney vitamin A (due primarily to increased retinol content) was only equal to 1/40 of total vitamin A which had disappeared from the liver. Although 3,3',4,4'-tetrachlorobiphenyl specifically induced certain drug-metabolizing enzyme activities, e.g. aryl hydrocarbon hydroxylase and UDP-glucuronosyltransferase (toward 4-nitrophenol), no highly significant correlations were found among the vitamin A levels and drug-metabolizing enzyme activities in the liver (aminopyrine N-demethylase, aryl hydrocarbon hydroxylase, aldrin epoxidase, microsomal epoxide hydrolase, UDP-glucuronosyltransferase toward 4-nitrophenol, glutathione transferase toward 1-chloro-2,4-dinitrobenzene and cytochrome P-450 content) as determined by multiple linear regression analysis.

Binding of a metabolite of 3,4,3',4'-tetrachlorobiphenyl to transthyretin reduces serum vitamin A transport by inhibiting the formation of the protein complex carrying both retinol and thyroxin

The mechanism of serum vitamin A reduction by polychlorinated biphenyls was studied at the level of the plasma transport protein system for vitamin A. Analysis of [3H]retinol-labeled serum proteins by polyacrylamide gel electrophoresis (PAGE) showed association of retinol with two proteins that were identified as retinol binding protein (RBP) and the RBP complex with transthyretin (TTR). The amount of [3H]retinol radioactivity in the serum as well as the label associated with the binding proteins was strongly reduced by 3,4,3',4'-tetrachlorobiphenyl (TCB). A possible interaction of TCB with the retinol binding proteins was investigated, using radiolabeled TCB. Analysis of the plasma proteins by PAGE revealed the presence of four peaks of 3H-TCB label, the major ones being associated with lipoproteins and TTR. No 3H-TCB radioactivity was found in the region of the gel where RBP or the RBP-TTR complex was located. HPLC analysis of the radioactive compound associated with TTR showed the presence of a metabolite of TCB, rather than the parent compound. These data indicate a direct interaction of a metabolite of TCB with TTR leading to an inhibition of formation of the serum transport protein complex carrying both retinol and thyroxin. A model is proposed, which may explain certain characteristic toxicopathological lesions observed in species exposed to polychlorinated biphenyls and related compounds (TCDD, PBBs, etc.).

The effects of nonadecafluoro-n-decanoic acid on serum retinol and hepatic retinyl palmitate hydrolase activity in male Sprague-Dawley rats

The effects of nonadecafluoro-n-decanoic acid (NDFDA) on serum retinol levels and hepatic retinyl palmitate hydrolase (RPH) activity were investigated in male Sprague-Dawley rats given a single intraperitoneal (IP) dose of 0, 50, or 100 mg/kg NDFDA and sacrificed at two, eight, or 11 days. Treated animals exhibited depressed serum retinol levels, lymphoid involution, and failure to gain weight in proportion to the dose. Hepatic RPH activities were depressed in both treatment groups at all time points and correlated with serum retinol levels. Hepatic retinol levels were also depressed by Day 11. Extraction of hepatic homogenates with acetone removed NDFDA and increased RPH activities twofold and threefold for the low- and high-dose groups, respectively. Analysis of partially purified RPH showed both NDFDA and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to be noncompetitive inhibitors: KI = 450 and 750 microM, respectively. We conclude that NDFDA causes a decrease in the mobilization of vitamin A from the liver by noncompetitive inhibition of RPH.

Purified rat liver fat-storing cells in culture divide and contain collagen

Primary cultures and cell lines were established from suspensions of purified fat-storing cells isolated from the rat liver. When seeded at a suitable density, fat-storing cells in primary culture reached confluency in 3 to 4 days and could be transferred and established as cell lines for at least two passages. The typical morphological characteristics of fat-storing cells in vivo were retained in the cells during primary culture. Vitamin A fluorescence was still associated with lipid droplets of cells in culture up to and including the second passage. Investigation of the cytoskeletal structure by indirect immunofluorescence showed the presence of vimentin, actin and tubulin in the cells; no alpha-prekeratin was present. The presence of vimentin suggested a fibroblastic or possible myogenic origin for fat-storing cells. The presence of connective tissue components in fat-storing cells in culture was demonstrated by indirect immunofluorescence. Collagen Types I and IV and laminin were present intracellularly in small granules in fat-storing cells in primary culture and in the first passage. Cells in the fourth passage contained only collagen Type 1. Fibronectin was only aligned extracellularly along the cell membrane, which did not exclude an extracellular source. Rat liver fat-storing cells in culture show a high proliferating capacity. Cell multiplication during prolonged culture was associated with phenotypic transition to a more fibroblastic appearance and gradual disappearance of vitamin A. These results indicate that fat-storing cells may be among the cell types involved in pathological changes observed during development of liver fibrosis.