2,3,7,8-Tetrachlorodibenzo-p-dioxin affects retinol esterification in rat hepatic stellate cells and kidney

Role of aryl hydrocarbon receptor (AHR) in overall retinoid metabolism: Response comparisons to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure between wild-type and AHR knockout mice

Young adult wild-type and aryl hydrocarbon receptor knockout (AHRKO) mice of both sexes and the C57BL/6J background were exposed to 10 weekly oral doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; total dose of 200 μg/kg bw) to further characterize the observed impacts of AHR as well as TCDD on the retinoid system. Unexposed AHRKO mice harboured heavier kidneys, lighter livers and lower serum all-trans retinoic acid (ATRA) and retinol (REOH) concentrations than wild-type mice. Results from the present study also point to a role for the murine AHR in the control of circulating REOH and ATRA concentrations. In wild-type mice, TCDD elevated liver weight and reduced thymus weight, and drastically reduced the hepatic concentrations of 9-cis-4-oxo-13,14-dihydro-retinoic acid (CORA) and retinyl palmitate (REPA). In female wild-type mice, TCDD increased the hepatic concentration of ATRA as well as the renal and circulating REOH concentrations. Renal CORA concentrations were substantially diminished in wild-type male mice exclusively following TCDD-exposure, with a similar tendency in serum. In contrast, TCDD did not affect any of these toxicity or retinoid system parameters in AHRKO mice. Finally, a distinct sex difference occurred in kidney concentrations of all the analysed retinoid forms. Together, these results strengthen the evidence of a mandatory role of AHR in TCDD-induced retinoid disruption, and suggest that the previously reported accumulation of several retinoid forms in the liver of AHRKO mice is a line-specific phenomenon. Our data further support participation of AHR in the control of liver and kidney development in mice.

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.

Retinoid metabolism in invertebrates: when evolution meets endocrine disruption

Recent genomic and biochemical evidence in invertebrate species pushes back the origin of the retinoid metabolic and signaling modules to the last common ancestor of all bilaterians. However, the evolution of retinoid pathways are far from fully understood. In the majority of non-chordate invertebrate lineages, the ongoing functional characterization of retinoid-related genes (metabolism and signaling pathways), as well as the characterization of the endogenous retinoid content (precursors and active retinoids), is still incomplete. Despite limited, the available data supports the presence of biologically active retinoid pathways in invertebrates. Yet, the mechanisms controlling the spatial and temporal distribution of retinoids as well as their physiological significance share similarities and differences with vertebrates. For instance, retinol storage in the form of retinyl esters, a key feature for the maintenance of retinoid homeostatic balance in vertebrates, was only recently demonstrated in some mollusk species, suggesting that such ability is older than previously anticipated. In contrast, the enzymatic repertoire involved in this process is probably unlike that of vertebrates. The suggested ancestry of active retinoid pathways implies that many more metazoan species might be potential targets for endocrine disrupting chemicals. Here, we review the current knowledge about the occurrence and functionality of retinoid metabolic and signaling pathways in invertebrate lineages, paying special attention to the evolutionary origin of retinoid storage mechanisms. Additionally, we summarize existing information on the endocrine disruption of invertebrate retinoid modules by environmental chemicals. Research priorities in the field are highlighted.

Vitamin A and E profiles as biomarkers of PCB exposure in beluga whales (Delphinapterus leucas) from the western Canadian Arctic

We evaluated the utility of vitamin A and E profiles as biomarkers of contaminant exposure in beluga whales (Delphinapterus leucas; n=66) harvested by the Inuvialuit in the Beaufort Sea. Blubber was an important repository for these vitamins, accounting for 76.8±2.6% of the total body store of vitamin A, and 98.5±0.4% of total vitamin E. While the free alcohol form of vitamin A (retinol) appeared highly regulated, the vitamin A esters were influenced by several biological factors including age, body condition and length. Vitamin E concentrations in liver and blubber were related to age, condition, length and feeding ecology, as described δ(15)N and δ(13)C. Despite the influence of these factors, collective results from univariate statistics, best fit multiple regressions, and principal component analysis (PCA) identified polychlorinated biphenyls (PCBs) as important determinants of vitamin concentrations and profiles in beluga tissues. Blubber PCB concentrations best explained variation of the first principal component in a PCA of hepatic vitamins (r(2)=0.13, p=0.014), and regression models found that vitamin A concentrations were negatively correlated with PCB levels in liver (esters: r(2)=0.19, p=0.001), but positively in plasma (retinol: r(2)=0.20, p=0.06) and blubber (retinol: r(2)=0.22, p=0.001, esters: r(2)=0.43, p<0.001). Our analyses provide a basis to propose an integrated toxicity reference value for disruption of vitamin A and E profiles in beluga of 1.6 mg/kg lw PCBs. This disruption of vitamin profiles by moderate levels of PCBs in an arctic cetacean highlights the global reach and impact of these legacy chemicals decades after their peak use.

Contrasting retinoid and thyroid hormone status in differentially-contaminated northern fulmar colonies from the Canadian Arctic, Svalbard and the Faroe Islands

The northern fulmar (Fulmarus glacialis) has previously been shown to accumulate a wide range, and occasionally high concentrations of organochlorines (OCs) (e.g., PCBs, chlorobenzenes, DDT- and chlordane-related compounds, dioxins and furans). The present study aimed to investigate, using a meta-analysis approach, the variations in cytochrome P450 (CYP) 1A-like enzyme induction based on ethoxyresorufin O-deethylase activity (EROD) and selected physiological variables (retinoids and thyroid hormones) in northern fulmar breeding in three differentially OC-exposed populations: Nunavut (Canadian Arctic), Svalbard (Norwegian Arctic) and the Faroe Islands. Substantially higher (roughly two-fold) OC levels were uncovered in the liver of this long-lived fulmarine petrel breeding in the Faroe Islands relative to Svalbard and Nunavut. Liver levels of PCDDs, PCDFs and non-ortho PCBs in Faroe Islands fulmars were amongst the highest reported thus far in any seabirds from the northern regions. Positive correlations were depicted in combined fulmars (all three populations) between hepatic EROD activity and concentrations of OCs, in which strongest associations were found for dioxin-like compound (PCDFs and PCDDs) and TEQ concentrations. Moreover, moderate to strong positive correlations were found between liver OC concentrations and plasma total thyroxin (TT(4)) levels and TT(4)/total triiodothyronine (TT(3)) level ratios, as well as strong negative correlations between the same suite of OCs and plasma TT(3) levels. Hepatic OC concentrations (PCBs, PCDDs, PCDFs, HCB, p,p'-DDE and oxychlordane) also were positively correlated with hepatic retinyl palmitate levels which, in turn, were associated with a significant decrease in plasma retinol levels and somewhat unchanged liver retinol levels. The present meta-analysis investigation on northern fulmar breeding in three geographically-distant sites illustrated that OC exposure (mainly PCBs and dioxins/furans) may be associated with modulation of the thyroid and retinoid homeostasis. However, the impact of confounding environmental factors (e.g., temperature and nutritional status) on current physiological variable variations could not be ruled out, and thus any cause-effect linkages between thyroid and retinoid system perturbation and OC exposure cannot be ascertained.

Effects of polychlorobiphenyls, polybromodiphenylethers, organochlorine pesticides and their metabolites on vitamin A status in lactating grey seals

Polychlorobiphenyls (PCBs), polybromodiphenylethers (PBDEs) and organochlorine pesticides (OCPs), such as dichlorodiphenyltrichloroethane (DDT) and hexachlorobenzene (HCB), are considered as endocrine disruptors in laboratory and wild animals. This study investigated whether these compounds and their hydroxylated metabolites (HO-PCBs and HO-PBDEs) may affect the homoeostasis of vitamin A, a dietary hormone, in the blubber and serum of twenty lactating grey seals sampled at early and late lactation on the Isle of May, Scotland. The effect of naturally produced compounds such as the methoxylated (MeO)-PBDEs was also examined. Vitamin A levels in inner blubber (37±9 μg/g wet weight (ww) and 92±32 μg/g ww at early and late lactation, respectively) and serum (408±143 and 390±98 ng/ml at early and late lactation, respectively) appeared to be positively related to ΣPCBs, ΣPBDEs and several individual PCB and PBDE congeners in inner blubber and serum. These findings may suggest enhanced mobilisation of hepatic retinoid stores and redistribution in the blubber, a storage site for vitamin A in marine mammals. We have also reported that serum concentrations of ΣHO-PCBs and 4-OH-CB107 tended to increase with circulating vitamin A levels. Although the direction of the relationships may sometimes differ from those reported in the literature, our results are in agreement with previous findings highlighting a disruption of vitamin A homoeostasis in the blubber and bloodstream following exposure to environmental pollutants. The fact that vitamin A and PCBs appeared to share common mechanisms of mobilisation and transfer during lactation in grey seals (Debier et al., 2004; Vanden Berghe et al., 2010) may also play a role in the different relationships observed between vitamin A and lipophilic pollutants.

Retinoid metabolism (LRAT, REH) in the liver and plasma retinoids of bullfrog, Rana catesbeiana, in relation to agricultural contamination

Retinoids have been extensively studied in birds, fish and mammals where their imbalances are associated with adverse effects on growth and reproduction along with decreased embryo survival and deformities. Organochlorine compounds may alter the retinoid system but little is known about the effects of agricultural contaminants on their metabolism. In the Yamaska River project, the retinoid system in bullfrogs is monitored to investigate the possible impact of agricultural contaminants on retinoid homeostasis. Retinoids were measured in liver and plasma of male bullfrogs collected from six locations subject to increasing agricultural activity in the Yamaska River watershed. Bullfrogs living in medium and high agricultural activity areas demonstrated lower hepatic retinyl palmitate and higher hepatic retinol levels when compared to frogs associated with low contaminated sites. Changes in the concentration of hepatic esters could be related to an altered activity of REH or LRAT, enzymes respectively linked to the hydrolysis of retinyl palmitate and the esterification of retinol. A partial characterization and the analysis of liver microsomial REH and LRAT showed significantly higher hydrolysis and lower esterification activities in highly contaminated sites. Enzymatic activities seemed to be influenced by plasma DROH but not by plasma retinol. Bullfrogs from the most contaminated sites showed altered retinoic metabolism that should increase concern for frogs living in intensive agricultural areas.

Disruption of retinoid transport, metabolism and signaling by environmental pollutants

Although the assessment of circulatory levels of retinoids has become a widely used biomarker of exposure to environmental pollutants, the adverse effects caused by imbalance of the retinoid metabolism and signaling in wildlife are not known in detail. Retinoids play an important role in controlling such vital processes as morphogenesis, development, reproduction or apoptosis. Unlike other signaling molecules, retinoids are not strictly endogenous but they are derived from dietary sources of vitamin A or its precursors and thus they are sometimes referred to as 'dietary' hormones. Some environmental pollutants that affect embryogenesis, immunity or epithelial functions were also shown to interfere with retinoid metabolism and signaling in animals. This suggests that at least some of their toxic effects may be related to interaction with the retinoid metabolism, transport or signal transduction. This review summarizes in vivo and in vitro studies on interaction of environmental complex samples, pesticides, polychlorinated dioxins, polychlorinated biphenyls, polycyclic aromatic compounds and other organic pollutants with physiology of retinoids. It sums up contemporary knowledge about levels of interaction and mechanisms of action of the environmental contaminants.

A 28-Day Oral Dose Toxicity Study Enhanced to Detect Endocrine Effects of Hexabromocyclododecane in Wistar Rats

A 28-day repeated dose study in rats (OECD407) enhanced for endocrine and immune parameters was performed with hexabromocyclododecane (HBCD). Rats were exposed by daily gavage to HBCD dissolved in corn oil in 8 dose groups with doses ranging between 0 and 200 mg/kg bw per day (mkd). Evaluation consisted of dose-response analysis with calculation of a benchmark dose at the lower 95% one-sided confidence bound (BMDL) at predefined critical effect sizes (CESs) of 10-20%. The most remarkable findings were dose-related effects on the thyroid hormone axis, that is, decreased total thyroxin (TT4, BMDL 55.5 mkd at CES--10%), increased pituitary weight (29 mkd at 10%) and increased immunostaining of TSH in the pituitary, increased thyroid weight (1.6 mkd at 10%), and thyroid follicle cell activation. These effects were restricted to females. Female rats also showed increased absolute liver weights (22.9 mkd at 20%) and induction of T4-glucuronyl transferase (4.1 mkd at 10%), suggesting that aberrant metabolization of T4 triggers feedback activation of the thyroid hormone system. These effects were accompanied by possibly secondary effects, including increased cholesterol (7.4 mkd at 10%), increased tibial bone mineral density (> 49 mkd at 10%), both in females, and decreased splenocyte counts (0.3-6.3 mkd at 20%; only evaluated in males). Overall, female rats appeared to be more sensitive to HBCD than male rats, and an overall BMDL is proposed at 1.6 mkd, based on a 10% increase of the thyroid weight, which was the most sensitive parameter in the sequence of events.

Changes in thyroid and vitamin A status in mink fed polyhalogenated-aromatic-hydrocarbon-contaminated carp from the Saginaw River, Michigan, USA

A study was conducted to determine whether changes in thyroid and vitamin A dynamics were induced in ranch mink exposed to environmentally relevant concentrations of polyhalogenated aromatic hydrocarbons. Adult female mink were fed diets that contained 0% (control), 10%, 20%, or 30% wild carp (Cyprinus carpio) collected from the Saginaw River, Michigan, USA. Total polychlorinated biphenyls concentrations were 0.03, 0.83, 1.05, and 1.69 mg/kg feed, respectively; the 2,3,7,8-tetrachlorodibenzo-p-diozin toxic equivalents were 3.4, 27.9, 47.6, and 73.2 ng/kg, respectively. Diets were fed 3 weeks prior to breeding and throughout gestation and lactation. When the kits were weaned at 6 weeks of age, they were continued on their respective diets until 27 weeks of age. Plasma thyroid hormone concentrations, thyroid gland activity and structure, and vitamin A dynamics were assessed in young mink at 6 and 27 weeks of age. Plasma total T4 and free T4 in 6-week-old female and male kits fed the 10% carp diet were significantly higher than those of controls, while kits fed the 20% and 30% carp diet had nonsignificant decreases relative to the control mink. Plasma total T3 concentrations in 27-week-old juvenile males fed the 30% carp diet were significantly lower than those in individuals fed the 10% carp diet. No overt thyroid toxicity was apparent as thyroid weight, activity, and structure in kits and juveniles of both sexes were similar among diet groups. Plasma retinol and total ester concentrations in both kits and juveniles were reduced in mink fed the 30% carp diet relative to controls. The ratio of retinol to retinyl palmitate in livers of juveniles fed the 30% carp diet was two times higher than that in control mink. Significant reductions in kidney retinol and fatty acyl retinyl esters were observed in kits and juveniles fed the 30% carp diet relative to control values.

Retinoid status and responsiveness to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice lacking retinoid binding protein or retinoid receptor forms

We have investigated the role of Vitamin A (retinoid) proteins in hepatic retinoid processing under normal conditions and during chemical stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a chemical known to interfere with retinoid turnover and metabolism. Three separate studies were performed in wildtype control mice and transgenic mice that lack one or more isoforms of retinoic acid receptors (RAR), retinoid X receptors (RXR), or intracellular retinoid-binding proteins (CRABP I, CRABP II, CRBP I). Body and organ weight development was monitored from 2 weeks of age to adult, and hepatic levels of retinyl esters, retinol, and retinoic acid were investigated. In addition, hepatic concentrations of 9-cis-4-oxo-13,14-dihydro-retinoic acid, a recently discovered retinoid metabolite that has proven sensitive to both TCDD exposure and Vitamin A status, were also determined. Mice absent in the three proteins CRBP I, CRABP I, and CRABP II (CI/CAI/CAII-/-) displayed significantly lower hepatic retinyl ester, retinol, and all-trans-retinoic acid levels compared to wildtype mice, whereas the liver concentrations of 9-cis-4-oxo-13,14-dihydro-retinoic acid was considerably higher. After treatment with TCDD, hepatic total retinoids were almost entirely depleted in the CI/CAI/CAII-/- mice, whereas wildtype mice and mice lacking CRABP I, and CRABP II (CAI/CAII-/-) retained approximately 60-70% of their Vitamin A content compared to controls at 28 days. RAR and RXR knockout mice responded similarly to wildtype mice with respect to TCDD-induced retinoid disruption, with the exception of RXRbeta-/- mice which showed no decrease in hepatic Vitamin A concentration, suggesting that the role of RXRbeta in TCDD-induced retinoid disruption should be further investigated. Overall, the abnormal retinoid profile in the triple knockout mice (CI/CAI/CAII-/-), but not double knockout (CAI/CAII-/-) mice, suggests that a loss of CRBP I may account for the difference in retinoid profile in CI/CAI/CAII-/- mice, and is likely to result in an increased susceptibility to hepatic retinoid depletion following dioxin exposure.

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

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

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.

2,3,7,8-tetrachlorodibenzo-p-dioxin induces lecithin: retinol acyltransferase transcription in the rat kidney

Vitamin A (retinoids) has an essential role in development and throughout life of humans and animals. Consequently, effects of the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on retinoid metabolism may be contributory to its toxicity. This study was performed to clarify the mechanism behind dioxin-induced retinyl ester formation in the rat kidney. In addition we investigated the possible role of CYP1A1 in dioxin-induced all-trans-retinoic acid (atRA) formation. Male Sprague-Dawley rats were exposed to a single oral dose of TCDD in a combined dose-response and time-course study, with doses ranging from 0.1 to 100 microg/kg bw and time points from 1 to 28 days. Levels of atRA and the expression of two potentially retinoic acid (RA)-controlled proteins critically involved in retinoid storage regulation, lecithin: retinol acyltransferase (LRAT) and cellular retinol binding protein I (CRBP I), were analyzed in liver and kidney. The expression and activity of cytochrome P4501A1 (assayed as ethoxyresorufin-O-deethylase activity) was assessed to gain insight into its potential role in RA synthesis. There was a significant increase in LRAT mRNA expression in the kidney, whereas no such increase could be observed in the liver, despite significantly increased atRA levels in both tissues. This suggests a tissue-specific regulation of LRAT by TCDD that may be dependent on other factors than atRA. Neither CRBP I mRNA nor protein levels were altered by TCDD. The time-course relationship between CYP1A1 activity and atRA levels in liver and kidney does not exclude a role of CYP1A1 in TCDD-induced RA synthesis. The observed altered regulation of the retinoid-metabolizing enzyme LRAT, together with the low doses and short time required by TCDD to change tissue RA levels, suggest that enzymes involved in retinoid metabolism are specific and/or direct targets of TCDD.

Retinoid metabolism (LRAT, REH) in the yolk-sac membrane of Japanese quail eggs and effects of mono-ortho-PCBs

Retinoids stored in the avian egg are essential for normal development, however, laboratory and field experiments suggest that they are affected by environmental contaminants. Lecithin:retinol acyltransferase (LRAT) activity was detected in the microsomal fraction of the yolk-sac membrane of the Japanese quail at day 6 of development. LRAT activity was maximal at pH 7.0 having apparent kinetic parameters of K(m)=1.35 microM and V(max)=0.21 nmol/mg protein/h and was inhibited by the sulfhydryl modifying agent N-ethyl-maleimide. Retinol ester hydrolase (REH) activity in the microsomal fraction of the yolk-sac membrane was stimulated by the bile salt analogue 3-[(3-cholamidopropyl) dimethyl-ammonio]-1-propane sulfonate and was maximal at pH 9.0 with apparent K(m)=77 microM and V(max)=34.3 nmol/mg protein/h. Injection of the PCB congener 2,3,3',4,4'-pentachlorobiphenyl increased both REH and LRAT activities, whereas 2,3,3',4-tetrachlorobiphenyl stimulated LRAT. Yolk retinol concentration and the molar ratio retinol:retinyl palmitate were lower in the exposed eggs. Yolk retinol concentration decreased as LRAT increased (R(2)=0.89) suggesting that certain PCB congeners may affect vitamin A mobilization in ovo by increasing LRAT activity in the yolk-sac membrane.

Subclinical hypervitaminosis A causes fragile bones in rats

Excessive intake of vitamin A has been associated with an increased risk of hip fracture in humans. This finding has raised the question of whether long-term intake of relatively moderate doses ("subclinical" hypervitaminosis A) contributes to fracture risk. Although it has been known for more than half a century that toxic doses of vitamin A lead to spontaneous fractures in rats, the lowest intake that induces adverse effects is not known, and the result of exposure to excessive doses that do not cause general toxicity has been rarely investigated. In this study, mature female rats were fed a standard diet with 12 IU vitamin A/g pellet (control, C), or standard diet supplemented with either 120 IU ("10 x C") or 600 IU ("50 x C") vitamin A/g pellet for 12 weeks. Fifteen animals were included in each group. The supplemented diets correspond to a vitamin A intake of approximately 1800 IU/day and 9000 IU/day, respectively. The latter dose is about one third of that previously reported to cause skeletal lesions. At the end of the study, serum retinyl esters were elevated 4- (p < 0.01) and 20-fold (p < 0.001) and the total amount of liver retinoid had increased 3- (p < 0.001) and 7-fold (p < 0.001) in the 10 x C and 50 x C group, respectively. The animals showed no clinical signs of general toxicity, and there were no significant bone changes in the 10 x C group. However, in the 50 x C group, a characteristic thinning of the cortex (cortical area -6.5% [p < 0.001]) and reduction of the diameter of the long bones were evident (bone cross-sectional area -7.2% [p < 0.01] at the midshaft and -11.0% [p < 0.01] at the metaphysis), as measured by peripheral quantitative computed tomography. In agreement with these data and a decreased polar strength strain index (-14.0%, p < 0.01), the three-point bending breaking force of the femur was reduced by 10.3% (p < 0.01) in the 50 x C group. These data indicate that the negative skeletal effects appear at a subchronic vitamin A intake of somewhere between 10 and 50 times the standard diet. This level is considerably lower than previously reported. Our results suggest that long-term ingestion of modest excesses of vitamin A may contribute to fracture risk.

The retinoid signaling system--a target in dioxin toxicity

This review summarizes the available data on the effects of dioxins on retinoid levels, retinoid-related enzyme activities, and toxicological endpoints that have been correlated to retinoid effects. Similarities between dioxin toxicity and retinoid deficiency as well as retinoid excess are pointed out. Several possible levels of interaction between the dioxin and the retinoid signaling pathways are discussed, including the involvement of the Ah receptor, altered retinoic acid homeostasis, and an altered set point for retinoid storage. A hypothesis for the effect of dioxins on retinoids is suggested. In this hypothesis, comprising two cascades of effects on the molecular level, the effect of dioxins on retinoic acid levels is central.

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

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

2,3,7,8-tetrachlorodibenzo-p-dioxin increases serum and kidney retinoic acid levels and kidney retinol esterification in the rat

Halogenatedorganic environmental contaminants such as dioxins are well-known to affect tissue levels of retinoids. To further investigate the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on retinoid homeostasis, adult male Sprague-Dawley rats were killed 1-112 days after a single oral dose of 10 microg TCDD/kg body wt. Additional groups of rats were killed three days after a single oral dose of 0.1, 1, 10, or 100 microg TCDD/kg body wt. Serum and renal retinoic acid levels were measured, as were levels of serum retinol-binding protein (RBP) in liver, kidneys, and serum. Hepatic and renal formation as well as hepatic hydrolysis of retinyl esters were determined, together with hepatic and renal retinoid levels. In addition, one of the retinyl ester hydrolase (REH) activities was investigated in isolated hepatocytes and hepatic stellate cells from rats killed 7 days after a single oral dose of 10 microg TCDD/kg body wt. No increased hepatic REH activity that could explain the decreased hepatic retinyl ester levels following TCDD treatment was found. In the liver, TCDD increased protein levels, but not mRNA levels, of RBP. A causal relationship is suggested for the increased renal lecithin:retinol acyltransferase (LRAT) activity and increased renal retinyl ester levels in TCDD-treated rats. Importantly, TCDD was shown to substantially increase serum and renal levels of retinoic acid. The ability of TCDD to cause increased tissue retinoic acid levels suggests that TCDD may alter the transcription of retinoic acid-responsive genes.

Change of bone tissue composition and impaired bone strength in rats exposed to 3,3',4,4',5-pentachlorobiphenyl (PCB126)

The aim of this study was to compare effects of estrogen depletion (ovariectomy) and exposure to 3,3',4,4',5-pentachlorobiphenyl (PCB126) on bone strength and bone tissue composition in the rat. Half of the rats were ovariectomized (n=20) and the remainder were sham-operated. Ten of the ovariectomized rats and ten of the sham operated were exposed to PCB126 (ip injections) for 3 months (total dose, 384 microgram/kg bodyweight), while those remaining received the vehicle. The humerus and femur were used for analysis of torsional strength and biochemical studies, respectively. Both sham-operated and ovariectomized animals showed a significantly shorter bone length, lower water content and a decreased torsional stiffness when exposed to PCB126. Sham-operated rats exposed to PCB126 had lower maximum torque when compared with sham operated controls. The PCB126-exposed rats also exhibited a significantly lower collagen concentration, but showed a higher pyridinoline concentration of cortical bone. PCB126 exposure decreased the hepatic level of vitamin A but increased vitamin A levels in serum and kidneys. Ovariectomy per se increased bone length and organic content and decreased the inorganic content significantly, but did not affect any of the tested biomechanical parameters. In conclusion, this study showed that the common environmental pollutant PCB126 impaired bone strength and altered bone composition. It is hypothesized that these effects might partly be explained by PCB-induced retinoid disturbances.

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.

Esterification and hydrolysis of vitamin A in the liver of brook trout (Salvelinus fontinalis) and the influence of a coplanar polychlorinated biphenyl

Recent reports of extremely low retinoid stores in fish living in contaminated river systems prompted an initial investigation of the mechanisms of hepatic storage and mobilization in brook trout. Enzyme characterization in microsomes revealed a lecithin:retinol acyltransferase activity (LRAT) optimum in the alkaline range (pH 9.0; Vmax = 0.6 nmol per mg prot. h(-1); Km = 10.2 microM) which is not known to occur in mammals, in addition to a secondary optimum at pH 6.5 typical of mammals. Acyl CoA:retinol acyltransferase (ARAT) kinetic parameters were quite different to those of mammals. The substrate affinity of trout ARAT (Km = 1.6 microM) was approximately 22-fold greater than that of the rat while maximal velocity (Vmax = 0.2 nmol per mg prot. h(-1)) was 18-fold less. Retinyl ester hydrolase activity (REH) was optimal under acid conditions (pH 4.2; Vmax = 6.6 nmol per mg prot. h(-1); Km = 0.6 mM), was inhibited by a bile salt analogue and was greater in males than females. This REH was tentatively categorized as a bile salt-independent, acid retinyl ester hydrolase (BSI-AREH). REH was inhibited in a dose-dependent manner following in vivo exposure to a representative environmental contaminant the coplanar polychlorinated biphenyl (PCB), 3,3',4,4'-tetrachlorobiphenyl (TCBP). Inhibition may be an indirect effect because enzyme activity was not affected by in vitro exposure of control microsomes. REH inhibition in the brook trout may affect the uptake of retinyl esters (REs) from chylomicron remnants as well as the mobilization of stored REs.

Morphology of liver stellate cells and liver vitamin A content in 3,4,3',4'-tetrachlorobiphenyl-treated rats

BACKGROUND/AIMS

Because xenobiotics decrease the vitamin A stores localized in the liver stellate cells, we investigated morphological alterations in the liver of rats exposed to 3,4,3',4'-tetrachlorobiphenyl. Special attention was given to the morphology of the liver stellate cells and to their relationship to the liver vitamin A content.

METHODS

Six rats received an intraperitoneal injection of 3,4,3',4'-tetrachlorobiphenyl (300 mumol/kg) in soyabean oil. A further six rats received the vehicle alone. After 7 days, all rats were killed and their livers assayed for vitamin A. Liver stellate cells were examined and counted on liver sections, stained with toluidine blue or immunocytochemically for desmin and, for some animals, for alpha-smooth muscle actin.

RESULTS

In the livers of 3,4,3',4'-tetrachlorobiphenyl-treated rats, we found spotty and bridging necrosis, with inflammation and accumulation of desmin-positive liver stellate cells. Steatosis and mild portal inflammation were also observed. 3,4,3',4'-Tetrachlorobiphenyl decreased the liver vitamin A content by 38%, whereas morphometric analyses showed a 40% decrease of the number of toluidine blue-detected liver stellate cells and an 11% increase of desmin-detected liver stellate cells, indicating a likely differentiation of liver stellate cells into myofibroblast-like cells. 3,4,3',4'-Tetrachlorobiphenyl treatment did not modify the expression of alpha-smooth muscle actin. Morphological alterations were more pronounced in periportal than in pericentral areas. The liver vitamin A content was positively correlated (r = 0.56, p < 0.005) with the number of toluidine-blue detected liver stellate cells.

CONCLUSIONS

3,4,3',4'-tetrachlorobiphenyl administration results in an accumulation of liver stellate cells that start differentiating into myofibroblast-like cells. The 3,4,3',4'-tetrachlorobiphenyl-induced decrease in liver vitamin A probably results from this differentiation, although other mechanisms cannot be excluded.