Vitamin A status, polybrominated biphenyl (PBB) toxicosis, and common bile duct hyperplasia in rats

Extraparenchymal Bile/Pancreatic Ducts and Duodenal Papillae: Pathologic Evaluation in Nonclinical Species--A Brief Review

This review focuses on the anatomy, histologic preparation, and pathologic evaluation of extraparenchymal bile and pancreatic ducts (BPDs) and their openings at the duodenal papillae in the cynomolgus macaque (Macaca fascicularis), the Beagle dog (Canis familiaris), the Wistar Hanover rat (Rattus norvegicus), and the CD1 mouse (Mus musculus). In nonclinical safety assessment, intraparenchymal BPDs (with sections of liver and pancreas, respectively) are evaluated routinely. However, detailed evaluation of the extraparenchymal BPDs or the duodenal papillae is not included. In the context of nonclinical safety assessment studies, this review describes situations in which evaluation of extraparenchymal ductal structures and duodenal papillae may be useful in characterizing test article-related changes; elucidates anatomic similarities between human, macaque, and dog and notable differences in rats and mice; and consolidates the information required for the histopathologic evaluation of these tissues.

Altered Retinoid Metabolism in Female Long-Evans and Han/Wistar Rats following Long-Term 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD)-Treatment

This study investigated the effects of long-term low-dose 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure on retinoid, thyroid hormone, and vitamin D homeostasis in Long-Evans and Han/Wistar rats using a tumor promotion exposure protocol. Female rats (ten/group) were partially hepatectomized, initiated with nitrosodiethylamine (NDEA), and given TCDD once per week by sc injection for 20 weeks at calculated daily doses of 0, 1, 10, 100, or 1000 ng/kg bw/day. Groups of nonhepatectomized/uninitiated rats (five/group) were identically maintained. After 20 weeks, the rats were killed, and apolar retinoid levels were determined in the liver and kidneys. No consistent differences were seen between partially hepatectomized/initiated and nonhepatectomized/uninitiated animals with respect to apolar retinoid levels or hepatic TCDD concentration. Further analyses of polar and apolar retinoid levels in liver, plasma, and kidney, as well as free thyroxine (FT4) and vitamin D (25-OH-D(3)) concentrations were carried out in partially hepatectomized/inititated animals. In Long-Evans rats, TCDD exposure dose-dependently decreased hepatic retinyl ester concentrations at doses of 1-100 ng/kg bw/day. Likewise, hepatic all-trans-retinoic acid (all-trans-RA) concentration was decreased 39 and 54% at 10 and 100 ng/kg bw/day respectively, whereas 9-cis-4-oxo-13,14-dihydro-retinoic acid (9-cis-4-oxo-13,14-dihydro-RA), a recently discovered retinoic acid metabolite, was decreased approximately 60% in the liver at 1 ng/kg bw/day. TCDD dose-dependently increased plasma retinol and kidney retinol concentrations, whereas all-trans-RA concentration was also increased in the plasma and kidney at 10 and 100 ng/kg bw/day. Plasma 9-cis-4-oxo-13,14-dihydro-RA was decreased to below detection limits from doses of 1 ng/kg bw/day TCDD. A qualitatively similar pattern of retinoid disruption was observed in the Han/Wistar rat strain following TCDD exposure. FT4 was decreased to a similar extent in both strains, whereas 25-OH-D(3) was decreased only at 100 ng/kg bw/day in Long-Evans rats. Together these results show that TCDD disrupts both retinoid storage and metabolism of retinoic acid and retinoic acid metabolites in liver, kidney, and plasma from doses as low as 1 ng/kg bw/day. Furthermore, 9-cis-4-oxo-13,14-dihydro-RA was identified as a novel and sensitive indicator of TCDD exposure, in a resistant and sensitive rat strain, thereby extending the database of low-dose TCDD effects.

Retinoids in eggs and embryos of birds fed fish from the Great Lakes

Retinoids were analyzed in 11-day chick embryos and eggs from white Leghorn hens (Gallus domesticus) fed environmentally-derived polychlorinated biphenyls (PCB) in carp (Cyprinus carpio) from Saginaw Bay. The yolks and the embryos contained all-trans-retinol, 3,4-didehydroretinol and retinyl esters. There was no significant difference in the total retinoid content in the yolks of 11-day incubated eggs among hens fed for 7 weeks diets containing 0.5-6.6 mg PCB/kg diet. However, the proportional amount of retinols in the high (6.6 mg) PCB group was significantly less than that in low (0.5 mg) PCB controls, while the amount of retinyl palmitate in the high PCB group was significantly greater than that in the controls. Retinoids in the embryos were not affected by any of the PCB levels fed to hens for 7 weeks prior to laying the eggs. The 50% reduction in the molar ratio of retinols to retinyl palmitate in the yolks of eggs as the result of the high PCB level fed to hens for 7 weeks can serve as an indicator for chronic exposure to PCB contamination at the level of 6.6 mg or higher PCB/kg diet.

Effects of fasting and 3,3',4,4',5,5'-hexabromobiphenyl on plasma transport of thyroxine and retinol: fasting reverses elevation of retinol

Male Wistar rats were injected ip with 0 or 20 mg/kg 3,3',4,4',5,5'-hexabromobiphenyl and blood samples were collected 1, 3, 6, 7, and 8 d later. At 8 d after the injection, serum retinol was increased 30% and serum thyroxine was decreased 26% relative to control values. These effects were apparently unrelated to transthyretin in that the biphenyl did not alter the proportion of thyroxine binding in vitro to the prealbumin fraction of serum proteins. Separate groups of control and HBBP-injected rats did not receive food on d 7 (i.e., 24-h fast) and d 8 after injection (i.e., 48-h fast). Fasting decreased the serum retinol and thyroxine concentrations as well as the proportion of thyroxine binding in vitro to the prealbumin fraction of serum. The decreases in retinol and thyroxine concentrations associated with fasting are therefore ascribed to a decrease in the concentration of transthyretin in circulation.

Polychlorinated and polybrominated biphenyl congeners and retinoid levels in rat tissues: structure-activity relationships

The purpose of this study was to examine the structural requirements of polychlorinated and polybrominated biphenyls (PCBs and PBBs) for altering tissue levels of retinoids. Seven congeneric PCBs and PBBs were studied: 3,3',4,4'-tetrachlorobiphenyl (TCB), 2',3,3',4,5- and 3,3',4,4',5-pentachlorobiphenyls (-PeCBs), 3,3',4,4'- and 3,3',5,5'-tetrabromobiphenyls (-TBBs), 2,2',3,3',5,5'-hexachlorobiphenyl (-HCB), and 3,3',4,4',5,5'-hexabromobiphenyl (-HBB). Male Sprague-Dawley rats were fed a vitamin A-adequate diet (1.3 mg/kg) for 30 days before being given a single IP injection of one of seven polyhalogenated biphenyls (150 mumol/kg) in corn oil (10 ml/kg) or vehicle alone. Rats were killed 1 week later. Except for 3,3',4,4',5,5'-HBB, all PCBs and PBBs studied significantly decreased serum retinol levels and, except for 3,3',4,4',5,5'-HBB and 2,2',3,3',5,5'-HCB, all PCBs and PBBs also lowered the serum retinol-binding-protein (RBP) content. The activity of hepatic retinyl ester hydrolase (REH) was reduced by the treatment of 3,3',4,4',5-PeCB, 3,3',4,4'-TBB, and 3,3',4,4',5,5'-HBB. The levels of hepatic retinol were decreased by 2,2',3,3',5,5'-HCB, 2',3,3',4,5-PeCB, and 3,3',4,4',5-PeCB, while levels of hepatic retinyl palmitate were decreased by 2',3,3',4,5-PeCB, 3,3',4,4',5-PeCB, 3,3',4,4'-TCB, 3,3',4,4'-TBB, and 3,3',4,4',5,5'-HBB. The substantial decreases in hepatic retinyl palmitate levels could not be explained solely on the basis of hepatomegaly caused by acutely toxic PCBs and PBBs. All halogenated biphenyls which caused a decrease in hepatic retinyl palmitate also caused an increase in renal retinyl palmitate except 3,3',4,4',5-PeCB. In summary, the acutely toxic (nonortho substituted) congeners had pronounced effects on hepatic, renal, and serum retinoids whereas other biphenyls only decreased serum retinol levels. The effects of these seven compounds on REH activity were not correlated with the effects on serum retinol or RBP levels. Therefore, this study shows that the structure-activity relationships for altering hepatic retinoids differ from those for serum retinol, implying the involvement of multiple mechanisms.

Elimination of PBBs in rats. Effect of mineral oil and/or feed restriction

Rats were fed polybrominated biphenyls (PBBs) at 0.1 to 100.0 ppm for 14 d and then treated to hasten the removal of PBBs with 0, 5, or 10% mineral oil (MO) and/or 0, 15, 30, or 45% feed restriction (FR) for 21 d. PBB body burdens were determined at d 14 and expressed on a log-log basis by Y = 0.91X + 2.179 (r2 = 0.974), where X = log of PBB concentration in diet (ppm) and Y = log of PBB body burden (micrograms). After 21 d withdrawal, body burdens were expressed by the equation Y = 0.787X + 2.218 (r2 = 0.95). The most effective withdrawal treatment was 10% MO + 45% FR producing a reduction of body burdens inversely related to prior body burdens (69% at 0.1 ppm to 23% at 100 ppm). Body weights and fat content were significantly (p less than or equal to .05) reduced by feed restriction, with fat content only 39% of controls at 21 d off. Mortality averaged 0, 13.6, and 35.8% for rats fed 0, 5, or 10% MO, and 25, 15, 8.6, and 3.7% for rats feed restricted at 0, 15, 30, and 45%, respectively. Histopathology of the dead and moribund rats indicated that the clinical signs were not characteristic of PBB toxicity. In a second experiment, safflower oil at 3.5% or excess vitamins prevented the mortality and clinical signs associated with MO during withdrawal from 100 ppm PBBs. Based on these data and those in the literature, PBBs interfere with vitamin utilization.

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 vitamin A metabolism by polyhalogenated aromatic hydrocarbons

Adequate stores and adequate tissue levels of vitamin A are maintained by a balance of tissue demands and dietary intake of the vitamin and are modified by many factors, including xenobiotics. It is well established that exposure to polyhalogenated aromatic hydrocarbons (PHAH) decreases hepatic content of vitamin A. Recent findings indicate that hepatic depletion of vitamin A is accompanied by an increase in serum and renal vitamin A content and enhanced excretion of vitamin A metabolites in urine and feces. Examination of tissue retinoid profiles reveals that PHAH exposure causes the generation of increased amounts of polar retinoids. It is very likely that PHAH affect enzymes crucial for regulation of vitamin A storage as well as enhance activities of specific enzymes in vitamin A metabolic pathway.

Effect of hexachlorobiphenyl on vitamin A homeostasis in the rat

Vitamin A status and turnover were examined in rats that had been exposed to chronic dietary treatment of 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), 1 mg/kg diet. HCB caused hepatic depletion and renal accumulation of vitamin A, and a 1.7-fold increase in the serum retinol concentration. Intravenously administered [3H]retinol bound to retinol binding protein-transthyretin complex (RBP-TTR complex) was used to study the dynamics of circulatory retinol in these rats. In HCB-treated rats, the plasma turnover rate of retinol was increased compared to vitamin A-adequate untreated controls. HCB caused a 50% reduction of total radioactivity in liver, and, except for 0.5 h after the [3H]retinol-RBP-TTR dose, the specific activity of the hepatic retinyl ester pool was greater compared to control rats. The kidneys of HCB-treated rats accumulated radioactivity in the retinyl ester fraction. HCB also caused a 50% reduction in adrenal radioactivity compared with control rats. Urinary and fecal excretion of radioactivity was 3-fold higher in HCB-treated rats as compared to controls. Our findings demonstrate that chronic HCB feeding results in expansion of plasma vitamin A mass, in changes of liver and kidney retinol and retinyl ester pool dynamics and in an increased metabolism of vitamin A.

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.

Relationship of basic research in toxicology to environmental standard setting: the case of polybrominated biphenyls in Michigan

The accidental contamination of dairy cattle feed in Michigan in 1973-74 with polybrominated biphenyls (PBB) led to the contamination of cattle and people consuming their products. This led to an extensive animal and product monitoring and disposal program conducted by the Michigan Department of Agriculture and the Department of Natural Resources. It also led to several studies of the people of Michigan, extensive research on the chemicals, and an unprecedented establishment by the Legislature of a Toxic Substance Control Commission. Only a few relatively minor components of the PBB mixture that contaminated Michigan are metabolized and another group of minor components seem responsible for the toxicity, which, similar to that caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), includes induction of microsomal enzymes, liver hypertrophy, thymic involution, porphyria, anorexia and chloracne. PBB were found to produce the "dioxin-like" toxicity with roughly 0.01% the potency of TCDD. Both non-toxic as well as toxic congeners were found to be tumor promotors. To date it is impossible to unequivocally conclude that any human health effects can be attributed to PBB. The Toxic Substance Control Commission was established as an independent oversight body with responsibility to gather information, investigate, coordinate and make recommendations concerning toxic substances and the handling of toxic substances incidents. The Commission has declared two toxic substances emergencies and made several recommendations for regulating and solving toxic substances problems but its major activities have evolved towards a role as an environmental ombudsman.

Short-term oral administration of polybrominated biphenyls enhances the development of hepatic enzyme-altered foci in initiated rats

FireMaster BP-6 (FM), a commercial mixture of polybrominated biphenyls (PBB), has been shown to act as a tumor promoter in hepatocarcinogenesis assays in rats. Most hepatic tumor promoters must be administered for many weeks or months. Because FM is highly persistent in animal tissues, it was hypothesized that very short-term administration of FM would result in tumor promotion. Female Sprague-Dawley rats weighing 185-215 g were initiated by a two-thirds partial hepatectomy followed by 10 mg diethylnitrosamine/kg body weight (BW) 24 h later. Thirty days later, rats were gavaged with FM in corn oil, at total doses of 0, 13, or 130 mg FM/kg BW. Half the dose was given on d 30, and the remaining half was given 24 h later. At 120 d after gavage the rats were killed and necropsied. Five liver sections from each animal were histochemically stained for gamma-glutamyl transpeptidase-positive enzyme-altered foci (EAF). EAF were significantly increased over control values in initiated rats given 130 mg FM/kg. In animals given 13 mg FM/kg, EAF were increased to a lesser extent but not significantly above controls. Enhancement of these EAF in initiated rats reflects tumor-promoting activity. In this study, 24-h administration of FM in initiated rats was sufficient to enhance hepatic EAF measured 120 d later in an rats was sufficient to enhance hepatic EAF measured 120 d later in an initiation-promotion protocol, and a dose of 13 mg FM/kg was apparently close to a possible no-effect threshold level for enhancement of EAF.

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.