Alterations in vitamin A metabolism by polyhalogenated aromatic hydrocarbons

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.

Fatty acid alterations caused by PCBs (Aroclor 1242) and copper in adipose tissue around lymph nodes of mink

Fatty acid composition was determined in adipose tissue surrounding the mesenteric lymph nodes of mink (Mustela vison) exposed to polychlorinated biphenyls (PCBs: 1 mg Aroclor 1242 in food day-1 for 28 days) and/or copper (62 mg kg-1 food). These specific adipose tissues are known to have functional relationships with lymphocytes, and proliferation of cultured lymphocytes is influenced by the quality of fatty acids available in media. In six experimental groups the diet was based on freshwater fish, and in two groups it was based on marine fish. These basal diets differed in terms of fatty acid composition and content of fat-soluble vitamins A1 and E. The fatty acid composition of membrane phospholipids (PL) responded to PCBs more than that of triacylglycerols (TG). The effects of copper were small. In female minks fed a diet of freshwater fish, the proportion of highly unsaturated fatty acids in PL decreased by 5 wt.% due to PCBs, and the acids seemed to be replaced by monounsaturated fatty acids (9 wt.% increase of total). This decrease of highly unsaturated fatty acids in PL was milder in minks on the marine fish diet rich in fat-soluble vitamins. In TG of minks on the marine diet, however, PCBs decreased the proportion of docosahexaenoic acid (22:6n-3). The possibility that these alterations in the fatty acid metabolism of adipose tissue supporting the lymph nodes affect immune function during PCB exposure should be studied further. Interestingly, the quality of the fish diet affected the magnitude of the alterations. The fatty acid responses may also differ between males and females.

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.

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.

Effects of a technical PCB preparation and fractions thereof on ethoxyresorufin O-deethylase activity, vitamin A levels and thymic development in the mink (Mustela vison)

Clophen A50, a technical preparation of polychlorinated biphenyls (PCBs), was separated into four fractions; three containing chlorobiphenyls with 0, 1, or 2 to 4 ortho chlorines and one containing di- and tricyclic impurities such as naphthalenes and dibenzofurans. Clophen A50, the four fractions, and a synthetic mixture of the biologically most active non-ortho-chlorinated congeners (3,3',4,4'-tetra-, 3,3',4,4',5-penta-, and 3,3',4,4',5,5'-hexachlorobiphenyl), were separately mixed in the feed and given to female mink during the reproductive season. The concentration of a given compound in the feed mixture was equivalent to its concentration in the feed mixed with Clophen A50. Hepatic 7-ethoxyresorufin O-deethylase (EROD) activity in adults was enhanced 2-3 times by Clophen A50, the fractions containing non- or mono-ortho-chlorinated congeners, and the synthetic mixture. In neonatal kits delivered by females treated with non- or mono-ortho-chlorinated congeners, EROD was enhanced to about 30 times the control value. No live kits were delivered by the females treated with unfractionated Clophen A50. The fractions containing congeners with two to four ortho chlorines or di- and tricyclic compounds did not significantly induce EROD in either adults or kits. Clophen A50 reduced hepatic and pulmonary vitamin A contents in adult mink, while renal vitamin A was unaffected. Responses to the fractions containing the non- and mono-ortho-chlorinated congeners were similar to those obtained with Clophen A50. Their effects were, however, less pronounced, particularly with respect to the hepatic vitamin A reduction. The fractions containing congeners with two to four ortho chlorines and the di- and tricyclic compounds had no significant effects on tissue vitamin A contents.(ABSTRACT TRUNCATED AT 250 WORDS)