Retinoid Levels Research Articles

Distribution of all‐trans‐, 13–cis‐ and 9‐;cis;‐Retinoic Acid to Whole Rat Embryos and Maternal Serum Following Oral Administration of a Teratogenic Dose of all‐;trans‐Retinoic Acid

Maternal administration of 200 mg/kg all-trans-retinoic acid to rat embryos at early limb stages of development (day 12 to day 13.5 post coitum) results in limb reduction defects. In order to determine the duration of exposure of the embryo to raised levels of all-trans-retinoic acid, we have used high performance liquid chromatography to measure retinoid levels at a series of time intervals following maternal administration on day 12.5 post coitum. Raised levels of all-trans-retinoic acid and 13-cis-retinoic acid were detectable in embryos after 30 min., reached a peak at 2 hr, and had fallen sharply by 4 hr. 13-cis-Retinoic acid levels were undetectable after 4 hr, and all-trans-retinoic acid levels after 8 hr. 9-cis-retinoic acid levels rose more slowly, were less elevated, and fell more gradually than the other two retinoids. The retinoid profiles in maternal serum were similar. The results indicate that induction of limb abnormalities by all-trans-retinoic acid in rat embryos is associated with a relatively short-term rise in embryonic retinoid levels.

Effects of 3,3′,4,4′-tetrachlorobiphenyl on the dynamics of vitamin A in brook trout (Salvelinus fontinalis) and intestinal retinoid concentrations in lake sturgeon (Acipenser fulvescens)

A single i.p. injection of 5 μg 3,3′,4,4′-tetrachlorobiphenyl (TCBP)/g body mass in adult brook trout (Salvelinus fontinalis) resulted in decreased (p < 0.0001) growth rate despite pair feeding. Plasma retinol decreased (p < 0.0037). Intestinal retinyl palmitate (RP) and 3,4-dehydroretinyl palmitate (DRP) concentrations decreased in TCBP-injected males (RP, p < 0.0143; DRP, p < 0.0009), whereas retinoid levels did not decrease significantly in TCBP-injected females. The RP:DRP ratio in trout liver increased (p < 0.0001). These results suggested that DRP is more sensitive than RP to the effects of TCBP. No significant differences in ovarian retinoids occurred in post-spawning trout. Field validation of the effects on intestinal retinoids was conducted with lake sturgeon (Acipenser fulvescens) caught at a contaminated site on the Des Prairies River near Montréal (St. Lawrence River population) and reference sturgeon taken from a site near the origin of the Ottawa River in LaVerendrye Park. Intestinal retinoid concentrations were lower (RP, p < 0.0008; DRP, p < 0.0004) in the St. Lawrence River sturgeon. Our results demonstrate that a coplanar PCB is capable of altering vitamin A dynamics in several tissues and organs, and may cause a lowering of retinoids in the intestine.

Regulation of Differentiated Phenotype of Rat Hepatic Lipocytes by Retinoids in Primary Culture

Activation of hepatic lipocytes to myofibroblastlike cells observed in cell culture and during liver fibrogenesis is characterized by an increase in collagen formation and cell proliferation. These changes appear to be associated with the loss of intracellular retinoid in lipocytes, the principal storage site for vitamin A in the body. To evaluate whether retinoids have the capability to suppress lipocyte activation, we exposed cultured lipocytes in both native and myofibroblastlike states to retinoids and determined their effects on collagen production, intracellular retinoid level, and cell proliferation. Retinol (1 μM) and retinoic acid (1 μM) supplementation of primary rat lipocyte cultures inhibited the spontaneous increase in collagen synthesis associated with lipocyte activation; lower concentrations of retinol (10 or 100 nM) were also effective. Simultaneously, retinol addition prevented a precipitous decline in intracellular retinoid content in the absence of added retinoid. These retinoid effects were reversed by a change to unsupplemented control medium. When cells in the myofibroblastlike state were exposed to retinol (⩾ 1 μ M), a significant increase in intracellular retinoid levels and reduction in collagen synthesis occurred. Lipocytes in both native and myofibroblastlike states secreted four to five times higher amounts of type I collagen than type III collagen, but retinol and retinoic acid particularly inhibited production of type I collagen. Cell proliferation measured by [ 3H]thymidine incorporation was also inhibited by retinol. These results demonstrate that extracellular retinoids suppress lipocyte-activated collagen synthesis and cell proliferation and support the interpretation that retinoids themselves are regulatory factors in maintenance of the lipocyte in its native, differentiated state.

Effects of polyhalogenated aromatic hydrocarbons (PHAHs) on biochemical parameters in chicks of the common tern ( Sterna hirundo)

Common tern ( Sterna hirundo) eggs were collected from eight breeding colonies that differed in degree of polyhalogenated aromatic hydrocarbon (PHAH) pollution. The eggs were artificially incubated and the chicks sacrificed within 12 hours after hatching. Plasma thyroid hormones (total thyroxine (TT4), free thyroxine (FT4) and triiodothyronine (TT3)), hepatic T4-glucuronyltransferase (T4-UGT) activity, plasma retinol and yolksac retinoid levels were measured and compared with yolksac PHAH residues and hepatic ethoxyresorufin- and pentoxyresorufin- O-deethylase (EROD and PROD) activities. The yolksac PHAH residues measured were polychlorinated biphenyl (PCB), polychlorinated dibenzofuran (PCDF) and polychlorinated dibenzo- p-dioxin (PCDD). No significant differences were observed between colony average levels of plasma thyroid hormones (TT4, FT4 and TT3) nor plasma retinol levels and T4-UGT activities that could be related to colony average PHAH levels. However, average colony yolksac retinyl ester levels did show significant differences. When correlated for individual terns, significant correlations between all these parameters and PHAH levels or EROD or PROD activities were found. The correlation between hepatic T4-UGT and EROD and PROD activities was very strong, suggesting a concomitant induction of these drug metabolizing enzymes. The PHAH levels were expressed as dioxin equivalents (TEQs) based on toxic equivalence factors (TEFs) as proposed by Safe (1990, 1994). In addition to plasma retinol, another retinoid was found which resembles 3,4-didehydro-retinol (vitamin A2) in chromatographic and spectroscopic behaviour. This putative plasma vitamin A2 was significantly positive correlated with the yolksac TEQs and with the hepatic EROD activity. Esters of both retinol and putative vitamin A2 were detected in the yolksac. The average yolksac vitamin A2 levels of the cleanest Zeewolde colony were significantly higher than the average levels in the other colonies.

Lipoprotein lipase hydrolysis of retinyl ester. Possible implications for retinoid uptake by cells

Adipose tissue contains substantial stores of retinoid (retinol+retinyl ester) that, quantitatively, are second only to retinoid stores in the liver. Our studies show that retinoid levels in adipose tissue are markedly influenced by dietary retinoid intake. Because lipoprotein lipase (LPL) increases the uptake of lipoproteins and lipid emulsion particles by many cell types including adipocytes, we investigated whether LPL also increases retinoid uptake by adipocytes from lipid-containing particles. Addition of LPL (10 micrograms/ml) to BFC-1 beta adipocytes produced a 2-fold increase in cellular uptake of [3H]retinoid from a lipid emulsion containing [3H]retinyl ester. Heparin, which displaces LPL from binding sites on cell surface proteoglycans, increased [3H]retinoid uptake by an additional 2-fold. High performance liquid chromatography analyses showed that greater than 75% of the media and 85% of the cellular radioactivity was present as retinol. The conversion of retinyl ester to retinol by LPL was then assessed using model retinyl ester containing lipid emulsions. Although triglyceride appears to be the preferred substrate for LPL, after greater than 25% of the triglyceride was hydrolyzed, significant amounts of retinyl ester were hydrolyzed by LPL. Retinyl ester hydrolysis was increased approximately 20-fold in the presence of a source of apolipoprotein C-II. The physiologically significant palmitate, stearate, oleate, and linoleate esters of retinol were all hydrolyzed by LPL. When LPL was incubated with [3H]retinyl ester containing rabbit mesenteric chylomicrons and in the presence of heparin and apolipoprotein C-II, the LPL was able to completely hydrolyze the retinyl ester to retinol. Thus, LPL is able to catalyze the hydrolysis of retinyl esters and, through the process of hydrolysis, may facilitate uptake of retinoid by adipocytes.

Retinoids and vertebrate development.

In conclusion, it is obvious that many of the abnormalities in pattern formation and organ formation that result from the exogenous addition of RA during embryogenesis are related at least in part to the ability of RA to change the pattern of expression of the clusters of homeobox genes in the mammalian embryo. The majority of experimental protocols to study RA-induced changes in embryonic development have utilized treatments with excess, exogenous RA, but it has not yet been proven that endogenous RA acts as a key signaling molecule in developing embryo. Thus, we need to develop an understanding of the metabolic enzymes that control the levels of the important endogenous retinoids, including RA, in the developing embryo. The continuing study of retinoids and their receptors will provide us with significant new information about the regulatory programs that control differentiation and development. Moreover, these studies will lead to discoveries relevant to the clinical use of retinoids in the areas of cancer prevention and treatment, and dermatology.

Retinoic Acid Gradients during Limb Regeneration

Retinoids have been implicated in pattern formation processes in both developing chick limbs and in the regenerating limbs of urodele amphibians as well as in other aspects of embryonic development. Since chick wing buds have been shown to have a higher concentration of all-trans-retinoic acid (RA) in the posterior region than in the anterior region, we set out to look for a gradient of RA in the regenerating limb of the axolotl, Ambystoma mexicanum. We used high-performance liquid chromatography to separate, identify, and measure the concentration of retinoids present in the tissues. Our results show that the concentration of RA is about five times higher in posterior quarters of the limb regeneration blastema compared to anterior quarters. In addition, levels of RA are about two and a half times higher in blastomas from the radius-ulna level of the limb compared to those from the humerus level. By contrast, the limb regenerates of the African clawed frog, Xenopus laevis (which produce a regenerative outgrowth which is pattern-deficient), do not exhibit an anteroposterior gradient of any of the retinoids investigated. This paper also reports on the levels of retinoids in a number of other tissues as well. This anteroposterior gradient of RA in the axolotl limb regeneration blastema has significant implications for pattern formation during limb development and regeneration.

Pharmacokinetics relevant to the anti-carcinogenic and anti-tumor activities of glucarate and the synergistic combination of glucarate: retinoid in the rat

Alone and in synergistic combination with retinoids, dietary glucarate inhibits both the chemical induction and growth of rat mammary tumors. To investigate the pharmacokinetics of glucarate, [ 14C]glucarate was synthesized, converted to the calcium salt, and administered to rats bearing primary mammary tumors. When given by gavage, [ 14C]glucarate, as the calcium salt, showed a biphasic response in the blood. After peaking within 1 hr of administration at a level of 0.4 μmol/mL (normal endogenous level is approximately 0.04 μmol/mL), its plasma concentration dropped to 0.1 μmol/mL at 3 hr. In the second phase, there was a semilog increase to 0.6 μmol/mL at 15 hr, followed by a slow rise to 0.75 μmol/mL at 24 hr. Of the 38% of the administered glucarate that was recovered, 38% was excreted in the urine, and 30% remained in the gastrointestinal tract at 24 hr. Glucarate was concentrated 3- to 4-fold in the liver and intestinal mucosa, compared to the level in serum. With minor exception, the pharmacokinetics of [ 14C]13- cis-retinoic acid administered by gavage to rats was similar whether or not the semipurified diets were supplemented with 64 mmol/kg of calcium glucarate. During the interval between 5 and 10 hr post-administration of [ 14C]13- cis-retinoid, there was a transient 35–50% rise in the plasma level in rats on the glucarate-supplemented diet. This rise had no observable effect on the level of retinoid in major organs or in the tumor. A glucarate-binding protein was detected in the tumor cytosol. This potential receptor had a K a of 1.49 × 10 7 M −1.

A sustained elevation in retinoic acid receptor-β2 mRNA and protein occurs during retinoic acid-induced fetal dysmorphogenesis

We have previously shown that oral treatment of pregnant mice with all- trans retinoic acid (RA) at doses which cause 100% fetal dysmorphogenesis results in a rapid elevation in the mRNA of one specific isoform of the RA receptor-β, RAR-β2, in susceptible embryonic regions. To further investigate the involvement of RAR-β2 mRNA in teratogenesis, we have examined its expression in mouse embryos exposed to marginal/nonteratogenic and teratogenic dosing regimens of both 13- cis RA and all- trans RA. We have found that the mere elevation in embryonic RAR-β2 mRNA levels and free retinoid levels is not sufficient to result in dysmorphogenesis. Rather, retinoid-induced dysmorphogenesis of embryos appears to occur only when RAR-β2 mRNA and unbound retinoid levels remain elevated for at least 6–9 h following retinoid treatment resulting in a significant and prolonged elevation in RAR-β protein levels.

Repeated topical administration of all- trans-retinoic acid and plasma levels of retinoic acids in humans

Vitamin A ingestion raises plasma levels of several potentially teratogenic retinoic acids (RAs) with all-trans and 13-cis configuration, but definitive data concerning the consequences of topical administration of all-trans-RA are lacking. The study objective was to investigate the potential for inducing systemic activity after topical administration of 0.025% all-trans-RA by measuring plasma retinoid levels. Plasma levels of all-trans-RA, 13-cis-RA, and 4-oxo-13-cis-RA were measured in four healthy subjects before, during, and after 14 daily topical applications of all-trans-RA. Topical administration of all-trans-RA did not significantly increase plasma levels of all-trans-RA, 13-cis-RA, and 4-oxo-13-cis-RA. Significant decreases in levels of these RAs observed during the night may reflect diurnal variations of retinoid metabolism or lowered absorption of dietary vitamin A. Diurnal and nutritional factors influence plasma levels of endogenous retinoids to a greater extent than topical administration of all-trans-RA at doses used for acne therapy, which on the basis of these results appears unlikely to induce systemic effects.

Regulation of oligodendrocyte differentiation: a role for retinoic acid in the spinal cord.

During development, oligodendrocyte precursors undergo sequential stages of differentiation characterized by expression of distinct cell surface properties and proliferative responses. Although both PDGF and bFGF are mitogenic for these cells, the factors that regulate the progression of oligodendrocyte precursors through their differentiative program remain unclear. One factor present in the embryonic spinal cord that may regulate differentiation of oligodendrocyte precursors is retinoic acid. Here we show that retinoic acid inhibits the maturation of embryonic spinal cord oligodendrocyte precursors in vitro at an early, highly motile stage of differentiation, characterized by the expression of A2B5 immunoreactivity. Basic FGF acts both as a mitogen and an inhibitor of spinal cord oligodendrocyte precursor maturation, but at a significantly later stage of differentiation, characterized by the expression of O4 immunoreactivity. In the presence of RA both the mitogenic and differentiation inhibiting effects of bFGF are abolished, consistent with RA acting as an early regulator of oligodendrocyte differentiation. During embryonic development, oligodendrocyte precursors arise initially from a distinct group of cells at the ventral ventricular zone of the spinal cord. Myelination of the entire spinal cord is dependent on the migration of immature precursor cells to peripheral developing white matter. Since the embryonic spinal cord has the capacity to release relatively high levels of retinoids, we propose that RA inhibits oligodendrocyte differentiation during early embryonic development permitting their dispersal throughout the entire spinal cord.

Toxic and biochemical effects of 3,3′,4,4′-tetrachlorobiphenyl (CB-77) and clophen A50 on eider duckling ( Somateria mollissima) in a semi-field experiment

In this study the possible toxic and biochemical effects of one intraperitoneal dose of 5 or 50 mg kg −1 of 3,3′,4,4′-tetrachlorobiphenyl (CB-77) or 50 or 200 mg kg −1 of Clophen A50 (CloA50) on 28-day-old eider ducklings ( Somateria mollissima) were investigated. After ten days, no significant differences could be observed in any of the toxic and biochemical parameters studied, apart from ethoxyresorufin (EROD) activity, when comparing group average values of the dosed and control animals. However, significant correlations were observed at day 10 after exposure between the individual internal PCB concentration and body weight gain and beak length growth (negative correlations in the CloA50 groups); relative liver weight and cytochrome P4501A activity (positive correlations in CB-77 and CloA50 groups); plasma thyroid-hormone and hepatic retinoid levels (negative correlations in CB-77 groups); and plasma retinol levels and the ratio plasma retin/hepatic retinyl palmitate (positive correlations in CB-77 groups only). Animal activity was significantly reduced in the group that received 50 mg CB-77 kg −1. These observations indicate that eider ducks are a sensitive species to PCB toxicity and may be at risk for development of adverse health effects in relatively highly contaminated areas such as the Waddenzee.

A Concentration Gradient of Retinoids in the Early Xenopus laevis Embryo

Previous studies have postulated that Xenopus embryos contain an endogenous retinoic acid (RA) concentration gradient from posterior to anterior during the process of primary axis formation, since RA is able to alter profoundly primary axis formation in Xenopus embryos, to increase the expression of some posterior markers, and to inhibit the expression of some anterior markers, including homeobox-containing genes. Here, we provide direct evidence for this hypothesis. By using a reporter cell system, we demonstrate that the endogenous biologically active retinoid concentration in whole Xenopus embryos increases 3-fold from the two-cell stage to the neurula stage, and that the active retinoid concentration in the dorsal marginal zone, a region wherein the Spemann's organizer is located, increases about 5-fold from the early gastrula to late gastrula stages, suggesting the developmental regulation of the retinoid levels. In the early neurula stage (stage 13-14), endogenous active retinoids are present in a concentration gradient with the highest level at the posterior end, about 10-fold higher than that at the anterior end, of the embryo. This concentration gradient may be established during gastrulation and may provide positional cues for primary axis formation.

Vitamin A deficiency potentiates carbon tetrachloride-induced liver fibrosis in rats

Earlier studies have shown that retinoid administration suppresses the generation of hepatic fibrosis and stimulates its regression in normal (i.e., vitamin A-sufficient) carbon tetrachloride-treated rats. This study focuses on the possible role of a marginal or deficient vitamin A status on carbon tetrachloride-induced fibrosis. This experimental study in rats shows that vitamin A status, reflected by hepatic retinoid content (retinol and retinyl esters), modulates the development of hepatic fibrosis induced by carbon tetrachloride. In rats with low hepatic retinoid levels (12 +/- 0.9 micrograms/gm liver), carbon tetrachloride-induced liver fibrosis was more pronounced than in rats with sufficient hepatic retinoid levels (1,065 +/- 327 micrograms/gm liver). Enhanced liver fibrogenesis was confirmed both morphologically and by a higher hydroxyproline content of the liver. It was associated with a reduced liver weight and the development of parenchymal regeneration nodules. Furthermore, carbon tetrachloride treatment itself reduced the hepatic retinoid content in rats independently of the liver vitamin A status before treatment and increased serum retinol levels in vitamin A-sufficient rats. The results show that the vitamin A status of the liver plays an important role in hepatic fibrogenesis. Low hepatic vitamin A levels, which can be the result not only of low dietary intake but also of interference with vitamin A metabolism by agents such as ethanol and carbon tetrachloride, may be a risk factor for the development of liver fibrosis. We suggest that retinoids modulate collagen synthesis and deposition irrespective of the degree of hepatocellular necrosis induced by carbon tetrachloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoreceptor Recovery in Retinoid-deprived Rats After Vitamin A Replenishment

Dietary deficiency in the retinoid precursors of the visual pigment chromophore 11- cis retinal results in the synthesis of photoreceptor outer segments containing opsin in excess of the vitamin A available for rhodopsin regeneration. This suggests that vitamin A-free opsin may be incorporated into newly synthesized outer segment disc membranes. If this opsin is functionally intact, it should be possible to convert it to rhodopsin in vivo by providing the appropriate retinoids, and the resulting rhodopsin should be able to mediate visual transduction. Experiments were conducted to evaluate this possibility and to identify the rate-limiting steps in photoreceptor recovery from retinoid depletion. Rats were maintained on diets either containing or lacking retinoid precursors of 11- cis retinal for 23 weeks, at which time outer segment opsin content greatly exceeded the availability of visual cycle retinoids in the retina. The retinoid-deprived animals were then each given a single intramuscular injection of all- trans retinol. At various time intervals after retinol administration, electroretinograms (ERGs) were recorded on some rats, and retinal rhodopsin contents were determined in others. At similar time intervals, blood and retinal pigment epithelial (RPE) retinoid levels and photoreceptor outer segment size were also determined. No significant increase in retinal rhodopsin content was observed up to 8 hr after injection, despite the fact that by 3 hr, blood retinol levels had recovered to more than 30% of normal. By 1 day after injection, however, rhodopsin levels had recovered to 30% of normal and ERG responses showed increases in visual sensitivity commensurate with the recovery in rhodopsin. The lag in rhodopsin recovery was apparently due to delayed uptake of retinol from the blood by the RPE. Photoreceptor outer segment size was reduced by over 50% in the retinoid-deprived rats and did not begin to recover by 1 day. By 1 week, however, outer segment size had returned to an average of 65% of normal. Commensurate with this regrowth of the outer segments, both rhodopsin levels and visual sensitivity increased between 1 and 7 days after vitamin A administration. Because the rates of recovery in rhodopsin levels and visual sensitivity greatly exceeded the normal rate of new opsin synthesis at short time intervals after vitamin A repletion, it appears that the opsin incorporated into the disc membranes of retinoid-deprived rats is able to form functional rhodopsin in vivo when the chromophore is supplied. Regrowth of the outer segments back to their normal size is required for full recovery of visual sensitivity.

Tissue levels of retinoids in human embryos/fetuses

In nonhuman vertebrate embryos, two endogenous retinoids with significant morphogenic activities have been identified thus far: all- trans retinoic acid and 3,4-didehydroretinoic acid. To date, no information is available concerning endogenous retinoid levels in developing human embryos or fetuses. The purpose of the present study was to provide data relating to normal levels of retinoids in various human embryonic and fetal tissues at various stages of gestation measurable with HPLC techniques. Our investigations show that all- trans-retinoic acid, 13- cis-retinoic acid, retinol, all- trans-retinoylβ-glucuronide, and one unidentified metabolite were all presented and quantifiable in several human embryos and fetuses investigated. Tissue levels of retinol were consistently much higher than those of the other three detected metabolites; 4-oxo metabolites were below the levels of detection in all samples studied.

Embryotoxicity and teratogenicity of topical retinoic acid.

All-trans-retinoic acid is a potent developmental toxicant in all species examined. The teratogenic risk of topical all-trans-retinoic acid is reviewed. Experimental studies are limited because of the high maternal toxicity, including skin irritation, with doses below those resulting in significant teratogenic response with other application routes, such as oral application. The maximal systemic availability reported for transdermal exposure of all-trans-retinoic acid was 5-6% in the rat, 9.6% in the monkey (48% with dermatitic skin) and 5-7% in the human. Oral administration of threshold teratogenic doses of all-trans-retinoic acid (6 mg/kg/day) to Wistar rats and Swiss hare rabbits resulted in embryonic area under the concentration time curve levels (approximately 1,000 ng.h/g) which were 2- to 4-fold higher than the endogenous all-trans-retinoic acid levels; corresponding maternal plasma area under the concentration time curve values were 98 and 321 ng.h/ml in rat and rabbit, respectively. The 4-oxo-metabolite was also found in maternal plasma and embryo. Large, controlled studies on the possible developmental toxicity of topical all-trans-retinoic acid in the human are not available. Isolated case reports appeared in the literature claiming teratogenic outcome resembling effects after oral isotretinoin use or those observed in experimental studies with oral or parenteral all-trans-retinoic acid administration. The dose absorbed from daily cosmetic or therapeutic application of all-trans-retinoic acid is expected to be below 0.015 mg/kg, which is at least 30-fold lower than the lowest teratogenic dose of isotretinoin in the human. Topical all-trans-retinoic acid application did not appreciably alter endogenous plasma retinoid levels. The influence of nutrition, diurnal variation and in particular oral vitamin A supplements are more important determinants of plasma retinoic acid compounds than topical all-trans-retinoic acid. These results imply a low risk of therapeutic or cosmetic application of topical all-trans-retinoic acid. However, the highly specific spatial and temporal distribution of binding proteins and nuclear receptors in the embryo suggests that even small alterations in endogenous levels of all-trans-retinoic acid in the embryo may alter crucial developmental processes such as morphogenes; this aspect should be further investigated.

Retinoids: From metabolism to environmental monitoring

Adult herring gulls, Larus argentatus, were captured on their nests at various colonies on the Great Lakes and retinoids (vitamin A) analyzed in the liver by reverse-phase HPLC. All- trans-retinol and all- trans-retinyl palmitate concentrations showed significant differences between colonies. Colonies having the highest concentrations of 2,3,7,8-tetrachlorodibenzo- p-dioxin had the lowest retinoid concentrations. Particularly low levels of hepatic retinoids occurred in Lake Ontario gulls in 1987; and all- trans-retinol was negatively correlated with liver aryl hydrocarbon hydroxylase activity. Liver retinoids were identified in the white sucker, Catostomus commersoni, using reverse-phase HPLC with diode array detection. All- trans-retinol, all- trans-retinyl palmitate, and 3,4-didehydroretinyl esters were detected. In the case of adult white suckers sampled from a contaminated river near Montreal and a control site, significantly lower levels of all- trans-retinol and all- trans-retinyl palmitate were found in the contaminated population.

The duration of the effectiveness of vitamin A at causing proximodistal duplication in regenerating limbs of the axolotl,Ambystoma mexicanum, in relation to whole body retinoid levels

Urodele amphibians regenerate their limbs after amputation. Normally, only those parts of the limb distal to the plane of amputation are regenerated. However, retinoids are capable of inducing proximodistal (PD) duplication such that structures already present proximal to the plane of amputation are duplicated in the regenerating limb. PD duplication can be induced by retinoids even if amputation occurs some time after the retinoid treatment. Thus, the purpose of this investigation was to determine for how long after administration, retinoids were effective at inducing PD duplication, and to determine whether this persistent effectiveness was due to some change in the cells of the limb or to residual levels of retinoids in the body. Thus, forelimbs of larval axolotls, Ambystoma mexicanum, were amputated at various times in relation to retinol palmitate (RP) treatment to investigate the relationship between the induction of proximodistal duplications and the timing of the retinoid treatment. RP induced PD duplications when amputation was delayed for up to 2 weeks after cessation of the retinoid treatment. To measure the levels of retinoids actually present within the body, axolotls were treated with RP, and whole body retinoid levels were colorimetrically assayed during and after the RP treatment. Since high levels of retinoids were present for several weeks after the cessation of the treatment, we conclude that the induction of PD duplications, when amputation is delayed until after the RP treatment, is due to the persistence of retinoids in the axolotl body. © 1992 Wiley-Liss, Inc.

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 μmol/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 involvemetn of multiple mechanisms.