Retinoid Levels Research Articles

Sequestration of Retinyl Esters Is Essential for Retinoid Signaling in the Zebrafish Embryo

For vertebrate development, vitamin A (all-trans retinol) is required in quantitative different amounts and spatiotemporal distribution for the production of retinoic acid, a nuclear hormone receptor ligand, and 11-cis retinal, the chromophore of visual pigments. We show here for zebrafish that embryonic retinoid homeostasis essentially depends on the activity of a leci-thin:retinol acyltransferase (Lratb). During embryogenesis, lratb is expressed in mostly non-overlapping domains opposite to retinal dehydrogenase 2 (raldh2), the key enzyme for retinoic acid synthesis. Blocking retinyl ester formation by a targeted knock down of Lratb results in significantly increased retinoic acid levels, which lead to severe embryonic patterning defects. Thus, we provide evidence that a balanced competition between Lratb and Raldh2 for yolk vitamin A defines embryonic compartments either for retinyl ester or retinoic acid synthesis. This homeostatic mechanism dynamically adjusts embryonic retinoic acid levels for gene regulation, concomitantly sequestering excess yolk vitamin A in the form of retinyl esters for the establishment of larval vision later during development.

Ontogeny of rdh9 (Crad3) expression: Ablation causes changes in retinoid and steroid metabolizing enzymes, but RXR and androgen signaling seem normal

Crad3 ( cis-retinol/androgen dehydrogenase 3), a short-chain dehydrogenase/reductase, converts 9-cis-retinol into 9-cis-retinal and 3α-androstanediol into dihydrotestosterone. Crad3 may serve in biosynthesis of 9-cis-retinoic acid, a putative RXR ligand, and/or regeneration of potent androgens. RT-PCR showed that expression of the gene that encodes Crad3, rdh9, begins in liver by e11.5, and in kidney, testis, brain and intestine during e15.5–e16.5. In situ hybridization showed rdh9 expression in embryonic liver, ganglia, small intestine, lung, skin and vertebral cartilage. In adult, in situ hybridization revealed rdh9 expression intensely in hepatocytes, weakly in kidney glomerulus, and intensely in collecting tubules. In intestine, undifferentiated epithelia had greater expression than differentiated epithelia at the distal villus end. Testes expressed rdh9 in spermatogonia, and weakly in Leydig cells. Adult brain expressed rdh9 in the dentate gyrus and CA regions of the hippocampus, the cerebellum Purkinje cells, and the glomerular and mitral cell layers of the olfactory bulb. Rdh9-null mice, backcrossed against C57BL/6J mice, were born in Mendelian frequency, were healthy and fertile, and had normal tissue retinoid and serum dihydrotestosterone levels. Expression of rdh1, a gene that encodes an efficient retinol dehydrogenase, decreased 3- to 8-fold in rdh9-null mice, depending on dietary vitamin A. Microarray analysis and quantitative PCR revealed 2- to 4-fold increases in mRNA of enzymes that catalyze xenobiotic and steroid metabolism, including Cyp2, Cyp3, 11β-hydroxysteroid dehydrogenase type 2, and 17β-hydroxsteroid dehydrogenases types 4 and 5. These data indicate widespread Crad3 function(s) in steroid and/or retinoid metabolism starting mid embryogenesis.

Rescue of cytochrome P450 oxidoreductase ( Por) mouse mutants reveals functions in vasculogenesis, brain and limb patterning linked to retinoic acid homeostasis

Cytochrome P450 oxidoreductase (POR) acts as an electron donor for all cytochrome P450 enzymes. Knockout mouse Por − /− mutants, which are early embryonic (E9.5) lethal, have been found to have overall elevated retinoic acid (RA) levels, leading to the idea that POR early developmental function is mainly linked to the activity of the CYP26 RA-metabolizing enzymes (Otto et al., Mol. Cell. Biol. 23, 6103–6116). By crossing Por mutants with a RA-reporter lacZ transgene, we show that Por − /− embryos exhibit both elevated and ectopic RA signaling activity e.g. in cephalic and caudal tissues. Two strategies were used to functionally demonstrate that decreasing retinoid levels can reverse Por − /− phenotypic defects, (i) by culturing Por − /− embryos in defined serum-free medium, and (ii) by generating compound mutants defective in RA synthesis due to haploinsufficiency of the retinaldehyde dehydrogenase 2 ( Raldh2) gene. Both approaches clearly improved the Por − /− early phenotype, the latter allowing mutants to be recovered up until E13.5. Abnormal brain patterning, with posteriorization of hindbrain cell fates and defective mid- and forebrain development and vascular defects were rescued in E9.5 Por − /− embryos. E13.5 Por − /− ; Raldh2 +/− embryos exhibited abdominal/caudal and limb defects that strikingly phenocopy those of Cyp26a1 − /− and Cyp26b1 − /− mutants, respectively. Por − /− ; Raldh2 +/− limb buds were truncated and proximalized and the anterior–posterior patterning system was not established. Thus, POR function is indispensable for the proper regulation of RA levels and tissue distribution not only during early embryonic development but also in later morphogenesis and molecular patterning of the brain, abdominal/caudal region and limbs.

Retinoid receptors, transporters, and metabolizers as therapeutic targets in late onset Alzheimer disease

Vitamin A (retinoid) is required in the adult brain to enable cognition, learning, and memory. While brain levels of retinoid diminish over the course of normal ageing, retinoid deficit is greater in late onset Alzheimer disease (LOAD) brains than in normal-aged controls. This paper reviews recent evidence supporting these statements and further suggests that genes necessary for the synthesis, transport and function of retinoid to and within the ageing brain are appropriate targets for treatment of LOAD. These genes tend to be clustered with genes that have been proposed as candidates in LOAD, are found at chromosomal regions linked to LOAD, and suggest the possibility of an overall coordinated regulation. This phenomenon is termed Chromeron and is analogous to the operon mechanism observed in prokaryotes. Suggested treatment targets are the retinoic-acid inactivating enzymes (CYP26)s, the retinol binding and transport proteins, retinol-binding protein (RBP)4 and transthyretin (TTR), and the retinoid receptors. TTR as a LOAD target is the subject of active investigation. The retinoid receptors and the retinoid-inactivating enzymes have previously been proposed as targets. This is the first report to suggest that RBP4 is an amenable treatment target in LOAD. RBP4 is elevated in type-2 diabetes and obesity, conditions associated with increased risk for LOAD. Fenretinide, a novel synthetic retinoic acid (RA) analog lowers RBP4 in glucose intolerant obese mice. The feasibility of using fenretinide either as an adjunct to present LOAD therapies, or on its own as an early prevention strategy should be determined.

Cyclic Nucleotide-gated Ion Channels in Rod Photoreceptors Are Protected from Retinoid Inhibition

In vertebrate rods, photoisomerization of the 11-cis retinal chromophore of rhodopsin to the all-trans conformation initiates a biochemical cascade that closes cGMP-gated channels and hyperpolarizes the cell. All-trans retinal is reduced to retinol and then removed to the pigment epithelium. The pigment epithelium supplies fresh 11-cis retinal to regenerate rhodopsin. The recent discovery that tens of nanomolar retinal inhibits cloned cGMP-gated channels at low [cGMP] raised the question of whether retinoid traffic across the plasma membrane of the rod might participate in the signaling of light. Native channels in excised patches from rods were very sensitive to retinoid inhibition. Perfusion of intact rods with exogenous 9- or 11-cis retinal closed cGMP-gated channels but required higher than expected concentrations. Channels reopened after perfusing the rod with cellular retinoid binding protein II. PDE activity, flash response kinetics, and relative sensitivity were unchanged, ruling out pharmacological activation of the phototransduction cascade. Bleaching of rhodopsin to create all-trans retinal and retinol inside the rod did not produce any measurable channel inhibition. Exposure of a bleached rod to 9- or 11-cis retinal did not elicit channel inhibition during the period of rhodopsin regeneration. Microspectrophotometric measurements showed that exogenous 9- or 11-cis retinal rapidly cross the plasma membrane of bleached rods and regenerate their rhodopsin. Although dark-adapted rods could also take up large quantities of 9-cis retinal, which they converted to retinol, the time course was slow. Apparently cGMP-gated channels in intact rods are protected from the inhibitory effects of retinoids that cross the plasma membrane by a large-capacity buffer. Opsin, with its chromophore binding pocket occupied (rhodopsin) or vacant, may be an important component. Exceptionally high retinoid levels, e.g., associated with some retinal degenerations, could overcome the buffer, however, and impair sensitivity or delay the recovery after exposure to bright light.

Effect of Visible Light on Normal and P23H‐3 Transgenic Rat Retinas: Characterization of a Novel Retinoic Acid Derivative Present in the P23H‐3 Retina

Transgenic rats with the P23H mutation in rhodopsin exhibit increased susceptibility to light damage, compared with normal animals. It is known that light-induced retinal damage requires repetitive bleaching of rhodopsin and that photoreceptor cell loss is by apoptosis; however, the underlying molecular mechanism(s) leading to photoreceptor cell death are still unknown. Photoproducts, such as all-trans retinal or other retinoid metabolites, released by the extensive bleaching of rhodopsin could lead to activation of degenerative processes, especially in animals genetically predisposed to retinal degenerations. Using wild-type and transgenic rats carrying the P23H opsin mutation, we evaluated the effects of acute intense visible light on retinoid content, type and distribution in ocular tissues. Rats were exposed to green light (480-590 nm) for 0, 5, 10, 30 and 120 min. Following light treatment, rats were sacrificed and neural retinas were dissected free of the retinal pigment epithelium. Retinoids were extracted from retinal tissues and then subjected to HPLC and mass spectral analysis. We found that the light exposure affected relative levels of retinoids in the neural retina and retinal pigment epithelium of wild-type and P23H rat eyes similarly. In the P23H rat retina but not the wild-type rat retina, we found a retinoic acid-like compound with an absorbance maximum of 357 nm and a mass of 304 daltons. Production of this retinoic acid-like compound in transgenic rats is influenced by the age of the animals and the duration of light exposure. It is possible that this unique retinoid may be involved in the process of light-induced retinal degeneration.

Retinoid Signaling Determines Germ Cell Fate in Mice

Germ cells in the mouse embryo can develop as oocytes or spermatogonia, depending on molecular cues that have not been identified. We found that retinoic acid, produced by mesonephroi of both sexes, causes germ cells in the ovary to enter meiosis and initiate oogenesis. Meiosis is retarded in the fetal testis by the action of the retinoid-degrading enzyme CYP26B1, ultimately leading to spermatogenesis. In testes of Cyp26b1-knockout mouse embryos, germ cells enter meiosis precociously, as if in a normal ovary. Thus, precise regulation of retinoid levels during fetal gonad development provides the molecular control mechanism that specifies germ cell fate.

The Effects of Mainstream and Sidestream Cigarette Smoke on Sperm Production: Role Retinoic Acid

Vitamin A is an important signaling molecule for testicular cell growth and differentiation. Our preliminary studies have indicated that cigarette smoke (CS) causes a disturbance in retinoid metabolism and signaling in lung. However, nothing is known on the effects of smoking on retinoids in testis. In this study, we determined the effects of mainstream (MS) and sidestream (SS) smoke not only on sperm quantity and motility, but also on the levels of retinoids in the testis of guinea pigs in the presence or absence of retinoic acid (RA) exposure. Male guinea pigs (4 wk-old) were exposed to MS, SS or sham (SH) smoke twice daily from 3 cigarettes for 6 weeks. Once a day before CS exposure, half of the animals received RA via direct inhalation, while others were kept as room controls (RC). After 6 weeks of smoke exposure, testes were extracted and analyzed by HPLC to determine the levels of retinoids. CS altered sperm motility (MS<SS<SH<RC) and sperm count (RC=SH<MS<SS), respectively, whereas treatment with RA had no effect on sperm count and motility. Both MS and SS smoking caused a significant decrease in 13-cis, 9-cis and all-trans RA in testis whereas retinol and retinyl esters were increased. Our results indicate that CS not only affects sperm production, sperm motility but also retinoid levels. (Supported by R25 GM064319 and DAMD 17-03-2-0054).

Serum retinoic acid, retinol and retinyl palmitate levels in patients with lung cancer

Epidemiological studies have shown an inverse relationship between dietary vitamin A intake and the risk of developing lung cancer. The aim of this study was to investigate the vitamin A status in patients with lung cancer, by determining the serum levels of retinoic acid, retinol and retinyl palmitate. In total, 36 patients with lung cancer and 27 controls were assessed. Of the patients 14 had squamous cell carcinoma, 3 adenocarcinoma, 15 non-small cell lung cancer and 4 small cell lung cancer. Serum retinoic acid, retinol and retinyl palmitate levels were determined with HPLC and UV detection, after liquid extraction. Serum retinol levels did not differ between patients (733.5 +/- 326.4 ng/mL) and controls (734.5 +/- 337.1 ng/mL). The retinyl palmitate concentration tended to be lower in patients (14.3 +/- 9.7 ng/mL) than in controls (16.7 +/- 13.7 ng/mL). The serum retinoic acid levels were significantly lower in patients (1.9 +/- 0.6 ng/mL) than in controls (2.5 +/- 1.1 ng/mL, P < 0.05). A positive correlation was observed between the retinol and retinoic acid levels and retinyl palmitate and retinoic acid levels. The lower levels of retinoic acid in patients with lung cancer suggest there may be a deficiency or impairment in retinol metabolism in these patients. Further studies with larger numbers of patients are needed to evaluate the possible relationship between serum retinoid levels and lung cancer.

Incorporation of vitamin A into milk of wild type and mutant mice

Green et al. estimated that 40–70% of vitamin A delivered to the milk of lactating rats is derived via plasma retinol-binding protein (RBP) with the rest arriving postprandially (J. Nutr. 131: 1279-82, 2001). To explore the role of RBP in this process, we assessed retinoid levels in milk of lactating RBP−/− and wild type mice consuming a chow diet fed ad lib. No statistically significant differences in milk total retinol levels were observed between the RBP−/− and wild type mice. Total retinol levels at different postpartum days for RBP−/− and wild type mice respectively were: days 1-5, 5.7 ± 2.1 μM and 10 ± 5 μM; days 6–14, 14.5 ± 4.3 μM and 9.2 ± 6.6 μM; and day 15 onward, 10.4 ± 4.2 μM and 16 ± 6.4 μM. Hence, RBP is not required for maintaining delivery of retinoids for milk production suggesting that milk vitamin A can be acquired solely from dietary retinol. RBP-/- mice incorporate higher [3H]retinol into their milk than wild type mice when measured over 6 hours postprandially, indicating a compensatory up-regulation in the mutants. We investigated the role of enzymes needed for retinyl ester hydrolysis and synthesis in facilitating retinoid incorporation into milk using mice lacking lipoprotein lipase (LpL) and mice lacking lecithin:retinol acyltransferase (LRAT). The lipase inhibitor p407 significantly reduced the incorporation of post prandial [3H]retinol into milk of the RBP-/- demonstrating the importance of this pathway. Studies of milk formation in LRAT-deficient mice indicate that LRAT is not needed for incorporation of vitamin A into milk. Our studies help define the biochemical factors that are critically needed for milk formation. (Supported by NIH grant R01 DK068437)

Effects of dietary oxidized lipid and vitamin A on the early development and antioxidant status of Siberian sturgeon (Acipenser baeri) larvae

A 4-week feeding trial was conducted to investigate the effects of dietary oxidized lipid and vitamin A on the early ontogenesis and the antioxidant status of Siberian sturgeon larvae. Auto-oxidized capelin oil (peroxide value=245meq/kg) was added at 3 levels: 0, 40 and 80g/kg in semi-purified casein based diets containing either 22,500 or 772,500IU/kg vitamin A as retinyl acetate.Survival and growth were significantly reduced in larvae fed diets containing oxidized lipid with the lowest vitamin A level. A high percentage (25%) of deformed larvae was noted with diets containing 80g/kg oxidized lipid. These effects were not observed in larvae fed diets with higher vitamin A level indicating an interaction between dietary oxidized lipid and vitamin A. The highest dietary vitamin A level had negative effects in larvae fed fresh lipid and positive effects in larvae fed oxidized lipid, compared to the lowest supply of vitamin A.Retinyl palmitate was found to be the main storage form of vitamin A with 6.7μg/g in larvae fed diets with the highest vitamin A level and only 0.05μg/g in larvae fed diets with the lowest vitamin A level, irrespective of the dietary oxidized lipid level. Retinol contents in larvae were also significantly affected by dietary vitamin A level. Retinoid levels in larval bodies were not modified by dietary oxidized lipid level. Indicators of lipid peroxidation in larvae such as 8-isoprostanes were the highest in larvae fed 80g oxidized lipid/kg diet as activities of antioxidant enzymes such as superoxide dismutase, catalase and glutathione peroxidase poorly respond to dietary oxidized lipid and vitamin A.An increased oxidative stress in Siberian sturgeon larvae fed oxidized lipid may account for their poor growth and survival and high occurrence of deformed fish. A high dietary supplementation with vitamin A allowed to counteract partially the negative effects of dietary oxidized lipid, suggesting antioxidant properties of dietary vitamin A. However the precise role of dietary vitamin A on negative effects due to oxidized lipid in Siberian sturgeon larvae remains unclear.

Altered hepatic retinol and CYP26 levels in adult European common frogs (Rana temporaria) exposed to p,p′-DDE

The effects of environmental contamination on amphibians are of particular concern because there are reports of declining numbers of species and individuals in most parts of the world during the last 50 years. During the last decade there has been increased focus on the role of persistent organic pollutants as retinoid (vitamin A) disrupters, and their effects on development, growth and sexual differentiation. To study the effects of p,p'-DDE, one of the most persistent metabolites of the pesticide DDT, on retinol homeostasis, we subcutaneously exposed adult male European common frogs (Rana temporaria) to different doses of p,p'-DDE (0, 0.01, 0.1, 1 and 10 mg/kg body mass) and studied the effect of a short term exposure (14 days) on hepatic retinoid levels and CYP26 gene and protein expression. Hepatic retinol concentrations, CYP26 gene and protein levels were analysed using HPLC, quantitative RT-PCR and indirect ELISA, respectively. Our results showed a significant p,p'-DDE dose-specific increase in the hepatic retinol concentration. CYP26 gene and protein expression were reduced in an apparent p,p'-DDE dose-specific manner. The results suggest that p,p'-DDE may interfere with the hepatic metabolism of retinol in adult frogs by decreasing CYP26 expression patterns.

Retinoid Targets for the Treatment of Cancer

Retinoic acid (RA), the most potent natural retinoid, is essential for normal cell growth and differentiation. The RA signaling pathway is multistep, involving the precise regulation of retinoid levels and the control of RA-dependent gene expression in target cells. Within this complex scheme, there are many different aberrations in the RA signaling pathway of tumor cells that have been found to be associated with abnormal cell growth and tumorigenesis. This article reviews the normal pathways of RA signaling, followed by a discussion of the various sites that have been implicated in tumorigenesis and targeted for drug development. Currently, there are several retinoids and one rexinoid approved for the treatment of specific cancers. Future experimentation in drug discovery will continue to explore the efficacy of retinoids/rexinoids, either alone or in combination with other chemotherapeutic agents and/or chromatin remodeling agents, and the development of agents to modulate RA metabolism within cells. It is likely that different drug treatments will be developed that are specifically tailored to the unique point(s) in the RA signaling pathways that are aberrant in specific types of tumor cells.

Retinoids and steroids in bovine mammary gland immunobiology

Major variations in systemic retinoid and steroid levels during the periparturient period suggest that these potent signaling molecules play a role in the decreased innate immune response observed at early lactation. This period coincides with a marked increase in the incidence of coliform mastitis. Two critical parameters have been identified within the cow's first line of defense during acute coliform mastitis: efficient mammary epithelium remodeling and functional polymorphonuclear leukocytes. The overall aim of this review is to relate the actual knowledge on retinoids and steroids to bovine mammary gland immunobiology. Recent insights on the potential role of retinoic acid, estradiol and progesterone in the development and maintenance of a normal glandular epithelial architecture are provided. In parallel, their influence on myelopoiesis and PMN functionality is also discussed. Retinoic acid and steroid hormones predominantly serve as ligands that activate specific transcription factors belonging to a nuclear receptor superfamily and thereby control gene expression. Additionally, both retinoids and steroids may act via non-nuclear receptor mediated pathways. There are also indications on a subtle interplay between retinoic acid and estrogen pathways. These messengers probably affect cell proliferation and apoptosis through interaction with local growth factors e.g., the insulin-like growth factor /insulin-like growth factor binding protein-axis, and with the extracellular matrix e.g., by modulating matrix metalloproteinase activity. In conclusion, arguments are provided supporting the importance of retinoids and sex steroid hormones in the mammary gland innate immune defense.

Polychlorinated biphenyls, sex steroid hormones and liver retinoids in adult male European common frogs Rana temporaria

Declines in amphibian populations and species biodiversity during the last decades has called for an assessment of possible threats to these animals. Polychlorinated biphenyls (PCBs) are known endocrine disrupting contaminants and are found in high levels in some populations of wild living amphibians. To evaluate the endocrine disrupting potential of PCBs in adult frogs, Aroclor 1254 were subcutaneously injected into male European common frogs Rana temporaria. The injected doses ranged from 0.01 to 100 mg/kg body mass, resulting in liver concentrations between 74 and 133 619 μg/kg ww. After 14 days, serum testosterone (T), estradiol (E) and hepatic retinol (R) and retinylpalmitate (RP) were easured. No dose dependent effects were found on levels of hormones or retinoids. However, a significantly higher within-group variation in the E–T ratio in the exposed groups may indicate that the sex-hormone homeostasis of male R. temporaria is affected by PCBs shortly after arousal from hibernation, but that the effects are subtle and that several different mechanisms are involved. The lack of direct effect on T, E, R and RP may be due to the timing of exposure (shortly after arousal from hibernation), or due to a relatively short exposure time to Aroclor 1254. Based on the results, we propose that future research should focus on effects of PCBs in relation to the different physiological phases frogs experience throughout the year (hibernation, reproduction etc.).

Organochlorine contaminant and retinoid levels in blubber of common dolphins ( Delphinus delphis) off northwestern Spain

The effect of age, sex, nutritive condition and organochlorine concentration on blubber retinoid concentrations was examined in 74 common dolphins incidentally caught off northwestern Spain. Age and blubber lipid content were strong determinants of the retinoid concentrations in males, while these variables did not account for the variation found in females. Retinoids were positively correlated with organochlorines in males and negatively in females. However, pollution levels were moderate and likely to be below threshold levels above that a toxicological response is to be expected. Thus, a cause–effect relationship between organochlorine and retinoid concentrations could not be properly established, and the observed correlation may be the result of an independent association of the two variables with age. Further research on the influence of the best predictor variables on retinoid dynamics is required to implement the use of retinoids as biomarkers of pollutant exposure in cetaceans.

Effects of Caloric Restriction and Growth Hormone Resistance on the Expression Level of Peroxisome Proliferator-Activated Receptors Superfamily in Liver of Normal and Long-Lived Growth Hormone Receptor/Binding Protein Knockout Mice

Growth hormone receptor/binding protein knockout (GHR-KO) mice live approximately 40% longer than their normal siblings do. These mice have dramatically reduced plasma levels of insulin-like growth factor 1 (IGF1) and enhanced insulin sensitivity. We examined the expression level of peroxisome proliferator-activated receptors (PPARs) and retinoid X receptors family genes in the livers of normal and GHR-KO mice fed ad libitum or subjected to long-term 30% caloric restriction (CR). The levels of PPARgamma and PPARalpha messenger RNA and proteins and the levels of retinoid X receptors messenger RNA were elevated in long-lived GHR-KO mice as compared to normal mice with no major effect of CR in either genotype. These findings suggest that enhanced insulin sensitivity of GHR-KO mice may be related to the altered actions of PPARs family members in the liver. The results also indicate that CR may increase insulin sensitivity through a different mechanism.

Rat hepatic stellate cells become retinoid unresponsive during activation

Hepatic stellate cells (HSC) play an essential role in fibrogenesis. Many stimuli cause HSC to activate, lose their Vitamin A and produce collagen. It is unclear whether Vitamin A loss causes activation, potentiates it or is simply an event in the cascade of activation changes. We determine if exogenous retinoids prevent the activation of freshly isolated rat HSC activated by plating on plastic. We also determine if retinoids: (1) reverse HSC activation; (2) maintain/restore HSC intracellular retinoid levels; (3) maintain expression of HSC nuclear receptors for retinoic acid (RAR) in HSC that are becoming activated or are chronically activated. Markers of activation in freshly isolated HSC were decreased by either retinol or retinoic acid without increases in HSC retinoid concentration. mRNA levels for RAR-alpha, RAR-beta and RAR-gamma, the nuclear receptors for retinoic acid, decreased during activation of freshly isolated HSC even with retinoid supplementation. RAR-alpha, RAR-beta and RAR-gamma mRNA and RAR-beta protein was undetectable in chronically activated HSC and remained absent after retinoic acid supplementation. Activation markers in chronically activated HSC were only slightly decreased after retinoid exposure. We conclude that exposure of HSC to extracellular retinoids diminishes some markers of activation but does not prevent HSC activation.

Retinoid metabolism during development of liver cirrhosis

The changes in retinoid metabolism have been documented in liver cirrhosis. However, the dynamic alterations in levels of this vitamin between circulation and liver during development of the liver cirrhosis are not well understood. The aim of this study was to measure retinoids in the liver and circulation in parallel, during and after development of cirrhosis induced by carbon tetrachloride and thioacetamide. Retinoid levels were measured by HPLC. A decrease in retinaldehyde and total retinol, together with an increase in retinoic acid was evident in liver from both carbon tetrachloride or thioacetamide treated rats within a month after initiation of treatment. Activity of enzymes involved in retinoid metabolism such as retinaldehyde oxidase, retinaldehyde dehydrogenase, and retinaldehyde reductase were decreased in the liver. In parallel, levels of retinol and retinaldehyde in the serum were increased while retinoic acid was decreased. This study indicates that during development of cirrhosis, there is reciprocal transfer of retinoid metabolites between the circulation and the liver.

Relationship of retinoid and carotenoid metabolism with caecotrophy in rabbits.

The relationship of retinoid and carotenoid metabolism with caecotrophy was studied in adult female New Zealand White rabbits kept in individual metabolic cages. Caecotrophy was prevented by the use of plastic collars. The dry matter, crude protein, fibre, fat and ash contents of hard and soft faecal samples were determined. The retinoid (retinol and retinyl palmitate) and carotenoid (canthaxanthin, beta-carotene, beta-cryptoxanthin, lutein and zeaxanthin) levels of the blood, liver, kidney, caecal content and faeces were also measured by high-performance liquid chromatography (HPLC). The prevention of caecotrophy resulted in a significant decrease of blood retinol (P < 0.001) and retinyl palmitate (P < 0.01) concentration but it did not cause any significant change in the retinol and retinyl palmitate contents of the liver and kidney. The caecal content (25.78 +/- 6.87 microg/g) and the soft faeces (34.52 +/- 10.48 microg/g) contained the retinoids in similar amounts. Various carotenoids were found in considerable amounts in different types of faeces, while in the tissues (blood, liver and kidney) these pigments did not occur in substantial amounts. Total carotenoid concentration was similar in the caecal content (11.23 microg/g) and in the caecotroph (13.85 microg/g). On the basis of the results it can be assumed that the retinoid content of rabbit feed could be lowered in the presence of adequate caecal function and caecotrophy.