Analysis of water-soluble compounds: glucuronides

Filipino Children with High Usual Vitamin A Intakes and Exposure to Multiple Sources of Vitamin A Have Elevated Total Body Stores of Vitamin A But Do Not Show Clear Evidence of Vitamin A Toxicity

Background

Young children exposed to high-dose vitamin A supplements (VAS) and vitamin A (VA)-fortified foods may be at risk of high VA intake and high VA total body stores (TBS).

Objectives

TBS and estimated liver VA concentration were compared among children with adequate or high VA intake and different timing of exposure to VAS, and associations between estimated liver VA concentrations and biomarkers of VA toxicity were examined.

Methods

Children 12-18 mo of age (n = 123) were selected for 3 groups: 1) retinol intake >600 µg/d and VAS within the past mo, 2) retinol intake >600 µg/d and VAS in the past 3-6 mo, and 3) VA intake 200-500 µg retinol activity equivalents (RAE)/d and VAS in the past 3-6 mo. Dietary intake data were collected to measure VA intakes from complementary foods, breast milk, and low-dose, over-the-counter supplements. TBS were assessed by retinol isotope dilution, and VA toxicity biomarkers were measured. Main outcomes were compared by group.

Results

Mean (95% CI) VA intakes excluding VAS were 1184 (942, 1426), 980 (772, 1187), and 627 (530, 724) µg RAE/d, in groups 1-3, respectively; mean VA intake was higher in groups 1 and 2 compared with group 3 (P < 0.05). Geometric mean (GM) (95% CI) TBS were 589 (525, 661), 493 (435, 559), and 466 (411, 528) µmol, respectively. GM TBS and GM liver VA concentrations were higher in group 1 compared with group 3 (liver VA concentration: 1.62 vs. 1.33 µmol/g; P < 0.05). Plasma retinyl ester and 4-oxo-retinoic acid concentrations and serum markers of bone turnover and liver damage did not indicate VA toxicity.

Conclusions

In this sample, most children had retinol intakes above the Tolerable Upper Intake Level (UL) and liver VA concentrations above the proposed cutoff for "hypervitaminosis A" (>1 µmol/g liver). There was no evidence of chronic VA toxicity, suggesting that the liver VA cutoff value should be re-evaluated. This trial was registered at www.clinicaltrials.gov as NCT03030339.

UVA/B exposure promotes the biosynthesis of dehydroretinol in cultured human keratinocytes

Retinol and its metabolites modulate epithelial differentiation and serve as cellular UV sensors through changes in retinoid status. Of note is the dehydroretinol family which may serve functions distinct from parental retinol. This study focuses on the metabolism of this family and its potential participation in the response of normal epidermal human keratinocytes to UV irradiation. There were three findings. First, keratinocytes contain two pools of dehydroretinyl esters, one of which is shielded from UVB-, but not from UVA-induced decomposition. Second, using a novel in vitro assay we demonstrated that both UVA and UVB promote dehydroretinol biosynthesis in keratinocytes, but only UVB exposure promotes retinoid ester accretion by enhancing the activity of at least one acyl transferase. Finally, dehydroretinol sufficiency reduces UVA/B driven apoptosis more effectively than retinol sufficiency. This may in part be due to differences in the expression of Fas ligand, which we found to be upregulated by retinoic acid, but not dehydroretinoic acid. These observations implicate a role of dehydroretinol and its metabolites in UVA/B adaptation. Thus, the keratinocyte response to UV is jointly shaped by both the retinoids and dehydroretinoids.

Elevated serum concentrations of beta-glucuronide metabolites and 4-oxoretinol in lactating sows after treatment with vitamin A: a model for evaluating supplementation in lactating women

BACKGROUND

The effects of large doses of preformed vitamin A, such as those provided in supplementation programs for the prevention of deficiency, on total serum vitamin A have been inadequately investigated.

OBJECTIVE

This study characterized the time course of serum vitamin A metabolites in lactating sows after single high doses of retinyl ester.

DESIGN

Lactating sows were fitted with jugular catheters and subsequently fed either 1.05 or 2.1 mmol retinyl ester (n = 6/group) or a corn oil vehicle (n = 3). Blood was collected at baseline and at intervals to 48 h and analyzed by gradient HPLC for retinol, retinyl esters, and metabolites.

RESULTS

The mean (+/-SD) total serum vitamin A concentration peaked at 1 h (3.69 +/- 4.0 mumol/L) and 2 h (7.70 +/- 6.8 mumol/L) in the low- and high-dose groups, respectively (P < 0.05). Retinyl esters accounted for most of the serum vitamin A in both groups at peak time points. Mean serum retinol concentrations changed little and accounted for most of the serum vitamin A at baseline (94% and 97% for the low- and high-dose groups, respectively) but for only 22% and 14% at peak times for the low- and high-dose groups, respectively. Postdosage increases were observed for total vitamin A and retinyl esters, 4-oxoretinol, retinoyl beta-glucuronide, and retinyl beta-glucuronide but not for retinoic acid.

CONCLUSIONS

Serum retinol concentration remains relatively static after a large dose of preformed vitamin A and therefore is not an appropriate measure of intervention efficacy. The increases in beta-glucuronide metabolites and 4-oxoretinol suggest a preventive role against a rise in retinoic acid and retinol.

Post-irradiation dietary vitamin E does not affect the development of radiation-induced lung damage in rats

The purpose of this study was to investigate whether application of post-irradiation vitamin E, an anti-oxidant, could prevent the development of radiation induced lung damage. Wistar rats were given vitamin E enriched or vitamin E deprived food starting from 4 weeks after 18Gy single dose irradiation of the right thorax. Neither breathing frequencies nor CT density measurements revealed differences between the groups. It is concluded that post-irradiation vitamin E does not influence radiation-induced fibrosis to the lung.

Lung retinol storing cells synthesize and secrete retinoic acid, an inducer of alveolus formation

Retinoids play a key role in the formation of pulmonary alveoli. Lipid interstitial cells (LICs) of the alveolar wall store retinol and are concentrated at sites of alveolus formation, suggesting they are an endogenous source of retinoids for alveolus formation. We show in cultured rat lung cells that LICs synthesize and secrete all-trans retinoic acid (ATRA); its secretion is halved by dexamethasone, an inhibitor of alveolus formation. In a second alveolar wall cell, the pulmonary microvascular endothelial cell (PMVC), ATRA increases expression of the mRNA of cellular retinol binding protein-I (CRBP-I), a protein involved in ATRA synthesis. Serum-free, exogenous ATRA-free medium conditioned by LICs rich in retinol storage granules caused a 10-fold greater increase of CRBP-I mRNA in PMVCs than media conditioned by LICs with few retinol storage granules. This action of medium conditioned by retinol storage granule-rich LICs is decreased by a retinoic acid receptor pan-antagonist and by a retinoid X receptor pan-antagonist, suggesting the responsible molecule(s) is a retinoid and that retinoid signaling occurs in a paracrine fashion.

Analysis of retinoids and carotenoids: problems resolved and unsolved

Progress in nutritional biochemistry has always depended on progress in analysis of nutrients. Animal growth assays were fundamentally important in the discovery and initial isolation of the fat-soluble vitamins. Chromatography, initially introduced by Tswett for separation of plant pigments (including carotenoids), quickly became indispensable for separation of carotenoids and vitamin A compounds; the early open-column methods were eventually superseded by more efficient HPLC techniques, and reversed-phase HPLC has become the current method of choice for analysis of retinoids and carotenoids in biological tissues. Detection and quantitation of retinoids and carotenoids most often has depended on their unparalleled spectral properties; the conjugated polyene structures of these compounds give them unique light absorption spectra and high molar absorptivities, and hence outstanding lower limits of detection. Other techniques, such as gas chromatography (GC) and mass spectroscopy (coupled with GC and HPLC), immunoassays, supercritical fluid chromatography, and capillary electrophoresis, have proven useful in certain applications. Analysis of retinoid-binding proteins has been mostly by conventional protein methods, although the fluorescence of the retinol ligand has been useful in some instances to provide a highly specific assay. Current challenges in retinoid and carotenoid analysis include the resolution of stereoisomers, and quantitation of these compounds at ultratrace levels in biological tissues. Possible new approaches include accelerator mass spectroscopy, and use of gene expression assays to assess vitamin A status.

Epidermal growth factor signaling pathway influences retinoid metabolism by reduction of retinyl ester hydrolase activities in normal and malignant keratinocytes

The effects of EGFR signaling on retinol metabolism were evaluated in the squamous cell carcinoma cell lines defective in LRAT. In a 24-h incubation, the presence of EGF resulted in a 20-25% increase in retinyl ester accumulation. Assessment of retinol esterification and retinyl ester utilization (hydrolysis), in cell cultures and in cell homogenates, revealed that the increase in retinyl ester mass was the result of a reduction in retinyl ester hydrolysis. When grown in the absence of EGF, the cultures used about 40% of their retinyl esters, compared to about 21% in cultures grown with EGF. This effect of EGF was blocked by an EGF receptor-neutralizing antibody, an EGF receptor tyrosine-kinase inhibitor (PD153035), and a specific inhibitor of MEK kinase influencing the mitogen-activated protein kinase (MAPK) cascade (PD98059). Both transcription and translation were required, suggesting that signaling from the EGF receptor through the MAPK cascade controls the expression of modulators or inhibitors of the retinyl ester hydrolase(s). Thus EGFR signaling can alter the intracellular concentration of retinol by suppressing the access to the retinyl ester pool. Similar EGF effects were seen in cultures of normal keratinocytes.

Reduced lecithin:retinol acyl transferase activity in cultured squamous cell carcinoma lines results in increased substrate-driven retinoic acid synthesis

The uptake and metabolism of retinol was compared in squamous cell carcinoma lines, SCC12b and SCC13, and in normal human keratinocytes (NHK). Long chain fatty acid esters of retinol and 3,4-didehydroretinol were the predominant metabolites formed in both cell types. Lesser amounts of unesterified retinol, 3,4-didehydroretinol, and their respective active acid forms were also observed. Despite a qualitatively similar retinoid composition, there were significant quantitative differences between cell types. Most notable was that SCC formed only about one-fourth the retinoid ester as did normal cells. In parallel with this, unesterified retinol and retinoic acid concentrations in SCC were significantly elevated over those in normal cells. This altered pattern of retinoid metabolites in SCC was found to be due to very low lecithin:retinol acyltransferase (LRAT) activity. SCC exhibited less than one-tenth the LRAT activity of normal cells. Acyl-coenzyme A:retinol acyltransferase (ARAT) and retinyl ester hydrolase activities were not different between cell types. Challenging cells with increasing medium retinol concentrations resulted in dose-dependent increases in retinol and retinoic acid within SCC. In contrast, retinol and retinoic acid concentrations in similarly challenged normal cells remained relatively low across a wide retinol concentration range. This was accomplished by the storage of retinol, via LRAT activity, as retinyl ester. Consistent with increased substrate-driven retinoic acid synthesis in SCC, the expression of transglutaminase 1 was suppressed to a greater extent in the SCCs than in NHK, when cells were exposed to equivalent medium concentrations of retinol. The data demonstrate a central role of LRAT in regulating retinoic acid synthesis via its capacity to modulate cellular levels of substrate retinol.

Influence of fasting on circulating levels of alpha-tocopherol and beta-carotene. Effect of short-term supplementation

We investigated the influence of fasting on the levels of alpha-tocopherol in plasma, erythrocytes and platelets, and on plasma beta-carotene. Six apparently healthy adults were subjected to 17-h feed-fasting experiments at various days before, during and after supplementation with alpha-tocopherol (455 mg/day, 41 days) and beta-carotene (25 mg/day, 24 days). Adipose tissue alpha-tocopherol and beta-carotene were measured at regular intervals. Supplementation increased alpha-tocopherol and beta-carotene in all compartments, except for beta-carotene in adipose tissue. Discontinuation caused a rapid return to baseline, except for adipose tissue alpha-tocopherol and plasma beta-carotene. Fasting caused linear increases of free fatty acids, consistent (but small) increases of plasma alpha-tocopherol and inconsistent increases of plasma beta-carotene. There were no fasting-related changes in other compartments. We conclude that fasting is unable to increase alpha-tocopherol and beta-carotene in circulating lipoproteins and cells to a considerable extent, both at baseline levels and after short-term supplementation. Maintenance of high levels may necessitate regular high oral intakes.

Properties of retinoids. Structure, handling, and preparation

Retinoids are unstable compounds being readily oxidized and/or isomerized to altered compounds, especially in the presence of oxidants including air, light, and excessive heat. They are labile toward strong acids and solvents that have dissolved oxygen or peroxides. In this review, procedures for handling and storage of retinoids and biological samples containing them have been described. The physical and chemical properties of retinoids have been reported. Simplified procedures for derivatizations and purification, and methods for quantitation of retinoids have been presented.

Dermal fibroblasts actively metabolize retinoic acid but not retinol

The accessibility of plasma retinol and retinoic acid to the epidermis may be influenced by the number and metabolic capacity of fibroblasts in papillary dermis. The metabolism of retinol-binding protein-bound all-trans-retinol, and albumin-bound all-trans-retinoic acid, by fibroblasts cultured on plastic dishes or in type I collagen gels, was examined. There were no significant differences in the metabolism of either retinoid by fibroblasts as a function of culture condition. There were large differences between retinoids, however. Retinoic acid was rapidly taken up and metabolized to unidentified polar metabolites that were released to the medium. Metabolic capacity was not saturated up to a medium retinoic acid concentration of 1 microM, and was induced further by prior exposure to retinoic acid. In contrast, retinol, although readily taken up, was not metabolized, i.e., neither retinoic acid nor retinyl ester was formed. By immunohistochemistry, the average number of fibroblasts in a 100 microm thickness of papillary dermis was estimated to be 1 x 10(6) cells per cm2. Utilizing this value, the capacity of dermal fibroblasts to metabolize retinoic acid based on fibroblast abundance in the dermis was calculated. The results suggest that fibroblasts could limit delivery of plasma retinoic acid but not retinol to the epidermis on the basis of their metabolic capacity and abundance in the dermis.

Liposome encapsulated vitamin A compounds exhibit greater stability and diminished toxicity

Absorption and fluorescence studies of retinol (vitamin A alcohol) and retinol palmitate (vitamin A palmitate) intercalated in phosphatidylcholine (PC) liposomes show that these compounds are bound to the lipid bilayer. It is further found that retinol binds liposomes with greater affinity as compared to retinol palmitate. In addition, the delivery of liposome-incorporated retinoids to the blood has also been studied and it is found that these systems reduce blood viscosity and cause less lysis of red blood cells than retinoid compounds not complexed in liposomes.

Retinoic acid signaling cascade in differentiating murine epidermal keratinocytes: alterations in papilloma- and carcinoma-derived cell lines

The retinoic acid (RA) signaling pathway was investigated by transient transfection of a chloramphenicol acetyltransferase (CAT) reporter gene construct containing the RA response element (RARE) of the murine (m) RARbeta2 gene into murine primary epidermal keratinocytes (PEK), papilloma-derived SP1 cells, and carcinoma-derived 3P2 cells. Murine PEK transfected in a low-Ca2+ medium (0.05 mM Ca2+) exhibited a strong transactivation of the CATgene after exposure of the cells to 0.1 microM RA. Transactivation of the CATgene could, however, also be achieved by shifting RAREbeta2-transfected low-Ca2+ PEK to high-Ca2+ conditions (0.15-1.2 mM Ca2+). Concomitantly, the Ca2+ raise also led to the induction of both cellular retinol (ROL)-binding protein I (CRBPI) and cellular RA-binding protein II (CRABPII), whereas expression of cellular RA-binding protein I (CRABPI) was not observed. Moreover, induction of in vitro differentiation also activated the ROL-->RA converting enzyme system in PEK. These findings suggest the following sequence of events involved in the high Ca2+-mediated activation of RAREbeta2. First, high Ca2+ induces the synthesis of mCRBPI, which binds ROL released from retinyl ester stores and makes it accessible to the ROL-RA converting enzyme system. Enzymatically generated RA is taken over by mCRABPII and transported to the nucleus, where it acts as ligand for nuclear receptors, which complex with RAREbeta2 to activate the reporter gene. This hypothetical cascade of RA signaling was supported by our findings that inhibition of the ROL-->RA converting enzyme system by citral abolished the Ca2+-mediated transactivation of the CAT gene in a nontoxic manner. Studies in transformed murine cell lines revealed that Ca2+-induced activation of RAREbeta2 was essentially maintained in papilloma-derived SP1 cells, although all parameters of the Ca2+-dependent RAREbeta2 activation cascade were induced to a much lower extent. In contrast, strong RAREbeta2 activity was already observed in low-Ca2+ carcinoma-derived 3P2 cells. Low-Ca2+ 3P2 cells also expressed high levels of both mCRBPI and mCRABPII and possessed a highly active ROL-->RA converting enzyme system. Again, inhibition of the enzyme by citral abolished RAREbeta2 activity in low-Ca2+ 3P2 cells. Our data show that Ca2+-induced differentiation in cultured murine PEK entails a series of events that ultimately lead to the activation of RARE-containing genes. These properties are maintained in transformed epidermal keratinocytes. However, with increasing malignant potential of the cells, the respective signaling pathway becomes independent from a differentiation stimulus and leads to constitutive activation of RARE-controlled genes.

Retinoyl beta-glucuronide: a biologically active form of vitamin A

Retinoyl beta-glucuronide is a naturally occurring, biologically active metabolite of vitamin A. Although retinoyl beta-glucuronide is regarded as a detoxification product of retinoic acid, it plays several roles in the functions of vitamin A. It can serve as a source of retinoic acid, and it may be a vehicle for transport of retinoic acid to target tissues. Topically applied retinoyl beta-glucuronide is comparable in efficacy to retinoic acid in the treatment of acne in humans, without the same side effects. Retinoyl beta-glucuronide may or may not be teratogenic, depending on the mode of administration and the species in which it is used. It may be a valuable therapeutic compound for the treatment of skin disorders and certain types of cancers.

Comparative embryolethality and teratogenicity of the all-trans isomers of retinoic acid, 3,4-didehydroretinyl acetate, and retinyl acetate in pregnant rats

The teratogenic potencies of the all-trans isomers of retinoic acid (RA), 3,4-didehydroretinyl acetate (A2), and retinyl acetate (A1) were compared. Groups of eight timed-pregnant Sprague-Dawley rats were administered single equimolar doses (3.5-352 mumol/kg BW) of the retinoids orally in oil on day 8.5 of pregnancy, and dams and fetuses were sacrificed on day 19. The relative teratogenicity and embryolethality of the three tested retinoids were: RA > A2 > A1. The no-effect level of RA and A2 was 3.5 mumol/kg BW and of A1 was 35 mumol/kg BW. Whereas the adverse effects of RA and A1 were dose dependent, A2 showed biphasic effects, with a peak of embryolethality at 35 mumol/kg BW. Dams also exhibited weight loss and other toxic manifestations from doses of A2 and Ra > or = 35 mumol/kg BW. In dosed dams, (1) Liver concentrations of A1 and A2 increased with the doses of A1 and A2, respectively, (2) RA had little effect on liver A1 except for an increase at the highest toxic dose, and (3) A2 showed a sparing effect on liver A1. RA, although not detected in fetuses from dams treated with A1, was present in significant concentrations (0.5-4.1 nmol/g liver) in fetuses from dams treated with A2. The biphasic change in embryolethality with the dose of A2 correlates with this enhanced concentration of fetal RA. We hypothesize that the actual teratogen in the fetuses of A2-dosed dams is RA. A2 might induce this biphasic effect by inhibiting the catabolism of RA at lower doses and its formation at higher doses.

All-trans-retinoyl beta-glucuronide: new procedure for chemical synthesis and its metabolism in vitamin A-deficient rats

All-trans-retinoyl beta-glucuronide (RAG) was chemically synthesized in high yields (up to 79%) by a new procedure involving the reaction of the tetrabutylammonium salt of glucuronic acid with all-trans-retinoic acid (RA) via the imidazole or triazole derivative. When RAG was fed orally to vitamin A-deficient rats, RA was identified as the major metabolite in the serum within hours of administration of RAG. Very little or no RAG was detected in the serum. Thus RAG, which was not appreciably hydrolysed to RA in vitamin A-sufficient rats [Barua and Olson (1987) Biochem. J. 263, 403-409], was rapidly converted into RA in vitamin A-deficient rats.

all-trans-retinoic acid regulates retinol and 3,4-didehydroretinol metabolism in cultured human epidermal keratinocytes

The potential for all-trans-retinoic acid to regulate the metabolism of 3H-retinol and 3H-3,4-didehydroretinol was examined in cultured human epidermal keratinocytes. Confluent cultures were treated daily with medium containing 5% fetal bovine serum or the same medium supplemented with nanomolar concentrations of all-trans-retinoic acid for up to 3 d. During the last 24 of treatment, cells were incubated with 3H-retinol or 3H-3,4-didehydroretinol for 24 h (isotopic steady state) to label the endogenous retinoids. After the labeling period, one group of cells was harvested and another group was allowed to incubate for an additional 24 h in the absence of medium retinol for the determination of endogenous 3H-retinoid utilization. The 3H-retinoids present in cells were extracted and quantitated by reverse-phased high-pressure liquid chromatography. Keratinocytes treated with retinoic acid and labeled with 3H-retinol exhibited time- and concentration-dependent (i) increases in retinyl ester mass, (ii) increases in the rate of retinyl ester synthesis, (iii) decreases in retinyl ester utilization, and (iv) decreases in the cellular concentrations of retinoic and 3,4-didehydroretinoic acids. There was no effect of exogenous retinoic acid on its own metabolism. Cells labeled with 3H-3,4-didehydroretinol exhibited exclusive labeling of vitamin A2-related retinoids suggesting that the A1 to A2 conversion is not reversible. Treatment of cells with low nanomolar concentrations of retinoic acid decreased the utilization of 3,4-didehydroretinyl esters, decreased the production of 3,4-didehydroretinoic acid but had no effect on the synthesis of 3,4-didehydroretinol or its esters. The results demonstrate that keratinocytes respond to extracellular retinoic acid by decreasing endogenous production of active retinoids, sequestering extracellular substrate retinol as retinyl ester, and decreasing ester utilization.

Chromatographic and electrophoretic analysis of biomedically important retinoids

The determination of retinol (vitamin A) and its metabolites, as well as synthetic retinoids, in biological samples is a challenging task due to the sensitivity of these compounds to light, heat and oxygen, high protein binding, separation of geometric isomers and determination of low endogenous levels. Numerous procedures for sample preparation have been published for biological fluids and tissues, consisting of solvent extraction, solid-phase extraction (off-line) and HPLC with column switching (on-line solid-phase extraction). The last-mentioned technique has several advantages, including a high degree of automation, no evaporation of extraction solvents, protection from light and higher sensitivity. Due to the favourable UV characteristics of most retinoids, HPLC with UV detection is most often employed, and photodiode array detection is becoming more and more popular. Fluorescence and electrochemical detection have found only a limited field of application, but the use of LC-MS resulted in a few highly sensitive methods. Reconsideration of GC through the use of better deactivated columns and cold on-column injection and evaluation of new promising separation methods, such as supercritical fluid chromatography and capillary electrophoresis, have shown preliminary encouraging results, but appear to reach the required sensitivity only by coupling to MS. Therefore, HPLC with UV detection is still the method of choice for highly sensitive and selective retinoid determination, as well as for high sample throughput and robustness.