Véronique Azaïs-Braesco

CELLULAR RETINOL-BINDING PROTEIN I IS ESSENTIAL FOR VITAMIN A HOMEOSTASIS

The gene encoding cellular retinol (ROL, vitA)‐binding protein type I (CRBPI) has been inactivated. Mutant mice fed a vitA‐enriched diet are healthy and fertile. They do not present any of the congenital abnormalities related to retinoic acid (RA) deficiency, indicating that CRBPI is not indispensable for RA synthesis. However, CRBPI deficiency results in an ∼50% reduction of retinyl ester (RE) accumulation in hepatic stellate cells. This reduction is due to a decreased synthesis and a 6‐fold faster turnover, which are not related to changes in the levels of RE metabolizing enzymes, but probably reflect an impaired delivery of ROL to lecithin:retinol acyltransferase. CRBPI‐null mice fed a vitA‐deficient diet for 5 months fully exhaust their RE stores. Thus, CRBPI is indispensable for efficient RE synthesis and storage, and its absence results in a waste of ROL that is asymptomatic in vitA‐sufficient animals, but leads to a severe syndrome of vitA deficiency in animals fed a vitA‐deficient diet.

Vitamin A in pregnancy: requirements and safety limits.

Most of the functions of vitamin A are mediated through the binding of retinoic acid to specific nuclear receptors that regulate genomic expression. Recent experimental work in transgenic mice showed clearly that normal embryonic development depends on the correct spatial and temporal expression of the receptors in the differentiating cells and on the binding of specific forms of retinoic acid. This implies that the parent compound, vitamin A, is available in adequate forms and quantities. Excessive dietary intake of vitamin A has been associated with teratogenicity in humans in <20 reported cases over 30 y. However, caution must be exercised to avoid unnecessary supplementation of women of childbearing age. Hypovitaminosis A affects millions of women and children worldwide. The main consequence of a poor vitamin A supply during pregnancy is a low vitamin A status at birth and in the next few months. Vitamin A deficiency is strongly associated with depressed immune function and higher morbidity and mortality due to infectious diseases such as diarrhea, measles, and respiratory infections. Vitamin A deficiency is often associated with an increased mother-to-child transmission of HIV-1. The initiation of vitamin A supplementation should be carefully examined in each case according to the risk-to-benefit ratio. The final decision should take into account the estimated vitamin A status of the woman, the availability of vitamin A-rich foods in her diet, and whether supplementation can be supervised.

Incorporation of carotenoids in aqueous systems: uptake by cultured rat hepatocytes

The in vitro investigations about carotenoid functions and metabolism are hindered by their hydrophobicity. In order to mimic as close as possible the physiological events, we prepared by rapid and easy methods sterile liposomes and emulsions containing carotenoids which are absorbed by cultured rat hepatocytes as a function of time and temperature. The lipid composition of the vesicles was shown to influence the carotenoid encapsulation.

Effect of magnesium deficiency on triacylglycerol-rich lipoprotein and tissue susceptibility to peroxidation in relation to vitamin E content

Given the current interest in the cardiovascular complications of Mg deficiency, the aim of the present experiment was to investigate the effect of Mg deficiency on the time-course of lipoprotein oxidation and to assess whether short-term Mg deficiency results in vitamin E depletion that predisposes lipoproteins and tissues to subsequent oxidation. Weanling rats were pair-fed for 8 d with control and Mg-deficient diets respectively. Plasma triacylglycerol and alpha-tocopherol levels were significantly greater in Mg-deficient rats compared with control animals. The increase in plasma apolipoprotein B concentration indicated that a corresponding increase in plasma triacylglycerol-rich lipoproteins (TGRLP) occurred in Mg-deficient animals. Hyperlipaemia was associated with modifications in the composition of TGRLP. The proportion of triacylglycerols was elevated whereas that of cholesterol and protein was reduced, and Mg deficiency resulted in a slight significant reduction in alpha-tocopherol content. When the TGRLP fractions were subjected to in vitro Cu-induced oxidation the lipoprotein fractions from Mg-deficient rats were more susceptible to oxidative damage than lipoprotein fractions from control rats. Mg deficiency did not modify the alpha-tocopherol content of liver, heart and skeletal muscle. However, after exposure of tissue homogenates to Fe-induced lipid peroxidation, thiobarbituric acid-reactive substances were significantly higher in tissues from Mg-deficient rats compared with those from control rats. These results complement previous findings, showing that Mg deficiency increases the susceptibility of TGRLP and tissues to peroxidation and suggest that oxidative damage is not the result of a decrease in vitamin E antioxidant status.

Postprandial chylomicron and plasma vitamin E responses in healthy older subjects compared with younger ones

The effect of ageing on vitamin E bioavailability in humans was assessed by comparing chylomicron and plasma α‐tocopherol postprandial concentrations after a dose of vitamin E (432 or 937 IU as dl‐α‐tocopherol acetate), in eight young (20–30 years old) and eight healthy elderly men (64–72 years old). The fasting plasma α‐tocopherol concentration was significantly higher in the elderly (33 ± 2 μmol L−1) than in the young (22 ± 2 μmol L−1). In both groups, the plasma and chylomicron α‐tocopherol postprandial concentrations were significantly, approximately twofold, higher after the 937‐IU meal than after the 432‐IU meal. For both test meals, the chylomicron α‐tocopherol areas under the curve were significantly lower in the elderly than in the young subjects: 98.9 ± 16.5 (young group) vs. 55.3 ± 7.8 (elderly group) μmol L−1 h for the 937‐IU test meal and 60.4 ± 14.1 (young group) vs. 26.0 ± 7.6 (elderly group) μmol L−1 h for the 432‐IU test meal, whereas the plasma α‐tocopherol area under the curve was significantly higher in elderly than in young subjects: 337.56 ± 16.11 (937‐IU test meal) vs. 159.81 ± 35.55 (432‐IU test meal) μmol L−1 h in the young group and 709.55 ± 69.33 (937‐IU test meal) vs. 436.39 ± 41.08 (432‐IU test meal) μmol L−1 h in the elderly group. We concluded that (a) the amount of vitamin E appearing in plasma is proportional to the dose ingested (up to 937 IU); (b) the intestinal absorption of vitamin E is not increased, even possibly decreased, in the elderly; and (c) the amount of vitamin E transported by non‐chylomicron lipoproteins is apparently higher in the elderly. This suggests that vitamin E postprandial transport is affected by ageing, mainly as the consequence of age‐related modifications of lipoprotein metabolism.

Esterification of Vitamin A by the Human Placenta Involves Villous Mesenchymal Fibroblasts

Vitamin A (retinol) and its active derivatives (retinoic acids) are essential for growth and development of the mammalian fetus. Maternally derived retinol must pass the placenta to reach the developing fetus. Despite its apparent importance, little is known concerning placental transfer and metabolism of retinol, and particularly of placental production and storage of retinyl esters. To elucidate this metabolic pathway, we incubated, in the presence of retinol, 1) human full-term placental explants and 2) primary cultures of major cells types contributing to placental function: trophoblasts and villous mesenchymal fibroblasts. We used HPLC to determine the types and concentrations of retinyl esters produced by these explants and cells. About 14% of total cellular retinol in placental explants was esterified. The most abundant esters were myristate and palmitate. Primary cell cultures showed that fibroblasts efficiently produced retinyl esters, but trophoblasts did not. In both types of experiments, no retinyl esters were detected in the culture medium, suggesting that retinyl esters were produced for storage purpose. These results suggest that villous mesenchymal fibroblasts are primary sites of retinol esterification and storage in the placenta.

Comparative bioavailability of diet-, oil- and emulsion-based preparations of vitamin a and β-carotene in rat

Abstract The bioavailability of vitamin A and β-carotene delivered from the diet, from an oily solution or from a lipid emulsion was studied in female and male rats both in moderate and in severe vitamin A-deficient states. Weaning rats were fed a vitamin A-free diet for 3 or 5 weeks and were thereafter supplemented with vitamin A or β-carotene by oral gavage with oil- or emulsion-based solutions or by diet. Vitamin A repletion, as judged by growth rate and by vitamin A accumulation in tissues, was improved when aqueous- preparations of both micronutrients were selected, as compared with other treatments. The vitamin A equivalence of β-carotene ranged from 3.75 to 8.83 when emulsion- and oil- mediated supplementations were respectively achieved. Both vitamin A bioavailabity and vitamin A activity of β-carotene were shown to be related to the vitamin A status of the animal. No sex effect was observed on the measured parameters.

A review of total & added sugar intakes and dietary sources in Europe

Public health policies, including in Europe, are considering measures and recommendations to limit the intake of added or free sugars. For such policies to be efficient and monitored, a precise knowledge of the current situation regarding sugar intake in Europe is needed. This review summarizes published or re-analyzed data from 11 representative surveys in Belgium, France, Denmark, Hungary, Ireland, Italy, Norway, The Netherlands, Spain and the UK. Relative intakes were higher in children than in adults: total sugars ranged between 15 and 21% of energy intake in adults and between 16 and 26% in children. Added sugars (or non-milk extrinsic sugars (NMES), in the UK) contributed 7 to 11% of total energy intake in adults and represented a higher proportion of children’s energy intake (11 to 17%). Educational level did not significantly affect intakes of total or added sugars in France and the Netherlands. Sweet products (e.g. confectionery, chocolates, cakes and biscuits, sugar, and jam) were major contributors to total sugars intake in all countries, genders and age groups, followed by fruits, beverages and dairy products. Fruits contributed more and beverages contributed less to adults’ total sugars intakes than to children’s. Added sugars were provided mostly by sweet products (36 to 61% in adults and 40 to 50% in children), followed by beverages (12 to 31% in adults and 20 to 34% in children, fruit juices excluded), then by dairy products (4 to 15% in adults and 6 to 18% in children). Caution is needed, however, as survey methodologies differ on important items such as dietary data collection, food composition tables or estimation of added sugars. Cross-country comparisons are thus not meaningful and overall information might thus not be robust enough to provide a solid basis for implementation of policy measures. Data nevertheless confirm that intakes of total and added sugars are high in the European countries considered, especially in children, and point to sweet products and beverages as the major contributors to added sugar intakes.

Vitamin A contained in the lipid droplets of rat liver stellate cells is substrate for acid retinyl ester hydrolase

Vitamin A is stored in the lipid droplets of liver stellate cells (LSCs), as retinyl esters whose hydrolysis is necessary for the secretion of retinol into the blood. Here, we isolated these retinyl esters under their physiological form, i.e., in LSC lipid droplets, which had retained their morphological and biochemical characteristics. These retinyl esters are substrate for an hydrolytic enzyme, whose optimum pH is 4.1, and which is kinetically similar to the acidic retinyl ester hydrolase (aREH) we had previously described (Mercier et al., Biochim. Biophys. Acta (1994) 1212, 176-182). The cellular and subcellular localizations of aREH activity in rat liver suggest that this enzyme could be involved in the hydrolysis of the esterified vitamin A stores.

Hydrolysis of retinyl esters in rat liver. Description of a lysosomal activity

When incorporated into liposomes made of phospholipids, retinyl palmitate is an adequate substrate for an acidic REH (aREH). In rat liver, this activity is mainly localized in the lysosomal fraction. Kinetic parameters have been determined for retinyl palmitate (Km = 315 microM; maximal rate = 22.1 nmol retinol/h per mg protein). The aREH activity is different from the lysosomal acidic cholesteryl ester hydrolase (aCEH): cholesteryl oleate does not inhibit aREH activity, neither do some aCEH specific inhibitors, and aREH does not hydrolyse cholesteryl ester. Involvement of aREH in the hydrolysis of lipid droplets retinyl esters in fat storing cells is discussed.