Dietary vitamin A modulates the properties of retinoic acid and glucocorticoid receptors in rat liver

Vitamin A deficiency in rats induces anatomic and metabolic changes comparable with those of neurodegenerative disorders

Anatomic and metabolic changes in central nervous system induced by 14 wk of vitamin A deprivation (VAD) were monitored and quantified in rats. In vivo brain magnetic resonance imaging (4.7T) was performed at 5, 7, 9, 11, and 14 wk of each diet after weaning in the following: 1) VAD group; 2) control pair-fed group; and 3) control group that consumed the diet ad libitum (1.15 microg retinol/g diet). After 14 wk, high-resolution magic angle spinning proton NMR spectroscopy (11.7T) was performed on small samples of cortex, hippocampus, and striatum. Serum retinol concentrations remained stable and cerebral volume (CV) increased as a linear function of body weight in the ad libitum group (R(2) = 0.78; P = 0.047) and pair-fed controls (R(2) = 0.78; P = 0.046). In VAD rats, retinol decreased from the onset of deprivation (2.2 +/- 0.14 micromol/L) to reach 0.3 +/- 0.13 micromol/L at wk 5, followed by a stopping of body weight gain from wk 7. In VAD rats, the CV decreased from wk 5 and reached a value 11% lower than that of the control group (P < 0.001) at wk 14 and was correlated with retinol status (R(2) = 0.99; P = 0.002). The VAD hippocampal volume decreased beginning at wk 9 and was 22% lower than that of the control group at wk 14 (P < 0.001). Compared with the control, VAD led to lower N acetyl aspartate:creatine+phosphocreatine (Cr) in cortex (-36%), striatum (-22%), and hippocampus (-19%) and higher myoinositol:Cr in cortex (+127%) and striatum (+150%). VAD induced anatomic and metabolic changes comparable to those associated with neurodegenerative disorders. By wk 7 of deprivation, the slowing in cerebral growth that correlated with the retinol level could be considered as a predictive marker of brain disorders, confirmed by metabolic data from VAD rats after 14 wk.

Vitamin A modulates the effects of thyroid hormone on UDP-glucuronosyltransferase expression and activity in rat liver

We studied the influence of thyroid hormones and vitamin A status on the regulation of UDP-glucuronosyltransferase (UGT) expression and the glucuronidation of thyroid hormones by UGTs. For this, we used an original model of rats fed with different vitamin A diets and implanted subcutaneously by osmotic minipumps delivering vehicle or thyroid hormones, which permitted the control of plasma thyroid hormone concentrations. The activity and expression of family 1 UGTs are correlated and were significantly modified by both thyroid status and amounts of retinol in the diet. Dietary vitamin A did not perturbe the UGT1A expression in thyroidectomized animals. Thyroid hormones and dietary vitamin A did not affect the activity and expression of family 2 UGTs. We conclude that thyroid hormones and vitamin A are co-regulator of the UGT1 family expression, without affecting the UGT2 family; by modifying activity and expression of the bilirubin UOT isoform, a member of UGT1 family, thyroid hormone reduced the glucuronidation of T4 and rT3.

High-fat diets affect the expression of nuclear retinoic acid receptor in rat liver

The purpose of this study was to differentiate between the effects of the amount and the type of dietary lipids on the expression of the retinoic acid receptor (RAR), but also the peroxisome proliferator-activated receptor (PPAR) and the receptor of the 9-cis retinoic acid (retinoid X receptor (RXR)) in rat liver. Six groups of eight rats (5-weeks old) were fed during 4 weeks on the following diets: control 50 g vegetable oil/kg, high-fat diet 250 g vegetable oil/kg. These oils were either coconut oil (rich in saturated fatty acids) or olive oil (rich in monounsaturated fatty acids) or safflower oil (rich in polyunsaturated fatty acids, mainly as n-6). The three high-fat diets induced a significant decrease of the maximal binding capacity of RAR and of the abundance of RAR beta mRNA. Simultaneously, an increased expression of PPAR alpha mRNA was observed while no significant difference on abundance of RXR alpha mRNA was observed. The mechanisms involved are probably multiple, but one hypothesis is that a modification of the equilibrium between the nuclear receptors, resulting from an increased expression of PPAR, induces a decreased expression of RAR in rat liver.

Expression of retinoic acid, triiodothyronine, and glucocorticoid hormone nuclear receptors is decreased in the liver of rats fed a hypercholesterolemia-inducing diet

Several studies have shown that dietary factors modulate cell signaling pathways. The aim of this study was to determine whether a hypercholesterolemia-inducing diet rich in saturated fat and cholesterol modifies rat liver expression of the nuclear receptors of retinoic acid (RAR), triiodothyronine (TR), and glucocorticoid hormone (GR), which are transcriptional factors. The experimental diet contained coconut oil 25 g/100 g as a source of lipids, cholesterol 1 g/100 g, and cholic acid 0.5 g/100 g, and the control diet contained olive oil 5 g/100 g. After 26 days of feeding the hypercholesterolemia-inducing diet, a lower binding capacity of the nuclear receptors and a smaller amount of their mRNA were observed. Moreover, the activities of malic enzyme (ME) and tyrosine aminotransferase (TAT), whose gene promotors contain a response element to TR and GR, respectively, were significantly decreased. These changes occurred in a cellular environment characterized by a high level of cholesterol and free fatty acids (FFAs). Thus, two nonexclusive hypotheses can be proposed to explain this decreased expression of nuclear receptors, one emphasizing the effect of lipidic components on the cellular amount of receptor ligands (retinoic acid [RA] and triiodothyronine [T3]), the other emphasizing a modification of the balance between nuclear receptors that could impede the upregulation of TR and RAR.

Aging decreases retinoic acid and triiodothyronine nuclear expression in rat liver: exogenous retinol and retinoic acid differentially modulate this decreased expression

The expression of nuclear receptors of retinoic acid (RAR) and triiodothyronine (TR) was analyzed in the liver of rats aged 2.5 (young), 6 (adult) and 24 (aged) months. In aged rats, decreased binding properties, binding capacity (Cmax) and affinity (Ka), of nuclear receptors were observed. This resulted, at least in part, from decreased transcription of receptor genes in that the amount of their mRNA also decreased. Moreover, the activity of malic enzyme (ME) and tissue transglutaminase (tTG), whose genes are TR and RAR responsive, respectively, was reduced in aged rats. These results are in agreement with the decreased binding capacity of these receptors. An inducer-related increase of RAR and TR expression was observed 24 h after a single dose of retinoic acid administration (5 mg/kg), while retinol administration (retinyl palmitate, 13 mg/kg) was without incidence on nuclear receptor expression in aged rats.

Correlation between the effects of retinoic acid and dexamethasone on liver tyrosine aminotransferase

A single dose of 50 microg of trans-retinoic acid administered to rats significantly raised the level of hepatic tyrosine after a few hours. This effect was compared with that of dexamethasone and a possible correlation between these effectors was also investigated. An equal increase in enzyme activity level caused by retinoic acid was observed in adrenalectomized rats, leading to the suggestion that the effect of retinoic acid on liver tyrosine aminotransferase is independent of glucocorticoids. However, the study of the binding activity of the liver nuclear glucocorticoid receptors vs dexamethasone showed that this activity is favoured by retinoic acid, whereas no variation was evidenced for retinoic acid receptors caused by dexamethasone. In the adrenalectomized rat, the synergistic effect produced by the association of retinoic acid and dexamethasone at the lowest doses used led us to conclude that retinoic acid is an efficient effector of liver tyrosine aminotransferase. It probably affects tyrosine aminotransferase activity in a dependent and an independent way, regulated respectively by the glucorticoid status and by the provision of retinoic acid.

Effect of perinatal vitamin A or retinoic acid treatment (hormonal imprinting) on the sexual behavior of adult rats

Single neonatal treatment with vitamin A (retinol) dramatically reduced the sexual activity of adult male rats. In females there was a significant decrease in the Meyerson index and a non significant decrease in the lordosis quotient. The effect of three perinatal treatments (at the first, third and fifth day) with all-trans retinoic acid was much weaker, causing only a significant increase in the time of the first ejaculation in males and non-significant decrease in the lordosis quotient of females. The experiments call attention to the false imprinting provoking effect of materials acting on members of the steroid receptor superfamily with possible human health aspect.

Teratogenicity of a single oral dose of retinyl palmitate in the rat, and the role of dietary vitamin A status

Vitamin A, known for its teratogenic properties, may be present in high concentrations in consumption liver. It is as yet unclear whether congenital malformations can result from a single liver meal. In our first experiment, the teratogenicity of a single dose of retinyl palmitate was tested in the rat. Pregnant rats were treated at day 10 of gestation by gavage with 100, 300 or 1000 mg/kg body weight retinyl palmitate on a dietary background level of 5 mg/kg feed. At gestation day 11 the number of embryos with an open cranial neural tube had increased with the dose. At gestation day 21, the high dose group showed an increase in late resorptions, whereas both the high and the medium dose groups had a high incidence of foetuses with malformations typical of retinoid embryopathy. The data suggest that delayed neural tube closure had occurred in a large proportion of the embryos. In a second experiment, the high oral dose was applied on gestation day 10 in pregnant rats receiving retinyl palmitate at 1.5, 5, 15, or 50 mg/kg feed for 6 weeks. Delayed neural tube closure, post-implantation loss and the nature and incidence of malformations were similar between diet groups, as well as being reminiscent of the high dose group in the first experiment. Thus the dietary status of the animals did not seem to influence the teratogenic potential of a single high dose of retinyl palmitate.

The influence of dietary vitamin A on triiodothyronine, retinoic acid, and glucocorticoid receptors in liver of hypothyroid rats

The properties of nuclear receptors belonging to the superfamily of receptors acting as transcription factors are modulated by nutritional and hormonal conditions. We showed recently that retinoic acid (RA) restored to normal the expression of receptors attenuated by hypothyroidism. The present study was designed to find out whether dietary vitamin A (as retinol) had the same effect. Propylthiouracil in drinking water induced both hypothyroidism and a vitamin A-deficient status in rats. The maximum binding capacity (Cmax) of triiodothyronine nuclear receptors (TR) was unchanged, while that of nuclear RA receptors (RAR) and nuclear glucocorticoid hormone receptors (GRn) was reduced in the liver of these hypothyroid rats. The reduced Cmax of RAR stemmed from a lower level of RAR mRNA, while the reduced Cmax of GRn was assumed to be due to reduced translocation of the receptor from the cytosol to the nucleus. Feeding the hypothyroid rats with a vitamin A-rich diet did not restore the Cmax of either RAR or GRn to normal. The lack of effect of dietary retinol on RAR expression may be attributed to either genomic (unoccupied TR block the expression of RAR genes) and/or extragenomic (hypothyroidism decreases the availability of retinol and/or its metabolism to RA) mechanisms. Triiodothyronine is thought to favour the translocation of glucocorticoid hormone receptors from cytosol to nucleus. These findings provide more information on the relationship between vitamin A and hormonal status, showing that a vitamin A-rich diet is without apparent effect on the expression of nuclear receptors in hypothyroid rats.

Dexamethasone decreases the expression of retinoic acid receptors (RARs) in rat liver

Although adrenalectomy was without effect on the expression of retinoic acid (RA) receptors (RARs), administration of the glucocorticoid analog dexamethasone (Dex) to both control and adrenalectomized rats reduced the expression of these receptors in rat liver. This effect can be mainly attributed to the action of Dex on 4-hydroxylation of RA. Dex, by enhancing 4-hydroxylation of RA, reduces its intracellular concentration thereby leading to a decreased expression of RARs, since RARbeta, the main type of RARs in liver, are known to be up-regulated by RA.

Chronic ethanol administration enhances retinoic acid and triiodothyronine receptor expression in mouse liver

Chronic alcoholism induces perturbations of storage and metabolization of retinol and related compounds. After 6 months of ethanol consumption we have observed in mouse liver an increased expression of Tri-iodothyronine receptors (TR) while the expression of retinoic acid (RA) receptors (RAR) was unaffected. After 10 months of alcoholization the TR expression was strongly increased and the RAR expression was also increased. At this time the activity of aldehyde dehydrogenase and that of alcohol dehydrogenase, two enzymes involved in biosynthesis of RA from retinol, were similar in the liver of alcoholized and pair-fed mice. Thus it can be hypothesized that (i) the change of RAR expression was, at least in part, the result of a change of TR expression (result in agreement with previous data), (ii) the increased expression of RAR could induce apoptosis and subsequently liver necrosis.