Vitamin A-sufficient Diet Research Articles

Lycopene Accumulation in Transgenic Mice Lacking One or Both Carotenoid Cleaving Enzymes

Backgroundβ-carotene oxygenase 1 (BCO1) and β-carotene oxygenase 2 (BCO2) are responsible for the cleavage of carotenoids in mammals. Objective: The goals of this study were to (1) establish the relative contribution of each enzyme on lycopene accumulation in mice and (2) examine the role of lycopene on gene expression in the gut of wild type (WT) mice. MethodsWe utilized male and female WT, Bco1-/-, Bco2-/-, and Bco1-/-Bco2-/- double knockout (DKO) mice. We gavaged the mice with either 1 mg of lycopene resuspended in cottonseed oil or vehicle as a control group daily for 2 wk. In a second study, we evaluated the effect of dietary vitamin A on lycopene absorption and intestinal gene expression by RT-PCR. We also quantified lycopene concentration isomer distribution by high performance liquid chromatography. ResultsOf the 11 tissues measured, the liver accounted for 94 to 98% of the lycopene content across genotypes. We did not observe sex differences between genotypes, although hepatic lycopene levels in Bco1-/- mice were approximately half in comparison to the other genotypes; Bco1-/- verses Bco2-/- (P < 0.0001), DKO mice (P < 0.001), WT (ns). Analyses of mitochondrial lycopene content revealed a 3- to 5-fold enrichment compared with total hepatic content (P < 0.05) in all genotypes and sexes. In our second study, WT mice fed a vitamin A-deficient diet (VAD) accumulated greater amounts of lycopene in the liver than those fed a vitamin A-sufficient diet (VAS) (P < 0.01). These changes were accompanied by an upregulation of the vitamin A-responsive transcription factor intestine specific homeobox (ISX) in mice fed VAD + lycopene and VAS + lycopene diets compared with VAD control-fed mice (P < 0.05). ConclusionsOur data suggest that BCO2 is the primary lycopene cleavage enzyme in mice. Lycopene concentration was enriched in the mitochondria of hepatocytes independently of genotype, and lycopene stimulated vitamin A signaling in WT mice.

Mice Lacking the Systemic Vitamin A Receptor RBPR2 Show Decreased Ocular Retinoids and Loss of Visual Function

Simple SummaryThis work represents an initial evaluation of the second RBP4-vitamin A receptor RBPR2 in a mammalian model. We provide evidence that the membrane localized RBPR2 protein, under variable conditions of dietary vitamin A intake, plays an important role for dietary vitamin A transport to the eye for ocular retinoid homeostasis and visual function. These findings are of general interest, as disturbances in blood and ocular vitamin A homeostasis are linked to retinal degenerative diseases, which are blinding diseases. The animal model described here could also serve as an in vivo tool to study mechanisms related to retinal cell degeneration that are associated with vitamin A deficiency.The systemic transport of dietary vitamin A/all-trans retinol bound to RBP4 into peripheral tissues for storage is an essential physiological process that continuously provides visual chromophore precursors to the retina under fasting conditions. This mechanism is critical for phototransduction, photoreceptor cell maintenance and survival, and in the support of visual function. While the membrane receptor STRA6 facilitates the blood transport of lipophilic vitamin A into the eye, it is not expressed in most peripheral organs, which are proposed to express a second membrane receptor for the uptake of vitamin A from circulating RBP4. The discovery of a novel vitamin A receptor, RBPR2, which is expressed in the liver and intestine, but not in the eye, alluded to this long-sort non-ocular membrane receptor for systemic RBP4-ROL uptake and transport. We have previously shown in zebrafish that the retinol-binding protein receptor 2 (Rbpr2) plays an important role in the transport of yolk vitamin A to the eye. Mutant rbpr2 zebrafish lines manifested in decreased ocular retinoid concentrations and retinal phenotypes. To investigate a physiological role for the second vitamin A receptor, RBPR2, in mammals and to analyze the metabolic basis of systemic vitamin A transport for retinoid homeostasis, we established a whole-body Rbpr2 knockout mouse (Rbpr2−/−) model. These mice were viable on both vitamin A-sufficient and -deficient diets. Rbpr2−/− mice that were fed a vitamin A-sufficient diet displayed lower ocular retinoid levels, decreased opsins, and manifested in decrease visual function, as measured by electroretinography. Interestingly, when Rbpr2−/− mice were fed a vitamin A-deficient diet, they additionally showed shorter photoreceptor outer segment phenotypes, altogether manifesting in a significant loss of visual function. Thus, under conditions replicating vitamin A sufficiency and deficiency, our analyses revealed that RBPR2-mediated systemic vitamin A transport is a regulated process that is important for vitamin A delivery to the eye when RBP4-bound ROL is the only transport pathway in the fasting condition or under vitamin A deficiency conditions.

Interaction between dietary vitamin A, gut microbes, and host vitamin A status

Micronutrient deficiency during childhood, impairs microbiome development and establishment, ultimately affecting nutrient absorption and promoting inflammatory responses, especially in the gut, in a sort of vicious cycle1. Vitamin A is an essential micronutrient that regulates many critical biological functions, such as vision, reproduction and immunity, and that more generally contributes to the integrity of the epithelia by modulating cellular proliferation and differentiation 2. Vitamin A‐deficiency (VAD) is an overwhelming and still intractable public health problem3. The essential nutrient vitamin A regulates intestinal functions by modulating inflammation, gut immunity, and intestinal cell proliferation. Thus, the vitamin A status has a profound impact on intestinal health and, indeed, VAD is often associated with pathological conditions of the gut. On the other hand, a healthy intestine is critical to support a “healthy” microbiome which in turn helps maintaining proper intestinal functions in a feedback loop.To gain further understanding of the complex interplay between vitamin A, host physiology and microbiome, we used Lrat−/−Rbp−/−mice as a model of marginal VAD 4, when maintained on a vitamin A‐sufficient diet, and of severe VAD when deprived of dietary vitamin A. Due to the absence of lecithin:retinol acyltransferase (LRAT) and retinol‐binding protein (RBP), these mice cannot store and mobilize, respectively, hepatic vitamin A towards the periphery of the body. Thus, they rely exclusively on dietary vitamin A to support vitamin A‐dependent functions4. At 6 weeks of age, Lrat−/−Rbp−/− and WT mice were placed on either vitamin A‐sufficient or ‐deficient diet for 4 weeks and then returned to the initial regular chow regimen for 1 week. Various tissues, including intestine, and feces were collected at different time points throughout the experiment. We confirmed that VAD impairs intestinal vitamin A homeostasis and barrier functions (reduced antimicrobial defense and mucus production; increased oxidative stress; leaky gut). Genomic sequencing of the fecal 16S RNA showed that fecal dysbiosis is concomitant with the VAD status rather than the dietary vitamin A content per se, even though the degree of deficiency (marginal vs severe) did not further affect the dysbiosis. Instead, a preliminary analysis of microbial functionalities showed a significant separation of the microbial populations between vitamin A sufficient, marginal VAD and severe VAD.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Pathways of retinoid synthesis in mouse macrophages and bone marrow cells.

In vivo pathways of natural retinoid metabolism and elimination have not been well characterized in primary myeloid cells, even though retinoids and retinoid receptors have been strongly implicated in regulating myeloid maturation. With the use of a upstream activation sequence-GFP reporter transgene and retrovirally expressed Gal4-retinoic acid receptor α in primary mouse bone marrow cells, we identified 2 distinct enzymatic pathways used by mouse myeloid cells ex vivo to synthesize retinoic acid receptor α ligands from free vitamin A metabolites (retinyl acetate, retinol, and retinal). Bulk Kit(+) bone marrow progenitor cells use diethylaminobenzaldehyde-sensitive enzymes, whereas bone marrow-derived macrophages use diethylaminobenzaldehyde-insensitive enzymes to synthesize natural retinoic acid receptor α-activating retinoids (all-trans retinoic acid). Bone marrow-derived macrophages do not express the diethylaminobenzaldehyde-sensitive enzymes Aldh1a1, Aldh1a2, or Aldh1a3 but instead, express Aldh3b1, which we found is capable of diethylaminobenzaldehyde-insensitive synthesis of all trans-retinoic acid. However, under steady-state and stimulated conditions in vivo, diverse bone marrow cells and peritoneal macrophages showed no evidence of intracellular retinoic acid receptor α-activating retinoids, despite expression of these enzymes and a vitamin A-sufficient diet, suggesting that the enzymatic conversion of retinal is not the rate-limiting step in the synthesis of intracellular retinoic acid receptor α-activating retinoids in myeloid bone marrow cells and that retinoic acid receptor α remains in an unliganded configuration during adult hematopoiesis.

Quantitation of retinaldehyde in small biological samples using ultrahigh-performance liquid chromatography tandem mass spectrometry

We report an ultrahigh-performance liquid chromatography tandem mass spectrometry (UHPLC–MS/MS) method to quantify all-trans-retinal in biological samples of limited size (15–35mg), which is especially advantageous for use with adipose. To facilitate recovery, retinal and the internal standard 3,4-didehydroretinal were derivatized in situ into their O-ethyloximes. UHPLC resolution combined with high sensitivity and specificity of MS/MS allowed quantification of retinal-O-ethyloximes with a 5-fmol lower limit of detection and a linear range from 5fmol to 1pmol. This assay revealed that extraocular concentrations of retinal range from approximately 2 to 40pmol/g in multiple tissues—the same range as all-trans-retinoic acid. All-trans-retinoic acid has high affinity (kd⩽0.4nM) for its nuclear receptors (RARα, -β, and -γ), whereas retinal has low (if any) affinity for these receptors, making it unlikely that these retinal concentrations would activate RAR. We also show that the copious amount of vitamin A used in chow diets increases retinal in adipose depots 2- to 5-fold relative to levels in adipose of mice fed a vitamin A-sufficient diet, as recommended for laboratory rodents. This assay also is proficient for quantifying conversion of retinol into retinal in vitro and, therefore, provides an efficient method to study metabolism of retinol in vivo and in vitro.

Effects of diet and strain on mouse serum and tissue retinoid concentrations.

The relationship between dietary vitamin A and all-trans-retinoic acid levels in serum and tissues had not been quantified. We determined the impact of dietary vitamin A on retinoid levels in serum, liver, kidney, testis, and epididymal white adipose of five mouse strains: AKR/J; BALB/cByJ; C3H/HeJ; C57BL/6J; 129S1/SvImJ. Retinoids were quantified in mice fed copious vitamin A (lab chow, ≥20 IU/g) followed by one month feeding a vitamin A-sufficient diet (4 IU/g), or after three generations of feeding a vitamin A-sufficient diet. Retinol and retinyl esters were measured by high-performance liquid chromatography with ultraviolet absorbance detection. All-trans-retinoic acid was quantified by liquid chromatography tandem mass spectrometry. The amounts of dietary vitamin A had long-term strain-specific effects on tissue retinyl ester, retinol and all-trans-retinoic acid concentrations. Three generations of feeding a vitamin A-sufficient diet decreased all-trans-retinoic acid in most tissues of most strains, in some cases more than 60%, compared to a diet with copious vitamin A. With both diets, all-trans-retinoic acid concentrations maintained an order of liver ≈ testis > kidney > white adipose tissue ≈ serum. Neither retinol nor all-trans-retinoic acid in serum reflected all-trans-retinoic acid concentrations in tissues. Strain and tissue-specific differences in retinol and all-trans-retinoic acid altered by different amounts of dietary vitamin A could have profound effects on retinoid action. This would be the case especially with the increased all-trans-retinoic acid values associated with the amounts of vitamin A and its precursors (carotenoids) in chow diets.

Abstract 304: Global Transcriptional Effects of Vitamin A Deficiency on the Postnatal Heart

Vitamin A (VA) is the chemical precursor of retinoic acid (RA), which is critical for embryonic development and for growth, immunity, metabolism, and cell differentiation in postnatal regenerating systems such as skin, sensory organs, and stem cell niches in the brain. VA is also essential for embryonic heart development, and we hypothesized that Vitamin A might exert an effect on the postnatal heart similar to what is observed for other tissues. Here, we report the global transcriptional profiles of wild-type (WT) mice fed a VA sufficient diet (VAS) compared with retinyl acyl transferase (LRAT) knock-out mice fed either a VAS or VA deficient (VAD) diet. Knockout of the LRAT gene alone was sufficient to induce differential expression of 576 genes relative to WT. Feeding LRAT mutant mice a VAD diet resulted in a change in the relative expression levels of 257 genes relative to LRAT mutant mice fed a VAS diet. As expected, we observed transcriptional alterations related to Vitamin A metabolism, including an increase in the gene encoding cellular retinoid binding protein 7 and down-regulation of the retinol metabolic enzymes Cy1a2 and Cyp2a4. Importantly, several cardiac genes not previously known to require VA were perturbed, including the gene encoding B-type natriuretic peptide, which was down-regulated in mutant mice irrespective of diet, and A-type natriuretic peptide, which was decreased only in mice fed the VAD diet. There was also a striking effect of VAD on genes important for immune responses, which could have an impact on the wound healing process subsequent to injury of the heart. This is consistent with recent evidence that showed that Vitamin A deficiency influences post-infarct ventricular remodeling in rats. In summary, this is the first microarray study of Vitamin A deficiency in the postnatal heart, which suggests mechanisms by which Vitamin A depletion may alter myocardial maintenance and repair after injury.

The role of β‐carotene‐15,15′‐oxygenase (CMO1) during mammalian embryonic development

β‐carotene is the best‐known vitamin A precursor. By the action of β‐carotene‐15,15′‐oxygenase (CMO1), one molecule of β‐carotene can be cleaved symmetrically, ultimately resulting in the formation of two molecules of retinol. Vitamin A is essential for embryonic development. In order to meet vitamin A requirements, the mammalian embryo relies on maternal circulating retinoids (vitamin A and its derivatives). We previously showed that, upon maternal dietary vitamin A deprivation, normal embryonic development is impaired in mice lacking retinol‐binding protein (RBP), the sole specific carrier for retinol in the bloodstream. To investigate the relationship between carotenoid and retinoid metabolism during development, we generated a mouse strain that lacks both CMO1 and RBP (CMO1−/−RBP−/−). We show that CMO1−/−RBP−/− mice are viable and fertile when maintained on a vitamin A‐sufficient diet. However, E14.5 embryos from CMO1−/−RBP−/− dams bred on a vitamin A‐deficient diet are malformed, and the severity of malformations is greater than those seen in RBP−/− embryos. No differences in circulating retinoids levels were observed between CMO1−/−RBP−/− and RBP−/− dams. Also, embryonic development in CMO1−/− mice is normal. We are now investigating the reasons for the embryonic phenotype of the CMO1−/−RBP−/− mice to elucidate the physiological role played by CMO1 and β‐carotene during development.

Vitamin A: Overlapping Delivery Pathways to Tissues from the Circulation

Although retinol bound to retinol-binding protein (RBP) is the most abundant retinoid form present in the circulations of humans and most mammals, other retinoid and proretinoid forms are also present in the blood. We are interested in understanding to what extent each of these circulating retinoid forms contributes towards retinoid actions within cells and tissues. Here we report two studies focused on this question. First, we examined retinoid transport and storage in RBP-deficient mice that lack circulating RBP. These mice under normal laboratory conditions are phenotypically normal except for a visual impairment early in life that is corrected if the mice are maintained on a vitamin A-sufficient diet throughout life. The RBP-deficient mice take up vitamin A from the diet into most tissues at least as well as wild type mice. Compared to wild type mice, mice lacking RBP accumulate excess vitamin A in the liver, since there is no RBP to facilitate mobilization of stored retinol from hepatic stores. In a second study, we explored in vitro the actions of carotene cleavage enzyme (CCE) in facilitating beta-carotene cleavage to retinoid in the testis. CCE is most highly expressed in the testis. Pull-down experiments coupled with MALDI-MS analysis showed that mouse testis CCE is able to interact with the testis-specific lactate dehydrogenase-C (LDH-C) isoform. This may suggest that CCE and LDH-C act in concert to catalyze beta-carotene cleavage.

Retinol-binding protein-deficient mice: biochemical basis for impaired vision.

We reported previously that mice lacking plasma retinol-binding protein (RBP) are phenotypically normal except that they display impaired vision at the time of weaning. This visual defect is associated with greatly diminished eyecup levels of retinaldehyde and is reversible if the mutants are maintained for several months on a vitamin A-sufficient diet. Here we provide a biochemical basis for the visual phenotype of RBP-deficient mice. This phenotype does not result from inadequate milk total retinol levels since these are not different for RBP-deficient and wild-type mice. The eye, unlike all other tissues that have been examined, takes up dietary retinol very poorly. Moreover, compared to other tissues, the eye displays a strong preference for retinol uptake when retinol is delivered bound to RBP. The poor uptake of dietary retinol by the eye coupled with its marked ability to take up retinol from RBP, we propose, provides a basis for the impaired vision observed in weanling RBP-deficient mice. Further study of the mutants suggests that the impaired vision is reversible because the eyes of mutant mice slowly acquire sufficient retinol from the low levels of retinol present in their circulation either bound to albumin or present in lipoprotein fractions. Thus, the eye is unlike other tissues in the body in that it shows a very marked preference for acquiring retinol needed to support vision from the retinol-RBP complex and is unable to meet adequately its retinol need through uptake of recently absorbed dietary retinol. This provides an explanation for the impaired vision phenotype of RBP-deficient mice.

Vitamin A deficiency produces spatial learning and memory impairment in rats

Vitamin A and its derivatives (retinoids) play important roles in many physiological processes. The recent finding of high levels of cellular retinol-binding protein type 1 immunoreactivity, cellular retinoic acid-binding protein type 1 immunoreactivity and the presence of nuclear retinoid receptors in the central nervous system of adult rodents suggests that retinoids may carry out important roles in the adult brain. In consideration of the role of the hippocampus in spatial learning and memory we evaluated the effect of vitamin A deprivation in adult rats on these functions. Following 12 weeks of vitamin A-free diet, rats were trained to acquire a radial-arm maze task. Results show that this diet induced a severe deficit in the spatial learning and memory task. The cognitive impairment was fully restored when vitamin A was replaced in the diet. We also found a significant decrease in hippocampal acetylcholine release induced by scopolamine, assessed using microdialysis technique, and a reduction in the size of hippocampal nuclei of CA1 region in vitamin-deficient rats, compared to rats fed with a vitamin A-sufficient diet. These results demonstrate that vitamin A has a critical role in the learning and memory processes linked to a proper hippocampal functioning.

Impaired retinal function and vitamin A availability in mice lacking retinol-binding protein.

Retinol-binding protein (RBP) is the sole specific transport protein for retinol (vitamin A) in the circulation, and its single known function is to deliver retinol to tissues. Within tissues, retinol is activated to retinoic acid, which binds to nuclear receptors to regulate transcription of >300 diverse target genes. In the eye, retinol is also activated to 11-cis-retinal, the visual chromophore. We generated RBP knockout mice (RBP(-/-)) by gene targeting. These mice have several phenotypes. Although viable and fertile, they have reduced blood retinol levels and markedly impaired retinal function during the first months of life. The impairment is not due to developmental retinal defect. Given a vitamin A-sufficient diet, the RBP(-/-) mice acquire normal vision by 5 months of age even though blood retinol levels remain low. Deprived of dietary vitamin A, vision remains abnormal and blood retinol declines to undetectable levels. Another striking phenotype of the mutant mice is their abnormal retinol metabolism. The RBP(-/-) mice can acquire hepatic retinol stores, but these cannot be mobilized. Thus, their vitamin A status is extremely tenuous and dependent on a regular vitamin A intake. Unlike wild-type mice, serum retinol levels in adult RBP(-/-) animals become undetectable after only a week on a vitamin A-deficient diet and their retinal function rapidly deteriorates. Thus RBP is needed for normal vision in young animals and for retinol mobilization in times of insufficient dietary intake, but is otherwise dispensable for the delivery of retinol to tissues.

Carbon Flow through the Hepatic Folate-Dependent One-Carbon Pool Is Not Altered in Vitamin A-Deficient Rats

Vitamin A status can influence a number of enzymes and coenzymes involved in folate-dependent one-carbon metabolism as well as subsequent methyl group metabolism. Tracer kinetic techniques were used in the present study to assess the physiological importance of vitamin A deficiency on the de novo synthesis of methionine via the hepatic folate-dependent one-carbon pool. Vitamin A-deficient (0 retinol equivalents (RE) retinyl palmitate/g diet) rats were fed their respective diet for 11 wk, whereas control rats (1.2 RE retinyl palmitate/g diet) were food restricted to match the growth rate exhibited by the vitamin A-deficient group. After the dietary treatment period, duodenal cannulated rats were continuously infused with L-[3-14C] serine and L-[methyl-3H] methionine until a plateau specific radioactivity was exhibited with respect to the hepatic serine and methionine pools, indicating a steady state had been achieved. The hepatic concentration of both S-adenosylmethionine and S-adenosylhomocysteine were elevated in vitamin A-deficient rats. However, vitamin A-deficient rats exhibited similar kinetic values compared with control rats fed a vitamin A-sufficient diet. The irreversible loss rate of hepatic serine and methionine, the transfer quotient from serine to methionine and the folate-dependent flow of carbon to methionine from serine were unaffected by vitamin A status. These studies demonstrate that vitamin A deficiency does not affect the reductive carbon flow from serine to methionine because the ability to generate methionine via remethylation of homocysteine with the carbon group originating from serine was not altered in vitamin A-deficient rats. Furthermore, the data illustrate the importance of using tracer kinetic techniques to quantify metabolic flux under steady-state conditions in vivo, thereby evaluating the consequences of an abnormal condition on a physiological and functional basis.

Cell-type specific interactions between retinoic acid and thyroid hormone in the regulation of expression of the gene encoding ornithine aminotransferase.

The purposes of this study were to determine whether expression of the gene encoding ornithine aminotransferase (OAT) in the rat liver and kidney is regulated by retinoic acid (RA) and to characterize further the role of thyroid hormone in regulating the expression of this gene. The level of OAT messenger RNA (mRNA) was reduced 70% in the liver of animals fed a vitamin A-deficient diet relative to that in animals fed a vitamin A-sufficient diet. RA, administered at a dose of 20 micrograms/rat to A-deficient rats for 1 or 3 days, restored OAT mRNA to near the level observed in animals fed the A-sufficient diet. Retinol was also effective in this regard. T3, when injected alone at a dose of 10 micrograms/100 g BW, had no effect on the level of OAT mRNA in the liver. However, when injected concurrently with RA, T3 blocked the ability of RA to induce OAT mRNA in the liver of rats fed the vitamin A-deficient diet. Animals made both vitamin A deficient and hypothyroid responded to RA in a manner similar to vitamin A-deficient animals. The vitamin A-deficient, hypothyroid rats responded somewhat differently to T3, however. T3 was unable to block the induction of OAT mRNA in the liver of vitamin A-deficient, hypothyroid rats when injected concurrently with RA for 1 day, but did block the induction of OAT mRNA by RA when these two hormones were injected concurrently for 3 days. These data indicate that RA and T3 exert opposing effects on the level of OAT mRNA in the liver. The effects of RA and T3 on OAT mRNA were markedly different in the kidney. Neither vitamin A deficiency nor RA had any apparent affect on the level of OAT mRNA in the kidney. T3, however, increased the level of OAT mRNA in the kidney of vitamin A-deficient rats. In the kidney of vitamin A-deficient, hypothyroid rats, T3 was unable to increase OAT mRNA when injected for 1 day, but did increase this mRNA when injected for 3 days. Together, these data indicate cell-type specific effects of both RA and T3 on the OAT gene.

Cell-type specific interactions between retinoic acid and thyroid hormone in the regulation of expression of the gene encoding ornithine aminotransferase

The purposes of this study were to determine whether expression of the gene encoding ornithine aminotransferase (OAT) in the rat liver and kidney is regulated by retinoic acid (RA) and to characterize further the role of thyroid hormone in regulating the expression of this gene. The level of OAT messenger RNA (mRNA) was reduced 70% in the liver of animals fed a vitamin A-deficient diet relative to that in animals fed a vitamin A-sufficient diet. RA, administered at a dose of 20 micrograms/rat to A-deficient rats for 1 or 3 days, restored OAT mRNA to near the level observed in animals fed the A-sufficient diet. Retinol was also effective in this regard. T3, when injected alone at a dose of 10 micrograms/100 g BW, had no effect on the level of OAT mRNA in the liver. However, when injected concurrently with RA, T3 blocked the ability of RA to induce OAT mRNA in the liver of rats fed the vitamin A-deficient diet. Animals made both vitamin A deficient and hypothyroid responded to RA in a manner similar to vitamin A-deficient animals. The vitamin A-deficient, hypothyroid rats responded somewhat differently to T3, however. T3 was unable to block the induction of OAT mRNA in the liver of vitamin A-deficient, hypothyroid rats when injected concurrently with RA for 1 day, but did block the induction of OAT mRNA by RA when these two hormones were injected concurrently for 3 days. These data indicate that RA and T3 exert opposing effects on the level of OAT mRNA in the liver. The effects of RA and T3 on OAT mRNA were markedly different in the kidney. Neither vitamin A deficiency nor RA had any apparent affect on the level of OAT mRNA in the kidney. T3, however, increased the level of OAT mRNA in the kidney of vitamin A-deficient rats. In the kidney of vitamin A-deficient, hypothyroid rats, T3 was unable to increase OAT mRNA when injected for 1 day, but did increase this mRNA when injected for 3 days. Together, these data indicate cell-type specific effects of both RA and T3 on the OAT gene.

Carcinogenicity studies of tobacco extract in vitamin A-deficient Sprague-Dawley rats : Bhide SV, Ammigan N, Nair UJ, Lalitha VS. Carcinogeneis Division, Cancer Research Institute, Tata Memorial Centre, Parel, Bombay 400 012. Cancer Res 1991;51:3018–3023

Long-term carcinogenicity studies were carried out in male Sprague-Dawley rats maintained on vitamin A-sufficient (SLO+) and vitamin A-deficient (SLO-) diets and treated with tobacco extract (TE). Three-week-old rats received by gavage a total dose of 860 mg of TE at a daily dose of 3 mg/rat over a period of 21 months. Besides tumorigenicity, drug-metabolizing phase I and phase II enzymes in lung and liver as well as vitamin A and C levels in plasma and liver were measured at 12 and 21 months of age. The cumulative tumor incidence in TE-treated SLO- rats was significantly higher (77-100%) than that observed in TE-treated SLO+ rats (20-22%). Furthermore, SLO+ rats treated with TE showed lung and forestomach tumors, whereas TE-treated SLO- rats showed a preponderance of pituitary adenomas (87%). It was observed that TE treatment increased the activity of the hepatic and pulmonary phase I enzymes and decreased the glutathione/glutathione S-transferase detoxification system at both time points in SLO- rats. On TE treatment the vitamin A levels in the liver and plasma were significantly decreased with a concurrent increase in vitamin C levels. The data show that a vitamin A-deficient diet renders male Sprague-Dawley rats more susceptible to TE treatment than the vitamin A-sufficient diet, an effect which was associated with the augmented induction of P-450 content and activities and depletion of the glutathione/glutathione S-transferase pathway by TE.

Exposure to the carcinogen benzopyrene depletes tissue vitamin a:β‐carotene prevents depletion

Evidence in humans and laboratory animals supports a cancer-protective effect of vitamin A, but the mechanism remains unclear. While vitamin A deficiency causes squamous metaplasia, and lung cancer patients have lower vitamin A status, their serum vitamin A levels are not indicative of deficiency. We hypothesize that local enzymatic degradation of vitamin A can be induced by exposure to carcinogens such as benzopyrene found in cigarette smoke. This study was designed to determine if benzopyrene exposure depletes tissue vitamin A and whether beta-carotene might prevent the depletion. Weanling male Fischer rats were fed a nutritionally complete purified diet, supplemented with or without benzopyrene at 400 mg/kg feed or beta-carotene at 2 g/kg feed. Vitamin A content of the liver, small intestine, and serum was determined by high-performance liquid chromatography. There was no effect of benzopyrene feeding on serum retinol levels through four weeks. However, there was a decline in tissue retinol in the liver and small intestine by two weeks, with a 30% decline by four weeks (p less than 0.05). In rats fed beta-carotene, there was no effect of benzopyrene on tissue vitamin A level. These results indicate that exposure to benzopyrene induces a local tissue vitamin A depletion despite a vitamin A-sufficient diet and maintenance of serum vitamin A levels. A high intake of beta-carotene prevented the vitamin A depletion effect of benzopyrene exposure. Further studies appear warranted to determine whether some of the adverse effects of environmental carcinogens, as found in cigarette smoke, charcoal-broiled meats, and industrial wastes, might be alleviated by dietary intervention.