Vitamin A metabolism: an update

The retinol esterifying enzyme LRAT supports cell signaling by retinol-binding protein and its receptor STRA6

Vitamin A, retinol, circulates in blood bound to retinol-binding protein (RBP). At some tissues, holo-RBP is recognized by a plasma membrane receptor termed STRA6, which serves a dual role: it mediates transport of retinol from RBP into cells, and it functions as a cytokine receptor that, on binding holo-RBP, activates JAK2/STAT5 signaling. As STAT target genes include SOCS3, an inhibitor of insulin receptor, holo-RBP suppresses insulin responses in STRA6-expressing cells. We have shown previously that the two functions of STRA6 are interdependent. These observations suggest factors that regulate STRA6-mediated retinol transport may also control STRA6-mediated cell signaling. One such factor is retinol metabolism, which enables cellular uptake of retinol by maintaining an inward-directed concentration gradient. We show here that lecithin:retinol acyl transferase (LRAT), which catalyzes esterification of retinol to its storage species retinyl esters, is necessary for activation of the STRA6/JAK2/STAT5 cascade by holo-RBP. In accordance, LRAT-null mice are protected from holo-RBP-induced suppression of insulin responses. Hence, STRA6 signaling, which requires STRA6-mediated retinol transport, is supported by LRAT-catalyzed retinol metabolism. The observations demonstrate that STRA6 regulates key cellular processes by coupling circulating holo-RBP levels and intracellular retinol metabolism to cell signaling.

Retinol-binding protein 4 and its membrane receptor STRA6 control adipogenesis by regulating cellular retinoid homeostasis and retinoic acid receptor α activity

Retinoids are vitamin A (retinol) derivatives and complex regulators of adipogenesis by activating specific nuclear receptors, including the retinoic acid receptor (RAR) and retinoid X receptor (RXR). Circulating retinol-binding protein 4 (RBP4) and its membrane receptor STRA6 coordinate cellular retinol uptake. It is unknown whether retinol levels and the activity of RAR and RXR in adipocyte precursors are linked via RBP4/STRA6. Here, we show that STRA6 is expressed in precursor cells and, dictated by the apo- and holo-RBP4 isoforms, mediates bidirectional retinol transport that controls RARα activity and subsequent adipocyte differentiation. Mobilization of retinoid stores in mice by inducing RBP4 secretion from the liver activated RARα signaling in the precursor cell containing the stromal-vascular fraction of adipose tissue. Retinol-loaded holo-RBP4 blocked adipocyte differentiation of cultured precursors by activating RARα. Remarkably, retinol-free apo-RBP4 triggered retinol efflux that reduced cellular retinoids, RARα activity, and target gene expression and enhanced adipogenesis synergistically with ectopic STRA6. Thus, STRA6 in adipocyte precursor cells links nuclear RARα activity to the circulating RBP4 isoforms, whose ratio in obese mice was shifted toward limiting the adipogenic potential of their precursors. This novel cross talk identifies a retinol-dependent metabolic function of RBP4 that may have important implications for the treatment of obesity.

Analysis of circulating lipid-phase micronutrients in humans by HPLC: review and overview of new developments

Retinol, tocopherols, coenzyme Q10, carotenoids, and vitamin D are lipophilic compounds shown to function as important health-protective agents by mitigating the damaging effects of oxidative and other injury. Scientific interest in evaluating these compounds has resurfaced in recent years, particularly in the nutritional, clinical and epidemiologic fields, and has precipitated the development of a multitude of new analytical techniques. This review considers recent developments in HPLC-based assays since 2007 for the simultaneous determination of these lipid-phase compounds utilizing exclusively serum or plasma as these matrices are mostly used in clinical and epidemiological investigations. We also provide an overview of blood measurements for selected carotenoids, tocopherols, coenzyme Q10 and retinol from the last 15years of healthy umbilical cord blood, children, and adults.

Mefloquine use, psychosis, and violence: a retinoid toxicity hypothesis

Mefloquine use has been linked to severe gastrointestinal and neuropsychiatric adverse effects, including cognitive disturbances, anxiety, depression, psychosis, and violence. The adverse effects of the drug are thought to result from the secondary consequences of hepatocellular injury; in fact, mefloquine is known to cause a transient, anicteric chemical hepatitis. However, the mechanism of mefloquine-associated liver damage and the associated neuropsychiatric and behavioral effects of the drug are not well understood. Mefloquine and other 8-amino-quinolines are the only antimalarial drugs that target the liver-stage malaria parasites, which selectively absorb vitamin A from the host. Vitamin A is also stored mainly in the liver, in potentially poisonous concentrations. These observations suggest that both the therapeutic effectiveness of mefloquine and its adverse effects are related to the ability of the 8-aminoquinolines to alter the metabolism of retinoids (vitamin A and its congeners). Several lines of evidence support the hypothesis that mefloquine neurotoxicity and other adverse effects reflect an endogenous form of hypervitaminosis A due to a process involving: mefloquine-induced dehydrogenase inhibition; the accumulation of retinoids in the liver; retinoid-induced hepatocellular damage; the spillage of stored retinoids into the circulation; and the transport of these compounds to the gut and brain in toxic concentrations. The retinoid hypothesis could be tested clinically by comparing cases of mefloquine toxicity and untreated controls in terms of retinoid profiles (retinol, retinyl esters, percent retinyl esters, and retinoic acid). Subject to such tests, retinoid profiling could provide an indicator for assessing mefloquine-associated adverse effects.

Enzymology of retinoic acid biosynthesis and degradation

All-trans-retinoic acid is a biologically active derivative of vitamin A that regulates numerous physiological processes. The concentration of retinoic acid in the cells is tightly regulated, but the exact mechanisms responsible for this regulation are not completely understood, largely because the enzymes involved in the biosynthesis of retinoic acid have not been fully defined. Recent studies using in vitro and in vivo models suggest that several members of the short-chain dehydrogenase/reductase superfamily of proteins are essential for retinoic acid biosynthesis and the maintenance of retinoic acid homeostasis. However, the exact roles of some of these recently identified enzymes are yet to be characterized. The properties of the known contributors to retinoid metabolism have now been better defined and allow for more detailed understanding of their interactions with retinoid-binding proteins and other retinoid enzymes. At the same time, further studies are needed to clarify the interactions between the cytoplasmic and membrane-bound proteins involved in the processing of hydrophobic retinoid metabolites. This review summarizes current knowledge about the roles of various biosynthetic and catabolic enzymes in the regulation of retinoic acid homeostasis and outlines the remaining questions in the field.

Differential and isomer-specific modulation of vitamin A transport and the catalytic activities of the RBP receptor by retinoids

Retinoids are vitamin A derivatives with diverse biological functions. Both natural and artificial retinoids have been used as therapeutic reagents to treat human diseases, but not all retinoid actions are understood mechanistically. Plasma retinol binding protein (RBP) is the principal and specific carrier of vitamin A in the blood. STRA6 is the membrane receptor for RBP that mediates cellular vitamin A uptake. The effects of retinoids or related compounds on the receptor's vitamin A uptake activity and its catalytic activities are not well understood. In this study, we dissected the membrane receptor-mediated vitamin A uptake mechanism using various retinoids. We show that a subset of retinoids strongly stimulates STRA6-mediated vitamin A release from holo-RBP. STRA6 also catalyzes the exchange of retinol in RBP with certain retinoids. The effect of retinoids on STRA6 is highly isomer-specific. This study provides unique insights into the RBP receptor's mechanism and reveals that the vitamin A transport machinery can be a target of retinoid-based drugs.

Expression of Serum Retinol Binding Protein and Transthyretin within Mouse Gastric Ghrelin Cells

Ghrelin is an orexigenic peptide hormone produced mainly by a distinct group of dispersed endocrine cells located within the gastric oxyntic mucosa. Besides secreted gene products derived from the preproghrelin gene, which include acyl-ghrelin, desacyl-ghrelin and obestatin, ghrelin cells also synthesize the secreted protein nesfatin-1. The main goal of the current study was to identify other proteins secreted from ghrelin cells. An initial gene chip screen using mRNAs derived from highly enriched pools of mouse gastric ghrelin cells demonstrated high levels of serum retinol-binding protein (RBP4) and transthyretin (TTR), both of which are known to circulate in the bloodstream bound to each other. This high expression was confirmed by quantitative RT-PCR using as template mRNA derived from the enriched gastric ghrelin cell pools and from two ghrelin-producing cell lines (SG-1 and PG-1). RBP4 protein also was shown to be secreted into the culture medium of ghrelin cell lines. Neither acute nor chronic caloric restriction had a significant effect on RBP4 mRNA levels within stomachs of C57BL/6J mice, although both manipulations significantly decreased stomach TTR mRNA levels. In vitro studies using PG-1 cells showed no effect on RBP4 release of octanoic acid, epinephrine or norepinephrine, all of which are known to act directly on ghrelin cells to stimulate ghrelin secretion. These data provide new insights into ghrelin cell physiology, and given the known functions of RBP4 and TTR, support an emerging role for the ghrelin cell in blood glucose handling and metabolism.

Pharmacological inhibition to examine the role of DGAT1 in dietary lipid absorption in rodents and humans

Alterations in fat metabolism, in particular elevated plasma concentrations of free fatty acids and triglycerides (TG), have been implicated in the pathogenesis of Type 2 diabetes, obesity, and cardiovascular disease. Acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1), a member of the large family of membrane-bound O-acyltransferases, catalyzes the final step in triacylglycerol formation. In the intestine, DGAT1 is one of the acyltransferases responsible for the reesterficiation of dietary TG. Following a single dose of a selective pharmacological inhibitor of DGAT1, PF-04620110, a dose-dependent inhibition of TG and vitamin A absorption postprandially was demonstrated in rodents and human subjects. In C57/BL6J mice, acute DGAT1 inhibition alters the temporal and spatial pattern of dietary lipid absorption. To understand the impact of DGAT1 inhibition on enterocyte lipid metabolism, lipomic profiling was performed in rat intestine and plasma as well as human plasma. DGAT1 inhibition causes an enrichment of polyunsaturated fatty acids within the TG class of lipids. This pharmacological intervention gives us insight as to the role of DGAT1 in human dietary lipid absorption.

Genome-wide transcriptional reorganization associated with senescence-to-immortality switch during human hepatocellular carcinogenesis

Senescence is a permanent proliferation arrest in response to cell stress such as DNA damage. It contributes strongly to tissue aging and serves as a major barrier against tumor development. Most tumor cells are believed to bypass the senescence barrier (become “immortal”) by inactivating growth control genes such as TP53 and CDKN2A. They also reactivate telomerase reverse transcriptase. Senescence-to-immortality transition is accompanied by major phenotypic and biochemical changes mediated by genome-wide transcriptional modifications. This appears to happen during hepatocellular carcinoma (HCC) development in patients with liver cirrhosis, however, the accompanying transcriptional changes are virtually unknown. We investigated genome-wide transcriptional changes related to the senescence-to-immortality switch during hepatocellular carcinogenesis. Initially, we performed transcriptome analysis of senescent and immortal clones of Huh7 HCC cell line, and identified genes with significant differential expression to establish a senescence-related gene list. Through the analysis of senescence-related gene expression in different liver tissues we showed that cirrhosis and HCC display expression patterns compatible with senescent and immortal phenotypes, respectively; dysplasia being a transitional state. Gene set enrichment analysis revealed that cirrhosis/senescence-associated genes were preferentially expressed in non-tumor tissues, less malignant tumors, and differentiated or senescent cells. In contrast, HCC/immortality genes were up-regulated in tumor tissues, or more malignant tumors and progenitor cells. In HCC tumors and immortal cells genes involved in DNA repair, cell cycle, telomere extension and branched chain amino acid metabolism were up-regulated, whereas genes involved in cell signaling, as well as in drug, lipid, retinoid and glycolytic metabolism were down-regulated. Based on these distinctive gene expression features we developed a 15-gene hepatocellular immortality signature test that discriminated HCC from cirrhosis with high accuracy. Our findings demonstrate that senescence bypass plays a central role in hepatocellular carcinogenesis engendering systematic changes in the transcription of genes regulating DNA repair, proliferation, differentiation and metabolism.

Altered hepatic retinyl ester concentration and acyl composition in response to alcohol consumption

Retinoids (vitamin A and its metabolites) are essential micronutrients that regulate many cellular processes. Greater than 70% of the body's retinoid reserves are stored in the liver as retinyl ester (RE). Chronic alcohol consumption induces depletion of hepatic retinoid stores, and the extent of this has been correlated with advancing stages of alcoholic liver disease. The goal of this study was to analyze the mechanisms responsible for depletion of hepatic RE stores by alcohol consumption. A change in the fatty-acyl composition of RE in alcohol-fed mice was observed within two weeks after the start of alcohol consumption. Specifically, alcohol-feeding was associated with a significant decline in hepatic retinyl palmitate levels; however, total RE levels were maintained by a compensatory increase in levels of usually minor RE species, particularly retinyl oleate. Our data suggests that alcohol feeding initially stimulates a futile cycle of RE hydrolysis and synthesis, and that the change in RE acyl composition is associated with a change in the acyl composition of hepatic phosphatidylcholine. The alcohol-induced change in RE acyl composition was specific to the liver, and was not seen in lung or white adipose tissue. This shift in hepatic RE fatty acyl composition is a sensitive indicator of alcohol consumption and may be an early biomarker for events associated with the development of alcoholic liver disease.

Retinoic acid signaling in spinal cord development

Retinoic acid (RA) is an important signaling molecule mediating intercellular communication through vertebrate development. Here, we present and discuss recent information on the roles of the RA signaling pathway in spinal cord development. RA is an important player in the patterning and definition of the spinal cord territory from very early stages of development, even before the appearance of the neural plate and further serves a role in the patterning of the spinal cord both along the dorsoventral and anteroposterior axes, particularly in the promotion of neuronal differentiation. It is thus required to establish a variety of neuronal cell types at specific positions of the spinal cord. The main goal of this review is to gather information from vertebrate models, including fish, frogs, chicken and mice, and to put this information in a comparative context in an effort to visualize how the RA pathway was incorporated into the evolving vertebrate spinal cord and to identify mechanisms that are both common and different in the various vertebrate models. In doing so, we try to reconstruct how spinal cord development has been regulated by the RA signaling cascade through vertebrate diversification, highlighting areas which require further studies to obtain a better understanding of the evolutionary events that shaped this structure in the vertebrate lineage.

Retinoids: Potent regulators of metabolism

Retinoids (vitamin A and its analogs) are highly potent regulators of cell differentiation, cell proliferation, and apoptosis. Because of these activities, retinoids have been most extensively studied in the contexts of embryonic development and of proliferative diseases, especially cancer and skin disease. Recently, there has been considerable new research interest focused on gaining understanding of the roles that retinoids and/or retinoid-related proteins may have in the development of metabolic diseases, primarily obesity, diabetes, and dyslipidemia. This review will summarize recent advances that have been made in these areas, focusing on the role of retinoids in modulating adipogenesis, the roles of retinoids and retinoid-related proteins as signaling molecules linking obesity with the development of type II diabetes, the roles of retinoids in pancreatic β-cell biology/insulin secretion, and the actions of retinoids in hepatic steatosis.

The apolipoprotein m-sphingosine-1-phosphate axis: biological relevance in lipoprotein metabolism, lipid disorders and atherosclerosis

Apolipoprotein M (apoM) is a plasma apolipoprotein that mainly associates with high-density lipoproteins. Hence, most studies on apoM so far have investigated its effect on and association with lipid metabolism and atherosclerosis. The insight into apoM biology recently took a major turn. ApoM was identified as a carrier of the bioactive lipid sphingosine-1-phosphate (S1P). S1P activates five different G-protein-coupled receptors, known as the S1P-receptors 1–5 and, hence, affects a wide range of biological processes, such as lymphocyte trafficking, angiogenesis, wound repair and even virus suppression and cancer. The ability of apoM to bind S1P is due to a lipophilic binding pocket within the lipocalin structure of the apoM molecule. Mice overexpressing apoM have increased plasma S1P concentrations, whereas apoM-deficient mice have decreased S1P levels. ApoM-S1P is able to activate the S1P-receptor-1, affecting the function of endothelial cells, and apoM-deficient mice display impaired endothelial permeability in the lung. This review will focus on the putative biological roles of the new apoM–S1P axis in relation to lipoprotein metabolism, lipid disorders and atherosclerosis.

Effects of estradiol on the endocytic transport of vitamin D carrier protein in hepatocytes

BACKGROUND

The possible modulation of receptor-mediated endocytosis (RME) by sex steroids is not well understood, especially in terms of the different receptor-ligand systems and cell types that may exhibit such regulation. The main objective of the current study was to examine the short-term effects of 17β-estradiol (E2) on RME of an extracellular carrier protein for calciferols, vitamin D-binding protein (DBP).

METHODS

Murine male and female primary hepatocytes were treated for 30min in the absence (controls) or presence of Ε2 (1μM). Labeled DBP was then added, and its endocytosis was measured after an incubation of 10min at 37°C using standard ELISA techniques. To obtain further insight into potential molecular mechanisms, fulvestrant and 17α-ethinyl estradiol (EE) were also analyzed. And as part of comparative analyses, a second nutrient carrier protein, vitamin A-binding protein (RBP), was also analyzed.

RESULTS

The results provide the first evidence for an estradiol-dependent stimulation of DBP endocytosis (p<0.05 relative to controls without Ε2). This stimulation, however, was only observed in female hepatocytes. Uptake of RBP was enhanced to a similar extent as DBP by estradiol. In normal (non-estradiol treated) male and female hepatocytes such changes in DBP or RBP endocytosis were not observed. Both fulvestrant and EE exhibited a significant (p<0.05), but incomplete, inhibition of Ε2-dependent stimulation of endocytosis.

CONCLUSIONS

The results provide novel evidence for Ε2 effects on endocytic transport; and for gender-related differences in E2-enhanced transport. These Ε2 effects may be partly dependent on estrogen receptors; but possible, additional or alternative mechanisms are also proposed.

GENERAL SIGNIFICANCE

Endocytic transport is a fundamental function whose regulation has implications for cell signaling, growth, survival, differentiation, and death. This study helps delineate a possible endocrine regulatory pathway involving modulation of endocytosis by a steroid hormone. It also provides a potential, new relation between different hormonal regulators, e.g., estradiol effects on cellular assimilation of calciferols.

Genetics and diet regulate vitamin A production via the homeobox transcription factor ISX

Low dietary intake of β-carotene is associated with chronic disease and vitamin A deficiency. β-Carotene is converted to vitamin A in the intestine by the enzyme β-carotene-15,15'-monoxygenase (BCMO1) to support vision, reproduction, immune function, and cell differentiation. Considerable variability for this key step in vitamin A metabolism, as reported in the human population, could be related to genetics and individual vitamin A status, but it is unclear how these factors influence β-carotene metabolism and vitamin A homeostasis. Here we show that the intestine-specific transcription factor ISX binds to the Bcmo1 promoter. Moreover, upon induction by the β-carotene derivative retinoic acid, this ISX binding decreased expression of a luciferase reporter gene in human colonic CaCo-2 cells indicating that ISX acts as a transcriptional repressor of BCMO1 expression. Mice deficient for this transcription factor displayed increased intestinal BCMO1 expression and produced significantly higher amounts of vitamin A from supplemental β-carotene. The ISX binding site in the human BCMO1 promoter contains a common single nucleotide polymorphism that is associated with decreased conversion rates and increased fasting blood levels of β-carotene. Thus, our study establishes ISX as a critical regulator of vitamin A production and provides a mechanistic explanation for how both genetics and diet can affect this process.

Vitamin A-fortified milk increases total body vitamin A stores in Mexican preschoolers

Vitamin A (VA) deficiency (VAD) continues to be a major nutritional problem in developing countries, including Central America. In Mexico, milk is a well-accepted vehicle for the administration of micronutrients, including VA, to preschoolers. Thus, we conducted a randomized, controlled, clinical trial to investigate the efficacy of daily consumption of 250 mL of VA-fortified milk (which provided 196 retinol equivalents/d) for 3 mo on VA stores in mildly to moderately VAD (serum retinol concentration 0.35-0.7 μmol/L) preschoolers who were not enrolled in a food assistance program. Twenty-seven mildly to moderately VAD children were randomly assigned based on screening measurements to either the intervention (n = 14) or control group (n = 13) (children in the control group did not receive placebo). All children in the control group and 79% (n = 11) of the children in the intervention group completed the study. The total body VA (TBVA) pool size was estimated using the deuterated retinol dilution technique before and after the intervention. After 3 mo, median changes in the serum retinol concentration for the intervention and control groups were 0.13 and -0.21 μmol/L, respectively (P = 0.009). Median changes in the TBVA stores were 0.06 and 0.01 mmol, respectively (P = 0.006) and estimated median changes in the liver VA concentration were 0.09 and 0.01 μmol/g, respectively (P = 0.002). The VA-fortified milk was well accepted among preschoolers and significantly increased TBVA stores, liver VA stores, and serum retinol concentration, indicating that it may be an effective means to ameliorate VAD in young Mexican children.

Effects of polychlorobiphenyls, polybromodiphenylethers, organochlorine pesticides and their metabolites on vitamin A status in lactating grey seals

Polychlorobiphenyls (PCBs), polybromodiphenylethers (PBDEs) and organochlorine pesticides (OCPs), such as dichlorodiphenyltrichloroethane (DDT) and hexachlorobenzene (HCB), are considered as endocrine disruptors in laboratory and wild animals. This study investigated whether these compounds and their hydroxylated metabolites (HO-PCBs and HO-PBDEs) may affect the homoeostasis of vitamin A, a dietary hormone, in the blubber and serum of twenty lactating grey seals sampled at early and late lactation on the Isle of May, Scotland. The effect of naturally produced compounds such as the methoxylated (MeO)-PBDEs was also examined. Vitamin A levels in inner blubber (37±9 μg/g wet weight (ww) and 92±32 μg/g ww at early and late lactation, respectively) and serum (408±143 and 390±98 ng/ml at early and late lactation, respectively) appeared to be positively related to ΣPCBs, ΣPBDEs and several individual PCB and PBDE congeners in inner blubber and serum. These findings may suggest enhanced mobilisation of hepatic retinoid stores and redistribution in the blubber, a storage site for vitamin A in marine mammals. We have also reported that serum concentrations of ΣHO-PCBs and 4-OH-CB107 tended to increase with circulating vitamin A levels. Although the direction of the relationships may sometimes differ from those reported in the literature, our results are in agreement with previous findings highlighting a disruption of vitamin A homoeostasis in the blubber and bloodstream following exposure to environmental pollutants. The fact that vitamin A and PCBs appeared to share common mechanisms of mobilisation and transfer during lactation in grey seals (Debier et al., 2004; Vanden Berghe et al., 2010) may also play a role in the different relationships observed between vitamin A and lipophilic pollutants.

Encapsulation of antioxidants in gastrointestinal-resistant nanoparticulate carriers

Reactive oxygen species (ROS) are known to cause several human pathologies. For this reason, antioxidants have gained utmost importance because of their potential as prophylactic and therapeutic agents in many diseases. Examples of their application include their use in diabetic patients, as aging drugs, in cancer diseases, Parkinson's, Alzheimer's, autoimmune disorders, and also in inflammation. Antioxidants have limited absorption profiles, therefore low bioavailability and low concentrations at the target site. Efforts have been done towards loading antioxidant molecules in advanced nanoparticulate carriers, e.g., liposomes, polymeric nanoparticles, solid lipid nanoparticles, self-emulsifying drug delivery system. Examples of -successful achievements include the encapsulation of drugs and other active ingredients, e.g., coenzyme Q10, vitamin E and vitamin A, resveratrol and polyphenols, curcumin, lycopene, silymarin, and superoxide dismutase. This review focuses on the comprehensive analysis of using nanoparticulate carriers for loading these molecules for oral administration.

Retina, retinol, retinal and the natural history of vitamin A as a light sensor

Light is both the ultimate energy source for most organisms and a rich information source. Vitamin A-based chromophore was initially used in harvesting light energy, but has become the most widely used light sensor throughout evolution from unicellular to multicellular organisms. Vitamin A-based photoreceptor proteins are called opsins and have been used for billions of years for sensing light for vision or the equivalent of vision. All vitamin A-based light sensors for vision in the animal kingdom are G-protein coupled receptors, while those in unicellular organisms are light-gated channels. This first major switch in evolution was followed by two other major changes: the switch from bistable to monostable pigments for vision and the expansion of vitamin A's biological functions. Vitamin A's new functions such as regulating cell growth and differentiation from embryogenesis to adult are associated with increased toxicity with its random diffusion. In contrast to bistable pigments which can be regenerated by light, monostable pigments depend on complex enzymatic cycles for regeneration after every photoisomerization event. Here we discuss vitamin A functions and transport in the context of the natural history of vitamin A-based light sensors and propose that the expanding functions of vitamin A and the choice of monostable pigments are the likely evolutionary driving forces for precise, efficient, and sustained vitamin A transport.

Vitamin A metabolism in rod and cone visual cycles

The chromophore of all known visual pigments consists of 11-cis-retinal (derived from either vitamin A1 or A2) or a hydroxylated derivative, bound to a protein (opsin) via a Schiff base. Absorption of a photon results in photoisomerization of the chromophore to all-trans-retinal and conversion of the visual pigment to the signaling form. Regeneration of the 11-cis-retinal occurs in an adjacent tissue and involves several enzymes, several water-soluble retinoid-binding proteins, and intra- and intercellular diffusional processes. Rod photoreceptor cells depend completely on the output of 11-cis-retinal from adjacent retinal pigment epithelial (RPE) cells. Cone photoreceptors cells can use 11-cis-retinal from the RPE and from a second more poorly characterized cycle, which appears to involve adjacent Müller (glial) cells. Recent progress in the characterization of rod and cone visual cycle components and reactions will result in the development of approaches to the amelioration of blinding eye diseases associated with visual cycle defects.

Effect of retinoic acid on the nitrergic innervation of meibomian glands in rats

The purpose of this study was to investigate the effect of prenatal administration of retinoic acid (RA) on the development of nicotinamide adenine dinucleotide diaphorase (NADPH-d) positive structures in the rat Meibomian glands. One mg/kg of RA was applied to pregnant Wistar rats intraperitonaelly during the gestational period in each of the 12th-14th embryonic days (totally 3 mg/kg). Sections of the central upper eyelids were investigated in rat pups on the 14th postnatal day. They were processed histochemically for NADPH-d, to study the presence and distribution of nitric oxide synthase (NOS) positive nerve structures. NADPH-d staining of Meibomian glands was compared in two groups of rat pups. In the control group, eyelids of 14 day-old rats were studied with no experimental intervention. The second group consisted of rat pups which were prenatally administered the excess of RA. Histochemical analysis of control eyelids revealed numerous NADPH-d well-stained acini of Meibomian glands arranged tightly into groups. Intensively stained vessels and NADPH-d/NOS-positive nerve fibers bordered acini of Meibomian glands. These structures were present in the submucosal layer as well. The analysis of RA group showed less numerous, shrunken acini of Meibomian glands that were seen not only smaller in size, but also in density of their staining and the amount of nitrergic nerve fibers around acini were considerably lowered. In the submucosa differences were noticed compared to the control group, there were numerous NADPH-d stained vessels accompanied by NADPH-d/NOS-positive nerve fibers. The excess of RA during the prenatal period may influence on the development and morphology of NADPH-d positive structures of rat's Meibomian glands.

Contrasting effects of type 2 and type 1 diabetes on plasma RBP4 levels: the significance of transthyretin

Retinol-binding protein 4 (RBP4) is the principle carrier of retinol in the human plasma, which circulates as a complex with transthyretin (TTR), a homotetrameric thyroxine transport protein. Although this complex formation is thought to prevent glomerular filtration of RBP4, it also stabilizes the quaternary structure of TTR. Recent studies indicate elevated plasma levels of RBP4 in type 2 diabetes (T2D). In contrast, reduced RBP4 levels were observed in type 1 diabetes (T1D). Herein, we critically examine the probable mechanisms involved in the regulation of RBP4 and TTR levels during T2D and T1D. The available evidences point to the involvement of pancreatic factors in regulating the expression of both RBP4 and TTR. It appears that during T1D, TTR levels are reduced and it exists predominantly as a monomer that may interfere its interaction with RBP4 resulting in its loss through glomerular filtration. However, plasma TTR levels remain high under T2D conditions and thus reducing glomerular filtration of RBP4. Therefore, the plasma TTR levels appear to be an important determinant of plasma RBP4 levels in these two diabetic conditions.

Lipophilic micronutrients and adipose tissue biology

Lipophilic micronutrients (LM) constitute a large family of molecules including several vitamins (A, D, E, K) and carotenoids. Their ability to regulate gene expression is becoming increasingly clear and constitutes an important part of nutrigenomics. Interestingly, adipose tissue is not only a main storage site for these molecules within the body, but it is also subjected to the regulatory effects of LM. Indeed, several gene regulations have been described in adipose tissue that could strongly impact its biology with respect to the modulation of adipogenesis, inflammatory status, or energy homeostasis and metabolism, among others. The repercussions in terms of health effects of such regulations in the context of obesity and associated pathologies represent an exciting and emerging field of research. The present review will focus on the regulatory effects of vitamin A, D, E and K as well as carotenoids on adipose tissue biology and physiology, notably in the context of obesity and associated disorders.

Provitamin A metabolism and functions in mammalian biology

Vitamin A deficiency is a major public health problem in developing countries. Some studies also implicate a suboptimal vitamin A intake in certain parts of the population of the industrialized world. Provitamin A carotenoids such as β-carotene are the major source for retinoids (vitamin A and its derivatives) in the human diet. However, it is still controversial how much β-carotene intake is required and safe. An important contributor to this uncertainty is the lack of knowledge about the biochemical and molecular basis of β-carotene metabolism. Recently, key players of provitamin A metabolism have been molecularly identified and biochemically characterized. Studies in knockout mouse models showed that intestinal β-carotene absorption and conversion to retinoids is under negative feedback regulation that adapts this process to the actual requirement of vitamin A of the body. These studies also showed that in peripheral tissues a conversion of β-carotene occurs and affects retinoid-dependent physiologic processes. Moreover, these analyses provided a possible explanation for the adverse health effects of carotenoids by showing that a pathologic accumulation of these compounds can induce oxidative stress in mitochondria and cell signaling pathways related to disease. Genetic polymorphisms in identified genes exist in humans and also alter carotenoid homeostasis. Here, the advanced knowledge of β-carotene metabolism is reviewed, which provides a molecular framework for understanding the role of this important micronutrient in health and disease.

Evidence for metabolic imbalance of vitamin A2 in wild fish chronically exposed to metals

In a recent study on indigenous yellow perch chronically exposed to metals, we reported a negative correlation between liver metal concentration and liver transcription levels of genes encoding for enzymes involved in the metabolism of retinoids. We therefore speculated that metals, and especially the non-essential metal Cd, could alter the metabolism of retinoids in wild fish. Thus the present field study investigates the impact of in situ metal exposure on retinoid storage. A total of 55 yellow perch (Perca flavescens) were sampled in six lakes representing a metal contamination gradient (8≤N≤10 per lake). Our results show that yellow perch from Cd-contaminated lakes had significantly higher concentrations of liver dehydroretinol and dehydroretinyl esters than did fish from reference lakes. However, the increase in retinyl ester stores with increasing Cd concentrations was quantitatively much more important than the increase in free dehydroretinol. As a result, a significant decrease in the percentage of hepatic free dehydroretinol with increasing renal Cd concentrations was observed. These results suggest that the enzymes and the binding proteins involved in vitamin A homeostasis are inhibited by the presence of Cd. Alternatively, the increase in tissue vitamin A (antioxidant) levels could serve to better counteract the oxidative stress engendered by Cd exposure. Overall our findings illustrate that vitamin A(2) homeostasis can be altered as a consequence of chronic exposure to low Cd concentrations. Thus, in the context of environmental risk assessment, the percentage of liver free dehydroretinol can be considered as a biomarker of for in situ Cd exposure.

Toward a theory of childhood learning disorders, hyperactivity, and aggression

Learning disorders are often associated with persistent hyperactivity and aggression and are part of a spectrum of neurodevelopmental disorders. A potential clue to understanding these linked phenomena is that physical exercise and passive forms of stimulation are calming, enhance cognitive functions and learning, and are recommended as complementary treatments for these problems. The theory is proposed that hyperactivity and aggression are intense stimulation-seeking behaviors (SSBs) driven by increased brain retinergic activity, and the stimulation thus obtained activates opposing nitrergic systems which inhibit retinergic activity, induce a state of calm, and enhance cognition and learning. In persons with cognitive deficits and associated behavioral disorders, the retinergic system may be chronically overactivated and the nitrergic system chronically underactivated due to environmental exposures occurring pre- and/or postnatally that affect retinoid metabolism or expression. For such individuals, the intensity of stimulation generated by SSB may be insufficient to activate the inhibitory nitrergic system. A multidisciplinary research program is needed to test the model and, in particular, to determine the extent to which applied physical treatments can activate the nitrergic system directly, providing the necessary level of intensity of sensory stimulation to substitute for that obtained in maladaptive and harmful ways by SSB, thereby reducing SSB and enhancing cognitive skills and performance.

Cellular retinoic acid binding protein I mediates rapid non-canonical activation of ERK1/2 by all-trans retinoic acid

All-trans retinoic acid (atRA), one of the active ingredients of vitamin A, exerts canonical activities to regulate gene expression mediated by nuclear RA receptors (RARs). AtRA could also elicit certain non-canonical activities including, mostly, rapid activation of extracellular signal regulated kinase 1/2 (ERK1/2); but the mechanism was unclear. In this study, we have found that cellular retinoic acid binding protein I (CRABPI) mediates the non-canonical, RAR- and membrane signal-independent activation of ERK1/2 by atRA in various cellular backgrounds. In the context of embryonic stem cells (ESCs), atRA/CRABPI-dependent ERK1/2 activation rapidly affects ESC cell cycle, specifically to expand the G1 phase. This is mediated by ERK stimulation resulting in dephosphorylation of nuclear p27, which elevates nuclear p27 protein levels to block G1 progression to S phase. This is the first study to identify CRABPI as the mediator for non-canonical activation of ERK1/2 by atRA, and demonstrate a new functional role for CRABPI in modulating ESC cell cycle progression.

Serum antioxidant concentrations and metabolic syndrome are associated among U.S. adolescents in recent national surveys

Specific micronutrients, including retinol, retinyl esters, carotenoids [α-carotene, β-carotene (cis+trans), β-cryptoxanthin, lutein+zeaxanthin, and total lycopene], vitamin E, and vitamin C have antiinflammatory and antioxidant effects, properties shown to reduce oxidative stress, a process that accompanies the pathogenesis of many chronic diseases. It is still largely unknown whether they are associated with the occurrence of metabolic syndrome (MetS) in the adolescent U.S. population. MetS was defined by the International Diabetes Federation (IDF) criteria. Other non-MetS outcomes relying on blood measurements were elevated HOMA-IR, C-reactive protein (CRP), and hyperuricemia. We tested associations between serum antioxidants and MetS outcomes among adolescents aged 12-19 y using cross-sectional data from NHANES 2001-2006 (n = 782-4285). IDF MetS prevalence was estimated at 7% among boys and 3% among girls. In adjusted models, adolescents with MetS had consistently lower carotenoid concentrations compared with their counterparts without MetS. Total carotenoids were also inversely related to HOMA-IR and CRP. Vitamin C was inversely related to uric acid level and MetS binary outcome. Retinol+retinyl esters exhibited an inverse relationship with CRP and a positive relationship with uric acid and HOMA-IR as well as MetS binary outcome. Vitamin E had no association with MetS, particularly after controlling for serum cholesterol and TG. In conclusion, among U.S. adolescents, serum carotenoid concentrations were inversely associated with MetS status, HOMA-IR, and CRP, whereas serum vitamin C was inversely related to MetS status and serum uric acid. Vitamin E had no consistent association with MetS, whereas retinol+retinyl esters had a positive relationship with HOMA-IR, uric acid, and MetS, while being inversely related to CRP. These associations need further study.

Insulin resistance and adiposity in relation to serum β-carotene levels

OBJECTIVE

To determine the effects of placebo vs an encapsulated supplement of fruit and vegetable juice concentrate (FVJC) on serum β-carotene levels, insulin resistance, adiposity, and subclinical inflammation in boys.

STUDY DESIGN

Thirty age-matched prepubertal boys (9 lean and 21 overweight (OW); age range, 6-10 years) were studied. All participants received nutrition counseling and were randomized to receive FVJC or placebo capsules for 6 months. Total cholesterol, triglycerides, lipid corrected β-carotene, serum retinol, glucose, insulin, retinol binding protein-4, leptin, adiponectin, leptin-to-adiponectin ratio, high-sensitivity C-reactive protein, and interleukin-6 were measured before and after the 6-month intervention. Homeostasis model assessment-insulin resistance (HOMA-IR), acute insulin response to intravenous glucose, along with abdominal fat mass (dual-energy x-ray absorptiometry) were also determined.

RESULTS

Baseline β-carotene concentrations correlated inversely with HOMA-IR, leptin-to-adiponectin ratio, and abdominal fat mass (P ≤ .01). FVJC intake increased β-carotene concentrations (P ≤ .001) but did not influence retinol or retinol binding protein-4. Retinol insufficiency <1.047 μM was present in 18% of the entire cohort at baseline and in 37% at 6 months. HOMA-IR decreased after supplementation in the OW cohort, when adjusted for percent weight change (P = .014). The percent change in abdominal fat mass increased in the placebo group and decreased in the FVJC group (P = .029).

CONCLUSIONS

A 6-month supplementation with FVJC in the presence of nutritional counseling was associated with an increase in serum β-carotene concentrations and a reduction in adiposity in conjunction with an improvement in insulin resistance in OW boys.

Association of vitamin A and carotenoid intake with melanoma risk in a large prospective cohort

Laboratory data suggest that intake of vitamin A and carotenoids may have chemopreventive benefits against melanoma, but epidemiological studies examining the association have yielded conflicting results. We examined whether dietary and supplemental vitamin A and carotenoid intake was associated with melanoma risk among 69,635 men and women who were participants of the VITamins And Lifestyle (VITAL) cohort study in western Washington. After an average of 5.84 years of follow-up, 566 incident melanomas were identified. Cox proportional hazards regression models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for risk of melanoma associated with dietary, supplemental, and total vitamin A and carotenoid intake after adjusting for melanoma risk factors. Baseline use of individual retinol supplements was associated with a significant reduction in melanoma risk (HR: 0.60; 95% CI: 0.41-0.89). High-dose (>1,200 μg per day) supplemental retinol was also associated with reduced melanoma risk (HR: 0.74; 95% CI: 0.55-1.00), as compared with non-users. The reduction in melanoma risk was stronger in sun-exposed anatomic sites. There was no association of melanoma risk with dietary or total intake of vitamin A or carotenoids. Retinol supplementation may have a preventative role in melanoma among women.

Lecithin:retinol acyltransferase is critical for cellular uptake of vitamin A from serum retinol-binding protein

Vitamin A (all-trans-retinol) must be adequately distributed within the mammalian body to produce visual chromophore in the eyes and all-trans-retinoic acid in other tissues. Vitamin A is transported in the blood bound to retinol-binding protein (holo-RBP), and its target cells express an RBP receptor encoded by the Stra6 (stimulated by retinoic acid 6) gene. Here we show in mice that cellular uptake of vitamin A from holo-RBP depends on functional coupling of STRA6 with intracellular lecithin:retinol acyltransferase (LRAT). Thus, vitamin A uptake from recombinant holo-RBP exhibited by wild type mice was impaired in Lrat(-/-) mice. We further provide evidence that vitamin A uptake is regulated by all-trans-retinoic acid in non-ocular tissues of mice. When in excess, vitamin A was rapidly taken up and converted to its inert ester form in peripheral tissues, such as lung, whereas in vitamin A deficiency, ocular retinoid uptake was favored. Finally, we show that the drug fenretinide, used clinically to presumably lower blood RBP levels and thus decrease circulating retinol, targets the functional coupling of STRA6 and LRAT to increase cellular vitamin A uptake in peripheral tissues. These studies provide mechanistic insights into how vitamin A is distributed to peripheral tissues in a regulated manner and identify LRAT as a critical component of this process.

Dermatopathy in juvenile Angus cattle due to vitamin A deficiency

In juvenile cattle, vitamin A deficiency is reported most commonly as a neurological condition; only rarely are there dermatologic manifestations. In the current study, alopecia, severe epidermal and follicular orthokeratosis, and acanthosis due to hypovitaminosis A are reported in 2 of 32 Angus calves, with a third animal suspected. Affected animals responded to vitamin A supplementation, and no additional calves displayed signs. Vitamin A acts on skin by regulating DNA transcription in keratinocytes, reducing the number of tonofilaments and desmosomes, both involved in cell-to-cell adhesion. Hence, adequate levels of dietary vitamin A are necessary for normal keratinocyte turnover, and deficiencies result in retention of keratinized cells (orthokeratosis). The present report reminds diagnosticians to consider vitamin A deficiency in cases of orthokeratotic dermatopathy in cattle.

Retinoic acid signalling during development

Retinoic acid (RA) is a vitamin A-derived, non-peptidic, small lipophilic molecule that acts as ligand for nuclear RA receptors (RARs), converting them from transcriptional repressors to activators. The distribution and levels of RA in embryonic tissues are tightly controlled by regulated synthesis through the action of specific retinol and retinaldehyde dehydrogenases and by degradation via specific cytochrome P450s (CYP26s). Recent studies indicate that RA action involves an interplay between diffusion (morphogen-like) gradients and the establishment of signalling boundaries due to RA metabolism, thereby allowing RA to finely control the differentiation and patterning of various stem/progenitor cell populations. Here, we provide an overview of the RA biosynthesis, degradation and signalling pathways and review the main functions of this molecule during embryogenesis.

The adverse effects of alcohol on vitamin A metabolism

The objective of this review is to explore the relationship between alcohol and the metabolism of the essential micronutrient, vitamin A; as well as the impact this interaction has on alcohol-induced disease in adults. Depleted hepatic vitamin A content has been reported in human alcoholics, an observation that has been confirmed in animal models of chronic alcohol consumption. Indeed, alcohol consumption has been associated with declines in hepatic levels of retinol (vitamin A), as well as retinyl ester and retinoic acid; collectively referred to as retinoids. Through the use of animal models, the complex interplay between alcohol metabolism and vitamin A homeostasis has been studied; the reviewed research supports the notion that chronic alcohol consumption precipitates a decline in hepatic retinoid levels through increased breakdown, as well as increased export to extra-hepatic tissues. While the precise biochemical mechanisms governing alcohol's effect remain to be elucidated, its profound effect on hepatic retinoid status is irrefutable. In addition to a review of the literature related to studies on tissue retinoid levels and the metabolic interactions between alcohol and retinoids, the significance of altered hepatic retinoid metabolism in the context of alcoholic liver disease is also considered.

Co-cultures of enterocytes and hepatocytes for retinoid transport and metabolism

Dietary retinoid bioavailability involves the interplay of the intestine (transport and metabolism) and the liver (secondary metabolism). To reproduce these processes in vitro, differentiated human intestinal Caco-2/TC7 cells were co-cultured with two hepatocyte cell lines. Murine 3A cells and the more highly differentiated human HepaRG hepatocytes were both shown to respond to β-carotene (BC) and retinol (ROH) treatment by secreting Retinol Binding Protein 4 (RBP4). In co-culture experiments, Caco-2/TC7 were differentiated on filter inserts and transferred for the time of the experiment to culture wells containing confluent 3A or differentiated HepaRG cells. Functionality of the co-cultures was assayed using as endpoints the retinol-dependent secretion of RBP4 and the retinoic acid-dependent induction of CYP26A1 in hepatocytes. BC and ROH added to intestinal Caco-2/TC7 induced a reduction in intracellular RBP4 levels in the underlying hepatocytes and its secretion into the medium. HepaRG hepatocytes were also shown to up-regulate the expression of CYP26A1 mRNA in response to retinoid treatment. This in vitro model represents a useful tool to analyze the absorption and metabolism of retinoids and could be further developed to investigate other dietary compounds and molecules of pharmacological interest.

Novel phytonutrient contributors to antioxidant protection against cardiovascular disease

The associations linking endothelial inflammation, endothelial oxidative stress, and atherogenesis and the potential for dietary phytonutrients to decrease the impact of these associations were assessed. A detailed literature review was conducted and summarized. A large body of scientific evidence describes the interactions among endothelial inflammation, endothelial oxidative stress, and atherogenesis. A growing body of research indicates that several dietary phytonutrients (astaxanthin, lycopene, lutein, and glabridin) can decrease the risk for atherosclerosis by decreasing endothelial inflammation and oxidative stress. The consumption of foods or dietary supplements that provide astaxanthin, lycopene, lutein, and glabridin can ameliorate endothelial inflammation and oxidative stress, retard atherogenesis, and decrease the risk for atherogenic cardiovascular disease.

Retinoic acid synthesis and functions in early embryonic development

Retinoic acid (RA) is a morphogen derived from retinol (vitamin A) that plays important roles in cell growth, differentiation, and organogenesis. The production of RA from retinol requires two consecutive enzymatic reactions catalyzed by different sets of dehydrogenases. The retinol is first oxidized into retinal, which is then oxidized into RA. The RA interacts with retinoic acid receptor (RAR) and retinoic acid X receptor (RXR) which then regulate the target gene expression. In this review, we have discussed the metabolism of RA and the important components of RA signaling pathway, and highlighted current understanding of the functions of RA during early embryonic development.

A sensitive and specific method for measurement of multiple retinoids in human serum with UHPLC-MS/MS

Retinol (vitamin A) circulates at 1-4 μM concentration and is easily measured in serum. However, retinol is biologically inactive. Its metabolite, retinoic acid (RA), is believed to be responsible for biological effects of vitamin A, and hence the measurement of retinol concentrations is of limited value. A UHPLC-MS/MS method using isotope-labeled internal standards was developed and validated for quantitative analysis of endogenous RA isomers and metabolites. The method was used to measure retinoids in serum samples from 20 healthy men. In the fed state, the measured concentrations were 3.1 ± 0.2 nM for atRA, 0.1 ± 0.02 nM for 9-cisRA, 5.3 ± 1.3 nM for 13-cisRA, 0.4 ± 0.4 nM for 9,13-dicisRA, and 17.2 ± 6.8 nM for 4oxo-13-cisRA. The concentrations of the retinoids were not significantly different when measured after an overnight fast (3.0 ± 0.1 nM for atRA, 0.09 ± 0.01 nM for 9-cisRA, 3.9 ± 0.2 nM for 13-cisRA, 0.3 ± 0.1 nM for 9,13-dicisRA, and 11.9 ± 1.6 nM for 4oxo-13-cisRA). 11-cisRA and 4OH-RA were not detected in human serum. The high sensitivity of the MS/MS method combined with the UHPLC separation power allowed detection of endogenous 9-cisRA and 4oxo-atRA for the first time in human serum.

A single dose of c9,t11 or t10,c12 conjugated linoleic acid isomers perturbs vitamin A metabolism in mice

Conjugated linoleic acid (CLA) is a polyunsaturated fatty acid that has numerous biologic activities. Previous studies in rodents demonstrated that chronic intake of CLA t10,c12 or CLA c9,t11 isomers perturbs the metabolism of retinoids (vitamin A and its derivatives). Specifically, although both isomers increased liver retinoid levels, only CLA t10,c12 also stimulated hepatic retinol secretion into the bloodstream. Given that retinoid homeostasis in mammalian serum and tissues is crucial to maintain health, it is important to gain more insights into the mode of action of this nutrient-nutrient interaction. Here we hypothesized that an acute administration of either CLA isomer may also influence vitamin A metabolism. By gavaging wild-type and retinol-binding protein knockout mice with an oral bolus of radiolabeled retinol containing 1 of these 2 isomers, we showed that both CLA t10,c12 and CLA c9,t11 rapidly enhance hepatic uptake of dietary vitamin A and its resecretion from the liver in the form of retinol bound to retinol-binding protein. Indeed, in mice lacking this protein, the sole specific carrier for retinol in the circulation, this latter effect was blunted. In addition, by using a pharmacologic inhibitor of the clearance of chylomicrons, which distribute recently ingested vitamin A and lipids throughout the body, we provided evidence that CLA intake might rapidly enhance intestinal absorption of dietary vitamin A. These data demonstrate the existence of multiple levels of interaction between dietary CLA and retinoid metabolism and warrant further studies to understand the molecular mechanisms underlying these effects and their implications for human health.

Hepatic metabolism of retinoids and disease associations

The liver is the most important tissue site in the body for uptake of postprandial retinoid, as well as for retinoid storage. Within the liver, both hepatocytes and hepatic stellate cells (HSCs) are importantly involved in retinoid metabolism. Hepatocytes play an indispensable role in uptake and processing of dietary retinoid into the liver, and in synthesis and secretion of retinol-binding protein (RBP), which is required for mobilizing hepatic retinoid stores. HSCs are the central cellular site for retinoid storage in the healthy animal, accounting for as much as 50-60% of the total retinoid present in the entire body. The liver is also an important target organ for retinoid actions. Retinoic acid is synthesized in the liver and can interact with retinoid receptors which control expression of a large number of genes involved in hepatic processes. Altered retinoid metabolism and the accompanying dysregulation of retinoid signaling in the liver contribute to hepatic disease. This is related to HSCs, which contribute significantly to the development of hepatic disease when they undergo a process of cellular activation. HSC activation results in the loss of HSC retinoid stores and changes in extracellular matrix deposition leading to the onset of liver fibrosis. An association between hepatic disease progression and decreased hepatic retinoid storage has been demonstrated. In this review article, we summarize the essential role of the liver in retinoid metabolism and consider briefly associations between hepatic retinoid metabolism and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Distinct populations of hepatic stellate cells in the mouse liver have different capacities for retinoid and lipid storage

Hepatic stellate cell (HSC) lipid droplets are specialized organelles for the storage of retinoid, accounting for 50–60% of all retinoid present in the body. When HSCs activate, retinyl ester levels progressively decrease and the lipid droplets are lost. The objective of this study was to determine if the HSC population in a healthy, uninjured liver demonstrates heterogeneity in its capacity for retinoid and lipid storage in lipid droplets. To this end, we utilized two methods of HSC isolation, which leverage distinct properties of these cells, including their vitamin A content and collagen expression. HSCs were isolated either from wild type (WT) mice in the C57BL/6 genetic background by flotation in a Nycodenz density gradient, followed by fluorescence activated cell sorting (FACS) based on vitamin A autofluorescence, or from collagen-green fluorescent protein (GFP) mice by FACS based on GFP expression from a GFP transgene driven by the collagen I promoter. We show that GFP-HSCs have: (i) increased expression of typical markers of HSC activation; (ii) decreased retinyl ester levels, accompanied by reduced expression of the enzyme needed for hepatic retinyl ester synthesis (LRAT); (iii) decreased triglyceride levels; (iv) increased expression of genes associated with lipid catabolism; and (v) an increase in expression of the retinoid-catabolizing cytochrome, CYP2S1. Conclusion: Our observations suggest that the HSC population in a healthy, uninjured liver is heterogeneous. One subset of the total HSC population, which expresses early markers of HSC activation, may be “primed” and ready for rapid response to acute liver injury.

Emerging roles for retinoids in regeneration and differentiation in normal and disease states

The vitamin A (retinol) metabolite, all-trans retinoic acid (RA), is a signaling molecule that plays key roles in the development of the body plan and induces the differentiation of many types of cells. In this review the physiological and pathophysiological roles of retinoids (retinol and related metabolites) in mature animals are discussed. Both in the developing embryo and in the adult, RA signaling via combinatorial Hox gene expression is important for cell positional memory. The genes that require RA for the maturation/differentiation of T cells are only beginning to be cataloged, but it is clear that retinoids play a major role in expression of key genes in the immune system. An exciting, recent publication in regeneration research shows that ALDH1a2 (RALDH2), which is the rate-limiting enzyme in the production of RA from retinaldehyde, is highly induced shortly after amputation in the regenerating heart, adult fin, and larval fin in zebrafish. Thus, local generation of RA presumably plays a key role in fin formation during both embryogenesis and in fin regeneration. HIV transgenic mice and human patients with HIV-associated kidney disease exhibit a profound reduction in the level of RARβ protein in the glomeruli, and HIV transgenic mice show reduced retinol dehydrogenase levels, concomitant with a greater than 3-fold reduction in endogenous RA levels in the glomeruli. Levels of endogenous retinoids (those synthesized from retinol within cells) are altered in many different diseases in the lung, kidney, and central nervous system, contributing to pathophysiology. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Gene expression response of mouse lung, liver and white adipose tissue to β-carotene supplementation, knockout of Bcmo1 and sex

SCOPE

Little information is available on differences, commonalities and especially interactions in overall gene expression responses as a result of diet, differences in sex (male and female) and effects induced by differences in metabolism. Moreover, it is unknown whether such effects are tissue specific.

METHODS AND RESULTS

We investigated the gene expression effects induced by β-carotene (BC) supplementation, knockout of β-carotene 15,15'-monooxygenase 1 (Bcmo1) and differences between male and female mice in lung, liver and inguinal white adipose tissue (iWAT). Unsupervised principal component analysis showed that lung gene expression was most affected by knockout of Bcmo1. Liver was most affected by knockout of Bcmo1 and differences in sex. iWAT was most affected by differences in sex. Hardly any genes were commonly influenced by BC among the three tissues. The effect of BC supplementation and knockout of Bcmo1 were relatively sex specific, especially in iWAT.

CONCLUSION

These data demonstrate that gene expression differences induced by BC are limited to the tissue and sex that is analyzed, and that differences in metabolism induced by for example single nucleotide polymorphisms, should be taken into account as much as possible. Moreover, our results indicate that translation from one tissue to the other should be done with caution for any nutritional intervention.

Vitamin A in reproduction and development

The requirement for vitamin A in reproduction was first recognized in the early 1900's, and its importance in the eyes of developing embryos was realized shortly after. A greater understanding of the large number of developmental processes that require vitamin A emerged first from nutritional deficiency studies in rat embryos, and later from genetic studies in mice. It is now generally believed that all-trans retinoic acid (RA) is the form of vitamin A that supports both male and female reproduction as well as embryonic development. This conclusion is based on the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency induced either by nutritional or genetic means with RA, and the ability to recapitulate the majority of embryonic defects in retinoic acid receptor compound null mutants. The activity of the catabolic CYP26 enzymes in determining what tissues have access to RA has emerged as a key regulatory mechanism, and helps to explain why exogenous RA can rescue many vitamin A deficiency defects. In severely vitamin A-deficient (VAD) female rats, reproduction fails prior to implantation, whereas in VAD pregnant rats given small amounts of carotene or supported on limiting quantities of RA early in organogenesis, embryos form but show a collection of defects called the vitamin A deficiency syndrome or late vitamin A deficiency. Vitamin A is also essential for the maintenance of the male genital tract and spermatogenesis. Recent studies show that vitamin A participates in a signaling mechanism to initiate meiosis in the female gonad during embryogenesis, and in the male gonad postnatally. Both nutritional and genetic approaches are being used to elucidate the vitamin A-dependent pathways upon which these processes depend.

Higher baseline serum concentrations of vitamin E are associated with lower total and cause-specific mortality in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study

BACKGROUND

A meta-analysis of 19 trials suggested a small increase in the risk of all-cause mortality with high-dose vitamin E supplementation. Little is known, however, about the relation between mortality and circulating concentrations of vitamin E resulting from dietary intake, low-dose supplementation, or both.

OBJECTIVE

We examined whether baseline serum alpha-tocopherol concentrations are associated with total and cause-specific mortality.

DESIGN

A prospective cohort study of 29 092 Finnish male smokers aged 50-69 y who participated in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study was carried out. Fasting serum alpha-tocopherol was measured at baseline by using HPLC. Only 10% of participants reported vitamin E supplement use at baseline, and thus serum concentrations of vitamin E mainly reflected dietary intake and other host factors. Risks of total and cause-specific mortality were estimated by using proportional hazards models.

RESULTS

During up to 19 y of follow-up, 13 380 deaths (including 4518 and 5776 due to cancer and cardiovascular disease, respectively) were identified. Men in the higher quintiles of serum alpha-tocopherol had significantly lower risks of total and cause-specific mortality than did those in the lowest quintile [relative risk (RR) = 0.82 (95% CI: 0.78, 0.86) for total mortality and 0.79 (0.72, 0.86), 0.81 (0.75, 0.88), and 0.70 (0.63, 0.79) for deaths due to cancer, cardiovascular disease, and other causes, respectively; P for trend for all < 0.0001]. Cubic regression spline analysis of continuous serum alpha-tocopherol values indicated greater risk reductions with increasing concentrations up to approximately 13-14 mg/L, after which no further benefit was noted.

CONCLUSION

Higher circulating concentrations of alpha-tocopherol within the normal range are associated with significantly lower total and cause-specific mortality in older male smokers.