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Vitamin A, acting through its metabolite, all-trans-retinoic acid, is a potent transcriptional regulator affecting expression levels of hundreds of genes through retinoic acid response elements present within these genes. However, the literature is replete with claims that consider vitamin A to be an antioxidant vitamin, like vitamins C and E. This apparent contradiction in the understanding of how vitamin A acts mechanistically within the body is a major focus of this review. Vitamin E, which is generally understood to act as a lipophilic antioxidant protecting polyunsaturated fatty acids present in membranes, is often proposed to be a transcriptional regulator. The evaluation of this claim is another focus of the review. We conclude that vitamin A is an indirect antioxidant, whose indirect function is to transcriptionally regulate a number of genes involved in mediating the body's canonical antioxidant responses. Vitamin E, in addition to being a direct antioxidant, prevents the increase of peroxidized lipids that alter both metabolic pathways and gene expression profiles within tissues and cells. However, there is little compelling evidence that vitamin E has a direct transcriptional mechanism like that of vitamin A. Thus, we propose that the term antioxidant not be applied to vitamin A, and we discourage the use of the term transcriptional mediator when discussing vitamin E. [ABSTRACT FROM AUTHOR]

Retinol-binding protein 2 (RBP2; originally cellular retinol-binding protein, type II (CRBPII)) is a 16 kDa cytosolic protein that in the adult is localized predominantly to absorptive cells of the proximal small intestine. It is well established that RBP2 plays a central role in facilitating uptake of dietary retinoid, retinoid metabolism in enterocytes, and retinoid actions locally within the intestine. Studies of mice lacking Rbp2 establish that Rbp2 is not required in times of dietary retinoid-sufficiency. However, in times of dietary retinoid-insufficiency, the complete lack of Rbp2 gives rise to perinatal lethality owing to RBP2 absence in both placental (maternal) and neonatal tissues. Moreover, when maintained on a high-fat diet, Rbp2-knockout mice develop obesity, glucose intolerance and a fatty liver. Unexpectedly, recent investigations have demonstrated that RBP2 binds long-chain 2-monoacylglycerols (2-MAGs), including the canonical endocannabinoid 2-arachidonoylglycerol, with very high affinity, equivalent to that of retinol binding. Crystallographic studies establish that 2-MAGs bind to a site within RBP2 that fully overlaps with the retinol binding site. When challenged orally with fat, mucosal levels of 2-MAGs in Rbp2 null mice are significantly greater than those of matched controls establishing that RBP2 is a physiologically relevant MAG-binding protein. The rise in MAG levels is accompanied by elevations in circulating levels of the hormone glucose-dependent insulinotropic polypeptide (GIP). It is not understood how retinoid and/or MAG binding to RBP2 affects the functions of this protein, nor is it presently understood how these contribute to the metabolic and hormonal phenotypes observed for Rbp2-deficient mice. [ABSTRACT FROM AUTHOR]

Much evidence has accumulated in the literature over the last fifteen years that indicates vitamin A has a role in metabolic disease prevention and causation. This literature proposes that vitamin A can affect obesity development and the development of obesity-related diseases including insulin resistance, type 2 diabetes, hepatic steatosis and steatohepatitis, and cardiovascular disease. Retinoic acid, the transcriptionally active form of vitamin A, accounts for many of the reported associations. However, a number of proteins involved in vitamin A metabolism, including retinol-binding protein 4 (RBP4) and aldehyde dehydrogenase 1A1 (ALDH1A1, alternatively known as retinaldehyde dehydrogenase 1 or RALDH1), have also been identified as being associated with metabolic disease. Some of the reported effects of these vitamin A-related proteins are proposed to be independent of their roles in assuring normal retinoic acid homeostasis. This review will consider both human observational data as well as published data from molecular studies undertaken in rodent models and in cells in culture. The primary focus of the review will be on the effects that vitamin A per se and proteins involved in vitamin A metabolism have on adipocytes, adipose tissue biology, and adipose-related disease, as well as on early stage liver disease, including non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH). [ABSTRACT FROM AUTHOR]

Abstract: The majority of retinoid (vitamin A and its metabolites) present in the body of a healthy vertebrate is contained within lipid droplets present in the cytoplasm of hepatic stellate cells (HSCs). Two types of lipid droplets have been identified through histological analysis of HSCs within the liver: smaller droplets bounded by a unit membrane and larger membrane-free droplets. Dietary retinoid intake but not triglyceride intake markedly influences the number and size of HSC lipid droplets. The lipids present in rat HSC lipid droplets include retinyl ester, triglyceride, cholesteryl ester, cholesterol, phospholipids and free fatty acids. Retinyl ester and triglyceride are present at similar concentrations, and together these two classes of lipid account for approximately three-quarters of the total lipid in HSC lipid droplets. Both adipocyte-differentiation related protein and TIP47 have been identified by immunohistochemical analysis to be present in HSC lipid droplets. Lecithin:retinol acyltransferase (LRAT), an enzyme responsible for all retinyl ester synthesis within the liver, is required for HSC lipid droplet formation, since Lrat-deficient mice completely lack HSC lipid droplets. When HSCs become activated in response to hepatic injury, the lipid droplets and their retinoid contents are rapidly lost. Although loss of HSC lipid droplets is a hallmark of developing liver disease, it is not known whether this contributes to disease development or occurs simply as a consequence of disease progression. Collectively, the available information suggests that HSC lipid droplets are specialized organelles for hepatic retinoid storage and that loss of HSC lipid droplets may contribute to the development of hepatic disease. [Copyright &y& Elsevier]

Abstract: 9-Cis-retinoic acid (RA) suppresses cancer cell proliferation via binding and activation of nuclear receptors, retinoid X receptors (RXRs). In vivo, 9-cis-RA is formed through oxidation of 9-cis-retinol by cis-retinol dehydrogenase (cRDH), an enzyme that we characterized previously. Since 9-cis-RA is a potent inhibitor of breast cancer cell proliferation, we hypothesized that overexpression of cRDH in breast cancer cells would result in increased production of 9-cis-RA, which in turn would suppress cell proliferation. To investigate this hypothesis, MCF7 human breast carcinoma cells were transduced with cRDH cDNA (LRDHSN/MCF7), and the growth kinetics and retinoid profiles of cells were examined following treatment with 9-cis-retinol. LRDHSN/MCF7 cells showed a marked reduction in cell numbers (60–80%) upon treatment with 9-cis-retinol compared to vehicle alone. Within 24 h of treatment, approximately 75% of the 9-cis-retinol was taken up and metabolized by LRDHSN/MCF7 cells. Despite the rapid uptake and oxidation of 9-cis-retinol to 9-cis-retinal, 9-cis-RA was not formed in these cells. We detect at least one novel metabolite formed from both 9-cis-retinol and 9-cis-retinal that may play a role in inhibition of MCF7 cell proliferation. Our studies demonstrate that 9-cis-retinol in combination with cRDH inhibits breast cancer cell proliferation by production of retinol metabolites other than RA. [Copyright &y& Elsevier]

Retinoids comprise both naturally and synthetically occurring compounds that have been proven to be differentiation agents for a variety of neoplasias, including breast cancer and promyelocytic leukemia in animal models and humans. They offer a unique panoply of therapeutics for the prevention or treatment of breast cancer. Nonetheless, considerable controversy remains as to the efficacy and potential toxic side-effects and as to which group of patients may most benefit. In this article, we review evidence of retinoid efficacy in breast cancer in humans and in animal models and provide possible mechanisms of retinoid action in breast cancer treatment, focusing on the roles of the different retinoic acid receptors and the metabolic pathways necessary for gene activation and cellular homeostasis. [ABSTRACT FROM AUTHOR]

Investigates the role of opsins, retinal-based pigments, in circadian photoreception in mice. Circadian photoreceptors in mammals; Circadian photoreception in the absence of opsins; Identity of the non-opsin circadian photoreceptor.

Recent studies reported that retinoid metabolism was influenced by long-term dietary restriction (DR) in rats. Because plasma retinol was decreased in rats subjected to DR, it was thought that this dietary manipulation may have an effect on retinol-binding protein (RBP) metabolism. Thus, the aim of this study was to assess retinoids, RBP, and transthyretin (TTR) levels in plasma and liver of young (3 months), adult (12 months), and old (22 months) female Sprague-Dawley rats fed ad libitum (AL) or subjected to a 40% DR, enriched (DR+), or not (DR), with vitamins and minerals. Results indicate that hepatic total retinoid concentrations and content increased with age in all the groups. DR+ rats showed higher hepatic retinoid concentrations than age-matched AL and DR rats. Adult and old DR and DR+ rats exhibited significantly lower plasma RBP-retinol and higher total retinoic acid levels than corresponding controls, although these parameters were not influenced by aging. Liver RBP levels were also decreased in DR and DR+ rats when compared to respective AL controls. There was a slight age-related decline in plasma TTR levels in DR and DR+ rats which was not associated with modifications in liver TTR levels. Hepatic gene expression of RBP and TTR, as evaluated by Northern blot hybridization, did not change with age or diet, suggesting that the lower levels of plasma RBP-retinol and liver RBP in vitamin A-sufficient rats subjected to DR may reflect post-transcriptional alterations and/or accelerated degradation of hepatic RBP. The elevated plasma levels of retinoic acid may represent an adaptive mechanism developed by DR rats to maintain retinoid-dependent functions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The levels of retinoids, retinol-binding protein, cellular retinol-binding protein, cellular retinoic-acid-binding protein, transthyretin and the activities of retinyl palmitate hydrolase and cholesteryl oleate hydrolase were determined in purified parenchymal, fat-storing, endothelial and Kupifer cell preparations, and in liver homogenates from young adult (6-month-old) and old (36-month-old) rats. Retinoid levels were also determined in the plasma from young and old rats. Retinoid contents were determined by H PLC. The binding proteins and transthyretin were measured by specific radioimmunoassays; retinyl palmitate and cholesterol oleate hydrolases were measured by sensitive microassays. The retinoid content of both the liver homogenates and of the fat-storing, and parenchymal cell preparations increased between 6 months and 36 months of age. The cellular distribution of retinoids was similar for the two age groups analyzed with the fat-storing cells being the main retinoid storage sites in the rat liver. Concentrations of retinol-binding protein and transthyretin were high in parenchymal cell preparations. Cellular retinol-binding protein was enriched both in parenchymal and in fat-storing cell preparations; the highest concentrations of cellular retinoic-acid-binding protein were present in fat-storing cell preparations. No major differences were observed between the two age groups in the cellular concentrations and distributions of any of these binding proteins. High activity of cholesterol oleate hydrolase was measured in parenchymal and in Kupifer cell preparations; end othelial cell preparations also contained considerable activities. The distribution of this activity over the various cell types reflects their role in lipoprotein metabolism. Retinyl palmitate hydrolase activity was specifically enriched in parenchymal and in fat-storing cell preparations, consistent with the roles of these cells in retinoid metabolism. No major differences were observed between the two age groups in the cellular distributions of the two hydrolase activities. This study indicates that no major changes occur in the retinoid- related parameters analyzed with age, suggesting that rat liver retinoid metabolism does not change dramatically with age and that retinoid homeostasis is maintained. [ABSTRACT FROM AUTHOR]

Studies were conducted to examine the in vivo transfer of chylomicron (dietary) retinoid from rat liver parenchymal to stellate cells. We specifically addressed the question of whether chylomicron retinyl ester is transferred directly from hepatic parenchymal to stellate cells without first undergoing hydrolysis. [14C]Retinyl palmitate and its non-hydrolyzable ether analog, retinyl [3H]hexadecyl ether, were utilized to answer this question. Chylomicrons labeled with these retinoids were injected intravenously into rats. Liver cell fractions, highly enriched in parenchymal or in stellate cells, were isolated 0.5 h, 4.5 h and 24 h after chylomicron injection. The ratio of 3H: 14C found in parenchymal cell preparations 4.5 h after injection was 1.8 times the ratio for the injected chylomicrons, and 24 h postinjection the ratio had increased to 2.5 times that of the chyloimcrons. In the stellate-cell-enriched preparations the 3H: 14C ratio was found to be 0.39, 0.29, and 0.23 times the ratio found in the injected labeled chylomicrons at 0.5 h, 4.5 h and 24 h after injection respectively. From the levels of 14C observed in the isolated stellate cells, it is estimated that 0.5 h postinjection the stellate cells contained approximately 34% of the 14C (i.e. the retinol injected as chylomicron retinyl ester) present in the liver. By 4.5 h the 14C present in isolated stellate cells had risen to approximately 41% of that present in the total liver, and 24 h after injection approximately 55% of hepatic total 14C was found in the stellate cells. These findings suggest that chylomicron retinyl ester is not transferred dircetly from the parenehymal to stellate cells without first undergoing hydrolysis to retinol. [ABSTRACT FROM AUTHOR]

Retinoic acid, an active metabolite of dietary vitamin A, acts as a ligand for nuclear receptor transcription factors with more than 500 known target genes. It is becoming increasingly clear that alcohol has a significant impact on cellular retinoic acid metabolism, with resultant effects on its function. Here, we test the hypothesis that chronic alcohol consumption impairs retinoic acid signaling in brown adipose tissue (BAT), leading to impaired BAT function and thermoregulation. All studies were conducted in age-matched, male mice consuming alcohol-containing liquid diets. Alcohol's effect on BAT was assessed by histology, qPCR, HPLC, LC/MS and measures of core body temperature. Our data show that chronic alcohol consumption decreases BAT mass, with a resultant effect on thermoregulation. Follow-up mechanistic studies reveal a decreased triglyceride content in BAT, as well as impaired retinoic acid homeostasis, associated with decreased BAT levels of retinoic acid in alcohol-consuming mice. Our work highlights a hitherto uncharacterized effect of alcohol on BAT function, with possible implications for thermoregulation and energy metabolism in drinkers. Our data indicate that alcohol's effects on brown adipose tissue may be mediated through altered retinoic acid signaling. [ABSTRACT FROM AUTHOR]

Bisphenol A (BPA, 2,2-bis(4-hydroxyphenyl) propane) is a widely used industrial chemical. The extensive distribution of BPA in the environment poses risks to humans. However, the molecular mechanisms underlying BPA toxicity as well as its effective detoxification and elimination are not well understood. We have investigated specifically for BPA the notion raised in the literature that the optimal sensing, detoxification, and elimination of xenobiotics requires retinoid (natural derivatives and synthetic analogs of vitamin A) actions. The objective of the study was to explore how retinoids, both those stored in the liver and those originating from recent oral intake, help maintain an optimal xenobiotic detoxification response, affecting mRNA expression and activities of elements of xenobiotic detoxification system upon BPA administration to mice. Wild-type and mice lacking hepatic retinoid stores (Lrat-/-) were acutely treated with BPA (50 mg/kg body weight), with or without oral supplementation with retinyl acetate. Hepatic mRNA expression levels of the genes encoding nuclear receptors and their downstream targets involved in xenobiotic biotransformation, phase I and phase II enzyme activities, and levels of oxidative damage to cellular proteins and lipids in hepatic microsomes, mitochondria and cytosol, were assessed. BPA treatment induced hepatic activities needed for its detoxification and elimination in wild-type mice. However, BPA failed to induce these activities in the livers of Lrat-/- mice. Oral supplementation with retinyl acetate restored phase I and phase II enzyme activities, but accelerated BPA-induced oxidative damage through enhancement of non-mitochondrial ROS production. Thus, the activities of the enzymes involved in the hepatic elimination of BPA require hepatic retinoid stores. The extent of hepatic damage that arises from acute BPA intoxication is directly affected by retinoid administration during the period of BPA exposure and hepatic retinoid stores that have accumulated over the lifetime of the organism. [ABSTRACT FROM AUTHOR]

Perturbed vitamin-A metabolism is associated with type 2 diabetes and mitochondrial dysfunction that are pathophysiologically linked to the development of diabetic cardiomyopathy (DCM). However, the mechanism, by which vitamin A might regulate mitochondrial energetics in DCM has previously not been explored. To test the hypothesis that vitamin-A deficiency accelerates the onset of cardiomyopathy in diet-induced obesity (DIO), we subjected mice with lecithin retinol acyltransferase (Lrat) germline deletion, which exhibit impaired vitamin-A stores, to vitamin A-deficient high-fat diet (HFD) feeding. Wild-type mice fed with a vitamin A-sufficient HFD served as controls. Cardiac structure, contractile function, and mitochondrial respiratory capacity were preserved despite vitamin-A deficiency following 20 wk of HFD feeding. Gene profiling by RNA sequencing revealed that vitamin A is required for the expression of genes involved in cardiac fatty acid oxidation, glycolysis, tricarboxylic acid cycle, and mitochondrial oxidative phosphorylation in DIO as expression of these genes was relatively preserved under vitamin A-sufficient HFD conditions. Together, these data identify a transcriptional program, by which vitamin A preserves cardiac energetic gene expression in DIO that might attenuate subsequent onset of mitochondrial and contractile dysfunction. NEW & NOTEWORTHY The relationship between vitamin-A status and the pathogenesis of diabetic cardiomyopathy has not been studied in detail. We assessed cardiac mitochondrial respiratory capacity, contractile function, and gene expression by RNA sequencing in a murine model of combined vitamin-A deficiency and diet-induced obesity. Our study identifies a role for vitamin A in preserving cardiac energetic gene expression that might attenuate subsequent development of mitochondrial and contractile dysfunction in diet-induced obesity. [ABSTRACT FROM AUTHOR]

Efficient delivery of vitamin A to the retinal pigment epithelium is vital to the production of the light-sensitive visual chromophore 11-cis-retinal. Nevertheless, retinol binding protein 4 (RBP4) is the only known carrier of vitamin A in plasma. Here, we present new findings that further characterize the visual cycle in the presence of Rbp4 deficiency. In the face of impaired delivery of retinol in Rbp4-/- mice, we determined that 11-cis-retinaldehyde reached levels that were ~60% of WT at 4 months of age and all-irans-retinyl ester was 18% of normal yet photoreceptor cell loss was apparent by 8 months of age. The lack of Rbp4 appeared to have a greater impact on scotopic rod-mediated responses than on cone function at early ages. Also, despite severely impaired delivery of retinol, bisretinoid lipofuscin that forms as a byproduct of the visual cycle was measurable by HPLC and by quantitative fundus autofluorescence. In mice carrying an Rpe65 amino acid variant that slows visual cycle kinetics, Rbp4 deficiency had a less pronounced effect on 11-cis-retinal levels. Finally, we found that ocular retinoids were not altered in mice expressing elevated adipose-derived total Rbp4 protein (hRBP4+/+Adi-Cre+/+). In conclusion, our findings are consistent with a model in which vitamin A can be delivered to the retina by Rbp4-independent pathways. [ABSTRACT FROM AUTHOR]

Background: Lumbar spinal stenosis (LSS) is a common reason for spine surgery in which ligamentum flavum is resected. Transthyretin (TTR) amyloid is an often unrecognized and potentially modifiable mechanism for LSS that can also cause TTR cardiac amyloidosis. Accordingly, older adult patients undergoing lumbar spine (LS) surgery were evaluated for amyloid and if present, the precursor protein, as well as comprehensive characterization of the clinical phenotype. Methods: A prospective, cohort study in 2 academic medical centers enrolled 47 subjects (age 69 ± 7 years, 53% male) undergoing clinically indicated LS decompression. The presence of amyloid was evaluated by Congo Red staining and in those with amyloid, precursor protein was determined by laser capture microdissection coupled to mass spectrometry (LCM‐MS). The phenotype was assessed by disease‐specific questionnaires (Swiss Spinal Stenosis Questionnaire and Kansas City Cardiomyopathy Questionnaire) and the 36‐question short‐form health survey, as well as biochemical measures (TTR, retinol‐binding protein, and TTR stability). Cardiac testing included technetium‐99m‐pyrophosphate scintigraphy, electrocardiograms, echocardiograms, and cardiac biomarkers as well as measures of functional capacity. Results: Amyloid was detected in 16 samples (34% of participants) and was more common in those aged ≥ 75 years of age (66.7%) compared with those <75 years (22.3%, p < 0.05). LCM‐MS demonstrated TTR as the precursor protein in 62.5% of participants with amyloid while 37.5% had an indeterminant type of amyloid. Demographic, clinical, quality‐of‐life measures, electrocardiographic, echocardiographic, and biochemical measures did not differ between those with and without amyloid. Among those with TTR amyloid (n = 10), one subject had cardiac involvement by scintigraphy. Conclusions: Amyloid is detected in more than a third of older adults undergoing LSS. Amyloid is more common with advancing age and is particularly common in those >75 years old. No demographic, clinical, biochemical, or cardiac parameter distinguished those with and without amyloid. In more than half of subjects with LS amyloid, the precursor protein was TTR indicating the importance of pathological assessment. See related Editorial by Cyrille‐Superville et al. in this issue. [ABSTRACT FROM AUTHOR]

The alcohol-induced depletion of hepatic retinoid stores correlates with the progression of liver injury; however, the mechanisms underlying alcohol's effects have not been fully elucidated. Our goal was to gain a mechanistic understanding of alcohol-induced hepatic retinoid depletion. Wild-type and mutant mice were continuously fed alcohol through Lieber-DeCarli liquid diets, with matched control animals pair fed an isocaloric alcohol-free diet to ensure equal nutrient and calorie intake between groups. A systematic analysis of tissue retinol and retinyl ester levels was performed with HPLC, complemented by gene and protein expression analyses. Our results delineated 2 phases of alcohol-induced depletion of hepatic retinoid. Initially, ~15% of hepatic retinoid content was mobilized from the liver, causing extrahepatic tissue retinoid levels to increase. Subsequently, there was a precipitous drop in hepatic retinoid content (>60%), without further retinoid accumulation in the periphery. Follow-up studies in mutant mice revealed roles for RBP, CRBP1, and CD36 in retinoid mobilization and extrahepatic retinoid uptake, as well as a role for CYP2E1 in the catabolism of hepatic retinoid. In summary, alcohol has abiphasic effect on hepatic retinoid stores, characterized by an initial phase of rapid mobilization to extrahepatic tissues followed by extensive catabolism within the liver. [ABSTRACT FROM AUTHOR]

The intestine and liver are crucial organs for vitamin A uptake and storage. Liver accounts for 70% of total body retinoid stores. Vitamin A deficiency (VAD) is a major micronutrient deficiency around the world. The provitamin A carotenoid, β-carotene, is a significant source of vitamin A in the diet. β-Carotene 15,15′ oxygenase-1 (BCO1) and β-carotene 9′,10′ oxygenase-2 (BCO2) are the two known carotenoid cleavage enzymes in humans. BCO1 and BCO2 are highly expressed in liver and intestine. Hepatocytes and hepatic stellate cells are two main cell types involved in the hepatic metabolism of retinoids. Stellate-like cells in the intestine also show ability to store vitamin A. Liver is also known to accumulate carotenoids, however, their uptake, retention and metabolism in specific liver and intestinal cell types is still unknown. Hence, we studied the cellular and subcellular expression and localization of BCO1 and BCO2 proteins in rat liver and intestine. We demonstrate that both BCO1 and BCO2 proteins are localized in hepatocytes and mucosal epithelium. We also show that BCO1 is also highly expressed in hepatic stellate cells (HSC) and portal endothelial cells in liver. At the subcellular level in liver, BCO1 is found in cytosol, while BCO2 is found in mitochondria. In intestine, immunohistochemistry showed strong BCO1 immunoreactivity in the duodenum, particularly in Brunner’s glands. Both BCO1 and BCO2 showed diffuse presence along epithelia with strong immunoreactivity in endothelial cells and in certain epithelial cells which warrant further investigation as possible intestinal retinoid storage cells. [ABSTRACT FROM AUTHOR]

Chronic alcohol consumption leads to a spectrum of liver disease that is associated with significant global mortality and morbidity. Alcohol is known to deplete hepatic vitamin A content, which has been linked to the pathogenesis of alcoholic liver disease. It has been suggested that induction of Cytochrome P450 2E1 (CYP2E1) contributes to alcohol-induced hepatic vitamin A depletion, but the possible contributions of other retinoid-catabolizing CYPs have not been well studied. The main objective of this study was to better understand alcohol-induced hepatic vitamin A depletion and test the hypothesis that alcohol-induced depletion of hepatic vitamin A is due to CYP-mediated oxidative catabolism. This hypothesis was tested in a mouse model of chronic alcohol consumption, including wild type and Cyp2e1-/- mice. Our results show that chronic alcohol consumption is associated with decreased levels of hepatic retinol, retinyl esters, and retinoic acid. Moreover, the depletion of hepatic retinoid is associated with the induction of multiple retinoid catabolizing CYPs, including CYP26A1, and CYP26B1 in alcohol fed wild type mice. In Cyp2e1-/- mice, alcohol-induced retinol decline is blunted but retinyl esters undergo a change in their acyl composition and decline upon alcohol exposure like WT mice. In conclusion, the alcohol induced decline in hepatic vitamin A content is associated with increased expression of multiple retinoid-catabolizing CYPs, including the retinoic acid specific hydroxylases CYP26A1 and CYP26B1. [ABSTRACT FROM AUTHOR]

Abstract: Background: Prenatal micronutrient deficiency has been linked to later development of schizophrenia among offspring; however, no study has specifically investigated the association between vitamin A and this disorder. Vitamin A is an essential nutrient which is required by the early embryo and fetus for gene expression and regulation, cell differentiation, proliferation and migration. Previous work suggests that vitamin A deficiency in the second trimester may be particularly relevant to the etiopathogenesis of neurobehavioral phenotypes some of which are observed in schizophrenia. Methods: We examined whether low maternal vitamin A levels in the second trimester are associated with the risk of schizophrenia and other schizophrenia spectrum disorders (SSD) in the Prenatal Determinants of Schizophrenia study; third trimester vitamin A levels were also examined in relation to SSD. The cases were derived from a population-based birth cohort; all cohort members belonged to a prepaid health plan. Archived maternal serum samples were assayed for vitamin A in cases (N=55) and up to 2 controls per case (N=106) matched on length of membership in the health plan, date of birth (±28days), sex, and gestational timing and availability of archived maternal sera. Results: For the second trimester, low maternal vitamin A, defined as values in the lowest tertile of the distribution among controls, was associated with a greater than threefold increased risk of SSD, adjusting for maternal education and age (OR=3.04, 95% CI=1.06, 8.79, p=.039). No association between third trimester maternal vitamin A and SSD was observed. Conclusions: Although further investigations are warranted, this is the first birth cohort study to our knowledge to report an association between low maternal vitamin A levels and SSD among offspring. [Copyright &y& Elsevier]

Objective: To determine whether nonobese adolescents with polycystic ovary syndrome (PCOS) have higher levels of retinol-binding protein 4 (RBP4) and ectopic fat than controls and whether RBP4 and ectopic fat correlate with comorbidities of metabolic disease. Design: Cross-sectional case-control study. Setting: Pediatric clinical research center based in a quaternary care medical center. Patient(s): Twenty-four nonobese adolescents between the ages of 13 and 21 years, 13 with PCOS and 11 controls. Intervention(s): Measurement of RBP4, insulin resistance, lipids, and body composition. Main Outcome Measure(s): Retinol-binding protein 4, reproductive and adrenal hormones, insulin resistance, intrahepatic and intramyocellular lipid levels, and visceral adipose tissue. Result(s): Adolescents with PCOS had higher intrahepatic lipid content and a statistical trend for higher RBP4 compared with controls. Retinol-binding protein 4 correlated with body fat, triglycerides, insulin resistance, and androgens but not intrahepatic lipid content; however, when adjusted for body fat, the correlation between RBP4 and triglycerides weakened to a statistical trend and was no longer statistically significant for the other measures. Conclusion(s): This small preliminary study of nonobese adolescent girls suggests that RBP4 may be involved in the dyslipidemia associated with PCOS and that there may be an independent relationship between RBP4 and triglycerides but not between RBP4 and insulin resistance. Although intrahepatic lipid content was higher in PCOS, it did not correlate with RBP4, triglycerides, or insulin resistance. [Copyright &y& Elsevier]

Loss of retinoid-containing lipid droplets upon hepatic stellate cell (HSC) activation is one of the first events in the development of liver disease leading to hepatocellular carcinoma. Although retinoid stores are progressively lost from HSCs during the development of hepatic disease, how this affects hepatocarcinogenesis is unclear. To investigate this, we used diethylnitrosamine (DEN) to induce hepatic tumorigenesis in matched wild-type (WT) and lecithin:retinol acyltransferase (LRAT) knockout (KO) mice, which lack stored retinoid and HSC lipid droplets. Male 15-day-old WT or Lrat KO mice were given intraperitoneal injections of DEN (25 mg/kg body wt). Eight months later, Lrat KO mice showed significantly less liver tumor development compared with WT mice, characterized by less liver tumor incidence and smaller tumor size. Two days after DEN injection, lower serum levels of alanine aminotransferase and decreased hepatic levels of cyclin D1 were observed in Lrat KO mice. Lrat KO mice also exhibited increased levels of retinoic acid-responsive genes, including p21, lower levels of cytochrome P450 enzymes required for DEN bioactivation and higher levels of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT), both before and after DEN treatment. Our results indicate that Lrat KO mice are less susceptible to DEN-induced hepatocarcinogenesis due to increased retinoid signaling and higher expression of p21, which is accompanied by altered hepatic levels of DEN-activating enzymes and MGMT in Lrat KO mice also contribute to decreased cancer initiation and suppressed liver tumor development. [ABSTRACT FROM PUBLISHER]

The metabolism of vitamin A is a highly regulated process that generates essential mediators involved in the development, cellular differentiation, immunity, and vision of vertebrates. Retinol saturase converts all-trans-retinol to all-trans-13,14-dihydroretinol (Moise, A. R., Kuksa, V., Imanishi, Y., and Palczewski, K. (2004) J. Biol. Chem. 279, 50230–60242). Here we demonstrate that the enzymes involved in oxidation of retinol to retinoic acid and then to oxidized retinoic acid metabolites are also involved in the synthesis and oxidation of all-trans-13,14-dihydroretinoic acid. All-trans-13,14-dihydroretinoic acid can activate retinoic acid receptor/retinoid X receptor heterodimers but not retinoid X receptor homodimers in reporter cell assays. All-trans-13,14-dihydroretinoic acid was detected in vivo in Lrat-/- mice supplemented with retinyl palmitate. Thus, all-trans-13,14-dihydroretinoic acid is a naturally occurring retinoid and a potential ligand for nuclear receptors. This new metabolite can also be an intermediate in a retinal degradation pathway or it can serve as a precursor for the synthesis of bioactive 13,14-dihydroretinoid metabolites. [ABSTRACT FROM AUTHOR]

Retinoic acid exerts a variety of effects on gene transcription that regulate growth, differentiation, and inflammation in normal and neoplastic skin cells. Because there is a lack of information regarding the influence of metabolic transformation of retinoids on their pharmacologic effects in skin, we have analyzed the functional activity of all- trans-, 9- cis-, and 13- cis-retinoic acid and their 4-oxo-metabolites in normal human epidermal keratinocytes (NHEKs) and dermal fibroblasts using gene and protein expression profiling techniques, including cDNA microarrays, two-dimensional gel electrophoresis, and MALDI-MS. It was previously thought that the 4-oxo-metabolites of RA are inert catabolic end-products but our results indicate instead that they display strong and isomer-specific transcriptional regulatory activity in both NHEKs and dermal fibroblasts. Microarray and proteomic analyses identified a number of novel genes/gene products that are influenced by RA treatment of NHEKs or fibroblasts, including genes for enzymes catalyzing biotransformation of retinoids, corticosteroids, and antioxidants and structural and transport proteins known to be essential for homeostasis. Our results expand current knowledge regarding retinoic acid action within skin cells and the target tissue/cell regulatory systems that are important for modulating the physiological and pharmacological effects of this important class of dermatological drugs. [ABSTRACT FROM AUTHOR]

The physiologic role(s) of cellular retinol-binding protein (CRBP)-III, an intracellular retinol-binding protein that is expressed solely in heart, muscle, adipose, and mammary tissue, remains to be elucidated. To address this, we have generated and characterized CRBP-III-deficient (CRBP-III-/-) mice. Mice that lack CRBP-III were viable and healthy hut displayed a marked impairment in retinoid incorporation into milk. Milk obtained from CRBP-III-/- dams contains significantly less retinyl ester, especially retinyl palmitate, than milk obtained from wild type dams. We demonstrated that retinol bound to CRBP-III is an excellent substrate for lecithin-retinol acyltransferase, the enzyme responsible for catalyzing retinyl ester formation from retinol. Our data indicated that the diminished milk retinyl ester levels arise from impaired utilization of retinol by lecithin-retinol acyltransferase in CRBP-III-/- mice. Interestingly, CRBP-I and CRBP-III each appeared to compensate for the absence of the other, specifically in mammary tissue, adipose tissue, muscle, and heart. For CRBP-III-/- mice, CRBP-I protein levels were markedly elevated in adipose tissue and mammary gland. In addition, in CRBP-I-/- mice, CRBP-III protein levels were elevated in tissues that normally express CRBP-III but were not elevated in other tissues that do not normally express CRBP-III. Our data suggested that CRBP-I and CRBP-III share some physiologic actions within tissues and that each can compensate for the absence of the other to help maintain normal retinoid homeostasis. However, under conditions of high demand for retinoid, such as those experienced during lactation, this compensation was incomplete. [ABSTRACT FROM AUTHOR]

Presents a study which discussed the metabolism of vitamin A through a number of genetically engineered mouse strains with alterations in genes that affect its metabolism. Overview of vitamin A metabolism; Summary.

Retinoic acid stimulates the expression of tissue-type plasrninogen activator (t-PA) in vascular endothelial cells in vitro and enhances t-PA levels in plasma and tissues in vivo. Compared with the in vivo situation, high retinoic acid concentrations are required to induce optimally t-PA expression in vitro. These findings led us to study retinoic acid metabolism in cultured human endothelial cells. For comparison, these studies were also performed in the human hepatoma cell line, HepG2, and key experiments were repeated with human primary hepatocytes. Both hepatocyte cultures gave very similar results. Human endothelial cells were shown to possess an active retinoic acid metabolizing capacity, which is quantitatively comparable to that of hepatocytes, but different from that of hepatocytes in several qualitative aspects. Our results demonstrate that all-trans-retinoic acid :is quickly metabolized by both endothelial cells and hepatocytes. All-trans-retinoic acid induces its own metabolism in endothelial cells but not in hepatocytes. 9-cis-Retinoic acid is degraded slowly by endothelial cells, whereas hepatocytes metabolize 9-cis-retinoic acid very quickly. Furthermore, our data show that hepatocytes, but not endothelial cells, delectably isomerise all-trans-retinoic acid to 9-cis-retinoic acid and vice versa. In both endothelial cells and hepatocytes all-trans-retinoic acid metabolism was inhibitable by the cytochrome P-450 inhibitors liarozole (10 µM) and ketoconazole (10 µM), albeit to different extents and with different specificities. In the presence of the most potent retinoic acid metabolism inhibitor in endothelial cells, liarozole, at least 10-fold lower all-trans-retinoic acid concentrations were required than in the absence of the inhibitor to obtain the same induction of t-PA. In conclusion, our results clearly demonstrate that all-trans-retinoic acid and 9-cis retinoic acid are actively but differently metabolized and isomerised by human endothelial cells and hepatocytes. The rapid metabolism of retinoic acid explains the relatively high concentrations of retinoic acid required to induce t-PA in cultured endothelial cells. [ABSTRACT FROM AUTHOR]

All- trans -retinoic acid (ATRA) has been well described as a positive regulator for early stage of adipocyte differentiation and lipid metabolism and also linked to an in vivo fat-lowering effect in mice. However, not all studies support this association. Our objective was to characterize the action of ATRA in mature adipocytes of mice by ablating RAR signaling through overexpression of a well-characterized dominant negative RARα mutant (RARdn) form specifically in adipocytes. Altered RAR signaling in adipocytes resulted in a significant decrease in ATRA levels in visceral and brown adipose tissues as well as liver tissue. This was linked to significant impairments in glucose clearance and elevated hepatic lipid accumulation for chow diet fed mice, indicating the development of metabolic disease, including hepatic steatosis. In addition, we found that adipose RARdn expression in mice fed a chow diet decreased thermogenesis. We conclude that altered RAR signaling and ATRA levels in adipocytes impacts glucose and lipid metabolism in mice. • Altered RAR signaling in adipocytes decreases ATRA levels in visceral and brown adipose tissues. • Adipose RARdn expression shows glucose intolerance and elevated hepatic lipid accumulation for chow fed mice. • Adipose RARdn expression in mice decreases thermogenesis. [ABSTRACT FROM AUTHOR]

Abstract Hepatic stellate cells (HSCs) are non-parenchymal liver cells that characteristically contain multiple retinoid (vitamin A)-containing lipid droplets. In this study, we addressed the metabolic fate of non-retinoid lipids originating from lipid droplet loss during HSCs activation. UPLC/MS/MS and qRT-PCR were used to monitor the lipid composition and mRNA expression of selected genes regulating lipid metabolism in freshly isolated, overnight-, 3- and 7-day cultures or primary mouse HSCs. A preferential accumulation of specific C20-C24 fatty acid species, especially arachidonic (C20:4) and docosahexaenoic acids (C22:6), was revealed in culture-activated HSCs along with an upregulation of transcription of fatty acid desaturases (Scd1, Scd2) and elongases (Elovl5, Elovl6). This was accompanied with an enrichment of activated HSCs with 36:4 and 38:4 phosphatidylcholine species containing polyunsaturated fatty acids and associated accumulation of selective lipid mediators, including endocannabinoids and related N-acylethanolamides, as well as ceramides. An increase in 2-arachidonoylglycerol and N-arachydonoylethanolamide concentrations was observed along with an upregulation of Daglα mRNA expression in HSCs during culture activation. N-palmitoylethanolamide was identified as the most abundant endocannabinoid-like species in activated HSCs. An increase in total ceramide levels and enrichment with N-palmitoyl (C16:0), N-tetracosenoyl (C24:1), N-tetracosanoyl (C24:0) and N-docosanoyl (C22:0) ceramides was detected in activated HSC cultures and was preceded by increased mRNA expression of ceramide synthesizing enzymes (CerS2 , CerS5 and Smpd1). Our data suggest an active redistribution of non-retinoid lipids in HSCs underlying the formation of low abundance, highly bioactive lipid species that may affect signaling during HSC activation, as well as extracellularly within the liver. Graphical abstract Unlabelled Image Highlights • Culture-activated primary mouse HSCs are enriched with unesterified and phosphatidylcholine-incorporated AA and DHA. • 2-AG and AEA levels are increased during the early stage of HSCs activation. • 2-AG is the predominant endocannabinoid and PEA is the second most abundant endocannabinoid-like species in HSCs during culture activation. • During activation, HSC cultures are enriched with C16:0, C24:1, C24:0 and C22:0 ceramides. [ABSTRACT FROM AUTHOR]

Liver fatty acid binding protein (L-Fabp) modulates lipid trafficking in enterocytes, hepatocytes, and hepatic stellate cells (HSCs). We examined hepatocyte vs. HSC L-Fabp deletion in hepatic metabolic adaptation and fibrotic injury. Floxed L-Fabp mice were bred to different transgenic Cre mice or injected with adeno-associated virus type 8 (AAV8) Cre and fed diets to promote steatosis and fibrosis or were subjected to either bile duct ligation or CCl4 injury. Albumin-Cre-mediated L-Fabp deletion revealed recombination in hepatocytes and HSCs; these findings were confirmed with 2 other floxed alleles. Glial fibrillary acid protein-Cre and platelet-derived growth factor receptor β-Cre-mediated L-Fabp deletion demonstrated recombination only in HSCs. Mice with albumin promoter-driven Cre recombinase (Alb-Cre)-mediated or AAV8-mediated L-Fabp deletion were protected against food withdrawal-induced steatosis. Mice with Alb-Cre-mediated L-Fabp deletion were protected against high saturated fat-induced steatosis and fibrosis, phenocopying germline L-Fabp-/- mice. Mice with HSC-specific L-Fabp deletion exhibited retinyl ester depletion yet demonstrated no alterations in fibrosis. On the other hand, fibrogenic resolution after CCl4 administration was impaired in mice with Alb-Cre-mediated L-Fabp deletion. These findings suggest cell type-specific roles for L-Fabp in mitigating hepatic steatosis and in modulating fibrogenic injury and reversal. [ABSTRACT FROM AUTHOR]

Background: The first stage of alcoholic liver disease is hepatic steatosis. While alcohol is known to profoundly impact hepatic lipid metabolism, gaps in our knowledge remain regarding the mechanisms leading to alcohol‐induced hepatic triglyceride (TG) accumulation. As the sole enzymes catalyzing the final step in TG synthesis, diacylglycerol O‐acyltransferase (DGAT) 1 and 2 are potentially important contributors to alcoholic steatosis. Our goal was to study the effects of dietary fat content on alcohol‐induced hepatic TG accumulation, and the relative contribution of DGAT1 and DGAT2 to alcoholic steatosis. Methods: These studies were carried out in wild‐type (WT) mice fed alcohol‐containing high‐fat or low‐fat formulations of Lieber‐DeCarli liquid diets, as well as follow‐up studies in Dgat1−/− mice. Results: A direct comparison of the low‐fat and high‐fat liquid diet in WT mice revealed surprisingly similar levels of alcoholic steatosis, although there were underlying differences in the pattern of hepatic lipid accumulation and expression of genes involved in hepatic lipid metabolism. Follow‐up studies in Dgat1−/− mice revealed that these animals are protected from alcoholic steatosis when consumed as part of a high‐fat diet, but not a low‐fat diet. Conclusions: Dietary macronutrient composition influences the relative contribution of DGAT1 and DGAT2 to alcoholic steatosis, such that in the context of alcohol and a high‐fat diet, DGAT1 predominates. The pathogenesis of alcoholic fatty liver disease is an active area of research. As the only enzymes capable of catalyzing the final step in triglyceride synthesis, Diacylglycerol O‐acyltransferase (DGAT) 1 and 2 are important contributors to alcoholic steatosis. Our data indicate that the relative contribution of DGAT1 and DGAT2 to alcohol‐induced hepatic triglyceride accumulation depends on dietary macronutrient composition. Thus, in the context of chronic alcohol consumption and a high‐fat diet, DGAT1 is the predominant contributor to alcoholic steatosis. [ABSTRACT FROM AUTHOR]

RBP4 is produced mainly by hepatocytes. In type 2 diabetes and obesity, circulating RBP4 is increased and may act systemically to cause insulin resistance and glucose intolerance. Observations that adipocyte RBP4 mRNA increases in parallel with circulating RBP4 in these conditions, whereas liver RBP4 mRNA does not, led to a widely held hypothesis that elevated circulating RBP4 is a direct result of increased production by adipocytes. To test this, we generated mice with hepatocyte-specific deletion of RBP4 (liver RBP4 knockout or LRKO mice). Adipose tissue RBP4 expression and secretion remained intact in LRKO mice and increased as expected in the setting of diet-induced insulin resistance. However, circulating RBP4 was undetectable in LRKO mice. We conclude that adipocyte RBP4 is not a significant source of circulating RBP4, even in the setting of insulin resistance. Adipocyte RBP4, therefore, may have a more important autocrine or paracrine function that is confined within the adipose tissue compartment. [ABSTRACT FROM AUTHOR]

Vitamin A homeostasis is critical to normal cellular function. Retinol-binding protein (RBP) is the sole specific carrier in the bloodstream for hydrophobic retinol, the main form in which vitamin A is transported. The integral membrane receptor STRA6 mediates cellular uptake of vitamin A by recognizing RBP-retinol to trigger release and internalization of retinol. We present the structure of zebrafish STRA6 determined to 3.9-angstrom resolution by single-particle cryo-electron microscopy. STRA6 has one intramembrane and nine transmembrane helices in an intricate dimeric assembly. Unexpectedly, calmodulin is bound tightly to STRA6 in a noncanonical arrangement. Residues involved with RBP binding map to an archlike structure that covers a deep lipophilic cleft. This cleft is open to the membrane, suggesting a possible mode for internalization of retinol through direct diffusion into the lipid bilayer. [ABSTRACT FROM AUTHOR]

Retinol-binding protein 2 (RBP2, also known as cellular retinol-binding protein 2 (CRBP2)) is a member of the fatty acid-binding protein family and has been extensively studied for its role in facilitating dietary vitamin A (retinol) uptake and metabolism within enterocytes of the small intestine. RBP2 is present in highest concentrations in the proximal small intestine where it constitutes approximately 0.1–0.5% of soluble protein. Recent reports have established that RBP2 binds monoacylglycerols (MAGs) with high affinity, including the canonical endocannabinoid 2-arachidonoylglycerol (2-AG). Crystallographic studies reveal that retinol, 2-AG, or other long-chain MAGs alternatively can bind in the retinol-binding pocket of RBP2. It also has been demonstrated recently that Rbp2 -deficient mice are more susceptible to developing obesity and associated metabolic phenotypes when exposed to a high fat diet, or as they age when fed a conventional chow diet. When subjected to an oral fat challenge, the Rbp2 -deficient mice release into the circulation significantly more, compared to littermate controls, of the intestinal hormone glucose-dependent insulinotropic polypeptide (GIP). These new findings regarding RBP2 structure and actions within the intestine are the focus of this review. • RBP2 binds 2-monoacylglycerols with very high affinity, equivalent to that of retinol binding. • 2-Monoacylglycerol and retinol bind in the same binding pocket of RBP2. • RBP2 modulates the intestinal release of glucose-dependent insulinotropic polypeptide (GIP) in response to a fat challenge. • The absence of RBP2 renders mice more prone to the development of obesity and associated metabolic diseases. [ABSTRACT FROM AUTHOR]

We have recently characterized the role of lipocalin 2 (Lcn2) as a new adipose-derived cytokine in the regulation of adaptive thermogenesis via a non-adrenergic pathway. Herein, we explored a potential non-adrenergic mechanism by which Lcn2 regulates thermogenesis and lipid metabolism. We found that Lcn2 is a retinoic acid target gene, and retinoic acid concurrently stimulated UCP1 and Lcn2 expression in adipocytes. Lcn2 KO mice exhibited a blunted effect of all-trans-retinoic acid (ATRA) on body weight and fat mass, lipid metabolism, and retinoic acid signaling pathway activation in adipose tissue under the high fat diet-induced obese condition. We further demonstrated that Lcn2 is required for the full action of ATRA on the induction of UCP1 and PGC-1α, expression in brown adipocytes and the restoration of cold intolerance in Lcn2 KO mice. Interestingly, we discovered that Lcn2 KO mice have decreased levels of retinoic acid and retinol in adipose tissue. The protein levels of STRA6 responsible for retinol uptake were significantly decreased in adipose tissue. The retinol transporter RBP4 was increased in adipose tissue but decreased in the circulation, suggesting the impairment of RBP4 secretion in Lcn2 KO adipose tissue. Moreover, Lcn2 deficiency abolished the ATRA effect on RBP4 expression in adipocytes. All the data suggest that the decreased retinoid level and action are associated with impaired retinol transport and storage in adipose tissue in Lcn2KOmice. We conclude that Lcn2 plays a critical role in regulating metabolic homeostasis of retinoids and retinoidmediated thermogenesis in adipose tissue. [ABSTRACT FROM AUTHOR]

Increased visceral fat, rather than subcutaneous fat, during the onset of obesity is associated with a higher risk of developing metabolic diseases. The inherent adipogenic properties of human adipose-derived stem cells (ASCs) from visceral depots are compromised compared with those of ASCs from subcutaneous depots, but little is known about the underlying mechanisms. Using ontological analysis of global gene expression studies, we demonstrate that many genes involved in retinoic acid (RA) synthesis or regulated by RA are differentially expressed in human tissues and ASCs from subcutaneous and visceral fat. The endogenous level of RA is higher in visceral ASCs; this is associated with upregulation of the RA synthesis gene through the visceral-specific developmental factor WT1. Excessive RA-mediated activity impedes the adipogenic capability of ASCs at early but not late stages of adipogenesis, which can be reversed by antagonism of RA receptors or knockdown of WT1. Our results reveal the developmental origin of adipocytic properties and the pathophysiological contributions of visceral fat depots. [ABSTRACT FROM AUTHOR]

β-Apo-carotenoids, including β-apo-13-carotenone and β-apo-14′-carotenal, are potent retinoic acid receptor (RAR) antagonists in transactivation assays. We asked how these influence RAR-dependent processes in living cells. Initially, we explored the effects of β-apo-13-carotenone and β-apo-14′-carotenal on P19 cells, a mouse embryonal carcinoma cell line that differentiates into neurons when treated with all- trans -retinoic acid. Treatment of P19 cells with either compound failed to block all- trans -retinoic acid induced differentiation. Liquid chromatography tandem mass spectrometry studies, however, established that neither of these β-apo-carotenoids accumulates in P19 cells. All- trans -retinoic acid accumulated to high levels in P19 cells. This suggests that the uptake and metabolism of β-apo-carotenoids by some cells does not involve the same processes used for retinoids and that these may be cell type specific. We also investigated the effects of two β-apo-carotenoids on 3T3-L1 adipocyte marker gene expression during adipocyte differentiation. Treatment of 3T3-L1 adipocytes with either β-apo-13-carotenone or β-apo-10′-carotenoic acid, which lacks RAR antagonist activity, stimulated adipocyte marker gene expression. Neither blocked the inhibitory effects of a relatively large dose of exogenous all- trans -retinoic acid on adipocyte differentiation. Our data suggest that in addition to acting as transcriptional antagonists, some β-apo-carotenoids act through other mechanisms to influence 3T3-L1 adipocyte differentiation. [ABSTRACT FROM AUTHOR]

Retinoic acid signaling is required for maintaining a range of cellular processes, including cell differentiation, proliferation, and apoptosis. We investigated the actions of all-trans-retinoic acid (atRA) signaling in pancreatic β-cells of adult mice. atRA signaling was ablated in β-cells by overexpressing a dominant-negative retinoic acid receptor (RAR)-α mutant (RARdn) using an inducible Cre-Lox system under the control of the pancreas duodenal homeobox gene promoter. Our studies establish that hypomorphism for RAR in β-cells leads to an age-dependent decrease in plasma insulin in the fed state and in response to a glucose challenge. Glucose-stimulated insulin secretion was also impaired in islets isolated from mice expressing RARdn. Among genes that are atRA responsive, Glut2 and Gck mRNA levels were decreased in isolated islets from RARdn-expressing mice. Histologic analyses of RARdn-expressing pancreata revealed a decrease in β-cell mass and insulin per β-cell 1 mo after induction of the RARdn. Our results indicate that atRA signaling mediated by RARs is required in the adult pancreas for maintaining both β-cell function and mass, and provide insights into molecular mechanisms underlying these actions. [ABSTRACT FROM AUTHOR]

Dietary carote-noids like β-carotene are converted within the body either to retinoid, via β-carotene-15,15′-dioxygenase (BCO1), or to β-apo-carotenoids, via β-carotene-9′,10′-oxygenase 2. Some β-apo-carotenoids are po-tent antagonists of retinoic acid receptor (RAR)-mediated transcrip-tional regulation, which is required to ensure normal heart develop-ment and functions. We established liquid chromatography tandem mass spectrometery methods for measuring concentrations of 10 β-apo-carotenoids in mouse plasma, liver, and heart and assessed how these are influenced by Bco1 deficiency and β-carotene intake. Sur-prisingly, Bco1-/- mice had an increase in heart levels of retinol, nonesterified fatty acids, and ceramides and a decrease in heart triglycerides. These lipid changes were accompanied by elevations in levels of genes important to retinoid metabolism, specifically retinol dehydrogenase 10 and retinol-binding protein 4, as well as genes involved in lipid metabolism, including peroxisome proliferator-acti-vated receptor-7, lipoprotein lipase, Cd36, stearoyl-CoA desaturase 1, and fatty acid synthase. We also obtained evidence of compromised heart function, as assessed by two-dimensional echocardiography, in Bco1-/- mice. However, the total absence of Bco1 did not substan-tially affect β-apo-carotenoid concentrations in the heart. β-Carotene administration to matched Bco1-/- and wild-type mice elevated total β-apo-carotenal levels in the heart, liver, and plasma and total β-apo-carotenoic acid levels in the liver. Thus, BCO1 modulates heart metabolism and function, possibly by altering levels of cofactors required for the actions of nuclear hormone receptors. [ABSTRACT FROM AUTHOR]

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the final step in triglyceride synthesis, the conversion of diacylglycerol (DAG) to triglyceride. Dgat1-/- mice exhibit a number of beneficial metabolic effects including reduced obesity and improved insulin sensitivity and no known cardiac dysfunction. In contrast, failing human hearts have severely reduced DGAT1 expression associated with accumulation of DAGs and ceramides. To test whether DGAT1 loss alone affects heart function, we created cardiomyocyte-specific DGAT1 knock-out (hDgat1-/-) mice. hDgat1-/- mouse hearts had 95% increased DAG and 85% increased ceramides compared with floxed controls. 50% of these mice died by 9 months of age. The heart failure marker brain natriuretic peptide increased 5-fold in hDgat1-/- hearts, and fractional shortening (FS) was reduced. This was associated with increased expression of peroxisome proliferator-activated receptor α and cluster of differentiation 36. We crossed hDgat1-/- mice with previously described enterocyte-specific Dgat1 knock-out mice (hiDgat1-/-). This corrected the early mortality, improved FS, and reduced cardiac ceramide and DAG content. Treatment of hDgat1-/- mice with the glucagon-like peptide 1 receptor agonist exenatide also improved FS and reduced heart DAG and ceramide content. Increased fatty acid uptake into hDgat1-/- hearts was normalized by exenatide. Reduced activation of protein kinase Cα (PKCα), which is increased by DAG and ceramides, paralleled the reductions in these lipids. Our mouse studies show that loss of DGAT1 reproduces the lipid abnormalities seen in severe human heart failure. [ABSTRACT FROM AUTHOR]

The article presents a study on the effect of the null mutation in a hormone-sensitive lipase gene on the risk of having type 2 diabetes. Topics include the important enzyme for lipolysis called hormone-sensitive lipase (HSL), the use of genotyping and association analyses, and the mutation results like small adipocytes, insulin resistance, and inflammation. It also discusses the role of HSL in systemic lipid regulation and glucose homeostasis.

The literature indicates that retinoids can influence the metabolism and actions of xenobiotics and conversely that xenobiotics can influence the metabolism and actions of retinoids. We were interested in understanding the degree to which hepatic retinoid stores, accumulated over a lifetime, affect xenobiotic metabolism, and actions. To investigate this, we induced liver injury through administration of the hepatotoxin thioacetamide (TAA) to chow fed wild type (WT) mice and lecithin:retinol acyltransferase-deficient (Lrat−/−) mice that are genetically unable to accumulate hepatic retinoid stores. Within 48 h of TAA-treatment, WT mice develop liver injury as evidenced by focal necrotic areas and increases in serum ALT activity and myeloperoxidase activity in hepatic parenchyma. Simultaneously, features of hepatic encephalopathy develop, as evidenced by a 25% increase in blood ammonia and a threefold reduction of blood glucose levels. This is accompanied by reduced hepatic glutathione, and increased thiobarbituric acid reactive substances, protein carbonyl and sulfhydryl groups, and increased cytochrome P450-catalyzed hydroxylation activity and flavin-containing monooxygenase activity in microsomes prepared from WT liver. Strikingly, none of these TAA-induced effects were observed for matched Lrat−/− mice. To confirm that TAA hepatotoxicity depends on retinoid availability, we administered, over 48 h, four oral doses of 3000 IU retinyl acetate each to the mice. This led to the development of hepatotoxicity in Lrat−/− mice that was similar in extent to that observed in WT mice. Our findings establish that endogenous hepatic retinoid stores can modulate the toxicity of TAA in mice. [ABSTRACT FROM PUBLISHER]

Insulin signaling in osteoblasts has been shown recently to contribute to whole-body glucose homeostasis in animals fed a normal diet; however, it is unknown whether bone contributes to the insulin resistance that develops in animals challenged by a high-fat diet (HFD). Here, we evaluated the consequences of osteoblast-specific overexpression of or loss of insulin receptor in HFD-fed mice. We determined that the severity of glucose intolerance and insulin resistance that mice develop when fed a HFD is in part a consequence of osteoblast-dependent insulin resistance. Insulin resistance in osteoblasts led to a decrease in circulating levels of the active form of osteocalcin, thereby decreasing insulin sensitivity in skeletal muscle. Insulin resistance developed in osteoblasts as the result of increased levels of free saturated fatty acids, which promote insulin receptor ubiquitination and subsequent degradation. Together, these results underscore the involvement of bone, among other tissues, in the disruption of whole-body glucose homeostasis resulting from a HFD and the involvement of insulin and osteocalcin cross-talk in glucose intolerance. Furthermore, our data indicate that insulin resistance develops in bone as the result of lipotoxicity-associated loss of insulin receptors. [ABSTRACT FROM AUTHOR]

The plasma membrane protein STRA6 is thought to mediate uptake of retinol from its blood carrier retinol-binding protein (RBP) into cells and to function as a surface receptor that, upon binding of holo-RBP, activates a JAK/STAT cascade. It was suggested that STRA6 signaling underlies insulin resistance induced by elevated serum levels of RBP in obese animals. To investigate these activities in vivo, we generated and analyzed Stra6-null mice. We show that the contribution of STRA6 to retinol uptake by tissues in vivo is small and that, with the exception of the eye, ablation of Stra6 has only a modest effect on retinoid homeostasis and does not impair physiological functions that critically depend on retinoic acid in the embryo or in the adult. However, ablation of Stra6 effectively protects mice from RBP-induced suppression of insulin signaling. Thus one biological function of STRA6 in tissues other than the eye appears to be the coupling of circulating holo-RBP levels to cell signaling, in turn regulating key processes such as insulin response. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Alcohol, a major cause of human cardiomyopathy, decreases cardiac contractility in both animals and man. However, key features of alcohol-related human heart disease are not consistently reproduced in animal models. Accordingly, we studied cardiac histology, contractile function, cardiomyocyte long chain fatty acid (LCFA) uptake, and gene expression in male C57BL/6J mice consuming 0, 10, 14, or 18% ethanol in drinking water for 3months. At sacrifice, all EtOH groups had mildly decreased body and increased heart weights, dose-dependent increases in cardiac triglycerides and a marked increase in cardiac fatty acid ethyl esters. [3H]-oleic acid uptake kinetics demonstrated increased facilitated cardiomyocyte LCFA uptake, associated with increased expression of genes encoding the LCFA transporters CD36 and Slc27a1 (FATP1) in EtOH-fed animals. Although SCD-1 expression was increased, lipidomic analysis did not indicate significantly increased de novo LCFA synthesis. By echocardiography, ejection fraction (EF) and the related fractional shortening (FS) of left ventricular diameter during systole were reduced and negatively correlated with cardiac triglycerides. Expression of myocardial PGC-1α and multiple downstream target genes in the oxidative phosphorylation pathway, including several in the electron transport and ATP synthase complexes of the inner mitochondrial membrane, were down-regulated. Cardiac ATP was correspondingly reduced. The data suggest that decreased expression of PGC-1α and its target genes result in decreased cardiac ATP levels, which may explain the decrease in myocardial contractile function caused by chronic EtOH intake. This model recapitulates important features of human alcoholic cardiomyopathy and illustrates a potentially important pathophysiologic link between cardiac lipid metabolism and function. [Copyright &y& Elsevier]

Adipose fat storage is thought to require uptake of circulating triglyceride (TG)-derived fatty acids via lipoprotein lipase (LpL). To determine how LpL affects the biology of adipose tissue, we created adipose-specific LpL knock-out (ATLO) mice, and we compared them with whole body LpL knock-out mice rescued with muscle LpL expression (MCK/L0) and wild type (WT) mice.ATLOLpLmRNAand activity were reduced, respectively, 75 and 70% in gonadal adipose tissue (GAT), 90 and 80% in subcutaneous tissue, and 84 and 85% in brown adipose tissue (BAT). ATLO mice had increased plasma TG levels associated with reduced chylomicronTGuptake intoBATand lung.ATLO BAT, but not GAT, had altered TG composition. GAT from MCK/L0 was smaller and contained less polyunsaturated fatty acids in TG, although GAT from ATLO was normal unless LpL was overexpressed in muscle. High fat diet feeding led to less adipose in MCK/L0 mice but TG acyl composition in subcutaneous tissue and BAT reverted to that of WT. Therefore, adipocyte LpL inBATmodulates plasma lipoprotein clearance, and the greater metabolic activity of this depot makes its lipid composition more dependent on LpL-mediated uptake. Loss of adipose LpL reduces fat accumulation only if accompanied by greater LpL activity in muscle. These data support the role of LpL as the "gatekeeper" for tissue lipid distribution. [ABSTRACT FROM AUTHOR]

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. © 2012 BioFactors, 2013 [ABSTRACT FROM AUTHOR]

The plasma membrane protein STRA6 transports vitamin A from its blood carrier retinol binding protein (RBP) into cells, and it also functions as a cytokine receptor which activates JAK/STAT signaling. We show here that, unlike other cytokine receptors, phosphorylation of STRA6 is not simply induced upon binding of its extracellular ligand. Instead, activation of the receptor is triggered by STRA6-mediated translocation of retinol from serum RBP to an intracellular acceptor, the retinol-binding protein CRBP-I . The observations also demonstrate that the movement of retinol from RBP to CRBP-I , and thus activation of STRA6, is critically linked to the intracellular metabolism of the vitamin. Furthermore, the data show that STRA6 phosphorylation is required for retinol uptake to proceed. Hence, the observations demonstrate that STRA6 orchestrates a multicomponent "machinery" that couples vitamin A homeostasis and metabolism to activation of a signaling cascade and that, in turn, STRA6 signaling regulates the cellular uptake of the vitamin. STRA6 appears to be a founding member of a new class of proteins that may be termed "cytokine signaling transporters." [ABSTRACT FROM AUTHOR]