Differential effects of retinoic acid on uncoupling protein-1 and leptin gene expression

Dietary Vitamin A Affects the Function of Incretin-Producing Enteroendocrine Cells in Male Mice Fed a High-Fat Diet

BACKGROUND

Retinol-binding protein 2 (RBP2) is an intracellular carrier for vitamin A in the absorptive enterocytes. Mice lacking RBP2 (Rbp2-/-) display an unexpected phenotype of obesity, glucose intolerance, and elevated glucose-dependent insulinotropic polypeptide (GIP) levels. GIP and glucagon-like peptide 1 (GLP-1) are incretin hormones secreted by enteroendocrine cells (EECs). We recently demonstrated the presence of RBP2 and other retinoid-related proteins in EECs.

OBJECTIVES

Given RBP2's role in intracellular retinoid trafficking, we aimed to evaluate whether dietary vitamin A affects incretin-secreting cell function and gene expression.

METHODS

Male Rbp2-/- mice and sex- and age-matched controls (n = 6-9) were fed a high-fat diet (HFD) for 18 wk containing normal (VAN, 4000 IU/kg of diet) or low (VAL, 25% of normal) vitamin A concentrations. Body weight was recorded biweekly. Plasma GIP and GLP-1 levels were obtained fasting and 30 min after an oral fat gavage at week 16. Glucose tolerance tests were also performed. Mice were killed at week 18, and blood and tissue samples were obtained.

RESULTS

Rbp2-/- mice displayed greater weight gain on the VAN compared with the VAL diet from week 7 of the intervention (P ≤ 0.01). Stimulated GIP levels were elevated in Rbp2-/- mice compared with their controls fed the VAN diet (P = 0.02), whereas their GIP response was lower when fed the VAL diet (P = 0.03). Although no differences in GLP-1 levels were observed in the VAN diet group, a lower GLP-1 response was seen in Rbp2-/- mice fed the VAL diet (P = 0.02). Changes in incretin gene expression and that of other genes associated with EEC lineage and function were consistent with these observations. Circulating and hepatic retinoid levels revealed no systemic vitamin A deficiency across dietary groups.

CONCLUSIONS

Our data support a role for RBP2 and dietary vitamin A in incretin secretion and gene expression in mice fed a HFD.

β-Carotene induces UCP1-independent thermogenesis via ATP-consuming futile cycles in 3T3-L1 white adipocytes

The activation of brown fat and induction of beige adipocytes, so-called non-shivering thermogenesis, is emerging as a promising target for therapeutic intervention in obesity management. Our previous report demonstrated that β-carotene (BC) induces beige adipocytes to increase UCP1-dependent thermogenic activity. However, the UCP1-independent thermogenic effect of BC on adipose tissues remains unexplored. In this study, we examined the effects of BC on UCP1-independent thermogenic activity with a focus on the ATP-consuming futile cycles in 3T3-L1 adipocytes. BC increased intracellular calcium levels and stimulated the expression of calcium cycling-related proteins, including sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) 2b, ryanodine receptor 2 (RyR2), voltage-dependent anion channel (VDAC), mitochondrial calcium uniporter (MCU), and Ca²⁺/calmodulin-dependent protein kinase 2 (CaMK2) in 3T3-L1 white adipocytes. In addition, BC stimulated thermogenesis by activating the creatine metabolism-related thermogenic pathway. Moreover, BC activated β-carotene oxygenase 1 (BCO1), which efficiently cleaved BC to retinal and consequently converted to its transcriptionally active form retinoic acid. These BC conversion products also exhibited thermogenic effects comparable to a similar level of BC. The mechanistic study revealed that retinal exhibited thermogenic activity independently of retinoic acid and retinoic acid-mediated thermogenesis was resulted partly from conversion of retinal. Moreover, BC activated α1-AR and UCP1-independent thermogenic effectors independently of UCP1 expression. In conclusion, the thermogenic response to BC and its conversion products in 3T3-L1 white adipocytes involves two interacting pathways, one mediated via β3-adrenergic receptors (β3-AR) and cyclic adenosine monophosphate (cAMP) and the other via α1-AR and increases in cytosolic Ca²⁺ levels activated by calcium regulatory proteins.

Retinoic acid promotes tissue vitamin A status and modulates adipose tissue metabolism of neonatal rats exposed to maternal high-fat diet-induced obesity

Maternal obesity may compromise the micronutrient status of the offspring. Vitamin A (VA) is an essential micronutrient during neonatal development. Its active metabolite, retinoic acid (RA), is a key regulator of VA homeostasis, which also regulates adipose tissue (AT) development in obese adults. However, its role on VA status and AT metabolism in neonates was unknown and it was determined in the present study. Pregnant Sprague-Dawley rats were randomised to a normal fat diet (NFD) or a high fat diet (HFD). From postnatal day 5 (P5) to P20, half of the HFD pups received oral RA every 3 d (HFDRA group). NFD pups and the remaining HFD pups (HFD group) received placebo. Six hours after dosing on P8, P14 and P20, n 4 pups per group were euthanised for different measures. It was found that total retinol concentration in neonatal liver and lung was significantly lower in the HFD group than the NFD group, while the concentrations were significantly increased in the HFDRA group. The HFD group exhibited significantly higher body weight (BW) gain, AT mass, serum leptin and adiponectin, and gene expression of these adipokines in white adipose tissue compared with the NFD group; these measures were significantly reduced in the HFDRA group. BAT UCP2 and UCP3 gene expression were significantly higher in pups receiving RA. In conclusion, repeated RA treatment during the suckling period improved the tissue VA status of neonates exposed to maternal obesity. RA also exerted a regulatory effect on neonatal obesity development by reducing BW gain and adiposity and modulating AT metabolism.

Screening of flavor compounds using Ucp1-luciferase reporter beige adipocytes identified 5-methylquinoxaline as a novel UCP1-inducing compound

Uncoupling protein 1 (UCP1) in brown or beige adipocytes is a mitochondrial protein that is expected to enhance whole-body energy expenditure. For the high-throughput screening of UCP1 transcriptional activity regulator, we established a murine inguinal white adipose tissue-derived Ucp1-luciferase reporter preadipocyte line. Using this reporter preadipocyte line, 654 flavor compounds were screened, and a novel Ucp1 expression-inducing compound, 5-methylquinoxaline, was identified. Adipocytes treated with 5-methylquinoxaline showed increased Ucp1 mRNA expression levels and enhanced oxygen consumption. 5-Methylquinoxaline induced Ucp1 expression through peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), and 5-methylquinoxaline-induced PGC1α activation seemed to be partially regulated by its phosphorylation or deacetylation. Thus, our Ucp1-luciferase reporter preadipocyte line is a useful tool for screening of Ucp1 inductive compounds.

Cyanidin-3-O-glucoside Regulates the Expression of Ucp1 in Brown Adipose Tissue by Activating Prdm16 Gene

(1) Background: Brown adipose tissue (BAT) burns energy to produce heat. Cyanidin-3-O-glucoside (C3G) can then enhance the thermogenic ability of BAT in vivo. However, the mechanism by which C3G regulates Ucp1 protein expression remains unclear. (2) Methods: In this study, C3H10T12 brown adipose cells and db/db mice and mice with high-fat, high-fructose, diet-induced obesity were used as the model to explore the effect of C3G on the expression of the Ucp1 gene. Furthermore, the 293T cell line was used for an in vitro cell transgene, a double luciferase reporting system, and yeast single hybridization to explore the mechanism of C3G in regulating Ucp1 protein. (3) Results: we identified that, under the influence of C3G, Prdm16 directly binds to the -500 to -150 bp promoter region of Ucp1 to activate its transcription and, thus, facilitate BAT programming. (4) Conclusions: This study clarified the mechanism by which C3G regulates the expression of the Ucp1 gene of brown fat to a certain extent.

Vitamin A Supplementation during Suckling and Postweaning Periods Attenuates the Adverse Metabolic Effects of Maternal High-Fat Diet Consumption in Sprague-Dawley Rats

BACKGROUND

Vitamin A (VA) has been demonstrated to be a regulator of adipose tissue (AT) development in adult obese models. However, little is known about the effect of VA on obesity-associated developmental and metabolic conditions in early life.

OBJECTIVES

We aimed to assess the effects of dietary VA supplementation during suckling and postweaning periods on the adiposity and metabolic health of neonatal and weanling rats from mothers consuming a high-fat diet (HFD).

METHODS

Pregnant Sprague-Dawley rats were fed a normal-fat diet (NFD; 25% fat; n = 2) or an HFD (50% fat; n = 2), both with 2.6 mg VA/kg. Upon delivery, half of the rat mothers were switched to diets with supplemented VA at 129 mg/kg, whereas the other half remained at 2.6 mg VA/kg. Four groups of rat pups were designated as NFD, NFD + VA, HFD, and HFD + VA, respectively. At postnatal day (P)14, P25, and P35, pups (n = 4 or 3/group) were killed. Body weight (BW), visceral white AT (WAT) mass, brown AT (BAT) mass, uncoupling protein 1 mRNA expression in BAT, serum glucose, lipids, adipokines, and inflammatory biomarkers, as well as serum and AT redox status were assessed.

RESULTS

Rat pups in the HFD group exhibited significantly higher BW, WAT mass, and serum glucose and leptin but reduced BAT mass compared with the NFD group. Without affecting the dietary intake, supplementing the HFD with VA significantly reduced the BW and WAT mass of pups but increased the BAT mass, significantly lowered the systemic and WAT oxidative stress, and modulated serum adipokines and lipids to some extent.

CONCLUSIONS

VA supplementation during suckling and postweaning periods attenuated metabolic perturbations caused by excessive fat intake. Supplementing maternal or infant obesogenic diets with VA or establishing a higher RDA of VA for specific populations should be studied further for managing overweight/obesity in early life.

Carotenoids and carotenoid conversion products in adipose tissue biology and obesity: Pre-clinical and human studies

Antiobesity activities of carotenoids and carotenoid conversion products (CCPs) have been demonstrated in pre-clinical studies, and mechanisms behind have begun to be unveiled, thus suggesting these compounds may help obesity prevention and management. The antiobesity action of carotenoids and CCPs can be traced to effects in multiple tissues, notably the adipose tissues. Key aspects of the biology of adipose tissues appear to be affected by carotenoid and CCPs, including adipogenesis, metabolic capacities for energy storage, release and inefficient oxidation, secretory function, and modulation of oxidative stress and inflammatory pathways. Here, we review the connections of carotenoids and CCPs with adipose tissue biology and obesity as revealed by cell and animal intervention studies, studies addressing the role of endogenous retinoid metabolism, and human epidemiological and intervention studies. We also consider human genetic variability influencing carotenoid and vitamin A metabolism, particularly in adipose tissues, as a potentially relevant aspect towards personalization of dietary recommendations to prevent or manage obesity and optimize metabolic health. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Programming mediated by fatty acids affects uncoupling protein 1 (UCP-1) in brown adipose tissue

Brown adipose tissue (BAT) has recently been given more attention for the part it plays in obesity. BAT can generate great amounts of heat through thermogenesis by the activation of uncoupling protein 1 (UCP-1), which can be regulated by many environmental factors such as diet. Moreover, the build-up of BAT relates to maternal nutritional changes during pregnancy and lactation. However, at present, there is a limited number of studies looking at maternal nutrition and BAT development, and it seems that the research trend in this field has been considerably declining since the 1980s. There is much to discover yet about the role of different fatty acids on the development of BAT and the activation of UCP-1 during the fetal and the postnatal periods of life. A better understanding of the impact of nutritional intervention on the epigenetic regulation of BAT could lead to new preventive care for metabolic diseases such as obesity. It is important to know in which circumstances lipids could programme BAT during pregnancy and lactation. The modification of maternal dietary fatty acids, amount and composition, during pregnancy and lactation might be a promising strategy for the prevention of obesity in the offspring and future generations.

Vitamin A signaling and homeostasis in obesity, diabetes, and metabolic disorders

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).

Retinoic acid induces white adipose tissue browning by increasing adipose vascularity and inducing beige adipogenesis of PDGFRα+ adipose progenitors

Formation of beige adipocytes within white adipose tissue enhances energy expenditure, which is a promising strategy to reduce obesity and prevent metabolic symptoms. Vitamin A and its bioactive metabolite, retinoic acid (RA), have regulatory roles in lipid metabolism. Here we report that RA induces white adipose tissue browning via activating vascular endothelial growth factor (VEGF) signaling. RA triggered angiogenesis and elicited de novo generation of platelet-derived growth factor receptor α positive (PDGFRα⁺) adipose precursor cells via VEGFA/VEGFR2 signaling. In addition, RA promoted beige/brown adipocyte formation from capillary networks in vitro. Using PDGFRα tracking mice, we found that the vascular system acted as an adipogenic repository by containing PDGFRα⁺ progenitors which differentiated into beige adipocytes under RA or VEGF164 treatments. Conditional knockout of VEGF receptors blocked RA-stimulated white adipose tissue browning. Moreover, the VEGFA and RA activated p38MAPK to enhance the binding of RA receptor to RA response elements of the Prdm16 promoter and upregulated Prdm16 transcription. In conclusion, RA induces white adipose tissue browning by increasing adipose vascularity and promoting beige adipogenesis of PDGFRα⁺ adipose progenitors.

Alterations in vitamin A/retinoic acid homeostasis in diet-induced obesity and insulin resistance

Vitamin A is an essential micronutrient for life and the phytochemical β-carotene, also known as pro-vitamin A, is an important dietary source of this vitamin. Vitamin A (retinol) is the parent compound of all bioactive retinoids but it is retinoic acid (RA) that is the active metabolite of vitamin A. The plasma concentration of retinol is maintained in a narrow range and its normal biological activities strictly regulated since excessive intake can lead to toxicity and thus also be detrimental to life. The present review will give an overview of how vitamin A homeostasis is maintained and move on to focus on the link between circulating vitamin A and metabolic disease states. Finally, we will examine how pharmacological or genetic alterations in vitamin A homeostasis and RA-signalling can influence body fat and blood glucose levels including a novel link to the liver secreted hormone fibroblast growth factor 21, an important metabolic regulator.

Vitamin A status affects the plasma parameters and regulation of hepatic genes in streptozotocin-induced diabetic rats

Vitamin A (VA) status regulates metabolism in rats. Whether VA status and availability of retinoic acid (RA) contribute to the insulin-regulated hepatic gene expression remains to be determined. Zucker lean rats with VA sufficient (VAS) or VA deficient (VAD) status were treated with streptozotocin (STZ) to induce insulin-dependent diabetes. They were treated with saline (STZ-VAS-C or STZ-VAD-C), RA (STZ-VAS-RA or STZ-VAD-RA), insulin (STZ-VAS-INS or STZ-VAD-INS), or insulin + RA (STZ-VAS-INS + RA or STZ-VAD-INS + RA) for 3 h. Insulin and insulin + RA treatments reduced tail tip blood glucose, raised plasma insulin and suppressed plasma β-hydroxybutyrate levels in both STZ-VAD and STZ-VAS rats. STZ-VAD-INS and STZ-VAD-INS + RA rats had lower plasma glucose levels than STZ-VAD-C rats had. STZ-VAD-INS and STZ-VAD-INS + RA rats had higher plasma leptin level and lower glucagon level than STZ-VAD-C rats did. Insulin treatment induced Gck, Srebp-1c and Fas and suppressed Pck1 expression levels in the liver of STZ-VAS and STZ-VAD rats. Interestingly, insulin treatment inhibited Cyp26a1 expression in STZ-VAD, but not STZ-VAS rats, whereas RA treatment induced it in both. RA treatment induced Gck expression only in STZ-VAD rats. Insulin + RA treatment further induced the Cyp26a1 and Gck expressions in STZ-VAD rats. The Srebp-1c expression levels of STZ-VAD-INS and STZ-VAD-INS + RA rats were higher than that of STZ-VAS-INS and STZ-VAS-INS + RA rats. The changes of Gck mRNA and glucokinase protein were consistent. In STZ-induced diabetic rats, VA is not required for insulin-regulated Gck, Srebp-1c, Fas and Pck1 expression. However, VA status altered responses of certain genes (Cyp26a1 and Srebp-1c) to insulin treatment.

All-trans-retinoic acid represses chemokine expression in adipocytes and adipose tissue by inhibiting NF-κB signaling

An effect of the Vitamin A metabolite all-trans-retinoic acid (ATRA) on body weight regulation and adiposity has been described, but little is known about its impact on obesity-associated inflammation. Our objective was to evaluate the overall impact of this metabolite on inflammatory response in human and mouse adipocytes, using high-throughput methods, and to confirm its effects in a mouse model. ATRA (2 μM for 24 h) down-regulated the mRNA expression of 17 chemokines in human adipocytes, and limited macrophage migration in a TNFα-conditioned 3 T3-L1 adipocyte medium (73.7%, P<.05). These effects were confirmed in mice (n=6-9 per group) subjected to oral gavage of ATRA (5 mg/kg of body weight) and subsequently injected intraperitoneally with lipopolysaccharide. In this model, both systemic and adipose levels of inflammatory markers were reduced. The antiinflammatory effect of ATRA was associated with a reduction in the phosphorylation levels of IκB and p65 (~50%, P<.05), two subunits of the NF-κB pathway, probably mediated by PGC1α, in 3 T3-L1 adipocytes. Taken together, these results show a significant overall antiinflammatory effect of ATRA on proinflammatory cytokine and chemokine production in adipocyte and adipose tissue and suggest that ATRA supplementation may represent a strategy of preventive nutrition to fight against obesity and its complications.

Carotenoids in Adipose Tissue Biology and Obesity

Cell, animal and human studies dealing with carotenoids and carotenoid derivatives as nutritional regulators of adipose tissue biology with implications for the etiology and management of obesity and obesity-related metabolic diseases are reviewed. Most studied carotenoids in this context are β-carotene, cryptoxanthin, astaxanthin and fucoxanthin, together with β-carotene-derived retinoids and some other apocarotenoids. Studies indicate an impact of these compounds on essential aspects of adipose tissue biology including the control of adipocyte differentiation (adipogenesis), adipocyte metabolism, oxidative stress and the production of adipose tissue-derived regulatory signals and inflammatory mediators. Specific carotenoids and carotenoid derivatives restrain adipogenesis and adipocyte hypertrophy while enhancing fat oxidation and energy dissipation in brown and white adipocytes, and counteract obesity in animal models. Intake, blood levels and adipocyte content of carotenoids are reduced in human obesity. Specifically designed human intervention studies in the field, though still sparse, indicate a beneficial effect of carotenoid supplementation in the accrual of abdominal adiposity. In summary, studies support a role of specific carotenoids and carotenoid derivatives in the prevention of excess adiposity, and suggest that carotenoid requirements may be dependent on body composition.

Chronic vitamin A-enriched diet feeding induces body weight gain and adiposity in lean and glucose-intolerant obese rats of WNIN/GR-Ob strain

NEW FINDINGS

What is the central question of this study? Previously, we reported that chronic feeding of a vitamin A-enriched diet to euglycaemic obese rats (WNIN/Ob) ameliorated obesity. Does this diet exert similar effects even with a different genetic background, i.e. obese rats of the WNIN/GR-Ob strain with impaired glucose tolerance? What is the main finding and its importance? Vitamin A-enriched diet aggravated weight gain and adiposity/obesity in both lean and glucose-intolerant obese rats of the WNIN/GR-Ob strain. Therefore, the role of genetic factors and their regulation by nutrients in determining health and disease conditions assumes greater significance in experimental and clinical research. Vitamin A and its metabolites are key regulators of the development of adipose tissue and its associated metabolic complications. Here, we tested, in a glucose-intolerant obese rat model (the WNIN/GR-Ob stain), whether feeding a vitamin A-enriched diet alters adiposity and its associated changes. For this purpose, 30-week-old male lean and obese rats were divided into two groups and received either stock diet or vitamin A-enriched diet [2.6 or 129 mg vitamin A (kg diet)(-1) , respectively] for 14 weeks. At the end, feeding of the vitamin A-enriched diet resulted in increased body weight gain/obesity and retroperitoneal white adipose tissue (RPWAT) in both lean and obese rats of the WNIN/GR-Ob strain, when compared with their respective control animals receiving stock diet, without affecting food intake. An improvement in hypertriglyceridaemia and circulatory non-esterified fatty acid levels and unaltered hepatic fatty acid oxidative and triglyceride secretory pathway proteins with vitamin A-enriched diet feeding are suggestive of enhanced hepatic clearance of circulatory lipids, resulting in increased hepatic triglyceride accumulation. Transcriptional analysis of RPWAT showed that feeding the vitamin A-enriched diet augmented the expression of adipogenic/adipose tissue-specific genes; peroxisome proliferator-activated receptor-γ, stearoyl CoA desaturase 1, retinol saturase, leptin and lipoprotein lipase and vitamin A metabolic pathway genes; retinoic acid receptors, retinoid X receptors and cytochrome P450 26B1. Besides, RPWAT-lipoprotein lipase-mediated clearance of triglyceride could also have contributed to increased adiposity and improved hypertriglyceridaemia. In conclusion, chronic feeding of vitamin A-enriched diet induces weight gain and adiposity in both lean and obese rats of the WNIN/GR-Ob strain, possibly through transcriptional regulation of key adipogenic pathway genes of RPWAT, but improves dyslipidaemia.

Low serum concentrations of carotenoids and vitamin E are associated with high adiposity in Mexican-American children

Mexican-American children have a high prevalence of overweight/obesity. Micronutrient deficiencies may be contributing to the development of greater adiposity in these children. This study investigated the relations between adiposity and serum concentrations of carotenoids, retinol, and vitamin E among Mexican-American children 8-15 y of age included in the 2001-2004 U.S. NHANES. Associations of the outcomes of children's body mass index (BMI), truncal fat mass (TrFM), and total body fat mass (TBFM) with serum concentrations of α-carotene, cis-β-carotene, trans-β-carotene, retinol, and α-tocopherol were determined by using linear, quantile, and multinomial regression models. BMI was inversely associated with serum concentrations of α-carotene (β = -0.88, P < 0.05), trans-β-carotene (β = -2.21, P < 0.01), cis-β-carotene (β = -2.10, P < 0.01), and α-tocopherol adjusted for total cholesterol ratio (β = -3.66, P < 0.01), respectively. Similar inverse associations were found with TrFM and TBFM. Higher cis-β-carotene and α-tocopherol serum concentrations were associated with reduced probability of overweight (OR: 0.57; 95% CI: 0.37, 0.89; P < 0.05; and OR: 0.56; 95% CI: 0.37, 0.86; P < 0.05; respectively) and obesity (OR: 0.39; 95% CI: 0.26, 0.58; P < 0.01; and OR: 0.38; 95% CI: 0.24, 0.60; P < 0.01; respectively). Higher retinol serum concentrations were associated with increased probability of overweight and obesity (OR: 2.01; 95% CI: 1.26, 3.22; P < 0.01; and OR: 2.90; 95% CI: 1.65, 5.09; P < 0.01; respectively). Significant inverse associations were found between serum concentrations of carotenoids and vitamin E and adiposity among Mexican-American children, but serum retinol concentrations were positively associated with adiposity. Future research is needed to understand the causes and consequences of micronutrient status on adiposity and comorbidities.

Retinoic acid has different effects on UCP1 expression in mouse and human adipocytes

Background

Increased adipose thermogenesis is being considered as a strategy aimed at preventing or reversing obesity. Thus, regulation of the uncoupling protein 1 (UCP1) gene in human adipocytes is of significant interest. Retinoic acid (RA), the carboxylic acid form of vitamin A, displays agonist activity toward several nuclear hormone receptors, including RA receptors (RARs) and peroxisome proliferator-activated receptor δ (PPARδ). Moreover, RA is a potent positive regulator of UCP1 expression in mouse adipocytes.

Results

The effects of all-trans RA (ATRA) on UCP1 gene expression in models of mouse and human adipocyte differentiation were investigated. ATRA induced UCP1 expression in all mouse white and brown adipocytes, but inhibited or had no effect on UCP1 expression in human adipocyte cell lines and primary human white adipocytes. Experiments with various RAR agonists and a RAR antagonist in mouse cells demonstrated that the stimulatory effect of ATRA on UCP1 gene expression was indeed mediated by RARs. Consistently, a PPARδ agonist was without effect. Moreover, the ATRA-mediated induction of UCP1 expression in mouse adipocytes was independent of PPARγ coactivator-1α.

Conclusions

UCP1 expression is differently affected by ATRA in mouse and human adipocytes. ATRA induces UCP1 expression in mouse adipocytes through activation of RARs, whereas expression of UCP1 in human adipocytes is not increased by exposure to ATRA.

Fenretinide treatment prevents diet-induced obesity in association with major alterations in retinoid homeostatic gene expression in adipose, liver, and hypothalamus

The synthetic retinoid, Fenretinide (FEN), inhibits obesity and insulin resistance in mice and is in early clinical trials for treatment of insulin resistance in obese humans. We aimed to determine whether alterations in retinoic acid (RA)-responsive genes contribute to the beneficial effects of FEN. We examined the effect of FEN on 3T3-L1 adipocyte differentiation and alterations in gene expression in C57Bl/6 and retinaldehyde dehydrogenase (RALDH) 1 knockout (KO) mice fed a high-fat (HF) diet. FEN completely inhibited adipocyte differentiation by blocking CCAAT/enhancer-binding protein (C/EBP) α/peroxisome proliferator-activated receptor (PPAR) γ-mediated induction of downstream genes and upregulating RA-responsive genes like cellular retinol-binding protein-1. In mice fed an HF diet, RA-responsive genes were markedly increased in adipose, liver, and hypothalamus, with short-term and long-term FEN treatment. In adipose, FEN inhibited the downregulation of PPARγ and improved insulin sensitivity and the levels of adiponectin, resistin, and serum RBP (RBP4). FEN inhibited hyperleptinemia in vivo and leptin expression in adipocytes. Surprisingly, hypothalamic neuropeptide Y expression was completely suppressed, suggesting a central effect of FEN to normalize hyperglycemia. Moreover, FEN induced RA-responsive genes in RALDH1 KO mice, demonstrating that FEN can augment RA signaling when RA synthesis is impaired. We show that FEN-mediated beneficial effects are through alterations in retinoid homeostasis genes, and these are strong candidates as therapeutic targets for the treatment of obesity and insulin resistance.

All-trans retinoic acid displays multiple effects on the growth, lipogenesis and adipokine gene expression of AML-I preadipocyte cell line

Adipose tissue is a potential site of retinoic acid (RA) action, but its physiological significance remains to be clarified. We have examined the effect of all-trans retinoic acid (ATRA) on growth and differentiation of preadipocytes, and on adipokine gene expression in mature adipocytes using human preadipocyte cell model, AML-I. Both ATRA and 9-cis RA induced growth arrest in AML-I preadipocyte at between 50 and 100 µM, which was accompanied by apoptosis. Western blotting showed a loss of NF-κB, Bcl-2 and p-Akt, and the accumulation of Bad and Akt in cytoplasm of ATRA-treated AML-I preadipocytes. Exposure of AML-I to ATRA or 9-cis RA increased intracellular lipid accumulation in a time-dependent manner compared to vehicle-treated cells. Expression of fatty acid synthase (FAS) and peroxisome proliferator-activated receptor-γ (PPAR-γ) proteins was increased in ATRA-treated cells. Thus, both ATRA and 9-cis RA promoted differentiation, inhibited proliferation and induced apoptosis in AML-I preadipocytes. ATRA also modulated adipokine expression by increasing the mRNA level of adipocytokines (adiponectin, leptin and LPL), and by inhibiting PAI-1 mRNA expression in mature AML-I adipocytes. The data suggest that ATRA exerts a wide range of effects--growth arrest, apoptosis, lipogenesis and modulation of adipokine gene expression--during the maturation of preadipocytes into adipocytes.

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.

Specific micronutrient concentrations are associated with inflammatory cytokines in a rural population of Mexican women with a high prevalence of obesity

It has been recognised recently that obese individuals have lower concentrations of micronutrients and this may affect the concentrations of inflammatory cytokines. A cross-sectional study was carried out to evaluate the association of specific micronutrients' status with chronic inflammation caused by obesity in 280 women (36·1 (SD 7·5) years) from seven rural communities in Mexico. Measurements of weight, height and waist circumference were made on all women and body composition was determined by dual-energy X-ray absorptiometry. Concentrations of the cytokines IL-1, TNF-α, IL-6, IL-10 and IL-12, lipid profile, and the micronutrients Zn and vitamins A, C and E were determined in fasting blood samples. Ordered logistic regression models were used to determine associations between categorised cytokine levels and micronutrients. It was found that 80% of women were overweight or obese, and had significantly higher concentrations of C-reactive protein than normal-weight women (P= 0·05). The risk of higher levels of TNF-α, IL-6, IL-10 and IL-12 was reduced significantly among women with higher Zn concentrations (OR 0·63, 95% CI 0·42, 0·96, P= 0·03; OR 0·57, 95% CI 0·39, 0·86, P= 0·025; OR 0·63, 95% CI 0·41, 0·96, P= 0·04; OR 0·62, 95% CI 0·41, 0·95, P= 0·03, respectively). Higher concentrations of vitamin A were slightly associated with reduced risks of higher levels of IL-1 and IL-12 (OR 0·97, 95% CI 0·95, 0·99, P= 0·03; OR 0·97, 95% CI 0·94, 0·99, P= 0·03, respectively); when adjusting for BMI, this association was lost. No associations were found between vitamin C or vitamin E:lipids concentrations and inflammatory cytokines. In conclusion, higher Zn concentrations are associated with reduced risks of higher concentration of inflammation markers in a population of women with a high prevalence of obesity.

Lipid metabolism in mammalian tissues and its control by retinoic acid

Evidence has accumulated that specific retinoids impact on developmental and biochemical processes influencing mammalian adiposity including adipogenesis, lipogenesis, adaptive thermogenesis, lipolysis and fatty acid oxidation in tissues. Treatment with retinoic acid, in particular, has been shown to reduce body fat and improve insulin sensitivity in lean and obese rodents by enhancing fat mobilization and energy utilization systemically, in tissues including brown and white adipose tissues, skeletal muscle and the liver. Nevertheless, controversial data have been reported, particularly regarding retinoids' effects on hepatic lipid and lipoprotein metabolism and blood lipid profile. Moreover, the molecular mechanisms underlying retinoid effects on lipid metabolism are complex and remain incompletely understood. Here, we present a brief overview of mammalian lipid metabolism and its control, introduce mechanisms through which retinoids can impact on lipid metabolism, and review reported activities of retinoids on different aspects of lipid metabolism in key tissues, focusing on retinoic acid. Possible implications of this knowledge in the context of the management of obesity and the metabolic syndrome are also addressed. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Retinoic acid reduces glucocorticoid sensitivity in C2C12 myotubes by decreasing 11beta-hydroxysteroid dehydrogenase type 1 and glucocorticoid receptor activities

Vitamin A is a nutrient with remarkable effects on adipose tissue and skeletal muscles, and plays a role in controlling energy balance. Retinoic acid (RA), the carboxylic form of vitamin A, has been associated with improved glucose tolerance and insulin sensitivity. In contrast, elevated glucocorticoids have been implicated in the development of insulin resistance and impaired glucose tolerance. Here, we investigated whether RA might counteract glucocorticoid effects in skeletal muscle cells by lowering 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1)-dependent local glucocorticoid activation and/or activation of glucocorticoid receptor (GR). We found a dose-dependent down-regulation of 11beta-HSD1 mRNA expression and activity upon incubation of fully differentiated mouse C2C12 myotubes with RA. In addition, RA inhibited GR transactivation by an 11beta-HSD1-independent mechanism. The presence of RA during myogenesis did not prevent myotube formation but resulted in relatively glucocorticoid-resistant myotubes, exhibiting very low 11beta-HSD1 expression and GR activity. The use of selective retinoic acid receptor (RAR) and retinoid X receptor ligands provided evidence that these effects were mediated through RARgamma. Importantly, short hairpin RNA against RARgamma abolished the effect of RA on 11beta-HSD1 and GR. In conclusion, we provide evidence for an important role of RA in the control of glucocorticoid activity during myogenesis and in myotubes. Disturbances of the nutrient and hormonal regulation of glucocorticoid action in skeletal muscles might be relevant for metabolic diseases.

Plasma leptin levels are related to body composition, sex, insulin levels and the A55V polymorphism of the UCP2 gene

OBJECTIVE

Circulating leptin levels show a high degree of individual variability even after the main effect of body fatness is accounted for. We therefore wanted to determine the roles of variation in body composition, age, sex and polymorphisms of the UCP2 gene and promoter region on levels of circulating leptin.

SUBJECTS

One hundred and fifty Caucasian subjects, which represented a cross-section of the population from NE, Scotland, were recruited.

MEASUREMENTS

Body composition was measured using dual X-ray absorptiometry. Fasted circulating leptin, insulin, T3 and T4 levels were measured, and all individuals were genotyped for the UCP2 polymorphisms A55V, -866G>A and exon-8 ins/del.

RESULTS

The results indicate that circulating leptin was significantly related to sex and principle component (PC) scores representing overall adipose tissue mass and a second representing the contrast of central to peripheral bone mineral content. Residual leptin was associated with the A55V polymorphism (P< 0.001) explaining 11.3% of the residual variance. There was a marginal effect associated with exon-8 ins/del (P=0.045) explaining 4.4% of the residual variance in leptin. Log(e) transformed circulating fasting insulin was related to PC scores representing general adiposity and sex. Residual Log(e) insulin was associated with the A55V and exon-8 ins/del polymorphisms explaining 5.7% (P=0.015) and 5% (P=0.026) of the residual variation, respectively. The -866G>A polymorphism was not significantly associated with residual leptin or insulin. Leptin and insulin were significantly (P=0.007) correlated. Statistically removing the effect of insulin on leptin still showed association between leptin and A55V (P=0.002). Removing the effect of leptin on insulin, the A55V polymorphism was no longer significant (P=0.120). After accounting for the correlation between insulin and leptin, the exon-8 ins/del was no longer significant for residual leptin (P=0.119) or Log(e) insulin (P=0.252).

CONCLUSION

These data suggest that the A55V polymorphism directly affected the levels of leptin but not via an effect on insulin.

Serum levels of albumin, triglycerides, total protein and glucose in rats are altered after oral treatment with low doses of 13-cis-retinoic acid or all-trans-retinoic acid

Currently used to treat severe acne, 13-cis-retinoic acid (13-cis-RA) is under investigation for its anticancer effects as is the isomer, all-trans-retinoic acid (all-trans-RA). Here, the effects of oral 13-cis-RA or all-trans-RA treatment on serum chemistry, leptin and adiponectin levels were evaluated. Adult Sprague-Dawley rats were gavaged once daily for 7 consecutive days with 13-cis-RA (7.5 or 15 mg kg(-1)), all-trans-RA (10 or 15 mg kg(-1)) (n=24/sex/dose), or soy oil (n=16/sex) and blood was sampled 30-480 min after the last gavage. The body weight was unaffected; however, the liver/body weight ratios were increased by both doses of all-trans-RA. Sex differences were noted for levels of cholesterol, creatine, triglycerides, albumin, alanine aminotransferase and total protein. Both doses of all-trans-RA reduced albumin levels to approximately 90% of the control and total protein levels to approximately 93% of the control while substantially elevating triglyceride levels to approximately 66%-99% above the control. Additionally, triglyceride levels of the 15 mg kg(-1) 13-cis RA group were approximately 62% higher than the controls and total protein levels were approximately 5% less. Glucose levels were affected by sex and RA treatment in that males treated with 15 mg kg(-1) of 13-cis-RA or 10 mg kg(-1) all-trans-RA had lower (13%-19%) levels than the same-sex controls; however, females were not similarly affected. Neither 13-cis-RA nor all-trans-RA treatment had significant effects on the levels of blood urea nitrogen, aspartate amino transferase, leptin or adiponectin. On a mg kg(-1) basis, all-trans-RA was more potent than 13-cis-RA. These results replicate previous findings of RA-induced increased triglyceride levels. Additionally, several new findings indicate there may be sex-specific effects of RA treatment. Finally, neither treatment appeared to alter the typical diurnal cycles of these endpoints.

Topical ER36009, a RARgamma-selective retinoid, decreases abdominal white adipose tissue and elicits changes in expression of genes related to adiposity and thermogenesis

Chronic topical treatment of rats with a new RARgamma-selective retinoid, ER36009, resulted in a significant reduction of epididymal white adipose tissue and a significant increase of interscapular brown adipose tissue without affecting food intake. ER36009 markedly decreased PPARgamma, 11beta-HSD1, and Bcl-2 mRNA levels, and increased Bax mRNA in white adipose tissue, while it upregulated UCP1 and UCP3 mRNAs in brown adipose tissue and UCP3 mRNA in gastrocnemial muscle. These results suggest that ER36009 has multiple effects on adipose tissue biology and the energy balance. Topically applied ER36009 may have potential for the treatment of obesity.

Cold exposure suppresses serum adiponectin levels through sympathetic nerve activation in mice

OBJECTIVE

Several lines of evidence suggest important roles for adiponectin in glucose and lipid metabolism and atherosclerosis. However, the mechanisms regulating serum adiponectin levels and adiponectin production are still not completely understood. Our aim was to determine whether adiponectin synthesis is physiologically regulated by the sympathetic nervous system (SNS).

RESEARCH METHODS AND PROCEDURES

Mice were exposed to cold (4 degrees C) for 12 hours and for 24 hours with or without inhibition of noradrenaline synthesis or pan-beta adrenergic function, followed by measurement of serum adiponectin concentrations and levels of adiponectin and uncoupling protein (UCP) 1 expressions in various white adipose tissues (WATs).

RESULTS

Cold exposure significantly reduced serum adiponectin concentrations without changing body weights or WAT sizes in either subcutaneous or intra-abdominal fat tissues. The serum adiponectin reduction was associated with a decrease in adiponectin mRNA expression in subcutaneous, epididymal, and mesenteric fat tissues. In these adipose tissues, UCP1 expression was markedly enhanced, suggesting SNS activation in these tissues. Administration of alpha-methyl-p-tyrosine or a combination of SR59230A and propranolol reversed the cold-exposure-induced decreases in serum adiponectin concentrations and adiponectin mRNA expression in these tissues. In contrast, in retroperitoneal fat, the effects of cold exposure on adiponectin and UCP1 expressions were strikingly weak but were not reversed by SNS inhibitors.

DISCUSSION

SNS physiologically regulates serum adiponectin levels and adiponectin synthesis in WATs in vivo, although responsiveness to SNS stimulation differs markedly among WATs. Sympathetic activation might be involved in development of the metabolic syndrome by modulation of serum adiponectin concentrations.

Once and again: Retinoic acid signaling in the developing and regenerating olfactory pathway

Retinoic acid (RA), a member of the steroid/thyroid superfamily of signaling molecules, is an essential regulator of morphogenesis, differentiation, and regeneration in the mammalian olfactory pathway. RA-mediated teratogenesis dramatically alters olfactory pathway development, presumably by disrupting retinoid-mediated inductive signaling that influences initial olfactory epithelium (OE) and bulb (OB) morphogenesis. Subsequently, RA modulates the genesis, growth, or stability of subsets of OE cells and OB interneurons. RA receptors, cofactors, and synthetic enzymes are expressed in the OE, OB, and anterior subventricular zone (SVZ), the site of neural precursors that generate new OB interneurons throughout adulthood. Their expression apparently accommodates RA signaling in OE cells, OB interneurons, and slowly dividing SVZ neural precursors. Deficiency of vitamin A, the dietary metabolic RA precursor, leads to cytological changes in the OE, as well as olfactory sensory deficits. Vitamin A therapy in animals with olfactory system damage can accelerate functional recovery. RA-related pathology as well as its potential therapeutic activity may reflect endogenous retinoid regulation of neuronal differentiation, stability, or regeneration in the olfactory pathway from embryogenesis through adulthood. These influences may be in register with retinoid effects on immune responses, metabolism, and modulation of food intake.

Chronic dietary vitamin A supplementation regulates obesity in an obese mutant WNIN/Ob rat model

OBJECTIVE

To understand the possible role of chronic dietary high vitamin A supplementation in body weight regulation and obesity using a novel WNIN/Ob obese rat model developed at the National Centre for Laboratory Animal Sciences of National Institute of Nutrition, India.

RESEARCH METHODS AND PROCEDURES

Thirty-six 7-month-old male rats of lean, carrier, and obese phenotypes were broadly divided into two groups; each group was subdivided into three subgroups consisting of six lean, six carrier, and six obese rats and received diets containing either 2.6 or 129 mg vitamin A/kg of diet for 2 months. Body weight gain, food intake, and weights of various organs were recorded. Adiposity index and BMI were calculated. Serum and liver retinol and brown adipose tissue (BAT)-uncoupling protein1 (UCP1) mRNA expression levels were quantified.

RESULTS

Chronic feeding of high but non-toxic doses of vitamin A through diet significantly reduced (P < or = 0.05) body weight gain, adiposity index, and retroperitoneal white adipose tissue mass (without affecting food intake) in obese rats compared with their lean and carrier counterparts. In general, vitamin A treatment significantly improved hepatic retinol stores (P < or = 0.05) in all phenotypes without affecting serum free retinol levels. However, augmented BAT-UCP1 expression was observed only in carrier and obese rats (whose basal expression was low).

DISCUSSION

Our data suggest that chronic dietary vitamin A supplementation at high doses effectively regulates obesity in obese phenotype of the WNIN/Ob strain, possibly through up-regulation of the BAT-UCP1 gene and associated adipose tissue loss. However, in vitamin A-supplemented lean and carrier rats, changes in adiposity could not be related to BAT-UCP1 expression levels.

Retinoic acid administration and vitamin A status modulate retinoid X receptor alpha and retinoic acid receptor alpha levels in mouse brown adipose tissue

BACKGROUND/AIMS

Retinoic acid (RA), the carboxylic acid form of vitamin A, through the activation of cognate receptors, stimulates the transcription of the gene encoding uncoupling protein 1 (UCP1), which is critical to brown adipose tissue (BAT) thermogenesis. RA was previously shown to down-regulate the steady state levels of retinoic acid receptor alpha (RARalpha) and retinoid X receptor alpha (RXRalpha) in primary brown adipocytes differentiated in culture. Our aim was to study the impact of RA-treatment and vitamin A status on the expression levels of these receptors in vivo.

METHODS

Three-week-old mice were fed standard chow or a vitamin A deficient diet for 10 weeks, after which animals on both diets were treated with all-trans RA (ATRA, 100 mg x Kg(-1) day(-1)) or vehicle during 4 days. Levels of UCP1, RARalpha and RXRalpha in BAT were determined by immunoblotting.

RESULTS

ATRA-treatment resulted in a reduction of the specific RARalpha and especially RXRalpha content in BAT that paralleled the induction of UCP1 appearance in the tissue. RARalpha and RXRalpha levels per gram of BAT were reduced in mice chronically fed the vitamin A-deficient diet.

CONCLUSION

RA modulates the expression of cognate receptors in BAT, suggesting auto regulation of the retinoid effect on the thermogenic system.

Tissue-specific regulation of leptin expression and secretion by all-trans retinoic acid

In addition to hormones and the sympathetic nervous system, specific nutrients may regulate leptin mRNA expression and adipose tissue metabolism. However, little is known regarding the effect of nutrients on leptin mRNA expression. Retinoic acid (RA) is a ligand of some nuclear receptors and previous reports have demonstrated contradictory effects on plasma leptin levels. Thus, we examined the effect of RA on expression of leptin in adipocytes of murine and human origin. After 48 h incubation of murine 3T3-L1 adipocytes with 1 and 10 microM all-trans RA, the expression of leptin mRNA was reduced by 56% and 65%, respectively, whereas the secretion of leptin was reduced by 38% and 77%, respectively. In human adipose tissue explants, 1 microM all-trans RA reduced leptin mRNA expression levels by 55% and leptin secretion by 25% after 24 h incubation. We observed an increased mRNA expression level of the transcription factors peroxisomal proliferator activated receptor gamma (PPAR gamma), retinoid X receptor alpha (RXR alpha), and RA receptor alpha (RAR alpha) in 3T3-L1 cells, whereas the mRNA level of these transcription factors was unchanged in human adipose tissue explants after incubation with RA. In two other leptin-expressing cell systems, the human placental trophoblast cell line BeWo and normal human primary osteoblasts, there was no effect of RA on leptin mRNA expression, but leptin secretion was reduced by 64% after 24 h incubation with 10 microM all-trans RA in BeWo cells. In conclusion, all-trans RA reduced both expression and secretion of leptin in human and rodent adipose tissue. In human BeWo cells or primary osteoblasts, leptin mRNA expression levels was not changed by all-trans RA, indicating a tissue-specific regulation of leptin mRNA expression by all-trans RA.

Effects of retinoic acid administration and dietary vitamin A supplementation on leptin expression in mice: lack of correlation with changes of adipose tissue mass and food intake

Retinoic acid (RA) administration and chronic vitamin A supplementation were reported to inhibit adipose tissue leptin expression in rodents, but the impact of this effect on food intake and its relationship with changes of body adiposity was not analyzed. Here, we have studied the effects of RA administration at three different doses on body weight, adipose tissue mass, food intake, adipose tissue leptin expression and circulating leptin levels in NMRI mice; the effects of chronic vitamin A supplementation with a 40-fold excess retinyl palmitate on the same parameters in NMRI and C57BL/6J mice; and the effects of RA and retinoid receptors agonists on leptin expression in brown and white adipocyte cell model systems. The results show that vitamin A down-regulates leptin expression in white and brown adipose tissue and circulating leptin levels independently of changes of adipose tissue mass and, for the first time to our knowledge, that this effect does not correlate with increased food intake. They also demonstrate a direct inhibitory effect of RA on leptin expression in both white and brown adipocyte cell cultures, and constitute first proof of the involvement of both RA receptors (RARs) and rexinoid receptors (RXRs) in this effect. Reduction of leptin levels by specific nutrients is of potential interest from a clinical point of view.

Modulation of resistin expression by retinoic acid and vitamin A status

This work identifies retinoic acid (RA), the acid form of vitamin A, as a signal that inhibits the expression of resistin, an adipocyte-secreted protein previously proposed to act as an inhibitor of adipocyte differentiation and as a systemic insulin resistance factor. Both 9-cis and all-trans RA reduced resistin mRNA levels in white and brown adipocyte cell model systems; the effect was time- and dose-dependent, was followed by a reduced secretion of resistin, and was reproduced by selective agonists of both RA receptors and rexinoid receptors. Association of CCAAT/enhancer-binding protein alpha (a positive regulator of the resistin gene) and its coactivators p300, cAMP response element-binding protein binding protein, and retinoblastoma protein with the resistin gene promoter was reduced in RA-treated adipocytes. RA administration to normal mice resulted in reduced resistin mRNA levels in brown and white adipose tissues, reduced circulating resistin levels, reduced body weight, and improved glucose tolerance. Resistin expression was also downregulated after dietary vitamin A supplementation in mice. The results raise the possibility that vitamin A status may contribute to modulate systemic functions through effects on the production of adipocyte-derived protein signals.

Up-regulation of muscle uncoupling protein 3 gene expression in mice following high fat diet, dietary vitamin A supplementation and acute retinoic acid-treatment

OBJECTIVE

To analyse the impact of vitamin A supplementation of both a normal fat (NF) diet and a high fat (HF) diet and of acute retinoic acid (RA)-treatment on the expression of uncoupling protein 3 (UCP3) in mice.

DESIGN

C57BL/6J mice were fed for 18 weeks a NF or a HF diet (10 and 45 energy% as fat, respectively), both with the normal vitamin A content or an excess vitamin A (8 mg and 320 mg retinyl palmitate/kg diet, respectively). Body weight and energy intake were recorded periodically. UCP3 mRNA and UCP3 protein levels in skeletal muscle (soleus/gastrocnemius) were analysed, as well as UCP1, UCP2 and UCP3 mRNA levels in interscapular brown adipose tissue (BAT), and UCP2 mRNA, UCP2 protein and leptin mRNA levels in white adipose tissue (WAT) depots. The effect of acute RA-treatment (100 mg/kg/day, 4 days) on UCP3 mRNA levels in skeletal muscle and BAT of NMRI mice was also assessed.

RESULTS

Vitamin A supplementation of a NF diet led to increased levels of UCP3 mRNA and UCP3 protein in muscle, UCP1 mRNA in BAT, and UCP2 mRNA in inguinal WAT, but had no impact on body weight or adiposity of B6 mice. HF diet promoted obesity and increased levels of UCP3 mRNA and UCP3 protein in skeletal muscle, and of the mRNAs for all three UCPs in BAT. Supplementing the HF diet with vitamin A had little effect on the final obesity reached and did not lead to further increases of muscle UCP3 mRNA nor BAT UCP1 mRNA over the levels achieved with the non-supplemented HF diet. Adipose leptin mRNA levels were down regulated after vitamin A supplementation, independently of the fat content of the diet. Up-regulation of muscle, but not BAT, UCP3 mRNA levels was also found after acute RA-treatment in NMRI mice.

CONCLUSION

The results provide evidence of a stimulatory effect of retinoids on muscle UCP3 expression in vivo, and a differential retinoid-regulation of the UCP3 gene in muscle and BAT.

Regulation of adiponectin and leptin gene expression in white and brown adipose tissues: influence of beta3-adrenergic agonists, retinoic acid, leptin and fasting

Circulating adiponectin levels fall whereas leptin levels rise with obesity, suggesting that regulation of these two adipocyte-derived hormones may be simultaneously influenced by common obesity-related factors. We examined adiponectin mRNA levels in WAT and in some instances, brown adipose tissue (BAT) following fasting and refeeding, acute and chronic administration of a beta(3)-adrenergic agonist, acute treatment with retinoic acid (RA) and a glucocorticoid, and following chronic infusion of leptin and compared the expression of adiponectin to that of leptin in each circumstance. Serum concentrations of adiponectin were also reported for most of the treatments. Fasting diminished and refeeding reversed both adiponectin and leptin gene expression. Peripheral injection of the beta(3)-adrenergic agonist, CL316,243, suppressed both leptin and adiponectin expression in WAT. A small but significant reduction in adiponectin expression in BAT was also observed following this treatment. Although CL316,23 lowered serum leptin levels markedly, it did not affect serum adiponectin levels. A chronic 7-day infustion of CL316,243 resulted in an elevation of adiponectin expression in WAT and serum concentrations in contrast to suppressions in both mRNA and serum levels of leptin by a similar treatment as previously reported. Chronic administration of leptin did not alter adiponectin synthesis in WAT compared to controls, but prevented the reduction in adiponectin synthesis associated with pair feeding. Food restriction through pair feeding also diminished adiponectin expression in BAT. Collectively, although leptin and adiponectin are inversely correlated with obesity, leptin does not appear to participate directly in adiponectin synthesis. The short-term regulation of the two adipokine expression in WAT is somewhat similar, perhaps subjective to common control of energy balance. The long-term regulation of adiponectin expression in WAT appears to be the opposite of that of leptin and may be more sensitive to changes in adiposity or insulin sensitivity.

Retinoic acid negatively regulates neuropeptide Y expression in human neuroblastoma cells

Retinoids are involved in the regulation of development and differentiation in many tissues, including the nervous system, where they have been associated with some neurotransmitter systems. In the present study, we evaluated the effects of all-trans retinoic acid (RA) on the biosynthesis and secretion of neuropeptide Y (NPY), a widely expressed neuroregulatory peptide. The SH-SY5Y human neuroblastoma cell line has been used as the in vitro model system. Treatment with 10 microM RA induced a marked decrease in NPY gene expression after as little as 3-6 h of incubation and resulted in its almost complete suppression at 12-24 h and after a 6-day differentiating treatment. The NPY content in cell extracts and the NPY secreted and accumulated in the culture medium were also reduced by exposure to 10 microM RA at 12 and 24 h and at 6 days. Moreover, RA treatment for 6 days, but not for 24 h, resulted in a marked stimulation of proNPY processing to mature NPY. The presence of negative retinoic acid-response elements in the human NPY promoter (up to -1078 bp) was excluded by a computer search. When SH-SY5Y cells were treated simultaneously with 20 nM TPA and 10 microM RA for 24 h, the marked stimulatory effect of TPA alone was completely suppressed. These observations suggest that the expression of NPY in SH-SY5Y human neuroblastoma cells is negatively regulated by RA at the level of gene expression, probably by mechanisms involving the interaction of activated RARs with transcription factors (such as AP-1).

Leptin induction of UCP1 gene expression is dependent on sympathetic innervation

We previously demonstrated that leptin increases uncoupling protein 1 (UCP1) and lipoprotein lipase (LPL) gene expression in brown adipose tissue (BAT) of rats. To determine whether the induction of these transcripts is dependent on sympathetic innervation of BAT, we unilaterally surgically denervated interscapular BAT in both pair-fed and leptin (0.9 mg/day by infusion)-treated rats. In pair-fed rats, the level of UCP1 mRNA in the denervated BAT pad was 30-47% less than in the innervated pad. In the intact BAT pad, leptin administration increased UCP1 mRNA levels by nearly 2.5-fold compared with pair-fed rats. In contrast, in the denervated BAT pad, there was no increase in UCP1 gene expression. When LPL mRNA was examined in pair-fed rats, there was no difference between innervated and denervated BAT pads. With leptin administration, LPL gene expression increased by 75% in both the innervated and denervated BAT pads. beta3-Adrenergic receptor mRNA was unaffected by either denervation or leptin, whereas uncoupling protein 2 mRNA levels were increased in epididymal white adipose tissue (WAT) but not in perirenal WAT. CGP-12177, a specific beta3-adrenergic receptor agonist, induced nearly a fourfold increase in UCP1 and a twofold increase in LPL gene expression in both the innervated and denervated BAT pads. These data indicate that the leptin induction of UCP1 gene expression in BAT is dependent on sympathetic innervation but that the leptin induction of LPL gene expression is not.