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

The role of leptin and ghrelin in the regulation of appetite in obesity

Leptin and ghrelin are two key hormones that play opposing roles in the regulation of appetite and energy balance. Ghrelin stimulates appetite and food intake following binding to receptors and the subsequent activation of orexigenic neurons in the arcuate nucleus. Leptin, conversely, has been demonstrated to suppress appetite and reduce food intake. This occurs through the inhibition of ghrelin-activated neurons, while simultaneously activating those that promote satiety and increase energy expenditure. A lack of biological response despite elevated leptin levels, which is known as leptin resistance, is observed in individuals with excess body weight and represents a significant challenge. As the dysregulation of ghrelin and leptin signalling has been linked to the development of obesity and other metabolic disorders, an in-depth understanding of the genetic determinants affecting these two hormones may facilitate a more comprehensive grasp of the intricate interactions that underpin the pathogenesis of obesity.

Vitamin A-Enriched Diet Increases Urothelial Cell Proliferation by Upregulating Itga3 and Areg After Cyclophosphamide-Induced Injury in Mice

Vitamin A (VitA) is an essential nutrient, affecting many cell functions, such as proliferation, apoptosis, and differentiation, all of which are important for the regeneration of various tissues. In this study, we investigated the effects of a VitA-enriched diet on the regeneration of the urothelium of the urinary bladder in mice after cyclophosphamide (CP)-induced injury. Female mice were fed VitA-enriched and normal diet for 1 week before receiving an intraperitoneal injection of CP (150 mg/kg). Urinary bladders were removed 1 and 3 days after CP. On Day 1, RNA sequencing showed that VitA upregulated two Kyoto Encyclopedia of Genes and Genomes (KEGG) signaling pathways: the cell cycle and the PI3K-Akt pathway. This was confirmed by qPCR, which showed significantly increased expression of the Itga3 and Areg genes. In addition, the effect of VitA on the proliferation of urothelial cells was analyzed by immunohistochemistry of Ki-67, which confirmed an increased proliferation rate. No significant effects of the VitA-enriched diet were observed on the expression of apoptosis-related genes and on differentiation-related markers of superficial urothelial cells. Our results suggest that a VitA-enriched diet improves early urothelial regeneration after CP-induced injury by promoting cell proliferation.

β-Carotene accelerates the resolution of atherosclerosis in mice

β-Carotene oxygenase 1 (BCO1) catalyzes the cleavage of β-carotene to form vitamin A. Besides its role in vision, vitamin A regulates the expression of genes involved in lipid metabolism and immune cell differentiation. BCO1 activity is associated with the reduction of plasma cholesterol in humans and mice, while dietary β-carotene reduces hepatic lipid secretion and delays atherosclerosis progression in various experimental models. Here we show that β-carotene also accelerates atherosclerosis resolution in two independent murine models, independently of changes in body weight gain or plasma lipid profile. Experiments in Bco1-/- mice implicate vitamin A production in the effects of β-carotene on atherosclerosis resolution. To explore the direct implication of dietary β-carotene on regulatory T cells (Tregs) differentiation, we utilized anti-CD25 monoclonal antibody infusions. Our data show that β-carotene favors Treg expansion in the plaque, and that the partial inhibition of Tregs mitigates the effect of β-carotene on atherosclerosis resolution. Our data highlight the potential of β-carotene and BCO1 activity in the resolution of atherosclerotic cardiovascular disease.

β-carotene accelerates the resolution of atherosclerosis in mice

β-carotene oxygenase 1 (BCO1) catalyzes the cleavage of β-carotene to form vitamin A. Besides its role in vision, vitamin A regulates the expression of genes involved in lipid metabolism and immune cell differentiation. BCO1 activity is associated with the reduction of plasma cholesterol in humans and mice, while dietary β-carotene reduces hepatic lipid secretion and delays atherosclerosis progression in various experimental models. Here we show that β-carotene also accelerates atherosclerosis resolution in two independent murine models, independently of changes in body weight gain or plasma lipid profile. Experiments in Bco1-/- mice implicate vitamin A production in the effects of β-carotene on atherosclerosis resolution. To explore the direct implication of dietary β-carotene on regulatory T cells (Tregs) differentiation, we utilized anti-CD25 monoclonal antibody infusions. Our data show that β-carotene favors Treg expansion in the plaque, and that the partial inhibition of Tregs mitigates the effect of β-carotene on atherosclerosis resolution. Our data highlight the potential of β-carotene and BCO1 activity in the resolution of atherosclerotic cardiovascular disease.

The Role of Organokines in Obesity and Type 2 Diabetes and Their Functions as Molecular Transducers of Nutrition and Exercise

Maintaining systemic homeostasis requires the coordination of different organs and tissues in the body. Our bodies rely on complex inter-organ communications to adapt to perturbations or changes in metabolic homeostasis. Consequently, the liver, muscle, and adipose tissues produce and secrete specific organokines such as hepatokines, myokines, and adipokines in response to nutritional and environmental stimuli. Emerging evidence suggests that dysregulation of the interplay of organokines between organs is associated with the pathophysiology of obesity and type 2 diabetes (T2D). Strategies aimed at remodeling organokines may be effective therapeutic interventions. Diet modification and exercise have been established as the first-line therapeutic intervention to prevent or treat metabolic diseases. This review summarizes the current knowledge on organokines secreted by the liver, muscle, and adipose tissues in obesity and T2D. Additionally, we highlighted the effects of diet/nutrition and exercise on the remodeling of organokines in obesity and T2D. Specifically, we investigated the ameliorative effects of caloric restriction, selective nutrients including ω3 PUFAs, selenium, vitamins, and metabolites of vitamins, and acute/chronic exercise on the dysregulation of organokines in obesity and T2D. Finally, this study dissected the underlying molecular mechanisms by which nutrition and exercise regulate the expression and secretion of organokines in specific tissues.

Short communication: In ovo injection of all-trans retinoic acid causes adipocyte hypertrophy in embryos but lost its effect in posthatch chickens

The regulation of adipose deposition in broiler chickens is an important factor for production efficiency to poultry producers and health concerns to customers. Although vitamin A and its metabolite [all-trans retinoic acid (atRA)] have been used for studies on adipogenesis in mammals and avian, effects of embryonic atRA on adipose development in embryonic (E) and posthatch (D) ages in broiler chickens have not been studied yet. Different concentrations of atRA (0 M-2 μM) were injected in broiler eggs at E10, and adipose tissues were sampled at E16. Percentages of adipose tissues in chicken embryos were significantly increased in the group injected with 500 nM of atRA compared to the 0 M group (P < 0.05). In addition, the adipocyte cross-sectional area (CSA) was significantly greater by in ovo injection of 500 nM atRA compared to the injection of 0 M (P < 0.01). Moreover, in ovo atRA-injected embryos were hatched and BWs were measured at D0, D7, and D14. BWs were not different from those of the 0 M group. Percentages of adipose tissues and CSA of the in ovo atRA-injected group (500 nM) were not different from those of the 0 M group at D14. Taken together, the current study clearly showed that in ovo injection of atRA promoted adipose deposition with hypertrophy during embryonic development, but its effects were not maintained in early posthatch age in broiler chickens, implying that embryonic atRA has an important role in the regulation of adipose development in chicken embryos.

Vitamin A regulates tissue-specific organ remodeling in diet-induced obesity independent of mitochondrial function

Background

Perturbed mitochondrial energetics and vitamin A (VitA) metabolism are associated with the pathogenesis of diet-induced obesity (DIO) and type 2 diabetes (T2D).

Methods

To test the hypothesis that VitA regulates tissue-specific mitochondrial energetics and adverse organ remodeling in DIO, we utilized a murine model of impaired VitA availability and high fat diet (HFD) feeding. Mitochondrial respiratory capacity and organ remodeling were assessed in liver, skeletal muscle, and kidney tissue, which are organs affected by T2D-associated complications and are critical for the pathogenesis of T2D.

Results

In liver, VitA had no impact on maximal ADP-stimulated mitochondrial respiratory capacity (VADP) following HFD feeding with palmitoyl-carnitine and pyruvate each combined with malate as substrates. Interestingly, histopathological and gene expression analyses revealed that VitA mediates steatosis and adverse remodeling in DIO. In skeletal muscle, VitA did not affect VADP following HFD feeding. No morphological differences were detected between groups. In kidney, VADP was not different between groups with both combinations of substrates and VitA transduced the pro-fibrotic transcriptional response following HFD feeding.

Conclusion

The present study identifies an unexpected and tissue-specific role for VitA in DIO that regulates the pro-fibrotic transcriptional response and that results in organ damage independent of changes in mitochondrial energetics.

TRPC3 Regulates Islet Beta-Cell Insulin Secretion

Insulin release is tightly controlled by glucose-stimulated calcium (GSCa) through hitherto equivocal pathways. This study investigates TRPC3, a non-selective cation channel, as a critical regulator of insulin secretion and glucose control. TRPC3's involvement in glucose-stimulated insulin secretion (GSIS) is studied in human and animal islets. TRPC3-dependent in vivo insulin secretion is investigated using pharmacological tools and Trpc3-/- mice. TRPC3's involvement in islet glucose uptake and GSCa is explored using fluorescent glucose analogue 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxy-D-glucose and calcium imaging. TRPC3 modulation by a small-molecule activator, GSK1702934A, is evaluated in type 2 diabetic mice. TRPC3 is functionally expressed in human and mouse islet beta cells. TRPC3-controlled insulin secretion is KATP -independent and primarily mediated by diacylglycerol channel regulation of the cytosolic calcium oscillations following glucose stimulation. Conversely, glucose uptake in islets is independent of TRPC3. TRPC3 pharmacologic inhibition and knockout in mice lead to defective insulin secretion and glucose intolerance. Subsequently, TRPC3 activation through targeted small-molecule enhances insulin secretion and alleviates diabetes hallmarks in animals. This study imputes a function for TRPC3 at the onset of GSIS. These insights strengthen one's knowledge of insulin secretion physiology and set forth the TRPC3 channel as an appealing candidate for drug development in the treatment of diabetes.

Carrot Juice Consumption Reduces High Fructose-Induced Adiposity in Rats and Body Weight and BMI in Type 2 Diabetic Subjects

Nutritional intervention is a key strategy in the control and management of non-communicable diseases. Here, initially, we evaluated the effects of carrot juice (CJ) on some of the physical and biochemical parameters in rats fed with high-fructose diet, then in type 2 diabetic subjects. For the animal study, weanling male Wistar rats were given control (n = 6) or high fructose (HFr; n = 24) diet for 8 weeks. Then, the HFr group rats were subdivided into 4 groups (n = 6 in each) and continued either on HFr diet or shifted to control diet, with or without CJ (0.3 mg β-carotene) ingestion orally for 8 weeks. At the end, the ingestion of CJ reversed the HFr-induced adiposity (23 ± 1.6 vs 18 ± 1.1, P = .038), hypertriglyceridemia (182 ± 18.2 vs 90 ± 10.5 mg/dL, P<0.001), and hyperinsulinemia (81 ± 14.7 vs 40 ± 7.5 µU/mL, P = .014), while increased the retinol levels in liver (240 ± 38.4 vs 492 ± 61.2 µg/g, P = .002) and adipose tissue (1.8 ± 0.09 vs 2.5 ± 0.18 µg/g, P = .026). On the other hand, in the diabetic subjects (7 males and females each, n = 14) compared to their baseline, the daily consumption of 50 mL CJ (~2400 µg β-carotene) for 6 weeks significantly reduced the body weight (69.4 ± 4.13 vs 69.0 ± 4.09 kg, P = .014), BMI (27.4 ± 1.07 vs 27.2 ± 1.06 kg/m², P = .007), and fat% (33.4 ± 1.87 vs 31.9 ± 2.13, P = .029) with an increase in plasma β-carotene levels (0.21 ± 0.045 vs 0.45 ± 0.089 µmol/L, P = .044). Although CJ increased the glucose (145 ± 10.4 vs 165 ± 11.4 mg/dL, P = .039), insulin, and glycated hemoglobin levels remained unaltered. In conclusion, the consumption of carrot juice reversed the HFr-induced metabolic abnormalities in a rat model and decreased body weight and BMI of diabetic subjects.

Hypertrophy of Adipose Tissues in Quail Embryos by in ovo Injection of All-Trans Retinoic Acid

Excessive adipose accretion causes health issues in humans and decreases feed efficiency in poultry. Although vitamin A has been known to be involved in adipogenesis, effects of all-trans retinoic acid (atRA), as a metabolite of vitamin A, on embryonic adipose development have not been studied yet. Avian embryos are developing in confined egg environments, which can be directly modified to study effects of nutrients on embryonic adipogenesis. With the use of quail embryos, different concentrations of atRA (0 M to 10 μM) were injected in ovo at embryonic day (E) 9, and adipose tissues were sampled at E14. Percentages of fat pad weights in embryo weights were significantly increased in the group injected with 300 nM of atRA. Also, among three injection time points, E5, E7, or E9, E7 showed the most significant increase in weight and percentage of inguinal fat at E14. Injection of atRA at E7 increased fat cell size in E14 embryos with up-regulation of pro-adipogenic marker genes (Pparγ and Fabp4) and down-regulation of a preadipocyte marker gene (Dlk1) in adipose tissues. These data demonstrate that atRA promotes hypertrophic fat accretion in quail embryos, implying important roles of atRA in embryonic development of adipose tissues.

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.

Low intakes of vitamins C and A are associated with obesity in early adulthood

To evaluate the relationships between the intake of individual antioxidants as well as the dietary antioxidant quality score and obesity-related measures. A cross-sectional study was conducted on 562 young adults. Fat mass, fat-mass percentage, and fat-free mass were measured using a body composition analyzer. The intake of antioxidant nutrients including vitamins C, E, and A, selenium, zinc, and magnesium were calculated based on a 72-hour diet recall interview. We observed significant differences in the vitamin C (88.6 ± 72.6 mg/day vs. 70.7 ± 60.6 mg/day, p = 0.010), vitamin A (635.8 ± 519.8 μg/day vs. 492.6 ± 318.9 μg/day, p = 0.014), and selenium (135.3 ± 88.7 μg/day vs. 139.3 ± 79.3 μg/day, p = 0.034) intake between normal-weight and overweight or obese young adults. When the Dietary Antioxidant Quality Score (DAQS) was analyzed, there were no significant differences between normal-weight versus overweight or obese young adults after adjusting for confounders. Logistic regression analysis revealed that vitamin C intake (odds ratio (OR) 0.995, 95% CI 0.992-0.999, p = 0.013) and vitamin A intake (OR 0.999, 95% CI 0.999-1.000, p = 0.016) were independent predictors of overweight/obesity after adjusting for age, sex and energy intake. In contrast, a higher selenium intake was associated with an increased risk of overweight/obesity (OR 1.003, 95% CI 1.000-1.006, p = 0.034). Future longitudinal investigations of dietary antioxidant intake in relation to the development of obesity would be of interest to better understand the effect of dietary antioxidants on obesity.

Vitamin A Rich Diet Diminishes Early Urothelial Carcinogenesis by Altering Retinoic Acid Signaling

Urinary bladder cancer is one of the leading malignancies worldwide, with the highest recurrence rates. A diet rich in vitamin A has proven to lower the risk of cancer, yet the molecular mechanisms underlying this effect are unknown. We found that vitamin A decreased urothelial atypia and apoptosis during early bladder carcinogenesis induced by N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN). Vitamin A did not alter urothelial cell desquamation, differentiation, or proliferation rate. Genes like Wnt5a, involved in retinoic acid signaling, and transcription factors Pparg, Ppara, Rxra, and Hoxa5 were downregulated, while Sox9 and Stra6 were upregulated in early urothelial carcinogenesis. When a vitamin A rich diet was provided during BBN treatment, none of these genes was up- or downregulated; only Lrat and Neurod1 were upregulated. The lecithin retinol acyltransferase (LRAT) enzyme that produces all-trans retinyl esters was translocated from the cytoplasm to the nuclei in urothelial cells as a consequence of BBN treatment regardless of vitamin A rich diet. A vitamin A-rich diet altered retinoic acid signaling, decreased atypia and apoptosis of urothelial cells, and consequently diminished early urothelial carcinogenesis.

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.

Long-term dietary intake of excessive vitamin A impairs spermatogenesis in mice

Vitamin A and its derivatives contribute to many physiological processes, including vision, neural differentiation, and reproduction. Vitamin A deficiency causes early cessation of spermatogenesis, characterized by a marked depletion of germ cells. However, there has been no clear understanding about the role of chronic intake of vitamin A excess (VAE) in spermatogenesis. The objective of this study was to investigate whether chronic intake of VAE diet causes arrest of spermatogenesis. To examine the effects of VAE on spermatogenesis, we used ICR male mice fed with control (AIN-93G purified diet: 4 IU/g) diet or VAE (modified AIN-93G diet with VAE: 1,000 IU/g) diet for 7 weeks (from 3 to 10 weeks of age). At 10 weeks of age, the retinol concentration in the testes of VAE mice was significantly higher than that of control mice. Testicular cross sections from control mice contained a normal array of germ cells, while the seminiferous tubules from VAE mice exhibited varying degrees of testicular degeneration. Daily sperm production in VAE testes was dramatically decreased compared to that in control testes. Sperm viability, motility, and morphology were also impaired in VAE mice. Furthermore, we examined the effects of VAE on the expression of genes involved in retinoid signaling and spermatogenesis to determine the underlying molecular mechanisms. Therefore, we are the first to present results describing the long-term dietary intake of VAE impairs spermatogenesis using a mouse model.

All-trans retinoic acid inhibits lipopolysaccharide-induced inflammatory responses in bovine adipocytes via TGFβ1/Smad3 signaling pathway

BACKGROUND

Dairy cows with metabolic disorder in peripartal period display high inflammatory levels. Adipose tissue is a major endocrine organ, which is closely related to systemic inflammation. Retinoic acid (RA), an active metabolite of vitamin A, has shown potential therapeutic immunomodulatory properties. The objective of the study was to examine the effect of all-trans-RA (ATRA), the biologically most active metabolite of vitamin A, on lipopolysaccharide (LPS)-induced bovine adipocytes inflammatory responses and elucidate the underlying mechanisms. Primary cultured bovine adipocytes were treated with ATRA in the presence or absence of LPS. The treated cells were examined for the inflammatory responses and the activity of transforming growth factor beta 1 (TGFβ1) /Smad3 signaling pathway.

RESULTS

LPS treatment significantly decreased the expression levels of TGFβ1/Smad3 components and increased the content of pro-inflammatory cytokines. Treatment with ATRA could over-activate TGFβ1/Smad3 signaling pathway in bovine adipocytes and reversed the over-production of pro-inflammatory cytokines and inhibition of anti-inflammatory cytokines induced by LPS. Importantly, inhibition of TGFβ1/Smad3 signaling diminished the effects of ATRA on suppressing the proinflammatory responses induced by LPS. Furthermore, activation of TGFβ1/Smad3 signaling further extended the effects of ATRA on suppressing the proinflammatory responses on LPS stimulation.

CONCLUSION

In conclusion, ATRA stimulates TGFβ1/Smad3 signaling pathway and further suppresses bovine adipocytes inflammatory responses induced by LPS.

All-trans retinoic acid induces mitochondria-mediated apoptosis of human adipose-derived stem cells and affects the balance of the adipogenic differentiation

The all-trans-retinoic acid (ATRA) is the most active form of vitamin A that helps to regulate the proliferation, differentiation and apoptosis of several types of cells, mainly the adipocytes, and causes weight loss through the reduction of adipogenesis and lipogenesis. In this present study we demonstrated that ATRA concentrations of 20.75, 50 and 100 μM decreased the cell viability in vitro of human adipose-derived stem cells (ADSCs), and in ADSCs during adipogenic differentiation. The cells cycle assessment showed that ATRA increased the cell frequency in Sub-G1 at 4.02x and decreased it in G1 in 2.54x. Moreover, the membrane integrity loss increased by 4.66x and apoptosis increased by 33.56x in ATRA-treated cultures. The gene expression assay suggested that the treatment using ATRA leads to mitochondrial membrane permeabilization and to consequent release of proapoptotic BAK and BAX molecules (increased expression 5.5 and 5.4x respectively); in addition, it increased CASP3 expression (by 8.8x). These events may activate the Bcl-2 (4.1x increase), GADD45 (increase 3.14x) and PPAR-γ (16x increase) expressions, as well as, to reduce the p53 (by -1.38x) expression; therefore, these events should be further mediated by increased RARα expression (by 3.8x). The results evidenced that ATRA may be a good proposal for mesotherapy strategies in order to control the development of subcutaneous adipose tissue; as this tissue have a higher development in some specific areas and ATRA interferes not only in the ADSCs differentiation but also in the apoptosis of ADSCs, preadipocytes and adipocytes.

A simple and robust quantitative analysis of retinol and retinyl palmitate using a liquid chromatographic isocratic method

Vitamin A is a vital nutritional substances that regulates biological activities including development, but is also associated with disease onset. The extent of vitamin A intake influences the retinoid content in the liver, the most important organ for the storage of vitamin A. Measurement of endogenous retinoid in biological samples is important to understand retinoid homeostasis. Here we present a reliable, highly sensitive, and robust method for the quantification of retinol and retinyl palmitate using a reverse-phase HPLC/ UV isocratic method. We determined the impact of chronic dietary vitamin A on retinoid levels in livers of mice fed an AIN-93G semi-purified diet (4 IU/g) compared with an excess vitamin A diet (1000 IU/g) over a period from birth to 10 weeks of age. Coefficients of variation for intra-assays for both retinoids were less than 5%, suggesting a higher reproducibility than any other HPLC/UV gradient method. Limits of detection and quantification for retinol were 0.08 pmol, and 0.27 pmol, respectively, which are remarkably higher than previous results. Supplementation with higher doses of vitamin A over the study period significantly increased liver retinol and retinyl palmitate concentrations in adult mice. The assays described here provide a sensitive and rigorous quantification of endogenous retinol and retinyl palmitate, which can be used to help determine retinoid homeostasis in disease states, such as toxic hepatitis and liver cancer.

Whole Blood RNA as a Source of Transcript-Based Nutrition- and Metabolic Health-Related Biomarkers

Blood cells are receiving an increasing attention as an easily accessible source of transcript-based biomarkers. We studied the feasibility of using mouse whole blood RNA in this context. Several paradigms were studied: (i) metabolism-related transcripts known to be affected in rat tissues and peripheral blood mononuclear cells (PBMC) by fasting and upon the development of high fat diet (HFD)-induced overweight were assessed in whole blood RNA of fasted rats and mice and of HFD-fed mice; (ii) retinoic acid (RA)-responsive genes in tissues were assessed in whole blood RNA of control and RA-treated mice; (iii) lipid metabolism-related transcripts previously identified in PBMC as potential biomarkers of metabolic health in a rat model were assessed in whole blood in an independent model, namely retinoblastoma haploinsufficient (Rb^+/-) mice. Blood was collected and stored in RNAlater^® at -80°C until analysis of selected transcripts by real-time RT-PCR. Comparable changes with fasting were detected in the expression of lipid metabolism-related genes when RNA from either PBMC or whole blood of rats or mice was used. HFD-induced excess body weight and fat mass associated with expected changes in the expression of metabolism-related genes in whole blood of mice. Changes in gene expression in whole blood of RA-treated mice reproduced known transcriptional actions of RA in hepatocytes and adipocytes. Reduced expression of Fasn, Lrp1, Rxrb and Sorl1 could be validated as early biomarkers of metabolic health in young Rb^+/- mice using whole blood RNA. Altogether, these results support the use of whole blood RNA in studies aimed at identifying blood transcript-based biomarkers of nutritional/metabolic status or metabolic health. Results also support reduced expression of Fasn, Lrp1, Rxrb and Sorl1 in blood cells at young age as potential biomarkers of metabolic robustness.

Leptin Effect on Acetylation and Phosphorylation of Pgc1α in Muscle Cells Associated With Ampk and Akt Activation in High-Glucose Medium

Leptin is crucial in energy metabolism, including muscle regulation. Peroxisome proliferator activated receptor gamma co-activator 1α (PGC1α) orchestrates energy metabolism and is tightly controlled by post-translational covalent modifications such as phosphorylation and acetylation. We aimed to further the knowledge of PGC1α control by leptin (at physiological levels) in muscle cells by time-sequentially analysing the activation of AMP activated protein kinase (AMPK), P38 mitogen-activated protein kinase (P38 MAPK) and Akt (Protein kinase B)--all known to phosphorylate PGC1α and to be involved in the regulation of its acetylation status--in C2C12 myotubes placed in a high-glucose serum-free medium. We also studied the protein levels of PGC1α, Sirtuin 1, adiponectin, COX IV, mitofusin 2 (Mfn2), and pyruvate dehydrogenase kinase 4 (PDK4). Our main findings suggest an important role of leptin regulating AMPK and Akt phosphorylation, Mfn2 induction and PGC1α acetylation status, with the novelty that the latter in transitorily increased in response to leptin, an effect dependent, at least in part, on AMPK regulation. These post-translational reversible changes in PGC1α in response to leptin, especially the increase in acetylation status, may be related to the physiological role of the hormone in modulating muscle cell response to the physiological/nutritional status.

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.

Inhibition of ASCT2 is essential in all-trans retinoic acid-induced reduction of adipogenesis in 3T3-L1 cells

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Asct2, an amino acid transporter, could be a target for obesity prevention and treatment.

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All-trans retinoic acid suppresses upregulation of Asct2 during adipogenesis of 3T3-L1 cells.

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The Asct2 inhibitor, l-γ-glutamyl-p-nitroanilide, suppresses adipogenesis at early time points.

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Treatment with l-γ-glutamyl-p-nitroanilide suppresses adipogenesis more effectively than l-glutamine-deficient conditions.

Vitamin A has preventive effects on obesity. All-trans retinoic acid (ATRA), the active form of vitamin A, inhibits lipid accumulation in 3T3-L1 cells in an experimental adipogenesis model. We found that ATRA suppressed up-regulation of the amino acid transporter, Asct2, in adipogenerating 3T3-L1 cells. We observed that Asct2 was up-regulated at 1 day after adipogenesis stimuli. The Asct2 inhibitor l-γ-glutamyl-p-nitroanilide (GPNA) decreased lipid accumulation. Glutamine-free conditions also suppressed adipogenesis. Suppression of adipogenesis by ATRA may be through Asct2 reduction. These results indicate that Asct2 could be a target for obesity prevention and treatment.

All-trans retinoic acid induces oxidative phosphorylation and mitochondria biogenesis in adipocytes

A positive effect of all-trans retinoic acid (ATRA) on white adipose tissue (WAT) oxidative and thermogenic capacity has been described and linked to an in vivo fat-lowering effect of ATRA in mice. However, little is known about the effects of ATRA on mitochondria in white fat. Our objective has been to characterize the effect of ATRA on mitochondria biogenesis and oxidative phosphorylation (OXPHOS) capacity in mature white adipocytes. Transcriptome analysis, oxygraphy, analysis of mitochondrial DNA (mtDNA), and flow cytometry-based analysis of mitochondria density were performed in mature 3T3-L1 adipocytes after 24 h incubation with ATRA (2 µM) or vehicle. Selected genes linked to mitochondria biogenesis and function and mitochondria immunostaining were analyzed in WAT tissues of ATRA-treated as compared with vehicle-treated mice. ATRA upregulated the expression of a large set of genes linked to mtDNA replication and transcription, mitochondrial biogenesis, and OXPHOS in adipocytes, as indicated by transcriptome analysis. Oxygen consumption rate, mtDNA content, and staining of mitochondria were increased in the ATRA-treated adipocytes. Similar results were obtained in WAT depots of ATRA-treated mice. We conclude that ATRA impacts mitochondria in adipocytes, leading to increased OXPHOS capacity and mitochondrial content in these cells.

The Roles of Vitamin A in the Regulation of Carbohydrate, Lipid, and Protein Metabolism

Currently, two-thirds of American adults are overweight or obese. This high prevalence of overweight/obesity negatively affects the health of the population, as obese individuals tend to develop several chronic diseases, such as type 2 diabetes and cardiovascular diseases. Due to obesity's impact on health, medical costs, and longevity, the rise in the number of obese people has become a public health concern. Both genetic and environmental/dietary factors play a role in the development of metabolic diseases. Intuitively, it seems to be obvious to link over-nutrition to the development of obesity and other metabolic diseases. However, the underlying mechanisms are still unclear. Dietary nutrients not only provide energy derived from macronutrients, but also factors such as micronutrients with regulatory roles. How micronutrients, such as vitamin A (VA; retinol), regulate macronutrient homeostasis is still an ongoing research topic. As an essential micronutrient, VA plays a key role in the general health of an individual. This review summarizes recent research progress regarding VA's role in carbohydrate, lipid, and protein metabolism. Due to the large amount of information regarding VA functions, this review focusses on metabolism in metabolic active organs and tissues. Additionally, some perspectives for future studies will be provided.

The one-two punch: Retinoic acid suppresses obesity both by promoting energy expenditure and by inhibiting adipogenesis

The vitamin A metabolite retinoic acid (RA) regulates gene transcription by activating the nuclear receptors RAR and PPARβ/δ and their cognate lipid binding proteins CRABP-II, which delivers RA to RAR, and FABP5, which shuttles the hormone to PPARβ/δ. In preadipocytes, RA signals predominantly through CRABP-II and the RAR isotype RARγ to induce the expression of hallmark markers of preadipocytes Pref-1, Sox9, and KLF2. RA thus maintains the preadipocyte phenotype and inhibits adipogenesis. In mature adipocytes, RA activates both of its receptors to upregulate expression of genes that enhance lipid oxidation, energy dissipation, and insulin responses. Consequently, RA potently protects mice from diet-induced obesity and insulin resistance by two distinct mechanisms: by counteracting adipogenesis, thereby moderating the formation of new fat cells, and by promoting energy expenditure, thereby preventing adipocyte hypertrophy.

Food components and the immune system: from tonic agents to allergens

The intestinal mucosa is the major site of contact with antigens, and it houses the largest lymphoid tissue in the body. In physiological conditions, microbiota and dietary antigens are the natural sources of stimulation for the gut-associated lymphoid tissues (GALT) and for the immune system as a whole. Germ-free models have provided some insights on the immunological role of gut antigens. However, most of the GALT is not located in the large intestine, where gut microbiota is prominent. It is concentrated in the small intestine where protein absorption takes place. In this review, we will address the involvement of food components in the development and the function of the immune system. Studies in mice have already shown that dietary proteins are critical elements for the developmental shift of the immature neonatal immune profile into a fully developed immune system. The immunological effects of other food components (such as vitamins and lipids) will also be addressed. Most of the cells in the GALT are activated and local pro-inflammatory mediators are abundant. Regulatory elements are known to provide a delicate yet robust balance that maintains gut homeostasis. Usually antigenic contact in the gut induces two major immune responses, oral tolerance and production of secretory IgA. However, under pathological conditions mucosal homeostasis is disturbed resulting in inflammatory reactions such as food hypersensitivity. Food allergy development depends on many factors such as genetic predisposition, biochemical features of allergens, and a growing array of environmental elements. Neuroimmune interactions are also implicated in food allergy and they are examples of the high complexity of the phenomenon. Recent findings on the gut circuits triggered by food components will be reviewed to show that, far beyond their role as nutrients, they are critical players in the operation of the immune system in health and disease.

Retinoids: Potent regulators of metabolism

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

Vitamin A supplementation in early life affects later response to an obesogenic diet in rats

OBJECTIVE

To assess the influence of supplementation with a moderate dose of vitamin A in early life on adipose tissue development and the response to an obesogenic diet later in life.

METHODS

During the suckling period, rat pups received a daily oral dose of retinyl palmitate corresponding to three times the vitamin A ingested daily from maternal milk. Control rats received the vehicle (olive oil). Short-term effects of treatment on gene expression and morphology of white adipose tissue (WAT) were analyzed in animals on the day after weaning (day 21). To study long-term effects, control and vitamin A-treated rats were fed, after weaning, a normal fat or a high-fat (HF) diet for 16 weeks.

RESULTS

WAT of vitamin A-treated young rats (day 21) was enriched in small adipocytes with a reduced expression of adipogenic markers (peroxisome proliferator-activated receptor γ and lipoprotein lipase) and an increased cell proliferation potential as indicated by increased expression of proliferating cell nuclear antigen. Increased retinoic acid (RA)-induced transcriptional responses were present in the tissues of vitamin A-treated young rats (day 21) including WAT. Vitamin A-treated rats developed higher adiposity than control rats on a HF diet as indicated by body composition analysis and increased WAT depot mass, adipocyte diameter, WAT DNA content, leptinemia and adipose leptin gene expression. Excess adiposity gain in vitamin A-treated rats developed in the absence of changes in body weight and was attributable to excess adipocyte hyperplasia. No differences in adiposity were observed between vitamin A-treated rats and control rats on a normal fat diet. Total retinol levels in WAT of vitamin A-treated rats were elevated at weaning (day 21) and normalized by day 135 of age.

CONCLUSION

Vitamin A intake in the early stages of postnatal life favors subsequent HF diet-induced adiposity gain through mechanisms that may relate to changes in adipose tissue development, likely mediated by RA.

Modulation of PPAR-γ by Nutraceutics as Complementary Treatment for Obesity-Related Disorders and Inflammatory Diseases

A direct correlation between adequate nutrition and health is a universally accepted truth. The Western lifestyle, with a high intake of simple sugars, saturated fat, and physical inactivity, promotes pathologic conditions. The main adverse consequences range from cardiovascular disease, type 2 diabetes, and metabolic syndrome to several cancers. Dietary components influence tissue homeostasis in multiple ways and many different functional foods have been associated with various health benefits when consumed. Natural products are an important and promising source for drug discovery. Many anti-inflammatory natural products activate peroxisome proliferator-activated receptors (PPAR); therefore, compounds that activate or modulate PPAR-gamma (PPAR-γ) may help to fight all of these pathological conditions. Consequently, the discovery and optimization of novel PPAR-γ agonists and modulators that would display reduced side effects is of great interest. In this paper, we present some of the main naturally derived products studied that exert an influence on metabolism through the activation or modulation of PPAR-γ, and we also present PPAR-γ-related diseases that can be complementarily treated with nutraceutics from functional foods.

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.

Zinc, vitamin A, and vitamin C status are associated with leptin concentrations and obesity in Mexican women: results from a cross-sectional study

Background

The prevalence of obesity among Mexican women is high and it could be related to micronutrient status. We evaluated in a cross-sectional study the associations of zinc and vitamins A, C and E concentrations with BMI, central adiposity, body fat and leptin concentration.

Methods

Women aged 37 ± 7.5 years (n = 580) from 6 rural communities in Mexico were evaluated. Anthropometric measurements included weight, height, waist and hip circumference. A fasting blood sample was taken for the analysis of glucose, lipid profile, leptin, zinc, and vitamins A, C and E. Body composition was determined by DEXA (Hologic Mod Explorer).

Results

The prevalence of overweight and obesity was 36% (BMI > 25 Kg/m²) and 44% (BMI > 30 Kg/m²), respectively. Prevalence of zinc and vitamins C and E deficiencies were similar in obese, overweight and normal weight women. No vitamin A deficiency was found. Vitamin C was negatively associated with BMI, waist-to-height ratio, and leptin concentrations (p < 0.05). Vitamin A was positively associated with leptin (p < 0.05). When stratifying by BMI, % body fat and waist circumference, high leptin concentrations were associated with lower zinc and lower vitamin C concentrations in women with obesity (p < 0.05) and higher vitamin A concentrations in women without obesity (p < 0.01). Vitamin E status was not associated with any markers of obesity.

Conclusion

Zinc and vitamins A and C are associated with obesity, adiposity and leptin concentration in women from rural Mexico, and may play an important role in fat deposition. The causality of these associations needs to be confirmed.

Retinoic acid upregulates preadipocyte genes to block adipogenesis and suppress diet-induced obesity

Retinoic acid (RA) protects mice from diet-induced obesity. The activity is mediated in part through activation of the nuclear receptors RA receptors (RARs) and peroxisome proliferator-activated receptor β/δ and their associated binding proteins cellular RA binding protein type II (CRABP-II) and fatty acid binding protein type 5 in adipocytes and skeletal muscle, leading to enhanced lipid oxidation and energy dissipation. It was also reported that RA inhibits differentiation of cultured preadipocytes. However, whether the hormone suppresses adipogenesis in vivo and how the activity is propagated remained unknown. In this study, we show that RA inhibits adipocyte differentiation by activating the CRABP-II/RARγ path in preadipose cells, thereby upregulating the expression of the adipogenesis inhibitors Pref-1, Sox9, and Kruppel-like factor 2 (KLF2). In turn, KLF2 induces the expression of CRABP-II and RARγ, further potentiating inhibition of adipocyte differentiation by RA. The data also indicate that RA suppresses adipogenesis in vivo and that the activity significantly contributes to the ability of the hormone to counteract diet-induced obesity.

Mammalian carotenoid-oxygenases: key players for carotenoid function and homeostasis

Humans depend on a dietary intake of lipids to maintain optimal health. Among various classes of dietary lipids, the physiological importance of carotenoids is still controversially discussed. On one hand, it is well established that carotenoids, such as β,β-carotene, are a major source for vitamin A that plays critical roles for vision and many aspects of cell physiology. On the other hand, large clinical trials have failed to show clear health benefits of carotenoids supplementation and even suggest adverse health effects in individuals at risk of disease. In recent years, key molecular players for carotenoid metabolism have been identified, including an evolutionarily well conserved family of carotenoid-oxygenases. Studies in knockout mouse models for these enzymes revealed that carotenoid metabolism is a highly regulated process and that this regulation already takes place at the level of intestinal absorption. These studies also provided evidence that β,β-carotene conversion can influence retinoid-dependent processes in the mouse embryo and in adult tissues. Moreover, these analyses provide an explanation for adverse health effects of carotenoids by showing that a pathological accumulation of these compounds can induce oxidative stress in mitochondria and cell signaling pathways related to disease. Advancing knowledge about carotenoid metabolism will contribute to a better understanding of the biochemical and physiological roles of these important micronutrients in health and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

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.

Beta-carotene reduces body adiposity of mice via BCMO1

Evidence from cell culture studies indicates that β-carotene-(BC)-derived apocarotenoid signaling molecules can modulate the activities of nuclear receptors that regulate many aspects of adipocyte physiology. Two BC metabolizing enzymes, the BC-15,15′-oxygenase (Bcmo1) and the BC-9′,10′-oxygenase (Bcdo2) are expressed in adipocytes. Bcmo1 catalyzes the conversion of BC into retinaldehyde and Bcdo2 into β-10′-apocarotenal and β-ionone. Here we analyzed the impact of BC on body adiposity of mice. To genetically dissect the roles of Bcmo1 and Bcdo2 in this process, we used wild-type and Bcmo1^-/- mice for this study. In wild-type mice, BC was converted into retinoids. In contrast, Bcmo1^-/- mice showed increased expression of Bcdo2 in adipocytes and β-10′-apocarotenol accumulated as the major BC derivative. In wild-type mice, BC significantly reduced body adiposity (by 28%), leptinemia and adipocyte size. Genome wide microarray analysis of inguinal white adipose tissue revealed a generalized decrease of mRNA expression of peroxisome proliferator-activated receptor γ (PPARγ) target genes. Consistently, the expression of this key transcription factor for lipogenesis was significantly reduced both on the mRNA and protein levels. Despite β-10′-apocarotenoid production, this effect of BC was absent in Bcmo1^-/- mice, demonstrating that it was dependent on the Bcmo1-mediated production of retinoids. Our study evidences an important role of BC for the control of body adiposity in mice and identifies Bcmo1 as critical molecular player for the regulation of PPARγ activity in adipocytes

Vitamin A metabolism and adipose tissue biology

In recent years, the importance of vitamin A in adipose tissue biology, obesity and type II diabetes has become apparent. This review focuses on recent developments within the area of vitamin A and adipose tissue biology. Adipose tissue has an active vitamin A metabolism as it not only stores vitamin A but retinol is also converted to its active metabolite retinoic acid. Several mouse models point to a relationship between vitamin A metabolism and the development of adiposity. Similarly, in vitro studies provide new molecular mechanisms for the function of different forms of vitamin A and retinol- or retinoic acid-binding proteins in adipose tissue.

Colors with functions: elucidating the biochemical and molecular basis of carotenoid metabolism

Carotenoids affect a rich variety of physiological functions in nature and are beneficial for human health, serving as antioxidants in lipophilic environments and blue light filters in the macula of human retina. These dietary compounds also serve as precursors of a unique set of apo-carotenoid cleavage products, including retinoids. Although knowledge about retinoid biology has tremendously increased, the metabolism of retinoids' parent precursors remains poorly understood. Recently, molecular players in carotenoid metabolism have been identified and biochemically characterized. Moreover, mutations in their corresponding genes impair carotenoid metabolism and induce various pathologies in animal models. Polymorphisms in these genes alter carotenoid and retinoid homeostasis in humans as well. This review summarizes our current knowledge about the molecular/biochemical basis of carotenoid metabolism and particularly the physiological role of carotenoids in retinoid-dependent physiological processes.

Beta,beta-carotene decreases peroxisome proliferator receptor gamma activity and reduces lipid storage capacity of adipocytes in a beta,beta-carotene oxygenase 1-dependent manner

Increasing evidence has been provided for a connection between retinoid metabolism and the activity of peroxisome proliferator receptors (Ppars) in the control of body fat reserves. Two different precursors for retinoids exist in the diet as preformed vitamin A (all-trans-retinol) and provitamin A (beta,beta-carotene). For retinoid production, beta,beta-carotene is converted to retinaldehyde by beta,beta-carotene monooxygenase 1 (Bcmo1). Previous analysis showed that Bcmo1 knock-out mice develop dyslipidemia and are more susceptible to diet-induced obesity. However, the role of Bcmo1 for adipocyte retinoid metabolism has yet not been well defined. Here, we showed that Bcmo1 mRNA and protein expression are induced during adipogenesis in NIH 3T3-L1 cells. In mature adipocytes, beta,beta-carotene but not all-trans-retinol was metabolized to retinoic acid (RA). RA decreased the expression of Ppar gamma and CCAAT/enhancer-binding protein alpha, key lipogenic transcription factors, and reduced the lipid content of mature adipocytes. This process was inhibited by the retinoic acid receptor antagonist LE450, showing that it involves canonical retinoid signaling. Accordingly, gavage of beta,beta-carotene but not all-trans-retinol induced retinoid signaling and decreased Ppar gamma expression in white adipose tissue of vitamin A-deficient mice. Our study identifies beta,beta-carotene as a critical physiological precursor for RA production in adipocytes and implicates provitamin A as a dietary regulator of body fat reserves.

Pharmacogenetic model of retinoic acid-induced dyslipidemia and insulin resistance

Aims: Therapeutic administration of retinoids is often accompanied with undesirable side effects, including an increase in lipid levels in up to 45% of treated patients. We tested the hypothesis of whether spontaneously hypertensive rat (SHR) and congenic SHR.PD-(D8Rat42-D8Arb23)/Cub (SHR-Lx) strains, differing only in a 14-gene region of chromosome 8 and previously shown to display differential sensitivity to the teratogenic effects of retinoic acid, could serve as a pharmacogenetic model set of the metabolic side effects of retinoid therapy. Materials & methods: Male, 15-week old rats (n = 12/strain) of SHR and SHR-Lx strains were fed a high-sucrose diet for 2 weeks and subsequently treated either with all-trans retinoic acid (15 mg/kg) or only with a vehicle for 16 days (n = 6/strain/treatment), while still on the high-sucrose diet. We assessed the morphometric and metabolic profiles of all groups, including glucose tolerance tests, levels of insulin, adiponectin, free fatty acids, concentrations of triglycerides and cholesterol in 20 lipoprotein fractions under conditions of both high-sucrose diet and high-sucrose diet plus all-trans retinoic acid administration. Results & conclusion: SHR-Lx displayed substantially greater sensitivity to a number of all-trans retinoic acid-induced metabolic dysregulations compared with SHR, resulting in impairment of glucose tolerance, increased visceral adiposity, and substantially greater increase of circulating triglyceride concentrations, accompanied by a shift towards their less favorable distribution into the lipoprotein fractions. These observations closely mimic the common side effects of retinoid therapy in humans, rendering SHR-Lx an experimental pharmacogenetic model of atRA-induced dyslipidemia.

Increased adiposity in the retinol saturase-knockout mouse

The enzyme retinol saturase (RetSat) catalyzes the saturation of all-trans-retinol to produce (R)-all-trans-13,14-dihydroretinol. As a peroxisome proliferator-activated receptor (PPAR) gamma target, RetSat was shown to be required for adipocyte differentiation in the 3T3-L1 cell culture model. To understand the mechanism involved in this putative proadipogenic effect of RetSat, we studied the consequences of ablating RetSat expression on retinoid metabolism and adipose tissue differentiation in RetSat-null mice. Here, we report that RetSat-null mice have normal levels of retinol and retinyl palmitate in liver, serum, and adipose tissue, but, in contrast to wild-type mice, are deficient in the production of all-trans-13,14-dihydroretinol from dietary vitamin A. Despite accumulating more fat, RetSat-null mice maintained on either low-fat or high-fat diets gain weight and have similar rates of food intake as age- and gender-matched wild-type control littermates. This increased adiposity of RetSat-null mice is associated with up-regulation of PPARgamma, a key transcriptional regulator of adipogenesis, and also its downstream target, fatty acid-binding protein 4 (FABP4/aP2). On the basis of these results, we propose that dihydroretinoids produced by RetSat control physiological processes that influence PPARgamma activity and regulate lipid accumulation in mice.-Moise, A. R., Lobo, G. P., Erokwu, B., Wilson, D. L., Peck, D., Alvarez, S., Domínguez, M., Alvarez, R., Flask, C. A., de Lera, A. R., von Lintig, J., Palczewski, K. Increased adiposity in the retinol saturase-knockout mouse.

beta-Carotene conversion products and their effects on adipose tissue

Recent epidemiological data suggest that beta-carotene may be protective against metabolic diseases in which adipose tissue plays a key role. Adipose tissue constitutes the major beta-carotene storage tissue and its functions have been shown to be modulated in response to beta-carotene breakdown products, especially retinal produced after cleavage by beta-carotene 15,15'-monooxygenase (BCMO1), and retinoic acid arising from oxidation of retinal. However, the possibility exists that beta-carotene in its intact form can also affect adipocyte function. Development of a knock out model and identification of a loss-of-function mutation have pointed out BCMO1 as being probably the sole enzyme responsible for provitamin A conversion into retinal in mammals. The utilisation of BCMO1(-/-)mice should provide insights on beta-carotene effect on its own in the future. In humans, intervention studies have highlighted the huge interindividual variation of beta-carotene conversion efficiency, possibly due to genetic polymorphisms, which might impact on response to beta-carotene. This brief review discusses the processes involved in beta-carotene conversion and the effect of cleavage products on body fat and adipose tissue function.

Retinoic acid treatment increases lipid oxidation capacity in skeletal muscle of mice

OBJECTIVE

All-trans retinoic acid (ATRA), a carboxylic form of vitamin A, favors in mice a mobilization of body fat reserves that correlates with an increment of oxidative and thermogenic capacity in adipose tissues. The objective of this study has been to investigate the effect of ATRA treatment on skeletal muscle capacity for fatty-acid catabolism.

METHODS AND PROCEDURES

Tissue composition and gene expression related to lipid and oxidative metabolism were analyzed in skeletal muscle of mice acutely treated with ATRA or vehicle (olive oil).

RESULTS

ATRA treatment triggered a dose-dependent increase in the muscle mRNA expression levels of selected enzymes, transporters and transcription factors involved in fatty-acid oxidation, respiration, and thermogenesis namely: muscle-type carnitine palmitoyltransferase 1, acyl CoA oxidase 1, subunit II of cytochrome oxidase, uncoupling protein 3, peroxisome proliferator-activated receptor-gamma co-activator -1alpha and peroxisome proliferator-activated receptor-delta (PPARdelta). The treatment also resulted in the upregulation of the mRNA levels of acetyl-CoA carboxylase 2 (ACC2), a key regulatory enzyme for mitochondrial fatty-acid oxidation in muscle. Skeletal muscle protein levels of PPARdelta and retinoid X receptor gamma, a partner for many nuclear receptors involved in lipid metabolism, were increased after ATRA treatment. Muscle lipid content was decreased.

DISCUSSION

These results indicate that ATRA treatment increases the capacity of skeletal muscle for fatty-acid oxidation. Knowledge of nutrients or nutrient-derivatives capable of enhancing oxidative metabolism in muscle and other tissues can contribute to new avenues of prevention and treatment of obesity and related disorders.

Adipose tissue failure and mitochondria as a possible target for improvement by bioactive food components

PURPOSE OF REVIEW

Adipose tissue is an essential, highly dynamic and metabolically active tissue that vigorously communicates to support its primary function: the storage of lipids. It performs this function to secure energy supply and prevent lipotoxicity. Adipose tissue is essential for maintaining a healthy glucose and lipid homeostasis and failure results in disease. This review discusses causes of adipose tissue failure and four categories of bioactive food components that may help to prevent this.

RECENT FINDINGS

Based on recent findings, it is argued that initial adipose failure following long-term excess energy intake may be the result of reduced mitochondrial capacity associated with altered mitochondrial reactive oxygen species signaling and adipose tissue hypoxia. Current data suggest that different classes of bioactive food components, including vitamin B3, retinoids, fatty acids and polyphenols, may have the potential to modulate mitochondrial function and consequently prevent adipose dysfunction in obesity.

SUMMARY

It seems most attractive to aim nutritional intervention at the prevention of initial adipose dysfunction and hence to target dietary intervention at improvement of mitochondrial function.