Inhibition of adipose conversion in 3T3-L2 cells by retinoic acid

Mammalian adipogenesis regulator (Areg) cells use retinoic acid signalling to be non- and anti-adipogenic in age-dependent manner

Adipose stem and precursor cells (ASPCs) give rise to adipocytes and determine the composition and plasticity of adipose tissue. Recently, several studies have demonstrated that ASPCs partition into at least three distinct cell subpopulations, including the enigmatic CD142⁺ cells. An outstanding challenge is to functionally characterise this population, as discrepant properties, from adipogenic to non- and anti-adipogenic, have been reported for these cells. To resolve these phenotypic ambiguities, we characterised mammalian subcutaneous CD142⁺ ASPCs across various experimental conditions, demonstrating that CD142⁺ ASPCs exhibit high molecular and phenotypic robustness. Specifically, we find these cells to be firmly non- and anti-adipogenic both in vitro and in vivo, with their inhibitory signals also impacting adipogenic human cells. However, these CD142⁺ ASPC-specific properties exhibit surprising temporal phenotypic alterations, and emerge only in an age-dependent manner. Finally, using multi-omic and functional assays, we show that the inhibitory nature of these adipogenesis-regulatory CD142⁺ ASPCs (Aregs) is driven by specifically expressed secretory factors that cooperate with the retinoic acid signalling pathway to transform the adipogenic state of CD142^- ASPCs into a non-adipogenic, Areg-like state.

CRABP1 in Non-Canonical Activities of Retinoic Acid in Health and Diseases

In this review, we discuss the emerging role of Cellular Retinoic Acid Binding Protein 1 (CRABP1) as a mediator of non-canonical activities of retinoic acid (RA) and relevance to human diseases. We first discuss the role of CRABP1 in regulating MAPK activities and its implication in stem cell proliferation, cancers, adipocyte health, and neuro-immune regulation. We then discuss an additional role of CRABP1 in regulating CaMKII activities, and its implication in heart and motor neuron diseases. Through molecular and genetic studies of Crabp1 knockout (CKO) mouse and culture models, it is established that CRABP1 forms complexes with specific signaling molecules to function as RA-regulated signalsomes in a cell context-dependent manner. Gene expression data and CRABP1 gene single nucleotide polymorphisms (SNPs) of human cancer, neurodegeneration, and immune disease patients implicate the potential association of abnormality in CRABP1 with human diseases. Finally, therapeutic strategies for managing certain human diseases by targeting CRABP1 are discussed.

Roles of vitamin A in the regulation of fatty acid synthesis

Dietary macronutrients and micronutrients play important roles in human health. On the other hand, the excessive energy derived from food is stored in the form of triacylglycerol. A variety of dietary and hormonal factors affect this process through the regulation of the activities and expression levels of those key player enzymes involved in fatty acid biosynthesis such as acetyl-CoA carboxylase, fatty acid synthase, fatty acid elongases, and desaturases. As a micronutrient, vitamin A is essential for the health of humans. Recently, vitamin A has been shown to play a role in the regulation of glucose and lipid metabolism. This review summarizes recent research progresses about the roles of vitamin A in fatty acid synthesis. It focuses on the effects of vitamin A on the activities and expression levels of mRNA and proteins of key enzymes for fatty acid synthesis in vitro and in vivo. It appears that vitamin A status and its signaling pathway regulate the expression levels of enzymes involved in fatty acid synthesis. Future research directions are also discussed.

Retinoic Acid Receptor Agonists Suppress Muscle Fatty Infiltration in Mice

BACKGROUND

The infiltration of fat tissue into skeletal muscle, a condition referred to as muscle fatty infiltration or fatty degeneration, is regarded as an irreversible event that significantly compromises the motor function of skeletal muscle.

PURPOSE

To investigate the effect of retinoic acid receptor (RAR) agonists in suppressing the adipogenic differentiation of fibroadipogenic progenitors (FAPs) in vitro and fatty infiltration after rotator cuff tear in mice.

STUDY DESIGN

Controlled laboratory study.

METHODS

FAPs isolated from mouse skeletal muscle were cultured in adipogenic differentiation medium in the presence or absence of an RAR agonist. At the end of cell culture, adipogenic differentiation was evaluated by gene expression analysis and oil red O staining. A mouse model of fatty infiltration-which includes the resection of the rotator cuff, removal of the humeral head, and denervation the supraspinatus muscle-was used to induce fatty infiltration in the supraspinatus muscle. The mice were orally or intramuscularly administered with an RAR agonist after the surgery. Muscle fatty infiltration was evaluated by histology and gene expression analysis.

RESULTS

RAR agonists effectively inhibited the adipogenic differentiation of FAPs in vitro. Oral and intramuscular administration of RAR agonists suppressed the development of muscle fatty infiltration in the mice after rotator cuff tear. In accordance, we found a significant decrease in the number of intramuscular fat cells and suppressed expression in adipogenic markers. RAR agonists also increased the expression of the transcripts for collagens; however, an accumulation of collagenous tissues was not histologically evident in the present model.

CONCLUSION

Muscle fatty infiltration can be alleviated by RAR agonists through suppressing the adipogenic differentiation of FAPs. The results also suggest that RAR agonists are potential therapeutic agents for treating patients who are at risk of developing muscle fatty infiltration. The consequence of the increased expression of collagen transcripts by RAR agonists needs to be clarified.

CLINICAL RELEVANCE

RAR agonists can be used to prevent the development of muscle fatty infiltration after rotator cuff tear. Nevertheless, further studies are mandatory in a large animal model to examine the safety and efficacy of intramuscular injection of RAR agonists.

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.

Cellular retinoic acid binding protein 1 protects mice from high-fat diet-induced obesity by decreasing adipocyte hypertrophy

Objectives

Obesity, an emerging global health issue, involves numerous factors; understanding its underlying mechanisms for prevention and therapeutics is urgently needed. Cellular retinoic acid binding protein 1 (Crabp1) knockout (CKO) mice exhibit an obese phenotype under normal diet feedings, which prompted us to propose that Crabp1 could play a role in modulating adipose tissue development/homeostasis. Studies were designed to elucidate the underlying mechanism of Crabp1’s action in reducing obesity.

Subjects/methods

In animal studies, 6 weeks old male wild type and CKO mice were fed with normal diet (ND) or high fat diet (HFD) for 10 weeks. Body weight and food intake were regularly monitored. Glucose tolerance test and biological parameters of plasma (glucose and insulin levels) were measured after 10 weeks of ND vs. HFD feedings. Visceral adipose tissues were collected for histological and molecular analyses to determine affected signaling pathways. In cell culture studies, the 3T3L1 adipocyte differentiation model was used to examine and validate relevant signaling pathways.

Results

CKO mice, compared to WT mice, gained more body weight, exhibited more elevated fasting plasma glucose levels, and developed more severe impaired glucose tolerance under both ND and HFD. Histological examination revealed readily increased adipocyte hypertrophy and adipose tissue inflammation under HFD feedings. In 3T3L1 adipocytes, Crabp1 silencing enhanced extracellular signal-regulated kinase 1/2 (ERK1/2) activation, accompanied by elevated markers and signaling pathways of lipid accumulation and adipocyte hypertrophy.

Conclusions

This study identifies Crabp1’s physiological role against the development of obesity. The protective function of CRABP1 is likely attributed to its classically proposed (canonical) activity as a trap for RA, which will reduce RA availability, thereby dampening RA-stimulated ERK1/2 activation and adipocyte hypertrophy. The results suggest Crabp1 as a potentially new therapeutic target in managing obesity and metabolic diseases.

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.

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.

Non-classical Transcriptional Activity of Retinoic Acid

It has long been established that the transcriptional activity of retinoic acid (RA) is mediated by members of the nuclear receptor family of ligand-activated transcription factors termed RA receptors (RARs). More recent observations have established that RA also activates an additional nuclear receptor, PPARβ/δ. Partitioning RA between RARs and PPARβ/δ is governed by different intracellular lipid-binding proteins: cellular RA binding protein 2 (CRABP2) delivers RA to nuclear RARs and a fatty acid binding protein (FABP5) delivers the hormone from the cytosol to nuclear PPARβ/δ. Consequently, RA signals through RARs in CRABP2-expressing cells, but activates PPARβ/δ in cells that express a high level of FABP5. RA elicits different and sometimes opposing responses in cells that express different FABP5/CRABP2 ratios because PPARβ/δ and RARs regulate the expression of distinct sets of genes. An overview of the observations that led to the discovery of this non-classical activity of RA are presented here, along with a discussion of evidence demonstrating the involvement of the dual transcriptional activities of RA in regulating energy homeostasis, insulin responses, and adipocyte and neuron differentiation.

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.

The transient expression of Klf4 and Klf5 during adipogenesis depends on GSK3β activity

Adipogenesis is regulated by a complex cascade of transcriptional factors, among them KLF4. This factor was previously shown to be necessary for adipose differentiation. We found that GSK3β activity was required for Klf4 and Klf5 expression during adipogenesis. In addition, retinoic acid inhibited Klf4 and Klf5 expression but not that of Cebpb. Protein synthesis inhibition showed that the transient expression of Klf4, Cebpb and Klf5 during early adipogenesis seemed to require a yet unknown protein for their repression. We also found that Klf4 forced expression in 3T3-F442A cells cultured under non-adipogenic conditions did not induce adipogenesis, nor the expression of Cebpb or Klf5, a Cebpb target gene, showing that KLF4 was not sufficient for adipose differentiation to take place. This would suggest that a more complex combination of molecular pathways not yet understood, is involved during early adipogenesis.

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

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

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.

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.

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.

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.

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.

Hormone-sensitive lipase (HSL) is also a retinyl ester hydrolase: evidence from mice lacking HSL

Here, we investigated the importance of hormone-sensitive lipase (HSL) as a retinyl ester hydrolase (REH). REH activity was measured in vitro using recombinant HSL and retinyl palmitate. The expression of retinoic acid (RA)-regulated genes and retinoid metabolites were measured in high-fat diet fed HSL-null mice using real-time quantitative PCR and triple-stage liquid chromatography/tandem mass spectrometry, respectively. Age- and gender-matched wild-type littermates were used as controls. The REH activity of rat HSL was found to be higher than that against the hitherto best known HSL substrate, i.e., diacylglycerols. REH activity in white adipose tissue (WAT) of HSL-null mice was completely blunted and accompanied by increased levels of retinyl esters and decreased levels of retinol, retinaldehyde and all-trans RA. Accordingly, genes known to be positively regulated by RA were down-regulated in HSL-null mice, including pRb and RIP140, key factors promoting differentiation into the white over the brown adipocyte lineage. Dietary RA supplementation partly restored WAT mass and the expression of RA-regulated genes in WAT of HSL-null mice. These findings demonstrate the importance of HSL as an REH of adipose tissue and suggest that HSL via this action provides RA and other retinoids for signaling events that are crucial for adipocyte differentiation and lineage commitment.

Commitment of 3T3-F442A cells to adipocyte differentiation takes place during the first 24-36 h after adipogenic stimulation: TNF-alpha inhibits commitment

We studied the commitment of 3T3-F442A cells during stimulation with adipogenic serum or growth hormone. Confluent 3T3-F442A preadipocytes were incubated with adipogenic medium for increasing times; the number of adipose clusters, GPDH activity, and lipid accumulation were evaluated. Results show that cell commitment took place during the first 24-36 h after stimulation under adipogenic conditions. Then, cultures underwent a 2-fold increase in total cell number through selective multiplication of committed cells, followed by a dramatic decrease in colony-forming ability and 300- to 1000-fold raise in GPDH activity. Cell commitment was not modulated by insulin, but this hormone stimulated clonal expansion of committed cells and lipogenesis. Commitment was inhibited by TNF-alpha at concentrations as low as 5 ng/ml, and by retinoic acid. The results show that TNF-alpha inhibits adipose conversion at two different levels: at concentrations as low as 5 ng/ml, it blocks commitment, and at concentrations of 100 ng/ml or higher the cytokine seems to block mitotic expansion and other steps of differentiation after cell commitment. The identification of a specific time for cell commitment would allow the study of the early genes that might regulate cell reprogramming into adipocytes.

Relationship between peroxisome proliferator-activated receptor gamma and retinoic acid receptor alpha gene expression in obese human adipose tissue

OBJECTIVE

To investigate in human adipose tissue a possible relationship between per oxisome proliferator-activated receptor gamma (PPARgamma) and retinoic acid receptor alpha (RARalpha) gene expression, two genes involved in the control of adipocyte differentiation.

SUBJECTS

Ten lean control women (age 31-60 y, body mass index (BMI) 18-24.7 kg/m(2)) and an obese group of 15 women (age 27-62 y, BMI 30-57.5 kg/m(2)), of whom 10 subjects were in weight-gain phase and five were in weight-loss phase.

MEASUREMENTS

We assessed the relative PPARgamma and RARalpha mRNA levels in subcutaneous abdominal adipose tissue using a real-time PCR method.

RESULTS

PPARgamma mRNA level were significantly increased (+91%; P<0.01) in obese women compared to lean control women. In the obese group, we observed a PPARgamma mRNA level 42% lower in weight-loss obese than in weight-gain obese subjects. We obtained a positive correlation (r=0.56; P<0.01) between PPARgamma mRNA level and the BMI of all subjects. Relative mRNA abundance level of RARalpha in subcutaneous adipose tissue of obese subjects is significantly lower than in control subjects (-56%, P<0.01), and a negative correlation was found between PPARgamma and RARalpha mRNA levels in subcutaneous adipose tissue of subjects study (r=-0.75; P<0.01).

CONCLUSION

Our findings suggest that obesity is associated with an inverse relationship between PPARgamma and RARalpha expressions in human subcutaneous adipose tissue. Modulations of nuclear receptor profile could be an important event in the body's early adaptive mechanisms promoting adipose tissue plasticity and leading to the onset of obesity.

Retinoids are positive effectors of adipose cell differentiation

Retinoids, especially all-trans retinoic acid (t-RA), have been reported in the last decade to inhibit the differentiation of preadipose cells. In those studies, however, the concentrations of t-RA were supraphysiological (0.1-10 microM range). In contrast we show that, when present at concentrations below or close to the Kd values of retinoic acid receptors, retinoids behave as potent adipogenic hormones (1 pM to 10 nM range). As shown by the use of specific ligands for each RAR subtype, these positive effects on adipose differentiation involve in particular the RAR alpha subtype, and have been observed in Ob17 cells exposed to serum-supplemented or serum-free medium, and in rat preadipocytes exposed to serum-free medium. Among the two classes of retinoid acid receptors (RARs) and retinoid X receptors (RXRs), RAR alpha, RAR gamma, RXR alpha and RXR beta mRNAs could be detected in growing adipoblasts and were found to be increased in committed preadipocytes and differentiated cells upon retinoid treatment. Like other adipogenic hormones, retinoids were only effective in the terminal differentiation process leading from preadipocytes to adipocytes.

Intracellular actions of vitamin A

Because the effects of vitamin A vary with tissue type and often with the form of vitamin A itself, a complete understanding of the mechanism(s) of action still has not been attained. The action of vitamin A may be at the level of genomic expression, at the membrane level, or both. Intercellular and intracellular transport of vitamin A are facilitated by specific binding proteins but probably not in the cellular uptake of vitamin A. Subcellularly, vitamin A may exert a direct effect on transit through the Golgi apparatus, as observed from both biochemical and morphological studies. In my laboratory, recent work using cell-free systems has shown that retinol stimulates transition vesicle formation from endoplasmic reticulum in a GTP-requiring step.

Physiological and clinical aspects of vitamin A and its metabolites

Retinoids, including retinol and retinoic acid (RA), are a group of naturally occurring and synthetic compounds that exhibit vitamin A-like biological activity. They achieve their effects by binding to intracellular proteins. Important sites of action are the nuclear retinoic acid receptors (RAR). These receptors, namely, RAR alpha, RAR beta, and RAR gamma, function as transcription factors by binding to RA-responsive elements (RARE) of multiple genes. Retinoids play a role in vision, embryogenesis, immune modulation, growth and differentiation of normal, premalignant and malignant tissues, the suppression of carcinogenesis, and the inhibition of tumor growth in experimental systems and humans. Reports of the significant antitumor effect of all-trans-RA in acute promyelocytic leukemia and the synthesis of new, less toxic, and more potent retinoids has generated renewed interest in these compounds. Retinoids may have an important role to play in the chemoprevention and therapy of cancer.

Physiological occurrence, biosynthesis and metabolism of retinoic acid: evidence for roles of cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) in the pathway of retinoic acid homeostasis

This article will address recent work on the physiological occurrence, biogenesis and metabolism of retinoic acid and summarize the data that retinoic acid is synthesized in situ in multiple tissues and cell types via enzymes or enzyme complexes that are distinct from the alcohol dehydrogenases. There is now considerable evidence that retinoic acid is an activated metabolite of retinol that supports the systemic functions of vitamin A in vivo. Many studies in vitro, for example, have shown that retinoic acid is the most potent naturally-occurring retinoid with an ED-50 in the range of 1 pM to 10 nM, depending on the assay system. This is below the tissue concentrations of retinoic acid which range from approximately 20-600 nM. Retinoic acid synthesis from retinol in the dog kidney cell line MDCK maintained in serum-free medium is inhibited by the prostanoid, PGE, and the phorbol ester, TPA. In tissues, one pathway of retinoic acid synthesis begins with apo-CRBP stimulating retinyl ester hydrolysis by a microsomal, cholate-independent retinyl ester hydrolase to form holo-CRBP. The holo-CRBP itself is used as substrate by an NADP-dependent, microsomal retinol dehydrogenase to generate retinal, which is converted into retinoic acid by a cytosolic NAD-dependent retinal dehydrogenase. Therefore, cellular retinol-binding protein (CRBP) apparently has at least 2 functions in retinoic acid synthesis: the apo form stimulates retinol mobilization from retinyl ester stores; the holo form delivers the retinol via direct transfer to dehydrogenase(s). Retinoic acid is converted into a mixture of at least 4 metabolites by testes microsomes which migrate closely on reverse-phase HPLC with 4-hydroxyretinoic acid, and may be mistaken for either 4-hydroxy or 4-oxo-retinoic acid. More rigorous analysis, however, shows that only one of them is 4-hydroxyretinoic acid, and another is 18-hydroxyretinoic acid. Two others remain unidentified. These metabolites are also formed in the presence of excess cellular retinoic acid-binding protein (CRABP), which increases the elimination half-life of retinoic acid, but does not prevent retinoic acid catabolism, suggesting that holo-CRABP may be a substrate for retinoic acid catabolism that modulates the steady-state concentrations of retinoic acid. Thus, both retinoid binding proteins, CRBP and CRABP, may each have direct roles as substrate in the biosynthesis and metabolism of retinoic acid, respectively.

The molecular basis for inhibition of adipose conversion of murine 3T3-L1 cells by retinoic acid

The effect of retinoic acid (RA) on the adipose conversion of 3T3 cells has been studied. Differentiation of 3T3-L1 cells was initiated by addition of 0.5 mM methylisobutylxanthine, 0.3 microM dexamethasone and 10 micrograms/ml insulin (MDI) to confluent monolayers of preadipocytes for 48 h. During this time, the cells underwent DNA replication and cell division prior to the expression of adipose specific genes. RA administration had no apparent effect on the rate or extent of cell growth, cell division, or DNA replication. However, RA treatment concomitant with MDI addition inhibited triacylglycerol accumulation (I0.5 = 6 nM) and the accumulation of the differentiation-dependent mRNAs encoding the adipocyte lipid-binding protein (ALBP) and stearoyl-CoA desaturase 1 (SCD1). No inhibition occurred with RA addition either prior to or after MDI treatment. Runoff transcription revealed that the inhibitory effects of RA occurred at the level of transcription and were persistent. Cells treated with RA during the MDI regimen did not appreciably transcribe ALBP or SCD1 mRNAs several days following RA withdrawal. The effects of RA were specific for differentiation-dependent transcripts: 10(-6) M RA did not inhibit expression of the mRNAs encoding beta-tubulin or glutamine synthase. Examination of immediate-early transcription factor expression during the MDI regimen revealed that RA mediated an elevated, prolonged expression of c-Jun mRNA accompanied by diminished expression of c-Fos and Jun-B mRNAs. Given the previously demonstrated role of transcription factor AP-1 in ALBP gene expression, our results suggest that the initiation of expression of this and other adipocyte-specific genes during adipose conversion is regulated by the relative composition of transcription factor AP-1.

Differential effects of retinoic acid upon early and late events in adipose conversion of 3T3 preadipocytes

When confluent 3T3-F442A cell cultures (Day 0) were grown for 3 days in fetal calf serum-supplemented medium containing isobutyl methyl xanthine and dexamethasone (induction phase) and then shifted to serum-free hormone-defined medium (expression phase), they spontaneously exhibited a sharp rise in lipoprotein lipase activity (LPL); at the peak (Day 7) the LPL activity was about 25 times higher than in control cultures and was further enhanced by insulin. Although this expression of LPL activity was spontaneous, the emergence of glycerophosphate dehydrogenase (G3PDH) activity was completely dependent upon insulin as well as upon the expression of the differentiated phenotype. In committed cells, insulin elicited sustained DNA synthesis associated with limited cell proliferation. The addition of retinoic acid during the phase of expression inhibited insulin-dependent terminal differentiation (i.e., the emergence of G3PDH activity and acquisition of the differentiated phenotype). In addition, retinoic acid counteracted the stimulating effect of insulin upon LPL activity, but affected neither the mitotic process nor the spontaneous emergence of LPL activity. When added during the phase of induction, it prevented the overall process of adipogenic differentiation. Thus, the use of retinoic acid can indicate independent control of the mitogenic and lipogenic effects of insulin following commitment to adipogenic differentiation.

Functional studies of newly synthesized benzoic acid derivatives: identification of highly potent retinoid-like activity

Three newly synthesized benzoic acid derivatives (terephthalic acid anilides, chalcone carboxylic acid, and azobenzene carboxylic acid), with a certain structural similarity to retinoic acid, were examined for their retinoid-like bioactivity and their capacity to bind to cellular retinoid binding proteins. Two in vitro systems were used to evaluate their retinoid-like bioactivity: inhibition of adipose conversion of ST 13 murine preadipose cells and growth promotion of murine sarcoma virus (MSV)-transformed 3T3 cells in serum-free culture. All three compounds tested inhibited ST 13 adipose conversion at nanomolar concentrations in a manner similar to classical retinoids such as retinoic acid. The growth-stimulating activity of these compounds on MSV-transformed 3T3 cells was one to two orders of magnitude greater than that of retinoic acid. Simultaneous treatment with these compounds and retinoic acid produced only a barely detectable additive effect, suggesting a common mechanism of action, whereas unrelated mitogens, thrombin, and insulin worked synergistically in combination with retinoic acid. None of the compounds competed with retinol for binding to cellular retinol binding protein. However, two of the three competed with retinoic acid for binding to cellular retinoic acid binding protein. This study provides evidence that the newly synthesized compounds should be included among the retinoids and that their strong biological activity will undoubtedly contribute to the biological and medical application of retinoids.

Commitment of adipocyte differentiation in 3T3 cells is inhibited by retinoic acid, and the expression of lipogenic enzymes is modulated through cytoskeleton stabilization

Retinoic acid (RA), at 1-10 microM, inhibited adipose conversion of 3T3-F442A cells as determined by the activities of lipogenic enzymes, glycerophosphate dehydrogenase (GPD) and malic enzyme. This inhibition was reversible by RA removal, but the increase in lipogenic enzyme activities was considerably delayed in a dose-dependent manner. The onset of the two lipogenic enzyme activities could be regulated somewhat independently, suggesting that expression of the two enzymes is subject to noncoordinated regulation. The RA-inhibited cells had a more flattened and elongated shape, suggesting cytoskeletal changes. Cytochalasin B (CB) did not prevent RA inhibition and did not promote adipose conversion in cultures supplemented with nonadipogenic medium. Reversion of inhibition was accelerated if cells were cultured for 3 days with adipogenic medium containing CB. The drug promoted an early increase in lipogenic enzyme activities. On the other hand, cells cultured on fibronectin-coated dishes, a condition that stabilizes actin cytoskeleton, do not undergo adipocyte differentiation. However, we show here that cells cultured on fibronectin and changed to nonadipogenic medium containing insulin underwent adipose conversion; in contrast, cells treated with RA and then supplemented with nonadipogenic medium containing insulin, but without the retinoid, did not undergo differentiation. We conclude that RA blocks adipose conversion probably before commitment to differentiation, and modulates lipogenic enzyme expression in a noncoordinated manner through changes in cytoskeletal elements, whereas fibronectin blocks phenotype expression in differentiating cells.

Control of the adipogenic differentiation of 3T3-F442A cells by retinoic acid, dexamethasone, and insulin: a topographic analysis

Differentiation of 3T3-F442A adipocytes, monitored by accumulation of neutral lipid and by using the sensitive marker glycerophosphate dehydrogenase, is inhibited by incubation of confluent 3T3-F442A fibroblasts in medium containing retinoic acid or dexamethasone. When added together, dexamethasone (0.25 microM) potentiates about 50-fold the inhibitory effect of retinoic acid (10 microM). Insulin cannot counteract the retinoic acid blockade; however, it can overcome the inhibition of differentiation elicited by dexamethasone. These differential effects of insulin are used for characterizing the adipose conversion cycle. We describe cell culture conditions where terminal differentiation of 3T3-F442A preadipocytes is achieved by low, physiological levels of insulin. They include the switch from a high-serum medium containing isobutyl methyl xanthine and dexamethasone to a serum-free, hormone-supplemented medium. The data reported establish the existence of two successive states for commitment to adipogenic differentiation: a first commitment point (CA) to differentiation which requires serum adipogenic factors, and a second commitment point (CH) controlled by lipogenic hormones, namely insulin, after which terminal maturation can resume. We demonstrate that retinoic acid can prevent and interrupt differentiation by blocking the cells within the early differentiation phase.

Alterations in polyamine metabolism during embryonal carcinoma cell differentiation in vitro

Murine embryonal carcinoma (EC) cells can be stimulated to differentiate by several chemical inducers. Since the response of EC cells to induction is likely to occur shortly after exposure to the inducer, we report here the changes that occur in polyamine levels in a number of EC cell lines shortly after exposure to two chemical stimuli, alpha-difluoromethylornithine (DFMO) and retinoic acid (RA). Our results suggest that polyamine levels are important in determining the state of EC cell differentiation, but that reduction in these levels alone is not sufficient to induce differentiation in all the EC cell lines tested. Also, it is apparent that RA does influence levels of polyamines. However, this influence does not seem to be mediated through direct interaction with ODCase.

Cyclic AMP effects on cell-to-cell junctional membrane permeability during adipocyte differentiation of 3T3-L1 fibroblasts

Mouse 3T3-L1 fibroblast cells, also know as preadipocytes, differentiate in vitro into adipocytes when treated with promoting agents and acquire numerous properties characteristic of mature fat cells. We studied junctional cell-to-cell communication by measuring the incidence of electrical coupling and transfer of carboxy-fluorescein among these cells. When 3T3-L1 cells were induced to differentiate into adipocytes, they lost virtually all cell-cell communication. Preadipocytes that remained nondifferentiated after the treatment maintained normal communication. Loss of communication in the adipocytes invariably coincided with appearance of lipid droplets and not with other phenotypic changes. In the differentiating cells, loss of cell-to-cell communication and lipid accumulation was prevented if dibutyryl cyclic AMP and caffeine were present in the culture medium. Addition of dibutyryl cyclic AMP and caffeine to already differentiated adipocytes resulted in loss of lipid and simultaneously improved junctional permeability. The results demonstrate that in the in vitro 3T3-L1 cell system, (a) cell-to-cell communication and lipid synthesis are intimately related during the adipose conversion and (b) cAMP affects the expression of the two phenotypes.

Differentiation of 3T3-F442A cells into adipocytes is inhibited by retinoic acid

Retinoic acid at a concentration higher than 10(-6) M inhibited adipose conversion of 3T3-F442A cells in surface cultures. Since cells treated with 10(-5) M of the vitamin A analogue and the non-treated cells showed the same capacity to form colonies when seeded at low density, adipose conversion was not inhibited by a general toxic action of the retinoid. The inhibition produced was accompanied by changes in cell shape; adipose morphology was absent in retinoic acid treated cells, and they were more elongated and flattened without piling up. Adipose conversion of 3T3-F442A in Methocel suspended cultures was also inhibited by retinoic acid, suggesting the inhibition exerted by this compound was not related to the changes observed in cell shape. The inhibitory action of retinoic acid is manifested only when the cultures are treated before differentiation takes place; if the retinoid was added after some early events of adipose conversion, differentiation was not blocked. The inhibition was reversed when the drug was removed from the cultures and they were further incubated in adipogenic medium; incubation in medium without adipogenic factor did not permit reversion. On the other hand, vitamin A depleted serum showed a higher capacity to support adipose conversion than non-depleted serum. It is concluded that this vitamin A analogue blocks some early events before the cells undergo the commitment to differentiate into adipocytes.