Fatty acids and retinoids act synergistically on adipose cell differentiation

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.

Retinoic acid exacerbates chlorpyrifos action in ensuing adipogenic differentiation of C3H10T½ cells in a GSK3β dependent pathway

The cell differentiation can be exploited as a paradigm to evaluate the effects of noxious chemicals, on human health, either alone or in combinations. In this regard, the effect of a known cell differentiation agent, retinoic acid (RA) was analyzed in the presence of a noxious chemical chlorpyrifos (CPF), an organophosphate (OP), the receptors of which have recently been localized to mesenchymal stem cells (MSCs). The observed imbalance of adipogenic to skeletal differentiation by CPF together with conundrum about adipogenic potential of RA prompted us to delineate their combinatorial effects on C₃H₁₀T½MSC-like undifferentiated cells. Based on MTT assay, the cellular viability was retained by CPF at concentrations ranging from 0.01–50μM, beyond which it caused cytotoxicity. These non-toxic concentrations also mildly interfered with adipogenesis of C₃H₁₀T½ cells following exposure to adipogenic cocktail. However, upon exposure to RA alone, these MSCs adopted elongated morphology and accumulated lipid vesicles, by day 20, as discerned by phase-contrast and transmission electron microscopy (TEM), in concert with enhanced Oil Red O stained cells. This effect got strongly augmented upon exposure to combination of CPF and RA in a dose-dependent manner. Simultaneous up-regulation in perilipin-1 (PLIN1) and adipsin (ADN) genes, additionally reiterated the adipogenic differentiation. Mechanistically, GSK3β pathway was found to be a major player, whereby inhibiting it with lithium chloride (LiCl) resulted in complete blockage of lipid accumulation, accompanied by complete down regulation of PLIN1 and ADN gene expression. In conclusion, these observations for the first time, lend evidence that exposure of CPF accompanied by RA directs commitment of C₃H₁₀T½ cells to adipogenic differentiation through a process involving a crosstalk at GSK3β signaling.

RARγ-C-Fos-PPARγ2 signaling rather than ROS generation is critical for all-trans retinoic acid-inhibited adipocyte differentiation

Obesity has become a worldwide public health problem, which is mainly determined by excess energy intake and adipose tissue expansion. Adipose tissue expansion can occur through hyperplasia (adipocyte differentiation) or hypertrophy. Retinoic acid was shown to inhibit adipocyte differentiation. However, the molecular mechanism is unclear. In the study, we found that all-trans-retinoic acid (ATRA) inhibited 3T3-L1 adipocyte differentiation. We did not observe significant apoptosis in differentiated adipocytes treated by ATRA. ATRA increased ROS generation and disturbed redox balance. However, antioxidant treatment did not ameliorate the reduction of lipid accumulation induced by ATRA, indicating that ROS generation was not involved in ATRA-inhibited adipocyte differentiation. ATRA reduced C/EBPα, PPARγ and its target gene expression. In the presence of ATRA, retinoic acid receptor (RAR) α/γ expression was increased. Inhibition of RARγ, but not RARα, blocked ATRA-induced reduction of PPARγ2 expression. ATRA induced a profound interaction between RARγ and C-Fos protein, reflected by Co-IP results. C-Fos was found to exhibit a differentiation-dependent DNA binding activity to PPARγ2 promoter. RARγ inhibitor significantly suppressed ATRA-inhibited DNA binding activity of C-Fos to PPARγ2 promoter, indicating that downregulation of C-Fos activity mediated activation of RARγ-exerted reduction of PPARγ2 expression and thus inhibition of adipocyte differentiation induced by ATRA. Taken together, these data demonstrates that RARγ-C-Fos-PPARγ2 signaling rather than ROS generation is critical for ATRA-inhibited adipocyte differentiation.

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

Background

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

Results

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

Conclusions

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

P311 functions in an alternative pathway of lipid accumulation that is induced by retinoic acid

Lipid droplets are complex and dynamic intracellular organelles that have an essential role in cholesterol and lipid homeostasis, and profoundly affect cellular structure and function. Variations in lipid-droplet composition exist between different cell types, but whether there are differences in the mechanisms of lipid-droplet accumulation remains to be elucidated. Here, we report that P311, previously identified to have a function in neuronal regeneration and a potential role in distal lung generation, regulates lipid droplet accumulation. P311 upregulates several classes of genes associated with lipid synthesis, significantly increases intracellular cholesterol and triglyceride levels, and increases intracellular lipid droplets. Interestingly, P311 expression is not necessary for lipogenesis in the well-established NIH3T3-L1 cell model of adipogenic differentiation. Instead, we demonstrate a novel role for P311 in an alternative pathway of lipid-droplet accumulation that is induced by the regeneration-inducing molecule retinoic acid.

Studies on structure-activity relationships of retinoic acid receptor ligands by means of molecular modeling

Vitamin A and its biologically active derivatives, retinal and retinoic acid, play an important role in vision, are required for reproduction, act as morphogenic agents during embryonic development, and regulate the growth and differentiation of a wide variety of cell types throughout the life of an organism. The biological action of retinoic acid and synthetic analogs, referred to as retinoids, is mediated by RAR alpha, RAR beta, or RAR gamma and/or by RXR alpha, RXR beta, or RXR gamma, all being nuclear receptors. Since retinoids exert profound effects on cell proliferation, differentiation, and apoptosis, these compounds seem to be promising agents for the treatment of cancer. Consequently, a large number of retinoids have been synthesized and examined to determine if they exert their biological activity according to retinoic acid receptor interaction. These screening methods are often expensive, time-consuming, and labor-intensive procedures. Since one can construct the pharmacophores of congeneric groups of drug molecules, molecular modeling techniques offer a new way to determine the binding abilities of different agents. We examined the structural properties of retinoids, which allow them to specifically bind to the different receptor subtypes. The thus-generated 3D pharmacophore models were used to predict the binding affinities of several retinoids to the retinoic receptor subtypes. Finally, the 3D models served as criteria for searching the Derwent World Drug Index for compounds that possess the features necessary for favorable ligand receptor interaction. The search resulted in a "hit list" containing 323 compounds, some of which are worth further investigation to determine if they act via retinoic acid receptor binding or not.

Understanding adipocyte differentiation

The adipocyte plays a critical role in energy balance. Adipose tissue growth involves an increase in adipocyte size and the formation of new adipocytes from precursor cells. For the last 20 years, the cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems. Committed preadipocytes undergo growth arrest and subsequent terminal differentiation into adipocytes. This is accompanied by a dramatic increase in expression of adipocyte genes including adipocyte fatty acid binding protein and lipid-metabolizing enzymes. Characterization of regulatory regions of adipose-specific genes has led to the identification of the transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein (C/EBP), which play a key role in the complex transcriptional cascade during adipocyte differentiation. Growth and differentiation of preadipocytes is controlled by communication between individual cells or between cells and the extracellular environment. Various hormones and growth factors that affect adipocyte differentiation in a positive or negative manner have been identified. In addition, components involved in cell-cell or cell-matrix interactions such as preadipocyte factor-1 and extracellular matrix proteins are also pivotal in regulating the differentiation process. Identification of these molecules has yielded clues to the biochemical pathways that ultimately result in transcriptional activation via PPAR-gamma and C/EBP. Studies on the regulation of the these transcription factors and the mode of action of various agents that influence adipocyte differentiation will reveal the physiological and pathophysiological mechanisms underlying adipose tissue development.

Regulation of retinoidal actions by diazepinylbenzoic acids. Retinoid synergists which activate the RXR-RAR heterodimers

In human HL-60 promyelocytic leukemia cells, diazepinylbenzoic acid derivatives can exhibit either antagonistic or synergistic effects on the differentiation-inducing activities of natural or synthetic retinoids, the activity depending largely on the nature of the substituents on the diazepine ring. Thus, a benzolog of the retinoid antagonist LE135 (6), 4-(13H-10,11,12,13-tetrahydro-10, 10,13,13,15-pentamethyldinaphtho[2,3-b][1,2-e]diazepin-7-yl) benzoic acid (LE540, 17), exhibits a 1 order of magnitude higher antagonistic potential than the parental LE135 (6). In contrast, 4-[5H-2,3-(2,5-dimethyl-2,5-hexano)-5-methyldibenzo[b,e] [1,4]diazepin-11-yl]-benzoic acid (HX600, 7), a structural isomer of the antagonistic LE135 (6), enhanced HL-60 cell differentiation induced by RAR agonists, such as Am80 (2). This synergistic effect was further increased for a thiazepine, HX630 (29), and an azepine derivative, HX640 (30); both synergized with Am80 (2) more potently than HX600 (7). Notably, the negative and positive effects of the azepine derivatives on retinoidal actions can be related to their RAR-antagonistic and RXR-agonistic properties, respectively, in the context of the RAR-RXR heterodimer.

Notch-1 controls the expression of fatty acid-activated transcription factors and is required for adipogenesis

Notch, a transmembrane receptor member of the homeotic epidermal growth factor-like family of proteins, participates in cell-to-cell signaling to control cell fate during development. Activated Notch-1 constructs lacking the extracellular region prevent differentiation of several mammalian cells in vitro. This effect, however, bypasses the normal mechanisms of cell-to-cell interactions in which Notch-1 participates. We investigated the role of Notch-1 in the hormone-induced adipocyte differentiation of 3T3-L1 fibroblasts, a paradigmatic model of adipogenesis that requires cell-to-cell contact. Unlike other differentiation models, Notch-1 expression and function were necessary conditions for adipogenesis. Impaired Notch-1 expression by antisense Notch-1 constructs prevented adipocyte differentiation. Strategies aimed at blocking putative Notch/ligand interactions also blocked adipogenesis, implicating Notch as a critical molecule in cell-to-cell signaling necessary for differentiation. Inhibition of Notch-1 expression or function decreased the expression of peroxisomal proliferator-activated receptors delta and gamma, transcription factors that control adipocyte differentiation and that are up-regulated at cell confluence. These results implicate Notch in the commitment of 3T3-L1 cells to undergo adipogenesis by controlling the expression of the principal regulators of this process.

Perilla oil prevents the excessive growth of visceral adipose tissue in rats by down-regulating adipocyte differentiation

We examined the effect of dietary oils with different fatty acid compositions on the growth of visceral adipose tissue in rats. Rats were fed for 4 mo starting at weaning a basal diet containing (12 g/100 g diet) perilla oil rich in (n-3) polyunsaturated fatty acids (PUFA), safflower oil rich in (n-6) PUFA, olive oil rich in monounsaturated fatty acid, or beef tallow rich in saturated fatty acids. The amount of food consumed and body weight gain did not differ among the four dietary groups. The weight of the epididymal fat pad and the serum triglyceride concentration in perilla oil-fed rats were significantly lower (P < 0.05) than those of olive oil- and beef tallow-fed groups. The product of [(volume of individual adipocytes) x (number of adipocytes in epididymal fat pad)], which presumably represents total adipocyte volume in the fat pad, was significantly lower (P < 0.05) in perilla oil-fed rats than in beef tallow- and olive oil-fed groups. Expression of the late genes of adipocyte differentiation, peroxisome proliferator-activated receptor alpha, adipocyte P2 and adipsin, was significantly (P < 0. 05) down-regulated in epididymal fat tissue of rats that had been fed perilla oil rather than beef tallow or olive oil, whereas expression of the early gene, lipoprotein lipase, was not significantly affected. Greater levels (P < 0.05) of (n-3) PUFA in the membrane phospholipid fraction of the fat tissue were observed in perilla oil-fed rats than in the other dietary groups. These results suggest that perilla oil or (n-3) PUFA prevents excessive growth of adipose tissue in rats at least in part by suppressing the late phase of adipocyte differentiation.

Action mechanism of retinoid-synergistic dibenzodiazepines

4-[5H-2,3-(2,5-Dimethyl-2,5-hexano)-5-methyldibenzo[b,e][1,4 ]diazepin-11-yl]benzoic acid (HX600), as well as its oxa- (HX620) and thia- (HX630) analogs, enhanced the activity of retinoic acid and a receptor alpha (RAR alpha)-selective agonist Am80 in HL-60 cell differentiation assays. HX600 synergizes with Am80 by binding to, and transactivating through, the RXR subunit of the RXR-RAR heterodimer. HX600 exhibited RXR pan-agonist activity in transient transfections with a DR1-based reporter gene and synergized with RA-bound RAR alpha and RAR beta in inducing transcription from a DR5-based reporter. In addition, all three compounds at high concentrations acted as RAR pan-antagonists in stably transfected RAR "reporter cells." These efficient synergists bind only weakly with RXRs in vitro, suggesting that they are RXR-RAR heterodimer-selective activators. These HX retinoids exhibited dual functionality, since they affected signalling through both retinoid receptor families (RARs and RXRs).

Calcitriol is a positive effector of adipose differentiation in the OB 17 cell line: relationship with the adipogenic action of triiodothyronine

In a previous report, we showed that physiological concentrations of calcitriol (1 alpha,25-(OH)2 vitamin D3 or VD), markedly stimulated the terminal adipose differentiation of Ob 17 preadipocytes cultured under standard conditions with fetal calf serum (FCS), and increased the differentiating effect of triiodothyronine (T3) reported as a necessary adipogenic factor in these cells. Here, we demonstrate, for the first time, that VD is an intrinsic strong adipogenic factor for the Ob 17 preadipocytes cultured in thyroid hormone-deprived medium (adipogenic concentrations: 0.025-0.25 nM in the presence of stripped FCS, 1-10 pM under serum-free conditions). VD action was potentiated by the coaddition of either T3, or arachidonic acid, two agents which also bear proper adipogenic properties. The efficient concentration ranges of other vitamin D3 metabolites suggest a mediation through the VD nuclear receptor (VDR). An expression of the VDR gene is here demonstrated in the Ob 17 cells, and evidence is given that VDR mRNA level increased during the differentiation process and that this increase is moderately amplified under long term treatment with adipogenic concentrations of VD. Our results strongly suggest that adipose differentiation is under the control of different closely related nuclear receptors acting at an early preadipocyte step and probably in an interchangeable manner depending on the availability of their respective ligands. The existence of an interplay between these receptors in exerting their adipogenic action is suggested.

Peroxisome proliferator-activated receptors: structures and function

We have been attempting to elucidate the molecular mechanisms through which peroxisome proliferators exert their pleiotropic effects, with particular emphasis on understanding why humans appear unresponsive to these compounds. There is a wealth of data to implicate the peroxisome proliferator-activated receptor alpha (PPAR alpha) in mediating these effects in rodent species; PPAR alpha is expressed in tissues that show physiological changes in response to PPs, is transcriptionally activated in vitro by a variety of PPs, and it has been recently demonstrated that mice lacking this receptor are refractory to the effects of clofibrate and Wy-14,643, at least in the short term. It is conceivable that differences in PPAR alpha between responsive rodent and unresponsive human subjects may provide the key to understanding the basis of this species variation in response, and with this in mind we have been studying the biology of PPAR alpha in humans and looking at interindividual variation. There is already published evidence, albeit on only two sequences, for structural and functional polymorphism in human PPAR alphas. We have extended these findings, and shown that: There is considerable variation in hPPAR alpha cDNAs obtained from different individuals, both at the gross structural level (lack of a coding exon) and of a more subtle nature (single base changes leading to amino acid substitutions). One such cDNA, the sequence of which differs at only three amino acids from that published, encodes a receptor that is incapable of transcriptional activation by potent PPs. The degree to which hPPAR alpha transcripts are expressed in human livers can vary by up to an order of magnitude between individuals. The tissue-specific expression profile of PPAR alpha in humans is very different from that in rat and mouse. In particular, the human liver contains generally low levels of PPAR alpha in contrast to the responsive rodents, in which potent PPs cause liver tumors. Taken together, these data suggest first that human and rodent PPAR alphas differ according to a number of molecular and biochemical criteria, and secondly that there is a degree of interindividual variation in PPAR alpha structure and function. Studies are ongoing to clarify this further, but human polymorphism may go some way towards explaining the apparent paradox that active PPAR alpha receptors can be isolated from an "unresponsive" species.

Essential cis-acting elements in rat uncoupling protein gene are in an enhancer containing a complex retinoic acid response domain

Transgenic mice were generated with a transgene containing the 211-base pair (bp) enhancer and 0.4 kilobase pairs of 5'-flanking DNA of the uncoupling protein (ucp) gene. Expression of this transgene was restricted to brown adipose tissue and was inducible by cold exposure or treatment of transgenic mice by norepinephrine, retinoic acid (RA), or CL-316,243 beta3-adrenoreceptor agonist. A search for retinoic acid response elements in the ucp gene enhancer was undertaken using mutagenesis and transfection of cultured cells with chloramphenicol acetyltransferase constructs. Deletion or mutations of several putative retinoic acid response elements were ineffective. Mutations of a TGAATCA region dramatically decreased the transcriptional activity in the presence of RA. In vitro this region was able to bind a complex containing proteins recognized by antibodies against Jun or Fos. Mutations of an adjacent region related to an inverted repeat of type 2 also markedly decreased RA effect. This region was able to bind in vitro retinoid X receptor alpha and retinoic acid receptor beta. The two regions form an activating region between bp -2421 and -2402 (referred to as the ucp gene-activating region), which has an enhancer activity but cannot confer RA response to a promoter. This response was obtained with a larger DNA fragment (bp -2489 to -2398) constituting a complex RA response domain.

Distinct stages in adipogenesis revealed by retinoid inhibition of differentiation after induction of PPARgamma

Retinoic acid (RA) inhibits adipocyte differentiation of 3T3-L1 preadipocytes but is effective only early in adipogenesis. RA prevented induction of the adipogenic factors PPARgamma and C/EBPalpha. Using receptor-specific ligands, we determined that the effects of RA were mediated by liganded RA receptors (RARs) rather than retinoid X receptors. Preadipocytes expressed primarily RARalpha and RARgamma; during adipocyte differentiation, RARalpha gene expression was nearly constant, whereas RARgamma1 mRNA and protein levels dramatically decreased. Ectopic expression of RARgamma1 extended the period of effectiveness of RA by 24 to 48h; RARalpha expression had a similar effect, suggesting functional redundancy of RAR subtypes. Remarkably, RA inhibited differentiation when added after PPARgamma1 and PPARgamma2 proteins had already been expressed and resulted in the loss of PPARgamma proteins from cells. By 72 to 96 h after the induction of differentiation, RA failed to prevent differentiation of even ectopic-RAR-expressing cells. Thus, the unresponsiveness of 3T3-L1 preadipocytes to RA after the induction of differentiation is initially due to the reduction in cellular RAR concentration rather than to the induction of PPARgamma. At later times cells continue along the differentiation pathway in a manner which is RA and RAR independent.