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

GPR55 inhibits the pro-adipogenic activity of anandamide in human adipose stromal cells

The endocannabinoid anandamide (AEA) stimulates adipogenesis via the cannabinoid receptor CB1 in adipose stromal cells (ASCs). However, AEA interacts also with nonclassical cannabinoid receptors, including transient receptor potential cation channel (TRPV)1 and G protein-coupled receptor (GPR)55. Their roles in AEA mediated adipogenesis of human ASCs have not been investigated. We examined the receptor-expressions by immunostaining on human ASCs and tested their functionality by measuring the expression of immediate early genes (IEGs) related to the transcription factor-complex AP-1 upon exposition to receptor agonists. Cells were stimulated with increasing concentrations of specific ligands to investigate the effects on ASC viability (proliferation and metabolic activity), secretory activity, and AEA mediated differentiation. ASCs expressed both receptors, and their activation suppressed IEG expression. TRPV1 did not affect viability or cytokine secretion. GPR55 decreased proliferation, and it inhibited the release of hepatocyte growth factor. Blocking GPR55 increased the pro-adipogenic activity of AEA. These data suggest that GPR55 functions as negative regulator of cannabinoid mediated pro-adipogenic capacity in ASCs.

Dietary and plasma retinoids are not associated with fatty acid desaturase indices in healthy young adults

Past research in rodents suggests that fatty acid (FA) desaturase expression and activity may be modified by vitamin A; however, this has not been investigated in humans. The primary objective of this study was to examine associations between dietary retinoid intakes, plasma retinoid concentrations, and FA desaturase indices in young adults. As a secondary objective, biological sex and estrogen-containing contraceptive (EC) use were investigated due to prior evidence demonstrating that both can influence plasma retinol concentration and FA desaturase indices. Dietary retinoid intake (food frequency questionnaire), plasma retinoid concentrations (high-performance liquid chromatography-tandem mass spectrometry), plasma FA (gas chromatography), and FA desaturase indices (product-to-precursor ratios) from 945 adults recruited for the cross-sectional Toronto Nutrigenomics and Health study were analyzed. Participants were stratified into quartiles based on plasma retinol concentration and data analyzed by one-way analysis of covariance. Dietary retinoid intakes were not associated with the overall n-3 pathway, overall n-6 pathway, delta-5 desaturase, delta-6 desaturase, or delta-9 desaturase indices (all r < 0.10, p > 0.05). The overall n-6 pathway index was significantly higher (p = 0.0004) and the delta-5 desaturase index was significantly lower (p = 0.0003) in individuals with higher plasma retinol levels; however, these differences were lost when participants were grouped by biological sex and EC use. Although weak relationships were observed between plasma retinol and some FA desaturase indices in the total population, these associations appear to be driven by biological sex and EC usage rather than retinoids. We therefore find little evidence of a relationship between retinoids and FA desaturase indices in young, healthy adults.

Pigment Epithelium-Derived Factor: Inhibition of Phosphorylation of Insulin Receptor (IR)/IR Substrate (IRS), Osteogeneration from Adipocytes, and Increased Levels Due to Doxorubicin Exposure

OBJECTIVES

Pigment epithelium-derived factor (PEDF) has been recently linked to insulin resistance and is capable of differentiating myocytes to bone. We examined in more detail the intricate signalling of the insulin pathway influenced by PEDF in skeletal myocytes. We tested whether this serpin is also capable of generating de novo bone from adipocytes in vitro and in vivo, and how the anticancer drug doxorubicin links with PEDF and cellular metabolism.

METHODS AND KEY FINDINGS

We demonstrate that PEDF can inhibit phosphorylation of insulin receptor (IR) and insulin receptor substrate (IRS) in skeletal myocytes. PEDF constitutively activates p42/44 MAPK/Erk, but paradoxically does not affect mitogenic signalling. PEDF did not perturb either mitochondrial activity or proliferation in cells representing mesenchymal stem cells, cardiomyocytes, and skeletal myocytes and adipocytes. PEDF induced transdifferentiation of adipocytes to osteoblasts, promoting bone formation in cultured adipocytes in vitro and gelfoam fatpad implants in vivo. Bone formation in white adipose tissue (WAT) was better than in brown adipose tissue (BAT). The frontline anticancer drug doxorubicin increased levels of PEDF in a human breast cancer cell line, mirroring the in vivo finding where cardiac muscle tissue was stained increasingly for PEDF as the dose of doxorubicin increased in mice. PEDF also increased levels of reactive oxygen species (ROS) and glutathione (GSH) in the breast cancer cell line.

CONCLUSIONS

PEDF may be used to regenerate bone from adipose tissue in cases of trauma such as fractures or bone cancers. The increased presence of PEDF in doxorubicin-treated tumour cells need further exploration, and could be useful therapeutically in future. The safety of PEDF administration in vivo was further demonstrated in this study.

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.

ALDH1A1 Inhibits Chicken Preadipocytes' Proliferation and Differentiation via the PPARγ Pathway In Vitro and In Vivo

ALDH1A1 (aldehyde dehydrogenase 1A1) is a crucial protein in retinoids’ metabolism, and the lack of ALDH1A1 inhibits the fat deposition in mice. However, whether ALDH1A1 has a similar effect on chickens’ fat-depot is still unknown. In this study, we investigate the role of ALDH1A1 in chickens’ adipogenesis. The immortalized chicken preadipocyte 1 (ICP1) cell line and chicken primary preadipocytes isolated from abdominal fat were used to perform a series of experiments in vitro to elucidate the effects of ALDH1A1. In addition, lentivirus was used to verify the results of cell experiments in vivo. The data showed that overexpression of ALDH1A1 significantly weakened the proliferation of preadipocytes and suppressed the differentiation of preadipocytes through the PPARγ pathway, and the knockdown experiments had the opposite results. Moreover, chickens injected with overexpression lentivirus had higher abdominal fat percentage, a bigger size of lipid droplets, and higher triglyceride content in abdominal fat, and chickens injected with interfering lentivirus had the opposite situation. We proved that ALDH1A1 not only inhibited the proliferation and differentiation of chickens’ preadipocytes in vitro, but also inhibited the fat-depot of chickens in vivo, which was completely opposite the function of ALDH1A1 in mice, indicating that ALDH1A1 may have a different mechanism that is still unknown.

RDH1 suppresses adiposity by promoting brown adipose adaptation to fasting and re-feeding

RDH1 is one of the several enzymes that catalyze the first of the two reactions to convert retinol into all-trans-retinoic acid (atRA). Here, we show that Rdh1-null mice fed a low-fat diet gain more weight as adiposity (17% males, 13% females) than wild-type mice by 20 weeks old, despite neither consuming more calories nor decreasing activity. Glucose intolerance and insulin resistance develop following increased adiposity. Despite the increase in white fat pads, epididymal white adipose does not express Rdh1, nor does muscle. Brown adipose tissue (BAT) and liver express Rdh1 at relatively high levels compared to other tissues. Rdh1 ablation lowered body temperatures during ambient conditions. Given the decreased body temperature, we focused on BAT. A lack of differences in BAT adipogenic gene expression between Rdh1-null mice and wild-type mice, including Pparg, Prdm16, Zfp516 and Zfp521, indicated that the phenotype was not driven by brown adipose hyperplasia. Rather, Rdh1 ablation eliminated the increase in BAT atRA that occurs after re-feeding. This disruption of atRA homeostasis increased fatty acid uptake, but attenuated lipolysis in primary brown adipocytes, resulting in increased lipid content and larger lipid droplets. Rdh1 ablation also decreased mitochondrial proteins, including CYCS and UCP1, the mitochondria oxygen consumption rate, and disrupted the mitochondria membrane potential, further reflecting impaired BAT function, resulting in both BAT and white adipose hypertrophy. RNAseq revealed dysregulation of 424 BAT genes in null mice, which segregated predominantly into differences after fasting vs after re-feeding. Exceptions were Rbp4 and Gbp2b, which increased during both dietary conditions. Rbp4 encodes the serum retinol-binding protein-an insulin desensitizer. Gbp2b encodes a GTPase. Because Gbp2b increased several hundred-fold, we overexpressed it in brown adipocytes. This caused a shift to larger lipid droplets, suggesting that GBP2b affects signaling downstream of the β-adrenergic receptor during basal thermogenesis. Thus, Rdh1-generated atRA in BAT regulates multiple genes that promote BAT adaptation to whole-body energy status, such as fasting and re-feeding. These gene expression changes promote optimum mitochondria function and thermogenesis, limiting adiposity. Attenuation of adiposity and insulin resistance suggests that RDH1 mitigates metabolic syndrome.

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

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

RXRG associated in PPAR signal regulated the differentiation of primordial germ cell

Primitive germ cells (PGCs) are the "ancestor" of sex germ cells. However, their regulation mechanism is not clear at present. The aim of this study is to investigate the effect of PPAR signaling pathway on the differentiation of chicken embryonic stem cells (ESCs) into PGCs, providing theoretical support for the further study of the mechanism of regulation of chicken PGCs. Based on the results of RNA-Seq analysis, the key signal pathway PPAR and the key signal molecule RXRG in the pathway were successfully screened. Also, we made the PPAR signal transduction pathway be activated in vivo and in vitro. The results showed that the expression of FABP3 and SCD-1 was up-regulated by high expression of RXRG, which significantly up-regulated the expression of Cvh, C-kit, and the ratio of Cvh + C-kit+ cells was higher than that of control group. The formation of PGCs was blocked by interfering with PPAR signaling pathway. We conclude that RXRG positively regulates the PPAR signaling pathway and promotes differentiation of chicken ESCs into PGCs. Thus, gene RXRG can be used to study the regulation of PGCs differentiation in chickens.

Combined effect of retinoic acid and calcium on the in vitro differentiation of human adipose-derived stem cells to adipocytes

CONTEXT

It has been shown that adipogenesis can be modulated by factors such as all-trans retinoic acid (ATRA) and calcium.

OBJECTIVE

To determine, the combined effect of ATRA and calcium on the differentiation of human adipose-derived stem cells (hADSCs).

METHODS

Mesenchymal stem cells (MSCs) were differentiated into the adipocytes by 0.5 and 1 µM of ATRA and 5 and 10 mM calcium separately or in combination. After MTS assay the differentiation of MSCs to adipocyte was evaluated, Oil Red O staining, GLUT4 concentration and gene expression of PPARG2, adiponectin, and GLUT4 were measured by Real-Time PCR.

RESULTS

Except 10 mM calcium treated group, other groups and more significantly combination treatments could reduce all adipocyte markers compared to the control.

CONCLUSION

These results suggest that ATRA and calcium together have significant inhibitory effect on adipogenesis that can be helpful for finding new mechanisms to prevent or control the adipogenesis.

Raldh1 promotes adiposity during adolescence independently of retinal signaling

All-trans-retinoic acid (RA) inhibits adipogenesis in established preadipocyte cell lines. Dosing pharmacological amounts of RA reduces weight gain in mice fed a high-fat diet, i.e. counteracts diet-induced obesity (DIO). The aldehyde dehydrogenase Raldh1 (Aldh1a1) functions as one of three enzymes that converts the retinol metabolite retinal into RA, and one of many proteins that contribute to RA homeostasis. Female Raldh1-ablated mice resist DIO. This phenotype contrasts with ablations of other enzymes and binding-proteins that maintain RA homeostasis, which gain adiposity. The phenotype observed prompted the conclusion that loss of Raldh1 causes an increase in adipose tissue retinal, and therefore, retinal functions independently of RA to prevent DIO. A second deduction proposed that low nM concentrations of RA stimulate adipogenesis, in contrast to higher concentrations. Using peer-reviewed LC/MS/MS assays developed and validated for quantifying tissue RA and retinal, we show that endogenous retinal and RA concentrations in adipose tissues from Raldh1-null mice do not correlate with the phenotype. Moreover, male Raldh1-null mice resist weight gain regardless of dietary fat content. Resistance to weight gain occurs during adolescence in both sexes. We show that RA concentrations as low as 1 nM, i.e. in the sub-physiological range, impair adipogenesis of embryonic fibroblasts from wild-type mice. Embryonic fibroblasts from Raldh1-null mice resist differentiating into adipocytes, but retain ability to generate RA. These fibroblasts remain sensitive to an RA receptor pan-agonist, and are not affected by an RA receptor pan-antagonist. Thus, the data do not support the hypothesis that retinal itself represses weight gain and adipogenesis independently of RA. Instead, the data indicate that Raldh1 functions as a retinal and atRA-independent promoter of adiposity during adolescence, and enhances adiposity through pre-adipocyte cell autonomous actions.

Vitamin A Improves Hyperglycemia and Glucose-Intolerance through Regulation of Intracellular Signaling Pathways and Glycogen Synthesis in WNIN/GR-Ob Obese Rat Model

Vitamin A and its metabolites modulate insulin resistance and regulate stearoyl-CoA desaturase 1 (SCD1), which are also known to affect insulin resistance. Here, we tested, whether vitamin A-mediated changes in insulin resistance markers are associated with SCD1 regulation or not. For this purpose, 30-week old male lean and glucose-intolerant obese rats of WNIN/GR-Ob strain were given either a stock or vitamin A-enriched diet, i.e. 2.6 mg or 129 mg vitamin A/kg diet, for 14 weeks. Compared to the stock diet, vitamin A-enriched diet feeding improved hyperglycemia and glucose-clearance rate in obese rats and no such changes were seen in lean rats receiving identical diets. These changes were corroborated with concomitant increase in circulatory insulin and glycogen levels of liver and muscle (whose insulin signaling pathway genes were up-regulated) in obese rats. Further, the observed increase in muscle glycogen content in these obese rats could be explained by increased levels of the active form of glycogen synthase, the key regulator of glycogen synthesis pathway, possibly inactivated through increased phosphorylation of its upstream inhibitor, glycogen synthase kinase. However, the unaltered hepatic SCD1 protein expression (despite decreased mRNA level) and increased muscle-SCD1 expression (both at gene and protein levels) suggest that vitamin A-mediated changes on glucose metabolism are not associated with SCD1 regulation. Chronic consumption of vitamin A-enriched diet improved hyperglycemia and glucose-intolerance, possibly, through the regulation of intracellular signaling and glycogen synthesis pathways of muscle and liver, but not associated with SCD1.

Inhibition of fat cell differentiation in 3T3-L1 pre-adipocytes by all-trans retinoic acid: Integrative analysis of transcriptomic and phenotypic data

The process of adipogenesis is controlled in a highly orchestrated manner, including transcriptional and post-transcriptional events. In developing 3T3-L1 pre-adipocytes, this program can be interrupted by all-trans retinoic acid (ATRA). To examine this inhibiting impact by ATRA, we generated large-scale transcriptomic data on the microRNA and mRNA level. Non-coding RNAs such as microRNAs represent a field in RNA turnover, which is very important for understanding the regulation of mRNA gene expression. High throughput mRNA and microRNA expression profiling was performed using mRNA hybridisation microarray technology and multiplexed expression assay for microRNA quantification. After quantitative measurements we merged expression data sets, integrated the results and analysed the molecular regulation of in vitro adipogenesis. For this purpose, we applied local enrichment analysis on the integrative microRNA-mRNA network determined by a linear regression approach. This approach includes the target predictions of TargetScan Mouse 5.2 and 23 pre-selected, significantly regulated microRNAs as well as Affymetrix microarray mRNA data. We found that the cellular lipid metabolism is negatively affected by ATRA. Furthermore, we were able to show that microRNA 27a and/or microRNA 96 are important regulators of gap junction signalling, the rearrangement of the actin cytoskeleton as well as the citric acid cycle, which represent the most affected pathways with regard to inhibitory effects of ATRA in 3T3-L1 preadipocytes. In conclusion, the experimental workflow and the integrative microRNA–mRNA data analysis shown in this study represent a possibility for illustrating interactions in highly orchestrated biological processes. Further the applied global microRNA–mRNA interaction network may also be used for the pre-selection of potential new biomarkers with regard to obesity or for the identification of new pharmaceutical targets.

Carrot Juice Administration Decreases Liver Stearoyl-CoA Desaturase 1 and Improves Docosahexaenoic Acid Levels, but Not Steatosis in High Fructose Diet-Fed Weanling Wistar Rats

Non-alcoholic fatty liver disease (NAFLD) is one of the most prevalent liver diseases associated with an altered lifestyle, besides genetic factors. The control and management of NAFLD mostly depend on lifestyle modifications, due to the lack of a specific therapeutic approach. In this context, we assessed the effect of carrot juice on the development of high fructose-induced hepatic steatosis. For this purpose, male weanling Wistar rats were divided into 4 groups, fed either a control (Con) or high fructose (HFr) diet of AIN93G composition, with or without carrot juice (CJ) for 8 weeks. At the end of the experimental period, plasma biochemical markers, such as triglycerides, alanine aminotransferase, and β-hydroxy butyrate levels were comparable among the 4 groups. Although, the liver injury marker, aspartate aminotransferase, levels in plasma showed a reduction, hepatic triglycerides levels were not significantly reduced by carrot juice ingestion in the HFr diet-fed rats (HFr-CJ). On the other hand, the key triglyceride synthesis pathway enzyme, hepatic stearoyl-CoA desaturase 1 (SCD1), expression at mRNA level was augmented by carrot juice ingestion, while their protein levels showed a significant reduction, which corroborated with decreased monounsaturated fatty acids (MUFA), particularly palmitoleic (C16:1) and oleic (C18:1) acids. Notably, it also improved the long chain n-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA; C22:6) content of the liver in HFr-CJ. In conclusion, carrot juice ingestion decreased the SCD1-mediated production of MUFA and improved DHA levels in liver, under high fructose diet-fed conditions. However, these changes did not significantly lower the hepatic triglyceride levels.

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.

Aromatase activity induction in human adipose fibroblasts by retinoic acids via retinoic acid receptor α

Estrogen synthesis in adipose tissue is associated with the development of breast cancer. Tumors are preferentially found in breast quadrants with strongest expression of the cytochrome P450 aromatase (encoded by the gene CYP19A1). Several promoters regulated by various hormonal factors drive aromatase expression in human breast adipose fibroblasts (BAFs). As adipose tissue is a major source of retinoids, in this study, we investigated their role in the regulation of aromatase expression. The retinoids all-trans-retinoic acid (at-RA) and 9-cis-RA induce aromatase activity in human BAFs. In BAFs, at-RA induces aromatase gene expression via promoter I.4. In 3T3-L1 cells, both retinoids specifically drive luciferase reporter gene expression under the control of aromatase promoter I.4, whereas other promoters active in human adipose tissue are insensitive. Activation by retinoids depends on a 467 bp fragment (-256/+211) of promoter I.4 containing four putative retinoic acid response elements (RAREs). Site-directed mutagenesis revealed that only RARE2 (+91/+105) mediates the retinoid-dependent induction of reporter gene activity. In 3T3-L1 preadipocytes and human BAFs, RA receptor α (RARα (RARA)) expression is predominant, whereas RARβ (RARB) or RARγ (RARG) expression is low. Electrophoretic mobility shift assays with nuclear extracts obtained from human BAFs and 3T3-L1 cells identified a specific RARE2-binding complex. Retinoids enhanced complex formation, whereas pre-incubation with anti-RARα antibodies prohibited the binding of RARα to RARE2. Chromatin immunoprecipitation showed RA-dependent binding of RARα to the RARE2-containing promoter region in vivo. Furthermore, we provide evidence that RARE2 is also necessary for the basal activation of promoter I.4 in these cells. Taken together, these findings indicate a novel retinoid-dependent mechanism of aromatase activity induction in adipose tissue.

Effect of retinoic acid on gene expression profiles of bovine intramuscular preadipocytes during adipogenesis

To investigate genes involved in intramuscular adipogenesis in ruminants, 16 genes with dramatic variable expression were selected. These were selected from the differentiation- and proliferation-phase libraries of our previous serial analysis of gene expression (SAGE) studies of a clonal bovine intramuscular preadipocyte (BIP) cell line. We harvested the BIP cells over 12 days after adipogenic stimulation with all-trans retinoic acid (ATRA). Quantitative real-time PCR confirmed the earlier SAGE study results of the expression patterns of 15 of the genes. On day 6, TG accumulation increased significantly in the BIP cells but was completely inhibited in the 3T3-L1 cells (the monogastric reference). ATRA enhanced expression levels of six genes whereas it suppressed expression of eight genes on day 3 of adipogenesis in the BIP cells. Forty-eight hours after transfection, the messenger RNA expression level of the adipose differentiation-related protein (ADFP), encoded by one of the upregulated genes, in the ADFP small interference RNA (siRNA)-transfected cells was 3.5% of that in negative control-transfected cells. Also, 6 days after induction the TG level in the ADFP siRNA-transfected cells was 21.8% lower than that in negative control-transfected cells. This analysis of gene expression profiles after ATRA treatment will contribute to our understanding of the molecular mechanisms involved in bovine intramuscular adipogenesis.

Retinoic acid inhibits adipogenesis via activation of Wnt signaling pathway in 3T3-L1 preadipocytes

Although retinoic acid (RA) is well known to inhibit the differentiation of 3T3-L1 cells into adipocytes both in vivo and in vitro, its molecular mechanism is not fully understood. In this report, we investigate the inhibitory mechanism of adipocyte differentiation by RA in 3T3-L1 cells. Because both RA and Wnt are known to inhibit adipogenesis at a common step involving the inhibition of PPAR-γ expression, we focused on the crosstalk between these two signaling pathways. We found that RA treatment resulted in a dramatic inhibition of adipogenesis, especially at an early phase of differentiation, and led to increased β-catenin protein expression. Moreover, RA enhances the transcriptional activity of β-catenin as well as Wnt gene expression during adipogenesis. Taken together, the present study demonstrated that Wnt/β-catenin signaling may be associated with the RA-induced suppression of adipogenesis and may cooperatively inhibit adipocyte differentiation.

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

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

Lipophilic micronutrients and adipose tissue biology

Lipophilic micronutrients (LM) constitute a large family of molecules including several vitamins (A, D, E, K) and carotenoids. Their ability to regulate gene expression is becoming increasingly clear and constitutes an important part of nutrigenomics. Interestingly, adipose tissue is not only a main storage site for these molecules within the body, but it is also subjected to the regulatory effects of LM. Indeed, several gene regulations have been described in adipose tissue that could strongly impact its biology with respect to the modulation of adipogenesis, inflammatory status, or energy homeostasis and metabolism, among others. The repercussions in terms of health effects of such regulations in the context of obesity and associated pathologies represent an exciting and emerging field of research. The present review will focus on the regulatory effects of vitamin A, D, E and K as well as carotenoids on adipose tissue biology and physiology, notably in the context of obesity and associated disorders.

The effects of retinoic acid on reversing the adipocyte differentiation into an osteoblastic tendency in ST2 cells, a murine bone marrow-derived stromal cell line

Although the mouse bone marrow stromal cell line ST2 has been known to be differentiated into osteoblasts, the differentiation characteristics of the cell into adipocyte and the concerned relationship between its adipogenesis and osteogenesis remains unknown. The adipogenic induction medium which is made up of insulin, dexamethasone (DEX) and 3-isobutyl-1-methylxanthine(IBMX), stimulated the expression of n early adipogenic marker PPAR gamma and a late marker GPDH in ST2 cells. The triglyceride accumulation and lipid stain level generated by the induction medium in ST2 cells was inhibited by RA with IC(50) at about 1 nM. The induction medium up-regulated expression of PPARgamma and GPDH was also inhibited by RA whereas RA (30 nM) exterted no effect on the cell growth. Interestingly, treatment of the cells with induction medium in the presense of RA caused a 3- or 10-fold higher in ALP activity respectively as compared to those treated with RA or the induction medium alone. RT-PCR analysis showed that such a synergistic effect of RA and the induction medium paralleled the process of inhibition on adipogenesis. Additional experiments showed that IBMX played a key role in increasing the effect of RA and ALP activity. Our results suggested that the relationship between adipogenesis and osteogenesis in ST2 cells was reciprocally interrelated and the process of adipogenesis could be potentially reversed into an osteoblastogenic tendency. This is the first report demonstrating that RA transforms adipogenic potential into an osteoblastic tendency.

Long-term Fenretinide treatment prevents high-fat diet-induced obesity, insulin resistance, and hepatic steatosis

The synthetic retinoid Fenretinide (FEN) increases insulin sensitivity in obese rodents and is in early clinical trials for treatment of insulin resistance in obese humans with hepatic steatosis (46). We aimed to determine the physiological mechanisms for the insulin-sensitizing effects of FEN. Wild-type mice were fed a high-fat diet (HFD) with or without FEN from 4-5 wk to 36-37 wk of age (preventive study) or following 22 wk of HF diet-induced obesity (12 wk intervention study). Retinol-binding protein-4 (RBP4) knockout mice were also fed the HFD with or without FEN in a preventive study. FEN had minimal effects on HFD-induced body weight gain but markedly reduced HFD-induced adiposity and hyperleptinemia in both studies. FEN-HFD mice gained epididymal fat but not subcutaneous or visceral fat mass in contrast to HFD mice without FEN. FEN did not have a measurable effect on energy expenditure, food intake, physical activity, or stool lipid content. Glucose infusion rate during hyperinsulinemic-euglycemic clamp was reduced 86% in HFD mice compared with controls and was improved 3.6-fold in FEN-HFD compared with HFD mice. FEN improved insulin action on glucose uptake and glycogen levels in muscle, insulin-stimulated suppression of hepatic glucose production, and suppression of serum FFA levels in HFD mice. Remarkably, FEN also reduced hepatic steatosis. In RBP4 knockout mice, FEN reduced the HFD-induced increase in adiposity and hyperleptinemia. In conclusion, long-term therapy with FEN partially prevents or reverses obesity, insulin resistance, and hepatic steatosis in mice on HFD. The anti-adiposity effects are independent of the RBP4 lowering effect.

Synergic effect of vitamin A and high-fat diet in adipose tissue development and nuclear receptor expression in young rats

In order to study the effects of dietary lipids and vitamin A on the development of adipose tissues, young rats were submitted for 8 d to a control or to two cafeteria diets with normal (Caf) or higher (Caf + ) vitamin A levels. Retinoid (retinoic acid receptor (RAR) a, RARg, retinoid X receptor(RXR) alpha) and fatty acid (PPARgamma) receptor mRNA was measured in the subcutaneous white adipose tissue (Swat) and in isolated mature adipocytes by RT-PCR. The stroma vascular fraction was cultured in vitro to test the capacities of the adipocyte precursors to proliferate and differentiate.The Caf diet enriched in vitamin A resulted in an increased adiposity, due to increased adipocyte hypertrophy. This was concomitant with a lower expression of RARa and RARg mRNA (234.6 and 238.6 %) and a higher expression of PPARgamma (+59 %) in the Swat and, to a less extent,in isolated adipocytes. Positive correlations were obtained between PPARgamma mRNA and Swat weights and between PPARgamma and RXRalpha mRNA. By contrast, RARgamma mRNA and Swat masses were negatively correlated. The adipocyte precursors from Caf + Swat proliferated more,in vitro, at the beginning of the culture. This difference progressively disappeared and was totally absent after 8 d of culture, but with a higher percentage of differentiated preadipocytes (+80.3 %) in the Caf + group. In conclusion, lipids and vitamin A act synergistically on the normal growth of the adipose tissue in young rats, concomitant with an imbalance in the pattern of the nuclear receptors. These changes influence the early normal development of the endogenous adipocyte precursors.

Identification of genes downregulated during differentiation of porcine mesenteric adipocytes

Adipose tissue development is a process that comprises not only hypertrophy, but also hyperplasia, of adipocytes. Although the proliferation of undifferentiated preadipocytes plays an important part in hyperplasia, this process is less well understood than the post-proliferation differentiation process. Despite the potential importance of porcine visceral adipose tissue to both meat production and biomedical research, there has been little study of this tissue and, in particular, its development and differentiation. To detect the genes involved in the maintenance of porcine visceral preadipocytes in an undifferentiated state or in the inhibition of adipocyte differentiation, we performed suppression subtractive hybridization using mesenteric preadipocytes in which fragments of the genes that are downregulated at 2 d of differentiation were enriched. We selected 672 clones and subjected them to differential screening and semiquantitative reverse transcription (RT)-PCR. As a result, we identified 34 downregulated genes. Among these, the detailed expression patterns of 6 genes were examined using real-time RT-PCR in both preadipocytes during in vitro differentiation and cell fractions directly isolated from pig mesenteric adipose tissue. The expressions of connective tissue growth factor, AXL receptor tyrosine kinase, stromal membrane-associated protein 1-like, and retinoic acid-induced 14 were significantly downregulated during adipocyte differentiation in vitro (P < 0.05), and the expressions of Rho/Rac guanine nucleotide exchange factor 2 and secreted frizzled-related protein 4 also tended to be decreased, although not significantly. Furthermore, all 6 genes showed significantly greater expression in stromal vascular cells, which contain preadipocytes, than in mature adipocytes (P < 0.05), raising the possibility that these genes are involved in adipocyte differentiation in vivo as well as in vitro.

Orphan nuclear receptor TR2, a mediator of preadipocyte proliferation, is differentially regulated by RA through exchange of coactivator PCAF with corepressor RIP140 on a platform molecule GRIP1

Orphan nuclear receptor TR2 is a preadipocyte proliferator. Knockdown of TR2 in 3T3-L1 preadipocytes reduced their proliferation efficiency, whereas specific elevation of TR2 in these cells facilitated their proliferation. All-trans retinoic acid (RA) stimulates cellular proliferation in 3T3-L1 preadipocytes by activating TR2 through an IR0-type RA response element, which further activates c-Myc expression. In post-differentiated adipocytes, RA becomes a repressive signal for TR2 and rapidly down-regulates its expression. The biphasic effect of RA on TR2 expression in 3T3-L1 is mediated by differential RA-dependent coregulator recruitment to the receptor/Glucocorticoid Receptor-Interacting Protein 1 (GRIP1) complex that binds IR0 on the TR2 promoter. RA induces the recruitment of histone acetyl transferase-containing/GRIP1/p300/CBP-associated factor (PCAF) complex to the TR2 promoter in undifferentiated cells, whereas it triggers recruitment of histone deacetylase-containing/GRIP1/receptor-interacting protein 140 (RIP140) complex in differentiated cells. GRIP1 directly interacts with RIP140 through its carboxyl terminal AD2 domain. GRIP1 interacts with PCAF and RIP140 directly and differentially, functioning as a platform molecule to mediate differential RA-induced coregulator recruitment to TR2 promoter target. This results in a biphasic effect of RA on the expression of TR2 in undifferentiated and differentiated cells, which is required for RA-stimulated preadipocyte proliferation.

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

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

Maturational factors modulate transcription factors CCAAT/enhancer-binding proteins alpha, beta, delta, and peroxisome proliferator-activated receptor-gamma in fetal rat lung epithelial cells

Previous investigations have evidenced the importance of CCAAT/enhancer-binding proteins (C/EBPs) and peroxisome proliferator-activated receptor (PPAR)gamma for lung development, especially for alveolar type II cells (ATII). This prompted us to explore whether ATII maturation-promoting mediators controlled their expression in isolated ATII. In whole rat lung, C/EBPalpha, beta, delta, and PPARgamma mRNAs increased 3-5 times between gestational day 18 and term (Day 22), dropped around birth, then reincreased. C/EBPbeta and delta, but not PPARgamma, displayed similar profile in isolated ATII; C/EBPalpha transcript disappeared and the protein became hardly detectable in isolated cells. In cultured ATII, dexamethasone increased C/EBPbeta and PPARgamma mRNAs 2-4 times, and cyclic AMP increased C/EBPbeta and delta mRNAs approximately 1.5 times. Whereas retinoic acid increased C/EBPbeta and PPARgamma mRNAs 1.5 times in ATII in vitro, vitamin-A deficiency strongly decreased fetal lung C/EBPalpha, beta, and PPARgamma transcripts in vivo. C/EBPbeta, delta, and PPARgamma mRNAs were also increased in vitro by epidermal growth factor and keratinocyte growth factor, whereas they were unchanged by the maturation inhibitor transforming growth factor-beta. C/EBPalpha expression was not reinduced by any mediator. Changes in transcripts were reflected in protein levels analyzed through Western blotting. These results argue for a role of these factors in ATII functional maturation, and indicate a multifactorial control of their ontogeny.

A dominant negative PPARgamma mutant shows altered cofactor recruitment and inhibits adipogenesis in 3T3-L1 cells

AIMS/HYPOTHESIS

PPARgamma, a member of the nuclear hormone receptor family of transcription factors, plays a key role in adipocyte differentiation and insulin sensitivity. The aim of this study was to identify a potential dominant negative murine PPARgamma mutant and to characterize the in vitro functional properties of this mutant.

METHODS

In vitro transient transfections and mammalian two-hybrid assays in TSA201 cells were used to characterize the transcriptional activity of the L466A mutant and to study the molecular interaction of transcriptional cofactors with the L466A mutant in an attempt to elucidate the mechanism of its dominant negative activity. Adenoviral constructs expressing PPARgamma wild-type (AdWT) or the L466A mutant (AdL466A) were infected into the murine 3T3-L1 cell line to study the mutant's effect on adipogenesis.

RESULTS

The L466A mutant alone is transcriptionally defective. However, it retains DNA binding and inhibits the ligand-dependent and -independent activity of the wild-type receptor, consistent with dominant negative properties. In mammalian two-hybrid studies, the L466A mutant does not bind nuclear receptor coactivators. However, it more avidly recruits corepressors due to enhanced binding to the corepressor ID1 domain, leading to pronounced transcriptional repression. The AdL466A mutant inhibits adipogenesis induced by either a differentiation cocktail or by thiazolidinedione ligand. AdL466A infection also blocked the upregulation of the adipocyte marker genes aP2 and adipsin.

CONCLUSION

We conclude that the L466A PPARgamma mutant possesses potent dominant negative activity based on preferential corepressor recruitment and it inhibits adipogenesis and the expression of adipocyte-specific genes.

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.

Isolation of up- or down-regulated genes in PPARgamma-expressing NIH-3T3 cells during differentiation into adipocytes

Adipocyte differentiation is a complex process in which the expression of many transcription factors and adipocyte-specific genes is regulated under a strict program. The peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the steroid/thyroid nuclear hormone receptor superfamily of ligand-activated transcription factors, is an important regulator of adipocyte gene expression and differentiation. In this study, we tried to identify downstream target genes of PPARgamma, by using PPARgamma-expressing cells and a subtractive cloning strategy, and isolated cDNA clones which were up-regulated or down-regulated by PPARgamma. Northern blot analyses revealed that the expression levels of the aldehyde dehydrogenase-2-like, type VI collagen alpha 3 subunit, cellular retinoic acid binding protein 1 and thrombospondin 1 are changed during the differentiation of mouse 3T3-L1 preadipocyte cells, indicating that these genes might be downstream targets of PPARgamma in adipocyte differentiation.

Transcriptional control of adipogenesis

The major transcriptional factors involved in the adipogenic process include proteins belonging to the CCAAT/enhancer binding protein family, peroxisome proliferator-activated receptor gamma, and adipocyte determination and differentiation dependent factor 1, also known as sterol regulatory element-binding protein 1. This process has been characterized with the aid of cell lines that represent various stages in the path of adipocyte commitment, ranging from pluripotent mesodermal fibroblasts to preadipocytes. Molecular analyses have led to a cascade model for adipogenesis based on timed expression of CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptor gamma. Gene targeting and transgenic-mouse technologies, which allow the manipulation of endogenous genes for these transcription factors, have also contributed to the understanding of adipogenesis. This review aims to integrate this information to gain an understanding of the transcriptional regulation of fat cell formation.

Induction of apoptosis by all-trans-retinoic acid and C2-ceramide treatment in rat stromal-vascular cultures

Apoptosis of preadipocytes and adipocytes contributes to the balance of adipose tissue mass by reducing adipocyte number. To address this phenomenon, we treated cultured rat S-V cells with all-trans-retinoic acid (RA) (10 microM) or C2-ceramide (50 microM) during adipogenesis. Gel electrophoresis of DNA from treated cells cultured in serum-free medium showed that 10 microM RA or 50 microM ceramide induced a distinct laddering pattern of DNA fragments. Cellular caspase 3 activity, another marker of apoptosis, was increased by RA (10 microM) (P < 0.05), but not by 50 microm C2-ceramide. RT-PCR results showed that RA (10 microM) decreased the expression of Bcl-2 mRNA. These results suggest that fat cell loss by apoptosis can be regulated, in part, by RA (10 microM) which increases caspase 3 activity and decreases Bcl-2 expression in rat S-V cells. C2-ceramide apparently works through a different cellular mechanism to induce apoptosis.

The expression of peroxisome proliferator-activated receptor gamma in pig fetal tissue and primary stromal-vascular cultures

OBJECTIVE

This study was designed to determine when peroxisome proliferator-activated receptor gamma (PPARgamma) is expressed in developing fetal adipose tissue and stromal-vascular adipose precursor cells derived from adipose tissue. In addition we examined developing tissue for CCAAT/enhancer-binding protein beta (C/EBPbeta) expression to see if it was correlated with PPARgamma expression. Pituitary function and hormones involved with differentiation (dexamethasone and retinoic acid) were also tested for their effects on PPARgamma expression to determine if hormones known to affect differentiation also effect PPARgamma expression in vivo and in cell culture.

RESEARCH METHODS AND PROCEDURES

Developing subcutaneous adipose tissues from the dorsal region of the fetal pig were collected at different gestation times and assayed using Western blot analysis to determine levels of PPARgamma and C/EBPbeta. Hypophysectomy was performed on 75-day pig fetuses and tissue samples were then taken at 105 days for Western blot analysis. Adipose tissue was also taken from postnatal pigs to isolate stromal-vascular (S-V) cells. These adipose precursor cells were grown in culture and samples were taken for Western blot analysis to determine expression levels of PPARgamma.

RESULTS

Our results indicate that PPARgamma is expressed as early as 50 days of fetal development in adipose tissue and continues through 105 days. Expression of PPARgamma was found to be significantly enhanced in adipose tissue from hypophysectomized fetuses at 105 days of fetal development (p<0.05). C/EBPbeta was not found in 50- or 75-day fetal tissues and was found only at low levels in 105-day tissues. C/EBPbeta was not found in hypophysectomized (hypoxed) 105-day tissue where PPARgamma was elevated. S-V cells freshly isolated from adipose tissue of 5- to 7-day postnatal pigs showed the expression of PPARgamma1. When S-V cells were cultured, both PPARgamma1 and 2 were expressed after the first day and continued as cells differentiated. High concentrations of retinoic acid decreased PPARgamma expression in early S-V cultures (p<0.05).

DISCUSSION

Our data indicate that PPARgamma is expressed in fetal adipose tissue very early before distinct fat cells are observed and can be expressed without the expression of C/EBPbeta. The increase in PPARgamma expression after hypophysectomy may explain the increase in fat cell size under these conditions. Adipose precursor cells (S-V cells) from 5- to 7-day postnatal pigs also express PPARgamma in the tissue before being induced to differentiate in culture. Thus S-V cells from newborn pig adipose tissue are probably more advanced in development than the 3T3-L1 cell model. S-V cells may be in a state where PPARgamma and C/EBPalpha are expressed but new signals or vascularization are needed before cells are fully committed and lipid filling begins.

Carotenoids and retinoids as suppressors on adipocyte differentiation via nuclear receptors

The adipocyte differentiation program is regulated by the sequential expression of transcriptional activators, mainly peroxisome proliferator activated receptor (PPAR) families. In the present study, we have decided to systematically examine the effects of vitamin A and its precursors, carotenoids and retinoids, on terminal differentiation from preadipocytes to adipocytes on the cellular and molecular aspects. The effects of active form of vitamin A, retinoic acid (RA), are believed to be mediated by specific nuclear receptor proteins [retinoic acid receptor (RAR)] which are members of the steroid and thyroid/retinoid receptor superfamily of ligand dependent transcriptional regulators, RARalpha, RARgamma, RXRalpha, and RXRbeta mRNA were abundant in adipose tissue and 3T3-L1 adipose cells. The autoregulated amplification of RARgamma mRNA was observed by these own ligands in 3T3-L1 cells. And, RA inhibited PPARgamma2 expression more effectively and caused concomitantly a greater inhibition of adipocyte differentiation. These results suggest that the inhibitory action of adipocyte differentiation by carotenoids and retinoids are exhibited through the RAR up-regulation and the suppression of PPARgamma2. The nature of the cross talk of vitamin A actions between the RARs, RXRs and PPARs via co-activator in adipose tissue will likely prove to be important for understanding the process of adipogenesis.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Dominant negative regulation by c-Jun of transcription of the uncoupling protein-1 gene through a proximal cAMP-regulatory element: a mechanism for repressing basal and norepinephrine-induced expression of the gene before brown adipocyte differentiation

The brown fat uncoupling protein-1 (ucp-1) gene is regulated by the sympathetic nervous system, and its transcription is stimulated by norepinephrine, mainly through cAMP-mediated pathways. Overexpression of the catalytic subunit of protein kinase A stimulated a chloramphenicol acetyltransferase expression vector driven by the 4.5-kb 5'-region of the rat ucp-1 gene. Mutant deletion analysis indicated the presence of the main cAMP-regulatory element (CRE) in the proximal region between -141 and -54. This region contains an element at -139/-122 able to confer enhancer and protein kinase A (PKA)-dependent activity to the basal thymidine kinase promoter. The potency of this element was much higher in differentiated than in nondifferentiated brown adipocytes. Gel shift analyses indicated that a complex array of proteins from brown fat nuclei bind to the -139/-122 element, among which CRE-binding protein (CREB) and Jun proteins were identified. In transfected brown adipocytes, c-Jun was a negative regulator of basal and PKA-induced transcription from the ucp-1 promoter acting through this proximal CRE region. A double-point mutation in the -139/-122 element abolished both PKA- and c-Jun-dependent regulation through this site, and overexpression of CREB blocked c-Jun repression. Thus, an opposite action of these two transcription factors on the -139/-122 CRE is proposed. c-Jun content in brown adipocytes differentiating in culture correlated negatively with both ucp-1 gene expression and the acquisition of the brown adipocyte morphology. These findings indicate that c-Jun provides a molecular mechanism to repress the basal and cAMP-mediated expression of the ucp-1 gene before the differentiation of the brown adipocyte.

Counteraction of retinoic acid and 1,25-dihydroxyvitamin D3 on up-regulation of adipocyte differentiation with PPARgamma ligand, an antidiabetic thiazolidinedione, in 3T3-L1 cells

Retinoic acid (RA) and 1,25-dihydroxyvitamin D3 (1,25 (OH)2 D3) inhibited adipocyte differentiation of 3T3-L1 preadipocytes in the presence of thiazolidinedione, a specific ligand for peroxisome proliferator-activated receptory (PPARgamma). These fat-soluble vitamins repressed the up-regulated protein expression of PPARgamma2 during the first 40 hours of initiation of 3T3-L1 preadipocyte differentiation. Compared with RA, 1,25 (OH)2 D3 inhibited PPARgamma2 expression more effectively and caused concomitantly a greater inhibition of adipocyte differentiation. These results suggest that the inhibitory action of adipocyte differentiation by RA or 1,25 (OH)2 D3 is exhibited through direct repression of the expression of PPARgamma2 protein, even in the presence of its ligand. They also raise the intriguing possibility that attenuation or amplification of the pharmacological effects of thiazolidinedione that are dependent on PPARgamma in adipose cells is caused by alteration of the levels of these fat-soluble vitamins.

Adipocyte differentiation and leptin expression

Adipose tissue has long been known to house the largest energy reserves in the animal body. Recent research indicates that in addition to this role, the adipocyte functions as a global regulator of energy metabolism. Adipose tissue is exquisitely sensitive to a variety of endocrine and paracrine signals, e.g. insulin, glucagon, glucocorticoids, and tumor necrosis factor (TNF), that combine to control both the secretion of other regulatory factors and the recruitment and differentiation of new adipocytes. The process of adipocyte differentiation is controlled by a cascade of transcription factors, most notably those of the C/EBP and PPAR families, which combine to regulate each other and to control the expression of adipocyte-specific genes. One such gene, i.e. the obese gene, was recently identified and found to encode a hormone, referred to as leptin, that plays a major role in the regulation of energy intake and expenditure. The hormonal and transcriptional control of adipocyte differentiation is discussed, as is the role of leptin and other factors secreted by the adipocyte that participate in the regulation of adipose homeostasis.

Peroxisome proliferator activated receptor gamma, CCAAT/enhancer-binding protein alpha, and cell cycle status regulate the commitment to adipocyte differentiation

Terminal differentiation of stem cells is characterized by cessation of cell proliferation as well as changes in cell morphology associated with the differentiated state. For adipocyte differentiation, independent lines of evidence show that the transcription factors peroxisome proliferator activated receptor gamma (PPARgamma) and CCAAT/enhancer-binding protein alpha (C/EBPalpha) as well as the tumor suppressor retinoblastoma (Rb) protein are essential. How these proteins promote adipocyte conversion and how they function cooperatively during the differentiation process remain unclear. We have used retinoic acid (RA) inhibition of adipogenesis to investigate these issues. RA blocked adipogenesis of 3T3-L1 cells induced to differentiate by ectopic expression of PPARgamma and C/EBPalpha independently or together. However, under these circumstances RA was only effective at preventing adipogenesis when added prior to confluence, suggesting that factors involved in regulation of the cell cycle might play a role in establishing the commitment state of adipogenesis that is insensitive to RA. During differentiation of wild type 3T3 L1 preadipocytes, we found that Rb protein is hyperphosphorylated early in adipogenesis, corresponding to previously quiescent cells re-entering the cell cycle, and later becomes hypophosphorylated. The data suggest that, together with the coexpression of PPARgamma and C/EBPalpha, permanent exit from the cell cycle establishes the irreversible commitment to adipocyte differentiation.

Retinoic acid blocks adipogenesis by inhibiting C/EBPbeta-mediated transcription

Adipocyte differentiation is thought to involve sequential induction of the transcription factors C/EBPbeta, peroxisome proliferator-activated receptor gamma (PPARgamma), and C/EBPalpha. C/EBPalpha expression is both necessary and sufficient for adipocyte differentiation. Here we report that ectopic expression of either C/EBPalpha or C/EBPbeta induces PPARgamma expression and adipogenesis and that retinoic acid (RA) completely inhibits adipogenesis by either form of C/EBP. In studies of normal preadipocytes, RA does not prevent C/EBPbeta induction but blocks induction of PPARgamma, C/EBPalpha, and adipogenesis. In transient transfection studies, liganded RA receptor (RAR) specifically blocks transcriptional activation by either C/EBPalpha or C/EBPbeta. These results strongly suggest that C/EBPalpha substitutes for C/EBPbeta to induce adipocyte differentiation and that liganded RAR inhibits adipogenesis by blocking C/EBPbeta-mediated induction of downstream genes.

CCAAT/enhancer-binding protein beta is not affected by tetrachlorodibenzo-p-dioxin (TCDD) inhibition of 3T3-L1 preadipocyte differentiation

The differentiation of 3T3-L1 preadipocytes is induced by the coordinate activation of trans-acting factors in response to inducers. Depending on the time of treatment, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was effective in inhibiting 3T3-L1 preadipocyte differentiation and the expression of differentiation-dependent trans-acting factors. Based on glycerol-3-phosphate dehydrogenase activity, in the differentiation of 3T3-L1 cells was decreased by 70% in cells treated with TCDD before the induction of differentiation, 25% during induction, and not at all after induction. This time-dependent inhibition of cell differentiation by TCDD was correlated with the levels of aryl hydrocarbon receptor (AhR). TCDD treatment decreased the mRNA levels of C/EBP alpha and PPAR gamma 2 but did not affect the mRNA levels of RXR alpha and RAR alpha. Furthermore, TCDD did not change the mRNA or protein levels of C/EBP beta, which is thought to play a role in inducing C/EBP alpha and PPAR gamma 2 expression. These results suggest that TCDD inhibited 3T3-L1 preadipocyte differentiation through the AhR pathway, and the change of C/EBP beta mRNA and protein was not involved in reducing mRNA expression of C/EBP alpha and PPAR gamma 2.