Role of peroxisome proliferator-activated receptor-gamma in maintenance of the characteristics of mature 3T3-L1 adipocytes

The positive effects of yerba maté (Ilex paraguariensis) in obesity

The prevalence of obesity has increased worldwide over the past three decades. Global anti-obesity strategies focus on dietary and lifestyle modifications to slow the development of obesity. Research in the nutrition field has recently aroused considerable interest based on the potential of natural products to counteract obesity. Several studies have identified yerba maté (Ilex paraguariensis) as an excellent candidate. In this review, we evaluated the impact of yerba maté on obesity and obesity-related inflammation. Cellular studies demonstrate that yerba maté suppresses adipocyte differentiation and triglyceride accumulation and reduces inflammation. Animal studies show that yerba maté modulates signaling pathways that regulate adipogenesis, antioxidant, anti-inflammatory and insulin signaling responses. In summary, the data presented here showed that the use of yerba maté might be useful against obesity, improving the lipid parameters in humans and animal models. In addition, yerba maté modulates the expression of genes that are changed in the obese state and restores them to more normal levels of expression. In doing so, it addresses several of the abnormal and disease-causing factors associated with obesity. Protective and ameliorative effects on insulin resistance were also observed. Thus, as a general conclusion, it seems that yerba maté beverages and supplements might be helpful in the battle against obesity.

Thyrotropin and obesity: increased adipose triglyceride content through glycerol-3-phosphate acyltransferase 3

Epidemiological evidence indicates that thyrotropin (TSH) is positively correlated with the severity of obesity. However, the mechanism remains unclear. Here, we show that TSH promoted triglyceride (TG) synthesis in differentiated adipocytes in a thyroid hormone-independent manner. Mice with subclinical hypothyroidism, which is characterized by elevated serum TSH but not thyroid hormone levels, demonstrated a 35% increase in the total white adipose mass compared with their wild-type littermates. Interestingly, Tshr KO mice, which had normal thyroid hormone levels after thyroid hormone supplementation, resisted high-fat diet-induced obesity. TSH could directly induce the activity of glycerol-3-phosphate-acyltransferase 3 (GPAT3), the rate-limiting enzyme in TG synthesis, in differentiated 3T3-L1 adipocytes. However, following either the knockdown of Tshr and PPARγ or the constitutive activation of AMPK, the changes to TSH-triggered GPAT3 activity and adipogenesis disappeared. The over-expression of PPARγ or the expression of an AMPK dominant negative mutant reversed the TSH-induced changes. Thus, TSH acted as a previously unrecognized master regulator of adipogenesis, indicating that modification of the AMPK/PPARγ/GPAT3 axis via the TSH receptor might serve as a potential therapeutic target for obesity.

Adipose tissue signaling by nuclear receptors in metabolic complications of obesity

In recent years white adipose tissue inflammation has been recognized to be associated with obesity. Adipocytes and adipose tissue associated macrophages (ATMs) secrete bioactive molecules, including adipokines, chemokines/cytokines and free fatty acids that modulate the development of low-grade inflammation and insulin resistance responsible for obesity-related metabolic and cardiovascular diseases. Nuclear receptors, notably peroxisome-proliferator-activated receptors, are sensors of dietary lipids and control transcriptional programs of key metabolic and inflammatory pathways in adipocytes and macrophages. This review focuses on mechanisms by which nuclear receptors maintain white adipose tissue homeostasis. The identification of ATMs as active players in the initiation of chronic inflammation and the links between inflammatory signaling and metabolic dysfunction will be presented, followed by discussion of recent evidence for nuclear receptors in ATM function, with an emphasis on the paracrine interaction between adipocytes and ATMs.

Elevated S-adenosylhomocysteine alters adipocyte functionality with corresponding changes in gene expression and associated epigenetic marks

Maternal deficiencies in micronutrients affecting one-carbon metabolism before and during pregnancy can influence metabolic status and the degree of insulin resistance and obesity of the progeny in adulthood. Notably, maternal and progeny plasma S-adenosylhomocysteine (SAH) levels are both elevated after vitamin deficiency in pregnancy. Therefore, we investigated whether this key one-carbon cycle intermediate directly affects adipocyte differentiation and function. We found that expansion and differentiation of murine 3T3-L1 preadipocytes in the presence of SAH impaired both basal and induced glucose uptake as well as lipolysis compared with untreated controls. SAH did not alter preadipocyte factor 1 (Dlk1) or peroxisome proliferator-activated receptor-γ 2 (Pparγ2) but significantly reduced expression of CAAT enhancer-binding protein-α (Cebpα), Cebpβ, and retinoid x receptor-α (Rxrα) compared with untreated adipocytes. SAH increased Rxrα methylation on a CpG unit (chr2:27,521,057+, chr2:27,521,049+) and CpG residue (chr2:27,521,080+), but not Cebpβ methylation, relative to untreated adipocytes. Trimethylated histone H3-Lys27 occupancy was significantly increased on Cebpα and Rxrα promoters in SAH-treated adipocytes, consistent with the reduction in gene expression. In conclusion, SAH did not affect adipogenesis per se but altered adipocyte functionality through epigenetic mechanisms, such that they exhibited altered glucose disposal and lipolysis. Our findings implicate micronutrient imbalance in subsequent modulation of adipocyte function.

CCAAT/enhancer-binding protein α is a crucial regulator of human fat mass and obesity associated gene transcription and expression

Several susceptibility loci have been reported associated with obesity and T2DM in GWAS. Fat mass and obesity associated gene (FTO) is the first gene associated with body mass index (BMI) and risk for diabetes in diverse patient populations. FTO is highly expressed in the brain and pancreas, and is involved in regulating dietary intake and energy expenditure. While much is known about the epigenetic mutations contributing to obesity and T2DM, less is certain with the expression regulation of FTO gene. In this study, a highly conserved canonical C/EBPα binding site was located around position −45~−54 bp relative to the human FTO gene transcriptional start site. Site-directed mutagenesis of the putative C/EBPα binding sites decreased FTO promoter activity. Overexpression and RNAi studies also indicated that C/EBPα was required for the expression of FTO. Chromatin immunoprecipitation (ChIP) experiment was carried out and the result shows direct binding of C/EBPα to the putative binding regions in the FTO promoter. Collectively, our data suggest that C/EBPα may act as a positive regulator binding to FTO promoter and consequently, activates the gene transcription.

Isolation, culture and differentiation of duck (Anas platyrhynchos) preadipocytes

In the present study, we isolated preadipocytes from the adipose tissue of Peking duck and subsequently cultured them in vitro. Cell counting kit-8 assay was employed to establish the growth curve of duck primary preadipocytes. Meanwhile, after the cells reaching full confluency, they were induced to differentiate into mature adipocytes by the addition of a cocktail containing dexamethasone, insulin, 3-isobutyl-1-methylxanthine, and oleic acid for 8 days. Successful differentiation was demonstrated by the development of lipid droplets and the expression of key marker genes including peroxisome proliferator-activated receptor-γ (PPARγ), CCAAT/enhancer binding protein-α (CEBP/α) and adipocyte fatty acid-binding protein (FABP4). Our results showed that duck primary preadipocytes began to adhere 12 h after seeding as short spindle shapes or litter triangles, which grew quickly 3 days post attachment and maintained stable after day 7. After 8 days the preadipocytes were induced to differentiate into mature adipocytes, which were stained red by oil red O. Additionally, it showed that during preadipocyte differentiation PPARγ mRNA was highly expressed at day 3, while CEBP/α and FABP4 mRNA peaked at day 5 and 8, respectively. These results indicate that we have successfully isolated and cultured Peking duck preadipocytes and successfully induced them to differentiate into mature adipocytes. This work could lay a foundation for further research into waterfowl adipogenesis.

Terminalia paniculata bark extract attenuates non-alcoholic fatty liver via down regulation of fatty acid synthase in high fat diet-fed obese rats

BACKGROUND

This study was performed to understand the possible therapeutic activity of Terminalia paniculata ethanolic extract (TPEE) on non alcoholic fatty liver in rats fed with high fat diet.

METHODS

Thirty six SD rats were divided into 6 groups (n = 6): Normal control (NC), high fat diet (HFD), remaining four groups were fed on HFD along with different doses of TPEE (100,150 and 200 mg/kg b.wt) or orlistat, for ten weeks. Liver tissue was homogenized and analyzed for lipid profiles, activities of superoxide dismutase (SOD), catalase (CAT) and malondialdehyde (MDA) content. Further, the expression levels of FAS and AMPK-1α were also studied in addition to histopathology examination of liver tissue in all the groups.

RESULTS

HFD significantly increased hepatic liver total cholesterol (TC), triglycerides (TG), free fatty acids (FFA) and MDA but decreased the activities of SOD and CAT which were subsequently reversed by supplementation with TPEE in a dose-dependent manner. In addition, TPEE administration significantly down regulated hepatic mRNA expression of FAS but up regulated AMPK-1α compared to HFD alone fed group. Furthermore, western blot analysis of FAS has clearly demonstrated decreased expression of FAS in HFD + TPEE (200 mg/kg b.wt) treated group when compared to HFD group at protein level.

CONCLUSIONS

Our biochemical studies on hepatic lipid profiles and antioxidant enzyme activities supported by histological and expression studies suggest a potential therapeutic role for TPEE in regulating obesity through FAS.

Distinction of white, beige and brown adipocytes derived from mesenchymal stem cells

Adipose tissue is a major metabolic organ, and it has been traditionally classified as either white adipose tissue (WAT) or brown adipose tissue (BAT). WAT and BAT are characterized by different anatomical locations, morphological structures, functions, and regulations. WAT and BAT are both involved in energy balance. WAT is mainly involved in the storage and mobilization of energy in the form of triglycerides, whereas BAT specializes in dissipating energy as heat during cold- or diet-induced thermogenesis. Recently, brown-like adipocytes were discovered in WAT. These brown-like adipocytes that appear in WAT are called beige or brite adipocytes. Interestingly, these beige/brite cells resemble white fat cells in the basal state, but they respond to thermogenic stimuli with increased levels of thermogenic genes and increased respiration rates. In addition, beige/brite cells have a gene expression pattern distinct from that of either white or brown fat cells. The current epidemic of obesity has increased the interest in studying adipocyte formation (adipogenesis), especially in beige/brite cells. This review summarizes the developmental process of adipose tissues that originate from the mesenchymal stem cells and the features of these three different types of adipocytes.

Peroxisome proliferator-activated receptor γ regulates genes involved in insulin/insulin-like growth factor signaling and lipid metabolism during adipogenesis through functionally distinct enhancer classes

The nuclear receptor peroxisome proliferator-activated receptor (PPAR) is a transcription factor whose expression is induced during adipogenesis and that is required for the acquisition and control of mature adipocyte functions. Indeed, PPAR induces the expression of genes involved in lipid synthesis and storage through enhancers activated during adipocyte differentiation. Here, we show that PPAR also binds to enhancers already active in preadipocytes as evidenced by an active chromatin state including lower DNA methylation levels despite higher CpG content. These constitutive enhancers are linked to genes involved in the insulin/insulin-like growth factor signaling pathway that are transcriptionally induced during adipogenesis but to a lower extent than lipid metabolism genes, because of stronger basal expression levels in preadipocytes. This is consistent with the sequential involvement of hormonal sensitivity and lipid handling during adipocyte maturation and correlates with the chromatin structure dynamics at constitutive and activated enhancers. Interestingly, constitutive enhancers are evolutionary conserved and can be activated in other tissues, in contrast to enhancers controlling lipid handling genes whose activation is more restricted to adipocytes. Thus, PPAR utilizes both broadly active and cell type-specific enhancers to modulate the dynamic range of activation of genes involved in the adipogenic process.

Association of PPARγ2 gene variant Pro12Ala polymorphism with hypertension and obesity in the aboriginal Qatari population known for being consanguineous

AIM

The aim of this study was to investigate the association of the Pro12Ala polymorphism of the human peroxisome proliferator-activated receptor gamma 2 (PPARγ2) gene with hypertension and obesity in a highly consanguineous aboriginal Qatari population.

DESIGN

A cross-sectional survey conducted from January 2011-December 2012.

SETTING

Primary health care clinics.

SUBJECTS

A random sample of 1,528 Qatari male and female population older than 20 years of age.

MATERIALS AND METHODS

Data on age, sex, income, level of education, occupation status, body mass index, and blood pressure and lipid profile were obtained. The Pro12Ala in the PPARγ2 gene was detected on the LightCycler® using two specific probes: (Sensor [G] 5'-CTC CTA TTG ACG CAG AAA GCG-FL and PPAR Anchor 5' LC Red 640- TCC TTC ACT GAT ACA CTG TCT GCA AAC ATA TC-PH). Univariate and multivariate logistic regression were performed.

RESULT

Out of a total 1,528 participants, 220 were diagnosed with essential hypertension, and 420 were obese. Participants with consanguinity were significantly higher among hypertensive than normotensive (41.9% versus 30.8%; P=0.001). Altogether, more than three-fourths (89%) of the participants had a wild genotype (Pro12Pro), 9.8% were heterozygous with Pro12Ala, and only 1.2% was homozygous with the Ala12Ala genotype. The frequency of the Pro allele was 94.5% in normotensive versus 90.5% in hypertensive, while the distribution of the Ala allele was 5.5% in normotensive versus 9.5% in the hypertensive group (P=0.001). The odds of hypertension were 1.7 times higher among the participants with the Ala allele as compared to those with the Pro, while adjusting for other potential confounders (adjusted odds ratio 1.69; 95% confidence interval 1.12-2.55; P=0.012). There was no association between the PPARγ2Ala allele and obesity (P=0.740).

CONCLUSION

The current study revealed an association between the PPARγ2Ala allele and hypertension in Qatar's population. On the other hand, this study found no association between the Ala allele and obesity.

Hsp90 chaperones PPARγ and regulates differentiation and survival of 3T3-L1 adipocytes

Adipose tissue dysregulation has a major role in various human diseases. The peroxisome proliferator-activated receptor-γ (PPARγ) is a key regulator of adipocyte differentiation and function, as well as a target of insulin-sensitizing drugs. The Hsp90 chaperone stabilizes a diverse set of signaling 'client' proteins, thereby regulates various biological processes. Here we report a novel role for Hsp90 in controlling PPARγ stability and cellular differentiation. Specifically, we show that the Hsp90 inhibitors geldanamycin and novobiocin efficiently impede the differentiation of murine 3T3-L1 preadipocytes. Geldanamycin at higher concentrations also inhibits the survival of both developing and mature adipocytes, respectively. Further, Hsp90 inhibition disrupts an Hsp90-PPARγ complex, leads to the destabilization and proteasomal degradation of PPARγ, and inhibits the expression of PPARγ target genes, identifying PPARγ as an Hsp90 client. A similar destabilization of PPARγ and a halt of adipogenesis also occur in response to protein denaturing stresses caused by a single transient heat-shock or proteasome inhibition. Recovery from stress restores PPARγ stability and adipocyte differentiation. Thus, our findings reveal Hsp90 as a critical stress-responsive regulator of adipocyte biology and offer a potential therapeutic target in obesity and the metabolic syndrome.

Roles of activin receptor-like kinase 7 signaling and its target, peroxisome proliferator-activated receptor γ, in lean and obese adipocytes

We recently discovered a novel signaling pathway involving activin receptor-like kinase 7 (ALK7), one of the type I transforming growth factor-β receptors. ALK7 and activated Smads 2, 3, and 4 inhibit the master regulators of adipogenesis, CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ) specifically in differentiated adipocytes, but surprisingly increase both the adipocyte size and lipid content by suppressing lipolysis. Here, we show that, although both transcription factors are suppressed by ALK7 in either the obese or lean state, PPARγ, but not C/EBPα, is further suppressed under obesity through an ALK7-independent pathway. As a result, PPARγ and adipose lipolytic activities are severely downregulated in obesity. Reactivation of PPARγ by ALK7 inactivation leads to downregulation of inflammatory adipocytokines and upregulation of adiponectin. We propose that PPARγ promotes lipid turnover and remodeling by stimulating both triglyceride synthesis and breakdown in differentiated adipocytes. Finally, we discuss the physiological and evolutionary roles of the ALK7-signaling pathway and consider it as a potential target of therapy for obesity.

Get the fat out!

Obesity is associated with increased risk of a number of cancers in humans, but the mechanism(s) responsible for these associations have not been established. It is estimated that 68% of adults are overweight or obese and that obesity may be causative in 4% to 7% of cancers in the United States. Several hypotheses have been put forward to explain the association between obesity and cancer including adipose-directed signaling (e.g., mTOR, AMPK), production of factors (e.g., insulin growth factor 1, fibroblast growth factor 1, and/or chronic inflammation associated with obesity. Huffman and colleagues used surgical methods to determine if visceral fat was causally related to intestinal tumorigenesis in the Apc(1638/N+) mouse in a manner independent of confounding factors such as caloric restriction. They found that caloric restriction could extend survival in both male and female Apc(1638/N+) mice but found that surgical removal of visceral fat was only effective in reducing macroadenomas in females. The results of this study do not identify the specific mechanism of association between visceral fat and intestinal carcinogenesis in female mice but do support the rationale for future cancer prevention trials that evaluate pharmacologic and behavioral strategies to reduce abdominal obesity in humans. Cancer Prev Res; 6(3); 161-4. ©2013 AACR.

Endogenous oxidative stress, but not ER stress, induces hypoxia-independent VEGF120 release through PI3K-dependent pathways in 3T3-L1 adipocytes

OBJECTIVE

Expressions of vascular endothelial growth factor (VEGF) are increased in obese adipocytes and is secreted from obese adipose tissue through hypoxia-independent pathways. Therefore, we investigated the hypoxia-independent mechanism underlying increased expression and release of VEGF in obese adipocytes.

DESIGN AND METHODS

We compared signal transduction pathways regulating VEGF with those regulating monocyte chemoattractant protein-1 (MCP-1), which is increased in obese adipocytes, in an in vitro model of artificially hypertrophied 3T3-L1 adipocytes preloaded with palmitate, without the influence of hypoxia.

RESULTS

Palmitate-preloaded cells exhibited significantly enhanced oxidative stress (P < 0.01) and showed increased VEGF120 and MCP-1 release (P < 0.01, respectively), while endoplasmic reticulum (ER) stress was not induced. Increased VEGF120 release was significantly decreased with PI3K inhibitor LY294002 (P < 0.01). In addition, antioxidant N-acetyl-cysteine (NAC) markedly diminished not only VEGF120 secretion (P < 0.01) but also augmented Akt phosphorylation on Ser473 (P < 0.01). In contrast, increased MCP-1 release was suppressed with JNK inhibitor SP600125 and p38 MAPK inhibitor SB203580 (P < 0.01).

CONCLUSIONS

VEGF120 release from hypertrophied adipocytes can be enhanced through PI3K pathways activated by oxidative stress but not by ER stress, suggesting that VEGF120 secretion is regulated through oxidative stress-dependent pathways distinct from those involved in MCP-1 release through either JNK or p38 MAPK activation.

Blueberry peel extracts inhibit adipogenesis in 3T3-L1 cells and reduce high-fat diet-induced obesity

This study examined the anti-obesity effect and mechanism of action of blueberry peel extracts (BPE) in 3T3-L1 cells and high-fat diet (HFD)-induced obese rats. The levels of lipid accumulation were measured, along with the changes in the expression of genes and proteins associated with adipocyte differentiation in 3T3-L1 cells. Evidenced by Oil-red O staining and triglyceride assay, BPE dose-dependently inhibited lipid accumulation at concentrations of 0, 50, and 200 µg/ml. BPE decreased the expression of the key adipocyte differentiation regulator C/EBPβ, as well as the C/EBPα and PPARγ genes, during the differentiation of preadipocytes into adipocytes. Moreover, BPE down-regulated adipocyte-specific genes such as aP2 and FAS compared with control adipocytes. The specific mechanism mediating the effects of BP revealed that insulin-stimulated phosphorylation of Akt was strongly decreased, and its downstream substrate, phospho-GSK3β, was downregulated by BPE treatment in 3T3-L1 cells. Together, these data indicated that BP exerted anti-adipogenic activity by inhibiting the expression of PPARγ and C/EBPβ and the Akt signaling pathway in 3T3-L1 adipocytes. Next, we investigated whether BP extracts attenuated HFD-induced obesity in rats. Oral administration of BPE reduced HFD-induced body weight gain significantly without affecting food intake. The epididymal or perirenal adipose tissue weights were lower in rats on an HFD plus BPE compared with the tissue weights of HFD-induced obese rats. Total cholesterol and triglyceride levels in the rats fed BPE were modestly reduced, and the HDL-cholesterol level was significantly increased in HFD plus BP-fed rats compared with those of HFD-fed rats. Taken together, these results demonstrated an inhibitory effect of BP on adipogenesis through the down-regulation of C/EBPβ, C/EBPα, and PPARγ and the reduction of the phospho-Akt adipogenic factor in 3T3-L1 cells. Moreover, BPE reduced body weight gain and inhibited fat accumulation in an HFD-induced animal model of obesity.

CTLA-4Ig immunotherapy of obesity-induced insulin resistance by manipulation of macrophage polarization in adipose tissues

It has been established that obesity alters the metabolic and endocrine function of adipose tissue and, together with accumulation of adipose tissue macrophages, contributes to insulin resistance. Although numerous studies have reported that shifting the polarization of macrophages from M1 to M2 can alleviate adipose tissue inflammation, manipulation of macrophage polarization has not been considered as a specific therapy. Here, we determined whether cytotoxic T-lymphocyte-associated antigen-4IgG1 (CTLA-4Ig) can ameliorate insulin resistance by induction of macrophages from proinflammatory M1 to anti-inflammatory M2 polarization in the adipose tissues of high fat diet-induced insulin-resistant mice. CTLA4-Ig treatment prevented insulin resistance by changing gene expression to M2 polarization, which increased the levels of arginase 1. Furthermore, flow cytometric analysis confirmed the alteration of polarization from CD11c (M1)- to CD206 (M2)-positive cells. Concomitantly, CTLA-4Ig treatment resulted in weight reductions of epididymal and subcutaneous adipose tissues, which may be closely related to overexpression of apoptosis inhibitors in macrophages. Moreover, proinflammatory cytokine and chemokine levels decreased significantly. In contrast, CCAAT enhancer binding protein α, peroxisome proliferator-activated receptor γ, and adiponectin expression increased significantly in subcutaneous adipose tissue. This novel mechanism of CTLA-4lg immunotherapy may lead to an ideal anti-obesity/inflammation/insulin resistance agent.

Effect of dietary lipid structure in early postnatal life on mouse adipose tissue development and function in adulthood

Obese individuals have more (hyperplastic) and larger (hypertrophic) adipocytes in their white adipose tissue (WAT) than normal-weight individuals. The difference in cell number emerges early in childhood, suggesting that this is a critical period for being susceptible to obesity. Breast-feeding has been shown to be protective against obesity, and we have previously shown in mice that the physical structure of lipids in human milk may contribute to this protective effect. In the present study, we investigated how differences in the physical structure of lipids in the early diet may modulate adipose tissue development. Male mice were fed a diet containing control infant milk formula (Control IMF; Danone Research) or Nuturis® (Concept IMF with large phospholipid-coated lipid droplets; Danone Research) from postnatal day (PN)16 to 42. Subsequently, mice were challenged with a moderate Western-style diet (WSD) until PN98, and body composition was monitored by dual-energy X-ray absorptiometry. Epididymal WAT was analysed for adipocyte size, number and gene expression of metabolic transcription factors. Early Concept IMF exposure reduced fat accumulation during the WSD challenge by 30 % compared with the Control IMF. It reduced adipocyte size without affecting adipocyte number in adult mice. The Concept IMF decreased the expression of PPARγ, CCAAT/enhancer-binding protein and retinoid X receptor α in WAT in adulthood, key regulators of metabolic activity. In conclusion, Concept IMF exposure in early life reduced susceptibility to obesity in adult life, by preventing adipocyte hypertrophia upon adult dietary challenge without affecting adipogenesis. These data emphasise the importance of the physical properties of dietary lipids in early life in obesity risk later in life.

The role of microRNAs in adipocyte differentiation

Adipocytes differentiate from mesenchymal stem cells (MSCs) in a process known as adipogenesis. The programme of adipogenesis is regulated by the sequential activation of transcription factors and several signaling pathways. There is growing evidence indicating that a class of small non-coding single-stranded RNAs known as "microRNAs (miRNAs)" also are involved in this process. In this review, we summarize the biology and functional mechanisms of miRNAs in adipocyte differentiation. In addition, we further discuss the miRNAs profiling, the miRNAs function and miRNAs target prediction in the adipogenesis.

Origins and actions of tumor necrosis factor α in postmenopausal breast cancer

Tumor necrosis factor α (TNFα) has many roles in both physiological and pathological states. Initially thought to cause necrosis of tumors, research has shown that in many tumor types, including breast cancer, TNFα contributes to growth and proliferation. The presence of TNFα-derived from the tumor and infiltrating immune cells-within a breast tumor microenvironment has been correlated with a more aggressive phenotype, and the postmenopausal ER+ subtype of breast cancers appears to strongly respond to its many pro-growth signaling functions. We discuss how TNFα regulates estrogen biosynthesis within the breast, affecting the activity of the key estrogen-synthesizing enzymes aromatase, estrone sulfatase, and 17β-HSD type 1. Additionally, we describe the anti-adipogenic actions of TNFα that are critical in preventing adjacent estrogen-producing adipose fibroblasts from differentiating, ensuring that the tumor maintains a constant source of estrogen-producing cells. We examine how the increased risk of developing breast cancer in older and obese individuals may be linked to the levels of TNFα in the body. Finally, we evaluate the feasibility of targeting TNFα and its associated pathways as a novel approach to breast cancer therapeutics.

Effects of SOCS 1/3 gene silencing on the expression of C/EBPα and PPARγ during differentiation and maturation of rat preadipocytes

BACKGROUND

Suppressor of cytokine signaling-1 and -3 (SOCS-1 and SOCS-3) are two important negative regulators in the insulin-signaling pathway, and their overexpression may aggravate insulin resistance. Subjects with insulin resistance are often obese and have increased expressions of SOCS-1 and SOCS-3. We speculated that SOCS-1 and SOCS-3 may be involved in abnormal deposition of adipose tissues during insulin resistance.

METHODS

A catch-up growth intrauterine growth retardation (CG-IUGR) rat model with insulin resistance was established; mRNA and protein expression of SOCS-1, SOCS-3, the CCAAT/enhancer binding protein (C/EBPα), and peroxisome proliferator-activated receptor (PPARγ) in adipose tissue were measured by real-time PCR and western blot; plasmids carrying small hairpin RNAs (shRNAs) targeting the SOCS-1 and SOCS-3 genes were constructed and transfected into preadipocytes, which were then induced to mature. At 72 h after differentiation was induced, the expressions of C/EBPα and PPARγ, two important molecules promoting the differentiation of preadipocytes, were detected.

RESULTS

Expressions of SOCS-1, SOCS-3, C/EBPα, and PPARγ were markedly increased in adipose tissues of CG-IUGR rats, whereas the expressions of C/EBPα and PPARγ were significantly reduced after gene silencing of SOCS-1 or SOCS-3 in adipocytes.

CONCLUSION

Overexpression of SOCS-1 and SOCS-3 may enhance the expression of C/EBPα and PPARγ, resulting in abnormal deposition of adipose tissues during insulin resistance.

Dietary supplementation with long-chain monounsaturated fatty acids attenuates obesity-related metabolic dysfunction and increases expression of PPAR gamma in adipose tissue in type 2 diabetic KK-Ay mice

The objective of present study was to examine the effect of long-chain monounsaturated fatty acids (LC-MUFAs) with chain lengths longer than 18 (i.e., C20:1 and C22:1 isomers combined) on obesity-related metabolic dysfunction and its molecular mechanisms. Type-2 diabetic KK-Ay mice (n = 20) were randomly assigned to the 7% soybean oil-diet group (control group) and 4% LC-MUFA concentrate-supplemented-diet group (LC-MUFA group). At 8 weeks on the diet, the results showed that plasma, liver and adipose tissue levels of C20:1 and C22:1 isomers increased significantly with LC-MUFA treatment. Supplementation with LC-MUFAs markedly reduced white fat pad weight as well as adipocyte size in the mice. The levels of plasma free fatty acids, insulin, and leptin concentration in the obese diabetic mice of the LC-MUFA group were also decreased as compared with the mice in the soybean oil-diet control group. Dietary LC-MUFAs significantly increased the mRNA expression of peroxisome proliferator-activated receptor gamma (Pparg), lipoprotein lipase (Lpl), fatty acid transport protein (Fatp), fatty acid translocase/CD36 (Cd36), as well as mRNA expression of genes involved in lipid oxidation such as carnitine palmitoyltransferase-1A (Cpt1a) and citrate synthase (Cs), and decreased the mRNA expression of inflammatory marker serum amyloid A 3 (Saa3) in the adipose tissues of diabetic mice. The results suggest that LC-MUFAs may ameliorate obesity-related metabolic dysfunction partly through increased expression of Pparg as well as its target genes, and decreased inflammatory marker expression in white adipose tissue.

The human lipodystrophy protein seipin is an ER membrane adaptor for the adipogenic PA phosphatase lipin 1

Disruption of the gene BSCL2 causes a severe, generalised lipodystrophy, demonstrating the critical role of its protein product, seipin, in human adipose tissue development. Seipin is essential for adipocyte differentiation, whilst the study of seipin in non-adipose cells has suggested a role in lipid droplet formation. However, its precise molecular function remains poorly understood. Here we demonstrate that seipin can inducibly bind lipin 1, a phosphatidic acid (PA) phosphatase important for lipid synthesis and adipogenesis. Knockdown of seipin during early adipogenesis decreases the association of lipin 1 with membranes and increases the accumulation of its substrate PA. Conversely, PA levels are reduced in differentiating cells by overexpression of wild-type seipin but not by expression of a mutated seipin that is unable to bind lipin 1. Together our data identify lipin as the first example of a seipin-interacting protein and reveals a novel molecular function for seipin in developing adipocytes.

Adipogenic differentiation of scaffold-bound human adipose tissue-derived stem cells (hASC) for soft tissue engineering

Adipose tissue engineering, instead of tissue substitution, often uses autologous adipose tissue-derived stem cells (hASC). These cells are known to improve graft integration and to support neovascularization of scaffolds when seeded onto biomaterials. In this study we thought to engineer adipose tissue using scaffold-bound hASC, since they can be differentiated into the adipocyte cell lineage and used for soft tissue regeneration. We show here by microscopy and gene expression of the peroxysome proliferator-activated receptor gene (PPARγ2) that hASC growing on polypropylene fibrous scaffolds as well as on three-dimensional nonwoven scaffolds can be turned into adipose tissue within 19 days. Freshly isolated hASC displayed a higher differentiation potential than hASC cultured for eight passages. In addition, we proved a modified alginate microcapsule to directly induce adipogenic differentiation of incorporated hASC. The results may help to improve long-term success of adipose tissue regeneration, especially for large-scale soft tissue defects, and support the development of cell-scaffold combinations which can be shaped individually and directly induce the adipogenic differentiation of incorporated hASC at the site of implantation.

Association between PPARγ2 Pro12Ala polymorphism and myocardial infarction and obesity in Han Chinese in Hohhot, China

Activation of the peroxisome proliferator-activated receptor g (PPARg) improves insulin sensitivity and inhibits atherosclerosis. Whether PPARg2 Pro12Ala polymorphism affects myocardial infarction is not clearly understood. We investigated a possible association of PPARg2 Pro12Ala polymorphism with obesity and myocardial infarction in Han Chinese in Hohhot, Inner Mongolia, China. We included 121 subjects with myocardial infarction and 137 healthy controls in our study. Triglycerides, total cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol were measured. The following information was recorded for each subject: age, gender, body height, body weight, systolic blood pressure, and diastolic blood pressure; the body mass index was calculated. PCR-RFLP was used to examine Pro12Ala polymorphism. There were significant differences in clinical characteristics between myocardial infarction patients and healthy controls, except for diastolic blood pressure and triglycerides. The PP, PA/AA genotype frequencies were 88.4 and 11.6% in myocardial infarction patients and 95.6 and 4.4% in controls, respectively (P = 0.031). Individuals with the A allele had a significantly higher risk of myocardial infarction. The A allele was not an independent risk factor for obesity. We conclude that PPARg2 Pro12Ala polymorphisms are associated with increased risk for myocardial infarction in Han Chinese in Hohhot.

MLR-1023 is a potent and selective allosteric activator of Lyn kinase in vitro that improves glucose tolerance in vivo

2(1H)-pyrimidinone,5-(3-methylphenoxy) (MLR-1023) is a candidate for the treatment of type 2 diabetes. The current studies were aimed at determining the mechanism by which MLR-1023 mediates glycemic control. In these studies, we showed that MLR-1023 reduced blood glucose levels without increasing insulin secretion in vivo. We have further determined that MLR-1023 did not activate peroxisome proliferator-activated α, δ, and γ receptors or glucagon-like peptide-1 receptors or inhibit dipeptidyl peptidase-4 or α-glucosidase enzyme activity. However, in an in vitro broad kinase screen MLR-1023 activated the nonreceptor-linked Src-related tyrosine kinase Lyn. MLR-1023 increased the V(max) of Lyn with an EC(50) of 63 nM. This Lyn kinase activation was ATP binding site independent, indicating that MLR-1023 regulated the kinase through an allosteric mechanism. We have established a link between Lyn activation and blood glucose lowering with studies showing that the glucose-lowering effects of MLR-1023 were abolished in Lyn knockout mice, consistent with existing literature linking Lyn kinase and the insulin-signaling pathway. In summary, these studies describe MLR-1023 as a unique blood glucose-lowering agent and show that MLR-1023-mediated blood glucose lowering depends on Lyn kinase activity. These results, coupled with other results (J Pharmacol Exp Ther 342:23-32, 2012), suggest that MLR-1023 and Lyn kinase activation may be a new treatment modality for type 2 diabetes.

Phenolic compounds from maté (Ilex paraguariensis) inhibit adipogenesis in 3T3-L1 preadipocytes

Leaves of Ilex paraguariensis are used to prepare a tea known as maté which is a common beverage in several South American countries. The ethanol extract was fractionated to identify the compounds responsible for the anti-adipogenic activity in 3T3-L1 cells. Extracts of both fresh and dried maté leaves were subjected to column chromatography using molecular permeation to obtain the saponin (20 % yields) and the polyphenol extracts (40 % yields) from the fresh and dried leaves. The phenolic content was determined using high-performance liquid chromatography analysis and the Folin-Ciocalteau method. Also, maté extracts (50 μg/ml to 1,000 μg/ml) did not display citotoxicity using MTT. The polyphenol extract from the dried leaves was the most effective (50 μg/ml) in the inhibition of triglyceride accumulation in 3T3-L1 adipocytes, and rutin (100 μg/ml) likely accounted for a large portion of this activity. Additionally, maté extracts had a modulatory effect on the expression of genes related to the adipogenesis as PPARγ2, leptin, TNF-α and C/EBPα.

Genetic polymorphisms in fatty acid metabolism genes and colorectal cancer

Colorectal cancer (CRC) is a leading cause of cancer death worldwide. Epidemiological risk factors for CRC included dietary fat intake; consequently, the role of genes in the fatty acid biosynthesis and metabolism pathways is of particular interest. Moreover, hyperlipidaemia has been associated with different type of cancer and serum lipid levels could be affected by genetic factors, including polymorphisms in the lipid metabolism pathway. The aim of this study is to assess the association between single-nucleotide polymorphisms (SNPs) in fatty acid metabolism genes, serum lipid levels, body mass index (BMI) and dietary fat intake and CRC risk; 30 SNPs from 8 candidate genes included in fatty acid biosynthesis and metabolism pathways were genotyped in 1780 CRC cases and 1864 matched controls from the Molecular Epidemiology of Colorectal Cancer study. Information on clinicopathological characteristics, lifestyle and dietary habits were also obtained. Logistic regression and association analysis were conducted. Several LIPC (lipase, hepatic) polymorphisms were found to be associated with CRC risk, although no particular haplotype was related to CRC. The SNP rs12299484 showed an association with CRC risk after Bonferroni correction. We replicate the association between the T allele of the LIPC SNP rs1800588 and higher serum high-density lipoprotein levels. Weak associations between selected polymorphism in the LIPC and PPARG genes and BMI were observed. A path analysis based on structural equation modelling showed a direct effect of LIPC gene polymorphisms on colorectal carcinogenesis as well as an indirect effect mediated through serum lipid levels. Genetic polymorphisms in the hepatic lipase gene have a potential role in colorectal carcinogenesis, perhaps though the regulation of serum lipid levels.

Conjugated linoleic acid supplementation caused reduction of perilipin1 and aberrant lipolysis in epididymal adipose tissue

Perilipin1, a coat protein of lipid droplet, plays a key role in adipocyte lipolysis and fat formation of adipose tissues. However, it is not clear how the expression of perilipin1 is affected in the decreased white adipose tissues (WAT) of mice treated with dietary supplement of conjugated linoleic acids (CLA). Here we obtained lipodystrophic mice by dietary administration of CLA which exhibited reduced epididymal (EPI) WAT, aberrant adipocytes and decreased expression of leptin in this tissue. We found both transcription and translation of perilipin1 was suppressed significantly in EPI WAT of CLA-treated mice compared to that of control mice. The gene expression of negative regulator tumor necrosis factor α (TNFα) and the positive regulator Peroxisome Proliferator-Activated Receptor-γ (PPARγ) of perilipin1 was up-regulated and down-regulated, respectively. In cultured 3T3-L1 cells the promoter activity of perilipin1 was dramatically inhibited in the presence of CLA. Using ex vivo experiment we found that the basal lipolysis was elevated but the hormone-stimulated lipolysis blunted in adipose explants of CLA-treated mice compared to that of control mice, suggesting that the reduction of perilipin1 in white adipose tissues may at least in part contribute to CLA-mediated alternation of lipolysis of WAT.

Controlling a master switch of adipocyte development and insulin sensitivity: covalent modifications of PPARγ

Adipocytes are highly specialized cells that play a central role in lipid homeostasis and the maintenance of energy balance. Obesity, an excessive accumulation of adipose tissue, is a major risk factor for the development of Type 2 diabetes mellitus (T2DM), cardiovascular disease, and hypertension. A variety of studies suggest that obesity and T2DM can be linked to a breakdown in the regulatory mechanisms that control the expression and transcriptional activity of PPARγ. PPARγ is a nuclear hormone receptor that functions as a master switch in controlling adipocyte differentiation and development. Also important in controlling glucose homeostasis and insulin sensitivity, PPARγ is a ligand-dependent transcription factor that is the functional receptor for the anti-diabetic thiazolidinediones (TZDs). In the last fifteen years, a variety of covalent modifications of PPARγ activity have been identified and studied. These covalent modifications include phosphorylation, ubiquitylation, O-GlcNAcylation and SUMOylation. Covalent modifications of PPARγ represent key regulatory mechanisms that control both PPARγ protein stability and transcriptional activity. A variety of PPARγ transgenic models, including mice heterozygous for PPARγ, have demonstrated the importance of PPARγ expression in glucose homeostasis and insulin resistance. In the following review, we have highlighted the regulation of PPARγ by covalent modifications, the interplay between these interactions and how these post-translational modifications impact metabolic disease states.

Mechanisms underlying the osteo- and adipo-differentiation of human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) are considered a promising cell source for regenerative medicine, because they have the potential to differentiate into a variety of lineages among which the mesoderm-derived lineages such adipo- or osteogenesis are investigated best. Human MSCs can be harvested in reasonable to large amounts from several parts of the patient's body and due to this possible autologous origin, allorecognition can be avoided. In addition, even in allogenic origin-derived donor cells, hMSCs generate a local immunosuppressive microenvironment, causing only a weak immune reaction. There is an increasing need for bone replacement in patients from all ages, due to a variety of reasons such as a new recreational behavior in young adults or age-related diseases. Adipogenic differentiation is another interesting lineage, because fat tissue is considered to be a major factor triggering atherosclerosis that ultimately leads to cardiovascular diseases, the main cause of death in industrialized countries. However, understanding the differentiation process in detail is obligatory to achieve a tight control of the process for future clinical applications to avoid undesired side effects. In this review, the current findings for adipo- and osteo-differentiation are summarized together with a brief statement on first clinical trials.

Mammalian ste20-like kinase and SAV1 promote 3T3-L1 adipocyte differentiation by activation of PPARγ

The mammalian ste20 kinase (MST) signaling pathway plays an important role in the regulation of apoptosis and cell cycle control. We sought to understand the role of MST2 kinase and Salvador homolog 1 (SAV1), a scaffolding protein that functions in the MST pathway, in adipocyte differentiation. MST2 and MST1 stimulated the binding of SAV1 to peroxisome proliferator-activated receptor γ (PPARγ), a transcription factor that plays a key role in adipogenesis. The interaction of endogenous SAV1 and PPARγ was detected in differentiating 3T3-L1 adipocytes. This binding required the kinase activity of MST2 and was mediated by the WW domains of SAV1 and the PPYY motif of PPARγ. Overexpression of MST2 and SAV1 increased PPARγ levels by stabilizing the protein, and the knockdown of SAV1 resulted in a decrease of endogenous PPARγ protein in 3T3-L1 adipocytes. During the differentiation of 3T3-L1 cells into adipocytes, MST2 and SAV1 expression began to increase at 2 days when PPARγ expression also begins to increase. MST2 and SAV1 significantly increased PPARγ transactivation, and SAV1 was shown to be required for the activation of PPARγ by rosiglitazone. Finally, differentiation of 3T3-L1 cells was augmented by MST2 and SAV1 expression and inhibited by knockdown of MST1/2 or SAV1. These results suggest that PPARγ activation by the MST signaling pathway may be a novel regulatory mechanism of adipogenesis.

White adipocytes: more than just fat depots

Globally 30% of adults are overweight or obese. The white adipocyte is a major component of adipose tissue, and as the obesity epidemic increases it is critically important to understand the factors determining adipocyte development and function. Adipogenesis has two distinct phases; determination of the adipocyte from a multipotent stem cell, and terminal differentiation of a pre-adipocyte into a mature adipocyte. The environment encountered in early life can alter adipocyte number and size and potentially impact upon adipocyte endocrine function in adulthood. These alterations may contribute to the pathophysiology of chronic diseases and thus targeted therapy of the adipocyte has great potential for treating the current obesity epidemic.

PPARgamma-independent increase in glucose uptake and adiponectin abundance in fat cells

Although thiazolidinediones (TZD) effectively improve hyperglycemia and increase adiponectin, a proinsulin-sensitizing adipokine, they also increase adipogenesis via peroxisome proliferator-activated receptor (PPAR)γ induction, which may be undesirable. Recent safety concerns about some TZD have prompted the search for next generation agents that can enhance glycemic control and adiponectin independent of PPARγ or adipogenesis. Reminiscent of TZD action, a human adenovirus, adenovirus 36 (Ad36), up-regulates PPARγ, induces adipogenesis, and improves systemic glycemic control in vivo. We determined whether this effect of Ad36 requires PPARγ and/or adipogenesis. Glucose uptake and relevant cell signaling were determined in mock-infected or human adenoviruses Ad36 or Ad2-infected cell types under the following conditions: 1) undifferentiated human-adipose-tissue-derived stem cells (hASC), 2) hASC differentiated as adipocytes, 3) hASC in presence or absence of a PPARγ inhibitor, 4) NIH/3T3 that have impaired PPARγ expression, and 5) PPARγ-knockout mouse embryonic fibroblasts. Mouse embryonic fibroblasts with intact PPARγ served as a positive control. Additionally, to determine natural Ad36 infection, human sera were screened for Ad36 antibodies. In undifferentiated or differentiated hASC, or despite the inhibition, down-regulation, or the absence of PPARγ, Ad36 significantly enhanced glucose uptake and PPARγ, adiponectin, glucose transporter 4, and glucose transporter 1 protein abundance, compared with mock or Ad2-infected cells. This indicated that Ad36 up-regulates glucose uptake and adiponectin secretion independent of adipogenesis or without recruiting PPARγ. In humans, natural Ad36 infection predicted greater adiponectin levels, suggesting a human relevance of these effects. In conclusion, Ad36 provides a novel template to metabolically remodel human adipose tissue to enhance glycemic control without the concomitant increase in adiposity or PPARγ induction associated with TZD actions.

Subchronic treatment of rats with oxytocin results in improved adipocyte differentiation and increased gene expression of factors involved in adipogenesis

BACKGROUND AND PURPOSE

Treatment with thiazolidinediones, insulin-sensitizing drugs, enhances adipogenesis, which may result in unwanted increase in adiposity. Based on the suggested metabolic effects of oxytocin, the aims of the present study were to: (i) determine whether chronic treatment with oxytocin exerts positive effects on white adipose tissue growth without increasing adiposity; (ii) investigate possible mechanisms of action of oxytocin by measuring the level of gene expression of adipogenic factors; and (iii) test the hypothesis that oxytocin's effect on adipose tissue involves specific activation of eukaryotic elongation factor 2 (eEF2).

EXPERIMENTAL APPROACH

Adult rats were subcutaneously treated with oxytocin (3.6 µg·100 g⁻¹ body weight day⁻¹) via osmotic minipumps for 2 weeks. Adipocytes from epididymal adipose tissue were isolated and their size evaluated by light microscopy. Gene expression of adipogenic and angiogenic factors was determined by real-time PCR and dephosphorylation of eEF2 by immunoblotting.

KEY RESULTS

Oxytocin treatment decreased the diameter of adipocytes and increased the epididymal adipose tissue protein content without changing the adipose tissue mass. Increases in fatty acid binding protein, peroxisome proliferator-activated receptor γ, insulin-sensitive glucose transporter 4, leptin and CD31 mRNA levels were noted in the epididymal and/or retroperitoneal fat tissue of oxytocin-treated rats. Oxytocin enhanced the dephosphorylation of eEF2 in the epididymal adipose tissue.

CONCLUSIONS AND IMPLICATIONS

The present results demonstrate that subchronic treatment with oxytocin induces adipogenic and angiogenic effects and that the eEF2 signalling pathway is involved in these effects of oxytocin on adipose tissue in vivo. These findings are likely to motivate further research and indicate new approaches for modulating adipose tissue morphology and metabolism.

The environmental chemical tributyltin chloride (TBT) shows both estrogenic and adipogenic activities in mice which might depend on the exposure dose

Exposure during early development to chemicals with hormonal action may be associated with weight gain during adulthood because of altered body homeostasis. It is known that organotins affect adipose mass when exposure occurs during fetal development, although no knowledge of effects are available for exposures after birth. Here we show that the environmental organotin tributyltin chloride (TBT) exerts adipogenic action when peripubertal and sexually mature mice are exposed to the chemical. The duration and extent of these effects depend on the sex and on the dose of the compound, and the effects are relevant at doses close to the estimated human intake (0.5μg/kg). At higher doses (50-500μg/kg), TBT also activated estrogen receptors (ERs) in adipose cells in vitro and in vivo, based on results from acute and longitudinal studies in ERE/luciferase reporter mice. In 3T3-L1 cells (which have no ERs), transiently transfected with the ERE-dependent reporter plus or minus ERα or ERβ, TBT (in a dose range of 1-100nM) directly targets each ER subtype in a receptor-specific manner through a direct mechanism mediated by ERα in undifferentiated preadipocytic cells and by ERβ in differentiating adipocytes. The ER antagonist ICI-182,780 inhibits this effect. In summary, the results of this work suggest that TBT is adipogenic at all ages and in both sexes and that it might be an ER activator in fat cells. These findings might help to resolve the apparent paradox of an adipogenic chemical being also an estrogen receptor activator by showing that the two apparently opposite actions are separated by the different doses to which the organism is exposed.

PPARs are a unique set of fatty acid regulated transcription factors controlling both lipid metabolism and inflammation

Cells are constantly exposed to a large variety of lipids. Traditionally, these molecules were thought to serve as simple energy storing molecules. More recently it has been realized that they can also initiate and regulate signaling events that will decisively influence development, cellular differentiation, metabolism and related functions through the regulation of gene expression. Multicellular organisms dedicate a large family of nuclear receptors to these tasks. These proteins combine the defining features of both transcription factors and receptor molecules, and therefore have the unique ability of being able to bind lipid signaling molecules and transduce the appropriate signals derived from lipid environment to the level of gene expression. Intriguingly, the members of a subfamily of the nuclear receptors, the peroxisome proliferator-activated receptors (PPARs) are able to sense and interpret fatty acid signals derived from dietary lipids, pathogenic lipoproteins or essential fatty acid metabolites. Not surprisingly, Peroxisome proliferator-activated receptors were found to be key regulators of lipid and carbohydrate metabolism. Unexpectedly, later studies revealed that Peroxisome proliferator-activated receptors are also able to modulate inflammatory responses. Here we summarize our understanding on how these transcription factors/receptors connect lipid metabolism to inflammation and some of the novel regulatory mechanisms by which they contribute to homeostasis and certain pathological conditions. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

MicroRNAs in adipogenesis and as therapeutic targets for obesity

INTRODUCTION

Obesity and obesity-related disease have reached pandemic proportions and are prevalent even in developing countries. Adipose tissue is increasingly being recognized as a key regulator of whole-body energy homeostasis and consequently as a prime therapeutic target for metabolic syndrome. This review discusses the roles of miRNAs, small endogenously expressed RNAs that regulate gene expression at a post-transcriptional level, in the development and function of adipose tissue and other relevant metabolic tissues impacted by obesity. Several high-throughput studies have identified hundreds of miRNAs that are differentially expressed during the development of metabolic tissues or as an indication of pathophysiology. Further investigation has functionalized the regulatory capacity of individual miRNAs and revealed putative targets for these miRNAs. Therefore, as with several other pathologies, miRNAs are emerging as feasible therapeutic targets for metabolic syndrome.

AREAS COVERED

This review provides a comprehensive view of miRNAs involved in adipogenesis, from mesenchymal stem cell lineage determination through terminal adipocyte differentiation. We also discuss the differential expression of miRNAs among adipose depots and the dysregulation of miRNAs in other metabolic tissues during metabolic pathophysiology. Finally, we discuss the therapeutic potential of targeting miRNAs in obesity and give a perspective on the challenges and advantages of miRNA-based drugs.

EXPERT OPINION

miRNAs are extensive regulators of adipocyte development and function and are viable therapeutic targets for obesity. Despite the broad-spectrum and redundancy of miRNA-target interactions, sophisticated bioinformatic approaches are making it possible to determine the most physiologically relevant miRNAs to target in disease. In vivo delivery of miRNAs for therapeutic purposes is rapidly developing and has been successful in other contexts. Additionally, miRNAs can be used as prognosis markers for disease onset and progression. Ultimately, miRNAs are prime therapeutic targets for obesity and its consequent pathologies in other metabolic tissues.

Cyclooxygenase-2 deficiency attenuates adipose tissue differentiation and inflammation in mice

Obesity is associated with a variety of disorders and is a significant health problem in developed countries. One factor controlling the level of adiposity is the differentiation of cells into adipocytes. Adipocyte differentiation requires expression of peroxisome proliferator-activated receptor γ (PPARγ), which is activated by ligands to regulate expression of genes involved in adipocyte differentiation. Although 15-deoxy-Δ(12,14)-prostaglandin (PG) J(2) (15d-PGJ(2)) has long been known to be a potent activator of PPARγ, the importance of its synthesis in adipose tissue in vivo is not clear. The current study utilized mice deficient in cyclooxygenase-2 (COX-2) to examine the role of COX-2-derived PGs as in vivo modulators of adiposity. As compared with strain- and age-matched wild-type controls, the genetic deficiency of COX-2 resulted in a significant reduction in total body weight and percent body fat. Although there were no significant differences in food consumption between groups, COX-2-deficient mice showed increased metabolic activity. Epididymal adipose tissue from wild-type mice produced a significantly greater level of 15d-PGJ(2), as compared with adipose tissue isolated from mice deficient in COX-2. Furthermore, production of the precursor required for 15d-PGJ(2) formation, PGD(2), was also significantly reduced in COX-2-deficient adipose tissue. The expression of markers for differentiated adipocytes was significantly reduced in adipose tissue from COX-2-deficient mice, whereas preadipocyte marker expression was increased. Macrophage-dependent inflammation was also significantly reduced in adipose tissue of COX-2-deficient mice. These findings suggest that reduced adiposity in COX-2-deficient mice results from attenuated PPARγ ligand production and adipocyte differentiation.

Shikonin inhibits adipogenesis by modulation of the WNT/β-catenin pathway

AIM

Our previous study showed for the first time that shikonin, a natural compound isolated from Lithospermun erythrorhizon Sieb. Et Zucc, inhibits adipogenesis and fat accumulation. This study was conducted to investigate the molecular mechanism of the anti-adipogenic effects of shikonin.

MAIN METHODS

Gene knockdown experiments using small interfering RNA (siRNA) transfection were conducted to elucidate the crucial role of β-catenin in the anti-adipogenic effects of shikonin.

KEY FINDINGS

Shikonin prevented the down-regulation of β-catenin and increased the level of its transcriptional product, cyclin D1, during adipogenesis of 3T3-L1 cells, preadipocytes originally derived from mouse embryo. β-catenin was a crucial mediator of the anti-adipogenic effects of shikonin, as determined by siRNA-mediated knockdown. Shikonin-induced reductions of the major transcription factors of adipogenesis including peroxisome proliferator-activated receptor γ and CCAAT/enhancer binding protein α, and lipid metabolizing enzymes including fatty acid binding protein 4 and lipoprotein lipase, as well as intracellular fat accumulation, were all significantly recovered by siRNA-mediated knockdown of β-catenin. Among the genes located in the WNT/β-catenin pathway, the levels of WNT10B and DVL2 were significantly up-regulated, whereas the level of AXIN was down-regulated by shikonin treatment.

SIGNIFICANCE

This study clearly shows that shikonin inhibits adipogenesis by the modulation of WNT/β-catenin pathway in vitro, and also suggests that WNT/β-catenin pathway can be used as a therapeutic target for obesity and related diseases using a natural compound like shikonin, even though the in vivo effects of shikonin and its clinical significance remain to be elucidated.

A nonthiazolidinedione peroxisome proliferator-activated receptor α/γ dual agonist CG301360 alleviates insulin resistance and lipid dysregulation in db/db mice

Activation of peroxisome proliferator-activated receptors (PPARs) have been implicated in the treatment of metabolic disorders with different mechanisms; PPARα agonists promote fatty acid oxidation and reduce hyperlipidemia, whereas PPARγ agonists regulate lipid redistribution from visceral fat to subcutaneous fat and enhance insulin sensitivity. To achieve combined benefits from activated PPARs on lipid metabolism and insulin sensitivity, a number of PPARα/γ dual agonists have been developed. However, several adverse effects such as weight gain and organ failure of PPARα/γ dual agonists have been reported. By use of virtual ligand screening, we identified and characterized a novel PPARα/γ dual agonist, (R)-1-(4-(2-(5-methyl-2-p-tolyloxazol-4-yl)ethoxy)benzyl)piperidine-2-carboxylic acid (CG301360), exhibiting the improvement in insulin sensitivity and lipid metabolism. CG301360 selectively stimulated transcriptional activities of PPARα and PPARγ and induced expression of their target genes in a PPARα- and PPARγ-dependent manner. In cultured cells, CG301360 enhanced fatty acid oxidation and glucose uptake and it reduced pro-inflammatory gene expression. In db/db mice, CG301360 also restored insulin sensitivity and lipid homeostasis. Collectively, these data suggest that CG301360 would be a novel PPARα/γ agonist, which might be a potential lead compound to develop against insulin resistance and hyperlipidemia.

Literature mining for the discovery of hidden connections between drugs, genes and diseases

The scientific literature represents a rich source for retrieval of knowledge on associations between biomedical concepts such as genes, diseases and cellular processes. A commonly used method to establish relationships between biomedical concepts from literature is co-occurrence. Apart from its use in knowledge retrieval, the co-occurrence method is also well-suited to discover new, hidden relationships between biomedical concepts following a simple ABC-principle, in which A and C have no direct relationship, but are connected via shared B-intermediates. In this paper we describe CoPub Discovery, a tool that mines the literature for new relationships between biomedical concepts. Statistical analysis using ROC curves showed that CoPub Discovery performed well over a wide range of settings and keyword thesauri. We subsequently used CoPub Discovery to search for new relationships between genes, drugs, pathways and diseases. Several of the newly found relationships were validated using independent literature sources. In addition, new predicted relationships between compounds and cell proliferation were validated and confirmed experimentally in an in vitro cell proliferation assay. The results show that CoPub Discovery is able to identify novel associations between genes, drugs, pathways and diseases that have a high probability of being biologically valid. This makes CoPub Discovery a useful tool to unravel the mechanisms behind disease, to find novel drug targets, or to find novel applications for existing drugs.

The biomedical literature is an important source of knowledge on the function of genes and on the mechanisms by which these genes regulate cellular processes. Several text mining approaches have been developed to leverage this rich source of information by automatically extracting associations between concepts such as genes, diseases and drugs from a large body of text. Here, we describe a new method that extracts novel, not yet recognized associations between genes, diseases, drugs and cellular processes from the biomedical literature. Our method is built on the assumption that even if two concepts do not have a direct connection in literature, they may be functionally related if they are both connected to an overlapping set of concepts. Using this approach we predicted several novel connections between genes, diseases, drugs and pathways. Our results imply that our method is able to predict novel relationships from literature and, most importantly, that these newly identified relationships are biologically relevant. Our method can aid the drug discovery process where it can be used to find novel drug targets, increase insight in mode of action of a drug or find novel applications for known drugs.