Activation of peroxisome proliferator-activated receptor-α enhances fatty acid oxidation in human adipocytes

Peroxisome proliferator-activated receptor-α (PPARα) is a key regulator for maintaining whole-body energy balance. However, the physiological functions of PPARα in adipocytes have been unclarified. We examined the functions of PPARα using human multipotent adipose tissue-derived stem cells as a human adipocyte model. Activation of PPARα by GW7647, a potent PPARα agonist, increased the mRNA expression levels of adipocyte differentiation marker genes such as PPARγ, adipocyte-specific fatty acid-binding protein, and lipoprotein lipase and increased both GPDH activity and insulin-dependent glucose uptake level. The findings indicate that PPARα activation stimulates adipocyte differentiation. However, lipid accumulation was not changed, which is usually observed when PPARγ is activated. On the other hand, PPARα activation by GW7647 treatment induced the mRNA expression of fatty acid oxidation-related genes such as CPT-1B and AOX in a PPARα-dependent manner. Moreover, PPARα activation increased the production of CO₂ and acid soluble metabolites, which are products of fatty acid oxidation, and increased oxygen consumption rate in human adipocytes. The data indicate that activation of PPARα stimulates both adipocyte differentiation and fatty acid oxidation in human adipocytes, suggesting that PPARα agonists could improve insulin resistance without lipid accumulation in adipocytes. The expected effects of PPARα activation are very valuable for managing diabetic conditions accompanied by obesity, because PPARγ agonists, usually used as antidiabetic drugs, induce excessive lipid accumulation in adipocytes in addition to improvement of insulin resistance.     

Genistein inhibits differentiation of primary human adipocytes

Genistein, a major soy isoflavone, has been reported to exhibit antiadipogenic and proapoptotic potential in vivo and in vitro. It is also a phytoestrogen which has high affinity to estrogen receptor beta. In this study, we determined the effect of genistein on adipogenesis and estrogen receptor (ER) alpha and beta expression during differentiation in primary human preadipocytes. Genistein inhibited lipid accumulation in a dose-dependent manner at concentrations of 6.25 microM and higher, with 50 microM genistein inhibiting lipid accumulation almost completely. Low concentrations of genistein (3.25 microM) increased cell viability and higher concentrations (25 and 50 microM) decreased it by 16.48+/-1.35% (P<.0001) and 50.68+/-1.34% (P<.0001). Oil Red O staining was used to confirm the effects on lipid accumulation. The inhibition of lipid accumulation was associated with inhibition of glycerol-3-phosphate dehydrogenase activity and down-regulation of expression of adipocyte-specific genes, including peroxisome proliferator-activated receptor gamma, CCAAT/enhancer binding protein alpha, glycerol-3-phosphate dehydrogenase, adipocyte fatty acid binding protein, fatty acid synthase, sterol regulatory element-binding protein 1, perilipin, leptin, lipoprotein lipase and hormone-sensitive lipase. These effects of genistein during the differentiation period were associated with down-regulation of ERalpha and ERbeta expression. This study adds to the elucidation of the molecular pathways involved in the inhibition of adipogenesis by phytoestrogens.     

Anti-adipogenic effect of dioxinodehydroeckol via AMPK activation in 3T3-L1 adipocytes

Dioxinodehydroeckol (DHE) isolated from Ecklonia cava, has previously been investigated for its inhibition of the differentiation of 3T3-L1 preadipocytes into adipocytes. Levels of lipid accumulation were measured, along with changes in the expression of genes and proteins associated with adipogenesis and lipolysis. Confluent 3T3-L1 preadipocytes in medium with or without different concentrations of DHE for 7 days were differentiated into adipocytes. Lipid accumulation was quantified by measuring direct triglyceride contents and Oil-Red O staining. The expression of genes and proteins associated with adipogenesis and lipolysis was measured using RT-PCR, quantitative real-time RT-PCR and Western blotting analysis. It was found that the presence of DHE significantly reduced lipid accumulation and down-regulated the expression of peroxisome proliferator-activated receptor-γ (PPARγ), sterol regulatory element-binding protein 1 (SREBP1) and CCAAT/enhancer-binding proteins (C/EBPα) in a dose-dependent manner. Moreover, DHE suppressed regulation of the adipocyte-specific gene promoters such as fatty acid binding protein (FABP4), fatty acid transport protein (FATP1), fatty acid synthase (FAS), lipoprotein lipase (LPL), acyl-CoA synthetase 1 (ACS1), leptin, perilipin and HSL compared to control adipocytes. The specific mechanism mediating the effects of DHE was confirmed by activation of phosphorylated AMP-activated protein kinase (pAMPK). Therefore, these results suggest that DHE exerts anti-adipogenic effect on adipocyte differentiation through the activation and modulation of the AMPK signaling pathway.     

Dehydroabietic acid activates peroxisome proliferator-activated receptor-γ and stimulates insulin-dependent glucose uptake into 3T3-L1 adipocytes

Dehydroabietic acid (DAA) is a food-derived terpenoid with various bioactivities. Our previous study has revealed that DAA activates peroxisome proliferator-activated receptor-γ (PPARγ) in luciferase assay and suppresses chronic inflammation in obese adipose tissues. In this study, we examined the effects of DAA on adipocyte differentiation. DAA treatment stimulated the adipocyte differentiation of 3T3-L1 preadipocytes. The DAA treatment increased the mRNA expression levels of adipocyte differentiation marker genes such as aP2, lipoprotein lipase (LPL), and PPARγ. In particular, the expression level of adiponectin, which is an adipocytokine with stimulatory effects on insulin sensitivity, was increased at both the mRNA and protein levels by the DAA treatment. Moreover, the DAA treatment stimulated insulin-dependent glucose uptake into differentiated 3T3-L1 adipocytes. These findings indicate that DAA stimulates adipocyte differentiation and insulin sensitivity in 3T3-L1 cells, suggesting that DAA is a valuable food-derived compound for the management of metabolic syndrome.     

Phloretin promotes adipocyte differentiation in vitro and improves glucose homeostasis in vivo

Adipocyte dysfunction is associated with many metabolic diseases such as obesity, insulin resistance and diabetes. Previous studies found that phloretin promotes 3T3-L1 cells differentiation, but the underlying mechanisms for phloretin's effects on adipogenesis remain unclear. In this study, we demonstrated that phloretin enhanced the lipid accumulation in porcine primary adipocytes in a time-dependent manner. Furthermore, phloretin increased the utilization of glucose and nonesterified fatty acid, while it decreased the lactate output. Microarray analysis revealed that genes associated with peroxisome proliferator-activated receptor-γ (PPARγ), mitogen-activated protein kinase and insulin signaling pathways were altered in response to phloretin. We further confirmed that phloretin enhanced expression of PPARγ, CAAT enhancer binding protein-α (C/EBPα) and adipose-related genes, such as fatty acids translocase and fatty acid synthase. In addition, phloretin activated the Akt (Thr308) and extracellular signal-regulated kinase, and therefore, inactivated Akt targets protein. Wortmannin effectively blocked the effect of phloretin on Akt activity and the protein levels of PPARγ, C/EBPα and fatty acid binding protein-4 (FABP4/aP2). Oral administration of 5 or 10 mg/kg phloretin to C57BL BKS-DB mice significantly decreased the serum glucose level and improved glucose tolerance. In conclusion, phloretin promotes the adipogenesis of porcine primary preadipocytes through Akt-associated signaling pathway. These findings suggested that phloretin might be able to increase insulin sensitivity and alleviate the metabolic diseases.     

Propyl Gallate Inhibits Adipogenesis by Stimulating Extracellular Signal-Related Kinases in Human Adipose Tissue-Derived Mesenchymal Stem Cells

Propyl gallate (PG) used as an additive in various foods has antioxidant and anti-inflammatory effects. Although the functional roles of PG in various cell types are well characterized, it is unknown whether PG has effect on stem cell differentiation. In this study, we demonstrated that PG could inhibit adipogenic differentiation in human adipose tissue-derived mesenchymal stem cells (hAMSCs) by decreasing the accumulation of intracellular lipid droplets. In addition, PG significantly reduced the expression of adipocyte-specific markers including peroxisome proliferator-activated receptor-γ (PPAR-γ), CCAAT enhancer binding protein-α (C/EBP-α), lipoprotein lipase (LPL), and adipocyte fatty acid-binding protein 2 (aP2). PG inhibited adipogenesis in hAMSCs through extracellular regulated kinase (ERK) pathway. Decreased adipogenesis following PG treatment was recovered in response to ERK blocking. Taken together, these results suggest a novel effect of PG on adipocyte differentiation in hAMSCs, supporting a negative role of ERK1/2 pathway in adipogenic differentiation.     

Resveratrol inhibits cell differentiation in 3T3-L1 adipocytes via activation of AMPK

Resveratrol (Res) is a natural polyphenolic compound with anti-inflammatory and antioxidant properties. Also, Res can inhibit lipogenesis and adipocyte differentiation. However, the underlying mechanisms of Res's functions remain largely unknown. AMP-activated protein kinase (AMPK) is a key player in adipocyte differentiation. Therefore, the purpose of our study was to determine the role played by AMPK in the Res-mediated regulation of adipocyte differentiation. Incubation of 3T3-L1 cells with Res confirmed that Res inhibited adipocyte differentiation. The phosphorylation of AMPKα was increased by Res in a dose-dependent manner, while total AMPKα levels were unchanged, and peroxisome proliferator-activated receptor γ (PPARγ), CCAAT-enhancer-binding protein α (C/EBPα), and sterol regulatory element-binding protein 1c (SREBP-1c) levels were decreased. Interestingly, pretreatment with AMPKα siRNA and Res promoted adipocyte differentiation, while the decrease of p-AMPKα increased PPARγ, C/EBPα, and SREBP-1c protein expression. Our study shows that Res is capable of inhibiting lipogenesis and differentiation of 3T3-L1 adipocytes via activation of AMPK, suggesting its potential therapeutic application in the treatment or prevention of obesity.     

Impaired lipid accumulation by trans10, cis12 CLA during adipocyte differentiation is dependent on timing and length of treatment

Conjugated linoleic acids (CLAs) are a group of polyunsaturated fatty acids found in ruminant products, where the predominant isomers are cis9, trans11 (c9,t11) and trans10, cis12 (t10,c12) CLA. We have previously shown that t10,c12 CLA prevents lipid accumulation in mature adipocytes in part by acting as a peroxisome proliferator-activated receptor gamma (PPAR gamma) modulator. The objective of this study was to further establish the molecular mechanisms underlying the attenuating effect on lipid accumulation by t10,c12 CLA, with focus on time point and duration of treatment during adipogenesis. We have shown that t10,c12 CLA treatment has its most attenuating effect early (day (D) 0-6) during differentiation. Treatment during this period is sufficient to prevent lipid accumulation in mature adipocytes. The adipogenic marker genes PPAR gamma and CCAAT/enhancer binding protein alpha (C/EBP alpha) are both down-regulated after treatment within the period from D0-6, while additional treatment also down-regulates the expression of sterol regulatory element binding protein-1c (SREBP-1c), liver X receptor alpha (LXR alpha), fatty acid binding protein (aP2), fatty acid translocase (CD36) and insulin-sensitive glucose transporter 4 (GLUT4). These effects of t10,c12 CLA reflect the subsequent attenuation of lipid accumulation observed in mature adipocytes. Interestingly, the early B-cell factor (O/E-1), which is known to promote adipogenesis and to be involved in control of genes important for terminal adipocyte differentiation, is unaffected by treatment of t10,c12 CLA. Taken together, our data indicate that inhibition of lipid accumulation induced by t10,c12 CLA treatment during adipocyte differentiation is associated with a tight regulatory cross-talk between early (PPAR gamma and C/EBP alpha) and late (LXR alpha, aP2 and CD36) adipogenic marker genes.     

Suppression of adipocyte differentiation by Cordyceps militaris through activation of the aryl hydrocarbon receptor

Mycelial extracts have a wide range of biological activities that modulate functions of mammalian cells. In this report, we sought to identify antiadipogenic mycelia with the use of 3T3-L1 cells and found that the extract of Cordyceps militaris exclusively suppressed differentiation of 3T3-L1 preadipocytes into mature adipocytes without affecting cell viability. This inhibitory effect was dose dependent, reversible, and associated with 1) a decrease in lipid accumulation, 2) blunted induction of adipocyte markers including adiponectin, peroxisome proliferator-activated receptor-gamma, and CCAAT/enhancer binding protein-alpha, and 3) sustained expression of a preadipocyte marker, monocyte chemoattractant protein-1. C. militaris also significantly decreased accumulation of lipid and hypertrophy in mature adipocytes and preserved their response to insulin (phosphorylation of Akt) during prolonged culture. Subsequent experiments revealed that C. militaris has the potential to activate the aryl hydrocarbon receptor (AhR). In 3T3-L1 cells, treatment with AhR agonists including benzo[a]pyrene and 3-methylcholanthrene reproduced the antiadipogenic effect of C. militaris. Furthermore, dominant-negative inhibition of AhR abrogated the suppressive effect of C. militaris on adipocyte differentiation. These results suggest that C. militaris has the potential to interfere with adipocyte differentiation through activation of AhR.     

MicroRNA-302a inhibits adipogenesis by suppressing peroxisome proliferator-activated receptor γ expression

The present study explored the involvement of miR-302a in adipocyte differentiation via interaction with 3′-untranslated region of peroxisome proliferator-activated receptor gamma (PPARγ) mRNA. In differentiating 3T3-L1 adipocytes, expression of miR-302a was negatively correlated with that of the adipogenic gene aP2 and PPARγ. Overexpression of miR-302a inhibited adipogenic differentiation with lipid accumulation, and inversely anti-miR-302a increased the differentiation. In silico analysis revealed a complementary region of miR-302a seed sequence in 3′-UTR of PPARγ mRNA. Luciferase assay showed the direct interaction of miR-302a with PPARγ at the cellular level. The miR-302a inhibition of adipocyte differentiation was reversed by PPARγ overexpression. These findings suggest that miR-302a might be a negative regulator of adipocyte differentiation and that the dysregulation of miR-302a should lead to metabolic disorders.     

Trigonelline attenuates the adipocyte differentiation and lipid accumulation in 3T3-L1 cells

Trigonelline is a natural alkaloid mainly found in Trigonella Foenum Graecum (fenugreek) Fabaceae and other edible plants with a variety of medicinal applications. Therefore, we investigated the molecular mechanism of trigonelline (TG) on the inhibition of adipocyte differentiation and lipid accumulation in 3T3-L1 cells. Trigonelline suppressed lipid droplet accumulation in a concentration (75 and 100 μM) dependent manner. Treatment of adipocyte with of TG down regulates the peroxisome proliferator-activated receptor (PPARγ) and CCAAT element binding protein (C/EBP-α) mRNA expression, which leads to further down regulation of other gene such as adiponectin, adipogenin, leptin, resistin and adipocyte fatty acid binding protein (aP2) as compared with respective control cells on 5th and 10th day of differentiation. Further, addition of triognelline along with troglitazone to the adipocyte attenuated the troglitazone effects on PPARγ mediated differentiation and lipid accumulation in 3T3-L1 cells. Trigonelline might compete against troglitazone for its binding to the PPARγ. In addition, adipocyte treated with trigonelline and isoproterenol separately. Isoproterenol, a lipolytic agent which inhibits the fatty acid synthase and GLUT-4 transporter expression via cAMP mediated pathway, we found that similar magnitude response of fatty acid synthase and GLUT-4 transporter expression in trigonelline treated adipocyte. These results suggest that the trigonelline inhibits the adipogenesis by its influences on the expression PPARγ, which leads to subsequent down regulation of PPAR-γ mediated pathway during adipogenesis. Our findings provide key approach to the mechanism underlying the anti-adipogenic activity of trigonelline.