Prostacyclin as a potent effector of adipose-cell differentiation

Generation of monoclonal antibody against 6-Keto PGF1α and development of ELISA for its quantification in culture medium

Prostacyclin or prostaglandin I2 (PGI2), a metabolite of arachidonic cyclooxygenase pathway, has been demonstrated as an effector of adipocyte differentiation. However, due to its instability in biological fluid, it is difficult to evaluate the role of PGI2 in regulating adipocyte differentiation in different stages in culture. Therefore, this study aimed to establish a simple and rapid method for the production of monoclonal antibody against 6-Keto PGF1α, a stable PGI2 metabolite, and its quantification to determine the role of PGI2 in culture medium. Eight-week-old female BALB/c mice were immunized with the hapten of 6-Keto PGF1α and BSA for several weeks until a higher antibody titer (absorbance value > 0.9 at 1000-times dilution) against 6-Keto PGF1α was found. Then, fusion of antibody-producing spleen lymphocytes with SP-2 myeloma cells and thymocytes was performed and cultured in HAT-medium supplemented with hypoxanthine, aminopterin, and thymine. Specific antibody-producing cells (M2-A4-B8-D10) against 6-Keto PGF1α were identified and separated. A standard ELISA calibration curve was developed with 100% reactivity for 6-Keto-PGF 1 α ranging from 0.26 pg to 6.44 ng corresponding to 90% and 10% of the maximum binding capacity for the immobilized antigen respectively. This method can easily be applied to monitor PGI2 regulation in different stages of cultured adipocytes to reveal the regulatory roles of PGI2 in maintaining homeostasis and adipocyte differentiation.

Perinatal Polyunsaturated Fatty Acid Status and Obesity Risk

High obesity rates in almost all regions of the world prompt an urgent need for effective obesity prevention. Very good scientific evidence from cell culture and rodent studies show that the availability of essential polyunsaturated fatty acids (PUFA) and their long-chain polyunsaturated derivatives, namely, arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid, influence adipogenesis; for this reason, early life status may influence later obesity risk. The respective PUFA effects could be mediated via their eicosanoid derivatives, their influence on cell membrane properties, the browning of white adipose tissue, changes to the offspring gut microbiome, their influence on developing regulatory circuits, and gene expression during critical periods. Randomized clinical trials and observational studies show divergent findings in humans, with mostly null findings but also the positive and negative effects of an increased n-3 to n-6 PUFA ratio on BMI and fat mass development. Hence, animal study findings cannot be directly extrapolated to humans. Even though the mechanistic data basis for the effects of n-3 PUFA on obesity risk appears promising, no recommendations for humans can be derived at present.

Fighting Fat With Fat: n-3 Polyunsaturated Fatty Acids and Adipose Deposition in Broiler Chickens

Modern broiler chickens are incredibly efficient, but they accumulate more adipose tissue than is physiologically necessary due to inadvertent consequences of selection for rapid growth. Accumulation of excess adipose tissue wastes feed in birds raised for market, and it compromises well-being in broiler-breeders. Studies driven by the obesity epidemic in humans demonstrate that the fatty acid profile of the diet influences adipose tissue growth and metabolism in ways that can be manipulated to reduce fat accretion. Omega-3 polyunsaturated fatty acids (n-3 PUFA) can inhibit adipocyte differentiation, induce fatty acid oxidation, and enhance energy expenditure, all of which can counteract the accretion of excess adipose tissue. This mini-review summarizes efforts to counteract the tendency for fat accretion in broilers by enriching the diet in n-3 PUFA.

Involvement of prostacyclin synthase in high-fat-diet-induced obesity

Prostacyclin (PGI₂) synthase (PGIS) functions downstream of inducible cyclooxygenase COX-2 in the PGI₂ biosynthetic pathway. Although COX-2 and PGI₂ receptor (IP) are known to be involved in adipogenesis and obesity, the involvement of PGIS has not been fully elucidated. In this study, we examined the role of PGIS in adiposity by using PGIS-deficient mice. Although PGIS deficiency did not affect in vitro adipocyte differentiation, when fed a high-fat diet (HFD), PGIS knockout (KO) mice showed reductions in both body weight gain and epididymal fat mass relative to wild-type (WT) mice. PGIS deficiency might reduce HFD-induced obesity by suppressing PGI₂ production. We further found that additional gene deletion of microsomal prostaglandin (PG) E synthase-1 (mPGES-1), one of the other PG terminal synthases that also functions downstream of COX-2, emphasized the metabolic phenotypes of PGIS-deficient mice. More marked reduction in obesity and improved insulin resistance were observed in PGIS/mPGES-1 double KO (DKO) mice. Since an additive increase in PGF_2α level in epididymal fat was observed in DKO mice, mPGES-1 deficiency might affect adiposity by enhancing the production of PGF_2α. Our immunohistochemical analysis further revealed that in adipose tissues, PGIS was expressed in vascular and stromal cells but not in adipocytes. These results suggested that PGI₂ produced from PGIS-expressed stromal tissues might enhance HFD-induced obesity by acting on IP expressed in adipocytes. The balance of expressions of PG terminal synthases and the subsequent production of prostanoids might be critical for adiposity.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

Overexpression of cyclooxygenase-2 in adipocytes reduces fat accumulation in inguinal white adipose tissue and hepatic steatosis in high-fat fed mice

Cyclooxygenases are known as important regulators of metabolism and immune processes via conversion of C20 fatty acids into various regulatory lipid mediators, and cyclooxygenase activity has been implicated in browning of white adipose tissues. We generated transgenic (TG) C57BL/6 mice expressing the Ptgs2 gene encoding cyclooxygenase-2 (COX-2) in mature adipocytes. TG mice fed a high-fat diet displayed marginally lower weight gain with less hepatic steatosis and a slight improvement in insulin sensitivity, but no difference in glucose tolerance. Compared to littermate wildtype mice, TG mice selectively reduced inguinal white adipose tissue (iWAT) mass and fat cell size, whereas the epididymal (eWAT) fat depot remained unchanged. The changes in iWAT were accompanied by increased levels of specific COX-derived lipid mediators and increased mRNA levels of interleukin-33, interleukin-4 and arginase-1, but not increased expression of uncoupling protein 1 or increased energy expenditure. Epididymal WAT (eWAT) in TG mice exhibited few changes except from increased infiltration with eosinophils. Our findings suggest a role for COX-2-derived lipid mediators from adipocytes in mediating type 2 immunity cues in subcutaneous WAT associated with decreased hepatic steatosis, but with no accompanying induction of browning and increased energy expenditure.

Fatty Acid Metabolites as Novel Regulators of Non-shivering Thermogenesis

Fatty acids are essential contributors to adipocyte-based non-shivering thermogenesis by acting as activators of uncoupling protein 1 and serving as fuel for mitochondrial heat production. Novel evidence suggests a contribution to this thermogenic mechanism by their conversion to bioactive compounds. Mammalian cells produce a plethora of oxylipins and endocannabinoids, some of which have been identified to affect the abundance or thermogenic activity of brown and brite adipocytes. These effectors are produced locally or at distant sites and signal toward thermogenic adipocytes via a direct interaction with these cells or indirectly via secondary mechanisms. These interactions are evoked by the activation of receptor-mediated pathways. The endogenous production of these compounds is prone to modulation by the dietary intake of the respective precursor fatty acids. The effect of nutritional interventions on uncoupling protein 1-derived thermogenesis may thus at least in part be conferred by the production of a supportive oxylipin and endocannabinoid profile. The manipulation of this system in future studies will help to elucidate the physiological potential of these compounds as novel, endogenous regulators of non-shivering thermogenesis.

Prostacyclin: A major prostaglandin in the regulation of adipose tissue development

Prostaglandins (PGs) belong to the group lipid mediators and can act as local hormones. They contain 20 carbon atoms, including a 5-carbon ring, and are biosynthesized from membrane phospholipid derived arachidonic acid through the arachidonate cyclooxygenase (COX) pathway with the help of various terminal synthase enzymes. Prostacyclin (prostaglandin I₂ ) is one of the major prostanoids produced with the help of prostacyclin synthase (prostaglandin I₂ synthase) enzyme and rapidly hydrolyzed into 6-keto-PGF_1α in biological fluids. Obesity indicates an excess of body adiposity, which is globally considered as one of the major health disasters responsible for developing complex pathological situations in the human body. Adipose tissues can produce various PGs, and thus, the level and the molecular activity of these endogenously synthesized PGs are considered critical for the development of obesity. In this regard, the involvement of prostacyclin in adipogenesis has been studied in the last few decades. The current review, along with the background of other related PGs, presents the several molecular aspects of endogenous prostaglandin I₂ in adipose tissue development. Especially, the regulation of life cycle of adipocytes, impact on terminal differentiation, activity through prostacyclin receptor (IP), autocrine-paracrine manner, thermogenic adipose tissue remodeling and some future experimental aspects of prostacyclin have been focused upon in this study. This discussion might assist to develop new drug molecules acting on the signaling pathways of prostacyclin and devise therapeutic strategies for treating obesity.

Indomethacin Enhances Brown Fat Activity

Indomethacin, a nonsteroidal anti-inflammatory drug, has been shown to induce white adipocyte differentiation; however, its roles in brown adipocyte differentiation and activation in brown adipose tissue (BAT) and obesity are unknown. To address this issue, we treated mouse brown preadipocytes with different doses of indomethacin, and delivered indomethacin to interscapular BAT (iBAT) of obese mice using implanted osmotic pumps. Indomethacin dose dependently increased brown preadipocyte differentiation and upregulated both mRNA and protein expression of uncoupling protein 1 (UCP1) and peroxisome proliferator-activated receptor (PPAR) γ coactivator 1-alpha. The mechanistic study showed that indomethacin significantly activated the reporter driven by the PPAR response element, indicating that indomethacin may work as a PPARγ agonist in this cell line. Consistently, indomethacin significantly decreased iBAT mass and fasting blood glucose levels in high-fat diet-induced obesity (DIO) mice. Histologic analysis showed that brown adipocytes of indomethacin-treated mice contained smaller lipid droplets compared with control mice, suggesting that indomethacin alleviated the whitening of BAT induced by the high-fat diet. Moreover, indomethacin significantly increased UCP1 mRNA expression in iBAT. Taken together, this study indicates that indomethacin can promote mouse brown adipocyte differentiation, and might increase brown fat and glucose oxidation capacity in DIO mice.

Maternal consumption of fish oil programs reduced adiposity in broiler chicks

Maternal intake of eicosapentaenoic acid (EPA; 20:5 n-3) and docosahexaenoic acid (22:6 n-3) has been associated with reduced adiposity in children, suggesting the possibility to program adipose development through dietary fatty acids before birth. This study determined if enriching the maternal diet in fish oil, the primary source of EPA and DHA, affected adipose development in offspring. Broiler chickens were used because they are obesity-prone, and because fatty acids provided to the embryo can be manipulated through the hen diet. Hens were fed diets supplemented (2.8% wt:wt) with corn oil (CO; n-6) or fish oil (FO; n-3) for 28 d. Chicks from both maternal diet groups were fed the same diet after hatch. Maternal FO consumption enriched chick adipose tissue in EPA and DHA and reduced adiposity by promoting more, but smaller, adipocytes. This adipocyte profile was paralleled by lower expression of the adipogenic regulator PPARG and its co-activator PPARGC1B, and elevated expression of LPL. Proteomics identified 95 differentially abundant proteins between FO and CO adipose tissue, including components of glucose metabolism, lipid droplet trafficking, and cytoskeletal organization. These results demonstrate that the maternal dietary fatty acid profile programs offspring adipose development.

The effect of supplementing rumen undegradable unsaturated fatty acids on marbling in early-weaned steers

The objective of this study was to determine if supplementation with a rumen undegradable unsaturated fatty acids (FA) source improved marbling deposition in early-weaned steers. All steers (Angus, = 23; Angus × Hereford, = 24) were weaned at 150 ± 5 d of age. Steers were blocked by BW and breed and then randomly assigned to either control (CON; average 1.5 kg of corn gluten feed [CGF], = 23) or isocaloric supplementation containing a rumen undegradable fat source (RUF; 200 g of Megalac-R added to an average of 1.06 kg of CGF, = 24) offered 5 d/wk for 110 d. All steers had ad libitum access to pastures throughout treatment and received supplements individually. Steer BW and blood samples were collected at 0, 55, and 110 d of supplementation, and real-time ultrasound measurements were collected at d 110. Following treatment, steers were transported to Oklahoma State University for finishing and subsequent harvesting at a commercial plant. All data were analyzed using the PROC MIXED procedure of SAS either as repeated measures or ANOVA depending on parameters. There were no significant changes in BW from the beginning of treatment to harvest due to treatment. Ultrasound data showed that RUF steers tended ( = 0.08) to have more i.m. fat than CON at d 110. Serum concentrations of FA showed a treatment × day interaction ( < 0.02) for 16:0, 18:0, 18:1-9, 18:2, 20:4, and total FA. These specific FA concentrations slightly increased in CON steers, but there was a more pronounced increase in the concentration of these FA across the supplementation period in RUF steers. Serum triglyceride and cholesterol concentrations were increased ( < 0.01) on d 55 and 110 in RUF steers compared with those in CON steers. Serum leptin concentration in RUF steers was greater ( < 0.01) than CON steers at d 110. After harvest, RUF carcasses had greater ( = 0.01) marbling scores than those of CON carcasses. All other carcass measures were similar between treatments. The percentage of total lipids was increased ( = 0.011) in steaks from RUF compared to CON. There was a decrease ( < 0.05) in adipocyte diameter in i.m. fat depot of RUF steers compared to CON. There was also a tendency ( = 0.06) for RUF steers to have a greater percentage of 20 to 30 μm adipocytes in their i.m. depot than CON steers. This study indicates that supplementation of unsaturated FA can positively impact marbling deposition in early-weaned steers without impacting other carcass measures.

Combined Angiotensin Receptor Modulation in the Management of Cardio-Metabolic Disorders

Cardiovascular and metabolic disorders, such as hypertension, insulin resistance, dyslipidemia or obesity are linked with chronic low-grade inflammation and dysregulation of the renin–angiotensin system (RAS). Consequently, RAS inhibition by ACE inhibitors or angiotensin AT1 receptor (AT1R) blockers is the evidence-based standard for cardiovascular risk reduction in high-risk patients, including diabetics with albuminuria. In addition, RAS inhibition reduces the new onset of diabetes mellitus. Yet, the high and increasing prevalence of metabolic disorders, and the high residual risk even in properly treated patients, calls for additional means of pharmacological intervention. In the past decade, the stimulation of the angiotensin AT2 receptor (AT2R) has been shown to reduce inflammation, improve cardiac and vascular remodeling, enhance insulin sensitivity and increase adiponectin production. Therefore, a concept of dual AT1R/AT2R modulation emerges as a putative means for risk reduction in cardio-metabolic diseases. The approach employing simultaneous RAS blockade (AT1R) and RAS stimulation (AT2R) is distinct from previous attempts of double intervention in the RAS by dual blockade. Dual blockade abolishes the AT1R-linked RAS almost completely with subsequent risk of hypotension and hypotension-related events, i.e. syncope or renal dysfunction. Such complications might be especially prominent in patients with renal impairment or patients with isolated systolic hypertension and normal-to-low diastolic blood pressure values. In contrast to dual RAS blockade, the add-on of AT2R stimulation does not exert significant blood pressure effects, but it may complement and enhance the anti-inflammatory and antifibrotic/de-stiffening effects of the AT1R blockade and improve the metabolic profile. Further studies will have to investigate these putative effects in particular for settings in which blood pressure reduction is not primarily desired.

Stimulation of fat storage by prostacyclin and selective agonists of prostanoid IP receptor during the maturation phase of cultured adipocytes

We have previously shown that cultured adipocytes have the ability to biosynthesize prostaglandin (PG) I₂ called alternatively as prostacyclin during the maturation phase by the positive regulation of gene expression of PGI synthase and the prostanoid IP receptor. To clarify how prostacyclin regulates adipogenesis, we investigated the effects of prostacyclin and the specific agonists or antagonists for the IP receptor on the storage of fats during the maturation phase of cultured adipocytes. Exogenous PGI₂ and the related selective agonists for the IP receptor including MRE-269 and treprostinil rescued the storage of fats attenuated by aspirin, a cyclooxygenase inhibitor. On the other hand, selective antagonists for IP such as CAY10441 and CAY10449 were effective to suppress the accumulation of fats as GW9662, a specific antagonist for peroxisome proliferator-activated receptor (PPAR)γ. Thus, pro-adipogenic action of prostacyclin can be explained by the action mediated through the IP receptor expressed at the maturation stage of adipocytes. Cultured adipocytes incubated with each of PGI₂ and MRE-269 together with troglitazone, an activator for PPARγ, exhibited additively higher stimulation of fats storage than with either compound alone. The combined effect of MRE-269 and troglitazone was almost abolished by co-incubation with GW9662, but not with CAY10441. Increasing concentrations of troglitazone were found to reverse the inhibitory effect of CAY10441 in a dose-dependent manner while those of MRE-269 failed to rescue adipogenesis suppressed by GW9662, indicating the critical role of the PPARγ activation as a downstream factor for the stimulated adipogenesis through the IP receptor. Treatment of cultured adipocytes with cell permeable stable cAMP analogues or forskolin as a cAMP elevating agent partly restored the inhibitory effect of aspirin. However, excess levels of cAMP stimulated by forskolin attenuated adipogenesis. Supplementation with H-89, a cell permeable inhibitor for protein kinase A (PKA), had no effect on the promoting action of PGI₂ or MRE-269 along with aspirin on the storage of fats, suggesting that the promotion of adipogenesis mediated by the IP receptor does not require the PKA activity.

Pretreatment of cultured preadipocytes with arachidonic acid during the differentiation phase without a cAMP-elevating agent enhances fat storage after the maturation phase

Arachidonic acid (AA) and the related prostanoids exert complex effects on the adipocyte differentiation depending on the culture conditions and life stages. Here, we investigated the effect of the pretreatment of cultured 3T3-L1 preadipocytes with exogenous AA during the differentiation phase without 3-isobutyl-1-methylxanthine (IBMX), a cAMP-elevating agent, on the storage of fats after the maturation phase. This pretreatment with AA stimulated appreciably adipogenesis after the maturation phase as evident with the up-regulated gene expression of adipogenic markers. The stimulatory effect of the pretreatment with AA was attenuated by the co-incubation with each of cyclooxygenase (COX) inhibitors. Among exogenous prostanoids and related compounds, the pretreatment with MRE-269, a selective agonist of the IP receptor for prostaglandin (PG) I2, strikingly stimulated the storage of fats in adipocytes. The gene expression analysis of arachidonate COX pathway revealed that the transcript levels of inducible COX-2, membrane-bound PGE synthase-1, and PGF synthase declined more greatly in cultured preadipocytes treated with AA. By contrast, the expression levels of COX-1, cytosolic PGE synthase, and PGI synthase remained constitutive. The treatment of cultured preadipocytes with AA resulted in the decreased synthesis of PGE2 and PGF2α serving as anti-adipogenic PGs although the biosynthesis of pro-adipogenic PGI2 was up-regulated during the differentiation phase. Moreover, the gene expression levels of EP4 and FP, the respective prostanoid receptors for PGE2 and PGF2α, were gradually suppressed by the supplementation with AA, whereas that of IP for PGI2 remained relatively constant. Collectively, these results suggest the predominant role of endogenous PGI2 in the stimulatory effect of the pretreatment of cultured preadipoccytes with AA during the differentiation phase without IBMX on adipogenesis after the maturation phase.

Transcriptional Pathways in cPGI2-Induced Adipocyte Progenitor Activation for Browning

De novo formation of beige/brite adipocytes from progenitor cells contributes to the thermogenic adaptation of adipose tissue and holds great potential for the therapeutic remodeling of fat as a treatment for obesity. Despite the recent identification of several factors regulating browning of white fat, there is a lack of physiological cell models for the mechanistic investigation of progenitor-mediated beige/brite differentiation. We have previously revealed prostacyclin (PGI2) as one of the few known endogenous extracellular mediators promoting de novo beige/brite formation by relaying β-adrenergic stimulation to the progenitor level. Here, we present a cell model based on murine primary progenitor cells defined by markers previously shown to be relevant for in vivo browning, including a simplified isolation procedure. We demonstrate the specific and broad induction of thermogenic gene expression by PGI2 signaling in the absence of lineage conversion, and reveal the previously unidentified nuclear relocalization of the Ucp1 gene locus in association with transcriptional activation. By profiling the time course of the progenitor response, we show that PGI2 signaling promoted progenitor cell activation through cell cycle and adhesion pathways prior to metabolic maturation toward an oxidative cell phenotype. Our results highlight the importance of core progenitor activation pathways for the recruitment of thermogenic cells and provide a resource for further mechanistic investigation.

The ω6-fatty acid, arachidonic acid, regulates the conversion of white to brite adipocyte through a prostaglandin/calcium mediated pathway

Objective

Brite adipocytes are inducible energy-dissipating cells expressing UCP1 which appear within white adipose tissue of healthy adult individuals. Recruitment of these cells represents a potential strategy to fight obesity and associated diseases.

Methods/Results

Using human Multipotent Adipose-Derived Stem cells, able to convert into brite adipocytes, we show that arachidonic acid strongly inhibits brite adipocyte formation via a cyclooxygenase pathway leading to secretion of PGE2 and PGF2α. Both prostaglandins induce an oscillatory Ca⁺⁺ signaling coupled to ERK pathway and trigger a decrease in UCP1 expression and in oxygen consumption without altering mitochondriogenesis. In mice fed a standard diet supplemented with ω6 arachidonic acid, PGF2α and PGE2 amounts are increased in subcutaneous white adipose tissue and associated with a decrease in the recruitment of brite adipocytes.

Conclusion

Our results suggest that dietary excess of ω6 polyunsaturated fatty acids present in Western diets, may also favor obesity by preventing the “browning” process to take place.

Endogenous synthesis of prostacyclin was positively regulated during the maturation phase of cultured adipocytes

Prostacyclin alternatively called prostaglandin (PG) I2 is an unstable metabolite synthesized by the arachidonate cyclooxygenase pathway. Earlier studies have suggested that prostacyclin analogues can act as a potent effector of adipose differentiation. However, biosynthesis of PGI2 has not been determined comprehensively at different life stages of adipocytes. PGI2 is rapidly hydrolyzed to the stable product, 6-keto-PGF1α, in biological fluids. Therefore, the generation of PGI2 can be quantified as the amount of 6-keto-PGF1α. In this study, we attempted to develop a solid-phase enzyme-linked immunosorbent assay (ELISA) using a mouse antiserum specific for 6-keto-PGF1α. According to the typical calibration curve of our ELISA, 6-keto-PGF1α can be quantified from 0.8 pg to 7.7 ng in an assay. The evaluation of our ELISA revealed the higher specificity of our antiserum without the cross-reaction with other related prostanoids while it exhibited only the cross-reaction of 1.5 % with PGF2α. The resulting ELISA was applied to the quantification of 6-keto-PGF1α generated endogenously by cultured 3T3-L1 cells at different stages. The cultured cells showed the highest capability to generate 6-keto-PGF1α during the maturation phase of 4-6 days, which was consistent with the coordinated changes in the gene expression of PGI synthase and the IP receptor for PGI2. Following these events, the accumulation of fats was continuously promoted up to 14 days. Thus, our immunological assay specific for 6-keto-PGF1α is useful for monitoring the endogenous levels of the unstable parent PGI2 at different life stages of adipogenesis and for further studies on the potential association with the up-regulation of adipogenesis in cultured adipocytes.

Insulin suppresses distal-less homeobox 5 expression through the up-regulation of microRNA-124 in 3T3-L1 cells

Distal-less homeobox 5 (Dlx5) is a pro-osteogenic but anti-adipogenic transcription factor that regulates lineage commitment in mesenchymal stem cells. Although the expression of Dlx5 is known to be decreased by adipogenic stimuli, the mechanism of Dlx5 down-regulation has not yet been clarified. MicroRNAs (miRNAs) are small regulatory RNAs that post-transcriptionally regulate many biological functions, including cell differentiation. In this study, we examined whether miRNAs are involved in down-regulation of Dlx5 following adipogenic stimuli. We screened candidate miRNAs that have a direct target site in the Dlx5 3'UTR using computational prediction programs, selected seven miRNA candidates with the highest binding score and observed their expression levels in 3T3-L1 murine pre-adipocytes. Among the miRNAs examined, only miR-124 was significantly up-regulated by 24-h incubation in adipogenic medium. Among the four components of adipogenic stimuli (1-methy-3-isobutyl xanthine, insulin, indomethacin and dexamethasone), insulin exhibited the highest stimulatory effect on miR-124 expression. Insulin significantly increased the expression of miR-124 precursors including pri-miR-124-1, pri-miR124-2 and pri-miR-124-3. LY294002, an inhibitor of phosphatidylinositol-3-kinase, prevented the regulatory effect of insulin on the expression levels of miR-124 and Dlx5. Over-expression of a miR-124 mimic decreased the expression of Dlx5 while increasing adipogenic differentiation in 3T3-L1 cells. Blocking miR-124 with anti-miR-124, a hairpin inhibitor of miR-124, increased the expression level of Dlx5 and suppressed adipogenic differentiation. When reporter assays were performed with a reporter construct containing the Dlx5 3'UTR sequence downstream of a luciferase gene, miR-124 mimic suppressed, but anti-miR-124 enhanced, luciferase activity in an miR-124 binding site-dependent manner. These results suggest that insulin-induced miR-124 plays a pivotal role in post-transcriptional regulation of Dlx5 during adipogenic differentiation and that miR-124 exerts pro-adipogenic effects by targeting Dlx5, at least in part.

Angiotensin-Converting EnzymeGene Polymorphisms and Obesity: An Examination of Three Black Populations

We examined the association between obesity and 13 angiotensin-converting enzyme (ACE) gene polymorphisms, including the presence (I) or absence (D) of an Alu element in intron 16 (I/D polymorphism), and performed haplotype analysis using data collected from participants of a community survey of hypertension among blacks living in Ibadan, Nigeria; Spanish Town, Jamaica; and Chicago, IL. Transmission distortion of ACE gene polymorphisms and haplotypes from heterozygous parents to affected offspring was examined in each study population. To estimate haplotypes, polymorphisms were divided into three groups based on their position on the ACE gene. No ACE gene polymorphism was consistently overtransmitted from parents to obese offspring among the three populations. However, the haplotype ACE1-ACE5 TACAT, located in the promoter region, was significantly overtransmitted from parents to obese offspring in both the U.S. and Nigerian populations. No haplotype was significantly overtransmitted from parents to obese offspring among the Jamaicans. In conclusion, we noted the overtransmission of a particular ACE gene promoter region haplotype from parents to obese offspring in two separate black populations. These data suggest that ACE gene polymorphisms may influence the development of weight gain.

Hormonal influences of early postnatal indomethacin and ibuprofen in neonatal rats

OBJECTIVE

Indomethacin and ibuprofen are administered to preterm neonates for symptomatic patent ductus arteriosus. The drugs suppress prostaglandins (PGs) which modulate growth and secretion of various hormones. We examined the hypothesis that early postnatal indomethacin and ibuprofen influence growth and GH-IGF-I-insulin and HPA axes in neonatal rats.

DESIGN

Rat pups received IP injections of saline (Sal) on P1, P2, and P3; 10mg/kg ibuprofen on P1 followed by 5mg/kg on P2 and P3; or 0.2mg/kg indomethacin on P1 followed by 0.1mg/kg on P2 and P3. Serum and hepatic GH, GHBP and IGF-I; and serum corticosterone and insulin levels were determined.

RESULTS

Ibuprofen suppressed somatic growth in the sucking rats, but the effect was transient, resolving by P14. Indomethacin had an opposite, latent effect on body weight and liver to body weight ratios in weanling rats. Both indomethacin and ibuprofen had profound hormonal effects that differed in magnitude and timing. Indomethacin resulted in a sustained elevation in corticosterone levels at P21, while ibuprofen increased serum and hepatic GH levels. Both drugs suppressed GHBP in serum at P7 and P14; and liver at P4 and P7, but a rebound increase in serum GHBP was noted at P21 with Ibuprofen only. Both drugs increased serum IGF-I at P7. The effect remained sustained with indomethacin.

CONCLUSIONS

These results provide evidence for an involvement of PGs in the regulation of growth as well as the GH-IGF and HPA axes. Therefore, early postnatal exposure to PG inhibitors may further exacerbate postnatal growth restriction and ability to cope with stress.

Mechanisms of obesity and related pathologies: role of apolipoprotein E in the development of obesity

Apolipoprotein E is a polymorphic glycoprotein in humans with a molecular mass of 34.5 kDa. It is a component of chylomicron remnants, very low density lipoprotein, low density lipoprotein and high density lipoprotein, and is primarily responsible for maintaining plasma lipid homeostasis. In addition to these well-documented functions, recent studies in experimental mouse models, as well as population studies, show that apolipoprotein E also plays an important role in the development of obesity and insulin resistance. It is widely accepted that disruption in homeostasis between food intake and energy expenditure, and the subsequent deposition of excess fatty acids into fat cells in the form of triglycerides, leads to the development of obesity. Despite the pivotal role of obesity and dyslipidemia in the development of the metabolic syndrome and heart disease, the functional interactions between adipose tissue and components of the lipoprotein transport system have not yet been investigated thoroughly. In this minireview, we focus on the current literature pertinent to the involvement of apolipoprotein E in the development of pathologies associated with the metabolic syndrome.

Guardian of corpulence: a hypothesis on p53 signaling in the fat cell

Adipocytes provide an organism with fuel in times of caloric deficit, and are an important type of endocrine cell in the maintenance of metabolic homeostasis. In addition, as a lipid-sink, adipocytes serve an equally important role in the protection of organs from the damaging effects of ectopic lipid deposition. For the organism, it is of vital importance to maintain adipocyte viability, yet the fat depot is a demanding extracellular environment with high levels of interstitial free fatty acids and associated lipotoxic effects. These surroundings are less than beneficial for the overall health of any resident cell, adipocyte and preadipocyte alike. In this review, we discuss the process of adipogenesis and the potential involvement of the p53 tumor-suppressor protein in alleviating some of the cellular stress experienced by these cells. In particular, we discuss p53-mediated mechanisms that prevent damage caused by reactive oxygen species and the effects of lipotoxicity. We also suggest the potential for two p53 target genes, START domain-containing protein 4 (StARD4) and oxysterol-binding protein (OSBP), with the concomitant synthesis of the signaling molecule oxysterol, to participate in adipogenesis.

Apolipoprotein E predisposes to obesity and related metabolic dysfunctions in mice

Obesity is a central feature of the metabolic syndrome and is associated with increased risk for insulin resistance and typeII diabetes. Here, we investigated the contribution of human apoliproteinE3 and mouse apoliproteinE to the development of diet-induced obesity in response to western-type diet. Our data show that apolipoproteinE contributes to the development of obesity and other related metabolic disorders, and that human apolipoproteinE3 is more potent than mouse apolipoproteinE in promoting obesity in response to western-type diet. Specifically, we found that apolipoproteinE3 knock-in mice fed western-type diet for 24 weeks became obese and developed hyperglycemia, hyperinsulinemia, hyperleptinemia, glucose intolerance and insulin resistance that were more severe than in C57BL/6 mice. In contrast, apolipoproteinE-deficient mice fed western-type diet for the same period were resistant to diet-induced obesity, had normal plasma glucose, leptin and insulin levels, and exhibited normal responses to glucose tolerance and insulin resistance tests. Furthermore, low-density lipoprotein receptor-deficient mice were more sensitive to the development of diet-induced obesity and insulin resistance than apolipoprotein E-deficient mice, but were still more resistant than C57BL/6 mice, raising the possibility that low-density lipoprotein receptor mediates, at least in part, the effects of apolipoproteinE on obesity. Taken together, our findings suggest that, in addition to other previously identified mechanisms of obesity, apolipoproteinE and possibly the chylomicron pathway are also important contributors to the development of obesity and related metabolic dysfunctions in mice.

An emerging risk factor for obesity: does disequilibrium of polyunsaturated fatty acid metabolism contribute to excessive adipose tissue development?

A positive energy balance (energy intake>energy expenditure), in which total fat intake plays an important role, is commonly regarded as a major factor contributing to obesity. Adipose tissue development, i.e. both size (hypertrophy) and number (hyperplasia), is stimulated by high dietary fat intake during early postnatal development, a susceptibility that now appears to continue well into adulthood. Recent human and animal studies suggest that by altering rates of adipocyte differentiation and proliferation, differences in the composition of dietary fat may also contribute to adipose tissue development. At least in rodent models, the relative intake of n-6 to n-3 PUFA is clearly emerging as a new factor in this development. In these models, higher linoleate intake raises tissue arachidonic acid, which increases prostacyclin production and, in turn, stimulates signalling pathways implicated in adipogenesis. Signalling pathways stimulated by arachidonic acid probably include phospholipase and/or cyclo-oxygenase activation and may be linked as much to relatively low intake of n-3 PUFA as to excessive dietary linoleate. One factor potentially contributing to oversight about the apparent role of dietary n-6 PUFA (especially excess dietary linoleate) in adipose tissue development is the historical overestimation of linoleate requirements and the enthusiasm for higher intake of 'essential fatty acids'. More research is needed to address whether disequilibration of dietary PUFA intake contributes to the risk of obesity in humans.

Prostaglandins differentially affect osteogenic differentiation of human adipose tissue-derived mesenchymal stem cells

Adipose tissue-derived mesenchymal stem cells (AT-MSCs) are currently used for bone tissue engineering. AT-MSCs undergoing osteogenic differentiation respond to mechanical loading with increased cyclooxygenase-2 gene expression, a key enzyme in prostaglandin (PG) synthesis. PGs are potent multifunctional regulators in bone, exhibiting stimulatory and inhibitory effects on bone formation and resorption. PGE(2), but not PGI(2) or PGF(2), recruits osteoprogenitors from the bone marrow space and influences their differentiation. We hypothesize that PGE(2), PGI(2), and PGF(2) may differentially regulate osteogenic differentiation of human AT-MSCs. PGE(2), PGI(2), and PGF(2) (0.01-10 microM) affected osteogenic differentiation, but not proliferation of AT-MSCs after 4-14 days. Only PGF(2) (0.01-10 microM) increased alkaline phosphatase (ALP) activity at day 4. PGE(2) (10 microM), PGI(2) (0.01-10 microM), and PGF(2) (10 microM) decreased ALP activity, whereas PGF(2) (0.1 microM) increased ALP activity at day 14. PGF(2) (0.01-0.1 microM) and PGI(2) (0.01 microM) upregulated osteopontin gene expression, and PGF(2) (0.01 microM) upregulated alpha1(I)procollagen gene expression at day 4. PGE(2) and PGF(2) (10 microM) at day 4 and PGF(2) (1 microM) at day 14 downregulated runt-related transcription factor-2 gene expression. We conclude that PGE(2), PGI(2), and PGF(2) differentially affect osteogenic differentiation of AT-MSCs, with PGF(2) being the most potent. Thus, locally produced PGF(2) might be most beneficial in promoting osteogenic differentiation of AT-MSCs, resulting in enhanced bone formation for bone tissue engineering.

Neonatal dietary supplementation of arachidonic acid increases prostaglandin levels in adipose tissue but does not promote fat mass development in guinea pigs

The role of arachidonic acid (AA) on the development of adipose tissue is still controversial since its metabolites, i.e., prostaglandins, can either stimulate or inhibit preadipocyte differentiation in vitro. In the present study, we evaluated the effects of early postnatal supplementation of AA on body weight and adipose tissue development in guinea pigs. Male newborn guinea pigs were fed for 21 days ( day 21) with diets (milk and pellet) supplemented (+AA) or not (−AA) with 1.2% (total fatty acids) AA. From day 21 to day 105 both groups were fed a chow diet. The 21-days-old +AA pups showed a twofold higher AA accretion in phospholipids associated with a two- to sixfold increase in several prostaglandins, such as 6-keto PGF1α(the stable hydrolysis product of PGI2), PGF2α, PGE2, and PGD2in adipose tissue, compared with the −AA group. No difference in fat pad and body weight, aP2, and leptin gene expression in adipose tissue, fasting plasma glucose, free-fatty acids, and triglyceride concentration was observed between groups at day 21 or day 105. These results show that dietary supplementation of AA during the suckling/weaning period increases prostaglandin levels in adipose tissue but does not influence early fat mass development in the guinea pig.

Down-regulation of microsomal prostaglandin E2 synthase-1 in adipose tissue by high-fat feeding

OBJECTIVE

Prostaglandin (PG)E2 is a lipid mediator implicated in inflammatory diseases and in the regulation of lipolysis and adipocyte differentiation. This work was, thus, undertaken to study the regulation of the various PGE2 synthases (PGESs) in obesity.

RESEARCH METHODS AND PROCEDURES

C57Bl/6 mice were subjected to a high-fat or regular diet for 12 weeks. The levels of PGE2 in white adipose tissue (WAT) of lean and obese mice were quantified by liquid chromatography-mass spectrometry, and the change in expression of the three major PGES caused by diet-induced obesity was characterized by Western blotting. Human preadipocytes and 3T3-L1 cells were used to assess the expression of microsomal prostaglandin E2 synthase-1 (mPGES-1) during adipogenesis.

RESULTS

mPGES-1, mPGES-2, and cytosolic PGES proteins were all detected in WAT of lean animals. mPGES-1 was expressed at higher levels in WAT than in any other tissues examined and was more abundant (3- to 4-fold) in epididymal (visceral) compared with inguinal (subcutaneous) WAT. Expression of mPGES-1 was also detected in undifferentiated and differentiated 3T3-L1 cells and in human primary subcutaneous preadipocytes at all stages of adipogenesis. The mPGES-1 protein was substantially down-regulated in epididymal and inguinal WAT of obese mice, whereas mPGES-2 and cytosolic PGES remained relatively stable. Concordantly, the PGE2 levels in obese inguinal WAT were significantly lower than those of lean animals.

DISCUSSION

These data suggest that mPGES-1 is the major form of PGESs contributing to the synthesis of PGE2 in WAT and that its down-regulation might be involved in the alterations of lipolysis and adipogenesis associated with obesity.

Adipose tissue as a secretory organ: from adipogenesis to the metabolic syndrome

Adipose tissue contains various types of cells that include preadipocytes and adipocytes. Studies have emphasized that (i) preadipocytes secrete factors involved in their own differentiation and (ii) adipocytes acquire the ability to communicate systemically with other organs (brain, liver, skeletal muscle) and locally with other cells (preadipocytes, endothelial cells and monocytes/macrophages). Adipocytes secrete proteins exhibiting either beneficial (leptin, adiponectin) or deleterious effects (angiotensinogen). Associated to the effect of secretory products from macrophages (cytokines), a disturbance in the balance between these various secreted factors leads to the development of a metabolic syndrome.

Regulation of adipocyte differentiation and function by polyunsaturated fatty acids

A diet enriched in PUFAs, in particular of the n-3 family, decreases adipose tissue mass and suppresses development of obesity in rodents. Although several nuclear hormone receptors are identified as PUFA targets, the precise molecular mechanisms underlying the effects of PUFAs still remain to be elucidated. Here we review research aimed at elucidating molecular mechanisms governing the effects of PUFAs on the differentiation and function of white fat cells. This review focuses on dietary PUFAs as signaling molecules, with special emphasis on agonistic and antagonistic effects on transcription factors currently implicated as key players in adipocyte differentiation and function, including peroxisome proliferator activated receptors (PPARs) (alpha, beta and gamma), sterol regulatory element binding proteins (SREBPs) and liver X receptors (LXRs). We review evidence that dietary n-3 PUFAs decrease adipose tissue mass and suppress the development of obesity in rodents by targeting a set of key regulatory transcription factors involved in both adipogensis and lipid homeostasis in mature adipocytes. The same set of factors are targeted by PUFAs of the n-6 family, but the cellular/physiological responses are dependent on the experimental setting as n-6 PUFAs may exert either an anti- or a proadipogenic effect. Feeding status and hormonal background may therefore be of particular importance in determining the physiological effects of PUFAs of the n-6 family.

Fatty acid composition of fats is an early determinant of childhood obesity: a short review and an opinion

The importance of dietary fat in human obesity remains a controversial issue as the prevalence of overweight and obesity has increased despite no dramatic change in the amount of ingested fats over the past few decades. However, qualitative changes (i.e. the fatty acid composition of fats) have been largely disregarded. In this review, we summarize experimental evidence which supports polyunsaturated fatty acids of the omega6 series as being potent promoters of both adipogenesis in vitro and adipose tissue development in vivo during the gestation/lactation period. This conclusion is also supported by epidemiological data from infant studies as well as by the assessment of the fatty acid composition of mature breast milk and formula milk. It is proposed that unnoticed changes in fatty acid composition of ingested fats over the last decades have been important determinants in the increasing prevalence of childhood overweight and obesity.

Arachidonic acid-dependent inhibition of adipocyte differentiation requires PKA activity and is associated with sustained expression of cyclooxygenases

Arachidonic acid inhibits adipocyte differentiation of 3T3-L1 cells via a prostaglandin synthesis-dependent pathway. Here we show that this inhibition requires the presence of a cAMP-elevating agent during the first two days of treatment. Suppression of protein kinase A activity by H-89 restored differentiation in the presence of arachidonic acid. Arachidonic acid treatment led to a prolonged activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), and suppression of ERK1/2 activity by the addition of U0126 rescued differentiation. Upon induction of differentiation, expression of cyclooxygenase-2 (COX-2) was transiently induced and then declined, whereas COX-1 expression declined gradually as differentiation progressed. Treatment with arachidonic acid led to sustained expression of COX-1 and COX-2. Omission of a cAMP-elevating agent or addition of H-89 or U0126 prevented sustained expression of COX-2. Unexpectedly, we observed that selective COX-1 or COX-2 inhibitors rescued adipocyte differentiation in the presence of arachidonic acid as effectively as did the nonselective COX-inhibitor indomethacin. De novo fatty acid synthesis, diacylglycerol acyltransferase (DGAT) activity, and triacylglycerol accumulation were repressed in cells treated with arachidonic acid. Indomethacin restored DGAT activity and triacylglycerol accumulation without restoring de novo fatty acid synthesis, resulting in an enhanced incorporation of arachidonic acid into cellular triacylglycerols.

Selective cyclo-oxygenase-2 inhibitors impair adipocyte differentiation through inhibition of the clonal expansion phase

Selective cyclo-oxygenase-2 (COX-2) inhibitors are nonsteroidal antiinflammatory drugs used in the management of inflammatory diseases. We demonstrate here that inhibition of the COX-2 enzyme impairs adipocyte differentiation. The inhibition of adipogenesis occurs in the early clonal expansion phase. In particular, COX-2 inhibition limits cell cycle reentry required before terminal adipocyte differentiation. This inhibition of adipogenesis is independent of the production of the peroxisome proliferator activated receptor gamma ligand prostaglandin J2, but dependent on the production of proliferative prostaglandins, such as prostaglandin E2. Modulation of the activity of the COX-2 enzyme via COX-2 selective inhibitors might open up new perspectives in the control of obesity and related metabolic diseases.

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

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

Possible involvement of the adipose tissue renin-angiotensin system in the pathophysiology of obesity and obesity-related disorders

Angiotensin II (Ang II), acting on the AT1 and AT2 receptors in mammalian cells, is the vasoactive component of the renin-angiotensin system (RAS). Several components of the RAS have been demonstrated in different tissues, including adipose tissue. Although the effects of Ang II on metabolism have not been studied widely, it is intriguing to assume that components of the RAS produced by adipocytes may play an autocrine, a paracrine and/or an endocrine role in the pathophysiology of obesity and provide a potential pathway through which obesity leads to hypertension and type 2 diabetes mellitus. In the first part of this review, we will describe the production of Ang II, the different receptors through which Ang II exerts its effects and summarize the concomitant intracellular signalling cascades. Thereafter, potential Ang II-induced mechanisms, which may be associated with obesity and obesity-related disorders, will be considered. Finally, we will focus on the different pharmaceutical agents that interfere with the RAS and highlight the possible implications of these drugs in the treatment of obesity-related disorders.

Arachidonic acid and prostacyclin signaling promote adipose tissue development: a human health concern?

High fat intake is associated with fat mass gain through fatty acid activation of peroxisome proliferator-activated receptors delta and gamma, which promote adipogenesis. We show herein that, compared to a combination of specific agonists to both receptors or to saturated, monounsaturated, and omega-3 polyunsaturated fatty acids, arachidonic acid (C20:4, omega-6) promoted substantially the differentiation of clonal preadipocytes. This effect was blocked by cyclooxygenase inhibitors and mimicked by carbacyclin, suggesting a role for the prostacyclin receptor and activation of the cyclic AMP-dependent pathways that regulate the expression of the CCAAT enhancer binding proteins beta and delta implicated in adipogenesis. During the pregnancy-lactation period, mother mice were fed either a high-fat diet rich in linoleic acid, a precursor of arachidonic acid (LO diet), or the same isocaloric diet enriched in linoleic acid and alpha-linolenic acid (LO/LL diet). Body weight from weaning onwards, fat mass, epididymal fat pad weight, and adipocyte size at 8 weeks of age were higher with LO diet than with LO/LL diet. In contrast, prostacyclin receptor-deficient mice fed either diet were similar in this respect, indicating that the prostacyclin signaling contributes to adipose tissue development. These results raise the issue of the high content of linoleic acid of i) ingested lipids during pregnancy and lactation, and ii) formula milk and infant foods in relation to the epidemic of childhood obesity.

Polymorphism of angiotensin-converting enzyme gene and BMI in obese Korean women

BACKGROUND

The renin-angiotensin system (RAS) plays a role in the pathogenesis of metabolic diseases. This system was recently found to be completely expressed in human adipose tissue. Especially angiotensin II, the active component of RAS, may affect adipogenesis and adipocyte metabolism. We examined whether obese and non-obese subjects differ from angiotensin-converting enzyme (ACE) genotype distribution, and whether the ACE genotypes affect the anthropometric parameters or the degrees of body mass index (BMI).

METHODS

The study included 155 obese healthy women (BMI > or = 25 kg/m(2), range 25-54.7, age range 15-40 years), 82 non-obese women (BMI < 25 kg/m(2), range 15-40 years), and 613 random controls. Total fat mass and percent body fat (PBF) were determined by dual-energy X-ray absorptiometry (DEXA). Genomic DNA was extracted and used for polymerase chain reaction (PCR)-based genotyping of ACE.

RESULTS

Age, percent body fat, waist-to-hip ratio (WHR), body mass index, and cholesterol concentrations did not differ from ACE genotype. No differences were observed for allelic and genotype frequencies between obese women (BMI > or = 25) and 82 non-obese women or 613 random controls. In addition, no association of ACE polymorphism was observed with BMI for genotype in obese women.

CONCLUSIONS

ACE polymorphism is not a significant factor for BMI and does not contribute to the odds of obesity in obese healthy women from Korea.

Thiazolidinediones as a novel class of NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase inhibitors

NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH) catalyzes NAD(+)-dependent oxidation of prostaglandins and other nonprostanoid compounds. This enzyme was found to be dramatically induced in hormone-responsive human prostate cancer cells by androgens [M. Tong, and H. H. Tai, 2000, Biochem. Biophys. Res. Commun. 276, 77-81] and could be involved in prostate tumorigenesis. Inhibitors of this enzyme may be of value in determining the utility of these compounds in cancer chemoprevention. Previously, ciglitazone, an antidiabetic thiazolidinedione, was found to be a potent inhibitor of 15-PGDH. Structure-activity analysis of available thiazolidinediones indicated that the nature of the moiety linking to phenyl ring through ether linkage and benzylidene configuration play important roles in inhibitory potency. Furthermore, N-methylation of 2,4-thiazolidinedione abolished the inhibitory activity. A series of benzylidene thiazolidinediones with varied ring structure and methylene bridge to phenyl ring through ether linkage were synthesized and assayed for inhibitory activity. It was found that compound CT-8 (5-[4-(cyclohexylethoxy)benzylidene]-2,4-thiazolidinedione) was the most potent inhibitor effective at nanomolar range. Kinetic studies revealed that inhibition by this compound was noncompetitive with respect to NAD(+) and uncompetitive with respect to prostaglandin E(2), indicating that the inhibitor interacts with the enzyme at a site distinct from the substrate binding site. This regulatory site appears to overlap with the activator site occupied by imipramine since activation of the enzyme by this activator is competitively inhibited by compound CT-8.

The roles of PPARs in adipocyte differentiation

Adipose tissue development takes place primarily around birth but adipose cell number can increase throughout life in response to nutritional changes. At the molecular level, adipogenesis is the result of transcriptional remodeling that leads to activation of a considerable number of genes. Several transcription factors act cooperatively and sequentially in this process. This article attempts to review the roles of peroxisome proliferator-activated receptors gamma and delta in the control of preadipocyte proliferation and differentiation during adipose tissue development or during the adaptive response of adipose tissue mass to high-fat feeding.

Molecular regulation of adipogenesis

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.

Prostaglandin F(2)alpha enhances glucose consumption through neither adipocyte differentiation nor GLUT1 expression in 3T3-L1 cells

Arachidonic acid (AA) at 0.2 mM enhances glucose uptake through increased levels of glucose transporter (GLUT) 1 protein in 3T3-L1 adipocytes. Since AA is a precursor of prostaglandins (PGs), we investigated the effect of PGs on glucose consumption in 3T3-L1 cells. Among several PGs, only prostaglandin F(2)alpha (PGF(2)alpha) enhanced glucose consumption in 3T3-L1 cells treated with dexamethasone (DEX), 3-isobutyl-1-methyl-xanthine (IBMX), and insulin. To study the mechanism of PGF(2)alpha-enhanced glucose consumption, we investigated the effect of PGF(2)alpha on glycerol-3-phosphate dehydrogenase (GPDH) activity, triglycerides (TGs) content, and the expression of GLUT1 protein. PGF(2)alpha suppressed GPDH activity and did not increase the expression of GLUT1 protein in 3T3-L1 cells treated with DEX, IBMX, and insulin. These results suggest that AA-stimulated glucose uptake is not through the effect of PGF(2)alpha. Our results indicate that PGF(2)alpha is a unique regulator of adipocyte differentiation (suppression) and glucose consumption (enhancement) in 3T3-L1 cells.

Angiotensin II as a trophic factor of white adipose tissue: stimulation of adipose cell formation

White adipose tissue is known to contain the components of the renin-angiotensin system giving rise to angiotensin II (AngII). In vitro, prostacyclin is synthesized from arachidonic acid through the activity of cyclooxygenases 1 and 2 and is released from AngII-stimulated adipocytes. Prostacyclin, in turn, is able to favor adipocyte formation. Based upon in vivo and ex vivo experiments combined to immunocytochemical staining of glycerol-3-phosphate dehydrogenase (GPDH), an indicator of adipocyte formation, it is reported herein that AngII favors the appearance of GPDH-positive cells. In the presence of a cyclooxygenase inhibitor, this adipogenic effect is abolished, whereas that of (carba)prostacyclin, a stable analog of prostacyclin that bypasses this inhibition, appears unaltered. Taken together, these results are in favor of AngII acting as a trophic factor implicated locally in adipose tissue development. It is proposed that AngII enhances the formation of GPDH-expressing cells from preadipocytes in response to prostacyclin released from adipocytes.

Physiology and pathophysiology of the adipose tissue renin-angiotensin system

The renin-angiotensin system has long been recognized as an important regulator of systemic blood pressure and renal electrolyte homeostasis, and local renin-angiotensin systems have also been implicated in pathological changes of organ structure and function by modulation of gene expression, growth, fibrosis, and inflammatory response. Recently, substantial data have been accumulated in support of the notion that adipose tissue, besides other endocrine functions, also hosts a local renin-angiotensin system. In the first part of this review, we describe the components of the adipose tissue renin-angiotensin system in human and rodent animal models with respect to regulation of angiotensinogen expression and secretion, formation of angiotensin peptides, and the existence of angiotensin II receptors. In the second part, we describe the role of the adipose tissue renin-angiotensin system in the process of adipogenic differentiation and in the regulation of body weight. We also detail the differential regulation of the adipose tissue renin-angiotensin system in obesity and hypertension and thereby also speculate on its possible role in the development of obesity-associated hypertension. Although some findings on the adipose tissue renin-angiotensin system appear to be confusing, its involvement in the physiology and pathophysiology of adipose tissue has been confirmed by several functional studies. Nevertheless, future studies with more carefully described phenotypes are necessary to conclude whether obesity (by stimulation of adipogenic differentiation) and hypertension are associated with changes of renin-angiotensin system activity in adipose tissue. If so, the physiological relevance of this system in animal models and humans may warrant further interest.

Rapamycin inhibits human adipocyte differentiation in primary culture

OBJECTIVE

The immunosuppressant drug rapamycin, has been reported to inhibit 3T3-L1 adipocyte differentiation by interfering with critical postconfluent mitoses that are required early on for successful differentiation of this cell line (clonal expansion phase). In contrast to the murine 3T3-L1 preadipocyte cell line, human preadipocytes in primary culture do not undergo clonal expansion during differentiation. We investigated whether rapamycin could inhibit human adipocyte differentiation.

RESEARCH METHODS AND PROCEDURES

The effect of rapamycin on the induction of differentiation of human preadipocytes in primary culture into adipocytes was measured using Oil Red O staining and glycerol phosphate dehydrogenase activity.

RESULTS

We have observed that rapamycin severely curtails human adipocyte differentiation of both omental and abdominal subcutaneous preadipocytes (to 14% and 19% of standard differentiation, respectively). The rapamycin-mediated inhibition of human adipocyte differentiation could be reversed in the presence of excess amounts of FK-506, which displaces rapamycin from its intracellular receptor, FKPB12. Measurement of cytosolic protein and [3H]thymidine incorporation into DNA confirmed the absence of proliferation during differentiation of human preadipocytes in primary culture.

DISCUSSION

Our data indicate that rapamycin exerts important negative regulatory effects on adipogenesis in human preadipocytes, through a mechanism that does not depend on interruption of clonal expansion.

Prostacyclin IP receptor up-regulates the early expression of C/EBPbeta and C/EBPdelta in preadipose cells

Prostacyclin (PGI(2)) and its stable analogue carbacyclin (cPGI(2)) are known to trigger the protein kinase A pathway after binding to the cell surface IP receptor and to promote or enhance terminal differentiation of adipose precursor cells to adipose cells. The early expression of C/EBPbeta and C/EBPdelta is known to be critical for adipocyte differentiation in vitro as well as in vivo. We report herein that in Ob1771 and 3T3-F442A preadipose cells, activation of the IP receptor by specific agonists (PGI(2), cPGI(2) and BMY 45778) is sufficient to up-regulate rapidly the expression of C/EBPbeta and C/EBPdelta. Cyclic AMP-elevating agents are able to substitute for IP receptor agonists, in agreement with the coupling of IP receptor to adenylate cyclase. Consistent with the fact that PGI(2) is released from preadipose cells and behaves as a paracrine/autocrine effector of adipose cell differentiation, the present results favor a key role of prostacyclin by means of the IP receptor and its intracellular signaling pathway in eliciting the critical early expression of both transcription factors.