Potentiation of glucose uptake in 3T3-L1 adipocytes by PPAR gamma agonists is maintained in cells expressing a PPAR gamma dominant-negative mutant: evidence for selectivity in the downstream responses to PPAR gamma activation

Liquid Crystal Monomer: A Potential PPARγ Antagonist

Liquid crystal monomers (LCMs) are a large family of artificial ingredients that have been widely used in global liquid crystal display (LCD) industries. As a major constituent in LCDs as well as the end products of e-waste dismantling, LCMs are of growing research interest with regard to their environmental occurrences and biochemical consequences. Many studies have analyzed LCMs in multiple environmental matrices, yet limited research has investigated the toxic effects upon exposure to them. In this study, we combined in silico simulation and in vitro assay validation along with omics integration analysis to achieve a comprehensive toxicity elucidation as well as a systematic mechanism interpretation of LCMs for the first time. Briefly, the high-throughput virtual screen and reporter gene assay revealed that peroxisome proliferator-activated receptor gamma (PPARγ) was significantly antagonized by certain LCMs. Besides, LCMs induced global metabolome and transcriptome dysregulation in HK2 cells. Notably, fatty acid β-oxidation was conspicuously dysregulated, which might be mediated through multiple pathways (IL-17, TNF, and NF-kB), whereas the activation of AMPK and ligand-dependent PPARγ antagonism may play particularly important parts. This study illustrated LCMs as a potential PPARγ antagonist and explored their toxicological mode of action on the trans-omics level, which provided an insightful overview in future chemical risk assessment.

Macroalgal protein hydrolysates from Palmaria palmata influence the 'incretin effect' in vitro via DPP-4 inhibition and upregulation of insulin, GLP-1 and GIP secretion

PURPOSE

This study investigated metabolic benefits of protein hydrolysates from the macroalgae Palmaria palmata, previously shown to inhibit dipeptidylpeptidase-4 (DPP-4) activity in vitro.

METHODS

Previously, Alcalase/Flavourzyme-produced P. palmata protein hydrolysate (PPPH) improved glycaemia and insulin production in streptozotocin-induced diabetic mice. Here the PPPH, was compared to alternative Alcalase, bromelain and Promod-derived hydrolysates and an unhydrolysed control. All PPPH's underwent simulated gastrointestinal digestion (SGID) to establish oral bioavailability. PPPH's and their SGID counterparts were tested in pancreatic, clonal BRIN-BD11 cells to assess their insulinotropic effect and associated intracellular mechanisms. PPPH actions on the incretin effect were assessed via measurement of DPP-4 activity, coupled with GLP-1 and GIP release from GLUTag and STC-1 cells, respectively. Acute in vivo effects of Alcalase/Flavourzyme PPPH administration on glucose tolerance and satiety were assessed in overnight-fasted mice.

RESULTS

PPPH's (0.02-2.5 mg/ml) elicited varying insulinotropic effects (p < 0.05-0.001). SGID of the unhydrolysed protein control, bromelain and Promod PPPH's retained, or improved, bioactivity regarding insulin secretion, DPP-4 inhibition and GIP release. Insulinotropic effects were retained for all SGID-hydrolysates at higher PPPH concentrations. DPP-4 inhibitory effects were confirmed for all PPPH's and SGID counterparts (p < 0.05-0.001). PPPH's were shown to directly influence the incretin effect via upregulated GLP-1 and GIP (p < 0.01-0.001) secretion in vitro, largely retained after SGID. Alcalase/Flavourzyme PPPH produced the greatest elevation in cAMP (p < 0.001, 1.7-fold), which was fully retained post-SGID. This hydrolysate elicited elevations in intracellular calcium (p < 0.01) and membrane potential (p < 0.001). In acute in vivo settings, Alcalase/Flavourzyme PPPH improved glucose tolerance (p < 0.01-0.001) and satiety (p < 0.05-0.001).

CONCLUSION

Bioavailable PPPH peptides may be useful for the management of T2DM and obesity.

An insight into anti-adipogenic properties of an Oroxylum indicum (L.) Kurz extract

Background

Oroxylum indicum fruit extract (OIE) has been reported to inhibit the development of adipocytes. However, the exact mechanism of its metabolic activity is not clearly defined. This study attempted to investigate whether OIE was involved in disrupting the cell cycle, glucose metabolism, and mitochondrial function in 3 T3-L1 cells.

Methods

The effect of the OIE on cell cycle progression was measured by flow cytometry along with observing the expression of the cycle regulator by immunoblotting. The effect of the OIE on glucose metabolism was investigated. The amount of glucose uptake (2-NBDG) influenced by insulin was determined as well as the protein tyrosine phosphorylation (PY20), and glucose transporter4 (GLUT4) expression was determined by immunoblotting assay. Mitochondria are also essential to metabolic processes. This study investigated mitochondrial activity using fluorescent lipophilic carbocyanine dye (JC-1) and mitochondria mass by MitoTracker Green (MTG) staining fluorescence dyes. Finally, cellular ATP concentration was measured using an ATP chemiluminescence assay.

Results

Treatment with OIE plus adipogenic stimulators for 24 h arrested cell cycle progression in the G2/M phase. Moreover, 200 μg/mL of OIE significantly diminished the expression of the insulin receptor (IR) and GLUT4 protein compared to the untreated-adipocytes (P < 0.05). The mitochondrial membrane potential (MMP) was significantly reduced (24 h) and increased (day 12) by OIE compared to untreated-adipocytes (P < 0.05). However, OIE maintained MMP and ATP at a similar level compared to the pre-adipocytes (day 12). Transmission electron microscope (TEM) results demonstrated that OIE could protect mitochondria deformation compared to the untreated-adipocytes.

Conclusion

These results suggest that the inhibitory effect of the OIE on adipogenesis may potentially inhibit the cell cycle and phosphorylation of IR, leading to a decrease in glucose uptake to the cells. The OIE also slows down the mitochondrial activity of the early phase of cell differentiation, which can also inhibit the development of fat cells.

Supplementary information

The online version contains supplementary material available at 10.1186/s12906-020-03111-2.

PPAR-γ and Akt regulate GLUT1 and GLUT3 surface localization during Mycobacterium tuberculosis infection

The success of Mycobacterium tuberculosis (Mtb) as a pathogen stems from its ability to manipulate the host macrophage towards increased lipid biogenesis and lipolysis inhibition. Inhibition of lipolysis requires augmented uptake of glucose into the host cell causing an upregulation of the glucose transporters GLUT1 and GLUT3 on the cell surface. Mechanism behind this upregulation of the GLUT proteins during Mtb infection is hitherto unknown and demands intensive investigation in order to understand the pathways linked with governing them. Our endeavor to investigate some of the key proteins that have been found to be affected during Mtb infection led us to investigate host molecular pathways such as Akt and PPAR-γ that remain closely associated with the survival of the bacilli by modulating the localization of glucose transporters GLUT1 and GLUT3.

Betulinic acid is a PPARγ antagonist that improves glucose uptake, promotes osteogenesis and inhibits adipogenesis

PPAR antagonists are ligands that bind their receptor with high affinity without transactivation activity. Recently, they have been demonstrated to maintain insulin-sensitizing and antidiabetic properties, and they serve as an alternative treatment for metabolic diseases. In this work, an affinity-based bioassay was found to be effective for selecting PPAR ligands from the dried extract of an African plant (Diospyros bipindensis). Among the ligands, we identified betulinic acid (BA), a compound already known for its anti-inflammatory, anti-tumour and antidiabetic properties, as a PPARγ and PPARα antagonist. Cell differentiation assays showed that BA inhibits adipogenesis and promotes osteogenesis; either down-regulates or does not affect the expression of a series of adipogenic markers; and up-regulates the expression of osteogenic markers. Moreover, BA increases basal glucose uptake in 3T3-L1 adipocytes. The crystal structure of the complex of BA with PPARγ sheds light, at the molecular level, on the mechanism by which BA antagonizes PPARγ, and indicates a unique binding mode of this antagonist type. The results of this study show that the natural compound BA could be an interesting and safe candidate for the treatment of type 2 diabetes and bone diseases.

Antidiabetic Potential of Purple and Red Rice (Oryza sativa L.) Bran Extracts

Pigmented rice contains anthocyanins and proanthocyanidins that are concentrated in the bran layer. In this study, we determined the phenolic, flavonoid, anthocyanin, and proanthocyanidin content of five rice bran (1 brown, 2 red, and 2 purple) extracts. Each bran extract was evaluated for inhibitory effects on α-amylase and α-glucosidase activity, two key glucosidases required for starch digestion in humans. All purple and red bran extracts inhibited α-glucosidase activity, however only the red rice bran extracts inhibited α-amylase activity. Additionally, each bran extract was examined for their ability to stimulate glucose uptake in 3T3-L1 adipocytes, a key function in glucose homeostasis. Basal glucose uptake was increased between 2.3- and 2.7-fold by exposure to the red bran extracts, and between 1.9- and 3.1-fold by exposure to the purple bran extracts. In red rice bran, the highest enzyme inhibition and glucose uptake was observed with a proanthocyanidin-enriched fraction. Both IITA red bran and IAC purple bran increased expression of GLUT1 and GLUT4 mRNA, and genes encoding insulin-signaling pathway proteins.

Limonin, a Component of Dictamni Radicis Cortex, Inhibits Eugenol-Induced Calcium and cAMP Levels and PKA/CREB Signaling Pathway in Non-Neuronal 3T3-L1 Cells

Limonin, one of the major components in dictamni radicis cortex (DRC), has been shown to play various biological roles in cancer, inflammation, and obesity in many different cell types and tissues. Recently, the odorant-induced signal transduction pathway (OST) has gained attention not only because of its function in the perception of smell but also because of its numerous physiological functions in non-neuronal cells. However, little is known about the effects of limonin and DRC on the OST pathway in non-neuronal cells. We investigated odorant-stimulated increases in Ca²⁺ and cAMP, major second messengers in the OST pathway, in non-neuronal 3T3-L1 cells pretreated with limonin and ethanol extracts of DRC. Limonin and the extracts significantly decreased eugenol-induced Ca²⁺ and cAMP levels and upregulated phosphorylation of CREB and PKA. Our results demonstrated that limonin and DRC extract inhibit the OST pathway in non-neuronal cells by modulating Ca²⁺ and cAMP levels and phosphorylation of CREB.

PFOS induces adipogenesis and glucose uptake in association with activation of Nrf2 signaling pathway

PFOS is a chemical of nearly ubiquitous exposure in humans. Recent studies have associated PFOS exposure to adipose tissue-related effects. The present study was to determine whether PFOS alters the process of adipogenesis and regulates insulin-stimulated glucose uptake in mouse and human preadipocytes. In murine-derived 3T3-L1 preadipocytes, PFOS enhanced hormone-induced differentiation to adipocytes and adipogenic gene expression, increased insulin-stimulated glucose uptake at concentrations ranging from 10 to 100μM, and enhanced Glucose transporter type 4 and Insulin receptor substrate-1 expression. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2), NAD(P)H dehydrogenase, quinone 1 and Glutamate-cysteine ligase, catalytic subunit were significantly induced in 3T3-L1 cells treated with PFOS, along with a robust induction of Antioxidant Response Element (ARE) reporter in mouse embryonic fibroblasts isolated from ARE-hPAP transgenic mice by PFOS treatment. Chromatin immunoprecipitation assays further illustrated that PFOS increased Nrf2 binding to ARE sites in mouse Nqo1 promoter, suggesting that PFOS activated Nrf2 signaling in murine-derived preadipocytes. Additionally, PFOS administration in mice (100μg/kg/day) induced adipogenic gene expression and activated Nrf2 signaling in epididymal white adipose tissue. Moreover, the treatment on human visceral preadipocytes illustrated that PFOS (5 and 50μM) promoted adipogenesis and increased cellular lipid accumulation. It was observed that PFOS increased Nrf2 binding to ARE sites in association with Nrf2 signaling activation, induction of Peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding protein α expression, and increased adipogenesis. This study points to a potential role of PFOS in dysregulation of adipose tissue expandability, and warrants further investigations on the adverse effects of persistent pollutants on human health.

Promoting effect of triterpenoid compound from Agrimonia pilosa Ledeb on preadipocytes differentiation via up-regulation of PPARγ expression

Background:

Agrimonia Pilosa Ledeb (APL), a traditional Chinese medicine, has been reported a variety of biological activities, including treating T2DM.

Objective:

Triterpenoid compound (TC) was collected from APL. The aim of this study was to investigate the effects of TC on 3T3-L1 preadipocytes differentiation and genes related to differentiation and IR.

Materials and Methods:

Column chromatography was used to collect TC from ALP. 3T3-L1 cell differentiation was induced typically in the presence of various concentrations of TC or pioglitazone. Oil red O staining and measurement of intracellular TG content were performed on the seventh day of differentiation. Then quantitative polymerase chain reaction (Q-PCR) was used to test the expressions of three transcription factors (PPARγ, CCAAT enhancer binding protein-α (C/EBP-α), and sterol regulatory element-binding protein 1 (SREBP-1)) and the target genes of PPARγ including glucose transporter (GLUT4), lipoprotein lipase (LPL), fat acid binding protein (AP2), and adiponectin in 3T3-L1 cells.

Results:

At the concentration of 5, 25 and 125 μg/mL, TC significantly promoted triglyceride accumulation. Further study showed that TC could promote the expression of PPARγ, C/EBPα and ADD1/SREBP1 significantly at 125 μg/mL. As for downstream genes controlled by PPARγ, TC at 25 and 125 μg/mL could significantly promote the expression of GLUT4 and adiponectin. However, the expression of aP2 related to lipid metabolism and adiposity in the TC group was significantly lower than that in the pioglitazone group.

Conclusion:

TC could promote preadipocytes differentiation through activating PPARγ and downstream controlled genes. TC has the ideal insulin sensitization with lower adipogenic action than classical TZDs in vitro. So TC from Agrimonia Pilosa Ledeb has a good prospect as a natural drug for IR and T2DM.

Antidiabetic property of Symplocos cochinchinensis is mediated by inhibition of alpha glucosidase and enhanced insulin sensitivity

The study is designed to find out the biochemical basis of antidiabetic property of Symplocos cochinchinensis (SC), the main ingredient of ‘Nisakathakadi’ an Ayurvedic decoction for diabetes. Since diabetes is a multifactorial disease, ethanolic extract of the bark (SCE) and its fractions (hexane, dichloromethane, ethyl acetate and 90% ethanol) were evaluated by in vitro methods against multiple targets relevant to diabetes such as the alpha glucosidase inhibition, glucose uptake, adipogenic potential, oxidative stress, pancreatic beta cell proliferation, inhibition of protein glycation, protein tyrosine phosphatase-1B (PTP-1B) and dipeptidyl peptidase-IV (DPP-IV). Among the extracts, SCE exhibited comparatively better activity like alpha glucosidase inhibition (IC₅₀ value-82.07±2.10 µg/mL), insulin dependent glucose uptake (3 fold increase) in L6 myotubes, pancreatic beta cell regeneration in RIN-m5F (3.5 fold increase) and reduced triglyceride accumulation (22% decrease) in 3T3L1 cells, protection from hyperglycemia induced generation of reactive oxygen species in HepG2 cells (59.57% decrease) with moderate antiglycation and PTP-1B inhibition. Chemical characterization by HPLC revealed the superiority of SCE over other extracts due to presence and quantity of bioactives (beta-sitosterol, phloretin 2′glucoside, oleanolic acid) in addition to minerals like magnesium, calcium, potassium, sodium, zinc and manganese. So SCE has been subjected to oral sucrose tolerance test to evaluate its antihyperglycemic property in mild diabetic and diabetic animal models. SCE showed significant antihyperglycemic activity in in vivo diabetic models. We conclude that SC mediates the antidiabetic activity mainly via alpha glucosidase inhibition, improved insulin sensitivity, with moderate antiglycation and antioxidant activity.

CMHX008, a novel peroxisome proliferator-activated receptor γ partial agonist, enhances insulin sensitivity in vitro and in vivo

The peroxisome proliferator-activated receptor γ (PPARγ) plays an important role in adipocyte differentiation and insulin sensitivity. Its ligand rosiglitazone has anti-diabetic effect but is frequently accompanied with some severe unwanted effects. The aim of the current study was to compare the anti-diabetic effect of CMHX008, a novel thiazolidinedione-derivative, with rosiglitazone. A luciferase assay was used to evaluate in vitro PPARγ activation. 3T3-L1 cells were used to examine adipocyte differentiation. High fat diet (HFD) mice were used to examine in vivo insulin sensitivity. The mRNA levels were evaluated by real-time RT-PCR. Serum biochemical and hormonal variables were assessed using a clinical chemistry analyser. CMHX008 displayed a moderate PPARγ agonist activity, and promoted 3T3-L1 preadipocyte differentiation with lower activity than rosiglitazone. CMHX008 regulated the expression of PPARγ target genes in a different manner from rosiglitazone. CMHX008 increased the expression and secretion of adiponectin with the similar efficacy as rosiglitazone, but only 25% as potent as rosiglitazone for the induction of adipocyte fatty acid binding protein. Treatment of CMHX008 and rosiglitazone protected mice from high fat diet (HFD)-induced glucose intolerance, hyperinsulinemia and inflammation. CMHX008 reduced the mRNA expression of M1 macrophage markers, and significantly increased the expressions of M2 markers. In conclusion, CMHX008 shared the comparable insulin-sensitizing effects as rosiglitazone with lower adipogenic capacity and might potentially be developed into an effective agent for the treatment of diabetes and metabolic disorders.

The effects of propionate and valerate on insulin responsiveness for glucose uptake in 3T3-L1 adipocytes and C2C12 myotubes via G protein-coupled receptor 41

Since insulin resistance can lead to hyperglycemia, improving glucose uptake into target tissues is critical for regulating blood glucose levels. Among the free fatty acid receptor (FFAR) family of G protein-coupled receptors, GPR41 is known to be the Gα_i/o-coupled receptor for short-chain fatty acids (SCFAs) such as propionic acid (C3) and valeric acid (C5). This study aimed to investigate the role of GPR41 in modulating basal and insulin-stimulated glucose uptake in insulin-sensitive cells including adipocytes and skeletal muscle cells. Expression of GPR41 mRNA and protein was increased with maximal expression at differentiation day 8 for 3T3-L1 adipocytes and day 6 for C2C12 myotubes. GPR41 protein was also expressed in adipose tissues and skeletal muscle. After analyzing dose-response relationship, 300 µM propionic acid or 500 µM valeric acid for 30 min incubation was used for the measurement of glucose uptake. Both propionic acid and valeric acid increased insulin-stimulated glucose uptake in 3T3-L1 adipocyte, which did not occur in cells transfected with siRNA for GPR41 (siGPR41). In C2C12 myotubes, these SCFAs increased basal glucose uptake, but did not potentiate insulin-stimulated glucose uptake, and siGPR41 treatment reduced valerate-stimulated basal glucose uptake. Therefore, these findings indicate that GPR41 plays a role in insulin responsiveness enhanced by both propionic and valeric acids on glucose uptake in 3T3-L1 adipocytes and C2C12 myotubes, and in valerate-induced increase in basal glucose uptake in C2C12 myotubes.

Adaptive expression of microRNA-125a in adipose tissue in response to obesity in mice and men

MicroRNAs are emerging as new mediators in the regulation of adipose tissue biology and the development of obesity. An important role of microRNA-125a has been suggested in the pathogenesis of insulin resistance (IR). Here, we characterized the function of microRNA-125a in adipose tissue in a context of experimentally-induced IR and obesity in mice and in obese patients. We showed time dependent overexpression of the microRNA in adipose tissue of BALB/c and C57BL/6J mice in response to high fat diet (HFD) feeding. MicroRNA-125a expression was downregulated in vitro in insulin resistant 3T3-L1 adipocytes and ex vivo in adipose tissue of obese patients. In vitro modulation of microRNA-125a expression in 3T3-L1 adipocytes did not affect glucose uptake. Gene set enrichment analysis (GSEA) identified significantly altered expression patterns of predicted microRNA-125a gene targets in transcriptomic datasets of adipose tissue from HFD-fed mice and obese patients. Among genes that contributed to global enrichment of altered expression of microRNA-125a targets, Thyrotroph embryonic factor (Tef), Mannan-binding lectin serine peptidase 1, Reticulon 2 and Ubiquitin-conjugating enzyme E2L3 were significantly differentially expressed in adipose tissue in these groups. We showed that Tef expression is reduced in adipose tissue of obese patients following gastric bypass surgery. Our findings indicate that microRNA-125a expression in adipose tissue adapts to IR and may play a role in the development of obesity in mice and obese subjects through uncoupled regulation of the expression of microRNA-125a and its targets.

Ficus deltoidea: A Potential Alternative Medicine for Diabetes Mellitus

Ficus deltoidea from the Moraceae family has been scientifically proven to reduce hyperglycemia at different prandial states. In this study, we evaluate the mechanisms that underlie antihyperglycemic action of Ficus deltoidea. The results had shown that hot aqueous extract of Ficus deltoidea stimulated insulin secretion significantly with the highest magnitude of stimulation was 7.31-fold (P < 0.001). The insulin secretory actions of the hot aqueous extract involved K⁺  _ATP channel-dependent and K⁺  _ATP-channel-independent pathway. The extract also has the ability to induce the usage of intracellular Ca²⁺ to trigger insulin release. The ethanolic and methanolic extracts enhanced basal and insulin-mediated glucose uptake into adipocytes cells. The extracts possess either insulin-mimetic or insulin-sensitizing property or combination of both properties during enhancing glucose uptake into such cells. Meanwhile, the hot aqueous and methanolic extracts augmented basal and insulin-stimulated adiponectin secretion from adipocytes cells. From this study, it is suggested that Ficus deltoidea has the potential to be developed as future oral antidiabetic agent.

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.

A peroxisome proliferator-activated receptor gamma (PPARgamma)/PPARgamma coactivator 1beta autoregulatory loop in adipocyte mitochondrial function

Peroxisome proliferator-activated receptor γ (PPARγ) activation induces adipogenesis and also enhances lipogenesis, mitochondrial activity, and insulin sensitivity in adipocytes. Whereas some studies implicate PPARγ coactivator 1α (PGC-1α) in the mitochondrial effect, the mechanisms involved in PPARγ regulation of adipocyte mitochondrial function are not resolved. PPARγ-activating ligands (thiazolidinediones (TZDs)) are important insulin sensitizers and were recently shown to indirectly induce PGC-1β transcription in osteoclasts. Here, we asked whether similar effects occur in adipocytes and show that TZDs also strongly induce PGC-1β in cultured 3T3-L1 cells. This effect, however, differs from the indirect effect proposed for bone and is rapid and direct and involves PPARγ interactions with an intronic PPARγ response element cluster in the PGC-1β locus. TZD treatment of cultured adipocytes results in up-regulation of mitochondrial marker genes, and increased mitochondrial activity and use of short interfering RNA confirms that these effects require PGC-1β. PGC-1β did not participate in PPARγ effects on adipogenesis or lipogenesis, and PGC-1β knockdown did not alter insulin-responsive glucose uptake into 3T3-L1 cells. Similar effects on PGC-1β and mitochondrial gene expression are seen in vivo; fractionation of obese mouse adipose tissue reveals that PPARγ and PGC-1β, but not PGC-1α, are coordinately up-regulated in adipocytes relative to preadipocytes and that TZD treatment induces PGC-1β and mitochondrial marker genes in adipose tissue of obese mice. We propose that PPARγ directly induces PGC-1β expression in adipocytes and that this effect regulates adipocyte mitochondrial activity.

Adipose-specific disruption of autotaxin enhances nutritional fattening and reduces plasma lysophosphatidic acid

Autotaxin (ATX) is a secreted lysophospholipase D that generates the lipid mediator lysophosphatidic acid (LPA). ATX is secreted by adipose tissue and its expression is enhanced in obese/insulin-resistant individuals. Here, we analyzed the specific contribution of adipose-ATX to fat expansion associated with nutritional obesity and its consequences on plasma LPA levels. We established ATX(F/F)/aP2-Cre (FATX-KO) transgenic mice carrying a null ATX allele specifically in adipose tissue. FATX-KO mice and their control littermates were fed either a normal or a high-fat diet (HFD) (45% fat) for 13 weeks. FATX-KO mice showed a strong decrease (up to 90%) in ATX expression in white and brown adipose tissue, but not in other ATX-expressing organs. This was associated with a 38% reduction in plasma LPA levels. When fed an HFD, FATX-KO mice showed a higher fat mass and a higher adipocyte size than control mice although food intake was unchanged. This was associated with increased expression of peroxisome proliferator-activated receptor (PPAR)γ2 and of PPAR-sensitive genes (aP2, adiponectin, leptin, glut-1) in subcutaneous white adipose tissue, as well as in an increased tolerance to glucose. These results show that adipose-ATX is a negative regulator of fat mass expansion in response to an HFD and contributes to plasma LPA levels.

Somatic cell plasticity and Niemann-pick type C2 protein: adipocyte differentiation and function

The phenotypic stability of somatic cells is essential for the maintenance of both structural and functional organ integrity of the adult human body. Deregulated cell plasticity could result in the development of debilitating diseases such as cancer, fibrosis, atherosclerosis, obesity, and type 2 diabetes. We have previously demonstrated that a nonsense mutation in the NPC2 gene, which encodes ubiquitous, highly conserved, secretory protein with unknown function, leads to activation of human skin fibroblasts. The activated fibroblasts, also known as myofibroblasts, have the properties of mesenchymal stem cells and are able to differentiate along the mesodermal and endodermal lineages. Here we show that NPC2-null, but not the normal skin fibroblasts, possess characteristics of adipogenic progenitors as demonstrated by their specific gene expression pattern as well as the ability for efficient differentiation into white adipocytes. The presence of NPC2 in mature white adipocytes was also necessary for their maintenance because silencing NPC2 in differentiated cells by siRNA stimulated PPARG expression, which was followed by a shift toward a more favorable, brown adipocyte-like metabolic state characterized by up-regulated lipolysis and increased insulin sensitivity. It appears that NPC2 controls both the adipogenesis and the metabolic state of mature white adipocytes through a common mechanism that is linked to activation of FGFR2 that could be followed by induction of PPARG expression. Altogether, the current study highlights NPC2 as a novel intracrine/autocrine factor that controls adipocyte differentiation and function as well as potential therapeutic target for the treatment of type 2 diabetes and related metabolic disorders.

Rosiglitazone enhances glucose uptake in glomerular podocytes using the glucose transporter GLUT1

AIMS/HYPOTHESIS

Peroxisome proliferator-activated receptor (PPAR) gamma agonists are used increasingly in the treatment of type 2 diabetes. In the context of renal disease, PPARgamma agonists reduce microalbuminuria in diabetic nephropathy; however, the mechanisms underlying this effect are unknown. Glomerular podocytes are newly characterised insulin-sensitive cells and there is good evidence that they are targeted in diabetic nephropathy. In this study we investigated the functional and molecular effects of the PPARgamma agonist rosiglitazone on human podocytes.

METHODS

Conditionally immortalised human podocytes were cultured with rosiglitazone and functional effects were measured with glucose-uptake assays. The effect of rosiglitazone on glucose uptake was also measured in 3T3-L1 adipocytes, nephrin-deficient podocytes, human glomerular endothelial cells, proximal tubular cells and podocytes treated with the NEFA palmitate. The role of the glucose transporter GLUT1 was investigated with immunofluorescence and small interfering RNA knockdown and the plasma membrane expression of GLUT1 was determined with bis-mannose photolabelling.

RESULTS

Rosiglitazone significantly increased glucose uptake in wild-type podocytes and this was associated with translocation of GLUT1 to the plasma membrane. This effect was blocked with GLUT1 small interfering RNA. Nephrin-deficient podocytes, glomerular endothelial cells and proximal tubular cells did not increase glucose uptake in response to either insulin or rosiglitazone. Furthermore, rosiglitazone significantly increased basal and insulin-stimulated glucose uptake when podocytes were treated with the NEFA palmitate.

CONCLUSIONS/INTERPRETATION

In conclusion, rosiglitazone has a direct and protective effect on glucose uptake in wild-type human podocytes. This represents a novel mechanism by which PPARgamma agonists may improve podocyte function in diabetic nephropathy.

Gamma-synuclein is an adipocyte-neuron gene coordinately expressed with leptin and increased in human obesity

Recently, we characterized tumor suppressor candidate 5 (Tusc5) as an adipocyte-neuron PPARgamma target gene. Our objective herein was to identify additional genes that display distinctly high expression in fat and neurons, because such a pattern could signal previously uncharacterized functional pathways shared in these disparate tissues. gamma-Synuclein, a marker of peripheral and select central nervous system neurons, was strongly expressed in white adipose tissue (WAT) and peripheral nervous system ganglia using bioinformatics and quantitative PCR approaches. Gamma-synuclein expression was determined during adipogenesis and in subcutaneous (SC) and visceral adipose tissue (VAT) from obese and nonobese humans. Gamma-synuclein mRNA increased from trace levels in preadipocytes to high levels in mature 3T3-L1 adipocytes and decreased approximately 50% following treatment with the PPARgamma agonist GW1929 (P < 0.01). Because gamma-synuclein limits growth arrest and is implicated in cancer progression in nonadipocytes, we suspected that expression would be increased in situations where WAT plasticity/adipocyte turnover are engaged. Consistent with this postulate, human WAT gamma-synuclein mRNA levels consistently increased in obesity and were higher in SC than in VAT; i.e. they increased approximately 1.7-fold in obese Pima Indian adipocytes (P = 0.003) and approximately 2-fold in SC and VAT of other obese cohorts relative to nonobese subjects. Expression correlated with leptin transcript levels in human SC and VAT (r = 0.887; P < 0.0001; n = 44). Gamma-synuclein protein was observed in rodent and human WAT but not in negative control liver. These results are consistent with the hypothesis that gamma-synuclein plays an important role in adipocyte physiology.

Effect of a novel non-thiazolidinedione peroxisome proliferator-activated receptor alpha/gamma agonist on glucose uptake

AIMS/HYPOTHESIS

The effect of the benzopyran derivative T33, a novel non-thiazolidinedione agent, was studied on peroxisome proliferator-activated receptors (PPARs), insulin signalling and glucose uptake in adipocytes and skeletal muscle. We hypothesised that T33 could activate PPARgamma and exert a beneficial effect on insulin action on glucose uptake and lipid metabolism.

MATERIALS AND METHODS

Using a cell-based reporter gene assay, T33 was identified as a PPARalpha/gamma dual agonist, which activated human PPARgamma and PPARalpha with EC50 values of 19 and 148 nmol/l, respectively. The effect of T33 on glucose metabolism was studied in cultured 3T3-L1 adipocytes and L6 myotubes. In vivo effects of T33 on skeletal muscle were determined in ob/ob mice treated with 8 mg/kg T33. The effect of T33 on metabolic abnormalities was observed in diet-induced obese mice.

RESULTS

Exposure of 3T3-L1 adipocytes to T33 for 4 days increased basal and insulin-stimulated glucose uptake, with no effect noted in L6 myotubes. Treatment of ob/ob mice for 20 days with T33 normalised basal and insulin-stimulated glucose uptake and increased phosphorylation of Akt and p38 mitogen-activated protein kinase in skeletal muscle. In contrast, phosphorylation of AMP-activated protein kinase was unaltered. Moreover, T33 improved insulin sensitivity and lipid metabolism in diet-induced obese mice.

CONCLUSIONS/INTERPRETATION

T33 is non-thiazolidinedione PPARalpha/gamma dual agonist which directly increases basal and insulin-stimulated glucose uptake in adipocytes and secondarily improves insulin action on insulin signalling and glucose metabolism in skeletal muscle from diabetic ob/ob mice.

Pharmacological inhibition of glucosylceramide synthase enhances insulin sensitivity

A growing body of evidence implicates ceramide and/or its glycosphingolipid metabolites in the pathogenesis of insulin resistance. We have developed a highly specific small molecule inhibitor of glucosylceramide synthase, an enzyme that catalyzes a necessary step in the conversion of ceramide to glycosphingolipids. In cultured 3T3-L1 adipocytes, the iminosugar derivative N-(5'-adamantane-1'-yl-methoxy)-pentyl-1-deoxynojirimycin (AMP-DNM) counteracted tumor necrosis factor-alpha-induced abnormalities in glycosphingolipid concentrations and concomitantly reversed abnormalities in insulin signal transduction. When administered to mice and rats, AMP-DNM significantly reduced glycosphingolipid but not ceramide concentrations in various tissues. Treatment of ob/ob mice with AMP-DNM normalized their elevated tissue glucosylceramide levels, markedly lowered circulating glucose levels, improved oral glucose tolerance, reduced A1C, and improved insulin sensitivity in muscle and liver. Similarly beneficial metabolic effects were seen in high fat-fed mice and ZDF rats. These findings provide further evidence that glycosphingolipid metabolites of ceramide may be involved in mediating the link between obesity and insulin resistance and that interference with glycosphingolipid biosynthesis might present a novel approach to the therapy of states of impaired insulin action such as type 2 diabetes.

The Wnt antagonist Dickkopf-1 and its receptors are coordinately regulated during early human adipogenesis

Secretion of Wnts by adipose cells has an important role in the control of murine adipogenesis. We present the first evidence that a Wnt antagonist, Dickkopf 1 (Dkk1), is secreted by human preadipocytes and promotes adipogenesis. DKK1 mRNA increases six hours after onset of human adipogenesis and this is followed by an increase in Dkk1 protein. With further differentiation, the mRNA and protein levels progressively decline such that they are undetectable in mature adipocytes. The transient induction in DKK1 correlates with downregulation of cytoplasmic and nuclear beta-catenin levels, this being a surrogate marker of canonical Wnt signalling, and Wnt/beta-catenin transcriptional activity. In addition, constitutive expression of Dkk1 in 3T3-L1 preadipocytes promotes their differentiation, further supporting the functional significance of increased Dkk1 levels during human adipogenesis. Concomitant downregulation of the Dkk1 receptors LRP5 and LRP6 is likely to potentiate the ability of Dkk1 to inhibit Wnt signalling and promote differentiation. Notably, Dkk1 is not expressed in primary murine preadipocytes or cell lines. The involvement of Dkk1 in human but not murine adipogenesis indicates that inter-species differences exist in the molecular control of this process. Given the public health importance of disorders of adipose mass, further knowledge of the pathways involved specifically in human adipocyte differentiation might ultimately be of clinical relevance.

WNT10B mutations in human obesity

AIMS/HYPOTHESIS

Recent studies suggest that wingless-type MMTV integration site family, member 10B (WNT10B) may play a role in the negative regulation of adipocyte differentiation in vitro and in vivo. In order to determine whether mutations in WNT10B contribute to human obesity, we screened two independent populations of obese subjects for mutations in this gene.

SUBJECTS AND METHODS

We studied 96 subjects with severe obesity of early onset (less than 10 years of age) from the UK Genetics of Obesity Study and 115 obese Italian subjects of European origin.

RESULTS

One proband with early-onset obesity was found to be heterozygous for a C256Y mutation, which abrogated the ability of WNT10B to activate canonical WNT signalling and block adipogenesis and was not found in 600 control alleles. All relatives of the proband who carried this allele were either overweight or obese. Three other rare missense variants were found in obese probands, but these did not clearly cosegregate with obesity in family studies and one (P301S), which was found in three unrelated subjects with early-onset obesity, had normal functional properties.

CONCLUSIONS/INTERPRETATION

These mutations represent the first naturally occurring missense variants of WNT10B. While the pedigree analysis in the case of C256Y WNT10B does not provide definitive proof of a causal link of this variant with obesity, the finding of a non-functioning WNT10B allele in a human family affected by obesity should encourage further study of this gene in other obese populations.

Suppression of oxidative metabolism and mitochondrial biogenesis by the transcriptional corepressor RIP140 in mouse adipocytes

Using an siRNA-based screen, we identified the transcriptional corepressor RIP140 as a negative regulator of insulin-responsive hexose uptake and oxidative metabolism in 3T3-L1 adipocytes. Affymetrix GeneChip profiling revealed that RIP140 depletion upregulates the expression of clusters of genes in the pathways of glucose uptake, glycolysis, TCA cycle, fatty acid oxidation, mitochondrial biogenesis, and oxidative phosphorylation in these cells. Conversely, we show that reexpression of RIP140 in mouse embryonic fibroblasts derived from RIP140-null mice downregulates expression of many of these same genes. Consistent with these microarray data, RIP140 gene silencing in cultured adipocytes increased both conversion of [14C]glucose to CO2 and mitochondrial oxygen consumption. RIP140-null mice, previously reported to resist weight gain on a high-fat diet, are shown here to display enhanced glucose tolerance and enhanced responsiveness to insulin compared with matched wild-type mice upon high-fat feeding. Mechanistically, RIP140 was found to require the nuclear receptor ERRalpha to regulate hexose uptake and mitochondrial proteins SDHB and CoxVb, although it likely acts through other nuclear receptors as well. We conclude that RIP140 is a major suppressor of adipocyte oxidative metabolism and mitochondrial biogenesis, as well as a negative regulator of whole-body glucose tolerance and energy expenditure in mice.

PPARgamma regulates adipocyte cholesterol metabolism via oxidized LDL receptor 1

In addition to its role in energy storage, adipose tissue also accumulates cholesterol. Concentrations of cholesterol and triglycerides are strongly correlated in the adipocyte, but little is known about mechanisms regulating cholesterol metabolism in fat cells. Here we report that antidiabetic thiazolidinediones (TZDs) and other ligands for the nuclear receptor PPARgamma dramatically upregulate oxidized LDL receptor 1 (OLR1) in adipocytes by facilitating the exchange of coactivators for corepressors on the OLR1 gene in cultured mouse adipocytes. TZDs markedly stimulate the uptake of oxidized LDL (oxLDL) into adipocytes, and this requires OLR1. Increased OLR1 expression, resulting either from TZD treatment or adenoviral gene delivery, significantly augments adipocyte cholesterol content and enhances fatty acid uptake. OLR1 expression in white adipose tissue is increased in obesity and is further induced by PPARgamma ligand treatment in vivo. Serum oxLDL levels are decreased in both lean and obese diabetic animals treated with TZDs. These data identify OLR1 as a novel PPARgamma target gene in adipocytes. While the physiological role of adipose tissue in cholesterol and oxLDL metabolism remains to be established, the induction of OLR1 is a potential means by which PPARgamma ligands regulate lipid metabolism and insulin sensitivity in adipocytes.

Deletion of PPARgamma in adipose tissues of mice protects against high fat diet-induced obesity and insulin resistance

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a crucial role in adipocyte differentiation, glucose metabolism, and other physiological processes. To further explore the role of PPARgamma in adipose tissues, we used a Cre/loxP strategy to generate adipose-specific PPARgamma knockout mice. These animals exhibited marked abnormalities in the formation and function of both brown and white adipose tissues. When fed a high-fat diet, adipose-specific PPARgamma knockout mice displayed diminished weight gain despite hyperphagia, had diminished serum concentrations of both leptin and adiponectin, and did not develop glucose intolerance or insulin resistance. Characterization of in vivo glucose dynamics pointed to improved hepatic glucose metabolism as the basis for preventing high-fat diet-induced insulin resistance. Our findings further illustrate the essential role for PPARgamma in the development of adipose tissues and suggest that a compensatory induction of hepatic PPARgamma may stimulate an increase in glucose disposal by the liver.

Rosiglitazone ameliorates insulin resistance in brown adipocytes of Wistar rats by impairing TNF-alpha induction of p38 and p42/p44 mitogen-activated protein kinases

AIMS/HYPOTHESIS

TNF-alpha caused insulin resistance on glucose uptake and on insulin signalling in fetal brown adipocytes. Since treatment with TNF-alpha activates stress kinases, including c-jun NH2 terminal kinase (JNK), and p42/p44 and p38 mitogen-activated protein kinases (MAPK), we explored the contribution of these pathways to insulin resistance by TNF-alpha. Rosiglitazone is used to treat Type 2 diabetes as it improves insulin sensitivity in vivo. However, its ability to ameliorate TNF-alpha-induced insulin resistance in brown adipocytes remains to be explored.

METHODS

We used fetal rat primary brown adipocytes cultured with TNF-alpha, with or without stress kinase inhibitors or rosiglitazone, and further stimulated with insulin. Then, we measured glucose uptake and GLUT4 translocation. To determine the insulin signalling cascade, we submitted cells to lysis, immunoprecipitation and immunoblotting.

RESULTS

Exposure to TNF-alpha for 24 h impairs insulin stimulation of the phosphatidylinositol (PI) 3-kinase activity associated with IRS-2 and Akt activity. Pretreatment with PD98059 or PD169316, which inhibit p42/p44MAPK and p38MAPK respectively, restored insulin signalling and insulin-induced glucose uptake in the presence of TNF-alpha. However, in the presence of SP600125, an inhibitor of JNK, TNF-alpha still produced insulin resistance. Rosiglitazone ameliorated insulin resistance by TNF-alpha in brown adipocytes, restoring completely insulin-stimulated glucose uptake and insulin-induced GLUT4 translocation to plasma membrane in parallel to the insulin signalling cascade IRS-2/PI 3-kinase/Akt.

CONCLUSIONS/INTERPRETATION

Rosiglitazone treatment impaired TNF-alpha activation of p38 and p42/p44MAPK, restoring insulin signalling and leading to normalisation of glucose uptake.

Role of the POZ zinc finger transcription factor FBI-1 in human and murine adipogenesis

Poxvirus zinc finger (POZ) zinc finger domain transcription factors have been shown to play a role in the control of growth arrest and differentiation in several types of mesenchymal cells but not, as yet, adipocytes. We found that a POZ domain protein, factor that binds to inducer of short transcripts-1 (FBI-1), was induced during both murine and human preadipocyte differentiation with maximal expression levels seen at days 2-4. FBI-1 mRNA was expressed in human adipose tissue with the highest levels found in samples from morbidly obese subjects. Murine cell lines constitutively expressing FBI-1 showed evidence for accelerated adipogenesis with earlier induction of markers of differentiation and enhanced lipid accumulation, suggesting that FBI-1 may be an active participant in the differentiation process. Consistent with the properties of this family of proteins in other cell systems, 3T3L1 cells stably overexpressing FBI-1 showed reduced DNA synthesis and reduced expression of cyclin A, cyclin-dependent kinase 2, and p107, proteins known to be involved in the regulation of mitotic clonal expansion. In addition, FBI-1 reduced the transcriptional activity of the cyclin A promoter. Thus, FBI-1, a POZ zinc finger transcription factor, is induced during the early phases of human and murine preadipocyte differentiation where it may contribute to adipogenesis through influencing the switch from cellular proliferation to terminal differentiation.

Mitochondrial biogenesis and remodeling during adipogenesis and in response to the insulin sensitizer rosiglitazone

White adipose tissue is an important endocrine organ involved in the control of whole-body metabolism, insulin sensitivity, and food intake. To better understand these functions, 3T3-L1 cell differentiation was studied by using combined proteomic and genomic strategies. The proteomics approach developed here exploits velocity gradient centrifugation as an alternative to isoelectric focusing for protein separation in the first dimension. A 20- to 30-fold increase in the concentration of numerous mitochondrial proteins was observed during adipogenesis, as determined by mass spectrometry and database correlation analysis. Light and electron microscopy confirmed a large increase in the number of mitochondrion profiles with differentiation. Furthermore, mRNA profiles obtained by using Affymetrix GeneChips revealed statistically significant increases in the expression of many nucleus-encoded mitochondrial genes during adipogenesis. Qualitative changes in mitochondrial composition also occur during adipose differentiation, as exemplified by increases in expression of proteins involved in fatty acid metabolism and of mitochondrial chaperones. Furthermore, the insulin sensitizer rosiglitazone caused striking changes in mitochondrial shape and expression of selective mitochondrial proteins. Thus, although mitochondrial biogenesis has classically been associated with brown adipocyte differentiation and thermogenesis, our results reveal that mitochondrial biogenesis and remodeling are inherent to adipose differentiation per se and are influenced by the actions of insulin sensitizers.