Protein kinase A suppresses the differentiation of 3T3-L1 preadipocytes

Nootkatone (NK), a grapefruit-derived aromatic compound, inhibited lipid accumulation by regulating JAK2-STAT signaling and antioxidant response in adipocyte

Nootkatone (NK) is an aromatic compound derived from grapefruit. This study aimed to investigate the inhibitory effect of NK on lipid accumulation and its underlying mechanism in adipocytes. NK effectively inhibited adipogenic lipid storage by downregulating C/EBPα and PPARγ, while upregulating KLF2, an early inhibitory factor, downregulating C/EBPβ, an early promoting factor. In addition, NK inhibited the JAK2-STAT signaling pathway by decreasing the phosphorylation of STAT3 and STAT5 in the early adipogenic stage. NK significantly reduced ROS generation while elevating antioxidant enzymes such as catalase and glutathione peroxidase. It activated NRF2-HO-1 signaling, responsible for antioxidant response, by increasing protein levels. Furthermore, NK regulated adipokines, increasing adiponectin and visfatin, while downregulating resistin. Collectively, NK inhibited adipogenic lipid accumulation through the suppression of JAK2-STAT signaling and the augmentation of antioxidant response. This study highlights the potential of NK as an edible agent to alleviate obesity and its associated metabolic diseases.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-024-01522-2.

Phosphodiesterase 4 activity uniquely regulates ciliary cAMP-dependent 3T3-L1 adipogenesis

Recent evidence indicates that the presence of a primary cilium (PC), and of selective cAMP signaling within this smallest of organelles, promotes adipogenic differentiation of 3T3-L1 preadipocytes incubated in media supplemented with either a natural (docosahexaenoic acid, DHA), or a synthetic (TUG-891), free fatty acid receptor 4 (FFAR4) agonist. Indeed, in this earlier work, activation of ciliary FFAR4 in 3T3-L1 cells was correlated with selective increases in PC cAMP and adipogenesis in these cells. However, this study was silent on the role of local PC cAMP phosphodiesterases (PDEs)-mediated events in regulating these adipogenic responses and on the identity of cAMP PDEs that could regulate the "pool" of ciliary cAMP accessed by FFAR4 agonists. In this context, we have identified the PDEs expressed by 3T3-L1 preadipocytes and showed that of these, only PDE4 inhibition promotes FFAR4-mediated adipogenesis. We propose that this work will identify more selective therapeutic targets through which to control adipogenesis, and perhaps the differentiation of other stem cells in which ciliary cAMP is critical.

Sex Hormone-Binding Globulin (SHBG) Maintains Proper Equine Adipose-Derived Stromal Cells (ASCs)' Metabolic Functions and Negatively Regulates their Basal Adipogenic Potential

BACKGROUND

Sex hormone binding globulin (SHBG) deteriorated expression has been recently strongly correlated to increased level of circulating pro-inflammatory cytokines and insulin resistance, which are typical manifestations of equine metabolic syndrome (EMS). Despite previous reports demonstrated the potential therapeutic application of SHBG for liver-related dysfunctions, whether SHBG might modulate equine adipose-derived stem/stromal cells (EqASCs) metabolic machinery remains unknown. Therefore, we evaluated for the first time the impact of SHBG protein on metabolic changes in ASCs isolated from healthy horses.

METHODS

Beforehand, SHBG protein expression has been experimentally lowered using a predesigned siRNA in EqASCs to verify its metabolic implications and potential therapeutic value. Then, apoptosis profile, oxidative stress, mitochondrial network dynamics and basal adipogenic potential have been evaluated using various molecular and analytical techniques.

RESULTS

The SHBG knockdown altered the proliferative and metabolic activity of EqASCs, while dampening basal apoptosis via Bax transcript suppression. Furthermore, the cells treated with siRNA were characterized by senescent phenotype, accumulation of reactive oxygen species (ROS), nitric oxide, as well as decreased mitochondrial potential that was shown by mitochondrial membrane depolarization and lower expression of key mitophagy factors: PINK, PARKIN and MFN. The addition of SHBG protein reversed the impaired and senescent phenotype of EMS-like cells that was proven by enhanced proliferative activity, reduced apoptosis resistance, lower ROS accumulation and greater mitochondrial dynamics, which is proposed to be related to a normalization of Bax expression. Crucially, SHBG silencing enhanced the expression of key pro-adipogenic effectors, while decreased the abundance of anti-adipogenic factors namely HIF1-α and FABP4. The addition of exogenous SHBG further depleted the expression of PPARγ and C/EBPα and restored the levels of FABP4 and HIF1-α evoking a strong inhibitory potential toward ASCs adipogenesis.

CONCLUSION

Herein, we provide for the first time the evidence that SHBG protein in importantly involved in various key metabolic pathways governing EqASCs functions, and more importantly we showed that SHBG negatively affect the basal adipogenic potential of tested ASCs through a FABP4-dependant pathway, and provide thus new insights for the development of potential anti-obesity therapeutic approach in both animals and humans.

Eicosapentaenoic Acid Induces the Inhibition of Adipogenesis by Reducing the Effect of PPARγ Activator and Mediating PKA Activation and Increased COX-2 Expression in 3T3-L1 Cells at the Differentiation Stage

Obesity has received increasing attention in recent years because it is a factor in the development of non-communicable diseases. The current study aimed to analyze how representative fatty acids (FAs) such as palmitic acid, stearic acid, oleic acid, α-linolenic acid (ALA), and eicosapentaenoic acid (EPA) affected adipogenesis when/if introduced at the differentiation stage of 3T3-L1 cell culture. These FAs are assumed to be potentially relevant to the progression or prevention of obesity. EPA added during the differentiation stage reduced intracellular triacylglycerol (TAG) accumulation, as well as the expression of the established adipocyte-specific marker genes, during the maturation stage. However, no other FAs inhibited intracellular TAG accumulation. Coexistence of Δ12-prostaglandin J2, a peroxisome proliferator-activated receptor γ activator, with EPA during the differentiation stage partially attenuated the inhibitory effect of EPA on intracellular TAG accumulation. EPA increased cyclooxygenase-2 (COX-2) expression and protein kinase A (PKA) activity at the differentiation stage, which could explain the inhibitory actions of EPA. Taken together, exposure of preadipocytes to EPA only during the differentiation stage may be sufficient to finally reduce the mass of white adipose tissue through increasing COX-2 expression and PKA activity.

Pituitary Adenylate Cyclase-activating Polypeptide (PACAP) -derived Peptide MPAPO Stimulates Adipogenic Differentiation by Regulating the Early Stage of Adipogenesis and ERK Signaling Pathway

Regenerative medicine and tissue engineering have delivered new healing possibilities to the treatment of soft tissue defects, but the selection of seed cells is critical for treatment. Adipose-derived stem cells have perpetually been a preferred candidate for seed cells due to their wealthy sources, simple access, high plasticity, and powerful value-added capabilities. How to improve the efficiency of adipogenic differentiation is the key to the treatment. Pituitary adenylate cyclase-activating peptide, as a biologically active peptide secreted by the pituitary, is widely involved in regulating the body's sugar metabolism and lipid metabolism. However, the effects of MPAPO in ADSCs adipogenic differentiation remain unknown. Our results reveal that MPAPO treatment improves the adipogenic differentiation efficiency of ADSCs, including promoting the accumulation of lipid droplets and triglycerides, and the expression of adipocyte protein biomarkers PPARγ and C/EBPa. Additionally, the mechanism studies showed that the effective window of MPAPO-induced adipogenesis was the first 3 days during ADSCs differentiation. MPAPO selectively binds to the PAC1 receptor and promotes adipogenic differentiation of ADSCs by activating the ERK signaling pathway and elevating cell proliferation during postconfluent mitosis stage. Altogether, we demonstrate that MPAPO plays a crucial role in ADSCs adipogenesis, providing experimental basis and data for exploring therapeutic options in tissue defect repair.

Myokine musclin alleviates lipid accumulation in 3T3-L1 adipocytes through PKA/p38-mediated upregulation of lipolysis and suppression of lipogenesis

Musclin (MUS), an exercise-responsive myokine, has been documented to attenuate inflammation and enhance physical endurance. However, the effects of MUS on differentiation and related molecular mechanisms in adipocytes have not yet been studied. In this study, we found that treatment with MUS attenuated lipid accumulation in fully differentiated 3T3-L1 cells. Furthermore, MUS treatment enhanced lipolysis assessed by glycerol release, and caused apoptosis, whereas it reduced the expression of lipogenic proteins, such as PPARγ and processed SREBP1. Treatment with MUS augmented phosphorylated PKA expression, whereas suppressed p38 phosphorylation in 3T3-L1 adipocytes. H89, a selective PKA inhibitor reduced the effects of MUS on lipogenic lipid accumulation as well as lipolysis except for apoptosis. These results suggest that MUS promotes lipolysis and suppresses lipogenesis through a PKA/p38-dependent pathway, thereby ameliorating lipid deposition in cultured adipocytes. The current study offers the potential of MUS as a therapeutic approach for treating obesity with few side effects.

Arachidonic Acid Added during the Differentiation Phase of 3T3-L1 Cells Exerts Anti-Adipogenic Effect by Reducing the Effects of Pro-Adipogenic Prostaglandins

A linoleic acid (LA) metabolite arachidonic acid (AA) added to 3T3-L1 cells is reported to suppress adipogenesis. The purpose of the present study aimed to clarify the effects of AA added during the differentiation phase, including adipogenesis, the types of prostaglandins (PG)s produced, and the crosstalk between AA and the PGs produced. Adipogenesis was inhibited by AA added, while LA did not. When AA was added, increased PGE₂ and PGF_2α production, unchanged Δ¹²-PGJ₂ production, and reduced PGI₂ production were observed. Since the decreased PGI₂ production was reflected in decreased CCAAT/enhancer-binding protein-β (C/EBPβ) and C/EBPδ expression, we expected that the coexistence of PGI₂ with AA would suppress the anti-adipogenic effects of AA. However, the coexistence of PGI₂ with AA did not attenuate the anti-adipogenic effects of AA. In addition, the results were similar when Δ¹²-PGJ₂ coexisted with AA. Taken together, these results indicated that the metabolism of ingested LA to AA is necessary to inhibit adipogenesis and that exposure of AA to adipocytes during only the differentiation phase is sufficient. As further mechanisms for suppressing adipogenesis, AA was found not only to increase PGE₂ and PGF_2α and decrease PGI₂ production but also to abrogate the pro-adipogenic effects of PGI₂ and Δ¹²-PGJ₂.

The Novel Inhibitory Effect of YM976 on Adipocyte Differentiation

The pyrimidine derivative YM976 (4-(3-chlorophenyl)-1,7-diethylpyrido(2,3-d)-pyrimidin-2(1H)-one) exerts anti-inflammatory and anti-asthmatic effects. Considering that accumulation of lipids in adipose tissue is accompanied by inflammation, we investigated whether YM976 affects adipocyte differentiation. We found that YM976 significantly decreased lipid accumulation without cytotoxicity and reduced the expression levels of peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT/enhancer-binding protein α (C/EBPα) as well as their lipogenic regulators including fatty acid synthase (FASN) and fatty acid-binding protein 4 (FABP4) in 3T3-L1 cells induced for differentiation. YM976 mainly inhibited the early stage of adipocyte differentiation. Furthermore, intracellular cAMP level was elevated by YM976 resulting in increased phosphorylation of adenosine monophosphate-activated protein kinase (AMPK). Conversely, decreasing the levels of AMPK or treatment with Compound C, an AMPK inhibitor, lessened the suppressive effects of YM976 on PPARγ transcriptional activity and adipogenesis. Thus, our results suggest YM976 as a novel potential compound for controlling lipid accumulation and formation of adipocytes in obesity.

PGE2 -EP3 axis promotes brown adipose tissue formation through stabilization of WTAP RNA methyltransferase

Brown adipose tissue (BAT) functions as a thermogenic organ and is negatively associated with cardiometabolic diseases. N⁶ -methyladenosine (m⁶ A) modulation regulates the fate of stem cells. Here, we show that the prostaglandin E₂ (PGE₂ )-E-prostanoid receptor 3 (EP3) axis was activated during mouse interscapular BAT development. Disruption of EP3 impaired the browning process during adipocyte differentiation from pre-adipocytes. Brown adipocyte-specific depletion of EP3 compromised interscapular BAT formation and aggravated high-fat diet-induced obesity and insulin resistance in vivo. Mechanistically, activation of EP3 stabilized the Zfp410 mRNA via WTAP-mediated m⁶ A modification, while knockdown of Zfp410 abolished the EP3-induced enhancement of brown adipogenesis. EP3 prevented ubiquitin-mediated degradation of WTAP by eliminating PKA-mediated ERK1/2 inhibition during brown adipocyte differentiation. Ablation of WTAP in brown adipocytes abrogated the protective effect of EP3 overexpression in high-fat diet-fed mice. Inhibition of EP3 also retarded human embryonic stem cell differentiation into mature brown adipocytes by reducing the WTAP levels. Thus, a conserved PGE₂ -EP3 axis promotes BAT development by stabilizing WTAP/Zfp410 signaling in a PKA/ERK1/2-dependent manner.

TCR activation directly stimulates PYGB-dependent glycogenolysis to fuel the early recall response in CD8+ memory T cells

Glycolysis facilitates the rapid recall response of CD8⁺ memory T (Tm) cells. However, it remains unclear whether Tm cells uptake exogenous glucose or mobilize endogenous sugar to fuel glycolysis. Here, we show that intracellular glycogen rather than extracellular glucose acts as the major carbon source for the early recall response. Following antigenic stimulation, Tm cells exhibit high glycogen phosphorylase (brain form, PYGB) activity, leading to glycogenolysis and release of glucose-6-phosphate (G6P). Elevated G6P mainly flows to glycolysis but is also partially channeled to the pentose phosphate pathway, which maintains the antioxidant capacity necessary for later recall stages. Mechanistically, TCR signaling directly induces phosphorylation of PYGB by LCK-ZAP70. Functionally, the glycogenolysis-fueled early recall response of CD8⁺ Tm cells accelerates the clearance of OVA-Listeria monocytogenes in an infected mouse model. Thus, we uncover a specific dependency on glycogen for the initial activation of memory T cells, which may have therapeutic implications for adaptive immunity.

Prostaglandin D2 and its analog, 11d-11m-PGD2, added during the differentiation phase contribute to adipogenic program inhibition in 3T3-L1 cells

We previously reported that prostaglandin (PG)D2 and its isosteric analog, 11-deoxy-11-methylene-PGD2 (11d-11m-PGD2), promote adipogenesis in 3T3-L1 cells during the maturation phase. Focusing on the differentiation phase, although both PGs inhibited adipogenesis, this effect was canceled out by PGI2 and PGJ2 derivatives. Thus, PGD2 and 11d-11m-PGD2 play different roles during the phases, but do not affect PGI2- and PGJ2-derivative-induced adipogenesis.

The regulation of PKA signaling in obesity and in the maintenance of metabolic health

The cAMP-dependent protein kinase (PKA) system represents a primary cell-signaling pathway throughout systems and across species. PKA facilitates the actions of hormones, neurotransmitters and other signaling molecules that bind G-protein coupled receptors (GPCR) to modulate cAMP levels. Through its control of synaptic events, exocytosis, transcriptional regulation, and more, PKA signaling regulates cellular metabolism and emotional and stress responses making it integral in the maintenance and dysregulation of energy homeostasis. Neural PKA signaling is regulated by afferent and peripheral efferent signals that link specific neural cell populations to the regulation of metabolic processes in adipose tissue, liver, pancreas, adrenal, skeletal muscle, and gut. Mouse models have provided invaluable information on the roles for PKA subunits in brain and key metabolic organs. While limited, human studies infer differential regulation of the PKA system in obese compared to lean individuals. Variants identified in PKA subunit genes cause Cushing syndrome that is characterized by metabolic dysregulation associated with endogenous glucocorticoid excess. Under healthy physiologic conditions, the PKA system is exquisitely regulated by stimuli that activate GPCRs to alter intracellular cAMP concentrations, and by PKA cellular localization and holoenzyme stability. Adenylate cyclase activity generates cAMP while phosphodiesterase-mediated cAMP degradation to AMP decreases cAMP levels downstream of GPCRs. Chronic perturbations in PKA signaling appear to be capable of resetting PKA regulation at several levels; in addition, sex differences in PKA signaling regulation, while not well understood, impact the physiologic consequences of metabolic dysregulation and obesity. This review explores the roles for PKA signaling in the pathogenesis of metabolic diseases including obesity, type 2 diabetes mellitus and associated co-morbidities through neural-peripheral crosstalk and cAMP/PKA signaling pathway targets that hold therapeutic potential.

Novel anti-adipogenic effect of CF3-allylated indole in 3T3-L1 cells

Indole derivatives from various plants are known to have health benefits because of their anti-cancer, anti-oxidant, anti-inflammatory, and anti-tubercular effects. However, their effects on adipogenesis have not been fully elucidated yet. Herein, we show that a newly synthesized indole derivative, CF₃-allylated indole, [(E)-1-(pyrimidin- 2-yl)-2-(4,4,4- trifluorobut-2-enyl)-1H-indole], effectively inhibits adipogenesis. We found that CF₃-allylated indole inhibited lipid accumulation and suppressed the expression of CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator activated receptor γ (PPARγ) in 3T3-L1 cells. The inhibitory effect of CF₃-allylated indole primarily occurred at the early phase of adipocyte differentiation by increasing intracellular cyclic adenosine monophosphate (cAMP) levels and enhancing protein kinase A (PKA) and adenosine monophosphate-activated protein kinase (AMPK) signaling. Conversely, depletion of PKA or treatment with a protein kinase A inhibitor (H89) reversed such inhibitory effects of CF₃-allylated indole on adipogenesis and PPARγ expression. These results suggest that CF₃-allylated indole inhibits early stages of adipogenesis by increasing phosphorylation of PKA/AMPK, leading to decreased expression of adipogenic genes in 3T3-L1 cells. These results indicate that CF₃-allylated indole has potential for controlling initial adipocyte differentiation in metabolic disorders such as obesity.

The effects of neuronostatin on proliferation and differentiation of rat primary preadipocytes and 3T3-L1 cells

Neuronostatin is a peptide hormone encoded by the somatostatin gene. Biological effects of neuronostatin are mediated through activation of GPR107. There is evidence indicating that neuronostatin modulates energy homeostasis by suppressing food intake and insulin secretion, while stimulating glucagon secretion. While it was found that neuronostatin receptor is expressed in white adipose tissue, the role of neuronostatin in controlling adipose tissue formation is unknown. The aim of this study is to investigate the effects of neuronostatin on proliferation and differentiation of rat primary preadipocytes and 3T3-L1 cells. We found that neuronostatin receptor GPR107 is expressed in rat preadipocytes and 3T3-L1 cells. Neuronostatin promotes proliferation of preadipocytes via AKT activation. Downregulation of GPR107 mRNA expression and protein production results in an attenuation of neuronostatin-induced stimulation of preadipocyte proliferation. Moreover, neuronostatin reduces intracellular lipid content and the expression of adipogenesis-modulating genes C/ebpα, C/ebpβ, Pparγ, and Fabp4. In summary, these results show that neuronostatin, AKT-dependently, stimulates the proliferation of preadipocytes via GPR107. In contrast, neuronostatin inhibits the differentiation of preadipocytes into mature adipocytes.

Direct Effect of the Synthetic Analogue of Glucagon-Like Peptide Type 1, Liraglutide, on Mature Adipocytes Is Realized through Adenylate-Cyclase-Dependent Enhancing of Insulin Sensitivity

Incretin hormones analogues, including glucagon-like peptide type 1 (GLP-1), exhibit complex glucose-lowering, anorexigenic, and cardioprotective properties. Mechanisms of action of GLP-1 and its analogues are well known for pancreatic β-cells, hepatocytes, and other tissues. Nevertheless, local effects of GLP-1 and its analogues in adipose tissue remain unclear. In the present work effects of the GLP-1 synthetic analogue, liraglutide, on adipogenesis and insulin sensitivity of the 3T3-L1 adipocytes were examined. Enhancement of insulin sensitivity of mature adipocytes by the GLP-1 synthetic analogue liraglutide mediated by adenylate cyclase was demonstrated. The obtained results imply existence of the positive direct insulin-sensitizing effect of liraglutide on mature adipocytes.

Therapeutic potential of phosphodiesterase inhibitors in the treatment of osteoporosis: Scopes for therapeutic repurposing and discovery of new oral osteoanabolic drugs

Cyclic nucleotide phosphodiesterases (PDEs) are ubiquitously expressed enzymes that hydrolyze phosphodiester bond in the second messenger molecules including cAMP and cGMP. A wide range of drugs blocks one or more PDEs thereby preventing the inactivation of cAMP/cGMP. PDEs are differentially expressed in bone cells including osteoblasts, osteoclasts and chondrocytes. Intracellular increases in cAMP/cGMP levels in osteoblasts result in osteogenic response. Acting via the type 1 PTH receptor, teriparatide and abaloparatide increase intracellular cAMP and induce osteoanabolic effect, and many PDE inhibitors mimic this effect in preclinical studies. Since all osteoanabolic drugs are injectable and that oral drugs are considered to improve the treatment adherence and persistence, osteogenic PDE inhibitors could be a promising alternative to the currently available osteogenic therapies and directly assessed clinically in drug repurposing mode. Similar to teriparatide/abaloparatide, PDE inhibitors while stimulating osteoblast function also promote osteoclast function through stimulation of receptor activator of nuclear factor kappa-B ligand production from osteoblasts. In this review, we critically discussed the effects of PDE inhibitors in bone cells from cellular signalling to a variety of preclinical models that evaluated the bone formation mechanisms. We identified pentoxifylline (a non-selective PDE inhibitor) and rolipram (a PDE4 selective inhibitor) being the most studied inhibitors with osteogenic effect in preclinical models of bone loss at ≤ human equivalent doses, which suggest their potential for post-menopausal osteoporosis treatment through therapeutic repurposing. Subsequently, we treated pentoxifylline and rolipram as prototypical osteogenic PDEs to predict new chemotypes via the computer-aided design strategies for new drugs, based on the structural biology of PDEs.

The adenosinergic pathway in mesenchymal stem cell fate and functions

Mesenchymal stem cells (MSCs) play an important role in tissue homeostasis and damage repair through their ability to differentiate into cells of different tissues, trophic support, and immunomodulation. These properties made them attractive for clinical applications in regenerative medicine, immune disorders, and cell transplantation. However, despite multiple preclinical and clinical studies demonstrating beneficial effects of MSCs, their native identity and mechanisms of action remain inconclusive. Since its discovery, the CD73/ecto-5'-nucleotidase is known as a classic marker for MSCs, but its role goes far beyond a phenotypic characterization antigen. CD73 contributes to adenosine production, therefore, is an essential component of purinergic signaling, a pathway composed of different nucleotides and nucleosides, which concentrations are finely regulated by the ectoenzymes and receptors. Thus, purinergic signaling controls pathophysiological functions such as proliferation, migration, cell fate, and immune responses. Despite the remarkable progress already achieved in considering adenosinergic pathway as a therapeutic target in different pathologies, its role is not fully explored in the context of the therapeutic functions of MSCs. Therefore, in this review, we provide an overview of the role of CD73 and adenosine-mediated signaling in the functions ascribed to MSCs, such as homing and proliferation, cell differentiation, and immunomodulation. Additionally, we will discuss the pathophysiological role of MSCs, via CD73 and adenosine, in different diseases, as well as in tumor development and progression. A better understanding of the adenosinergic pathway in the regulation of MSCs functions will help to provide improved therapeutic strategies applicable in regenerative medicine.

Effect of Forskolin on Body Weight, Glucose Metabolism and Adipocyte Size of Diet-Induced Obesity in Mice

The purpose of this study was to investigate the effects of forskolin on body weight, glucose metabolism and fat cell diameter in high-fat diet-induced obese mice. Four-week-old male mice (C57BL/6) were randomly assigned to 1 of 3 treatment groups: a high-fat diet plus 5% dimethyl sulfoxide (vehicle), high-fat diet plus 2 mg/kg of forskolin (dissolved in 5% dimethyl sulfoxide) and high-fat diet plus 4 mg/kg of forskolin (dissolved in 5% dimethyl sulfoxide). Forskolin or dimethyl sulfoxide was administered intraperitoneally every two days. The results indicated that no significant difference was observed in the body weight, feed intake and serum lipid parameters among groups at 20 weeks of age. The blood glucose levels were significantly reduced in the groups treated with 2 mg/kg of forskolin before glucose tolerance test. Forskolin administration linearly decreased blood glucose levels of high-fat diet-fed mice at 90 min and total area under curve (AUC) after insulin tolerance test. The subcutaneous adipocyte diameter was significantly reduced in the groups treated with 2 mg/kg of forskolin. Forskolin administration linearly reduced the gonadal adipocyte diameter of high-fat diet-fed mice. Forskolin significantly reduced the differentiation of murine mesenchymal stem cells into adipocytes and this was accompanied by a decrease in intracellular triglyceride content and an increase in glycerol concentration in the culture medium. The subcutaneous adipocyte diameter, gonadal adipocyte diameter and total AUC of insulin tolerance test were moderately negatively correlated with the concentration of forskolin in the high-fat diet-induced obese model. These results demonstrate that forskolin can regulate glucose metabolism and reduce fat cell diameter of high-fat diet-fed mice and inhibit the adipocyte differentiation of murine mesenchymal stem cells.

Zyflamend, a unique herbal blend, induces cell death and inhibits adipogenesis through the coordinated regulation of PKA and JNK

The prevalence of obesity and its comorbidities has sparked a worldwide concern to address rates of adipose tissue accrual. Recent studies have demonstrated a novel role of Zyflamend, a blend of natural herbal extracts, in regulating lipid metabolism in several cancer cell lines through the activation of the AMPK signalling pathway. Yet, the role of Zyflamend in adipogenic differentiation and lipid metabolism remains largely unexplored. The objective of this study is to investigate the effects of Zyflamend on white 3T3-MBX pre-adipocyte differentiation and elucidate the molecular mechanisms. We demonstrate that Zyflamend treatment altered cell cycle progression, attenuated proliferation, and increased cell death of 3T3-MBX pre-adipocytes. In addition, treatment with Zyflamend inhibited lipid accumulation during the differentiation of 3T3-MBX cells, consistent with decreased expression of lipogenic genes and increased lipolysis. Mechanistically, Zyflamend-induced alterations in adipogenesis were mediated, at least in part, through the activation of AMPK, PKA, and JNK. Inhibition of AMPK partially reversed Zyflamend-induced inhibition of differentiation, whereas the inhibition of either JNK or PKA fully restored adipocyte differentiation and decreased lipolysis. Taken together, the present study demonstrates that Zyflamend, as a novel anti-adipogenic bioactive mix, inhibits adipocyte differentiation through the activation of the PKA and JNK pathways.

Abbreviation: 7-AAD: 7-amino-actinomycin D; ACC: acetyl-CoA carboxylase; AKT: protein kinase B; AMPK: AMP-activated protein kinase; ATGL: adipose triglyceride lipase; C/EBPα: CCAAT-enhancer binding protein alpha; DMEM: Dulbecco’s Modified Eagle Medium; DMSO: dimethyl sulphoxide; DTT: dithiothreitol; EGTA: ethylene glycol-bis-(2-aminoethyl)-N,N,N’,N’-tetraacetic acid; ERK: extracellular signal–regulated kinases; FASN: fatty acid synthase; FBS: foetal bovine serum; GLUT: glucose transporter; HSL: hormone-sensitive lipase; IR: insulin receptor; IRS: insulin receptor substrate; JNK: c-JUN N-terminal kinase; MGL: monoacylglycerol lipase; NaF: sodium fluoride; NF-κB: nuclear factor kappa-light-chain-enhancer of activated B cells; PBS: phosphate buffered- saline; PCB: pyruvate carboxylase; PDE: phosphodiesterase; PKA: protein kinase cAMP-dependent; PMSF: phenylmethylsulfonyl fluoride; PPARγ: perilipin peroxisome proliferator-activated receptor gamma; PREF-1: pre-adipocyte factor 1; PVDF: polyvinylidene fluoride; RIPA: radio-immunoprecipitation assay; SDS-PAGE: sodium dodecyl sulphate polyacrylamide gel electrophoresis; SEM: standard error of the mean; SOX9: suppressor of cytokine signalling 9; TGs: triacylglycerols.

Optimization of Co-Culture Conditions for a Human Vascularized Adipose Tissue Model

In vitro adipose tissue models can be used to provide insight into fundamental aspects of adipose physiology. These systems may serve as replacements for animal models, which are often poor predictors of obesity and metabolic diseases in humans. Adipose tissue consists of a rich vasculature that is essential to its function. However, the study of endothelial cell–adipocyte interactions has been challenging due to differences in culture conditions required for the survival and function of each cell type. To address this issue, we performed an extensive evaluation of the cell culture media composition to identify the conditions optimal for the co-culture of endothelial cells and adipocytes. The effects of individual media factors on cell survival, proliferation, and differentiation were systematically explored. Several media factors were determined to disrupt the co-culture system. Optimized culture conditions were identified and used to generate a vascularized human adipose microtissue. An interconnected vascular network was established within an adipose micro-tissue, and the networks were anastomosed with perfused channels to form a functional network. In conclusion, media conditions were identified that enabled endothelial cell–adipocyte co-culture and were used to support the formation of a vascularized adipose tissue within a microfluidic device.

The Gintonin-Enriched Fraction of Ginseng Regulates Lipid Metabolism and Browning via the cAMP-Protein Kinase a Signaling Pathway in Mice White Adipocytes

Obesity is a major health concern and is becoming an increasingly serious societal problem worldwide. The browning of white adipocytes has received considerable attention because of its potential protective effect against obesity-related metabolic disease. The gintonin-enriched fraction (GEF) is a non-saponin, glycolipoprotein component of ginseng that is known to have neuroprotective and anti-inflammatory effects. However, the anti-obesity and browning effects of GEF have not been explored to date. Therefore, we aimed to determine whether GEF has a preventive effect against obesity. We differentiated 3T3-L1 cells and mouse primary subcutaneous adipocytes for 8 days in the presence or absence of GEF, and then measured the expression of intermediates in signaling pathways that regulate triglyceride (TG) synthesis and browning by Western blotting and immunofluorescence analysis. We found that GEF reduced lipid accumulation by reducing the expression of pro-adipogenic and lipogenic factors, and increased lipolysis and thermogenesis, which may be mediated by an increase in the phosphorylation of protein kinase A. These findings suggest that GEF may induce fat metabolism and energy expenditure in white adipocytes and therefore may represent a potential treatment for obesity.

Filbertone Ameliorates Adiposity in Mice Fed a High-Fat Diet via Activation of cAMP Signaling

The aim of this research was to estimate the preventive effects of filbertone, the main flavor compound in hazelnuts, on lipid accumulation in the adipose tissue of mice fed a high-fat diet (HFD) and to reveal the underlying molecular mechanisms. Male C57BL/6N mice were fed chow, a HFD, or a 0.025% filbertone-supplemented HFD for 14 weeks. We found that filbertone supplementation resulted in significant reductions in body weight gain and lipid accumulation in adipose tissue, with parallel improvements in plasma lipid levels (triglycerides, total cholesterol, and free fatty acids) and proinflammatory cytokines (interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-α)). Molecular analysis revealed that filbertone treatment led to reprogramming of metabolic signatures in the cyclic adenosine monophosphate (cAMP) pathway. Filbertone supplementation significantly increased the cAMP level and increased downstream protein kinase A catalytic subunit (PKA) signaling in mouse adipose tissue. The mRNA level of adipogenesis-related genes was downregulated in the adipose tissue of filbertone-fed mice compared to control mice fed the HFD alone. Furthermore, filbertone treatment elevated the expression of thermogenic genes in mouse adipose tissue. Filbertone reduced intracellular lipid accumulation and increased the oxygen consumption rate in 3T3-L1 cells and these filbertone-induced changes were abrogated by the adenylate cyclases (ADCY) inhibitor. Taken together, our results suggest that the beneficial effects of filbertone on lipid accumulation may be associated with the activation of cAMP signaling.

Role of bone marrow adipocytes in leukemia and chemotherapy challenges

Adipose tissue (AT) is an extramedullary reservoir of normal hematopoietic stem cells (HSCs). Adipocytes prevent the production of normal HSCs via secretion of inflammatory factors, and adipocyte-derived free fatty acids may contribute to the development and progression of leukemia via providing energy for leukemic cells. In addition, adipocytes are able to metabolize and inactivate therapeutic agents, reducing the concentrations of active drugs in adipocyte-rich microenvironments. The aim of this study was to detect the role of adipocytes in the progression and treatment of leukemia. Relevant literature was identified through a PubMed search (2000-2018) of English-language papers using the following terms: leukemia, adipocyte, leukemic stem cell, chemotherapy, and bone marrow. Findings suggest the striking interplay between leukemic cells and adipocytes to create a unique microenvironment supporting the metabolic demands and survival of leukemic cells. Based on these findings, targeting lipid metabolism of leukemic cells and adipocytes in combination with standard therapeutic agents might present novel treatment options.

Regulation of Adipogenesis and Thermogenesis through Mouse Olfactory Receptor 23 Stimulated by α-Cedrene in 3T3-L1 Cells

Olfactory receptors (ORs) are G protein-coupled receptors that perform important physiological functions beyond their role as odorant detectors in the olfactory sensory neurons. In the present study, we describe a novel role for one of these ORs, mouse olfactory receptor 23 (MOR23), as a regulator of adipogenesis and thermogenesis in 3T3-L1 cells. Downregulation of MOR23 by small interfering RNA in 3T3-L1 cells enhanced intracellular lipid accumulation and reduced the oxygen consumption rate. In agreement with this phenotype, MOR23 deletion significantly decreased intracellular cyclic adenosine monophosphate (cAMP) levels and protein amounts of adenylyl cyclase 3 (ADCY3), protein kinase A catalytic subunit (PKA Cα), phospho-5′-adenosine monophosphate (AMP)-activated protein kinase (AMPK), and phospho-cAMP-responsive element-binding protein (CREB), along with upregulation of adipogenic genes and downregulation of genes involved in thermogenesis. Activation of MOR23 by α-cedrene, a novel natural ligand of MOR23, significantly reduced lipid content, increased the oxygen consumption rate, and stimulated reprogramming of the metabolic signature of 3T3-L1 cells, and these changes elicited by α-cedrene were absent in MOR23-deficient cells. These findings point to the role of MOR23 as a regulator of adipogenesis and thermogenesis in adipocytes.

cAMP/Protein Kinase A Signaling Inhibits Dlx5 Expression via Activation of CREB and Subsequent C/EBPβ Induction in 3T3-L1 Preadipocytes

Distal-less homeobox 5 (Dlx5) is a negative regulator of adipogenesis. Dlx5 expression is decreased by adipogenic stimuli, but the mechanisms of Dlx5 downregulation by adipogenic stimuli have not yet been determined. Here, we tested the impact of cAMP/PKA (protein kinase A) signaling induced by 3-isobutyl-1 methyl xanthine (IBMX), forskolin, and 8-CPT-cAMP on the expression of Dlx5 in 3T3-L1 preadipocytes. Significant downregulation of Dlx5 mRNA expression and protein production levels were observed via cAMP/PKA-dependent signaling. Forced expression of cAMP-responsive element-binding protein (CREB) and CCAAT/enhancer-binding protein β (C/EBPβ) was sufficient for downregulation of Dlx5 expression and revealed that CREB functions upstream of C/EBPβ. In addition, C/EBPβ knockdown by siRNA rescued Dlx5 expression in IBMX-treated 3T3-L1 preadipocytes. Luciferase assays using a Dlx5-luc-2935 reporter construct demonstrated the requirement of the Dlx5 promoter region, ranging from −774 to −95 bp that contains two putative C/EBPβ binding elements (site-1: −517 to −510 bp and site-2: −164 to −157 bp), in the suppression of Dlx5 transcription. Consequently, chromatin immunoprecipitation analysis confirmed the importance of site-1, but not site-2, in C/EBPβ binding and transcriptional suppression of Dlx5. In conclusion, we elucidated the underling mechanism of Dlx5 downregulation in IBMX-induced adipogenesis. IBMX activated cAMP/PKA/CREB signaling and subsequently upregulated C/EBPβ, which binds to the Dlx5 promoter to suppress Dlx5 transcription.

Phoenixin-14 stimulates differentiation of 3T3-L1 preadipocytes via cAMP/Epac-dependent mechanism

Phoenixin-14 (PNX) is a newly discovered peptide produced by proteolytic cleavage of the small integral membrane protein 20 (Smim20). Previous studies showed that PNX is involved in controlling reproduction, pain, anxiety and memory. Furthermore, in humans, PNX positively correlates with BMI suggesting a potential role of PNX in controlling fat accumulation in obesity. Since the influence of PNX on adipose tissue formation has not been so far demonstrated, we investigated the effects of PNX on proliferation and differentiation of preadipocytes using 3T3-L1 and rat primary preadipocytes. We detected Smim20 and Gpr173 mRNA in 3T3-L1 preadipocytes as well as in rat primary preadipocytes. Furthermore, we found that PNX peptide is produced and secreted from 3T3-L1 and rat primary adipocytes. PNX increased 3T3-L1 preadipocytes proliferation and viability. PNX stimulated the expression of adipogenic genes (Pparγ, C/ebpβ and Fabp4) in 3T3-L1 adipocytes. 3T3-L1 preadipocytes differentiated in the presence of PNX had increased lipid content. Stimulation of cell proliferation and differentiation by PNX was also confirmed in rat preadipocytes. PNX failed to induce AKT phosphorylation, however, PNX increased cAMP levels in 3T3-L1 cells. Suppression of Epac signalling attenuated PNX-induced Pparγ expression without affecting cell proliferation. Our data show that PNX stimulates differentiation of 3T3-L1 and rat primary preadipocytes into mature adipocytes via cAMP/Epac-dependent pathway. In conclusion our data shows that phoenixin promotes white adipogenesis, thereby may be involved in controlling body mass regulation.

The Vitamin K Epoxide Reductase Vkorc1l1 Promotes Preadipocyte Differentiation in Mice

OBJECTIVE

Identification of novel regulators involved in adipose development is important to understand the molecular mechanism underlying obesity and associated metabolic disorders. Through isolation and analysis of a vitamin K epoxide reductase Vkorc1l1 mutant, this study aimed to disclose its function and underlying mechanism in adipose development and to obtain valuable insights regarding the mechanism of obesity.

METHODS

A Vkorc1l1 mutation recovered from a forward genetic screen for obesity-related loci in mice was characterized to explore its effects in gene expression, animal metabolism, and adipose development. Adipogenesis was evaluated in both Vkorc1l1 mutant stromal vascular fraction and Vkorc1l1 knockdown preadipocytes. Intracellular vitamin K₂ level and the effect of vitamin K₂ on adipogenesis were tested in primary preadipocytes.

RESULTS

Vkorc1l1 mutants displayed a considerably lower fat to body weight ratio, substantially decreased plasma leptin, and significantly underdeveloped white adipose tissue. Adipogenic defects related with Vkorc1l1 deficiency were observed both in vivo and in vitro. Vitamin K₂ could inhibit adipogenesis in stromal vascular fraction. Increased intracellular vitamin K₂ level was detected in Vkorc1l1 mutant preadipocytes.

CONCLUSIONS

Vkorc1l1 promotes adipogenesis and possibly obesity. Downregulation of Vkorc1l1 increases intracellular vitamin K₂ level and impedes preadipocyte differentiation.

Piperonal attenuates visceral adiposity in mice fed a high-fat diet: potential involvement of the adenylate cyclase-protein kinase A dependent pathway

SCOPE

Piperonal is an aromatic compound found in vanilla and has a floral odor resembling vanillin. This study was aimed to test whether piperonal attenuates visceral adiposity induced by a high-fat diet (HFD) in mice and to explore the underlying molecular mechanisms.

METHODS AND RESULTS

Male C57BL/6N mice were fed a normal diet, HFD, or 0.05% piperonal-supplemented HFD (PSD) for 10 weeks. PSD-fed mice showed attenuation of body weight gain, total visceral fat pad weights, and plasma lipid levels compared to HFD-fed mice. Piperonal supplementation of the HFD increased the mRNA expression of certain isotypes of adenylate cyclase (Adcy) and protein kinase A (PKA) in the white adipose tissue (WAT) of mice. The adipogenesis-related genes were downregulated, whereas fatty acid oxidation- and thermogenesis-related genes were upregulated in the WAT of PSD-fed mice compared to those in HFD-fed mice. Piperonal directly activated Adcy by decreasing the K_m for its substrate (ATP) in plasma membranes prepared from the WAT of mice. Furthermore, piperonal-induced inhibition of adipocyte differentiation and elevation of Adcy and PKA activities in 3T3-L1 cells were abrogated by an Adcy inhibitor.

CONCLUSION

The anti-adipogenic effect of piperonal in mice fed the high-fat diet appears to be associated with increased Adcy-PKA signaling in WAT.

Nuclear phosphoproteome analysis of 3T3-L1 preadipocyte differentiation reveals system-wide phosphorylation of transcriptional regulators

Adipocytes (fat cells) are important endocrine and metabolic cells critical for systemic insulin sensitivity. Both adipose excess and insufficiency are associated with adverse metabolic function. Adipogenesis is the process whereby preadipocyte precursor cells differentiate into lipid-laden mature adipocytes. This process is driven by a network of transcriptional regulators (TRs). We hypothesized that protein PTMs, in particular phosphorylation, play a major role in activating and propagating signals within TR networks upon induction of adipogenesis by extracellular stimulus. We applied MS-based quantitative proteomics and phosphoproteomics to monitor the alteration of nuclear proteins during the early stages (4 h) of preadipocyte differentiation. We identified a total of 4072 proteins including 2434 phosphorylated proteins, a majority of which were assigned as regulators of gene expression. Our results demonstrate that adipogenic stimuli increase the nuclear abundance and/or the phosphorylation levels of proteins involved in gene expression, cell organization, and oxidation-reduction pathways. Furthermore, proteins acting as negative modulators involved in negative regulation of gene expression, insulin stimulated glucose uptake, and cytoskeletal organization showed a decrease in their nuclear abundance and/or phosphorylation levels during the first 4 h of adipogenesis. Among 288 identified TRs, 49 were regulated within 4 h of adipogenic stimulation including several known and many novel potential adipogenic regulators. We created a kinase-substrate database for 3T3-L1 preadipocytes by investigating the relationship between protein kinases and protein phosphorylation sites identified in our dataset. A majority of the putative protein kinases belong to the cyclin-dependent kinase family and the mitogen-activated protein kinase family including P38 and c-Jun N-terminal kinases, suggesting that these kinases act as orchestrators of early adipogenesis.

Identification of small molecules that improve ATP synthesis defects conferred by Leber's hereditary optic neuropathy mutations

Inherited mitochondrial complex I mutations cause blinding Leber's hereditary optic neuropathy (LHON), for which no curative therapy exists. A specific biochemical consequence of LHON mutations in the presence of trace rotenone was observed: deficient complex I-dependent ATP synthesis (CIDAS) and mitochondrial O2 consumption, proportional to the clinical severity of the three primary LHON mutations. We optimized a high-throughput assay of CIDAS to screen 1600 drugs to 2, papaverine and zolpidem, which protected CIDAS in LHON cells concentration-dependently. TSPO and cAMP were investigated as protective mechanisms, but a conclusive mechanism remains to be elucidated; next steps include testing in animal models.

Targeting Tumor Mitochondrial Metabolism Overcomes Resistance to Antiangiogenics

Epithelial malignancies are effectively treated by antiangiogenics; however, acquired resistance is a major problem in cancer therapeutics. Epithelial tumors commonly have mutations in the MAPK/Pi3K-AKT pathways, which leads to high-rate aerobic glycolysis. Here, we show how multikinase inhibitor antiangiogenics (TKIs) induce hypoxia correction in spontaneous breast and lung tumor models. When this happens, the tumors downregulate glycolysis and switch to long-term reliance on mitochondrial respiration. A transcriptomic, metabolomic, and phosphoproteomic study revealed that this metabolic switch is mediated by downregulation of HIF1α and AKT and upregulation of AMPK, allowing uptake and degradation of fatty acids and ketone bodies. The switch renders mitochondrial respiration necessary for tumor survival. Agents like phenformin or ME344 induce synergistic tumor control when combined with TKIs, leading to metabolic synthetic lethality. Our study uncovers mechanistic insights in the process of tumor resistance to TKIs and may have clinical applicability.

Cellular mechanisms of MR regulation of adipose tissue physiology and pathophysiology

In addition to the well-documented expression and activity of the mineralocorticoid receptor (MR) in the kidney, in the last decade research on MR has also revealed its important role in regulating functions of extrarenal tissues, including adipose tissue, where MR is involved in adipocyte fundamental processes such as differentiation, autophagy and adipokine secretion. MR expression is increased in adipose tissue of murine models of obesity and in obese human subjects, suggesting that over-activation of the mineralocorticoid signaling leads to dysfunctional adipocyte and associated metabolic disorders. Notably, pharmacological blockade of MR prevents metabolic dysfunctions observed in obese mice and suggests a potential therapeutic use of MR antagonists in the treatment of obesity and metabolic syndrome. However, the molecular pathways affected by MR blockade have been poorly investigated. This review summarizes the functions of MR in the adipocyte, discusses potential signaling pathways mediating MR action, and describes post-translational modifications regulating its activity.

Protein kinase inhibitor γ reciprocally regulates osteoblast and adipocyte differentiation by downregulating leukemia inhibitory factor

The protein kinase inhibitor (Pki) gene family inactivates nuclear protein kinase A (PKA) and terminates PKA-induced gene expression. We previously showed that Pkig is the primary family member expressed in osteoblasts and that Pkig knockdown increases the effects of parathyroid hormone and isoproterenol on PKA activation, gene expression, and inhibition of apoptosis. Here, we determined whether endogenous levels of Pkig regulate osteoblast differentiation. Pkig is the primary family member in murine embryonic fibroblasts (MEFs), murine marrow-derived mesenchymal stem cells, and human mesenchymal stem cells. Pkig deletion increased forskolin-dependent nuclear PKA activation and gene expression and Pkig deletion or knockdown increased osteoblast differentiation. PKA signaling is known to stimulate adipogenesis; however, adipogenesis and osteogenesis are often reciprocally regulated. We found that the reciprocal regulation predominates over the direct effects of PKA since adipogenesis was decreased by Pkig deletion or knockdown. Pkig deletion or knockdown also simultaneously increased osteogenesis and decreased adipogenesis in mixed osteogenic/adipogenic medium. Pkig deletion increased PKA-induced expression of leukemia inhibitory factor (Lif) mRNA and LIF protein. LIF neutralizing antibodies inhibited the effects on osteogenesis and adipogenesis of either Pkig deletion in MEFs or PKIγ knockdown in both murine and human mesenchymal stem cells. Collectively, our results show that endogenous levels of Pkig reciprocally regulate osteoblast and adipocyte differentiation and that this reciprocal regulation is mediated in part by LIF. Stem Cells 2013;31:2789-2799.

Functional characterization of cytochrome P450-derived epoxyeicosatrienoic acids in adipogenesis and obesity

Adipogenesis plays a critical role in the initiation and progression of obesity. Although cytochrome P450 (CYP)-derived epoxyeicosatrienoic acids (EETs) have emerged as a potential therapeutic target for cardiometabolic disease, the functional contribution of EETs to adipogenesis and the pathogenesis of obesity remain poorly understood. Our studies demonstrated that induction of adipogenesis in differentiated 3T3-L1 cells (in vitro) and obesity-associated adipose expansion in high-fat diet (HFD)-fed mice (in vivo) significantly dysregulate the CYP epoxygenase pathway and evoke a marked suppression of adipose-derived EET levels. Subsequent in vitro experiments demonstrated that exogenous EET analog administration elicits potent anti-adipogenic effects via inhibition of the early phase of adipogenesis. Furthermore, EET analog administration to mice significantly mitigated HFD-induced weight gain, adipose tissue expansion, pro-adipogenic gene expression, and glucose intolerance. Collectively, these findings suggest that suppression of EET bioavailability in adipose tissue is a key pathological consequence of obesity, and strategies that promote the protective effects of EETs in adipose tissue offer enormous therapeutic potential for obesity and its downstream pathological consequences.

Possible targets for nonimmunosuppressive therapy of Graves' orbitopathy

CONTEXT

Graves' orbitopathy (GO) is caused by expansion of the orbital contents by excess adipogenesis and overproduction of hyaluronan (HA). Immunosuppressive and antiinflammatory treatments of GO are not always effective and can have side effects, whereas targeting GO-associated tissue remodeling might be a more logical therapeutic strategy. Previously we reported that signaling cascades through IGF1 receptor and thyrotropin receptor within orbital preadipocytes/fibroblasts drove adipogenesis and HA production. Our current study combined the stimulation of IGF1 receptor and thyrotropin receptor increase of HA accumulation, which we hypothesize is by activation of phosphatidylinositol 3-kinase (PI3K)-1A/PI3K1B, respectively. The central aim of this study was to investigate whether PI3K/mammalian target of rapamycin complex 1 (mTORC1) inhibitors affected adipogenesis and/or HA production within orbital preadipocyte/fibroblasts.

METHODS

Human orbital preadipocytes were treated with/without inhibitors, LY294002 (PI3K1A/mTORC1), AS-605240 (PI3K1B), or PI103 (PI3K1A/mTORC1) in serum-free medium for 24 hours or cultured in adipogenic medium for 15 days. Quantitative PCR was used to measure hyaluronan synthases (HAS2) transcripts and the terminal adipogenesis differentiation marker lipoprotein lipase. HA accumulation in the medium was measured by an ELISA.

RESULTS

Unlike AS-605240, both LY294002 (10 μM) and PI-103 (5 μM) significantly decreased HAS2 transcripts/HA accumulation and adipogenesis. Because PI-103 and LY294002 are dual PI3K/mTOR inhibitors, we investigated the inhibition of mTORC1 (rapamycin 100 nM), which significantly decreased adipogenesis but had no effect on HAS2 transcripts/HA, implicating PI3K-1A in the latter.

CONCLUSIONS

The combined inhibition of PI3K1A and mTORC1 signaling in vitro decreased both HA accumulation and adipogenesis. Because PI3K and mTOR inhibitors are clinically used to treat other conditions, they have the potential to be repositioned to be used as an alternative nonimmunosuppressive therapy of GO.

Termination mechanism of CREB-dependent activation of COX-2 expression in early phase of adipogenesis

We elucidated the molecular mechanism of prostaglandin (PG) E2- and PGF2α-mediated suppression of the early phase of adipogenesis through enhanced COX-2 expression in 3T3-L1 cells. 3-Isobutyl-1-methylxanthine, an inhibitor of phosphodiesterase which catalyzes the conversion of cAMP to AMP, enhanced the activity of protein kinase A (PKA). Dibutyryl cAMP activated PKA and enhanced the phosphorylation of cAMP response element (CRE)-binding protein (CREB). The ability of CREB binding to the CRE of the COX-2 promoter was elevated for enhancement of the expression of the COX-2 gene. CREB siRNA suppressed the expression of the COX-2 gene. Furthermore, okadaic acid, a protein phosphatase (PP) 1/2A inhibitor, suppressed the progression of adipogenesis by preventing PP1/2A-mediated suppression of CREB-dependent COX-2 expression, thus resulting in increased production of anti-adipogenic PGE2 and PGF2α. These results indicate that CREB-dependent expression of COX-2 for the production of anti-adipogenic PGs is critical for the regulation of the early phase of adipogenesis.

G protein-coupled receptors and adipogenesis: a focus on adenosine receptors

G-protein coupled receptors (GPCRs) are a large family of proteins that coordinate extracellular signals to produce physiologic outcomes. Adenosine receptors (AR) are one class of GPCRs that have been shown to regulate functions as diverse as inflammation, blood flow, and cellular differentiation. Adenosine signals through four GPCRs that either inhibit (A1AR and A3AR) or activate (A2aAR and A2bAR) adenylyl cyclase. This review will focus on the role of GPCRs, and in particular, adenosine receptors, in adipogenesis. Preadipocytes differentiate to mature adipocytes as the adipose tissue expands to compensate for the consumption of excess nutrients. These newly generated adipocytes contribute to maintaining metabolic homeostasis. Understanding the key drivers of this differentiation process can aid the development of therapeutics to combat the growing obesity epidemic and associated metabolic consequences. Although much literature has covered the transcriptional events that culminate in the formation of an adipocyte, less focus has been on receptor-mediated extracellular signals that direct this process. This review will highlight GPCRs and their downstream messengers as significant players controlling adipocyte differentiation.

Exchange protein activated by cyclic AMP is involved in the regulation of adipogenic genes during 3T3-L1 fibroblasts differentiation

Adipogenesis is stimulated in 3T3-L1 fibroblasts by a combination of insulin, dexamethasone and isobutylmethylxanthine, IBMX, (I+D+M). Two transcription factors are important for the acquisition of the adipocyte phenotype, C/EBP beta (CCAT enhancer-binding protein beta) and PPAR gamma (peroxisome proliferator-activated receptor gamma). IBMX increases cAMP content, which can activate protein kinase A (PKA) and/or EPAC (exchange protein activated by cAMP). To investigate the importance of IBMX in the differentiation mixture, we first evaluated the effect of the addition of IBMX on the increase of C/EBP beta and PPAR gamma and found an enhancement of the amount of both proteins. IBMX addition (I+D+M) or its replacement with a cAMP analogue, dibutyryl-cAMP or 8-(4-chlorophenylthio)-2-O'-methyl-cAMP (8CPT-2-Me-cAMP), the latter activates EPAC and not PKA, remarkably increased PPAR gamma mRNA. However, neither I+D nor any of the inducers alone, increased PPAR gamma mRNA to a similar extent, suggesting the importance of the presence of both IBMX and I+D. It was also found that the addition of IBMX or 8CPT-2-Me-cAMP was able to increase the content of C/EBP beta with respect to I+D. In agreement with these findings, a microarray analysis showed that the presence of either 8CPT-2-Me-cAMP or IBMX in the differentiation mixture was able to upregulate PPAR gamma and PPAR gamma-activated genes as well as other genes involved in lipid metabolism. Our results prove the involvement of IBMX-cAMP-EPAC in the regulation of adipogenic genes during differentiation of 3T3-L1 fibroblasts and therfore contributes to elucidate the role of cyclic AMP in this process.

PKA regulatory subunit R2B is required for murine and human adipocyte differentiation

ADIPOGENESIS IS A COMPLEX PROCESS MODULATED BY SEVERAL FACTORS, INCLUDING CAMP SIGNALING. THE MAIN CAMP TARGET IS PROTEIN KINASE A (PKA), A TETRAMERIC ENZYME WITH FOUR REGULATORY SUBUNITS SHOWING TISSUE-SPECIFIC EXPRESSION AND FUNCTION: PRKAR2B is the main regulatory subunit in adipose tissue in mice and in adult humans. This study aimed to evaluate the expression of PKA regulatory subunits in human adipose tissue during fetal development and to investigate their role in the differentiation of 3T3-L1 and primary human preadipocytes. The expression of PKA regulatory subunits was evaluated in fetal adipose tissue (immunohistochemistry) and in cultured 3T3-L1 and primary human preadipocytes (western blot analysis). Cultured cells were transiently transfected with siRNA against PRKAR2B and induced to differentiate. Differentiation was evaluated by intracellular triglyceride staining (Oil Red O) and expression of molecular markers of adipocyte differentiation. In this study, we found that PRKAR2B is the main regulatory subunit in human adipose tissue during fetal development, from 12 weeks of gestation to the end of gestation, as well as in 3T3-L1 and primary human preadipocytes. The expression of PRKAR2B increases progressively during in vitro differentiation. The silencing of PRKAR2B abolishes the increase in the expression of peroxisome proliferator-activated receptor gamma (PPARγ (PPARG)), fatty acid synthase, aP2 (FABP4), and lipoprotein lipase, as well as intracellular triglyceride accumulation, resulting in impaired adipocyte differentiation in both mouse and human cell systems. In conclusion, PRKAR2B is the key PKA regulatory subunit involved in mouse and human adipose tissue development. The physiological increase in the expression of PRKAR2B is an essential event in adipogenesis in both mice and humans, and it might represent a possible target for future strategies for obesity treatment.

Consecutive positive feedback loops create a bistable switch that controls preadipocyte-to-adipocyte conversion

Adipogenesis, or the conversion of proliferating preadipocytes into nondividing adipocytes, is an important part of the vertebrate weight-maintenance program. It is not yet understood how and when an irreversible transition occurs into a distinct state capable of accumulating lipid. Here, we use single-cell fluorescence imaging to show that an all-or-none switch is induced before lipid accumulation occurs. Conversion begins by glucocorticoid and cAMP signals raising C/EBPβ levels above a critical threshold, triggering three consecutive positive feedback loops: from PPARγ to C/EBPα, then to C/EBPβ, and last to the insulin receptor. Experiments and modeling show that these feedbacks create a robust, irreversible transition to a terminally differentiated state by rejecting short- and low-amplitude stimuli. After the differentiation switch is triggered, insulin controls fat accumulation in a graded fashion. Altogether, our study introduces a regulatory motif that locks cells in a differentiated state by engaging a sequence of positive feedback loops.

Gene expression of mesoderm-specific transcript is upregulated as preadipocytes differentiate to adipocytes in vitro

Mesoderm-specific transcript (Mest) is a distinct gene associated with adipocyte differentiation and proliferation. The mechanisms regulating expression of the Mest gene are not established. Therefore, we investigated Mest gene expression during adipogenic differentiation in murine 3T3-L1 preadipocytes and adipose-derived stromal cells (ADCs) from C57BL/6J mouse adipose tissue. Expression of Mest mRNA increased significantly in 3T3-L1 cells during differentiation. Additionally, Mest mRNA expression levels were additively enhanced by the inhibition of DNA methylation. Expression levels of the Mest gene were also markedly elevated in differentiating ADCs in vitro. Additionally, we showed that Mest mRNA can be upregulated by increasing intracellular cAMP, and that Mest expression is suppressed by inhibition of protein kinase A (PKA). Mest expression was regulated through cAMP-dependent PKA pathways during differentiation of preadipocytes into adipocytes in vitro, supporting the critical role of Mest in proliferation and differentiation of adipocytes.

Diverse effects of cyclic AMP variants on osteogenic and adipogenic differentiation of human mesenchymal stromal cells

Osteogenic differentiation of human mesenchymal stromal cells (hMSCs) may potentially be used in cell-based bone tissue-engineering applications to enhance the bone-forming potential of these cells. Osteogenic differentiation and adipogenic differentiation are thought to be mutually exclusive, and although several signaling pathways and cues that induce osteogenic or adipogenic differentiation, respectively, have been identified, there is no general consensus on how to optimally differentiate hMSCs into the osteogenic lineage. Some pathways have also been reported to be involved in both adipogenic and osteogenic differentiation, as for example, the protein kinase A (PKA) pathway, and the aim of this study was to investigate the role of cAMP/PKA signaling in differentiation of hMSCs in more detail. We show that activation of this pathway with dibutyryl-cAMP results in enhanced alkaline phosphatase expression, whereas another cAMP analog induces adipogenesis in long-term mineralization cultures. Adipogenic differentiation, induced by 8-bromo-cAMP, was accompanied by stronger PKA activity and higher expression of cAMP-responsive genes, suggesting that stronger activation correlates with adipogenic differentiation. In addition, a whole-genome expression analysis showed an increase in expression of adipogenic genes in 8-br-cAMP-treated cells. Furthermore, by means of quantitative polymerase chain reaction, we show differences in peroxisome proliferator-activated receptor-γ activation, either alone or in combination with dexamethasone, thus demonstrating differential effects of the PKA pathway, most likely depending on its mode of activation.

EGF receptor (ERBB1) abundance in adipose tissue is reduced in insulin-resistant and type 2 diabetic women

CONTEXT

Indications of adipose tissue dysfunction correlate with systemic insulin resistance and type 2 diabetes. It has been suggested that a defect in adipose tissue turnover may be involved in the development of these disorders. Whether this dysfunction causes or exacerbates systemic insulin resistance is not fully understood. OBJECTIVES, PARTICIPANTS, AND MEASURES: We tested whether the expression of members of the mitogenic ErbB family was reduced in adipose tissue of insulin-resistant individuals and whether ErbB1 and ErbB2 were involved in adipogenesis. Thirty-two women covering a wide range of body mass index values and insulin sensitivity participated in the cross-sectional portion of this study. We also studied preadipocytes isolated from 12 insulin-sensitive individuals to evaluate the impact of ErbB1 or ErbB2 inhibition on adipogenesis in vitro. For this purpose, we measured phospho-ErbB1 and phospho-ErbB2 levels using ELISA and the expression of peroxisome proliferator-activated receptor γ (PPARγ) and PPARγ-regulated genes by real-time PCR.

RESULTS

Among the ErbB family members, only ErbB1 expression was correlated with insulin sensitivity. Additionally, ErbB1 levels correlated positively with PPARγ and several PPARγ-regulated genes including acyl-coenzyme A synthetase long-chain family member 1 (ACSL1), adiponectin, adipose tissue triacylglycerol lipase (ATGL), diacylglycerol acyl transferase 1 (DGAT1), glycerol-3-phosphate dehydrogenase 1 (GPD1), and lipoprotein lipase (LPL), but negatively with CD36 and fatty acid-binding protein 4 (FABP4). In preadipocyte culture, ErbB1, but not ErbB2, inhibition was associated with a reduction in the expression of all the above-mentioned genes.

CONCLUSIONS

These findings demonstrate a key role for ErbB1 in adipogenesis and suggest that lower ErbB1 protein abundance may lead to adipose tissue dysfunction.

cAMP antagonizes ERK-dependent antiapoptotic action of insulin

Insulin has antiapoptotic activity in various cell types. However, the signaling pathways underlying the antiapoptotic activity of insulin is not yet known. This study was conducted to determine if cAMP affects the antiapoptotic activity of insulin and the activity of PI3K and ERK in CHO cells expressing human insulin receptors (CHO-IR). Insulin-stimulated ERK activity was completely suppressed by cAMP-elevating agents like as pertussis toxin (Ptx) and cholera toxin (Ctx) after 4 h treatment. Insulin-stimulated PKB/Akt activity was not affected at all. Ptx treatment together with insulin increased the number of apoptotic cells and the degree of DNA fragmentation. Ctx or 8-brcAMP treatment also increased the number of apoptotic cells and stimulated the cleavage of caspase-3 and the hydrolysis of PARP. Taken together, cAMP antagonizes the antiapoptotic activity of insulin and the main target molecule of cAMP in this process is likely ERK, not PI3K-dependent PKB/Akt.

CCAAT/enhancer-binding protein-β participates in oxidized LDL-enhanced proliferation in 3T3-L1 cells

Increased circulating oxidized LDL (oxLDL) have been found in obese subjects. Obesity is characterized by an excess of fat mass resulting from an increase in adipocyte number and size. The generation of new adipocytes is a tightly controlled process where multiple factors acting in a signaling cascade follow a precise temporal expression pattern; oxLDL appear to have a role in the impairment of this process. The purpose of this study was to examine the effects of oxLDL on the mechanisms involved in the proliferative stage of the differentiation process in 3T3-L1 cells. After hormonal induction, 3T3-L1 cells undergo approximately two rounds of mitotic clonal expansion (MCE), a process required for adipogenesis. CCAAT/enhancer-binding protein β (C/EBPβ) is immediately expressed after induction, and plays a crucial role in MCE, but its expression must decrease to allow preadipocytes to mature into adipocytes. We found that, in the presence of stimuli to differentiate, oxLDL induced a higher proliferation rate in this cell line, associated with a sustained up-regulation of C/EBPβ, which remained activated inside the nucleus for several days. RNAi-mediated knockdown of C/EBPβ 24 h after oxLDL treatment counteracted the increase in proliferation rate. Both C/EBPβ expression and proliferation processes appear to be influenced by cAMP/protein kinase A (PKA) and extracellular signal-regulated kinases1/2 (ERK1/2) pathways. OxLDL treatment led to increased levels of cAMP, and to a strong, prolonged phosphorylation of ERK1/2 and C/EBPβ. The addition of cAMP and PKA inhibitors, SQ22536 and H-89, respectively, reduced proliferation only in oxLDL-treated cells, whereas the addition of ERK1/2 inhibitor U0126 blocked proliferation in both control and oxLDL-treated cells. C/EBPβ nuclear expression and DNA-binding activity were reduced by U0126, under all tested conditions. These findings show that the altered expression pattern of C/EBPβ is involved in the increase in the number of proliferating cells induced by oxLDL, in hormone-stimulated 3T3-L1 cells.

Chromatin accessibility and transcription factor binding at the PPARγ2 promoter during adipogenesis is protein kinase A-dependent

The nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-activated transcription factor that specifies formation of the adipocyte lineage. PPARγ also serves as a primary target for the treatment of type 2 diabetes, illustrating both its medical relevance as well as the need to understand fundamental aspects of PPARγ expression and function. Here, we characterize molecular changes that occur at the PPARγ2 promoter within the first several hours of adipocyte differentiation in culture. Our results demonstrate that changes in chromatin accessibility at the PPARγ2 promoter and occupancy of the promoter by the c-Fos transcription factor occur within an hour of the onset of differentiation, followed closely by the binding of the CCAAT/enhancer binding protein beta (C/EBPβ) transcription factor. All three events show a remarkable dependency on protein kinase A (PKA) activity. These results reflect novel requirements for the PKA signaling pathway and reinforce the importance of PKA function during the onset of adipocyte differentiation.

Mechanism of increased PLD1 gene expression during early adipocyte differentiation process of mouse cell line 3T3-L1

A mouse cell line 3T3-L1 is differentiated into adipocytes when treated with an inducer cocktail (IDX) (insulin, dexametahsone, and a cAMP phosphodiesterase inhibitor of isobutyl-methylxanthine (IBMX)). Here, we report that PLD1, but not PLD2, mRNA and protein increased during the early differentiation process. Our analysis shows that IDX resulted in a sequential induction of C/EBPbeta, PLD1, and C/EBPalpha which is a key transcription factor of late adipocyte differentiation. Among the three inducers, IBMX + any other inducer induced mild adipocyte differentiation, whereas insulin + dexamethasone did not. IBMX increased PLD1 but not PLD2 mRNA. Forskolin, an adenylate cyclase activator, and dbcAMP also increased PLD1 mRNA, suggesting the cellular cAMP as the inducer of both adipocyte differentiation and PLD1 transcription. We focused on the regulatory mechanism of PLD1 transcription during this differentiation process. IDX or a combination of inducers including IBMX increased PLD1 promoter activity, which is consistent with mRNA analysis. Promoter analysis identified two adjacent C/EBP motifs located between -338 and -231 bp from the first exon as the IBMX responsive elements. Furthermore, overexpression of C/EBPbeta, but not C/EBPalpha, increased PLD1 mRNA and PLD1 5' promoter activity. EMSA and chromatin immunoprecipitation assay confirmed the direct binding of C/EBPbeta, but not C/EBPalpha, to these C/EBP motifs of PLD1 5' promoter. Our results show that PLD1 is a target gene of C/EBPbeta through the increased cellular cAMP during early adipocyte differentiation of 3T3-L1 cells.

Possible role of the exchange protein directly activated by cyclic AMP (Epac) in the cyclic AMP-dependent functional differentiation and syncytialization of human placental BeWo cells

BACKGROUND

The mononuclear villous cytotrophoblast (CTB) differentiates and fuses to the multinucleated syncytiotrophoblast (STB), which produces hCG and progesterone. cAMP-mediated intracellular pathways are involved in the process of endocrine differentiation and fusion (syncytialization). The exchange protein directly activated by cAMP (Epac) is a mediator of cAMP signaling. We examined the differential roles of Epac and protein kinase A (PKA) signaling in the cell fusion and differentiation of trophoblast-derived BeWo cells.

METHODS

Epac1 and Epac2 were localized in human placental tissue (n = 9) by immunohistochemistry. The PKA-selective cAMP analog (N(6)-phenyl-cAMP, Phe) or Epac-selective cAMP analog (CPT) was tested for effects on hCG and progesterone production, and syncytialization in BeWo cells. The effect of knockdown of Epac or its downstream target molecule (Rap1) on syncytialization was evaluated.

RESULTS

Epac1 and Epac2 proteins were expressed in villous CTB, STB, stroma, blood vessels and extravillous CTB of the placenta. Phe increased the expression of hCG alpha/beta mRNA and secretion of hCG protein in BeWo cells (P < 0.01 versus control). CPT-stimulated production of hCG (P < 0.05), albeit to a lesser extent than Phe. Progesterone production was also enhanced by Phe or CPT (P < 0.01 and P < 0.05, respectively). CPT or a stable cAMP analog (dibutyryl-cAMP: Db) increased the number of syncytialized BeWo cells (P < 0.01), whereas Phe did not stimulate fusion. CPT- or Db-induced syncytialization was observed, even in the presence of a PKA inhibitor. Knockdown of Epac1 or Rap1 repressed the Db-, CPT- or forskolin-induced cell fusion.

CONCLUSIONS

The Epac signaling pathway may be associated with the cAMP-mediated functional differentiation and syncytialization of human trophoblasts.