Activation of signal transducer and activator of transcription-3 during proliferative phases of 3T3-L1 adipogenesis

Weighted Gene Co-Expression Network Based on Transcriptomics: Unravelling the Differentiation Dynamics of 3T3-L1 Preadipocytes and the Regulatory Mechanism of Protopanaxatriol

The intricate regulatory mechanisms governing adipocyte differentiation are pivotal in elucidating the complex pathophysiology underlying obesity. This study aims to explore the dynamic changes in gene expression during the differentiation of 3T3-L1 adipocytes using transcriptomics methods. Protopanaxatriol (PPT) significantly inhibited adipocyte differentiation. To uncover the molecular mechanisms, we conducted an extensive transcriptomic analysis of adipocytes throughout various differentiation stages, comparing gene expression profiles before and after PPT treatment. The construction of 16 co-expression modules was achieved using weighted gene co-expression network analysis (WGCNA). The 838 differentially expressed genes in the blue module were highly correlated with PPT treatment. Further analysis revealed that PIKfyve, STAT3, JAK1, CTTN, TYK2, JAK3, STAT2, STAT5b, SOCS3, and IRF9 were core genes closely associated with adipocyte differentiation. This discovery underscores the potential pivotal function of these ten genes in regulating adipocyte differentiation. This study elucidated that PPT, an active ingredient in ginseng, could reduce lipid accumulation by inhibiting the differentiation of adipocyte precursors through the negative regulation of genes such as PIKfyve, STAT3, and JAK1. Finally, molecular docking identified potential binding sites for PPT on PIKfyve and JAK1. This study provides potential drug targets for preventing obesity and related metabolic diseases.

Differential chromatin accessibility and Gene Expression Associated with Backfat Deposition in pigs

BACKGROUND

Backfat serves as a vital fat reservoir in pigs, and its excessive accumulation will adversely impact pig growth performance, farming efficiency, and pork quality. The aim of this research is to integrate assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and RNA sequencing (RNA-seq) to explore the molecular mechanisms underlying porcine backfat deposition.

RESULTS

ATAC-seq analysis identified 568 genes originating from 698 regions exhibiting differential accessibility, which were significantly enriched in pathways pertinent to adipocyte differentiation and lipid metabolism. Besides, a total of 283 transcription factors (TFs) were identified by motif analysis. RNA-seq analysis revealed 978 differentially expressed genes (DEGs), which were enriched in pathways related to energy metabolism, cell cycle and signal transduction. The integration of ATAC-seq and RNA-seq data indicates that DEG expression levels are associated with chromatin accessibility. This comprehensive study highlights the involvement of critical pathways, including the Wnt signaling pathway, Jak-STAT signaling pathway, and fatty acid degradation, in the regulation of backfat deposition. Through rigorous analysis, we identified several candidate genes (LEP, CTBP2, EHHADH, OSMR, TCF7L2, BCL2, FGF1, UCP2, CCND1, TIMP1, and VDR) as potentially significant contributors to backfat deposition. Additionally, we constructed TF-TF and TF-target gene regulatory networks and identified a series of potential TFs related to backfat deposition (FOS, STAT3, SMAD3, and ESR1).

CONCLUSIONS

This study represents the first application of ATAC-seq and RNA-seq, affording a novel perspective into the mechanisms underlying backfat deposition and providing invaluable resources for the enhancement of pig breeding programs.

Epigallocatechin gallate suppresses mitotic clonal expansion and adipogenic differentiation of preadipocytes through impeding JAK2/STAT3-mediated transcriptional cascades

BACKGROUND

Mitotic clonal expansion (MCE) is a prerequisite for preadipocyte differentiation and adipogenesis. Epigallocatechin gallate (EGCG) has been shown to inhibit preadipocyte differentiation. However, the exact molecular mechanisms are still elusive.

PURPOSE

This study investigated whether EGCG could inhibit adipogenesis and lipid accumulation by regulating the cell cycle in the MCE phase of adipogenesis and its underlying molecular mechanisms.

METHOD

3T3-L1 preadipocytes were induced to differentiate by a differentiation cocktail (DMI) and were treated with EGCG (25-100 μM) for 9, 18, and 24 h to examine the effect on MCE, or eight days to examine the effect on terminal differentiation. C57BL/6 mice were fed a high-fat diet (HFD) for three months to induce obesity and were given EGCG (50 or 100 mg/kg) daily by gavage.

RESULTS

We showed that EGCG significantly inhibited terminal adipogenesis and lipid accumulation in 3T3-L1 cells and decreased expressions of PPARγ, C/EBPα, and FASN. Notably, at the MCE phase, EGCG regulated the cell cycle in sequential order, induced G0/G1 arrest at 18 h, and inhibited the G2/M phase at 24 h upon DMI treatment. Meanwhile, EGCG regulated the expressions of cell cycle regulators (cyclin D1, cyclin E1, CDK4, CDK6, cyclin B1, cyclin B2, p16, and p27), and decreased C/EBPβ, PPARγ, and C/EBPα expressions at MCE. Mechanistic studies using STAT3 agonist Colivelin and antagonist C188-9 revealed that EGCG-induced cell cycle arrest in the MCE phase and terminal adipocyte differentiation was mediated by the inhibition of JAK2/STAT3 signaling cascades and STAT3 (Tyr705) nuclear translocation. Furthermore, EGCG significantly protected mice from HFD-induced obesity, reduced body weight and lipid accumulations in adipose tissues, reduced hyperlipidemia and leptin levels, and improved glucose intolerance and insulin sensitivity. Moreover, RNA sequencing (RNA-seq) analysis showed that the cell cycle changes in epididymal white adipose tissue (eWAT) were significantly enriched upon EGCG treatment. We further verified that EGCG treatment significantly reduced expressions of adipogenic factors, cell cycle regulators, and p-STAT3 in eWAT.

CONCLUSION

EGCG inhibits MCE, resulting in the inhibition of early and terminal adipocyte differentiation and lipid accumulation, which were mediated by inhibiting p-STAT3 nucleus translocation and activation.

Effect of Lauric vs. Oleic Acid-Enriched Diets on Leptin Autoparacrine Signalling in Male Mice

High-fat diets enriched with lauric acid (SOLF) do not enhance leptin production despite expanding white adipose tissue (WAT). Our study aimed at identifying the influence of SOLF vs. oleic acid-enriched diets (UOLF) on the autoparacrine effect of leptin and was carried out on eight-week-old mice consuming control chow, UOLF or SOLF. Phosphorylation of kinases integral to leptin receptor (LepR) signalling pathways (705Tyr-STAT3, 473Ser-Akt, 172Thr-AMPK), adipocyte-size distribution, fatty acid content, and gene expression were analyzed in WAT. SOLF enhanced basal levels of phosphorylated proteins but reduced the ability of leptin to enhance kinase phosphorylation. In contrast, UOLF failed to increase basal levels of phosphorylated proteins and did not modify the effect of leptin. Both SOLF and UOLF similarly affected adipocyte-size distribution, and the expression of genes related with adipogenesis and inflammation. WAT composition was different between groups, with SOLF samples mostly containing palmitic, myristic and lauric acids (>48% w/w) and UOLF WAT containing more than 80% (w/w) of oleic acid. In conclusion, SOLF appears to be more detrimental than UOLF to the autoparacrine leptin actions, which may have an impact on WAT inflammation. The effect of SOLF and UOLF on WAT composition may affect WAT biophysical properties, which are able to condition LepR signaling.

Role of lncRNAs into Mesenchymal Stromal Cell Differentiation

Currently, findings support that 75% of the human genome is actively transcribed, but only 2% is translated into a protein, according to databases such as ENCODE (Encyclopedia of DNA Elements) [1]. The development of high-throughput sequencing technologies, computational methods for genome assembly and biological models have led to the realization of the importance of the previously unconsidered non-coding fraction of the genome. Along with this, noncoding RNAs have been shown to be epigenetic, transcriptional and post-transcriptional regulators in a large number of cellular processes [2]. Within the group of non-coding RNAs, lncRNAs represent a fascinating field of study, given the functional versatility in their mode of action on their molecular targets. In recent years, there has been an interest in learning about lncRNAs in MSC differentiation. The aim of this review is to address the signaling mechanisms where lncRNAs are involved, emphasizing their role in either stimulating or inhibiting the transition to differentiated cell. Specifically, the main types of MSC differentiation are discussed: myogenesis, osteogenesis, adipogenesis and chondrogenesis. The description of increasingly new lncRNAs reinforces their role as players in the well-studied field of MSC differentiation, allowing a step towards a better understanding of their biology and their potential application in the clinic.

STAT3 Is the Master Regulator for the Forming of 3D Spheroids of 3T3-L1 Preadipocytes

To elucidate the currently unknown mechanisms responsible for the diverse biological aspects between two-dimensional (2D) and three-dimensional (3D) cultured 3T3-L1 preadipocytes, RNA-sequencing analyses were performed. During a 7-day culture period, 2D- and 3D-cultured 3T3-L1 cells were subjected to lipid staining by BODIPY, qPCR for adipogenesis related genes, including peroxisome proliferator-activated receptor γ (Pparγ), CCAAT/enhancer-binding protein alpha (Cebpa), Ap2 (fatty acid-binding protein 4; Fabp4), leptin, and AdipoQ (adiponectin), and RNA-sequencing analysis. Differentially expressed genes (DEGs) were detected by next-generation RNA sequencing (RNA-seq) and validated by a quantitative reverse transcription–polymerase chain reaction (qRT–PCR). Bioinformatic analyses were performed on DEGs using a Gene Ontology (GO) enrichment analysis and an Ingenuity Pathway Analysis (IPA). Significant spontaneous adipogenesis was observed in 3D 3T3-L1 spheroids, but not in 2D-cultured cells. The mRNA expression of Pparγ, Cebpa, and Ap2 among the five genes tested were significantly higher in 3D spheroids than in 2D-cultured cells, thus providing support for this conclusion. RNA analysis demonstrated that a total of 826 upregulated and 725 downregulated genes were identified as DEGs. GO enrichment analysis and IPA found 50 possible upstream regulators, and among these, 6 regulators—transforming growth factor β1 (TGFβ1), signal transducer and activator of transcription 3 (STAT3), interleukin 6 (IL6), angiotensinogen (AGT), FOS, and MYC—were, in fact, significantly upregulated. Further analyses of these regulators by causal networks of the top 14 predicted diseases and functions networks (IPA network score indicated more than 30), suggesting that STAT3 was the most critical upstream regulator. The findings presented herein suggest that STAT3 has a critical role in regulating the unique biological properties of 3D spheroids that are produced from 3T3-L1 preadipocytes.

Stimulatory Effect of Tofacitinib on Bone Marrow Adipocytes Differentiation

BACKGROUND

Systemic inflammation is the main factor underlying secondary osteoporosis in patients with rheumatoid arthritis (RA). Janus kinase inhibitors (JAKi), such as tofacitinib (Tofa), can control systemic inflammation and may have beneficial effects on bone in various models. This might be due to direct effects on the bone microenvironment and not exclusively based on their anti-inflammatory function. Bone marrow adipocytes (BMAds) are abundant in the bone microenvironment. The effect of JAKi on BMAds is unknown, but evidence suggests that there is competition between human bone marrow-derived stromal cell (hBMSC) differentiation routes towards BMAds and osteoblasts (Ob) in osteoporosis.

OBJECTIVES

The aims of the study are to determine whether Tofa influences BMAds and Ob derived from hBMSCs and to investigate the potential effects of Tofa on bone marrow adiposity in RA patients.

METHODS

To determine the effect of Tofa on cellular commitment, hBMSCs were differentiated to BMAds or OBs for 3 days together with Tofa at 200, 400, or 800 nM and TNFα. This study was also conducted using differentiated BMAds. The impact of Tofa was determined by gene and protein expression analysis and cell density monitoring. In parallel, in a pilot study of 9 RA patients treated with Tofa 5 mg twice a day (NCT04175886), the proton density fat fraction (PDFF) was measured using MRI at the lumbar spine at baseline and at 6 months.

RESULTS

In non-inflammatory conditions, the gene expression of Runx2 and Dlx5 decreased in Ob treated with Tofa (p <0.05). The gene expression of PPARγ2, C/EBPα, and Perilipin 1 were increased compared to controls (p <0.05) in BMAds treated with Tofa. Under inflammatory conditions, Tofa did not change the expression profiles of Ob compared to TNFα controls. In contrast, Tofa limited the negative effect of TNFα on BMAd differentiation (p <0.05). An increase in the density of differentiated BMAds treated with Tofa under TNFα was noted (p <0.001). These findings were consolidated by an increase in PDFF at 6 months of treatment with Tofa in RA patients (46.3 ± 7.0% versus 53.2 ± 9.2% p <0.01).

CONCLUSION

Together, these results suggest a stimulatory effect of Tofa on BMAd commitment and differentiation, which does not support a positive effect of Tofa on bone.

Tofacitinib Blocks Entheseal Lymphocyte Activation and Modulates MSC Adipogenesis, but Does Not Directly Affect Chondro- and Osteogenesis

Entheseal spinal inflammation and new bone formation with progressive ankylosis may occur in ankylosing spondylitis (AS) and psoriatic arthritis (PsA). This study evaluated whether JAK inhibition with tofacitinib modulated the key disease associated cytokines, TNF and IL-17A, and whether tofacitinib also modulated bone marrow stromal cell-derived mesenchymal stem cell (MSCs) function, including osteogenesis, since post inflammation new bone formation occurs under these conditions. Methods: Conventional entheseal derived αβ CD4+ and CD8+ T-cells were investigated following anti-CD3/CD28 bead stimulation to determine IL-17A and TNF levels in tofacitinib treated (1000 nM) peri-entheseal bone (PEB) and peripheral blood mononuclear cells (PBMC) using ELISA. Bone marrow stromal cell-derived mesenchymal stem cell (MSC) colony forming units (CFU-F) and multi-lineage potential were evaluated using tofacitinib (dosages ranging between 100, 500, 1000 and 10,000 nM). Results: Induced IL-17A and TNF cytokine production from both entheseal CD4+ T-cells and CD8+ T-cells was effectively inhibited by tofacitinib. Tofacitinib treatment did not impact on CFU-F potential or in vitro chondro- and osteogenesis. However, tofacitinib stimulation increased MSC adipogenic potential with greater Oil Red O stained areas. Conclusion: Inducible IL-17A and TNF production by healthy human entheseal CD4+ and CD8+ T-cells was robustly inhibited in vitro by tofacitinib. However, tofacitinib did not impact MSC osteogenesis, but stimulated in vitro MSC adipogenesis, the relevance of which needs further evaluation given that the adipocytes are associated with new bone formation in SpA.

Torreya nucifera seed oil improves 3T3-L1 adipocyte differentiation

BACKGROUND

Adipose tissue is a critical regulator of lipid storage and endocrine function. Impairment of the recruitment of new adipocytes in the adipose tissue is associated with ectopic fat accumulation, diabetes and insulin resistance. Torreya nucifera, an evergreen conifer that grows in warm temperate climates, has been found to exert beneficial effects against inflammation, infection and diabetes. However, the molecular mechanisms responsible for these effects at the cellular level remain unknown. This study aimed to investigate effects of Torreya nucifera seed oil (TNSO) on 3T3-L1 adipocyte differentiation and its underlying regulatory mechanism.

METHODS

To investigate the effects of TNSO on adipocyte differentiation, 3T3-L1 cells were induced to differentiate for 5 days in the presence of 0.75 μL/mL TNSO. Oil Red O staining and an assay for intracellular triglyceride were performed to determine the extent of lipid accumulation in 3T3-L1 cells. To elucidate the underlying mechanism of TNSO, adipogenic gene expression was analyzed using quantitative real-time PCR. Moreover, we monitored TNSO-derived activation of PPARγ and STAT3 with 3T3-L1 reporter cell lines engineered to secrete Gaussia luciferase upon the interaction of a transcription factor to its DNA binding element.

RESULTS

Oil Red O staining revealed that TNSO improved the differentiation of 3T3-L1 preadipocytes into mature adipocytes. The mRNA levels of adipogenic genes, including adiponectin, fatty acid synthase (FAS) and adipocyte fatty acid-binding protein (FABP4), were upregulated and intracellular triglyceride levels increased upon TNSO treatment. We also established that adipocyte differentiation was improved by TNSO-derived activation of PPARγ and STAT3.

CONCLUSIONS

Our results suggest that TNSO improves adipocyte differentiation by regulating the activation of adipogenic transcription factors, indicating that it may serve as a potential treatment strategy for adipocyte dysfunction.

Effects of 4-nonylphenol on adipogenesis in 3T3-L1 preadipocytes and C3H/10T1/2 mesenchymal stem cells

Obesogens are a subset of endocrine disruptor chemicals (EDCs) that cause obesity. The typical EDC 4-nonylphenol (4-NP) has been identified as an obesogen. However, the in vitro effects of 4-NP on adipogenesis remain unclear. In this study, 3T3-L1 preadipocytes and C3H/10T1/2 mesenchymal stem cells (MSCs) were used to investigate the influence of 4-NP on adipogenesis. The differentiation protocols for 3T3-L1 preadipocytes and C3H/10T1/2 MSCs took 8 and 12 days, respectively, beginning at Day 0. In differentiated 3T3-L1 preadipocytes, 20 μM 4-NP decreased cell viability on Days 4 and 8. Exposure to 4-NP inhibited triglyceride (TG) accumulation and adipogenic marker expression on Days 0-8, but the inhibitory effects were weaker on Days 2-8. The protein expression of pSTAT3 or STAT3 decreased on Days 0-8 and 2-8. Conversely, 4-NP promoted TG accumulation and the adipogenic marker expression in C3H/10T1/2 adipocytes. The opposing effects were attributed to physiological differences between the two cell lines. The 3T3-L1 preadipocytes are dependent on mitotic clonal expansion (MCE) to drive differentiation, while C3H/10T1/2MSCs and human preadipocytes are not. Additionally, 4-NP downregulated β-catenin expression in C3H/10T1/2 adipocytes. Accordingly, we hypothesized that 4-NP promotes adipogenesis. The role of the canonical Wnt pathway in the promotion of adipogenesis by 4-NP requires further validation. This study provides new insights into the mechanisms and appropriate risk management of 4-NP.

Garcinia cambogia attenuates adipogenesis by affecting CEBPB and SQSTM1/p62-mediated selective autophagic degradation of KLF3 through RPS6KA1 and STAT3 suppression

The overexpansion of adipose tissues leads to obesity and eventually results in metabolic disorders. Garcinia cambogia (G. cambogia) has been used as an antiobesity supplement. However, the molecular mechanisms underlying the effects of G. cambogia on cellular processes have yet to be fully understood. Here, we discovered that G. cambogia attenuated the expression of CEBPB (CCAAT/enhancer binding protein (C/EBP), beta), an important adipogenic factor, suppressing its transcription in differentiated cells. In addition, G. cambogia inhibited macroautophagic/autophagic flux by decreasing autophagy-related gene expression and autophagosome formation. Notably, G. cambogia markedly elevated the expression of KLF3 (Kruppel-like factor 3 (basic)), a negative regulator of adipogenesis, by reducing SQSTM1/p62-mediated selective autophagic degradation. Furthermore, increased KLF3 induced by G. cambogia interacted with CTBP2 (C-terminal binding protein 2) to form a transcriptional repressor complex and inhibited Cebpa and Pparg transcription. Importantly, we found that RPS6KA1 and STAT3 were involved in the G. cambogia-mediated regulation of CEBPB and autophagic flux. In an obese animal model, G. cambogia reduced high-fat diet (HFD)-induced obesity by suppressing epididymal and inguinal subcutaneous white adipose tissue mass and adipocyte size, which were attributed to the regulation of targets that had been consistently identified in vitro. These findings provide new insight into the mechanism of G. cambogia-mediated regulation of adipogenesis and suggest molecular links to therapeutic targets for the treatment of obesity.

Control of adipogenic commitment by a STAT3-VSTM2A axis

White adipose tissue (WAT) is a dynamic organ that plays crucial roles in controlling metabolic homeostasis. During development and periods of energy excess, adipose progenitors are recruited and differentiate into adipocytes to promote lipid storage capability. The identity of adipose progenitors and the signals that promote their recruitment are still incompletely characterized. We have recently identified V-set and transmembrane domain-containing protein 2A (VSTM2A) as a novel protein enriched in preadipocytes that amplifies adipogenic commitment. Despite the emerging role of VSTM2A in promoting adipogenesis, the molecular mechanisms regulating Vstm2a expression in preadipocytes are still unknown. To define the molecular mechanisms controlling Vstm2a expression, we have treated preadipocytes with an array of compounds capable of modulating established regulators of adipogenesis. Here, we report that Vstm2a expression is positively regulated by PI3K/mTOR and cAMP-dependent signaling pathways and repressed by the MAPK pathway and the glucocorticoid receptor. By integrating the impact of all the molecules tested, we identified signal transducer and activator of transcription 3 (STAT3) as a novel downstream transcription factor affecting Vstm2a expression. We show that activation of STAT3 increased Vstm2a expression, whereas its inhibition repressed this process. In mice, we found that STAT3 phosphorylation is elevated in the early phases of WAT development, an effect that strongly associates with Vstm2a expression. Our findings identify STAT3 as a key transcription factor regulating Vstm2a expression in preadipocytes.NEW & NOTEWORTHY cAMP-dependent and PI3K-mTOR signaling pathways promote the expression of Vstm2a. STAT3 is a key transcription factor that controls Vstm2a expression in preadipocytes. STAT3 is activated in the early phases of WAT development, an effect that strongly associates with Vstm2a expression.

Garcinia cambogia suppresses adipogenesis in 3T3-L1 cells by inhibiting p90RSK and Stat3 activation during mitotic clonal expansion

Obesity is associated with an increase in adipose tissue, which is mediated by hyperplasia and hypertrophy. Therefore, inhibiting cell proliferation during mitotic clonal expansion (MCE) is one of the major strategies for preventing obesity. The antagonistic effects of Garcinia cambogia (G. cambogia) on obesity have been studied in animal experimental models. However, the effects of G. cambogia extract on MCE, and the underlying molecular mechanisms, are poorly understood. In this study, 3T3-L1 cells were used to investigate whether G. cambogia extract affected cell proliferation during MCE and to identify target molecules for any anti-adipogenic activity. G. cambogia extract suppressed isobutylmethylxanthine and dexamethasone-and-insulin (MDI)-induced adipogenesis at an early stage by attenuating MCE. In G. cambogia extract-treated preadipocytes, MDI-induced cell proliferation and cell cycle progression were inhibited by G₀ /G₁ arrest due to an increase in p21 and p27 expression, and inhibition of cyclin-dependent kinase 2, cyclin E1 expression, and retinoblastoma (Rb) phosphorylation. In addition, the MDI-induced phosphorylation and subsequent translocation into the nucleus of p90 ribosomal S6 kinase (p90RSK) and signal transducer and activator of transcription (Stat) 3 were suppressed. Specific inhibitors of p90RSK (FMK) and Stat3 (stattic) regulated cell proliferation and adipogenesis. In conclusion, this study demonstrated that G. cambogia extract inhibited MCE by regulating p90RSK, Stat3, and cell cycle proteins, leading to G₀ /G₁ arrest. These findings provide new insight into the mechanism by which G. cambogia suppresses adipocyte differentiation and show that p90RSK is critical for adipogenesis as a new molecular target.

Vanillic Acid Improves Comorbidity of Cancer and Obesity through STAT3 Regulation in High-Fat-Diet-Induced Obese and B16BL6 Melanoma-Injected Mice

Obesity is known to be associated with risk and aggressiveness of cancer. Melanoma, the most lethal type of skin cancer, is also closely related to the prevalence of obesity. In this study, we established a cancer–obesity comorbidity (COC) model to investigate the effects of vanillic acid (VA). After a five-week administration with a high-fat diet (HFD) to induce obesity, subcutaneous allograft of B16BL6 cells were followed, and VA was orally administrated for an additional two weeks. VA-fed mice showed significantly decreased body weight and white adipose tissue (WAT) weight, which were due to increased thermogenesis and AMPK activation in WATs. Growth of cancer was also suppressed. Mechanistic studies revealed increased apoptosis and autophagy markers by VA; however, caspase 3 was not involved. Since signal transducer and activator of transcription 3 (STAT3) is suggested as an important pathway linking obesity and cancer, we further investigated to find out if STAT3 phosphorylation was repressed by VA treatment, and this was again confirmed in a COC cell model of adipocyte conditioned medium-treated B16BL6 melanoma cells. Overall, our results show VA induces STAT3-mediated autophagy to inhibit cancer growth and thermogenesis to ameliorate obesity in COC. Based on these findings, we suggest VA as a candidate therapeutic agent for COC treatment.

Wu-Mei-Wan prevents high-fat diet-induced obesity by reducing white adipose tissue and enhancing brown adipose tissue function

BACKGROUND

Wu-Mei-Wan, a classic traditional Chinese herb medicine, is one of the most important formulations to treat digestive diseases from ancient times to the present. Our previous study showed that WMW treatment can prevent T2DM in db/db mice, which motivating the application of WMW on metabolic disorders.

PURPOSE

Obesity and its comorbid diseases have increased dramatically and are now a worldwide health problem. There is still a lack of satisfactory treatment strategies for obesity. This work was designed to assess the effect and related mechanism of WMW on high fat diet (HFD)-induced obese mice model.

METHODS

Obese mice were induced by HFD. Thetherapeutic effect of WMW were analyzed by examining body and adipose tissue weight, metabolic profile and energy expenditure. Adipose tissue phenotype was determined by histological staining and the mitochondrial content was examined by transmission electron microscopy (TEM). Immunohistochemical and immunofluorescence staining, RT-qPCR and Western blot analysis were used to evaluate expression of key molecules in adipose tissue.

RESULTS

WMW treatment significantly protects HFD-induced obesity. Here we showed that WMW limits weight gain, improves metabolic profile and increases energy expenditure. WMW inhibits the hypertrophy and hyperplasia of white adipocytes, the mechanism involving the inhibition of TLR3/IL-6/JAK1/STAT3 pathway. In brown adipose tissue (BAT), WMW promotes thermogenicprogramme without affecting cell proliferation. The activated BMP7/ Smad1/5/9 pathway is considered to be one of the explanations for the effect of WMW on BAT.

CONCLUSION

Our results suggested that WMW can prevent obesity and its underlying mechanisms are associated with reducing white adipose tissue and enhancing brown adipose tissue function.

m6A methylation modulates adipogenesis through JAK2-STAT3-C/EBPβ signaling

N⁶-methyladenosine (m⁶A), the most abundant internal mRNA modification in eukaryotes, plays a vital role in regulating adipogenesis. However, its underlying mechanism remains largely unknown. Here, we reveal that deletion of m⁶A demethylase FTO in porcine and mouse preadipocytes inhibits adipogenesis through JAK2-STAT3-C/EBPβ signaling. Mechanistically, FTO deficiency suppresses JAK2 expression and STAT3 phosphorylation, leading to attenuated transcription of C/EBPβ, which is essential for the early stage of adipocyte differentiation. Using dual-luciferase assay, we validate that knockdown of FTO reduces expression of JAK2 in an m⁶A-dependent manner. Furthermore, we find that m⁶A "reader" protein YTHDF2 directly targets m⁶A-modified transcripts of JAK2 and accelerates mRNA decay, which results in decreased JAK2 expression and inactivated JAK2-STAT3-C/EBPβ signaling, thereby inhibiting adipogenesis. Collectively, our results provide a novel insight into the molecular mechanism of m⁶A methylation in post-transcriptional regulation of JAK2-STAT3-C/EBPβ signaling axis and highlight the crucial role of m⁶A modification and its modulators in adipogenesis.

STAT3 suppresses Wnt/β-catenin signaling during the induction phase of primary Myf5+ brown adipogenesis

Thermogenic fat is a promising target for new therapies in diabetes and obesity. Understanding how thermogenic fat develops is important to develop rational strategies to treat obesity. Previously, we have shown that Tyk2 and STAT3, part of the JAK-STAT pathway, are necessary for proper development of classical brown fat. Using primary preadipocytes isolated from newborn mice we demonstrate that STAT3 is required for differentiation and robust expression of Uncoupling Protein 1 (UCP1). We also confirm that STAT3 is necessary during the early induction stage of differentiation and is dispensable during the later terminal differentiation stage. The inability of STAT3-/- preadipocytes to differentiate can be rescued using Wnt ligand secretion inhibitors when applied during the induction stage. Through chemical inhibition and RNAi, we show that it is the canonical β-catenin pathway that is responsible for the block in differentiation; inhibition or knockdown of β-catenin can fully rescue adipogenesis and UCP1 expression in the STAT3-/- adipocytes. During the induction stage, Wnts 1, 3a, and 10b have increased expression in the STAT3-/- adipocytes, potentially explaining the increased levels and activity of β-catenin. Our results for the first time point towards an interaction between the JAK/STAT pathway and the Wnt/β-catenin pathway during the early stages of in-vitro adipogenesis.

The fruits of Gleditsia sinensis Lam. inhibits adipogenesis through modulation of mitotic clonal expansion and STAT3 activation in 3T3-L1 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Gleditsia sinensis Lam. (G. sinensis) has been used in Oriental medicine for tumor, thrombosis, inflammation-related disease, and obesity.

AIM OF THE STUDY

The pharmacological inhibitory effects of fruits of G. sinensis (GFE) on hyperlipidemia have been reported, but its inhibitory effects on adipogenesis and underlying mechanisms have not been elucidated. Herein we evaluated the anti-adipogenic effects of GFE and described the underlying mechanisms.

MATERIALS AND METHODS

The effects of ethanol extracts of GFE on adipocyte differentiation were examined in 3T3-L1 cells using biochemical and molecular analyses.

RESULTS

During the differentiation of 3T3-L1 cells, GFE significantly reduced lipid accumulation and downregulated master adipogenic transcription factors, including CCAAT/enhancer-binding protein-α and peroxisome proliferator-activated receptor-γ, at mRNA and protein levels. These changes led to the suppression of several adipogenic-specific genes and proteins, including fatty acid synthase, sterol regulatory element-binding protein 1, stearoyl-CoA desaturase-1, and acetyl CoA carboxylase. However, the inhibitory effects of GFE on lipogenesis were only shown when GFE is treated in the early stage of adipogenesis within the first two days of differentiation. As a potential mechanism, during the early stages of differentiation, GFE inhibited cell proliferation by a decrease in the expression of DNA synthesis-related proteins and increased p27 expression and suppressed signal transducer and activator of transcription 3 (STAT3) activation induced in a differentiation medium.

CONCLUSIONS

GFE inhibits lipogenesis by negative regulation of adipogenic transcription factors, which is associated with GFE-mediated cell cycle arrest and STAT3 inhibition.

Transcriptional Regulation of Adipogenesis

Adipocytes are the defining cell type of adipose tissue. Once considered a passive participant in energy storage, adipose tissue is now recognized as a dynamic organ that contributes to several important physiological processes, such as lipid metabolism, systemic energy homeostasis, and whole-body insulin sensitivity. Therefore, understanding the mechanisms involved in its development and function is of great importance. Adipocyte differentiation is a highly orchestrated process which can vary between different fat depots as well as between the sexes. While hormones, miRNAs, cytoskeletal proteins, and many other effectors can modulate adipocyte development, the best understood regulators of adipogenesis are the transcription factors that inhibit or promote this process. Ectopic expression and knockdown approaches in cultured cells have been widely used to understand the contribution of transcription factors to adipocyte development, providing a basis for more sophisticated in vivo strategies to examine adipogenesis. To date, over two dozen transcription factors have been shown to play important roles in adipocyte development. These transcription factors belong to several families with many different DNA-binding domains. While peroxisome proliferator-activated receptor gamma (PPARγ) is undoubtedly the most important transcriptional modulator of adipocyte development in all types of adipose tissue, members of the CCAAT/enhancer-binding protein, Krüppel-like transcription factor, signal transducer and activator of transcription, GATA, early B cell factor, and interferon-regulatory factor families also regulate adipogenesis. The importance of PPARγ activity is underscored by several covalent modifications that modulate its activity and its ability to modulate adipocyte development. This review will primarily focus on the transcriptional control of adipogenesis in white fat cells and on the mechanisms involved in this fine-tuned developmental process. © 2017 American Physiological Society. Compr Physiol 7:635-674, 2017.

Mitochondrial STAT3 and reactive oxygen species: A fulcrum of adipogenesis?

The balance between cellular lineages can be controlled by reactive oxygen species (ROS). Cellular differentiation into adipocytes is highly dependent on the production of ROS to initiate the process through activation of multiple interlinked factors that stimulate mitotic clonal expansion and cellular maturation. The signal transducer and activator of transcription family of signaling proteins have accepted roles in adipogenesis and associated lipogenesis. Non-canonical mitochondrial localization of STAT3 and other members of the STAT family however opens up new avenues for investigation of its role in the aforementioned processes. Following recent observations of differences in mitochondrially localized serine 727 phosphorylated STAT3 (mtSTAT3-pS727) in preadipocytes and adipocytes, here, we hypothesize and speculate further on the role of mitochondrial STAT3 in adipogenesis.

The regulation of oxytocin receptor gene expression during adipogenesis

Although it has been reported that oxytocin stimulates lipolysis in adipocytes, changes in the expression of oxytocin receptor (OTR) mRNA in adipogenesis are still unknown. The present study aimed to investigate the expression of OTR mRNA during adipocyte differentiation and fat accumulation in adipocytes. OTR mRNA was highly expressed in adipocytes prepared from mouse adipose tissues compared to stromal-vascular cells. OTR mRNA expression was increased during the adipocyte differentiation of 3T3-L1 cells. OTR expression levels were higher in subcutaneous and epididymal adipose tissues of 14-week-old male mice compared to 7-week-old male mice. Levels of OTR mRNA expression were higher in adipose tissues at four different sites of mice fed a high-fat diet than in those of mice fed a normal diet. The OTR expression level was also increased by refeeding for 4 h after fasting for 16 h. Oxytocin significantly induced lipolysis in 3T3-L1 adipocytes. In conclusion, a new regulatory mechanism is demonstrated for oxytocin to control the differentiation and fat accumulation in adipocytes via activation of OTR as a part of the hypothalamic-pituitary-adipose axis.

Identification of STAT target genes in adipocytes

Adipocytes play important roles in lipid storage, energy homeostasis and whole body insulin sensitivity. Studies in the last two decades have identified the hormones and cytokines that activate specific STATs in adipocytes in vitro and in vivo. Five of the seven STAT family members are expressed in adipocyte (STATs 1, 3, 5A, 5B and 6). Many transcription factors, including STATs, have been shown to play an important role in adipose tissue development and function. This review will summarize the importance of adipocytes, indicate the cytokines and hormones that utilize the JAK-STAT signaling pathway in fat cells and focus on the identification of STAT target genes in mature adipocytes. To date, specific target genes have been identified for STATs, 1, 5A and 5B, but not for STATs 3 and 6.

Interleukin-4 regulates lipid metabolism by inhibiting adipogenesis and promoting lipolysis

Long-term cytokine-mediated inflammation is a risk factor for obesity and type 2 diabetes mellitus (T2DM). Our previous studies reveal significant associations between promoter single nucleotide polymorphisms (SNPs) of interleukin (IL)-4 and T2DM, as well as between SNPs in genes encoding IL-4/IL-4 receptor and high density lipoproteins. Our animal study reveals that IL-4 regulates glucose/lipid metabolism by promoting glucose tolerance and inhibiting lipid deposits. The above results strongly suggest the involvement of IL-4 in energy homeostasis. In the present study, we focus on examining the regulatory mechanism of IL-4 to lipid metabolism. Our results show that IL-4 inhibits adipogenesis by downregulating the expression of peroxisome proliferator-activated receptor-γ and CCAAT/enhancer-binding protein-α. Additionally, IL-4 promotes lipolysis by enhancing the activity and translocation of hormone sensitive lipase (HSL) in mature adipocytes, which suggests that IL-4 plays a pro-lipolytic role in lipid metabolism by boosting HSL activity. Our results demonstrate that IL-4 harbors pro-lipolysis capacity by inhibiting adipocyte differentiation and lipid accumulation as well as by promoting lipolysis in mature adipocytes to decrease lipid deposits. The above findings uncover the novel roles of IL-4 in lipid metabolism and provide new insights into the interactions among cytokine/immune responses, insulin sensitivity, and metabolism.

The role of JAK-STAT signaling in adipose tissue function

Adipocytes play important roles in lipid storage, energy homeostasis and whole body insulin sensitivity. The JAK-STAT (Janus Kinase-Signal Transducer and Activator of Transcription) pathway mediates a variety of physiological processes including development, hematopoiesis, and inflammation. Although the JAK-STAT signaling pathway occurs in all cells, this pathway can mediate cell specific responses. Studies in the last two decades have identified hormones and cytokines that activate the JAK-STAT signaling pathway. These cytokines and hormones have profound effects on adipocytes. The content of this review will introduce the types of adipocytes and immune cells that make up adipose tissue, the impact of obesity on adipose cellular composition and function, and the general constituents of the JAK-STAT pathway and how its activators regulate adipose tissue development and physiology. A summary of the identification of STAT target genes in adipocytes reveals how these transcription factors impact various areas of adipocyte metabolism including insulin action, modulation of lipid stores, and glucose homeostasis. Lastly, we will evaluate exciting new data linking the JAK-STAT pathway and brown adipose tissue and consider the future outlook in this area of investigation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Dimethylfumarate suppresses adipogenic differentiation in 3T3-L1 preadipocytes through inhibition of STAT3 activity

The excessive accumulation of adipocytes contributes to the development of obesity and obesity-related diseases. The interactions of several transcription factors, such as C/EBPβ, PPARγ, C/EBPα, Nrf2, and STAT3, are required for adipogenic differentiation. Dimethylfumarate (DMF), an immune modulator and antioxidant, may function as an inhibitor of STAT3 and an activator of Nrf2. This study examined whether DMF inhibits adipogenic differentiation of 3T3-L1 preadipocytes by inhibiting STAT3 or activating Nrf2. DMF suppressed 3T3-L1 preadipocyte differentiation to mature adipocytes in a dose-dependent manner as determined by Oil Red O staining. The mRNA and protein levels of adipogenic genes, including C/EBPβ, C/EBPα, PPARγ, SREBP-1c, FAS, and aP2, were significantly lower in DMF-treated 3T3-L1 preadipocytes. Suppression of adipogenic differentiation by DMF treatment resulted primarily from inhibition of the early stages of differentiation. DMF inhibits clonal expansion during adipogenic differentiation through induction of a G1 cell cycle arrest. Additionally, DMF regulates cell cycle-related proteins, such as p21, pRb, and cyclin D. DMF treatment markedly inhibited differentiation medium-induced STAT3 phosphorylation and inhibited STAT3 transcriptional activation of a reporter construct composed of four synthetic STAT3-response elements. Moreover, inhibition of endogenous Nrf2 activity using a dominant negative Nrf2 did not abolish the DMF-induced inhibition of adipogenic differentiation of 3T3-L1 preadipocytes. In summary, DMF is a negative regulator of adipogenic differentiation based on its regulation of adipogenic transcription factors and cell cycle proteins. This negative regulation by DMF is mediated by STAT3 inhibition, but is unlikely to involve Nrf2 activation.

Adipogenesis

Adipose tissue is an important site for lipid storage, energy homeostasis, and whole-body insulin sensitivity. It is important to understand the mechanisms involved in adipose tissue development and function, which can be regulated by the endocrine actions of various peptide and steroid hormones. Recent studies have revealed that white and brown adipocytes can be derived from distinct precursor cells. This review will focus on transcriptional control of adipogenesis and its regulation by several endocrine hormones. The general functions and cellular origins of adipose tissue and how the modulation of adipocyte development pertains to metabolic disease states will also be considered.

Emerging roles of JAK-STAT signaling pathways in adipocytes

Twenty years ago, adipocytes were largely considered to be inert energy-storage depots. We now know that fat cells are highly insulin-sensitive with significant endocrine functions. Alterations in adipocyte development or function can contribute to metabolic disease, in particular type 2 diabetes. The current obesity epidemic that plagues many nations provides a strong rationale for understanding basic adipocyte biology. The JAK-STAT signaling pathway mediates the action of a variety of hormones that have profound effects on adipocyte development and function. In addition, adipocytes secrete hormones that utilize this signaling pathway. This review summarizes research on the expression and function of JAKs and STATs in adipocytes and highlights the roles of JAK-STAT-activating cytokines in adipose tissue.

Catalytically inactive SHIP2 inhibits proliferation by attenuating PDGF signaling in 3T3-L1 preadipocytes

Inadequate proliferation and/or differentiation of preadipocytes may lead to adipose tissue dysfunction characterized by hypertrophied, insulin-resistant adipocytes. Platelet-derived growth factor (PDGF) may alter adipose tissue function by promoting proliferation of preadipocytes. Two principal signaling pathways that regulate proliferation are PI3K/PI(3,4,5)P3/Akt and Shc/Ras/ERK1/2. SH2 domain-containing inositol 5-phosphatase 2 (SHIP2) dephosphorylates PI(3,4,5)P3, and also binds to Shc. Our goal was to determine how SHIP2 affects these PDGF signaling routes. To assess the role of the 5-phosphatase domain, we expressed wild-type or catalytically inactive dominant-negative SHIP2 (P686A-D690A-R691A; PDR/AAA) in 3T3-L1 preadipocytes. Surprisingly, SHIP2 PDR/AAA inhibited proliferation more potently than wild-type SHIP2. After three days of proliferation, phospho-Akt, phospho-ERK1/2, and PDGF receptor (PDGFR) levels were reduced in PDR/AAA-expressing preadipocytes. SHIP2 PDR/AAA interference with PDGFR signaling was demonstrated using imatinib, an inhibitor of PDGFR tyrosine kinase. The anti-proliferative effect of imatinib observed in control preadipocytes was not significant in SHIP2 PDR/AAA-expressing preadipocytes, indicating a pre-existing impairment of PDGFR-dependent mitogenesis in these cells. The inhibition of PDGF-activated mitogenic pathways by SHIP2 PDR/AAA was consistent with a decrease in PDGFR phosphorylation caused by a drop in receptor levels in SHIP2 PDR/AAA-expressing cells. SHIP2 PDR/AAA promoted ubiquitination of the PDGFR and its degradation via the lysosomal pathway independently of the association between the E3 ubiquitin ligase c-Cbl and PDGFR. Overall, our findings indicate that SHIP2 PDR/AAA reduces preadipocyte proliferation by attenuating PDGFR signaling.

Regulation of the O-linked beta-N-acetylglucosamine transferase by insulin signaling

O-Linked beta-N-acetylglucosamine (O-GlcNAc) transferase (OGT) catalyzes the addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues in response to stimuli or stress analogous to phosphorylation by Ser/Thr-kinases. Like protein phosphatases, OGT appears to be targeted to myriad specific substrates by transiently interacting with specific targeting subunits. Here, we show that OGT is activated by insulin signaling. Insulin treatment of 3T3-L1 adipocytes stimulates both tyrosine phosphorylation and catalytic activity of OGT. A subset of OGT co-immunoprecipitates with the insulin receptor. Insulin stimulates purified insulin receptor to phosphorylate OGT in vitro. OGT is a competitive substrate with reduced and carboxyamidomethylated lysozyme (RCAM-lysozyme), a well characterized insulin receptor substrate. Insulin stimulation of 3T3-L1 adipocytes results in a partial translocation of OGT from the nucleus to the cytoplasm. The insulin activation of OGT results in increased O-GlcNAc modification of OGT and other proteins including, signal transducer and activator of transcription 3 (STAT3). We conclude that insulin stimulates the tyrosine phosphorylation and activity of OGT.

Adipose-specific disruption of signal transducer and activator of transcription 3 increases body weight and adiposity

To determine the role of STAT3 in adipose tissue, we used Cre-loxP DNA recombination to create mice with an adipocyte-specific disruption of the STAT3 gene (ASKO mice). aP2-Cre-driven disappearance of STAT3 expression occurred on d 6 of adipogenesis, a time point when preadipocytes have already undergone conversion to adipocytes. Thus, this knockout model examined the role of STAT3 in mature but not differentiating adipocytes. Beginning at 9 wk of age, ASKO mice weighed more than their littermate controls and had increased adipose tissue mass, associated with adipocyte hypertrophy, but not adipocyte hyperplasia, hyperphagia, or reduced energy expenditure. Leptin-induced, but not isoproterenol-induced, lipolysis was impaired in ASKO adipocytes, which may partially explain the increased cell size. Despite reduced adiponectin and increased liver triacylglycerol, ASKO mice displayed normal glucose tolerance. Overall, these findings demonstrate that adipocyte STAT3 regulates body weight homeostasis in part through direct effects of leptin on adipocytes.

Elevated sensitivity to diet-induced obesity and insulin resistance in mice lacking 4E-BP1 and 4E-BP2

The most common pathology associated with obesity is insulin resistance, which results in the onset of type 2 diabetes mellitus. Several studies have implicated the mammalian target of rapamycin (mTOR) signaling pathway in obesity. Eukaryotic translation initiation factor 4E-binding (eIF4E-binding) proteins (4E-BPs), which repress translation by binding to eIF4E, are downstream effectors of mTOR. We report that the combined disruption of 4E-BP1 and 4E-BP2 in mice increased their sensitivity to diet-induced obesity. Increased adiposity was explained at least in part by accelerated adipogenesis driven by increased expression of CCAAT/enhancer-binding protein delta (C/EBPdelta), C/EBPalpha, and PPARgamma coupled with reduced energy expenditure, reduced lipolysis, and greater fatty acid reesterification in the adipose tissue of 4E-BP1 and 4E-BP2 double KO mice. Increased insulin resistance in 4E-BP1 and 4E-BP2 double KO mice was associated with increased ribosomal protein S6 kinase (S6K) activity and impairment of Akt signaling in muscle, liver, and adipose tissue. These data clearly demonstrate the role of 4E-BPs as a metabolic brake in the development of obesity and reinforce the idea that deregulated mTOR signaling is associated with the development of the metabolic syndrome.

Midkine is an autocrine activator of signal transducer and activator of transcription 3 in 3T3-L1 cells

Mitotic clonal expansion is believed to be necessary for 3T3-L1 adipocyte formation. Signal transducer and activator of transcription 3 (STAT3), a mitogenic signaling protein, is activated through tyrosine phosphorylation during the proliferative phases of adipogenesis. We hypothesize that this signaling protein plays a key role in mitotic clonal expansion and differentiation. Here we determined that the adipocyte differentiation cocktail containing isobutylmethylxanthine, dexamethasone, and insulin (MDI) induced STAT3 tyrosine phosphorylation indirectly through the synthesis of an autocrine/paracrine factor. We further determined that the factor has heparin binding properties and identified the factor as midkine, a pleiotrophic growth factor previously associated with neuronal development and oncogenesis. Recombinant midkine induced STAT3 tyrosine phosphorylation in a time- and dose-dependent manner and stimulated the proliferation of postconfluent 3T3-L1 cells. Midkine neutralizing antibodies inhibited differentiation-induced STAT3 tyrosine phosphorylation as well as adipogenesis. These results show that MDI-induced synthesis and release of midkine explains the delayed activation of STAT3 during adipogenesis and that the midkine-STAT3 signaling pathway plays a necessary role in mitotic clonal expansion and differentiation.

Simultaneous identification of differential gene expression and connectivity in inflammation, adipogenesis and cancer

MOTIVATION

Biological differences between classes are reflected in transcriptional changes which in turn affect the levels by which essential genes are individually expressed and collectively connected. The purpose of this communication is to introduce an analytical procedure to simultaneously identify genes that are differentially expressed (DE) as well as differentially connected (DC) in two or more classes of interest.

RESULTS

Our procedure is based on a two-step approach: First, mixed-model equations are applied to obtain the normalized expression levels of each gene in each class treatment. These normalized expressions form the basis to compute a measure of (possible) DE as well as the correlation structure existing among genes. Second, a two-component mixture of bi-variate distributions is fitted to identify the component that encapsulates those genes that are DE and/or DC. We demonstrate our approach using three distinct datasets including a human systemic inflammation oligonucleotide data; a spotted cDNA data dealing with bovine in vitro adipogenesis and SAGE database on cancerous and normal tissue samples.

Protein inhibitor of activated STAT3 inhibits adipogenic gene expression

Protein inhibitor of activated STAT3 (PIAS3), a cytokine-induced repressor of signal transducer and activator of transcription 3 (STAT3) and a modulator of a broad array of nuclear proteins, is expressed in white adipose tissue, but its role in adipogenesis is not known. Here, we determined that PIAS3 was constitutively expressed in 3T3-L1 cells at all stages of adipogenesis. However, it translocated from the nucleus to the cytoplasm 4 days after induction of differentiation by isobutylmethylxanthine, dexamethasone, and insulin (MDI). In ob/ob mice, PIAS3 expression was increased in white adipose tissue depots compared to lean mice and was found in the cytoplasm of adipocytes. Overexpression of PIAS3 in differentiating preadipocytes, which localized primarily to the nucleus, inhibited mRNA level gene expression of adipogenic transcription factors C/EBPalpha and PPARgamma, as well as their downstream target genes aP2 and adiponectin. PIAS3 also inhibited C/EBPalpha promoter activation mediated specifically by insulin, but not dexamethasone or isobutylmethylxanthine. Taken together, these data suggest that PIAS3 may play an inhibitory role in adipogenesis by modulating insulin-activated transcriptional activation events. Increased PIAS3 expression in adipose tissue may play a role in the metabolic disturbances of obesity.

Cell-to-cell adhesion modulates Stat3 activity in normal and breast carcinoma cells

Stat3 (signal transducer and activator of transcription-3) activity is required for transformation by a number of oncogenes, while a constitutively active form of Stat3 alone is sufficient to induce neoplastic transformation. Although in most instances Stat3 is growth-promoting, the impact of cell density on Stat3 activation status and the biological importance of Stat3 during growth arrest have not been characterized. Previous results indicated that cell density alters tyrosine phosphorylation levels of cultured cells. Since signalling through Stat3 is determined by a key phosphorylation at tyr705, we examined the effects of cell density upon Stat3 activity in normal breast epithelial cells, breast carcinoma lines and normal mouse fibroblasts. Intriguingly, the results revealed a dramatic increase in Stat3, tyr705 phosphorylation and activity with cell density, which gradually declined at later stages. This activation was dependent upon cell-cell contact, since it was eliminated if cell adhesion was disrupted through calcium chelation, while it was reinstated through cell aggregation. Furthermore, this activation was suppressed following inhibition of JAKs (Janus kinases) but not inhibition of Fer, IGF1-R, or kinases of the c-Src family. On the other hand, constitutively active Stat3 in carcinoma lines, known to harbor activated Src, was blocked by pharmacological inhibitors of Src as well as JAKs. These results point to the existence of two distinct pathways of Stat3 activation in breast carcinomas, based on Src dependence. More importantly, our results suggest that Stat3 activity is upregulated during the confluence-mediated growth arrest by a signalling mechanism that requires JAKs.

Constitutively active signal transducer and activator of transcription 5 can replace the requirement for growth hormone in adipogenesis of 3T3-F442A preadipocytes

Although it is the best characterized in vitro model of GH action, the mechanisms used by GH to induce differentiation of murine 3T3-F442A preadipocytes remain unclear. Here we have examined the role of three transcriptional regulators in adipogenesis. These regulators are either rapidly induced in response to GH [Stra13, signal transducer and activator of transcription (Stat)3] or of central importance to GH signaling (Stat5). Retroviral transfection of 3T3-F442A preadipocytes was used to increase expression of Stra13, Stat3, and Stat5a. Only Stat5a transfection increased the expression of adipogenic markers peroxisome proliferator-activated receptor gamma, CCAAT enhancer binding protein (C/EBP)alpha, and adipose protein 2/fatty acid-binding protein in response to GH, as determined by quantitative RT-PCR. Transfection with constitutively active Stat3 and Stat5a revealed that constitutively active Stat5a but not Stat3 was able to replace the GH requirement for adipogenesis. Constitutively active Stat5a but not Stat3 was able to increase the formation of lipid droplets and expression of alpha-glycerol phosphate dehydrogenase toward levels seen in mature adipocytes. Constitutively active Stat5a was also able to increase the expression of transcripts for C/EBPalpha to similar levels as GH, and of C/EBPbeta, peroxisome proliferator-activated receptor gamma, and adipose protein 2/fatty acid-binding protein transcripts to a lesser extent. An in vivo role for GH in murine adipogenesis is supported by significantly decreased epididymal fat depot size in young GH receptor-deleted mice, before manifestation of the lipolytic actions of GH. We conclude that Stat5 is a critical factor in GH-induced, and potentially prolactin-induced, murine adipogenesis.

Proadipogenic effect of leptin on rat preadipocytes in vitro: activation of MAPK and STAT3 signaling pathways

Because leptin has recently been shown to induce proliferation and/or differentiation of different cell types through different pathways, the aim of the present study was to investigate, in vitro, the influence of leptin on adipogenesis in rat preadipocytes. A prerequisite to this study was to identify leptin receptors (Ob-Ra and Ob-Rb) in preadipocytes from femoral subcutaneous fat. We observed that expressions of Ob-Ra and Ob-Rb increase during adipogenesis. Furthermore, leptin induces an increase of p42/p44 mitogen-activated protein kinase phosphorylated isoforms in both confluent and differentiated preadipocytes and of STAT3 phosphorylation only in confluent preadipocytes. Moreover, exposure to leptin promoted activator protein-1 complex DNA binding activity in confluent preadipocytes. Finally, exposure of primary cultured preadipocytes from the subcutaneous area to leptin (10 nM) resulted in an increased proliferation ([(3)H]thymidine incorporation and cell counting) and differentiation (glycerol-3-phosphate dehydrogenase activity and mRNA levels of lipoprotein lipase, peroxisome proliferator-activated receptor-gamma2, and c-fos). Altogether, these results indicate that, in vitro at least, leptin through its functional receptors exerts a proadipogenic action in subcutaneous preadipocytes.

Agouti regulates adipocyte transcription factors

Agouti is a secreted paracrine factor that regulates pigmentation in hair follicle melanocytes. Several dominant mutations cause ectopic expression of agouti, resulting in a phenotype characterized by yellow fur, adult-onset obesity and diabetes, increased linear growth and skeletal mass, and increased susceptibility to tumors. Humans also produce agouti protein, but the highest levels of agouti in humans are found in adipose tissue. To mimic the human agouti expression pattern in mice, transgenic mice (aP2-agouti) that express agouti in adipose tissue were generated. The transgenic mice develop a mild form of obesity, and they are sensitized to the action of insulin. We correlated the levels of specific regulators of insulin signaling and adipocyte differentiation with these phenotypic changes in adipose tissue. Signal transducers and activators of transcription (STAT)1, STAT3, and peroxisome proliferator-activated receptor (PPAR)-gamma protein levels were elevated in the transgenic mice. Treatment of mature 3T3-L1 adipocytes recapitulated these effects. These data demonstrate that agouti has potent effects on adipose tissue. We hypothesize that agouti increases adiposity and promotes insulin sensitivity by acting directly on adipocytes via PPAR-gamma.

Differential effects of flavonoids on 3T3-L1 adipogenesis and lipolysis

Flavonoids, polyphenolic compounds that exist widely in plants, inhibit cell proliferation and increase cell differentiation in many cancerous and noncancerous cell lines. Because terminal differentiation of preadipocytes to adipocytes depends on proliferation of both pre- and postconfluent preadipocytes, we predicted that flavonoids would inhibit adipogenesis in the 3T3-L1 preadipocyte cell line. The flavonoids genistein and naringenin inhibited proliferation of preconfluent preadipocytes in a time- and dose-dependent manner. When added to 2-day postconfluent preadipocytes at the induction of differentiation, genistein inhibited mitotic clonal expansion, triglyceride accumulation, and peroxisome proliferator-activated receptor-gamma expression, but naringenin had no effect. The antiadipogenic effect of genistein was not due to inhibition of insulin receptor subtrate-1 tyrosine phosphorylation. When added 3 days after induction of differentiation, neither flavonoid inhibited differentiation. In fully differentiated adipocytes, genistein increased basal and epinephrine-induced lipolysis, but naringenin had no significant effects. These data demonstrate that genistein and naringenin, despite structural similarity, have differential effects on adipogenesis and adipocyte lipid metabolism.

Differential expression of signal transducers and activators of transcription during human adipogenesis

Signal Transducers and Activators of Transcription (STATs) display unique expression patterns upon induction of differentiation of murine 3T3-L1 preadipocytes into adipocytes. During differentiation, expression of STAT1 and STAT5 increase, while STAT3 and STAT6 remain relatively unchanged. Here, we determined whether human subcutaneous preadipocytes expressed STATs and if the pattern of expression changed during adipogenesis. We found by Western blot analysis that freshly isolated preadipocytes expressed STAT1, STAT3, STAT5, and STAT6, but not STAT2 and STAT4. Induction of preadipocyte differentiation with 1-methyl-3-isobutylxanthine, dexamethasone, insulin, and BRL49653 decreased expression of STAT1, and increased expression of STAT3 and STAT5. STAT6 expression did not change during adipogenesis. Changes in expression of CCAAT/enhancer binding protein beta (C/EBPbeta), C/EBPdelta, C/EBPalpha, and peroxisome proliferator-activated receptor gamma were similar to murine cell lines. These results suggest that unlike the traditional adipogenic transcription factors, unique differences exist in STAT expression patterns between murine and human adipose cells.

Interferon-gamma-induced regulation of peroxisome proliferator-activated receptor gamma and STATs in adipocytes

Interferon-gamma (IFN-gamma) is known primarily for its roles in immunological responses but also has been shown to affect fat metabolism and adipocyte gene expression. To further investigate the effects of IFN-gamma on fat cells, we examined the effects of this cytokine on the expression of adipocyte transcription factors in 3T3-L1 adipocytes. Although IFN-gamma regulated the expression of several adipocyte transcription factors, IFN-gamma treatment resulted in a rapid reduction of both peroxisome proliferator-activated receptor (PPAR) protein and mRNA. A 48-h exposure to IFN-gamma also resulted in a decrease of both CCAAT/enhancer-binding alpha and sterol regulatory element binding protein (SREBP-1) expression. The short half-life of both the PPARgamma mRNA and protein likely contributed to the rapid decline of both cytosolic and nuclear PPARgamma in the presence of IFN-gamma. Our studies clearly demonstrated that the IFN-gamma-induced loss of PPARgamma protein is partially inhibited in the presence of two distinct proteasome inhibitors. Moreover, IFN-gamma also inhibited the transcription of PPARgamma, which was accompanied by a decrease in PPARgamma mRNA accumulation. In addition, exposure to IFN-gamma resulted in a substantial increase in STAT 1 expression and a small increase in STAT 3 expression. IFN-gamma treatment of 3T3-L1 adipocytes (48-96 h) resulted in a substantial inhibition of insulin-sensitive glucose uptake. These data clearly demonstrate that IFN-gamma treatment results in the development of insulin resistance, which is accompanied by the regulation of various adipocyte transcription factors, in particular the synthesis and degradation of PPARgamma.