Gene expression during 3T3-L1 adipocyte differentiation. Characterization of initial responses to the inducing agents and changes during commitment to differentiation

Adipose tissue-derived stem cells, in vivo and in vitro models for metabolic diseases

The primary role of adipose tissue stem cells (ADSCs) is to support the function and homeostasis of adipose tissue in physiological and pathophysiological conditions. However, when ADSCs become dysfunctional in diseases such as obesity and cancer, they become impaired, undergo signalling changes, and their epigenome is altered, which can have a dramatic effect on human health. In more recent years, the therapeutic potential of ADSCs in regenerative medicine, wound healing, and for treating conditions such as cancer and metabolic diseases has been extensively investigated with very promising results. ADSCs have also been used to generate two-dimensional (2D) and three-dimensional (3D) cellular and in vivo models to study adipose tissue biology and function as well as intracellular communication. Characterising the biology and function of ADSCs, how it is altered in health and disease, and its therapeutic potential and uses in cellular models is key for designing intervention strategies for complex metabolic diseases and cancer.

Cinnamyl Alcohol Attenuates Adipogenesis in 3T3-L1 Cells by Arresting the Cell Cycle

Cinnamyl alcohol (CA) is an aromatic compound found in several plant-based resources and has been shown to exert anti-inflammatory and anti-microbial activities. However, the anti-adipogenic mechanism of CA has not been sufficiently studied. The present study aimed to investigate the effect and mechanism of CA on the regulation of adipogenesis. As evidenced by Oil Red O staining, Western blotting, and real-time PCR (RT-PCR) analyses, CA treatment (6.25-25 μM) for 8 d significantly inhibited lipid accumulation in a concentration-dependent manner and downregulated adipogenesis-related markers (peroxisome proliferator-activated receptor γ (PPARγ), CCAAT/enhancer-binding protein α (C/EBPα), fatty acid binding protein 4 (FABP4), adiponectin, fatty acid synthase (FAS)) in 3-isobutyl-1-methylxanthine, dexamethasone, and insulin(MDI)-treated 3T3-L1 adipocytes. In particular, among the various differentiation stages, the early stage of adipogenesis was critical for the inhibitory effect of CA. Cell cycle analysis using flow cytometry and Western blotting showed that CA effectively inhibited MDI-induced initiation of mitotic clonal expansion (MCE) by arresting the cell cycle in the G0/G1 phase and downregulating the expression of C/EBPβ, C/EBPδ, and cell cycle markers (cyclin D1, cyclin-dependent kinase 6 (CDK6), cyclin E1, CDK2, and cyclin B1). Moreover, AMP-activated protein kinase α (AMPKα), acetyl-CoA carboxylase (ACC), and extracellular signal-regulated kinase 1/2 (ERK1/2), markers of upstream signaling pathways, were phosphorylated during MCE by CA. In conclusion, CA can act as an anti-adipogenic agent by inhibiting the AMPKα and ERK1/2 signaling pathways and the cell cycle and may also act as a potential therapeutic agent for obesity.

3D culture induction of adipogenic differentiation in 3T3-L1 preadipocytes exhibits adipocyte-specific molecular expression patterns and metabolic functions

Adipose tissues are closely related to physiological functions and pathological conditions in most organs. Although differentiated 3T3-L1 preadipocytes have been used for in vitro adipose studies, the difference in cellular characteristics of adipogenic differentiation in two-dimensional (2D) culture and three-dimensional (3D) culture remain unclear. In this study, we evaluated gene expression patterns using RNA sequencing and metabolic functions using an extracellular flux analyzer in 3T3-L1 preadipocytes with and without adipogenic induction in 2D culture and 3D culture. In 2D culture, 565 up-regulated genes and 391 down-regulated genes were identified as differentially expressed genes (DEGs) by adipogenic induction of 3T3-L1 preadipocytes, whereas only 69 up-regulated genes and 59 down-regulated genes were identified as DEGs in 3D culture. Ingenuity Pathway Analysis (IPA) revealed that genes associated with lipid metabolism were identified as 2 out of the top 3 causal networks related to diseases and function in 3D spheroids, whereas only one network related to lipid metabolism was identified within the top 9 of these causal networks in the 2D planar cells, suggesting that adipogenic induction in the 3D culture condition exhibits a more adipocyte-specific gene expression pattern in 3T3-L1 preadipocytes. Real-time metabolic analysis revealed that the metabolic capacity shifted from glycolysis to mitochondrial respiration in differentiated 3T3-L1 cells in the 3D culture condition but not in those in the 2D cultured condition, suggesting that adipogenic differentiation in 3D culture induces a metabolic phenotype of well-differentiated adipocytes. Consistently, expression levels of mitochondria-encoded genes including mt-Nd6, mt-Cytb, and mt-Co1 were significantly increased by adipogenic induction of 3T3-L1 preadipocytes in 3D culture compared with those in 2D culture. Taken together, the findings suggest that induction of adipogenesis in 3D culture provides a more adipocyte-specific gene expression pattern and enhances mitochondrial respiration, resulting in more adipocyte-like cellular properties.

Opposing Actions of TLR2 and TLR4 in Adipocyte Differentiation and Mature-Onset Obesity

Understanding the signaling cascades that govern adipocyte metabolism and differentiation is necessary for the development of therapies for obesity. Toll-like receptors (TLRs) are key mediators in adipogenesis, but their specific role is not completely understood. In this study, siRNA knockdown of Tlr2 in 3T3-L1 cells allowed them to differentiate more efficiently into adipocytes, whereas the opposite was observed for the knockdown of Tlr4. At the same time, we show that TLR2 knock-out mice spontaneously developed mature-onset obesity and insulin resistance. Besides a higher incidence of hyperplasia and hypertrophy in white adipose tissue (WAT), we found a significantly increased number of adipocyte precursor cells in TLR2^-/- mice compared to TLR4^-/- mice. Interestingly, genetic inactivation of Tlr4 in TLR2^-/- mice reverted their increased adiposity, insulin resistance, and restored normal levels of adipocyte precursor cells. These findings provide evidence that TLR2 and TLR4 play opposing roles in WAT homeostasis and point to the existence of cross-regulation among TLR2 and TLR4 during adipocyte differentiation both in vitro and in vivo.

Stromal Co-Cultivation for Modeling Breast Cancer Dormancy in the Bone Marrow

Cancers metastasize to the bone marrow before primary tumors can be detected. Bone marrow micrometastases are resistant to therapy, and while they are able to remain dormant for decades, they recur steadily and result in incurable metastatic disease. The bone marrow microenvironment maintains the dormancy and chemoresistance of micrometastases through interactions with multiple cell types and through structural and soluble factors. Modeling dormancy in vitro can identify the mechanisms of these interactions. Modeling also identifies mechanisms able to disrupt these interactions or define novel interactions that promote the reawakening of dormant cells. The in vitro modeling of the interactions of cancer cells with various bone marrow elements can generate hypotheses on the mechanisms that control dormancy, treatment resistance and reawakening in vivo. These hypotheses can guide in vivo murine experiments that have high probabilities of succeeding in order to verify in vitro findings while minimizing the use of animals in experiments. This review outlines the existing data on predominant stromal cell types and their use in 2D co-cultures with cancer cells.

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.

Coordinated reprogramming of metabolism and cell function in adipocytes from proliferation to differentiation

Adipose tissue plays a major role in regulating lipid and energy homeostasis by storing excess nutrients, releasing energetic substrates through lipolysis, and regulating metabolism of other tissues and organs through endocrine and paracrine signaling. Adipocytes within fat tissues store excess nutrients through increased cell number (hyperplasia), increased cell size (hypertrophy), or both. The differentiation of pre-adipocytes into mature lipid-accumulating adipocytes requires a complex interaction of metabolic pathways that is still incompletely understood. Here, we applied parallel labeling experiments and ¹³C-metabolic flux analysis to quantify precise metabolic fluxes in proliferating and differentiated 3T3-L1 cells, a widely used model to study adipogenesis. We found that morphological and biomass composition changes in adipocytes were accompanied by significant shifts in metabolic fluxes, encompassing all major metabolic pathways. In contrast to proliferating cells, differentiated adipocytes 1) increased glucose uptake and redirected glucose utilization from lactate production to lipogenesis and energy generation; 2) increased pathway fluxes through glycolysis, oxidative pentose phosphate pathway and citric acid cycle; 3) reduced lactate secretion, resulting in increased ATP generation via oxidative phosphorylation; 4) rewired glutamine metabolism, from glutaminolysis to de novo glutamine synthesis; 5) increased cytosolic NADPH production, driven mostly by increased cytosolic malic enzyme flux; 6) increased production of monounsaturated C16:1; and 7) activated a mitochondrial pyruvate cycle through simultaneous activity of pyruvate carboxylase, malate dehydrogenase and malic enzyme. Taken together, these results quantitatively highlight the complex interplay between pathway fluxes and cell function in adipocytes, and suggest a functional role for metabolic reprogramming in adipose differentiation and lipogenesis.

Matrigel® enhances 3T3-L1 cell differentiation

Culturing cells on bio-gels are believed to provide a more in vivo-like extracellular matrix. 3T3-L1 cells cultured on Matrigel® significantly alteregd their proliferation and differentiation as compared to growth on tissue culture-coated polystyrene surfaces. Growth on a 250-μm thick layer of Matrigel® facilitated the formation of cellular aggregates of 3T3-L1 cells. Differentiation of 3T3-L1 cells cultured on Matrigel® demonstrated increased levels of mRNA levels for key adipogenic transcription factors (PPARγ, C/EBPα, SREBP1), lipogenic markers (FAS, FABP4, LPL, PLIN1) and markers of adipocyte maturity (LEP), compared to cells cultured directly on a polystyrene tissue culture surface. The gene expression of extracellular matrix proteins (FN1, COL1A1, COL4A1, COL6, LAM) was decreased in 3T3-L1 cells cultured on Matrigel®. Furthermore, growth on Matrigel® increased lipid accumulation in 3T3-L1 cells in the presence and absence of rosiglitazone, a thiazolidinedione routinely used to optimize differentiation in these cells. These changes in adipocyte gene expression and lipid accumulation patterns may be a result of the increased cell-cell and cell-ECM interactions occurring on the Matrigel®, a scenario that is more reflective of an in vivo model. Taken together, our data advance the understanding of the value of culturing 3T3-L1 cells on Matrigel®.

Activation of SphK1 by adipocytes mediates epithelial ovarian cancer cell proliferation

Background

Adipocytes, active facilitators of epithelial ovarian cancer (EOC) growth, have been implicated in the link between obesity and EOC. However, the current understanding of the mechanisms underlying adipocyte-induced EOC cell proliferation remains incomplete.

Results

We provide the first evidence showing that sphingosine kinase (SphK) 1 is critical for adipocyte-induced EOC cell proliferation. Adipocytes are capable of activating SphK1, which then leads to extracellular signal-regulated kinase (ERK) phosphorylation. Moreover, adipocyte-induced SphK1 activation is ERK dependent. Furthermore, sphingosine 1-phosphate receptor (S1PR) 1 and S1PR3, key components of the SphK1 signalling pathway, participate in adipocyte-mediated growth-promoting action in EOC cells.

Conclusions

Our study reveals a previously unrecognized role of SphK1 in adipocyte-induced growth-promoting action in EOC, suggesting a new target for EOC therapy.

In-Vitro-Generated Hypertrophic-Like Adipocytes Displaying PPARG Isoforms Unbalance Recapitulate Adipocyte Dysfunctions In Vivo

Reduced neo-adipogenesis and dysfunctional lipid-overloaded adipocytes are hallmarks of hypertrophic obesity linked to insulin resistance. Identifying molecular features of hypertrophic adipocytes requires appropriate in vitro models. We describe the generation of a model of human hypertrophic-like adipocytes directly comparable to normal adipose cells and the pathologic evolution toward hypertrophic state. We generate in vitro hypertrophic cells from mature adipocytes, differentiated from human mesenchymal stem cells. Combining optical, confocal, and transmission electron microscopy with mRNA/protein quantification, we characterize this cellular model, confirming specific alterations also in subcutaneous adipose tissue. Specifically, we report the generation and morphological/molecular characterization of human normal and hypertrophic-like adipocytes. The latter displays altered morphology and unbalance between canonical and dominant negative (PPARGΔ5) transcripts of PPARG, paralleled by reduced expression of PPARγ targets, including GLUT4. Furthermore, the unbalance of PPARγ isoforms associates with GLUT4 down-regulation in subcutaneous adipose tissue of individuals with overweight/obesity or impaired glucose tolerance/type 2 diabetes, but not with normal weight or glucose tolerance. In conclusion, the hypertrophic-like cells described herein are an innovative tool for studying molecular dysfunctions in hypertrophic obesity and the unbalance between PPARγ isoforms associates with down-regulation of GLUT4 and other PPARγ targets, representing a new hallmark of hypertrophic adipocytes.

[Molecular Mechanisms of Early-stage Adipocyte Differentiation and Multi-functional Roles of Newly Isolated Adipogenic Factors]

Obesity is a major risk factor for diabetes, hypertension, hyperlipidemia, and arteriosclerosis. Although the middle and late stages of adipocyte differentiation are well characterized, the earliest step in the differentiation process has remained largely unknown. We isolated 102 genes expressed at the beginning of the differentiation of a mouse preadipocyte cell line, 3T3-L1 cells. Because approximately half of these genes were unknown, we named them factor for adipocyte differentiation (fad) genes. I first show how these genes regulate the early stage of adipocyte differentiation. We next generated fad104-deficient mice, and demonstrated that fad104-deficient mice died due to cyanosis-associated lung dysplasia with atelectasis. We also found that fad104 positively regulated adipocyte differentiation and negatively regulated osteoblast differentiation. We then demonstrated that fad24-knockdown inhibited mitotic clonal expansion (MCE) and that FAD24 contributed to the regulation of DNA replication by recruiting histone acetyltransferase binding to ORC1 (HBO1) to DNA replication origins. In vitro culture experiments revealed that fad24-null embryos developed normally to the morula stage, but acquired growth defects in subsequent stages. These results strongly suggest that fad24 is essential for pre-implantation in embryonic development, particularly for progression to the blastocyst stage. These findings together indicate that both fad104 and fad24 contribute not only to adipogenesis but also to other physiological events. The multi-functional roles of these genes are discussed.

Adipocytes can induce epithelial-mesenchymal transition in breast cancer cells

Adipocytes are known to be involved in epithelial-mesenchymal transition (EMT) in several cancers. However, the role of adipocytes in the EMT of breast cancer cells is poorly understood. The purpose of this study was to investigate the involvement of adipocytes in the EMT in breast cancer. Breast cancer cell lines MCF-7, MDA-MB-453, MDA-MB-435S, MDA-MB-231, and MDA-MB-468 were co-cultured with adipocytes and analyzed for morphological changes, proliferation activity, EMT markers, migration, and invasion. In addition, 296 human breast cancer specimens were classified according to the presence of the fibrous or adipose stroma and analyzed by immunohistochemistry for the expression of estrogen and progesterone receptors, human epidermal growth factor receptor 2, antigen Ki-67, N-cadherin, Twist-related protein 1 (TWIST1), high-mobility group AT-hook 2, TGFβ, and S100 calcium-binding protein A4 using tissue microarray. After co-culture with adipocytes, MCF-7, MDA-MB-435S, and MDA-MB-231 cells exhibited elongated spindle-like morphology and increased proliferation; MDA-MB-435S and MDA-MB-231 cells also showed increased dispersion. In all tested breast cancer cells, adipocytes induced migration and invasion. The EMT-like phenotypic changes and increased cell migration and invasion were accompanied by the upregulation of matrix metallopeptidase 9 and TWIST1. Consistently, breast cancer tumors with the adipose stroma showed higher TWIST1 expression than those with the adipose stroma; however, no difference was observed in the levels of other EMT-related proteins. Adipocytes stimulate breast cancer cells to acquire aggressive tumor phenotype by inducing EMT-associated traits, and breast cancer with the adipose stroma expresses EMT markers as breast cancer with the fibrous stroma.

Inhibitory effects of hydroxylated cinnamoyl esters on lipid absorption and accumulation

Obesity is a risk factor associated with several lifestyle-related diseases, for example, diabetes, high blood pressure, hyperlipidemia and cancer. Caffeic acid 2-phenylethyl ester (CAPE, 1), a naturally-occurring compound found in various plants and propolis, which exhibits anti-inflammatory, immunomodulatory and cytotoxic activities and inhibits 3T3-L1 differentiation to adipocytes. As part of our efforts to moderate lifestyle-related diseases, we synthesized analogs of 1 and studied their effects on pancreatic lipase activities, lipid absorption, and 3T3-L1 differentiation. We found that catechols 1-4 show inhibitory activities against pancreatic lipase in a dose-dependent manner in vitro. Compounds 1-3 proved to be more potent inhibitors of pancreatic lipase than 5, 6, 8, and 9, which have one hydroxyl group, respectively. Compound 7 has three aromatic hydroxyl groups and restrains greater lipase inhibitory activity than the other compounds. In addition, 7 and 3 significantly suppress a rise in blood triglyceride (TG) levels in mice given corn oil orally. Furthermore, 2 and 3 are more potent at preventing 3T3-L1 differentiation (lipid accumulation) than 1, while 7 is more potent than 3, 8, and 9 in these assays. Compounds 2, 3, and 7 inhibit lipid absorption and accumulation, with new compound 7 being the most potent. These results indicate that 7 may have potential benefits as a health agent with anti-obesity properties.

5-azacytidine promotes the transdifferentiation of cardiac cells to skeletal myocytes

The DNA methylation inhibitor 5-azacytidine is widely used to stimulate the cardiac differentiation of stem cells. However, 5-azacytidine has long been employed as a tool for stimulating skeletal myogenesis. Yet, it is unclear whether the ability of 5-azacytidine to promote both cardiac and skeletal myogenesis is dependent strictly on the native potential of the starting cell population or if this drug is a transdifferentiation agent. To address this issue, we examined the effect of 5-azacytidine on cultures of adult mouse atrial tissue, which contains cardiac but not skeletal muscle progenitors. Exposure to 5-azacytidine caused atrial cells to elongate and increased the presence of fat globules within the cultures. 5-Azacytidine also induced expression of the skeletal myogenic transcription factors MyoD and myogenin. 5-Azacytidine pretreatments allowed atrial cells to undergo adipogenesis or skeletal myogenesis when subsequently cultured with either insulin and dexamethasone or low-serum media, respectively. The presence of skeletal myocytes in atrial cultures was indicated by dual staining for myogenin and sarcomeric α-actin. These data demonstrate that 5-azacytidine converts cardiac cells to noncardiac cell types and suggests that this drug has a compromised efficacy as a cardiac differentiation factor.

β-adrenergic receptor-stimulated lipolysis requires the RAB7-mediated autolysosomal lipid degradation

Hormone-stimulated lipolysis is a rapid way to mobilize fat from its storage depot for use in peripheral tissues. By convention, activation of cytosolic lipases via the β-adrenergic receptor (ADRB2)-cAMP signaling pathway is the only molecular mechanism considered to liberate fatty acids from triglycerides stored in lipid droplets (LDs) of cells. Herein, we provide evidence that, aside from the activation of cytosolic lipases, autophagy contributes to this hormone-stimulated lipolysis. The ADRB2-stimulated lipolysis was reduced after inhibition of early or late autophagy using either pharmacological inhibitors or shRNA-mediated autophagic gene knockdown. ADRB2 stimulation has caused a marked increase in the autophagy-targeted LDs for lysosomal degradation, which is dependent on the LD-associated RAB7 as evidenced by the use of both shRNA-mediated RAB7 knockdown and a dominant-negative RAB7 mutant. In addition, RAB7 is involved in unstimulated (basal) lipolysis, and mediates the enhanced basal lipolysis in PLIN1/perilipin 1 knockdown fat cells. In conclusion, our results showed a contribution of lipophagy to both basal and hormone-stimulated lipolysis and that RAB7 plays a pivotal role in the regulation of this autolysosome-mediated lipid degradation in fat cells.

Studying lipolysis in adipocytes by combining siRNA knockdown and adenovirus-mediated overexpression approaches

3T3-L1 adipocytes are widely used as a model system for studying hormone-stimulated lipolysis. However, these cells were limited in their utility for gain- and loss-of-function studies due to the low efficiency of their transfection with plasmid DNA or small interfering RNA (siRNA) oligos. In this chapter, we provide a review of two methods established for manipulation of protein expression in differentiated mature adipocytes. The use of electroporation allows a high-efficiency delivery of siRNA oligos and subsequent knockdown of specific gene expression. A centrifugation-assisted infection with recombinant adenovirus, on the other hand, enables robust overexpression of ectopic proteins. Most importantly, by combining siRNA electroporation with adenovirus infection, simultaneous manipulation of levels of two different proteins can be achieved in differentiated adipocytes. Through subsequent analyses of lipase activity in cell extracts and fatty acid or glycerol release from living cells, mutual interdependence between the two proteins in the context of basal and hormone-stimulated adipocyte lipolysis can be evaluated.

Knockdown of the fat mass and obesity gene disrupts cellular energy balance in a cell-type specific manner

Recent studies suggest that FTO variants strongly correlate with obesity and mainly influence energy intake with little effect on the basal metabolic rate. We suggest that FTO influences eating behavior by modulating intracellular energy levels and downstream signaling mechanisms which control energy intake and metabolism. Since FTO plays a particularly important role in adipocytes and in hypothalamic neurons, SH-SY5Y neuronal cells and 3T3-L1 adipocytes were used to understand how siRNA mediated knockdown of FTO expression alters cellular energy homeostasis. Cellular energy status was evaluated by measuring ATP levels using a luminescence assay and uptake of fluorescent glucose. FTO siRNA in SH-SY5Y cells mediated mRNA knockdown (−82%), increased ATP concentrations by up to 46% (P = 0.013) compared to controls, and decreased phosphorylation of AMPk and Akt in SH-SY5Y by −52% and −46% respectively as seen by immunoblotting. In contrast, FTO siRNA in 3T3-L1 cells decreased ATP concentration by −93% (p<0.0005), and increased AMPk and Akt phosphorylation by 204% and 70%, respectively suggesting that FTO mediates control of energy levels in a cell-type specific manner. Furthermore, glucose uptake was decreased in both SH-SY5Y (−51% p = 0.015) and 3T3-L1 cells (−30%, p = 0.0002). We also show that FTO knockdown decreases NPY mRNA expression in SH-SY5Y cells (−21%) through upregulation of pSTAT3 (118%). These results provide important evidence that FTO-variant linked obesity may be associated with altered metabolic functions through activation of downstream metabolic mediators including AMPk.

Adipocytes Promote B16BL6 Melanoma Cell Invasion and the Epithelial-to-Mesenchymal Transition

Metastatic melanoma is one of the most deadly and evasive types of cancer. On average, cancer patients with metastatic melanoma survive only 6-9 months after diagnosis. Epidemiological and animal studies suggest that obesity increases the metastatic ability of malignant melanoma, though the mechanism is not known. In the present studies, we assessed the ability of 3T3L1 adipocytes to modulate B16BL6 melanoma cell invasion and the Epithelial-to-Mesenchymal Transition (EMT). For this purpose, we induced the differentiation of 3T3L1 fibroblasts to adipocytes. Then, we collected the cell culture media from both fibroblasts and adipocytes and determined their effect on the invasive ability and EMT gene expression of B16BL6 melanoma cells. Results show that adipocyte media increased that ability of B16BL6 cells to invade. The higher invasive ability of B16BL6 melanoma cells was associated with increased expression of EMT genes such as Snai1, MMP9, Twist, and Vimentin. Additionally, the expression of the cell-to-cell adhesion protein E-cadherin and the metastasis suppressor gene Kiss1 were down-regulated in these B16BL6 cells. Also, adipocytes had high levels of the pro-inflammatory cytokine Interleukin 6 (IL-6). Treatment of B16BL6 cells with IL-6 elicited similar effects as the adipocyte media; IL-6 promoted the invasive ability of B16BL6 melanoma cells, increased the expression of Snai1, and decreased Kiss1 expression. IL-6 neutralization, however, did not have a visible effect on adipocyte media-induced invasion and snai1 staining. In summary, adipocytes may increase the invasive ability of B16BL6 melanoma cells by promoting EMT and decreasing the expression of genes such as E-cadherin and Kiss1.

RNA interference-mediated knockdown of the mouse gene encoding potassium channel subfamily K member 10 inhibits hormone-induced differentiation of 3T3-L1 preadipocytes

Intramuscular fat (IMF) is an economically important trait of domestic meat animals; thus, it is important to identify the factors that influence the IMF content. In this study, we identified the gene associated with adipogenesis from all the positional candidate genes located in the quantitative trait loci (QTL) for IMF content on porcine chromosome 7. We analyzed the expression of the abovementioned genes during differentiation of mouse 3T3-L1 preadipocytes by using real-time polymerase chain reaction (PCR). Total cellular RNA was extracted before and 6, 12, 36, and 48 h and 4, 6, and 8d after treatment with standard hormonal inducers of differentiation-insulin, dexamethasone, and 3-isobutyl-1-methylxanthine (IBMX). Six hours after induction, potassium channel subfamily K member 10 (KCNK10) gene expression in the preadipocytes was found to be 100-fold greater than that at the baseline; this expression declined until day 4 after the induction. Moreover, knockdown of the KCNK10 gene by transfection with short-hairpin RNA (shRNA) significantly decreased triacylglycerol accumulation on day 8 after the induction. An RNA interference study revealed that KCNK10 knockdown inhibited the differentiation of 3T3-L1 cells. These results indicate that KCNK10 plays an important role in the early stages of preadipocyte differentiation.

Heterogeneity in the physiological states and pharmacological responses of differentiating 3T3-L1 preadipocytes

A systems biology–based analysis shows that differentiating adipocytes look very different at the single-cell level and form distinct cellular subpopulations.

Increases in key components of adipogenesis and lipolysis pathways correlate at the population-averaged level during adipogenesis. However, differentiating preadipocytes are highly heterogeneous in cellular and lipid droplet (LD) morphologies, and the degree to which individual cells follow population-averaged trends is unclear. In this study, we analyze the molecular heterogeneity of differentiating 3T3-L1 preadipocytes using immunofluorescence microscopy. Unexpectedly, we only observe a small percentage of cells with high simultaneous expression of markers for adipogenesis (peroxisome proliferator-activated receptor γ [PPARγ], CCAAT/enhancer-binding protein α, and adiponectin) and lipid accumulation (hormone-sensitive lipase, perilipin A, and LDs). Instead, we identify subpopulations of cells with negatively correlated expressions of these readouts. Acute perturbation of adipocyte differentiation with PPARγ agonists, forskolin, and fatty acids induced subpopulation-specific effects, including redistribution of the percentage of cells in observed subpopulations and differential expression levels of PPARγ. Collectively, our results suggested that heterogeneity observed during 3T3-L1 adipogenesis reflects a dynamic mixture of subpopulations with distinct physiological states.

An approach for extensibly profiling the molecular states of cellular subpopulations

Microscopy often reveals the existence of phenotypically distinct cellular subpopulations. However, additional characterization of observed subpopulations can be limited by the number of biomolecular markers that can be simultaneously monitored. Here we present a computational approach for extensibly profiling cellular subpopulations by freeing one or more imaging channels to monitor additional probes. In our approach, we trained classifiers to re-identify subpopulations accurately based on an enhanced collection of phenotypic features extracted from only a subset of the original markers. Then we constructed subpopulation profiles step-wise from replicate experiments, in which cells were labeled with different but overlapping marker sets. We applied our approach to identify molecular differences among subpopulations and to identify functional groupings of markers, in populations of differentiating mouse preadipocytes, polarizing human neutrophil-like cells and dividing human cancer cells.

Metabolic phenotyping of a model of adipocyte differentiation

The 3T3-L1 murine cell line is a robust and widely used model for the study of adipogenesis and processes occurring in mature adipocytes. The fibroblastic like cells can be induced by hormones to differentiate into mature adipocytes. In this study, the metabolic phenotype associated with differentiation of the 3T3-L1 cell line has been studied using gas chromatography-mass spectrometry, (1)H nuclear magnetic resonance spectroscopy, liquid chromatography-mass spectrometry, direct infusion-mass spectrometry, and 13C substrate labeling in conjunction with multivariate statistics. The changes in metabolite concentrations at distinct periods during differentiation have been defined including alterations in the TCA cycle, glycolysis, the production of odd chain fatty acids by alpha-oxidation, fatty acid synthesis, fatty acid desaturation, polyamine biosynthesis, and trans-esterification to produce complex lipids. The metabolic changes induced during differentiation of the 3T3-L1 cell line were then compared with the metabolic differences between pre- and postdifferentiation primary adipocytes. These metabolic alterations reflect the changing role of the 3T3-L1 cells during differentiation, as well as possibly providing metabolic triggers to stimulate the processes which occur during differentiation.

Proteomics-based identification of differentially-expressed proteins including galectin-1 in the blood plasma of type 2 diabetic patients

Type 2 diabetes (T2D) is a very heterogeneous and multifactorial disease. The pathophysiology of T2D is presumed to occur with an alteration in the levels of plasma proteins. To identify these differentially expressed proteins, plasma samples from normal and T2D humans were subjected to two-dimensional gel electrophoresis, quantitative densitometry, and mass spectrometry. Up to 200 protein spots were visible on each gel, of which 57 appeared modulated in diabetic individuals. Subsequently, 31 spots with > or =2-fold change in their expression were analyzed by MALDI-TOF mass spectrometry leading to the identification of 11 proteins with average sequence coverage of approximately 38%. The expression of apolipoprotein A-I was reduced by 4.2-fold, and galectin-1 was increased 4.8 times in diabetic samples. Induction of galectin-1 in T2D samples was confirmed by ELISA. In addition, the dose-dependent treatment of rat L6 skeletal muscle cells with glucose resulted in an upregulation of galectin-1. These data implicate the association of galectin-1 with the pathophysiology of diabetes and identify galectin-1 as a novel diagnostic marker protein in T2D patients.

Incorporation of 3T3-L1 Cells To Mimic Bioaccumulation in a Microscale Cell Culture Analog Device for Toxicity Studies

Deficiencies in the early ADMET (absorption, distribution, metabolism, elimination, and toxicity) information on drug candidates extract a significant economic penalty on pharmaceutical firms. We have developed a microscale cell culture analog (microCCA) device that can potentially provide better, faster, and more efficient prediction of human and animal responses to a wide range of chemicals. The system described in this paper is a simple four-chamber microCCA ("lung"-"liver"-"fat"-"other tissue") designed on the basis of a physiologically based pharmacokinetics (PBPK) model of a rat. Cultures of L2, HepG2/C3A, and differentiated 3T3-L1 adipocytes were selected to mimic the key functions of the lung, liver, and fat compartments, respectively. Here, we have demonstrated the application of the microCCA system to study bioaccumulation, distribution, and toxicity of selected compounds. Results from the bioaccumulation study reveal that hydrophobic compounds such as fluoranthene preferentially accumulated in the fat chamber. Only a small amount of fluoranthene was observed in the liver and lung chambers. In addition, the presence of the differentiated 3T3-L1 adipocytes in the microCCA device significantly reduced naphthalene and naphthoquinone-induced glutathione (GSH) depletion. These findings suggest the potential utilization of the microCCA system to assess ADMET characteristics of the compound of interest prior to animal or human trials.

Comparative expression analysis of isolated human adipocytes and the human adipose cell lines LiSa-2 and PAZ6

OBJECTIVE

To obtain insight in the extent to which the human cell lines LiSa-2 and PAZ6 resemble isolated primary human adipocytes.

DESIGN

A combination of cDNA subtraction (representative difference analysis; RDA) and cDNA microarray analysis was used to select adipose specific genes to compare isolated (pre-)adipocytes with (un)differentiated LiSa-2 and PAZ6 cells.

MEASUREMENTS

RDA was performed on adipose tissue against lung tissue. A total of 1400 isolated genes were sequenced and cDNA microarray technology was used for further adipose related gene selection. 30 genes that were found to be enriched in adipose tissue were used to compare isolated human adipocytes and LiSa-2 and PAZ6 cells in the differentiated and undifferentiated states.

RESULTS

RDA and microarray analysis resulted in the identification of adipose enriched genes, but not in adipose specific genes. Of the 30 most differentially expressed genes, as expected, most were related to lipid metabolism. The second category consisted of methyltransferases, DNMT1, DNMT3a, RNMT and SHMT2, of which the expression was differentiation dependent and higher in differentiated adipocytes. Using the 30 adipose expressed genes, it was found that isolated adipocytes on one hand, and PAZ6 and LiSa-2 adipocytes on the other, differ primarily in lipid metabolism. Furthermore, LiSa-2 cells seem to be more similar to isolated adipocytes than PAZ6 cells.

CONCLUSION

The LiSa-2 cell line is a good model for differentiated adipocytes, although one should keep in mind that the lipid metabolism in these cells deviates from the in vivo situation Furthermore, our results imply that methylation may have an important function in terminal adipocyte differentiation.

Histone H3 Lysine 4 Dimethylation Signals the Transcriptional Competence of the Adiponectin Promoter in Preadipocytes

Adipogenesis is regulated by a coordinated cascade of sequence-specific transcription factors and coregulators with chromatin-modifying activities that are between them responsible for the establishment of the gene expression pattern of mature adipocytes. Here we examine the histone H3 post-translational modifications occurring at the promoters of key adipogenic genes during adipocyte differentiation. We show that the promoters of apM1, glut4, gpd1, and leptin are enriched in dimethylated histone H3 Lys4 (H3-K4) in 3T3-L1 fibroblasts, where none of these genes are yet expressed. A detailed study of the apM1 locus shows that H3-K4 dimethylation is restricted to the promoter region in undifferentiated cells and associates with RNA polymerase II (pol II) loading. The beginning of apM1 transcription at the early stages of adipogenesis coincides with promoter H3 hyperacetylation and H3-K4 trimethylation. At the coding region, H3 acetylation and dimethylation, as well as pol II binding, are found in cells at later stages of differentiation, when apM1 transcription reaches its maximal peak. This same pattern of histone modifications is detected in mouse primary preadipocytes and adipocytes but not in a related fibroblast cell line that is not committed to an adipocyte fate. Inhibition of H3-K4 methylation by treatment of 3T3-L1 cells with methylthioadenosine results in decreased apM1 gene expression as well as decreased adipogenesis. Taken together, our data indicate that H3-K4 dimethylation and pol II binding to the promoter of key adipogenic genes are distinguishing marks of cells that have undergone determination to a preadipocyte stage.

Down-regulation of histone deacetylases stimulates adipocyte differentiation

Specific cell type differentiation is driven by programmed regulation of gene expression, which is the result of coordinated modulation of the transcription machinery and chromatin-remodeling factors. We present evidence here that the down-regulation of histone deacetylases is an important process during adipocyte differentiation. In 3T3-L1 cells, histone hyperacetylation was selectively induced at the promoter regions of adipogenic genes during adipocyte differentiation. Interestingly, this was accompanied by a dramatic decrease in the expression level of several histone deacetylases including HDAC1, -2, and -5 and a reduction in overall histone deacetylase enzyme activity. Inhibition of histone deacetylase activity using sodium butyrate resulted in stimulation of adipogenic gene expression and adipocyte differentiation. Consistently, HDAC1 knock-down promoted adipogenesis whereas HDAC1 overexpression attenuated adipocyte differentiation in 3T3-L1 cells. Together, these results suggest that the regulation of not only adipogenic transcription factors, but also chromatin-modifying enzymes is crucial for the execution of bona fide adipogenesis.

The vanadyl (VO2+) chelate bis(acetylacetonato)oxovanadium(IV) potentiates tyrosine phosphorylation of the insulin receptor

We have compared the insulin-like activity of bis(acetylacetonato)oxovanadium(IV) [VO(acac)2], bis(maltolato)oxovanadium(IV) [VO(malto)2], and bis(1-N-oxide-pyridine-2-thiolato)oxovanadium(IV) [VO(OPT)2] in differentiated 3T3-L1 adipocytes. The insulin-like influence of VO(malto)2 and VO(OPT)2 was decreased compared with that of VO(acac)2. Also, serum albumin enhanced the insulin-like activity of all three chelates more than serum transferrin. Each of the three VO2+ chelates increased the tyrosine phosphorylation of proteins in response to insulin, including the beta-subunit of the insulin receptor (IRbeta) and the insulin receptor substrate-1 (IRS1). However, VO(acac)2 exhibited the greatest synergism with insulin and was therefore further investigated. Treatment of 3T3-L1 adipocytes with 0.25 mM VO(acac)2 in the presence of 0.25 mM serum albumin synergistically increased glycogen accumulation stimulated by 0.1 and 1 nM insulin, and increased the phosphorylation of IRbeta, IRS1, protein kinase B, and glycogen synthase kinase-3beta. Wortmannin suppressed all of these classical insulin-signaling activities exerted by VO(acac)2 or insulin, except for tyrosine phosphorylation of IRbeta and IRS1. Additionally, VO(acac)2 enhanced insulin signaling and metabolic action in insulin-resistant 3T3-L1 adipocytes. Cumulatively, these results provide evidence that VO(acac)2 exerts its insulin-enhancing properties by directly potentiating the tyrosine phosphorylation of the insulin receptor, resulting in the initiation of insulin metabolic signaling cascades in 3T3-L1 adipocytes.

Regulation of apelin mRNA expression by insulin and glucocorticoids in mouse 3T3-L1 adipocytes

The novel 36-amino acid peptide, apelin, is the endogenous ligand for the orphan receptor APJ. Apelin may play important roles in the regulation of the cardiovascular system and the hypothalamic-pituitary axis. It is a potent hypotensive agent and one of the most potent stimulators of cardiac contractility. In this study, we investigated the roles of apelin derived from adipocytes in the regulation of cardiovascular homeostasis. We found that both apelin and APJ mRNAs were expressed in isolated mouse adipocytes and that apelin mRNA levels increased during the differentiation of 3T3-L1 cells to adipocytes. We also found that the administration of insulin (1 nM-100 nM) increased, while that of dexamethasone (0.1 nM-100 nM) decreased the apelin mRNA levels in 3T3-L1 adipocytes in a dose-dependent manner, suggesting that insulin and glucocorticoids regulate apelin gene expression in adipocytes. We speculate that high glucocorticoid levels suppress apelin production and stimulate angiotensin II production in adipocyte, decreasing the counter-regulatory activity of apelin against the pressor action of angiotensin II, which might partly be involved in the mechanism underlying the development of obesity-related hypertension.

Desnutrin, an adipocyte gene encoding a novel patatin domain-containing protein, is induced by fasting and glucocorticoids: ectopic expression of desnutrin increases triglyceride hydrolysis

We have used rat cDNA microarrays to identify adipocyte-specific genes that could play an important role in adipocyte differentiation or function. Here, we report the cloning and identification of a 2.0-kb mRNA coding for a putative protein that we have designated as desnutrin. The novel gene is expressed predominantly in adipose tissue, and its expression is induced early during 3T3-L1 adipocyte differentiation. Desnutrin mRNA levels were regulated by the nutritional status of animals, being transiently induced during fasting. In vitro desnutrin gene expression was up-regulated by dexamethasone in a dose-dependent manner but not by cAMP, suggesting that glucocorticoids could mediate the increase in desnutrin mRNA levels observed during fasting. Desnutrin mRNA codes for a 486-amino acid putative protein containing a patatin-like domain, characteristic of many plant acyl hydrolases belonging to the patatin family. Confocal microscopy of enhanced green fluorescent protein-tagged desnutrin protein-transfected cells showed that the fusion protein localized in the cytoplasm. Moreover, cells overexpressing desnutrin by transfection showed an increase in triglyceride hydrolysis. Interestingly, we also found that the desnutrin gene expression level was lower in ob/ob and db/db obese mouse models. Overall, our data suggest that the newly identified desnutrin gene codes for an adipocyte protein that may function as a lipase and play a role in the adaptive response to a low energy state, such as fasting, by providing fatty acids to other tissues for oxidation. In addition, decreased expression of desnutrin in obesity models suggests its possible contribution to the pathophysiology of obesity.

Simple flow cytometric method used to assess lipid accumulation in fat cells

Adipogenesis of preadipocytes in culture has been frequently used to study the molecular basis and effect of drugs on fat cell conversion. However, after adipogenic induction, cells respond to the inducing agent with various speeds of conversion and fat accumulation, which complicates direct molecular and biochemical analyses. Here we present a simple and sensitive method to detect and quantify fat accumulation inside cells by flow cytometry. Using this method, we detected elevated levels of cytoplasmic granularity that correlated well with an increased level of fat accumulated inside cells after adipogenic conversion. We further demonstrated the ability of this method to monitor and quantify fat cell maturation within a complex population of cells and to identify and collect the fat cells with similar fat storage for further analysis. Flow cytometry offers distinct advantages over existing detection systems for cytoplasmic lipid staining and lipid extraction and could represent a powerful analytical tool to monitor the effect of chemicals and biological molecules on fat cell conversion and maturation. Moreover, in combination with a cell sorting facility, our method offers a simple and efficient means of collecting fat cells of specific status for further analysis.

Carnitine profiles during differentiation and effects of carnitine on differentiation of 3T3-L1 cells

These experiments were conducted to investigate carnitine profiles during differentiation and the effects of carnitine on the differentiation of 3T3-L1 cells. To induce cell differentiation, undifferentiated 3T3-L1 cells were treated with dexamethasone, 1-methyl-3-isobutylxanthine, and d-biotin. Carnitine was also exogenously added to the cells to test its effect on cell differentiation. Triglyceride, total lipid, total protein, nonesterified carnitine, acid-soluble acylcarnitine, and acid-insoluble acylcarnitine were analyzed during the differentiation of 3T3-L1 cells. Total lipid, triglyceride, and total protein increased during the 3T3-L1 cell differentiation. However, nonesterified carnitine, acid-soluble acylcarnitine, and acid-insoluble acylcarnitine concentrations were lower in the differentiated 3T3-L1 cells. In addition, the exogenously added carnitine inhibited the increases in triglyceride and total lipid levels. These results suggest that carnitine may have an inhibitory role on the early stage of 3T3-L1 cell differentiation.

Expression of the murine glutathione S-transferase α3 (GSTA3) subunit is markedly induced during adipocyte differentiation: activation of the GSTA3 gene promoter by the pro-adipogenic eicosanoid 15-deoxy-Δ12,14-prostaglandin J2

The expression of class alpha, micro, and pi glutathione S-transferases (GSTs) has been examined during the adipose conversion of mouse 3T3-L1 cells. The GSTA4, GSTM1, and GSTP1/2 subunits are expressed constitutively in confluent 3T3-L1 cells, and their levels remain essentially unchanged during adipocyte differentiation. By contrast, the GSTA3 subunit is virtually undetectable in confluent 3T3-L1 cells under basal conditions, but its expression is markedly induced during adipose conversion. Inhibition of the 3T3-L1 adipogenic program demonstrated that GSTA3 expression is associated specifically with acquisition of the adipocytic phenotype. Reporter gene assays demonstrated that the mouse GSTA3 5(')-upstream region is transcriptionally activated by 15-deoxy-Delta(12,14)-prostaglandin J(2) through an antioxidant response element, suggesting that this pro-adipogenic eicosanoid may be involved in regulating GSTA3 expression during adipogenesis. These data suggest a previously unrecognised role for GSTs in mouse adipocytes.

Expression of p68 RNA helicase is closely related to the early stage of adipocyte differentiation of mouse 3T3-L1 cells

We have previously identified the genes expressed early in the differentiation of mouse 3T3-L1 cells into adipocytes. Since these genes were isolated as small fragments, many were unknown. In this study, we have cloned two full-length cDNAs and identified them as p68 RNA helicase and mc3s5/mtCLIC. The expression of these genes was rapidly induced, and specific to the adipocyte differentiation. When the expression of p68 RNA helicase was inhibited using an inducible antisense system, the differentiation into adipocytes was partially blocked, and the expression levels of some marker genes decreased. These findings strongly indicate that the expression of the above two genes was closely related to the adipocyte differentiation, and p68 RNA helicase in particular is crucial to the differentiation.

Adiponutrin, a transmembrane protein corresponding to a novel dietary- and obesity-linked mRNA specifically expressed in the adipose lineage

We have used a mRNA differential display technique to identify new genes involved in the reprogramming of gene expression during the adipose conversion of mouse 3T3 preadipocyte cell lines. We report here on the identification and cloning of a novel adipose-specific cDNA encoding a predicted membrane protein of 413 amino acids. The level of the corresponding 3.2-kilobase mRNA is tremendously increased during 3T3-L1 and 3T3-F442A differentiation into adipocytes. A single, very abundant 3.2-kilobase transcript is also found in inguinal and epididymal white adipose tissues and in interscapular brown adipose tissue but not in any other tissues examined. Its expression in adipose tissue is under tight nutritional regulation. The level of this novel 3.2-kilobase transcript becomes virtually nondetectable during fasting but is dramatically increased when fasted mice are refed a high carbohydrate diet. Based on its adipose specificity and dietary regulation, the novel gene product has been designated adiponutrin. The expression of adiponutrin mRNA is also 50-fold elevated in genetically obese fa/fa rats, indicating a link between adiponutrin and obesity. Western blot and confocal imagery analyses with epitope-tagged protein transiently expressed in 3T3-L1 adipocytes, and COS cells show that adiponutrin strictly localizes to membranes and is absent from the cytosol. Sequence analysis reveals homologies with several other members of related eukaryotic proteins, including a human paralog, which has been recently described in vesicular transport mechanisms. This leads us to suggest that adiponutrin could be involved in vesicular targeting and protein transport restricted to the adipocyte function.

RGS2 promotes adipocyte differentiation in the presence of ligand for peroxisome proliferator-activated receptor gamma

The events at the earliest stage of adipocyte differentiation are yet to be fully elucidated. Previously, we cloned the genes that are induced at the beginning of the differentiation of mouse 3T3-L1 preadipocyte cells. We found that the gene expression of regulators of G protein signaling-2 (RGS2) rapidly increased after the addition of inducers and decreased at 3-12 h. The expression pattern of RGS2 mRNAs differed among growth-arrested and proliferating 3T3-L1 cells and NIH-3T3 cells, indicating a specificity for adipogenesis. Here we report that the ectopic expression of RGS2 using a retroviral system in mouse NIH-3T3 cells promotes adipogenesis only in the presence of BRL49653, which is a ligand for the peroxisome proliferator-activated receptor gamma (PPARgamma). These results strongly suggest that RGS2 play a crucial role in the program of adipocyte differentiation and may contribute to the function of PPARgamma.

The molecular mechanism of the stimulation of adipocyte differentiation by a glucocorticoid

The glucocorticoid dexamethasone (dex) stimulates differentiation of cultured preadipocytes of the 3T3-L1 cell line to adipocytes. The preadipocytes express a gene termed preadipocyte factor 1 (pref-1). The pref-1 gene product is abundant in preadipocytes but completely absent in adipocytes. Constitutive expression of pref-1 blocks differentiation of adipocytes, as does a soluble 50 kDa fragment of the pref-1 protein. It appears that dex mediates adipogenesis by down-regulating expression of pref-1.

Transcriptional repression of pref-1 by glucocorticoids promotes 3T3-L1 adipocyte differentiation

Pref-1 is an epidermal growth factor-like domain-containing transmembrane protein that is cleaved to generate a soluble factor. It is abundant in 3T3-L1 preadipocytes but absent in mature adipocytes. Constitutive expression of pref-1 or the addition of its ectodomain inhibits adipogenesis. We find that the pref-1 gene is an early target of dexamethasone, a component of the dexamethasone/methylisobutylxanthine differentiation mixture used routinely for adipoconversion. The time course of the decrease in pref-1 mRNA by dexamethasone reflected the pref-1 mRNA half-life determined by actinomycin D treatment. Nuclear run-on assays showed that dexamethasone attenuates pref-1 transcription. We demonstrate a correlation between pref-1 down-regulation and adipoconversion by varying the time period and concentration of dexamethasone. Increasing the dexamethasone treatment from 2 to 4 days resulted in a time-dependent pref-1 down-regulation and increased differentiation as measured by adipocyte marker mRNAs. The dexamethasone concentration between 1 and 10 nM showed a dose-dependent decrease in pref-1 mRNA and an enhancement of adipogenesis. To test the hypothesis that dexamethasone initiation of adipoconversion may be via down-regulation of pref-1, we lowered endogenous pref-1 mRNA levels by stably transfecting 3T3-L1 preadipocytes with antisense pref-1. At 1 microM, antisense cells had enhanced adipose conversion; a similar degree of differentiation occurred with 2 nM dexamethasone, a concentration that does not support differentiation of control 3T3-L1 cells. We conclude that dexamethasone-mediated repression of pref-1 contributes to the mechanisms whereby glucocorticoids promote adipogenesis.

Identification of inducible genes at the early stage of adipocyte differentiation of 3T3-L1 cells

Adipocyte differentiation takes place via a complex series of steps. While PPARgamma2 and C/EBPalpha are known to be master regulators, the events at the earliest stage of adipocyte differentiation are not yet known. In this study, we cloned the genes which are induced at the beginning of differentiation of 3T3-L1 preadipocyte cells. Of 58 clones obtained, only a few were already reported as the genes that are expressed differentially during adipocyte development. More than 30 clones are known but have been newly identified here as differentially expressed genes. Nineteen clones seemed to be unknown genes. The expression of RGS2, HSP105, Rho (TC10), VDR, and HIF-1alpha genes isolated here rapidly increased after the addition of inducers, and after 3-12 h the levels of expression decreased. The expression patterns of these mRNAs were different among growth-arrested and proliferating 3T3-L1 cells and NIH-3T3 cells, strongly indicating that some of the proteins identified here have crucial roles in the program of adipocyte differentiation.

Transcriptional control of the pref-1 gene in 3T3-L1 adipocyte differentiation. Sequence requirement for differentiation-dependent suppression

Preadipocyte factor-1 (Pref-1) is a transmembrane epidermal growth factor-like domain-containing protein highly expressed in 3T3-L1 preadipocytes, but is undetectable in mature fat cells; this down-regulation is required for adipocyte differentiation. We show here that pref-1 transcription is markedly suppressed during adipose conversion and results in decreased Pref-1 RNA levels. Using 3T3-L1 cells stably transfected with Pref-1 5'-deletion constructs truncated at -6000, -2100, -1300, -692, -300, -235, -193, -183, -170, -93, and -45 base pairs, we determined that the -183 to -170 region is responsible for the suppression of the pref-1 gene during adipogenesis. This is distinct from the -93 to -45 sequence important for pref-1 promoter activity in preadipocytes. The placement of a 40-base pair -193 to -154 pref-1 sequence containing the putative SAD (suppression in adipocyte differentiation) element upstream of the SV40 promoter decreased promoter activity by 85% upon adipocyte differentiation, compared with 40% observed with the SV40 promoter alone. The SAD element is therefore sufficient for adipocyte differentiation-dependent down-regulation of a heterologous promoter. A DNA-protein complex was observed when the -193 to -174 sequence was used with 3T3-L1 nuclear extracts in gel mobility shift assays. Competition with oligonucleotides harboring base substitution mutations identified a core sequence of -183AAAGA-179 as crucial for DNA-protein complex formation. UV cross-linking predicts that an approximately 63-kDa protein specifically binds the SAD element.

Human milk and infant formula can induce in vitro adipocyte differentiation in murine 3T3-L1 preadipocytes

The potential of infant diet to influence fat cell development has largely been examined in clinical studies with conflicting results. In this study, the direct effects of two standard infant formulas, Enfamil and Similac, as well as human milk were examined using a well characterized model of adipocyte differentiation, the 3T3-L1 murine preadipocyte cell line. After exposure to a hormonal regimen of insulin, dexamethasone, and 1-methyl-3-isobutylmethylxanthine, these cells undergo a mitotic expansion phase followed by terminal differentiation. On d 4 of hormonal exposure, greater than 95% of 3T3-L1 cells exhibit the morphologic and biochemical characteristics of mature adipocytes. In this study, cells were exposed to control medium, or control medium supplemented with either 10% Enfamil, 10% Similac, 10% human milk (skim or whole), or the standard hormonal regimen. Oil Red O-detectable lipid accumulation, immunocytochemical cell proliferation assays, and activated expression of adipocyte differentiation-specific mRNAs by Northern blot analysis were used to assess the effects of treatment on adipocyte differentiation. Results from each level of assessment revealed that both Enfamil and human milk were as effective as the standard hormonal regimen at stimulating adipocyte differentiation. In contrast, results from treatment with Similac or human skim milk were indistinguishable from control unstimulated cells. This study, demonstrating that Enfamil and human milk are capable of independently inducing in vitro adipocyte differentiation, suggests that diet during infancy could influence body fat development.

Leukoregulin induction of protein expression in human orbital fibroblasts: evidence for anatomical site-restricted cytokine-target cell interactions

The molecular basis for the profound inflammatory response and the accumulation of hyaluronan in orbital connective tissues seen in thyroid-associated ophthalmopathy is unknown. Moreover, the link between the orbital manifestations of Graves' disease and those in the pretibial skin, localized dermopathy, has yet to be established. We have reported recently that leukoregulin, an activated T lymphocyte-derived cytokine, dramatically induces hyaluronan synthesis and prostaglandin-endoperoxide H synthase 2 in human orbital fibroblasts in culture. In the current studies, utilizing giant two-dimensional gel electrophoresis, we find that orbital fibroblasts express constitutively a protein profile that distinguishes them from skin fibroblasts derived from the abdominal wall and from the pretibium. We further demonstrate that leukoregulin, when present in culture medium for 16 hr, up-regulates a set of orbital fibroblast proteins not present in untreated cultures or in fibroblasts from the abdominal wall. However, some of the same protein inductions are present in the pretibial fibroblasts. These leukoregulin-induced changes in protein expression are completely blocked by dexamethasone (10 nM). Our findings are the first to identify proteins that appear to be expressed and differentially regulated in an anatomical site-restricted manner in orbital and pretibial fibroblasts and seem to establish a molecular link between fibroblasts from the orbit and those in pretibial skin.

Developmental profile of homeobox gene expression during 3T3-L1 adipogenesis

The homeobox family of proteins are transcription factors are known to be important during the differentiation of a variety of mammalian tissues, however, expression of the genes encoding homeobox proteins during adipogenesis or in adipose tissue has not been described. To investigate whether members of the homeobox gene family are expressed and regulated during adipocyte differentiation, RNA was isolated from 3T3-L1 preadipocyte cells during the hormonal induced differentiation of this cell line into adipocytes. A reverse transcriptase-polymerase chain reaction strategy using degenerate oligonucleotide primers complementary to the highly conserved homeodomain resulted in the identification of 10 different homeobox genes expressed during 3T3-L1 adipogenesis. One of the clones appears to be unique and 9 of the clones represented known members of the homeobox gene family. Examination of the relative mRNA levels encoding these proteins by ribonuclease protection assay during adipocyte differentiation revealed that 3 members, Hox a4, Hox a7, and Hox d4, are regulated as a function of adipocyte development. Further examination of RNA isolated from murine retroperitoneal adipose tissue revealed that these three regulated homeobox mRNAs are expressed in vivo. Combined, these results suggest that members of the homeobox gene family may serve an important role during the differentiation of adipocytes.

Regulation and expression of retinoblastoma proteins p107 and p130 during 3T3-L1 adipocyte differentiation

During 3T3-L1 adipocyte differentiation, growth-arrested, postconfluent preadipocytes are required to reenter the cell cycle and proceed through a mitotic clonal expansion phase prior to terminal differentiation. The retinoblastoma proteins (pRB, p107, and p130) are thought to be critical in controlling cell cycle progression by binding to and regulating the activity of the E2F transcription factors. We show here that p130/p107 protein levels, p107 mRNA levels, and E2F DNA binding complexes are regulated during 3T3-L1 adipogenesis. The predominant E2F binding complex in day 0 preadipocytes was p130-E2F with no detectable free E2F or p107. On Day 1, during mitotic clonal expansion, there was a distinct switch to free E2F and p107-E2F complexes associated with increased p107 mRNA and protein along with decreased p130 protein levels. Following differentiation, the day 0 pattern is reestablished. The switch is not just a consequence of reentry into the cell cycle, in that p107 protein levels are both detectable and unchanged in dividing, serum-restricted, or serum restimulated preconfluent cells. Interestingly, hormonal stimulation of 3T3-C2 cells, a related nondifferentiating cell line, also induces a mitotic clonal expansion phase that is associated with the p130:p107 switch in a pattern very similar to 3T3-L1 cells, suggesting the block in differentiation observed in 3T3-C2 cells occurs after clonal expansion. Combined, these findings suggest that the regulatory mechanisms of the p130:p107 switch are not specific to differentiation but may play a key role in regulating the mitotic clonal expansion necessary for adipocyte differentiation in 3T3-L1 cells.

cAMP activates the expression of stearoyl-CoA desaturase gene 1 during early preadipocyte differentiation

Stearoyl-CoA desaturase gene 1 (scd1) mRNA levels were induced during the 1st day of 3T3-L1 differentiation. scd1 expression had previously been observed only in late-stage differentiation, during the period of triacylglycerol accumulation. The induction of 3T3-L1 differentiation requires treatment with insulin, dexamethasone, and an agent that increases intracellular cAMP levels. Treatment of preadipocytes with cAMP-elevating agents caused an increase in scd1 mRNA concentrations. Insulin and dexamethasone had no effect on scd1 mRNA expression in preadipocytes. The increase in mRNA resulted from transcriptional activation of the scd1 gene. 8-Bromo-cAMP treatment of differentiated adipocytes had no effect on scd1 mRNA levels, suggesting a preadipocyte-specific effect. The increase in scd1 mRNA occurred maximally after 6 h of cAMP treatment and was shown to require protein synthesis. Deletion and mutagenesis analyses have localized the cAMP response to sequence within the first 250 base pairs of the 5'-flanking region of the scd1 gene. Treatment with phorbol ester enhanced the induction of scd1 mRNA by cAMP, suggesting the involvement of AP-2 as a mediator of the cAMP response. The induction of scd1 expression by cAMP during early differentiation was distinct from that observed during late adipocyte development. This early expression presents a novel regulated function of scd1 during the differentiation process, independent of its lipogenic role in late adipocyte development. The induction of scd1 also introduces a role for cAMP in 3T3-L1 differentiation.

Evidence for a role of insulin in hepatocytic differentiation of human hepatoma BC1 cells

To examine the effect of insulin on hepatocytic differentiation, we took advantage of the properties of the newly established human hepatoma BC1 cell line to maintain quiescence after confluency and to progressively acquire in culture (3 weeks after confluency) an hepatocytic phenotype, as assessed by expression of specific hepatic genes (Le Jossicet al., 1995). In BC1 cells cultured in the presence of insulin (1 μM: ), expression of albumin and transferrin mRNA and protein occurs earlier than in cells cultured in its absence (1 weekvs 2 weeks). Moreover, at any time considered, the level of the two hepatic markers was higher (2- to 3-fold) in the former than in untreated cells. The beneficial effect of insulin on hepatocytic differentiation of BC1 cells was paralleled by: i) modest increases in insulin receptor (IR) mRNA level and IR binding activity, and ii) a 6-fold increase in sensitivity to insulin for stimulation of glycogenesis. These results provide the first evidence for insulin's ability to exert a positive effect on hepatocytic differentiation. The beneficial effect of insulin probably results both from increased IR expression and binding activity and from alteration at post-receptor levels.