Insulin-like growth factor-1/insulin bypasses Pref-1/FA1-mediated inhibition of adipocyte differentiation

Cloning of goat PGAM2 gene and its overexpression promotes the differentiation of intramuscular preadipocytes

As a member of the PGAMs family, PGAM2 has been proved to catalyze the reversible reaction of 3-phosphoglycerate (3-PGA) to 2-phosphoglycerate (2-PGA) in the glycolytic pathway. However, it is unclear whether PGAM2 has a role in regulating differentiation in goat intramuscular preadipocytes. Here, this study was carried to clone the open reading frame (ORF) of goat PGAM2, elucidate its molecular and expressional characteristics, and evaluate the involvement in adipogenesis of intramuscular preadipocytes. According to our findings, the cloned goat PGAM2 gene was 784 bp in full length, including 762 bp of ORF and encoding a protein of 253 amino acids. The expressional level of PGAM2 peaked at 48 hours after induced adipogenic differentiation and was highest in the skeletal muscle of triceps. Moreover, overexpression of PGAM2 transfected by its overexpression plasmid promotes lipid accumulation of goat intramuscular adipocyte as shown by Oil Red O and bodipy staining, accompanied by up-regulating the mRNA levels of peroxisome proliferator-activated receptor γ (PPARγ) (p < 0.001), sterol regulatory element-binding protein 1 (SREBP1) (p < 0.001), CCAAT/Enhancer-binding protein α (C/EBPα) (p < 0.01) and lipoprotein lipase (LPL) (p < 0.01). Taken together, these findings indicate that PGAM2 is a positive regulator for goat intramuscular adipocytes and provide new insights into improvement intramuscular fat deposition in goat meat.

Lipedema: Insights into Morphology, Pathophysiology, and Challenges

Lipedema is an adipofascial disorder that almost exclusively affects women. Lipedema leads to chronic pain, swelling, and other discomforts due to the bilateral and asymmetrical expansion of subcutaneous adipose tissue. Although various distinctive morphological characteristics, such as the hyperproliferation of fat cells, fibrosis, and inflammation, have been characterized in the progression of lipedema, the mechanisms underlying these changes have not yet been fully investigated. In addition, it is challenging to reduce the excessive fat in lipedema patients using conventional weight-loss techniques, such as lifestyle (diet and exercise) changes, bariatric surgery, and pharmacological interventions. Therefore, lipedema patients also go through additional psychosocial distress in the absence of permanent treatment. Research to understand the pathology of lipedema is still in its infancy, but promising markers derived from exosome, cytokine, lipidomic, and metabolomic profiling studies suggest a condition distinct from obesity and lymphedema. Although genetics seems to be a substantial cause of lipedema, due to the small number of patients involved in such studies, the extrapolation of data at a broader scale is challenging. With the current lack of etiology-guided treatments for lipedema, the discovery of new promising biomarkers could provide potential solutions to combat this complex disease. This review aims to address the morphological phenotype of lipedema fat, as well as its unclear pathophysiology, with a primary emphasis on excessive interstitial fluid, extracellular matrix remodeling, and lymphatic and vasculature dysfunction. The potential mechanisms, genetic implications, and proposed biomarkers for lipedema are further discussed in detail. Finally, we mention the challenges related to lipedema and emphasize the prospects of technological interventions to benefit the lipedema community in the future.

Different Expression Levels of DLK2 Inhibit NOTCH Signaling and Inversely Modulate MDA-MB-231 Breast Cancer Tumor Growth In Vivo

NOTCH signaling is implicated in the development of breast cancer tumors. DLK2, a non-canonical inhibitor of NOTCH signaling, was previously shown to be involved in skin and breast cancer. In this work, we studied whether different levels of DLK2 expression influenced the breast cancer characteristics of MDA-MB-231 cells. We found that DLK2 overexpression inhibited NOTCH activation in a dose-dependent manner. Moreover, depending on the level of inhibition of NOTCH1 activation generated by different levels of DLK2 expression, cell proliferation, cell cycle dynamics, cell apoptosis, cell migration, and tumor growth in vivo were affected in opposite directions. Low levels of DLK2 expression produced a slight inhibition of NOTCH1 activation, and enhanced MDA-MB-231 cell invasion in vitro and cell proliferation both in vitro and in vivo. In contrast, MDA-MB-231 cells expressing elevated levels of DLK2 showed a strong inhibition of NOTCH1 activation, decreased cell proliferation, increased cell apoptosis, and were unable to generate tumors in vivo. In addition, DLK2 expression levels also affected some members of other cell signaling pathways implicated in cancer, such as ERK1/2 MAPK, AKT, and rpS6 kinases. Our data support an important role of DLK2 as a protein that can finely regulate NOTCH signaling and affect the tumor properties and growth dynamics of MDA-MB-231 breast cancer cells.

Emerging Potential of Exosomes on Adipogenic Differentiation of Mesenchymal Stem Cells

The mesenchymal stem cells have multidirectional differentiation potential and can differentiate into adipocytes, osteoblasts, cartilage tissue, muscle cells and so on. The adipogenic differentiation of mesenchymal stem cells is of great significance for the construction of tissue-engineered fat and the treatment of soft tissue defects. Exosomes are nanoscale vesicles secreted by cells and widely exist in body fluids. They are mainly involved in cell communication processes and transferring cargo contents to recipient cells. In addition, exosomes can also promote tissue and organ regeneration. Recent studies have shown that various exosomes can influence the adipogenic differentiation of stem cells. In this review, the effects of exosomes on stem cell differentiation, especially on adipogenic differentiation, will be discussed, and the mechanisms and conclusions will be drawn. The main purpose of studying the role of these exosomes is to understand more comprehensively the influencing factors existing in the process of stem cell differentiation into adipocytes and provide a new idea in adipose tissue engineering research.

Adipogenesis: A Complex Interplay of Multiple Molecular Determinants and Pathways

The formation of adipocytes during embryogenesis has been largely understudied. However, preadipocytes appear to originate from multipotent mesenchymal stromal/stem cells which migrate from the mesoderm to their anatomical localization. Most studies on adipocyte formation (adipogenesis) have used preadipocytes derived from adult stem/stromal cells. Adipogenesis consists of two phases, namely commitment and terminal differentiation. This review discusses the role of signalling pathways, epigenetic modifiers, and transcription factors in preadipocyte commitment and differentiation into mature adipocytes, as well as limitations in our understanding of these processes. To date, a limited number of transcription factors, genes and signalling pathways have been described to regulate preadipocyte commitment. One reason could be that most studies on adipogenesis have used preadipocytes already committed to the adipogenic lineage, which are therefore not suitable for studying preadipocyte commitment. Conversely, over a dozen molecular players including transcription factors, genes, signalling pathways, epigenetic regulators, and microRNAs have been described to be involved in the differentiation of preadipocytes to adipocytes; however, only peroxisome proliferator-activated receptor gamma has proven to be clinically relevant. A detailed understanding of how the molecular players underpinning adipogenesis relate to adipose tissue function could provide new therapeutic approaches for addressing obesity without compromising adipose tissue function.

The Role of Pref-1 during Adipogenic Differentiation: An Overview of Suggested Mechanisms

Obesity contributes significantly to the global health burden. A better understanding of adipogenesis, the process of fat formation, may lead to the discovery of novel treatment strategies. However, it is of concern that the regulation of adipocyte differentiation has predominantly been studied using the murine 3T3-L1 preadipocyte cell line and murine experimental animal models. Translation of these findings to the human setting requires confirmation using experimental models of human origin. The ability of mesenchymal stromal/stem cells (MSCs) to differentiate into adipocytes is an attractive model to study adipogenesis in vitro. Differences in the ability of MSCs isolated from different sources to undergo adipogenic differentiation, may be useful in investigating elements responsible for regulating adipogenic differentiation potential. Genes involved may be divided into three broad categories: early, intermediate and late-stage regulators. Preadipocyte factor-1 (Pref-1) is an early negative regulator of adipogenic differentiation. In this review, we briefly discuss the adipogenic differentiation potential of MSCs derived from two different sources, namely adipose-derived stromal/stem cells (ASCs) and Wharton’s Jelly derived stromal/stem cells (WJSCs). We then discuss the function and suggested mechanisms of action of Pref-1 in regulating adipogenesis, as well as current findings regarding Pref-1’s role in human adipogenesis.

Adipose Stem Cells from Lipedema and Control Adipose Tissue Respond Differently to Adipogenic Stimulation In Vitro

BACKGROUND

Lipedema is characterized by localized accumulation of fat in the extremities, which is typically unresponsive to dietary regimens or physical activity. Although the disease is well described and has a high incidence, little is known regarding the molecular and cellular mechanisms underlying its pathogenesis. The aim of this study was to investigate the pathophysiology of lipedema adipose cells in vitro.

METHODS

Adipose-derived stem cells were isolated from lipoaspirates derived from lipedema and nonlipedema patients undergoing tumescent liposuction. In vitro differentiation studies were performed for up to 14 days using adipogenic or regular culture medium. Supernatants and cell lysates were tested for adiponectin, leptin, insulin-like growth factor-1, aromatase (CYP19A1), and interleukin-8 content at days 7 and 14, using enzyme-linked immunosorbent assays. Adipogenesis was evaluated by visualizing and measuring cytoplasmic lipid accumulation.

RESULTS

Lipedema adipose-derived stem cells showed impeded adipogenesis already at early stages of in vitro differentiation. Concomitant with a strongly reduced cytoplasmic lipid accumulation, significantly lower amounts of adiponectin and leptin were detectable in supernatants from lipedema adipose-derived stem cells and adipocytes compared with control cells. In addition, lipedema and nonlipedema cells differed in their expression of insulin-like growth factor-1, aromatase (CYP19A1), and interleukin-8 and in their proliferative activity.

CONCLUSIONS

The authors' findings indicate that in vitro adipogenesis of lipedema adipose-derived stem cells is severely hampered compared with nonlipedema adipose-derived stem cells. Lipedema adipose cells differ not only in their lipid storage capacity but also in their adipokine expression pattern. This might serve as a valuable marker for diagnosis of lipedema, probably from an early stage on.

Distinct roles of Dlk1 isoforms in bi-potential differentiation of hepatic stem cells

BACKGROUND

Fully understanding the developmental process of hepatic stem cells (HSCs) and the mechanisms of their committed differentiation is essential for optimizing the generation of functional hepatocytes for cell therapy in liver disease. Delta-like 1 homolog (Dlk1), primarily the membrane-bound form (Dlk1^M), is generally used as a surface marker for fetal hepatic stem cell isolation, while its soluble form (Dlk1^S) and the functional roles of different Dlk1 isoforms in HSC differentiation remain to be investigated.

METHODS

Hepatic spheroid-derived cells (HSDCs) were isolated from E12.5 mouse livers to obtain Dlk1⁺ and Dlk1^-subpopulations. Colony formation, BrdU staining, and CCK8 assays were used to evaluate the cell proliferation capacity, and hepatic/cholangiocytic differentiation and osteogenesis/adipogenesis were used to assess the multipotency of the two subpopulations. Transformation of Dlk1⁺ cells into Dlk1^- cells was detected by FACS, and the expression of Dlk1 isoforms were measured by western blot. The distinct roles and regulatory mechanisms of Dlk1 isoforms in HSC differentiation were investigated by overexpressing Dlk1^M.

RESULTS

HSDCs were capable of differentiating into liver and mesenchymal lineages, comprising Dlk1⁺ and Dlk1^- subpopulations. Dlk1⁺ cells expressed both Dlk1^M and Dlk1^S and lost expression of Dlk1^M during passaging, thus transforming into Dlk1^- cells, which still contained Dlk1^S. Dlk1^- cells maintained a self-renewal ability similar to that of Dlk1⁺ cells, but their capacity to differentiate into cholangiocytes was obviously enhanced. Forced expression of Dlk1^M in Dlk1^- cells restored their ability to differentiate into hepatocytes, with an attenuated ability to differentiate into cholangiocytes, suggesting a functional role of Dlk1 in regulating HSC differentiation in addition to acting as a biomarker. Further experiments illustrated that the regulation of committed HSC differentiation by Dlk1 was mediated by the AKT and MAPK signaling pathways. In addition, bFGF was found to serve as an important inducement for the loss of Dlk1^M from Dlk1⁺ cells, and autophagy might be involved.

CONCLUSIONS

Overall, our study uncovered the differential expression and regulatory roles of Dlk1 isoforms in the commitment of HSC differentiation and suggested that Dlk1 functions as a key regulator that instructs cell differentiation rather than only as a marker of HSCs. Thus, our findings expand the current understanding of the differential regulation of bi-potential HSC differentiation and provide a fine-tuning target for cell therapy in liver disease.

Different expression levels of DLK1 inversely modulate the oncogenic potential of human MDA-MB-231 breast cancer cells through inhibition of NOTCH1 signaling

NOTCH receptors participate in cancer cell proliferation and survival. Accumulated evidence indicates that, depending on the cellular context, these receptors can function as oncogenes or as tumor-suppressor genes. The epidermal growth factor-like protein delta-like homolog (DLK)1 acts as a NOTCH inhibitor and is involved in the regulation of normal and tumoral growth. In this work, we focused on the role of DLK1 in the control of breast cancer cell growth, a tumor type in which NOTCH receptors have been shown to play both opposite roles. We found that human DLK1 inhibits NOTCH signaling in MDA-MB-231 breast cancer cells. The proliferation rate and invasion capabilities of these cells depended on the level of NOTCH activation and signaling, as regulated by DLK1. High levels of DLK1 expression led to a significant decrease in NOTCH signaling, which was associated with a decrease in breast cancer cell proliferation and invasion. On the contrary, lower levels of NOTCH inhibition, caused by lower levels of DLK1 overexpression, led to enhanced in vitro MDA-MB-231 cell invasion, and to both in vitro and in vivo increased cell proliferation. The data presented in this work suggest that a fine regulation of NOTCH signaling plays an important role in the control of breast cancer cell proliferation and invasion.-Nueda, M.-L., Naranjo, A.-I., Baladrón V., Laborda, J. Different expression levels of DLK1 inversely modulate the oncogenic potential of human MDA-MB-231 breast cancer cells through inhibition of NOTCH1 signaling.

Antagonistic roles in fetal development and adult physiology for the oppositely imprinted Grb10 and Dlk1 genes

Background

Despite being a fundamental biological problem the control of body size and proportions during development remains poorly understood, although it is accepted that the insulin-like growth factor (IGF) pathway has a central role in growth regulation, probably in all animals. The involvement of imprinted genes has also attracted much attention, not least because two of the earliest discovered were shown to be oppositely imprinted and antagonistic in their regulation of growth. The Igf2 gene encodes a paternally expressed ligand that promotes growth, while maternally expressed Igf2r encodes a cell surface receptor that restricts growth by sequestering Igf2 and targeting it for lysosomal degradation. There are now over 150 imprinted genes known in mammals, but no other clear examples of antagonistic gene pairs have been identified. The delta-like 1 gene (Dlk1) encodes a putative ligand that promotes fetal growth and in adults restricts adipose deposition. Conversely, Grb10 encodes an intracellular signalling adaptor protein that, when expressed from the maternal allele, acts to restrict fetal growth and is permissive for adipose deposition in adulthood.

Results

Here, using knockout mice, we present genetic and physiological evidence that these two factors exert their opposite effects on growth and physiology through a common signalling pathway. The major effects are on body size (particularly growth during early life), lean:adipose proportions, glucose regulated metabolism and lipid storage in the liver. A biochemical pathway linking the two cell signalling factors remains to be defined.

Conclusions

We propose that Dlk1 and Grb10 define a mammalian growth axis that is separate from the IGF pathway, yet also features an antagonistic imprinted gene pair.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-014-0099-8) contains supplementary material, which is available to authorized users.

Inhibition of Pref-1 (preadipocyte factor 1) by oestradiol in adolescent girls with anorexia nervosa is associated with improvement in lumbar bone mineral density

OBJECTIVE

Adolescents with anorexia nervosa (AN) are amenorrheic and have decreased bone mass accrual and low bone mineral density (BMD). The regulation of mesenchymal stem cell differentiation is an important factor governing bone formation. Preadipocyte factor 1 (Pref-1), an inhibitor of adipocyte and osteoblast differentiation, is elevated in states of oestrogen deficiency. In this study, we aim to (i) investigate effects of transdermal oestradiol on Pref-1 in adolescent girls with AN, and (ii) examine associations of changes in Pref-1 with changes in lumbar BMD and bone turnover markers.

DESIGN

Adolescent girls with AN and normal-weight controls were studied cross-sectionally. Girls with AN were examined longitudinally in a double-blind study and received transdermal oestradiol (plus cyclic medroxyprogesterone) or placebo for 12 months.

PATIENTS

Sixty-nine girls (44 with AN, 25 normal-weight controls) 13-18 years were studied at baseline; 22 AN girls were followed prospectively.

MEASUREMENTS

Pref-1 levels, bone formation and resorption markers, and BMD.

RESULTS

Pref-1 levels decreased in girls with AN after treatment with transdermal oestradiol compared with placebo (-0·015 ± 0·016 vs 0·060±0·026 ng/ml, P = 0·01), although at baseline, levels did not differ in AN vs controls (0·246 ± 0·015 vs 0·267 ± 0·022 ng/ml). Changes in Pref-1 over 12 months correlated inversely with changes in lumbar BMD (r = -0·48, P = 0·02) and positively with changes in CTX (r = 0·73, P = 0·006).

CONCLUSIONS

For the first time, we show that Pref-1 is negatively regulated by oestradiol in adolescent girls with AN. Inhibition of Pref-1 may mediate the beneficial effects of transdermal oestradiol replacement on BMD in girls with AN.

Horse serum reduces expression of membrane-bound and soluble isoforms of the preadipocyte marker Delta-like 1 homolog (Dlk1), but is inefficient for adipogenic differentiation of mouse preadipocytes

Downregulation of the preadipocyte marker Delta-like 1 homologue (Dlk1), an inhibitor of adipogenesis, has been suggested to be a prerequisite for adipogenic differentiation to occur, and low Dlk1 levels are often used to verify adipogenesis. Mouse preadipocytic cell lines such as 3T3-L1, as well as primary derived preadipocytes, are important models to study adipogenic differentiation and obesity. However, in vitro adipogenic differentiation of primary derived preadipocytes remains incomplete, and identification of factors that will improve the adipogenic differentiation process is thus of high value. In this study we show that horse serum fails to improve adipogenic differentiation of mouse preadipocytes (both 3T3-L1 cells and primary derived mouse preadipocytes) as otherwise reported for bone marrow derived adipogenic precursors. Unexpectedly, while Dlk1 levels were indeed decreased using horse serum, this did not correlate with a high degree of adipogenic differentiation. In conclusion, our novel results thus reveal that horse serum clearly is insufficient for adipogenic differentiation of mouse preadipocytes and that low levels of Dlk1 alone are a poor marker of mouse in vitro adipogenesis. We would also like to emphasize that it is very important for the field of cellular differentiation that researchers thoroughly investigate the effect of individual reagents in their protocols. Such data will increase understanding of the limitations and possibilities of individual systems.

Pref-1, a gatekeeper of adipogenesis

Preadipocyte factor 1 (Pref-1, also called Dlk1/FA1) is a molecular gatekeeper of adipogenesis which acts by maintaining the preadipocyte state and preventing adipocyte differentiation. Pref-1 is made as an epidermal growth factor-like repeat containing transmembrane protein, and is cleaved by TNFα-converting enzyme (TACE) to generate a soluble form, which acts as an autocrine/paracrine factor. Pref-1 upregulates Sox9 expression by activating the ERK/MAPK pathway and the Pref-1 interaction with fibronectin is required for inhibition of adipogenesis. Pref-1 also prevents brown adipocyte differentiation and its thermogenic function. Here, we highlight the recent evidence for the role of Pref-1 in adipogenesis.

Conditional deletions refine the embryonic requirement for Dlk1

Numerous studies have implicated Delta-like 1 (DLK1), a transmembrane protein that shares homology with Notch ligands, in embryonic growth and differentiation. Dlk1 expression is widespread, though not ubiquitous, during early development, but is confined to a few specific cell types in adults. Adult Dlk1-expressing tissues include the Insulin-producing β-cells of the pancreas and the Growth hormone-producing somatotrophs of the pituitary gland. Previously generated Dlk1 null mice (Dlk1(Sul-pat)), display a partially penetrant neonatal lethality and a complex pattern of developmental and adult phenotypes. Here we describe the generation of a conditional Dlk1 mouse line (Dlk1(flox)) to facilitate cell type-specific deletion of the Dlk1 gene, providing a powerful system to explore each aspect of the Dlk1 null phenotype. Four tissue-specific Cre mouse lines were used to produce individual Dlk1 deletions in pancreatic β-cells, pituitary somatotrophs and the endothelial cells of the embryo and placenta, key candidates for the Dlk1 phenotype. Contrary to expectations, all of these conditional mice were fully viable, and none recapitulated any aspect of the Dlk1(Sul-pat) null mice. Dlk1 expression is therefore not essential for the normal development of β-cells, somatotrophs and endothelial cells, and the tissues responsible for the Dlk1 null phenotype remain to be identified. Dlk1(flox) mice will continue to provide an important tool for further research into the function of Dlk1.

Serum levels of fetal antigen 1 in extreme nutritional States

Objective. Recent data suggest that fetal antigen (FA1) is linked to disorders of body weight. Thus, we measured FA1 serum levels in two extreme nutritional states of morbid obesity (MO) and anorexia nervosa (AN) and monitored its response to weight changes. Design. FA1 and insulin serum concentrations were assessed in a cross-sectional study design at defined time points after gastric restrictive surgery for 25 MO patients and 15 women with AN. Results. Absolute FA1 serum levels were within the assay normal range and were not different between the groups at baseline. However, the ratio of FA1/BMI was significantly higher in AN. FA1 was inversely correlated with BMI before and after weight change in AN, but not in MO patients. In addition, MO patients displayed a significant concomitant decrease of FA1 and insulin with the first 25% of EWL, while in AN patients a significant increase of FA1 was observed in association with weight gain. Conclusion. FA1 is a sensitive indicator of metabolic adaptation during weight change. While FA1 serum levels in humans generally do not correlate with BMI, our results suggest that changes in FA1 serum levels reflect changes in adipose tissue turnover.

The crosstalk between transforming growth factor-β1 and delta like-1 mediates early chondrogenesis during embryonic endochondral ossification

Delta like-1 (Dlk1)/preadipocyte factor-1 (Pref-1)/fetal antigen-1 (FA1) is a novel surface marker for embryonic chondroprogenitor cells undergoing lineage progression from proliferation to prehypertrophic stages. However, mechanisms mediating control of its expression during chondrogenesis are not known. Thus, we examined the effect of a number of signaling molecules and their inhibitors on Dlk1 expression during in vitro chondrogenic differentiation in mouse embryonic limb bud mesenchymal micromass cultures and mouse embryonic fibroblast (MEF) pellet cultures. Dlk1/Pref-1 was initially expressed during mesenchymal condensation and chondrocyte proliferation, in parallel with expression of Sox9 and Col2a1, and was downregulated upon the expression of Col10a1 by hypertrophic chondrocytes. Among a number of molecules that affected chondrogenesis, transforming growth factor-β1 (TGF-β1)-induced proliferation of chondroprogenitors was associated with decreased Dlk1 expression. This effect was abolished by TGF-β signaling inhibitor SB431542, suggesting regulation of Dlk1/FA1 by TGF-β1 signaling in chondrogenesis. TGF-β1-induced Smad phosphorylation and chondrogenesis were significantly increased in Dlk1(-/-) MEF, while they were blocked in Dlk1 overexpressing MEF, in comparison with wild-type MEF. Furthermore, overexpression of Dlk1 or addition of its secreted form FA1 dramatically inhibited TGF-β1-induced Smad reporter activity. In conclusion, our data identified Dlk1/FA1 as a downstream target of TGF-β1 signaling molecule that mediates its function in embryonic chondrogenesis. The crosstalk between TGF-β1 and Dlk1/FA1 was shown to promote early chondrogenesis during the embryonic endochondral ossification process.

Possible roles of DLK1 in the Notch pathway during development and disease

The Delta-Notch pathway is an evolutionarily conserved signaling pathway which controls a broad range of developmental processes including cell fate determination, terminal differentiation and proliferation. In mammals, four Notch receptors (NOTCH1-4) and five activating canonical ligands (JAGGED1, JAGGED2, DLL1, DLL3 and DLL4) have been described. The precise function of noncanonical Notch ligands remains unclear. Delta-like 1 homolog (DLK1), the best studied noncanonical Notch ligand, has been shown to act as an inhibitor of Notch signaling in vitro, but its function in vivo is poorly understood. In this review we summarize Notch signaling during development and highlight recent studies in DLK1expression that reveal new insights into its function.

New factors controlling the balance between osteoblastogenesis and adipogenesis

The majority of conditions associated with bone loss, including aging, are accompanied by increased marrow adiposity possibly due to shifting of the balance between osteoblast and adipocyte differentiation in bone marrow stromal (skeletal) stem cells (MSC). In order to study the relationship between osteoblastogenesis and adipogenesis in bone marrow, we have characterized cellular models of multipotent MSC as well as pre-osteoblastic and pre-adipocytic cell populations. Using these models, we identified two secreted factors in the bone marrow microenviroment: secreted frizzled-related protein 1 (sFRP-1) and delta-like1 (preadipocyte factor 1) (Dlk1/Pref-1). Both exert regulatory effects on osteoblastogenesis and adipogenesis. Our studies suggest a model for lineage fate determination of MSC that is regulated through secreted factors in the bone marrow microenvironment that mediate a cross-talk between lineage committed cell populations in addition to controlling differentiation choices of multipotent MSC.

The IGF-I regulatory system and its impact on skeletal and energy homeostasis

Insulin-like growth factor (IGF)-I is important in the acquisition and maintenance of both soft and hard tissues. Skeletal remodeling requires energy and recent work has demonstrated that bone can influence insulin sensitivity and thereby regulate metabolic processes. New insights from mouse models into the role of IGF-binding proteins (IGFBPs) as more than mere depots for the IGFs has reignited investigations into the metabolic targets influenced by the IGF regulatory system and the pathways that link bone to adipose tissue. Although there remains continued uncertainty about the relative balance between the effects of circulating versus tissue IGF-I actions, the role of the IGFBPs has been redefined both as modulators of IGF-I action and as independent signaling factors. This review highlights several recent findings that shed new light on the physiologic role of the IGF regulatory system and its influence on skeletal and fat metabolism.

Protein tyrosine kinase pathway-derived ROS/NO productions contribute to G2/M cell cycle arrest in evodiamine-treated human cervix carcinoma HeLa cells

A previous study indicated that reactive oxygen species (ROS) and nitric oxide (NO) played pivotal roles in mediating cytotoxicity of evodiamine in human cervix carcinoma HeLa cells. This study suggested that G2/M cell cycle arrest was triggered by ROS/NO productions with regulations of p53, p21, cell division cycle 25C (Cdc25C), Cdc2 and cyclin B1, which were able to be prevented by protein tyrosine kinase (PTK) activity inhibitor genistein or JNK inhibitor SP600125. The decreased JNK phosphorylation by addition of Ras or Raf inhibitor, as well as the increased cell viability by addition of insulin-like growth factor-1 receptor (IGF-1R), Ras, Raf or c-Jun N-terminal kinase (JNK) inhibitor, further demonstrated that the Ras-Raf-JNK pathway was responsible for this PTK-mediated signalling. These observations provide a distinct look at PTK pathway for its suppressive effect on G2/M transition by inductions of ROS/NO generations.

Cross talk between insulin and bone morphogenetic protein signaling systems in brown adipogenesis

Both insulin and bone morphogenetic protein (BMP) signaling systems are important for adipocyte differentiation. Analysis of gene expression in BMP7-treated fibroblasts revealed a coordinated change in insulin signaling components by BMP7. To further investigate the cross talk between insulin and BMP signaling systems in brown adipogenesis, we examined the effect of BMP7 in insulin receptor substrate 1 (IRS-1)-deficient brown preadipocytes, which exhibit a severe defect in differentiation. Treatment of these cells with BMP7 for 3 days prior to adipogenic induction restored differentiation and expression of brown adipogenic markers. The high level of adipogenic inhibitor preadipocyte factor 1 (Pref-1) in IRS-1-null cells was markedly reduced by 3 days of BMP7 treatment, and analysis of the 1.3-kb pref-1 promoter revealed 9 putative Smad binding elements (SBEs), suggesting that BMP7 could directly suppress Pref-1 expression, thereby allowing the initiation of the adipogenic program. Using a series of sequential deletion mutants of the pref-1 promoter linked to the luciferase gene and chromatin immunoprecipitation, we demonstrate that the promoter-proximal SBE (-192/-184) was critical in mediating BMP7's suppressive effect on pref-1 transcription. Together, these data suggest cross talk between the insulin and BMP signaling systems by which BMP7 can rescue brown adipogenesis in cells with insulin resistance.

MicroRNA-15a fine-tunes the level of Delta-like 1 homolog (DLK1) in proliferating 3T3-L1 preadipocytes

Delta like 1 homolog (Dlk1) exists in both transmembrane and soluble molecular forms, and is implicated in cellular growth and plays multiple roles in development, tissue regeneration, and cancer. Thus, DLK1 levels are critical for cell function, and abnormal DLK1 expression can be lethal; however, little is known about the underlying mechanisms. We here report that miR-15a modulates DLK1 levels in preadipocytes thus providing a mechanism for DLK1 regulation that further links it to cell cycle arrest and cancer since miR-15a is deregulated in these processes. In preadipocytes, miR-15a increases with cell density, and peaks at the same stage where membrane DLK1(M) and soluble DLK1(S) are found at maximum levels. Remarkably, miR-15a represses the amount of all Dlk1 variants at the mRNA level but also the level of DLK1(M) protein while it increases the amount of DLK1(S) supporting a direct repression of DLK1 and a parallel effect on the protease that cleaves off the DLK1 from the membrane. In agreement with previous studies, we found that miR-15a represses cell numbers, but additionally, we report that miR-15a also increases cell size. Conversely, anti-miR-15a treatment decreases cell size while increasing cell numbers, scenarios that were completely rescued by addition of purified DLK1(S). Our data thus imply that miR-15a regulates cell size and proliferation by fine-tuning Dlk1 among others, and further emphasize miR-15a and DLK1 levels to play important roles in growth signaling networks.

Role of preadipocyte factor 1 in adipocyte differentiation

Preadipocyte factor 1 (Pref-1) is an EGF-repeat-containing transmembrane protein that inhibits adipogenesis. The extracellular domain of Pref-1 is cleaved by TNF-α converting enzyme to generate the biologically active soluble form of Pref-1. The role of Pref-1 in adipogenesis has been firmly established by in vitro and in vivo studies. Pref-1 activates ERK/MAPK and upregulates Sox9 expression to inhibit adipocyte differentiation. Sox9 directly binds to the promoter regions of CCAAT/enhancer-binding protein-β and CCAAT/enhancer-binding protein-δ in order to suppress their promoter activities in preventing adipocyte differentiation. Here, we describe the function of Pref-1 in adipocyte differentiation and the recent findings on the mechanisms by which Pref-1 inhibits adipocyte differentiation.

Establishment of a mammary stromal fibroblastic cell line for in vitro studies in mice of mammary adipocyte differentiation

Mammary stromal adipose tissue remodeling is important for appropriate mammary gland development during pregnancy, lactation, and involution. However, the precise mechanisms underlying mammary stromal adipose tissue remodeling remain unclear. We have established a mammary stromal, fibroblastlike cell line (MSF) from primary mouse mammary culture by introducing a temperature-sensitive simian virus-40 large tumor antigen. Among several hormones related to mammary gland development, hydrocortisone was found to commit MSF cells to a preadipocyte lineage, whereas insulin was found to induce extracellular matrix-dependent adipogenic differentiation of the cells, as assessed by lipid accumulation and marker gene expression. Interestingly, such hormone-induced adipogenic differentiation of MSF cells, but not 3T3-L1 cells, was suppressed by prolactin through its receptor and downstream STAT5. Furthermore, coculture of MSF cells with mammary epithelial HC11 cells and culture in HC11-conditioned medium also suppressed adipogenic differentiation of MSF cells. We have demonstrated that adipogenic differentiation of at least some populations of mammary stromal cells is modulated by lactogenic hormones and humoral factors from epithelial cells, suggesting that the response of these mammary cells may differ from adipocytes at other sites. We believe that the MSF cell line will prove a useful model to elucidate mammary stromal adipose development in vitro as well as represent an important first step toward developing stable adipocyte cell lines that faithfully represent their site of origin.

"The preadipocyte factor" DLK1 marks adult mouse adipose tissue residing vascular cells that lack in vitro adipogenic differentiation potential

Delta-like 1 (Dlk1) is expressed in 3T3-L1 preadipocytes and has frequently been referred to as "the" preadipocyte marker, yet the phenotype of DLK1(+) cells in adipose tissue remains undetermined. Herein, we demonstrate that DLK1(+) cells encompass around 1-2% of the adult mouse adipose stromal vascular fraction (SVF). Unexpectedly, the DLK1(+)SVF population was enriched for cells expressing genes generally ascribed to the vascular lineage and did not possess any adipogenic differentiation potential in vitro. Instead, DLK1(+) cells comprised an immediate ability for cobblestone formation, generation of tube-like structures on matrigel, and uptake of Acetylated Low Density-Lipoprotein, all characteristics of endothelial cells. We therefore suggest that DLK1(+)SVF cells are of a vascular origin and not them-selves committed preadipocytes as assumed hitherto.

Characterization of DLK1+ cells emerging during skeletal muscle remodeling in response to myositis, myopathies, and acute injury

Delta like 1 (DLK1) has been proposed to act as a regulator of cell fate determination and is linked to the development of various tissues including skeletal muscle. Herein we further investigated DLK1 expression during skeletal muscle remodeling. Although practically absent in normal adult muscle, DLK1 was upregulated in all human myopathies analyzed, including Duchenne- and Becker muscular dystrophies. Substantial numbers of DLK1(+) satellite cells were observed in normal neonatal and Duchenne muscle, and furthermore, myogenic DLK1(+) cells were identified during muscle regeneration in animal models in which the peak expression of Dlk1 mRNA and protein coincided with that of myoblast differentiation and fusion. In addition to perivascular DLK1(+) cells, interstitial DLK1(+) cells were numerous in regenerating muscle, and in agreement with colocalization studies of DLK1 and CD90/DDR2, qPCR of fluorescence-activated cell sorting DLK1(+) and DLK1(-) cells revealed that the majority of DLK1(+) cells isolated at day 7 of regeneration had a fibroblast-like phenotype. The existence of different DLK1(+) populations was confirmed in cultures of primary derived myogenic cells, in which large flat nonmyogenic DLK1(+) cells and small spindle-shaped cells coexpressing DLK1 and muscle-specific markers were observed. Myogenic differentiation was achieved when sorted DLK1(+) cells were cocultured together with primary myoblasts revealing a myogenic potential that was 10% of the DLK1(-) population. Transplantation of DLK1(+) cells into lacerated muscle did, however, not give rise to DLK1(+) cell-derived myofibers. We suggest that the DLK1(+) subpopulations identified herein each may contribute at different levels/time points to the processes involved in muscle development and remodeling.

Minireview: Pref-1: role in adipogenesis and mesenchymal cell fate

Preadipocyte factor-1 [Pref-1; also called Dlk1 (Delta-like protein 1)] is made as an epidermal growth factor-repeat containing transmembrane protein that produces a biologically active soluble form by TNF-alpha-converting enzyme (TACE)-mediated cleavage. Soluble Pref-1 activates the MAPK kinase/ERK pathway. In adipose tissue, Pref-1 is specifically expressed in preadipocytes but not in adipocytes and thus is used as a preadipocyte marker. Inhibition of adipogenesis by Pref-1 has been well established in vitro as well as in vivo by ablation and overexpression of Pref-1. SRY (sex determining region Y)-box 9 (Sox9), a transcription factor expressed in preadipocytes to suppress CCAAT enhancer binding protein beta and (C/EBP) delta expression, is required to be down-regulated before adipocyte differentiation can proceed. By activating MAPK kinase/ERK, Pref-1 prevents down-regulation of Sox9, resulting in inhibition of adipogenesis. Furthermore, by inducing Sox9, Pref-1 promotes chondrogenic induction of mesenchymal cells but prevents chondrocyte maturation as well as osteoblast differentiation. Thus, Pref-1 directs multipotent mesenchymal cells toward the chondrogenic lineage but inhibits differentiation into adipocytes as well as osteoblasts and chondrocytes. Pref-1, encoded by an imprinted gene, has also been detected in progenitor cells in various tissues during regeneration and therefore may have a more general role in maintaining cells in an undifferentiated state.

Single-cell profiling reveals the origin of phenotypic variability in adipogenesis

Phenotypic heterogeneity in a clonal cell population is a well-observed but poorly understood phenomenon. Here, a single-cell approach is employed to investigate non-mutative causes of phenotypic heterogeneity during the differentiation of 3T3-L1 cells into fat cells. Using coherent anti-Stokes Raman scattering microscopy and flow cytometry, adipogenic gene expression, insulin signaling, and glucose import are visualized simultaneously with lipid droplet accumulation in single cells. Expression of adipogenic genes PPARγ, C/EBPα, aP2, LP2 suggests a commitment to fat cell differentiation in all cells. However, the lack of lipid droplet in many differentiating cells suggests adipogenic gene expression is insufficient for lipid droplet formation. Instead, cell-to-cell variability in lipid droplet formation is dependent on the cascade responses of an insulin signaling pathway which includes insulin sensitivity, kinase activity, glucose import, expression of an insulin degradation enzyme, and insulin degradation rate. Increased and prolonged insulin stimulation promotes lipid droplet accumulation in all differentiating cells. Single-cell profiling reveals the kinetics of an insulin signaling cascade as the origin of phenotypic variability in drug-inducible adipogenesis.

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

cAMP and protein kinase A (PKA) are widely known as signaling molecules that are important for the induction of adipogenesis. Here we show that a strong increase in the amount of cAMP inhibits the adipogenesis of 3T3-L1 fibroblast cells. Stimulation of PKA activity suppresses adipogenesis and, in contrast, inhibition of PKA activity markedly accelerates the adipogenic process. As adipogenesis progresses, there is a significant increase in the expression level of PKA regulatory subunits and a corresponding decrease in PKA activity. Moreover, treatment of 3T3-L1 cells with epidermal growth factor (EGF) stimulates PKA activity and blocks adipogenesis. Inhibition of PKA activity abolishes this suppressive effect of EGF on adipogenesis. Moreover, activation of PKA induces serine/threonine phosphorylation, reduces tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1) and the association between PKA and IRS-1. Taken together, our study demonstrates that PKA has a pivotal role in the suppression of adipogenesis. cAMP at high concentrations can suppress adipogenesis through PKA activation. These findings could be important and useful for understanding the mechanisms of adipogenesis and the relevant physiological events.

dlk1/FA1 Regulates the Function of Human Bone Marrow Mesenchymal Stem Cells by Modulating Gene Expression of Pro-inflammatory Cytokines and Immune Response-related Factors

dlk1/FA1 (delta-like 1/fetal antigen-1) is a member of the epidermal growth factor-like homeotic protein family whose expression is known to modulate the differentiation signals of mesenchymal and hematopoietic stem cells in bone marrow. We have demonstrated previously that Dlk1 can maintain the human bone marrow mesenchymal stem cells (hMSC) in an undifferentiated state. To identify the molecular mechanisms underlying these effects, we compared the basal gene expression pattern in Dlk1-overexpressing hMSC cells (hMSC-dlk1) versus control hMSC (negative for Dlk1 expression) by using Affymetrix HG-U133A microarrays. In response to Dlk1 expression, 128 genes were significantly up-regulated (with >2-fold; p < 0.001), and 24% of these genes were annotated as immune response-related factors, including pro-inflammatory cytokines, in addition to factors involved in the complement system, apoptosis, and cell adhesion. Also, addition of purified FA1 to hMSC up-regulated the same factors in a dose-dependent manner. As biological consequences of up-regulating these immune response-related factors, we showed that the inhibitory effects of dlk1 on osteoblast and adipocyte differentiation of hMSC are associated with Dlk1-induced cytokine expression. Furthermore, Dlk1 promoted B cell proliferation, synergized the immune response effects of the bacterial endotoxin lipopolysaccharide on hMSC, and led to marked transactivation of the NF-kappaB. Our data suggest a new role for Dlk1 in regulating the multiple biological functions of hMSC by influencing the composition of their microenvironment "niche." Our findings also demonstrate a role for Dlk1 in mediating the immune response.

Pref-1 (preadipocyte factor 1) activates the MEK/extracellular signal-regulated kinase pathway to inhibit adipocyte differentiation

Preadipocyte factor 1 (Pref-1) is found in preadipocytes but is absent in adipocytes. Pref-1 is made as a transmembrane protein but is cleaved to generate a biologically active soluble form. Although Pref-1 inhibition of adipogenesis has been well studied in vitro and in vivo, the signaling pathway for Pref-1 is not known. Here, by using purified soluble Pref-1 in Pref-1 null mouse embryo fibroblasts (MEF), we show that Pref-1 increases MEK/extracellular signal-regulated kinase (ERK) phosphorylation in a time- and dose-dependent manner. Compared to wild-type MEF, differentiation of Pref-1 null MEF into adipocytes is enhanced, as judged by lipid accumulation and adipocyte marker expression. Both wild-type and Pref-1 null MEF show a transient burst of ERK phosphorylation upon addition of adipogenic agents. Wild-type MEF show a significant, albeit lower, second increase in ERK phosphorylation peaking at day 2. This ERK phosphorylation, corresponding to Pref-1 abundance, is absent during differentiation of Pref-1 null MEF. Prevention of this second increase in ERK1/2 phosphorylation in wild-type MEF by the MEK inhibitor PD98059 or by transient depletion of ERK1/2 via small interfering RNA-enhanced adipocyte differentiation. Furthermore, treatment of Pref-1 null MEF with Pref-1 restores this ERK phosphorylation, resulting in inhibition of adipocyte differentiation primarily by preventing peroxisome proliferator-activated receptor gamma2 induction. However, in the presence of PD98059 or depletion of ERK1/2, exogenous Pref-1 cannot inhibit adipocyte differentiation in Pref-1 null MEF. We conclude that Pref-1 activates MEK/ERK signaling, which is required for Pref-1 inhibition of adipogenesis.

Ectodomain shedding of preadipocyte factor 1 (Pref-1) by tumor necrosis factor alpha converting enzyme (TACE) and inhibition of adipocyte differentiation

Preadipocyte factor 1 (Pref-1), an epidermal growth factor repeat containing transmembrane protein found in the preadipocytes, inhibits adipocyte differentiation in vitro and in vivo. Here, we examined the processing of membrane form of Pref-1A to release the 50-kDa soluble form that inhibits adipocyte differentiation. The ectodomain cleavage of Pref-1 is markedly enhanced by phorbol 12-myristate 13-acetate in a dose- and time-dependent manner. The basal and stimulated cleavage is inhibited by the broad metalloproteinase inhibitor GM6001, a fact that suggests that cleavage of membrane Pref-1A is dependent on a metalloproteinase. Next, we showed that release of soluble Pref-1A is inhibited by TAPI-0 and by a tissue inhibitor of metalloproteinase-3, TIMP-3, that can inhibit tumor necrosis factor alpha converting enzyme (TACE), but not by TIMP-1 or TIMP-2. On the other hand, overexpression of TACE increases Pref-1 cleavage to produce the 50-kDa soluble form. Furthermore, this cleavage was not detected in cells with TACE mutation or with TACE small interfering RNA. TACE-mediated shedding of Pref-1 ectodomain inhibits adipocyte differentiation of 3T3-L1 cells and in Pref-1-null mouse embryo fibroblasts transduced with Pref-1A. Identification of TACE as the major protease responsible for conversion of membrane-bound Pref-1 to the biologically active diffusible form provides a new insight into Pref-1 function in adipocyte differentiation.

Increased adipogenicity of cells from regenerating skeletal muscle

Adipose tissue development is observed in some muscle pathologies, however, mechanisms that induce accumulation of this tissue as well as its cellular origin are unknown. The adipogenicity of cells from bupivacaine hydrochloride (BPVC)-treated and untreated muscle was compared in vitro. Culturing cells from both BPVC-treated and untreated muscles in adipogenic differentiation medium (ADM) for 10 days resulted in the appearance of mature adipocytes, but their number was 3.5-fold higher in cells from BPVC-treated muscle. Temporal expressions of PPARgamma and the presence of lipid droplets during adipogenic differentiation were examined. On day 2 of culture in ADM, only cells from BPVC-treated muscle were positive both for PPARgamma and lipid droplets. Pref-1 was expressed in cells from untreated muscle, whereas its expression was absent in cells from BPVC-treated muscle. In ADM, the presence of insulin, which negates an inhibitory effect of Pref-1 on adipogenic differentiation, was required for PPARgamma2 expression in cells from untreated muscle, but not for cells from BPVC-treated muscle. These results indicate that BPVC-induced degenerative/regenerative changes in muscle lead to increased adipogenicity of cells, and suggest that this increased adipogenicity not only involves an increase in the number of cells having adipogenic potential, but also contributes to the progression of these cells toward adipogenic differentiation.

Growth hormone and prolactin receptors in adipogenesis: STAT-5 activation, suppressors of cytokine signaling, and regulation of insulin-like growth factor I

Growth hormone GH stimulates lipolysis in mature adipocytes and primary preadipocytes but promotes adipogenesis in preadipocyte cell lines. The lactogenic hormones (prolactin PRL and placental lactogen) also stimulate adipogenesis in preadipocyte cell lines but have variable lipolytic and lipogenic effects in mature adipose tissue. We hypothesized that differences in expression of GH receptors GHR and PRL receptors PRLR during adipocyte development might explain some of the differential effects of the somatogens and lactogens on fat metabolism. To that end, we compared: (a) the expression of GHR and PRLR mRNAs in 3T3-L1 preadipocytes during the course of adipocyte differentiation; (b) the induction of STAT-5 activity by GH and PRL during adipogenesis; and (c) the acute effects of GH and PRL on the suppressors of cytokine signaling (SOCS-1-3 and cytokine-inducible SH2-domain-containing protein CIS) and IGF-I. In confluent, undifferentiated 3T3-L1 cells, the levels of GHR mRNA were approximately 250-fold higher than the levels of PRLR mRNA. Following induction of adipocyte differentiation the levels of PRLR mRNA rose 90-fold but GHR mRNA increased only 0.8-fold. Expression of both full-length (long) and truncated (short) isoforms of the PRLR increased during differentiation but the long isoform predominated at all time points. Mouse GH mGH stimulated increases in STAT-5a and 5b activity in undifferentiated as well as differentiating 3T3-L1 cells; mouse PRL mPRL had little or no effect on STAT-5 activity in undifferentiated cells but stimulated increases in STAT-5a and 5b activity in differentiating cells. mGH stimulated increases in SOCS-2 and SOCS-3 mRNAs in undifferentiated cells and SOCS-1-3 and CIS mRNAs in differentiating cells; mPRL induced CIS in differentiating cells but had no effect on SOCS-1-3. mPRL and mGH stimulated increases in IGF-I mRNA in differentiating cells but not in undifferentiated cells; the potency of mGH (3-6-fold increase, p < 0.01) exceeded that of mPRL (40-90% increase, p < 0.05). Our findings reveal disparities in the expression of PRLR and GHR during adipocyte development and differential effects of the hormones on STAT-5, the SOCS proteins, CIS, and IGF-I. These observations suggest that somatogens and lactogens regulate adipocyte development and fat metabolism through distinct but overlapping cellular mechanisms.

Identification of DLK1 variants in pituitary- and neuroendocrine tumors

In a gene chip analysis of common pituitary tumor types, one of the genes with the most impressive tissue-specific expression regulation was delta-like 1 (DLK1), which was strongly expressed in GH-secreting (GH-S) pituitary tumors. In addition to pituitary adenomas, various endocrine tumors were subjected to real-time-quantitative PCR revealing high expression of DLK1 in normal pituitary tissue, in GH-S-, in one prolactin-secreting pituitary adenoma and in pheochromocytomas. Additionally, three DLK1 gene-derived subvariants were identified. The first, lacking 204 bp--coding for epidermal growth factor-like domain 6 and parts of the juxtamembrane region--was named Secredeltin. In the other two splice variants (named Brevideltin and Brevideltinin), a stop codon is introduced due to a frame-shift, leading to truncated proteins of 204 and 213 aas, respectively.

Microarray analysis of early adipogenesis in C3H10T1/2 cells: cooperative inhibitory effects of growth factors and 2,3,7,8-tetrachlorodibenzo-p-dioxin

C3H10T1/2 mouse embryo fibroblasts differentiate into adipocytes when stimulated by a standard hormonal mixture (IDMB). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), via the aryl hydrocarbon receptor (AhR), inhibits induction of the key adipogenic gene peroxisome proliferator-activated receptor gamma (PPARgamma) and subsequent adipogenesis. This TCDD-mediated inhibition requires activation of the extracellular signal-regulated kinase (ERK) pathway, which can be accomplished by serum, epidermal growth factor (EGF), or fibroblast growth factor (FGF). In the absence of serum or growth factors, IDMB induced adipogenesis without mitosis. Microarray analysis identified 200 genes that exhibited expression changes of at least twofold after 24 h of IDMB treatment. This time precedes most PPARgamma stimulation but follows the period of TCDD/ERK cooperation and periods of increased cell contraction and DNA synthesis. Functionally related gene clusters include genes associated with cell structure, triglyceride and cholesterol metabolism, oxidative regulation, and secreted proteins. In the absence of growth factors TCDD inhibited 30% of these IDMB responses without inhibiting the process of differentiation. A combination of EGF and TCDD that blocks differentiation cooperatively blocked a further 44 IDMB-responsive genes, most of which have functional links to differentiation, including PPARgamma. Cell cycle regulators that are stimulated by EGF were substantially inhibited by IDMB but these responses were unaffected by TCDD. By contrast, TCDD and EGF cooperatively reversed IDMB-induced changes in cell adhesion complexes immediately prior to increases in PPARgamma1 expression. Changes in adhesion-linked signaling may play a key role in TCDD affects on differentiation.

Regulation of adipocyte differentiation and function by polyunsaturated fatty acids

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

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.

Notch 1 impairs osteoblastic cell differentiation

Notch receptors are single pass transmembrane receptors activated by membrane-bound ligands with a role in cell proliferation and differentiation. As Notch 1 and 2 mRNAs are expressed by osteoblasts and induced by cortisol, we postulated that Notch could regulate osteoblastogenesis. We investigated the effects of retroviral vectors directing the constitutive expression of the Notch 1 intracellular domain (NotchIC) in murine ST-2 stromal and in MC3T3 cells. NotchIC overexpression was documented by increased Notch 1 transcripts and activity of the Notch-dependent Hairy Enhancer of Split promoter. In the presence of bone morphogenetic protein-2 (BMP-2), ST-2 cells differentiated toward osteoblasts forming mineralized nodules, and Notch 1 opposed this effect and decreased the expression of osteocalcin, type I collagen, and alkaline phosphatase transcripts and Delta2Delta FosB protein. Further, NotchIC decreased Wnt/beta-catenin signaling. As cells differentiated in the presence of BMP-2, they underwent apoptosis, and Notch opposed this event. In the presence of cortisol, NotchIC induced the formation of mature adipocytes and enhanced the effect of cortisol on adipsin, peroxisome proliferator-activated receptor-gamma2 and CCAAT enhancer binding protein alpha and delta mRNA levels. NotchIC also opposed MC3T3 cell differentiation and the expression of a mature osteoblastic phenotype. In conclusion, NotchIC impairs osteoblast differentiation and enhances adipogenesis in stromal cell cultures.