SEARCH

SEARCH BY CITATION

Keywords:

  • Mesenchymal stem cells;
  • Differentiation;
  • Hedgehog signaling;
  • Adipocytes

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

Human stem cells are powerful tools by which to investigate molecular mechanisms of cell growth and differentiation under normal and pathological conditions. Hedgehog signaling, the dysregulation of which causes several pathologies, such as congenital defects and cancer, is involved in several cell differentiation processes and interferes with adipocyte differentiation of rodent cells. The present study was aimed at investigating the effect of Hedgehog pathway modulation on adipocyte phenotype using different sources of human mesenchymal cells, such as bone marrow stromal cells and human multipotent adipose-derived stem cells. We bring evidence that Hedgehog signaling decreases during human adipocyte differentiation. Inhibition of this pathway is not sufficient to trigger adipogenesis, but activation of Hedgehog pathway alters adipocyte morphology as well as insulin sensitivity. Analysis of glycerol-3-phosphate dehydrogenase activity and expression of adipocyte marker genes indicate that activation of Hedgehog signaling by purmorphamine impairs adipogenesis. In sharp contrast to reports in rodent cells, the maturation process, but not the early steps of human mesenchymal stem cell differentiation, is affected by Hedgehog activation. Hedgehog interferes with adipocyte differentiation by targeting CCAAT enhancer-binding protein α and peroxisome proliferator-activated receptor (PPAR) γ2 expression, whereas PPARγ1 level remains unaffected. Although Hedgehog pathway stimulation does not modify the total number of adipocytes, adipogenesis appears dramatically impaired, with reduced lipid accumulation, a decrease in adipocyte-specific markers, and acquisition of an insulin-resistant phenotype. This study indicates that a decrease in Hedgehog signaling is necessary but not sufficient to trigger adipocyte differentiation and unveils a striking difference in the adipocyte differentiation process between rodent and human mesenchymal stem cells.

Disclosure of potential conflicts of interest is found at the end of this article.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

Alterations in fat mass are frequently associated with modulation of adipocyte differentiation and lead to dysregulation of metabolic pathways, such as cholesterol, fatty acid, and glucose metabolism. This results in the development of dyslipidemia, insulin resistance, diabetes, and obesity. To investigate this phenomenon, various cellular and animal models have been developed. Up to now, preadipocyte cell lines from rodents have been used to gain insights into the molecular mechanisms of adipogenesis and as a tool to develop therapeutic drugs. In regard to human cell models presently available, primary cultures of stroma vascular fraction of adipose tissue are widely used. However, after a few passages, they undergo a dramatic decrease in their ability to differentiate before stopping growth and entering replicative senescence [1]. Recently, we isolated human stem cells derived from adipose tissue. These cells are called human multipotent adipose-derived stem (hMADS) cells, or adipose stem cells according to the new international nomenclature [2]. hMADS cells exhibit a normal karyotype, multipotency at the clonal level, and high self-renewal [3, 4]. They offer a limitless supply of adipocytes to investigate the molecular pathways of adipocyte differentiation and drug metabolism [5].

Adipocyte differentiation is a complex biological process, which is reflected at the cellular level by morphological changes, ability to accumulate cytoplasmic lipid droplets, and acquisition of insulin sensitivity. The nuclear receptor peroxisome proliferator-activated receptor (PPAR) γ and the CCAAT enhancer-binding protein (C/EBP) α are the two main key transcription factors that regulate this process [6]. Adipocyte differentiation is highly controlled by stimulatory and inhibitory cues, and modifications in their balance give rise to lipodystrophy or obesity, an epidemic health problem.

Hedgehog, the function of which during embryogenesis has been abundantly described, has emerged as a crucial modulator of cell differentiation processes, including adipogenesis. Hedgehog signaling is initiated by the binding of one of the three mammalian ligands: Sonic Hedgehog (Shh), Indian Hedgehog (Ihh), or Desert Hedgehog (Dhh) to the 12-transmembrane protein Patched (Ptch), relieving suppression of the 7-transmembrane protein Smoothened (Smo). In turn, Smo activates an intracellular cascade that results in activation of the Gli2 and Gli3 transcription factors. Gli1 is one of their target genes and has been characterized as a reliable marker of Hedgehog signaling activity [7, [8]9]. Several pharmacological modulators of Hedgehog signaling are available. Cyclopamine is a well-described inhibitor of the signaling [10], and purmorphamine, a purine derivative, has been found to activate Hedgehog pathway by directly targeting Smo [11, 12].

Since molecules are developed to modulate Hedgehog signaling in the treatment of various pathologies, such as cancer and alopecia, evaluation of potential side effects of this signaling pathway is of importance [13]. For instance, the observation that Hedgehog signaling is modified in rodent situation of obesity has pointed to a possible physiological implication of this pathway in this syndrome [14]. Antiadipogenic effects of Hedgehog signaling have been evidenced in vitro on murine mesenchymal cells able to differentiate into both adipocyte and osteoblast lineages, such as calvaria cells and C3H10T1/2 and KS483 cell lines [15, [16]17], and in the classic preadipocyte cell line 3T3-L1 [14]. In these cells, activation of Hedgehog signaling favored osteoblastic differentiation at the expense of adipogenesis. However, the precise mechanism of Hedgehog-mediated inhibition of adipocyte differentiation has not been clearly determined in rodents, and no evaluation of the role of the Hedgehog pathway on human adipocyte differentiation has been reported. For this purpose, human bone marrow stromal cells (hBMSCs) [18, 19] and adipocyte-derived stem cells, such as hMADS cells, are invaluable models with which to study human adipogenesis.

Together, our results show that Hedgehog signaling activation leads to a decrease in human adipocyte maturation and point out a major difference in the effect of Hedgehog on human and rodent adipocyte differentiation. This suggests that drugs developed to target Hedgehog signaling should be monitored for potential side effects using appropriate human cell models.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

Growth and Adipocyte Differentiation of hMADS Cells

Establishment and characterization of the multipotency of hMADS cells have been extensively described [4]. In the experiments reported herein, hMADS-1, hMADS-2, and hMADS-3 cells—established from male and female infants—were used. Cells were grown in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal calf serum, 2.5 ng/ml human fibroblast growth factor (hFGF)-2, 60 μg/ml penicillin, and 50 μg/ml streptomycin. The medium was changed 2 days later in the absence of hFGF-2 until cells reached confluence. At day 2 postconfluence (designated day 0), cells were then induced to differentiate into adipocytes in the presence of DMEM/Ham's F12 media supplemented with 0.85 μM insulin, 10 μg/ml transferrin, 0.2 nM triiodothyronine, 1 μM dexamethasone (DEX), 100 μM isobutyl-methylxanthine (IBMX), and 0.1 μM rosiglitazone. Three days later, the medium was changed (DEX and IBMX omitted). The media were then changed every other day, and cells were used at the indicated days.

Cyclopamine was dissolved in dimethyl sulfoxide, used at 5 μM and added every other day. Purmorphamine was dissolved in ethanol and added in triplicate wells at 2 μM (or at the indicated concentration) for different periods of time before cells were collected. Both drugs were found not to have any obvious toxic effect. The specific Hedgehog activator purmorphamine was used instead of bacteria-produced mutant form of Shh (NShh) or conditioned medium from Shh-producing cells (obtained from American Type Culture Collection, Manassas, VA, http://www.atcc.org; CRL-2782). Indeed, recombinant NShh is not covalently modified by cholesterol and palmitate, is 30 times less potent than native Shh, and appears unable to form multimers, prompting some authors to question the physiological relevance of this protein [20, 21]. Conditioned medium from Shh-producing cells was found to exert the same effect on aP2, PPARγ1, and PPARγ2 gene expression as purmorphamine. However, conditioned medium also contained undefined molecules interfering with adipocyte differentiation, and reproducibility between batches was loose (supplemental online Fig. 1).

RNA Extraction and Analysis

Total RNA was extracted using the TRI-Reagent kit (Euromedex, Souffelweyersheim, France, http://www.euromedex.com) according to the manufacturer's instructions. Total RNA, digested with DNase I (Promega, Charbonnières-les-bains, France, http://www.promega.com), was subjected to real-time quantitative reverse transcription (RT)-polymerase chain reaction (PCR) analysis. The RT reactions were carried out in the presence of the pd(N)6 random hexamer (Roche Molecular Biochemicals, Meylan, France, http://www.roche-applied-science.com), Moloney murine leukemia virus reverse transcriptase (100 U), supplied buffer, dNTPs (0.5 mM), and RNase inhibitor (10 U) at 37°C for 1 hour in a 20-μl reaction volume.

Real-time quantitative PCRs were performed on an ABI Prism 7000 instrument (Applied Biosystems, Foster City, CA, http://www.appliedbiosystems.com). For each PCR run, a master mix composed of qPCR MasterMix Plus for SYBR Green I (Eurogentec, Seraing, Belgium, http://www.eurogentec.be) was prepared. Primers were designed using Primer Express software (Applied Biosystems; supplemental online Table 1) and validated by testing PCR efficiency using standard curves (85% ≤ efficiency ≤115%). Gene expression was quantified using the comparative CT (threshold cycle) method; TBP was used as reference.

Determination of Cell Size, Lipid Accumulation, GPDH Activity, and Glucose Transport

To examine lipid accumulation, cells were fixed with paraformaldehyde (4% in phosphate-buffered saline) and stained with oil red O. Three pictures per single well were realized in triplicate for each condition and were imported into ImageJ (available at http://rsb.info.nih.gov/nih-image). Adipocytes (at least 300 cells for each conditions) were outlined, and the area was calculated with the Analyze tool. The area measurements were imported into Microsoft Excel (Redmond, WA, http://www.microsoft.com), and the mean ± SD of the areas was calculated for control and purmorphamine-treated cells. To evaluate cell morphology, more than 100 single cells were randomly chosen. To determine cell number, three random pictures of each surface were analyzed for each sample.

GPDH activity was assayed spectrophotometrically, as previously described [22], on cell homogenates obtained using a 20 mM Tris/HCl buffer, pH 7.5, containing 1 mM EDTA and 1 mM β-mercaptoethanol. Assays were carried out in triplicate at day 14 of differentiation. Protein concentrations were determined by the Bradford method, with bovine serum albumin as the standard. Enzyme activity is expressed in mU/mg (i.e., nmol of product formed per minute per mg of protein).

Insulin-stimulated glucose transport was determined using nonmetabolizable [3H]2-deoxy-d-glucose. Briefly, adipocytes were starved overnight in DMEM 0.2% bovine serum albumin (BSA) and thereafter incubated in Kreb's Ringer phosphate medium 0.2% BSA (10 mM HEPES, pH 7.5, 2.5 mM Na2HPO4, 1.25 mM MgSO4, 2 mM CaCl2, 130 mM NaCl, 5 mM KCl) for 4 hours. Human recombinant insulin (10−7 M) was added for 45 minutes. During the last 10 minutes, 0.5 μCi/ml 2-deoxy-D-[2,6-3H] glucose (Amersham Biosciences, Piscataway, NJ, http://www.amersham.com) and 0.1 mM 2-deoxy-d-glucose were added. Cells were washed twice in phosphate-buffered saline and lysed in KRPH 1% Triton X-100. Radioactivity was measured using a β-scintillation counter. The data were expressed in dpm/minute/mg of protein.

Cell Proliferation Assay

hMADS cells were plated into six-well plates (4,500 cells per cm2). After the appropriate time, adherent cells were dissociated in 0.25% trypsin EDTA and counted with a Coulter counter.

Statistical Analysis

Statistical differences between groups were analyzed by Student's t test and are indicated on figures as follow: * p ≤ .05, ** p ≤ .01, and *** p ≤ .001.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

Hedgehog Signaling Is Downregulated During Adipocyte Differentiation

We studied the role of Hedgehog signaling in human adipocyte differentiation using hMADS cells. These mesenchymal cells present several advantages. They display a normal karyotype and are not transformed, and differentiation takes place in a chemically defined medium, with no addition of serum. So far, they exhibit properties similar, if not identical, to those of human adipocytes [5]. We first investigated the status of Hedgehog signaling during human adipocyte differentiation through Gli1. Indeed, in the absence of Hedgehog signaling, Gli1 is transcriptionally silent, and since Hedgehog is the only known inducer of Gli1, its expression is considered as a marker of pathway activity [7, [8]9, 23, [24]25]. We observed that adipocyte differentiation of hMADS cells was associated with a 90% decrease in Gli1 expression (Fig. 1A), indicating that undifferentiated hMADS cells are endowed with an active Hedgehog signaling pathway that decreases during adipocyte differentiation. Consistently, the expression of Hedgehog receptor Ptch, another Hedgehog-target gene, was also found to be downregulated during adipocyte differentiation (Fig. 1 B). Ihh and Dhh were found to be expressed at low levels, and their expression was not modified during adipocyte differentiation. Shh messenger was also detected, but at levels too low to allow quantification. We then studied other elements of Hedgehog signaling: Smo, Gli2, and Gli3. The expression of Smo was not modified. Interestingly, both Gli2 and Gli3 expression decreased. Since Gli2 is the primary mediator of Hedgehog signaling [7, [8]9, 26], it can be envisioned that a decrease in its expression could be responsible for the decrease in Hedgehog signaling observed during adipocyte differentiation.

thumbnail image

Figure Figure 1.. The decrease in Hedgehog signaling observed during adipogenesis is neither sufficient nor beneficial to trigger adipocyte differentiation of human mesenchymal stem cells. Cyclopamine (5 μM) was added every other day from day 0 to the end of the differentiation (day 14) in the presence or absence of the adipogenic cocktail. Shown is real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) of Gli1 (A) and Ihh, Dhh, Smo, Ptch, Gli2, and Gli3 (B). (C): Oil red O staining. (D): GPDH activity. (E): Expression of the aP2 gene using real-time quantitative RT-PCR. Data are mean ± SE of triplicates from an experiment representative of three independent experiments or the mean ± SE of three independent experiments (B). Abbreviations: C, control; ctrl, control; D, day 14 of differentiation.

Download figure to PowerPoint

We then determined whether the decrease in Hedgehog signaling observed during adipocyte differentiation was necessary and/or sufficient for adipocyte differentiation using the potent and specific Hedgehog inhibitor cyclopamine. hMADS cells appear particularly well suited for this purpose, since they differentiated in the absence of serum that could contain undefined molecules affecting Hedgehog signaling. Cells were treated or not treated with an adipogenic cocktail in the presence or absence of cyclopamine. As observed, cyclopamine addition induced a decrease in Gli1 expression, comparable to the one associated with adipocyte differentiation (Fig. 1A). After 14 days, cells were stained with oil red O to visualize lipid accumulation. As illustrated in Figure 1C, cyclopamine did not alter lipid accumulation of hMADS in the presence or absence of the adipogenic cocktail. Accordingly, chronic treatment of cyclopamine during differentiation did not modify either the GPDH activity (Fig. 1D) or aP2 gene expression (Fig. 1E). These data indicate that Hedgehog signaling inhibition is not sufficient to trigger human adipogenesis.

Hedgehog Signaling Impairs Adipogenesis and Lipid Accumulation

We then investigated whether a decrease in Hedgehog signaling was necessary for adipocyte differentiation. We first tested the ability of purmorphamine, a potent and highly specific inducer of Hedgehog signaling that directly targets Smo [11, 12, 27, 28], to activate the Hedgehog pathway in hMADS cells. Purmorphamine exhibits an optimal biological effect at 2 μM (Fig. 2A), as observed in other cell types [12]. Kinetics studies demonstrated that Gli1 gene induction is maintained at least 72 hours after purmorphamine treatment (2 μM) in hMADS cells (Fig. 2B). Interestingly, the use of purmorphamine during adipogenesis induces Gli1 mRNA levels similar to those present in undifferentiated cells, demonstrating that purmorphamine did not induce a massive overactivation of Hedgehog signaling under these conditions (Fig. 2C). Adipocyte-differentiated cells were fixed and stained with oil red O. As observed in Figure 3A, chronic purmorphamine treatment induced a decrease in the global oil red O staining of hMADS. Microscopic analysis and a count of oil red O-labeled cells in the different conditions indicate that purmorphamine did not modify the total number of adipocytes (Fig. 3B). However, hMADS cells differentiated into adipocytes in the presence of purmorphamine have a reduced cell size compared with control adipocytes (Fig. 3A, 3C). Consistently, adipocytes from purmorphamine-treated cells presented smaller lipid droplets (Fig. 3A, 3D). We then tested the expression of perilipin, the major protein associated with lipid droplets in adipocytes [29, 30], and found that its expression was decreased in purmorphamine-treated cells (Fig. 3E).

thumbnail image

Figure Figure 2.. Hedgehog signaling is downregulated during differentiation but remains inducible in adipocytes. Expression of Gli1 using real-time quantitative RT-PCR in nondifferentiated cells (A, B) or at the indicated days of adipocyte differentiation (C). Cells were treated with increasing concentrations of purmorphamine for 24 h (A) or for the indicated times at 2 μM (B, C). Results are representative of three independent experiments. Abbreviation: h, hour.

Download figure to PowerPoint

thumbnail image

Figure Figure 3.. Chronic purmorphamine treatment during differentiation decreases adipocyte and lipid droplet size but has no effect on adipocytes cell number. Purmorphamine (2 μM) was added every other day from day 0 to completion of differentiation. (A): Oil red O staining. (B): Mean number of cells per cm2. (C, D): Mean size of at least 300 cells per condition (C) and lipid droplets surface area of at least 100 single cells randomly chosen per condition (D) were determined using NIH ImageJ software. (E): Real-time quantitative reverse transcription-polymerase chain reaction of perilipin (mean ± SE of three independent experiments). Abbreviations: Ctrl, control; NS, not significant; PLN, perilipin.

Download figure to PowerPoint

These morphological changes were accompanied by a 40% reduction of GPDH activity (Fig. 4A), as well as a significant decrease in the induction of the expression of adipocyte-specific markers aP2 (Fig. 4B), adipsin (Fig. 4C), CD36 (Fig. 4D), adiponectin (Fig. 4E), and leptin (Fig. 4F). Similar effects were observed with hBMSCs (supplemental online Fig. 2). Altogether, analysis of triglyceride storage, GPDH activity, and expression of adipocyte markers showed that chronic activation of Hedgehog signaling pathway by purmorphamine impairs normal differentiation, leading to less lipid accumulation, but does not alter adipocyte number.

thumbnail image

Figure Figure 4.. Chronic purmorphamine treatment during adipocyte differentiation decreases GPDH activity and expression of adipocyte marker genes. Purmorphamine (2 μM) was added every other day from day 0 to completion of differentiation. (A): GPDH activity. Results are representative of three independent experiments. Each column is the mean value ± SE of triplicate determinations. (B–F): Gene expression analyzed using real-time quantitative RT-PCR. The data presented are means ± SE of at least four independent experiments performed in triplicate. Abbreviations: Ctrl, control; prot, protein.

Download figure to PowerPoint

Hedgehog Signaling Pathway Activation Does Not Inhibit the Early Steps of Adipogenesis

To determine the mechanism by which Hedgehog signaling modulates the adipogenic process, we tested whether it could induce the expression of a series of known negative regulators of adipogenesis. GATA-2 [31], GILZ [32], and CHOP-10 [33] gene expression was quantified by real-time quantitative RT-PCR, and induction of Gli1 mRNA level was analyzed as a positive response of Hedgehog activation (Fig. 5A). No change in the expression of these antiadipogenic factors was observed when comparing control hMADS cells (day 0) with purmorphamine-treated cells (2 μM, 48 hours) (Fig. 5A). Two other factors proposed to drive mesenchymal stem cells toward the osteoblastic at the expense of the adipogenic lineage were analyzed: the morphogen Wnt10b [34] and the transcriptional modulator TAZ [35]. Purmorphamine did not modulate the expression of these factors (Fig. 5A). Consistent with this observation, purmorphamine did not induce gene expression of osteoblastic markers such as alkaline phosphatase or osteoprotegerin (supplemental online Fig. 3).

thumbnail image

Figure Figure 5.. Hedgehog signaling does not inhibit the early step of adipogenesis of human mesenchymal stem cells. (A): hMADS cells (day 0) were treated with purmorphamine (48 hours, 2 μM) or vehicle, and gene expression was analyzed using real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). (B): Counting of hMADS cells grown in the presence or absence of FGF-2 (2 ng/ml) and treated every other day with purmorphamine or vehicle. (C, D): hMADS cells were differentiated for 14 days in the absence (−) or presence of purmorphamine from day 0 to day 14 (0–14), day 0 to day 3 (0–3), or day 3 to day 14 (3–14). (C): Oil red O staining. (D): Gene expression. (E, F): hMADS were differentiated for 21 days in the absence (−) or presence of purmorphamine from day 0 to day 21 (0–21) or from day 14 to day 21 (14–21). (E): Oil red O staining. (F): Real-time quantitative RT-PCR presented in fold induction relative to undifferentiated hMADS cells. Results are representative of three (E, F) or four (A–D) independent experiments. Each column is the mean value ± SE of triplicate determinations. Abbreviations: C/EBPα, CCAAT enhancer-binding protein α; ctrl, control; FGF, fibroblast growth factor; hMADS, human multipotent adipose-derived stem; NS, not significant.

Download figure to PowerPoint

Since Hedgehog signaling has been linked to cell proliferation in a variety of systems [36], we tested this effect on hMADS cells. As shown in Figure 5B, there was no difference in cell proliferation between control and purmorphamine-treated cells in the presence or absence of FGF-2, which is critical for self-renewal of hMADS cells [3]. This indicates that Hedgehog does not interfere with adipocyte differentiation through cell growth modulation.

The early steps of adipocyte differentiation take place during the first 3 days, when cells are treated with the complete differentiation medium. To determine whether these early steps are affected by Hedgehog, purmorphamine was added along the differentiation process (days 0–14), during only the first 3 days (days 0–3), or from day 3 to the end of the experiment (days 3–14). Purmorphamine had no effect on hMADS adipocyte differentiation when added only from day 0 to day 3, as reflected by triglyceride staining (Fig. 5C) and aP2 gene induction (Fig. 5D). Nevertheless, treatment of purmorphamine added chronically since day 3 was sufficient to alter adipocyte differentiation (Fig. 5C, 5D).

We assessed the effect of purmorphamine on differentiated adipocytes. A 48-hour purmorphamine treatment of 14-day-differentiated adipocytes did not interfere with marker expression (supplemental online Fig. 4). Cells were then differentiated for 14 days, and purmorphamine was added for 7 days (days 14–21). Cells were then compared with adipocytes differentiated without (−) or with purmorphamine for 21 days (days 0–21). As observed, whereas increase in Gli1 expression indicated that the Hedgehog signaling pathway was activated, purmorphamine treatment of differentiated cells for 7 days (days 14–21) did not affect oil red O staining (Fig. 5E) or adipsin, aP2, or C/EBPα expression (Fig. 5F). These data indicated that Hedgehog signaling is inefficient to induce dedifferentiation of human adipocytes.

Cells Differentiated in the Presence of Purmorphamine Are Resistant to Insulin

We next determined whether the defect in triglyceride accumulation was associated with a decrease in glucose metabolism. hMADS cells were treated under adipogenic conditions in the presence or absence of purmorphamine, and glucose uptake was monitored after 14 days of differentiation. Although basal glucose uptake was not altered by Hedgehog activation pathway, insulin-stimulated glucose uptake was abolished in the presence of purmorphamine during adipogenesis (Fig. 6A). In addition, the insulin-sensitive glucose transporter Glut4 expression dramatically decreased under purmorphamine treatment during adipogenesis (Fig. 6B). Thus, the mechanism by which purmorphamine impaired triglyceride accumulation during human adipogenesis may be accounted for, at least by part, by a decrease in insulin-stimulated glucose uptake.

thumbnail image

Figure Figure 6.. Glucose transport in response to insulin is inhibited by chronic treatment with purmorphamine during adipogenesis. Purmorphamine (2 μM) was added every other day from day 0 to completion of differentiation. (A): [3H]2-Deoxyglucose uptake performed in adipocytes stimulated or not stimulated by insulin. The data presented are mean ± SE of triplicates from an experiment representative of three independent experiments. (B): Glut4 expression analyzed using real-time quantitative reverse transcription-polymerase chain reaction. The data presented are mean ± SE of four independent experiments performed in triplicate and are expressed as fold induction relative to undifferentiated hMADS cells (day 0). Abbreviation: Ctrl, control.

Download figure to PowerPoint

Hedgehog Activation Targets Adipocyte Differentiation Through C/EBPα and PPARγ2

The transcriptional pathway of adipogenesis and insulin sensitivity are regulated by cross-regulation of PPARγ and C/EBPα [37]. Analysis of these two key transcription factors expression indicated that the induction of C/EBPα mRNA level was markedly reduced by Hedgehog activation (Fig. 7A), whereas no significant effect was observed on PPARγ expression when primers allowing the detection of total PPARγ (i.e., PPARγ1 and PPARγ2) were used (Fig. 7B).

thumbnail image

Figure Figure 7.. Purmorphamine treatment during adipogenesis decreases C/EBPα and PPARγ2 gene induction. Purmorphamine (2 μM) was added every other day from day 0 to completion of differentiation. C/EBPα (A), PPARγ (primers common to both PPARγ isoforms) (B), PPARγ coactivators (C), and PPARγ1 and PPARγ2 isoform expression (D) were analyzed by real-time quantitative reverse transcription-polymerase chain reaction. The data presented are means ± SE of at least three independent experiments performed in triplicate. Abbreviations: C/EBPα, CCAAT enhancer-binding protein α; ctrl, control; NS, not significant; PPAR, peroxisome proliferator-activated receptor.

Download figure to PowerPoint

Nuclear receptors, such as PPARγ, are known to activate transcription through docking of specific coactivator proteins. Moreover, the relative cellular levels of these cofactors affect the ability of PPARγ ligands to induce adipocyte differentiation [38]. Therefore, we assessed whether purmorphamine could modulate the expression of PPARγ coactivators during adipogenesis. As shown in Figure 7C, no change was observed in the expression of the PPARγ cofactors tested, such as TRAP220, SRC-1, p300, and PGC1α.

Finally, because PPARγ1 and PPARγ2 have distinct patterns of induction during adipocyte differentiation [39, 40] and could exert specific functions in adipocytes [41], it was of interest to examine the expression of each PPARγ isoform using specific primers. Interestingly, we found that expression of the most potent proadipogenic isoform, PPARγ2, decreased under purmorphamine treatment (Fig. 7D), whereas PPARγ1 levels remained unmodified (Fig. 7D). Similar results were observed using hBMSCs (supplemental online Fig. 5), demonstrating a uniform mechanism of Hedgehog on adipocyte differentiation of human mesenchymal stem cells.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

Although they are more difficult to handle than classic rodent preadipocyte cell lines, human mesenchymal stem cells present several significant advantages for the study of human adipocyte differentiation because they display a normal karyotype, and, in the case of hMADS cells developed in our laboratory, differentiation takes place in a serum-free, chemically defined medium. We show here that human mesenchymal stem cells are endowed with active Hedgehog signaling. Since these cells express several partners of Hedgehog signaling pathway (Ptch, Smo, Gli1, Gli2, and Gli3), it can be envisioned that the expression of Ihh and Dhh is responsible for the maintenance of active signaling in these cells, through an autocrine mechanism. Interestingly, adipocyte differentiation of these cells is associated with a decrease in Hedgehog signaling. This is similar to what has been observed in murine cells [14, 42]. Since we did not detect any modulation in Dhh and Ihh expression during adipocyte differentiation, the decrease in Hedgehog signaling could be associated with a decrease in Gli2 expression (Fig. 1B). Indeed, Gli2 is the primary activator of Hedgehog signaling, and a decrease in its expression in vivo and in vitro leads to a decreased signaling [7, [8]9, 26]. So far, Hedgehog signaling was thought to be regulated primarily through modulation of its ligands. Downregulation of Hedgehog signaling through Gli2 expression could be an original mechanism, specific to adipocyte differentiation. Interestingly, a decrease in Gli2 expression was also observed during adipocyte differentiation of murine cells [14].

Modulation in Hedgehog signaling pathway during adipogenesis suggested that it could play a role in adipocyte differentiation of human mesenchymal stem cells. Experiments performed using cyclopamine indicated that a decrease in this pathway is not sufficient to trigger adipocyte differentiation (Fig. 1). Although debated [14], this is similar to what we observed in murine 3T3-L1 cells [42]. On the other hand, chronic activation of Hedgehog pathway impairs human adipocyte differentiation. Together, this indicates that a decrease in Hedgehog signaling is a necessary but not a sufficient phenomenon for adipocyte differentiation. Under our conditions, purmorphamine maintained a level of Hedgehog signaling similar to the one observed in undifferentiated cells, suggesting that the antiadipogenic effect of Hedgehog can occur without massive overactivation of the pathway. A close examination indicated that the initial events of adipocyte differentiation are not modified by purmorphamine. Indeed, purmorphamine does not modify the total number of adipocytes but interferes with the late steps of differentiation, such as lipid accumulation and the expression of late adipocyte markers. Moreover, activation of the Hedgehog signaling pathway during only the first 3 days did not affect adipogenesis (Fig. 5C, 5D), indicating that Hedgehog signaling does not interfere with the induction but instead with the maturation of human adipocytes. Interestingly, Hedgehog had no effect on differentiated hMADS (Fig. 5E, 5F). This suggests that the Hedgehog effect is concomitant with the increase in the expression levels of adipocyte master genes, such as PPARγ and C/EBPα [5]. It is likely that when these factors reach critical levels, the cells become resistant to Hedgehog.

These effects of Hedgehog on the maturation of human adipocyte differ from those reported for rodent cells. Indeed, in C3H10T1/2, KS483, and 3T3-L1 cell lines, Hedgehog signaling inhibits the early steps of adipocyte differentiation and diverts cells to an osteoblastic phenotype [14, [15], [16]17]. This has also been observed in murine primary culture of calvaria [15] and in primary bone marrow stromal cells, using 20(S)-hydroxycholesterol as a Hedgehog activator [43, 44], suggesting that the difference is not specific to a cell model but is indeed a species difference.

Suh et al. proposed that Hedgehog-induced adipocyte inhibition of murine cells is mediated through GATA-2 [14], although clear evidence of GATA-2 induction by Hedgehog remained to be strengthened [45]. In human mesenchymal stem cells, purmorphamine did not affect GATA expression and was not found to favor osteoblastic differentiation (supplemental online Fig. 3). Furthermore, it must be stressed that these interspecies differences in Hedgehog signaling on human adipocytes are not specific to the age of the donor or to a given cell line. Indeed, two other hMADS cells, isolated from biopsies from different donors (4 months old and 5 years old), were tested, with similar results (data not shown); in addition, these results were recapitulated in hBMSCs from a 23-year-old patient.

Cells differentiated in the presence of purmorphamine exhibited a smaller cell size, with reduced lipid accumulation, and were insulin-resistant. The difference in morphology could be caused, by part, by the decrease in perilipin expression (Fig. 3E). Indeed, perilipin is the major protein associated with lipid droplets in adipocytes [29, 30], and although perilipin does not interfere per se in adipocyte differentiation, adipocytes from perilipin knockout mice have smaller adipocytes than their wild-type counterparts [46, 47]. Interestingly, although it is assumed that large adipocytes found in situation of obesity are less sensitive to insulin than “normal-sized” adipocytes, it has been reported that, in the situation of obesity, insulin resistance is associated with an expanded population of small adipocytes, with a decreased expression in adipocyte-specific markers in human patients [48]. It would be of interest to determine the degree to which immature adipocytes from purmorphamine-treated cells share phenotypic similarities with small adipocytes from these patients.

The later stages of adipocyte differentiation are controlled by C/EBPα and PPARγ, two transcription factors positively regulating each other. Expression of C/EBPα was found to be downregulated by 70%. Strikingly, although total PPARγ was not significantly modified, PPARγ2 expression decreased by 40%. PPARγ is expressed as two isoforms, PPARγ1 and PPARγ2, which are derived from the same gene by alternative promoter usage and differential mRNA splicing, so that PPARγ2 has an additional 30 amino acids at the NH2 terminus. PPARγ1 can be found in several tissues, including fat, skeletal muscle, heart, and liver, whereas PPARγ2 is found almost exclusively in adipose tissue. Although it is debated [49], it is generally thought that PPARγ1 is the predominant PPARγ isoform in human adipocytes [50, 51], which could explain why PPARγ2 expression does not significantly affect the expression of total PPARγ (Fig. 7). In vitro, PPARγ2 is more efficient for promoting adipocyte differentiation; however, the role of PPARγ1 on adipogenesis remains controversial [52, 53]. It can be noted that in vitro studies were performed using overexpression of PPARγ isoforms, which did not exactly match the physiological levels of these proteins. Our data show that maintenance of total PPARγ levels is not sufficient to maintain full adipocyte differentiation when C/EBPα and PPARγ2 expression are downregulated.

By which mechanism does purmorphamine inhibit adipocyte maturation? C/EBPα and PPARγ2 are required for the normal induction of key adipogenic genes, such as aP2, adipsin, CD36, and Glut4. Cells treated with purmorphamine during differentiation display defective expression of these genes and become insulin-resistant adipocytes. Adipogenesis of hMADS cells takes place in serum-free defined conditions. Insulin stimulates adipocytes to increase glucose uptake, whose energy is then stored in glycogen and triacylglycerol. The decrease in insulin sensitivity is likely to be one of the causes for the decrease in lipid accumulation. Moreover, since insulin could modulate PPARγ transcription in an isoform-specific manner [54, 55], it is tempting to speculate that the decrease in insulin sensitivity could be responsible in part for the downregulation of PPARγ2 expression. Together, purmorphamine could then trigger a vicious circle: C/EBPα and PPARγ positively regulate each other and control insulin sensitivity, which in return regulates PPARγ levels. Perturbation in one of these elements ultimately would lead to a global decrease in adipocyte maturation. Incidentally, this indicates that therapeutic treatments targeting the Hedgehog signaling pathway could engender insulin resistance.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

Together, our results indicate that a decrease in Hedgehog signaling is necessary but not sufficient for a complete adipocyte differentiation. Hedgehog signaling inhibits adipocyte maturation of human mesenchymal stem cells, leading to small and insulin-resistant adipocytes without modifying the total level of PPARγ. Moreover, these results reveal a species-specific effect of Hedgehog signaling activation on adipocyte differentiation and thus highlight the importance of using human mesenchymal stem cells to evaluate side effects of drugs modifying this pathway.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

We are grateful to Drs. G. Ailhaud, A. Ladoux, S. Giorgetti Peraldi, and Ez-Zoubir Amri for helpful comments. This work was supported by grants from the Centre National de la Recherche Scientifique and the Fondation pour la Recherche Medicale. C.F. and M.P. were supported by the Fondation pour la Recherche Medicale, and W.C. was supported by the Association pour la Recherche sur le Cancer 7696.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Disclosure of Potential Conflicts of Interest
  9. Acknowledgements
  10. References
  11. Supporting Information
FilenameFormatSizeDescription
SC-07-0974_Supplemental_Data.pdf81KSupplemental Data
SC-07-0974_Supplemental_Figures.pdf221KSupplemental Figures
SC-07-0974_Supplemental_Table_1.pdf34KSupplemental Table 1

Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.