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(Circulation. 2004;109:1392-1400.)
© 2004 American Heart Association, Inc.

Basic Science Reports

Rosiglitazone Facilitates Angiogenic Progenitor Cell Differentiation Toward Endothelial Lineage

A New Paradigm in Glitazone Pleiotropy

Chao-Hung Wang, MD; Nadia Ciliberti, BSc; Shu-Hong Li, MSc; Paul E. Szmitko, BSc; Richard D. Weisel, MD; Paul W.M. Fedak, MD; Mohammed Al-Omran, MD, MSc; Wen-Jin Cherng, MD; Ren-Ke Li, MD, PhD; William L. Stanford, PhD; Subodh Verma, MD, PhD

From the Division of Cardiovascular Surgery, Toronto General Hospital, Toronto, Ontario, Canada (C.W., S.L., P.E.S., R.D.W., P.W.M.F., M.A.-O., R.L., S.V.); Institute of Biomaterials and Biomedical Engineering, University of Toronto, and Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario, Canada (N.C., W.L.S.); Cardiology Section, Department of Medicine, Chang Gung Memorial Hospital, Keelung, Taiwan (C.W., W.C.); and Department of Pharmacology and Therapeutics, University of Calgary, Calgary, Alberta, Canada (S.V.).

Correspondence to Subodh Verma, MD, PhD, Division of Cardiac Surgery, 14EN-215, 200 Elizabeth St, Toronto General Hospital, Toronto, Ontario, Canada M5G 2C4. E-mail Subodh.Verma{at}sympatico.ca

Received September 18, 2003; de novo received December 8, 2003; revision received January 22, 2004; accepted January 30, 2004.

	Abstract

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Background— Peroxisome proliferator–activated receptor- (PPAR-) agonists inhibit vascular smooth muscle proliferation and migration and improve endothelial function. It is unknown whether PPAR- agonists favorably modulate bone marrow (BM)–derived angiogenic progenitor cells (APCs) to promote endothelial lineage differentiation and early reendothelialization after vascular intervention.

Methods and Results— C57/BL6 mice, treated with or without rosiglitazone (8 mg/kg per day), a PPAR- agonist, underwent femoral angioplasty. Rosiglitazone treatment attenuated neointimal formation (intima/media ratio: 0.98±0.12 [rosiglitazone] versus 3.1±0.5 [control]; P<0.001; n=10 per group). Using a BM transplantation model, we identified that 58±12% of the cells within the neointima at 4 weeks were derived from the BM. Pure endothelial marker–positive, pure -smooth muscle actin (SMA)–positive, or double-positive APCs could be found both in mouse BM and in human peripheral blood after culture in conditional medium enriched with vascular endothelial growth factor. Rosiglitazone caused a 6-fold (P<0.001) increase in colony formation by human endothelial progenitor cells, promoted the differentiation of APCs toward the endothelial lineage in mouse BM in vivo (0.66±0.06% [control] to 0.95±0.08% [rosiglitazone]; P<0.05) and in human peripheral blood in vitro (13.2±1.5% [control] to 28.4±3.3% [rosiglitazone]; P<0.05), and inhibited the differentiation toward the smooth muscle cell lineage. Within the neointima, rosiglitazone also stimulated APCs to differentiate into mature endothelial cells and caused earlier reendothelialization compared with controls (31±5 versus 8±2 CD31-positive cells per millimeter of neointimal surface on day 14; P<0.01).

Conclusions— Similar to embryonic stem cell–derived progenitors, the adult BM and peripheral blood harbor APCs that are at least bipotential and able to differentiate into endothelial and smooth muscle lineages. The PPAR- agonist rosiglitazone promotes the differentiation of these APCs toward the endothelial lineage and attenuates restenosis after angioplasty.

Key Words: cells • endothelium, vascular • vessels • receptors, cytoplasmic and nuclear • thiazolidinediones

	Introduction

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Intimal hyperplasia and resulting restenosis remain the Achilles’ heel of vascular intervention. In the past, vascular smooth muscle cells within the neointima were considered to be derived from the vascular media layer, while the regenerated endothelial cells on the neointimal surface were thought to have migrated from the adjacent vascular wall. However, recent studies have suggested a new paradigm in which more than half of these cells are from bone marrow (BM)–derived progenitor cells.¹ Currently, the best-investigated circulating vascular progenitor cell is the endothelial progenitor cell (EPC).² Manipulation of EPCs results in early reendothelialization and attenuates neointimal formation.³ However, in response to different factors, these putative EPCs can transform into cells with different phenotypes, such as smooth muscle cells.⁴ This suggests that these cells may represent a population of angiogenic progenitor cells (APCs), which in this report are defined as cells with the ability to differentiate along the various vascular lineages. Manipulation of cell signaling has an important effect on embryonic stem cell development toward hematopoietic, endothelial, and smooth muscle lineages.⁵ However, it is unknown whether these putative EPCs can also be programmed for differentiation into a variety of vascular cell types.

Peroxisome proliferator–activated receptor- (PPAR-) agonists, a new class of insulin sensitizers, are used clinically to treat diabetes. PPAR- agonists appear to improve endothelial function independent of their insulin sensitization effects.^6,7 They also favorably limit vascular inflammation⁸ and decrease circulating levels of C-reactive protein,⁹ which serves not only as a marker but also as a mediator of atherosclerosis.¹⁰ PPAR- agonists inhibit intimal hyperplasia after balloon injury in both diabetic and nondiabetic animal models.^11–13 However, it is unknown whether PPAR- agonists have effects on the bioactivity and maturity of putative EPCs. In this study we demonstrate that the PPAR- agonist rosiglitazone stimulates the differentiation of APCs toward the endothelial cell lineage both in vitro and in vivo. These data identify a novel mechanism of vascular protection by PPAR- agonists.

	Methods

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Mouse Model of Femoral Artery Angioplasty
Male C57/BL6 mice (Charles River, Quebec, Canada) under general anesthesia underwent transluminal mechanical injury of the femoral arteries by insertion of a straight spring wire (0.38 mm in diameter; Cook) for >5 mm toward the iliac artery, as previously described.¹ All procedures involving experimental animals were approved by the institutional committee for animal research of the Toronto General Hospital and Mount Sinai Hospital.

Rosiglitazone Therapy
Animals received rosiglitazone (8 mg/kg per day; GlaxoSmithKline)¹³ or saline by oral gavage (n=10 for each group) 2 weeks before surgery. Treatments were continued until the mice were killed (at 1, 2, 3, or 4 weeks).

BM Transplantation Model
Recipient 129S6 mice at 8 weeks of age were lethally irradiated with a total dose of 950 rad, 9.5 Gy. TgN(ActbEYFP) (or eYFP) transgenic mice (129S6 background) that ubiquitously express enhanced YFP were used as the donors.¹⁴ After irradiation, the recipient mice received unfractionated BM cells (3x10⁶) from eYFP mice by tail vein injection. At 8 weeks after injection, angioplasty was performed. Repopulation by eYFP-positive BM cells was measured by flow cytometry to be 74%. The chicken -actin and CMV enhancer that drives eYFP in the TgN(ActbEYFP) mice demonstrates some silencing in BM-derived cells (N. Anderson, MSc, W.L. Stanford, PhD, unpublished data, 2003), suggesting that the contribution of BM cells to neointimal formation is greater than observed.

Mouse Progenitor Cell Purification and Culture
A SpinSep kit (Stem Cell Technologies) was used to enrich murine hematopoietic progenitor cells. After the enriched murine hematopoietic progenitor cells were resuspended in EGM-2 medium (Clonetics), 1x10⁶ cells were plated on 60-mm plates coated with fibronectin and treated with or without rosiglitazone (1 µmol/L). After 4 weeks, cells were evaluated by morphology and immunofluorescent expression analysis.

Human Mononuclear Cell Isolation and Culture
Mononuclear cells were isolated from the blood of healthy young volunteers by density gradient centrifugation with Ficoll separating solution (Becton Dickinson). After resuspension in EGM-2 medium (containing vascular endothelial growth factor [VEGF] 10 ng/mL), 10⁶ mononuclear cells/cm² were plated on fibronectin-coated 60-mm dishes and separated into subgroups treated with rosiglitazone (1 µmol/L), 15d-PGJ2 (1 and 10 µmol/L; Calbiochem), or platelet-derived growth factor (PDGF) (10 ng/mL; Sigma). In the PDGF subgroup, PDGF was added 4 days after cell plating. Cells were evaluated by flow cytometry, colony formation, or immunofluorescent expression analysis at the indicated time points.

Colony Formation Assay
After 4 days in culture with or without rosiglitazone (1 µmol/L), adherent mononuclear cells were gently detached with cell dissociation solution (Sigma). Cells (1x10⁵) were seeded in methylcellulose plates (Stem Cell Technologies) containing 100 ng/mL human recombinant VEGF with or without rosiglitazone (1 µmol/L). Plates were studied under phase-contrast microscopy, and colonies were counted after 9 days of incubation by 2 independent investigators. A colony included at least 50 cells.

Fluorescence-Activated Cell Sorter Analysis
In both human and murine cells, fluorescence-activated cell sorting (FACS) (FacScan, Becton Dickinson) was performed to identify both cell-surface and intracellular antigens. Intracellular antigens were exposed with the use of the Cytofix/Cytoperm kit (Pharmingen). For cultured human mononuclear cells, Cy3-conjugated anti–-smooth muscle actin (SMA) (Sigma) and FITC-conjugated anti-human VE-cadherin antibodies (Serotec) were used. In mouse experiments, FACS was performed on both peripheral blood and BM cells collected from mice treated with or without rosiglitazone (8 mg/kg per day) for 16 days (n=7 to 8 for each group). Cy3-conjugated anti-SMA, PE-conjugated anti-mouse Sca-1 (Pharmingen), and biotin-conjugated anti-mouse KDR antibodies (eBioscience) with secondary detection by FITC-conjugated streptavidin were used.

Confocal Immunofluorescent and Histological Analysis
For in vitro studies, cells were analyzed by immunofluorescent staining for von Willebrand factor (vWF), SMA, caldesmon, and calponin. For in vivo studies, frozen sections of femoral arteries were stained with primary antibodies (-SMA, Sigma; vWF, Dako; VE-cadherin, CD31, Mac-3, Pharmingen; caldesmon, calponin, Santa Cruz) followed by incubation with FITC-, PE-, or Alexa Fluor 647–conjugated secondary antibodies. The ratio of intimal area to medial area (I/M ratio) was calculated. Cells positive for endothelial markers were counted in at least 10 different cross sections from different animals and expressed as the average number of positive cells per luminal surface length (in millimeters).

Statistical Analysis
Data were compared by unpaired t tests. A probability value of <0.05 was considered significant. All data presented in the text and figures are expressed as mean±SEM.

	Results

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Rosiglitazone Attenuates Neointimal Formation
Femoral arteries were harvested 4 weeks after angioplasty. Compared with the control group (treated with saline), there was a 3-fold decrease in I/M ratio in the rosiglitazone-treated group (3.1±0.5 [control] versus 0.98±0.12 [rosiglitazone]; P<0.001; Figure 1).

View larger version (84K): [in this window] [in a new window]   Figure 1. Rosiglitazone attenuates intimal hyperplasia. C57/BL6 mice were treated with rosiglitazone (Rosi) (8 mg/kg per day) or saline (control [Con]) 2 weeks before angioplasty and for 4 weeks after vessel injury. The I/M ratio at 4 weeks after angioplasty was significantly lower in the rosiglitazone group than in controls (n=10 for each group; *P<0.001). Arrows indicate vascular medial layer.

Angiogenic Progenitor Cells From Mouse BM Are Multipotential Cells
In 129S6 mice after BM reconstitution from eYFP mice, most of the cells in the neointima that formed 2 weeks after angioplasty were eYFP⁺SMA⁺ (Figure 2A), suggesting that they were BM-derived vascular smooth muscle cells. Four weeks after angioplasty, 58±12% of the cells within the neointima were eYFP⁺SMA⁺. To investigate the contribution of BM to neointimal formation, lineage-negative progenitor cells were purified from mouse BM and cultured in EGM-2 medium. Although 1x10⁶ cells were initially plated in each 60-mm dish, only 2 to 3 colonies formed after culture for 2 weeks. In the colonies, cells could be generally sorted into 2 different morphologies: oval or spindle shaped (Figure 2B and 2C). After the cells were cultured for a total of 4 weeks in EGM-2, immunofluorescent analysis with anti-SMA and anti-vWF (markers of vascular smooth muscle and endothelial populations, respectively) was performed, which revealed 4 different cell groups: pure SMA⁺ (vWF^-; Figure 2D), pure vWF⁺ (SMA^-; Figure 2E), double-positive (vWF⁺SMA⁺; Figure 2F and 2G), and double-negative cells. Double-positive cells also expressed caldesmon and calponin, 2 additional markers of the vascular smooth muscle lineage (Figure 2F and 2G, respectively). Differentiated smooth muscle–like cells stained positive for SMA, caldesmon, and calponin (Figure 2H and 2I).

View larger version (83K): [in this window] [in a new window]   Figure 2. APCs in mouse BM. A, Immunofluorescence staining at 14 days after angioplasty in mice with BM transplantation from eYFP mice. Arrows indicate vascular medial layer. B and C, Purified progenitor cells from mice BM cultured in EGM-2 medium for 14 days developed into spindle (B) or oval (C) cells. D to I, At 28 days, immunofluorescence staining revealed pure SMA+ (D), pure vWF+ (E), vWF+SMA+caldesmon+ (F), vWF+SMA+calponin+ (G), and differentiated smooth muscle–like cells (H and I) that expressed SMA, calponin, and H-caldesmon.

Human Circulating APCs
Ten days after culture in EGM-2 medium, human mononuclear cells transformed into either spindle- or polygonal-shaped cells (Figure 3A and 3B). In general, the cells in the rosiglitazone group appeared to undergo a more dramatic transformation compared with the control group. Via the colony formation assay assessed on day 9, the number of colonies that formed in the rosiglitazone group was 6-fold over that in the control group (Figure 3C). After the cells were cultured for 4 weeks in EGM-2 medium alone, mononuclear cells in a few colonies transformed into endothelial cells (Figure 3D). In cells supplemented with PDGF (10 ng/mL) 4 days after being cultured in EGM-2, mononuclear cells proliferated greatly over the next 4 weeks (Figure 3E). However, these cells did not transform. In plates treated with 15d-PGJ2, a natural PPAR- agonist, a great deal of cell detachment was observed, and the remaining cells did not transform (Figure 3F).

View larger version (93K): [in this window] [in a new window]   Figure 3. APCs in human peripheral blood. A and B, Human mononuclear cells were cultured in EGM-2 medium for 10 days without (A) or with (B) rosiglitazone (1 µmol/L). C, EPC colony formation assay was evaluated at day 9. The numbers of colonies were 2.3±1.5 and 18.6±1.5 per well in the control and rosiglitazone (Rosi) groups, respectively (P<0.01). D to F, Mononuclear cells were cultured in EGM-2 alone (D), with PDGF (E, started 4 days after cells cultured in EGM-2), or with 15d-PGJ2 (F) for 28 days. G to I, PDGF was supplemented after the cells were cultured in EGM-2 for 28 days. Some cells maintained their oval shape (G), while some started to elongate (H) and transform into smooth muscle–like cells (I). J to M, Immunofluorescence staining shows that some of the oval cells were pure vWF+ (J), and others were both vWF+SMA+ (K). L and M, In some elongated cells, filament-like SMA was developing from SMA+ granules (magnified in M).

After the APCs were cultured for 4 weeks in EGM-2, cells were supplied with PDGF to stimulate transformation into smooth muscle cells. After 4 additional weeks, the initially oval and endothelial-like cells followed different fates. Some cells maintained an oval shape (Figure 3G), while others became elongated (Figure 3H) and transformed into smooth muscle–like cells (Figure 3I). Immunofluorescence double staining showed that some of the oval-shaped cells were pure vWF⁺ (Figure 3J) and others were double positive (vWF⁺SMA⁺; Figure 3K). Some elongated cells developed filament-shaped SMA (Figure 3L and 3M).

Rosiglitazone Promotes APC Differentiation Toward the Endothelial Lineage
FACS was performed 7 and 28 days after the plating of mononuclear cells. The percentages of pure SMA⁺, pure VE-cadherin⁺, and SMA⁺VE-cadherin⁺ cells were 0.33±0.05%, 5.78±0.61%, and 0.81±0.1%, respectively, on day 7 (Figure 4A) and increased to 1.27±0.23%, 13.16±1.5%, and 8.92±1.21%, respectively, on day 28 (Figure 4B). Rosiglitazone treatment markedly increased pure VE-cadherin⁺ cells to 28.39±3.28% and double-positive cells up to 18.97±2.32% on day 28 (Figure 4C) but decreased pure SMA⁺ cells to 0.24±0.01% (all P<0.01 compared with groups without rosiglitazone).

View larger version (34K): [in this window] [in a new window]   Figure 4. Rosiglitazone (Rosi) modulation of APCs. Four-quadrant analysis for VE-cadherin (VE-Cad) (x axis) and SMA (y axis) was done in human mononuclear cells cultured in EGM-2 media for 7 (A) and 28 days (B, without rosiglitazone; C, with rosiglitazone). D shows the isotype control. n=5 for each group.

In the mouse in vivo study, rosiglitazone treatment significantly increased the percentage of Sca-1⁺KDR⁺ cells (putative EPCs) in the BM (0.66±0.06% [control] to 0.95±0.08% [rosiglitazone]; P<0.05) and also had a tendency to increase Sca-1⁺KDR⁺ cells in peripheral blood (1.19±0.39% [control] to 2.36±0.37% [rosiglitazone]; P=NS). Rosiglitazone did not decrease the percentage of SMA⁺ cells in BM (1.46±0.32% [control] to 0.4±0.24% [rosiglitazone]; P=NS) but significantly decreased SMA⁺ cells in peripheral blood (1.2±0.34% [control] to 0.26±0.09% [rosiglitazone]; P<0.05).

Rosiglitazone Promotes APC Maturation to Endothelial Cells
As shown in Figure 5A and 5C, the majority of the cells in the neointima on day 14 expressed vWF, SMA, and eYFP, although some neointimal cells expressed SMA only. However, on day 28 the majority of the neointimal cells were SMA⁺ (Figure 5B). Only cells on the surface of the neointima were vWF⁺, suggesting the phenotypic maturation of these 2 cell populations. Additional staining with other endothelial markers was done to investigate the process of endothelial cell maturation. Vessels were stained with anti–VE-cadherin, an early marker for endothelial cells, on days 8 and 21 (Figure 5D), and stained with anti-CD31, a late and mature endothelial marker, on days 14 and 21 (Figure 6). On day 8, the injured vessel surface was covered with a monolayer of cells that expressed both VE-cadherin and SMA, supporting that they are APCs rather than pure EPCs. With neointimal formation, most of the APCs in the neointima turned off the expression of endothelial markers and maintained the expression of SMA. However, gradually, more and more cells on the neointimal surface expressed more mature endothelial markers and differentiated into mature endothelial cells. On day 21, rosiglitazone treatment significantly increased the cells expressing VE-cadherin on the surface of the neointima compared with the control group (43±5 versus 17±3 cells/mm; P<0.01; n=8). In addition, rosiglitazone treatment increased the number of cells expressing CD31 on the surface of injured vessels compared with controls on both day 14 (31±5 versus 8±2 cells/mm; P<0.01; n=7) and day 21 (36±6 versus 11±3 cells/mm; P<0.01; n=8). The majority of these BM-derived cells within the neointima expressed not only SMA but also calponin and caldesmon (Figure 7A, 7B, 7C). The SMA⁺ cells within the neointima were negative for Mac-3, a marker for macrophages, further suggesting they were not inflammatory cells (Figure 7D).

View larger version (100K): [in this window] [in a new window]   Figure 5. Rosiglitazone promotes the in vivo differentiation of APCs toward the endothelial lineage. A and B, Double-immunofluorescent labeling of mouse femoral artery with SMA and vWF at 14 (A) and 28 (B) days after angioplasty. C, Triple-fluorescent labeling with SMA, eYFP, and vWF at 14 days after angioplasty. Arrows indicate internal elastic lamina. D, Double labeling with SMA and VE-cadherin (VE-Cad) (arrows) at 8 days after angioplasty.

View larger version (95K): [in this window] [in a new window]   Figure 6. Rosiglitazone promotes early reendothelialization by APCs. A and B, Double-immunofluorescent labeling of mouse femoral artery at 14 and 21 days after angioplasty. Red indicates SMA; green, CD31 (arrows). Con indicates control group; Rosi, rosiglitazone-treated group.

View larger version (104K): [in this window] [in a new window]   Figure 7. BM-derived angiogenic cells express smooth muscle cell markers. A to C, Double-immunofluorescent labeling of mouse femoral artery with eYFP and either SMA (A), calponin (B), or caldesmon (C). D, Double-immunofluorescent labeling with SMA and Mac-3, a macrophage marker.

	Discussion

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We demonstrate that, similar to embryonic stem cell–derived progenitors, the adult BM and peripheral blood contain APCs that are at least bipotential and able to differentiate into endothelial and smooth muscle lineages. These APCs may be mobilized and contribute to neointimal formation after vascular interventions. In this study we show that rosiglitazone, a PPAR- agonist, promotes the differentiation of APCs toward the endothelial lineage, in vitro and in vivo, both before and after mobilization from the BM or after homing to injured vascular sites. Furthermore, we demonstrate that the PPAR- agonist rosiglitazone attenuates restenosis after angioplasty, possibly as a result of its favorable effect on APCs.

Early reendothelialization plays an important role in preventing complications and attenuating intimal hyperplasia after vascular interventions. Reports demonstrate that PPAR- ligands inhibit smooth muscle proliferation and migration,¹⁵ improve endothelial dysfunction,^6,7 and attenuate intimal hyperplasia after balloon injury in diabetic and nondiabetic animal models.^11–13 Alternatively, the beneficial effect of PPAR- agonists may be ascribed to their ability to increase the release of nitric oxide by endothelial cells.⁷ However, direct effects of rosiglitazone on progenitor cell differentiation and mobilization may also play a key role in inhibiting neointimal formation after vascular injury. It is unknown whether PPAR- ligands can enhance BM-derived APC-mediated reendothelialization. In the present study we define APCs as a population of progenitor cells that have the multipotential to differentiate into either endothelial or smooth muscle cells, and we highlight the role of rosiglitazone to favorably promote the former.

By using embryonic stem cell differentiation as a developmental model, we previously demonstrated that manipulation of cell signaling parameters can regulate the cell fate in early development toward hematopoietic, endothelial, and smooth muscle lineages.^5,16 Embryonic stem cell–derived APCs differentiate into distinct vascular lineages in response to different signaling queues. In the present study we show that bipotential APCs are also present in adult tissues, although the markers of APCs are still not well defined. The relationship between adult APCs and embryonic primitive mesodermal progenitors, often termed hemangioblasts, capable of generating hematopoietic progenitors as well as APCs, is currently unclear. In progenitor cells purified from adult mouse BM, APCs could be easily cultured, although their numbers are few. In addition to cells expressing markers specific for either endothelial or smooth muscle lineages, there is a population of double-positive progenitor cells that were also found in human peripheral blood. Our report suggests that double-positive progenitor cells hold dual potential to develop into either endothelial or smooth muscle cells in response to different stimuli. However, the timing for programming of this differentiation response is not clear. For example, when VEGF was added to selectively guide cells toward both endothelial and smooth muscle lineages, early stimulation with PDGF promoted only cell proliferation rather than differentiation. Interestingly, in the in vitro experiments, rosiglitazone was shown to stimulate the differentiation of human mononuclear cells toward the endothelial lineage. In the in vivo experiments, in mice without surgical intervention, rosiglitazone increased the amount of EPCs in the BM and decreased the number of smooth muscle progenitor cells in the circulation. An increase in EPCs (versus smooth muscle progenitors) by rosiglitazone treatment may serve to facilitate early reendothelialization.

Our in vivo studies lend further support to this notion. Mouse femoral angioplasty causes local vascular smooth muscle apoptosis and mobilizes BM cells to repair the resultant injury. In accord with the report by Sata et al,¹ our data showed a significant amount of BM-derived cells contributing to neointimal formation. Although the first monolayer of cells that formed on the injured vascular surface may have been smooth muscle cells migrating from the media layer, our confocal figures demonstrated that this was unlikely because these cells expressed both -SMA and VE-cadherin, an early marker of endothelial cells. The location of VE-cadherin was observed right at the tight junctions between cells, excluding the possibility of false-positive staining. These findings suggest that these SMA⁺ cells were not derived from the local vessel medial wall but may have been APCs from the circulation. As expected, these cells gradually expressed vWF, which is a marker expressed later than VE-cadherin in the maturation course of EPCs. However, coinciding with this reendothelialization was neointimal growth, in which the majority of the participating APCs differentiated into smooth muscle–like cells and extinguished expression of endothelial markers. Although the molecular controls of APC differentiation are not well characterized, our report shows that rosiglitazone accelerates the development of cells with the mature endothelial marker CD31 that cover the neointimal surface.

It is interesting to speculate why the PPAR- ligand 15d-PGJ2 did not have the same effects as rosiglitazone. Rosiglitazone is approximately 20-fold more efficacious than 15d-PGJ2 or troglitazone in binding to PPAR- and in increasing transcriptional activity of PPAR-, which may be one reason for this discrepancy.¹⁷ In addition, the biological effects of 15d-PGJ2 are complex because of its potential to activate prostaglandin receptors, induce apoptosis and Waf1 gene expression,^18,19 and inhibit aromatase,²⁰ which might be detrimental for growing APCs, as observed by the increased cell detachment and absent differentiation after 15d-PGJ2 administration. Although PPAR- ligands inhibit endothelial proliferation in vitro, inhibit retinal angiogenesis in vivo, and promote endothelial apoptosis in vitro,²¹ these effects may be concentration specific¹² and may be attributed to pharmacological activity independent of PPAR-,²² the potency of PPAR- receptors,¹⁷ or the differing transcriptional activation in different cell types,¹⁵ such as BM-derived progenitor cells.

The present study reinforces the role of VEGF and PDGF as critical mediators that guide the fate of APCs.^23–26 VEGF increases in both blood and local tissues after vascular injury.²⁷ Previous reports showed that PPAR- agonists increase VEGF expression in human vascular smooth muscle,²³ increase nitric oxide production by endothelial cells, upregulate endothelial nitric oxide synthase expression by APCs (S. Verma, MD, PhD, unpublished data, 2003), and increase blood VEGF concentrations in patients with diabetes.²⁸ On the other hand, PPAR- ligands, through a variety of pathways,^24–26 have been shown to attenuate the effects of PDGF, a factor that has long been known to stimulate smooth muscle proliferation and neointimal growth. The increased cellular and systemic VEGF levels may facilitate the differentiation of APCs to the endothelial lineage. In contrast, the attenuated effect of PDGF and upregulated expression of endothelial nitric oxide synthase in APCs may minimize the number of progenitor cells pursuing the path toward the smooth muscle lineage and also contribute to slower proliferation and migration of local smooth muscle cells.

Currently, the most commonly used method for culturing EPCs from peripheral blood is to isolate mononuclear cells by gradient centrifugation and to stimulate EPC maturation by conditional culture medium. However, these putative EPCs have been successfully transformed into cells with different phenotypes, such as smooth muscle cells and cardiomyocytes.^4,29 Accompanied with our findings, we suggest that cells isolated by this method include a variety of progenitor cells with multiple potentials. Although it is the direction of the future to transplant these putative EPCs for therapeutic purposes, the present information suggests that biochemical and molecular manipulations may be required to guide the fate of these cells before using them for regeneration therapy or to attenuate restenosis.

In summary, in addition to hematopoietic stem and progenitor cells, the adult BM and peripheral blood harbor bipotential progenitors capable of developing endothelial and smooth muscle lineages. The PPAR- agonist rosiglitazone promotes the differentiation of these APCs toward the endothelial lineage and attenuates restenosis after angioplasty.

	Acknowledgments

 
This study was supported by GlaxoSmithKline Canada and the Heart and Stroke Foundation of Canada (Drs Verma and Weisel). Dr Wang is supported by Chang Gung Memorial Hospital, Taiwan. Dr Stanford is a Canadian Research Chair in Stem Cell Biology and Functional Genomics. We would like to thank Nicole Anderson for performing FACS on the repopulated mice and George Cheong for performing tail vein injections.

	Footnotes

 
This article originally appeared Online on March 1, 2003 (Circulation. 2004;109:r67–r75).

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