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Published online July 10, 2007
Diabetes 56:2609-2615, 2007
DOI: 10.2337/db07-0069
© 2007 by the American Diabetes Association
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The Peroxisome Proliferator–Activated Receptor-{gamma} Agonist Pioglitazone Increases Number and Function of Endothelial Progenitor Cells in Patients With Coronary Artery Disease and Normal Glucose Tolerance

Christian Werner, Christel Hermann Kamani, Christoph Gensch, Michael Böhm, and Ulrich Laufs

From the Klinik für Innere Medizin III, Kardiologie, Angiologie und Internistische Intensivmedizin, Universitätsklinikum des Saarlandes, Homburg/Saar, Germany

Address correspondence and reprint requests to Ulrich Laufs, Klinik für Innere Medizin III, Universitätsklinikum des Saarlandes, 66421 Homburg/Saar, Germany. E-mail: ulrich{at}laufs.com

Abbreviations: CFU, colony-forming unit; CRP, C-reactive protein; DiLDL, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine–labeled acetylated LDL; EBM, endothelial cell basal medium; FACS, fluorescence-activated cell sorting; FITC, fluorescein isothiocyanate; HOMA, homeostasis model assessment; hsCRP, high-sensitivity CRP; KDR, kinase insert domain receptor; MNC, mononuclear cell; PE, phycoerythrin; PMA, phorbol myristate acetate; PPAR, peroxisome proliferator–activated receptor; TZD, thiazolidinedione


   ABSTRACT
TOP
 ABSTRACT
RESEARCH DESIGN AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
OBJECTIVE—Peroxisome proliferator–activated receptor-{gamma} (PPAR{gamma}) agonists (thiazolidinediones [TZDs]) are used for the treatment of diabetes. Bone marrow–derived endothelial progenitor cells (EPCs) improve vascular function and predict cardiovascular risk. The effect of pioglitazone therapy on EPCs was examined.

RESEARCH DESIGN AND METHODS AND RESULTS—We performed a prospective, randomized, double-blind study on patients with documented stable coronary artery disease and normal glucose tolerance. Of 54 patients with normal fasting glucose levels, 18 showed impaired glucose tolerance and 36 patients with normal glucose tolerance were randomized to 30-day treatment with pioglitazone (45 mg) or placebo in addition to optimal medical therapy. All patients in the TZD group showed an increase of adiponectin levels as an indicator of compliance (11.4 ± 1.1 to 36.8 ± 2.1 µg/ml; P < 0.001). TZD, but not placebo, decreased mean high-sensitivity C-reactive protein to 43 ± 19% (P < 0.05). Pioglitazone increased CD34+/kinase insert domain receptor+ EPCs to 142 ± 9% and cultured 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine–labeled acetylated LDL+/lectin+ EPCs to 180 ± 3% (P < 0.05). EPC numbers were not changed in the placebo group. TZD increased the SDF-1–induced migratory capacity to 146 ± 9% per EPC number (P < 0.05) and upregulated the clonogenic potential of EPCs, increasing the colony-forming units to 172 ± 12% (P < 0.001). In cultured human EPCs, TZD increased EPC numbers and migration and reduced NADPH-oxidase activity. The TZD effect was reversed by the PPAR{gamma} antagonist GW9662 and mimicked by treatment with adiponectin.

CONCLUSIONS—The PPAR{gamma} agonist pioglitazone increases the number and function of EPCs in patients with coronary artery disease. The effect represents a potential regenerative mechanism in atherosclerosis and is observed in normoglycemic individuals with stable coronary artery disease.

Peroxisome proliferator–activated receptor-{gamma} (PPAR{gamma}) agonists (thiazolidinediones [TZDs]) lower serum glucose levels in patients with type 2 diabetes (1). In addition to their insulin-sensitizing effects, increasing evidence suggests that these drugs improve endothelium-dependent vascular function and prevent atherosclerotic disease progression (2–7). Furthermore, experiments on cultured vascular cells support a direct and beneficial modulation of key regulators of atherosclerosis, such as cellular adhesion molecules, tissue factor, plasminogen activator inhibitor, and matrix metalloproteinases (rev. in 8–10). These findings have led to the hypothesis that TZDs may exert vasculoprotective effects independently of their metabolic action (9).

Cardiovascular function and angiogenesis have been shown to be significantly modulated by circulating premature cells derived from the bone marrow (11). A subset of these stem cells named endothelial progenitor cells (EPCs) (12) enhances angiogenesis, promotes vascular repair, and improves endothelial function (13–19). Recently, it was shown that reduced levels of circulating EPCs represent a cellular marker that independently predicts outcome in patients with vascular disease (20,21). The circulating numbers and function of EPCs are regulated. Vascular risk factors, and especially type 2 diabetes, have been shown to reduce EPC numbers and impair EPC function (14,22–27). On the other hand, lipid lowering with statins or physical activity are interventions capable of raising EPC numbers and improving their function (13,15,24,28,29). Similarly, glucose-lowering treatment increases EPC numbers in diabetic individuals (30,31). Interestingly, experiments in cultured cells and in mice have suggested that treatment with TZD upregulates EPCs (32–34).

On the basis of these preclinical data, we hypothesized that the PPAR{gamma} agonist pioglitazone may modulate the number and function of EPCs. We further speculated that these effects may be independent of serum glucose lowering. The effect of treatment with pioglitazone on EPCs was tested in a prospective, randomized, double-blind clinical trial on patients with stable coronary artery disease and documented normal glucose tolerance. We examined the level of CD34+/kinase insert domain receptor (KDR)+ mononuclear cells because this cell population was shown to predict the occurrence of cardiovascular events and death from cardiovascular causes (20). Furthermore, the effects of TZD on parameters of EPC function were characterized.


   RESEARCH DESIGN AND METHODS
TOP
 ABSTRACT
 RESEARCH DESIGN AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The study was designed as a prospective, randomized, double-blind clinical trial on patients with stable coronary artery disease and normal glucose tolerance. The trial was conducted with approval of the local ethics committee (Ethikvotum 130/05) and the German Federal Institute for Drugs and Medical Devices (Bfarm no. 4030855; Eudora-CT no. 2005-003939-42). Informed consent was obtained from all subjects.

All patients had angiographically documented and clinically stable coronary artery disease. Medication was not changed during the course of the study. Exclusion criteria included functional New York Heart Association class III or IV heart failure, left ventricular dysfunction measured as left ventricular ejection fraction <40%, elevated serum creatinine level, active liver disease, alanine transaminase levels of ≥2.5 times the upper limit of normal, regular use of nonsteroidal anti-inflammatory drugs, psychiatric diseases, and pregnancy.

Fifty-four eligible patients with normal fasting glucose underwent an oral glucose tolerance test (Dextro O.G-T.; Roche). Eighteen patients showed impaired glucose tolerance, and 36 patients with normal glucose tolerance were enrolled in the study. They were randomized to a 30-day treatment with pioglitazone (45 mg) or matching placebo in addition to an optimized cardiovascular medication. Randomization was performed by the pharmacy of the Universitätsklinikum des Saarlandes independently from the investigators. Venous blood samples were taken on the day of enrolment in the study and after 30 days of placebo/TZD therapy. Adiponectin serum levels were quantified by the human adiponectin radio immunoassay (Linco). High-sensitivity C-reactive protein (hsCRP) was measured by a turbidimetric latex test (CRP Dynamik; Biomed).

Flow cytometry analysis of EPCs.
Fluorescence-activated cell sorter (FACS) analyses were used to characterize mononuclear cells (MNCs) as previously described (20). MNCs were selected using Ficoll density gradient centrifugation (Biocoll Separating Solution; Biochrom) from 20 ml human blood drawn in sodium citrate and resuspended in 1 ml endothelial cell basal medium (EBM; CellSystems) with supplements (1 µg/ml hydrocortisone, 3 µg/ml bovine brain extract, 30 µg/ml gentamicin, 50 µg/ml amphotericin B, 10 µg/ml human endothelial growth factor, and 20% FCS). One hundred microliters was diluted in FACS buffer (PBS, 0.1% bovine albumin, and 15 units/l aprotinine) and immediately used for assessment of isotype controls and CD34-fluorescein isothiocyanate (CD34-FITC; Becton Dickinson)/goat anti-human KDR (R&D Systems) with the conjugated secondary antibody anti-goat streptavidin-phycoerythrin (PE) (DAKO). All antibody incubations were kept on ice in the dark. Isotype identical antibodies (IgG 2a-FITC and IgG 2a-PE; Becton Dickinson) served as controls in every experiment, and unspecific binding was blocked by addition of human serum. Cells were washed twice in FACS buffer and fixed with 2% paraformaldehyde. Each measurement was performed in two separate tubes by assessment of 105 MNCs in the lymphocyte gate using the Becton Dickinson FACSCalibur and Cell Quest Pro software. The intra-assay variability (coefficient of variation) was similar between groups and assays: FACS analysis of CD34+/KDR+ EPCs, placebo 28% and TZD 29%; 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine–labeled acetylated LDL (DiLDL)+/lectin+ EPCs, placebo 29% and TZD 30%. The interassay correlation between CD34+/KDR+ EPCs and DiLDL+/lectin+ EPCs was R2 = 0.54 (P < 0.01).

Cell culture of circulating EPCs.
MNCs were isolated by Ficoll density gradient centrifugation, the cell pellet was resuspended in 5 ml EBM (Cell Systems), and 4 x 106 mononuclear cells were cultured on fibronectin-coated dishes in EBM (CellSystems) (18,35). After 4 days in culture, adherent cells were incubated with DiLDL (2.4 µg/ml; CellSystems) and stained using FITC-labeled Ulex europaeus agglutinin I (lectin, 10 µg/ml) (Sigma). All measurements were performed in triplicates. Double-positive cells were counted manually by two independent observers blinded to the study (Nikon Eclipse E600; magnification x10). Expression of PPAR{gamma} in human EPCs was examined by RT-PCR: PPAR1 forward, 5'-CTGGCCTCCTTGATGAATAA; PPAR1 reverse, 5'-GGCGGTCTCCACTGAGAATA; PPAR2 forward, 5'-AGGGCGATCTTGACAGGAAAG; PPAR2 reverse, 5'-CCCATCATTAAGGAATTCATGTCA. To test the effects of pioglitazone in vitro, EPCs isolated and cultured from healthy volunteers were treated with pioglitazone (10 µmol/l; Takeda Pharmaceuticals), the PPAR{gamma} antagonist GW9662 (1 µmol/l per l; Alexis Biochemicals), or adiponectin (20 µg/ml; Alexis Biochemicals) for 24 h.

Migration assay.
Modified Boyden chambers were used to assess the migratory capacity of EPCs (25,35). On day 1, 4 x 106 MNCs were plated on noncoated six-well culture dishes and cultured for 4 days. Culture medium was removed, and cells were harvested by trypsination and centrifugation. The pellet was resuspended in 300 µl EBM and counted. Cells (1 x 105) in 500 µl EBM without supplements were placed in the upper chamber (HTS Fluoroblock, 8 µm pore size, in triplicate; BD Biosciences), and the chamber was placed in a 24-well plate containing EBM without supplements and 100 ng/ml SDF-1 (R&D Systems). After 24 h, the filter was carefully removed, washed, fixed, and incubated with labeled DiLDL as described above. SDF-1–stimulated migratory capacity was then quantified by counting the migrated EPCs on the lower surface of the filter using fluorescence microscopy (magnification x40, in triplicate).

Colony-forming units.
Colony-forming units (CFUs) were counted to examine the clonal expansion capacity of EPCs. After density gradient centrifugation, 5 x 106 MNCs were plated in a fibronectin-coated six-well plate for 48 h (in triplicate). Then, the nonadherent cells in the supernatant were centrifuged and resuspended, and 106 cells were plated into 24-well plates. After 5 more days in culture, EPC colonies (defined as clusters of more than 15 cells) were counted under a light microscope. Values are expressed as absolute colony number per well.

NADPH oxidase activity.
NADPH oxidase activity was measured by a lucigenin-enhanced chemiluminescence assay in buffer B containing 50 mmol/l phosphate (pH 7.0), 1 mmol/l EGTA, protease inhibitors (Complete; Roche), 150 mmol/l sucrose, 0.005 mmol/l lucigenin, and 0.1 mmol/l NADPH, as described previously (36). EPCs were lysed after washing with PBS in ice-cold buffer B, lacking lucigenin and substrate. Basal and phorbol myristate acetate (PMA)-stimulated NADPH oxidase activity was determined in 100 µg aliquots of the sample measured over 10 min in quadruplicates using NADPH as substrate in a scintillation counter (Berthold Lumat LB 9501) in 1-min intervals.

Statistical analysis.
Results are represented as mean ± SE. Statistical analysis was performed using the GraphPad Prism software (version 3.02). ANOVA and Student's t test were applied where applicable. Post hoc comparisons were made by using the Bonferroni test. Values of P < 0.05 were considered statistically significant.


   RESULTS
TOP
 ABSTRACT
 RESEARCH DESIGN AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Patient characteristics.
The baseline characteristics of the patients are summarized in Table 1 and did not differ between groups. There was no difference in medication between groups. The medication was not changed during the course of the study. Serum glucose and lipid concentrations were not different between groups and were not affected by treatment with pioglitazone. TZD treatment significantly reduced insulin concentrations and the homeostasis model assessment (HOMA) index (Table 1). None of the smokers stopped smoking during the course of the trial. Nine TZD and 10 placebo patients underwent coronary angioplasty or stenting (one drug-eluting stent in each group). The effect of TZD on EPC parameters did not differ between percutaneous coronary intervention (PCI) and non-PCI patients. Six individuals in the placebo group and four in the TZD group met the current International Diabetes Federation criteria for metabolic syndrome.


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  		TABLE 1 Patient characteristics

 
Adiponectin and hsCRP serum levels.
Adiponectin is a peptide hormone regulator produced in adipocytes, which is known to be increased by TZD treatment (1,37). The mean adiponectin level in the placebo group was 12.2 ± 1.2 µg/ml before and 12.2 ± 1.4 µg/ml after the study. In contrast, the adiponectin serum concentrations of the TZD group showed an increase to 322% after 4 weeks (11.4 ± 1.1 vs. 36.8 ± 2.1 µg/ml; P < 0.001) (Fig. 1A). Importantly, in every individual TZD patient, but no patient of the placebo group, the adiponectin level increased by more than twofold.


Figure 1
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  		FIG. 1. Effect of TZD treatment for 30 days on serum concentrations of adiponectin (A) and hsCRP (B).

 
hsCRP serum concentration is a marker of vascular inflammation and independent predictor of vascular events. Treatment with the PPAR{gamma} agonist reduced hsCRP levels from 3.9 ± 0.7 to 1.69 ± 0.3 mg/l (P < 0.05; Fig. 1B), whereas placebo treatment had no effect (3.8 ± 0.7 vs. 2.9 ± 0.7 mg/l).

Number of EPCs in normoglycemic patients.
CD34+/KDR+ EPCs have been shown to predict cardiovascular events (20). Figure 2A depicts a representative FACS scan for CD34-FITC/KDR-PE in peripheral blood. Treatment with TZD for 30 days resulted in upregulation of CD34+/KDR+ EPC numbers to 142% (55 ± 5 vs. 78 ± 6/105 cells; P < 0.01; Fig. 2B). In the placebo group, the number of CD34+/KDR+ EPCs did not change.


Figure 2
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  		FIG. 2. A: Example of a FACS scan depicting the gating (top row) and scatter graph (bottom row) for a representative patient from the TZD group showing CD34+/KDR+ EPCs at baseline (pre, left column) and after 1 month (post, right column) of TZD treatment. B: Quantification of the CD34+/KDR+ EPCs before and after therapy. The whiskers comprise minimum and maximum values and the box shows the range of 25–75% quartiles and the median in between. *P < 0.01 in the TZD group. Each measurement was in duplicate. SSC, sideward scatter; FSC, forward scatter.

 
Number of cultured DiLDL+/lectin+ EPCs.
Quantification of DiLDL uptake and lectin staining was used as an established second method to quantify and characterize EPCs (Fig. 3A) (11). Placebo treatment had no effect on the number of DiLDL+/lectin+ double-positive cells per microscopic field (122 ± 5 before treatment vs. 103 ± 4 after treatment; P > 0.05). In contrast, DiLDL+/lectin+ EPCs increased from 86 ± 4 to 154 ± 5 cells/microscopic field after TZD treatment (P < 0.05).


Figure 3
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  		FIG. 3. A: Representative fluorescence microscopy showing DiLDL+/lectin+ EPCs in a patient from the placebo versus one of the TZD group (magnification x10). Blue, DAPI (nuclei); green, FITC-lectin; red, DiLDL. B: Quantification of DiLDL+/lectin+ EPCs shown as mean ± SE. Each assay was in triplicate, with three random fields counted from each coverslip. Lectin, U. europaeus agglutinin I. (Please see http://dx.doi.org/10.2337/db07-0069 for a high-quality digital representation of this figure.)

 
Migratory capacity.
Migration is an important functional property of EPCs that can be regulated independent of EPC numbers (11,25). After 1 month of therapy, there was no change in placebo controls, whereas TZD treatment resulted in an increase of EPC migration to 146 ± 9% per EPC number (P < 0.05; Fig. 4).


Figure 4
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  		FIG. 4. A: Representative fluorescence microscopic images (magnification x40) showing the effects of TZD and placebo on EPC migration stimulated by 100 ng/ml SDF-1 in a Boyden chamber. B: Quantification. Each assay was performed in triplicate.

 
Colony-forming potential.
The ability to clonally expand and to create colonies in an endothelial-specific medium is a key functional feature of EPCs (14). There was no significant change in CFUs in patients treated with placebo (99 ± 9 CFUs at baseline vs. 119 ± 8 after 1 month; P > 0.05). After treatment with TZD, however, the development of CFUs per number of EPCs was upregulated to 172 ± 12%; P < 0.001; Fig. 5A).


Figure 5
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  		FIG. 5. A: Effect of TZD or placebo on the number of CFUs. Six wells were counted for each patient. B: Cultured human EPCs. NADPH oxidase activity measured by a lucigenin-enhanced chemiluminescence assay after treatment of cultured human EPCs with TZD (10 µmol/l pioglitazone). Basal NADPH oxidase activity was compared with PMA-stimulated NADPH oxidase activity (n = 4, *P < 0.05).

 
NADPH oxidase activity.
The NADPH oxidase is an important source of endothelial superoxide anions (38). Reactive oxygen species impair the function of both mature and immature endothelial cells (16,39,40). Treatment of cultured EPCs from healthy volunteers (n = 4) with pioglitazone (10 µmol/l, 24 h) decreased basal NADPH oxidase activity from 40 ± 3.29 to 16 ± 8.34 relative light units (RLU)/µg. Similarly, TZD was able to prevent the PMA-induced increase of NADPH oxidase activity (from 72 ± 5.14 to 44 ± 5.45 RLU/µg) (Fig. 5B).

Regulation by PPAR{gamma} and adiponectin.
The RT-PCR analysis showed that PPAR{gamma} is expressed in human EPCs. To test whether upregulation of EPCs is mediated via PPAR{gamma}, the EPCs were treated with pioglitazone (10 µmol/l 24 h) alone and in the presence of the PPAR{gamma} antagonist GW9662 (1 µmol/l, 24 h). TZD treatment increased EPC numbers to 189 ± 15% (P < 0.05). Figure 6A shows that the increase of EPCs in the presence of TZD is completely inhibited by the PPAR{gamma} antagonist (n = 4; P < 0.05). In vitro treatment with pioglitazone (10 µmol/l, 24 h) increased the migratory capacity per number of EPCs to 158 ± 10%; the effect was completely prevented by cotreatment with GW9662 (1 µmol/l, 24 h). The upregulation of EPC numbers and the improvement of migratory function by TZD were mimicked by treatment of the cultured EPCs with adiponectin at a concentration similar to that observed in patients after TZD treatment (20 µg/ml, 24 h) (Fig. 6B).


Figure 6
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  		FIG. 6. Effect of 24-h treatment with pioglitazone (TZD; 10 µmol/l), the PPAR{gamma} antagonist GW9662 (GW) (1 µmol/l), and adiponectin (Adi) (20 µg/ml) in cultured human EPCs (n = 4, mean ± SE, *P < 0.05). A: Quantification of DiLDL+/lectin+ EPCs. Lectin, U. europaeus agglutinin I. B: EPCs migration was stimulated by 100 ng/ml SDF-1 and quantified in modified Boyden chambers.

 

   DISCUSSION
TOP
 ABSTRACT
 RESEARCH DESIGN AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The main finding of this prospective, randomized trial is the demonstration of a novel vascular effect of TZDs in normoglycemic individuals with coronary artery disease. Treatment with pioglitazone increased the number and the function of circulating EPCs. The effects of TZD on EPCs occurred on top of the treatment with aspirin, ß-blockers, and inhibitors of the renin-angiotensin system and, importantly, in the presence of statin treatment.

To evaluate the adherence to the study medication, serum adiponectin concentrations were measured. Adiponectin has been shown to be increased by TZD treatment mediated by an effect on adipocytes contributing to improved insulin sensitivity (1,37). Adiponectin levels at least doubled in all of the pioglitazone-treated individuals, but no placebo-administered patient exhibited a similar increase. These findings suggest a very good compliance. To test whether adiponectin contributes to the regulation of EPCs by TZD, cultured human EPCs were exposed to adiponectin at concentrations similar to those observed in the patients taking TZD. Adiponectin potently increased both EPC numbers and function, suggesting that adiponectin contributes as a mediator to the effects of TZD on EPCs.

In agreement with previous observations, TZD treatment significantly lowered the established marker of vascular inflammation hsCRP (8,41). Our data show that this effect can be observed in normoglycemic patients with coronary artery disease after 30 days of treatment. These data are in agreement with recent data showing that treatment of isolated EPCs with recombinant CRP inhibits EPC differentiation, survival, and function and caused a concentration-dependent increase in reactive oxygen species production and apoptosis. Interestingly, the PPAR{gamma} agonist rosiglitazone was able to inhibit the negative effects of CRP on EPC biology (42,43). Most of these effects, however, were observed at a CRP concentration of 15 µg/ml, which is significantly higher than in our study population with mean hsCRP values of ≤5 µg/ml.

Several markers have been used to identify EPCs. Here, CD34+/KDR+ EPCs were quantified because they have been shown to predict cardiovascular outcomes in patients with coronary artery disease. The recent EPCAD (Endothelial Progenitor Cells Coronary Artery Disease) study showed an association between this population of EPCs and death from cardiovascular causes that is independent of the severity of coronary artery disease, cardiovascular risk factors, and medication known to influence cardiovascular outcomes (20). In addition to FACS analysis, we used a second, independent method of EPC characterization by culturing mononuclear cells and selection of EPCs by endothelial growth factors, adhesion to fibronectin, the ability to uptake LDL, and staining for lectin (11,14). As a third established method, we quantitated EPC CFUs, which have been shown to predict endothelial function in humans (14). All three methods of EPC quantification showed a robust increase of EPC numbers in patients taking the PPAR{gamma} agonist.

In addition to EPC numbers, functional properties of EPCs have been shown to determine cardiovascular disease (14,20). Recent data suggest that EPC function may be impaired by cardiovascular risk factors independent of the EPC number (26,27). In mice and in cultured human EPCs, pioglitazone prevents apoptotic cell death of EPCs by a mechanism involving phosphatidylinositol 3-kinase (34). Intracellular reactive oxygen species, such as superoxide and H2O2, impair the function of mature and immature endothelial cells, e.g., by promoting cell death (16,39,40). Of the several sources within vascular cells, the multi-subunit NADPH oxidase is a predominant contributor of endothelial superoxide free radicals (38). Here, cell culture experiments show that TZD treatment reduces basal and PMA-stimulated NADPH oxidase activity in EPCs, pinpointing a mechanism by which TZD treatment reduces EPC apoptosis and increases EPC numbers. Two widely studied functional characteristics of EPCs are their potential to migrate and their capacity to replicate. These features are likely to be of great importance for the improvement of endothelial function, neoangiogenesis, and inhibition of atherogenesis (44). The data show that TZD treatment increased both the migratory and the colony forming capacity per number of EPCs in patients with normal glucose tolerance, suggesting that the biological effect of TZD on EPCs may be significantly greater than the extent of the increase of EPC numbers. These effects are mediated by PPAR{gamma} because GW9662 reversed the effects of pioglitazone on EPC numbers and migration.

Cardiovascular disease accounts for ~70% of mortality in patients with diabetes. Prospective studies show that compared with their nondiabetic counterparts, the relative risk of cardiovascular mortality for men with diabetes is two to three and for women with diabetes is three to four. Several large trials have demonstrated that optimal control of blood pressure and LDL cholesterol level can substantially reduce excess cardiovascular risk in patients with diabetes (45,46). Metabolic control alone is not sufficient for the prevention of cardiovascular events in patients with diabetes (47). However, even with optimal control of the potent cardiovascular risk factors blood pressure and LDL cholesterol, incremental risk for cardiovascular events remains high compared with individuals without diabetes (45,46). Beneficial effects of TZDs on vascular inflammation and function seem to be possibly independent of glucose lowering and have been demonstrated in nondiabetic, healthy individuals (2,4,48,49). EPCs are independent predictors of endothelial function, cardiovascular events, and cardiovascular mortality (14,20,21). Patients with diabetes are characterized by an impairment of EPC function that is observed independently of other cardiovascular risk factors (22,25,27). On the basis of our study, it is interesting to speculate that patients with diabetes may benefit from PPAR{gamma} agonists in addition to insulin sensitization by upregulation of EPCs. This needs to be confirmed in further trials.

Here, we show that treatment with pioglitazone powerfully improves number and function of EPCs in patients with vascular disease and normal glucose tolerance. Serum glucose and lipid concentrations were not affected by treatment with pioglitazone. TZD treatment reduced insulin concentrations and the HOMA index, suggesting that TZDs exert metabolic effects that occur in normoglycemic individuals and that do not result in significant changes of serum glucose (such as regulation of adiponectin and insulin). The effects of TZDs in mice and in isolated cultured EPCs suggest that some of these effects are effective on the level of progenitor cells. Until now, TZDs are only approved and used for individuals with elevated serum glucose levels. These clinical data viewed together with preclinical studies therefore evoke the provocative hypothesis that these agents may be also beneficial for patients with vascular disease despite normal glucose tolerance, such as normoglycemic individuals with stable coronary artery disease.

The design of the study has limitations. It does not allow answering the question of whether TZD therapy confers a prognostic benefit to nondiabetic coronary artery disease patients reducing cardiovascular events at long-term follow-up. Furthermore, subgroups of patients that may especially benefit from TZD treatment and the kinetics and time course of the effects of TZD on EPCs are not known. Finally, the intracellular signaling pathways mediating the effect of TZD on number and function of EPCs remain to be elucidated. These issues need to be addressed in further studies. However, this is a proof-of-concept study providing the first evidence for a new mechanism of TZD action and setting the stage for large outcome trials using risk factor stratification by EPC numbers and function.

In summary, EPCs have emerged as a new dimension of vascular biology. Increasing evidence suggests that bone marrow–derived adult stem cells significantly contribute to vascular and cardiac function. Improvement of EPC function may represent a novel and relevant protective mechanism of TZDs that could potentially benefit patients with vascular diseases in the absence of manifest diabetes.


   ACKNOWLEDGMENTS
 
U.L. has received support from the Deutsche Forschungsgemeinschaft. This work has received support from Universität des Saarlandes. The Klinik für Innere Medizin III, Universitätsklinikum des Saarlandes, has received an unrestricted grant from Takeda (Aachen, Germany).

We thank Simone Jäger and Ellen Becker for their excellent technical assistance.


   FOOTNOTES
 
Published ahead of print at http://diabetes.diabetesjournals.org on 10 July 2007. DOI: 10.2337/db07-0069.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received for publication January 23, 2007 and accepted in revised form July 6, 2007


   REFERENCES
TOP
 ABSTRACT
 RESEARCH DESIGN AND METHODS
 RESULTS
 DISCUSSION
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