Priming With Angiopoietin-1 Augments the Vasculogenic Potential of the Peripheral Blood Stem Cells Mobilized With Granulocyte Colony-Stimulating Factor Through a Novel Tie2/Ets-1 Pathway
Background— The low engraftment rate of stem/progenitor cells infused via the intracoronary route to the ischemic myocardium is one of the most important factors limiting the efficacy of cell therapy. We investigated the concept of priming peripheral blood stem cells enriched by granulocyte colony-stimulating factor mobilization and apheresis (mobPBSCs) with angiopoietin-1 (Ang1), to enhance the engraftment into the ischemic tissue and neovasculogenic potential.
Methods and Results— The expression of Tie2, the Ang1 receptor, was significantly higher in mobPBSCs than naïve peripheral blood mononuclear cells (19.2±3.0% versus 1.2±0.8% versus 1.2±0.2%; P<0.001 for mobPBSCs from acute myocardial infarction (AMI) patients with granulocyte colony-stimulating factor treatment for 3 days versus peripheral blood mononuclear cells from AMI patients versus peripheral blood mononuclear cells from stable angina patients). After 4 hours of cartilage oligomeric matrix protein (COMP)-Ang1 stimulation, mobPBSCs committed to the endothelial lineage with the induction of CD31 and VE-cadherin expression, mediated by Tie2/Ets-1 pathway. Priming of mobPBSCs with COMP-Ang1 induced the expression of α4β1 and α5β1 integrins, which are also Ets-1 downstream molecules, leading to enhanced adhesion to endothelial cells or fibronectin. In a rabbit ear ischemia/reperfusion model, priming of mobPBSCs with COMP-Ang1 improved first-pass engraftment to the distal vascular bed after intraarterial delivery. In a murine ischemic hind-limb model, intravascular delivery of primed mobPBSCs enhanced both engraftment and neovascularization.
Conclusions— The short-term priming with COMP-Ang1 may be a feasible and promising option to activate mobPBSCs by enhancing differentiation and adhesiveness and to improve the efficacy of cell therapy for ischemic diseases.
Received February 6, 2009; accepted September 21, 2009.
The identification of various types of bone marrow–derived stem/progenitor cells has raised the possibility that cell therapy can be a promising therapeutic modality to repair ischemic cardiovascular tissues.1 Recent clinical trials demonstrated that intracoronary infusion of bone marrow–derived cells may improve the outcome of acute myocardial infarction (AMI) patients.2,3 Among delivery strategies of cell therapy for patients with MI, the intracoronary delivery of the cells is the most convenient and commonly used. We have studied the characteristics of circulating peripheral blood stem/progenitor cells4 and have demonstrated clinical application of the intracoronary infusion of peripheral blood stem cells enriched by granulocyte colony-stimulating factor (G-CSF) mobilization and apheresis (mobPBSCs) in patients with MI.5 However, the low efficiency of engraftment (ie, 1% to 3%) to the ischemic myocardium is one of the most important hurdles in improving the efficacy of cell therapy.6,7 Accordingly, we aimed to find a new method of augmenting neovascularization by overcoming the poor engraftment of mobPBSCs into ischemic tissue and enhancing its endothelial lineage commitment.
Clinical Perspective on p 2250
Experimental studies have reported that ex vivo genetic modification and pretreatment with small molecules enhanced cellular function.8,9 Relative to the safety concern of genetic manipulation and the clinical applicability of small molecules, it would be an attractive method to augment engraftment and induce lineage commitment of mobPBSCs by the short-term, protein-based, ex vivo stimulation of stem/progenitor cells. This short-term stimulation for boosting the neovasculogenic potential of mobPBSC is called priming.
Angiopoietin-1 (Ang1) is a growth factor binding to the Tie2 receptor expressed on endothelial cells (ECs) and hematopoietic stem cells. Through Tie2 signaling, Ang1 plays an essential role in postnatal angiogenesis by mediating vessel maturation and maintaining vessel integrity.10 However, little is known about its role in the engraftment and lineage commitment of stem cells and in the modulation of its downstream pathway.
In the present study, we found that mobPBSCs highly expressed Tie2 compared with naïve peripheral blood mononuclear cells (PBMCs) obtained from patients with AMI. Therefore, we investigated the effects of priming naïve PBMCs and mobPBSCs using Ang1 in vitro and in vivo. The short-term Ang1 stimulation of mobPBSCs, but not naïve PBMCs, augmented endothelial lineage commitment and engraftment through the Tie2/Ets-1 signaling pathway and eventually enhanced the neovasculogenic potential in the ischemic tissue.
All experiments dealing with humans or human products were conducted with informed consent and approved by the Institutional Review Board of Seoul National University Hospital. All animal experiments were performed after receiving approval from the Institutional Animal Care and Use Committee of the Clinical Research Institute of the Seoul National University Hospital and complied with the National Research Council Guidelines for the Care and Use of Laboratory Animals.
One-way ANOVA with Tukey posthoc analysis was used to compare ≥3 groups with regard to continuous response variables, and the Pearson χ2 test was used to analyze categorical data. In the Table, the differences between 3 groups were analyzed with the Welch test for one-way ANOVA with the Tamhane method because of unequal within-group variances. In the experiment for in vivo perfusion recovery, repeated-measures ANOVA with Scheffé contrasts was used to quantify the main effects of 4 groups and time and significant interactions. The calculation was performed with SAS version 9.1 (SAS Institute, Inc, Cary, NC). The other calculations were performed with SPSS version 13.0 (SPSS Inc, Chicago, Ill), and values of P<0.05 were considered statistically significant. Detailed experimental methods are described in the online-only Data Supplement.
mobPBSCs Highly Express Tie2 Compared With PBMCs
We obtained mobPBSCs that were PBMCs after 3 days of G-CSF subcutaneous injection from 33 AMI patients enrolled in the Myocardial Regeneration in Acute and Old MI With G-CSF Mobilization and Intra-Coronary Stem Cell Infusion (MAGIC-Cell) program.5 We also selected PBMCs from 15 AMI patients without G-CSF treatment (PBMC_AMI) as a control for mobilized PBSCs from AMI patients. In addition, we selected PBMCs from 20 patients with stable angina (PBMC_SA) as another control for the disease of AMI. The characteristics of the patients are shown in Table I of the online-only Data Supplement.
We characterized mobPBSCs and PBMCs by flow cytometry analysis. We found that mobPBSCs contained more cells positive for KDR, CD34, and CD133, which are major surrogate markers of endothelial progenitor cells (EPCs), than nonmobilized PBMCs did (the Table). Interestingly, reverse-transcription polymerase chain reaction (RT-PCR) and flow cytometry showed significantly greater Tie2 expression in mobPBSCs than in PBMCs from AMI or SA patients (Tie2-positive fraction in flow cytometry analysis: 19.2±3.0% versus 1.2±0.8% versus 1.2±0.2%; P<0.001 for mobPBSCs versus PBMC_AMI versus PBMC_SA; Figure 1A and 1B; quantitative RT-PCR data are given in Figure I of the online-only Data Supplement). These results encouraged us to investigate the utility of the Tie2 agonist cartilage oligomeric matrix protein (COMP)-Ang1 (an Ang1 recombinant chimera) as a priming agent.
Long-Term Stimulation of mobPBSCs With COMP-Ang1 Increases the Number of Endothelial Lineage Cell Clusters
We tested the effect of COMP-Ang1 on the commitment of naïve PBMCs and mobPBSCs, both from AMI patients, into endothelial lineage cells. They were seeded in EPC culture media4 containing COMP-Ang1 (400 ng/mL), and 5 to 7 days later, clusters of attached cells were observed. The cluster number of naïve PBMCs did not change even after COMP-Ang1 treatment for 7 days compared with vehicle (n=12; P=0.65; Figure 2A). However, the cluster number of mobPBSCs after COMP-Ang1 treatment for 7 days increased ≈2-fold compared with vehicle (94±18% increase; n=12; P<0.01; Figure 2B). These clusters were composed of spindle-shaped cells similar to EPCs. To determine the phenotype of the increased clusters in mobPBSC group, we performed immunocytochemical staining using antibodies for CD31, CD34, VE-cadherin, Tie2, CXCR4, and KDR. Clusters were stained positive 7 days after cultivation with COMP-Ang1 (Figure 2C).
Short-Term Stimulation of mobPBSCs With COMP-Ang1 Increases Endothelial Lineage Commitment Through Tie2/Ets-1 Signaling
Among the positively stained surface markers of the clusters, KDR, VE-cadherin, and Tie2 are all known to contain Ets-1 transcription factor binding sites in their promoter regions.11 Thus, we investigated whether the effect of COMP-Ang1 on the lineage commitment of mobPBSCs is mediated via the transcriptional regulation of Ets-1. The gene expression of Ets-1 did not change significantly after the stimulation of naïve PBMCs with COMP-Ang1 (Figure 3A). In mobPBSCs, however, expression of Ets-1 mRNA increased 2 hours after stimulation with COMP-Ang1 (Figure 3B; quantitative RT-PCR data are given in Figure II of the online-only Data Supplement). Furthermore, we pretreated mobPBSCs with anti-Tie2 neutralizing antibody 1 hour before COMP-Ang1 treatment and found that blockage of Tie2 receptor reduced the expression of Ets-1 mRNA (Figure 3C; quantitative RT-PCR data are given in Figure III of the online-only Data Supplement).
Next, we examined whether short-term stimulation of naïve PBMCs or mobPBSCs with COMP-Ang1 might induce endothelial lineage commitment. In naïve PBMCs, COMP-Ang1 treatment for 4 hours did not affect the expression of endothelial specific genes such as CD31 and VE-cadherin compared with vehicle treatment (Figure 3D and 3E). In mobPBSCs, however, COMP-Ang1 treatment resulted in stronger expression of CD31 and VE-cadherin, which was reversed by antisense Ets-1 oligodeoxynucleotides (Figure 3F; quantitative RT-PCR data are given in Figure IV of the online-only Data Supplement). These results were confirmed at the protein level by flow cytometry analysis (Figure 3G) in which short-term treatment with COMP-Ang1 increased the expression of CD31 and VE-cadherin by 51±10% and 118±13%, which was abrogated by transfection of mobPBSCs with antisense Ets-1 oligodeoxynucleotides by 56±7% and 46±5%.
Priming of mobPBSCs With COMP-Ang1 Activates α4β1 and α5β1 Integrins
As shown above, the short-term stimulation of mobPBSCs with COMP-Ang1 induced Ets-1 expression. Moreover, a previous report showed that β3 integrin was downstream of Ets-1 activation.11 Therefore, we postulated that other integrins that contain an Ets-1 binding motif in their promoter region may be activated by COMP-Ang1 such as α4β1 and α5β1 integrins, both of which are known to play key roles in the homing and retention of stem/progenitor cells in bone marrow.12 Thus, we measured the expression of these integrins after short-term treatment with COMP-Ang1. The stimulation of naïve PBMCs with COMP-Ang1 for 4 hours did not change the mRNA and protein expression of α4, α5, and β1 integrin (Figure 4A and 4B). However, stimulation of mobPBSCs with COMP-Ang1 increased the expression of α4, α5, and β1 integrin (flow cytometry analysis: 107±39%, 51±24%, and 78±21% increase for α4, α5, and β1 integrin, respectively; Figure 4C and 4D; quantitative RT-PCR data are given in Figure V of the online-only Data Supplement), and antisense Ets-1 oligodeoxynucleotides attenuated this expression (flow cytometry analysis: 45±3%, 29±7%, and 36±9% decrease for α4, α5, and β1 integrin, respectively). We also investigated the expression of β2 integrin, which is another key molecule for EPC homing,13 but its mRNA and protein expression did not change significantly after COMP-Ang1 treatment in both the naïve PBMC and mobPBSC groups even though it was highly expressed at baseline.
To correlate these findings with functional changes in mobPBSCs, we performed in vitro adhesion assays between α4β1 on mobPBSCs and vascular cell adhesion molecule-1 on human umbilical vein ECs (HUVECs) and between α5β1 on mobPBSCs and fibronectin. COMP-Ang1 stimulation for 4 hours increased mobPBSC adhesion to HUVECs (150±19% increase) and fibronectin (152±9% increase; Figure 4E and 4F), whereas pretreatment with anti-Tie2 neutralizing antibody or anti-integrin antibody (anti-α4 or -α5) before COMP-Ang1 stimulation abrogated the increased adhesion, suggesting a Tie2-dependent increase in cell-to-cell and cell-to-matrix adhesion after COMP-Ang1 stimulation.
Priming of mobPBSCs With COMP-Ang1 Increases Matrigel Tube Formation and Incorporation Ability of mobPBSCs
Next, we investigated whether short-term stimulation of mobPBSCs with COMP-Ang1 might induce vasculogenesis even though there was no continuous stimulation with the growth factor. When primed mobPBSCs were cocultured with HUVECs, mobPBSCs and HUVECs produced significantly more complete tubes as determined by tube number, area, and length after 3 days (59±5%, 35±10%, and 37±5% increase for tube number, area, and length, respectively; Figure 5A). Again, pretreatment with anti-Tie2 neutralizing antibody before COMP-Ang1 stimulation abrogated tube formation of mobPBSCs treated with COMP-Ang1 (44±8%, 35±11%, and 36±9% decrease for tube number, area, and length, respectively). In addition, more mobPBSCs were incorporated into Matrigel tubes when primed with COMP-Ang1, which was retarded when pretreated with anti-Tie2 neutralizing antibody (207±28% increase by priming and 66±2% decrease by Tie2 pre-blocking; Figure 5B).
Priming of mobPBSCs With COMP-Ang1 Enhances the First-Pass Engraftment Into the Distal Vascular Bed of a Rabbit Ear Ischemia/Reperfusion Model After Intraarterial Delivery
Next, we investigated whether short-term stimulation with COMP-Ang1 may enhance the engraftment of PBMCs in the distal ischemic tissue after intraarterial delivery. For this examination, we developed a rabbit ear model of ischemia and reperfusion (Figure 6A). Scintigrams of ischemic rabbit ear after infusion of naïve PBMCs and unprimed mobPBSCs showed scanty engraftment in the ischemic ear (Figure 6B and 6C). In contrast, mobPBSCs primed with COMP-Ang1 showed markedly increased engraftment by 53±33% (n=6; P<0.01), which was reversed by pretreatment of mobPBSCs with anti-Tie2 neutralizing antibody before COMP-Ang1 priming. Specific examinations of the tissue with immunofluorescent staining showed the same results (Figure VI of the online-only Data Supplement).
Priming of mobPBSCs With COMP-Ang1 Enhances Engraftment and Neovascularization of the Ischemic Limb After Arterial Infusion
We next investigated whether injection of COMP-Ang1–primed mobPBSCs enhanced neovascularization in vivo using a hind-limb ischemia model. COMP-Ang1–primed mobPBSCs from patients were administered via intracardiac puncture into athymic nude mice after the induction of hind-limb ischemia. COMP-Ang1–primed mobPBSC infusion improved limb perfusion compared with unprimed mobPBSC infusion (perfusion ratio of ischemic to nonischemic limbs at day 21, 49.3±13.8% versus 34.9±3.0%; n=10 each, P<0.01 for primed versus unprimed mobPBSC infusion), which was reversed by pretreatment with anti-Tie2 neutralizing antibody (perfusion ratio at day 21, 33.6±7.4%; n=10; P<0.05 versus COMP-Ang1–primed group; Figure 7A). Moreover, we assessed the neovasculogenic effect of PBMC_AMI. The naïve PBMCs were inferior to even unprimed mobPBSCs in terms of blood flow recovery (perfusion ratio ischemic to nonischemic limbs at day 21, 22.6±7.3% versus 34.9±3%; n=10 each; P<0.05 for naïve PBMCs versus unprimed mobPBSC infusion).
Histologically, we observed more capillaries in the ischemic limb that received COMP-Ang1–primed cells (192±15% increase; n=6; P<0.01; Figure 7B and 7C). Next, to determine the contribution of engrafted cells to neovascularization, we checked cellular engraftment using carboxyfluorescein succinimidyl ester (CFSE) –labeled mobPBSCs and performed immunohistochemical staining with human-specific and vessel markers. More CFSE-labeled mobPBSCs (Figure 7D and 7E) and human-specific von Willebrand factor– and HLA-positive cells (Figure 7F and Figure VII of the online-only Data Supplement) were detected in the ischemic hind limb treated with COMP-Ang1–primed cells than with the unprimed cells. Furthermore, confocal analysis of immunofluorescent staining demonstrated that there were more cells double positive for human HLA and CD31 (Figure 7G) in the COMP-Ang1 priming group. Because cell-to-cell adhesion through binding of α4β1 integrins with vascular cell adhesion molecule-1 plays a greater role in the homing of injected PBSCs than cell-to-fibronectin adhesion,14 we examined the expression of α4 integrin. We found that α4-positive cells increased in the ischemic hind limb treated with COMP-Ang1–primed mobPBSCs. The cells positive for α4 integrin were colocalized with human-specific HLA and engrafted into areas of vascular regeneration. Pretreatment of anti-Tie2 neutralizing antibody reversed this phenomenon (Figure 7H).
Systemic Administration of G-CSF Does Not Enhance Vasculogenesis in Mouse Ischemic Hind Limb but COMP-Ang1 Does
We also compared the effects of systemic administration of COMP-Ang1 and G-CSF on the vasculogenesis in the mouse ischemic hind-limb model. On day 3 after the intravenous injection of recombinant adenovirus-expressing COMP-Ang1 or LacZ (vehicle), C57BL/6 mice underwent unilateral femoral artery excision (n=6 in each group). Then, G-CSF or sterile saline was injected subcutaneously for 3 days (vehicle group, Ade-LacZ plus saline injection; COMP-Ang1 group, Ade-COMP-Ang1 plus saline injection; G-CSF group, Ade-LacZ plus G-CSF injection; combined cytokine group, Ade-COMP-Ang1 plus G-CSF injection).
When followed up at 14 days, the systemic administration of G-CSF did not improve neovascularization compared with vehicle, whereas COMP-Ang1 significantly improved perfusion. The combined cytokine group showed a result similar to the COMP-Ang1 alone group, suggesting no positive interaction between systemic administration of G-CSF and COMP-Ang1 (the perfusion ratio of ischemic to nonischemic limbs at day 14, 59.1±3.6% versus 71.4±4.8% versus 56.8±4.1% versus 71.8±3.6%; n=6 each; P<0.01 for vehicle versus COMP-Ang1 versus G-CSF versus combined cytokine; Figure 8A and 8B). In the histological analysis, systemic administration of G-CSF did not increase capillary density, whereas COMP-Ang1 or combined cytokines increased it compared with vehicle (78±10% increase by COMP-Ang1 and 67±10% increase by combined cytokines; n=6 each; P<0.01 versus vehicle by 1-way ANOVA with Tukey analysis; Figure 8C).
Ang1 has been reported to be important for blood vessel formation in the embryonic period, and blood vessels in the Ang1−/− mice had fewer ECs and showed defects in the association of the endothelium with the extracellular matrix and vessel rupture.15 This phenomenon suggests the possibility that Ang1 contributes to the differentiation and homing of primitive stem cells that participate in new vessel formation. The recently developed synthetic COMP-Ang1, an Ang1 variant that is both soluble and stable, is known to be more potent than native Ang1 in activating Tie2 in ECs16 and to augment new vessel formation in various disease models.16,17 The rationale for using COMP-Ang1 as a priming agent was our new finding that mobPBSCs express high levels of Tie2. We hypothesized that because of the high expression of the Tie2 receptor in mobPBSCs, these cells would be excellent candidates to embrace the various beneficial effects of COMP-Ang1. Our data showed that endothelial lineage commitment and functional upregulation of integrins occurred within 4 hours of treatment in the mobPBSC group, which means that priming of mobPBSCs can be performed in a short-term and simple manner and may be a clinically feasible method.
Neovascularization can be mediated by the vasculogenic potential of mobPBSCs, which can differentiate and acquire endothelial phenotype. G-CSF is well known to potently mobilize hematopoietic stem cells from bone marrow.18 Moreover, AC133+ CD34+ cells, subsets of mobPBSCs, are considered to have the potential to differentiate to ECs and the ability to home to ischemic sites, resulting in neovascularization in adults.19 Thus, we hypothesized that enhancement of the endothelial commitment of mobPBSCs could augment neovascularization in ischemic tissue. In this study, the culture of mobPBSCs with COMP-Ang1 induced more EPC clusters compared with vehicle. In addition, priming of mobPBSCs with COMP-Ang1 for 4 hours enhanced the endothelial commitment of mobPBSCs and vasculogenesis in the Matrigel. In a nude mouse hind-limb model, animals treated with mobPBSCs primed by COMP-Ang1 showed more capillaries that were composed of injected mobPBSCs than animals treated with unprimed mobPBSCs. To the best of our knowledge, this is the first report to demonstrate the effect of COMP-Ang1 on mobPBSC commitment to endothelial lineage cells.
The direct physical incorporation of mobPBSCs into the ischemic site is another important process for neovascularization.20 However, the mechanisms whereby mobPBSCs home to sites of ischemia are unclear. Before stem/progenitor cells are mobilized to the circulation, they interact with the bone marrow microenvironment.21 Ang1 is released from the microenvironment and contributes to the retention of PBSCs in the marrow by Tie2 and integrin signaling.22,23 Here, we hypothesized that the interaction between Ang1 and Tie2 and subsequent activation of integrins would be important not only for retainment in bone marrow but also for stem-cell homing to injured myocardium. In this study, the transcription of α4, α5, and β1 integrins was upregulated after short-term COMP-Ang1 treatment. Moreover, COMP-Ang1–primed mobPBSCs were incorporated more abundantly into the tube formation process. In addition, COMP-Ang1 priming increased first-pass engraftment of mobPBSCs in the distal vascular bed after intraarterial delivery in a rabbit ear model. In a murine hind-limb ischemia model, we showed greater engraftment of injected human mobPBSCs treated with COMP-Ang1 in areas of new vessel growth, and this phenomenon was mediated by the expression of α4 integrin in response to COMP-Ang1. These findings suggest that priming with Ang1 would play an important role in the initial engraftment of mobPBSCs to ischemic tissue by upregulating those integrins and would help to overcome the poor engraftment of mobPBSCs, which would be useful for clinical settings when cells are delivered through artery.
Among the homing mechanisms of stem/progenitor cells, the interaction between stromal cell–derived factor 1 and CXCR4 also has been known to play an important role in homing to injured myocardium.24 Recently, Honold et al25 reported that in the situation of mobilization with G-CSF, CXCR4 on the surface of mobilized EPCs may be degraded, leading to the decreased efficiency of homing to ischemic tissue driven by stromal cell–derived factor 1. However, our results showed that Ang1-primed mobPBSCs were very effective in promoting neovascularization even if they were mobilized by G-CSF. The reason for the difference between 2 studies may be explained as follows. First, we selected patients with AMI rather than with chronic ischemic heart disease. Second, freshly isolated mobPBSCs were different from EPCs obtained several days after ex vivo cultivation. Third, they did not present the total amount of CXCR4 in EPCs, even though some might be disrupted after G-CSF mobilization. We found that the total number of CXCR4 receptors in mobPBSCs was ≈3-fold higher than in nonmobilized PBMCs (CXCR4-positive fraction in flow cytometry analysis: 34.2±16.7% versus 11.7±8.8%; P<0.01 for mobPBSCs versus PBMC_AMI; Figure VIII of the online-only Data Supplement). Therefore, the total number of the functionally active CXCR4 receptors in mobPBSCs may not be lower or even higher than that of naïve PBMCs. Finally, this new priming strategy with COMP-Ang1 for G-CSF–mobilized stem/progenitor cells, including EPCs, may turn on the other homing mechanism mediated by integrin–vascular cell adhesion molecule-1 or integrin-fibronectin, which can overcome the possible impaired interaction between stromal cell–derived factor 1 and CXCR4.
Another novel finding of the present study is that the effect of COMP-Ang1 is mediated through Tie2/Ets-1 signaling. Ets-1 is the prototype of the Ets family transcription factors and is well known to be expressed in ECs during angiogenesis.11 However, few studies have addressed the effect of Ets-1 transcription factor in mobPBSCs on adult vasculogenesis. Previous reports documented that Ets-1 increased the expression of some matrix metalloproteinases, urokinase-type plasminogen activator, and integrin β3 in ECs.26 Other reports showed that VE-cadherin and endothelium-specific receptor-type tyrosine kinases such as Flt-1, KDR, Tie1, and Tie2 contained the Ets binding motif in their promoter/enhancer regions in ECs, and Ets family transcription factors were suggested to stimulate their promoter activities.11 In the present study, the transcription of Ets-1 in mobPBSCs significantly increased after COMP-Ang1 treatment, which was reversed after Tie2 blocking, indicating that Ets-1 is downstream of the Tie2 signaling. In addition, CD31 and VE-cadherin, markers of endothelial commitment, which were upregulated after the treatment of mobPBSCs with COMP-Ang1, were downregulated after antisense Ets-1 treatment, suggesting that the endothelial commitment of mobPBSCs by COMP-Ang1 is dependent on Ets-1 signaling. Relative to the expression of various integrins, Ets-1 blockade reversed the upregulation of α4, α5, and β1 integrins by COMP-Ang1, suggesting that the expression of integrins is also under the regulation of Ets-1 signaling. Therefore, Ets-1 may be an important mediator of the proangiogenic effects of priming of mobPBSCs with COMP-Ang1.
Our results show that priming of mobPBSCs with COMP-Ang1, via the Tie2/Ets-1 pathway, significantly induces their endothelial commitment and upregulates their expression of integrins, leading to enhancement of engraftment even after intravascular delivery and new vessel formation in the ischemic tissue (Figure IX of the online-only Data Supplement). These data suggest that short-term priming of stem/progenitor cells with COMP-Ang1 can be a feasible and promising option to functionally augment mobPBSCs and to enhance the therapeutic efficacy of cell therapy in clinical situations.
Sources of Funding
This study was supported by the National Research Laboratory for Cardiovascular Stem Cell program and by a grant (A062260) from the Korea Healthcare Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea.
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Although intracoronary cell infusion is the most popular method of cell delivery for cardiovascular disease because of its convenience, its efficacy is limited by the poor engraftment rate to ischemic myocardium after intracoronary delivery. We found that peripheral blood stem cells mobilized with granulocyte colony-stimulating factor (mobPBSCs) expressed high levels of the Tie2 receptor compared with naïve peripheral blood mononuclear cells, suggesting that angiopoietin-1 could be a very specific priming agent for mobPBSCs. Primed cells with angiopoietin-1 increased the expression of adhesion molecules and commitment to endothelial lineage, leading to improved engraftment and vasculogenesis. These results imply that priming with angiopoietin-1 can significantly improve the therapeutic efficacy of stem cells. The approach outlined here in which angiopoietin-1 is used for priming cells ex vivo before infusion requires further clinical studies to demonstrate efficacy.
The online-only Data Supplement is available with this article at http://circ.ahajournals.org/cgi/content/full/CIRCULATIONAHA.109.856815/DC1.
↵*Drs M.-S. Kim and C.-S. Lee contributed equally to this work.