Plasma vascular endothelial growth factor (VEGF) levels after intramuscular and intramyocardial gene transfer of VEGF-1 plasmid DNA

The purpose of this study was to document the kinetics of vascular endothelial growth factor (VEGF) protein release into the systemic circulation after phVEGF gene transfer for therapeutic angiogenesis. VEGF plasma levels were measured by ELISA in 64 patients undergoing gene transfer of plasmid DNA: intramuscular in 34 patients with peripheral artery disease, and intramyocardial in 30 patients with coronary disease. Baseline plasma VEGF was highly variable and not normally distributed. After intramuscular gene transfer, median plasma VEGF rose slightly, although significantly, by 7 days (38 to 41 pg/ml, p < 0.05), but was not different from baseline at 14, 21, or 28 days. After intramyocardial gene transfer, median plasma VEGF levels were significantly elevated compared with baseline on days 2, 3, and 7 (39, 38, and 45 pg/ml, respectively, each p < 0.05 vs. baseline value of 21 pg/ml). Day 7 plasma levels did not differ significantly as a function of phVEGF dose, or between intramyocardial and intramuscular injections (1.8 and 1.3 times baseline levels, respectively, p = 0.6), despite an almost 10-fold difference in mean phVEGF dose. Intramuscular and intramyocardial phVEGF injections result in significant, although modest, elevations of circulating gene product for <14 days, with no relationship to injected dose. While a statistically significant increase in circulating VEGF level can provide evidence of successful gene transfer for groups of patients, interpretation of results for individual subjects is complicated by wide variation in baseline VEGF and low circulating levels compared with baseline after gene transfer.

Determination of bone marrow-derived endothelial progenitor cell significance in angiogenic growth factor-induced neovascularization in vivo

OBJECTIVE

Our laboratory and others recently provided evidence indicating that endothelial progenitor cells (EPCs) participate in postnatal neovascularization. However, the extent to which EPCs contribute to adult neovascularization remains unclear. To address this issue, we investigated the quantitative contribution of EPCs to newly formed vascular structures in an in vivo Matrigel plug assay and corneal micropocket assay.

MATERIALS AND METHODS

Lethally irradiated FVB mice were transplanted with bone marrow (BM) mononuclear cells from transgenic mice constitutively expressing beta-galactosidase (beta-gal) encoded by the lacZ gene regulated by an endothelial-specific tie-2 promoter. Reconstitution of the transplanted BM leads to the expression of lacZ in mice, which is restricted to BM cells expressing tie-2.

RESULTS

Four weeks after BM transplantation (BMT), tie-2/lacZ/BMT mice were implanted with either Matrigel containing fibroblast growth factor-2 subcutaneously or with a vascular endothelial growth factor pellet into the cornea. After 7 days, the Matrigel plug or the cornea was removed and analyzed by X-gal staining or immunostaining for beta-gal. X-gal staining of the Matrigel plug identified 5.7% +/- 1.2% of endothelial cells (ECs) as cells originated from BM-derived EPCs, whereas the more sensitive technique of immunofluorescence identified 26.5% +/- 0.9% of ECs. Similarly, EPC-derived cells comprised 5.0% +/- 2.4% and 17.7% +/- 3.6% of the ECs in corneal neovascularization identified by X-gal staining and immunohistochemistry, respectively. Ki67 staining of the corneal tissue documented that the majority of EPC-derived cells were actively proliferating in situ.

CONCLUSION

These findings suggest that BM-derived EPCs make a significant contribution to angiogenic growth factor-induced neovascularization that may account for up to 26% of all ECs.

Endothelial progenitor cell vascular endothelial growth factor gene transfer for vascular regeneration

BACKGROUND

Previous studies have established that bone marrow-derived endothelial progenitor cells (EPCs) are present in the systemic circulation. In the current study, we investigated the hypothesis that gene transfer can be used to achieve phenotypic modulation of EPCs.

METHODS AND RESULTS

In vitro, ex vivo murine vascular endothelial growth factor (VEGF) 164 gene transfer augmented EPC proliferative activity and enhanced adhesion and incorporation of EPCs into quiescent as well as activated endothelial cell monolayers. To determine if such phenotypic modulation may facilitate therapeutic neovascularization, heterologous EPCs transduced with adenovirus encoding VEGF were administered to athymic nude mice with hindlimb ischemia; neovascularization and blood flow recovery were both improved, and limb necrosis/autoamputation were reduced by 63.7% in comparison with control animals. The dose of EPCs used for the in vivo experiments was 30 times less than that required in previous trials of EPC transplantation to improve ischemic limb salvage. Necropsy analysis of animals that received DiI-labeled VEGF-transduced EPCs confirmed that enhanced EPC incorporation demonstrated in vitro contributed to in vivo neovascularization as well.

CONCLUSIONS

In vitro, VEGF EPC gene transfer enhances EPC proliferation, adhesion, and incorporation into endothelial cell monolayers. In vivo, gene-modified EPCs facilitate the strategy of cell transplantation to augment naturally impaired neovascularization in an animal model of experimentally induced limb ischemia.

Bone marrow as a source of endothelial cells for natural and iatrogenic vascular repair

Postnatal neovascularization has previously been considered synonymous with angiogenesis, but the finding that circulating endothelial progenitor cells (EPCs) may home to sites of neovascularization and there differentiate into endothelial cells (ECs) is consistent with "vasculogenesis," through which the primordial vascular network is established in the embryo. Our findings suggest that growth and development of new blood vessels in the adult are not restricted to angiogenesis but encompass vasculogenesis as well, although the proportional contributions remain to be clarified. Likewise, augmented or retarded neovascularization probably involves enhancement or impariment of the vasculogenesis process.

Endothelial progenitor cells for regeneration

Endothelial progenitor cells (EPCs) have been recently isolated from peripheral blood and bone marrow (BM), and shown to be incorporated into sites of physiological and pathological neovascularization in vivo. In contrast to differentiated endothelial cells (ECs), transplantation of EPCs successfully enhanced vascular development by in situ differentiation and proliferation within ischemic organs. Based on such a novel concept of closed up function on EPCs in postnatal neovascularization, the beneficial property of EPC is attractive for cell therapy as well as cell-mediated gene therapy applications targeting regeneration of ischemic tissue.

VEGF gene transfer mobilizes endothelial progenitor cells in patients with inoperable coronary disease

BACKGROUND

Direct transfection of ischemic myocardium with naked plasmid DNA encoding for vascular endothelial growth factor-165 (VEGF165) has been shown to mobilize endothelial progenitor cells (EPCs). This study examined the kinetics of circulating EPCs isolated from peripheral blood mononuclear cells after gene transfer, and their role in neovascularization of ischemic myocardium.

METHODS

The mononuclear cell population was isolated from peripheral venous blood samples of patients with functional class III or IV angina receiving intramyocardial VEGF165 gene transfer. Peripheral blood mononuclear cells were examined by an in vitro EPC culture assay and fluorescent-activated cell sorting. The data were compared with a control group consisting of patients who had undergone off-pump coronary artery bypass grafting without receiving gene transfer.

RESULTS

Coinciding with a rise in VEGF levels, mobilization of EPCs increased significantly over base line for 9 weeks after the treatment (121+/-14 cells/mm2 versus 36.8+/-8 cells/mm2, p < 0.0005), followed by a subsequent decrease. Fluorescent-activated cell sorting analysis confirmed culture assay data, with a statistically significant rise in cells expressing vascular endothelial-cadherin, CD51/61 [alphavbeta3], CD62E [E-selectin], CD34, and KDR. The control group failed to show significant mobilization of EPCs.

CONCLUSIONS

Mobilization of EPCs with resultant postnatal vasculogenesis, may play a role in revascularizing ischemic myocardium following human gene transfer with VEGF165.

[Angiogenesis and vasculogenesis. Therapeutic strategies for stimulation of postnatal neovascularization]

The formation of new blood vessel is essential for a variety of physiological processes like embryogenesis and the female reproduction as well as wound healing and neovascularization of ischemic tissue. Major progress in understanding the underlying mechanisms regulating blood vessel growth has offered novel therapeutic options in the treatment of a variety of diseases including ischemic cardiovascular disorders. Vasculogenesis and angiogenesis are the mechanisms responsible for the development of the blood vessels. Angiogenesis refers to the formation of capillaries from preexisting vessels in the embryo and adult organism. While pathologic angiogenesis includes the role of post-natal neovascularization in the pathogenesis of arthritis, diabetic retinopathy, and tumor growth and metastasis, therapeutic angiogenesis, either endogenously or in response to administered growth factors, includes the development of collateral blood vessels in tissue ischemia. Preclinical studies established that angiogenic growth factors could promote collateral artery development in animal models of peripheral and myocardial ischemia. Subsequent clinical trials using gene transfer of naked DNA encoding for VEGF for the treatment of critical limb and myocardial ischemia documented the safety and clinical benefit of this novel therapeutic approach. Several objective methods indicated marked improvement in collateral vessel development. Vasculogenesis describes the development of new blood vessels from in situ differentiating endothelial cells. Recently considered to be restricted to embryogenesis, there exists now striking evidence that endothelial progenitor cells (EPC) circulate also in adult peripheral blood able to participate in ongoing neovascularization. Different cytokines and growth factors have a stimulatory effect on these bone-marrow derived EPC. Granulocyte macrophage colony stimulating factor (GM-CSF) and vascular endothelial growth factor (VEGF) mobilize EPC from the bone marrow into the peripheral circulation. While their endogenous contribution to postnatal neovascularization needs to be documented, the iatrogenic expansion and mobilization of EPC might represent an effective means to augment the resident population of endothelial cells (ECs). This kind of cell therapy for tissue regeneration in ischemic cardiovascular diseases opens a novel and challenging clinical option besides or in addition to the use of growth factors in gene therapy.

Vascular endothelial growth factor(165) gene transfer augments circulating endothelial progenitor cells in human subjects

Preclinical studies in animal models and early results of clinical trials in patients suggest that intramuscular injection of naked plasmid DNA encoding vascular endothelial growth factor (VEGF) can promote neovascularization of ischemic tissues. Such neovascularization has been attributed exclusively to sprout formation of endothelial cells derived from preexisting vessels. We investigated the hypothesis that VEGF gene transfer may also augment the population of circulating endothelial progenitor cells (EPCs). In patients with critical limb ischemia receiving VEGF gene transfer, gene expression was documented by a transient increase in plasma levels of VEGF. A culture assay documented a significant increase in EPCs (219%, P<0.001), whereas patients who received an empty vector had no change in circulating EPCs, as was the case for volunteers who received saline injections (VEGF versus empty vector, P<0.001; VEGF versus saline, P<0.005). Fluorescence-activated cell sorter analysis disclosed an overall increase of up to 30-fold in endothelial lineage markers KDR (VEGF receptor-2), VE-cadherin, CD34, alpha(v)beta(3), and E-selectin after VEGF gene transfer. Constitutive overexpression of VEGF in patients with limb ischemia augments the population of circulating EPCs. These findings support the notion that neovascularization of human ischemic tissues after angiogenic growth factor therapy is not limited to angiogenesis but involves circulating endothelial precursors that may home to ischemic foci and differentiate in situ through a process of vasculogenesis.

Transplantation of ex vivo expanded endothelial progenitor cells for therapeutic neovascularization

Animal studies and preliminary results in humans suggest that lower extremity and myocardial ischemia can be attenuated by treatment with angiogenic cytokines. The resident population of endothelial cells that is competent to respond to an available level of angiogenic growth factors, however, may potentially limit the extent to which cytokine supplementation enhances tissue neovascularization. Accordingly, we transplanted human endothelial progenitor cells (hEPCs) to athymic nude mice with hindlimb ischemia. Blood flow recovery and capillary density in the ischemic hindlimb were markedly improved, and the rate of limb loss was significantly reduced. Ex vivo expanded hEPCs may thus have utility as a "supply-side" strategy for therapeutic neovascularization.

Stem cell therapy and gene transfer for regeneration

The committed stem and progenitor cells have been recently isolated from various adult tissues, including hematopoietic stem cell, neural stem cell, mesenchymal stem cell and endothelial progenitor cell. These adult stem cells have several advantages as compared with embryonic stem cells as their practical therapeutic application for tissue regeneration. In this review, we discuss the promising gene therapy application of adult stem and progenitor cells in terms of modifying stem cell potency, altering organ property, accelerating regeneration and forming expressional organization.

Bone marrow origin of endothelial progenitor cells responsible for postnatal vasculogenesis in physiological and pathological neovascularization

Circulating endothelial progenitor cells (EPCs) have been isolated in peripheral blood of adult species. To determine the origin and role of EPCs contributing to postnatal vasculogenesis, transgenic mice constitutively expressing beta-galactosidase under the transcriptional regulation of an endothelial cell-specific promoter (Flk-1/LZ or Tie-2/LZ) were used as transplant donors. Localization of EPCs, indicated by flk-1 or tie-2/lacZ fusion transcripts, were identified in corpus luteal and endometrial neovasculature after inductive ovulation. Mouse syngeneic colon cancer cells (MCA38) were implanted subcutaneously into Flk-1/LZ/BMT (bone marrow transplantation) and Tie-2/LZ/BMT mice; tumor samples harvested at 1 week disclosed abundant flk-1/lacZ and tie-2/lacZ fusion transcripts, and sections stained with X-gal demonstrated that the neovasculature of the developing tumor frequently comprised Flk-1- or Tie-2-expressing EPCs. Cutaneous wounds examined at 4 days and 7 days after skin removal by punch biopsy disclosed EPCs incorporated into foci of neovascularization at high frequency. One week after the onset of hindlimb ischemia, lacZ-positive EPCs were identified incorporated into capillaries among skeletal myocytes. After permanent ligation of the left anterior descending coronary artery, histological samples from sites of myocardial infarction demonstrated incorporation of EPCs into foci of neovascularization at the border of the infarct. These findings indicate that postnatal neovascularization does not rely exclusively on sprouting from preexisting blood vessels (angiogenesis); instead, EPCs circulate from bone marrow to incorporate into and thus contribute to postnatal physiological and pathological neovascularization, which is consistent with postnatal vasculogenesis.

VEGF contributes to postnatal neovascularization by mobilizing bone marrow-derived endothelial progenitor cells

Vascular endothelial growth factor (VEGF) has been shown to promote neovascularization in animal models and, more recently, in human subjects. This feature has been assumed to result exclusively from its direct effects on fully differentiated endothelial cells, i.e. angiogenesis. Given its regulatory role in both angiogenesis and vasculogenesis during fetal development, we investigated the hypothesis that VEGF may modulate endothelial progenitor cell (EPC) kinetics for postnatal neovascularization. Indeed, we observed an increase in circulating EPCs following VEGF administration in vivo. VEGF-induced mobilization of bone marrow-derived EPCs resulted in increased differentiated EPCs in vitro and augmented corneal neovascularization in vivo. These findings thus establish a novel role for VEGF in postnatal neovascularization which complements its known impact on angiogenesis.

[Vascular endothelial factor (VEGF): therapeutic angiogenesis and vasculogenesis in the treatment of cardiovascular disease]

The formation of new blood vessel is essential for a variety of physiological processes like embryogenesis and the female reproduction as well as pathological processes like tumor growth, wound healing and neovascularization of ischemic tissue. Vasculogenesis and angiogenesis are the mechanisms responsible for the development of the blood vessels. While angiogenesis refers to the formation of capillaries from pre-existing vessels in the embryo and adult organism, vasculogenesis, the development of new blood vessels from in situ differentiating endothelial cells, has been previously considered restricted to embryogenesis. Recent investigations, however, show the existence of endothelial progenitor cells (EPCs) in the peripheral blood of the adult and their participation in ongoing neovascularization. Molecular and cell-biological experiments suggest that different cytokines and growth factors have a stimulatory effect on these bone-marrow derived EPCs. Results with GM-CSF (granulocyte macrophage-colony stimulating factor) and VEGF (vascular endothelial growth factor) open a new insight into the clinical use of cytokines and in particular the use of growth factors in gene therapy. The administration via protein or plasmid-DNA for neovascularization seems to enhance both pathways, angiogenesis and vasculogenesis.

Ischemia- and cytokine-induced mobilization of bone marrow-derived endothelial progenitor cells for neovascularization

Endothelial progenitor cells (EPCs) have been isolated from circulating mononuclear cells in human peripheral blood and shown to be incorporated into foci of neovascularization, consistent with postnatal vasculogenesis. We determined whether endogenous stimuli (tissue ischemia) and exogenous cytokine therapy (granulocyte macrophage-colony stimulating factor, GM-CSF) mobilize EPCs and thereby contribute to neovascularization of ischemic tissues. The development of regional ischemia in both mice and rabbits increased the frequency of circulating EPCs. In mice, the effect of ischemia-induced EPC mobilization was demonstrated by enhanced ocular neovascularization after cornea micropocket surgery in mice with hindlimb ischemia compared with that in non-ischemic control mice. In rabbits with hindlimb ischemia, circulating EPCs were further augmented after pretreatment with GM-CSF, with a corresponding improvement in hindlimb neovascularization. There was direct evidence that EPCs that contributed to enhanced corneal neovascularization were specifically mobilized from the bone marrow in response to ischemia and GM-CSF in mice transplanted with bone marrow from transgenic donors expressing beta-galactosidase transcriptionally regulated by the endothelial cell-specific Tie-2 promoter. These findings indicate that circulating EPCs are mobilized endogenously in response to tissue ischemia or exogenously by cytokine therapy and thereby augment neovascularization of ischemic tissues.

Nitric oxide synthase modulates angiogenesis in response to tissue ischemia

We tested the hypothesis that endothelial nitric oxide synthase (eNOS) modulates angiogenesis in two animal models in which therapeutic angiogenesis has been characterized as a compensatory response to tissue ischemia. We first administered L-arginine, previously shown to augment endogenous production of NO, to normal rabbits with operatively induced hindlimb ischemia. Angiogenesis in the ischemic hindlimb was significantly improved by dietary supplementation with L-arginine, compared to placebo-treated controls; angiographically evident vascularity in the ischemic limb, hemodynamic indices of limb perfusion, capillary density, and vasomotor reactivity in the collateral vessel-dependent ischemic limb were all improved by oral L-arginine supplementation. A murine model of operatively induced hindlimb ischemia was used to investigate the impact of targeted disruption of the gene encoding for ENOS on angiogenesis. Angiogenesis in the ischemic hindlimb was significantly impaired in eNOS-/- mice versus wild-type controls evaluated by either laser Doppler flow analysis or capillary density measurement. Impaired angiogenesis in eNOS-/- mice was not improved by administration of vascular endothelial growth factor (VEGF), suggesting that eNOS acts downstream from VEGF. Thus, (a) eNOS is a downstream mediator for in vivo angiogenesis, and (b) promoting eNOS activity by L-arginine supplementation accelerates in vivo angiogenesis. These findings suggest that defective endothelial NO synthesis may limit angiogenesis in patients with endothelial dysfunction related to atherosclerosis, and that oral L-arginine supplementation constitutes a potential therapeutic strategy for accelerating angiogenesis in patients with advanced vascular obstruction.