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

Newly Emerging Concepts in Blood Vessel Growth: Recent Discovery of Endothelial Progenitor Cells and Their Function in Tissue Regeneration

It has recently been established that bone marrow-derived endothelial progenitor cells (EPCs) are recruited to the systemic circulation and, in response to various cytokines, pharmacologic agents, and/or tissue ischemia, incorporate into sites of new blood vessel growth (neovascularization). These findings have changed our understanding of adult neovascularization by demonstrating that both preexisting endothelial cells and EPCs contribute to blood vessel formation during adult life. The following review article highlights the discovery of EPCs, their relationship to various clinical diseases, and their therapeutic potential for augmenting blood vessel formation.

Proliferation, differentiation, and tube formation by endothelial progenitor cells in response to shear stress

Endothelial progenitor cells (EPCs), circulating in peripheral blood, migrate toward target tissue, differentiate, and contribute to the formation of new vessels. In this study, we report that shear stress generated by blood flow or tissue fluid flow can accelerate the proliferation, differentiation, and capillary-like tube formation of EPCs. When EPCs cultured from human peripheral blood were subjected to laminar shear stress, the cells elongated and oriented their long axes in the direction of flow. The cell density of the EPCs exposed to shear stress was higher, and a larger percentage of these cells were in the G2-M phase of the cell cycle, compared with EPCs cultured under static conditions. Shear stress markedly increased the EPC expression of two vascular endothelial growth factor receptors, kinase insert domain-containing receptor and fms-like tyrosine kinase-1, and an intercellular adhesion molecule, vascular endothelial-cadherin, at both the protein and mRNA levels. Assays for tube formation in the collagen gels showed that the shear-stressed EPCs formed tubelike structures and developed an extensive tubular network significantly faster than the static controls. These findings suggest that EPCs are sensitive to shear stress and that their vasculogenic activities may be modulated by shear stress.

Vascular endothelial growth factor modulates skeletal myoblast function

Vascular endothelial growth factor (VEGF) expression is enhanced in ischemic skeletal muscle and is thought to play a key role in the angiogenic response to ischemia. However, it is still unknown whether, in addition to new blood vessel growth, VEGF modulates skeletal muscle cell function. In the present study immunohistochemical analysis showed that, in normoperfused mouse hindlimb, VEGF and its receptors Flk-1 and Flt-1 were expressed mostly in quiescent satellite cells. Unilateral hindlimb ischemia was induced by left femoral artery ligation. At day 3 and day 7 after the induction of ischemia, Flk-1 and Flt-1 were expressed in regenerating muscle fibers and VEGF expression by these fibers was markedly enhanced. Additional in vitro experiments showed that in growing medium both cultured satellite cells and myoblast cell line C2C12 expressed VEGF and its receptors. Under these conditions, Flk-1 receptor exhibited constitutive tyrosine phosphorylation that was increased by VEGF treatment. During myogenic differentiation Flk-1 and Flt-1 were down-regulated. In a modified Boyden Chamber assay, VEGF enhanced C2C12 myoblasts migration approximately fivefold. Moreover, VEGF administration to differentiating C2C12 myoblasts prevented apoptosis, while inhibition of VEGF signaling either with selective VEGF receptor inhibitors (SU1498 and CB676475) or a neutralizing Flk-1 antibody, enhanced cell death approximately 3.5-fold. Finally, adenovirus-mediated VEGF(165) gene transfer inhibited ischemia-induced apoptosis in skeletal muscle. These results support a role for VEGF in myoblast migration and survival, and suggest a novel autocrine role of VEGF in skeletal muscle repair during ischemia.

Oxidized low-density lipoprotein inhibits vascular endothelial growth factor-induced endothelial progenitor cell differentiation

1. Bone marrow-derived endothelial progenitor cells (EPC) in the peripheral blood of adult animals and humans have been shown to be incorporated into neovascularization. In contrast, hypercholesterolaemia impairs angiogenesis and collateral vessel formation in response to regional tissue ischaemia. We investigated whether oxidized LDL (oxLDL) affected human EPC differentiation. 2. When isolated human mononuclear cells (MNC) were incubated with vascular endothelial growth factor (VEGF), the number of differentiated, adherent EPC, as assessed by an in vitro culture assay, was increased in a dose-dependent manner (P < 0.01). When MNC were incubated with oxLDL at 1, 5 and 10 microg/mL in the presence of 100 ng/mL VEGF for 24 h, oxLDL dose-dependently reduced the number of differentiated, adherent EPC. 3. Vascular endothelial growth factor-induced EPC differentiation was significantly inhibited by pharmacological phosphatidylinositol 3-kinase blockers (either 10 nmol/L wortmannin or 10 micromol/L LY294002). Interestingly, immunoblotting analysis revealed that oxLDL dose-dependently led to dephosphorylation and, thus, deactivation of Akt in the presence of VEGF. Finally, these inhibitory effects induced by oxLDL were abolished by pretreatment with 1 micromol/L atorvastatin (P < 0.01). 4. Our data indicate that oxLDL inhibits VEGF-induced EPC differentiation through the dephosphorylation of Akt.

Circulating numbers of endothelial progenitor cells in patients with gastric and breast cancer

Angiogenic factors like VEGF or G-CSF were reported to mobilize endothelial progenitor cells (EPCs) from the bone marrow. These EPCs were shown to be incorporated in the neovessels of developing tumors. Although the concentrations of angiogenic factors in the peripheral blood were reported to be elevated in cancer patients, the number of circulating EPCs has not been previously investigated. In this study, the number of EPCs circulating in the blood in 16 healthy controls and 71 newly diagnosed cancer patients was examined by a culture assay of peripheral blood mononuclear cells. The number of circulating EPCs was not found to be increased in cancer patients, although the plasma levels of VEGF were elevated. It is suggested that VEGF, at concentrations typical of those observed in the blood of cancer patients, does not mobilize EPCs into the peripheral blood.

Gene therapy for cardiovascular angiogenesis

Atherosclerosis and endothelial dysfunction are responsible for the pathophysiologic basis of the spectrum of cardiovascular disorders including ischaemic heart disease (IHD), the leading cause of morbidity and mortality in the US. There have been major advances, including the use of pharmacotherapy, coronary and peripheral percutaneous transluminal interventions (PTI), coronary and peripheral bypass surgery and primary/secondary prevention measures. There are, however, multiple unmet needs: IHD refractory to medical therapy and unsuitable for revascularisation; critical limb ischaemia unsuitable for PTI or surgery; restenosis; ischaemic/diabetic neuropathy and heart failure. Cardiovascular gene therapy (GT) with vascular endothelial growth factor (VEGF) has yielded improved perfusion and reduced ischaemia in preclinical models of IHD. Several preclinical studies and Phase I and II clinical trials have shown the safety and therapeutic potential of GT in the treatment of IHD, peripheral arterial disease (PAD), restenosis, and ischaemic and diabetic neuropathy, pointing to the need for Phase III clinical trials.

Placental growth factor and its receptor, vascular endothelial growth factor receptor-1: novel targets for stimulation of ischemic tissue revascularization and inhibition of angiogenic and inflammatory disorders

In contrast to VEGF and its receptor VEGFR-2, PlGF and its receptor VEGFR-1 have been largely neglected and therefore their potential for therapy has not been previously explored. In this review, we describe the molecular properties of PlGF and VEGFR-1 and how this translates into an important role for PlGF in the angiogenic switch in pathological angiogenesis, by interacting with VEGFR-1 and synergizing with VEGF. PlGF was effective in the growth of new and stable vessels in cardiac and limb ischemia, through its action on different cell types (i.e. endothelial, smooth muscle and inflammatory cells and their precursors) that play a cardinal role in blood vessel formation. Accordingly, blocking its receptor VEGFR-1 with monoclonal antibodies (anti-VEGFR-1 mAb), expressed on al these cell types, successfully attenuated blood vessel formation during cancer, ischemic retinopathy and rheumatoid arthritis. In addition, while blocking this receptor was effective in reducing inflammatory disorders like atherosclerosis and rheumatoid arthritis, blocking the anti-angiogenic receptor VEGFR-2 was without effect. This indicates that in the latter diseases the beneficial effects of anti-VEGFR1 mAb were mainly due to its effect on inflammatory cells. Importantly, VEGFR-1 was also present on hematopoietic stem/progenitor cells, the precursors of inflammatory cells. Thus, these preclinical studies show proof-of-principle that PlGF and VEGFR-1 are promising therapeutic targets to treat angiogenesis and inflammation related disorders. Clinical trials will reveal whether this is also true for patients.

Multistep nature of microvascular recruitment of ex vivo-expanded embryonic endothelial progenitor cells during tumor angiogenesis

Tissue neovascularization involves recruitment of circulating endothelial progenitor cells that originate in the bone marrow. Here, we show that a class of embryonic endothelial progenitor cells (Tie-2+, c-Kit+, Sca-1+, and Flk-1-/low), which were isolated at E7.5 of mouse development at the onset of vasculogenesis, retain their ability to contribute to tumor angiogenesis in the adult. Using intravital fluorescence videomicroscopy, we further defined the multistep process of embryonic endothelial progenitor cell (eEPC) homing and incorporation. Circulating eEPCs are specifically arrested in "hot spots" within the tumor microvasculature, extravasate into the interstitium, form multicellular clusters, and incorporate into functional vascular networks. Expression analysis and in vivo blocking experiments provide evidence that the initial cell arrest of eEPC homing is mediated by E- and P-selectin and P-selectin glycoprotein ligand 1. This paper provides the first in vivo insights into the mechanisms of endothelial progenitor cell recruitment and, thus, indicates novel ways to interfere with pathological neovascularization.

Enhanced external counterpulsation for ischemic heart disease: what's behind the curtain?

Enhanced external counterpulsation (EECP) has been shown to reduce angina and to improve objective measures of myocardial ischemia in patients with refractory angina. Prospective clinical studies and large treatment registries suggest that a course of EECP is associated with prolongation of the time to exercise-induced ST-segment depression and resolution of myocardial perfusion defects, as well as with enhanced exercise tolerance and quality of life. With a growing knowledge base supporting the safety and beneficial clinical effects associated with EECP, this therapy can be considered a valuable treatment option, particularly in patients who have exhausted traditional revascularization methods and yet remain symptomatic despite optimal medical care. However, although the concept of external counterpulsation was introduced almost four decades ago, and despite growing evidence supporting the clinical benefit and safety of this therapeutic modality, little is firmly established regarding the mechanisms responsible for the beneficial effects associated with this technique. Suggested mechanisms contributing to the clinical benefit of EECP include improvement in endothelial function, promotion of coronary collateralization, enhancement of ventricular function, peripheral effects similar to those observed with regular physical exercise, and nonspecific placebo effects. This review summarizes the current evidence for a contribution of these mechanisms to the clinical benefit associated with EECP.

Localized arteriole formation directly adjacent to the site of VEGF-induced angiogenesis in muscle

We have shown previously that implantation of myoblasts constitutively expressing the VEGF-A gene into nonischemic mouse skeletal muscle leads to overgrowth of capillary-like blood vessels and hemangioma formation. These aberrant effects occurred directly at the implantation site. We show here that these regions result from angiogenic capillary growth and involve a change in capillary growth pattern and that smooth muscle-coated vessels similar to arterioles form directly adjacent to the implantation site. Myoblasts genetically engineered to produce VEGF were implanted into mouse leg muscles. Implantation sites were surrounded by a zone of dense capillary-sized vessels, around which was a second zone of muscle containing larger, smooth-muscle-covered vessels but few capillaries, and an outer zone of muscle exhibiting normal capillary density. The lack of capillaries in the middle region suggests that the preexisting capillaries adjacent to the implantation site underwent enlargement and/or fusion and recruited a smooth muscle coat. Capillaries at the implantation site were frequently wrapped around VEGF-producing muscle fibers and were continuous with the circulation and were not observed to include bone-marrow-derived endothelial cells. In contrast with the distant arteriogenesis resulting from VEGF delivery described in previous studies, we report here that highly localized arterioles also form adjacent to the site of delivery.

Therapeutic angiogenesis: protein-based therapy for coronary artery disease

Therapeutic angiogenesis is a promising treatment for ischaemic heart disease, particularly for patients who are not candidates for current methods of revascularisation. The goal of angiogenic therapy is the relief of symptoms of coronary artery disease and improvement of cardiac function by increasing perfusion to the ischaemic myocardium. Angiogenic cytokines such as fibroblast growth factor and vascular endothelial growth factor have been studied extensively in preclinical studies. Protein-based therapy with these growth factors has produced functionally significant angiogenesis in several animal models. Enthusiasm following these preclinical results led the way to clinical trials, which so far have shown only modest improvements in myocardial perfusion and clinical outcome. The attenuated angiogenic response to growth factor therapy observed in patients with coronary artery disease may be related to associated conditions such as endothelial dysfunction, regimens of single as opposed to multiple angiogenic agents and inefficiency of current delivery modalities, as illustrated by the disappointing results of the Phase II clinical trials using intravascular techniques of administration. The ultimate role angiogenesis will play clinically in the treatment of ischaemic heart disease will be determined by adequately powered, randomised, double-blind, placebo-controlled trials that include multi-agent angiogenic therapy and intramyocardial methods of delivery.

The role of VEGF in normal and neoplastic hematopoiesis

VEGF is a secreted growth factor that mediates its biological effects by binding to two transmembrane tyrosine kinase receptors, VEGFR-1 and VEGFR-2. The VEGF/receptor signaling system is involved in the regulation of two fundamental processes in vertebrates: the formation of blood vessels (angiogenesis) and of blood cells (hematopoiesis). Hematopoietic stem cells, capable of giving rise to all blood cell lineages, are often found in clusters with endothelial cells, the key cell type involved in the formation of blood vessels. Despite such proximity of VEGF-responsive cells, hematopoiesis occurs independently of neoangiogenesis in the adult bone marrow, suggesting that VEGF regulates the two processes by different mechanisms. In support of this hypothesis, the recently identified autocrine loop by which VEGF may control hematopoietic stem cell survival and repopulation, is fundamentally different from its paracrine effects regulating angiogenesis. Furthermore, coexpression of VEGF and its receptors, the prerequisite for autocrine loops, is frequently found in lymphomas and myelomas, suggesting that autocrine loops also play a role in hematological malignancies. Several therapeutic strategies blocking VEGF or VEGF-induced signaling are currently being investigated for the treatment of neoplastic diseases. They differ in their potential to interfere with the autocrine or paracrine effector functions of VEGF during angiogenesis, hematopoiesis, and tumor cell proliferation, properties which may ultimately determine their therapeutic potential.

Hemangioblasts, angioblasts, and adult endothelial cell progenitors

After decades of speculation, proof of embryonic hemangioblasts finally emerged a few years ago. Surprisingly, at about the same time, evidence for adult hemangioblasts began to appear, and recent single-cell bone marrow transplants have confirmed their existence. Embryonic and adult hemangioblasts appear to share antigenic determinants, including CD34, ACC133, and VEGFR2, although their phenotype may be plastic. They also respond to similar factors, prominent among them vascular endothelial growth factor (VEGF). In the adult, hemangioblasts reside principally in the bone marrow, although they may subsequently leave that niche to reside in nonhematopoietic tissues. A number of studies indicate that these cells or their progeny may be a significant source of endothelial cells in adult pathologic and nonpathologic vascularization, and may participate in vascular repair. In addition to hemangioblasts, a more differentiated source of endothelial cell progenitors may be present in the blood, namely, monocytes or monocytic-like cells. The relative importance of the two cell types in vivo is not clear, though endothelial cells derived from the two sources may not be identical, and hemangioblasts seem to provide a stimulus for differentiation of the monocytes. Treatment with exogenous bone marrow-derived cells can promote neovascularization, accelerate restoration of blood flow to ischemic tissues, and improve cardiac function after infarct. Hence, there is great hope that either alone, in combination with angiogenic factors, or as gene therapy vectors, we can harness these cells to treat ischemic and vascular diseases in the relatively near future.

Adeno-associated viral vector-mediated gene transfer of VEGF normalizes skeletal muscle oxygen tension and induces arteriogenesis in ischemic rat hindlimb

Critical limb ischemia is an important clinical problem that often leads to disability and limb loss. Vascular endothelial growth factor (VEGF), delivered either as recombinant protein or as gene therapy, has been shown to promote both collateral artery formation (arteriogenesis) and capillary angiogenesis in animal models of hindlimb ischemia. However, none of the previous studies has demonstrated an improvement in tissue hypoxia, the condition that drives the molecular response to ischemia. Furthermore, the optimal vector and route of gene delivery have not been determined. Recently, adeno-associated viral (AAV) vectors, which efficiently transduce skeletal muscle and produce sustained transgene expression, have been used as gene therapy vectors. We asked whether an intra-arterial injection of AAV-VEGF(165) normalizes muscle oxygen tension by increasing skeletal muscle oxygen tension, and promotes arteriogenesis and angiogenesis in a rat model of severe hindlimb ischemia. We found that AAV-VEGF treatment normalized muscle oxygen tension in the ischemic limb. In contrast, vehicle and AAV-lacZ-treated limbs remained ischemic. Collateral arteries were more numerous in AAV-VEGF-treated rats, but, surprisingly, capillaries were not. We conclude that intra-arterial AAV-mediated gene transfer of AAV-VEGF(165) normalizes muscle oxygen tension and leads to arteriogenesis in rats with severe hindlimb ischemia.

Circulating endothelial progenitor cells are reduced in hemodialysis patients

OBJECTIVE

The risk of cardiovascular disease in hemodialysis patients is far greater than in the general population. Endothelial progenitor cells (EPCs) circulating in the peripheral blood contribute to neovascularization in the ischemic tissue. EPCs are considered to be included in CD34 positive (CD34+) or AC133 positive (AC133+) mononuclear cells (MNCs). This study's aim was to determine the number and functional activity of EPCs in hemodialysis patients and age-matched control subjects.

METHODS

The numbers of CD34+ MNCs and AC133+ MNCs in the peripheral blood were quantified by flow cytometry. The peripheral blood EPCs were also examined by an in vitro culture assay. The levels of serum vascular endothelial growth factor (VEGF) were measured by sandwich enzyme immunoassay.

RESULTS

The numbers of CD34+ MNCs and AC133+ MNCs were significantly reduced by 56% and 49%, respectively, in hemodialysis patients (n = 50) compared with control subjects (n = 36). The number of EPCs determined by the culture assay was also significantly reduced by 41% in hemodialysis patients compared with control subjects. Multivariate analysis revealed that none of the atherosclerotic risk factors were independent predictors of reduced CD34+ MNC counts. The serum VEGF levels in hemodialysis patients were not different from those in control subjects and did not correlate with CD34+ MNC counts.

CONCLUSION

Circulating EPCs are significantly reduced in hemodialysis patients, which might be related to impaired neovascularization and cardiovascular disease in these patients.

VEGF increases engraftment of bone marrow-derived endothelial progenitor cells (EPCs) into vasculature of newborn murine recipients

Recent evidence suggests that bone marrow-derived angioblasts or endothelial progenitor cells circulate in peripheral blood and can incorporate at sites of pathologic neovascularization or during the ovarian cycle. However, the incorporation of endothelial progenitor cells into vessels of nonischemic tissues in adult animals has not been observed. We hypothesized that the vascular microenvironment differs between newborn and adult animals, and that donor endothelial cell progenitors would engraft in rapidly growing normal tissues during the neonatal period. After nonablative administration of bone marrow cells either at birth or at 4 weeks of age, donor-derived endothelial cells were found only in the neovasculature of the newborn recipients. Both the incorporation of donor endothelial cells into the newborn neovasculature as well as tissue vascularity were significantly increased by coadministering vascular endothelial growth factor with bone marrow cells. These findings suggest that bone marrow-derived endothelial progenitor cells can contribute to neovascularization during the newborn period and are responsive to vascular endothelial growth factor.

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.

Effect of transfection time on the survival of epigastric skin flaps pretreated with adenovirus encoding the VEGF gene

An experimental study was conducted to investigate the effect of time of adenovirus-mediated vascular endothelial growth factor (VEGF) gene therapy on the viability of epigastric skin flaps. Eighty-four male Sprague-Dawley rats were used. Skin flaps measuring 8 x 8 cm were marked on the ventral abdominal wall. The upper border of the flap was 1 cm above the costal margin, and the lower border was at the pubis and the inguinal fold. The lateral borders of the flap corresponded to the location of the distinct conversion of the thin ventral skin to the thick dorsal skin. Seven sites in the predicted area of necrosis on the outlined skin flaps were chosen for subdermal injections. All injections were administered by an individual who was blinded to the different treatment groups. The rats received either saline (control group I, N = 28) or adenovirus encoding green fluorescent protein (Ad-GFP; group II, N = 28) or Ad-VEGF (group III, N = 28). The epigastric island skin flaps based solely on the right inferior epigastric vessels were elevated either on the same day of injection (day 0 = 12 hours after transfection, N = 7) or on day 3 (N = 7), day 7 (N = 7), or day 14 (N = 7) after subdermal gene therapy. Flaps were sutured back to their native configuration. Flap viability was evaluated on day 7 after surgery. Sections of the flaps were examined histologically after undergoing hematoxylin-eosin staining. There was a significant reduction in mean percentage of necrotic flap area by 56%, 67%, 70%, and 54% in flaps transfected with Ad-VEGF, 12 hours, 3 days, 7 days, and 14 days before flap elevation, respectively ( < 0.05). There was no evidence that the mean percentage of skin necrosis in the Ad-GFP group was different than in the control group ( = 0.26). There was evidence of mild inflammation in flaps pretreated with Ad-GFP and Ad-VEGF compared with the control group. The authors demonstrated that adenovirus-mediated gene therapy of the abdominal skin after subdermal injections was technically feasible. This was demonstrated by the visualization of GFP expression in control experiments using a fluorescence microscope. In this study, adenovirus-mediated VEGF gene therapy promoted epigastric flap survival, which was not related to the time of transfection. These findings raise the possibility that pretreatment with VEGF gene therapy using an adenovirus vector may be applicable in patients at risk for plastic surgery.

Transplantation of endothelial progenitor cells for therapeutic neovascularization

Endothelial progenitor cells (EPCs), which were first identified in adult peripheral blood mononuclear cells (MNCs), play an important role in postnatal neovascularization. Tissue ischemia augments mobilization of EPCs from bone marrow into the circulation and enhances incorporation of EPCs at sites of neovascularization. Two methods to obtain EPCs from bone marrow, peripheral blood or cord blood MNCs have been evaluated for therapeutic neovascularization: (1) fresh isolation using anti-CD34, anti-KDR or anti-AC133 antibody, and (2) ex vivo expansion of total MNCs. In an immunodeficient mouse model of hindlimb ischemia, systemic transplantation of human ex vivo expanded EPCs improves limb survival through the enhancement of blood flow in the ischemic tissue. A similar strategy also leads to histological and functional preservation of ischemic myocardium of nude rats. Recently, a preclinical study of catheter-based, intramyocardial transplantation ofautologous EPCs in a swine model of chronic myocardial ischemia demonstrated the therapeutic potential of cell-based therapy, with attenuation of myocardial ischemia and improvement in left ventricular function. These favorable outcomes strongly suggest a therapeutic impact of EPC transplantation in clinical settings. Further basic research, with improved understanding of the mechanisms governing homing and incorporation of EPCs, will be still necessary to optimize the methodology of the cell therapy.

Evaluation of angiogenesis and side effects in ischemic rabbit hindlimbs after intramuscular injection of adenoviral vectors encoding VEGF and LacZ

BACKGROUND

Recent studies have suggested the therapeutic potential of vascular endothelial growth factor (VEGF) gene therapy in ischemic skeletal muscle. However, only limited information is available about the effects of VEGF gene therapy in different regions of ischemic limbs, effects of control adenoviruses, and biodistribution of the transgenes after intramuscular (i.m.) administration. Here we studied angiogenesis and side effects of adenovirus-mediated VEGF and beta-galactosidase (LacZ) gene transfers in ischemic rabbit hindlimbs.

METHODS AND RESULTS

Ten days after induction of ischemia, rabbits were treated with i.m. injections of saline, LacZ adenovirus (AdLacZ; 2x10(10) pfu) or adenovirus encoding mouse VEGF(164) (AdVEGF; 2x10(10) pfu). In rabbits treated with AdVEGF an increase in serum VEGF(164) levels was detected by ELISA three and seven days after the gene transfer. 30 days after the gene transfer a positive effect on capillary density was observed in the thigh region both in rabbits treated with AdVEGF and AdLacZ compared with animals that received saline. On the other hand, AdVEGF and AdLacZ gene transfers had no effect on the capillary density in the calf region on day 30. A positive correlation between the capillary density and the number of collateral arteries was observed in the thigh. Hindlimb and testis edema and excess non-physiological growth of capillaries were detected as adverse effects of the AdVEGF gene therapy. Biodistribution analysis showed that the transgene was present not only in the target muscle, but also in ectopic tissues seven days after i.m. gene transfer.

CONCLUSIONS

The results suggest that a high dose of adenoviral vector encoding either AdVEGF or AdLacZ induces angiogenesis in the rabbit hindlimb ischemia model; i.m. injection of adenovirus leads to the transfection of ectopic organs; and AdVEGF gene transfer induces edema in ischemic skeletal muscle.

Adult hematopoietic stem cells provide functional hemangioblast activity during retinal neovascularization

Adults maintain a reservoir of hematopoietic stem cells that can enter the circulation to reach organs in need of regeneration. We developed a novel model of retinal neovascularization in adult mice to examine the role of hematopoietic stem cells in revascularizing ischemic retinas. Adult mice were durably engrafted with hematopoietic stem cells isolated from transgenic mice expressing green fluorescent protein. We performed serial long-term transplants, to ensure activity arose from self-renewing stem cells, and single hematopoietic stem-cell transplants to show clonality. After durable hematopoietic engraftment was established, retinal ischemia was induced to promote neovascularization. Our results indicate that self-renewing adult hematopoietic stem cells have functional hemangioblast activity, that is, they can clonally differentiate into all hematopoietic cell lineages as well as endothelial cells that revascularize adult retina. We also show that recruitment of endothelial precursors to sites of ischemic injury has a significant role in neovascularization.

Enhancement of epigastric skin flap survival by adenovirus-mediated VEGF gene therapy

A novel approach to treat ischemic tissues by using gene therapy has recently been introduced on the basis of the angiogenic potential of certain growth factors. The authors investigated the effect of adenovirus-mediated gene therapy with vascular endothelial growth factor (VEGF) delivered into the subdermal space to treat compromised skin flaps. For this purpose, the epigastric skin flap model in rats, based solely on the right inferior epigastric vessels, was used. Thirty male Sprague-Dawley rats were divided into five groups of six rats each. Viral transfection with 108 plaque-forming units was performed 2 days before the epigastric flap elevation. Rats received subdermal injections of adenovirus encoding VEGF (Ad-VEGF) or green fluorescent protein (Ad-GFP) as treatment control. Another set of animals (n = 6) received no injections and were designated as control. To determine whether site of injection had an impact on flap viability, injections were given into the predicted local ischemic area (Ad-VEGF local, n = 6; Ad-GFP local, n = 6) and into the midline of the flap (Ad-VEGF midline, n = 6; Ad-GFP midline, n = 6). A flap measuring 8 x 8 cm was outlined on the abdominal skin extending from the xiphoid process proximally and the pubic region distally, to the anterior axillary lines bilaterally. Then, the epigastric flap was elevated as an island on the right inferior epigastric vessels and sutured back to its bed. Flap viability was evaluated at 7 and 14 days after the first operation. The epigastric flaps were scanned to the computer and areas of hypoxic and/or necrotic zones relative to total flap surface area were measured and expressed as percentages by using Image Pro Plus software. Specimens were taken for histologic evaluation at day 14 before the animals were killed. Combined area of necrotic and hypoxic zones as well as necrotic zone were decreased to 9.7 +/- 1.4 percent and 1.4 +/- 0.9 percent in Ad-VEGF local, and 11.8 +/- 1.9 percent and 3.5 +/- 1.64 percent in Ad-VEGF midline compared with the control and Ad-GFP treatment groups (control, 23 +/- 3.6 percent and 20.1 +/- 3.3 percent; Ad-GFP local, 24.8 +/- 4.8 percent and 16.2 +/- 5.9 percent; and Ad-GFP midline, 23.4 +/- 6.9 percent and 19.5 +/- 7.7 percent; p < 0.05). Histologic evaluation by light microscopy failed to demonstrate any quantitative difference in vascularity of skin flaps between the treatment groups. In this study, the authors demonstrated that adenovirus-mediated gene therapy using VEGF enhanced epigastric skin flap survival, as confirmed by the significant reduction in combined area of necrotic and hypoxic zones of the flap. Compared with the control, both local and midline subdermal injections of Ad-VEGF showed improvement in overall flap survival by 57.9 and 48.7 percent, respectively. The results of this study raise the possibility of using adenovirus-mediated therapeutic angiogenesis for safer flap surgery in high-risk patients.

Endothelial progenitor cells for vascular regeneration

The basis for native as well as therapeutic neovascularization is not restricted to angiogenesis but includes postnatal vasculogenesis. Our laboratory and others' have established that bone marrow-derived endothelial progenitor cells (EPCs) are present in the systemic circulation, are augmented in response to certain cytokines and/or tissue ischemia, and home to as well as incorporate into sites of neovascularization. Given the background, EPCs have been investigated as therapeutic agents in these studies of supply-side angiogenesis under pathological as well as physiological conditions. This review discusses the therapeutic potential of EPCs for cardiovascular ischemic diseases.

Expression of vascular endothelial growth factor and vascular endothelial growth factor receptor-2 (KDR/Flk-1) in ischemic skeletal muscle and its regeneration

Vascular endothelial growth factor (VEGF) is a hypoxia-inducible endothelial cell mitogen and survival factor. Its receptor VEGFR-2 (KDR/Flk-1) mediates these effects. We studied the expression of VEGF and VEGFR-2 in ischemic human and rabbit skeletal muscle by immunohistochemistry and in situ hybridization. Human samples were obtained from eight lower limb amputations because of acute or chronic critical ischemia. In chronically ischemic human skeletal muscle VEGF and VEGFR-2 expression was restricted to atrophic and regenerating skeletal myocytes, whereas in acutely ischemic limbs VEGF and VEGFR-2 were expressed diffusely in the affected muscle. Hypoxia-inducible factor-1alpha was associated with VEGF and VEGFR-2 expression both in acute and chronic ischemia but not in regeneration. Hindlimb ischemia was induced in 20 New Zealand White rabbits by excising the femoral artery. Magnetic resonance imaging and histological sections revealed extensive ischemic damage in the thigh and leg muscles of ischemic rabbit hindlimbs with VEGF expression similar to acute human lower limb ischemia. After 1 and 3 weeks of ischemia VEGF expression was restricted to regenerating myotubes and by 6 weeks regeneration and expression of VEGF was diminished. VEGFR-2 expression was co-localized with VEGF expression in regenerating myotubes. Macrophages and an increased number of capillaries were associated with areas of ischemic muscle expressing VEGF and VEGFR-2. In conclusion, two patterns of VEGF and VEGFR-2 expression in human and rabbit ischemic skeletal muscle are demonstrated. In acute skeletal muscle ischemia VEGF and VEGFR-2 are expressed diffusely in the affected muscle. In chronic skeletal muscle ischemia and in skeletal muscle recovering from ischemia VEGF and VEGFR-2 expression are restricted to atrophic and regenerating muscle cells suggesting the operation of an autocrine pathway that may promote survival and regeneration of myocytes.

Gene therapy in plastic surgery

Recent developments in gene therapy have shown promise in the treatment of soft-tissue repair, bone formation, nerve regeneration, and cranial suture development. This special topic article reviews commonly used methods of gene therapy and discusses their various advantages and disadvantages. In addition, an overview of new developments in gene therapy as they relate to plastic surgery is provided.

Gene transfer therapy in vascular diseases

Somatic gene therapy of vascular diseases is a promising new field in modern medicine. Recent advancements in gene transfer technology have greatly evolved our understanding of the pathophysiologic role of candidate disease genes. With this knowledge, the expression of selective gene products provides the means to test the therapeutic use of gene therapy in a multitude of medical conditions. In addition, with the completion of genome sequencing programs, gene transfer can be used also to study the biologic function of novel genes in vivo. Novel genes are delivered to targeted tissue via several different vehicles. These vectors include adenoviruses, retroviruses, plasmids, plasmid/liposomes, and oligonucleotides. However, each one of these vectors has inherent limitations. Further investigations into developing delivery systems that not only allow for efficient, targeted gene transfer, but also are stable and nonimmunogenic, will optimize the clinical application of gene therapy in vascular diseases. This review further discusses the available mode of gene delivery and examines six major areas in vascular gene therapy, namely prevention of restenosis, thrombosis, hypertension, atherosclerosis, peripheral vascular disease in congestive heart failure, and ischemia. Although we highlight some of the recent advances in the use of gene therapy in treating vascular disease discovered primarily during the past two years, many excellent studies published during that period are not included in this review due to space limitations. The following is a selective review of practical uses of gene transfer therapy in vascular diseases. This review primarily covers work performed in the last 2 years. For earlier work, the reader may refer to several excellent review articles. For instance, Belalcazer et al. (6) reviewed general aspects of somatic gene therapy and the different vehicles used for the delivery of therapeutic genes. Gene therapy in restenosis and stimulation of angiogenesis in the cardiac muscle are discussed in reviews by several investigators (13,26,57,74,83). In another review, Meyerson et al. (43) discuss advances in gene therapy for vascular proliferative disorders and chronic peripheral and cardiac 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.

Clinical applications of cardiovascular angiogenesis

Angiogenesis is fundamental to both normal physiologic (wound healing) and pathologic (cancer) processes. Manipulation of divergent angiogenic signals promises effective therapy of atherosclerotic cardiovascular disease. Positive proangiogenic strategies promise collateral circulation to ischemic territories, while negative antiangiogenic strategies starve the fibromuscular proliferation within the atherosclerotic lesion. Indeed, recent phase 1 trials suggest that delivering DNA or recombinant protein to the site of vascular occlusion may stimulate physiologically significant collateral circulation in chronically ischemic myocardium. While symptomatic and functional improvement has been documented, toxicity profiles and effects on long-term patient survival are still unclear. The purposes of this article are as follows: (1) to review the pathophysiologic basis for pro- and antiangiogenic strategies in the treatment of cardiovascular disease, (2) to examine the clinical trials of proangiogenic gene or recombinant protein delivery into ischemic beds, and conversely, (3) to explore antiangiogenic strategies in the prevention and treatment of intimal neovascularization and smooth muscle proliferation within the vessel wall.

NHLBI workshop report: endothelial cell phenotypes in heart, lung, and blood diseases

Endothelium critically regulates systemic and pulmonary vascular function, playing a central role in hemostasis, inflammation, vasoregulation, angiogenesis, and vascular growth. Indeed, the endothelium integrates signals originating in the circulation with those in the vessel wall to coordinate vascular function. This highly metabolic role differs significantly from the historic view of endothelium, in which it was considered to be merely an inert barrier. New lines of evidence may further change our understanding of endothelium, in regard to both its origin and function. Embryological studies suggest that the endothelium arises from different sites, including angiogenesis of endothelium from macrovascular segments and vasculogenesis of endothelium from microcirculatory segments. These findings suggest an inherent phenotypic distinction between endothelial populations based on their developmental origin. Similarly, diverse environmental cues influence endothelial cell phenotype, critical to not only normal function but also the function of a diseased vessel. Consequently, an improved understanding of site-specific endothelial cell function is essential, particularly with consideration to environmental stimuli present both in the healthy vessel and in development of vasculopathic disease states. The need to examine endothelial cell phenotypes in the context of vascular function served as the basis for a recent workshop sponsored by the National Heart, Lung, and Blood Institute (NHLBI). This report is a synopsis of pertinent topics that were discussed, and future goals and research opportunities identified by the participants of the workshop are presented.

Therapeutic angiogenesis in critical limb and myocardial ischemia

Research in animal models of ischemia has shown that administration of angiogenic growth factors, either as a recombinant protein or by gene transfer, can augment nutrient perfusion through neovascularization to promote the development of supplemental collateral blood vessels that will constitute endogenous bypass conduits around occluded native arteries; a strategy termed "therapeutic angiogenesis." In animal models and clinical trials, the best studied cytokines with angiogenic activity are vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). Clinical trials of therapeutic angiogenesis in patients with critical limb ischemia demonstrated resolution of rest pain and/or improved limb integrity, increased pain-free walking time and ankle-brachial index, newly visible collateral vessels by digital subtraction angiography, and qualitative evidence of improved distal flow by magnetic resonance imaging. Initial clinical trials in patients with end-stage coronary artery disease using direct myocardial injection via thoracotomy resulted in large increases in exercise time and marked reductions in anginal symptoms, as well as objective evidence of improved perfusion and left ventricular function. Larger scale placebo-controlled trials have been limited to intracoronary and intravenous administration of recombinant protein, and have not shown significant improvement in exercise time or angina compared to placebo. Larger scale placebo-controlled studies of gene transfer using catheter-based endocardial delivery are in progress. Future clinical studies are required to determine the optimal dose, formulation, route of administration, and combinations of growth factors, as well as the requirement for endothelial progenitor cell or stem cell supplementation, to provide effective and safe therapeutic angiogenesis for patients with critical limb ischemia and chronic myocardial ischemia who are not candidates for conventional revascularization procedures.

[Growth factors for therapeutic angiogenesis in cardiovascular diseases]

Therapeutic angiogenesis based on the administration of growth factors with angiogenic activity allows enhancement of collateral vessels able to palliate insufficient tissue perfusion secondary to obstruction of native arteries. At present, this type of therapy is addressed to patients that fail to respond to conventional treatment (surgical or percutaneous revascularization). The most extensively investigated angiogenic growth factors are vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). These cytokines can be administered either as recombinant proteins or as the genes encoding for these proteins. Both approaches have pros and cons that are under investigation in animal models and in clinical studies. Although clinical trials consist so far of small, often non-randomized series, preliminary results are promising. For example, administration of VEGF or FGF has been associated to objective evidence of increased tissue perfusion in patients with myocardial ischemia, and to a significant improvement of pain and ischemia in patients with peripheral arterial disease. Contrarily to expected, these interventions have been associated to scant adverse side effects, although larger clinical trials will be necessary in order to prove the safety and effectiveness of these interventions. Nevertheless, it seems clear that it is feasible to induce effective therapeutic angiogenesis in selected patients without significant associated toxicity.

Functional small-diameter neovessels created using endothelial progenitor cells expanded ex vivo

Arterial conduits are increasingly preferred for surgical bypass because of inherent functional properties conferred by arterial endothelial cells, especially nitric oxide production in response to physiologic stimuli. Here we tested whether endothelial progenitor cells (EPCs) can replace arterial endothelial cells and promote patency in tissue-engineered small-diameter blood vessels (4 mm). We isolated EPCs from peripheral blood of sheep, expanded them ex vivo and then seeded them on decellularized porcine iliac vessels. EPC-seeded grafts remained patent for 130 days as a carotid interposition graft in sheep, whereas non-seeded grafts occluded within 15 days. The EPC-explanted grafts exhibited contractile activity and nitric-oxide-mediated vascular relaxation that were similar to native carotid arteries. These results indicate that EPCs can function similarly to arterial endothelial cells and thereby confer longer vascular-graft survival. Due to their unique properties, EPCs might have other general applications for tissue-engineered structures and in treating vascular diseases.

HMG-CoA reductase inhibitors (statins) increase endothelial progenitor cells via the PI 3-kinase/Akt pathway

HMG-CoA reductase inhibitors (statins) have been developed as lipid-lowering drugs and are well established to reduce morbidity and mortality from coronary artery disease. Here we demonstrate that statins potently augment endothelial progenitor cell differentiation in mononuclear cells and CD34-positive hematopoietic stem cells isolated from peripheral blood. Moreover, treatment of mice with statins increased c-kit(+)/Sca-1(+)--positive hematopoietic stem cells in the bone marrow and further elevated the number of differentiated endothelial progenitor cells (EPCs). Statins induce EPC differentiation via the PI 3-kinase/Akt (PI3K/Akt) pathway as demonstrated by the inhibitory effect of pharmacological PI3K blockers or overexpression of a dominant negative Akt construct. Similarly, the potent angiogenic growth factor VEGF requires Akt to augment EPC numbers, suggesting an essential role for Akt in regulating hematopoietic progenitor cell differentiation. Given that statins are at least as potent as VEGF in increasing EPC differentiation, augmentation of circulating EPC might importantly contribute to the well-established beneficial effects of statins in patients with coronary artery disease.

HMG-CoA reductase inhibitor mobilizes bone marrow--derived endothelial progenitor cells

Endothelial progenitor cells (EPCs) have been isolated from circulating mononuclear cells in peripheral blood and shown to incorporate into foci of neovascularization, consistent with postnatal vasculogenesis. These circulating EPCs are derived from bone marrow and are mobilized endogenously in response to tissue ischemia or exogenously by cytokine stimulation. We show here, using a chemotaxis assay of bone marrow mononuclear cells in vitro and EPC culture assay of peripheral blood from simvastatin-treated animals in vivo, that the HMG-CoA reductase inhibitor, simvastatin, augments the circulating population of EPCs. Direct evidence that this increased pool of circulating EPCs originates from bone marrow and may enhance neovascularization was demonstrated in simvastatin-treated mice transplanted with bone marrow from transgenic donors expressing beta-galactosidase transcriptionally regulated by the endothelial cell-specific Tie-2 promoter. The role of Akt signaling in mediating effects of statin on EPCs is suggested by the observation that simvastatin rapidly activates Akt protein kinase in EPCs, enhancing proliferative and migratory activities and cell survival. Furthermore, dominant negative Akt overexpression leads to functional blocking of EPC bioactivity. These findings establish that augmented mobilization of bone marrow-derived EPCs through stimulation of the Akt signaling pathway constitutes a novel function for HMG-CoA reductase inhibitors.

HMG-CoA reductase inhibitor mobilizes bone marrow--derived endothelial progenitor cells

Endothelial progenitor cells (EPCs) have been isolated from circulating mononuclear cells in peripheral blood and shown to incorporate into foci of neovascularization, consistent with postnatal vasculogenesis. These circulating EPCs are derived from bone marrow and are mobilized endogenously in response to tissue ischemia or exogenously by cytokine stimulation. We show here, using a chemotaxis assay of bone marrow mononuclear cells in vitro and EPC culture assay of peripheral blood from simvastatin-treated animals in vivo, that the HMG-CoA reductase inhibitor, simvastatin, augments the circulating population of EPCs. Direct evidence that this increased pool of circulating EPCs originates from bone marrow and may enhance neovascularization was demonstrated in simvastatin-treated mice transplanted with bone marrow from transgenic donors expressing beta-galactosidase transcriptionally regulated by the endothelial cell-specific Tie-2 promoter. The role of Akt signaling in mediating effects of statin on EPCs is suggested by the observation that simvastatin rapidly activates Akt protein kinase in EPCs, enhancing proliferative and migratory activities and cell survival. Furthermore, dominant negative Akt overexpression leads to functional blocking of EPC bioactivity. These findings establish that augmented mobilization of bone marrow-derived EPCs through stimulation of the Akt signaling pathway constitutes a novel function for HMG-CoA reductase inhibitors.

Gene therapy in vascular medicine: recent advances and future perspectives

Gene therapy is emerging as a potential strategy for the treatment of cardiovascular diseases, such as restenosis after angioplasty, vascular bypass graft occlusion, and transplant coronary vasculopathy, for which no known effective therapy exists. The first human trial in cardiovascular disease was started in 1994 to treat peripheral vascular disease using vascular endothelial growth factor. In addition, therapeutic angiogenesis using the vascular endothelial growth factor gene was applied in the treatment of ischemic heart disease. The results from these clinical trials seem to exceed expectation. Improvement of clinical symptoms in peripheral arterial disease and ischemic heart disease has been reported. At least five different potent angiogenic growth factors have been tested in clinical trials to treat peripheral arterial disease or ischemic heart disease. In addition, another strategy for combating disease processes, to target the transcriptional process, has been tested in a human trial. Transfection of cis-element double-stranded oligodeoxynucleotides is an especially powerful tool in a new class of antigen strategies for gene therapy. Transfection of double-stranded oligodeoxynucleotides corresponding to the cis sequence will result in the attenuation of the authentic cis-trans interaction, leading to the removal of trans-factors from the endogenous cis-elements, with subsequent modulation of gene expression. Genetically modified vein grafts transfected with a decoy against E2F, an essential transcription factor in cell cycle progression, revealed apparent long-term potency in human patients. This review focuses on the future potential of gene therapy for the treatment of cardiovascular disease.

HMG-CoA reductase inhibitor mobilizes bone marrow--derived endothelial progenitor cells

Endothelial progenitor cells (EPCs) have been isolated from circulating mononuclear cells in peripheral blood and shown to incorporate into foci of neovascularization, consistent with postnatal vasculogenesis. These circulating EPCs are derived from bone marrow and are mobilized endogenously in response to tissue ischemia or exogenously by cytokine stimulation. We show here, using a chemotaxis assay of bone marrow mononuclear cells in vitro and EPC culture assay of peripheral blood from simvastatin-treated animals in vivo, that the HMG-CoA reductase inhibitor, simvastatin, augments the circulating population of EPCs. Direct evidence that this increased pool of circulating EPCs originates from bone marrow and may enhance neovascularization was demonstrated in simvastatin-treated mice transplanted with bone marrow from transgenic donors expressing beta-galactosidase transcriptionally regulated by the endothelial cell-specific Tie-2 promoter. The role of Akt signaling in mediating effects of statin on EPCs is suggested by the observation that simvastatin rapidly activates Akt protein kinase in EPCs, enhancing proliferative and migratory activities and cell survival. Furthermore, dominant negative Akt overexpression leads to functional blocking of EPC bioactivity. These findings establish that augmented mobilization of bone marrow-derived EPCs through stimulation of the Akt signaling pathway constitutes a novel function for HMG-CoA reductase inhibitors.

Gene therapy for therapeutic angiogenesis in critically ischaemic lower limb - on the way to the clinic

Currently, no effective pharmacological treatment is available for vascularisation defects in lower limbs. Many patients presenting with persistent pain and ischaemic ulcers are not suitable candidates for surgical or endovascular approaches. Further refinement of the available methods will undoubtedly lead to a more active approach towards treatment of peripheral arterial occlusive disease (PAOD). Recently, therapeutic angiogenesis, in the form of recombinant growth factor administration or gene therapy, has emerged as a novel tool to treat these patients. However, improved gene transfer methods and better understanding of blood vessel formation are required to bring therapeutic angiogenesis to clinical practice. Here we review the clinical problem (PAOD), mechanisms of blood vessel formation (angiogenesis, vasculogenesis and arteriogenesis), experimental evidence and clinical trials for therapeutic angiogenesis in critically ischaemic lower limbs. Also, angiogenic growth factors, including vascular endothelial growth factors (VEGFs) and fibroblast growth factors (FGFs), delivery methods, and vectors for gene transfer in skeletal muscle, are discussed. In addition to vascular growth, gene transfer of growth factors may enhance regeneration, survival, and innervation of ischaemic skeletal muscle. Nitric oxide (NO) appears to be a key mediator in vascular homeostasis and growth, and a reduction in its production by age, hypercholesterolemia or diabetes leads to the impairment of ischaemic disorders.

HMG-CoA reductase inhibitors (statins) increase endothelial progenitor cells via the PI 3-kinase/Akt pathway

HMG-CoA reductase inhibitors (statins) have been developed as lipid-lowering drugs and are well established to reduce morbidity and mortality from coronary artery disease. Here we demonstrate that statins potently augment endothelial progenitor cell differentiation in mononuclear cells and CD34-positive hematopoietic stem cells isolated from peripheral blood. Moreover, treatment of mice with statins increased c-kit(+)/Sca-1(+)--positive hematopoietic stem cells in the bone marrow and further elevated the number of differentiated endothelial progenitor cells (EPCs). Statins induce EPC differentiation via the PI 3-kinase/Akt (PI3K/Akt) pathway as demonstrated by the inhibitory effect of pharmacological PI3K blockers or overexpression of a dominant negative Akt construct. Similarly, the potent angiogenic growth factor VEGF requires Akt to augment EPC numbers, suggesting an essential role for Akt in regulating hematopoietic progenitor cell differentiation. Given that statins are at least as potent as VEGF in increasing EPC differentiation, augmentation of circulating EPC might importantly contribute to the well-established beneficial effects of statins in patients with coronary artery disease.

Stroke genomics: approaches to identify, validate, and understand ischemic stroke gene expression

Sequencing of the human genome is nearing completion and biologists, molecular biologists, and bioinformatics specialists have teamed up to develop global genomic technologies to help decipher the complex nature of pathophysiologic gene function. This review will focus on differential gene expression in ischemic stroke. It will discuss inheritance in the broader stroke population, how experimental models of spontaneous stroke might be applied to humans to identify chromosomal loci of increased risk and ischemic sensitivity, and also how the gene expression induced by stroke is related to the poststroke processes of brain injury, repair, and recovery. In addition, we discuss and summarise the literature of experimental stroke genomics and compare several approaches of differential gene expression analyzes. These include a comparison of representational difference analysis we have provided using an experimental stroke model that is representative of stroke evolution observed most often in man, and a summary of available data on stroke differential gene expression. Issues regarding validation of potential genes as stroke targets, the verification of message translation to protein products, the relevance of the expression of neuroprotective and neurodestructive genes and their specific timings, and the emerging problems of handling novel genes that may be discovered during differential gene expression analyses will also be addressed.

Synergism between vascular endothelial growth factor and placental growth factor contributes to angiogenesis and plasma extravasation in pathological conditions

Vascular endothelial growth factor (VEGF) stimulates angiogenesis by activating VEGF receptor-2 (VEGFR-2). The role of its homolog, placental growth factor (PlGF), remains unknown. Both VEGF and PlGF bind to VEGF receptor-1 (VEGFR-1), but it is unknown whether VEGFR-1, which exists as a soluble or a membrane-bound type, is an inert decoy or a signaling receptor for PlGF during angiogenesis. Here, we report that embryonic angiogenesis in mice was not affected by deficiency of PlGF (Pgf-/-). VEGF-B, another ligand of VEGFR-1, did not rescue development in Pgf-/- mice. However, loss of PlGF impaired angiogenesis, plasma extravasation and collateral growth during ischemia, inflammation, wound healing and cancer. Transplantation of wild-type bone marrow rescued the impaired angiogenesis and collateral growth in Pgf-/- mice, indicating that PlGF might have contributed to vessel growth in the adult by mobilizing bone-marrow-derived cells. The synergism between PlGF and VEGF was specific, as PlGF deficiency impaired the response to VEGF, but not to bFGF or histamine. VEGFR-1 was activated by PlGF, given that anti-VEGFR-1 antibodies and a Src-kinase inhibitor blocked the endothelial response to PlGF or VEGF/PlGF. By upregulating PlGF and the signaling subtype of VEGFR-1, endothelial cells amplify their responsiveness to VEGF during the 'angiogenic switch' in many pathological disorders.

Therapeutic angiogenesis for ischemic cardiovascular disease

In animal models of ischemia, a large body of evidence indicates that administration of angiogenic growth factors, either as recombinant protein or by gene transfer, can augment nutrient perfusion through neovascularization. While many cytokines have angiogenic activity, the best studied both in animal models and clinical trials are vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). Clinical trials of therapeutic angiogenesis in patients with end-stage coronary artery disease have shown large increases in exercise time and marked reductions in symptoms of angina, as well as objective evidence of improved perfusion and left ventricular function. Larger scale placebo-controlled trials have been limited to intracoronary and intravenous administration of recombinant protein, and have not yet shown significant improvement in either exercise time or angina when compared to placebo. Larger scale placebo-controlled studies of gene transfer are in progress. Future clinical studies will be required to determine the optimal dose, formulation, route of administration and combinations of growth factors, as well as the requirement for endothelial progenitor cell or stem cell supplementation, to provide effective and safe therapeutic myocardial angiogenesis.

Immunotherapeutic gene transfer into muscle

Immuno-gene therapy can be advantageously performed with nonviral approaches. Genes that encode regulatory cytokines or inflammatory cytokine inhibitors can be delivered intramuscularly and expressed for weeks or months. This type of gene transfer into muscle has been shown to ameliorate several autoimmune diseases and is relevant to the development of effective DNA vaccines in autoimmune diseases, infectious diseases and cancer.