Cell therapies for therapeutic angiogenesis: back to the bench

Therapeutic angiogenesis in critical limb ischemia

Critical limb ischemia (CLI) is a severe form of peripheral artery disease associated with high morbidity and mortality. The primary therapeutic goals in treating CLI are to reduce the risk of adverse cardiovascular events, relieve ischemic pain, heal ulcers, prevent major amputation, and improve quality of life (QoL) and survival. These goals may be achieved by medical therapy, endovascular intervention, open surgery, or amputation and require a multidisciplinary approach including pain management, wound care, risk factors reduction, and treatment of comorbidities. No-option patients are potential candidates for the novel angiogenic therapies. The application of genetic, molecular, and cellular-based modalities, the so-called therapeutic angiogenesis, in the treatment of arterial obstructive diseases has not shown consistent efficacy. This article summarizes the current status related to the management of patients with CLI and discusses the current findings of the emerging modalities for therapeutic angiogenesis.

Human pericytes for ischemic heart repair

Human microvascular pericytes (CD146(+)/34(-)/45(-)/56(-)) contain multipotent precursors and repair/regenerate defective tissues, notably skeletal muscle. However, their ability to repair the ischemic heart remains unknown. We investigated the therapeutic potential of human pericytes, purified from skeletal muscle, for treating ischemic heart disease and mediating associated repair mechanisms in mice. Echocardiography revealed that pericyte transplantation attenuated left ventricular dilatation and significantly improved cardiac contractility, superior to CD56+ myogenic progenitor transplantation, in acutely infarcted mouse hearts. Pericyte treatment substantially reduced myocardial fibrosis and significantly diminished infiltration of host inflammatory cells at the infarct site. Hypoxic pericyte-conditioned medium suppressed murine fibroblast proliferation and inhibited macrophage proliferation in vitro. High expression by pericytes of immunoregulatory molecules, including interleukin-6, leukemia inhibitory factor, cyclooxygenase-2, and heme oxygenase-1, was sustained under hypoxia, except for monocyte chemotactic protein-1. Host angiogenesis was significantly increased. Pericytes supported microvascular structures in vivo and formed capillary-like networks with/without endothelial cells in three-dimensional cocultures. Under hypoxia, pericytes dramatically increased expression of vascular endothelial growth factor-A, platelet-derived growth factor-β, transforming growth factor-β1 and corresponding receptors while expression of basic fibroblast growth factor, hepatocyte growth factor, epidermal growth factor, and angiopoietin-1 was repressed. The capacity of pericytes to differentiate into and/or fuse with cardiac cells was revealed by green fluorescence protein labeling, although to a minor extent. In conclusion, intramyocardial transplantation of purified human pericytes promotes functional and structural recovery, attributable to multiple mechanisms involving paracrine effects and cellular interactions.

Molecular and cellular mechanisms of bone morphogenetic proteins and activins in the skin: potential benefits for wound healing

Bone morphogenetic proteins (BMPs) and activins are phylogenetically conserved proteins, belonging to the transforming growth factor-β superfamily, that signal through the phosphorylation of receptor-regulated Smad proteins, activating different cell responses. They are involved in various steps of skin morphogenesis and wound repair, as can be evidenced by the fact that their expression is increased in skin injuries. BMPs play not only a role in bone regeneration but are also involved in cartilage, tendon-like tissue and epithelial regeneration, maintain vascular integrity, capillary sprouting, proliferation/migration of endothelial cells and angiogenesis, promote neuron and dendrite formation, alter neuropeptide levels and are involved in immune response modulation, at least in animal models. On the other hand, activins are involved in wound repair through the regulation of skin and immune cell migration and differentiation, re-epithelialization and granulation tissue formation, and also promote the expression of collagens by fibroblasts and modulate scar formation. This review aims at enunciating the effects of BMPs and activins in the skin, namely in skin development, as well as in crucial phases of skin wound healing, such as inflammation, angiogenesis and repair, and will focus on the effects of these proteins on skin cells and their signaling pathways, exploring the potential therapeutic approach of the application of BMP-2, BMP-6 and activin A in chronic wounds, particularly diabetic foot ulcerations.

Angiogenic therapy for cardiac repair based on protein delivery systems

Cardiovascular diseases remain the first cause of morbidity and mortality in the developed countries and are a major problem not only in the western nations but also in developing countries. Current standard approaches for treating patients with ischemic heart disease include angioplasty or bypass surgery. However, a large number of patients cannot be treated using these procedures. Novel curative approaches under investigation include gene, cell, and protein therapy. This review focuses on potential growth factors for cardiac repair. The role of these growth factors in the angiogenic process and the therapeutic implications are reviewed. Issues including aspects of growth factor delivery are presented in relation to protein stability, dosage, routes, and safety matters. Finally, different approaches for controlled growth factor delivery are discussed as novel protein delivery platforms for cardiac regeneration.

Prospective biomarkers of stem cells of human endometrium and fallopian tube compared with bone marrow

The applicability of stem cells from the human endometrium and fallopian tube for regeneration is a fascinating area of research because of the role of these cells in dynamic tissue remodelling and their cyclical regenerative property during the menstrual cycle and pregnancy. Nevertheless, studies on the identity of biomarkers of these stem cells are limited and need to be extended. The present study has aimed at exploring the tissue-specific biomarkers of stem cells derived from the human endometrium and fallopian tube compared with those from bone marrow. Cells were isolated from human endometrium and fallopian tubes and characterized for biomarkers, including CD34, CD133, CD117, CD90, CD105, CD73, nestin, CD29, CD44, CD31, CD54, CD166, CD106, CD49d, CD45, ABCG2, SSEA4, OCT4, SOX2, CD140b and CD146, by flowcytometry. Both endometrium and fallopian tube sources exhibited positivity over a wide range of markers, as did bone marrow. In particular, they exhibited pluripotency, perivascular and mesenchymal stem cell markers and cell adhesion molecules, thereby suggesting their relevance in tissue repair and regeneration. Overall, the results of this study provide evidence for the presence of stem cells in the human endometrium and fallopian tube, which could thus represent additional stem cell sources for regenerative medicine.

Soluble CD40 ligand impairs the anti-platelet function of peripheral blood angiogenic outgrowth cells via increased production of reactive oxygen species

Adult peripheral blood angiogenic early outgrowth cells (EOCs), also known as early endothelial progenitor cells, interact with other blood and vascular cells and may regulate atherothrombosis. We have previously shown that endothelial progenitor cells inhibit platelet function and thrombus formation. The CD40L/CD40 axis is a thrombo- inflammatory mediator that affects platelet and endothelial functions. It has been shown that EOCs express CD40, whereas platelets represent the major source of its soluble ligand (sCD40L), which impairs EOC function.We aimed to test the hypothesis that the sCD40L/CD40 axis affects the anti-platelet function of EOCs. Human peripheral blood mononuclear cell-derived EOCs in culture inhibited platelet aggregation. Pre-treatment of EOCs with sCD40L reduced their inhibitory effect on platelet aggregation in a CD40-dependent manner. EOCs viability and release of the anti-aggregating agents, prostacyclin and nitric oxide, were not affected by sCD40L. However, production of reactive oxygen species (ROS) was increased in sCD40L-treated EOCs. Blockade of ROS reversed the effects of sCD40L-treated EOCs on platelet aggregation. This study reveals that the sCD40L/CD40 axis impairs the anti-platelet properties of EOCs through increased production of ROS. These data may explain the link between elevated levels of sCD40L, impaired activity of EOCs and enhanced platelet reactivity, and consequently the occurrence of atherothrombotic disease.

Endothelial reconstitution by CD34+ progenitors derived from baboon embryonic stem cells

In this study, we used a large non-human primate model, the baboon, to establish a step-wise protocol to generate CD34+ endothelial progenitor cells (EPCs) from embryonic stem cells (ESCs) and to demonstrate their reparative effects. Baboon ESCs were sequentially differentiated from embryoid body cultures for 9 days and then were specified into EPCs by culturing them in monolayer for 12 days. The resulting EPCs expressed CD34, CXCR4 and UEA-1, but neither CD31 nor CD117. The EPCs were able to form intact lumen structures when seeded on Matrigel, took up Dil-LDL, and responded to TNF-α. Angioblasts specified in EGM-2 medium and ECGS medium had 6.41 ± 1.16% (n = 3) and 9.32 ± 3.73% CD34+ cells (n = 3). The efficiency of generating CD34+ EPCs did not differ significantly from ECGS to EGM-2 culture media, however, angioblasts specified in ECGS medium expressed a higher percentage of CD34+/CXCR4+ cells (3.49 ± 1.32%, n = 3) than those specified in EGM-2 medium (0.49 ± 0.52%, n = 3). To observe their reparative capacity, we purified CD34+ progenitors after specification by EGM-2 medium; inoculated fluorescently labelled CD34+ EPCs into an arterial segment denuded of endothelium in an ex vivo system. After 14 days of ex vivo culture, the grafted cells had attached and integrated to the denuded surface; in addition, they had matured further and expressed terminally differentiated endothelial markers including CD31 and CD146. In conclusion, we have proved that specified CD34+ EPCs are promising therapeutic agents for repairing damaged vasculature.

The effect of a heparin-based coacervate of fibroblast growth factor-2 on scarring in the infarcted myocardium

Effective delivery of exogenous angiogenic growth factors can provide a new therapy for ischemic diseases. However, clinical translation of growth factor therapies faces multiples challenges; the most significant one is the short half-life of the naked protein. We use heparin and a nontoxic polycation to form an injectable coacervate that protects growth factors and preserves their bioactivities. Here we report the effectiveness of fibroblast growth factor-2 (FGF2) coacervate in reducing scar burden in a mouse myocardial infarction model. The coacervate provides spatial and temporal control of the release of heparin-binding proteins. Coacervate treated animals show lower level of inflammation, fibrosis and cardiomyocyte death in the infarcted myocardium. Histological evaluation indicates that FGF2 coacervate significantly increases the number of endothelial and mural cells and results in stable capillaries and arterioles to at least 6 weeks post injection. Echocardiographic assessment shows that FGF2 coacervate promotes cardiac contractibility and inhibits ventricular dilation, suggesting that the improvement at the tissue level leads to better cardiac functions. On the contrary, identical dosage of free FGF2 shows no statistical difference from saline or vehicle control in histological or functional assessment. Overall, injection of FGF2 coacervate ameliorated the ischemic injury caused by myocardial infarction. The promising data in rodent warrant further examination of the potential of clinical translation of this technology.

Stem cell recruitment and angiogenesis of neuropeptide substance P coupled with self-assembling peptide nanofiber in a mouse hind limb ischemia model

For the successful treatment of ischemia, it is important to resupply sufficient blood into ischemic regions by inducing angiogenesis. Many stem cell transplantation studies have been reported to enhance angiogenesis, especially those relating to mesenchymal stem cells (MSCs); however cell transplantation has a number of limitations, such as the low rate of cell survival and donor cell shortage. In this study, we developed bioactive peptides by immobilizing substance P into self-assembling peptides, and their MSCs recruiting ability and therapeutic effects were evaluated by using ischemic hind limb models. Limb ischemia was produced in athymic mice, and 1% (wt/vol) peptides were injected into ischemic sites (n = 6 in each group: ischemia, substance P, RADA16-II, RADA16-II + substance P, and RADA16-II + RADA-SP (bioactive peptides)). The tissues were harvested for histological analysis and tissue perfusion measurement at 1, 3, 7, and 28 days after injection. We observed that bioactive peptides assembled themselves (<10 nm nanofibers) and formed 3-dimensional (3D) microenvironments within ischemic regions. In the animal study, it was observed that by applying bioactive peptides, substance P continued to be released at 28 days, and consequently, MSCs were successfully recruited into ischemic regions. Bioactive peptides could prevent fibrosis, promote neovascularization, enhance tissue perfusion, and prevent limb salvages. Our results demonstrated that bioactive peptides are one of the most powerful tools for the treatment of ischemia, through their recruitment of autologous MSCs and promotion of angiogenesis without cells transplantation.

Characterization of a distinct population of circulating human non-adherent endothelial forming cells and their recruitment via intercellular adhesion molecule-3

Circulating vascular progenitor cells contribute to the pathological vasculogenesis of cancer whilst on the other hand offer much promise in therapeutic revascularization in post-occlusion intervention in cardiovascular disease. However, their characterization has been hampered by the many variables to produce them as well as their described phenotypic and functional heterogeneity. Herein we have isolated, enriched for and then characterized a human umbilical cord blood derived CD133⁺ population of non-adherent endothelial forming cells (naEFCs) which expressed the hematopoietic progenitor cell markers (CD133, CD34, CD117, CD90 and CD38) together with mature endothelial cell markers (VEGFR2, CD144 and CD31). These cells also expressed low levels of CD45 but did not express the lymphoid markers (CD3, CD4, CD8) or myeloid markers (CD11b and CD14) which distinguishes them from ‘early’ endothelial progenitor cells (EPCs). Functional studies demonstrated that these naEFCs (i) bound Ulex europaeus lectin, (ii) demonstrated acetylated-low density lipoprotein uptake, (iii) increased vascular cell adhesion molecule (VCAM-1) surface expression in response to tumor necrosis factor and (iv) in co-culture with mature endothelial cells increased the number of tubes, tubule branching and loops in a 3-dimensional in vitro matrix. More importantly, naEFCs placed in vivo generated new lumen containing vasculature lined by CD144 expressing human endothelial cells (ECs). Extensive genomic and proteomic analyses of the naEFCs showed that intercellular adhesion molecule (ICAM)-3 is expressed on their cell surface but not on mature endothelial cells. Furthermore, functional analysis demonstrated that ICAM-3 mediated the rolling and adhesive events of the naEFCs under shear stress. We suggest that the distinct population of naEFCs identified and characterized here represents a new valuable therapeutic target to control aberrant vasculogenesis.

Systematic assessment in an animal model of the angiogenic potential of different human cell sources for therapeutic revascularization

INTRODUCTION

Endothelial progenitor cells (EPC) capable of initiating or augmenting vascular growth were recently identified within the small population of CD34-expressing cells that circulate in human peripheral blood and which are considered hematopoietic progenitor cells (HPC). Soon thereafter human HPC began to be used in clinical trials as putative sources of EPC for therapeutic vascular regeneration, especially in myocardial and critical limb ischemias. However, unlike HPC where hematopoietic efficacy is related quantitatively to CD34+ cell numbers implanted, there has been no consensus on how to measure EPC or how to assess cellular graft potency for vascular regeneration. We employed an animal model of spontaneous neovascularization to simultaneously determine whether human cells incorporate into new vessels and to quantify the effect of different putative angiogenic cells on vascularization in terms of number of vessels generated. We systematically compared competence for therapeutic angiogenesis in different sources of human cells with putative angiogenic potential, to begin to provide some rationale for optimising cell procurement for this therapy.

METHODS

Human cells employed were mononuclear cells from normal peripheral blood and HPC-rich cell sources (umbilical cord blood, mobilized peripheral blood, bone marrow), CD34+ enriched or depleted subsets of these, and outgrowth cell populations from these. An established sponge implant angiogenesis model was adapted to determine the effects of different human cells on vascularization of implants in immunodeficient mice. Angiogenesis was quantified by vessel density and species of origin by immunohistochemistry.

RESULTS

CD34+ cells from mobilized peripheral blood or umbilical cord blood HPC were the only cells to promote new vessel growth, but did not incorporate into vessels. Only endothelial outgrowth cells (EOC) incorporated into vessels, but these did not promote vessel growth.

CONCLUSIONS

These studies indicate that, since EPC are very rare, any benefit seen in clinical trials of HPC in therapeutic vascular regeneration is predominantly mediated by indirect proangiogenic effects rather than through direct incorporation of any rare EPC contained within these sources. It should be possible to produce autologous EOC for therapeutic use, and evaluate the effect of EPC distinct from, or in synergy with, the proangiogenic effects of HPC therapies.

Targets and delivery methods for therapeutic angiogenesis in peripheral artery disease

Therapeutic angiogenesis utilizing genetic and cellular modalities in the treatment of arterial obstructive diseases continues to evolve. This is, in part, because the mechanism of vasculogenesis, angiogenesis, and arteriogenesis (the three processes by which the body responds to obstruction of large conduit arteries) is a complex process that is still under investigation. To date, the majority of human trials utilizing molecular, genetic, and cellular modalities for therapeutic angiogenesis in the treatment of peripheral artery disease (PAD) have not shown efficacy. Consequently, the current available knowledge is yet to be translated into novel therapeutic approaches for the treatment of PAD. The aim of this review is to discuss relevant scientific and clinical advances in therapeutic angiogenesis and their potential application in the treatment of ischemic diseases of the peripheral arteries. Additionally, this review article discusses past and recent developments, such as some unconventional approaches that have the potential to be applied as therapeutic targets. The article also includes advances in the delivery of genetic, cellular, and bioactive endothelial growth factors.

Enhancement of cell-based therapeutic angiogenesis using a novel type of injectable scaffolds of hydroxyapatite-polymer nanocomposite microspheres

BACKGROUND

Clinical trials demonstrate the effectiveness of cell-based therapeutic angiogenesis in patients with severe ischemic diseases; however, their success remains limited. Maintaining transplanted cells in place are expected to augment the cell-based therapeutic angiogenesis. We have reported that nano-hydroxyapatite (HAp) coating on medical devices shows marked cell adhesiveness. Using this nanotechnology, HAp-coated poly(l-lactic acid) (PLLA) microspheres, named nano-scaffold (NS), were generated as a non-biological, biodegradable and injectable cell scaffold. We investigate the effectiveness of NS on cell-based therapeutic angiogenesis.

METHODS AND RESULTS

Bone marrow mononuclear cells (BMNC) and NS or control PLLA microspheres (LA) were intramuscularly co-implanted into mice ischemic hindlimbs. When BMNC derived from enhanced green fluorescent protein (EGFP)-transgenic mice were injected into ischemic muscle, the muscle GFP level in NS+BMNC group was approximate fivefold higher than that in BMNC or LA+BMNC groups seven days after operation. Kaplan-Meier analysis demonstrated that NS+BMNC markedly prevented hindlimb necrosis (P<0.05 vs. BMNC or LA+BMNC). NS+BMNC revealed much higher induction of angiogenesis in ischemic tissues and collateral blood flow confirmed by three-dimensional computed tomography angiography than those of BMNC or LA+BMNC groups. NS-enhanced therapeutic angiogenesis and arteriogenesis showed good correlations with increased intramuscular levels of vascular endothelial growth factor and fibroblast growth factor-2. NS co-implantation also prevented apoptotic cell death of transplanted cells, resulting in prolonged cell retention.

CONCLUSION

A novel and feasible injectable cell scaffold potentiates cell-based therapeutic angiogenesis, which could be extremely useful for the treatment of severe ischemic disorders.

Human blood-vessel-derived stem cells for tissue repair and regeneration

Multipotent stem/progenitor cells with similar developmental potentials have been independently identified from diverse human tissue/organ cultures. The increasing recognition of the vascular/perivascular origin of mesenchymal precursors suggested blood vessels being a systemic source of adult stem/progenitor cells. Our group and other laboratories recently isolated multiple stem/progenitor cell subsets from blood vessels of adult human tissues. Each of the three structural layers of blood vessels: intima, media, and adventitia has been found to include at least one precursor population, that is, myogenic endothelial cells (MECs), pericytes, and adventitial cells (ACs), respectively. MECs and pericytes efficiently regenerate myofibers in injured and dystrophic skeletal muscles as well as improve cardiac function after myocardial infarction. The applications of ACs in vascular remodeling and angiogenesis/vasculogenesis have been examined. Our recent finding that MECs and pericytes can be purified from cryogenically banked human primary muscle cell culture further indicates their potential applications in personalized regenerative medicine.

Effective mobilisation of bone marrow-derived cells through proteolytic activity: a new treatment strategy for age-related macular degeneration

Selective targeting of bone marrow-derived cells (BMCs) has been heralded as a promising avenue for age-related macular degeneration (AMD) therapeutics. Many researchers have demonstrated that the function of circulating BMCs is related to disease severity in patients with AMD. Transplanted BMCs are able to transdifferentiate into retina-specific cells to replace those lost due to damage or degeneration in the pathologic process of experimental models of AMD, which may provide beneficial effects in patients with AMD. However, a major barrier to transferring the use of BMCs into clinical practice is the limited quantity of BMCs in the peripheral circulation. Technology has not yet reached a stage where ex vivo-expanded BMCs can be routinely used for cell therapy. A feasible strategy to circumvent this issue of BMC scarcity is to increase the mobilisation of autologous BMCs from the patient's bone marrow into the blood circulation. Extensive studies have demonstrated that the SDF-1/CXCR4 axis is a key regulator for BMC mobilisation. Moreover, abrogation of the SDF-1/CXCR4 axis by proteolytic modification can efficiently increase BMC mobilisation. We speculate that BMC mobilisation by proteolytic enzymes may supply a sufficient amount of autologous cells to repair and regenerate injured and degenerated the retinal pigment epithelium (RPE), photoreceptors, or other retina-specific cells, which could prevent AMD progression. If the BMC mobilisation strategy is used to treat AMD, it may overcome the existing problems of transferring BMC-based therapy into the clinic and become a particularly feasible therapeutic approach for AMD.

Cell-based therapies for diabetic retinopathy

Autologous endothelial progenitor cell (EPC) populations represent a novel treatment for therapeutic revascularization and vascular repair for diabetic patients with complications including diabetic retinopathy. Current therapies are applicable to late-stage disease and carry significant side effects, whereas cell-based therapy may provide an alternative by repairing areas of vasodegeneration and reversing ischemia. However, EPCs from diabetic patients with vascular complications are dysfunctional. Moreover, the diabetic environment poses its own challenges and complicates the use of autologous EPCs. Before EPCs become the ideal "cell therapy," the optimal EPC must be determined, any functional dysfunction must be corrected prior to use, and the diabetic milieu will require modification to accept the EPCs. This review describes the rationale for harnessing the vascular reparative properties of EPCs with emphasis on the molecular and phenotypic nature of healthy EPCs, how diabetes alters them, and novel strategies to improve dysfunctional EPCs.

Vascular stem cells and ischaemic retinopathies

Retinal ischaemic disorders such as diabetic retinopathy and retinal vein occlusion are common. The hypoxia-related stimuli from oxygen-deprived neural and glial networks can drive expression of growth factors and cytokines which induce leakage from the surviving vasculature and/or pre-retinal and papillary neovascularisation. If left untreated, retinal vascular stasis, hypoxia or ischaemia can lead to macular oedema or fibro-vascular scar formation which are associated with severe visual impairment, and even blindness. Current therapies for ischaemic retinopathies include laser photocoagulation, injection of corticosteroids or VEGF-antibodies and vitreoretinal surgery, however they carry significant side effects. As an alternative approach, we propose that if reparative intra-retinal angiogenesis can be harnessed at the appropriate stage, ischaemia could be contained or reversed. This review provides evidence that reperfusion of ischaemic retina and suppression of sight-threatening sequelae is possible in both experimental and clinical settings. In particular, there is emphasis on the clinical potential for endothelial progenitor cells (EPCs) to promote vascular repair and reversal of ischaemic injury in various tissues including retina. Gathering evidence from an extensive published literature, we outline the molecular and phenotypic nature of EPCs, how they are altered in disease and provide a rationale for harnessing the vascular reparative properties of various cell sub-types. When some of the remaining questions surrounding the clinical use of EPCs are addressed, they may provide an exciting new therapeutic option for treating ischaemic retinopathies.

Factors Released from Embryonic Stem Cells Stimulate c-kit-FLK-1(+ve) Progenitor Cells and Enhance Neovascularization

We examined whether factors released from embryonic stem (ES) cells inhibit cardiac and vascular cell apoptosis and stimulate endogenous progenitor cells that enhance neovascularization with improved cardiac function. We generated and transplanted ES-conditioned medium (CM) in the infarcted heart to examine effects on cardiac and vascular apoptosis, activation of endogenous c-kit and FLK-1(+ve) cells, and their role in cardiac neovascularization. TUNEL, caspase-3 activity, immunohistochemistry, H&E, and Masson's trichrome stains were used to determine the effect of transplanted ES-CM on cardiac apoptosis and neovascularization. TUNEL staining and caspase-3 activity confirm significantly (p < 0.05) reduced apoptosis in MI+ES-CM compared with MI+ cell culture medium. Immunohistochemistry demonstrated increased (p < 0.05, 53%) c-kit(+ve) and FLK-1(+ve) positive cells, as well as increased (p < 0.05, 67%) differentiated CD31-positive cells in ES-CM groups compared with respective controls. Furthermore, significantly (p < 0.05) increased coronary artery vessels were observed in ES-CM transplanted hearts compared with control. Heart function was significantly improved following ES-CM transplantation. Next, we observed significantly increased (p < 0.05) levels of c-kit activation proteins (HGF and IGF-1), anti-apoptosis factors (IGF-1 and total antioxidants), and neovascularization protein (VEGF). In conclusion, we suggest that ES-CM following transplantation in the infarcted heart inhibits apoptosis, activates cardiac endogenous c-kit and FLK-1(+ve) cells, and differentiates them into endothelial cells (ECs) that enhances neovascularization with improved cardiac function.

Granulocyte colony-stimulating factor (G-CSF) depresses angiogenesis in vivo and in vitro: implications for sourcing cells for vascular regeneration therapy

SUMMARY BACKGROUND

The most common source of hematopoietic progenitor cells (HPCs) for hematopoietic reconstitution comprises granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood stem cells (PBSCs). It has been proposed that endothelial progenitor cells (EPCs) share precursors with HPCs, and that EPC release may accompany HPC mobilization to the circulation following G-CSF administration.

OBJECTIVE

To investigate EPC activity following HPC mobilization, and the direct effects of exogenous G-CSF administration on human umbilical vein endothelial cells (HUVECs) and endothelial outgrowth cells (EOCs), using in vitro and in vivo correlates of angiogenesis.

PATIENTS/METHODS

Heparinized venous blood samples were collected from healthy volunteers and from cord blood at parturition. G-CSF-mobilized samples were collected before administration, at apheresis harvest, and at follow-up. PBSCs were phenotyped by flow cytometry, and cultured in standard colony-forming unit (CFU)-EPC and EOC assays. The effect of exogenous G-CSF was investigated by addition of it to HUVECs and EOCs in standard tubule formation and aortic ring assays, and in an in vivo sponge implantation model.

RESULTS

Our data show that G-CSF mobilization of PBSCs produces a profound, reversible depression of circulating CFU-EPCs. Furthermore, G-CSF administration did not mobilize CD34+CD133- cells, which include precursors of EOCs. No EOCs were cultured from any mobilized PBSCs studied. Exogenous G-CSF inhibited CFU-EPC generation, HUVEC and EOC tubule formation, microvessel outgrowth, and implanted sponge vascularization in mice.

CONCLUSIONS

G-CSF administration depresses both endothelial cell angiogenesis and monocyte proangiogenic activity, and we suggest that any angiogenic benefit observed following implantation of cells mobilized by G-CSF may come only from a paracrine effect from HPCs.

Androgens, angiogenesis and cardiovascular regeneration

PURPOSE OF REVIEW

Striking sex differences exist not only in the incidence of cardiovascular disease, but also in the clinical outcomes. Although cardiovascular events occur earlier in men, in women, it appears they have poorer short-term and long-term outcomes following these events compared to men. Thus, intrinsic sex differences may exist not only in atherogenesis, but also with respect to cardiovascular adaptation/repair in response to ischemia and/or infarction. Angiogenesis, the growth of new blood vessels, is essential for organ development and is critical to cardiovascular repair/regeneration. Although the effect of estrogen on angiogenesis has been studied extensively, the role of androgens has remained largely unexplored.

RECENT FINDINGS

Multiple lines of evidence now suggest an important role for androgens in cardiovascular repair and regeneration. Studies suggest that androgens stimulate angiogenesis via vascular endothelial growth factor-related mechanisms and by the stimulation of erythropoietin production. Furthermore, endothelial progenitor cells, important in angiogenesis, appear to be hormonally regulated and an important target of androgen action.

SUMMARY

Given the age-related decline in androgens, the findings discussed here have implications for therapeutic angiogenesis and androgen replacement therapies in aging and hypogonadal men.

Vascular potential of human pluripotent stem cells

Cardiovascular disease is the number one cause of death and disability in the US. Understanding the biological activity of stem and progenitor cells, and their ability to contribute to the repair, regeneration and remodeling of the heart and blood vessels affected by pathological processes is an essential part of the paradigm in enabling us to achieve a reduction in related deaths. Both human embryonic stem (ES) cells and induced pluripotent stem (iPS) cells are promising sources of cells for clinical cardiovascular therapies. Additional in vitro studies are needed, however, to understand their relative phenotypes and molecular regulation toward cardiovascular cell fates. Further studies in translational animal models are also needed to gain insights into the potential and function of both human ES- and iPS-derived cardiovascular cells, and enable translation from experimental and preclinical studies to human trials.

Mechanisms of angiogenesis: role of hydrogen sulphide

1. Hydrogen sulphide (H(2)S) has recently been recognized as a gasotransmitter that regulates angiogenesis in vitro and in vivo under physiological and ischaemic conditions. 2. In the present review, the mechanisms underlying angiogenesis are summarized briefly and the most recent progress in H(2)S-induced angiogenesis in vivo and in vitro is described. The anti-angiogenic effects of garlic extracts, which may serve as substrates for H(2)S-generating enzymes in vivo, are also discussed. 3. Hydrogen sulphide increases cell growth, migration and the formation of tube-like structures in cultured endothelial cells. These effects are dependent on activation of the phosphatidylinositol 3-kinase-Akt-survivin signalling pathway. Neovascularization in vivo has also been demonstrated to be promoted in the mouse Matrigel plug assay, as well as in chicken chorioallantoic membranes. In a rat unilateral hindlimb ischaemic model, treatment with sodium hydrosulphide (NaHS), an H(2)S donor, promotes significant angiogenesis and improves regional blood flow. These effects may be mediated by interactions between upregulated vascular endothelial growth factor (VEGF) in skeletal muscle cells and VEGF receptor 2 and the downstream signalling element Akt in vascular endothelial cells. However, H(2)S does not exhibit a pro-angiogenic effect at a high concentrations/doses. 4. Based on the studies reviewed in the present article, we assume that, at physiologically relevant doses/concentrations, H(2)S/HS(-) promote angiogenesis at least partly via the VEGF signalling pathway. At high doses, H(2)S/HS(-) may act on additional cellular targets to evoke mechanisms that counteract the pro-angiogenic pathways. More studies need to be performed analysing the general interactions between H(2)S/HS(-) and other molecules, including other gasotransmitters, such as nitric oxide and carbon monoxide (CO).

Comparison between Culture Conditions Improving Growth and Differentiation of Blood and Bone Marrow Cells Committed to the Endothelial Cell Lineage

The aim of this study was to compare different cell sources and culture conditions to obtain endothelial progenitor cells (EPCs) with predictable antigen pattern, proliferation potential and in vitro vasculogenesis. Pig mononuclear cells were isolated from blood (PBMCs) and bone marrow (BMMCs). Mesenchymal stem cells (MSCs) were also derived from pig bone marrow. Cells were cultured on fibronectin in the presence of a high concentration of VEGF and low IGF-1 and FGF-2 levels, or on gelatin with a lower amount of VEGF and higher IGF-1 and FGF-2 concentrations. Endothelial commitment was relieved in almost all PBMCs and BMMCs irrespective of the protocol used, whilst MSCs did not express a reliable pattern of EPC markers under these conditions. BMMCs were more prone to expand on gelatin and showed a better viability than PBMCs. Moreover, about 90% of the BMMCs pre-cultured on gelatin could adhere to a hyaluronan-based scaffold and proliferate on it up to 3 days. Pre-treatment of BMMCs on fibronectin generated well-shaped tubular structures on Matrigel, whilst BMMCs exposed to the gelatin culture condition were less prone to form vessel-like structures. MSCs formed rough tubule-like structures, irrespective of the differentiating condition used. In a relative short time, pig BMMCs could be expanded on gelatin better than PBMCs, in the presence of a low amount of VEGF. BMMCs could better specialize for capillary formation in the presence of fibronectin and an elevated concentration of VEGF, whilst pig MSCs anyway showed a limited capability to differentiate into the endothelial cell lineage.

A sex-specific role for androgens in angiogenesis

Mounting evidence suggests that in men, serum levels of testosterone are negatively correlated to cardiovascular and all-cause mortality. We studied the role of androgens in angiogenesis, a process critical in cardiovascular repair/regeneration, in males and females. Androgen exposure augmented key angiogenic events in vitro. Strikingly, this occurred in male but not female endothelial cells (ECs). Androgen receptor (AR) antagonism or gene knockdown abrogated these effects in male ECs. Overexpression of AR in female ECs conferred androgen sensitivity with respect to angiogenesis. In vivo, castration dramatically reduced neovascularization of Matrigel plugs. Androgen treatment fully reversed this effect in male mice but had no effect in female mice. Furthermore, orchidectomy impaired blood-flow recovery from hindlimb ischemia, a finding rescued by androgen treatment. Our findings suggest that endogenous androgens modulate angiogenesis in a sex-dependent manner, with implications for the role of androgen replacement in men.