Mechanical, cellular, and molecular factors interact to modulate circulating endothelial cell progenitors

Mechanical stretch: physiological and pathological implications for human vascular endothelial cells

Vascular endothelial cells are subjected to hemodynamic forces such as mechanical stretch due to the pulsatile nature of blood flow. Mechanical stretch of different intensities is detected by mechanoreceptors on the cell surface which enables the conversion of external mechanical stimuli to biochemical signals in the cell, activating downstream signaling pathways. This activation may vary depending on whether the cell is exposed to physiological or pathological stretch intensities. Substantial stretch associated with normal physiological functioning is important in maintaining vascular homeostasis as it is involved in the regulation of cell structure, vascular angiogenesis, proliferation and control of vascular tone. However, the elevated pressure that occurs with hypertension exposes cells to excessive mechanical load, and this may lead to pathological consequences through the formation of reactive oxygen species, inflammation and/or apoptosis. These processes are activated by downstream signaling through various pathways that determine the fate of cells. Identification of the proteins involved in these processes may help elucidate novel mechanisms involved in vascular disease associated with pathological mechanical stretch and could provide new insight into therapeutic strategies aimed at countering the mechanisms’ negative effects.

Chronic endurance exercise affects paracrine action of CD31+ and CD34+ cells on endothelial tube formation

We aimed to determine if chronic endurance-exercise habits affected redox status and paracrine function of CD34(+) and CD34(-)/CD31(+) circulating angiogenic cells (CACs). Subjects were healthy, nonsmoking men and women aged 18-35 yr and categorized by chronic physical activity habits. Blood was drawn from each subject for isolation and culture of CD34(+) and CD34(-)/CD31(+) CACs. No differences in redox status were found in any group across either cell type. Conditioned media (CM) was generated from the cultured CACs and used in an in vitro human umbilical vein endothelial cell-based tube assay. CM from CD34(+) cells from inactive individuals resulted in tube structures that were 29% shorter in length (P < 0.05) and 45% less complex (P < 0.05) than the endurance-trained group. CD34(-)/CD31(+) CM from inactive subjects resulted in tube structures that were 26% shorter in length (P < 0.05) and 42% less complex (P < 0.05) than endurance-trained individuals. Proteomics analyses identified S100A8 and S100A9 in the CM. S100A9 levels were 103% higher (P < 0.05) and S100A8 was 97% higher in the CD34(-)/CD31(+) CM of inactive subjects compared with their endurance-trained counterparts with no significant differences in either protein in the CM of CD34(+) CACs as a function of training status. Recombinant S100A8/A9 treatment at concentrations detected in inactive subjects' CD34(-)/CD31(+) CAC CM also reduced tube formation (P < 0.05). These findings are the first, to our knowledge, to demonstrate a differential paracrine role in CD34(+) and CD34(-)/CD31(+) CACs on tube formation as a function of chronic physical activity habits and identifies a differential secretion of S100A9 by CD34(-)/CD31(+) CACs due to habitual exercise.

Thrombotic responses of endothelial outgrowth cells to protein-coated surfaces

There is significant clinical need for viable small-diameter vascular grafts. While there are many graft biomaterials in development, few have been clinically successful. Evaluation of grafts with a clinically relevant model is needed to drive development. This work examined extracellular matrix coatings on the thrombotic phenotype of endothelial outgrowth cells (EOCs). EOCs were tested on flat plates and tubular grafts. Flat plate studies examined collagen I, collagen IV, fibronectin and α-elastin coatings. EOCs attached or proliferated more readily on collagen I and fibronectin surfaces as determined by total DNA. The production of activated protein C (APC) by EOCs was also dependent on the surface coating, with collagen I and fibronectin displaying a higher activity than both collagen IV and α-elastin on flat plate studies. Based on these results, only collagen I and fibronectin coatings were tested on expanded polytetrafluoroethylene (ePTFE) in the ex vivo model. Tubular samples showed significantly greater tissue factor pathway inhibitor gene expression on collagen I than on fibronectin. Platelet adhesion was not significantly different, but EOCs on collagen I produced significantly lower APC than on fibronectin, suggesting that differences exist between the flat plate and tubular cultures. Overall, while the hemostatic phenotype of EOCs displayed some differences, cell responses were largely independent of the matrix coating. EOCs adhered strongly to both fibronectin- and collagen-I-coated ePTFE grafts under ex vivo (100 ml/min) flow conditions suggesting the usefulness of this clinically relevant cell source, testing modality, and shunt model for future work examining biomaterials and cell conditioning before implantation.

Effects of acute and chronic endurance exercise on intracellular nitric oxide and superoxide in circulating CD34⁺ and CD34⁻ cells

We investigated the influence of acute and chronic endurance exercise on levels of intracellular nitric oxide (NO), superoxide (O₂·⁻), and expression of genes regulating the balance between these free radicals in CD34⁺ and CD34⁻ peripheral blood mononuclear cells (PBMCs; isolated by immunomagnetic cell separation). Blood samples were obtained from age- and body mass index (BMI)-matched endurance-trained (n = 10) and sedentary (n = 10) men before and after 30 min of exercise at 75% maximal oxygen uptake (·VO(₂max)). Baseline levels of intracellular NO (measured by DAF-FM diacetate) and O₂·⁻ (measured by dihydroethidium) were 26% (P < 0.05) and 10% (P < 0.05) higher, respectively, in CD34⁺ PBMCs from the sedentary group compared with the endurance-trained group. CD34⁺ PBMCs from the sedentary group at baseline had twofold greater inducible nitric oxide synthase (iNOS) mRNA and 50% lower endothelial NOS (eNOS) mRNA levels compared with the trained group (P < 0.05). The baseline group difference in O₂·⁻ was eliminated by acute exercise. Experiments with apocynin indicated that the training-related difference in O₂·⁻ levels was explained by increased NADPH oxidase activity in the sedentary state. mRNA levels of additional angiogenic and antioxidant genes were consistent with a more angiogenic profile in CD34⁺ cells of trained subjects. CD34⁻ PBMCs, examined for exploratory purposes, also displayed a more angiogenic mRNA profile in trained subjects, with vascular endothelial growth factor (VEGF) and eNOS being more highly expressed in trained subjects. Overall, our data suggest an association between the sedentary state and increased nitro-oxidative stress in CD34⁺ cells.

Endothelial progenitor cells (EPCs) mobilized and activated by neurotrophic factors may contribute to pathologic neovascularization in diabetic retinopathy

Diabetic retinopathy is characterized by pathological retinal neovascularization. Accumulating evidence has indicated that high levels of circulating endothelial progenitor cells (EPCs) are an important risk factor for neovascularization. Paradoxically, the reduction and dysfunction of circulating EPCs has been extensively reported in diabetic patients. We hypothesized that EPCs are differentially altered in the various vasculopathic complications of diabetes mellitus, exhibiting distinct behaviors in terms of angiogenic response to ischemia and growth factors and potentially playing a potent role in motivating vascular precursors to induce pathological neovascularization. Circulating levels of EPCs from diabetic retinopathy patients were analyzed by flow cytometry and by counting EPC colony-forming units, and serum levels of neurotrophic factors were measured by enzyme-linked immunosorbent assay. We found increased levels of nerve growth factor and brain-derived neurotrophic factor in the blood of diabetic retinopathy patients; this increase was correlated with the levels of circulating EPCs. In addition, we demonstrated that retinal cells released neurotrophic factors under hypoxic conditions to enhance EPC activity in vitro and to increase angiogenesis in a mouse ischemic hindlimb model. These results suggest that neurotrophic factors may induce neoangiogenesis through EPC activation, leading to the pathological retinal neovascularization. Thus, we propose that neovascularization in the ischemic retina might be regulated by overexpression of neurotrophic factors.

Endothelial progenitor cells, late stent thrombosis and delayed re-endothelialisation

A decade ago, the description of a primitive novel cell type capable of differentiating into cells expressing a mature endothelial cell -phenotype and their capacity to incorporate into regions of active angiogenesis, witnessed the emergence of endothelial progenitor cell (EPC) biology1. The development and maturation of this new concept in vascular biology has resulted in numerous studies describing the role of EPCs in a myriad of disease states where abnormalities of the vasculature have been implicated. Thus, from pre-eclampsia to pulmonary hypertension, erythropoietin administration to erectile dysfunction and cancer to coronary disease the discovery of EPCs has added greatly to the understanding of basic pathophysiology. However, it is in the study of coronary artery -disease where this paradigm shift has had greatest impact, not only regarding basic disease mechanisms, but in the rapid translation of these findings into a clinical context. The purpose of this review is to outline the current understanding of the EPC phenotypes and their relationship with risk factors for coronary disease. In addition, the potential problems of EPC dysfunction and its impact on percutaneous intervention will be appraised together with both pharmacological and stent based strategies to augment EPC -number and function.

Biology of bone marrow-derived endothelial cell precursors

Over the past decade, the old idea that the bone marrow contains endothelial cell precursors has become an area of renewed interest. While some still believe that there are no endothelial precursors in the blood, even among those who do, there is no consensus as to what they are or what they do. In this review, we describe the problems in identifying endothelial cells and conclude that expression of endothelial nitric oxide synthase may be the most reliable antigenic indicator of the phenotype. The evidence for two different classes of endothelial precursors is also presented. We suggest that, though there is no single endothelial cell precursor, we may be able to use these phenotypic variations to our advantage in better understanding their biology. We also discuss how a variety of genetic, epigenetic, and methodological differences can account for the seemingly contradictory findings on the physiological relevance of bone marrow-derived precursors in normal vascular maintenance and in response to injury. Data on the impact of tumor type and location on the contribution of bone marrow-derived cells to the tumor vasculature are also presented. These data provide hope that we may ultimately be able to predict those tumors in which bone marrow-derived cells will have a significant contribution and design therapies accordingly. Finally, factors that regulate bone marrow cell recruitment to and function in the endothelium are beginning to be identified, and several of these, including stromal derived factor 1, monocyte chemoattractant factor-1, and vascular endothelial growth factor are discussed.

Stem cells for the ischaemic heart

The discovery of adult progenitor cells capable of generating new vascular and myocardial tissue offers the promise of salvage of ischaemically threatened or irreversibly damaged cardiac tissue. Not surprisingly, great interest has focused on the use of a variety of cell types to treat both acute myocardial infarction and chronic ischaemic heart disease. This review focuses on the treatment of these two categories of disease, the cell types being considered, our understanding of timing and methods of cellular administration, and possible mechanisms of myocardial salvage.

Obese diabetic mouse environment differentially affects primitive and monocytic endothelial cell progenitors

Two classes of adult bone marrow-derived endothelial cell (EC) progenitors have been described, primitive hematopoietic stem cell-related cells and monocytic cells. Both differentiate into ECs and promote vascular growth in vivo but have distinct characteristics. Despite the association of obesity and type 2 diabetes with cardiovascular disease, their effects on primitive EC progenitors (prECPs) have not been examined, and the limited data on monocytic EC progenitors are conflicting. We investigated functional parameters of primitive and monocytic EC progenitors from obese diabetic (Lepr(db)) mice. The viability, proliferation, and differentiation of EC progenitors were unaffected in Lepr(db) cell cultures under basal condition. However, Lepr(db)-derived prECPs, but not monocytic EC progenitors, were less able to cope with hypoxia and oxidative stress, conditions likely present when EC progenitors are most needed. Intrinsic prECP dysfunction was also apparent in vivo. Whereas injection of nondiabetic prECPs promoted vascularization of skin wounds, Lepr(db)-derived progenitors inhibited it in nondiabetic mice. Additionally, although treatment with Lepr(db)-derived prECPs did not significantly reduce blood flow restoration to ischemic limbs, it resulted in increased tissue necrosis and autoamputation. Thus, type 2 diabetes coupled with obesity seems to induce intrinsic EC progenitor dysfunction that is exacerbated by stress. prECPs are more affected than monocytic progenitors, exhibiting a reduced ability to survive or proliferate. The proangiogenic phenotype of prECPs also seems to convert to an antiangiogenic phenotype in obese diabetic mice. These data suggest that therapies involving prECPs or stem-like cells in diabetic patients may be inadvisable at this time.

Differential healing activities of CD34+ and CD14+ endothelial cell progenitors

OBJECTIVE

Peripheral blood contains primitive (stem cell-like) and monocytic-like endothelial cell progenitors. Diabetes apparently converts these primitive progenitors, from a pro-angiogenic to anti-angiogenic phenotype. Monocytic progenitors seem to be less affected by diabetes, but potential pro-angiogenic activities of freshly isolated monocytic progenitors remain unexplored. We compared the ability of primitive and monocytic endothelial cell progenitors to stimulate vascular growth and healing in diabetes and investigated potential molecular mechanisms through which the cells mediate their in vivo effects.

METHODS AND RESULTS

Human CD34+ primitive progenitors and CD14+ monocytic progenitors were injected locally into the ischemic limbs of diabetic mice. CD14+ cell therapy improved healing and vessel growth, although not as rapidly or effectively as CD34+ cell treatment. Western blot analysis revealed that cell therapy modulated expression of molecules in the VEGF, MCP-1, and angiopoietin pathways.

CONCLUSIONS

Injection of freshly isolated circulating CD14+ cells improves healing and vascular growth indicating their potential for use in acute clinical settings. Importantly, CD14+ cells could provide a therapeutic option for people with diabetes, the function of whose CD34+ cells may be compromised. At least some progenitor-induced healing probably is mediated through increased sensitivity to VEGF and increases in MCP-1, and possibly modulation of angiopoietins.

Time to abandon dogma: CD14 is expressed by non‐myeloid lineage cells

CD14 is a pattern recognition receptor; its important role in innate immunity is reviewed here. Since its discovery and subsequent classification at the first leucocyte typing workshop in 1982, CD14 has been thought of as a leucocyte differentiation antigen. However, it has become clear that CD14 is also expressed by many non-myeloid cells, and the evidence for this is presented. The possible role of the presence of low copy number CD14 on non-myeloid cells is discussed. It is time to acknowledge CD14 as an ubiquitous molecule and abandon the position that it is expressed by myeloid cells alone.

Differentiation and Proliferation of Endothelial Progenitor Cells from Canine Peripheral Blood Mononuclear Cells1,2

BACKGROUND

The isolation, differentiation, and expansion of endothelial progenitor cells (EPCs) from peripheral blood have potential applicability in areas of therapeutic neovascularization, vascular repair, and tissue engineering. The purpose of the current study was to elucidate a simple method of isolation and differentiation of EPCs by defining the endothelial morphology, surface marker expression, and proliferative capacity of EPC outgrowth from canine peripheral blood mononuclear cells (PBMCs).

MATERIALS AND METHODS

PBMCs were isolated from fresh canine blood and cultured in fibronectin-coated plates in which EPCs were identified from cell morphology and outgrowth characteristics. Cell surface markers were determined with flow cytometry analysis to identify differentiation of cultured and subcultured colonies. A hematologic counter with phase contrast microscopy was used to study cell growth curves of EPCs as compared with mature human coronary artery endothelial cells.

RESULTS

During the first week of canine PBMC culture, cells were morphologically round and varied in size, but in the course of the second and third week of culture, the cells, respectively, became spindle-shaped and displayed an endothelium-like cobblestone morphology with outgrowth. CD34 was significantly decreased at 21 days as compared with 7 days culture (36.04% to 21.37%), whereas vWF (from 77.26% to 96.37%) and eNOS (from 0% to 14.97%) were significantly increased. VEGFR-2 was slightly increased, and P1H12 (CD146) was unchanged. Subcultured canine EPCs displayed a higher proliferation rate as compared to mature human coronary artery endothelial cells in the same culture conditions.

CONCLUSIONS

These data demonstrate that canine EPCs can be isolated and cultured from the canine PBMC fraction. These outgrowth cells displayed characteristics of endothelial morphology with endothelial cell-specific surface markers. Furthermore, it was revealed that canine EPCs have a greater growth potential as compared to mature endothelial cells. This study suggests that PBMCs could be used as a source of EPCs for potential applications in tissue engineering and vascular therapy.

A novel role for vascular endothelial growth factor as an autocrine survival factor for embryonic stem cells during hypoxia

Vascular endothelial growth factor (VEGF) is best known for its angiogenic activity on endothelial cells, but it also affects neurons, pneumocytes, and other mature cell types as well as endothelial, neural, and hematopoietic progenitors. Here, we examined its effect on pluripotential embryonic stem (ES) cells under hypoxic stress. ES cells were found to produce VEGF and to express VEGF receptor-2 and neuropilin-1 (Nrp-1), a VEGF165 isoform-specific receptor. During hypoxia, expression levels of VEGF, Flk-1, and Nrp-1 were elevated. Inhibition or targeted gene inactivation of VEGF increased ES cell apoptosis during prolonged hypoxia (48 h) by about 10-fold. The survival activity of VEGF was specific since inhibition of other growth factors (including basic fibroblast growth factor, epidermal growth factor, insulin-like growth factor, platelet-derived growth factor, and placental growth factor) had no effect. Neuropilin-1 was involved in the VEGF-survival activity since overexpression of Nrp-1 decreased hypoxia-induced apoptosis about 3-fold. The hypoxia-response element, via which hypoxia-inducible transcription factors up-regulate VEGF expression under hypoxic conditions, was critical since targeted deletion of this element in the VEGF promoter enhanced hypoxia-induced ES cell apoptosis to the same extent as VEGF inhibition or gene inactivation. Thus, VEGF plays a critical role in survival of ES cells during prolonged hypoxia.

Adult Bone Marrow-Derived Hemangioblasts, Endothelial Cell Progenitors, and EPCs

Long before their existence was proven, work with blood islands pointed to the existence of hemangioblasts in the embryo, and it was widely accepted that such cells existed. In contrast, though evidence for adult hemangioblasts appeared at least as early as 1932, until quite recently, it was commonly assumed that there were no adult hemangioblasts. Over the past decade, these views have changed, and it is now generally accepted that a subset of bone marrow cells or their progeny can and do function as adult hemangioblasts. This chapter will examine the basic biology of bone marrow-derived hemangioblasts and endothelial cell progenitors (angioblasts) and the relationship of these adult cells to their embryonic counterparts. Efforts to define the endothelial cell progenitor phenotype will also be discussed, though to date, there is no consensus on the definitive adult phenotype, probably because there are multiple phenotypes and because the cells are plastic. Also examined are the putative roles of bone marrow-derived cells in vascular homeostasis and repair, including both their ability to differentiate and contribute directly to vascular repair, as well as to promote vascular growth by secreting pro-angiogenic factors. Finally, the use of bone marrow cells as therapeutic tools will be addressed.