True Endothelial Progenitor Cells Research Articles

Endothelial Progenitors in the Tumor Microenvironment.

Tumor vascularization refers to the formation of new blood vessels within a tumor and is considered one of the hallmarks of cancer. Tumor vessels supply the tumor with oxygen and nutrients, required to sustain tumor growth and progression, and provide a gateway for tumor metastasis through the blood or lymphatic vasculature. Blood vessels display an angiocrine capacity of supporting the survival and proliferation of tumor cells through the production of growth factors and cytokines. Although tumor vasculature plays an essential role in sustaining tumor growth, it represents at the same time an essential way to deliver drugs and immune cells to the tumor. However, tumor vasculature exhibits many morphological and functional abnormalities, thus resulting in the formation of hypoxic areas within tumors, believed to represent a mechanism to maintain tumor cells in an invasive state.Tumors are vascularized through a variety of modalities, mainly represented by angiogenesis, where VEGF and other members of the VEGF family play a key role. This has represented the basis for the development of anti-VEGF blocking agents and their use in cancer therapy: however, these agents failed to induce significant therapeutic effects.Much less is known about the cellular origin of vessel network in tumors. Various cell types may contribute to tumor vasculature in different tumors or in the same tumor, such as mature endothelial cells, endothelial progenitor cells (EPCs), or the same tumor cells through a process of transdifferentiation. Early studies have suggested a role for bone marrow-derived EPCs; these cells do not are true EPCs but myeloid progenitors differentiating into monocytic cells, exerting a proangiogenic effect through a paracrine mechanism. More recent studies have shown the existence of tissue-resident endothelial vascular progenitors (EVPs) present at the level of vessel endothelium and their possible involvement as cells of origin of tumor vasculature.

Recent Advances in Endothelial Progenitor Cells Toward Their Use in Clinical Translation.

Since the discovery of Endothelial Progenitor Cells (EPC) by Asahara and colleagues in 1997, an increasing number of preclinical studies have shown that EPC based therapy is feasible, safe, and efficacious in multiple disease states. Subsequently, this has led to several, mainly early phase, clinical trials demonstrating the feasibility and safety profile of EPC therapy, with the suggestion of efficacy in several conditions including ischemic heart disease, pulmonary arterial hypertension and decompensated liver cirrhosis. Despite the use of the common term “EPC,” the characteristics, manufacturing methods and subset of the cell type used in these studies often vary significantly, rendering clinical translation challenging. It has recently been acknowledged that the true EPC is the endothelial colony forming cells (ECFC). The objective of this review was to summarize and critically appraise the registered and published clinical studies using the term “EPC,” which encompasses a heterogeneous cell population, as a therapeutic agent. Furthermore, the preclinical data using ECFC from the PubMed and Web of Science databases were searched and analyzed. We noted that despite the promising effect of ECFC on vascular regeneration, no clinical study has stemmed from these preclinical studies. We showed that there is a lack of information registered on www.clinicaltrials.gov for EPC clinical trials, specifically on cell culture methods. We also highlighted the importance of a detailed definition of the cell type used in EPC clinical trials to facilitate comparisons between trials and better understanding of the potential clinical benefit of EPC based therapy. We concluded our review by discussing the potential and limitations of EPC based therapy in clinical settings.

Regulation of EPCs: The Gateway to Blood Vessel Formation

Endothelial progenitor cells (EPCs) are primitive endothelial precursors which are known to functionally contribute to the pathogenesis of disease. To date a number of distinct subtypes of these cells have been described, with differing maturation status, cellular phenotype, and function. Although there is much debate on which subtype constitutes the true EPC population, all subtypes have endothelial characteristics and contribute to neovascularisation. Vasculogenesis, the process by which EPCs contribute to blood vessel formation, can be dysregulated in disease with overabundant vasculogenesis in the context of solid tumours, leading to tumour growth and metastasis, and conversely insufficient vasculogenesis can be present in an ischemic environment. Importantly, it is widely known that transcription factors tightly regulate cellular phenotype and function by controlling the expression of particular target genes and in turn regulating specific signalling pathways. This suggests that transcriptional regulators may be potential therapeutic targets to control EPC function. Herein, we discuss the observed EPC subtypes described in the literature and review recent studies describing the role of a number of transcriptional families in the regulation of EPC phenotype and function in normal and pathological conditions.

Authors′ Reply

Background Although in the general population circulating vascular progenitor cell levels have been implicated in the homeostasis of the vascular wall through differentiation into endothelium and/or smooth muscle cells, it has not yet been assessed in HIV-infected patients. We herein investigated the number of progenitor cell subpopulations in HIV-infected patients and its relationship to carotid intima-media thickness (c-IMT). Methods Blood samples were collected from 200 HIV-infected patients and CD34/KDR, CD34/VE-cadherin, and CD14/Endoglin progenitor cells were identified by flow cytometry. c-IMT was determined by ultrasonography. A group of 27 healthy subjects was used as control group. Results In our population (20 ART-naive patients and 180 treated patients), traditional cardiovascular risk factors were not found predictive of vascular progenitor cell levels. However, antiretroviral therapy (ART)-treatment was identified as the main predictive value for low CD34/KDR cells and high CD14/Endoglin cells after adjustment by cardiovascular risk factors (age, sex, hypertension, diabetes, and hyperlipidaemia) and HIV-related characteristics (HIV duration and ART treatment). Low levels of circulating CD34/KDR or CD34/VE-cadherin endothelial progenitor cells tended to be associated with increased c-IMT. However, a positive association was found between CD14/Endoglin cells and c-IMT. Low number of CD34/KDR cells was also associated with the longest exposure to nucleoside reverse transcriptase inhibitors and/or protease inhibitors. Conclusions ART exposure is the main predictor of circulating vascular progenitor cell levels. However, their levels are only partially associated with high c-IMT in HIV-infected patients. ART has already been found to have proatherogenic effect, but our data first describe its relationship with vascular progenitor cells and c-IMT.

Roles of aberrant endothelial progenitor cells in pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is a multisystem connective tissue disease characterized by excessive fibrosis and microvasculopathy, along with poor vascular formation and repair. The maintenance of the postnatal vascular system requires constant remodeling through vasculogenesis, which is mediated by the de novo differentiation of mature endothelial cells from endothelial progenitor cells (EPCs). However, a great deal of controversy about EPCs and their roles in postnatal vascular formation has arisen because of discrepancies in how EPCs are defined. The current consensus is that EPCs are heterogeneous cell population containing an extremely small count of "true EPCs", and pro-angiogenic hematopoietic cells (PHCs) that promotes vascular formation and repair through secretion of pro-angiogenic factors, and differentiation into endothelial cells and mural cells. In 2004, we reported a reduced number and impaired function of circulating CD34(+)CD133(+)CD309(+)CD45(dim)CD14(-) EPCs, which are now regarded as an immature subset of PHCs, in patients with SSc, and proposed a theory that defective vascular repair machinery as one of important mechanisms contributing to SSc vasculopathy. In addition, we showed that in SSc patients, circulating monocytic PHCs were increased and have enhanced angiogenic potency and differentiation potential to fibroblast-like cells. In summary, EPCs are involved in the pathogenesis of SSc by participating in two major pathological features, microvasculopathy and excessive fibrosis. Understanding the roles of EPCs in disease process of SSc may be key to dissecting its pathogenesis and to developing novel therapeutic strategies for this intractable condition.

Effect of Exercise on Circulating Endothelial Progenitor Cells in Microvascular Angina

Circulating endothelial progenitor cells (EPCs) might limit endothelial dysfunction in patients with microvascular angina (MVA). Endothelial colony-forming cells (ECFCs; displaying the CD34+/KDR+/CD45- phenotype) are currently regarded as true EPCs. The aim of this study was to evaluate exercise-induced ECFC mobilization and platelet reactivity in patients with MVA or with obstructive coronary artery disease (CAD). Exercise stress test (EST) was performed in 20 MVA patients, 20 CAD patients and 20 controls. Platelet reactivity was assessed before and after EST as formation of monocyte-platelet aggregates (MPAs) and CD41 platelet expression, without and with adenosine diphosphate (ADP) stimulation. ECFC number was measured before and 24h after EST. At rest, MPAs and CD41 platelet expression increased more with ADP in MVA patients (+71±11.0% and +37±7.5%, respectively), than in CAD patients (+37±8.6% and +19±4.5%, respectively) and controls (+29±3.5% and +21±3.1%, respectively; P<0.001 for both). At rest, ECFCs tended to be lower in CAD patients, compared to MVA patients and controls (4.1±5.0%, 7.2±6.0% and 7.3±7.0% cells/10(5), respectively; P=0.056). After EST, ECFCs increased less in MVA patients (+2.8±11) compared to CAD patients (+3.3±15; P<0.05) and controls (+7.4±24; P<0.01). In MVA patients, EST is able to blunt the peculiar increase of platelet reactivity to ADP present at rest; in contrast, no potential protective response of ECFCs to exercise was seen in these patients.

Effects of the renin angiotensin system on vasculogenesis-related progenitor cells

The current concept is that there are both cells that integrate into the vasculature, true endothelial progenitor cells (EPC), and cells with hematopoietic markers that support neovascularisation. As identification of the EPC is controversial and studies refer cells that might fall into either pools, we will use the term, vasculogenesis-related progenitor cells (VRPC), for this review. VRPC are considered to be an important target for the treatment of cardiovascular diseases (CVD). Angiotensin II is known to be an important player in neovascularisation and the modulation of renin angiotensin system (RAS) is one of the major pharmacotherapeutic strategies for the treatment of CVD. We will review the effects of different components of the RAS on such VRPC under physiological conditions and in CVD. The reviewed research strongly supports a critical role of the RAS in vasculogenesis and vascular regeneration. Therefore, pharmacological intervention on the components of the RAS does not only target directly end-organ remodelling and blood pressure but also influence tissue healing and/or regeneration by influencing specific progenitor cells. Thus, the interrogation of RAS effects on VRPC will be important in the optimisation of RAS intervention or regenerative therapy.

Is It Possible to Obtain “True Endothelial Progenitor Cells” by In Vitro Culture of Bone Marrow Mononuclear Cells?

In vitro-cultured bone marrow cells have been shown to contain some low-density lipoprotein (LDL) uptake-positive cells. Although a small portion of LDL uptake-positive cells had expression for endothelial markers, all of them demonstrated a phagocytosis function similar to monocyte/macrophages and expression of the panleukocyte surface marker CD45 and monocyte marker CD14. These LDL uptake-positive cells did not show significant proliferative capacity and died out gradually in long-term culture. In contrast, the bone marrow-derived LDL uptake-negative cells showed strong proliferation and expression of typical mesenchymal surface markers CD29 and CD44. Although cultured under endothelial promoting conditions, these mesenchymal stem cells (MSCs) did not show any sign of differentiation toward endothelial cells. In conclusion, adult bone marrow-derived LDL uptake-positive cells that have been reported so far actually are monocytes/macrophages that can express some endothelial markers but are not "true endothelial progenitor cells" (EPCs). MSCs, which are the only cell type that shows strong proliferation during long-term adherent culture for bone marrow cells, do not differentiate toward the endothelial lineage when grown under endothelial promoting conditions.