The Rabbit Corneal Pocket Assay for the Study of Angiogenesis

Generation of Human Pluripotent Stem Cell-derived Endothelial Cells and Their Therapeutic Utility

PURPOSE OF REVIEW

Human pluripotent stem cell-derived endothelial cells (hPSC-ECs) emerged as an important source of cells for cardiovascular regeneration. This review summarizes protocols for generating hPSC-ECs and provides an overview of the current state of the research in clinical application of hPSC-derived ECs.

RECENT FINDINGS

Various systems were developed for differentiating hPSCs into the EC lineage. Stepwise two-dimensional systems are now well established, in which various growth factors, small molecules, and coating materials are used at specific developmental stages. Moreover, studies made significant advances in clinical applicability of hPSC-ECs by removing undefined components from the differentiation system, improving the differentiation efficiency, and proving their direct vascular incorporating effects, which contrast with adult stem cells and their therapeutic effects in vivo. Finally, by using biomaterial-mediated delivery, investigators improved the survival of hPSC-ECs to more than 10 months in ischemic tissues and described long-term behavior and safety of in vivo transplanted hPSC-ECs at the histological level. hPSC-derived ECs can be as a critical source of cells for treating advanced cardiovascular diseases. Over the past two decades, substantial improvement has been made in the differentiation systems and their clinical compatibility. In the near future, establishment of fully defined differentiation systems and proof of the advantages of biomaterial-mediated cell delivery, with some additional pre-clinical studies, will move this therapy into a vital option for treating those diseases that cannot be managed by currently available therapies.

A proangiogenic peptide derived from vascular endothelial growth factor receptor-1 acts through α5β1 integrin

Vascular endothelial growth factor receptor-1 (VEGFR-1) is a tyrosine kinase receptor for growth factors of the VEGF family. Endothelial cells express a membrane-bound and a soluble variant of this protein, the latter being mainly considered as a negative regulator of VEGF-A signaling. We previously reported that the soluble form is deposited in the extracellular matrix produced by endothelial cells in culture and is able to promote cell adhesion and migration through binding to α5β1 integrin. In this study, we demonstrate that the Ig-like domain II of VEGFR-1, which contains the binding determinants for the growth factors, is involved in the interaction with α5β1 integrin. To identify domain regions involved in integrin binding, we designed 12 peptides putatively mimicking the domain II surface and tested their ability to inhibit α5β1-mediated endothelial cell adhesion to soluble VEGFR-1 and directly support cell adhesion. One peptide endowed with both these properties was identified and shown to inhibit endothelial cell migration toward soluble VEGFR-1 as well. This peptide directly binds α5β1 integrin, but not VEGF-A, inducing endothelial cell tubule formation in vitro and neoangiogenesis in vivo. Alanine scanning mutagenesis of the peptide defined which residues were responsible for its biologic activity and integrin binding.

Pyrazolo-pyrimidine-derived c-Src inhibitor reduces angiogenesis and survival of squamous carcinoma cells by suppressing vascular endothelial growth factor production and signaling

Src tyrosine kinase family cooperates with activated growth factor receptors to regulate growth, invasion and metastasis. The authors examined the influence of a novel c-Src inhibitor, 1l, derived from 4-amino-substituted-pyrazolo-pyrimidines, on tumor angiogenesis and on the angiogenic output of squamous carcinoma cells, A431 and SCC-4. The effect of 1l was assessed on growth and microvessel density in A431 tumors and its effect compared with the established c-Src inhibitor PP-1. The effects of c-Src inhibition were investigated on vascular endothelial growth factor (VEGF) expression and activity in tumor cells grown in vivo and in vitro, as well as on VEGF mediated signaling and on endothelial cell functions. Nanomolar concentrations of 1l decreased tumor volume promoted by A431 implanted in nude mice, without affecting in vitro cell tumor survival. This effect was related to 1l inhibition of VEGF production, and secondary to an effect on tumor microvessel density. The rabbit cornea assay confirmed that 1l markedly decreased neovessel growth induced by VEGF. In cultured endothelial cells, 1l inhibited the VEGF-induced phosphorylation on tyr416 of c-Src, resulting in a reduced cell proliferation and invasion. Consistently, 1l dowregulated endothelial nitric oxide synthase, MAPK-extracellular receptor kinase 1-2 (ERK1-2) activity and matrix metalloproteinases (MMP-2/MMP-9), while the tissue inhibitors of metalloproteinases (TIMP2/TIMP-1) were upregulated. These results demonstrate that nM concentrations of c-Src kinase inhibitors (1l and PP-1), by reducing the production of VEGF released by tumor cell and its endothelial cell responses, have a highly selective antiangiogenesis effect, which might be useful in combination therapies.

Divergent effects of quercetin conjugates on angiogenesis

The present study reports the activities of quercetin and its main circulating conjugates in man (quercetin-3'-sulphate (Q3'S) and quercetin-3-glucuronide (Q3G)) on in vivo angiogenesis induced by vascular endothelial growth factor (VEGF) and examines the effects of these molecules on cultured endothelial cells. We found opposing effects of quercetin and its metabolites on angiogenesis. While quercetin and Q3G inhibited VEGF-induced endothelial cell functions and angiogenesis, Q3'S per se promoted endothelial cell proliferation and angiogenesis. The inhibitory effect elicited by Q3G was linked to inhibition of extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation elicited by VEGF. The activation of endothelial cells by Q3'S was associated to stimulation of VEGF receptor-2 and to downstream signalling activation (phosphatidylinositol-3 kinase/Akt and nitric oxide synthase pathways), ultimately responsible for ERK1/2 phosphorylation. These data indicate that the effects of circulating quercetin conjugates on angiogenesis are different depending on the nature of the conjugate. Q3G and Q3'S are the two major conjugates in plasma, but their ratio is dependent on several factors, so that inhibition or activation of angiogenesis could be subtly shifted as a result of metabolism in vivo.