Signal transduction in angiogenesis

Evaluation of Vascular Endothelial Growth Factor and Its Receptors in Human Neointima

OBJECTIVE

The potential contribution of vascular endothelial growth factor (VEGF) in neointima development has been evaluated in numerous animal studies. However, its role remains controversial. Moreover, little is known about neointima formation in humans. In this study we assessed the expression of VEGF-A and its receptors in the human neointima formed within vascular anastomosis.

METHODS

The studied material comprised neointima samples harvested during secondary vascular operations from patients with chronic limb ischemia after aorto-/iliofemoral bypass grafting who developed vascular graft occlusion at 6-18 months after the initial surgical treatment. The control material consisted of segments of femoral arteries without visible macroscopic lesions collected from organ donors. The expression and content of VEGF-A, VEGFR-1 and VEGFR-2 were analyzed with PCR and ELISA methods, respectively.

RESULTS

We observed a significantly increased expression of VEGF-A and VEGFR-2 mRNA in neointima compared to the normal aorta. A significantly higher protein content of VEGF-A and VEGFR-2 in neointima samples compared to the controls was also observed. No significant difference of VEGFR-1 content and VEGFR-1 mRNA expression was found in the studied material.

CONCLUSION

These results indicate a possible involvement of the VEGF-A and VEGFR-2 system in the pathologic process of human neointima formation after vascular interventions.

Lysosomal stress and lipid peroxidation products induce VEGF-121 and VEGF-165 expression in ARPE-19 cells

BACKGROUND

Pathologically increased VEGF-A expression is considered a major pathogenic factor in exudative AMD. Since VEGF-A can exist in isoforms with different individual functions, for a detailed understanding of the role of VEGF-A in normal and disease associated processes, in particular in wet AMD, the expression pattern of VEGF-A isoforms has to be taken into account. Therefore in the present study, adressing the effects of lysosomal dysfunction on VEGF expression and secretion by RPE cells induced by lipid peroxidation products and an inhibitor of lysosomal acidification, we applied quantitative methods discriminating the major VEGF-A isoforms.

METHODS

ARPE-19 cells were treated with the primary lipid peroxidation products 4-hydroxynonenal (HNE), malondialdehyde (MDA) or the lysosomal inhibitor bafilomycin A. VEGF-A isoforms were determined by splice-variant-specific RT-PCR. For detection of protein levels, a protein prefractionation strategy based on the strikingly different isoelectric points of VEGF isoforms was used prior to quantification of VEGF-A 121, -165, -189 and -206 expression by ELISA.

RESULTS

On mRNA level, VEGF-A 165 represents the major isoform (60%), VEGF-A 121 accounts for about one-third, and VEGF-A 189 for about 10% of total mRNA detected in untreated cells. No VEGF-A 206 mRNA was detected. Treatment with bafilomycin A increased VEGF-A 121 and VEGF-A 165 mRNA levels. VEGF-A 189 expression remained unaffected, and no induction of VEGF-A 206 mRNA was detectable. Similar effects were observed when cells were stressed with HNE or MDA. On protein level, bafilomycin A as well as the lipid peroxidation products caused an increase of total VEGF-A protein secretion into the culture medium. In analysis of VEGF-A for different splice variants, only VEGF-A 121 and VEGF-A 165 were detected, the latter representing the major secreted isoform, with the ratio of both isoforms being slightly changed in favour of VEGF-A 165 secretion.

CONCLUSION

Lysosomal dysfunction and lipid peroxidation damage might be an inducer of VEGF-A 121 and VEGF-A 165 expression in the retina. Furthermore, the novel technique used to analyze the protein expression pattern of VEGF- A isoforms in biological samples may represent a valuable tool in future analyses of specific VEGF-A isoforms in normal and pathogenic functions.

Axon guidance molecules in vascular patterning

Endothelial cells (ECs) form extensive, highly branched and hierarchically organized tubular networks in vertebrates to ensure the proper distribution of molecular and cellular cargo in the vertebrate body. The growth of this vascular system during development, tissue repair or in disease conditions involves the sprouting, migration and proliferation of endothelial cells in a process termed angiogenesis. Surprisingly, specialized ECs, so-called tip cells, which lead and guide endothelial sprouts, share many feature with another guidance structure, the axonal growth cone. Tip cells are motile, invasive and extend numerous filopodial protrusions sensing growth factors, extracellular matrix and other attractive or repulsive cues in their tissue environment. Axonal growth cones and endothelial tip cells also respond to signals belonging to the same molecular families, such as Slits and Roundabouts, Netrins and UNC5 receptors, Semaphorins, Plexins and Neuropilins, and Eph receptors and ephrin ligands. Here we summarize fundamental principles of angiogenic growth, the selection and function of tip cells and the underlying regulation by guidance cues, the Notch pathway and vascular endothelial growth factor signaling.

Anchorage of VEGF to the extracellular matrix conveys differential signaling responses to endothelial cells

Matrix-bound VEGF elicits more distinct vascular effects than soluble VEGF, including prolonged VEGFR2 activation with altered patterns of tyrosine activation and downstream enhancement of the p38/MAPK pathway.

VEGF can be secreted in multiple isoforms with variable affinity for extracellular proteins and different abilities to induce vascular morphogenesis, but the molecular mechanisms behind these effects remain unclear. Here, we show molecular distinctions between signaling initiated from soluble versus matrix-bound VEGF, which mediates a sustained level of VEGFR2 internalization and clustering. Exposure of endothelial cells to matrix-bound VEGF elicits prolonged activation of VEGFR2 with differential phosphorylation of Y1214, and extended activation kinetics of p38. These events require association of VEGFR2 with β1 integrins. Matrix-bound VEGF also promotes reciprocal responses on β1 integrin by inducing its association with focal adhesions; a response that is absent upon exposure to soluble VEGF. Inactivation of β1 integrin blocks the prolonged phosphorylation of Y1214 and consequent activation of p38. Combined, these results indicate that when in the context of extracellular matrix, activation of VEGFR2 is distinct from that of soluble VEGF in terms of recruitment of receptor partners, phosphorylation kinetics, and activation of downstream effectors.

Systems biology of vascular endothelial growth factors

Several cytokine families have roles in the development, maintenance, and remodeling of the microcirculation. Of these, the vascular endothelial growth factor (VEGF) family is one of the best studied and one of the most complex. Five VEGF ligand genes and five cell-surface receptor genes are known in the human, and each of these may be transcribed as multiple splice isoforms to generate an extensive family of proteins, many of which are subject to further proteolytic processing. Using the VEGF family as an example, we describe the current knowledge of growth-factor expression, processing, and transport in vivo. Experimental studies and computational simulations are being used to measure and predict the activity of these molecules, and we describe avenues of research that seek to fill the remaining gaps in our understanding of VEGF family behavior.

Tubular sprouting as a mode of vascular formation in a colonial ascidian (Tunicata)

Although phylogenetically related to vertebrates, invertebrate chordate tunicates possess an open circulatory system, with blood flowing in lacunae among organs. However, the colonial circulatory system (CCS) of the ascidian Botryllus schlosseri runs in the common tunic and forms an anastomized network of vessels, defined by simple epithelium, connected to the open circulatory system of the zooids. The CCS originates from epidermal evagination, grows, and increases its network accompanying colony propagation. New vessels are formed by means of mechanisms of tubular sprouting which, in their morphogenesis and molecular regulation, are very similar to those occurring in other metazoans, particularly during vertebrate angiogenesis. From the apex of new vessels, epithelial cells detach and migrate into the tunic, while exploring filopodia extend toward the tunic and possibly guide vessel growth. Immunohistology showed that growth factors fibroblast growth factor-2 and vascular endothelial growth factor and the receptor vascular endothelial growth factor receptor-1 participate in sprouting, associated with cell proliferation. As in vertebrates, these factors may regulate cell migration, proliferation, sprouting, and tube formation. Our data indicate that similar, conserved signals were co-opted in the sprouting processes of two nonhomologous circulatory systems, that of ascidian CCS, and vertebrate circulatory systems, by recruitment of the same signaling pathway.