Enhanced capillary formation stimulated by a chimeric vascular endothelial growth factor/vascular endothelial growth factor-C silk domain fusion protein

Proteolytic Cleavages in the VEGF Family: Generating Diversity among Angiogenic VEGFs, Essential for the Activation of Lymphangiogenic VEGFs

Specific proteolytic cleavages turn on, modify, or turn off the activity of vascular endothelial growth factors (VEGFs). Proteolysis is most prominent among the lymph-angiogenic VEGF-C and VEGF-D, which are synthesized as precursors that need to undergo enzymatic removal of their C- and N-terminal propeptides before they can activate their receptors. At least five different proteases mediate the activating cleavage of VEGF-C: plasmin, ADAMTS3, prostate-specific antigen, cathepsin D, and thrombin. All of these proteases except for ADAMTS3 can also activate VEGF-D. Processing by different proteases results in distinct forms of the "mature" growth factors, which differ in affinity and receptor activation potential. The "default" VEGF-C-activating enzyme ADAMTS3 does not activate VEGF-D, and therefore, VEGF-C and VEGF-D do function in different contexts. VEGF-C itself is also regulated in different contexts by distinct proteases. During embryonic development, ADAMTS3 activates VEGF-C. The other activating proteases are likely important for non-developmental lymphangiogenesis during, e.g., tissue regeneration, inflammation, immune response, and pathological tumor-associated lymphangiogenesis. The better we understand these events at the molecular level, the greater our chances of developing successful therapies targeting VEGF-C and VEGF-D for diseases involving the lymphatics such as lymphedema or cancer.

Efficient activation of the lymphangiogenic growth factor VEGF-C requires the C-terminal domain of VEGF-C and the N-terminal domain of CCBE1

The collagen- and calcium-binding EGF domains 1 (CCBE1) protein is necessary for lymphangiogenesis. Its C-terminal collagen-like domain was shown to be required for the activation of the major lymphangiogenic growth factor VEGF-C (Vascular Endothelial Growth Factor-C) along with the ADAMTS3 (A Disintegrin And Metalloproteinase with Thrombospondin Motifs-3) protease. However, it remained unclear how the N-terminal domain of CCBE1 contributed to lymphangiogenic signaling. Here, we show that efficient activation of VEGF-C requires its C-terminal domain both in vitro and in a transgenic mouse model. The N-terminal EGF-like domain of CCBE1 increased VEGFR-3 signaling by colocalizing pro-VEGF-C with its activating protease to the lymphatic endothelial cell surface. When the ADAMTS3 amounts were limited, proteolytic activation of pro-VEGF-C was supported by the N-terminal domain of CCBE1, but not by its C-terminal domain. A single amino acid substitution in ADAMTS3, identified from a lymphedema patient, was associated with abnormal CCBE1 localization. These results show that CCBE1 promotes VEGFR-3 signaling and lymphangiogenesis by different mechanisms, which are mediated independently by the two domains of CCBE1: by enhancing the cleavage activity of ADAMTS3 and by facilitating the colocalization of VEGF-C and ADAMTS3. These new insights should be valuable in developing new strategies to therapeutically target VEGF-C/VEGFR-3-induced lymphangiogenesis.

Overexpression of the chimeric plasmin-resistant VEGF165/VEGF183 (132-158) protein in murine breast cancer induces distinct vascular patterning adjacent to tumors and retarded tumor growth

A chimeric plasmin‑resistant vascular endothelial growth factor (VEGF)165/VEGF183 (132-158) protein, named as VEGF183 (according to the nomenclature of VEGF), designed by a previous study, was demonstrated to have an enhanced affinity for the extracellular matrix (ECM) amongst other bioactivities. However, it is now accepted that mutant VEGFs frequently demonstrate different angiogenic activities and produce different vascular patterning from the parental molecule. The present study hypothesized that VEGF183, due to its enhanced binding affinity to the ECM, would exhibit a different angiogenic activity and produce a different vascular patterning compared to those of VEGF165. Murine breast cancer EMT‑6 cells were manipulated to stably overexpress VEGF165 or VEGF183. These cells were then inoculated intradermally into BALB/c mice in order to monitor the formation of vascular patterning in skin proximal to tumors. In vivo angiogenesis experiments revealed that overexpression of VEGF183 in murine breast cancer cells resulted in irregular, disorganized and dense vascular patterning as well as induced a significant inhibition of tumor growth compared with that of VEGF165. In addition, allograft tumor immunochemical assays of VEGF183‑overexpressing tumors demonstrated significantly lower vascular densities than those of VEGF165‑overexpressing tumors; however, VEGF183 tumors had a significantly enlarged vascular caliber. Conversely, cell wound healing experiments revealed that VEGF183‑overexpressing EMT‑6 cells had significantly decreased migration rates compared with those of VEGF165‑overexpressing EMT‑6 cells. In conclusion, the results of the present study supported the hypothesis that the altered ECM affinity of VEGF induced structural alterations to vasculature. In addition, these results provided a novel insight into VEGF design and indirect evidence for the function of exon 8 in VEGF. [Corrected]

Reciprocal modulation of Aβ42 aggregation by copper and homocysteine

Hyperhomocysteinemia is a risk factor for Alzheimer’s disease (AD). Both homocysteine (Hcy) and amyloid β (Aβ), which accumulates in the brain of AD patients, bind copper. Aim of this study was to test the hypothesis that the association of Hcy and AD results from a molecular interaction between Hcy and Aβ that is mediated by copper. We established a microtiter plate format thioflavin T aggregation assay to monitor Aβ42 fibrillization. Copper (5 μM) completely prevented Aβ42 (5 μM) fibrillization. Homocysteine in the absence of copper did not impact Aβ42 fibrillization, but physiological concentrations of Hcy (10–100 μM) attenuated the inhibitory effect of copper on Aβ42 fibril formation. These results were qualitatively confirmed by electron microscopy, which did not reveal morphological differences. To compare the toxicity of fibrillar and non-fibrillar Aβ42 exposed to copper or Hcy, rat primary cortical neurons were treated in vitro with 5 μM Aβ42 for 72 h. After incubation with 5 μM Aβ42 that had been aggregating in the absence of Hcy or copper, cell viability was reduced to 40%. Incubation with 5 μM Aβ42, in which fibril formation had been prevented or reverted by the addition of 5 μM copper, resulted in cell viability of approximately 25%. Accordingly, viability was reduced to 25% after incubation with 5 μM monomeric, i.e., non-fibrillized, Aβ42. The addition of Hcy plus copper to 5 μM Aβ42 yielded 50% viability. In conclusion, copper prevents and reverts Aβ fibril formation leading rather to formation of lower order oligomers or amorphous aggregates, and Hcy reduces these effects. Such mechanisms may explain the association of hyperhomocysteinemia and AD, leading to novel therapeutic strategies in the prevention and treatment of this disease.

Vascular remodeling in cancer

The growth and dissemination of tumors rely on an altered vascular network, which supports their survival and expansion and provides accessibility to the vasculature and a route of transport for metastasizing tumor cells. The remodeling of vascular structures through generation of new vessels (for example, via tumor angiogenesis) is a well studied, even if still quite poorly understood, process in human cancer. Antiangiogenic therapies have provided insight into the contribution of angiogenesis to the biology of human tumors, yet have also revealed the ease with which resistance to antiangiogenic drugs can develop, presumably involving alterations to vascular signaling mechanisms. Furthermore, cellular and/or molecular changes to pre-existing vessels could represent subtle pre-metastatic alterations to the vasculature, which are important for cancer progression. These changes, and associated molecular markers, may forecast the behavior of individual tumors and contribute to the early detection, diagnosis and prognosis of cancer. This review, which primarily focuses on the blood vasculature, explores current knowledge of how tumor vessels can be remodeled, and the cellular and molecular events responsible for this process.

Vascular endothelial growth factor-angiopoietin chimera with improved properties for therapeutic angiogenesis

BACKGROUND

There is an unmet need for proangiogenic therapeutic molecules for the treatment of tissue ischemia in cardiovascular diseases. However, major inducers of angiogenesis such as vascular endothelial growth factor (VEGF/VEGF-A) have side effects that limit their therapeutic utility in vivo, especially at high concentrations. Angiopoietin-1 has been considered to be a blood vessel stabilization factor that can inhibit the intrinsic property of VEGF to promote vessel leakiness. In this study, we have designed and tested the angiogenic properties of chimeric molecules consisting of receptor-binding parts of VEGF and angiopoietin-1. We aimed at combining the activities of both factors into 1 molecule for easy delivery and expression in target tissues.

METHODS AND RESULTS

The VEGF-angiopoietin-1 (VA1) chimeric protein bound to both VEGF receptor-2 and Tie2 and induced the activation of both receptors. Detailed analysis of VA1 versus VEGF revealed differences in the kinetics of VEGF receptor-2 activation and endocytosis, downstream kinase activation, and VE-cadherin internalization. The delivery of a VA1 transgene into mouse skeletal muscle led to increased blood flow and enhanced angiogenesis. VA1 was also very efficient in rescuing ischemic limb perfusion. However, VA1 induced less plasma protein leakage and myeloid inflammatory cell recruitment than VEGF. Furthermore, angioma-like structures associated with VEGF expression were not observed with VA1.

CONCLUSIONS

The VEGF-angiopoietin-1 chimera is a potent angiogenic factor that triggers a novel mode of VEGF receptor-2 activation, promoting less vessel leakiness, less tissue inflammation, and better perfusion in ischemic muscle than VEGF. These properties of VA1 make it an attractive therapeutic tool.

A truncation allele in vascular endothelial growth factor c reveals distinct modes of signaling during lymphatic and vascular development

Vascular endothelial growth factor C (Vegfc) is a secreted protein that guides lymphatic development in vertebrate embryos. However, its role during developmental angiogenesis is not well characterized. Here, we identify a mutation in zebrafish vegfc that severely affects lymphatic development and leads to angiogenesis defects on sensitized genetic backgrounds. The um18 mutation prematurely truncated Vegfc, blocking its secretion and paracrine activity but not its ability to activate its receptor Flt4. When expressed in endothelial cells, vegfc(um18) could not rescue lymphatic defects in mutant embryos, but induced ectopic blood vessel branching. Furthermore, vegfc-deficient endothelial cells did not efficiently contribute to tip cell positions in developing sprouts. Computational modeling together with assessment of endothelial cell dynamics by time-lapse analysis suggested that an autocrine Vegfc/Flt4 loop plays an important role in migratory persistence and filopodia stability during sprouting. Our results suggest that Vegfc acts in two distinct modes during development: as a paracrine factor secreted from arteries to guide closely associated lymphatic vasculature and as an autocrine factor to drive migratory persistence during angiogenesis.

Activated forms of VEGF-C and VEGF-D provide improved vascular function in skeletal muscle

The therapeutic potential of vascular endothelial growth factor (VEGF)-C and VEGF-D in skeletal muscle has been of considerable interest as these factors have both angiogenic and lymphangiogenic activities. Previous studies have mainly used adenoviral gene delivery for short-term expression of VEGF-C and VEGF-D in pig, rabbit, and mouse skeletal muscles. Here we have used the activated mature forms of VEGF-C and VEGF-D expressed via recombinant adeno-associated virus (rAAV), which provides stable, long-lasting transgene expression in various tissues including skeletal muscle. Mouse tibialis anterior muscle was transduced with rAAV encoding human or mouse VEGF-C or VEGF-D. Two weeks later, immunohistochemical analysis showed increased numbers of both blood and lymph vessels, and Doppler ultrasound analysis indicated increased blood vessel perfusion. The lymphatic vessels further increased at the 4-week time point were functional, as shown by FITC-lectin uptake and transport. Furthermore, receptor activation and arteriogenic activity were increased by an alanine substitution mutant of human VEGF-C (C137A) having an increased dimer stability and by a chimeric CAC growth factor that contained the VEGF receptor-binding domain flanked by VEGF-C propeptides, but only the latter promoted significantly more blood vessel perfusion when compared to the other growth factors studied. We conclude that long-term expression of VEGF-C and VEGF-D in skeletal muscle results in the generation of new functional blood and lymphatic vessels. The therapeutic value of intramuscular lymph vessels in draining tissue edema and lymphedema can now be evaluated using this model system.

Lymphatic vasculature: a molecular perspective

The lymphatic vasculature comprises an intricate network of vessels critical for fluid homeostasis, immune surveillance and fat absorption. Recent studies have provided insights into the developmental processes and molecular mechanisms controlling the formation and remodelling of the lymphatic vessels. These studies have further demonstrated the essential and active role of the lymphatic vessels in various pathological conditions and advanced our understanding of the progression of human diseases, such as inflammation and tumorigenesis. In the context of the latest exciting findings, we review here the current understanding of the mechanisms of lymphatic development and contribution of lymphatic vessels to pathological conditions.