Vascular endothelial growth factor C is required for sprouting of the first lymphatic vessels from embryonic veins

Molecular regulation of the VEGF family – inducers of angiogenesis and lymphangiogenesis

The vascular endothelial growth factor (VEGF) family of secreted glycoproteins are critical inducers of angiogenesis (growth of blood vessels) and lymphangiogenesis (growth of lymphatic vessels). These proteins are attractive therapeutic targets for blocking growth of blood vessels and lymphatics in tumors and thereby inhibiting the growth and spread of cancer -- in fact, the first VEGF inhibitor has recently entered the clinic for treatment of cancer. In addition, the VEGFs are being considered for stimulation of angiogenesis in the context of ischemic disease and lymphangiogenesis for treatment of lymphedema. These therapeutic possibilities have focused great interest on the molecular regulation of VEGF family members. Much has been learned in the past five years about the mechanisms controlling the action of the VEGFs, including the importance of hypoxia, proteolysis, transcription factors and RNA splicing. An understanding of these mechanisms offers broader opportunities to manipulate expression and activity of the VEGFs for treatment of disease.

Angiogenesis and lymphangiogenesis: highlights of the past year

PURPOSE OF REVIEW

The purpose of this review is not to provide an extensive overview of well-established mechanisms of angiogenesis and lymphangiogenesis but rather to highlight several recent key studies that constituted a significant conceptual or medical advancement to the field during the past year or so. The authors apologize for their inability, because of space restrictions, to reference all other relevant work of the past or previous years.

RECENT FINDINGS

In 1993, fewer than 400 studies on angiogenesis were published. During the past year alone, more than 4000 angiogenesis studies were reported, making angiogenesis one of the most rapidly growing fields. Moreover, the first studies on lymphangiogenesis were published only a couple of years ago. A milestone in the field in the past year has been the first successful report that the angiogenesis inhibitor bevacizumab (Avastin), an antibody against vascular endothelial growth factor, prolonged the survival of colorectal and renal cancer patients in phase 3 clinical trials. This remarkable achievement provides great promise and hope for the future development of therapeutic strategies to inhibit or stimulate angiogenesis.

SUMMARY

The intensive search for antiangiogenic and proangiogenic mechanisms during the past decade is starting to translate into clinical promise. Further discovery of novel pathways and concepts in angiogenesis may lead to the optimization and refinement of current strategies to improve the clinical benefit and therapeutic safety for a vast number of patients with angiogenesis-related disease.

Role of lymphangiogenic factors in tumor metastasis

Nearly four centuries after the discovery of lymphatic vessels, the molecular mechanisms underlying their development are beginning to be elucidated. Vascular endothelial growth factor C (VEGF-C) and VEGF-D, via signaling through VEGFR-3, appear to be essential for lymphatic vessel growth. Observations from clinicopathological studies have suggested that lymphatic vessels serve as the primary route for the metastatic spread of tumor cells to regional lymph nodes. Recent studies in animal models have provided convincing evidence that tumor lymphangiogenesis facilitates lymphatic metastasis. However, it is not clear how tumor-associated lymphangiogenesis is regulated, and little is known about how tumor cells escape from the primary tumor and gain entry into the lymphatics. This review examines some of these issues and provides a brief summary of the recent developments in this field of research.

Vascular morphogenesis and the formation of vascular networks

A recent workshop organized by Luisa Iruela-Arispe and Brant Weinstein and sponsored by the North American Vascular Biology Organization (NAVBO) brought 200 developmental biologists together at the Asilomar Conference Center in Pacific Grove, California, to share some of their latest findings. This superb meeting synthesized data from a variety of model systems ranging from Urbilateria (a common ancestor to vertebrates and invertebrates), fruit flies, frogs, zebrafish, and mice to human genetic disorders. Participants enjoyed lively discussions on developmental vascular biology while experiencing the natural beauty of the Pacific Coast.

Lymphatics at the crossroads of angiogenesis and lymphangiogenesis

The lymphatic system is implicated in interstitial fluid balance regulation, immune cell trafficking, oedema and cancer metastasis. However, the sequence of events that initiate and coordinate lymphatic vessel development (lymphangiogenesis) remains obscure. In effect, the understanding of physiological regulation of lymphatic vasculature has been overshadowed by the greater emphasis focused on angiogenesis, and delayed by a lack of specific markers, thereby limiting this field to no more than a descriptive characterization. Recently, new insights into lymphangiogenesis research have been due to the discovery of lymphatic-specific markers and growth factors of vascular endothelial growth factor (VEGF) family, such as VEGF-C and VEGF-D. Studies using transgenic mice overexpressing VEGF-C and VEGF-D have demonstrated a crucial role for these factors in tumour lymphangiogenesis. Knowledge of lymphatic development has now been redefined at the molecular level, providing an interesting target for innovative therapies. This review highlights the recent insights and advances into the field of lymphatic vascular research, outlining the most important aspects of the embryo development, structure, specific markers and methods applied for studying lymphangiogenesis. Finally, molecular mechanisms involved in the regulation of lymphangiogenesis are described.

Gene Transfer for Therapeutic Vascular Growth in Myocardial and Peripheral Ischemia

Therapeutic vascular growth in the treatment of peripheral and myocardial ischemia has not yet fulfilled its expectations in clinical trials. Randomized, double-blinded placebo-controlled trials have predominantly shown the safety and feasibility but not the clear-cut clinically relevant efficacy of angiogenic gene or recombinant growth factor therapy. It is likely that growth factor levels achieved with single injections of recombinant protein or naked plasmid DNA are too low to induce any relevant angiogenic effects. Also, the route of administration of gene transfer vectors has not been optimal in many cases leading to low gene-transfer efficacy. Animal experiments using intramuscular or intramyocardial injections of adenovirus encoding vascular endothelial growth factor (VEGF, VEGF-A), the mature form of VEGF-D, and fibroblast growth factors (FGF-1, -2, and -4) have shown high angiogenic efficacy. Adenoviral overexpression of VEGF receptor-2 ligands, VEGF-A and the mature form of VEGF-D, enlarge the preexisting capillaries in skeletal muscle and myocardium via nitric oxide(NO)-mediated mechanisms and via proliferation of both endothelial cells and pericytes, resulting in markedly increased tissue perfusion. VEGF also enhances collateral growth, which is probably secondary to increased peripheral capillary blood flow and shear stress. As a side effect of VEGF overexpression and rapid microvessel enlargement, vascular permeability increases and may result in substantial tissue edema and pericardial effusion in the heart. Because of the transient adenoviral gene expression, the majority of angiogenic effects and side effects return to baseline by 2 weeks after the gene transfer. In contrast, VEGF overexpression lasting over 4 weeks has been shown to induce the growth of a persistent vascular network in preclinical models. To improve efficacy, the choice of the vascular growth factor, gene transfer vector, and route of administration should be optimized in future clinical trials. This review is focused on these issues.

Vascular Endothelial Growth Factor and Its Receptors in Embryonic Zebrafish Blood Vessel Development

There is intense interest in how blood vessel development is regulated. A number of vascular growth factors and their receptors have been described. The vascular endothelial growth factor (VEGF) and its receptors are major contributors to normal mammalian vascular development. These receptors include VEGFR-1, VEGFR-2, VEGFR-3, neuropilin-1 (NRP1), and NRP2. The function of these genes have been determined to some degree in mouse gene targeting studies. These knockouts are embryonically lethal, and early death can be attributed in part to lack of normal blood and lymphatic vessel development. More recently, it has been demonstrated that zebrafish are an excellent model for studying the genes and proteins that regulate embryonic vascular development. Zebrafish have a number of advantages compared to mice, including rapid embryonic development and the ability to examine and manipulate embryos outside of the animal. In this review, we describe some of the earlier mouse VEGF/receptor functional studies and emphasize the development of the zebrafish vasculature. We describe the zebrafish vasculature, zebrafish VEGF and VEGF receptors, advantages of the zebrafish model, resources, and methods of determining growth factor and receptor function.

[Experimental animal models of diabetes mellitus]

Vascular endothelial growth factor (VEGF) plays a critical role in angiogenesis, which is required for tumor growth and metastasis. In this article, a review of the functional and biological roles of the VEGF pathway in driving angiogenesis and growth of gynecologic malignancies was performed. Based on the biological functions of VEGF, multiple approaches for targeting the VEGF/VEGF-receptor complex have been developed and many of these have demonstrated substantial activity in preclinical models. These promising data have led to rapid clinical development of VEGF-targeted agents. Therefore, we also assessed the status of VEGF-targeted therapies and associated toxicities in gynecologic malignancies. However, many questions remain related to optimal dosing, sequencing of therapies, management of toxicities, appropriate patient selection, and assessment of response, which will require further studies. Nevertheless, VEGF-targeted therapies offer hope for improving the outcome of cancer patients.