Vascular Endothelial Growth Factor: The Good, the Bad, and the Ugly

Discovered in the late 1970s as a secretory product of tumor cells and characterized as a potent vascular permeability factor (VPF), the vascular endothelial growth factor (VEGF) was soon found to play a physiological role in angiogenesis and, thus, set the stage for the exciting drama in vascular biology of the 1990s. The discrepance in terminology (VPF/VEGF) reflects well on paradoxical effects of this growth factor, its role in the disease processes, and emerging paradigms in therapeutic trials to suppress or enhance its action. VEGF is a key regulator of physiological angiogenesis during embryogenesis, skeletal growth and for reproductive functions. The VEGF family consists of six VEGF molecules designated A, B, C, D, and E and the placenta growth factor (PIGF). VEGF-A is the best characterized of these factors. It is expressed in different isoforms derived from alternative splicing of the mRNA. The smaller isoforms, VEGF-121, VEGF-145, and VEGF-165 (numbers denote number of amino acids), are secreted while the larger isoforms, VEGF-189 and VEGF-205, are cell bound. The receptors for VEGF possess tyrosine kinase activity and are expressed almost exclusively on endothelial cells. The current nomenclature for the VEGF receptors lists three receptors: VEGFR-1 (flt-1), VEGFR-2 (kdr/flk-1), and VEFGR-3 (flt-4). In addition, co-receptors for some members of the VEGF family, neuropilin-1 and neuropilin-2, have been discovered. VEGF is essential for establishing a functional vascular system during embryogenesis. Deletion of a single VEGF allele results in abnormal blood vessel development and lethality in murine embryos. VEGF inactivation during early postnatal development has been shown to be lethal as well. The importance of VEGF for endochondral bone formation (the mechanism by which bone grows longitudinally) has been proven in an animal model that showed a growth restriction after VEGF inhibition. In adults, VEGF has limited physiological activities. Studies in mice indicate that disruption of VEGF is not associated with any apparent abnormalities. Its function appears to be limited to wound healing (new blood vessels transport nutrients and oxygen to the growing tissues, allowing inflammatory cells to the site of injury via the endothelial basement membrane), reproductive follicular growth and development of the corpus luteum (depend on the proliferation of new capillary vessels). The potential benefit of VEGF for therapeutic angiogenesis has been evaluated in studies administering the VEGF protein, such as in patients with limited options for coronary revascularization, as well as in gene therapy trials for patients with severe coronary artery disease and myocardial ischemia (for review, see [1]). Given the role of VEGF as a key mediator of normal physiological angiogenesis, abnormalities of VEGF expression have the potential to be important in disease processes. A large body of evidence supports the role of VEGF in the pathogenesis of diseases characterized by neovascularization, including ocular diseases and inflammatory conditions. Additionally, biopsies representing a large number of tumor types have been shown to overexpress VEGF, and, therefore, VEGF has been recognized to be fundamental to tumorigenesis and disease progression in a wide range of human cancers. Without an adequate vascular supply, solid tumors can grow only to a critical size of 1 to 2 mm (or about 106 cells), primarily due to lack of oxygen and nutrients. Folkman (1971) hypothesized that tumor blood vessel formation is dependent on a tumor angiogenic factor (TAF), and that its blockade during the period when a tumor is most vulnerable (i.e. prior to angiogenesis) may restrict tumor growth (2). VEGF was later identified as one of the most potent TAF molecules. Tumors may remain dormant, in an avascular phase, maintaining a steady state between cell proliferation and apoptosis before converting to an angiogenic phenotype. This conversion, which is known as the “angiogenic switch,” is due to an alteration in the balance of inhibitory and stimulatory factors. Preliminary evidence suggests that overexpression of certain isoforms of VEGF may cause different effects on tumor angiogenesis. VEGF-165 seems to be the predominant isoform in most tumors. Several factors have been shown to up-regulate VEGF expression including hypoxia, low pH, growth factors, intrinsic up-regulation of signal transduction pathways, and loss of tumor suppressor gene activity. Several studies have also demonstrated that VEGF is expressed in tumor-associated stromal cells, where it is probably upregulated by environmental conditions caused by the tumor. Increased VEGF expression correlates with poor prognosis, including aggressive tumor growth, tumor recurrence, tumor metastasis and decreased survival, in many tumor types including colon cancer, gastric carcinoma, non-small cell lung cancer, breast cancer, renal cell and pancreatic cancer. VEGF expression also correlates with increased tumor microvessel density, which itself is a prognostic indicator in certain types of cancer (for review, see [1, 3]). In all of the above cancers, anti-angiogenic therapy has been shown to inhibit tumor growth in preclinical models. However, the distinction between inhibition of tumor growth (significant in preclinical studies) and tumor regression is essential to comprehend when translating preclinical studies into clinical trial design. It is generally accepted that anti-angiogenic therapy will be part of multi-component regimens rather than being used as single agent therapy. Efficacy of single agent anti-angiogenic therapy has been ineffective in Phase I and II clinical trials for gastrointestinal malignancies. The addition of anti-angiogenic therapy to standard regimens for patients with metastatic disease will likely be the appropriate use of antiangiogenic therapy at this stage. Phase III clinical trials are already completed in patients with metastatic colon cancer treated with standard therapy versus anti-angiogenic therapy. Trial results show that the addition of bevacizumab (Avastin), the first anti-angiogenic agent with demonstrated anticancer benefit, to first-line chemotherapy provides improvement in progression-free and median overall survival in patients with metastatic colorectal cancer when combined with IFL (Irinotecan, 5-FU/Leucovorin) (3). Recent studies engage the growing interest in VEGF as an important factor in ischemia/reperfusion (I/R) events. However, the available data are controversial. Since ischemia is a potent stimulus for the expression of VEGF, this angiogenic factor is suggested to play a role in the process of the I/R injury. Boros et al. (4) reported that endogenous VEGF is expressed and released from the liver after transplantation and that the treatment of neutralizing antibody against VEGF during reperfusion attenuate liver damage. They suggested that endogenous VEGF, especially in the early phase of I/R injury, might promote organ damage and dysfunction. On the other hand, Luo et al. (5) described that exogenously administrated recombinant VEGF reduced myocardial I/R injury in an isolated rat-heart model. This group speculated that the observed beneficial effect might be dependent in part on nitric oxide release from the coronary vasculature stimulated by VEGF. Similar results were published by Addya et al. (6) in a model of ischemic preconditioned myocardium, where they could confirm the importance of VEGF-Flt-1 receptor signaling for the ischemic preconditioning pathway and the deleterious effects of interrupting this way in Flt-1 knockout mice. On the other hand, van Bruggen et al. (7) described a reduction of edema formation and tissue damage after I/R injury in the mouse brain after VEGF antagonism by administration of the novel fusion protein mFlt (1–3)-IgG. Best described are these discrepancies in a study by Tsurui et al. (8). They found an upregulation of local VEGF expression after 60 min of liver ischemia. VEGF was expressed predominantly in CD11b+ cells infiltrating into the ischemic liver associated with profound tissue injury. In contrast, administration of exogenous rhVEGF had a significant protective effect on this ischemic injured liver. Particularly, due to its role as a potent permeability factor, VEGF is of considerable interest in the pathophysiological events of pancreatic I/R. The pancreas is a low-flow organ and, therefore, very susceptible for the microcirculatory disturbances during reperfusion or in acute pancreatitis. Warzecha et al. (9) found an increased immunohistochemical presence of fibroblast growth factor-2 (FGF-2), VEGF, platelet-derived growth factor-A (PDGF-A), and transforming growth factor-b type II receptor (TGF-b RII) in the course of I/R-induced acute pancreatitis. Maximal content of these factors has been observed in early stage of pancreatic regeneration suggesting the involvement of these factors in pancreatic recovery. In this issue, Dobschütz et al. present data regarding the influence of a tyrosinekinase inhibitor (VEGF receptor blocker) on pancreatic microcirculation in a I/R model in rats. In the I/R animals, an increase in VEGF tissue levels was found associated with detrimental effects on microcirculation. In a further group with VEGF superfusion these damaging effects could be confirmed. The tyrosinekinase inhibitor treatment resulted in a significant reduction of microvascular albumin permeability, but failed to improve standard parameters for evaluation of the microcirculatory state like functional capillary density and leukocyte-endothelium interactions significantly. Thus, this investigation is a first step to support the importance of VEGF-evoked microvascular damage in the pancreas. However, a clear therapeutic potency of thyrosinekinase inhibitor treatment in this setting has to be confirmed by further studies. In summary, the development of new blood vessels from an existing vascular bed or from circulating precursor cells is tightly governed by the relative balance of pro- and anti-angiogenic stimuli. Understanding how these molecules interact with one another and with other mediators, is likely to lead to better management of conditions where angiogenesis plays a central role. Further studies are required to achieve a comprehensive understanding of the VEGF/VEGFR system; however, the recent progress in the molecular and biological study of this system provides us with novel and promising therapeutic strategies for a variety of diseases.