Angiogenesis

An Overview of VEGF

Vascular endothelial growth factor (VEGF) plays an important role in normal angiogenesis as well as pathological neovascularization and vascular exudation in oncology and exudative eye diseases. While normal and pathologic angiogenesis involve a complex balance of positive and negative regulators, VEGF-A, also referred to as VEGF, is one of the most important positive regulators of angiogenesis [7] and vascular permeability [8, 9].

VEGF-A Isoforms

VEGF-A is a member of the VEGF family of growth factors that also includes VEGF-B, VEGF-C, VEGF-D, platelet derived growth factor (PDGF), and placental growth factor (PlGF), which have different binding affinities for the three VEGF receptors: VEGFR1, VEGFR2, and VEGFR3 [10, 11]. VEGF-A also binds to neuro- pilin-1, a membrane protein on developing neurons that plays a role in embryonic neural blood vessel formation as well as neural tip guidance [12, 13]. Alternative RNA splicing of the human VEGF-A gene results in the formation of four major isoforms (VEGF121, VEGF165, VEGF189, and VEGF206) and at least five minor isoforms

(VEGF145, VEGF148, VEGF162, VEGF165b, andVEGF183). VEGF121 is freely diffusible, while

VEGF189 is found in the extracellular matrix; VEGF165 exists in both diffusible and matrixbound forms [14]. Physiologic protease degradation of VEGF also results in biologically active breakdown products that contain fewer than 121 amino acids [14, 15].

Vascular Endothelial Growth Factor-A:

Physiological and Pathological

Response

VEGF-A Physiological Response

VEGF has different functions in the angiogenesis cascade. VEGF regulates both endothelial cell mitosis [7] and survival rate [16], and acts as

a chemo-attractant for bone marrow-derived endothelial progenitor cells [17]. Moreover, VEGF induces the upregulation of extracellular matrix-degrading enzymes, such as metalloproteinases (MMPs) [18] and plasminogen activator [19], as well as nitric oxide [20], a downstream mediator of VEGF signaling [21].

In addition to promoting angiogenesis, VEGF-A also affects vascular permeability. VEGF-A is the most potent known inducer of vascular permeability, approximately 50,000 times more potent than histamine [9]. VEGFmediated vascular permeability results from the formation of pores in the vascular endothelial cells [22] and the disruption of intercellular junctions between these cells [23]. The angiogenic and vascular permeability effects of VEGF-A on the endothelium are mediated by the transmembrane receptor VEGFR2 [flk-1/kinase insert domain receptor (KDR)] and involve diverse downstream signaling partners, such as the Src family kinases and/or protein tyrosine phosphatases, which result in the disruption and uncoupling of junctions between endothelial cells [24]. This, in turn, leads to the extravasation of fluid, proteins, and circulating inflammatory cells [24]. In neovascular ocular diseases, the edema from new, permeable blood vessels as well as established vessels can disrupt the retinal anatomy and separate the retina from underlying structures, which is in part responsible for the vision loss associated with CNV and macular exudation.

VEGF also plays a role in promoting inflammation. Inflammation involves the release of various cytokines at specific sites in the body by inflammatory cells such as T cells, B cells, macrophages, natural killer cells, neutrophils, and granulocytes. These proinflammatory cytokines include tumor necrosis factor (TNF)-a, interleukin (IL)-6, IL-8, and IL-1a, IL-1b, and oncostatin M, which participate in a cascade of events leading to increased levels of VEGF-A, the promotion of local angiogenesis, and worsening inflammation.

VEGF-A Response in Retinal Diseases

Evidence from preclinical and clinical studies implicates VEGF-A in the pathogenesis of neovascular eye diseases. In streptozotocin-induced diabetic rats, VEGF-A gene expression was