Vascular endothelial growth factor of the lung: friend or foe

The discovery of vascular endothelial growth factor (VEGF) changed the field of angiogenesis. We have learned that VEGF has broader actions than merely a driver of tumor angiogenesis, particularly that VEGF controlled several fundamental functions and properties of endothelial cells and nonendothelial cells. The lung is one of the main organs where VEGF controls several crucial physiological functions. These actions rely on tightly regulated temporal and concentration gradients of VEGF and VEGF receptor expression in the lung. Excessive or diminished VEGF have been linked to abnormal lung phenotypes and, in humans, linked to several diseases. The beneficial and detrimental actions of VEGF underscore that therapeutic targeting of VEGF in disease has to carefully consider the lung biology of VEGF.

Introduction

VEGF-A, also known as vascular permeability factor (VPF) [1^••], plays a fundamental role in physiological and pathophysiological forms of angiogenesis and regulation of endothelial cell differentiation (Figure 1). In contrast to fibroblast growth factor, which requires cell damage or basement membrane proteolysis for its release and binding to multiple cell targets, vascular endothelial growth factor (VEGF) is actively secreted and has high specificity for endothelial cells. The lung contains the highest level of transcripts [2] amongst a wide range of organs that express VEGF. VEGF is necessary for the formation of vascular beds of several organs during embryo development, as demonstrated by the lethality of VEGF knockout mice and abnormal vasculogenesis of the heart and large vessels with the loss of only a single copy of the VEGF gene [3^••]. VEGF contributes to endothelial cell nitric oxide (NO) production in coronary arteries and cultured umbilical vein endothelial cells [4]. The increase in endothelial cell NO synthase activity relies on the activation of Src and MAP kinase [5] and the PI3K/Akt pathway [6]. VEGF has an anti-inflammatory action, decreasing leukocyte adhesion in an NO-dependent manner [7].

VEGF prevents death of endothelial cells, both in vitro and in animal models of oxygen-mediated retinopathy [8]. VEGF-dependent survival of endothelial cells relies on the activation of PI3 kinase, Akt, and Src [5]. The discovery of the parent VEGF-A molecule led to the subsequent identification of the subforms B–E. In this review, we will focus on VEGF-A (designated hereafter as VEGF).

VEGF binds to two tyrosine-kinase receptors present on endothelial cells, Flt or VEGF receptor 1 (VEGFR-1) and KDR or VEGFR-2. Dominant negative mutant forms of VEGFR-2 can abrogate VEGF-induced signal transduction in vitro and reduce blood vessel proliferation in vivo models of brain tumors [9]. VEGFR-1 binds VEGF with approximately 10-fold higher affinity than KDR [10], undergoes receptor autophosphorylation, and stimulates Ca⁺⁺ influx. VEGF binding to VEGFR-2 results in cell ruffling, mitosis, chemotaxis, and actin rearrangement [11]. VEGFR-2 undergoes autophosphorylation more efficiently than receptor 1 upon ligand binding. Inhibition of VEGFR-2 blocks proliferation of cultured umbilical vein endothelial cells, in vivo angiogenesis, and vascular permeability [12]. VEGFR-2 is also stimulated in an autocrine manner by endothelial VEGF, which was recently found to be essential for endothelial survival [13]. VEGFR-1 plays a role in the organization of development of embryonic blood vessels [14] and in enhanced monocyte adhesion to endothelial cells [15]. There is the potential for crossinteraction of both VEGFRs as they are approximately 70% homologous and possibly heterodimerize in vivo. VEGFR-1 might act as a silent receptor for VEGF since it has a poor kinase activity. However, its downstream cell signaling remains poorly delineated [16]. It may also serve as a decoy for VEGF [17], as documented by the excess numbers of endothelial cells in amniotic membrane vessels of embryonic bodies lacking VEGFR-1 in the presence of intact VEGFR-2 [18]. On the contrary, VEGFR-1 enhances VEGF-induced VEGFR-2 signaling during abnormal angiogenesis, because it prevents endothelial cell apoptosis. As there are no conditional knockouts of VEGFR-1 and VEGFR-2, loss-of-function experiments have relied on neutralizing antibodies (such as DC101 against VEGFR-2 and MF1 against VEGFR-1), soluble chimeric molecules with the ligand-binding domain of VEGFR-1 (VEGF traps), or chemical inhibition with small-molecule inhibitors such as SU5416, which prevents VEGF-induced phosphorylation of VEGFR-2 [12] and, subsequently shown, to also block VEGFR-1 [19] (Figure 1).