Development - VEGF-family signaling

VEGF-family signaling

The vascular endothelial growth factor (VEGF) family of ligands and receptors is crucial for vascular development and neovascularization in physiological and pathological processes in both embryos, and in adults [1].

VEGFs belong to a family of homodimeric glycoproteins that contains five members (VEGF-A, VEGF-B, VEGF-C, VEGF-D, and Placenta growth factor PLGF). VEGFs bind to three different VEGF-receptor tyrosine kinases (VEGFR-1, VEGFR-2 and VEGFR-3). Upon ligation, VEGF-receptors dimerize, autophosphorylate and, thereby transduce signals that direct cellular function [2].

VEGFR-1 is a high-affinity receptor for VEGF-A, VEGF-B and PLGF [3], [4], [5]. It is expressed in vascular endothelial and some non-endothelial cells including haematopoietic stem cells, macrophages and monocytes [1], [6].

VEGFR-2 is highly specific towards VEGF-A [1]. However, it also binds the processed forms of VEGF-C and VEGF-D [7]. VEGFR-2 is expressed in both vacular endothelial and lymphatic endothelial cells. Its expression has also been demonstrated in several other cell types such as megakaryocytes and haematopoietic stem cells.

VEGFR-3 is highly specific towards VEGF-C and VEGF-D [8], [9]. It is expressed at high levels in lymphatic endothelial cells, but also is important for vascular development [10].

VEGF-receptor function is enhanced by interaction with co-receptors of VEGFs Neuropilin-1 and Neuropilin-2 [11], [12], [13]. VEGF-A, VEGF-B and PLGF bind to Neuropilin-1, whereas VEGF-A, VEGF-C and PLGF bind to Neuropilin-2 [14], [15]. Neuropilin-1 stabilizes the VEGFR-2 complex with VEGF-A [13], [16], whereas Neuropilin-2 might be required for stabilizing the complex of VEGFR-3 with its ligands [17].

L1 cell adhesion molecule (L1CAM) and VEGF-A bind to alpha-V/beta-3 integrin and to VEGFR-2 to induce endothelial cell adhesion, migration, and survival [18], [19].

Extracellular matrix protein Fibronectin binds to alpha-5/beta-1 integrin and VEGFR-3 and induces association of the alpha-5/beta-1 integrin with VEGFR-3. Both Fibronectin and VEGF-A bind to VEGFR-3 and selectively promote the growth of lymphatic endothelial cells [20].

VEGFR-1 binds to Src homology 2 domain containing transforming protein (Shc) and Growth factor receptor bound 2 (GRB2). VEGFR-1 also phosphorylates Phospholipase C gamma (PLC-gamma) [21], [22], [23]. VEGFR-1 also can interact with the regulatory subunit of Phosphatidylinositol 3-kinase (PI3K reg class 1A) [24].

VEGFR-2 is considered to be a major mediator of several physiological and pathological effects of VEGF-A on endothelial cells. The activated VEGFR-2 phosphorylates and activates PLC-gamma, which in turn results in hydrolysis of the membrane Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and generation of the second messengers 1,2-Diacylglycerol (DAG) and Inositol (1,4,5)-trisphosphate (IP3). DAG is a physiological activator of conventional isoforms of Protein kinase C, such as PKC-alpha, whereas binds to a specific present on the endoplasmic reticulum (IP3 receptor), resulting in the release of intracellular stored Ca(2+) [2].

Activation of ERKs by VRGFR-2 proceeds via a major pathway that involves association with the adapter proteins Shc and GRB2, subsequent stimulation of the guanine nucleotide exchange factor, Son of sevenless proteins (SOS) and activation of the v-Ha-ras Harvey rat sarcoma viral oncogene homolog (H-Ras). H-Ras in turn activates v-Raf-1 murine leukemia viral oncogene homolog 1 (c-Raf-1)/ Mitogen-activated protein kinase kinase 1 and 2 (MEK1 and MEK2)/ Mitogen-activated protein kinases 1 and 3 (ERK1/2) cascade, resulting in cell proliferation [1].

VEGFR-2 also binds and activates PI3K reg class 1A [25] followed by the activation of the catalytic subunits of PI3K (PI3K cat class 1A). This results in increase in the lipid Phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) and in activation of the v-AKT murine thymoma viral oncogene homolog (AKT). AKT-signaling pathway regulates cellular survival by inhibiting pro-apoptotic pathways [2].

The activated VEGFR-3 phosphorylates adapter proteins Shc and GRB2, and Shc/GRB2 protein complex can mediate the signal for lymphatic endothelial cell growth [26], [27]. The activation of the classical ERK1/2 pathway by VEGFR-3 is considered to be independent of H-Ras. Incidentally, PLC-gamma/ PKC-alpha-dependent activation of the ERK1/2 cascade has indeed been reported [28]. VEGFR-3 activation also leads to induction of PI3K and stimulation of AKT. AKT signaling is important for lymphatic and blood endothelial cell survival [29].

It is suggested that VEGFR-1 has a dual function in angiogenesis where it plays negative and positive roles depending on the circumstances.

VEGFR-1 possesses weak kinase activity that is about 10 times lower than that of VEGFR-2 [6]. However, VEGFR-1 is capable of transducing signals in endothelial cells [6], [30] as well as monocytes and macrophages [2], [31], [32], [33].

VEGFR-1 (or its soluble form sVEGFR-1) [2], [34] is possibly a "decoy" receptor that sequesters VEGF-A and thus renders it less available to the functional VEGFR-2 [13]. VEGFR-1 also can directly bind to Neuropilin-1, thus competing with VEGF-A. Such inactivated Neuropilin-1 is unable to interact with VEGFR-2 [11].

On the other hand, binding of PLGF to VEGFR-1 in endothelial cells leads to displacement of VEGF-A from VEGFR-1. As a result, increased amounts of VEGF-A are available to bind to the mitogenic response-inducing receptor VEGFR-2 [35]. Moreover, activation of VEGFR-1 by PLGF results in intermolecular transphosphorylation of VEGFR-2, and thereby amplification of the angiogenesis through VEGFR-2 [36].

The signal-transduction capacity of VEGFR-3 is directly enhanced by heterodimeric-complex formation with VEGFR-2 in primary human endothelial cells expressing both receptors [37].

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