Development - VEGF signaling via VEGFR2 - generic cascades

VEGF signaling via VEGFR2 - generic cascades

Abstract

VEGF family of ligands and receptors is crucial for vascular development and neovascularization in physiological and pathological processes in both embryo and adult. VEGFR-2 is a high-affinity receptor for VEGF-A. Activated VEGFR-2 can activate p38 MAPK, PI3K reg class IA/ eNOS/ NO, PLC-gamma 1/ PKC/ ERK1/2 and PLC-gamma 1/ Ca('2+) cytosol pathways in ECs. This contributes to cell proliferation, angiogenesis, survival, cytoskeleton remodeling, migration and activation of vascular permeability in ECs.

Details

Vascular endothelial growth factor family of ligands and receptors is crucial for vascular development and neovascularization in physiological and pathological processes in both embryo and adult [1]. VEGFs denote a family of homodimeric glycoproteins, which currently consists of five members (VEGF-A, VEGF-B, VEGF-C, VEGF-D, and Placenta growth factor (PLGF)).

VEGFR-2 is a high-affinity receptor for VEGF-A [1]. Activated VEGFR-2 can activate PLC-gamma 1 directly or through the c-Src and TSAD adapter proteins [2], [3], [4]. PLC-gamma 1 activation 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 Protein kinase C beta 1 (PKC-beta), whereas IP3 binds to a specific receptor present on endoplasmic reticulum, resulting in the release of intracellular stored Ca('2+) cytosol. [5], [6], [7], [8].

PKC-beta phosphorylates and activates c-Raf-1 triggering MEK1 (MAP2K1) and MEK2 (MAP2K2)/ ERK1/2 signaling cascade. ERK1/2 can also be activated through PKC/ Sphingosine kinase 1 (SPHK1) pathway [9]. SPHK1 is an enzyme which catalyses Spingosine 1 phosphate formation from Sphingosine. Decrease of Sphingosine concentration and increase of sphingosine 1-phosphate may lead to activation of V-Ha-ras Harvey rat sarcoma viral oncogene homolog (H-Ras) through inhibition of Neurofibromin and RAS p21 protein activator 1 (p120GAP). H-Ras in turn binds to and activates c-Raf-1 leading to ERK1/2 activation. Activated ERK1/2 activates Jun oncogene (c-Jun) by phosphorylation. The latter forms a complex with V-fos FBJ murine osteosarcoma viral oncogene homolog (c-Fos) protein leading to DNA synthesis and cell proliferation [4], [6]. VEGFR-2 can also activate Prostaglandin I2 production through the ERK1/2 pathway contributing to VEGF-A-induced vascular permeability and angiogenesis in endothelial cells (ECs) [2].

DAG is also a physiological activator of PKC-alpha which can signal through IKK-alpha and IKK-beta to I-kB/ NF-kB p50/p65 pathway. NF-kB p50/p65 together with c-Jun/c-Fos activate transcription of CCL2 [10], [11].

VEGFR-2 also binds and activates PI3K reg class IA [12], followed by activation of PI3K cat class IA, which then results in an increase in lipid PtdIns(3,4,5)P3 and activation of PDK (PDPK1)/ AKT(PKB) pathway. AKT(PKB) signaling pathway regulates cellular survival by inhibiting pro-apoptotic pathways and inducing survival of ECs [8], [13]. PI3K reg class IA through PtdIns(3,4,5)P3 also activates Rac1. Rac1 can contribute to AKT(PKB) phosphorylation likely through the PAK1-dependent PDK (PDPK1) activation [7], [14].

AKT(PKB) can directly phosphorylate eNOS leading to nitric oxide production [7], [15], [16]. Another mechanism of eNOS activation involves phosphorylation of PLC-gamma 1, which leads to increase in intracellular levels of IP3 and elevation of Ca('2+) cytosol. Increase in Ca('2+) cytosol concentration stimulates eNOS to produce nitric oxide (NO) [2], [7]. Ca('2+) cytosol also activates PKC-alpha, which can directly induce AKT(PKB)/ eNOS signaling [5]. VEGF-A through VEGFR-2 also induce upregulation of both eNOS and iNOS protein levels supporting the important role of NO in VEGF-A-mediated ECs function and angiogenesis [17].

Release of Ca('2+) cytosol also activates Calcineurin A (catalytic)/ NF-AT2(NFATC1)/ COX-2 (PTGS2) pathway, which leads to upregulation of COX-2 (PTGS2) activity and induction of proliferation and angiogenesis of ECs [18], [19], [20], [21]. VEGF-A/ VEGFR-2 signaling upregulates protein expression of COX-1 (PTGS1) which also contributes to cell proliferation and angiogenesis in ECs [19].

VEGF-A-induced stimulation of VEGFR-2 also results in its association with Shc and GRB2 and the induction of Shc/ GRB2 complex formation. It is possible that Shc then mediates coupling of the GRB2/ SOS complex, promoting mitogenic H-Ras/ ERK1/2 signaling in ECs [22], [23], [24].

Activated VEGFR-2 can associate with Fyn, which forms a complex with NCK1 and PAK2. This leads to activation of CDC42/ MEKK1(MAP3K1)/ MEK3(MAP2K3)/ p38 MAPK pathway. Activation of p38 MAPK triggers phosphorylation of the Actin cytoskeletal polymerization modulator, HSP27 and subsequent cytoskeleton remodeling [23], [25], [26], [27], [28], [29].

Activated p38alpha (MAPK14) and ERK1/2 induce GSK3 beta/ Beta-catenin pathway and transcriptional activation of PLAUR (uPAR) most likely through the transcription factor TCF7L2 (TCF4). VEGF-A through VEGFR-2 can also activate PLAU (UPA), which leads to the activation of the PLAU (UPA)/ PLAUR (uPAR) system and increase in vascular permeability of ECs [30], [31], [32], [33].

Ligand-activated VEGFR-2 associates with HSP90, which induces phosphorylation of FAK1 in a RhoA/ ROCK1-dependent manner, leading to the recruitment of Vinculin and Paxillin. This leads to assembly of focal adhesions and stress fiber formation and subsequent migration of ECs [27], [34]. VEGFR-2 also associates with c-Src and SHB adapter proteins, which directly activate FAK1 signaling [34], [35].

VEGF-A triggers a synergistic interaction between the VEGFR-2 and the clustered alpha-V/beta-3 integrin. The interaction between VEGFR-2 and alpha-V/beta-3 integrin is required for full phosphorylation of VEGFR-2 and drives the activation of FAK1 and p38 MAPK pathways and migratory activity of ECs [27].

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