No abstract

No abstract

VEGFs (vascular endothelial growth factors) control vascular development during embryogenesis and the function of blood vessels and lymphatic vessels in the adult. There are five related mammalian ligands, which act through three receptor tyrosine kinases. Signalling is modulated through neuropilins, which act as VEGF co-receptors. Heparan sulfate and integrins are also important modulators of VEGF signalling. Therapeutic agents that interfere with VEGF signalling have been developed with the aim of decreasing angiogenesis in diseases that involve tissue growth and inflammation, such as cancer. The present review will outline the current understanding and consequent biology of VEGF receptor signalling.

The vascular endothelial growth factors VEGFA and VEGFC are crucial regulators of vascular development. They exert their effects by dimerization and activation of the cognate receptors VEGFR2 and VEGFR3. Here, we have used in situ proximity ligation to detect receptor complexes in intact endothelial cells. We show that both VEGFA and VEGFC potently induce formation of VEGFR2/-3 heterodimers. Receptor heterodimers were found in both developing blood vessels and immature lymphatic structures in embryoid bodies. We present evidence that heterodimers frequently localize to tip cell filopodia. Interestingly, in the presence of VEGFC, heterodimers were enriched in the leading tip cells as compared with trailing stalk cells of growing sprouts. Neutralization of VEGFR3 to prevent heterodimer formation in response to VEGFA decreased the extent of angiogenic sprouting. We conclude that VEGFR2/-3 heterodimers on angiogenic sprouts induced by VEGFA or VEGFC may serve to positively regulate angiogenic sprouting.

No abstract

Vascular endothelial growth factor (VEGF) guides the path of new vessel sprouts by inducing VEGF receptor-2 activity in the sprout tip. In the stalk cells of the sprout, VEGF receptor-2 activity is downregulated. Here, we show that VEGF receptor-2 in stalk cells is dephosphorylated by the endothelium-specific vascular endothelial-phosphotyrosine phosphatase (VE-PTP). VE-PTP acts on VEGF receptor-2 located in endothelial junctions indirectly, via the Angiopoietin-1 receptor Tie2. VE-PTP inactivation in mouse embryoid bodies leads to excess VEGF receptor-2 activity in stalk cells, increased tyrosine phosphorylation of VE-cadherin and loss of cell polarity and lumen formation. Vessels in ve-ptp−/− teratomas also show increased VEGF receptor-2 activity and loss of endothelial polarization. Moreover, the zebrafish VE-PTP orthologue ptp-rb is essential for polarization and lumen formation in intersomitic vessels. We conclude that the role of Tie2 in maintenance of vascular quiescence involves VE-PTP-dependent dephosphorylation of VEGF receptor-2, and that VEGF receptor-2 activity regulates VE-cadherin tyrosine phosphorylation, endothelial cell polarity and lumen formation.

The mechanism of cell transformation by Fms-like tyrosine kinase 3 (FLT3) in acute myeloid leukemia (AML) is incompletely understood. The most prevalent activated mutant FLT3 ITD exhibits an altered signaling quality, including strong activation of the STAT5 transcription factor. FLT3 ITD has also been found partially retained as a high-mannose precursor in an intracellular compartment. To analyze the role of intracellular retention of FLT3 for transformation, we have generated FLT3 versions that are anchored in the perinuclear endoplasmic reticulum (ER) by appending an ER retention sequence containing a RRR (R3) motif. ER retention of R3, but not of corresponding A3 FLT3 versions, is shown by biochemical, fluorescence-activated cell sorting, and immunocytochemical analyses. ER anchoring reduced global autophosphorylation and diminished constitutive activation of ERK1/2 and AKT of the constitutively active FLT3 versions. ER anchoring was, however, associated with elevated signaling to STAT3. Transforming activity of the FLT3 D835Y mutant was suppressed by ER anchoring. In contrast, ER-anchored FLT3 ITD retained STAT5-activating capacity and was transforming in vitro and in vivo. The findings highlight another aspect of the different signaling quality of FLT3 ITD: It can transform cells from an intracellular location.

Neuropilin 1 (NRP1) modulates angiogenesis by binding vascular endothelial growth factor (VEGF) and its receptor, VEGFR2. We examined the consequences when VEGFR2 and NRP1 were expressed on the same cell (cis) or on different cells (trans). In cis, VEGF induced rapid VEGFR2/NRP1 complex formation and internalization. In trans, complex formation was delayed and phosphorylation of phospholipase Cγ (PLCγ) and extracellular regulated kinase 2 (ERK2) was prolonged, whereas ERK1 phosphorylation was reduced. Trans complex formation suppressed initiation and vascularization of NRP1-expressing mouse fibrosarcoma and melanoma. Suppression in trans required high-affinity, steady-state binding of VEGF to NRP1, which was dependent on the NRP1 C-terminal domain. Compatible with a trans effect of NRP1, quiescent vasculature in the developing retina showed continuous high NRP1 expression, whereas angiogenic sprouting occurred where NRP1 levels fluctuated between adjacent endothelial cells. Therefore, through communication in trans, NRP1 can modulate VEGFR2 signaling and suppress angiogenesis.