Stuck (and Inhibited) at the Surface

Abstract

Vascular endothelial cadherin (VEC) inhibits the signaling of the vascular endothelial growth factor receptor (VEGFR), which may contribute to contact-mediated growth inhibition, a phenomenon that is lost in cancerous cells. Lampugnani et al. show that VEC sequesters VEGFR at the plasma membrane, preventing internalization. When sequestered at the plasma membrane at the site of cell-cell junctions, VEC promotes the dephosphorylation of VEGFR through the recruitment of the phosphatase DEP-1 to the complex. The authors studied VEGFR endocytosis and signaling in human umbilical vein endothelial cells (HUVECs) and VEC-null or VEC-positive cultured cell lines. In HUVECs, VEGF-stimulated internalization of VEGFR was decreased in confluent cultures compared with that observed in sparse cultures. A similar lower amount of internalization was observed in VEC-positive cultured cells compared with that in VEC-null cells. Internalized VEGFR was active, and the VEC-null cells showed greater abundance of tyrosine-phosphorylated VEGFR and activated phospholipase C-γ on internal membranes than did the VEC-positive cells. Internalization was clathrin mediated, and inhibition of clathrin-mediated endocytosis in the VEC-null cells returned VEGFR signaling--measured as tyrosine phosphorylation of mitogen-activated protein kinases (MAPKs) p44 and p42--to levels similar to those in VEC-positive cells. VEC was not endocytosed with VEGFR, suggesting that VEGFR activity was inhibited at the plasma membrane. VEC-positive cells in which DEP-1 was knocked down with silencing RNA showed enhanced VEGFR internalization and VEGFR phosphorylation and MAPK activation. Thus, VEC appears to stabilize VEGFR at the cell surface by stimulating VEGFR dephosphorylation at sites of cell-cell contact through the recruitment of DEP-1. Signaling by VEGFR after internalization appears to be an important mechanism of signal transduction by this growth factor receptor, and blocking this internalization is one mechanism by which activity can be controlled. M. G. Lampugnani, F. Orsenigo, M. C. Gagliani, C. Tacchetti, E. Dejana, Vascular endothelial cadherin controls VEGFR-2 internalization and signaling from intracellular compartments. J. Cell Biol. 174 , 593-604 (2006). [Abstract] [Full Text]