Ο ρόλος της διαμερισματοποίησης του υποδοχέα VEGFR2 στη μεταγωγή του σήματος του VEGF

Abstract

The vascular system represents one of the primitive systems formed in a developing organism,embracing all tissues and organs. It consists of a complex tubular network of blood vessels comprisedof endothelial cells (ECs), the main cellular unit that orchestrates the formation of the vascular plexus.The homeostasis of endothelium and thus, the transition from quiescence to angiogenesis, and viceversa, is balanced by the response of ECs to angiogenic stimuli. Aberration of this tightly controlledprogram is linked to a variety of disorders, with abnormal angiogenesis being a hallmark of cancerprogression and metastasis. Despite their diversity and complexity, ECs functions are predominantlydirected by the interplay between vascular endothelial growth factor receptors (VEGFRs) and theirligand, VEGF.VEGF is the most potent pro-angiogenic cytokine and a key molecular player in thepathophysiology of vascular system. Binding of VEGF to the extracellular domain of VEGFR2 promotesthe activation of downstream molecules and subsequent initiation of signalling cascades that propagateECs survival, proliferation, migration, differentiation, inflammation, vascular permeability and stem cellmobilization. Given the broad spectrum of action and the pivotal role in the regulation of ECs biology, unraveling of the underlying mechanisms that govern the VEGF-VEGFR2 system is imperative for thecomprehension of vascular pathophysiology and for targeted therapy application.At the cellular level, the response to extracellular stimulants is fine-tuned through thecoordinated regulation of the cellular compartmentalization and the intracellular trafficking of activatedreceptors. Unambiguously, the significance of membrane localization is set by the intrinsic traffickingand signalling of VEGFR2. Here, we employed a comprehensive screening to map all possibleinternalization routes of VEGFR2 and tested their functional significance in vitro and in vivo. Our datarevealed that, in the absence of ligand, VEGFR2 follows solely dynamin-dependent endocytosis, viawhich the receptor escapes from plasma membrane shedding and preserves its functional state untilligand activation. The presence of VEGF introduces macropinocytosis as an alternative internalizationroute for VEGFR2. Experimental approaches that uncouple signalling from trafficking revealed thatVEGFR2 internalization via macropinocytosis is critical for ERK1/2 activation by VEGF, while, despitethe current view, dynamin-mediated endocytosis of VEGFR2 is not essential. However, dynamindependenttrafficking of downstream components of the VEGFR2 cascade, such as PLCγ, is aprerequisite for macropinocytosis-mediated signal propagation, both in vitro and in vivo. These datareveal that propagation of growth factor receptor signalling requires coordinated trafficking of both thereceptor and downstream signalling molecules.