The endothelial cells (EC) are often regulated by the signaling from the peptide growth factors and the cellular adhesion receptors. The functional responses include cell adhesion, migration and proliferation, which, in turn, are essential for more complex processes such as formation of the endothelial tube network during angiogenesis. The process of angiogenesis, in turn, plays a crucial role in the pathogenesis of numerous diseases, including but not limited to tumor growth/metastasis, diabetic retinopathy, and tissue remodeling upon injury. During angiogenesis newly produced growth factors, including Vascular Endothelial Growth Factor (VEGF), initiate a series of intricate intracellular events which induce an """"""""inside-out"""""""" signal that alters the ligand binding affinity/avidity of the extracellular domains of integrins, the most studied group of heterodimeric transmembrane receptors that connect cells to the extracellular matrix (ECM). Such growth-factor-mediated modulation of integrin ligand binding function, commonly referred to as activation, is most clearly evident in the regulation of blood and vascular cell responses by the ? integrin family. Integrin binding triggers conformational changes and clustering of receptors, ultimately leading to the generation of large intracellular protein complexes linked to the cytoskeleton. This physical linkage between extracellular and intracellular compartments allows dynamic regulation of many cellular processes including cell migration, shape change, proliferation and differentiation. Integrin aV?3, originally identified as the vitronectin receptor, is expressed at variable density on many types of vascular cells, blood cell, tumors and osteocells. It has been demonstrated that one of the key physiological mechanisms of aV?3 activation on endothelium is via VEGF/VEGF receptor 2 (VEGFR2). EC stimulation by VEGF induces formation of the high affinity state of aV?3 and, moreover, activated aV?3 interacts with VEGFR2. A relationship between VEGFR2 and ?3 integrin appears to be synergistic, since VEGFR2 activation induces ?3 integrin tyrosine phosphorylation, which, in turn, is crucial for VEGF induced tyrosine phosphorylation of VEGFR2. It has been also shown that the complex formation between VEGFR2 and aV?3 is crucial event in regulation of angiogenesis, but the impact and structural requirements for this crosstalk have yet to be determined. In preliminary studies, we have obtained some novel and exciting preliminary data, which suggest that VEGFR2/aV?3 cytoplasmic tails (CT) are involved in direct interaction. This complex formation between two major EC receptors may underlie a major regulatory mechanism for the regulation of the integrin-dependent functions and for the overall angiogenic response.
This proposal presents the evidence that there is a complex between VEGFR2 and integrin ? cytoplasmic tails. We will investigate the exact mechanisms and the role of this interaction: the VEGF-integrin partnership may serve as an important model for fundamental studies of the communication between growth factors and cell adhesion systems. Our data may ultimately lead to a new paradigm for understanding how VEGF and integrin cross talk and regulate the complex cell adhesion events and angiogenic response. Moreover, our structural approach should contribute to the development of new therapeutic agents designed to inhibit integrin (and other receptors) cytoplasmic domain protein-protein interactions.
