Vascular endothelial growth factor-A (VEGF) is the master determinant for the activation of the angiogenic program leading to the formation of new blood vessels to sustain solid tumor growth and metastasis. VEGF specific binding to VEGF receptor-2 (VEGFR-2) triggers different signaling pathways including phospholipase C (PLC) and Akt cascades, crucial for endothelial proliferation, permeability and survival. By combining biological experiments, theoretical insights and mathematical modeling, we found that: (i) cell density influences VEGFR-2 protein level, as receptor number is 2-fold higher in long-confluent than in sparse cells; (ii) cell density affects VEGFR-2 activation by reducing its affinity for VEGF in long-confluent cells; (iii) despite reduced ligand-receptor affinity, high VEGF concentrations provide long-confluent cells with a larger amount of active receptors; (iv) PLC and Akt are not directly sensitive to cell density, but simply transduce downstream the upstream difference in VEGFR-2 protein level and activation; (v) the mathematical model correctly predicts the existence of at least one protein tyrosine phosphatase directly targeting PLCand counteracting the receptor-mediated signal. Our data-based mathematical model quantitatively describes VEGF signaling in quiescent and angiogenic endothelium, and is suitable to identify new molecular determinants and therapeutic targets.
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