Research Projects

Vascular Endothelial Growth Factor Receptor (VEGFR) is the key regulator of angiogenesis and vasculogenesis. VEGFR has emerged as an important target to treat various malignant and non-malignant diseases. The receptor comprises an extracellular ligand-binding domain, linked to the intracellular kinase domain, connected to the C-terminal tail. Ligand-binding leads to the phosphorylation of multiple tyrosine residues in the C-tail, initiating diverse cellular processes. The lifetime of the phosphotyrosine (pY) residues is crucial for generating biased receptor response. A long-lived pY residue will lead to sustained signaling, compared to transient signaling by a short-lived pY residue. The VEGF receptors are activated by a repertoire of ligands that function as proangiogenic (VEGF-165a) and antiangiogenic (VEGF-165b). How the context-dependent receptor response is regulated is unknown. We hypothesized that the lifetime of the pY residues is determined by the cellular component, such as phosphatase, receptor density, or the type of ligand. In this proposal, we will comparatively study two VEGFR family members, VEGFR1 and VEGFR2, to determine how cellular phosphatase levels and the receptor density generate biased responses in cells. As a final objective, we will re-evaluate the structure and function of proangiogenic and antiangiogenic ligands to redesign the VEGFR ligand that will generate a predictable response. The VEGFR2 is the central regulator of angiogenesis and modulates multiple signaling pathways upon ligand binding. VEGFR1 is an elusive member of the VEGFR family and functions as a decoy receptor. However, the receptor is activated spontaneously under pathological conditions like diabetes and cancer. Recently, we showed how stimulating the VEGFR1 and VEGFR2 with VEGF-A produces distinct phosphorylation patterns. The VEGFR1 is transiently phosphorylated by VEGF-165a, compared to sustained phosphorylation of VEGFR2. An electrostatic hook on the kinase domain of VEGFR1 is critical for stabilizing the autoinhibited conformation of the kinase domain. The VEGFR1 is upregulated in various cancers, including pancreatic, colon, and renal carcinoma. In spite of an extensive search of the cancer patient database, we could not find a mutation that helps release the VEGFR1 autoinhibition. How the autoinhibition of VEGFR1 is overcome under pathological conditions is puzzling. Our preliminary data point towards a reduced expression of tyrosine phosphatase in renal carcinoma cells. Suggesting an important role of cellular phosphatase balance in remodeling the final outcome of ligand-dependent activation of VEGFR1. Therefore, one objective of this project is to investigate the role of intracellular phosphatase load in remodeling VEGFR1 signaling. The context-dependent activation and signaling of cell surface receptors is an evolving concept. How one receptor generates diverse responses when activated by a single ligand in various cell types is an open question. Our preliminary data suggest that low receptor density induces a transient tyrosine phosphorylation in the C-tail of VEGFR2, in contrast to sustained phosphorylation seen in cells overexpressing VEGFR2. These observations suggest that the receptor density on the plasma membrane is an important determinant for biased receptor signaling. How receptor density remodels VEGFR activation is unknown. Here we will investigate the life-time of VEGFR2 pY residues upon ligand stimulation at the plasma membrane and various trafficking vesicles. Finally, we will investigate how the signaling output is remodeled by proangiogenic and antiangiogenic ligands. Elucidating the mechanisms will not only advance our fundamental understanding of receptor tyrosine kinase biology but also pave the way for novel therapeutic strategies to modulate VEGFR activity in diseases. The new ligand design will provide an alternative strategy compared to targeting VEGFR with a monoclonal antibody.