Nader Rahimi, PhD
MS and PhD, Queens University (Canada)
Schepens Eye Research Institute, Harvard Medical School
Angiogenesis, growth of new blood vessels, is an important natural process in the body used for healing and reproduction. Abnormal blood vessel growth, either excessive or insufficient, is now recognized as a “common denominator” underlying many deadly and debilitating human diseases, including cancer, age-related blindness, diabetic ulcers, cardiovascular disease, stroke, and many others. Indeed, the list of diseases that has angiogenesis as an underlying mechanism grows longer every year. Angiogenesis-based treatment is a novel approach to treating many human diseases. Angiogenesis-based treatments either inhibit or stimulate angiogenesis, now being used for prolonging the lives of cancer patients, and reversing vision loss. Our laboratory long has been interested in unraveling the molecular signaling of angiogenesis. A brief description of the current projects is outlined below.
Elucidation of angiogenic signaling of VEGFR-2: Our systematic analysis of VEGFR-2 during the past 10 years has identified key signaling pathways activated by VEGFR-2. Activation of VEGFR-2 stimulates a number of key signal transduction pathways in endothelial cells. Activation of PI3 kinase (phosphoinositide 3-OH kinase), requires tyrosine phosphorylation of Y799 and Y1173 on mouse VEGFR-2 (Y801 and Y1175 on human VEGFR-2) which stimulates endothelial cell survival and proliferation. Phosphorylation of Y1173 on VEGFR-2 also is responsible for activation of PLCg1 (phospholipase Cg1), which stimulates endothelial cells tubulogenesis and cell growth. Src kinases are also activated by VEGFR-2 and contribute to VEGFR-2 mediated cellular events. In particular, c-Src kinase directly through its Src homology 2 (SH2) domain and indirectly via c-Cbl binds to phospho-Y1057 (Y1059 in human VEGFR-2) of VEGFR-2. In turn, c-Src kinase phosphorylates VEGFR-2 at multi-docking site, Y1173 and also catalyzes tyrosine phosphorylation of IQGAP1 and acts as an adaptor to bridge IQGAP1 to VEGFR-2. IQGAP1 activates b-Raf and mediates proliferation of endothelial cells. c-Cbl, an ubiquitin E3 ligase also is activated by VEGFR-2 and mediates ubiquitination of PLCg1 resulting in the inhibition of its activity and with it angiogenesis . The current projects in our laboratory are focused in the identification of additional signaling molecules involved in VEGFR-2 signaling, and to determine molecular mechanisms of their activation by VEGFR-2 and their biological importance in pathological angiogenesis.
Role of ubiquitination pathway in angiogenesis: Attachment of ubiquitin to proteins regulates a broad range of key cellular events such as proteosomal degradation, tumor suppression, inflammation, cell cycle progression, and modulation of signaling pathways. Together with ubiquitin activating enzyme E1 and ubiquitin-conjugating enzyme E2, E3 ubiquitin ligases catalyze the ubiquitination of a variety of biologically significant protein substrates for targeted degradation through the 26S proteasome, as well as for non-proteolytic regulation of their functions or subcellular localizations. Our recent work has identified Casitas B-lineage lymphoma (c-Cbl) E3 ubiquitin ligase as a negative regulator of angiogenesis. Upon stimulation by VEGF, VEGFR-2 recruits and activates c-Cbl. As a result of activation by VEGFR-2, c-Cbl ubiquitinates PLCg1 and inhibits VEGFR-2/ PLCg1 driven angiogenesis. We are currently investigating role of c-Cbl ubiquitin E3 ligase and other related proteins in angiogenesis and mechanisms involved in this process. In particular, we are studying role of ubiquitin E3 ligases in ubiquitination of VEGFR-2, and its major substrate, PLCg1. Various in vivo and in vitro models of angiogenesis including genetically engineered mouse models are used to determine role of ubiquitin pathway in VEGFR-2 signaling and angiogenesis.
Amrik J. Singh, Rosana D. Meyer, Hamid Band and Nader Rahimi. The carboxyl terminus of VEGFR-2 is required for protein kinase C-mediated downregulation. Molecular Biology of the Cell, 2005, 16(4):2106-18., 2005.
Meyer RD, Mossa Mohammadi, and Rahimi N. A single amino acid substitution in the activation loop defines the decoy characteristic of VEGFR-1. Journal of Biological Chemistry, 2006, 281(2):867-75.
Rosana D. Meyer, and Nader Rahimi. Leucine motif-dependent tyrosine autophosphorylation of type III receptor tyrosine kinases. Journal of Biological Chemistry, 2006, 281(13):8620-7.
N. Rahimi. VEGFR-1 and VEGFR-2: Two non-identical twins with a unique physiognomy. Frontiers in Bioscience, 2006, 11:818-29.
Singh AJ, Meyer RD, Navruzbekov G, Shelke R, Duan L, Band H, Leeman SE, Rahimi N. A critical role for the E3-ligase activity of c-Cbl in VEGFR-2-mediated PLCgamma1 activation and angiogenesis. Proceeding National Academy of Science USA, 2007, 104: 5413-8.
Rosana D. Meyer, David B. Sacks, and Nader Rahimi. IQGAP1-Dependent Signaling Pathway Regulates Endothelial Cell Proliferation and Angiogenesis. PLoS ONE, 2008, 3(12):e3848.
Rahimi N, Golde TE and Meyer RD. Identification of ligand-induced proteolytic cleavage and ectodomain shedding of VEGFR-1/FLT-1 in leukemic cancer Cells. Cancer Research, 2009; 69:2607–14.