Affiliation: UCSD SOM
Professor of Pathology
2005-Present, Professor of Pathology, UCSD School of Medicine
1996-2005 Professor, Department of Immunology, The Scripps Research Institute
1989-1996 Associate Professor, Department of Immunology, The Scripps Research Institute
1985-1989 Assistant Professor, Department of Immunology, The Scripps Research Institute
1984-1985 Senior Research Associate, Research Institute of Scripps Clinic
1982-1984 Postdoctoral Fellow, Research Institute of Scripps Clinic
1979-1982 Graduate Assistant Instructor, Department of Microbiology, University of Miami, Florida
Molecular Basis of Angiogenesis
SIGNALING EVENTS IN ANGIOGENESIS. Angiogenesis depends on signals propagated by both growth factor receptors and integrins expressed on the surface of endothelial cells. We recently identified two angiogenic pathways that could be distinguished by their dependency on the distinct av integrins, avb3 and avb5. Angiogenesis induced by basic fibroblast growth factor (bFGF) depends on ligation of integrin avb3, while that induced with vascular endothelial cell growth factor (VEGF) depends on integrin avb5. Evidence is provided that the non-receptor tyrosine kinase, Src, is functionally required for VEGF but not bFGF-mediated angiogenesis in both chick embryos and mice. In fact, Src kinase activity is required to ensure the survival of VEGF stimulated blood vessels in chick embryos. This has important implications for tumor growth as many tumors develop angiogenesis in a VEGF-dependent manner. Using a gene delivery strategy we have delivered mutant forms of Src to tumor-associated blood vessels. This lead to decreased tumor angiogenesis and regression of preestablished tumors. We have also targeted activated Src to preexisting blood vessels in the absence of growth factors and found this is sufficient for new blood vessel growth.
An alternative gene delivery strategy involves the targeting of components of the Ras-MAP kinase pathway. For example, delivery of a kinase defective Raf to tumor-associated blood vessels disrupts tumor-induced angiogenesis leading to the regression of preestablished tumors. However, delivery of activated form of Raf to preexisting blood vessels induced new blood vessel growth that was refractory to the effects of av integrin antagonists. These findings suggest that Raf kinase may act downstream of the av integrin-dependent signals required for angiogenesis.
ROLE OF RAF-1 IN ENDOTHELIAL CELL SURVIVAL AND ANGIOGENESIS. Previous studies from our laboratory have revealed that basic fibroblast growth factor (FGF) and vascular endothelial cell growth factor (VEGF) promote distinct pathways of angiogenesis that depend on integrins avb3 and avb5, respectively. These pathways can be further distinguished based on their specific signaling activity. For example, both pathways lead to activation of Raf-1 kinase, but do so via different upstream kinases. Specifically, VEGF/avb5 requires activation of Src kinases which phosphorylate Raf-1 on YY340-341 within its activation domain, whereas, bFGF/avb3 promotes p21 activated kinase 1 (PAK-1) mediated phosphorylation of SS 338/339. Importantly, when Raf-1 is activated by bFGF and PAK, Raf-1 can be localized to the mitochondria where it protects endothelial cells from apoptosis triggered by cell stress. In contrast, VEGF-mediated Src activation of Raf-1 promotes endothelial cell resistance to receptor-mediated apoptosis. Therefore, bFGF and VEGF activate two pathways of angiogenesis that can be characterized by distinct mechanisms of Raf-1 activation, which serves as a common downstream effector, protecting endothelial cells from stress or receptor-mediated death, respectively.
ROLE OF INTEGRINS IN CELL SURVIVAL AND MIGRATION. Our laboratory is interested in the mechanism(s) by which integrin-mediated signals can regulate cellular migration and survival. Recently, we described a novel mechanism of cell death that can be activated by the cell surface expression of unligated or antagonist-occupied integrins. This process, referred to Integrin-mediated Death (IMD) depends on the capacity of the unligated integrin to recruit active caspase 8 to the cell membrane in the absence of death receptors. Preliminary findings in our laboratory indicate that IMD is a principle mechanism by which integrin antagonists regulate angiogenesis in vivo. Integrin antagonists also activate death via p53 dependent mechanisms, and the loss of both p53 and caspase 8 provides strong protection against IMD, promoting tumor growth in vivo. In particular, the loss of caspase 8 is prevalent in neuroendocrine tumors such as small cell carcinoma and neuroblastoma, and is associated with poor prognosis. Re-expression of caspase 8 in neuroblastoma promotes susceptibility to IMD and increases cellular dependence on integrin ligation.
Concomitant with survival, integrins also regulate the cells’ migration machinery. We are currently evaluating this process by examining integrin cooperation with growth factors leading to cell movement. For example, we recently identified a Src-dependent pathway promoting coupling of focal adhesion kinase to the integrin avb5. Our preliminary findings suggest that this pathway is important for epithelial cell migration/invasion, and may contribute to the malignant properties of various carcinomas.
ANTI-ANGIOGENESIS CLINIC WITH ANTAGONISTS OF INTEGRIN AVB3. Cancer, inflammatory disease and diseases leading to adult blindness depend on angiogenesis, the growth of new blood vessels. We previously observed that antibody or peptide antagonists of av integrins disrupt angiogenesis resulting in the suppression of tumor growth, decreased arthritis and suppressed retinal neovascularization in vivo. Moreover, Vitaxin, a humanized form of an anti-avb3 monoclonal antibody, is currently being evaluated in Phase II clinical trials in cancer patients. The Vitaxin Phase I clinical trials showed disease stabilization or regression in more than 50% of the patients without causing side effects. In addition, a small peptide antagonist which targets angiogenic blood vessels is also being tested in cancer patients and early results are promising. New trials for rheumatoid arthritis will begin by late 1999.
Vascular-Targeted Gene Delivery
REGRESSION OF TUMORS VIA TARGETED GENE DELIVERY TO THE NEOVASCULATURE. Efforts to influence the biology of blood vessels by gene delivery have been hampered by a lack of targeting vectors specific for endothelial cells in diseased tissues. We found that by coupling a cationic nanoparticle to a ligand that targets the integrin a vb3, we can selectively deliver genes to angiogenic blood vessels in tumor-bearing mice. We tested the therapeutic effectiveness of this approach by generating nanoparticles conjugated to a mutant gene for Raf kinase; the protein produced by this gene blocks endothelial signaling and angiogenesis in response to multiple growth factors. Systemic injection of the conjugated nanoparticle into tumor-bearing mice resulted in apoptosis of the tumor-associated endothelium, ultimately leading to apoptosis of tumor cells and sustained regression of established primary and metastatic tumors.
To further evaluate Raf signaling, we examined the mechanisms by which blood vessels stimulated with bFGF or VEGF induce activation of Raf-1. Surprisingly, we found that during angiogenesis, bFGF and VEGF, together with integrins avb3 and avb5, respectively, activate Raf-1 through distinct mechanisms. VEGF-dependent activation not only depends on avb5-mediated Ras activity but also requires activation of Src via VEGF receptors, a step in which tyrosines 340 and 341 of Raf are phosphorylated. In contrast, bFGF-dependent activation requires Ras and a vb3-mediated induction of p21-activated kinase 1, a step in which serines 338 and 339 of Raf are phosphorylated. Phosphorylation of these serines promotes Raf-1 association with mitochondria, accounting for bFGF-mediated, but not VEGF-mediated, resistance of endothelial cells to stress-induced apoptosis. Therefore, bFGF and VEGF differentially regulate endothelial cell survival during angiogenesis through distinct pathways of Raf-1 activation.
Myocardial Infarction and Stroke
BLOCKADE OF SRC PREVENTS VEGF-MEDIATED VASCULAR PERMEABILITY THEREBY REDUCING INFARCT VOLUME FOLLOWING ISCHEMIC INJURY IN THE BRAIN OR HEART. Stroke or myocardial infarction following ischemic injury is a leading cause of death within the western world. Within minutes of an ischemic event, VEGF is produced, promoting a vascular permeability response in the affected vasculature. Recent studies in our laboratory demonstrate that VEGF-mediated Src kinase activity regulates this pathway of vascular permeability. Importantly, blocking Src via genetic or pharmacological means not only prevents VEGF-mediated vascular permeability, but results in a marked decrease in infarct volume in mice and rats. Most recently we have established that Src, when activated by VEGF, leads to phosphorylation of vascular endothelial cell cadherin (VE-cadherin) and b-catenin causing a transient disruption in endothelial cell barrier function. Src inhibition prevents VEGF-mediated phosphorylation of VE-cadherin and b-catenin stabilizing endothelial cell junctional integrity and blocking vascular permeability. To our surprise, myocardial infarction or systemic injection of VEGF produced the same Src-dependent disruption of endothelial cell barrier function, as determined by transmission electron microscope ultrastructural analysis. Importantly, the prevention of vascular permeability via systemic application of a Src kinase inhibitor provided greater than 50% increase in cardiac or neurological tissue viability following myocardial infarction or stroke, respectively. These findings reveal Src kinase to be a new therapeutic target for tissue injury following ischemic disease in the brain and heart.
References From PubMed (NCBI)