title: Combined inhibition of VEGF and PDGF signaling enforces tumor vessel regression by interfering with pericyte-mediated endothelial cell survival mechanisms.
abstract: Destruction of existing tumor blood vessels may be achieved by targeting vascular endothelial growth factor (VEGF) signaling, which mediates not only endothelial cell proliferation but also endothelial cell survival. In this study, however, intravital microscopy failed to demonstrate that targeting of VEGFR-2 (by the tyrosine kinase inhibitor SU5416) induces significant regression of experimental tumor blood vessels. Immunohistochemistry, electron microscopy, expression analyses, and in situ hybridization provide evidence that this resistance of tumor blood vessels to VEGFR-2 targeting is conferred by pericytes that stabilize blood vessels and provide endothelial cell survival signals via the Ang-1/Tie2 pathway. In contrast, targeting VEGFR-2 plus the platelet-derived growth factor receptor (PDGFR)-beta system (PDGFR-beta) signaling (by SU6668) rapidly forced 40% of tumor blood vessels into regression, rendering these tumors hypoxic as shown by phosphorescence quenching. TUNEL staining, electron microscopy, and apoptosis blocking experiments suggest that VEGFR-2 plus PDGFR-beta targeting enforced tumor blood vessel regression by inducing endothelial cell apoptosis. We further show that this is achieved by an interference with pericyte-endothelial cell interaction. This study provides novel insights into the mechanisms of how 1) pericytes may provide escape strategies to anti-angiogenic therapies and 2) novel concepts that target not only endothelial cells but also pericyte-associated pathways involved in vascular stabilization and maturation exert potent anti-vascular effects.
authors.affiliation: Department of Neurosurgery, Medical Faculty of the University of Heidelberg, Mannheim, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Erber
authors.forename: Andreas
authors.initials: A
authors.lastname: Thurnher
authors.forename: Alice D
authors.initials: AD
authors.lastname: Katsen
authors.forename: Gesine
authors.initials: G
authors.lastname: Groth
authors.forename: Heinz
authors.initials: H
authors.lastname: Kerger
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Hammes
authors.forename: Michael D
authors.initials: MD
authors.lastname: Menger
authors.forename: Axel
authors.initials: A
authors.lastname: Ullrich
authors.forename: Peter
authors.initials: P
authors.lastname: Vajkoczy
mesh_terms: D000818:Animals; D017209:Apoptosis; D002470:Cell Survival / Q000187:drug effects; D000860:Hypoxia; D007211:Indoles / Q000494:pharmacology; D008833:Microcirculation / Q000187:drug effects; D008954:Models, Biological; D009369:Neoplasms / Q000098:blood supply*; D009389:Neovascularization, Pathologic / Q000188:drug therapy; D000078183:Oxindoles; D020286:Pericytes / Q000166:cytology* / Q000187:drug effects*; D010982:Platelet-Derived Growth Factor / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D011422:Propionates; D011758:Pyrroles / Q000494:pharmacology; D020797:Receptor, Platelet-Derived Growth Factor beta / Q000037:antagonists & inhibitors / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D042461:Vascular Endothelial Growth Factor A / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D040301:Vascular Endothelial Growth Factor Receptor-2 / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: D007211:Indoles; D000078183:Oxindoles; D010982:Platelet-Derived Growth Factor; D011422:Propionates; D011758:Pyrroles; D042461:Vascular Endothelial Growth Factor A; C116890:Semaxinib; C412603:orantinib; D020797:Receptor, Platelet-Derived Growth Factor beta; D040301:Vascular Endothelial Growth Factor Receptor-2