ABSTRACT
Inhibitors of alpha(v)beta(3) and alpha(v)beta(5) integrin have entered clinical trials as antiangiogenic agents for cancer treatment but generally have been unsuccessful. Here we present in vivo evidence that low (nanomolar) concentrations of RGD-mimetic alpha(v)beta(3) and alpha(v)beta(5) inhibitors can paradoxically stimulate tumor growth and tumor angiogenesis. We show that low concentrations of these inhibitors promote VEGF-mediated angiogenesis by altering alpha(v)beta(3) integrin and vascular endothelial growth factor receptor-2 trafficking, thereby promoting endothelial cell migration to VEGF. The proangiogenic effects of low concentrations of RGD-mimetic integrin inhibitors could compromise their efficacy as anticancer agents and have major implications for the use of RGD-mimetic compounds in humans.
Subject(s)
Angiogenesis Inhibitors/therapeutic use , Integrin alphaVbeta3/therapeutic use , Lung Neoplasms/drug therapy , Melanoma, Experimental/drug therapy , Receptors, Vitronectin/therapeutic use , Animals , Disease Models, Animal , Humans , Lung Neoplasms/blood supply , Lung Neoplasms/pathology , Melanoma, Experimental/blood supply , Melanoma, Experimental/pathology , Mice , Neoplasms/blood supply , Neoplasms/drug therapy , Neovascularization, Pathologic/prevention & control , Oligopeptides/pharmacology , Oligopeptides/therapeutic use , Vascular Endothelial Growth Factor A/pharmacologyABSTRACT
Endothelial cells respond to vascular endothelial growth factor (VEGF) to produce new blood vessels. This process of angiogenesis makes a critical contribution during embryogenesis and also in the response to ischemia in adult tissues. We have studied the intracellular trafficking of the major VEGF receptor KDR (VEGFR2). Unlike other related growth factor receptors, we find that a significant proportion of KDR is held in an endosomal storage pool within endothelial cells. We find that KDR can be delivered to the plasma membrane from this intracellular pool and that VEGF stimulates this recycling to the cell surface. KDR recycling appears to be distinct from the previously characterized Rab4- and Rab11-dependent pathways, but, instead, KDR(+) recycling vesicles contain Src tyrosine kinase and VEGF-stimulated recycling requires Src activation. Taken together, these data show that intracellular trafficking of KDR is markedly different from other receptor tyrosine kinases and suggest that the regulation of KDR trafficking by VEGF provides a novel mechanism for controlling the sensitivity of endothelial cells to proangiogenic signals.
