Endothelial Control of Vascular Growth and Quiescence: Functional Characterization of Potential Targets

Abstract

Blood vessels evolved to carry oxygen to distant organs. Not surprisingly, they are crucial for organ growth in embryos and for repair of wounded tissue in adults. Imbalances in angiogenesis, the growth of new blood vessels, promote malignancies and ocular and inflammatory disorders (1). In the past 15 years, an explosion of research interest in vascular biology has generated insights leading to the first clinically approved anti-angiogenic agents that combat cancer and blindness (1). The endothelium lines all the blood vessels and is the main target of those therapeutics, controlling the structure, tone, and interaction of the blood vessels with circulating blood elements and vascularized tissue and organs. Thus, the endothelium has remarkable, yet largely unexploited, diagnostic and therapeutic potential (2). With this habilitation, I present my efforts to characterize endothelial targets that have the potential to be used in treatments for vascular-dependent pathologies. I present four studies in which I assessed the function of signal-modulating proteins in the endothelium. A master regulator of cell growth, the signaling pathway centering on mammalian target of rapamycin (mTOR) is the focus of the first study (3), which defined the specific requirement for mTOR in angiogenesis under hypoxic conditions for the first time. The study contributed considerably to recognizing the potential of targeting mTOR to prevent restenosis in coronary stents by rapamycinelution, to reduce hypoxia-mediated pulmonary hypertension, and, particularly, to reduce tumor angiogenesis and growth (4). mTOR inhibition is now used clinically to treat specific cancers. Only a subset of patients respond to treatment, however. In the second study, a potential mechanism of rapamycin resistance is described. The growth-promoting Serine/Threonine kinase Pim1 is induced by mTOR-inhibition. Pim1 shuttles to the endothelial cell nucleus, where it apparently compensates and overcomes endothelial growth inhibition (5). Further study revealed, that Pim1 deletion generates a hyperadhesive endothelial phenotype, suggesting that PIM1 is crucial for endothelial cell proliferation and adhesion. Our results demonstrate that Pim1 deletion promotes the expression of an adhesion gene cluster with the deposition of a COL6A3-rich matrix as a starting point of specific outside-in signaling via integrins, which may increase vinculincontaining focal adhesion and enlarge membrane-associated β-catenin structures, finally leading to the hyperadhesive endothelial phenotype(6). Finally, mechanisms that restore, stabilize, and maintain the endothelial barrier function are necessary for normal function in established vessels. I show that Sprouty2, an FGF2-response modulating protein, is highly expressed only during quiescence and is required to sequester imbalanced angiogenic signaling and to redirect it from vessel growth to the maintenance of vascular integrity (7). These four original articles were generated as part of my PhD thesis at the Department of Biomedicine in Basel and under my guidance as Oberassistent at the Division of Internal Medicine at the University Hospital Zurich.