Molecular characterization of glioblastoma vessels: Identification of insulin-like growth factor binding protein 7 (IGFBP7) as modulator of angiogenesis and tumor growth

Angiogenesis is a major requirement for tumor growth. Inhibition of angiogenesis is considered a promising strategy for the treatment of highly vascularized tumors, such as glioblastoma multiforme (GBM). Tumor vessels exhibit an aberrant molecular and functional phenotype; therefore, identification and functional characterization of molecular differences between normal and tumor vessels could lead to the development of more targeted and effective anti-angiogenic therapies. The objective of this thesis was to identify molecular biomarkers of GBM vessels and evaluate their potential role in angiogenesis. Molecular differences between GBM and non-malignant brain vessels were identified using a combination of laser capture microdissection of brain vessels, RNA amplification and transcriptional profiling using microarray technology. Among differentially expressed genes, insulin-like growth factor binding protein 7 (IGFBP7) was validated as a selective biomarker of GBM vessels, induced in and secreted by tumor endothelial cells, and deposited into the vascular basement membrane. Analyses of tumor microenvironment properties and mediators revealed that IGFBP7 induction in human brain endothelial cells (HBEC) is triggered by members of TGF-beta family released by GBM cells through ALK5/Smad-2 signaling pathway, associated with the late phase angiogenesis. Functional in vitro and in vivo studies using a chicken chorioallantoic membrane (CAM) tumor model indicated that IGFBP7 has pericyte-recruiting and tumor suppressive roles in GBMs. The findings presented in this thesis demonstrated novel functions of IGFBP7 in angiogenesis and tumorigenesis that may lead to future development of diagnostic or anti-angiogenic/anti-tumorigenic therapeutic approaches to treat GBMs