**3.2. RTK and ECM/integrin signaling**

ECM/ integrins are key components mediating the dialog between cells and the microenvironment. Integrins are composed of two noncovalently associated α and β subunits, which are featured by a large extracellular domain, a short transmembrane domain and a small intracellular noncatalytic cytoplasmic tail [46]. These receptors play a role in the regulation of cell adhesion to ECM proteins or cell surface proteins [47]. Binding of ECM to integrins result in cell adhesion and activation of focal adhesion (FA)-associated signaling pathways [48] and thereafter cascades of intensive activation of downstream signaling that involved in cell proliferation and invasion [49]. Within FA complexes, further auto-phosphorylation of focal adhesion kinase (FAK) leads to its binding to SRC kinase and formation of activated FAK-SRC complexes. Consequently, FAK-SRC complexes activate cascades of downstream pathways including the Ras/Raf/MAPK, RAF/JNK, Rho/Rac/PAK and PI3K/Akt/mTOR [50]. Notably, FAK protein is overexpressed in many tumors including GBM, and its expression level is greatly correlated with poor clinical prognosis [51, 52].

ECM dysregulation is essential for establishing and maintaining a functional tumor microenvironment. ECM in GBM is stiffer and more cross-linked than that in the normal brain tissue, inducing abnormal cell behaviors such as aggressive cell invasion [35]. Dysfunction of ECM and its cognate receptor integrin may lead to aberrant signaling transduction pathways including Ras/Raf/MAPK, Raf/JNK, Rho/Rac/PAK and PI3K/Akt/mTOR, shaping a tumor microenvironment to promote tumor survival, angiogenesis, and invasion [31]. Importantly, many of these cellular signaling pathways are convergent with downstream signaling pathways of RTKs, implicating interaction and cross-talk of RTK- and ECM/integrin-mediated function in GBM invasiveness and aggressiveness [53]. We demonstrated that hypoxia tumor microenvironment and ECM vitronectin could enhance tumor cell invasion and EGFRvIII activity via EGFRvIII and integrin β3 complex, emphasizing key roles of TME in tumor progression and metastasis [54]. Furthermore, as ECM may act as a reservoir for multiple growth factors such as VEGF, EGF, PDGF and TGF-β, release of these factors and their binding to their cognate receptors may also converge and further strengthen the activation of these signaling cascades, leading to uncontrolled cell behaviors in tumor growth and survival, angiogenesis, and invasion [55].

Knockout studies show the role of integrins in overactive GBM angiogenesis, which highly depends on VEGF and bFGF [56], αvβ3, αvβ5, β1, and αvβ8 notably play an important role during the process. For example, endothelial cells-expressed αvβ3/αvβ5 can provide survival signals and traction for invading cells, which are necessary to angiogenesis [57, 58]. αvβ3/αvβ5 associated neovascularization is respectively dependent on tumor cell-secreted bFGF/TNFα and VEGF and involved in a process leading to active interaction between tumor cells and endothelial cells [59]. Overexpression of αvβ3/αvβ5 in endothelial cells facilitates adhesive interactions with ECM proteins such as vitronectin, fibronectin, fibrinogen, osteopontin, etc. In cooperation with bFGF/VEGF, αvβ3/avβ5 also activates signaling pathways including FAK/ ILK, PI3K/Akt, and SDF1-CXCR4 [60] that promote EC proliferation, survival, and migration [61], and initiation of tumor angiogenesis. Collectively, several key integrins such as αvβ3, αvβ5, β1, and αvβ8 appear to be potential targets in GBM to reduce tumor angiogenesis [62].
