**2.3. Heterogeneity of RTK expression within the TME**

Human GBM is characterized with high degrees of intertumoral and intratumoral heterogeneity. For example, individual GBM tumors display striking histological variations. As a hallmark of GBM development, oncogenic RTK activation is highly responsible for malignant behaviors of multiple cells in the TME other than GBM cells, that is, endothelial cells, epithelial cells, astrocytes, infiltrated immune cells, glioma stem cells (GSC), etc. [2]. The malignant grade in human astrocytoma was associated with an upregulation of the PDGFRβ on vessel endothelial cells indicating the role of paracrine activation in tumor angiogenesis [21, 22]. Besides EGF, five other respective ligands activate EGFR including transforming growth factor alpha (TGF-α), amphiregulin, betacellulin, heparin-binding EGF-like growth factor (HB-EGF), and epiregulin, respectively. These ligands are secreted by glioma cells and received by tumor microenvironmental cells such as microglia and reactive astrocytes [2]. Axl/Gas6 signaling has multiple functions to regulate survival, proliferation, and migration in a variety of cells in vitro including tumor-derived cell lines of epithelial, mesenchymal, and hematopoietic origin [23]. Moreover, the Eph/ephrin system plays a role in many biological processes such as cell adhesion and migration during development, especially in the central nervous system [24]. In glioma, different Eph receptors are overexpressed not only in tumor cells but also in the surrounding tumor-infiltrating cells like tumor-associated macrophages (TAMs) [25], endothelial cells, stromal cells [26], as well as GSCs [27].

hypoxic (**Figure 1**). Axl is predominantly expressed in the pseudopalisading cells, along with other markers such as VEGFR, etc. Furthermore, an accumulation of Axl positive tumor cells appeared adjacent to microvascular neoformations, which is a characteristic feature of invad-

**Figure 1.** The hypoxic tumor cells stimulate neovascularization in GBM. Under hypoxic conditions, tumor cells secrete enhanced levels of VEGF family members (VEGF-A, VEGF-C). Endothelial cell-specific RTKs (VEGFR-2,VEGFR-3) via ligand (VEGF-A, VEGF-C) binding to stimulate proliferation and migration of endothelial cells. The peri-vascular

Receptor Tyrosine Kinase Interaction with the Tumor Microenvironment in Malignant…

http://dx.doi.org/10.5772/intechopen.76873

35

As one of the most prominent features in human GBM, pseudopalisading necrosis, the area of hypercellularity surrounding necrotic regions, and associated active vascular proliferation and tumor invasion are driven by hypoxia [32, 33]. Tumor cells reside in these regions have a high expression of HIF-1α and release VEGF, which is one of the most important regulators of angiogenesis and neovascularization (**Figure 1**). VEGF family members signal predominantly through the cognate RTKs, VEGFR-1, VEGFR-2, and VEGFR-3, in association with the co-receptors [34] via both hypoxia-dependent and hypoxia-independent mechanisms. Moreover, pseudopalisading necrosis regions protect glioma stem cells (GSC) in the region from therapeutic agents, and this facilitates the GSC niche to expand and contribute to tumor growth [35]. HIF-1α is a transcription factor that regulates the expression of a variety of genes

ing glioma tumor cells spreading along perivascular regions [2].

**3. Active interactions between RTK and TME**

**3.1. RTK, hypoxia and angiogenesis**

regions contain glioma stem cells (GSCs).

Activation of RTK pathways can lead to cellular transformation and result in genetic alteration in GSCs. Fully differentiated neural cells were able to generate malignant glioma upon PDGFA overexpression and showed high expression of stem and progenitor cell markers [28]. Growth factors such as PDGF, bFGF, and EGF were usually added to the serum-free media to maintain properties of cancer stem cells derived from patient tumor biopsies [29]. HGF/c-Met pathway was involved in brain tumorigenesis and malignant progression, and thus, HGF/c-Met signaling may maintain GSC properties [30]. Moreover, RTKs show various regional expression pattern within tumor in situ during tumor progression, for example, histopathological analysis on in vivo human glioma biopsies showed that Ang-2, MMP-2, MT1-MMP, and laminin5γ2 are co-overexpressed in the invasive areas but not in the central regions of the glioma tissues [31]. GBM is characterized with the unique pattern showing that necrotic areas are typically surrounded by "pseudopalisading" glioma cells, which are highly Receptor Tyrosine Kinase Interaction with the Tumor Microenvironment in Malignant… http://dx.doi.org/10.5772/intechopen.76873 35

**Figure 1.** The hypoxic tumor cells stimulate neovascularization in GBM. Under hypoxic conditions, tumor cells secrete enhanced levels of VEGF family members (VEGF-A, VEGF-C). Endothelial cell-specific RTKs (VEGFR-2,VEGFR-3) via ligand (VEGF-A, VEGF-C) binding to stimulate proliferation and migration of endothelial cells. The peri-vascular regions contain glioma stem cells (GSCs).

hypoxic (**Figure 1**). Axl is predominantly expressed in the pseudopalisading cells, along with other markers such as VEGFR, etc. Furthermore, an accumulation of Axl positive tumor cells appeared adjacent to microvascular neoformations, which is a characteristic feature of invading glioma tumor cells spreading along perivascular regions [2].
