Netrin-1 in Glioblastoma Neovascularization: The New Partner in Crime?

Abstract

Glioblastoma (GBM) is the most aggressive and common primary tumor of the central nervous system. It is characterized by having an infiltrating growth and by the presence of an excessive and aberrant vasculature. Some of the mechanisms that promote this neovascularization are angiogenesis and the transdifferentiation of tumor cells into endothelial cells or pericytes. In all these processes, the release of extracellular microvesicles by tumor cells plays an important role. Tumor cell-derived extracellular microvesicles contain pro-angiogenic molecules such as VEGF, which promote the formation of blood vessels and the recruitment of pericytes that reinforce these structures. The present study summarizes and discusses recent data from different investigations suggesting that Netrin-1, a highly versatile protein recently postulated as a non-canonical angiogenic ligand, could participate in the promotion of neovascularization processes in GBM. The relevance of determining the angiogenic signaling pathways associated with the interaction of Netrin-1 with its receptors is posed. Furthermore, we speculate that this molecule could form part of the microvesicles that favor abnormal tumor vasculature. Based on the studies presented, this review proposes Netrin-1 as a novel biomarker for GBM progression and vascularization.

Keywords: Netrin-1; exosomes; glioblastoma; neovascularization; pericytes; transdifferentiation.

Figure 1

In silico analysis of NTN1 expression in gliomas. (A) Representative images of immunohistochemical staining of high-grade glioma samples using a Netrin-1 specific antibody. (B) RNA-seq data in 17 cancer types are reported as median FPKM (number Fragments Per Kilobase of exon per Million reads), generated by The Cancer Genome Atlas (TCGA). Data adapted under creative commons license from The Human Protein Atlas [66,67]. Images available from https://www.proteinatlas.org/ENSG00000065320-NTN1/pathology, accessed on 12 May 2021. (C) Pearson’s correlation analysis between the expression of NTN1 and VEGFA, ANGPT1, ANGPT2, using data from the Chinese Glioma Genome Atlas (CGGA) cohort visualized in the GlioVis data portal [68] (http://gliovis.bioinfo.cnio.es, accessed on 12 May 2021). The correlation coefficient (r) is shown on the right. Scale bar in A: 200 μm.

Figure 2

Signaling pathways that could regulate the expression of NTN1 in GBM. (A) NTN1 expression could be regulated by the canonical SHH signaling pathway, as described recently on medullobastoma, specifically through the transcriptional function of activator isoforms of the GLI zinc finger proteins (Falcon et al., unpublished observations) [72]. (B) Alternatively, the NF-κB pathway could modulate NTN1 expression, directly through binding to the promoter region of this gene, or indirectly through the regulation of SHH expression [74,75]. GLIa, GLI 1, and GLI2 activator forms; SHH, Sonic Hedgehog protein; PTCH, Patched protein receptor; SMO, Smoothened protein; IκB, inhibitor nuclear factor kappa B; NF-κB, nuclear factor kappa B; NTN1, Netrin-1 gene.

Figure 3

Schematic cartoon summarizing hypothetical models of Netrin-1 signaling that could promote expression of C-MYC. (A) Interaction of Netrin-1 with its receptor UNC5A may trigger the phosphorylation of NF-κB and the activation of the expression of C-MYC [10]. (B) Interaction of Netrin-1 with its receptors NEO-1 and UNC5B could promote the phosphorylation of Gsk3α/β and the stabilization of β-catenin, which in turn could trigger the expression of C-MYC [84]. (C) Interaction of Netrin-1 with its receptor NEO-1 could promote the activation of an Integrin heterodimer, formed by α and β1 subunits, and trigger the expression of C-MYC through regulation of the MAPK/SRC axis. STAT3 and ERK 1/2 may promote the induction of C-MYC expression by acting as transcription factors in the promoting region of the gene [80].

Figure 4

Schematic of possible participation of GBM cell-derived exosomes in promoting angiogenesis in the tumor niche. The figure depicts the possible content of exosomes derived from GSC and/or those derived from GBM tumor cells (growth factors, proteins, proteases, and ncRNAs) [5,6,8,9,89,90,91]. Besides, the uptake of these microvesicles by EC in the tumor vasculature, may promote the increase in the formation of new blood vessels through the promotion of cell proliferation, migration, and invasiveness [6,8,9,91,92,93].