Glioblastoma Microenvironment and Invasiveness: New Insights and Therapeutic Targets

Abstract

Glioblastoma (GBM) is the most common and malignant primary brain cancer in adults. Without treatment the mean patient survival is approximately 6 months, which can be extended to 15 months with the use of multimodal therapies. The low effectiveness of GBM therapies is mainly due to the tumor infiltration into the healthy brain tissue, which depends on GBM cells' interaction with the tumor microenvironment (TME). The interaction of GBM cells with the TME involves cellular components such as stem-like cells, glia, endothelial cells, and non-cellular components such as the extracellular matrix, enhanced hypoxia, and soluble factors such as adenosine, which promote GBM's invasiveness. However, here we highlight the role of 3D patient-derived glioblastoma organoids cultures as a new platform for study of the modeling of TME and invasiveness. In this review, the mechanisms involved in GBM-microenvironment interaction are described and discussed, proposing potential prognosis biomarkers and new therapeutic targets.

Keywords: brain tumors; glioblastoma; glioblastoma stem-like cells; invasiveness; tumor infiltration; tumor microenvironment.

Figure 1

The migration and invasion of GBM cells is related to the inherent capacity and the cellular interactions that occur on the migratory front. (a) The invasiveness of GSCs is greater when it they are enriched in CD144, CD44, and KLRC3 markers. Along with the above, the activation of the signaling pathways SOX2, Wnt5a, LIF, TGF-β, and Hh has been related to the invasive phenotype. (b) GBM cells infiltrate vascular areas and interact with ECs. These cells secrete factors such as VEGF, Ang-1, BK, IL-8, and SDF-1, which promote cell invasion by expressing adhesion proteins such as N-cadherin and integrin on GSCs and non-GSCs. (c) Glial cells secrete IL-6, GNDF, CTGF, and ANGPT2, which promote the invasion of GBM cells (not GSC). On the other hand, connexin 43, whose expression is elevated in astrocytes associated with gliomas, is an important gap junction protein that allows direct communication between astrocytes and GBM cells. Activation of CX3CL1/CX3CR1 in tumor-associated microglia/macrophages (TAMs) increases the adhesion/migration capacity of GBM cells through the expression of MMP-2, -9, and -14.Finally, the secretion of Wnt3 by non-GSCs towards microglial cells, induce the transition to profile like M2; this activation increases the invasion of GBM cells.

Figure 2

Relationship between tumor microenvironment components and GBM invasiveness. The main constituents of the brain ECM are collagen, osteopontin, fibronectin, tenascin-C, hyaluronic acid, brevican, laminin-2, -5, and -8, which have a role key in modulating invasiveness. The hypoxic microenvironment of GBM, characteristic of this tumor, promotes invasion mainly through the induction of one of the family of transcription factors induced under conditions of low O₂ pressure (HIF-1 and 2), the increase in the expression of MMP-2 and MMP-9, plasminogen complex system (PAI-1), plasminogen receptor (S100A10), Upa receptor (uPAR), and markers of the epithelial–mesenchymal transition (EMT) process; expression and regulation of transcription factors such as ZEB1, Twist1, FAT1; as well as the induction of the epigenetic regulation of ODZ1. Adenosine has been recognized as one of the molecules that increases in the hypoxic niche of GBM. Proteins such as CD73, PAP, and ADA participate in its extracellular metabolism. This nucleoside regulates biological processes through the A₁, A_2B, and A₃ adenosine receptors, ultimately promoting the invasive phenotype of GSCs and non-GSCs.

Figure 3

Patient-derived glioblastoma organoids (PD-GBOs) generation and models to study GBM invasiveness. The two mean models for establishment included resected fresh tumor samples, which are dissociated or finely minced and grown in Matrigel in Neurobasal complete medium (NBM) in the presence of exogenous growth factor (EGF/bFGF) [186], or finely minced (without-single cell dissociation) and grown on ultra-low attachment plates in a Neurobasal medium without exogenous growth factor alone to support the extracellular matrix from the tissue [184]. The main characteristics and challenges of these models are showed in the boxes at right. Below are shown the main models proposed for the study in vitro and in vivo of invasiveness and aggressive infiltration that have been used to study GSC invasion and that could be applied for the study of invasive ability of different PD-GBOs and screening of anti-invasive drugs. The figure was created with Biorender.