Tumor Microenvironment and Glioblastoma Cell Interplay as Promoters of Therapeutic Resistance

Abstract

The invasive nature of glioblastoma is problematic in a radical surgery approach and can be responsible for tumor recurrence. In order to create new therapeutic strategies, it is imperative to have a better understanding of the mechanisms behind tumor growth and invasion. The continuous cross-talk between glioma stem cells (GSCs) and the tumor microenvironment (TME) contributes to disease progression, which renders research in this field difficult and challenging. The main aim of the review was to assess the different possible mechanisms that could explain resistance to treatment promoted by TME and GSCs in glioblastoma, including the role of M2 macrophages, micro RNAs (miRNAs), and long non-coding RNAs (lncRNAs) from exosomes from the TME. A systematic review of the literature on the role of the TME in developing and promoting radioresistance and chemoresistance of GBM was performed according to PRISMA-P (Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols) guidelines. A dedicated literature review search was also performed on the immunotherapeutic agents against the immune TME. We identified 367 papers using the reported keywords. The final qualitative analysis was conducted on 25 studies. A growing amount of evidence in the current literature supports the role of M2 macrophages and non-coding RNAs in promoting the mechanisms of chemo and radioresistance. A better insight into how GBM cells interact with TME is an essential step towards comprehending the mechanisms that give rise to resistance to standard treatment, which can help to pave the way for the development of novel therapeutic strategies for GBM patients.

Keywords: M2-macrophages; chemoresistance; immunotherapy; non-coding RNA; radioresistance; stem cells; temozolomide; tumor microenvironment; tumor-associated macrophages.

Figure 1

PRISMA diagram showing the research strategy and selection of papers about TME and its role in GBM radioresistance and chemoresistance.

Figure 2

PRISMA diagram showing the research strategy and selection of papers about immunotherapeutic strategies for GBM.

Figure 3

Schematic representation of the mutual communication between GSCs and GBM-TME through EVs. GSCs can release EVs directed toward the TME that carry various signaling molecules, including growth factors, cytokines, and chemokines, and promote tumor growth, angiogenesis, and immune suppression. GSCs can also communicate with each other through EVs containing miRNAs, lncRNAs, and other molecules. Similarly, cells in the TME can communicate with each other through EVs carrying cytokines, growth factors, and other signaling molecules. The cells in the TME, including immune cells, astrocytes, and endothelial cells, can also release EVs containing signaling molecules that can promote GSC self-renewal, differentiation, or chemo- and radioresistance. Understanding these pathways and the molecules involved in EV-mediated communication could provide new targets for the development of novel therapies for GBMs.

Figure 4

The transition of TAMs from M1 to M2 phenotype is linked to the advancement of tumors. TAMs are thought to drive the progression of GBM through various cytokines and factors, promoting proliferation of GBM cells, migration and invasion of GBM cells, angiogenesis within GBM, breakdown of the extracellular matrix (ECM), and an immunosuppressive TME. ↑ = increase in production; ↓ = reduction of production.