Belonging to a network--microRNAs, extracellular vesicles, and the glioblastoma microenvironment

Abstract

The complexity of glioblastoma multiforme (GBM) and its distinct pathophysiology belong to a unique brain microenvironment and its cellular interactions. Despite extensive evidence of a role for microRNAs in GBM cells, little is known about microRNA-dependent communication between different cellular compartments of the microenvironment that may contribute to the tumor phenotype. While the majority of microRNAs are found intracellularly, a significant number of microRNAs have been observed outside of cells, often encapsulated in secreted extracellular vesicles (EVs). The function of these circulating/secreted microRNAs has not been explored in the context of the brain tumor microenvironment. Establishing how microRNAs are involved in the regulation of oncogenic signaling networks between tumor cells and stroma is likely to add a needed additional layer of complexity to the tumor network, consisting of intercellular communication. More importantly, microRNA/EV signaling may provide an additional therapeutic target for this deadly disease.

Keywords: exosomes; extracellular vesicles; glioblastoma multiforme; microRNA; tumor microenvironment.

Fig. 1.

The glioblastoma microenvironment. The microenvironment of glioblastoma is composed of several specialized cell types, which may contribute to tumor growth and invasion providing growth factors, neutrophic factors, and chemokines. Cells from the tumor microenvironment may also be recruited by cancer cells for their own benefit. Different cell types from the tumor microenvironment communicate via the release and uptake of EVs. Such communication can take place both locally and at distant ranges and contributes to tumor progression by transferring of bioactive molecules, including microRNAs.

Fig. 2.

Schematic representation of microRNA network action (example: microRNA-128). MiR-128 is one of the most downregulated microRNAs in glioblastoma cells. When reintroduced, either transiently by oligonucleotide precursors or permanently by lentiviral vector, it directly targets a broad array of oncogenes. The “blocked” function of many oncogenic pathways results in numerous indirect changes in level/activity of downstream effectors. Both direct and indirect effects of miR-128 result in antiproliferative, anti-angiogenic, and pro-apoptotic phenotypic rearrangements. Such phenotypes may be transferred to other cells (including glioblastoma stem cells, GSC) in the tumor microenvironment by EVs, either by direct transfer of miR-128 or by modulation of vesicular cargo, potentiating the effect of miR-128 replacement.

Fig. 3.

Mechanisms of microRNA cellular release and extracellular transfer. In the nucleus, microRNAs are transcribed from DNA by pol II. A precursor hairpin microRNA (pre-microRNA) is formed after cleavage of primary transcript by the RNase III enzyme Drosha. Then, pre-microRNAs are transported into the cytoplasm and are further cleaved into 19- to 23-nucleotide mature microRNA duplexes by the enzyme Dicer. One strand of the microRNA duplex is loaded into the RNA-induced silencing complex (RISC), where it guides the RISC to specific mRNA targets, preventing the translation of mRNA into protein. In the cytoplasm, mature and pre-microRNAs can be incorporated into endosomes and are released from cells as exosomes when multivesicular bodies fuse with the cell membrane. Cytoplasmic microRNAs can also be secreted by microvesicles, which are released from the cell through blebbing of the cell membrane. MicroRNAs are also secreted in a microvesicle-free form associated with high-density lipoproteins or RNA-binding proteins such as Argonaute 2. Extracellular vesicles release microRNAs into recipient cells by either endocytosis or fusion with cell membrane.

Fig. 4.

The modes of action of EV/microRNAs in the microenvironment. (A) Direct reprogramming of cells in the tumor microenvironment by microRNA transfer, (B) indirect reprogramming of cells in the tumor microenvironment by miR-dependent targeting of EV cargo, (C) modification of extracellular microenvironment by miR-dependent alteration of EV release, (D) therapy sensitization by delivering therapeutic microRNA/anti-microRNA.