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Glioblastoma is the brain tumor with the worst prognosis. This is mainly due to a cell subpopulation with an extremely aggressive potential, called glioblastoma stem-like cells (GSCs). These cells produce high levels of extracellular adenosine, which are increased even more under hypoxic conditions. Under hypoxia, adenosine signaling is related to HIF-2α expression, enhancing cell aggressiveness. Adenosine can be degraded using recombinant adenosine deaminase (ADA) to revert its pathological effects. The aim of this study was to degrade adenosine using ADA in order to decrease malignancy of GSCs. Adenosine depletion was performed using recombinant ADA. Migration and invasion were measured by transwell and matrigel-coated transwell assay, respectively. HIF-2α-dependent cell migration/invasion decreased in GSCs treated with ADA under hypoxia. MRPs-mediated chemoresistance and colony formation decreased in treatment with ADA. In conclusion, adenosine depletion using adenosine deaminase decreases GSCs aggressiveness.

Glioblastoma multiforme (GBM) is the most common glial cell-derived brain tumour, with one of the worst prognoses among all cancers. GBM cells are infiltrative and extremely resistant to radio- and chemotherapy, which inevitably leads to recurrence after surgical resection. These inherent GBM properties are the reasons that patient treatment has not seen major improvements in decades. Studies have consistently shown that glioblastoma stem-like cells (GSCs) are responsible for the tumourigenic properties in the GBM population. In fact, their self-renewal and proliferative potential are required for tumour growth, and their extreme chemoresistance leads to early recurrence of this tumour. Among those mechanisms associated with chemoresistance and having the greatest clinical impact in cancer treatment, are the activities of plasma membrane transporters that extrude antitumour drugs from the cell, thus notably decreasing the pharmacological efficiency of these drugs. The multiple drug resistance associated protein-1 (Mrp1) transporter has been shown to be particularly important in GBM, as inhibition of Mrp1 activity notably chemosensitises cells to antiproliferative drugs. As current therapeutic options for GBM offer only a poor improvement in overall survival rate, alternative strategies for overcoming tumour resistance are urgently sought after. To this end, it is of major clinical relevance to know more about the endogenous modulators that control Mrp1 expression within the pathological environment of the tumour. This review describes the particular properties of glioblastoma cells that overcome multimodal therapy and relapse, with an emphasis on the microenvironmental tumour properties that influence the chemoresistance phenotype to antiproliferative drugs. We also discuss alternative methods of reversal of Mrp1-mediated chemoresistance in these cells by targeting extracellular adenosine production or signalling through particular plasma membrane receptors.

Poor response to current treatments for glioblastoma has been attributed to the presence of glioblastoma stem-like cells (GSCs). GSCs are able to expel antitumor drugs to the extracellular medium using the multidrug resistance-associated protein 1 (MRP1) transporter. Tacrolimus (FK506) has been identified as an MRP1 regulator in differentiated glioblastoma (GBM) cells (non-GSCs); however, the effect of FK506 on GSCs is currently unknown. The objective of the following research is to evaluate the effect of FK506 on the MRP1-related chemo-resistant phenotype of GSCs. For this, U87MG and C6 glioma cell lines were used to generate non-GSCs and GSCs. mRNA and MRP1-positive cells were evaluated by RT-qPCR and flow cytometry, respectively. A Carboxyfluorescein Diacetate (CFDA)-retention assay was performed to evaluate the MRP1 activity. Apoptosis and MTT assays were employed to evaluate the cytotoxic effects of FK506 plus Vincristine (MRP1 substrate). GSC-derived subcutaneous tumors were generated to evaluate the in vivo effect of FK506/Vincristine treatment. No differences in transcript levels and positive cells for MRP1 were observed in FK506-treated cells. Lesser cell viability, increased apoptosis, and CFDA-retention in the FK506/Vincristine-treated cells were observed. In vivo, the FK506/Vincristine treatment decreased the tumor size as well as ki67, Glial Fibrillary Acidic Protein (GFAP), and nestin expression. We conclude that FK506 confers a chemo-sensitive phenotype to MRP1-drug substrate in GSCs.