Oncotarget 67373 www.impactjournals.com/oncotarget www.impactjournals.com/oncotarget/ Oncotarget, Vol. 7, No. 41 Adenosine A 3 receptor elicits chemoresistance mediated by multiple resistance-associated protein-1 in human glioblastoma stem-like cells Angelo Torres 1 , Yosselyn Vargas 1 , Daniel Uribe 1 , Catherine Jaramillo 1 , Alejandra Gleisner 2 , Flavio Salazar-Onfray 2 , Mercedes N. López 2 , Rómulo Melo 3 , Carlos Oyarzún 1 , Rody San Martín 1 , Claudia Quezada 1 1 Laboratorio de Patología Molecular, Instituto de Bioquímica y Microbiología, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile 2 Instituto Milenio de Inmunología e Inmunoterapia, Facultad de Medicina, Universidad de Chile, Santiago, Chile 3 Servicio de Neurocirugía, Instituto de Neurocirugía Dr. Asenjo, Santiago, Chile Correspondence to: Claudia Quezada, email: claudiaquezada@uach.cl Keywords: adenosine receptors, ATP-binding cassette (ABC) transporter superfamily, glioblastoma stem-like cells, multiple drug resistance Received: February 18, 2016 Accepted: August 29, 2016 Published: September 15, 2016 ABSTRACT MRP1 transporter correlates positively with glioma malignancy and the Multiple Drug Resistance (MDR) phenotype in Glioblastoma Multiforme (GBM). Evidence shows that the MRP1 transporter is controlled by the adenosine signalling axis. The aim of this study was to identify the role of adenosine on the MDR phenotype in Glioblastoma Stem-like Cells (GSCs), the cell population responsible for the tumorigenic and chemoresistance capabilities of this tumour. We found that GSCs have increased intrinsic capacity to generate extracellular adenosine, thus controlling MRP1 transporter expression and activity via activation of the adenosine A 3 receptor (A 3 AR). We showed PI3K/Akt and MEK/ERK1/2 signaling pathways downstream A 3 AR to control MRP1 in GSCs. In vitro pharmacological blockade of A 3 AR had a chemosensitizing effect, enhancing the actions of antitumour drugs and decreasing cell viability and proliferation of GSCs. In addition, we produced an in vivo xenograft model by subcutaneous inoculation of human GSCs in NOD/SCID-IL2Rg null mice. Pharmacological blockade of A 3 AR generated a chemosensitizing effect, enhancing the effectiveness of the MRP1 transporter substrate, vincristine, reducing tumour size and the levels of CD44 and Nestin stem cell markers as well as the Ki-67 proliferation indicator. In conclusion, we demonstrated the chemosensitizing effect of A 3 AR blockade on GSCs. INTRODUCTION Glioblastoma Multiforme (GBM), classifed as a grade IV astrocytoma by the World Health Organization (WHO), is considered the most common and aggressive tumour of the Central Nervous System (CNS) [1, 2]. Standard management for GBM patients involves surgical resection of the tumour, followed by radiation and chemotherapy with temozolomide (TMZ) which results in a survival rate up to 15 months [3]. Anti-angiogenic agents, and more recently immunotherapeutic approaches, are being developed to improve GBM prognostics [3, 4]. However, since it is a highly infltrative tumour, cancer cells often invade healthy brain tissue and evade surgical resection which inevitably leads to early reoccurrence [5]. In addition, the intrinsic properties of tumour cell populations produce poor outcomes in almost all evaluated therapeutic alternatives. A key features of GBM is its high chemoresistance to a broad spectrum of antitumor drugs, a phenomenon known as Multiple Drug Resistance (MDR) [6]. Acquisition of the MDR phenotype correlates with overexpression of members from the ATP-binding cassette (ABC) transporter superfamily [7]. These Research Paper