THE A3 ADENOSINE RECEPTOR: A LINK BETWEEN INFLAMMATION AND CANCER.

Adenosine is an endogenous and ubiquitous nucleoside that exerts many biological functions through interaction with 4 distinct subtypes of G protein-coupled receptors divided into A1, A2A, A2B, and A3. This nucleoside plays an important role in modulating inflammation and tumorigenesis. In the past most of the anti-inflammatory effects of this nucleoside were thought to be due to the activation of the A2A subtype, however more recently, the involvement of the A3 subtype has been also considered relevant for the outcome of inflammation. The figurative elements of blood are important substrates on which adenosine plays multiple physiological functions. In these reviews we summarized the status of the art on the role of the A3 receptor in different types of immune cells including neutrophils, eosinophils, lymphocytes, monocytes, macrophages and dendritic cells. It is known that the interaction with the adenosine A3 receptor inhibits several activities of these cells including the release of TNF-alpha and other potent inflammatory cytokines such as IL-6 and IL-8 and increases the production of IL-10 with anti-inflammatory activity. Recent studies have shown the presence of A3 receptors on neutrophils, which represent the majority of circulating leukocytes and are the first cells to be recruited into a site of tissue inflammation in defending the body against infection. In particular, the activation of the A3 receptor on this cellular type leads to the inhibition of degranulation and superoxide anion production with consequent anti-inflammatory effects. Finally, in lymphocytes, activation of A3 receptor subtype would result in a reduction of the accession of killer T cells to tumor cells by exerting an immunosuppressive effect and suggesting a role for antagonists of this receptor as anti-tumoral drugs. In particular, it was recognized more than two centuries ago a close relationship between inflammation and tumorigenesis, because the tumors arise more easily where there are foci of inflammation; the inflammatory cells are present in tumors and the overexpression of cytokines and chemokines may lead to the onset of tumors; also similar molecular targets and intracellular pathways are activated or dysregulated inflammation and in tumors. Several studies in literature and obtained in our laboratory have shown that adenosine exerts important modulatory function in the growth of tumors, giving an essential role in this to the A3 receptor. Therefore the aim of this study was to evaluate the involvement of adenosine on the regulation of metalloproteinases and in particular of MMP9 in U87MG glioblastoma cells. The metalloproteinases (MMPs) are a family of enzymes able to degrade and remodel the extracellular matrix. They are involved in many physiological and pathological processes, including inflammation and tumor growth. In particular, the degradation of extracellular matrix (ECM), which exerts a mechanical and biochemical barrier to cell movement, was demonstrated to be an important biological process in the invasion and the tumor metastatic process. In particular, it was demonstrated that MMP9 facilitates in vitro invasion of glioblastoma cells and the up-regulation of this metalloproteinase is associated with the progression of malignant glioma in vivo. The results of this study, obtained by using real time RT-PCR and Western blotting, show that adenosine is able to increase both MMP9 mRNA and protein levels through the activation of the A3 adenosine receptor. We also noted that the A3 receptor stimulation led to increased levels of MMP9 protein in cellular extracts of U87MG cells, through phosphorylation of ERK1 / 2, JNK, Akt / PKB and the transcription factor AP-1. The A3 receptor activation also led to an increase in extracellular levels of MMP9 in the supernatants of glioblastoma cells as evaluated by ELISA and gelatine zymography assays. Finally, as for the physiological relevance of the A3 receptor-mediated stimulation of MMP-9 we found that the A3 agonist was responsible for an increase of the invasive ability of U87MG cells. Overall, these results suggest that adenosine, through activation of the A3 receptor, modulates MMP9 protein levels and plays a role in the invasion of U87MG cells.