A3 adenosine receptors are overexpressed in pleura from patients with mesothelioma and reduce malignant mesothelioma cell growth

Malignant mesothelioma (MM) of the pleura is a highly aggressive neoplasm whose incidence is increasing due to asbestos exposure. The capacity of asbestos to induce MM has been linked to its ability to cause the release of TNF-α that promotes mesothelial cells survival via NF-kB pathway leading to transcription of multiple pro-survival genes that promote tumor development. To date, there is no standard curative therapy for MM that is largely unresponsive to conventional chemotherapy or radiotherapy. As a consequence, more effective therapeutic strategies are needed for this fatal disease with the aim to identify new candidates for targeted therapies. Increasing evidence demonstrates that adenosine affects numerous pathophysiological processes including the regulation of cell death and proliferation. Adenosine interacts with four G-proteins coupled receptors named as A1, A2A, A2B and A3 adenosine receptors (ARs). A1 and A3ARs inhibit adenylate cyclase activity and decrease cAMP production whilst A2A and A2BARs exert an increase of cAMP accumulation. It has been accepted that A3ARs are highly expressed in tumor cells showing a pivotal role in the development of cancer. The primary aim of this study was to investigate the presence of adenosine receptors (ARs) in human MM pleura (MMP) and healthy mesothelial pleura (HMP). To shed some light on the interaction between adenosine and MM, the presence and the functionality of ARs were explored in human healthy mesothelial cells (HMC) and in malignant mesothelioma cells (MMC). ARs were analyzed by using RT-PCR, Western blotting and saturation binding assays. HMC were treated with crocidolite asbestos which is the principal risk factor of MM. The role of A3ARs on these cellular models, evaluating cAMP production, Akt phosphorylation and NF-kB activation was investigated. The dual effect of A3AR stimulation on healthy and cancer cell growth was studied by means of proliferation, apoptosis and cytotoxicity assays. In conclusion, this is the first study reporting the up-regulation of A3AR in pleura from MM patients when compared with healthy pleura. A3AR mRNA and protein expression was significantly increased in MMC and in asbestos-treated HMC respect to untreated HMC. Furthermore, the A3AR agonist Cl-IB-MECA decreases proliferation and exerts cytotoxic and pro-apoptotic effect on HMC exposed to asbestos and TNF-α and on tumoral MMC, but not in control HMC. These effects appear to be related to the de-regulation of the Akt/NF-kB cell survival pathways contributing to the processes that lead to the malignant transformation of asbestos-exposed mesothelial cells. Taken together, these new findings suggest that A3AR could represent a possible target for pharmacological intervention to prevent mesothelial tumor development after asbestos exposure and to treat full blown MM.