Tumor microenvironment composition strongly conditions cancer growth and progression, acting not only at cancer itself but also modifying its interactions with immune, endothelial and nervous cells. Extracellular ATP and its receptors recently gained increasing attention in the oncological field. ATP accumulates in cancer milieu through spontaneous release, tumor necrosis or chemotherapy exerting a trophic activity on cancer cells, modulating the cross talk among tumor, and surrounding tissues. Accordingly, ATP gated P2X receptors emerged as central players in tumor development, invasion, progression and related symptoms. Indeed, P2X receptors are expressed and functional on tumor cells itself-but also in immune-infiltrate and nearby neurons. In this review, we summarize recent findings on P2X receptors role in tumor cell differentiation, bioenergetics, angiogenesis, metastasis and associated pain, giving an outline of the potential anti-neoplastic activity of receptor agonists and antagonists.
The P2X7 receptor (P2X7R) is attracting increasing attention for its involvement in cancer. Several recent studies have shown a crucial role of P2X7R in tumour cell growth, angiogenesis and invasiveness. In this study, we investigated the role of the two known human P2X7R functional splice variants, the full length P2X7RA and the truncated P2X7RB, in osteosarcoma cell growth. Immunohistochemical analysis of a tissue array of human osteosarcomas showed that forty-four, of a total fifty-four tumours (81.4%), stained positive for both P2X7RA and B, thirty-one (57.4%) were positive using an anti-P2X7RA antibody, whereas fifteen of the total number (27.7%) expressed only P2X7RB. P2X7RB positive tumours showed increased cell density, at the expense of extracellular matrix. The human osteosarcoma cell line Te85, which lacks endogenous P2X7R expression, was stably transfected with either P2X7RA, P2X7RB, or both. Receptor expression was a powerful stimulus for cell growth, the most efficient growth-promoting isoform being P2X7RB alone. Growth stimulation was matched by increased Ca(2+) mobilization and enhanced NFATc1 activity. Te85 P2X7RA+B cells presented pore formation as well as spontaneous extracellular ATP release. The ATP release was sustained in all clones by P2X7R agonist (BzATP) and reduced following P2X7R antagonist (A740003) application. BzATP also increased cell growth and activated NFATc1 levels. On the other hand cyclosporin A (CSA) affected both NFATc1 activation and cell growth, definitively linking P2X7R stimulation to NFATc1 and cell proliferation. All transfected clones also showed reduced RANK-L expression, and an overall decreased RANK-L/OPG ratio. Mineralization was increased in Te85 P2X7RA+B cells while it was significantly diminished in Te85 P2X7RB clones, in agreement with immunohistochemical results. In summary, our data show that the majority of human osteosarcomas express P2X7RA and B and suggest that expression of either isoform is differently coupled to cell growth or activity.
The P2X7 receptor is an ATP-gated ion channel known for its cytotoxic activity. However, recent evidence suggests a role for P2X7 in cell proliferation. Here, we found that P2X7 exhibits significant growth-promoting effects in vivo. Human embryonic kidney cells expressing P2X7 exhibited a more tumorigenic and anaplastic phenotype than control cells in vivo, and the growth rate and size of these tumors were significantly reduced by intratumoral injection of the P2X7 inhibitor-oxidized ATP. The accelerated growth of P2X7-expressing tumors was characterized by increased proliferation, reduced apoptosis, and a high level of activated transcription factor NFATc1. These tumors also showed a more developed vascular network than control tumors and secreted elevated amounts of VEGF. The growth and neoangiogenesis of P2X7-expressing tumors was blocked by intratumoral injection of the VEGF-blocking antibody Avastin (bevacizumab), pharmacologic P2X7 blockade, or P2X7 silencing in vivo. Immunohistochemistry revealed strong P2X7 positivity in several human cancers. Together, our findings provide direct evidence that P2X7 promotes tumor growth in vivo.
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