ATP-gated P2X 7 receptors (P2X 7 R) are unusual plasma membrane ion channels that have been extensively studied in immune cells. More recently, P2X 7 R have been described as potential cancer cell biomarkers. However, mechanistic links between P2X 7 R and cancer cell processes are unknown. Here, we show, in the highly aggressive human breast cancer cell line MDA-MB-435s, that P2X 7 receptor is highly expressed and fully functional. Its activation is responsible for the extension of neurite-like cellular prolongations, of the increase in cell migration by 35% and in cell invasion through extracellular matrix by 150%. The change in cancer cell morphology and the increased migration appeared to be due to the activation of Ca 2 þ -activated SK3 potassium channels. The enhanced invasion through the extracellular matrix was related to the increase of mature forms of cysteine cathepsins in the extracellular medium, which was independent of SK3 channel activity and not associated with cell death. Pharmacological targeting of P2X 7 R in vivo was crucial for cell invasiveness in a zebrafish model of metastases. Our results demonstrate a novel mechanistic link between P2X 7 R functionality in cancer cells and invasiveness, a key parameter in tumour growth and in the development of metastases. They also suggest a potential therapeutic role for the newly developed P2X 7 R antagonists.

P2X7 is an intriguing ionotropic receptor for which the activation by extracellular ATP induces rapid inward cationic currents and intracellular signalling pathways associated with numerous physiological processes such as the induction of the inflammatory cascade, the survival and proliferation of cells. In contrast, long-term stimulation of P2X7 is generally associated with membrane permeabilisation and cell death. Recently, P2X7 has attracted great attention in the cancer field, and particularly in the neoplastic transformation and the progression of solid tumours. A growing number of studies were published; however they often appeared contradictory in their results and conclusions. As such, the involvement of P2X7 in the oncogenic process remains unclear so far. The present review aims to discuss the current knowledge and hypotheses on the involvement of the P2X7 receptor in the development and progression of solid tumours, and highlight the different aspects that require further clarification in order to decipher whether P2X7 could be considered as a cancer biomarker or as a target for pharmacological intervention. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

The adenosine 5'-triphosphate (ATP)-gated Ca(2+)-permeable channel P2X7 receptor (P2X7R) is strongly upregulated in many tumors and cancer cells, and has an important role in cancer cell invasion associated with metastases. Emodin (1,3,8-trihydroxy-6-methylanthraquinone) is an anthraquinone derivative originally isolated from Rheum officinale Baill known for decades to possess anticancer properties. In this study, we examined the effects of emodin on P2X7R-dependent Ca(2+) signaling, extracellular matrix degradation, and in vitro and in vivo cancer cell invasiveness using highly aggressive human cancer cells. Inclusion of emodin at doses ≤10 µM in cell culture had no or very mild effect on the cell viability. ATP elicited increases in intracellular Ca(2+) concentration were reduced by 35 and 60% by 1 and 10 µM emodin, respectively. Emodin specifically inhibited P2X7R-mediated currents with an IC50 of 3 µM and did not inhibit the currents mediated by the other human P2X receptors heterologously expressed in human embryonic kidney (HEK293T) cells. ATP-induced increase in gelatinolytic activity, in cancer cell invasiveness in vitro and in cell morphology changes were prevented by 1 µM emodin. Furthermore, such ATP-evoked effects and inhibition by emodin were almost completely ablated in cancer cells transfected with P2X7R-specific small interfering RNA (siRNA) but not with scrambled siRNA. Finally, the in vivo invasiveness of the P2X7R-positive MDA-MB-435s breast cancer cells, assessed using a zebrafish model of micrometastases, was suppressed by 40 and 50% by 1 and 10 µM emodin. Taken together, these results provide consistent evidence to indicate that emodin inhibits human cancer cell invasiveness by specifically antagonizing the P2X7R.

Adenosine 5'-triphosphate (ATP) is found in high concentrations in the extracellular microenvironment of tumours and is postulated to play critical roles in cancer progression. In the present study, we found that stimulation of human MCF-7 breast cancer cells with 30 µM ATP increased their migration by 140 ± 31%, whereas it had minor or no effect on their proliferation. This effect was prevented by the ectonucleotidase apyrase and was antagonized by suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, consistently with the participation of P2 receptors. MCF-7 cells expressed messenger RNA for all known P2Y receptors and for P2X2, P2X4, P2X5, P2X6 and P2X7 receptors. Brief applications (20 s) of external ATP resulted in a 50 pA P2X-like inward current. ATP, but not adenosine diphosphate or uridine diphosphate, increased the intracellular calcium concentration in absence of extracellular calcium, and this effect was prevented by the inhibition of phospholipase C. Uridine triphosphate (UTP) (10 µM) and 2-thio-UTP (10 µM) increased intracellular calcium concentration and cell migration to the same extent as ATP. The UTP-dependent increase in cell migration was absent in cells knocked-down for P2Y2. It was inhibited by MEK inhibitor PD98059. UTP induced a time-dependent phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), which was prevented by the incubation with PD98059. Taken together, these results highlight the importance of the purinergic signalling in cancer cells and indicate that the activation of P2Y2 receptors enhances breast cancer cells migration through the activation of a MEK-ERK1/2-dependent signalling pathway.

The P2X7 receptor is an ATP-gated cation channel with a still ambiguous role in cancer progression, proposed to be either pro- or anti-cancerous, depending on the cancer or cell type in the tumour. Its role in mammary cancer progression is not yet defined. Here, we show that P2X7 receptor is functional in highly aggressive mammary cancer cells, and induces a change in cell morphology with fast F-actin reorganization and formation of filopodia, and promotes cancer cell invasiveness through both 2- and 3-dimensional extracellular matrices in vitro. Furthermore, P2X7 receptor sustains Cdc42 activity and the acquisition of a mesenchymal phenotype. In an immunocompetent mouse mammary cancer model, we reveal that the expression of P2X7 receptor in cancer cells, but not in the host mice, promotes tumour growth and metastasis development, which were reduced by treatment with specific P2X7 antagonists. Our results demonstrate that P2X7 receptor drives mammary tumour progression and represents a pertinent target for mammary cancer treatment.