P2X7 is the largest member of the P2X subfamily of purinergic receptors. A typical feature is the carboxyl tail, which allows formation of a large pore. Recently a naturally occurring truncated P2X7 splice variant, isoform B (P2X7B), has been identified. Here we show that P2X7B expression in HEK293 cells, a cell type lacking endogenous P2X receptors, mediated ATP-stimulated channel activity but not plasma membrane permeabilization, raised endoplasmic reticulum Ca(2+) content, activated the transcription factor NFATc1, increased the cellular ATP content, and stimulated growth. In addition, P2X7B-transfected HEK293 cells (HEK293-P2X7B), like most tumor cells, showed strong soft agar-infiltrating ability. When coexpressed with full-length P2X7 (P2X7A), P2X7B coassembled with P2X7A into a heterotrimer and potentiated all known responses mediated by this latter receptor. P2X7B mRNA was found to be widely distributed in human tissues, especially in the immune and nervous systems, and to a much higher level than P2X7A. Finally, P2X7B expression was increased on mitogenic stimulation of peripheral blood lymphocyte. Altogether, these data show that P2X7B is widely expressed in several human tissues, modulates P2X7A functions, participates in the control of cell growth, and may help understand the role of the P2X7 receptor in the control of normal and cancer cell proliferation.

The P2X 7 receptor is known for the cytotoxic activity because of its ability to cause opening of non-selective pores in the plasma membrane and activate apoptotic caspases. A key factor of P2X 7 -dependent cytotoxicity is the massive intracellular Ca 2؉ increase triggered by its activation. Here we show that P2X 7 transfection increased the ability of the endoplasmic reticulum to accumulate, store, and release Ca 2؉ . This caused a larger agonist-stimulated increase in cytosol and mitochondrial Ca 2؉ in P2X 7 transfectants than in mock transfected cells. P2X 7 transfectants survived and even proliferated in serum-free conditions and were resistant to apoptosis triggered by ceramide, staurosporin, or intracellular Zn 2؉ chelation. Finally, the nuclear factor of activated T cells complex 1 (NFATc1) was strongly activated in the P2X 7 transfectants. These observations support our previous finding that the P2X 7 receptor under tonic conditions of stimulation, i.e. those observed in response to basal ATP release, has an anti-apoptotic or even growth promoting rather than cytotoxic activity.Cell responses to extracellular ATP are mediated by P2 receptors: ionotropic P2X and metabotropic P2Y (1). The P2X 7 receptor (P2X 7 R) 4 subtype stands out in the P2X subfamily for its ability to trigger a host of physiologic or pathologic responses: plasma membrane blebbing (2, 3), rapid release of interleukin-1␤ via microvesicle shedding (2, 4), cell fusion (5), lymphoid cell proliferation (6), cell death (7,8), and bone formation/ resorption (9). This receptor is characterized by low affinity for ATP and by two states of permeability (10, 11). At high micromolar ATP concentrations, P2X 7 behaves as a cation-selective channel, whereas a prolonged exposure to nearly millimolar concentrations triggers the transition to a nonselective pore that admits hydrophilic solutes of molecular mass up to 900 Da (12-14). A common feature of both conductance states is a strong elevation of free cytoplasmic calcium levels ([Ca 2ϩ ] i ), a response critical for the biological role of this receptor.Recent evidence from our group shows that basal levels of ATP, naturally present in the extracellular milieu, cause a tonic activation of the P2X 7 R, which in turn triggers [Ca 2ϩ ] i increase and Ca 2ϩ entry into the mitochondria. The increased intramitochondrial Ca 2ϩ concentration ([Ca 2ϩ ] mt ) then enhances mitochondrial potential, increases ATP synthesis, and promotes survival and proliferation in the absence of serum (15). Increased mitochondrial potential and ability to grow under serum-free conditions are hallmark of tumor transformation (16). Furthermore, overexpression of the P2X 7 R is associated with several cancers or growth disturbances such as chronic lymphocytic leukemia (17), prostate (18)