Cancer in humans constitutes a set of about a hundred different diseases characterised by the uncontrolled growth of the affected cells often combined with the ability to spread, through metastasis, to distant sites. In examining this set of common characteristics, one is struck by the possibility that the inability of the affected cells to die at their allotted time may suggest that a common factor or master mechanism is present. The search for one or more elusive magic bullets as proposed by the Nobel Laureate Paul Ehrlich and others over a century ago [1
] was always predicated on the hope of identifying a common target or pathway. Such a discovery would enable the apparent innate complexity in cancer to be greatly simplified. The current consensus view is that widely observed cancer cell population heterogeneity makes the targeting either of individual mutations on cells or single signalling pathways too limited for such approaches to be fully curative.
The current complexity is best illustrated by a recent report [2
] in which the authors highlighted the prodigious effort expended on developing and testing treatments for the multitude of different human cancers for which there remains an enormous unmet need. A total of 981 were either in clinical trial or under review by the US FDA. Of the 84 actively sought cancer targets in current development, the top 8 are the focus of about 235 of the drugs that are now in advanced clinical development. These include VEGFR, PI3K, HER2, EGFR, PDGFR, c-Kit, cMet and mTor. Targeting of up-regulated receptors or pathway components present on normal cells is sub-optimal given the propensity for such procedures to generate unwanted side effects. The search for a genuine target specific for cancer cells is, therefore, vitally important, whether it is confined to a single tissue sub-indication or is present on a wide variety of cancer types. Specificity for the target is paramount, both for avoiding unwanted side effects and for minimising the required therapeutic dose that is otherwise lost through unwanted binding to normal (non-target) tissue.
It should not be surprising that in searching for a common target, an investigation was made of events central to the failure of cancer cells to die on schedule via normal activation of apoptosis. One potential common target receptor, P2X7
, is involved both in proliferation and in apoptosis. P2X7
is a purinergic receptor that forms an ATP-gated cation-selective channel mediating cell death in haematopoietic and immune cells such as thymocytes [3
], dendritic cells [4
], lymphocytes [5
], macrophages [7
] and monocytes [8
is also expressed on other cell types such as those possessing epithelial, mesenchymal and neural lineages [9
]. These receptors have two transmembrane domains separating an extracellular domain that exhibits extensive homology with other P2X subtypes. P2X7
has a short N-terminal intracellular segment and, unlike other P2X subtypes, a long intracellular C-terminal domain [13
] that confers pore-forming properties on the homotrimeric, assembled channel in the plasma membrane [14
]. Prolonged exposure of the assembled channel to ATP results in additional pore dilation [15
]. A rapid influx of calcium ensues, activating various proteases, such as caspases [16
], leading to apoptosis or programmed cell death [17
Over 80% of human tumours arise from epithelial cells, most frequently at mucosal surfaces. Early studies provided preliminary evidence that the onset of cancer in these tissues is universally accompanied by P2X7
receptor expression [9
]. Importantly, these cancer cell-expressed receptors are deployed in a non-functional conformation [18
], rendering them unable to form apoptotic pores either through an inability to bind ATP to each of the separate sites on the receptor or due to the inability of all three monomers to pack appropriately in the cell membrane [19
]. They are therefore unable to initiate apoptosis, preventing cells expressing this receptor from undergoing programmed cell death. However, while unable to form an apoptotic pore, the deployed receptors on cancer cells, referred to as nfP2X7
, maintain residual calcium signalling activity as a result of one of the ATP binding sites maintaining integrity [20
]. Thus, not only is nfP2X7
expressed in increasing amounts in a failed attempt to clear the aberrant cell, but the effect of an unwanted residual non-selective Ca2+
channel on the plasma membrane of these cells serves to eventually provide them with their undesirable metastatic potential [20
]. Residual channel activity leads to morphological changes including membrane blebbing [17
], a general rounding up of the cell [21
] and the shedding of various anchorage or adhesion proteins otherwise capable of holding the cell within the primary tissue. These include L-selectin, matrix metalloproteinase [15
] and cathepsins [22
]. The most metastatic cells, those with the highest invasion potential, appear to express more of the aberrant nfP2X7
on their surfaces.
The inability to undergo apoptotic pore formation once the receptors are up-regulated on the target cell surfaces could result from rare splice isoforms or single nucleotide polymorphisms [25
]. Alternatively, one of the monomers in the assembled trimer may be improperly packed due to problems with one or more of the eleven intracellular accessory binding proteins. Irrespective of the causes, two of the three ATP binding sites on the assembled receptor, formed only through correct packing of the monomers at the intermonomer interface, are exposed. This leaves just one site that is assembled and functioning correctly to provide the measured residual channel activity. The loss of the remaining two ATP sites through faulty packing of one of the monomers prevents the binding of all three ATP molecules required to activate pore formation. This provides a selective target on nfP2X7
that is unavailable on P2X7
]. Selective targeting of an otherwise hidden epitope allows for exquisite discrimination betweennormal cells and those expressing non-functional receptors.
Here we initially catalogue the expression of nfP2X7
receptor in a panel of human cancer tissues and show this receptor is expressed ubiquitously on the surface of cancer cells and thus has potential as an entirely novel and broad therapeutic target.