Purinergic Signalling (2006) 2:471–480 vessel wall to promote arterial inflammation associated with atherosclerosis. Recent studies revealed that a Src homology-3 (SH3)- binding domain in the C-terminal tail of the P2Y2 receptor promotes nucleotide-induced association of Src with the P2Y2 receptor, leading to transactivation of growth factor receptors, such as the EGF and VEGF receptors, and nu- cleotide-induced up-regulation of VCAM-1 [72, 73]. Since leukocyte infiltration and migration are key processes involved in atherosclerosis, these findings suggest that P2Y2 receptors represent a novel target for reducing arterial inflammation associated with cardiovascular disease. P2 receptors in vascular apoptosis Apoptosis has been reported to occur in various vascular diseases, such as atherosclerosis, restenosis, and hyperten- sion [74, 75]. The major cell types undergoing apoptosis in human atherosclerotic lesions are arterial SMC [11, 76–78] and macrophages [79]. In restenosis following balloon angioplasty, there is a peak in the proliferation and apoptosis of rat vascular SMC 14 days postangioplasty [76]. Furthermore, apoptosis of arterial SMC has been described in animal models of intimal thickenings [80] and probably takes part in the normal process involved in the control of hyperplasia. In contrast, apoptosis of SMC in advanced human atherosclerotic plaques may destabilize the fibrous lesion to promote plaque rupture and its clinical consequences. As a mediator of cell-to-cell communication, ATP can trigger a variety of biological responses after being rapidly released in large amounts from various sources, including activated platelets, endothelial cells, nerve terminals, antigen-stimulated T cells, and other cell types following hypoxia, stress, and tissue damage. For example, in human umbilical cord vein endothelial cells (HUVEC), substantial release of ATP (and UTP) is induced by shear stress [81], which may lead to alterations in the balance between proliferation and apoptosis regulated by P1 adenosine and P2 (particularly P2X7) nucleotide receptors [82]. P2X7 and P1 receptors have been previously linked to apoptosis in other cell types, including immune cells, astrocytes, and thymocytes [83–85]. The P2X7R also has been shown to mediate ATP-induced cell death in human embryonic kidney cells [86], human cervical epithelial cells [87], and primary rat cortical neurons [88]. In human arterial SMC, adenosine-induced apoptosis is essentially mediated via the A2b-adenosine receptor subtype and involves a cAMP- dependent pathway [89]. As an important constituent of atherosclerotic plaques, fibroblasts share several features with SMC. In human fibroblasts, P2X7 was identified as the main nucleotide 475 receptor involved in the high glucose concentration- dependent responses modulated by ATP, including morphological changes, enhanced apoptosis, caspase-3 activation, and IL-6 release [90]. In the immune system, ATP also plays important roles through nucleotide receptors in leukocyte functions. P2X7-receptor-mediated apoptosis has been demonstrated in various types of leukocytes, including a lymphocytic cell line, murine thymocytes, mu- rine peritoneal macrophages, human macrophages, mesan- gial cells, dendritic cells, and microglial cells [83, 91–96]. Extracellular ATP acting via the P2X7 receptor activates the transcription factor NF-κB by selectively targeting NF-κB p65 (Rel A) in the N9 mouse microglial cell line [97]. It also has been reported that the P2X7 receptor modulates macrophage production of TNF-α, IL-1β and NO follow- ing LPS exposure [98], consistent with a role for the P2X7 receptor in inflammation. In HUVEC, TNF-α markedly increases apoptotic cell death via the activation of caspase-3 [74]. Recent reports indicate that ATP/ADP activate NF-κB and induce apoptosis, probably through P2X7 receptors in porcine aortic endothelial cells [99]. These studies have provided compelling evidence suggesting a role for P2- and P1-receptor-mediated apoptosis in vascular diseases; how- ever, further studies are needed to determine the precise pathways involved and to accumulate direct evidence that these pathways contribute significantly to the development of atherosclerosis, hypertension, and restenosis. Modulation of P2 receptors in vascular injury Experimental arterial intimal hyperplasia can be induced by balloon angioplasty or by perivascular placement of a silicone collar around an artery. An influx of leukocytes precedes migration and proliferation of vascular SMC into the intima in both these models [100]. In normal adult rat aorta, P2Y2 mRNA was found in endothelial cell lining while a sustained expression of P2Y2 mRNA was detected in a few medial SMC [20]. In contrast, P2Y2 mRNA was detected in all medial SMC of rat fetal aortas and in most aortic SMC of intimal lesions after balloon angioplasty, with an overexpression in cells lining the lumen both 1 and 3 weeks after injury. In the collar model, neointimal formation appears to be triphasic [100]. The first phase is characterized by vascular infiltration of leukocytes beginning 2 h after collar placement around a rabbit carotid artery. The second phase begins within 12 h of collar placement and is characterized by medial replication of SMC. The third phase is characterized by the appearance, beginning at day 3 after collar placement, of subendothelial SMC. In situ hybrid- ization with sham-operated rabbit carotid arteries indicated that P2Y2 mRNA expression was localized to CD31-