Angiogenesis plays an important role in pathological events such as tumor growth, wound healing, psoriasis, and the ischemic retinopathies that occur in diabetes and sickle cell disease. The main modulators of the angiogenesis process in adults, is achieved through signaling by peptide growth factors, however, recent researches also emphasize the contribution of purines and pyrimidines to this process. Synergistic actions of purines and pyrimidines with growth factors aid in promoting cell proliferation (Ref.1). ATP (Adenosine Triphosphate), ADP (Adenosine Diphosphate), UTP (Uridine Triphosphate), UDP (Uridine Diphosphate) and adenosine play pivotal signaling roles in these long-term events, referred to as purinergic signaling, mediated via P1 and P2 receptors. Extracellular nucleotides exert their physiological effects through fast, ionotropic P2X, containing seven distinct receptors that function as cationic-gated channels; and P2Y, comprised of seven distinct receptors coupled to heteromeric G-Proteins. Physiologically, P2Y receptors are responsive to adenine or uridine nucleotides or, in some cases, to both and thus, can be characterized as purinoceptors, pyrimidinoceptors, or, receptors with mixed selectivity. These receptors are coupled to multiple specific cellular functions in addition to angiogenesis, as diverse as neurotransmission, wound healing, morphogenesis, and apoptosis (Ref.2).
The P2Y family consists of seven functional mammalian P2Y receptors: P2Y1, P2Y2, P2Y4, P2Y5, P2Y6, P2Y11, and P2Y12 . P2Y1 is activated preferentially by ADP, P2Y2 by ATP and UTP, P2Y4 by UTP, P2Y6 by UDP, P2Y11 by ATP and P2Y12 by ADP activates P2Y1 preferentially. P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 receptors all couple to the activation of PLC (Phospholipase-C), mobilization of intracellular Ca2+ and activation of PKC (Protein Kinase-C), whereas the newly cloned P2Y12 receptor couples solely to the inhibition of AC (Adenyl Cyclase). The P2Y11 receptor is dually coupled to activation of PLC and AC. The P2Y5 receptor does not show a nucleotide response through PLC or AC but this does not exclude other types of transduction, and the human P2Y5 receptor has recently been shown to give a functional response to ATP, of atypical character, in oocyte expression (Ref.3). The P2Y receptors are also closely related through GPCRs (G-Protein Coupled Receptors) for peptides, such as somatostatin, angiotensin, and platelet activating factor (Ref.4).
The P2Y receptors are expressed ubiquitously, but specific tissue responses are achieved by cell-specific expression profiles. Endothelial cells release ATP and UTP during shear stress and hypoxia to act on P2Y1, P2Y2, and sometimes P2Y4 purinoceptors, leading to production of NO (Nitric Oxide) and subsequent vasodilatation. ATP released as a cotransmitter from sympathetic nerves and sensory-motor nerves (during axon reflex activity) stimulates smooth muscle cell proliferation via P2Y2 and/or P2Y4 receptors via a MAPK (Mitogen-Activated Protein Kinase) cascade. ATP and UTP released from endothelial cells stimulate endothelial and smooth muscle cell proliferation via P2Y1, P2Y2 and P2Y4 receptors. Blood-borne platelets possess P2Y1 and P2Y12 ADP-selective purinoceptors (Ref.5). ADP promotes platelet aggregation through two distinct P2Y receptors: the P2Y1 receptor and the recently identified P2Y12 receptor. Activation of the P2Y1 receptor alone causes platelet shape change, but no aggregation occurs unless the P2Y12 receptor is activated concomitantly. ATP, after its release from aggregating platelets, also acts on these endothelial receptors. The concomitant activation of both the P2Y1 and P2Y12 receptors initiate signaling pathways that ultimately trigger the activation of GPIIB/IIIA (Glycoprotein-IIB/IIIA). The activation of GPIIB/IIIA promotes high-affinity binding to fibrinogen and platelet aggregation (Ref.4).
Mitogenesis/cell proliferation is one of the important functions of P2Y Receptors , affected by G-Proteins and c-Fos gene expression by the regulation of c-Fos, c-Jun, c-Myc and CREB (cAMP Responsive Element Binding Protein) transcription factors. G-Protein-mediated activation of PLC by P2Y receptors stimulates the breakdown of PIP2 (Phosphatidylinositol-4, 5-Bisphosphate) to form IP3 (Inositol-1, 4, 5-Trisphosphate) and DAG (Diacylglycerol). IP3 releases Ca2+ from intracellular stores and DAG activates PKC (Protein Kinase-C), which together modify the activity of cellular proteins. Further, P2Y-regulated formation of growth factor acts via the extracellular compartment, to activate RTKs (Receptor Tyrosine Kinases) and, hence, the Ras-MEK cascade. Mitogenesis is also regulated by other events, including the action of PI3K (Phosphatidylinositol- 3-Kinase) by the direct activation of the BetaGamma subunit of the dissociated G-Protein from P2Y receptor that activates NF-KappaB (Nuclear Factor-KappaB) pathway. A fourth mechanism operates via the P2Y11 receptor; by activation of AC that subsequently releases cAMP (Cyclic Adenosine-3, 5’-Monophosphate). cAMP acts intracellularly to activate PKA (Protein Kinase-A), which then phosphorylates substrates such as members of the CREB protein family (Ref.6).
Mucus hypersecretion and the inability to clear mucus from the airways are cardinal manifestations of chronic bronchitis that in most cases proceeds to infection and lung damage. P2Y2 receptor agonists are currently under development, as therapeutic agents that would help patients to clear their lungs and thus, solve this long-standing problem of chronic bronchitis. Because of the therapeutic potential of drugs targeted to the platelet P2Y12 receptor for the treatment of thromboembolisms and other clotting disorders, the cloning of this receptor has been an intensive area of research for many years (Ref.4). In a similar manner, since the P2Y receptors are coupled to multiple specific cellular functions, their efficient and tremendous therapeutic applications for the treatment of diseases are awaited (Ref.7).