Platelet Cyclic Adenosine Monophosphate Phosphodiesterases: Targets for Regulating Platelet-Related Thrombosis

Abstract

Platelets contain two cyclic adenosine monophosphate (cAMP) phosphodiesterases (PDEs) that regulate the level of cAMP, the major inhibitor of platelet activation pathways. PDE3A hydrolyzes cAMP to 5' AMP with a low K (m). PDE3A is inhibited by cyclic guanosine monophosphate (cGMP), which provides a feedback control and controls basal levels of cAMP. In contrast, PDE2A hydrolyzes both cAMP and cGMP with a high K (m), is allosterically stimulated by cGMP at moderate levels, and may control the stimulated levels of cAMP. Using affinity labeling, chemical modification, and site-directed mutagenesis of highly conserved amino acids, the amino acids required for catalytic activity and/or metal binding are H752 and H756. The singular binding sites for cAMP include N845, E971, and F972, whereas the unique amino acids interacting with cGMP are Y751, H836, H849, and D950. Residues E866 and F1004 are present in both the overlapping cGMP and cAMP sites. Two inhibitors of PDE3A are used in clinical medicine: milrinone and cilostazol. Three amino acids, Y751, D950, and F1004, show decreased sensitivity to both inhibitors (increased K (i)). These inhibitors mimic cGMP as an inhibitor of PDE3A rather than compete for cAMP binding. New nonhydrolyzable affinity labels inactivate PDE3A and are protected by Sp-cAMPS, a nonhydrolyzable substrate of the enzyme. These compounds have the potential to identify amino acids that are unique for PDE3A. An inhibitor of platelet PDE2A increases cAMP more than inhibitors of PDE3A but has much less effect on platelet activation, suggesting that these enzymes are present in different compartments of the cell.

Platelet Glycoprotein VI Dimerization, an Active Process Inducing Receptor Competence, Is an Indicator of Platelet Reactivity

“…Increased concentrations of intracellular cAMP induce potent inhibition of platelet functions. 27 The activity of adenylate cyclase is controlled by Gi-and Gs-coupled receptors, and indeed, opposite effects were produced by activation of the Gicoupled ADP receptor P2Y 12 and activation of the Gscoupled prostacycline receptor on 9E18 binding to platelets. Similarly, it is generally accepted that NO donors inhibit platelet activation and increase intraplatelet cGMP concentration 27 ; accordingly, SNAP also inhibited GPVI dimerization.…”

“…27 The activity of adenylate cyclase is controlled by Gi-and Gs-coupled receptors, and indeed, opposite effects were produced by activation of the Gicoupled ADP receptor P2Y 12 and activation of the Gscoupled prostacycline receptor on 9E18 binding to platelets. Similarly, it is generally accepted that NO donors inhibit platelet activation and increase intraplatelet cGMP concentration 27 ; accordingly, SNAP also inhibited GPVI dimerization. In healthy vessels, the continual production of prostacyclin and NO by endothelial cells maintains the platelets in a hyporesponsive state, and preservation of GPVI dimerization might contribute to this control process.…”

Time-Limited Hyporesponsiveness to Inhaled Nitric Oxide and Pulmonary Phosphodiesterase Activity in Endotoxemic Rats

“…cGMP activates cGMP-dependent protein kinases, which decrease pulmonary vascular smooth muscle tone. A family of enzymes known as the phosphodiesterases (PDE) inactivate cGMP by converting it to GMP [16]. The impact of PDE activity in reduced pulmonary vascular response to iNO in experimental endotoxemia is actually not completely understood.…”

Better Safe Than Sorry

“…Furthermore, endothelial cell-derived nitric oxide and prostacyclin inhibit platelet activation by increasing the intracellular cGMP concentration. 1,12 Loyau et al 9 now provide evidence that this pathway also inhibits GPVI dimerization, thereby maintaining platelets in a hyporeactive state. A loss of this protective effect due to endothelial dysfunction, damage of the vessel wall, or high shear-induced GPIb␣/von Willebrand factor interaction would then exert a priming effect, permitting a more rapid or more efficient interaction of platelets with collagen.…”

“…cAMP is known as a powerful inhibitor of platelet aggregation, 11 and adenylate cyclase is inhibited by the G i -coupled ADP receptor P2Y 12 and activated by the G s -coupled prostacyclin receptor. Furthermore, endothelial cell-derived nitric oxide and prostacyclin inhibit platelet activation by increasing the intracellular cGMP concentration.…”