Regulators of Platelet cAMP Levels: Clinical and Therapeutic Implications

Abstract

Platelets are indispensable for primary haemostasis, but their function needs to be tightly regulated to prevent excessive platelet activity, possibly leading to atherothrombotic events. An important mediator of the platelet activity is cyclic AMP (cAMP), which inhibits platelet aggregation. Intracellular cAMP levels are regulated via the Gs and Gi alpha subunits of heterotrimeric G proteins, which couple to adenylyl cyclase to respectively stimulate or inhibit cAMP production. Binding of a ligand to its G protein-coupled seven-transmembrane receptor activates these G proteins. In this review, we discuss a Gs-coupled receptor on platelets, VPAC1, and 2 important Gi-coupled receptors, the ADP receptor P2Y(12) and the prostaglandin E(2) receptor EP3. The regulation of platelet cAMP levels at the level of the receptors themselves or the G proteins coupled to them is analyzed. Alterations in Gsα and Giα function are associated with altered platelet reactivity. An increase in Gs function, or alternatively a defective Gi signaling, can be a risk factor for bleeding, while a loss of Gs function can result in a prothrombotic state. Regulator of G protein signaling (RGS) proteins accelerate the rate of inactivation of G protein-mediated signaling. One of the RGS proteins, RGS2, inhibits Gs signaling by interacting directly with adenylyl cyclase. The thienopyridine class of antiplatelet agents is based on cAMP-mediated regulation of platelet function through modification of the P2Y(12) receptor. Clopidogrel and some other novel cAMP regulators are discussed. Secondly, we review the use of prostacyclin derivatives to treat pulmonary arterial hypertension.

Ticagrelor inhibits human platelet aggregation via adenosine in addition to P2Y12 antagonism

“…However, when added in combination with dipyridamole or P2Y 12 antagonists, adenosine contributed an additional antiplatelet effect, which was greater in combination with ticagrelor than with PAM or dipyridamole and in combination with dipyridamole than with PAM (n = 12 Milan, n = 6-12 M€ olndal, Fig. 1A and Table 1).…”

“…Three independent series of experiments were performed in whole blood samples from each of two patients with inherited, severe deficiency of P2Y 12 . As expected, platelet aggregation in ) contribution to inhibition of platelet aggregation induced by collagen (added immediately after adenosine) in whole blood from healthy donors when combined with dipyridamole, ticagrelor, or PAM Comparisons for collagen-induced platelet aggregation in WB Relative difference* (%) P-value response to collagen was lower than in healthy subjects (not shown), due to lack of amplification of platelet aggregation mediated by the interaction of released ADP with P2Y 12 , and the inhibition of platelet aggregation by ticagrelor and PAM was negligible. However, the combination of ticagrelor and adenosine further inhibited platelet aggregation and was reversed by ZM241385, indicating that this effect of ticagrelor is adenosine mediated and independent of P2Y 12 .…”

“…Compared with the thienopyridine drug clopidogrel, which irreversibly inhibits P2Y 12 , ticagrelor decreased the incidence of major adverse cardiovascular events (MACE) and total mortality in patients with ACS [3].…”

“…Because one of the biological effects of adenosine is to inhibit the function of platelets [12][13][14], which express A 2A (and, to a lesser extent, A 2B ), the aim of this in vitro study was to investigate whether ticagrelor inhibits platelet function not only by antagonizing P2Y 12 but also by increasing the levels of adenosine. Experiments were performed in two institutions (Universit a degli Studi di Milano, Milan, Italy, and AstraZeneca R&D, M€ olndal, Sweden) under conditions in which adenosine uptake by ENT1 expressed on erythrocytes does (whole blood) or does not occur (platelet-rich plasma [PRP]).…”

“…Experiments were performed in two institutions (Universit a degli Studi di Milano, Milan, Italy, and AstraZeneca R&D, M€ olndal, Sweden) under conditions in which adenosine uptake by ENT1 expressed on erythrocytes does (whole blood) or does not occur (platelet-rich plasma [PRP]). The effects of ticagrelor were compared with those of two reference compounds, a wellcharacterized ENT1 inhibitor, dipyridamole, and another P2Y 12 antagonist, the active metabolite of prasugrel (PAM) [15].…”