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Background: We have previously shown that ADP-induced TXA2 generation requires signaling from αIIbβ3 integrin in platelets. Here we observed that, unlike ADP, protease-activated receptor (PAR)-mediated TXA2 generation occurs independently of αIIbβ3. PAR agonists, but not ADP, activate G12/13 signaling pathways. Hence, we evaluated the role of these pathways in TXA2 generation. Principal Findings: Inhibition of ADP-induced thromboxane generation by fibrinogen receptor antagonist SC57101 was rescued by co-stimulation of G12/13 pathways with YFLLRNP. This observation suggested an existence of a common signaling effector downstream of integrins and G12/13 pathways. Hence, we evaluated role of three potential tyrosine kinases; c-Src, Syk and FAK (Focal Adhesion Kinase) that are known to be activated by integrins. c-Src and Syk kinase did not play a role in ADP-induced functional responses in platelets. Selective activation of G12/13 pathways resulted in the activation of FAK, in the absence of integrin signaling. Interestingly, αIIbβ3-mediated FAK activation occurred in a Src family kinase (SFK)-independent manner whereas G12/13 pathway caused FAK activation in a SFK and RhoA-dependent manner. A FAK selective inhibitor TAE-226, blocked TXA2 generation. However, in comparison to WT mice, Pf4-Cre/Fak-Floxed mice did not show any difference in platelet TXA2 generation. Conclusions: Therefore, we conclude that differential activation of FAK occurs downstream of Integrins and G12/13 pathways. However, the common effector molecule, possibly a tyrosine kinase downstream of integrins and G12/13 pathways contributing to TXA2 generation in platelets remains elusive. [ABSTRACT FROM AUTHOR]

Antiplatelet therapy for the management of patients with cardiovascular risks often includes a combination therapy of aspirin and clopidogrel, acting through inhibition of thromboxane generation and blockade of Gi-coupled P2Y12 receptor, respectively. We hypothesized that ADP acting through P2Y12 regulates physiological thromboxane levels. The serum thromboxane levels in mice ( n = 3) dosed with clopidogrel and prasugrel were decreased by 83.1 ± 5.3% and 94.26 ± 1.75% respectively compared to untreated mice. Pre-treatment of human blood ( n = 3) ex vivo with active metabolites of clopidogrel or prasugrel led to a reduction in thromboxane levels to 16.3 ± 3.2% and 4.9 ± 0.8% respectively, compared to untreated human serum. We also evaluated serum thromboxane levels in P2Y receptor null mice ( n = 4). Whereas serum thromboxane levels in P2Y1 null mice were similar to those in wild type littermates, those in the P2Y12 null mice were inhibited by 83.15 ± 3.8%. Finally, in a pilot study, serum thromboxane levels were reduced by 76.05 ± 8.41% in healthy human volunteers ( n = 6) upon dosing with clopidogrel, compared to the levels before dosing. In conclusion, P2Y12 antagonism alone can decrease physiological thromboxane levels. Thus, this study could pave way the for newer/modified treatment regimens for the management of patients with thrombotic complications who are allergic or non-responsive to aspirin. [ABSTRACT FROM AUTHOR]

PI3Ks (phosphoinositide 3-kinases) play a critical role in platelet functional responses. PI3Ks are activated upon P2Y12 receptor stimulation and generate pro-aggregatory signals. P2Y12 receptor has been shown to play a key role in the platelet aggregation and thromboxane A2 generation caused by co-stimulation with Gq or Gz, or super-stimulation of Gi pathways. In the present study, we evaluated the role of specific PI3K isoforms alpha, beta, gamma and delta in platelet aggregation, thromboxane A2 generation and ERK (extracellular-signal-regulated kinase) activation. Our results show that loss of the PI3K signal impaired the ability of ADP to induce platelet aggregation, ERK phosphorylation and thromboxane A2 generation. We also show that Gq plus Gi- or Gi plus Gz-mediated platelet aggregation, ERK phosphorylation and thromboxane A2 generation in human platelets was inhibited by TGX-221, a PI3Kbeta-selective inhibitor, but not by PIK75 (a PI3Kalpha inhibitor), AS252424 (a PI3Kgamma inhibitor) or IC87114 (a PI3Kdelta inhibitor). TGX-221 also showed a similar inhibitory effect on the Gi plus Gz-mediated platelet responses in platelets from P2Y1-/- mice. Finally, 2MeSADP (2-methyl-thio-ADP)-induced Akt phosphorylation was significantly inhibited in the presence of TGX-221, suggesting a critical role for PI3Kbeta in Gi-mediated signalling. Taken together, our results demonstrate that PI3Kbeta plays an important role in ADP-induced platelet aggregation. Moreover, PI3Kbeta mediates ADP-induced thromboxane A2 generation by regulating ERK phosphorylation.

Antiplatelet therapy is a crucial component of disease management for patients with acute coronary syndromes or chronic stable coronary disease, as well as individuals undergoing percutaneous coronary intervention with stent implantation. Antiplatelet therapy includes the administration of aspirin and clopidogrel, either alone or in combination, which act through the inhibition of thromboxane A2 generation and blockade of the Gi-coupled P2Y12 purinergic receptor, respectively. Because of the selective expression and specific functions of P2Y12 , this receptor has become an important antiplatelet drug target. P2Y12 antagonists can be broadly classified as either irreversible or reversible. Clopidogrel, an irreversible P2Y12 antagonist, has a delayed onset of action and high inter-individual variability. Limitations of clopidogrel have necessitated the discovery of novel P2Y12 antagonists with superior pharmacological profiles. This review summarizes recent studies on novel P2Y12 antagonists and the clinical implications and limitations of these agents.

Antiplatelet therapy for the management of patients with cardiovascular risks often includes a combination therapy of aspirin and clopidogrel, acting through inhibition of thromboxane generation and blockade of G(i)-coupled P2Y₁₂ receptor, respectively. We hypothesized that ADP acting through P2Y₁₂ regulates physiological thromboxane levels. The serum thromboxane levels in mice (n = 3) dosed with clopidogrel and prasugrel were decreased by 83.1 ± 5.3% and 94.26 ± 1.75% respectively compared to untreated mice. Pre-treatment of human blood (n = 3) ex vivo with active metabolites of clopidogrel or prasugrel led to a reduction in thromboxane levels to 16.3 ± 3.2% and 4.9 ± 0.8% respectively, compared to untreated human serum. We also evaluated serum thromboxane levels in P2Y receptor null mice (n = 4). Whereas serum thromboxane levels in P2Y₁ null mice were similar to those in wild type littermates, those in the P2Y₁₂ null mice were inhibited by 83.15 ± 3.8%. Finally, in a pilot study, serum thromboxane levels were reduced by 76.05 ± 8.41% in healthy human volunteers (n = 6) upon dosing with clopidogrel, compared to the levels before dosing. In conclusion, P2Y₁₂ antagonism alone can decrease physiological thromboxane levels. Thus, this study could pave way the for newer/modified treatment regimens for the management of patients with thrombotic complications who are allergic or non-responsive to aspirin.