Abstract
Thrombin-mediated activation of platelets is critical for hemostasis, but the signaling pathways responsible for this process are not completely understood. In addition, signaling within this cascade can also lead to thrombosis. In this study, we have defined a new signaling pathway for the thrombin receptor protease activated receptor-1 (PAR1) in human platelets. We show that PAR1 couples to Gi/o in human platelets and activates phosphoinositide-3 kinase (PI3K). PI3K activation regulates platelet integrin αIIbβ3 activation and platelet aggregation and potentiates the PAR1-mediated increase in intraplatelet calcium concentration. PI3K inhibitors eliminated these effects downstream of PAR1, but they had no effect on PAR4 signaling. This study has identified an important role for the direct activation of Gi/o by PAR1 in human platelets. Given the efficacy of clopidogrel, which blocks the Gi/o-coupled P2Y purinoceptor 12, as an antiplatelet/antithrombotic drug, our data suggest that specifically blocking only PAR1-mediated Gi/o signaling could also be an effective therapeutic approach with the possibility of less unwanted bleeding.
Footnotes
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This work was supported by National Institutes of Health grants HL060678, EY010291, and 1P50-HL081009-01 (to H.E.H.), American Heart Association Predoctoral Fellowship (to B.V.), and National Research Service Award (to M.H.), HL082068-01. R.Z. is funded by a MERIT Award from the Department of Veterans Affairs and by National Institutes of Health grant R01-DK069921.
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This work was presented in part at the annual American Society for Biochemistry and Molecular Biology; April 1–5, 2006; San Francisco, CA (Voss B, McLaughlin JN, and Hamm HE, Calcium mobilization in human platelets is differentially modulated by PAR1 and PAR4 through Gi/o and P13K).
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Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
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doi:10.1124/mol.106.033365.
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ABBREVIATIONS: PAR, protease-activated receptor; PKC, protein kinase C; PLC, phospholipase C; PI3K, phosphoinositide-3 kinase; FITC, fluorescein isothiocyanate; FACS, fluorescence-activated cell sorting; Bis-1, bisindolymaleimide-1; U-73122, 1-[6-[[17β-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione; Y-27632, 4-[(1R)-1-aminoethyl]-N-pyridin-4-yl-cyclohexane-1-carboxamide; AP, activating peptide; DMSO, dimethyl sulfoxide; GPIIbIIIa, integrin αIIbβ3; LY294002, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; TRP, transient receptor potential.
- Received December 11, 2006.
- Accepted February 15, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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PAR1, but Not PAR4, Activates Human Platelets through a Gi/o/Phosphoinositide-3 Kinase Signaling Axis
