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Biochemistry - Hematology


Integrin aIIb-Mediated PI3K/Akt Activation in Platelets
Published: Wednesday, October 17, 2012
Author: Haixia Niu et al.

by Haixia Niu, Xue Chen, Ralph A. Gruppo, Ding Li, Yanhua Wang, Lin Zhang, Kemin Wang, Weiran Chai, Yueping Sun, Zhongren Ding, T. Kent Gartner, Junling Liu

Integrin aIIbß3 mediated bidirectional signaling plays a critical role in thrombosis and haemostasis. Signaling mediated by the ß3 subunit has been extensively studied, but aIIb mediated signaling has not been characterized. Previously, we reported that platelet granule secretion and TxA2 production induced by aIIb mediated outside-in signaling is negatively regulated by the ß3 cytoplasmic domain residues R724KEFAKFEEER734. In this study, we identified part of the signaling pathway utilized by aIIb mediated outside-in signaling. Platelets from humans and gene deficient mice, and genetically modified CHO cells as well as a variety of kinase inhibitors were used for this work. We found that aggregation of TxA2 production and granule secretion by ß3?724 human platelets initiated by aIIb mediated outside-in signaling was inhibited by the Src family kinase inhibitor PP2 and the PI3K inhibitor wortmannin, respectively, but not by the MAPK inhibitor U0126. Also, PP2 and wortmannin, and the palmitoylated ß3 peptide R724KEFAKFEEER734, each inhibited the phosphorylation of Akt residue Ser473 and prevented TxA2 production and storage granule secretion. Similarly, Akt phosphorylation in mouse platelets stimulated by the PAR4 agonist peptide AYPGKF was aIIbß3-dependent, and blocked by PP2, wortmannin and the palmitoylated peptide p-RKEFAKFEEER. Akt was also phosphorylated in response to mAb D3 plus Fg treatment of CHO cells in suspension expressing aIIbß3-?724 or aIIbß3E724AERKFERKFE734, but not in cells expressing wild type aIIbß3. In summary, SFK(s) and PI3K/Akt signaling is utilized by aIIb-mediated outside-in signaling to activate platelets even in the absence of all but 8 membrane proximal residues of the ß3 cytoplasmic domain. Our results provide new insight into the signaling pathway used by aIIb-mediated outside-in signaling in platelets.
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