Platelets activation: tyrosine kinase phosphorylation

Emerging Concepts of Platelet Activation Tyrosine Kinase Phosphorylation. 247... 

The integrins do not have any enzyme activity in their own cytoplasmic domain, but on hgand binding, stimulation of tyrosine phosphorylation is observed on the cytoplasmic side of many cells, such as fibroblasts and platelets. The exact configuration of protein-protein interactions on the cytosolic side of the integrins is not clear and the mechanism of stimulation of protein tyrosine kinases is unknown. Some components of the focal adhesion points, such as the structural protein tensin, have SH2 and SH3 domains that may serve as specific attachment points for tyrosine kinases and other signal proteins. 

Imatinib (STI571) is an inhibitor of the tyrosine kinase domain of the Bcr-Abl oncoprotein and prevents the phosphorylation of the kinase substrate by ATP. It is indicated for the treatment of chronic myelogenous leukemia (CML), a pluripotent hematopoietic stem cell disorder characterized by the t(9 22) Philadelphia chromosomal translocation. This translocation results in the Bcr-Abl fusion protein, the causative agent in CML, and is present in up to 95% of patients with this disease. This agent inhibits other activated receptor tyrosine kinases for platelet-derived growth factor receptor (PDGFR), stem cell factor (SCF), and c-kit. 

A group of receptors exists that responds to so-called growth factors such as insulin, epidermal growth factor, platelet-derived growth factor, etc. These receptors have an extracellular domain that binds the growth factor and an intracellular domain that possesses latent kinase activity. The interaction of insulin, for example, results in autophosphorylation of the intracellular domain and subsequent internalization of the insulin-receptor complex. The internalized complex now possesses the properties of a tyrosine kinase and can phosphorylate cell substrates that produce the appropriate intracellular effect. However, these kinases differ from the usual protein kinases in that they phosphorylate proteins exclusively on tyrosine hydroxyl residues. The ensemble of proteins phosphorylated by the insulin receptor has not yet been identified, but there is supportive evidence that tyrosine kinase activity is required for the major actions of insulin. For example, it is possible that a membrane-linked glucose transport system becomes activated following insulin-stimulated phosphorylation. 

In addition to the serine-threonine phosphorylation sites modulated by PKC and PKA, a basal state of tyrosine phosphorylation exists in platelet TP receptors, as in bradykinin receptors, and this phosphorylation increases on agonist stimulation (166). The tyrosine kinase responsible for this pho horylation, as well as that of the parallel phosphorylation of phosphatidylinositol 34dnase (PIj4dnase), that occurs on agonist stimulation, is unknown, but p27 known to be activated by TP receptor agonists (167), is a candidate (166). It is also possible that PI, kinase itself might phosphorylate TP receptors. The functional role of tyrosine kinase-mediated phosphorylation of TP receptors is unknown. 

In platelets, signaling is initiated primarily through members of the heterotrimeric G protein-coiqtled femily of leceptois (seven transmembrane domains) and through adhesion receptors, and the signaling involves activation of both Ser/Thr kinases and tyrosine kinases. Neither G protein-coiqtled receptors nor adhesion receptors have intrinsic tyrosine kinase activity. However, NRTKs (with SH2-, SH3-, and proline-rich domains) are activated and initiate tyrosine phosphorylation reactions that in turn lead to the recmitment of signaling molecules to certain locations in the cell. These tyrosine kinases may phosphorylate submembranous proteins including receptors for cytoplasmic domains or components of the submembranous cytoskeleton of adhesion receptor-cytoskeleton... 

Alternatively, the release of platelet phospholipid AA is mediated by the activation of phospholipase Aj (PIA) coupled to PLC, intracellular Ca rise and protein kinase C (PKC) and tyrosine kinase-mediated protein phosphorylations (23,24) and/or by the activation of PLA that is not coupled to these events in platelets (25-29). Furthermore, the hydrolysis of phosphatidic acid (PA) by a PA-specific PLAj may also contribute to eicosanoid synthesis (30,31). It is evident that the differential sensitivity of platelets to multiple agonists and their signals, have contributed to the complexity in understanding the regulation of PLAj. 

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