title: Late signaling in the activated platelets upregulates tyrosine phosphatase SHP1 and impairs platelet adhesive functions: Regulation by calcium and Src kinase.
abstract: Sustained stimulation of platelets with protease-activated receptor agonists in presence of extracellular calcium was associated with tyrosine dephosphorylation of specific proteins of relative mobilities 35, 67, and 75 kDa. From phosphatase assays and inhibitor studies SHP1, a Src homology 2 (SH2) domain-containing tyrosine phosphatase expressed abundantly in hemopoietic cells, was found to be upregulated in platelets between 25 and 30 min following thrombin stimulation. Concomitantly, SHP1 was tyrosine phosphorylated by, and coprecipitated with, Src tyrosine kinase. SHP1 activation, association with Src and dephosphorylation of specific proteins were dependent on extracellular calcium and maintenance of a higher cytosolic calcium plateau. There was progressive impairment of platelet functions like aggregability and clot retraction, associated with downregulation of fibrinogen-binding affinity of integrin alpha(IIb)beta(3), in the platelets exposed to thrombin for 45 min. This could reflect the late physiological changes in platelets when the cells are consistently exposed to stimulatory signals under thrombogenic environment in vivo.
authors.affiliation: Department of Biochemistry, Institute of Medical Sciences, Banaras Hindu University, Varanasi-221005, India.
authors.forename: Ramkrishna
authors.initials: R
authors.lastname: Gupta
authors.forename: Partha
authors.initials: P
authors.lastname: Chakrabarti
authors.forename: Madhu
authors.initials: M
authors.lastname: Dikshit
authors.forename: Debabrata
authors.initials: D
authors.lastname: Dash
mesh_terms: D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D002021:Buffers; D002118:Calcium / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects; D003600:Cytosol / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D010766:Phosphorylation / Q000187:drug effects; D019000:Phosphotyrosine / Q000378:metabolism; D010973:Platelet Adhesiveness / Q000187:drug effects / Q000502:physiology*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D013917:Thrombin / Q000494:pharmacology; D013997:Time Factors; D015854:Up-Regulation / Q000187:drug effects; D019061:src-Family Kinases / Q000378:metabolism*
chemical_list: D002021:Buffers; D004791:Enzyme Inhibitors; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D019000:Phosphotyrosine; D005340:Fibrinogen; D019061:src-Family Kinases; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6; D013917:Thrombin; D002118:Calcium