title: Dysfunction of the PI3 kinase/Rap1/integrin α(IIb)β(3) pathway underlies ex vivo platelet hypoactivity in essential thrombocythemia.
abstract: Patients with myeloproliferative disorders (MPDs), such as essential thrombocythemia (ET) have increased risk of thrombosis and bleeding, which are major sources of morbidity and mortality. Most MPD patients have a gain of function mutation in Janus kinase 2 (JAK2V617F), but little is known how JAK2V617F affects platelet function. Here, we demonstrate that platelets from ET patients have impaired SFLLRN-mediated fibrinogen binding and have lost the potentiating effect of thrombopoietin (which couples to JAK2) on this pathway. In contrast, SFLLRN-mediated P-selectin expression, ATP secretion, phosphorylation of the PKC substrate pleckstrin, and Ca(2+) mobilization were unaffected in JAK2V617F positive platelets. In addition, thrombopoietin-mediated JAK2 phosphorylation was unchanged, suggesting that signaling pathways activated downstream of JAK2 are impaired. Indeed, we found that platelets from JAK2V617F positive ET patients have significantly reduced phosphorylation of the PI3 kinase substrate Akt, and have reduced activation of Rap1 in response to thrombopoietin, IGF-1,ADP, SFLLRN, and thrombin. This effect was independent of Giα P2Y12 purinergic receptor function as ADP-mediated inhibition of VASP phosphorylation was unchanged. These results demonstrate that the PI3 kinase/Rap1 pathway is intrinsically impaired in platelets from JAK2V617F-positive ET patients, resulting in diminished thrombin and thrombopoietin-mediated integrin α(IIb)β(3) activation.
authors.affiliation: School of Physiology and Pharmacology, School of Medical Sciences, University of Bristol, Bristol, United Kingdom.
authors.forename: Samantha F
authors.initials: SF
authors.lastname: Moore
authors.forename: Roger W
authors.initials: RW
authors.lastname: Hunter
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Harper
authors.forename: Joshua S
authors.initials: JS
authors.lastname: Savage
authors.forename: Samreen
authors.initials: S
authors.lastname: Siddiq
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Westbury
authors.forename: Alastair W
authors.initials: AW
authors.lastname: Poole
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Mumford
authors.forename: Ingeborg
authors.initials: I
authors.lastname: Hers
mesh_terms: D000328:Adult; D000368:Aged; D019943:Amino Acid Substitution; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology*; D016022:Case-Control Studies; D005260:Female; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D053614:Janus Kinase 2 / Q000037:antagonists & inhibitors / Q000097:blood / Q000235:genetics; D008297:Male; D008875:Middle Aged; D020125:Mutation, Missense; D010446:Peptide Fragments / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000097:blood*; D010766:Phosphorylation; D015539:Platelet Activation / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism*; D000089804:Shelterin Complex; D015398:Signal Transduction / Q000187:drug effects; D034501:Telomere-Binding Proteins / Q000097:blood*; D013917:Thrombin / Q000494:pharmacology; D013920:Thrombocythemia, Essential / Q000097:blood* / Q000235:genetics; D013926:Thrombopoietin / Q000494:pharmacology
chemical_list: D005340:Fibrinogen; D053614:Janus Kinase 2; D010446:Peptide Fragments; D019869:Phosphatidylinositol 3-Kinases; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D000089804:Shelterin Complex; D034501:Telomere-Binding Proteins; D013917:Thrombin; D013926:Thrombopoietin; C440809:TERF2IP protein, human; C082835:thrombin receptor peptide (42-47); C507924:JAK2 protein, human