The serum- and glucocorticoid-inducible kinase 1 (SGK1) influences platelet calcium signaling and function by regulation of Orai1 expression in megakaryocytes

Abstract

Platelets are activated on increase of cytosolic Ca2+ activity ([Ca2+]i), accomplished by store-operated Ca2+ entry (SOCE) involving the pore-forming ion channel subunit Orai1. Here, we show, for the first time, that the serum- and glucocorticoid-inducible kinase 1 (SGK1) is expressed in platelets and megakaryocytes. SOCE and agonist-induced [Ca2+]i increase are significantly blunted in platelets from SGK1 knockout mice (sgk1-/-). Similarly, Ca2+- dependent degranulation, integrin αIIbβ 3activation, phosphatidylserine exposure, aggregation, and in vitro thrombus formation were significantly impaired in sgk1-/-platelets, whereas tail bleeding time was not significantly enhanced. Platelet and megakaryocyte Orai1 transcript levels and membrane protein abundance were significantly reduced in sgk1-/- mice. In human megakaryoblastic cells (MEG-01), transfection with constitutively active S422DSGK1 but not with inactive K127NSGK1 significantly enhanced Orai1 expression and SOCE, while effects reversed by the SGK1 inhibitor GSK650394 (1μM). Transfection of MEG-01 cells with S422DSGK1 significantly increased phosphorylation of IκB kinase α/β and IκBαresulting in nuclear translocation of NF-κB subunit p65. Treatment of S422DSGK1-transfected MEG-01 cells with the IκB kinase inhibitor BMS-345541 (10μM) abolished SGK1-induced increase of Orai1 expression and SOCE. The present observations unravel SGK1 as novel regulator of platelet function, effective at least in part by NF-κB-dependent transcriptional up-regulation of Orai1 in megakaryocytes and increasing platelet SOCE. © 2012 by The American Society of Hematology.