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

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 alpha(IIb)beta(3) activation, 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 (S422D)SGK1 but not with inactive (K127N)SGK1 significantly enhanced Orai1 expression and SOCE, while effects reversed by the SGK1 inhibitor GSK650394 (1 mu M). Transfection of MEG-01 cells with S422DSGK1 significantly increased phosphorylation of I kappa B kinase alpha/beta and I kappa B alpha resulting in nuclear translocation of NF-kappa B subunit p65. Treatment of (S422D)SGK1-transfected MEG-01 cells with the I kappa B kinase inhibitor BMS-345541 (10 mu M) 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-kappa B-dependent transcriptional up-regulation of Orai1 in megakaryocytes and increasing platelet SOCE. (Blood. 2012; 119(1): 251-261)