Platelets are activated upon increase of cytosolic Ca²⁺ activity ([Ca²⁺]_i), accomplished by store operated Ca²⁺ entry (SOCE) involving the pore forming ion channel subunit Orai1. However, regulation of Orai1 in platelets is poorly understood.

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 a_IIbb₃ activation, phosphatidylserine exposure, aggregation and in vitro thrombus formation were significantly impaired in sgk1^-/- platelets, while 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, effects reversed by the SGK1 inhibitor GSK650394 (1 mM). Transfection of MEG-01 cells with ^S422DSGK1 significantly increased phoshorylation of IkB kinase (IKK) a/b and IkBa resulting in nuclear translocation of NF-kB subunit p65. Treatment of ^S422DSGK1-transfected MEG-01 cells with the IKK inhibitor BMS-345541 (10 mM) 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-kB-dependent transcriptional upregulation of Orai1 in megakaryocytes and increasing platelet SOCE.