Translational regulation of the serum- and glucocorticoid-inducible kinase-1 (SGK1) in platelets.

Activation of platelets by thrombin opens pore forming channel protein Orai1 with subsequent store operated Ca(2+) entry (SOCE) and Ca(2+) dependent platelet granule release, integrin α(IIb)β(3) activation, adhesion, aggregation and thrombus formation. Orai1 and thus SOCE as well as platelet activation are up-regulated by the serum- and glucocorticoid-inducible kinase-1 (SGK1), which transcriptionally regulates Orai1 expression in megakaryocytes and thus determines Orai1 protein abundance in mature, circulating platelets. As platelets are devoid of nuclei, they are unable to modify protein abundance by regulation of transcription. However, they contain mRNA and thus could express novel protein by stimulation of protein translation. Translation is sensitive to actin polymerization and phosphoinositide-3-kinase (PI3K). Translational regulation of SGK1 expression has never been described before. The present study thus explored whether thrombin regulates SGK1 expression in platelets. As a result, according to RT-PCR mRNA encoding SGK1 is present in circulating platelets and significantly decreased by activation of platelets with thrombin (1 U/ml). The protein abundance of SGK1 is significantly enhanced by thrombin treatment, an effect significantly decreased by inhibition of translation with puromycin (100 nM) but not by inhibition of transcription with actinomycin (4 μg/ml). The increase of SGK1 protein abundance is blunted by inhibition of PI3K with wortmannin (100 nM) or LY294002 (25 μM), and by disruption of the cytoskeleton with cytochalasin B (1 μM). In conclusion, activation of platelets with thrombin stimulates the translation of SGK1.