Effect of TGFβ on Na+/K+ ATPase activity in megakaryocytes.

The Na(+)/K(+) ATPase generates the Na(+) and K(+) concentration gradients across the plasma membrane and is thus essential for cellular electrolyte homeostasis, cell membrane potential and cell volume maintenance. A powerful regulator of Na(+)/K(+) ATPase is the serum- and glucocorticoid-inducible kinase 1 (SGK1). The most powerful known regulator of SGK1 expression is TGFß1, which is pivotal in the regulation of megakaryocyte maturation and platelet formation. Signaling involved in the upregulation of SGK1 by TGFß1 includes p38 mitogen activated protein (MAP) kinase. SGK1 in turn phosphorylates the IκB kinase (IKKα/β), which phosphorylates the inhibitor protein IκBα thus triggering nuclear translocation of nuclear factor kappa B (NF-κB). The present study explored whether TGFβ influences Na(+)/K(+) ATPase activity in megakaryocytes, and if so, whether the effect of TGß1 requires p38 MAP kinase, SGK1 and/or NF-κB. To this end, murine megakaryocytes were treated with TGFß1 and Na(+)/K(+) ATPase activity determined from K(+) induced current utilizing whole cell patch clamp. The pump current (Ipump) was determined in the absence and presence of Na(+)/K(+) ATPase inhibitor ouabain (100μM). TGFß1 (60ng/ml) was added in the absence or presence of p38 MAP kinase inhibitor skepinone-L (1μM), SGK1 inhibitor EMD638683 (50μM) or NF-κB inhibitor wogonin (50nM). As a result, the Ipump was significantly increased by pretreatment of the megakaryocytes with TGFß1, an effect reaching statistical significance within 16 and 24h and virtually abrogated in the presence of skepinone-L, EMD638683 or wogonin. In conclusion, TGFß1 is a powerful regulator of megakaryocytic Na(+)/K(+) ATPase activity. Signaling mediating the effect of TGFß1 on Na(+)/K(+) ATPase activity involves p38 MAP kinase, SGK1 and NF-κB.
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