Up-Regulation of Na+/Ca2+ Exchange in Megakaryocytes Following TGFβ1 Treatment

Abstract

Background: Blood platelets are activated by increase of cytosolic Ca²⁺ activity ([Ca²⁺]_i). Ca²⁺ entry is accomplished in part by store operated Ca²⁺ entry (SOCE) involving Ca²⁺ release activated Ca²⁺-channel (CRAC) moiety Orai1 and its regulator STIM1, which are stimulated by depletion of intracellular Ca²⁺ stores. An increase of [Ca²⁺]_i is terminated by Na⁺/Ca²⁺-exchange. The expression of both, Orai1 and STIM1 in megakaryocytes is up-regulated by tumor growth factor TGFβ1, a powerful regulator of megakaryocyte differentiation. The present study explored whether TGFβ1 similarly modifies megakaryocyte Na⁺/Ca²⁺-exchanger activity. Methods: [Ca²⁺]_i was determined utlizing Fura-2 fluorescence, SOCE from increase of [Ca²⁺]_i, following readdition of extracellular Ca²⁺ after store depletion, and Na⁺/Ca²⁺-exchanger activity from increase of [Ca²⁺]_i and whole cell currents following removal of extracellular Na⁺. Results: TGFβ1 treatment not only augments the increase of [Ca²⁺]_i following store depletion and SOCE, but significantly up-regulates Na⁺/Ca²⁺-exchanger activity as apparent from [Ca²⁺]_i measurements and whole cell currents. Conclusions: TGFβ1 is a powerful stimulator of both, SOCE and Na⁺/Ca²⁺-exchanger activity in megakaryocytes.