A role for plasma membrane Ca²⁺ ATPases in regulation of cellular Ca²⁺ homeostasis by sphingosine kinase-1

Sphingosine-1-phosphate (S1P) is a ubiquitous lipid mediator, acting via specific G-protein-coupled receptors (GPCR) and intracellularly. Previous work has shown that deletion of S1P lyase caused a chronic elevation of cytosolic [Ca²⁺]_i and enhanced Ca²⁺ storage in mouse embryonic fibroblasts. Here, we studied the role of sphingosine kinase (SphK)-1 in Ca²⁺ signaling, using two independently generated EA.hy926 cell lines with stable knockdown of SphK1 (SphK1-KD1/2). Resting [Ca²⁺]_i and thapsigargin-induced [Ca²⁺]_i increases were reduced in both SphK1-KD1 and -KD2 cells. Agonist-induced [Ca²⁺]_i increases, measured in SphK1-KD1, were blunted. In the absence of extracellular Ca²⁺, thapsigargin-induced [Ca²⁺]_i increases declined rapidly, indicating enhanced removal of Ca²⁺ from the cytosol. In agreement, plasma membrane Ca²⁺ ATPase (PMCA)-1 and -4 and their auxiliary subunit, basigin, were strongly upregulated. Activation of S1P-GPCR by specific agonists or extracellular S1P did not rescue the effects of SphK1 knockdown, indicating that S1P-GPCR were not involved. Lipid measurements indicated that not only S1P but also dihydro-sphingosine, ceramides, and lactosylceramides were markedly depleted in SphK1-KD2 cells. SphK2 and S1P lyase were upregulated, suggesting enhanced flux via the sphingolipid degradation pathway. Finally, histone acetylation was enhanced in SphK1-KD2 cells, and the histone deacetylase inhibitor, vorinostat, induced upregulation of PMCA1 and basigin on mRNA and protein levels in EA.hy926 cells. These data show for the first time a transcriptional regulation of PMCA1 and basigin by S1P metabolism. It is concluded that SphK1 knockdown in EA.hy926 cells caused long-term alterations in cellular Ca²⁺ homeostasis by upregulating PMCA via increased histone acetylation.