Sphingosine kinase regulates oxidized low density lipoprotein-mediated calcium oscillations and macrophage survival

Abstract

We recently reported that oxidized LDL (oxLDL) induces an oscillatory increase in intracellular calcium ([Ca2+]i) levels in macrophages. Furthermore, we have shown that these [Ca2+]i oscillations mediate oxLDL's ability to inhibit macrophage apoptosis in response to growth factor deprivation. However, the signal transduction pathways by which oxLDL induces [Ca2+]i oscillations have not been elucidated. In this study, we show that these oscillations are mediated in part by intracellular mechanisms, as depleting extracellular Ca2+ did not completely abolish the effect. Inhibiting sarco-endoplasmic reticulum ATPase (SERCA) completely blocked [Ca2+]i oscillations, suggesting a role for Ca2+ reuptake by the ER. The addition of oxLDL resulted in an almost immediate activation of sphingosine kinase (SK), which can increase sphingosine-1-phosphate (S1P) levels by phosphorylating sphingosine. Moreover, S1P was shown to be as effective as oxLDL in blocking macrophage apoptosis and producing [Ca2+]i oscillations. This suggests that the mechanism in which oxLDL generates [Ca2+]i oscillations may be 1) activation of SK, 2) SK-mediated increase in S1P levels, 3) S1P-mediated Ca2+ release from intracellular stores, and 4) SERCA-mediated Ca2+ reuptake back into the ER.

Keywords

• sphingosine-1-phosphate
• calcium
• foam cells
• plaque stability