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

Johnny H Chen,Maziar Riazy,Shih Wei Wang,Jiazhen Minnie Dai,Vincent Duronio,Urs P Steinbrecher

doi:10.1194/jlr.m000398

Abstract

We recently reported that oxidized LDL (oxLDL) induces an oscillatory increase in intracellular calcium ([Ca(2+)](i)) levels in macrophages. Furthermore, we have shown that these [Ca(2+)](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 [Ca(2+)](i) oscillations have not been elucidated. In this study, we show that these oscillations are mediated in part by intracellular mechanisms, as depleting extracellular Ca(2+) did not completely abolish the effect. Inhibiting sarco-endoplasmic reticulum ATPase (SERCA) completely blocked [Ca(2+)](i) oscillations, suggesting a role for Ca(2+) 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 [Ca(2+)](i) oscillations. This suggests that the mechanism in which oxLDL generates [Ca(2+)](i) oscillations may be 1) activation of SK, 2) SK-mediated increase in S1P levels, 3) S1P-mediated Ca(2+) release from intracellular stores, and 4) SERCA-mediated Ca(2+) reuptake back into the ER.

Highlights

We have mediate Oxidized LDL (oxLDL)’s ability to inhibit macrophage apoptosis in response to growth factor deprivation
We describe a mechanism in which oxLDL generates [intracellular calcium (Ca2+]i) oscillations by 1) oxLDL-mediated activation of sphingosine kinase (SK), 2) SK-mediated increase in S1P levels, 3) S1Pmediated Ca2+ release from intracellular stores, and 4) sarco-endoplasmic reticulum ATPase (SERCA)-mediated Ca2+ reuptake back into the endoplasmic reticulum (ER)
Even though PC is converted to lysoPC during the LDL oxidation reaction [45], it is unlikely that the lysoPC content in oxLDL is responsible for the observed [Ca2+]i oscillations

Summary

Introduction

We have mediate oxLDL’s ability to inhibit macrophage apoptosis in response to growth factor deprivation. The signal transduction pathways by which oxLDL induces [Ca2+]i oscillations have not been elucidated. Our group has recently reported that oxLDL inhibits macrophage apoptosis by activating eukaryotic elongation factor-2 (eEF2) kinase ( known as Ca 2+/calmodulin-dependent kinase III) [16]. EEF2 kinase activation and inhibition of macrophage apoptosis is mediated by an oscillatory increase in intracellular calcium ([Ca2+]i). The signal transduction pathways involved in oxLDL-mediated [Ca2+]i oscillations have not been elucidated. Ca2+ is a ubiquitous intracellular signal responsible for controlling numerous cellular processes. These processes range from muscle contraction to synaptic transmission and from cellular proliferation to apoptosis [17].

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