Apolipoprotein M modulates erythrocyte efflux and tubular reabsorption of sphingosine-1-phosphate

Abstract

This article is available online at http://www.jlr.org Supplementary key words high density lipoprotein • kidney • megalin • chloride-proton exchanger ClC5 • cystinosin Sphingosine-1-phosphate (S1P) acts both as an intracellular signaling molecule and an extracellular agonist of at least fi ve different G protein-coupled receptors. By its dual functions, S1P regulates the survival, proliferation, and migration as well as the functionality of many cells, eventually in opposite directions ( 1-4 ). Therefore the absolute and relative abundance of S1P in intracellular and extracellular compartments appears to be important for its biological functionality ( 4 ).Most cells form S1P by the phosphorylation of sphingosine, a degradation product of ceramides, through sphingosine kinase and degrade S1P to phosphoethanolamine and fatty aldehyde through S1P-lyase ( 1-4 ). By contrast, not only erythrocytes and platelets, but also other cells which have low or no lyase activity, release S1P ( 4-6 ), probably by an as yet unidentifi ed ABC transporter ( 4,7,8 ). Within the plasma compartment, the majority of S1P is transported by HDLs in which it exerts many cyto-protective and anti-infl ammatory effects, for example, on endothelial cells ( 8-10 ). The enrichment of S1P in HDL has been explained by the presence of a specifi c S1P binding protein, namely apoM ( 10 ). Purifi ed and recombinant apoM binds S1P with an IC 50 of 0.9 mol/l, which is in the range of physiological S1P plasma concentrations ( 11 ). X-ray crystallography of apoM identifi ed an S1P binding domain which was confi rmed by the recombinant generation of non-S1P binding apoM mutants ( 11 ). In agreement with these physicochemical data, S1P was copurifi ed with apoM containing HDL, but not apoM-free HDL, from both human Abstract Sphingosine-1-phosphate (S1P) mediates several cytoprotective functions of HDL. apoM acts as a S1P binding protein in HDL. Erythrocytes are the major source of S1P in plasma. After glomerular fi ltration, apoM is endocytosed in the proximal renal tubules. Human or murine HDL elicited time-and dose-dependent S1P effl ux from erythrocytes. Compared with HDL of wild-type (wt) mice, S1P effl ux was enhanced in the presence of HDL from apoM transgenic mice, but not diminished in the presence of HDL from apoM knockout ( Apom ؊ / ؊ ) mice. Artifi cially reconstituted and apoM-free HDL also effectively induced S1P effl ux from erythrocytes. S1P and apoM were not measurable in the urine of wt mice. Apom ؊ / ؊ mice excreted signifi cant amounts of S1P. apoM was detected in the urine of mice with defective tubular endocytosis because of knockout of the LDL receptor-related protein, chloride-proton exchanger ClC-5 ( Clcn5 ؊ / ؊ ), or the cysteine transporter cystinosin. Urinary levels of S1P were signifi cantly elevated in Clcn5 ؊ / ؊ mice. In contrast to Apom ؊ / ؊ mice, these mice showed normal plasma concentrations for apoM and S1P. In conclusion, HDL facilitates S1P effl ux from erythrocytes by both apoMdependent and apoM-independent mechanisms...

Sources, metabolism, and regulation of circulating sphingosine-1-phosphate

“…Surprisingly, HDLs isolated from apoM-defi cient mice do not exhibit impaired effectiveness in this regard. This fi nding strongly indicates the presence of an additional apoM-independent mechanism (most likely involving apoA-I) responsible for HDL-induced release of S1P from erythrocytes ( 69 ). It was also demonstrated that S1P transfer between RBCs and HDL is facilitated by phospholipid transfer protein, and phospholipid transfer protein-defi cient mice show a 60% decrease in plasma S1P concentration ( 84 ).…”

“…They showed that the apoM-defi cient fraction of human HDL contains no S1P, and that apoM Ϫ / Ϫ mice are characterized by a lack of HDL-bound S1P, whereas transgenic mice overexpressing human apoM show increased S1P content in HDL. However, according to a recent study, apoM may not be the only HDL protein able to bind S1P ( 69 ). It was estimated that, on average, only 1-10% of HDL particles in human plasma transport S1P, and in mice the molar ratio between HDL-bound S1P and plasma apoM is ‫ف‬ 1:3 ( 56,68 ).…”

Dysfunctional high-density lipoproteins in coronary heart disease: implications for diagnostics and therapy

“…HDL also elicits efflux of S1P from erythrocytes by apoM-dependent and -independent mechanisms. 103 Although present on only one of 10 to 20 HDL particles, S1P contributes to several of the protective functions of HDL. For example, HDL-associated S1P appears to be important for the vasoprotective effects on endothelial cell survival, 104 109 and nitric oxide-dependent vasorelaxation.…”

“…116 This loading may not only be an experimental model but may also happen in vivo, because HDL induces S1P efflux from erythrocytes and other cells by apoM dependent and -independent mechanisms. 103 It may hence well be that HDL are locally uploaded with S1P by endothelial cells or circulating blood cells so that the ex vivo measured S1P levels in HDL underestimate the in vivo situation. microRNAs HDL was also found to contain miRNAs.…”

Uncleaved ApoM Signal Peptide Is Required for Formation of Large ApoM/Sphingosine 1-Phosphate (S1P)-enriched HDL Particles

“…3C). Collectively, our data suggest that overexpression of apoM WT , but not apoM Q22A , increases plasma HDL particle size without affecting plasma HDL-C concentrations.Ad-apoM-treated Mice Have Higher Plasma S1P Concentrations than Ad-Q22A-treated Mice-We (15) and others (11,16,26,27) have shown that apoM overexpression leads to increased plasma S1P concentrations. Here, we measured FIGURE 2.…”