Objective-Plasma high-density lipoproteins (HDL) are potent antiatherogenic and anti-inflammatory particles. However, HDL particles are highly heterogenic in composition, and different HDL-mediated functions can be ascribed to different subclasses of HDL. Only a small HDL population contains apolipoprotein M (ApoM), which is the main plasma carrier of the bioactive lipid mediator sphingosine-1-phosphate (S1P). Vascular inflammation is modulated by S1P, but both proand anti-inflammatory roles have been ascribed to S1P. The goal of this study is to elucidate the role of ApoM and S1P in endothelial anti-inflammatory events related to HDL. Approach and Results-Aortic or brain human primary endothelial cells were challenged with tumor necrosis factor-α (TNF-α) as inflammatory stimuli. The presence of recombinant ApoM-bound S1P or ApoM-containing HDL reduced the abundance of adhesion molecules in the cell surface, whereas ApoM and ApoM-lacking HDL did not. Specifically, ApoM-bound S1P decreased vascular adhesion molecule-1 (VCAM-1) and E-selectin surface abundance but not intercellular adhesion molecule-1. Albumin, which is an alternative S1P carrier, was less efficient in inhibiting VCAM-1 than ApoM-bound S1P. The activation of the S1P receptor 1 was sufficient and required to promote anti-inflammation. Moreover, ApoM-bound S1P induced the rearrangement of the expression of S1P-related genes to counteract TNF-α. Functionally, HDL/ApoM/S1P limited monocyte adhesion to the endothelium and maintained endothelial barrier integrity under inflammatory conditions. Conclusions-ApoM-bound S1P is a key component of HDL and is responsible for several HDL-associated protective functions in the endothelium, including regulation of adhesion molecule abundance, leukocyte-endothelial adhesion, and endothelial barrier. Ruiz et al ApoM-Containing HDL Reduces Vascular Inflammation 119secretion and used by the mature ApoM protein to anchor to the phospholipid surface of HDL. 7,8 Five different membrane-bound G-protein-coupled S1P receptors (S1PRs) are known, and binding of S1P to the receptors activates multiple receptor-specific downstream signaling pathways. In this way, S1P is able to regulate several biological processes, such as immune cell trafficking, angiogenesis, endothelial cell migration, and endothelial barrier function. 9 The role of S1P in the regulation of vascular inflammation has been studied, and contradictory results have been obtained, for example, direct stimulation by S1P is reported to increase the abundance of adhesion molecules, whereas other studies [10][11][12] show that S1P inhibits tumor necrosis factor-α (TNF-α) induction of adhesion molecules, such as E-selectin, ICAM-1, or VCAM-1. 11,13The aim of this study is to characterize the role of S1P in the regulation of human endothelium inflammation taking into account that S1P is mostly bound to ApoM in plasma. Using recombinant human ApoM with or without bound S1P and isolated HDL containing or HDL lacking human ApoM (HDL +ApoM and HDL −ApoM , respectively), w...

Sphingosine 1-phosphate (S1P) is a signalling sphingolipid affecting multiple cellular functions of vascular and immune systems. It circulates at submicromolar levels bound to HDL-associated apolipoprotein M (apoM) or to albumin. S1P in blood is mainly produced by platelets and erythrocytes, making blood sampling for S1P quantification delicate. Standardisation of sampling is thereby of great importance to obtain robust data. By optimising and characterising the extraction procedure and the LC-MS/MS analysis, we have developed and validated a highly specific and sensitive method for S1P quantification. Blood was collected from healthy individuals (n=15) to evaluate the effects of differential blood sampling on S1P levels. To evaluate correlation between S1P and apoM in different types of plasma and serum, apoM was measured by ELISA. The method showed good accuracy and precision in the range of 0.011 to 0.9 μM with less than 0.07 % carryover. We found that the methanol precipitation used to extract S1P co-extracted apoM and several other HDL-proteins from plasma. The plateletassociated S1P was released during coagulation, thus increasing the S1P concentration to double in serum as compared to that in plasma. Gel filtration chromatography revealed that the platelet-released S1P was mainly bound to albumin. This explains why the strong correlation between S1P and apoM levels in plasma is lost upon the clotting process and hence not observed in serum. We have developed, characterised and validated an efficient, highly sensitive and specific method for the quantification of S1P in biological material.

Background: Apo (apolipoprotein) M mediates the physical interaction between high-density lipoprotein (HDL) particles and sphingosine-1-phosphate (S1P). Apo M exerts anti-inflammatory and cardioprotective effects in animal models. Methods: In a subset of PHFS (Penn Heart Failure Study) participants (n=297), we measured apo M by Enzyme-Linked ImmunoSorbent Assay (ELISA). We also measured total S1P by liquid chromatography–mass spectrometry and isolated HDL particles to test the association between apo M and HDL-associated S1P. We confirmed the relationship between apo M and outcomes using modified aptamer-based apo M measurements among 2170 adults in the PHFS and 2 independent cohorts: the Washington University Heart Failure Registry (n=173) and a subset of TOPCAT (Treatment of Preserved Cardiac Function Heart Failure With an Aldosterone Antagonist Trial; n=218). Last, we examined the relationship between apo M and ≈5000 other proteins (SomaScan assay) to identify biological pathways associated with apo M in heart failure. Results: In the PHFS, apo M was inversely associated with the risk of death (standardized hazard ratio, 0.56 [95% CI, 0.51–0.61]; P <0.0001) and the composite of death/ventricular assist device implantation/heart transplantation (standardized hazard ratio, 0.62 [95% CI, 0.58–0.67]; P <0.0001). This relationship was independent of HDL cholesterol or apo AI levels. Apo M remained associated with death (hazard ratio, 0.78 [95% CI, 0.69–0.88]; P <0.0001) and the composite of death/ventricular assist device/heart transplantation (hazard ratio, 0.85 [95% CI, 0.76–0.94]; P =0.001) in models that adjusted for multiple confounders. This association was present in both heart failure with reduced and preserved ejection fraction and was replicated in the Washington University cohort and a cohort with heart failure with preserved ejection fraction only (TOPCAT). The S1P and apo M content of isolated HDL particles strongly correlated ( R =0.81, P <0.0001). The top canonical pathways associated with apo M were inflammation (negative association), the coagulation system (negative association), and liver X receptor/retinoid X receptor activation (positive association). The relationship with inflammation was validated with multiple inflammatory markers measured with independent assays. Conclusions: Reduced circulating apo M is independently associated with adverse outcomes across the spectrum of human heart failure. Further research is needed to assess whether the apo M/S1P axis is a suitable therapeutic target in heart failure.