To better define the metabolism of sphingosine-1-phosphate (S1P), its transport in plasma and its interactions with S1P receptors on vascular cells, and to evaluate the effect of statin treatment on the subnormal plasma levels of high-density lipoprotein (HDL)-bound S1P characteristic of the atherogenic dyslipidemia of metabolic syndrome (MetS).

Neither clinical intervention trials targeted to raising high-density lipoprotein-cholesterol (HDL-C) levels nor human genome-wide association studies (GWAS) studies have provided evidence to support an atheroprotective role of HDL. Recently however a large monogenic univariable Mendelian randomization on the N396S mutation in the gene encoding endothelial lipase revealed a causal protective effect of elevated HDL-C on coronary artery disease conferred by reduced enzyme activity. Given the complexity of the HDL lipidome and proteome, components of HDL other than cholesterol may in all likelihood contribute to such a protective effect. Among HDL lipids, S1P is a bioactive sphingolipid present in a small proportion of HDL particles (about 5%); indeed, S1P is preferentially enriched in small dense HDL3. As S1P is bound to apolipoprotein (apo) M in HDL, such enrichment is consistent with the elevated apoM concentration in HDL3. When HDL/apoM-bound S1P acts on S1P1 or S1P3 receptors in endothelial cells, potent antiatherogenic and vasculoprotective effects are exerted; those exerted by albumin-bound S1P at these receptors are typically weaker. When HDL/apoM-bound S1P binds to S1P2 receptors, proatherogenic effects may potentially be induced. Subnormal plasma levels of HDL-associated S1P are typical of dyslipidemic individuals at high cardiovascular risk and in patients with coronary heart disease. International Guidelines recommend statin treatment as first-line lipid lowering therapy in these groups. The cardiovascular benefits of statin therapy are derived primarily from reduction in low-density lipoprotein (LDL)-cholesterol, although minor contributions from pleiotropic actions cannot be excluded. Might statin treatment therefore normalize, directly or indirectly, the subnormal levels of S1P in dyslipidemic subjects at high cardiovascular risk? Our unpublished findings in the CAPITAIN study (ClinicalTrials.gov: NCT01595828), involving a cohort of obese, hypertriglyceridemic subjects (n = 12) exhibiting the MetS, showed that pitavastatin calcium (4 mg/day) treatment for 180days was without effect on either total plasma or HDL-associated S1P levels, suggesting that statin-mediated improvement of endothelial function is not due to normalization of HDL-bound S1P. Statins may however induce the expression of S1P1 receptors in endothelial cells, thereby potentiating increase in endothelial nitric oxide synthase response to HDL-bound S1P, with beneficial downstream vasculoprotective effects.

Current evidence indicates that S1P in small dense HDL3 containing apoM exerts antiatherogenic effects and that statins exert vasculoprotective effects through activation of endothelial cell S1P1 receptors in response to HDL/apoM-bound S1P.