Reduced Serine palmitoyltransferase activity in macrophages ameliorates atherosclerosis in mouse model.

Abstract

Sphingolipid de novo synthesis pathway is considered a promising target for pharmacological intervention against atherosclerosis. Mammalian Serine palmitoyltransferase (SPT) - the key enzyme of sphingomyelin (SM) biosynthetic pathway - is a heterodimer, mainly composed of two subunits Sptlc1 and Sptlc2 in macrophages, both of which are needed for its catalytic function. In this study, we investigated the effect of Sptlc2 haplo-insufficiency on macrophage functions that are involved in the development of atherosclerosis. Initially we confirmed that bone marrow derived macrophages from Sptlc2+/- mice have significantly reduced amounts of Sptlc2 transcripts and protein. This leads to reduced SPT activity in macrophages, resulting in decreased levels of intracellular SM compared to wild type controls. Furthermore, to evaluate Sptlc2 haplo-insufficiency-mediated anti-atherogenic consequences, we transplanted Sptlc2+/- mouse bone marrow cells into LDL receptor knockout mice (Sptlc2+/– → Ldlr–/–), creating a mouse model of Sptlc2 haplo-insufficiency in hematopoietically-derived cells, including macrophages. We found that, after 3 months on a Western diet, Sptlc2+/– → Ldlr–/– mice showed decrease of atherosclerotic lesions in aortic arches, aortic roots (34%), and entire aortae (47%), compared with WT → Ldlr–/– mice. Analysis of plaque morphology revealed that Sptlc2+/– → Ldlr–/– mice had significantly less necrotic cores (30%) and reduced macrophage content (37%) in the atherosclerotic lesions. Moreover, Sptlc2+/– → Ldlr–/– mice exhibited reduced IL-6 levels in their circulation compared to WT → Ldlr–/– mice on Western diet. To elucidate the mechanism, we further investigated the roles of macrophage Sptlc2 haplo-insufficiency in inflammation and reverse cholesterol transport (RCT). We found that Sptlc2 haplo-insufficiency significantly dampened LPS or palmitate-mediated inflammatory responses in macrophages, as reflected by attenuation of Toll like 4 receptor recruitment, and NFКB and MAP kinase activation. Also, Sptlc2 haplo-insufficiency promoted reverse cholesterol transport, an anti-atherogenic process, as demonstrated by elevated ATP-Binding Cassette Transporters- ABCA1, ABCG1 and ApoE- mediated cholesterol efflux in vitro and in vivo. And these effects are primarily attributable to diminished levels of SM, glycosphingolipids and GM3 (raft marker) on macrophage plasma membranes and lipid rafts leading to possible raft alterations, affecting inflammatory cell signaling and cholesterol efflux. Therefore, we conclude that SPT haplo-insufficiency attenuates atherosclerosis by reducing inflammation and augmenting reverse cholesterol transport. Thus macrophage SPT could be a novel therapeutic target for treating atherosclerosis.