Sphingomyelin synthase 2 is one of the determinants for plasma and liver sphingomyelin levels in mice.

BACKGROUND: It has been proposed that plasma sphingomyelin (SM) plays a very important role in plasma lipoprotein metabolism and atherosclerosis. Sphingomyelin synthase (SMS) is the last enzyme for SM de novo biosynthesis. Two SMS genes, SMS1 and SMS2, have been cloned and characterized. METHODS AND RESULTS: To evaluate the in vivo role of SMS2 in SM metabolism, we prepared SMS2 knockout (KO) and SMS2 liver-specific transgenic (LTg) mice and studied their plasma SM and lipoprotein metabolism. On a chow diet, SMS2 KO mice showed a significant decrease in plasma SM levels (25%, P<0.05), but no significant changes in total cholesterol, total phospholipids, or triglyceride, compared with wild-type (WT) littermates. On a high-fat diet, SMS2 KO mice showed a decrease in plasma SM levels (28%, P<0.01), whereas SMS2LTg mice showed a significant increase in those levels (29%, P<0.05), but no significant changes in other lipids, compared with WT littermates. Atherogenic lipoproteins from SMS2LTg mice displayed a significantly stronger tendency toward aggregation after mammalian sphingomyelinase treatment, compared with controls. Moreover, SMS2 deficiency significantly increased plasma apoE levels (2.0-fold, P<0.001), whereas liver-specific SMS2 overexpression significantly decreased those levels (1.8-fold, P<0.01). Finally, SMS2 KO mouse plasma promoted cholesterol efflux from macrophages, whereas SMS2LTg mouse plasma prevented it. CONCLUSIONS: We therefore believe that regulation of liver SMS2 activity could become a promising treatment for atherosclerosis.

Sphingomyelin synthase 2 deficiency attenuates NFkappaB activation.

BACKGROUND: NFkappaB has long been regarded as a proatherogenic factor, mainly because of its regulation of many of the proinflammatory genes linked to atherosclerosis. Metabolism of sphingomyelin (SM) has been suggested to affect NFkappaB activation, but the mechanism is largely unknown. SMS2 regulates SM levels in cell plasma membrane and lipid rafts and has a potential to regulate NFkappaB activation. METHODS AND RESULTS: To investigate the role of SMS2 in NFkappaB activation we used macrophages from SMS2 knockout (KO) mice and SMS2 siRNA-treated HEK 293 cells. We found that NFkappaB activation and its target gene expression are attenuated in macrophages from SMS2 KO mice in response to lipopolysaccharide (LPS) stimulation and in SMS2 siRNA- treated HEK 293 cells after tumor necrosis factor (TNF)-alpha simulation. In line with attenuated NFkappaB activation, we found that SMS2 deficiency substantially diminished the abundance of toll like receptor 4 (TLR4)-MD2 complex levels on the surface of macrophages after LPS stimulation, and SMS2 siRNA treatment reduced TNF-alpha-stimulated lipid raft recruitment of TNF receptor-1 (TNFR1) in HEK293 cells. SMS2 deficiency decreased the relative amounts of SM and diacylglycerol (DAG) and increased ceramide, suggesting multiple mechanisms for the decrease in NFkappaB activation. CONCLUSIONS: SMS2 is a modulator of NFkappaB activation, and thus it could play an important role in NFkappaB-mediated proatherogenic process.