Sphingomyelin Synthase 2 Is One of the Determinants for Plasma and Liver Sphingomyelin Levels in Mice
ABSTRACT 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.
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.
We therefore believe that regulation of liver SMS2 activity could become a promising treatment for atherosclerosis.
SourceAvailable from: De-Yong Ye
[Show abstract] [Hide abstract]
ABSTRACT: Sphingomyelin synthase related protein (SMSr) synthesizes the sphingomyelin analog ceramide phosphoethanolamine (CPE) in cells. Previous cell studies indicated that SMSr is involved in ceramide homeostasis and is crucial for cell function. To furhter examine SMSr function in vivo, we generated Smsr KO mice which were fertile and had no obvious phenotypic alterations. Quantitative MS analyses of plasma, liver, and macrophages from the KO mice revealed only marginal changes in CPE and ceramide, as well as other sphingolipid levels. Because SMS2 also has CPE synthase activity, we prepared Smsr/Sms2 double KO mice. We found that CPE levels were not significantly changed in macrophages, suggesting that CPE levels are not exclusively dependent on SMSr and SMS2 activities. We then measured CPE levels in Sms1 KO mice and found that Sms1 deficiency also reduced plasma CPE levels. Importantly, we found that expression of Sms1 or Sms2 in SF9 insect cells not only significantly increased SM but also CPE formation, indicating that SMS1 also has CPE synthase activity. Our study reveals that all mouse SMS family members (SMSr, SMS1, and SMS2) have CPE synthase activity. However, neither CPE nor SMSr appears to be a critical regulator of ceramide levels in vivo. Copyright © 2015, The American Society for Biochemistry and Molecular Biology.The Journal of Lipid Research 01/2015; 56(3). DOI:10.1194/jlr.M054627 · 4.73 Impact Factor
[Show abstract] [Hide abstract]
ABSTRACT: Besides bulk amounts of sphingomyelin (SM), mammalian cells produce small quantities of the SM analog ceramide phosphoethanolamine (CPE). Little is known about the biological role of CPE or enzymes responsible for CPE production. Heterologous expression studies revealed that SM synthase SMS2 is a bifunctional enzyme producing both SM and CPE whereas SM synthase-related enzyme SMSr serves as monofunctional CPE synthase. Acute disruption of SMSr catalytic activity in cultured cells causes a rise in ER ceramides, fragmentation of ER exit sites and induction of mitochondrial apoptosis. To address the relevance of CPE biosynthesis in vivo, we analyzed the tissue-specific distribution of CPE in mice and generated mouse lines lacking SMSr and SMS2 catalytic activity. We find that CPE levels are >300-fold lower than SM in all tissues examined. Unexpectedly, combined inactivation of SMSr and SMS2 significantly reduced but did not eliminate tissue-specific CPE pools and had no obvious impact on mouse development or fertility. While SMSr is widely expressed and serves as the principal CPE synthase in brain, blocking its catalytic activity did not affect ceramide levels or secretory pathway integrity in brain or any other tissue. Our data provide a first inventory of CPE species and CPE-biosynthetic enzymes in mammals. Copyright © 2015, The American Society for Biochemistry and Molecular Biology.The Journal of Lipid Research 02/2015; DOI:10.1194/jlr.M055269 · 4.73 Impact Factor