Sphingomyelin Synthase 2 Is One of the Determinants for Plasma and Liver Sphingomyelin Levels in Mice

Abstract

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%, PϽ0.05), 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%, PϽ0.01), whereas SMS2LTg mice showed a significant increase in those levels (29%, PϽ0.05), 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, PϽ0.001), whereas liver-specific SMS2 overexpression significantly decreased those levels (1.8-fold, PϽ0.01). 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. Key Words: SMS2 knockout mice Ⅲ SMS2 liver-specific transgenic mice Ⅲ plasma sphingomyelin Ⅲ plasma lipoproteins Ⅲ plasma cholesterol S phingomyelin (SM), which is the second most abundant phospholipid in mammalian plasma, appears in all major lipoproteins, where it is part of the monolayer of polar lipids and cholesterol that surrounds a core of neutral lipids. Up to 18% of total plasma phospholipid exists as SM, 1 with the ratio of phosphatidylcholine (PC)/SM varying widely among lipoprotein subclasses. 2 Atherogenic lipoproteins such as VLDL remnants accumulating in cholesterol-fed rabbits tend to be SM-enriched. 3 The SM content of atherosclerotic lesions is higher than that of normal arterial tissue, and SM accumulation is disproportionate to that of PC. 4 However, the role of SM deposited or synthesized in atheromata still remains undefined.It has been suggested that subendothelial retention and aggregation of atherogenic lipoproteins play a very important role in atherogenesis. 5 SM-rich LDL retained in atherosclerotic lesions is acted on by an arterial wall sphingomyelinase, which appears to promote aggregation, initiating the early phase of atherosclerosis development. 4 We have found that plasma SM levels in apoE KO mice are 4-fold higher than in WT mice, 6 and this may partially explain the increased atherosclerosis found in these animals. 7 Our laboratory and others have also discovered that inhibition of SM biosynthesis significantly decreases plasma SM and ...

Dynamic Modification of Sphingomyelin in Lipid Microdomains Controls Development of Obesity, Fatty Liver, and Type 2 Diabetes

“…Critical roles for lipid microdomains in fatty acid uptake and maturation of lipid droplets have been reported (28,34). In addition, SMS2 deficiency reportedly reduced sensi-tivity to lysenin-mediated cytolysis, indicating that SMS2 alters the lipid microdomain (35). Methyl-␤-cyclodextrin (M␤CD) is known to reduce cholesterol levels, disrupt lipid microdomains, and induce cell death (36).…”

Functional characterization of enzymes catalyzing ceramide phosphoethanolamine biosynthesis in mice

“…Ceramides serve as "metabolic hub" and cells are equipped with a multitude of enzymes to modulate their levels. This may explain why genetic ablation of SMS1 and SMS2 in mice causes only a modest increase in tissue ceramide levels [ ( 13,15,27 ), and this study], even though these enzymes normally consume the bulk of newly synthesized ceramides ( 10 ). As tissues of SMSrD348E mutant mice did not contain any signifi cantly elevated levels of sphingoid bases, hexosylceramides, or SM, it is unclear how embryonic cells with disrupted SMSr catalytic activity were able to overcome a deregulation of ER ceramides associated with a disruption in SMSr catalytic activity.…”

“…In the current study, we generated mouse lines lacking SMSr catalytic activity and crossbred these with SMS2-null mice ( 27 ) of wild-type heterozygous (+/SMSrD348E or +/SMSrdelEx6) and homozygous (SMSrD348E or SMSrdelEx6) mice. For the SMSrD348E mice, DNA was digested with Bcl I and a 5 ′ external probe binding downstream of exon 1 and upstream of the 5 ′ HR was used.…”

“…Because SMS2 is known to have dual activity as CPE synthase ( 23 ), we crossed the SMSr mutant mice with SMS2gt mice ( 27 ) and bred double mutants to determine the relative contribution of each enzyme to CPE biosynthesis.…”

Effects of sepsis on the metabolism of sphingomyelin and cholesterol in mice with liver dysfunction

“…In the present study, it was indicated that hepatic SMS activity increased following LPS treatment, and this implied that the cause of the increase in SMS activity and SM content was the overexpression of SMS2, but not of SMS1. SMS2 is the principal enzyme that takes charge of the biosynthesis of SM in the liver (26).…”