Objective: Sphingomyelin synthase (SMS) catalyzes the conversion of ceramide to sphingomyelin and sits at the crossroads of sphingolipid biosynthesis. SMS has 2 isoforms: SMS1 and SMS2. Although they have the same SMS activity, they are different enzymes with distinguishable subcellular localizations and cell expression patterns. It is conceivable that these differences could yield different consequences, in terms of sphingolipid metabolism and its related atherogenesis.
Methods and results: We created Sms1 gene knockout mice and found that Sms1 deficiency significantly decreased plasma, liver, and macrophage sphingomyelin (59%, 45%, and 54%, respectively), but only had a marginal effect on ceramide levels. Surprisingly, we found that Sms1 deficiency dramatically increased glucosylceramide and GM3 levels in plasma, liver, and macrophages (4- to 12-fold), whereas Sms2 deficiency had no such effect. We evaluated the total SMS activity in tissues and found that Sms1 deficiency causes 77% reduction in SMS activity in macrophages, indicating SMS1 is the major SMS in macrophages. Moreover, Sms1-deficient macrophages have a significantly higher glucosylceramide synthase activity. We also found that Sms1 deficiency significantly attenuated toll-like 4 receptor-mediated nuclear factor-κB and mitogen-activated protein kinase activation after lipopolysaccharide treatment. To evaluate atherogenicity, we transplanted Sms1 knockout mouse bone marrow into low-density lipoprotein receptor knockout mice (Sms1(-/-)→Ldlr(-/-)). After 3 months on a western diet, these animals showed a significant decrease of atherosclerotic lesions in the root and the entire aorta (35% and 44%, P<0.01, respectively) and macrophage content in lesions (51%, P<0.05), compared with wild-type→Ldlr(-/-) mice.
Conclusions: Sms1 deficiency decreases sphingomyelin, but dramatically increases the levels of glycosphingolipids. Atherosclerosis in Sms1(-/-)→Ldlr(-/-) mice is significantly decreased.