Niemann-Pick C1 Modulates Hepatic Triglyceride Metabolism and Its Genetic Variation Contributes to Serum Triglyceride Levels

Objective-To study how Niemann-Pick disease type C1 (NPC1) influences hepatic triacylglycerol (TG) metabolism and to determine whether this is reflected in circulating lipid levels. Methods and Results-In Npc1(-/-) mice, the hepatic cholesterol content is increased but the TG content is decreased. We investigated lipid metabolism in Npc1(-/-) mouse hepatocytes and the association of NPC1 single-nucleotide polymorphisms with circulating TGs in humans. TGs were reduced in Npc1(-/-) mouse serum and hepatocytes. In Npc1(-/-) hepatocytes, the incorporation of [H-3] oleic acid and [H-3] acetate into TG was decreased, but shunting of oleic acid- or acetate-derived [H-3] carbons into cholesterol was increased. Inhibition of cholesterol synthesis normalized TG synthesis, content, and secretion in Npc1(-/-) hepatocytes, suggesting increased hepatic cholesterol neogenesis as a cause for the reduced TG content and secretion. We found a significant association between serum TG levels and 5 common NPC1 single-nucleotide polymorphisms in a cohort of 1053 men, with the lowest P=8.7 x 10(-4) for the single-nucleotide polymorphism rs1429934. The association between the rs1429934 A allele and higher TG levels was replicated in 2 additional cohorts, which included 8041 individuals. Conclusion-This study provides evidence of the following: (1) in mice, loss of NPC1 function reduces hepatocyte TG content and secretion by increasing the metabolic flux of carbons into cholesterol synthesis; and (2) common variation in NPC1 contributes to serum TG levels in humans. (Arterioscler Thromb Vasc Biol. 2010;30:1614-1620.)