Elucidation of novel functions of cholesterol and its metabolites using genetic manipulation in mice

胆固醇的生物合成途径分为两条途径：类固醇途径和非类固醇途径。然后非甾醇途径提供各种物质，如异戊二烯、二十二醇、血红素A和泛醌。HMG-CoA还原酶(HMGCR)产生甲伐他酸，它是固醇和非固醇途径的前体，而角鲨烯合成酶(SS)产生角鲨烯，而角鲨烯只参与固醇途径。在本研究中，我们使用Cre-loxP系统以肝脏特异的方式干扰HMGCR或SS的基因，以剖析肝脏中各自酶的功能。肝脏特异性HMGCR基因敲除小鼠(L-HMGCRKO)出生后1个月因暴发性肝功能衰竭而死亡。血浆胆固醇水平仅降低了10%，肝脏胆固醇的生物合成没有减少。诱导SREBP2基因表达，下调Cyp7A基因表达。肝细胞内充满甘油三酯。肝脏特异性SS基因敲除小鼠(L-SSKO)表现为一过性肝肿大并伴有肝功能障碍。尽管肝脏极低密度脂蛋白的生成率降低，但至少在体内环境下，肝脏胆固醇的生物合成并没有减少。综上所述，HMGCR对于正常的肝功能和生存是必不可少的，而SS是必不可少的。在两种模型中，正常的胆固醇生物合成都表明存在代谢旁路，在没有这些胆固醇生物合成关键酶的情况下，该途径会产生胆固醇。

Cholesterol biosynthetic pathway is divided into two pathways: sterol pathway and non-sterol pathway. Then non-sterol pathway supplies various substances such as isoprenoids, dolichol, Haem A and ubiquinones. HMG-CoA reductase (HMGCR) produces mevalonate which is a precursor of both sterol and non-sterol pathways, whereas squalene synthase (SS) produces squalene which contributes only to sterol pathway. In the present study, we used Cre-LoxP system to disrupt genes of HMGCR or SS in a liver-specific manner to dissect out the functions of the respective enzymes in the liver. Liver-specific HMGCR knockout mice (L-HMGCRKO) were lethal at 1 month after the birth due to fulminant hepatic failure. Plasma cholesterol levels were reduced only by 10% and hepatic cholesterol biosynthesis was not reduced. mRNA expression of SREBP2 was induced, while expression of Cyp7A was reduced. The hepatocytes were filled with triglycerides. Liver-specific SS knockout mice (L-SSKO) showed transient hepatomegaly with liver dysfunction. Although hepatic VLDL production rate was reduced, hepatic cholesterol biosynthesis was not reduced at least in in vivo setting. In conclusion, HMGCR is essential for normal liver function and survival, while SS is dispensable. Normal cholesterol biosynthesis in both models suggests the presence of metabolic bypass pathway which produces cholesterol in the absence of these key enzymes in cholesterol biosynthesis.