Srivastava R A
Department of Internal Medicine, Washington University School of Medicine, Saint Louis, MO 63110, USA.
Mol Cell Biochem. 1996 Feb 23;155(2):153-62. doi: 10.1007/BF00229312.
The aim of the present investigation was to study the regulation of apolipoprotein E by two dietary nutrients, saturated fat and cholesterol, known to raise plasma cholesterol levels. ApoE is a protein component of several classes of lipoproteins including VLDL and HDL, and dietary lipids may regulate VLDL and apoE-containing HDL particles through their effects on apoE gene. Male rats and mice were fed the following 4 diets: control diet (C); high cholesterol diet with 0.5% cholesterol (HC); high fat diet with 20% hydrogenated coconut oil (HF); and high fat plus high cholesterol diet with 0.5% cholesterol and 20% fat (HF/ C). Plasma cholesterol levels remained unchanged on HC diet, but in mice VLDL-cholesterol increased by 31%. HF diet increased VLDL and LDL by 15-17% in rats, and 21% in mice. A combination of fat and cholesterol diet showed pronounced effects on plasma lipoprotein concentrations, raising apoB-containing particles by 21% and 44% in mice and rats, respectively. Plasma apoE levels increased significantly on all diets. The mechanism of regulation of increased plasma apoB and apoE levels was examined. Quantification of hepatic apoB mRNA showed a lack of correlation between plasma apoB and hepatic apoB mRNA levels, suggesting that posttranscriptional regulation increased plasma apoB-containing lipoproteins in animals fed saturated fat diets. Hepatic apoE mRNA levels increased significantly in animals fed cholesterol-rich diets. However, despite increased plasma apoE levels on diet containing only saturated fat, hepatic apoE mRNA did not change. Synthesis of apoE on the liver polysomes increased selectively on cholesterol-rich diets. These results suggest that cholesterol-rich diets altered apoE, in part, by transcriptional mechanism, and saturated fat-rich diets increased plasma apoE levels by posttranscriptional mechanism, possibly decreased receptor-mediated uptake of apoE-containing particles. The regulation of LDL receptor was also studied since plasma apoB and E levels may be altered by LDL receptor-mediated uptake by the hepatocytes. As expected, high cholesterol diet decreased LDL receptor mRNA by 30-40%. However, the LDL receptor protein on liver membranes did not change on any of the test diets in both animal species. Hepatic cholesterol content increased several fold selectively on high cholesterol diets. These findings suggest that: 1) both transcriptional and posttranscriptional mechanisms are important in regulating plasma apoB and E containing lipoproteins; 2) dietary cholesterol regulates apoE gene by a transcriptional mechanism and dietary saturated fat by posttranscriptional mechanism; and 3) changes in the hepatic apoE and LDL receptor mRNA are associated with the changes in intracellular cholesterol concentrations.
本研究的目的是探讨两种已知会升高血浆胆固醇水平的膳食营养素——饱和脂肪和胆固醇对载脂蛋白E的调节作用。载脂蛋白E是包括极低密度脂蛋白（VLDL）和高密度脂蛋白（HDL）在内的几类脂蛋白的蛋白质成分，膳食脂质可能通过对载脂蛋白E基因的作用来调节VLDL和含载脂蛋白E的HDL颗粒。给雄性大鼠和小鼠喂食以下4种饮食：对照饮食（C）；含0.5%胆固醇的高胆固醇饮食（HC）；含20%氢化椰子油的高脂肪饮食（HF）；以及含0.5%胆固醇和20%脂肪的高脂肪加高胆固醇饮食（HF/C）。HC饮食组的血浆胆固醇水平保持不变，但小鼠的VLDL胆固醇增加了31%。HF饮食使大鼠的VLDL和低密度脂蛋白（LDL）增加了15 - 17%，小鼠增加了21%。脂肪和胆固醇组合饮食对血浆脂蛋白浓度有显著影响，使小鼠和大鼠中含载脂蛋白B的颗粒分别增加了21%和44%。所有饮食组的血浆载脂蛋白E水平均显著升高。研究了血浆载脂蛋白B和载脂蛋白E水平升高的调节机制。肝脏载脂蛋白B mRNA的定量分析表明，血浆载脂蛋白B与肝脏载脂蛋白B mRNA水平之间缺乏相关性，这表明转录后调节增加了喂食饱和脂肪饮食动物的血浆含载脂蛋白B的脂蛋白。喂食富含胆固醇饮食的动物肝脏载脂蛋白E mRNA水平显著升高。然而，尽管仅含饱和脂肪的饮食使血浆载脂蛋白E水平升高，但肝脏载脂蛋白E mRNA并未改变。在富含胆固醇的饮食中，肝脏多核糖体上载脂蛋白E的合成选择性增加。这些结果表明，富含胆固醇的饮食部分通过转录机制改变载脂蛋白E，而富含饱和脂肪的饮食通过转录后机制增加血浆载脂蛋白E水平，可能是减少了含载脂蛋白E颗粒的受体介导摄取。还研究了低密度脂蛋白受体的调节，因为血浆载脂蛋白B和E水平可能会因肝细胞的低密度脂蛋白受体介导摄取而改变。正如预期的那样，高胆固醇饮食使低密度脂蛋白受体mRNA降低了30 - 40%。然而，在两种动物中，任何一种测试饮食对肝细胞膜上的低密度脂蛋白受体蛋白均无影响。高胆固醇饮食使肝脏胆固醇含量选择性增加了几倍。这些发现表明：1）转录和转录后机制在调节血浆含载脂蛋白B和E的脂蛋白中都很重要；2）膳食胆固醇通过转录机制调节载脂蛋白E基因，膳食饱和脂肪通过转录后机制调节；3）肝脏载脂蛋白E和低密度脂蛋白受体mRNA的变化与细胞内胆固醇浓度的变化相关。
