Leighton J K, Joyner J, Zamarripa J, Deines M, Davis R A
Cell and Molecular Biology Unit, University of Colorado Health Sciences Center, Denver 80262.
J Lipid Res. 1990 Sep;31(9):1663-8.
Two different molecular weight forms of apoB are produced from a common initial transcript via editing of a Gln codon (CAA) to a stop codon (UAA), leading to a truncated translation product (apo BS) that consists of the amino terminal half of the larger form (apoBL). Previous studies have shown that fasting coordinately decreases lipogenesis and the secretion of very low density lipoprotein (VLDL) lipids and apoBS. Secretion of the apoBL is unaffected by fasting. We studied whether editing of apoB RNA is repressed by fasting, thus accounting for the selective decreased secretion of apoBS. Column chromatography of [35S]methionine-labeled lipoproteins secreted by hepatocytes from fed rats showed that essentially all of apoBL is secreted in the VLDL fraction, whereas a significant amount (15%) of apoBS is secreted associated as lipoproteins eluting in the HDL fractions. Fasting decreased the relative amount of apoBS that eluted in the VLDL fractions and increased the amount secreted in the HDL fractions. Consistent with previous results, hepatocytes from fasted rats show a selective twofold decrease in apoBS secretion. Fasting did not affect the relative abundance of apoB RNA, determined by slot blot hybridization assays using two different 32P-labeled cDNA probes coding either for both molecular weight forms or for only the large molecular weight form. However, quantitative of the editing of apoB RNA showed that fasting caused a 60% decrease in the amount of apoB RNA possessing the stop codon. These data show that the editing of apoB RNA is sensitive to metabolic state (i.e., fasting) resulting in a selective decrease in the secretion of apoBS. However, since the total secretion of apoB was decreased by fasting, while apoB mRNA levels remained constant, additional (post-transcriptional) mechanisms play a role in regulating apoB secretion.
载脂蛋白B（apoB）的两种不同分子量形式由一个共同的初始转录本产生，这是通过将谷氨酰胺密码子（CAA）编辑为终止密码子（UAA）实现的，从而产生一种截短的翻译产物（apoBS），它由较大形式（apoBL）的氨基末端一半组成。先前的研究表明，禁食会协同降低脂肪生成以及极低密度脂蛋白（VLDL）脂质和apoBS的分泌。apoBL的分泌不受禁食影响。我们研究了apoB RNA的编辑是否被禁食抑制，从而解释了apoBS分泌的选择性降低。对喂食大鼠肝细胞分泌的[35S]甲硫氨酸标记的脂蛋白进行柱色谱分析表明，基本上所有的apoBL都分泌到VLDL组分中，而相当数量（15%）的apoBS作为脂蛋白分泌，在HDL组分中洗脱。禁食降低了在VLDL组分中洗脱的apoBS的相对量，并增加了在HDL组分中分泌的量。与先前的结果一致，禁食大鼠的肝细胞显示apoBS分泌选择性地减少了两倍。禁食不影响apoB RNA的相对丰度，这是通过使用两种不同的32P标记的cDNA探针进行狭缝印迹杂交分析确定的，这两种探针分别编码两种分子量形式或仅编码大分子质量形式。然而，对apoB RNA编辑的定量分析表明，禁食导致具有终止密码子的apoB RNA量减少了60%。这些数据表明，apoB RNA的编辑对代谢状态（即禁食）敏感，导致apoBS分泌选择性降低。然而，由于禁食导致apoB的总分泌减少，而apoB mRNA水平保持不变，额外的（转录后）机制在调节apoB分泌中起作用。
