Jäckle S, Runquist E, Brady S, Hamilton R L, Havel R J
Cardiovascular Research Institute, University of California, San Francisco 94143-0130.
J Lipid Res. 1991 Mar;32(3):485-98.
In earlier research we isolated and characterized three endosomal fractions from livers of estradiol-treated rats (1987. Belcher et al. Proc. Natl. Acad. Sci. USA. 84: 6785; 1989. Jäckle et al. Proc. Natl. Acad. Sci. USA. 86: 1880). We now describe the isolation of comparable endosomal fractions from untreated rats. The fraction of lowest density was composed almost exclusively of lipoprotein-filled multivesicular bodies (MVBs); the intermediate density fraction was composed of smaller lipoprotein-filled vesicles that were also multivesicular and had more numerous membranous appendages; the fraction of highest density was composed of membranes resembling the appendages of the two vesicular fractions. The paucity of contamination of these fractions with nonendosomal organelles is supported by their ultrastructural characteristics and by the proteins, lipids, and marker enzymes of their membranes. Only small amounts of MVBs could be separated from untreated rats not given a load of lipoproteins. However, after injection of large amounts of beta-very low density lipoproteins (beta-VLDL) from cholesterol-fed rabbits, the mass of MVBs increased dramatically. Under these conditions radiolabeled beta-VLDL and epidermal growth factor taken up by the liver accumulated in isolated endosomes at rates similar to those found for LDL in estradiol-treated rats. Although chylomicrons and chylomicron remnants were rapidly taken up by the liver of normal rats, chylomicrons and chylomicron remnants accumulated in endosomes at a lower rate than beta-VLDL. These findings, which differ from earlier data in estradiol-treated rats (1989 Jäckle et al., Proc. Natl. Acad. Sci. USA. 86: 1880) that showed equivalent rates of processing of chylomicron remnants and beta-VLDL, suggest that extracellular processing of chylomicron remnants in the liver normally ; precedes endocytosis.
在早期研究中，我们从经雌二醇处理的大鼠肝脏中分离并鉴定了三种内体组分（1987年。贝尔彻等人。美国国家科学院院刊。84: 6785；1989年。杰克勒等人。美国国家科学院院刊。86: 1880）。我们现在描述从未经处理的大鼠中分离出类似内体组分的过程。密度最低的组分几乎完全由充满脂蛋白的多囊泡体（MVBs）组成；中等密度组分由较小的、也充满脂蛋白的多囊泡且有更多膜性附属物的囊泡组成；密度最高的组分由类似于两个囊泡组分附属物的膜组成。这些组分中未被非内体细胞器污染的情况，通过它们的超微结构特征以及其膜上的蛋白质、脂质和标记酶得到了证实。从未经处理且未给予脂蛋白负荷的大鼠中只能分离出少量的MVBs。然而，在注射了大量来自喂食胆固醇的兔子的β-极低密度脂蛋白（β-VLDL）后，MVBs的数量急剧增加。在这些条件下，肝脏摄取的放射性标记的β-VLDL和表皮生长因子在分离的内体中积累的速率，与在经雌二醇处理的大鼠中发现的低密度脂蛋白（LDL）的积累速率相似。尽管乳糜微粒和乳糜微粒残粒被正常大鼠的肝脏迅速摄取，但乳糜微粒和乳糜微粒残粒在内体中的积累速率低于β-VLDL。这些发现与早期在经雌二醇处理的大鼠中的数据（1989年杰克勒等人，美国国家科学院院刊。86: 1880）不同，早期数据显示乳糜微粒残粒和β-VLDL的处理速率相当，这表明肝脏中乳糜微粒残粒的细胞外处理通常先于内吞作用。
