Attie A D, Pittman R C, Steinberg D
Proc Natl Acad Sci U S A. 1980 Oct;77(10):5923-7. doi: 10.1073/pnas.77.10.5923.
Human low density lipoprotein (LDL) covalently conjugated with 200-250 residues of lactose per LDL particle (Lac-LDL) was bound and rapidly taken up by the galactose-specific receptor of rat hepatocytes. Uptake of Lac-LDL was associated with inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase and stimulation of cholesterol esterification. Uptake of native human LDL had no significant effects on these enzyme activities even when the rates of LDL uptake equaled those of Lac-LDL. When injected into rats, Lac-LDL was selectively removed by the liver (98% of injected dose). The hepatic subcellular distribution of simultaneously injected native 125I-labeled LDL and 131I-labeled Lac-LDL differed significantly, Lac-LDL was associated with fractions enriched in lysosomal hydrolases whereas native LDL was found predominantly in the supernatant fraction enriched in lactate dehydrogenase. Chloroquine (0.1 mM) markedly suppressed uptake of Lac-LDL by cultured rat hepatocytes (> 80%) but had only a small effect on uptake of native LDL. Leupeptin (0.625 mM) inhibited degradation of Lac-LDL more than it did degradation of native LDL. Colchicine (0.25 microM) dramatically suppressed uptake of Lac-LDL (> 70%) but did not affect native LDL uptake even at concentrations as high as 10 microM. Uptake of human LDL by rat hepatocytes occurs largely by nonspecific mechanisms, including fluid endocytosis, whereas Lac-LDL, as shown here, is taken up by a specific receptor-mediated mechanism. The results show further that native human LDL, representing an example of a protein taken up nonspecifically, is processed intracellularly by a pathway qualitatively distinct from that for Lac-LDL, an example of a protein taken up by a specific mechanism. Lac-LDL may serve as a vehicle for specifically delivering drugs, hormones, or radioactive compounds to hepatocytes for therapeutic or diagnostic purposes.
每一个低密度脂蛋白（LDL）颗粒共价结合200 - 250个乳糖残基的人源低密度脂蛋白（Lac - LDL），可被大鼠肝细胞的半乳糖特异性受体结合并迅速摄取。Lac - LDL的摄取与3 - 羟基 - 3 - 甲基戊二酰辅酶A还原酶的抑制及胆固醇酯化的刺激有关。天然人源LDL的摄取对这些酶活性无显著影响，即使LDL摄取速率与Lac - LDL相等时也是如此。当注入大鼠体内时，Lac - LDL被肝脏选择性清除（注射剂量的98%）。同时注入的天然125I标记的LDL和131I标记的Lac - LDL在肝脏亚细胞分布上有显著差异，Lac - LDL与富含溶酶体水解酶的组分相关，而天然LDL主要存在于富含乳酸脱氢酶的上清液组分中。氯喹（0.1 mM）显著抑制培养的大鼠肝细胞对Lac - LDL的摄取（> 80%），但对天然LDL的摄取影响较小。亮抑蛋白酶肽（0.625 mM）对Lac - LDL降解的抑制作用比对天然LDL降解的抑制作用更强。秋水仙碱（0.25 microM）显著抑制Lac - LDL的摄取（> 70%），但即使在高达10 microM的浓度下也不影响天然LDL的摄取。大鼠肝细胞对人源LDL的摄取主要通过非特异性机制，包括液相内吞作用，而如本文所示，Lac - LDL是通过特异性受体介导的机制被摄取。结果进一步表明，天然人源LDL作为非特异性摄取蛋白质的一个例子，其在细胞内的加工途径与Lac - LDL（通过特异性机制摄取的蛋白质的一个例子）在性质上截然不同。Lac - LDL可作为一种载体，用于将药物、激素或放射性化合物特异性地递送至肝细胞，以达到治疗或诊断目的。
