Lougheed M, Zhang H F, Steinbrecher U P
Department of Medicine, University of British Columbia, Vancouver, Canada.
J Biol Chem. 1991 Aug 5;266(22):14519-25.
The rate of uptake of oxidized low density lipoprotein (LDL) by mouse peritoneal macrophages is similar to that of acetyl LDL; but only approximately 50% of the internalized oxidized LDL is ultimately degraded, in contrast to the near-complete degradation seen with acetyl LDL. The objectives of this study were to determine if this was due to increased surface binding of oxidized LDL, different uptake pathways for oxidized LDL and acetyl LDL, lysosomal dysfunction caused by oxidized LDL, or resistance of oxidized LDL to hydrolysis by lysosomal proteinases. LDL binding studies at 4 degrees C showed that the increased cell association with oxidized LDL could not be explained by differences in cell-surface binding. Immunofluorescence microscopy confirmed intracellular accumulation of apoB-immunoreactive material in macrophages incubated with oxidized LDL, but not with acetyl LDL. The scavenger receptor ligand polyinosinic acid inhibited both the cell association and degradation of oxidized LDL in macrophages by greater than 75%, suggesting a common uptake pathway for degraded LDL and nondegraded LDL. Studies in THP-1 cells also did not reveal more than one specific uptake pathway for oxidized LDL. LDL derivatized by incubation with oxidized arachidonic acid (under conditions that prevented oxidation of the LDL itself) showed inefficient degradation, similar to oxidized LDL. When macrophages were incubated with oxidized LDL together with acetyl 125I-LDL, the acetyl LDL was degraded normally, excluding lysosomal dysfunction as the explanation for the accumulation of oxidized LDL. Generation of trichloroacetic acid-soluble products from oxidized 125I-LDL by exposure to cathepsins B and D was less than that observed with native 125I-LDL. LDL modified by exposure to reactive products derived from oxidized arachidonic acid was also degraded more slowly than native 125I-LDL by cathepsins. In contrast, acetyl 125I-LDL was degraded more rapidly by cathepsins than native 125I-LDL, and aggregated LDL and malondialdehyde-modified LDL were degraded at the same rate as native 125I-LDL. It is concluded that the intracellular accumulation of oxidized LDL in macrophages can be explained at least in part by the resistance of oxidatively modified apolipoprotein B to cathepsins. This resistance to cathepsins does not appear to be due to aggregation of oxidized LDL, but may be a consequence of modification of apolipoprotein B by lipid peroxidation products.
小鼠腹腔巨噬细胞对氧化型低密度脂蛋白（LDL）的摄取速率与乙酰化LDL相似；但内化的氧化型LDL最终只有约50%被降解，这与乙酰化LDL几乎完全降解形成对比。本研究的目的是确定这是否是由于氧化型LDL的表面结合增加、氧化型LDL和乙酰化LDL的摄取途径不同、氧化型LDL导致的溶酶体功能障碍，或氧化型LDL对溶酶体蛋白酶水解的抗性。4℃下的LDL结合研究表明，细胞与氧化型LDL结合增加不能用细胞表面结合的差异来解释。免疫荧光显微镜证实，在用氧化型LDL而非乙酰化LDL孵育的巨噬细胞中，载脂蛋白B免疫反应性物质在细胞内积累。清道夫受体配体聚肌苷酸抑制巨噬细胞中氧化型LDL的细胞结合和降解超过75%，表明降解的LDL和未降解的LDL有共同的摄取途径。在THP-1细胞中的研究也未揭示氧化型LDL有不止一种特定的摄取途径。与氧化型花生四烯酸孵育衍生的LDL（在防止LDL自身氧化的条件下）显示出低效降解，类似于氧化型LDL。当巨噬细胞与氧化型LDL和乙酰化125I-LDL一起孵育时，乙酰化LDL正常降解，排除了溶酶体功能障碍是氧化型LDL积累的原因。通过暴露于组织蛋白酶B和D，氧化型125I-LDL产生的三氯乙酸可溶性产物少于天然125I-LDL。暴露于氧化型花生四烯酸衍生的反应产物而修饰的LDL也比天然125I-LDL被组织蛋白酶降解得更慢。相反，乙酰化125I-LDL被组织蛋白酶降解比天然125I-LDL更快，聚集的LDL和丙二醛修饰的LDL与天然125I-LDL以相同速率降解。结论是，巨噬细胞中氧化型LDL的细胞内积累至少部分可以用氧化修饰的载脂蛋白B对组织蛋白酶的抗性来解释。这种对组织蛋白酶的抗性似乎不是由于氧化型LDL的聚集，而是载脂蛋白B被脂质过氧化产物修饰的结果。
