Podrez E A, Hoppe G, O'Neil J, Sayre L M, Sheibani N, Hoff H F
Department of Cell Biology, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, OH 44195, USA.
J Lipid Res. 2000 Sep;41(9):1455-63.
Oxidation of low density lipoproteins (LDL) induced by incubation with Cu(2+) ions results in the formation of a heterogeneous group of aldehydic adducts on lysyl residues (Lys) of apolipoprotein B (apoB) that are thought to be responsible for the uptake of oxidized LDL (oxLDL) by macrophages. To define the structural and chemical criteria governing such cell recognition, we induced two modifications of lysines in LDL that mimic prototypic adducts present in oxLDL; namely, epsilon-amino charge-neutralizing pyrrolation by treatment with 2,5-hexanedione (hdLDL), and epsilon-amino charge-retaining pyridinium formation via treatment with 2,4,6-trimethylpyrylium (tmpLDL). Both modifications led to recognition by receptors on mouse peritoneal macrophages (MPM). To assess whether the murine scavenger receptor class A-I (mSR-A) was responsible for recognition of hdLDL or tmpLDL in MPM, we measured binding at 4 degrees C and degradation at 37 degrees C of these modified forms of (125)I-labeled LDL by mSR-A-transfected CHO cells. Although uptake and degradation of hdLDL by mSR-A-transfected CHO cells was quantitatively similar to that of the positive control, acLDL, tmpLDL was not recognized by these cells. However, both tmpLDL and hdLDL were recognized by 293 cells that had been transfected with CD36. In the human monocytic cell line THP-1 that had been activated with PMA, uptake of tmpLDL was significantly inhibited by blocking monoclonal antibodies to CD36, further suggesting recognition of tmpLDL by this receptor. Macrophage uptake and degradation of LDL oxidized by brief exposure to Cu(2+) was inhibited more effectively by excess tmpLDL and hdLDL than was more extensively oxidized LDL, consistent with the recognition of the former by CD36 and the latter primarily by SR-A.Collectively, these studies suggest that formation of specific pyrrole adducts on LDL leads to recognition by both the mSR-A and mouse homolog of CD36 expressed on MPM, while formation of specific pyridinium adducts on LDL leads to recognition by the mouse homolog of CD 36 but not by mSR-A. As such, these two modifications of LDL may represent useful models for dissecting the relative contributions of specific modifications on LDL produced during oxidation, to the cellular uptake of this heterogeneous ligand.
低密度脂蛋白（LDL）与铜离子（Cu(2+)）孵育诱导的氧化作用会导致载脂蛋白B（apoB）赖氨酸残基（Lys）上形成一组异质的醛加合物，这些加合物被认为是巨噬细胞摄取氧化型LDL（oxLDL）的原因。为了确定控制这种细胞识别的结构和化学标准，我们对LDL中的赖氨酸进行了两种修饰，模拟oxLDL中存在的原型加合物；即，通过用2,5 -己二酮处理使ε-氨基电荷中和的吡咯化（hdLDL），以及通过用2,4,6 -三甲基吡啶鎓处理形成ε-氨基电荷保留的吡啶鎓（tmpLDL）。这两种修饰都导致了小鼠腹腔巨噬细胞（MPM）上的受体识别。为了评估小鼠A-I类清道夫受体（mSR-A）是否负责MPM中hdLDL或tmpLDL的识别，我们测量了mSR-A转染的CHO细胞在4℃下对这些（125）I标记的LDL修饰形式的结合以及在37℃下的降解情况。虽然mSR-A转染的CHO细胞对hdLDL的摄取和降解在数量上与阳性对照乙酰化LDL（acLDL）相似，但这些细胞不能识别tmpLDL。然而，tmpLDL和hdLDL都能被转染了CD36的293细胞识别。在用佛波酯（PMA）激活的人单核细胞系THP-1中，tmpLDL的摄取被抗CD36阻断单克隆抗体显著抑制，这进一步表明该受体对tmpLDL的识别。与更广泛氧化的LDL相比，过量的tmpLDL和hdLDL能更有效地抑制短暂暴露于Cu(2+)氧化的LDL被巨噬细胞摄取和降解，这与前者被CD36识别而后者主要被SR-A识别一致。总的来说，这些研究表明LDL上特定吡咯加合物的形成导致MPM上表达的mSR-A和CD36的小鼠同源物的识别，而LDL上特定吡啶鎓加合物的形成导致CD 36的小鼠同源物的识别，但不被mSR-A识别。因此，LDL的这两种修饰可能代表了有用的模型，用于剖析氧化过程中LDL上特定修饰对这种异质配体细胞摄取的相对贡献。
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