Hammad S M, Barth J L, Knaak C, Argraves W S
Department of Cell Biology and Anatomy, Medical University of South Carolina, Charleston, South Carolina 29425-2204, USA.
J Biol Chem. 2000 Apr 21;275(16):12003-8. doi: 10.1074/jbc.275.16.12003.
Cubilin has recently been shown to function as an endocytic receptor for high density lipoproteins (HDL). The lack of apparent transmembrane and cytoplasmic domains in cubilin raises questions as to the means by which it can mediate endocytosis. Since cubilin has been reported to bind the endocytic receptor megalin, we explored the possibility that megalin acts in conjunction with cubilin to mediate HDL endocytosis. While megalin did not bind to HDL, delipidated HDL, or apoA-I, it was found to copurify with cubilin isolated by HDL-Sepharose affinity chromatography. Cubilin and megalin exhibited coincident patterns of mRNA expression in mouse tissues including the kidney, ileum, thymus, placenta, and yolk sac endoderm. The expression of both receptors in yolk sac endoderm-like cells was inducible by retinoic acid treatment but not by conditions of sterol depletion. Suppression of megalin activity or expression by treatment with either megalin antibodies or megalin antisense oligodeoxynucleotides resulted in inhibition of cubilin-mediated endocytosis of HDL. Furthermore, megalin antisense oligodeoxynucleotide treatment resulted in reduced cell surface expression of cubilin. These data demonstrate that megalin acts together with cubilin to mediate HDL endocytosis and further suggest that megalin may play a role in the intracellular trafficking of cubilin.
最近研究表明， cubilin可作为高密度脂蛋白（HDL）的内吞受体。 cubilin缺乏明显的跨膜和胞质结构域，这引发了关于其介导内吞作用方式的疑问。由于据报道cubilin可结合内吞受体巨蛋白（megalin），我们探讨了megalin与cubilin协同作用介导HDL内吞的可能性。虽然megalin不与HDL、脱脂HDL或载脂蛋白A-I结合，但发现它与通过HDL-琼脂糖亲和层析分离的cubilin共纯化。在包括肾脏、回肠、胸腺、胎盘和卵黄囊内胚层在内的小鼠组织中，cubilin和megalin呈现出一致的mRNA表达模式。在卵黄囊内胚层样细胞中，视黄酸处理可诱导这两种受体的表达，但固醇耗竭条件下则不能。用megalin抗体或megalin反义寡脱氧核苷酸处理抑制megalin活性或表达，会导致cubilin介导的HDL内吞作用受到抑制。此外，megalin反义寡脱氧核苷酸处理导致cubilin的细胞表面表达减少。这些数据表明，megalin与cubilin共同作用介导HDL内吞，并进一步表明megalin可能在cubilin的细胞内运输中发挥作用。
