Orlando R A, Rader K, Authier F, Yamazaki H, Posner B I, Bergeron J J, Farquhar M G
Department of Pathology, University of California, San Diego, La Jolla 92093-0651, USA.
J Am Soc Nephrol. 1998 Oct;9(10):1759-66. doi: 10.1681/ASN.V9101759.
Renal clearance is a major pathway for regulating the levels of insulin and other low molecular weight polypeptide hormones in the systemic circulation. Previous studies have shown that the reabsorption of insulin from the glomerular filtrate occurs by binding to as yet unidentified sites on the luminal surface of proximal tubule cells followed by endocytosis and degradation in lysosomes. In this study, an insulin binding site was identified in renal microvillar membranes by chemical cross-linking procedures. By immunoprecipitation it was demonstrated that this binding site is megalin, the large multiligand binding endocytic receptor that is abundantly expressed in clathrin-coated pits on the apical surface of proximal tubule cells. Moreover, using cytochemical procedures, it was also shown that megalin is able to internalize insulin into endocytic vesicles. In ligand blotting assays, megalin also bound several other low molecular weight polypeptides, including beta2-microglobulin, epidermal growth factor, prolactin, lysozyme, and cytochrome c. These data suggest that megalin may play a significant role as a renal reabsorption receptor for the uptake of insulin and other low molecular weight polypeptides from the glomerular filtrate.
肾脏清除是调节全身循环中胰岛素和其他低分子量多肽激素水平的主要途径。先前的研究表明，胰岛素从肾小球滤液中的重吸收是通过与近端小管细胞管腔表面尚未确定的位点结合，随后通过内吞作用并在溶酶体中降解来实现的。在本研究中，通过化学交联程序在肾微绒毛膜中鉴定出一个胰岛素结合位点。通过免疫沉淀证明该结合位点是巨膜蛋白，它是一种大型多配体结合内吞受体，在近端小管细胞顶端表面的网格蛋白包被小窝中大量表达。此外，使用细胞化学方法还表明，巨膜蛋白能够将胰岛素内化到内吞小泡中。在配体印迹分析中，巨膜蛋白还结合了其他几种低分子量多肽，包括β2-微球蛋白、表皮生长因子、催乳素、溶菌酶和细胞色素c。这些数据表明，巨膜蛋白可能作为一种肾脏重吸收受体，在从肾小球滤液中摄取胰岛素和其他低分子量多肽方面发挥重要作用。
