Truncated apo B-70.5–containing lipoproteins bind to megalin but not the LDL receptor
Zhouji Chen, Jeffrey E. Saffitz, Mickey A. Latour, Gustav Schonfeld
Zhouji Chen, Jeffrey E. Saffitz, Mickey A. Latour, Gustav Schonfeld
View: Text | PDF
Article

Truncated apo B-70.5–containing lipoproteins bind to megalin but not the LDL receptor

Abstract

Apo B-100 of LDL can bind to both the LDL receptor and megalin, but the molecular interactions of apo B-100 with these 2 receptors are not completely understood. Naturally occurring mutant forms of apo B may be a source of valuable information on these interactions. Apo B-70.5 is uniquely useful because it contains the NH2-terminal portion of apo B-100, that includes only one of the two putative LDL receptor–binding sites (site A). The lipoprotein containing apo B-70.5 (Lp B-70.5) was purified from apo B-100/apo B-70.5 heterozygotes by sequential ultracentrifugation combined with immunoaffinity chromatography. Cell culture experiments, ligand blot analysis, and in vivo studies all consistently showed that Lp B-70.5 is not recognized by the LDL receptor. The kidney was identified as a major organ in catabolism of Lp B-70.5 in New Zealand white rabbits. Autoradiographic analysis revealed that renal proximal tubular cells selectively removed Lp B-70.5. On ligand blotting of renal cortical membranes, Lp B-70.5 bound only to megalin. The ability of megalin to mediate cellular endocytosis of Lp B-70.5 was confirmed using retinoic acid/dibutyryl cAMP–treated F9 cells. This study suggests that the putative LDL receptor–binding site A on apo B-100 might not by itself be a functional binding domain and that the apo B–binding sites recognized by the LDL receptor and by megalin may be different. Moreover, megalin may play an important role in renal catabolism of apo B truncations, including apo B-70.5.

Authors

Zhouji Chen, Jeffrey E. Saffitz, Mickey A. Latour, Gustav Schonfeld

×

Figure 6

Options: View larger image (or click on image) Download as PowerPoint
Ligand blots demonstrating the ability of Lp B-70.5 to bind to megalin f...
Ligand blots demonstrating the ability of Lp B-70.5 to bind to megalin from rabbit kidney cortical membrane extracts. Approximately 200 μg of proteins of solubilized membranes from rabbit kidney cortex (K) or adrenal gland (A) of the cow was subjected to SDS-PAGE (3–12% gradient gel) under nonreducing conditions and transferred onto Immobilon-P membranes. Ligand blot was performed as described in Methods. Membrane strips 1, 2, and 3 were incubated with the indicated labeled ligands (1 μg/mL) alone, whereas strips 4–8 were incubated with 125I-Lp B-70.5 (1 μg/mL) in the absence or presence of the indicated unlabeled inhibitors (i.e., 10 mM EDTA, 100 μg/mL LDL, 30 μg/mL GST-RAP, or 15 μg/mL GST). The exposure time of the films was 3 hours for strip 2 and 10 hours for all other strips.