Evidence for a heterodimeric structure of megalin in liver macrophages

Megalin (LRP2) is an endocytotic transmembrane receptor of about 600 kDa and belongs to the low density lipoprotein receptor family. It is located at the apical surface of several epithelia, where it binds and internalizes many physiologically relevant molecules. Recently, we identified megalin-expressing macrophages in fibrotic liver of mice [1]. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) of liver homogenates under reducing conditions and immunoblotting using an antibody directed against the cytoplasmic tail of megalin revealed only a single 35 kDa-band corresponding to the megalin C-terminal fragment (MCTF), whereas in homogenates of kidney, both the 600 kDa-megalin band as well as the MCTF, which results from ectodomain shedding, were visualized. In contrast, gene expression studies using RNA from isolated liver macrophages revealed transcription of the complete megalin gene. We could now substantiate translation of the entire megalin transcript in liver. SDS-PAGE of liver homogenates under non-reducing conditions and immunoblotting using the antibody directed against the cytoplasmic tail of megalin demonstrated a protein band of 600 kDa in addition to the protein band corresponding to the MCTF. These findings suggest that megalin in liver macrophages, contrary to expression as a single molecule in kidney, consists of two polypeptides, the ectodomain and the MCTF, which are joined by disulfide bridges. We speculate that the heterodimeric structure of megalin in liver macrophages is associated with its likely function in the regulation of inflammation.