Receptor-mediated endocytosis of polymeric IgA and galactosylated serum albumin in rat liver. Evidence for intracellular ligand sorting and identification of distinct endosomal compartments

Abstract

Rat polymeric IgA (pIgA) and galactosylated bovine serum albumin (GalBSA), once injected to rats, are avidly taken up by hepatocytes via receptor-mediated endocytosis. Of injected pIgA, 64% was transferred undigested into bile within 3 h, with a peak at 30-45 min. GalBSA was essentially digested in lysosomes. By EM using ligand-peroxidase conjugates, both ligands were internalized through coated pits/coated vesicles into similar electron-lucent vesicles and tubules. Subsequently, pIgA remained mostly associated with small vesicles clustering around and fusing with bile canaliculi, while GalBSA was predominantly found in large, heterogeneous endocytic structures and in lysosomes. By subcellular fractionation, they were associated at 3 min after injection with structures that similarly sedimented in the P fraction (250,000-3 .cntdot. 106 .times. g .times. min) and equilibrated at densities of .apprx. 1.13 g/ml in sucrose gradients. At 10 min and 20 min, pIgA distribution remained mostly in the P fraction at the same equilibrium density. A minor component of the pIgA distribution was found at the density of lysosomes, but contrary to lysosomal enzymes, its distribution was not affected by Triton WP 1339. In contrast to pIgA, GalBSA was progessively recovered in the L fraction (33,000-250,000 .times. g .times. min) with organelles equilibrating around 1.11 g/ml, and, by 20-45 min, was found in the ML fraction (10,000-250,000 .times. g .times. min), around 1.20 g/ml, i.e., in lysosomes. Chloroquine did not reduce the efficiency but delayed the secretion of pIgA into bile. Similarly, it did not affect the uptake of GalBSA but apparently delayed GalBSA transfer along successive populations of host organelles. The low density, GalBSA-containing structures were devoid of proteolytic activity. Antisecretory component IgG and F(ab'')2 were selectively excreted into bile, partially or totally as compounds of lower molecular mass. These antibody fragments probably result from a disulfide reduction activity along the pIgA pathway. PIgA and GalBSA evidently are sorted between 3 min and 10 min after injection in nonlysosomal acidic organelles. Two successive and physically distinct endosomal populations containing GalBSA were identified. The 1st evidence for a disulfide reduction activity along the transcytotic pathway of rat hepatocytes was provided.