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

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 electron microscopy 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 X 10(6) X g X min) and equilibrated at densities of about 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 WR 1339. In contrast to pIgA, GalBSA was progressively recovered in the L fraction (33 000 - 250 000 X g X min) with organelles equilibrating around 1.11 g/ml, and, by 20-45 min, was found in the ML fraction (10 000 - 250 000 X g X 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. Anti-secretory components 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. In conclusion, our data (a) strongly suggest that pIgA and GalBSA are sorted between 3 min and 10 min after injection in non-lysosomal acidic organelles, (b) identify two successive and physically distinct endosomal populations containing GalBSA, and (c) provide the first evidence for a disulfide reduction activity along the transcytotic pathway of rat hepatocytes.