Transcytotic vesicular carriers for polymeric iga receptors accumulate in rat hepatocytes after bile duct ligation

Abstract

In rat hepatocytes, transcytotic vesicular carriers transport the mature 120 x 10(3) Mr form of the polymeric IgA receptor (pIgA-R), with or without its ligand, pIgA, from the sinusoidal to the biliary plasmalemma, where the ectodomain of the receptor is cleaved to produce an 80 x 10(3) Mr fragment that is secreted into the bile. Here we show that cholestasis induced by bile duct ligation results in the accumulation of transcytotic carriers, identified by the 120 x 10(3) Mr pIgA-R and pIgA, in the pericanalicular cytoplasm of hepatocytes. To determine the extent of pIgA-R accumulation, hepatic total microsomes (TM) were prepared from control and cholestatic rats. Solubilized TM proteins were separated by SDS-PAGE and receptor forms were detected by immunoblotting and autoradiography. Quantitative densitometry of these autoradiograms showed that after duct ligation the 120 x 10(3) Mr receptor accumulated to a level approximately threefold higher than the control. Concomitantly, immunologically related, novel 124, 90 and 80 x 10(3) Mr proteins (cholestatic antigens) became detectable. Immunoblot analyses of biliary and serum proteins showed that cholestasis resulted in: (1) a marked decrease in the concentrations of the 80 x 10(3) Mr receptor and pIgA in the bile, whereas albumin concentrations remained at control levels; and (2) a marked increase in the concentration of the 80 x 10(3) Mr receptor in the serum. Positive sites for pIgA-R were localized to the pericanalicular cytoplasm of hepatocytes by indirect immunofluorescence on semithin frozen sections in cholestatic hepatocytes. The sites were more numerous and the positive signal stronger than in controls. One day post-ligation, pIgA-positive sites were located to the same pericanalicular cytoplasm of hepatocytes; by three days, however, most pIgA appeared in sinusoidal endothelia and Kupffer cells. To validate the vesicular character of the receptor-positive sites, sham-operated and cholestatic livers were processed for either transmission electron microscopy (TEM) or immunogold localization of receptors on thin frozen sections. TEM verified the accumulation of pericanalicular vesicles in cholestatic hepatocytes. Immunogold tests localized pIgA-R to pleiomorphic, pericanalicular vesicles, which were increased in number, size and concentration of antigenic sites in cholestatic hepatocytes. These findings indicate that bile duct ligation provides a method for manipulating the in vivo transcytotic pathway and for accumulating previously unstudied transcytotic carriers in hepatocytes.