Background & aims: The asialoglycoprotein receptor localizes to the basolateral membrane of hepatocytes and to the apical membrane of enterocytes. The aim of this study was to examine HT-29 cells as a polarized cell model for studying apically localized endogenous asialoglycoprotein receptor.
Methods: Subunits H1 and H2 (human) were detected by Western blot and immunoprecipitated using subunit-specific antisera against hepatic receptor peptides. Receptor function was assessed by uptake of iodinated asialo-orosomucoid, immunoglobulin (Ig) A1, and haptocorrin. Immunocytochemistry was analyzed by standard light and confocal microscopy.
Results: Receptor content of the minor subunit, H2, was predominant. HT-29 cells mediated specific uptake and degradation of 125I-asialo-orosomucoid. A high-affinity (0.6 x 10(-9) mmol/L) and a low-affinity binding site were present. The specific ligand binding capacity of the apical surface was approximately twice that of the basolateral surface. Immunocytochemistry revealed a predominant apical membrane location of the minor receptor subunit, with some intracellular receptor. The apical H2 subunit was preferentially labeled with amino acid precursors compared with basolaterally located subunit. Human IgA1 bound specifically to HT-29 cells with a molar ratio of 0.26 compared with asialo-orosomucoid; porcine haptocorrin bound with a molar ratio of 1.35.
Conclusions: HT-29 cells produce a functional apically located asialoglycoprotein receptor and provide a model for receptor trafficking in the enterocyte.