Ligand-induced modulation of the hepatic receptor for asialoglycoproteins in the human hepatoblastoma cell line, Hep G2.

Abstract

Ligand-induced modulation of the hepatic receptor for asialoglycoproteins has been examined in the human hepatoblastoma cell line, Hep G2. When grown to confluence, approximately 80% of the functional receptors are expressed at the cell surface. In contrast, exposure of these cells to saturating levels of galactose- or N-acetylgalactosamine-terminated ligands, for extended periods of time, resulted in a drastic reduction in the number of cell surface receptors, as determined by the binding of 125I-asialo-orosomucoid at 4 degrees C. Recovery of binding capacity was slow and incomplete despite retention of cellular viability and normal growth characteristics. Under these conditions, the decreased number of surface receptors could not be accounted for by changes in binding affinity or by internalization since the intracellular receptor number remained essentially constant. No corresponding decrease was noted in the ability of insulin or transferrin to bind to their respective cell surface receptors in the modulated cells. Similarly, Northern blot analysis revealed no changes in the steady state levels of the asialoglycoprotein receptor transcripts. However, antibody prepared against the purified human receptor bound equally well to both control and modulated cells, thereby indicating the presence of a proportionate number of inactive surface receptors on the latter cells. Two-dimensional gel electrophoresis failed to detect any abnormality in the molecular weight or isoelectric point of the modulated receptor. These findings are interpreted as indicating that the Hep G2 cells are able to regulate the functional expression of this surface receptor without altering its immunologic integrity. A defect in cell surface sialylation appears to be involved in the regulatory response.