Differential expression of asialoglycoprotein receptor subunits in the endocytic compartment during liver regeneration

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Abstract
Asialoglycoprotein receptors, responsible for the removal of circulating asialogly-coproteins by the liver, are located in at least two different membrane locations in hepatocytes. Receptors on the cell surface account only for a minor proportion (20–36%), for the majority of receptors in the liver are located intracellularly, mainly in the endocytic membrane networks. An understanding of the basis of receptor distribution and the underlying trafficking of receptors between the hepatocyte's polarised cell surface and the endocytic compartment would be aided if biochemical differences between the receptors in these pools were established. We now show, using three antibodies that recognise the receptor subunits in rat liver (RHL-1, RHL-2 and RHL-3), that the asialoglycoprotein receptors located in the plasma membrane domains and the endocytic compartment differ in oligomeric composition, sialic acid content, and solubility in Triton X-114 using two-phase systems. It is well established that the expression of the asialoglycoprotein receptor is down-regulated in livers regenerating after a partial hepatectomy. We demonstrate that the levels of the receptor subtype that is located mainly in the endocytic compartment (RHL-1, 42 kDa) was elevated in regenerating liver by agents that regulate cAMP production, whereas the levels of the other receptor subtypes remained unchanged. The asialoglycoprotein receptor subtypes that are present in different subcellular locations are thus regulated independently.