The macrophage/endothelial cell mannose receptor cDNA encodes a protein that binds oligosaccharides terminating with SO₄-4-GalNAcβ1,4GlcNAcβ or Man at independent sites

Abstract

Lutropin (LH) and other glycoproteins bearing oligosaccharides with the terminal sequence SO 4 -4-GalNAc␤1,4GlcNAc␤1,4Man-(S4GGnM) are rapidly removed from the circulation by an S4GGnM-specific receptor (S4GGnM-R) expressed at the surface of hepatic endothelial cells. The S4GGnM-R isolated from rat liver is closely related to the macrophage mannose-specific receptor (Man-R) isolated from rat lung both antigenically and structurally. The S4GGnM-R and Man-R isolated from these tissues nonetheless differ in their ability to bind ligands bearing terminal GalNAc-4-SO 4 or Man. In this paper, we have explored the structural relationship between the Man-R and the S4GGnM-R by examining the properties of the recombinant Man-R in the form of a transmembrane protein and a soluble chimeric fusion protein in which the transmembrane and cytosolic domains have been replaced by the Fc region of human IgG1. Like the S4GGnM-R isolated from liver, the chimeric fusion protein is able to bind ligands terminating with GalNAc-4-SO 4 and Man at independent sites. When expressed in CHO cells the recombinant Man-R is able to mediate the uptake of ligands bearing either terminal GalNAc-4-SO 4 or terminal Man. We propose that the Man-R be renamed the Man͞S4GGnM receptor on the basis of its multiple and independent specificities.Glycoproteins bearing Asn-linked oligosaccharides terminating with the sequence SO 4 -4-GalNAc␤1,4GlcNAc␤1,2Man (S4GGnM) are bound by a receptor found at the surface of hepatic endothelial cells that mediates their internalization and transport to lysosomes, where they are degraded (1). The S4GGnM-specific receptor (S4GGnM-R) accounts for the rapid removal of the glycoprotein hormone lutropin (LH) from the circulation and thereby the episodic rise and fall in circulating LH after release from gonadotrophs into the blood (2). The episodic rise and fall in circulating LH levels has been proposed to be essential for attaining maximal biologic activity in vivo (3).We recently described the isolation of a receptor from rat liver that can account for the binding and internalization of LH by hepatic endothelial cells (4). The receptor is closely related to the macrophage mannose receptor (Man-R) (5, 6) on the basis of antigenicity and peptide mapping studies. Yet the S4GGnM-R isolated from liver and the Man-R isolated from lung display markedly different binding properties, suggesting that the two receptors are not identical (4). We have examined this issue by expressing a cDNA, isolated from mouse lung, which encodes the Man-R (6) in Chinese hamster ovary (CHO) cells. We have also examined the properties of a secreted, chimeric fusion protein derived from this cDNA in which the transmembrane and cytosolic domains of the receptor have been replaced by the Fc region of human IgG1 (7). Our results indicate that the Man-R cDNA directs synthesis of a receptor that displays properties characteristic of both the S4GGnM-R and the Man-R when expressed in CHO cells. The properties of the secreted chimeric fusion protein ind...

The Mannose/N-Acetylgalactosamine-4-SO4Receptor Displays Greater Specificity for Multivalent than Monovalent Ligands

“…Rapid clearance in conjunction with stimulated release from dense core storage granules in LH-producing cells located in the anterior lobe of the pituitary produce the episodic rise and fall in serum LH levels that is important for the expression of LH bioactivity in vivo. A novel feature of the Man/GalNAc-4-SO 4 receptor is its capacity to bind carbohydrate moieties terminating with Man and GalNAc-4-SO 4 at physically distinct domains that are not structurally related. The binding site for GalNAc-4-SO 4 is located in the cysteine-rich domain found at the N terminus of the Man/GalNAc-4-SO 4 receptor (5).…”

“…with ␤1,4-linked GalNAc-4-SO 4 are recognized by a receptor located in hepatic endothelial cells, the Man/GalNAc-4-SO 4 receptor, and are rapidly removed from the circulation (1)(2)(3)(4). Rapid clearance in conjunction with stimulated release from dense core storage granules in LH-producing cells located in the anterior lobe of the pituitary produce the episodic rise and fall in serum LH levels that is important for the expression of LH bioactivity in vivo.…”

Multiple interactions between pituitary hormones and the mannose receptor

“…Binding of the neoglycoprotein SO % -4GalNAcβ1, 4GlcNAcβ1,2Manα-BSA to isolated liver cells is not decreased significantly in the presence of EDTA, but binding of this ligand to isolated mannose receptor is decreased by EDTA to approx. 60 % of the level in the presence of Ca# + [7,8]. These results are interpreted as indicating that the binding of SO % -4GalNAc is not dependent on Ca# + but is enhanced in the presence of Ca# + [7].…”

“…60 % of the level in the presence of Ca# + [7,8]. These results are interpreted as indicating that the binding of SO % -4GalNAc is not dependent on Ca# + but is enhanced in the presence of Ca# + [7].…”

“…Glycoconjugates terminating in mannose, fucose or GlcNAc are recognized in a Ca# + -dependent manner by several of the eight Ctype carbohydrate-recognition domains (CRDs) in the extracellular region of the receptor [3][4][5]. Recent studies have suggested a further function for the mannose receptor in the clearance of pituitary hormones such as lutropin (LH) and thyrotropin (TSH) [6,7]. These hormones must be removed from the blood after acting on their target cells, to prevent desensitization of the target cell receptors.…”