ASGR2

ASGR2 Full Name

asialoglycoprotein receptor 2

ASGR2 Introduction

ASGR2, also known as hepatic lectin H2 or HL-2, is one of the two subunits that constitute the asialoglycoprotein receptor (ASGPR), a C-type lectin expressed almost exclusively on the sinusoidal and basolateral surfaces of hepatocytes. ASGPR is the prototypical mammalian lectin receptor responsible for the clearance of desialylated glycoproteins (asialo-glycoproteins) from the circulation. While ASGR1 is the major subunit required for ligand binding, ASGR2 serves as an auxiliary subunit that modulates receptor stability, trafficking, and ligand-binding avidity. The ASGR2 gene is located on chromosome 17p13.1, adjacent to the ASGR1 gene, suggesting duplication from a common ancestral gene. Human ASGR2 encodes a 311-amino-acid polypeptide (approximately 50 kDa) with a single C-type carbohydrate recognition domain (CRD). Alternative splicing produces two isoforms: a major membrane-bound form and a minor soluble form lacking the transmembrane region, though the functional significance of the soluble isoform remains unclear.

Figure 1.The structure of ASGR2 protein.

Receptor Structure and Sugar-Binding Specificity
Both ASGR1 and ASGR2 are type II transmembrane proteins. Their CRDs recognize terminal galactose (Gal) and N-acetylgalactosamine (GalNAc) residues with millimolar affinity when presented as mono- or disaccharides. However, physiological ligands (desialylated glycoproteins such as asialo-orosomucoid, asialo-fetuin, and asialo-α1-acid glycoprotein) display multiple, clustered Gal/GalNAc residues, allowing high-avidity binding through cooperative interactions among receptor subunits. ASGPR is uniquely dependent on calcium ions for carbohydrate recognition; chelation of Ca²⁺ by EDTA completely abolishes binding. The receptor cycles constitutively between the plasma membrane and endosomal compartments via clathrin-coated pits. Upon ligand binding, the ASGR-ligand complex is internalized, and the ligand is delivered to lysosomes for degradation, while the receptor recycles to the cell surface — a process completed within 5–10 minutes, allowing a single receptor to clear thousands of ligands per hour.

Tissue Distribution and Expression Regulation
ASGR2 expression is highly restricted to hepatocytes, with very low or undetectable expression in other cell types. Within the liver, expression is uniform across the hepatic acinus. The promoter region contains binding sites for hepatocyte nuclear factors (HNF1α, HNF3β, and HNF4α), which are essential for liver-specific expression. Inflammatory cytokines, particularly interleukin-1β (IL-1β) and tumor necrosis factor-alpha (TNF-α), downregulate ASGR2 transcription during the acute phase response, leading to reduced clearance of asialo-glycoproteins. This downregulation is mediated in part by NF-κB binding to the ASGR2 promoter. Conversely, growth hormone and thyroid hormone upregulate ASGR2 expression, consistent with the observation that receptor activity declines in hypothyroidism and during aging. Chronic liver diseases, including cirrhosis and hepatocellular carcinoma (HCC), are associated with reduced ASGPR expression, which correlates with impaired clearance of circulating desialylated proteins such as haptoglobin and α1-antitrypsin.

Alternate Names for ASGR2

ASGR2; asialoglycoprotein receptor 2; HL-2; HBXBP; ASGPR2; ASGP-R2; CLEC4H2; hepatic lectin H2; HBxAg-binding protein; C-type lectin domain family 4 member H2;