Galectin-3: Integrator of Signaling via Hexosamine Flux

Abstract

Galectin-3 (Gal-3) is a β-galactoside-binding lectin that mediates diverse signaling events in multiple cell types, including immune cells. It is also a prognostic indicator for multiple clinically important disorders, including cardiovascular disease. Gal-3 binds to cell surface glycans to form lattices that modulate surface receptor signaling and internalization. However, the tissue-specific regulation of Gal-3 surface expression remains poorly understood. Here, we review evidence for the involvement of Gal-3 in cell surface signaling, intranuclear events, and intracellular trafficking. Our focus will be on the O-GlcNAc modification as a regulator of Gal-3 biosynthesis, non-canonical secretion, and recycling. We argue that the nutrient-driven cytoplasmic hexosamine biosynthetic pathway (HBP) and endomembrane transport mechanisms generate unique pools of nucleotide sugars. The differing levels of nucleotide sugars in the cytosol, endoplasmic reticulum (ER), and Golgi apparatus generate differential thresholds for the responsiveness of O-GlcNAc cycling, N- and O-linked glycan synthesis/branching, and glycolipid synthesis. By regulating Gal-3 synthesis and non-canonical secretion, O-GlcNAc cycling may serve as a nexus constraining Gal-3 cell surface expression and lattice formation. This homeostatic feedback mechanism would be critical under conditions where extensive glycan synthesis and branching in the endomembrane system and on the cell surface are maintained by elevated hexosamine synthesis. Thus, O-GlcNAc cycling and Gal-3 synergize to regulate Gal-3 secretion and influence cellular signaling. In humans, Gal-3 serves as an early-stage prognostic indicator for heart disease, kidney disease, viral infection, autoimmune disease, and neurodegenerative disorders. Since O-GlcNAc cycling has also been linked to these pathologic states, exploring the interconnections between O-GlcNAc cycling and Gal-3 expression and synthesis is likely to emerge as an exciting area of research.

Keywords: Galectin-3; HBP; N-Glycan; O-GlcNAcylation; UDP-GlcNAc.

Figure 1

Schematic presentation of galectin family members and the galectin–glycan lattice. Galectin members are divided into three types based on the organization of the galectin carbohydrate recognition domain (CRD). (A) Prototype galectins (Gal-1, -2, -5, -7, -10, -11, -13, -14, -15, and -16). (B) Tandem repeat type galectins (Gal-4, -6, -8, -9, and -12). (C) The unique chimeric type of galectin (Gal-3). (D) Schematic presentation of bivalent lattice formation of the Gal-3 pentamer with glycans.

Figure 2

Schematic presentation of Gal-3 binding glycans. Gal-3 recognizes and binds to the galactose moiety of the Gal β1-4 GlcNAc linkage present in (A) N-glycans, (B) O-glycans, (C) glycolipids, or (D) blood group antigens. Brackets represent Gal-3 binding N-acetyllactosamine (LacNAc) sites on the glycan species. Asn = Asparagine, ST6Gal1 = α (2-6) sialyltransferase 1, GCNT1 = N-acetylglucosaminyltransferase 1, GnT5 = N-acetylglucosaminyltransferase 5, ST3Gal1 = α (2-3) sialyltransferase 1.

Figure 3

Schematic presentation of Gal-3 expression and distribution in mammalian tissues. Gal-3 is ubiquitously and differentially expressed throughout all tissues except the spleen and lymph nodes.

Figure 4

Schematic presentation of Gal-3 trafficking, showing its cytoplasmic synthesis, nuclear translocation, apical sorting, noncanonical secretion, glycan-mediated clathrin-independent endocytosis (CIE), and endosomal recycling. (1) Cytoplasmic synthesis of Gal-3 followed by nuclear import and export; (2) Apical sorting of Gal-3; (3) Gal-3 transports through exosomes as a form of noncanonical secretion; (4) Gal-3 is secreted into the extracellular matrix; (5) Glycan-mediated Gal-3 recognition occurs at the cell surface; (6) Gal-3 internalizes through CIE; (7) Internalized Gal-3 is recycled at the endosomes.

Figure 5

The HBP generates the nucleotide sugar donor UDP-GlcNAc. Oligosaccharyltransferase (OST) transfers the preassembled donor Glc₃Man₉GlcNAc₂-pp-dolichol to NXS/T motifs on nascently synthesized proteins in the ER. Glycoproteins travel from the ER to the cis, medial, and trans Golgi en route to the cell surface. The Golgi-residing N-acetylglucosaminyltransferases, named after the names of their encoding genes (Mgat1, Mgat2, Mgat4, and Mgat5), generate branched N-glycans that display a range of affinities for galectins. The Michaelis constant (Km, a measure of enzyme affinity for its substrate) values for Mgat1, Mgat2, Mgat4, and Mgat5 are indicated for both UDP-GlcNAc and acceptor glycoproteins as measured in vitro. Gln = Glutamine; Glu = Glutamate; GlcN-6P = Glucosamine-6-phosphate; GlcNAc = N-Acetyl-D-glucosamine; UTP = Uridine triphosphate; PPi = Pyrophosphate; UDP = Uridine diphosphate; UMP = Uridine monophosphate; CMP = Cytidine monophosphate; Man II = Golgi mannosidase II; Mgat1, 2, 4, and 5 = N-acetylglucosaminyltransferases I, II, IV, and V.

Figure 6

Schematic presentation of the role of O-GlcNAcylation in Gal-3 secretion. Gal-3 is synthesized in the cytoplasm and then secreted by noncanonical mechanisms. The cytoplasmic origin and nuclear translocation allow interaction with the O-GlcNAcylation machinery. This model indicates that Gal-3 necessitates O-GlcNAcylation and deglycosylation for proper secretion.

Figure 7

Gal-3 provides synergy between nutrient-driven O-GlcNAcylation and surface glycan signaling. At reduced intracellular concentrations of UDP-GlcNAc, dysregulation of the O-GlcNAcylation cycle may occur, leading to changes in Gal-3 transcriptional control and its subsequent secretion. These alterations can manifest across the physiologically relevant range of cytoplasmic concentrations, which are tightly regulated via feedback inhibition of the HBP. Due to the activity of specific nucleotide sugar transporters, UDP-GlcNAc concentrations within the ER and Golgi apparatus are substantially elevated, as evidenced by the higher substrate thresholds required for extensive N-glycan branching and mucin-type O-glycan biosynthesis. At elevated UDP-GlcNAc concentrations, low-affinity N-glycosylation sites are modified, leading to the synthesis of low-occupancy N-glycans. The threshold-dependent changes illustrated suggest that as N-glycan synthesis and branching increase, Gal-3 expression and secretion are restrained to prevent excessive activation of Gal-3–mediated signaling pathways.