Protein-carbohydrate interactions play important roles in modulating cell-cell and cell-extracellular matrix (ECM) interactions in various biological processes of normal and disease development such as cell activation, growth regulation, cancer metastasis, and fibrogenesis. Thus, a detailed understanding of how these carbohydrate-binding proteins (lectins) interact with their partners (carbohydrate ligands) in normal and disease development is very important for the development of lectin-targeted therapeutics. Galectins are a family of at least fifteen β-galactoside-binding lectins that are involved in growth development, and the progression of various diseases such as cancer metastasis ^{[1][2][3][4][5]}[1,2,3,4,5], organ fibrosis ^{[6][7][8][9][10]}[6,7,8,9,10], and type 2 diabetes ^{[11][12][13][14][15][16]}[11,12,13,14,15,16]. Galectins are classified as Proto, Chimera, and Tandem-repeat based on their subunit structures [17] (Figure 1). Proto-type galectins comprise one carbohydrate-recognition domain (CRD) per subunit and these are either monomer (examples: galectins-10, -11, -13, -14, and -15) or dimer (examples: galectins-1, -2, and -7). Tandem-repeat type galectins have two similar, but not identical, CRDs joined by a linker peptide (examples: galectins-4, -6, -8, -9, and -12). The chimera type galectin (galectin-3 only) also contains one CRD at the C-terminal end, but its N-terminal end is rich with proline-glycine repeats. Galectin-3 (Gal3) is a monomer, but it can form a multimer (dimer or pentamer) at higher concentrations [17].

Gal3 is one of the most studied members of the galectin family ^{[2][18][19][20][21]}[2,18,19,20,21]. The literature search of Gal3 (using the previous name IgE binding protein) shows almost 10,000 publications at the time of this manuscript preparation. As a multifunctional protein involved in multiple pathways of many diseases, including cancer, fibrosis, and diabetes, Gal3 has generated significant interest in pharmaceutical industries [22].

2. Primary and Three-Dimensional (3D) Structure of Gal3

Gal3 (previously known as Mac-2, L-29, L-31, L-34, IgE binding-protein, CBP35, and CBP30) consists of three structurally distinct domains containing a highly conserved short N-terminal domain (ND) with 12-amino acids [19], a long ND rich with proline and glycine, and a C-terminal CRD [23] (Figure 2). The short ND has been shown to have roles in Gal3 secretion and Gal3-mediated apoptosis since deletion of the short ND abrogates secretion of Gal3 [24], and mutation of the conserved Ser6 in the short ND affects Gal3’s anti-apoptotic signaling activity [25]. The long ND of Gal3 is responsible for its multimerization and positive cooperativity in carbohydrate binding ^[24][26][24,26]. The C-terminal CRD of Gal3, comprising approximately 130 amino acids, forms a globular structure like other galectins [19] and accommodates a pocket for carbohydrate binding ^{[27][28][29][30]}[27,28,29,30]. The human Gal3 gene (LGALS3) approximately 17 kb long is located on locus q21–q22 of chromosome 14 [31] and contains six exons [32]—of which exons 1–3 represent the N- terminal domain, while exons 4–6 house the CRD. The open reading frame of human Gal3 mRNA is 753 bp long (NM_002306.3 for transcript variant 1). The Gal3 CRD co-crystallized with small molecule inhibitors such as Thomsen-Friedenreich (TF) antigen (Galβ1-3GalNAcα1-O-Ser/Thr), lactose, TFN (TF p-nitrophenyl), and GM1 ^[33][34][35][33,34,35] shows a high similarity to the previously reported structure [36]. The Gal3 CRD adopts a typical galectin fold consisting of eleven-stranded (S1–S6 and F1–F5) antiparallel β-sheets, jointly forming a β-sandwich structure where the S1–S6 β-strands form a concave surface on which a TF antigen and other glycans are bound.

3. Carbohydrate-Binding Properties of Gal3 and Its Endogenous Ligands

All galectins bind β-galactoside; however, subtle differences in their carbohydrate-binding properties are observed. For example, most galectins preferentially bind N-acetyllactosamine, Galβ1,4GlcNAc (5–10 times stronger) over lactose, Galβ1,4Glc ^{[37][38][39][40][41]}[37,38,39,40,41] and so N-glycans containing the N-acetyllactosamine are good ligands for most galectins. Interestingly, the interaction of Gal3 with the TF-disaccharide, Galβ1,3GalNAc found in O-glycans seems to be strikingly different compared to that of galectin-1 ^{[36][37][38][39]}[36,37,38,39]. On isothermal titration calorimetry (ITC) assays, Gal3 interacted with the TF-antigen with a 100-fold higher affinity compared to galectin-1 [35]. The basis for these subtle differences in galectins’ carbohydrate-binding properties can be explained by their 3-D structures ^[35][36][42][35,36,42]. Gal3 has both intracellular and extracellular ligands. Like other galectins, Gal3 lacks a typical secretory signal peptide [43], and it is present in the cytosol and also in the ECM ^[44][45][44,45]. The β-galactoside-containing glycoproteins of the ECM and cell surface, such as laminin ^[46][47][46,47], fibronectin [48], CD29 [49], CD66 [50], α1β1 integrin [48], and Mac-2 binding protein, [51] are known extracellular ligands of Gal3. Among intracellular ligands of Gal3, gemin 4 [52], Bcl-2 [52], nucling [53], synexin [54], and β−catenin ^[55][56][55,56] are known, and Gal3 binds to these ligands via protein-carbohydrate or protein-protein interactions.

4. Gal3 Mediates Cell-Cell and Cell-ECM Interactions

Gal3 exerts multiple biological roles intracellularly within the nucleus or the cytoplasm, or after its secretion, at the cell surface and/or the extracellular space ^[3][4][17][3,4,17]. Gal3 binds to cell surface β-galactose-containing glycoconjugates or glycolipids, thereby regulating cell proliferation, apoptosis, cell adhesion, invasion, angiogenesis, and metastasis in normal development, as well regulating the progression of disease processes such as tumorigenesis and fibrogenesis.

4.1. Expression of Gal3 in Normal Growth Development

Gal3 is developmentally regulated and expressed in all types of tissues ^[57][58][57,58]. During mouse embryogenesis, expression of galectin-3 was first observed in the trophectoderm on day 4 and then in the notochord cells between 8.5 and 11.5 days of gestation [57]. In later stages of mouse development, expression of Gal3 was observed in the cartilage, ribs, larynx, esophagus, facial bones, a suprabasal layer of the epidermis, and endodermal lining of the bladder [19]. In the adult stage, expression of Gal3 was mostly found in epithelial cells such as small intestine [59], colon [60], cornea ^[61][62][61,62], kidney [63], lung [64], thymus [65], breast [66], and prostate [67]; ductal cells such as the salivary glands [68], pancreas [69], kidney [70], and eye [71]; and in intrahepatic bile ducts [72]. Among various cell types, Gal3 is expressed in fibroblasts [73], chondrocytes and osteoblasts [74], osteoclasts [75], keratinocytes [76], Schwann cells [77] and gastric mucosa [78], endothelial cells [79], and immune-related cells such as neutrophils [80], eosinophils [81], basophils and mast cells [82], Langerhans cells ^[76][83][76,83], dendritic cells [84], monocytes [85], and macrophages from different tissues ^{[3][18][86][87]}[3,18,86,87].

4.2. Gal3 Is Involved in the Progression of Many Diseases

Gal3 mostly plays a pro-inflammatory role and is involved in many diseases associated with chronic inflammation, such as cancer, fibrosis, and type 2 diabetes (Figure 3).