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Author: Antimo Natale
Date: 21/06/2013



A short protein description with the molecular wheight, isoforms, etc...
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Galectin-3 is a member of the lectin family, of which 14 mammalian galectins have been identified.
Galectin-3 is approximately 30 kDa and, like all galectins, contains a carbohydrate-recognition-binding domain (CRD) of about 130 amino acids that enable the specific binding of β-galactosides.
Galectin-3 is encoded by a single gene, LGALS3, located on chromosome 14, locus q21–q22. It is expressed in the nucleus, cytoplasm, mitochondrion, cell surface, and extracellular space. This protein has been shown to be involved in the following biological processes: cell adhesion, cell activation and chemoattraction, cell growth and differentiation, cell cycle, and apoptosis. Given galectin-3’s broad biological functionality, it has been demonstrated to be involved in cancer, inflammation and fibrosis, heart disease, and stroke. Studies have also shown that the expression of galectin-3 is implicated in a variety of processes associated with heart failure, including myofibroblast proliferation, fibrogenesis, tissue repair, inflammation, and Ventricular remodeling. Elevated levels of galectin-3 have been found to be significantly associated with thyroid cancer.

In human genome galectin-3 is coded by a single gene LGALS3 which is suited on chromosome 14, locus q21, q22. Human LGALS3 genes, 17 kb, are composed of six exons and five inrons. Exon I encodes the major part of the 5' untranslated sequence of m RNA. Exons II contains the sequence enconding the remaining part of the 5' untranslated region, the translation initiation site and codon sequence for the first six amino acids including the initial methionine. The sequence encoding the N-terminal domain in both, murine and human LGALS3 was found entirely within exon III. In mouse, the CRD is enconded by three succeeding exons (IV, V, VI), whereas in human, the sequence enconding the CRD was found entirely within exon V. Two transcription initiation sites identified in human LGALS3 correspond to the β, δ, γ start site region in the mouse gene which also contains the α transcription initiation site.
Interestingly, the second intron of LGALS3 contains an internal promoter, which drives production of alternative transcripts. These transcripts arise from an internal gene embedded within LGALS3, named galig (galectin-3 internal gene). They are preferentially expressed in peripheral blood leukocytes, and cannot be used for production of galectin-3 or a modified galectin-3 because they contain two overlapping open-reading frames out-offrame within the lectin coding sequence. Recently, it was shown that galig is a novel cell death gene encoding mitogaligin, a protein that promotes cytochrome c release upon direct interaction with the mitochondria.

The structure of galectin-3 seems to be unique among all vertebrate galectins; its single polypeptide chain forms two structurally distinct domains, atypical N-terminal domain (ND) anc C-terminal carbohydrate-recognition domain. Many studies of physico-chemical characteristics of galectin-3 suggested not only profound structural, but also functional differences of these two domains.
The N-terminal domain (ND):
The N-terminal domain of Galectin-3 is composed of 110-130 amino acids, depending on species. This relatively flexible structure contains multiple homologues repeats, each of which includes a consensus sequence Pro-Gly-Ala-Tyr-Pro-Gly, followed by three additional amino acids. The ND is highly conserved among galectin-3 molecules isolated from different species- In addition, the amino acid sequence is approximately 25% homologous with some heterogeneous nuclear ribonicleoprotein (hnRNP) complexes what corresponds to the homology found among the core hnRNP proteins themselves. It has been reported that the ND has 33.5% identity with collagen α1 chain of bovine cartilage, so the ND is also designated as a collagen-like N-terminal domain. Although the ND was shown to lack carbohydrate-binding activity, it is essential for full biological activity of galectin-3. Futhermore, more recent results obtained by molecular modeling and mutagenesis analysis revealed the ND, throught Tyr102 and adjacent residues, participates togheter with the CRD in oligosaccharide binding. The ND is also responsible for multimer forming and shows positive cooperativity in lectin binding to immobilized ligand clusters. This property seems to be biologically regulared, because the ND is suscetible to selective proteolysis by certain matrix metallo-proteinase, MMP-2 and MMP-9. The cleavage at the position Ala62 Tyr63 of the recombinant human galectin-3 increases the affinity of the CRD (preserved in 22 kDa fragment) to the carbohydrate ligands, but reduces self-association of Galectin-3, abrogating in that way biological porperties dependant on multimerization. Thus, for example, proteolytically cleaved galectin-3 displays approximately 20-fold higher binding affinity for human umbilical vein endothelial cells as compared to the full-length protein. The ND has been also implicated in secretion of galectin-3 outside of cells. The initial 12 amino acid N-terminal peptide sequence preceding the proline/glycin-rich repetitive domain, also called small N-terminal domain, is highly conserved in all mammalian galectin-3. At least two functional characteristics were ascribed to this N-terminal portion of galectin-3; deletion of these first 11 amino acids (following the first methionine) blocks secretion of galectine-3, while mutation of the conserved Ser6 affects galectin-3 anti-apoptotic signaling activity.
The carbohydrate-recognition domain (CRD):
The C-terminal domain of galectin-3, composed of about 130 amino acids forming a globular structure, accommodates whole carbohydrate-binding site, thus being responsible for lectin activity of Galectin-3. The CRD of Galectin-3 has a topology and a three-dimensional structure very similar to the CRD Galectin-1 and -2, with which it shares the sequence to 20-25%. Furthermore, as galectin-1 and -2, you have to 12 β strands form (two anti-parallel β-sheets, composed of five and six β-strands). Within the carbohydrate-recognition domain particularly interesting amino acid sequence is NWGR; this motof is highly conserved within the BH1 domain of the Blc-2 family proteins, and it was shown to be responsible for the anti-apoptotic activity of both Bcl-2 and galectin-3. The NWGR motif is also involved in self-association of galectin-3 molecules through the CRDs in the absence of saccharide ligands. The replacement of tryptophan with leucine (W181L) within the NWGR motif abolishes homodimerization through the CRDs of galectin-3. However, this mutant can still bind wild-type galectin-3 through the interactions of N-terminal domains. The CRD is also involved in carbohydrate-dependent homophilic interactions of galectin-3. Single cysteine residue suited near NWGR motif (Cys186) was shown to be required for dimerization of murine galectin-3, which in that form binds laminin with higler affinity than monomeric form. The CRD, comparing to the intact galectin-3, exhibits stronger binding affinity for advanced glycation end-products (AGE), what suggest that the CRD may also contain the principal AGE-binding site which is stericaly hindered by the ND in the full-lenght galectin-3.

Galectin-3 is synthesized on free ribosomes in the cytoplasm and lacks any signal sequence for translocation into the endoplasmic reticulum (ER). Although it does not traverse the endoplasmic reticulum/Golgi network, there is abundant evidence for galectin-3 also having an extracellular location. This protein has been shown to be secreted from cells by a novel, incompletely understood mechanism called ectocytosis, which is independent of the classical secretory pathway through the ER and Golgi system. The N-terminus of galectin-3 has been proposed to contain targeting information for nonclassical secretion. It has been shown that a hamster galectin-3 CRD fragment lacking the N-terminal domains, when expressed in transfected Cos cells, is not secreted. Moreover, the addition of the N-terminal segment to a normally cytosolic protein such as chloramphenicol acetyltransferase (CAT) means the fusion protein is efficiently exported from transfected Cos cells.
A short segment of the galectin-3 N-terminal sequence comprising residues 89-96 (Tyr-Pro-Ser-Ala-Pro-Gly-Ala-Tyr) has been found to play a critical role in galectin-3 secretion. However, this sequence is not sufficient on its own to cause the direct secretion of the CAT fusion protein, indicating that it is operative in the context of a large stretch of the N-terminal sequence of galectin-3. Immunohistochemical studies have indicated that the first step in galectin-3 secretion is its accumulation at the cytoplasmic side of the plasma membrane. This step is rate limiting in galectin-3 secretion from macrophages and Cos cells transfected with galectin-3 constructs, and is strongly up-regulated by heat shock and calcium ion. The next step in galectin-3 secretion is the pinching off of evaginating membrane domains and the release of extracellular vesicles in which galectin-3 is protected against proteolysis.
Electron microscopy showed that the vesicles are morphologically heterogeneous and have a small size (up to about 0.5 mm). Under culture conditions, lectin release from extracellular vesicles was rather fast with a halflife about 1 h. However, isolated vesicles were much more stable, suggesting that the rapid breakdown of vesicles requires factor(s) released by cells. Hughes suggested that a good candidate is one or more members of the phospholipase A2 family that catalyse the hydrolysis of an sn-2 fatty acyl bond of phospholipids and liberate free fatty acids and lysophospholipids. Although this hypothesis is very intriguing, it needs to be checked.

The galectin-3 is found expressed in different cells and tissues. The Galectin-3 and was found in macrophages activated, eosinophils, neutrophils, mast cells, the epithelium of the gastrointestinal and respiratory tract, in the kidney and in some sensory neurons. Furthermore, and expressed in many tumors, such as the pancreas, colon and carcinomas thyroid. Although galectin-3 has predominantly in the cytoplasm, and was also detected in the nucleus, on the cell surface or in the extracellular environment, suggesting multifunctionality of this molecule. The intracellular localization of galectin-3 and connected with its role in the regulation of nuclear pre-mRNA splicing and protection against apoptosis. On the other hand, its position on the extracellular cell surface and in the extracellular medium indicates its participation in cell-cell adhesion and cell matrix.

Extracellular function
Due to its affinity for poly-lattosaminoglicani, galectin-3 binds the components of the matrix
extracellular glycosylated, such as laminin, fibronectin, tenascin (table). Even some molecules cell-surface adhesion are ligands for galectin-3, as the integrins. Preliminary evidence suggests that galectin-3, through binding to the extracellular domains of one or both subunits of a integrin, can positively or negatively modulate the activation of integrins, and alter the link with extracellular ligands. An important ligand for galectin-3 on macrophages of mice and the subunit integrin AMB2, otherwise known as CD11b/18. Furthermore, galectin-3 also interacts with integrin a1b1 through its CRD domain through a mechanism dependent lactose. Galectin-3 appears to mediate the activation of integrins favoring dimerization antigen CD98. In fact, the domain cytoplasmic CD98 binds intracellularly to the cytoplasmic tail of specific beta subunit integrin, and promoting the activation of integrins through intracellular signaling pathways not yet been fully established.
Potential ligands for galectin-3 are the lysosomal membrane proteins Lamp-1 and 2. They are mainly confined to the lysosomal membranes, and are rarely found on the plasma membrane of normal cells. Some authors suggest that an increase of their expression on the cell surface of tumor cells, particularly in those metastatic, carriers of poly-N-acetillattosamine. For example, Serafian et
al. have shown a strong link between the surface of Galectin-3 and metastatic melanoma cells Lamp+.
These results favor the hypothesis that the Lamp may be ligands for cell adhesion molecules and may participate in the complex process of tumor invasion and metastasis. Recent studies have shown that MP20, a member of the tetraspanin superfamily, also seems to be a ligand for galectin-3. MP20 has an N-linked glycosylation site in one of the extracellular loops with the which could tie the Galectin-3, but until now not been made clear what exactly the role of the MP20/Galectin-3 complex. It is possible that galectin-3 plays a key role in modulating ability of MP20 to form adhesive joints in the critical stage of cell development.

Intracellular function
All the ligands listed above for galectin-3 are extracellular matrix proteins or proteins membrane. However, galectin-3, and also known to have an intracellular location and to interact with several proteins located within the cell, such as cytokeratins, CBP70, CHrp, Gemin4, Alix/AIP-1, and Bcl-2. It is worth noting that almost all the mentioned ligands interact with intracellular galectin-3 via protein-protein interactions rather than interactions lectin-glycoconjugate. The only exception are the cytokeratins. In literature are found a series of studies that show between the intracellular functions of Galectin-3, the involvement in the inhibition of apoptosis. These shed light on how cells overexpressing Galectin-3 possess greater resistance to apoptotic stimuli induced by anti-Fas antibody, staurosporine (reagent chemotherapy), tumor necrosis factor, radiation and nitric oxide. In fact ,it was noticed as the Galectin-3 possesses a significant similarity with the sequence of the protein Bcl-2, a known suppressor of apoptosis. The lectin contains a reason to four amino acids, Asn-Trp-Gly-Arg, and that a highly conserved sequence within the BH1 domain of Bcl-2 family proteins and crucial for the function of Bcl-2 protein in the inhibition of programmed cell death. Yang et al. have shown that galectin-3 may interact with Bcl-2 even if Bcl-2 is not a glycoprotein. The authors have suggested that the reason Asn-Trp-Gly-Arg and present within the recognition domain of carbohydrates in Galectin-3, and closely involved in the interaction with Bcl-2. The binding of lactose with Galectin-3 can induce a conformational change in the critical region of this protein, which prevents its interaction with Bcl-2. The molecular mechanism by which galectin-3 regulates apoptosis and yet to be clarified. However, it is possible that this lectin replaces or mimics the protein Bcl-2. Bcl-2 and a mitochondrial protein located on the outer membranes. It regulates apoptosis by blocking the release of cytochromes from the mitochondria.Luna et al. have demonstrated that the inhibition by the Galectin-3 of the release nitrogen free radical mediated in breast cancer BT549 human cells is involved in the protection of the integrity mitochondrial, and inhibiting the release of cytochrome C and activation of caspases. Thus, galectin-3 appears to be a regulator for additional mitochondrial apoptotic Bcl-2. The protection of mitochondrial damage mediated by Galectin-3 and regulated by some proteins that interact with the lectin. One of these proteins and a synexina, an annexin protein of 51 kDa, that can bind to phospholipid membranes. The down-regulation of Synexina abolishes the anti-apoptotic activity of Galectin-3 suggesting that the translocation to mitochondrial membranes and crucial for the antiapoptotic function of Galectin-3.

The expression of galectin-3, on both transcriptional and translational level is affected by various stimulus.Although a large body of reported data about galectin-3 expression is available in the literature, the mechanisms of regulation of galectin-3 expression are still poorly understood. The increase of galectin-3 on both protein and mRNA level was observed in the proliferating fibroblasts comparing to the quiescent cells. Furthermore, galectin-3 expression could be consid- ered as a transformation marker since the galectin-3 mRNA content is increased in fully ras-transformed fibroblasts, with maximal expression occurring when cells have lost their growth anchorage-dependence. Galectin-3 expression was suggested to be also differentiation marker for certain cell types. Thus, for example, the differentiation of the human promyelocytic cell line HL-60 to macrophage-like cells induced by phorbol ester is accompanied by elevation of both galectin-3 and galectin-3 mRNA level. As well, in vitro differentiation of human monocytes to macrophages provokes significant increase of galectin-3 expression. In contrast, differentiation of dendritic cells from bone marrow progenitors is accompanied by decrease of galectin-3 expression. Galectin-3 is also considered a “macrophage activation marker” due to the fact that its expression is up-regulated in phagocytic macrophages. In addition, the activation of monocytic THP-1 cells provoked by phorbol ester or low density lipoproteins induces galectin-3 expression as well as the exposure of bone marrow-derived macrophages to 1,25-dihydroxyvitamin D3. In microglia and macro- phages exposed to garanulocyte-macrophage colony-stimulating factor, as well as in macrophages and microglia human immunodeficiency virus-1 (HIV-1) induces expression of galectin-3. The activation of B lymphocytes with IL-4 and CD40 cross-linking, signals that promote survival and block final differentiation of the cells, is also accompanied by significant increase of galectin-3 expression. In addition, it has been showed that B cells from Trypanosoma cruzi-infected mice express galectin-3. The promoter region of the human LGALS3 gene contains several regulatory elements: five putative Sp1 binding sites (GC boxes), five cAMP-dependent response element (CRE) motifs, four AP-1- and one AP-4-like sites, two NF-κB-like sites, one sis-inducible element (SIE) and a consensus basic helix–loop–helix (bHLH) core sequence. The presence of multiple GC box motifs for binding ubiquitous expressed Sp1 transcription factor is a common feature of constitutively expressed, so-called housekeeping genes. The activation of the Sp1 binding transcription factor was suggested to be responsible for galectin-3 induction by Tat protein of HIV. On the contrary, the expression of galectin-3 in serum- starved, quiescent fibroblasts can be induced by addition of serum, on both protein and mRNA level, what is a feature of immediately early gene. The SIE that binds sisinducible factors was suggested to be a possible candidate for the growth-induced activation of LGALS3 expression, caused by the addition of serum. The presence of CRE and NF-κB-like site in the promoter region implies that the activation of galectin-3 expression could be also regulated through the signaling pathways involving the cAMP-response element-binding protein (CREB) or the NF-κB transcription factor. Indeed, the activation of the LGALS3 expression by the Tax protein during HTLV-I infection of T cells, was shown to indepen- dently involve the CREB/ATF and the NF-κB/Rel transcription factors pathways.

Galectin-3, and then involved in multiple functions, both inside and outside the cell. While the extracellular functions are most likely due to the binding of this protein with glycoconjugates for the middle of the CDR to the C-terminus, the intracellular functions probably do not involve interactions with carbohydrates and rather are due to amino acid repeats located in the N-terminal region. Recent studies suggest that galectin-3 may play an important role in a variety of processes physiological and pathological conditions, including inflammation, neoplastic transformation, as well as innate immunity and acquired. Studies of galectin-3-deficient mice have provided evidence for a role of galectin-3 in the response inflammatory. The expression of Galectin-3 is implicated in transformation of normal cells to cells cancer. It, and in fact up-regulated in tumors induced by viruses, ultraviolet light or chemicals. In mouse fibroblasts, the expression of galectin-3 and adjusted in a manner similar to other genes activated by the mitogen, including the oncogenes c-fos and c-myc. Furthermore, the gene of Galectin-3 contains an element that responds to the p53 and the expression of galectin-3 and down-regulated by p53. It appears, in fact, that galectin-3, and highly expressed nell'epatoma, in some subtypes of thyroid carcinomas and lymphomas, although not expressed by corresponding normal cells. In particular, recent studies have shown that Galectin-3 is overexpressed in thyroid carcinomas, while its expression in the cells and in normal follicalari adenomas is absent or weak. Furthermore, Galectin-3 mRNA was found in all thyroid lesions malignant, whereas in normal tissues and benign, not detectable. Galectin-3 in thyroid cells and malignant was detected by immunohistochemistry in both cytological specimens obtained by fine-needle aspiration (FNAB) is on histological sections. Galectin-3, when expressed, was predominantly found in the cytoplasm of cells follicular and parafollicular. Kawachi et al. have reported differences in the expression of Galectin-3 between the primary lesions of papillary carcinoma with metastasis and those without metastasis: the expression of galectin-3 was significantly higher in the former group than the latter. Published observations allow to draw conclusions that the cytoplasmic expression of Galectin-3 and associated with a phenotype most malignant progression to metastatic potential. Yoshii et al. have shown that inhibition of the expression of Galectin-3 in papillary thyroid carcinoma cells involves a marked reduction of the malignant phenotype. Recently, Takenaka et al. have shown that the Overexpression of Galectin-3 in normal thyroid follicular cells transfected with Galectin-3 cDNA door the acquisition of the malignant phenotype. Furthermore, Takenaka et al. have tried to identify the genes that are associated with overexpression of galectin-3, including: retinoblastoma (RB), nuclear antigen of cell proliferation (PCNA) and replication factor C (RCF). From a biochemical point of view, the cytoplasmic evaluation of Galectin-3 in epithelial cells isolated from FNAB should serve to make a preoperative differential diagnosis between benign follicular adenomas and differentiated carcinomas. The use of routine of this analysis may lead to better selection of patients who really need.

Duminic J, Dabelic S, Flögel M, Galectin-3: an open-ended story. Biochim Biophys Acta. 2006 Apr;1760(4):616-35.
Krześlak A, Lipińska A, Galectin-3 as a multifunctional protein.Cell Mol Biol Lett.2004;9(2):305-28.

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