What is lectin ?

Contents

What is Lectin ?

What is Lectin ?

Because research into lectin is still in its infancy, there is some confusion with the definition of lectin. According to the National Cancer Institute, lectin is a complex molecule that has both protein and sugars ¹⁾. Lectins are able to bind to the outside of a cell and cause biochemical changes in it. Lectins are made by both animals and plants ²⁾.

The other more commonly found definition of lectins is: “Lectins are defined as proteins that preferentially recognize and bind carbohydrate complexes protruding from glycolipids and glycoproteins” ³⁾, ⁴⁾, ⁵⁾, ⁶⁾. In other words, lectins are carbohydrate-binding proteins, macromolecules that are highly specific for sugar moieties. Lectins are a diverse group of proteins that are attracted to carbohydrates and biomolecules that contain carbohydrate groups, including varieties of sugars and sugar-modified proteins and lipids. This attraction, which is highly specific, allows lectins to influence a wide range of biological processes. For example, many bacterial and mammalian lectins control how cells move and adhere to each other ⁷⁾, while some plant lectins, notably ricin and abrin, are renowned for their lethality ⁸⁾.

Lectins are universally distributed in nature, being established in plants, fungi, viruses, bacteria, crustacea, insects, and animals, but leguminacae (legumes) plants are rich source of lectins.

In 1919, James B. Sumner at Cornell University, well known for being the first to crystallize in 1926 an enzyme, urease, isolated from jack bean (Canavalia ensiformis) a crystalline protein that he named concanavalin A and in this way obtained a pure hemagglutinin for the first time. However, nearly two decades passed before Sumner and Howell (1936) reported that concanavalin A agglutinates cells such as erythrocytes (red blood cells) and yeasts and also precipitates glycogen from solution ⁹⁾. They further showed that hemagglutination by concanavalin A was inhibited by sucrose, demonstrating for the first time the sugar specificity of lectins. With much foresight, they suggested that the hemagglutination induced by concanavalin A might be a consequence of a reaction of the plant protein with carbohydrates on the surface of the red blood cells. The pioneering work of Watkins and Morgan found that the agglutination of type A red blood cells by lima bean lectin was best inhibited by α-linked N-acetyl-D-galactosamine and that of type O cells by the lectin of L. tetragonolobus was best inhibited by α-linked L-fucose. They provided the earliest evidence for the presence of sugars on cell surfaces and their potential roles as identity markers ¹⁰⁾.

Moreover, the ability of plant agglutinins to distinguish between erythrocytes of different blood types led Boyd and Shapleigh (1954) to propose for them the name lectins, from the Latin word legere meaning “to choose” or “select”. This term was generalized to embrace all sugar-specific agglutinins of nonimmune origin, irrespective of source and blood type specificity ¹¹⁾. The interaction of lectins with particular carbohydrates can be as specific as the interaction between those of antigen-antibody or substrate-enzyme ¹²⁾. Lectins bind not only to oligosaccharides (a carbohydrate whose molecules are composed of a relatively small number of monosaccharide units) on cells but also to free-floating glycans (compounds consisting of a large number of monosaccharides linked glycosidically) including monosaccharides. Lectin-monosaccharide interactions, however, are relatively weak with dissociation constants often on the order of micromolar to millimolar range ¹³⁾, ¹⁴⁾.

Two major discoveries made in the early 1960s were instrumental in bringing lectins into the limelight. The first of these was by Peter C. Nowell (1960), who found that the lectin of the red kidney bean (Phaseolus vulgaris), known as phytohemagglutinin, is mitogenic, that is, it possesses the ability to stimulate lymphocytes to undergo mitosis (the process by which the nucleus divides in eukaryotic organisms, producing two new nuclei that are genetically identical to the nucleus of the parent cell) ¹⁵⁾. Within a short time, several other lectins were proven to be mitogenic. Of special significance was the finding that concanavalin A acts as a mitogen because, in contrast to phytohemagglutinin, its activity could be inhibited by low concentrations of monosaccharides, for example, mannose. This finding provided proof that mitogenic stimulation is the result of binding of lectins to sugars on the surface of the lymphocytes and was among the earliest demonstrations for a biological role of cell surface sugars. Mitogenic lectins soon became tools for the study of signal transmission into cells and for the analysis of the biochemical events that occur during lymphocyte stimulation in vitro. A most valuable outcome of such studies was the discovery in the 1970s by Robert C. Gallo and his associates at the National Institutes of Health of T cell growth factor, now known as interleukin-2, in conditioned medium of normal human lymphocytes stimulated by phytohemagglutinin ¹⁶⁾. The second discovery was made by Joseph C. Aub at the Massachusetts General Hospital in Boston. He found that wheat germ agglutinin has the ability to preferentially agglutinate malignant cells. This was followed by the reports of Max M. Burger at Princeton University and Leo Sachs and Michael Inbar at the Weizmann Institute that concanavalin A exhibits the same ability. Together with Sachs and Ben-Ami Sela subsequently found that soybean agglutinin also possesses the same property ¹⁷⁾.

Although some lectins are polyclonal activators both in vitro and in vivo, others may display a broad range of activities toward human lymphocytes. Indeed, the same lectin (e.g. wheat germ agglutinin or Datura lectin ) may be mitogenic, comitogenic, or antimitogenic, depending on the experimental conditions. An individual lectin may bind to several glycoproteins on the lymphocyte surface, resulting in interactions that may or may not be functionally relevant, and that may have opposing effects. Studies with lectins and with monoclonal antibodies (MAbs) have established that a surprisingly large variety of cell-surface molecules can influence the initiation and regulation of lymphocyte activation and proliferation ¹⁸⁾.

Numerous studies concerning the biological activity of plant lectins have reported that these substances possess toxic, cytotoxic, antitumor, and anticarcinogenic properties ¹⁹⁾.

Sources of Lectins

Lectins were initially found and described in plants, but in subsequent years multiple lectins were isolated from microorganisms and also from animals ²⁰⁾. Interestingly plant and animal lectins show no primary structural homology, yet they demonstrate similar preferential binding to carbohydrates. This suggests that animal and plant lectin genes may have co-evolved, thus highlighting the importance of lectin-carbohydrate interactions in living systems ²¹⁾. During the past several years, however, many primary and three dimensional structures of lectins have been elucidated. It was observed that lectins from diverse sources lacked primary sequence similarity but shared similarities in their tertiary structures ²²⁾. Structural studies conducted on animal lectins suggested that the carbohydrate binding activity of most lectins was generated by limited amino acid residues designated as the carbohydrate recognition domain ²³⁾. The carbohydrate recognition domain typically recognizes the terminal non-reducing carbohydrate residues of cell membrane glycoproteins and glycolipids ²⁴⁾. Lectin carbohydrate recognition domains also may discriminate between anomeric isomers as a function of their specificities. For example, the lectin concanavalin A (Con A) specifically binds the α-anomer of glucose and mannose, but not the β-anomer of either ²⁵⁾.

Lectin families

Within the animal lectins, several highly conserved carbohydrate recognition domain amino acid sequences have been identified, thus allowing investigators to categorize the majority of these lectins into structurally related families and superfamilies ²⁶⁾. C-type lectins are the most abundant of all animal lectins, and the C-type lectins superfamily is grouped into three families: selectins, collectins and endocytic lectins ²⁷⁾, ²⁸⁾. A majority of C-type lectins are large, asymmetric, have one or more carbohydrate recognition domains and exist as Ca2+ (calcium) dependent proteins found in secreted or bound forms ²⁹⁾, ³⁰⁾. In contrast, the S-type lectins (galectins) in the CTL superfamily are generally small, non-glycosylated, soluble and exist as Ca2+ independent proteins found intracellularly and extracellularly (Drickamer and Taylor, 1993; Barondes et al., 1994; Drickamer, 1995; Cooper and Barondes, 1999; Minko, 2004; Sharon and Lis, 2004; Chou et al., 2009; Malik et al., 2009; Saravanan et al., 2009). Currently at least ten galectins have been identified and all bind N-acetyllactosamine (Gal-β-1-nGlcNAc-R) by recognizing the β-gal residue (Gorelik et al., 2001). The collectin family of CTLs includes collagenous lectins such as mannose binding proteins (MBPs), pulmonary surfactant SP-A and SP-D and conglutinin (Gorelik et al., 2001; Kerrigan and Brown, 2009; Ruseva et al., 2009). The number of C-type lectins has reached 50 and at least 10 galectins have been identified. The selectin family of C-type lectins includes the E-, L- and P-selectins. These selectins have a single epidermal growth like domain, an extracellular carbohydrate recognition domain, a cytoplasmic tail, a transmembrane domain, and two to nine short consensus repeat units that are homologous to complement binding proteins ³¹⁾, ³²⁾. Selectins specifically bind oligosaccharides such as sLea and sLex or their sulfated equivalents ³³⁾. Another lectin family of special interest is the siglecs. The siglecs are sialic acid binding Ig-like lectins and belong to the Ig superfamily. They carry unique expression patterns in different cells, indicating that they are involved in highly specialized and specific cellular processes ³⁴⁾.

Table 1. Major Lectins

Table of the major lectins
Lectin Symbol	Lectin name	Source	ligand motif
Mannose binding lectins
ConA	Concanavalin A	Canavalia ensiformis	α-D-mannosyl and α-D-glucosyl residues branched α-mannosidic structures (high α-mannose type, or hybrid type and biantennary complex type N-Glycans)
LCH	Lentil lectin	Lens culinaris	Fucosylated core region of bi- and triantennary complex type N-Glycans
GNA	Snowdrop lectin	Galanthus nivalis	α 1-3 and α 1-6 linked high mannose structures
Galactose / N-acetylgalactosamine binding lectins
RCA	Ricin, Ricinus communis Agglutinin, RCA120	Ricinus communis	Galβ1-4GalNAcβ1-R
PNA	Peanut agglutinin	Arachis hypogaea	Galβ1-3GalNAcα1-Ser/Thr (T-Antigen)
AIL	Jacalin	Artocarpus integrifolia	(Sia)Galβ1-3GalNAcα1-Ser/Thr (T-Antigen)
VVL	Hairy vetch lectin	Vicia villosa	GalNAcα-Ser/Thr (Tn-Antigen)
N-acetylglucosamine binding lectins
WGA	Wheat Germ Agglutinin, WGA	Triticum vulgaris	GlcNAcβ1-4GlcNAcβ1-4GlcNAc, Neu5Ac (sialic acid)
N-acetylneuraminic acid binding lectins
SNA	Elderberry lectin	Sambucus nigra	Neu5Acα2-6Gal(NAc)-R
MAL	Maackia amurensis leukoagglutinin	Maackia amurensis	Neu5Ac/Gcα2,3Galβ1,4Glc(NAc)
MAH	Maackia amurensis hemoagglutinin	Maackia amurensis	Neu5Ac/Gcα2,3Galβ1,3(Neu5Acα2,6)GalNac
Fucose binding lectins
UEA	Ulex europaeus agglutinin	Ulex europaeus	Fucα1-2Gal-R
AAL	Aleuria aurantia lectin	Aleuria aurantia	Fucα1-2Galβ1-4(Fucα1-3/4)Galβ1-4GlcNAc, R2-GlcNAcβ1-4(Fucα1-6)GlcNAc-R1

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What does lectin do ?

Lectins perform recognition on the cellular and molecular level and play numerous roles in biological recognition phenomena involving cells, carbohydrates, and proteins ³⁶⁾, ³⁷⁾. Lectins also mediate attachment and binding of bacteria and viruses to their intended targets.

Endogenous lectins are involved in an enormous variety of biological processes as indicated by an increasing volume of data concerning them ³⁸⁾, ³⁹⁾, ⁴⁰⁾, ⁴¹⁾, ⁴²⁾. A complete and in-depth discussion of the biological significance of lectins is not within the scope of this brief review, however, a discussion of a few mammalian system examples is warranted. Endogenous lectins mediate biological processes such as cell-cell self recognition, cell-extracellular matrix interactions, gamete fertilization, embryonic development, cell growth, cell differentiation, cell signaling, cell adhesion and migration, apoptosis, immunomodulation and inflammation, host-pathogen interactions, glycoprotein folding and routing, mitogenic induction and homeostasis ⁴³⁾, ⁴⁴⁾, ⁴⁵⁾, ⁴⁶⁾, ⁴⁷⁾, ⁴⁸⁾.

Lectins are ubiquitous in nature and are found in many foods. Some foods such as beans and grains need to be cooked or fermented to reduce lectin content, but the lectins consumed in a typical balanced diet are not harmful. Some lectins are beneficial, such as CLEC11A which promotes bone growth, while others may be powerful toxins such as ricin ⁴⁹⁾.

Lectins may be disabled by specific mono- and oligosaccharides, which bind to ingested lectins from grains, legume, nightshade plants and dairy; binding can prevent their attachment to the carbohydrates within the cell membrane. The selectivity of lectins means that they are very useful for analyzing blood type, and they are also used in some genetically engineered crops to transfer traits, such as resistance to pests and resistance to herbicides.

Recently, direct evidence that changes in cell surface carbohydrates are important for the metastatic behavior of tumor cells ⁵⁰⁾. Cell surface carbohydrates affect tumor cell interactions with normal cells or with the extracellular matrix during metastatic spread and growth ⁵¹⁾. These interactions can be mediated via tumor cell carbohydrates and their binding proteins known as endogenous lectins ⁵²⁾. The family of the discovered endogenous lectins is rapidly expanding. The biological significance of the endogenous lectins and their possible role in tumor growth and metastasis formation has started to unravel. Some lectins recognize the ‘foreign’ patterns of cell surface carbohydrates expressed by microorganisms and tumor cells, and play a role in innate and adaptive immunity ⁵³⁾. It was shown that lectins affect tumor cell survival, adhesion to the endothelium or extracellular matrix, as well as tumor vascularization and other processes that are crucial for metastatic spread and growth ⁵⁴⁾.

Figure 1. Schematic illustration of direct lectin targeting (A, left) and reverse lectin targeting (B, right).

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Figure 2. Schematic illustration of bacterial lectins binding to cell surface glycans of a host cell prior to infection (left) and specific carbohydrates or their analogs interfering with the bacterial lectin-host carbohydrate interactions (right).

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Figure 3. Cell surface lectin–carbohydrate interactions.

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Note: Lectins serve as means of attachment of different kinds of cell as well as viruses to other cells via the surface carbohydrates of the latter. In some cases, cell-surface lectins bind particular glycoproteins (e.g., asialoglycoproteins), whereas in other cases the carbohydrates of cell surface glycoproteins or glycolipids serve as sites of attachment for biologically active molecules that themselves are lectins (e.g. carbohydrate-specific bacterial and plant toxins, or galectins).

Table 2. Functions of Lectins

Lectin	Role in
Microorganisms
Amoeba	Infection
Bacteria	Infection
Influenza virus	Infection
Plants
Various	Defense
Legumes	Symbiosis with nitrogen-fixing bacteria
Animals
Calnexin, calreticulin, ERGIC-53	Control of glycoprotein biosynthesis
Collectins	Innate immunity
Dectin-1	Innate immunity
Galectins	Regulation of cell growth and apoptosis; regulation of the cell cycle; modulation of cell–cell and cell–substratum interactions
Macrophage mannose receptor	Innate immunity; clearance of sulfated glycoprotein hormones
Man-6-P receptors	Targeting of lysosomal enzymes
L-selectin	Lymphocyte homing
E- and P-selectins	Leukocyte trafficking to sites of inflammation
Siglecs	Cell-cell interactions in the immune and neural system
Spermadhesin	Sperm-egg interaction

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Table 3. Major Uses of Lectins

Cell identification and separation
Detection, isolation, and structural studies of glycoproteins
Investigation of carbohydrates on cells and subcellular organelles; histochemistry and cytochemistry
Mapping of neuronal pathways
Mitogenic stimulation of lymphocytes^b
Purging of bone marrow for transplantation^b
Selection of lectin-resistant mutants
Studies of glycoprotein biosynthesis

a Lectins from sources other than plants are rarely in use.

b In clinical use.

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Selectin functions

The discovery of the selectins and the demonstration that they play a crucial role in the control of lymphocyte homing and of leukocyte trafficking to sites of inflammation was a landmark in lectin research. Indeed, the selectins provide the best paradigm for the role of sugar–lectin interactions in biological recognition. They mediate the binding of leukocytes to endothelial cells and thereby initiate a rolling phase, in which the lectins interact transiently with glycan ligands, leading eventually to their extravasation. Prevention of adverse inflammatory reactions by inhibition of leukocyte–endothelium interactions, another application of antiadhesion therapy, has become a major aim of the biomedical and pharmacological industry. There are also indications that the selectins may function in the spread of cancer cells from the main tumor to other sites in the body and that by blocking their sugar-binding sites it may be possible to prevent the formation of metastases ⁶⁰⁾.

In the immune system, endogenous lectins are an important component of the host’s defense against invading pathogens ⁶¹⁾, ⁶²⁾, ⁶³⁾, ⁶⁴⁾, ⁶⁵⁾, ⁶⁶⁾. In the innate immune system lectins are able to directly kill microorganisms, or they may aid in the phagocytosis of invading pathogens by dendritic cells and macrophages. Phagocytosis is an actin-dependent mechanism by which cells (phagocytes) ingest large particles that are usually greater than 0.5μm in diameter ⁶⁷⁾. Phagocytic cells are involved in a number of biological processes, including the recognition and control of invading microbes. The phagocytosed pathogens are neutralized and their proteins are processed into small peptides that are presented to T lymphocytes as a peptide-major histocompatibility complex. This antigen presentation activates specific immune responses and, therefore, lectins are also involved in the adaptive immune system ⁶⁸⁾. This is an example of indirect lectin involvement in the adaptive immune system. Lectins, however, are also directly involved in adaptive immunity. Leukocytes express L-selectins, members of the C-type Leptin superfamily, and these L-selectins aid in the homing capabilities of leukocytes ⁶⁹⁾. Interestingly, naïve T lymphocyte expression of L-selectins is high but, once activated, the L-selectin expression is low or lacking altogether. The elevated levels of L-selectin expression by naïve T lymphocytes allow them to migrate to the lymph nodes by binding to specialized high endothelial venules where they mature and become activated when presented with the proper antigens. In the latter case, the lack of L-selectin expression by activated T lymphocytes allows them to migrate and exit at the site of inflammation via high affinity interaction between integrins and their specific ligands. This L-selectin expression level-dependent behavior of T lymphocytes has been demonstrated in studies in which T lymphocytes from tumor bearing mice were restimulated in vitro and selected for their L-selectin expression levels. It was found that T lymphocytes with low L-selectin expression levels efficiently eradicated brain and pulmonary tumors while T lymphocytes with elevated levels of L-selectins demonstrated noticeably reduced tumor clearance abilities ⁷⁰⁾, ⁷¹⁾.

Collectin functions

Collectins, also members of the C-type lectin superfamily, are thought to be involved in the pattern recognition of respiratory viruses and pathogenic bacteria ⁷²⁾. Mannose binding protein is an example of a protective collectin that is able to bind oligomannose residues of bacterial and fungal cell surface oligosaccharides. The structural homology between the C1q component of the complement system and the collagen-like domain of mannose binding protein allows for the initiation of complement fixation upon mannose binding protein-pathogen binding. The initiation of complement fixation is brought about by the activation of mannose binding protein associated serine proteases (MASP-1 and MASP-2). In turn, activated MASPs cleave and activate downstream complement components that eventually neutralize the invading pathogen ⁷³⁾, ⁷⁴⁾. Mannose binding protein is also able to activate the classical and the alternative complement pathways, thus adding additional significance to its host protection role. Furthermore, mannose binding protein activation of complement promotes the formation of C3b and C5b fragments that increase opsonization, phagocytosis and the neutralization of pathogens by macrophages ⁷⁵⁾, ⁷⁶⁾, ⁷⁷⁾. There is some evidence that suggests mannose binding protein may initiate phagocytosis by neutrophils and monocytes directly by binding to bacterial cells ⁷⁸⁾, ⁷⁹⁾. Mannose binding protein compromised individuals are more susceptible to infections, and this emphasizes the importance of mannose binding proteins in the host defense response to pathogenic invasion ⁸⁰⁾, ⁸¹⁾. Another endogenous collectin known as mannose receptor is expressed on macrophage and dendritic cell surfaces ⁸²⁾. Mannose receptors recognize and bind yeasts, mycobacteria and a wide variety of Gram-positive and Gram-negative bacteria ⁸³⁾. Macrophages and dendritic cells, both of which are potent antigen presenting cells ⁸⁴⁾, phagocytose antigen presenting cells expressed mannose receptor bound microorganisms and process the phagocytosed proteins into short peptides that are then presented by MHC (major histocompatibility complex) class I or MHC class II molecules. Antigen presentation by MHC molecules activates T lymphocytes, and activated T lymphocytes then stimulate the adaptive immune system. Collectins such as mannose binding proteins and mannose receptors play active roles in host defense systems against invading pathogens and infection ⁸⁵⁾. It is also possible that mannose binding proteins and mannose receptors are able to target and neutralize malignant cells due to their altered glycan moieties ⁸⁶⁾, ⁸⁷⁾.

Galectin functions

Galectins are a family of soluble lectins that bind β-galactoside-containing glycans and are defined by a conserved carbohydrate recognition domain and a common structural fold ⁸⁸⁾, ⁸⁹⁾. Among the various lectin types, galectins are probably the most conserved and ubiquitous family, with members identified in most animal taxa examined so far ⁹⁰⁾. As many as fifteen galectins have been identified in mammals and proposed to mediate diverse biological processes involved in the regulation of innate and adaptive immune responses, such as cell activation, differentiation, cytokine secretion and apoptosis ⁹¹⁾, ⁹²⁾.

The S-type lectins (galectins), another member of the C-type lectin superfamily, are known to be involved in a wide variety of cellular processes that include pre mRNA splicing, cell growth regulation, cell adhesion, embryogenesis, inflammation, immune function, apoptosis, angiogenesis and tumor metastasis ⁹³⁾. Neoplastic progression has been associated with increased galectin-3 expression in malignancies of the head, neck, gastric or anaplastic large cell lymphoma tumors, thyroid and central nervous system tumors. However, galectin-3 expression has been shown to be down regulated in carcinomas of the uterus, breast and ovary. This suggests that alterations in galectin-3 expression may affect malignant cell interactions with other normal and malignant cells via their corresponding ligands, and thus affect their local growth potential and their potential to metastasize into other anatomical locations ⁹⁴⁾, ⁹⁵⁾, ⁹⁶⁾. Galectin-3 was also shown to have anti-apoptotic effects in galectin-3 cDNA transfected human T cell leukemia Jurkat E6-1 cells ⁹⁷⁾, ⁹⁸⁾. The anti-apoptotic effects of galectin-3 are primarily associated with the C-terminus NWGR motif. This anti-apoptotic activity is abolished with a single amino acid substitution, such as glycine 182 to alanine ⁹⁹⁾, ¹⁰⁰⁾.

In other studies it was reported that some endogenous animal lectins, such as galectin-1 (in humans) and galectin-9 (in mice), have cytotoxic activities and are able to induce thymocyte apoptosis. Galectin induced apoptosis of thymocytes has been associated with the physiological selection processes of thymocyte maturation in the thymus . The cytotoxic activities of these galectins are not isolated to thymocytes and are also applicable to malignant cells as well. It was found that recombinant galectin-1 was able to activate apoptosis in several human B lymphoid cell lines, including Burkitt’s lymphoma, in addition to T cell Jurkat and MOLT-4 ¹⁰¹⁾.

Optimal signal transmission into cells and adhesion require the clustering of ligands and receptors in most systems. Lectin-carbohydrate interactions are no different and thermodynamically favorable assembly of highly ordered clustering arrays are seen. Galectins bind Gal-β-1-nGlcNAc-R by recognizing the β-gal residue, and the binding affinities to N-glycans are associated with the oligosaccharide content of N-acetyllactosamine (LacNAc) and the GlcNAc branching. GlcNAc branching-dependent affinity is important because, as described earlier, cell surface carbohydrates from human and experimental tumors showed that the most prominent alteration in glycoproteins is the presence of larger and extensively branched N-linked β-1,6-GlcNAc oligosaccharides. The β-1,6-GlcNAc branched N-glycans are tri- or tetra-antenna oligosaccharides that enable lattice formation through multivalent oligomerization by galectins and other lectins in general. Lattice formation is preceded by membrane components such as specific glycoproteins, glycolipids and receptors being rearranged into lipid raft microdomains. These lipid raft microdomains are then reorganized by galectin binding during lattice formation. Lattice formation effectively traps receptors at the cell surface and, therefore, regulates the cell surface distribution of these receptors, receptor endocytosis and their activation. For example, the T cell receptor (TCR) α/β is an N-glycan modified by the enzymatic activity of GlcNAc-TV. Glycosylation of the TCR inhibits random nonspecific clustering of TCRs by binding galectin-3. Multivalent TCR-galectin-3 lattices restrict the lateral movement of TCRs within the plasma membrane and, in turn, restrict TCR aggregation at the immune synapse. Multivalent TCR-galectin-3 lattices therefore increase the threshold for TCR activation and by doing so regulate immune response ¹⁰²⁾, ¹⁰³⁾.

Regulatory T cells over-express galectin-1 and galectin-10 that are vital for the suppressive activity of these cells. It is highly possible that the suppressive activity of regulatory T cells by over-expression of galectin-1 contributes to immune system evasion by malignant cells ¹⁰⁴⁾. Although, multivalent lattice formation may be one possible mechanism by which galectin-oligosaccharide interaction regulate cellular processes. The exact mechanistic nature of galectin involvement in cellular processes such as cell growth regulation, cell adhesion, embryogenesis, inflammation, immune function, apoptosis, angiogenesis and tumor metastasis remains unclear. However, a wide body of evidence strongly suggests their involvement in many of these cellular processes in normal and diseased states ¹⁰⁵⁾, ¹⁰⁶⁾, ¹⁰⁷⁾.

Lectin-mediated therapeutics

The concept of lectin-mediated specific drug delivery was proposed by Woodley and Naisbett in 1988 ¹⁰⁸⁾. Delivery of targeted therapeutics via direct and reverse drug delivery systems to specific sites provides numerous advantages over traditional non-targeted therapeutics ¹⁰⁹⁾, ¹¹⁰⁾. Targeted drug delivery increases the efficacy of treatment by enhancing drug exposure to targeted sites while limiting side effects of drugs on normal and healthy tissues. Limiting or preventing side effects in treatments is important because side effects typically lead to reduction in dosage, delay in treatment and therapy termination. Furthermore, specific drug delivery increases the uptake and internalization of therapeutics that have reduced cellular permeability. Direct or reverse targeting relies on identifying and utilizing unique moieties of the targeted site while protecting the active (drug) component during the delivery . In addition to specific moieties, other parameters such as the target environment and the path taken to reach the target must be considered in tailoring lectin-based drug delivery systems ¹¹¹⁾, ¹¹²⁾. Drugs passing through the gastrointestinal tract are susceptible to early activation and degradation by the acidic environment and pancreatic enzymes. Alternatively, drugs administrated via the colon are vulnerable to catabolic assault by enzymes of bacterial origin (e.g. dextranase, pectinase, β-D-xylosidase, β-D-galactosidase, amylase, xylanase and β-D-glucosidase) ¹¹³⁾. However, it is possible to develop drug delivery systems that take advantage of these bacterial enzymes. For example, a drug core in a fermentable carbohydrate coating, drug-carbohydrate conjugates (prodrugs) and drugs embedded in a biodegradable matrix are all possible designs of drugs that can utilize bacterial enzymes ¹¹⁴⁾, ¹¹⁵⁾.

Mistletoe Extracts and Cancer Treatment

Mistletoe is a semiparasitic plant that has been used for centuries to treat numerous human ailments. Mistletoe is used commonly in Europe, where a variety of different extracts are manufactured and marketed as injectable prescription drugs. These injectable drugs are not available commercially in the United States and are not approved as a treatment for people with cancer ¹¹⁶⁾.

Mistletoe holds interest as a potential anticancer agent because extracts derived from it have been shown to kill cancer cells in vitro (taking place in a test tube) ¹¹⁷⁾, ¹¹⁸⁾, ¹¹⁹⁾, ¹²⁰⁾, ¹²¹⁾, ¹²²⁾, ¹²³⁾, ¹²⁴⁾, ¹²⁵⁾, ¹²⁶⁾ and to stimulate immune system cells both in vitro and in vivo (performed or taking place in a living organism) ¹²⁷⁾, ¹²⁸⁾, ¹²⁹⁾, ¹³⁰⁾, ¹³¹⁾, ¹³²⁾, ¹³³⁾, ¹³⁴⁾, ¹³⁵⁾, ¹³⁶⁾, ¹³⁷⁾, ¹³⁸⁾.

Three components of mistletoe, namely viscotoxins, polysaccharides, and lectins, may be responsible for these effects ¹³⁹⁾, ¹⁴⁰⁾, ¹⁴¹⁾, ¹⁴²⁾, ¹⁴³⁾. Viscotoxins are small proteins that exhibit cell-killing activity and possible immune-system-stimulating activity ¹⁴⁴⁾, ¹⁴⁵⁾, ¹⁴⁶⁾, ¹⁴⁷⁾. Lectins are complex molecules made of both protein and carbohydrates that are capable of binding to the outside of cells (e.g., immune system cells) and inducing biochemical changes in them ¹⁴⁸⁾, ¹⁴⁹⁾, ¹⁵⁰⁾, ¹⁵¹⁾, ¹⁵²⁾. In view of mistletoe’s ability to stimulate the immune system, it has been classified as a type of biological response modifier ¹⁵³⁾. Biological response modifiers constitute a diverse group of biological molecules that have been used individually, or in combination with other agents, to treat cancer or to lessen the side effects of anticancer drugs. Mistletoe extracts have been demonstrated in preclinical settings to have other mechanisms of action, such as antiangiogenesis ¹⁵⁴⁾.

Mistletoe is one of the most widely studied complementary and alternative medicine therapies for cancer. In certain European countries, the preparations made from European mistletoe (Viscum album, Loranthaceae) are among the most prescribed drugs offered to cancer patients ¹⁵⁵⁾. Although mistletoe plants and berries are considered poisonous to humans, few serious side effects have been associated with mistletoe extract use.

The use of mistletoe as a treatment for people with cancer has been investigated in clinical studies. Reports of improved survival and/or quality of life have been common, but nearly all of the studies had major weaknesses that raise doubts about the reliability of the findings ¹⁵⁶⁾.

At present, the use of mistletoe cannot be recommended outside the context of well-designed clinical trials. Such trials will be valuable to determine more clearly whether mistletoe can be useful in the treatment of specific subsets of cancer patients ¹⁵⁷⁾.

Preparations from mistletoe extracts are most frequently used in the treatment of cancer patients in German-speaking countries ¹⁵⁸⁾. And the evidence from randomized clinical trials to support the application of mistletoe extracts impact on survival or improved ability to fight cancer or to withstand anticancer treatments is weak. Nevertheless, there is some evidence that mistletoe extracts may offer benefits during chemotherapy for breast cancer, but these results need replication ¹⁵⁹⁾. Overall, more high quality, independent clinical research is needed to truly assess the safety and effectiveness of mistletoe extracts ¹⁶⁰⁾. Patients receiving mistletoe therapy should be encouraged to take part in future trails.

Commercially available extracts are marketed under a variety of brand names, including Iscador, Helixor, Iscucin and Plenosol. Some extracts are marketed under more than one name. Iscador, Isorel, and Plenosol are also sold as Iscar, Vysorel, and Lektinol, respectively. All of these products are prepared from Viscum album (Loranthaceae) (Viscum album L. or European mistletoe). They are not sold as a drug in the United States ¹⁶¹⁾.

In addition to European mistletoe, extracts from a type of Korean mistletoe (Viscum album var. coloratum [Kom.] Ohwi) have demonstrated in vitro and in vivo cytotoxicity in laboratory studies ¹⁶²⁾, ¹⁶³⁾, ¹⁶⁴⁾, ¹⁶⁵⁾, ¹⁶⁶⁾.

Mistletoe grows on several types of trees, and the chemical composition of extracts derived from it depends on the species of the host tree (e.g., apple, elm, oak, pine, poplar, and spruce), the time of year harvested, how the extracts are prepared, and the commercial producer ¹⁶⁷⁾, ¹⁶⁸⁾, ¹⁶⁹⁾, ¹⁷⁰⁾, ¹⁷¹⁾, ¹⁷²⁾.

Mistletoe extracts are prepared as aqueous solutions or solutions of water and alcohol, and they can be fermented or unfermented. Some extracts are prepared according to homeopathic principles, and others are not. Accordingly, as homeopathic preparations, they are typically not chemically standardized extracts. In addition, the commercial products can be subdivided according to the species of host tree, which is typically indicated in the product name by a suffix letter. Iscador, a fermented aqueous extract of Viscum album L. that is prepared as a homeopathic drug, is marketed as IscadorM (from apple trees; Malus domestica), IscadorP (from pine trees; Pinus sylvestris), IscadorQu (from oak trees; Quercus robur), and IscadorU (from elm trees; Ulmus minor). Helixor, an unfermented aqueous extract of Viscum album L. that is standardized by its biological effect on human leukemia cells in vitro, is marketed as HelixorA (from spruce trees; Picea abies), HelixorM (from apple trees), and HelixorP (from pine trees; Pinus sylvestris).

Mistletoe extracts are usually given by subcutaneous injection, although administration by other routes (i.e., oral, intrapleural, intratumoral, and intravenous) has been described. In most reported studies, subcutaneous injections were given 2 to 3 times a week, but the overall duration of treatment varied considerably.

Viscum album is listed in the Homeopathic Pharmacopoeia of the United States, which is the officially recognized compendium for homeopathic drugs in this country ¹⁷³⁾. Although the U.S. Food and Drug Administration (FDA) has regulatory authority over homeopathic drugs, this authority is usually not exercised unless the drugs are formulated for injection or there is evidence of severe toxicity. At present, the FDA does not allow the importation or distribution of injectable preparations of mistletoe, including homeopathic formulations, except for the purpose of clinical research ¹⁷⁴⁾. The extracts are not available commercially in the United States and are not approved as a treatment for people with cancer ¹⁷⁵⁾.

Before researchers can conduct clinical drug research in the United States, they must file an Investigational New Drug (IND) application with the FDA. IND approval is also required for clinical investigation of homeopathic drugs. The FDA does not disclose information about IND applications or approvals; this information can be released only by the applicants. At least two U.S. investigators were given IND approval to study mistletoe as a treatment for people with cancer.

Lectin in Foods

Lectins are clusters of glycoproteins of nonimmune foundation that combine specifically and reversibly to carbohydrates, mainly the sugar moiety of glycoconjugates, resulting in cell agglutination and precipitation of glycoconjugates. Lectins are omnipresent proteins that are possibly there in all eukaryotic and numerous bacterial species as well as in several viruses. They play a vital function in the plants resistance against insect pests and have been found to be deadly to viruses, bacteria, fungi, insects, and prominent animals ¹⁷⁶⁾. Lectin are carbohydrate-binding proteins of non-immune temperament ¹⁷⁷⁾, ¹⁷⁸⁾.

Many plants contain carbohydrate-binding proteins that are commonly designated as lectins, agglutinins, or hemagglutinins. Due to the obvious differences in molecular structure, biochemical properties and carbohydrate-binding specificity, plant lectins are usually considered a complex and heterogeneous group of proteins ¹⁷⁹⁾. Recent advances in the structural analysis of lectins and molecular cloning of lectin genes enable subdividision of plant lectins in a limited number of subgroups of structurally and evolutionary related proteins.

Four major lectin families ¹⁸⁰⁾, namely:

the legume lectins,
the chitin-binding lectins composed of hevein domains,
the type 2 ribosome-inactivating proteins, and
the monocot mannose-binding lectins comprise the majority of all currently known plant lectins.

In addition to these four large families the jacalin-related lectins, the amaranthin family, and the Cucurbitaceae phloem lectins are now recognized as separate subgroups ¹⁸¹⁾.

Lectins are extensively dispersed in nature and ample in plants (see Table 4). Lectins are established in diverse species of major taxonomical groups and most of as seed storage proteins ¹⁸²⁾, ¹⁸³⁾. First lectin to be purified on a large scale and accessible on a trade basis is concanavalin A. Lectins also originated in vegetative organs like roots, leaves, rhizomes, bulbs, tubers, corms, stems, bark, flowers, fruits, phloem sap, latex, and nectar ¹⁸⁴⁾, ¹⁸⁵⁾. Plant lectins possess analogous biological activities and chemical properties. The physiological, biochemical, cellular, and molecular characteristics of lectin show its involvement in plant defense mechanism, e.g., lentil, soybean, garden pea, wheat germ, Griffonia simplicifolia, Viccia graminae, Madura pomifera, Hura crepitans, Psophocarpus tetragonolobus and Abrus precatorius.

Many plant lectins are thought to play a role in the plants defence against being eaten. Accordingly, plant lectins have an obvious preference for binding to sugar structures of animal, fungal or microbial origin, and are usually at highest concentrations in plant parts essential for reproductive success such as seed germs. The intensively studied lectin wheat germ agglutinin, which protects against insects and fungi ¹⁸⁶⁾, is present in wheat seed in both the germ and the gluten part of endosperm ¹⁸⁷⁾. Peptides behaving in a lectin-like manner have also been obtained upon cleavage of gliadin in gluten ¹⁸⁸⁾. Sourdough lactic acid bacteria hydrolyse gliadin peptides and inhibit their lectin-like behaviour ¹⁸⁹⁾, which perhaps explains some of the unexplained health effects of probiotics ¹⁹⁰⁾. White flour consumed by humans contains a high proportion of gluten and has agglutinating activity suggestive of lectins ¹⁹¹⁾, ¹⁹²⁾, ¹⁹³⁾. Thus, lectins are present in our food, they are heat-stable and resistant to breakdown in the gastrointestinal tract, they bind to the surface epithelium of the digestive tract and they can lead to anti-nutritional, mild allergic or other subclinical effects in humans and animals ¹⁹⁴⁾, ¹⁹⁵⁾. Lectins can also be transported through the gut wall into the blood circulation, where they directly influence peripheral tissues and body metabolism through the binding to glycosylated structures, such as the insulin receptor, the epidermal growth factor receptor and the interleukin 2 receptor ¹⁹⁶⁾, ¹⁹⁷⁾. Wheat germ agglutinin have effects on activation of the epidermal growth factor receptor ¹⁹⁸⁾, mitogenesis ¹⁹⁹⁾, agglutination of red blood cells ²⁰⁰⁾, activation of platelets and cell adhesion molecules ²⁰¹⁾ and vascular permeability ²⁰²⁾, ²⁰³⁾. Wheat germ agglutinin also have several effects related to autoimmunity, allergy and inflammation ²⁰⁴⁾, ²⁰⁵⁾. Wheat germ agglutinin binds to several types of mammalian cells including pancreatic duct epithelial cells ²⁰⁶⁾, prostatic cancer cells ²⁰⁷⁾, arterial macrophages and smooth muscle cells ²⁰⁸⁾, ²⁰⁹⁾, glomerular capillary walls, mesangial cells and tubules of human kidney ²¹⁰⁾. Human serum contains antibodies against wheat germ agglutinin and lectins of soybean and peanut ²¹¹⁾. Hence, lectins have sufficient properties to affect the leptin system indirectly, through effects on metabolism central to the proper function of the leptin system, and possibly also directly through interaction with leptin or the leptin receptor ²¹²⁾.

Table 4. Listing of plant lectins

Lectin icon	Lectin name		Source
ConA	Concanavalin A		Canavalia ensiformis
GNA	Snowdrop lectin		Galanthus nivalis
RCA	Ricinus communis Agglutinin,		Ricinus communis
AIL	Jacalin		Artocarpus integrifolia
VVL	Hairy vetch lectin		Vicia villosa
WGA	Wheat Germ agglutinin		Triticum vulgaris
SNA	Elderberry lectin		Sambucus nigra
MAL	Maackia amurensis leukoagglutinin		Maackia amurensis
MAH	Maackia amurensis hemoagglutinin		Maackia amurensis
UEA	Ulex europaeus agglutinin		Ulex europaeus
PNA		Peanut agglutinin	Arachis hypogaea
LCH		Lentil lectin	Lens culinaris
GalPhL		Galactose specific lectin	Phaseolus lunatus
ManPSL		Mannose specific lectin	Pisum sativum
GalDBL		Galactose specific lectin	Dolichos biflorus
GalCaL		Galactose specific lectin	Caragana arborescens

[Source ]

Legumes lectins are one of the chief lectin families out of 70 lectins were reported. The legume lectins are a family of carbohydrate binding lectins found in the seeds of plants belonging to the Fabacaea family. The accurate role of the legume lectins in vivo is unknown but they are perhaps implicated in the defense of plants against predators e.g. insects ²¹⁴⁾.

Toxicity of lectin

Lectins are present in a variety of plants, especially in seeds, where they serve as defense mechanisms against other plants and fungi. Because of their ability to bind to virtually all cell types and cause damage to several organs, lectins are widely recognized as anti-nutrients within food ²¹⁵⁾. Most lectins are resistant to heat and the effects of digestive enzymes, and are able to bind to several tissues and organs in vitro and in vivo ²¹⁶⁾.

The question of the possible physiological role of lectins has intrigued investigators from the start and focused on plant lectins, which for long time were virtually the only ones known. It was speculated, for example, that lectins may function as antibodies to protect plants against harmful soil bacteria, control seed germination, or be involved in the transport and storage of sugars, but no evidence for these speculations could be found. However, two proposals put forward in the 1970s still hold. According to first one, lectins protect plants against phytopathogenic microorganisms and insects as well as against predatory animals. The second theory assumes that they are involved in the association between leguminous plants and their symbiotic nitrogen-fixing bacteria ²¹⁷⁾.

Ingestion of the lectins present in certain improperly cooked vegetables can result in acute gastro inestinal tract distress, but the mechanism of toxicity is unknown ²¹⁸⁾. Plant lectins that are not efficiently degraded by digestive enzymes, and that have an affinity for the surface of gut epithelial cells, such as those present in the legumes, can be poisonous ²¹⁹⁾. Acute symptoms following ingestion include nausea, vomiting and diarrhea. Long-term intake in rodent models is characterized by increased cell turnover, gut hyperplasia and weight loss. Areas of epithelial cell necrosis and even zones of complete epithelial cell denudation are seen in biopsies of the stomach and intestine of rats ²²⁰⁾ and insects ²²¹⁾ fed plant lectins. Indeed, the plant lectin may function as a natural insecticide. Epithelial cell microvilli particularly are affected by lectin exposure, which initiates disruption and shedding of these membrane rich surface projections. Confusingly, however, when cells are treated with lectins in vitro, even at very high doses, necrosis is not observed, though many other responses have been noted including mitogenesis, vacuole formation and inhibition of exocytosis.

The administration of the lectin wheat germ agglutinin to experimental animals caused hyperplastic and hypertrophic growth of the small intestine, hypertrophic growth of the pancreas and thymus atrophy ²²²⁾. Lectin activity has been demonstrated in wheat, rye, barley, oats, corn and rice, however the best studied of the cereal grain lectins is wheat germ agglutinin ²²³⁾. After ingestion, wheat germ agglutinin is capable of crossing the intestinal barrier. In animal models, WGA has been shown to reach the basolateral membrane and walls of the small blood vessels in the subepithelium of the small intestine ²²⁴⁾.

Elevated concentration of lectin in foods, such as beans, cereal grains, seeds, and nuts may be injurious to human and animals ²²⁵⁾. In a laboratory animal study involving mice being fed raw and irradiated diets containing kidney beans as the sole source of protein and providing 10% crude protein produced severe weight loss, followed in some cases by death ²²⁶⁾.

Summary

Lectins are carbohydrate-binding proteins that preferentially recognize and bind carbohydrate complexes protruding from glycolipids and glycoproteins. Lectins play crucial roles in various biological processes such as cellular signaling, malignancy, scavenging of glycoproteins from the circulatory system, cell–cell interactions in the immune system, differentiation and protein targeting to cellular compartments and also, in host defence mechanisms, inflammation, and metastasis. Generally lectins are able to agglutinate erythrocytes and are often referred to as hemagglutinins.

Due to the property of selectivity and specificity, lectins have gained more attention from researchers in identification of cancer and degree of metastasis ²²⁷⁾. Several lectins have been found to possess anticancer properties in vitro, in vivo, and in human case studies; they are used as therapeutic agents, preferentially binding to cancer cell membranes or their receptors, causing cytotoxicity, apoptosis, and inhibition of tumor growth ²²⁸⁾. For example, lectin found in Mistletoe extracts have been used to treat certain cancers. In certain European countries, the preparations made from European mistletoe (Viscum album, Loranthaceae) are among the most prescribed drugs offered to cancer patients ²²⁹⁾. Although mistletoe plants and berries are considered poisonous to humans, few serious side effects have been associated with mistletoe extract use. However, Mistletoe extracts are not available commercially in the United States and are not approved as a treatment for people with cancer ²³⁰⁾. Although lectins seem to have great potential as anticancer agents, further research is still needed.

The galectins are believed to act as modulators of cell–substratum interactions and to be essential for the normal differentiation and growth of all multicellular animals. They are capable of inducing cell proliferation, cell arrest, or apoptosis (physiological cell death) and have been implicated in organ morphogenesis, tumor cell metastasis, leukocyte trafficking, immune response, and inflammation, as well as recognition of extracellular matrix.

Lectins are universally distributed in nature, being established in plants, fungi, viruses, bacteria, crustacea, insects, and animals, but rich source of lectins are present in most plants, especially seeds and tubers like cereals, potatoes, corn, soy and beans. Controversies arise when studies that have been conducted using lab animals are used to present cases of toxicity, leaky bowel syndrome and inflammatory diseases (e.g. rheumatoid arthritis, type 2 diabetes, wheat allergy, celiac disease) ²³¹⁾, ²³²⁾. However, existing evidence regarding food lectins toxicity in humans is inconclusive and is likely to remain so, because until now, human epidemiological and intervention studies investigating the health effects of whole grains and legumes intake were confounded by other dietary and lifestyle factors. The science of lectinology is just beginning.

Moreover, studies involing legumes (beans, peas and lentils) and whole grains have shown legumes and whole grains are a healthy source of proteins, potassium, and complex carbohydrates, including dietary fiber and resistant starch. Legume also possess several beneficial biological activities in humans, as an antioxidant source, cholesterol- and low-density lipoprotein-lowering properties, antimutagenic and anticancer effects as well as effects on cardiovascular disease, diabetes and obesity. Bean consumption has been related to numerous health benefits, such as a decrease in cholesterol levels and cardiac diseases. Beans also offer some protection against cancer, diabetes and obesity, because of their antioxidant, antimutagenic and antiproliferative properties ²³³⁾, ²³⁴⁾. Prospective studies suggest that weight gain and increases in abdominal adiposity over time are lower in people who consume more whole grains. Analyses of the Physicians’ Health Study ²³⁵⁾ and the Nurses’ Health Study ²³⁶⁾ showed that those who consumed more whole grain foods consistently weighed less than those who consumed fewer whole grain foods at each follow-up period of the study. Koh-Banerjee et al. ²³⁷⁾ estimated that for every 40-gram increase in daily whole grain intake, the 8-year weight gain was lower by 1.1 kg. It has also been shown that the intake of whole grains is associated with healthier dietary factors and a healthier lifestyle in general. In a Scandinavian cross-sectional study, the intake of whole grains was directly associated with the length of education, the intake of vegetables, fruits, dairy products, fish, shellfish, coffee, tea and margarine and inversely associated with smoking, BMI and the intake of red meat, white bread, alcohol, cakes and biscuits ²³⁸⁾. Observational prospective and cross-sectional studies show that the intake of whole grain products is associated with reduced risks for developing type 2 diabetes, cardiovascular diseases, obesity and some types of cancer ²³⁹⁾.

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