Plant Lectins Targeting O-Glycans at the Cell Surface as Tools for Cancer Diagnosis, Prognosis and Therapy

Abstract

Aberrant O-glycans expressed at the surface of cancer cells consist of membrane-tethered glycoproteins (T and Tn antigens) and glycolipids (Lewis a, Lewis x and Forssman antigens). All of these O-glycans have been identified as glyco-markers of interest for the diagnosis and the prognosis of cancer diseases. These epitopes are specifically detected using T/Tn-specific lectins isolated from various plants such as jacalin from Artocarpus integrifola, and fungi such as the Agaricus bisporus lectin. These lectins accommodate T/Tn antigens at the monosaccharide-binding site; residues located in the surrounding extended binding-site of the lectins often participate in the binding of more extended epitopes. Depending on the shape and size of the extended carbohydrate-binding site, their fine sugar-binding specificity towards complex O-glycans readily differs from one lectin to another, resulting in a great diversity in their sugar-recognition capacity. T/Tn-specific lectins have been extensively used for the histochemical detection of cancer cells in biopsies and for the follow up of the cancer progression and evolution. T/Tn-specific lectins also induce a caspase-dependent apoptosis in cancer cells, often associated with a more or less severe inhibition of proliferation. Moreover, they provide another potential source of molecules adapted to the building of photosensitizer-conjugates allowing a specific targeting to cancer cells, for the photodynamic treatment of tumors.

Keywords: Morniga G; O-glycosylation; T antigen; Tn antigen; cancer; diagnosis; lectin; peanut lectin; photodynamic therapy; prognosis.

Figure 1

Molecular structure of the O-glycans expressed on the cancer cell surface. T antigen also occurs as a component of the soluble mucin excreted by both healthy and cancer cells. GlcNAc, N-acetyl d-glucosamine; GalNAc, N-acetyl d-galactosamine; Gal, d-galactose; Neu5Ac, sialic acid; Fuc, l-fucose.

Figure 2

Cartoon showing the clustering of Tn antigens along the peptide chain of the tetra-O-GalNAc glycosylated mucin sequence of the human α-dystroglycan mucin domain peptide (residues 419-PPTTTTKKP-427) (PDB code 2MK7; Borgert A, Foley L, Live D). Cartoon drawn with Chimera [27].

Figure 3

(A,C,E,G) Network of hydrogen bonds and stacking interactions anchoring Tn antigen (Tn) to the monosaccharide-binding site of: Bauhinia forficata BfL lectin (A) (PDB code 5T5J) [46]; soybean lectin SBA (C) (PDB code 4D69) [89]; Sambucus nigra SNA-II lectin (E) (PDB code 3CA6) [74]; and Vicia villosa VVA-B₄ lectin (G) (PDB code 1N47) [90]. Amino acid residues involved in stacking interactions with the disaccharide are colored orange; (B,D,F,H) Docking of Tn antigen to the monosaccharide-binding cavity (green dashed circle) of: Bauhinia forficata BfL lectin (B); soybean lectin SBA (D); Sambucus nigra SNA-II lectin (F); and Vicia villosa VVA-B4 lectin (H). The white dashed lines delineate the extended binding sites at the molecular surface of the different lectins. Cartoons drawn with Chimera [91].

Figure 4

Monosaccharide-binding sites (green dashed circles) and extended binding sites (yellow dashed lines) of: jacalin (Artocarpus integrifolia) (PDB code 1M26) [92] (A); the mushroom Agaricus bisporus lectin ABL (PDB code 1Y2V) [34] (C); the Osage orange (Maclura pomifera) lectin MPA (PDB code 1JOT) [58] (E); and the bitter gourd (Momordica charantia) galactose-specific lectin BGSL (PDB code 4ZGR) [62] (G), in complex with T-antigen (Galβ1→3GalNAcα1→Ser/Thr). Network of hydrogen bonds (dashed lines) anchoring T-antigen (colored cyan) to the amino acid residues of the extended binding site of: jacalin (B); ABL (D); MPA (F); and BGSL (H). Amino acid residues involved in non-polar stacking interactions with the disaccharide are colored orange. Cartoons drawn with Chimera [91].

Figure 5

(A) Network of hydrogen bonds (dashed lines) anchoring Lewis b tetrasaccharide (colored cyan) to the amino acid residues of the monosaccharide-binding site (red dashed circle) of Gs I-A₄ (Griffonia simplicifolia) (PDB code 1LED) [88]. Amino acid residues involved in stacking interactions with the trisaccharide are colored orange. The Gal residue (Gal) of the Lewis b antigen occupies the monosaccharide-binding pocket of the lectin; (B) Molecular surface of Gs I-A₄ showing the monosaccharide-binding site (red dashed circle) and the extended binding site (yellow dashed lines) complexed to the Lewis b trisaccharide. The Gal residue (Gal) of the Lewis b antigen occupies the monosaccharide-binding pocket of the lectin; (C) Network of hydrogen bonds (dashed lines) anchoring the Forssman trisaccharide (colored cyan) to the amino acid residues of the carbohydrate-recognition domain of galectin-9 (PDB code 2EAL) [93]. Amino acid residues involved in stacking interactions with the trisaccharide are colored orange. The red dashed circle delineates the monosaccharide-binding site of the lectin; and (D) Molecular surface of galectin-9 showing the monosaccharide-binding pocket (red dashed circle) and the extended binding site (yellow dashed lines) complexed to the Forssman trisaccharide. The penultimate GalNAc residue (GalNAc) of the Forssman antigen occupies the monosaccharide-binding pocket of the lectin. Cartoons drawn with Chimera [91].

Figure 6

(A) Glycoprotein-microarray technology showing the spotted tumor glycoprotein (TGP) recognized by the lectin probe (L) and visualized by a fluorescent-labeled anti-lectin antibody (FLAB). (B) Lectin-microarray technology showing the spotted lectin (L) recognized by the tumor glycoprotein probe (TGP) and visualized by a fluorescent-labeled anti-glycoprotein antibody (FLAB) (adapted from [114]).

Figure 7

Mechanism of action of photosensitizers. Upon illumination at a selective wavelength (light), the photosensitizer becomes excited (excited singlet state) and reaches, after relaxation, a steady-excited state (excited triplet state) for a longer duration associated with the emission of fluorescence. Collisions with O₂ produce different forms of active oxygen (O₂⁻, ^·OH, and H₂O₂) able to kill the cancer cells.

Figure 8

Lectin conjugated phthalocyanine-PEG gold nanoparticle made of a gold nanoparticle (G) covered with Zn phthalocyanine molecules (P) and polyethylene glycol (PEG) covalently linked to jacalin molecules (J) (adapted from [207]).