Mistletoe Lectin I Is a Sialic Acid-Specific Lectin with Strict Preference to Gangliosides and Glycoproteins with Terminal Neu5Acα2−6Galβ1−4GlcNAc Residues

Abstract

Mistletoe lectin I (ML-I) is a type II ribosome-inactivating protein, which inhibits the protein biosynthesis at the ribosomal level. ML-I is composed of a catalytically active A-chain with rRNA N-glycosidase activity and a B-chain with carbohydrate binding specificities. Using comparative solid-phase binding assays along with electrospray ionization tandem mass spectrometry, ML-I was shown to preferentially bind to terminally alpha2-6-sialylated neolacto series gangliosides from human granulocytes. IV(6)Neu5Ac-nLc4Cer, VI(6)Neu5Ac-nLc6Cer, and VIII(6)Neu5Ac-nLc8Cer were identified as ML-I receptors, whereas the isomeric alpha2-3-sialylated neolacto series gangliosides were not recognized. Only marginal binding of ML-I to terminal galactose residues of neutral glycosphingolipids with a Galbeta1-4Glc or Galbeta1-4GlcNAc sequence was determined, whereas a distal Galalpha1-4Gal, GalNAcbeta1-3Gal, or GalNAcbeta1-4Gal disaccharide did not bind at all. Among the glycoproteins investigated in Western blot and microwell adsorption assays, only those carrying Neu5Acalpha2-6Galbeta1-4GlcNAc residues, exclusively, predominantly, or even as less abundant constituents in an assembly with Neu5Acalpha2-3Galbeta1-4GlcNAc-terminated glycans, displayed high ML-I binding capacity. From our data we conclude that (i) ML-I has to be considered as a sialic acid- and not a galactose-specific lectin and (ii) neolacto series gangliosides and sialoglycoproteins with type II glycans, which share the Neu5Acalpha2-6Galbeta1-4GlcNAc terminus, are true ML-I receptors. This strict preference might help to explain the immunostimulatory potential of ML-I toward certain leukocyte subpopulations and its therapeutic success as a cytotoxic anticancer drug.

Can a synergistic activation of pattern recognition receptors by plant immunomodulators enhance the effect of oncologic therapy? Case Report of a patient with uterus and ovary sarcoma

“…Therefore, it is noteworthy referring to previous case reports [3] of six patients with sarcoma, showing astonishing remission under an immunomodulatory treatment using plant extracts which were standardized as to their mistletoe lectin (ML) content. As reported previously [4,5], ML represents a Pathogenic Associated Molecular Pattern (PAMP) like structure originating from plant leaves and stem which can bind appropriate Pattern Recognition Receptor (gangliosides with terminal Neu5Ac alpha 2-6Gal beta 1-4GlcNAc residues) on membrane of the type-1 phagocytic cells (such as M1 macrophages or CD1a dentritic cells).As known, PAMP -PRR (in the case of MLit is a lectin-sugar) interaction on membrane of type-1 phagocytic cell generate proinflammatory cytokines and IL-12 activating cytotoxic effector cells, such as Natural Killer (NK) and NK-T cells which are potent inhibitor of tumor growth [6][7][8][9][10][11]. Since NK cells are regularly present in peripheral blood, they are very suitable for monitoring the activation level of type-1 cellular cascades in innate immune system.…”

Tumor-associated CD75s- and iso-CD75s-gangliosides are potential targets for adjuvant therapy in pancreatic cancer

“…The TLC immunodetection procedure using primary chicken anti-GSL antibodies in conjunction with secondary alkaline phosphatase -labeled anti-chicken IgY antibodies was employed as described previously (14,16,17,24). Binding of viscumin and rViscumin toward CD75s-gangliosides was detected with the monoclonal mouse IgG1 antibody TA5, which recognizes the A-chain of both lectins, and secondary alkaline phosphatase -labeled antimouse IgG antibodies (16,17).…”

“…Binding of viscumin and rViscumin toward CD75s-gangliosides was detected with the monoclonal mouse IgG1 antibody TA5, which recognizes the A-chain of both lectins, and secondary alkaline phosphatase -labeled antimouse IgG antibodies (16,17).…”

“…CD75s-gangliosides are specific receptors of the ribosomeinactivating protein viscumin and its recombinant counterpart rViscumin (16,17). As a potential anticancer drug, rViscumin is currently under clinical development and has successfully passed clinical phase I trials (18,19).…”

Riproximin’s activity depends on gene expression and sensitizes PDAC cells to TRAIL

“…The well-known type II RIP ricin, e.g., reacts with galactose residues, 3 but the type II RIP mistletoe lectin I is more specific and reacts preferentially with terminally sialylated neolacto series gangliosides. 4 For riproximin, a significant affinity was found for bi-and tri-antennary N-glycan structures including CEA, as well as to O-glycan structures with three consecutive O-linked GalNAcα moieties. 5 The high toxicity after internalization of these RIPs is related to cell death resulting from apoptosis induction.…”