Differences in amino acid sequences of mistletoe lectin I and III B-subunits determining carbohydrate binding specificity

Quantitative analysis of receptor-mediated uptake and pro-apoptotic activity of mistletoe lectin-1 by high content imaging

Ribosome inactivating proteins (RIPs) are highly potent cytotoxins that have potential as anticancer therapeutics. Mistletoe lectin 1 (ML1) is a heterodimeric cytotoxic protein isolated from European Mistletoe and belongs to RIP class II. The aim of this project was to systematically study ML1 cell binding, endocytosis pathway(s), subcellular processing and apoptosis activation. For this purpose, state of the art cell imaging equipment and automated image analysis algorithms were used. ML1 displayed very fast binding to sugar residues on the membrane and energy-dependent uptake in CT26 cells. The co-staining with specific antibodies and uptake blocking experiments revealed involvement of both clathrin-dependent and -independent pathways in ML1 endocytosis. Co-localization studies demonstrated the toxin transport from early endocytic vesicles to Golgi network; a retrograde road to the endoplasmic reticulum. The pro-apoptotic and antiproliferative activity of ML1 were shown in time lapse movies and subsequently quantified. ML1 cytotoxicity was less affected in multidrug resistant tumor cell line 4T1 in contrast to commonly used chemotherapeutic drug (ML1 resistance index 6.9 vs 13.4 for doxorubicin; IC₅₀: ML1 1.4 ng/ml vs doxorubicin 24000 ng/ml). This opens new opportunities for the use of ML1 as an alternative treatment in multidrug resistant cancers.

Difficulties and perspectives of immunomodulatory therapy with mistletoe lectins and standardized mistletoe extracts in evidence-based medicine

Viscum album preparations are aqueous mistletoe plant extracts used in complementary and alternative medicine as immunomodulators in cancer therapy. However, evidence of immunological efficacy of mistletoe extracts (MEs) used in clinical trials is often lacking. Mechanisms involved in anti-tumor properties of ME and mistletoe lectins (MLs) modify both innate and adaptive immune systems, according to animal model experiments. In the background of these effects, a selective binding of ML on CD75 ganglioside receptors of interleukin 12 (IL-12)-producing macrophages or dendritic cells can play an important role. Immunological effects of ME correlate with their lectin activity, showing a bell-shaped dose-response curve of efficacy. Therefore, a correct determination of MLs for the standardization of commercial ME is essential. However, plant MLs exhibit heterogeneity, which most likely results from post-translational processing. In addition, amino acid analysis of ML has revealed numerous conservative substitutions along their amino acid sequence. Consequently, ML research needs new perspectives, and the advantages and disadvantages of purified and biologically better defined ML preparations are also discussed in this article.

Unexpected Different Binding of Mistletoe Lectins from Plant Extracts to Immobilized Lactose and N-acetylgalactosamine

Mistletoe Extracts (ME) are of growing interest to pharmacological research because of their apoptosis-inducing/cytostatic and immunomodulatory effects. The standardization of the three different groups of Mistletoe Isolectins (ML-I, II and III) is often rendered more difficult since the primary structures are nearly identical. Their classification is based on their Galactose- and N-acetyl-D-galactosamine (GalNAc)-specificity which was measured by various inhibitory assays. The aim of the present study was to improve the characterization of the direct binding activity of the isolectins from ME to immobilized lactose, GalNAc and to the oligosaccharide asialofetuin. After careful ultrafiltration of fresh ME, affinity chromatography was carried out using lactose-agarose, GalNAc—agarose and asialofetuin—affigel 15 columns. MLs were further purified by Sephadex G-100 or by cation exchange chromatography which was adapted to a Fast Protein Liquid Chromatography (FPLC) system. Proteins from both fresh plants and commercial ME were able to bind immobilized lactose to a considerable extent. The majority of this lectin has a B-chain with a Molecular Weight (MW) of 34kD and an A-chain with a MW of 29 kD (ML-I). Only a minor part of the lactose-binding proteins has a lower MW, namely 32kD and 27kD (MLII). However, neither MLs which were eluted from lactose columns, nor the proteins from fresh plant or ME showed a direct binding to the immobilized GalNAc. In spite of this deficiency, GalNAc was able to induce a considerable (25% and 32%) inhibitory effect on their binding to immobilized asialofetuin indicating a discrepancy between the lectin binding and inhibiting effects of GalNAC. Consequently, for an improved standardization of ME more specific sugar molecules are necessary.