Biochemical, histochemical and cell biological investigations on the actions of mistletoe lectins I, II and III with human breast cancer cell lines

European Mistletoe (Viscum album) Extract Is Cytotoxic to Canine High-Grade Astrocytoma Cells In Vitro and Has Additive Effects with Mebendazole

Malignant gliomas are associated with extremely poor clinical outcomes in both humans and dogs, and novel therapies are needed. Glioma-bearing canine patients may serve as promising preclinical models for human therapies, including complementary medicine. The objective of this study was to evaluate the effects of mistletoe extract (Viscum album) alone and in combination with mebendazole in an in vitro model of canine high-grade astrocytoma using the cell line SDT-3G. SDT-3G cells were exposed to a range of concentrations of mistletoe extract alone to obtain an IC50. In separate experiments, cells were exposed to mebendazole at a previously determined IC50 (0.03 µM) alone or in conjunction with varying concentrations of mistletoe extract to determine the additive effects. The IC50 for mistletoe alone was 5.644 ± 0.09 SD μg/mL. The addition of mistletoe at 5 μg/mL to mebendazole at 0.03 µM led to increased cell death compared to what would be expected for each drug separately. The cytotoxicity of mistletoe in vitro and its additive effect with mebendazole support future expanded in vitro and in vivo studies in dogs and supply early evidence that this may be a useful adjunct therapeutic agent for use in glioma-bearing dogs. To the authors’ knowledge, this is the first published report of Viscum album extract in canine glioma.

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

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.

Medicinal Applications of Plant Lectins

Plant lectins are a unique group of proteins and glycoproteins with potent biological activity and have received widespread attention for many years. They can be found in wheat, corn, tomatoes, peanuts, kidney beans, bananas, peas, lentils, soybeans, mushrooms, tubers, seeds, mistletoe and potatoes among many others. Due to their ability to bind reversibly with specific carbohydrate structures and their abundant availability, plant lectins have commonly been used as a molecular tool in various disciplines of biology and medicine. Whilst once thought of being a dietary toxin, the focus on plant lectins has since shifted to understanding the useful properties of these lectins and utilizing them in medicinal applications to advance human health. This chapter reviews the current and potential applications of plant lectins in various areas of medically related research.

MDR-1-overexpression in HT 29 colon cancer cells grown in SCID mice

The multidrug-resistance 1 (MDR-1) P-glycoprotein (Pgp) is a transmembrane transporter system, which actively pumps cytotoxic drugs out of the cell. MDR-1 acquired in vitro differs from MDR-1 acquired in vivo, but has important consequences on the cellular phenotype and metastatic behavior. Here we report that the human colonic cancer cell line HT29 (MDR-1 negative) is more malignant than its MDR-1 overexpressing variant (HT29 MDR-1 positive). HT29 MDR-1 negative cells produce undifferentiated signet ring carcinomas when implanted subcutaneously into SCID mice, while HT29 MDR-1 positive cells form tumors with tubular structures, but without signet ring cells. Immunohistochemical proliferation marker analysis revealed that the MDR-1 positive cells proliferate much more slowly than the MDR-1 negative cells. MDR-1 overexpression results in a less differentiated phenotype at the cellular level (absence of mucin producing cells) but in a more differentiated phenotype at the tissue level (tubule formation). In addition, lectin binding patterns including that of Helix pomatia agglutinin (HPA), an indicator of metastatic potential, differed between the two cell lines. HT29 MDR-1 positive cells had less HPA binding sites than HT29 MDR-1 negative counterparts and metastasized less frequently in SCID mice. As slow proliferation, low degree of differentiation and multidrug-resistance is a hallmark of cancer stem cells and all were present in MDR-1 positive tumors, it is attractive to speculate that they represent a stem cell rich tumor. As shown by global gene expression analyses, genes involved, e.g. in cell adhesion, glycosylation and signal transduction, were deregulated in MDR-1 positive tumors compared to MDR-negative tumors. Overexpression of E-cadherin and carcinoembryonic antigen-related cell adhesion molecules 1 (CEACAM1) may provide clues to the mechanisms responsible for the reduced metastatic potential of MDR-1 overexpressing tumors. Since drug treatment shifted the cells towards a less metastatic phenotype in this in vivo model, it seems conceivable to achieve this using drug treatment also in a clinical situation.

Isolation and characterization of two Korean mistletoe lectins

Two isolectins (KML-IIU and the KML-IIL) were individually isolated from the previously reported Korean mistletoe lectin, KML-C, by using an immunoaffinity column. Molecular weights of the KML-IIU and the KML-IIL were 64 kDa and 60 kDa respectively. Both of the lectins were composed of heterogeneous A and B subunits linked with a disulfide bond, and showed the same carbohydrate-binding specificities for Gal and GalNAc. However, they are different not only in biophysical properties (glycosylation and amino acid compositions) but also bioactivities (cell killing and cytokine induction). The KML-IIL showed 17-145 times stronger in cytotoxicities to various human and mouse cancer cell lines than the KML-IIU. The KML-IIL also induced TNF-alpha secretion from mouse peritoneal macrophages 4.5 times better than the KML-IIU. The results demonstrated isolectins in Korean mistletoe were varied in bioactivities and the KML-IIL may be developed as an anti-cancer agent.

Tumor-associated CD75s gangliosides and CD75s-bearing glycoproteins with Neu5Acalpha2-6Galbeta1-4GlcNAc-residues are receptors for the anticancer drug rViscumin

The anticancer drug rViscumin, currently under clinical development, has been shown in previous studies to be a sialic acid specific ribosome inactivating protein (RIP). Comparative binding assays with the CD75s-specific monoclonal antibodies HB6 and J3-89 revealed rViscumin to be a CD75s-specific RIP due to identical binding characteristics toward CD75s gangliosides. The receptor gangliosides are IV6nLc4Cer, VI6nLc6Cer, and the newly characterized ganglioside VIII6nLc8Cer, all three carrying the Neu5Acalpha2-6Galbeta1-4GlcNAc motif. To elucidate the clinical potential of the rViscumin targets, CD75s gangliosides were determined in several randomly collected gastrointestinal tumors. The majority of the tumors showed an enhanced expression of CD75s gangliosides compared with the unaffected tissues. The rViscumin binding specificity was further investigated with reference glycoproteins carrying sialylated and desialylated type II N-glycans. Comparative Western blots of rViscumin and ricin, an rViscumin homologous but galactoside-specific RIP, revealed specific recognition of type II N-glycans with CD75s determinants by rViscumin, whereas ricin failed to react with terminally sialylated oligosaccharides such as CD75s motifs and others. This strict binding specificity of rViscumin and the increased expression of CD75s gangliosides in various tumors suggest this anticancer drug as a promising candidate for an individualised adjuvant therapy of human tumors.

Preparation of alginate/chitosan microcapsules and enteric coated granules of mistletoe lectin

The aqueous extract of European mistletoe (Viscum album, L.) has been used in cancer therapy. The purified mistletoe lectins, main components of mistletoe, have demonstrated cytotoxic and immune-system-stimulating activities. Korean mistletoe (Viscum album L. coloratum), a subspecies of European mistletoe, has also been reported to possess anticancer and immunological activities. A galactose- and N-acetyl-D-galactosamine-specific lectin (Viscum album L. coloratum agglutinin, VCA) with Mr 60 kDa was isolated from Korean mistletoe. Mistletoe preparations have been given subcutaneously due to the low stability of lectin in the gastrointestinal (GI) tract. In the present study, we investigated the possibility of alginate/chitosan microcapsules as a tool for oral delivery of mistletoe lectin. In addition, our strategy has been to develop a system composed of stabilizing cores (granules), which contain mistletoe lectin, extract or powder, coated by a biodegradable polymer wall. Our results indicated that successful incorporation of VCA into alginate/chitosan microcapsules has been achieved and that the alginate/chitosan microcapsule protected the VCA from degradation at acidic pH values. And coating the VCA with polyacrylic polymers, Eudragit, produced outstanding results with ideal release profiles and only minimal losses of cytotoxicity after manufacturing step. The granules prepared with extract or whole plant produced the best results due to the stability in the extract or whole plant during manufacturing process.

The cytotoxic effect of mistletoe lectins I, II and III on sensitive and multidrug resistant human colon cancer cell lines in vitro

Multidrug resistance glycoprotein1 (MDR-1) eliminates amphiphilic chemotherapeutic agents out of tumour cells leading to therapeutic failures. The aim of this study was to investigate the cytotoxic effect of mistletoe lectins (MLs) I, II and III on the sensitive human colon cancer cell line HT 29(mdr-), its multidrug resistant variant HT 29(mdr+), the variant HT 29(SF1m) transfected with the MDR-1 gene and its sensitive control cell line HT 29(deltaSF). Both cell proliferation and ML binding pattern were analysed. Marked quantitative differences concerning the cytotoxic effect of the three MLs on the different cell lines were observed. All MLs showed the greatest cytotoxicity towards the HT 29(mdr+) cells, in which multidrug resistance (MDR) was induced by increasing concentrations of a MDR inducing agent. In contrast, MDR-1 and mock-transfected cells showed almost the same sensitivity towards the three MLs as the control cells (HT 29(mdr-)). FACS analysis showed that the HT 29(mdr+) cells were the cells with the highest density of ML binding sites. Thus, higher sensitivity of HT 29(mdr+) cells are not caused by the overexpression of MDR-1, but are caused by the general changes of the cellular glycosylation during the acquisition of the MDR phenotype.

Comparative study of Korean (Viscum album var. coloratum) and European mistletoes (Viscum album)

A lectin (agglutinin, VCA) from Korean mistletoe (Viscum album L. coloratum) was isolated by affinity chromatograpy on a asialofetuin-Sepharose 4B. The molecular weights of A- and B-chains of VCA were differenf from those of VAAS. The VCA recognized the antibody of VAAs in the Western blot analysis and ELLA system. We also investigated the synergistic effects of the components in mistletoe by dividing the extract into different molecular weight fractions.

Recombinant mistletoe lectin (rML) is successful in treating human ovarian cancer cells transplanted into severe combined immunodeficient (SCID) mice

The main active constituents carrying the anti-cancer activities of aqueous mistletoe extracts have recently been identified as the mistletoe lectins (MLs). Although three different isolectins have been isolated from plant extracts, molecular biological techniques have revealed the presence of one gene only. Subsequently, recombinant mistletoe lectin (rML) has become available and the aim of the present study was to analyse its anti-cancer potential. SoTü 3 human ovarian cancer cells (2x10(7)) were injected intraperitoneally into SCID mice, while rML treatment was started on the following day. Three experimental groups (n=20 SCID mice) each received every working day an intraperitoneal injection of 30, 150, 500 ng rML per kg body weight, respectively, while 20 SCID mice in the control group received the vehicle solution only. The survival of the animals was taken as the principal outcome measure. In addition, the peritoneal cavity was searched for the presence of tumour cells. Animals were sacrificed when the weight increase due to the development of ascites exceeded 120% of the initial body weight. The treatment continued until day 83 and the surviving animals were sacrificed 84 days after inoculation. In the control group, only two animals survived and were free of tumour at the end of the experiment at 84 days. In contrast, thirteen animals in the 500 ng/kg rML group were still alive and no evidence for the presence of tumour cells in the peritoneum was found. Both, number of animals surviving and survival time were larger in this treatment group. The 30 ng/kg rML group showed an increased number of survivors, whilst the 150 ng rML per kg body weight group revealed the worst survival rates. The results of the present study indicate that rML has potent anti-tumour activity, if administered locally into the peritoneum of a human ovarian cancer harbouring SCID mouse. RML is a macromolecule and its instillation into the peritoneal cavity seems to be particularly effective to inhibit intraperitoneal growth of cancer cells. One explanation might be that altered glycosylation of the cancer cells leads to an increased affinity of rML towards tumour cells. Clinical studies with post-operative instillation of rML in ovarian cancer patients should, therefore, be encouraged to provide clinical evidence for the effectiveness of rML treatment.

Mistletoe lectins I, II and III induce the production of cytokines by cultured human monocytes

The three mistletoe (Viscum album L.) lectins. ML I, ML II and ML III, were tested on their ability to enhance the secretion of the cytokines tumor necrosis factor (TNF)alpha, interleukin (IL)-1 alpha, IL-1 beta and IL-6 by human monocytes obtained from healthy donors. At lectin concentrations from 0.02 to 20/pg ml (100-10,000-fold lower than those showing toxic effects), stimulations of cytokine production several-fold over control values were observed. The immunoactivating concentrations by the three lectins were found different for each donor. At toxic concentrations, the amounts of IL-1 alpha, IL-1 beta and to a less extent of TNF alpha in monocytes supernatants were particularly high. The data are discussed in relationship with the cytotoxic and immunoactivating effects of mistletoe lectins and their interest in cancer treatment.