Induction of apoptosis in human lymphocytes treated with Viscum album L. is mediated by the mistletoe lectins

Viscum album L. (VAL) is a phytopreparation used in adjuvant cancer therapy with both immunostimulatory and DNA stabilizing properties at low drug concentrations and cytostatic/cytotoxic properties at higher concentrations. The present work examines the cytotoxic effects of VAL extracts produced from mistletoes grown on different host trees and of purified toxic proteins from VAL, such as the D-galactose-specific lectin I (ML I), the N-acetyl-D-galactosamine-specific ML II and ML III, and crude viscotoxins towards cultured human lymphocytes. The decrease in the number of cultured lymphocytes and blast cells treated with whole plant extracts from VAL was host tree-specific. Nevertheless, there was no close correlation to the content of MLs or viscotoxins. Using the purified proteins, it became obvious that the cell killing was mediated by the induction of apoptosis, as measured by the appearance of a hypodiploid DNA peak using flow cytometry. ML III was the most effective to induce apoptosis, followed by ML II and ML I, while the viscotoxins and oligosaccharides from VAL did not. By measuring the surface expression of IL-2R alpha chains, transferrin receptors and APO-1/Fas molecules on non-apoptotic T cells, no significant changes were observed at low ML concentrations (1 ng/ml), but their decrease at higher ones. Our findings suggest that there might be at least two different ways of cell killing operative in VAL-mediated cytotoxicity: (a) the typical apoptotic cell death with the appearance of hypo-diploid nuclei, and (b) a direct or indirect killing by damaging the cell membrane with subsequent influx of Ca2+ and of the DNA intercalating dye propidium iodide and cell shrinkage. These effects might not be exclusive, as they probably occur simultaneously.

Molecular mechanisms of mistletoe plant extract-induced apoptosis in acute lymphoblastic leukemia in vivo and in vitro

Viscum album (Mistletoe) is one of the most widely used alternative cancer therapies. Aqueous mistletoe extracts (MT) contain the three mistletoe lectins I, II and III as one predominant group of biologically active agents. Although MT is widely used, there is a lack of scientifically sound preclinical and clinical data. In this paper, we describe for the first time the in vivo efficacy and mechanism of action of MT in lymphoblastic leukemia. For this purpose, we first investigated both the cytotoxic effect and the mechanism of action of two standardized aqueous MTs (MT obtained from fir trees (MT-A); MT obtained from pine trees (MT-P)) in a human acute lymphoblastic leukemia (ALL) cell line (NALM-6). MT-A, MT-P and ML-I inhibited cell proliferation as determined by Casy Count analysis at very low concentrations with MT-P being the most cytotoxic extract. DNA-fragmentation assays indicated that dose-dependent induction of apoptosis was the main mechanism of cell death. Finally, we evaluated the efficacy of MT-A and MT-P in an in vivo SCID-model of pre-B ALL (NALM-6). Both MTs significantly improved survival (up to 55.4 days) at all tested concentrations in contrast to controls (34.6 days) without side effects.

Characterization of recombinant and plant-derived mistletoe lectin and their B-chains

Mistletoe lectin I (pML) and its isoforms ML II and III constitute the active principle in extract preparations from mistletoe, commonly used as immunomodulator in adjuvant tumour therapy. The heterodimeric disulfide-linked cytotoxic protein is classified as type II ribosome inactivating protein (RIP). Recently, the sequence coding for the mistletoe lectin prepro-protein was identified and the existence of a single intron-free gene was shown [Eck, J., Langer, M., Möckel, B., Baur, A., Rothe, M., Zinke, H. & Lentzen, H. (1999) Eur. J. Biochem. 264, 775-784]. The aim of this study was to prepare pure and homogeneous rMLB-chain as well as rML heterodimer for studying the carbohydrate binding specificity of recombinant versus natural protein and its contribution to the observed cytotoxic effect. Expression in E. coli resulted in the production of insoluble protein (inclusion bodies). A procedure for generating correctly folded, biochemically and biologically active rMLB was established starting from the insoluble single chain. Carbohydrate binding and specificity of pMLB and rMLB were analysed by a competitive enzyme linked lectin assay (ELLA). Asialofetuin was able to compete with binding of both chains (50% at 0.8 microM). The specificity of the B-chains to lactose was more distinct with halfmaximal competition at 4.9 mM (pMLB) and > 90 mM (rMLB), respectively. Furthermore, in a coassociation process rMLA- and rMLB inclusion bodies were associated in one step by defined dilution yielding active rML-heterodimer. The activities of recombinant (rML) and plant derived mistletoe lectin (pML) were compared. Cytotoxicity was determined using MOLT-4 cells and enzymatic rRNA N-glycosidase activity was measured in a coupled transcription/translation assay. The IC50 values of the two heterodimers were similar in both assays; rMLB-chain did not show any cytotoxic effect. In the ELLA with lactose as a competitor 50% competition of binding to asialofetuin was achieved at 1.6 mM (rML) and 1.8 mM (pML). Hence, using three different assays we found no significant differences between the recombinant protein and the glycosylated form of ML. Comparing the biological activities of the single chains with those of the heterodimer we conclude, that both, lectin activity and the rRNA N-glycosidase activity, are prerequisites for the cytotoxic effects on target cells.

Probing the cons and pros of lectin-induced immunomodulation: case studies for the mistletoe lectin and galectin-1

When imagining to monitor animal cells through a microscope with resolution at the molecular level, a salient attribute of their surfaces will be the abundance of glycan chains. They present galactosides at their termini widely extending like tentacles into the extracellular space. Their spatial accessibility and their potential for structural variability endow especially these glycan parts with capacity to act as docking points for molecular sensors (sugar receptors such as lectins). Binding and ligand clustering account for transmission of post-binding signals into the cell interior. The range of triggered activities has turned plant lectins into popular tools in cell biology and immunology. Potential for clinical application has been investigated rigorously only in recent years. As documented in vitro and in vivo for the galactoside-specific mistletoe lectin, its apparent immunomodulatory capacity reflected in upregulation of production of proinflammatory cytokines will not necessarily be clinically favorable but a double-edged sword. In fact, lectin application has been shown to stimulate tumor growth in cell lines, histocultures of human tumors and in two animal models using chemical carcinogenesis or tumor transplantation. When testing immunological effects of the endogenous lectin galectin-1, protection against disorders mediated by activated T cells came up for consideration. Elimination of these cells via CD7-dependent induction of apoptosis, and a shift to the Th2 response by the galectin, are factors to ameliorate disease states. This result encourages further efforts with other galectins. Functional redundancy, synergism, diversity or antagonism among galectins are being explored to understand the actual role of this class of endogenous lectins in inflammation. Regardless of the results of further preclinical testing for galectin-1, these two case studies break new ground in our understanding how glycans as ligands for lectins convey reactivity to immune cells, with impact on the course of a tumor or autoimmune disease.

Mistletoe lectin induces apoptosis and telomerase inhibition in human A253 cancer cells through dephosphorylation of Akt

Mistletoe lectin has been reported to induce apoptosis in different cancer cell lines in vitro and to show antitumor activity against a variety of tumors in animal models. We previously demonstrated the Korean mistletoe lectin (Viscum album var. coloratum, VCA)-induced apoptosis by down-regulation of Bcl-2 and telomerase activity and by up-regulation of Bax through p53- and p21-independent pathway in hepatoma cells. In the present study, we observed the induction of apoptotic cell death through activation of caspase-3 and the inhibition of telomerase activity through transcriptional down-regulation of hTERT in the VCA-treated A253 cells. We also observed the inhibition of telomerase activity and induction of apoptosis resulted from dephosphorylation of Akt in the survival signaling pathways. In addition, combining VCA with the inhibitors of phosphatidylinositol 3-kinase (PI3-kinase) upstream of Akt, wortmannin and LY294002 showed an additive inhibitory effect of telomerase activity. In contrast, the inhibitor of protein phosphatase 2A (PP2A), okadaic acid inhibited VCA-induced dephosphorylation of Akt and inhibition of telomerase activity. Taken together, VCA induces apoptotic cell death through Akt signaling pathway in correlated with the inhibition of telomerase activity and the activation of caspase-3. From these results, together with our previous studies, we suggest that VCA triggers molecular changes that resulting in the inhibition of cell growth and the induction of apoptotic cell death of cancer cells, which suggest that VCA may be useful as chemotherapeutic agent for cancer cells.

Botanical medicine and cancer: a review of the safety and efficacy

It is currently estimated that > 50% of all patients diagnosed with cancer explore complementary and alternative medicine - especially herbal medicine. We conducted a comprehensive review to assess the safety and efficacy of herbal medicines commonly used by patients in an attempt to: prevent cancer; treat cancer; and treat adverse effects associated with conventional cancer treatments. Current evidence suggests that Asian ginseng, garlic, green tea, tomatoes and soy intake as part of the diet may be useful in preventing various cancers; additional research is needed in order to determine the efficacy of essiac, evening primrose oil, mistletoe, reishi, shiitake and turmeric as cancer treatments; and ginger may be effective in treating chemotherapy-induced nausea and vomiting.

Preclinical and clinical effects of mistletoe against breast cancer

Breast cancer is among the most frequent types of cancer in women worldwide. Current conventional treatment options are accompanied by side effects. Mistletoe is amongst the important herbal medicines traditionally used as complementary remedies. An increasing number of studies have reported anticancer activity of mistletoe extracts on breast cancer cells and animal models. Some recent evidence suggests that cytotoxic activity of mistletoe may be mediated through different mechanisms. These findings provide a good base for clinical trials. Various studies on mistletoe therapy for breast cancer patients revealed similar findings concerning possible benefits on survival time, health-related quality of life (HRQoL), remission rate, and alleviating adverse reactions to conventional therapy. This review provides an overview of the recent findings on preclinical experiments and clinical trials of mistletoe for its cytotoxic and antitumor activity and its effect on HRQoL in breast cancer patients. Moreover, studies investigating molecular and cellular mechanisms underlying antitumor activity of mistletoe are discussed in this paper. The analyzed trials provided evidence that there might be a combination of pharmacological and motivational aspects mediated by the mistletoe extract application which may contribute to the clinical benefit and positive outcome such as improved HRQoL and self-regulation in breast cancer patients.

Prophylactic effect of Korean mistletoe (Viscum album coloratum) extract on tumor metastasis is mediated by enhancement of NK cell activity

We here demonstrated the prophylactic effect of an extract (KM-110) from Viscum album coloratum, a Korean mistletoe, on tumor metastasis produced by highly metastatic tumor cells, colon 26-M3.1 carcinoma, B16-BL6 melanoma and L5178Y-ML25 lymphoma cells, using experimental models in mice. Intravenous (i.v.) administration of KM-110 (100 microg/mouse) 2 days before tumor inoculation significantly inhibited lung metastasis of B16-BL6 and colon 26-M3.1 cells, and liver and spleen metastasis of L5178Y-ML25 cells. The prophylactic effect of KM-110 on tumor metastasis was evident with various administration routes, i.e. subcutaneous, oral, intranasal as well as i.v., and was dependent upon the dose of KM-110 administered. Furthermore, mice given KM-110 (100 microg) 2 days before tumor inoculation showed significantly prolonged survival rates compared with the untreated mice. In a time course analysis of NK activity, i.v. administration of KM-110 (100 microg) significantly augmented NK cytotoxicity to Yac-a tumor cells from 1 to 3 days after KM-110 treatment. Furthermore, depletion NK cells by injection of rabbit anti-asialo GM1 serum completely abolished the inhibitory effect of KM-110 on lung metastasis of colon 26-M3.1 cells. These results suggest that KM-110 possesses immunopotentiating activity which enhances the host defense system against tumors, and that its prophylactic effect on tumor metastasis is mediated by NK cell activation.

Mechanism of stimulation of human natural killer cytotoxicity by arabinogalactan from Larix occidentalis

Cultures of human peripheral blood mononuclear cells (PBMC) as well as cultures of preseparated peripheral non-adherent cells (PNAC) and monocytes showed enhancement of natural killer (NK) cytotoxicity against K562 tumor cells when pretreated with arabinogalactan from Larix occidentalis for 48-72 h. Lack of enhanced responses of PBMC (37% of donors) did not necessarily mean that PNAC and monocyte cultures were also non-responsive to arabinogalactan treatment. Moreover, PBMC, PNAC and monocytes of individual donors could exhibit various responses to arabinogalactan when cultures derived from bleedings after intervals of several months were assayed. Arabinogalactan-mediated enhancement of NK cytotoxicity was not initiated directly but was found to be governed by the cytokine network. Generally, arabinogalactan pretreatment induced an increased release of interferon gamma (IFN gamma), tumor necrosis factor alpha, interleukin-1 beta (IL-1 beta) and IL-6 but only IFN gamma was involved in enhancement of NK cytotoxicity since cytotoxicity enhancement of PBMC and PNAC but not that of monocytes could be blocked when anti-IFN gamma antibodies were present during pretreatment. The presence of anti-IL-2 antibodies completely blocked NK cytotoxicity enhancement of PBMC and only moderately that of PNAC and monocytes. This blocking effect was also observed when no detectable increase of IL-2 release could be recorded. The receptor specificity of arabinogalactan is not well characterized. Initial information obtained from comparative studies indicated that arabinogalactan presumably interacts with a receptor that showed specificity for a NK-cytotoxicity-enhancing oligo-saccharide from Viscum album extracts since the action of both components was not synergistic but rather competitive.

Recombinant mistletoe lectin induces p53-independent apoptosis in tumour cells and cooperates with ionising radiation

Mistletoe extracts are used as alternative cancer treatment in addition to standard chemotherapy and radiation treatment and have an immunostimulatory and pain-relieving effect. A direct antitumour effect of mistletoe extracts against tumour cells of lymphoid origin has been linked to the D-galactoside-specific mistletoe lectin I. In this study, we investigated the cellular effect of bacterially expressed, recombinant mistletoe lectin alone or in combination with ionising radiation in a genetically defined p53-wild-type and p53-deficient E1A/ras-transformed murine tumour cells system. Downregulation of the proliferative activity and cell killing by recombinant mistletoe lectin occurred in a clear dose response (0.1-1 ng ml(-1)). Induction of apoptosis was p53-independent, but apoptosis-associated factor-1-dependent. Cellular treatment with lectin in combination with ionising radiation resulted in both p53-wild-type and p53-deficient tumour cells in an at least additive, antiproliferative effect and enhanced activation of caspase-3. Combined treatment with ionising radiation and lectin revealed a similar cytotoxic effect in human, p53-mutated adenocarcinoma cells. Thus, recombinant mistletoe lectin alone and in combination with ionising radiation bypasses often prevalent apoptotic deficiencies in treatment-resistant tumour cells.

Crystal structure of mistletoe lectin I from Viscum album

The crystal structure of the ribosome-inactivating protein (RIP) mistletoe lectin I (ML-I) from Viscum album has been solved by molecular replacement techniques. The structure has been refined to a crystallographic R-factor of 24.5% using X-ray diffraction data to 2.8 A resolution. The heterodimeric 63-kDa protein consists of a toxic A subunit which exhibits RNA-glycosidase activity and a galactose-specific lectin B subunit. The overall protein fold is similar to that of ricin from Ricinus communis; however, unlike ricin, ML-I is already medically applied as a component of a commercially available misteltoe extract with immunostimulating potency and for the treatment of human cancer. The three-dimensional structure reported here revealed structural details of this pharmaceutically important protein. The comparison to the structure of ricin gives more insights into the functional mechanism of this protein, provides structural details for further protein engineering studies, and may lead to the development of more effective therapeutic RIPs.

Inhibitory effect of Korean mistletoe (Viscum album coloratum) extract on tumour angiogenesis and metastasis of haematogenous and non-haematogenous tumour cells in mice

We examined the inhibitory effect of an aqueous extract (referred to as KM-110) from Viscum album coloratum, a Korean mistletoe, on tumour metastasis produced by highly metastatic murine tumour cells, B16-BL6 melanoma, colon 26-M3.1 carcinoma and L5178Y-ML25 lymphoma cells, using experimental and spontaneous metastasis models in syngeneic mice. In experimental metastasis of B16-BL6 and colon 26-M3.1 cells, intravenous (i.v.) administration of KM-110 (100 micrograms/mouse) 1 day after tumour inoculation significantly inhibited lung metastasis of both tumour cells. The administration of KM-110 also exhibited a therapeutic effect on liver and spleen metastasis of L5178Y-ML25 lymphoma cells. Furthermore, in spontaneous metastasis of B16-BL6 melanoma cells, multiple administration of KM-110 into tumour-bearing mice resulted in significant inhibition of lung metastasis by tumour cells, as well as the suppressive activity to the growth of primary tumour. In in vivo analysis for tumour-induced angiogenesis, the i.v. administration of KM-110 suppressed tumour growth and inhibited the number of blood vessels oriented towards the tumour mass. In a bioassay, the culture supernatant (KM-110-treated medium) of murine peritoneal macrophages that had been stimulated with KM-110 (1-10 micrograms/ml) for 30 min followed by 24 h incubation in fresh medium showed a strong tumour necrosis factor-alpha (TNF-alpha) activity. In addition, KM-110-treated medium significantly inhibited the growth of in vitro cultures of rat lung endothelial (RLE) cells. These results suggested that the extract of Korean mistletoe inhibits tumour metastasis caused by haematogenous as well as non-haematogenous tumour cells, and that its antimetastatic effect results from the suppression of tumour growth and the inhibition of tumour-induced angiogenesis by inducing TNF-alpha.

Oxygen and carbon isotope composition of parasitic plants and their hosts in southwestern Australia

We measured leaf dry matter delta(18)O and delta(13)C in parasitic plants and their hosts growing in southwestern Australia. Parasite/host pairs included two mistletoe species, three species of holoparasites, and five species of root hemiparasites. Among these parasite functional types, significant variation was observed in parasite/host isotopic differences for both delta(18)O ( P<0.0001, n=65) and delta(13)C ( P<0.0001, n=64). Mistletoes were depleted in both (18)O and (13)C compared to their hosts; parasite/host differences were -4.0 per thousand for delta(18)O ( P<0.0001) and -1.9 per thousand for delta(13)C ( P<0.0001). The lower delta(18)O in mistletoe leaf dry matter compared to their hosts is consistent with the frequently observed high transpiration rates of these parasites. Root hemiparasites were also depleted in (18)O and (13)C compared to their hosts, but not to the same extent as mistletoes; parasite/host differences were -1.0 per thousand for delta(18)O ( P=0.04) and -1.2 per thousand for delta(13)C ( P=0.0006). In contrast to mistletoes and root hemiparasites, holoparasites were enriched in both (18)O and (13)C compared to their hosts; parasite/host differences were +3.0 per thousand for delta(18)O ( P<0.0001) and +1.5 per thousand for delta(13)C ( P=0.02). The enrichment in (18)O for holoparasite dry matter did not result from more enriched tissue water; holoparasite tissue water delta(18)O was less than host leaf water delta(18)O by a difference of -3.8 per thousand when sampled at midday ( P=0.0003). Enrichment of holoparasites in (13)C compared to their hosts is consistent with a generally observed pattern of enrichment in heterotrophic plant tissues. Results provide insights into the ecology of parasitic plants in southwestern Australia; additionally, they provide a context for the formulation of specific hypotheses aimed at elucidating mechanisms underlying isotopic variations among plants.

Crystal structure at 3 A of mistletoe lectin I, a dimeric type-II ribosome-inactivating protein, complexed with galactose

The X-ray structure of mistletoe lectin I (MLI), a type-II ribosome-inactivating protein (RIP), cocrystallized with galactose is described. The model was refined at 3.0 A resolution to an R-factor of 19.9% using 21 899 reflections, with Rfree 24.0%. MLI forms a homodimer (A-B)2 in the crystal, as it does in solution at high concentration. The dimer is formed through contacts between the N-terminal domains of two B-chains involving weak polar and non-polar interactions. Consequently, the overall arrangement of sugar-binding sites in MLI differs from those in monomeric type-II RIPs: two N-terminal sugar-binding sites are 15 A apart on one side of the dimer, and two C-terminal sugar-binding sites are 87 A apart on the other side. Galactose binding is achieved by common hydrogen bonds for the two binding sites via hydroxy groups 3-OH and 4-OH and hydrophobic contact by an aromatic ring. In addition, at the N-terminal site 2-OH forms hydrogen bonds with Asp27 and Lys41, and at the C-terminal site 3-OH and 6-OH undergo water-mediated interactions and C5 has a hydrophobic contact. MLI is a galactose-specific lectin and shows little affinity for N-acetylgalactosamine. The reason for this is discussed. Structural differences among the RIPs investigated in this study (their quaternary structures, location of sugar-binding sites, and fine sugar specificities of their B-chains, which could have diverged through evolution from a two-domain protein) may affect the binding sites, and consequently the cellular transport processes and biological responses of these toxins.

Antimicrobial properties and phytochemical constituents of the leaves of African mistletoe (Tapinanthus dodoneifolius (DC) Danser) (Loranthaceae): an ethnomedicinal plant of Hausaland, Northern Nigeria

African mistletoe (Tapinanthus dodoneifolius (DC) Danser) called 'Kauchi' in Hausa is a hemi-plant parasite used ethnomedicinally by the Hausa and the Fulani tribes of Northern Nigeria as a remedy for several human and animal ailments that include stomach ache, diarrhoea, dysentery, wound and cancer. Screening of the plant, obtained from 14 different hosts, revealed a wide spectrum of antimicrobial activities against certain multiple drug resistant bacterial and fungal isolates of farm animals. Interestingly, the inhibition of the growth of Agrobacterium tumefaciens, Bacillus sp., Escherichia coli, Salmonella sp., Proteus sp. and Pseudomonas sp., bacterial sp. known to be associated with either crown gall or gastrointestinal tract and wound infections, by extracts of T. dodoneifolius gives credence to the ethnomedicinal usage of the plant. Phytochemical screening showed the common occurrence of anthraquinones, saponins, and tannins, a rare presence of alkaloids and the absence of phlobatannins in the hemi-parasite. Moreover, the antimicrobial activity and the presence or distribution of phytochemical substances in T. dodoneifolius appeared to be partly dependent on the host plant species.

Involvement of hydrogen peroxide in mistletoe lectin-II-induced apoptosis of myeloleukemic U937 cells

Mistletoe lectin-II, a major component of Korean mistletoe (Viscum album var. coloratum) induces apoptotic death in cancer cells. In this study, we demonstrated that lectin-II induced the generation of pro-oxidants and thus resulted in the apoptotic death of human myeloleukemic U937 cells. We observed that lectin-II-induced apoptotic death was inhibited by antioxidants including reduced glutathione (GSH), N-acetylcysteine (NAC), ebselen, mnTBP, catalase and pyrrolidine dithiocarbamate (PDTC). GSH and NAC also abolished the apoptotic DNA ladder pattern fragmentation of U937 cells after lectin-II stimulation. Obviously, lectin-II treatment of cells resulted in a remarkable generation of intracellular hydrogen peroxide (H2O2) as an early event, which was monitored fluorimetrically using scopoletin-horse radish peroxidase (HRP) assay and peroxide-sensitive fluorescent probe, DCF-DA. In addition, antioxidants inhibited the activation of c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) as well as cytosolic release of cytochrome c by mistletoe lectin-II. Moreover, lectin-II-induced activation of caspase-9 and 3-like protease and cleavage of poly(ADP-ribose) polymerase (PARP) were inhibited by pretreatment of cells with thiol antioxidants, GSH and NAC. Taken together, these results suggest that Korean mistletoe lectin-II is a strong inducer of pro-oxidant generation such as H2O2, which mediates the JNK/SAPK activation, cytochrome c release, activation of caspase-9 and caspase 3-like protease, and PARP cleavage in human myeloleukemic U937 cells.

Biological activity of mistletoe: in vitro and in vivo studies and mechanisms of action

Mistletoe has been used as treatment of many diseases in traditional and folk medicine. To date, anticancer, immunomodulatory, cardiac, antidiabetic, hepatoprotective, neuropharmacological, antibacterial and antifungal properties of mistletoe extracts have been studied the most. In this review, we summarized in vitro and in vivo studies on the pharmacological activity of Viscum species. Furthermore, we proposed the possible mechanisms of action of this herb, which might include many signalling pathways. Mistletoe could regulate either similar or different targets in various pathways that act on membrane receptors, enzymes, ion channels, transporter proteins and transcriptional targets. Still, pharmacological activities of mistletoe have been investigated mainly for crude extracts. It is a new field for scientists to determined which chemical compounds are responsible for the individual biological activities of mistletoe and how these activities are achieved. As a result, mistletoe might become a source of new complementary therapies supporting the treatment of many diseases.

AB-type lectin (toxin/agglutinin) from mistletoe: differences in affinity of the two galactoside-binding Trp/Tyr-sites and regulation of their functionality by monomer/dimer equilibrium

Viscumin of mistletoe (Viscum album L.) has a concentration-dependent activity profile unique to plant AB-toxins. It starts with lectin-dependent mitogenicity and then covers toxicity and cell agglutination, associated with shifts in the monomer/dimer equilibrium. Each lectin subunit harbors two sections for ligand contact. In the dimer, the B-chain sites in subdomain 2 gamma (designated as the Tyr-sites) appear fully accessible, whereas Trp-sites in subdomain 1 alpha are close to the dimer interface. It is unclear whether both types of sites operate similarly in binding glycoligands in solution. By systematically covering a broad range of lactose/lectin ratio in isothermal titration calorimetry, we obtained evidence for two sites showing dissimilar binding affinity. Intriguingly, the site with higher affinity was only partially occupied. To assign the observed properties to the Trp/Tyr-sites, we next performed chemically induced dynamic nuclear polarization measurements of Trp and Tyr accessibility. A Tyr signal, but not distinct Trp peaks, was recorded when testing the dimer. Lactose-quenchable Trp peaks became visible on the destabilization of the dimer by citraconylation, intimating Trp involvement in ligand contact in the monomer. Fittingly, Tyr acetylation but not mild Trp oxidation reduced the dimer hemagglutination activity and the extent of binding to asialofetuin-Sepharose 4B. Altogether, the results attribute lectin activity in the dimer primarily to Tyr-sites. Full access to Trp-sites is gained on dimer dissociation. Thus, the monomer/dimer equilibrium of viscumin regulates the operativity of these sites. Their structural divergence affords the possibility for differences in ligand selection when comparing monomers (Tyr- and Trp-sites) with dimers (primarily Tyr-sites).

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.

Complete amino acid sequence of the A chain of mistletoe lectin I

The complete amino acid sequence of the A chain of mistletoe lectin I was determined via Edman degradation sequencing of the N-terminus and tryptic and endoproteinase Asp-N overlapping fragments, amino acid analysis and MALDI-MS. The data obtained show a great homology with the chains of ribosome-inactivating proteins such as ricin and abrin with 111 (abrin-a) and 103 (ricin-D) amino acid residues conserved, respectively. The knowledge of the primary structure of MLA will have a fundamental impact on elucidating the biological function of medically applied mistletoe lectins on a molecular basis.

Complete amino acid sequence of the B chain of mistletoe lectin I

The primary structure of the B chain of mistletoe lectin I, the component of a commercially available extract from Viscum album exhibiting immunomodulatory capacity, was established based on amino acid sequence analysis of the protein and peptides derived from its enzymatic digestion. It is composed of 264 residues, including seven cysteine residues and three N-linked carbohydrate chains. The amino acid sequence of MLB shows a high homology with those from other structurally related galactoside-specific lectins such as ricin and abrin with 169 and 146 identities, respectively. These results are of crucial importance in order to understand the biological activity of ML-1.

Absence of tumor growth stimulation in a panel of 16 human tumor cell lines by mistletoe extracts in vitro

Extracts of Viscum album (mistletoe) are widely used as complementary cancer therapies in Europe. The mistletoe lectins have been identified as the main active principle of mistletoe extracts. They have been shown to exhibit cytotoxic effects as well as immunomodulatory activities. The latter is exemplified by induction of cytokine secretion and increased activity of natural killer cells. Recent reports, however, indicated possible tumor growth stimulation by mistletoe extracts. Therefore, the three aqueous mistletoe extracts (Iscador M special, Iscador Qu special and Iscador P) were evaluated for antiproliferative and/or stimulatory effects in a panel of 16 human tumor cell lines in vitro using a cellular proliferation assay. The results show no evidence of stimulation of tumor growth by any of the three Iscador preparations, comprising central nervous system, gastric, non-small cell lung, mammary, prostate, renal and uterine cancer cell lines, as well as cell lines from hematological malignancies and melanomas. On the contrary, Iscador preparations containing a high lectin concentration (Iscador M special and Iscador Qu special) showed antitumor activity in the mammary cancer cell line MAXF 401NL at the 15 microg/ml dose level with a more than 70% growth inhibition compared to untreated control cells. In addition, a slight antitumor activity (growth inhibition 30-70%) was found in three tumor cell lines for Iscador M special and in seven tumor cell lines for Iscador Qu special, respectively. Iscador P, which contains no mistletoe lectin I, showed no antiproliferative activity.

Isolation and partial characterization of a small chitin-binding lectin from mistletoe (Viscum album)

A novel lectin, called VisalbCBA, was isolated from European mistletoe (Viscum album). This lectin differs completely from the classical galactose/N-acetylgalactosamine-binding mistletoe lectins MLI, MLII and MLIII. Biochemical analyses indicated that VisalbCBA is a dimeric protein composed of two identical subunits of approx. 10 kDa. VisalbCBA exhibits specificity towards oligomers of N-acetylglucosamine and shows sequence homology to the previously isolated chitin-binding plant proteins. Although VisalbCBA is less toxic than the other mistletoe lectins, it definitely exhibits cytotoxic properties. The possible involvement of VisalbCBA in the biological and therapeutic effects of mistletoe is discussed.

Mistletoe alkaloid fractions alleviates carbon tetrachloride-induced liver fibrosis through inhibition of hepatic stellate cell activation via TGF-β/Smad interference

ETHNOPHARMACOLOGICAL RELEVANCE

Mistletoe (Viscum coloratum (Kom.) Nakai) has long been categorized as a traditional herbal medicine in Asia. In addition to its application in cancer therapy, mistletoe has also been used in the treatment of chronic hepatic disorders in China. In the present study, we investigated the antifibrotic effect and mechanisms of action of mistletoe extracts in a rat model of carbon tetrachloride (CCl4)-induced hepatotoxicity.

MATERIALS AND METHODS

An experimental model of hepatic fibrosis was established by intraperitoneal injection of rats with CCl4 for 8 weeks. Rats were subsequently treated with a mistletoe alkaloid fraction preparation via oral administration (120mg/kg daily for 8 weeks) or with distilled water as a control. Histopathological changes were observed by hematoxylin and eosin staining and Masson׳s trichrome staining. The expression of markers relevant to hepatic stellate cell (HSC) activation in the liver was assessed by real-time reverse transcription-polymerase chain reaction, immunohistochemistry and western blotting. The anti-fibrosis activity and mechanisms of action of mistletoe alkaloid fractions were further investigated in the HSC-T6 HSC line, following treatment with mistletoe alkaloid fractions (12mg/ml) for 48h.

RESULTS

Hepatic fibrosis decreased markedly in CCl4-treated animals following treatment with mistletoe alkaloid fractions, compared to controls. The mRNA levels of transforming growth factor-β1 (TGF-β1), procollagen I and tissue inhibitors of metalloproteinases (TIMPs) were significantly downregulated, by about 40%, 40% and 45%, respectively, in liver tissues from rats treated with mistletoe alkaloid fractions. Furthermore, significant downregulation of TGF-β1, TGF-β1 receptor, phosphorylated Smad 2 and alpha smooth muscle actin (α-SMA) proteins, by about 45%, 30% and 40%, respectively, was also observed in liver tissues from mistletoe alkaloid fractions-treated rats. In contrast, Smad 7 levels were significantly increased by about 30% in mistletoe alkaloid fractions-treated rats. Treatment of HSC-T6 cells with mistletoe alkaloid fractions significantly induced Smad 7 expression and inhibited the expression of α-SMA, TGFβ1, TGF-β1 receptor, Smad 2 and TIMP-1, in vitro.

CONCLUSION

We demonstrate that mistletoe alkaloid fractions decrease extracellular matrix accumulation by inhibiting HSC activation. Mechanistically, this may occur via inhibition of TGF-β1/Smad 2 and Smad 7 signal transduction, thereby blocking the synthesis of procollagen I and TIMP-1. These findings suggest that mistletoe alkaloid fractions may be a potential therapeutic agent for the treatment of hepatic fibrosis.