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Comparative membrane interaction study of viscotoxins A3, A2 and B from mistletoe (Viscum album) and connections with their structures.

ABSTRACT: Viscotoxins A2 (VA2) and B (VB) are, together with viscotoxin A3 (VA3), among the most abundant viscotoxin isoforms that occur in mistletoe-derived medicines used in anti-cancer therapy. Although these isoforms have a high degree of amino-acid-sequence similarity, they are strikingly different from each other in their in vitro cytotoxic potency towards tumour cells. First, as VA3 is the only viscotoxin whose three-dimensional (3D) structure has been solved to date, we report the NMR determination of the 3D structures of VA2 and VB. Secondly, to account for the in vitro cytotoxicity discrepancy, we carried out a comparative study of the interaction of the three viscotoxins with model membranes. Although the overall 3D structure is highly conserved among the three isoforms, some discrete structural features and associated surface properties readily account for the different affinity and perturbation of model membranes. VA3 and VA2 interact in a similar way, but the weaker hydrophobic character of VA2 is thought to be mainly responsible for the apparent different affinity towards membranes. VB is much less active than the other two viscotoxins and does not insert into model membranes. This could be related to the occurrence of a single residue (Arg25) protruding outside the hydrophobic plane formed by the two amphipathic alpha-helices, through which viscotoxins are supposed to interact with plasma membranes.

SUBMITTER: Coulon A

PROVIDER: S-EPMC1223566 | biostudies-other | 2003 Aug

REPOSITORIES: biostudies-other

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Project description:BACKGROUND:Viscum album L. (Santalaceae), commonly known as mistletoe, is a hemiparasitic plant traditionally used in complementary cancer treatment. Its antitumor potential is mostly attributed to the presence of aqueous soluble metabolites; however, the use of ethanol as solvent also permits the extraction of pharmacological compounds with antitumor potential. The clinical efficacy of mistletoe therapy inspired the present work, which focuses on ethanolic extracts (V. album "mother tinctures", MT) prepared from different host trees. METHODS:Samples from three European subspecies (album, austriacum, and abietis) were harvested, and five different V. album-MT strains were prepared. The following phytochemical analyses were performed: thin layer chromatography (TLC), high-performance liquid chromatography (HPLC) and liquid chromatography-high resolution mass spectrometry (LC-HRMS). The proliferation assay was performed with WST-1 after incubation of tumor (Yoshida and Molt-4) and fibroblast cell lines (NIH/3?T3) with different MT concentrations (0.5 to 0.05% v/v). The cell death mechanism was investigated by flow cytometry (FACS) using Annexin V-7AAD. RESULTS:Chemical analyses of MT showed the presence of phenolic acids, flavonoids and lignans. The MT flavonoid and viscotoxin contents (mg/g fresh weight) were highest in Quercus robur (9.67?±?0.85?mg/g) and Malus domestica (3.95?±?0.58?mg/mg), respectively. The viscotoxin isoform proportions (% total) were also different among the VA subspecies with a higher content of A3 in V. album growing on Abies alba (60.57?±?2.13). The phytochemical compounds as well as the viscotoxin contents are probably related to the antitumor effects of MT. The cell death mechanisms evaluated by colorimetric and FACS methodologies involved necrotic damage, which was host tree-, time- and dose- dependent, with different selectivity to tumor cells. Mother tincture from V. album ssp. abietis was the most effective at inducing in vitro cellular effects, even when incubated at the smallest concentration tested, probably because of the higher content of VT A3. CONCLUSION:Our results indicate the promising antitumor potential of Viscum album ethanolic extracts and the importance of botanical and phytochemical characterization for in vitro anti-proliferative effects.

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Comparative membrane interaction study of viscotoxins A3, A2 and B from mistletoe (Viscum album) and connections with their structures.

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