Antiproliferative potential from aqueous Viscum album L. preparations and their main constituents in comparison with ricin and purothionin on human cancer cells

Ethnopharmacological relevance: Mistletoe has been used since ancient times in Europe mostly for medicinal purposes. Since 1917, mistletoe preparations have been applied in cancer therapy and today are the most frequently used complementary medicine in tumor treatment. The main cytotoxic constituents of Viscum album are lectins and viscotoxins. Aim of the study: The aim of this in vitro study was to investigate the antiproliferative potential of Viscum album preparations from different host trees and to assess the impact of mistletoe lectin 1 (ML-1) and viscotoxin A (VT-A) in comparison to a structurally similar lectin and thionin. Materials and methods: By means of widely accepted 2D Alamar Blue Assay, based on population counting of living cells using a fluorescent cell viability dye, the potential impact to inhibit tumor cell of the mistletoe preparations (Iscucin (R)) and their single compounds (ML-1 and VT-A) on the cell growth of six human cancer cell lines were evaluated. Also the mixture of ML-1 and VT-A corresponding to the contents in the specific mistletoe preparations were monitored. Ricin and purothionin were used as reference lectin and reference thionin, respectively. Results: The lung carcinoma cell line HCC827 was very sensitive to the Iscucin (R) preparations. Very strong antiproliferative effects were found with Iscucin (R) Salicis and Tiliae and a strong with Iscucin (R) Crataegi, Mali and Populi. The IC50 concentrations of the Iscucin (R) preparations correlated with their respective ML-1 contents, but the ML-1 levels were much lower than the IC50 concentration of isolated ML-1 (1 ng/ml 56 ng/ml). ML-1 was much more effective than ricin. Iscucin (R) preparations, ML-1 and ricin showed antiproliferative activity on human tumor cells. VT-A and purothionin had no effect on cell viability in the concentration ranges tested. Conclusion: The complete mistletoe extract is more potent to inhibit tumor cell proliferation than isolated ML-1 at an equivalent concentration level. Phenolic compounds found in all Iscucin (R) preparations might contribute to uphold the cytotoxic activity of ML-1 by antioxidative action. However, further studies are necessary to evaluate the role of VT-A and possible synergistic actions to the antiproliferative effect of aqueous mistletoe extracts.