br SK N SH neuroblastoma br No

SK-N-SH, neuroblastoma
No
values
DLD-1, colorectal cancer
No values
No values
No values
2014; Liu and Zhou, 2017; Zhou et al., 2015). Possibly, 5,7-dimethoxy-4′-hydroxyflavanone and quercetin derivates played also a role in the tumor cell growth inhibition directly or by upholding the lectin activity.
Comparing the results of the Iscucin® preparations with those of ML-1 showed that the whole mistletoe preparations are much more potent, especially towards Caki-2, followed by HCC827 and LN229 (Fig. 3 A – F).
3.3. VT-A had no antiproliferative effects
The fact that in the present study the ML-1 contents in the IC50 concentrations of the Iscucin® preparations were lower than the re-spective IC50 concentrations of the isolated ML-1, indicates that not only ML-1 was responsible for the antiproliferative effect of the extracts on the tumor cells. Therefore another putatively potent ingredient of mistletoe, VT-A, was tested in comparison with the reference thionin. The amount of VT is very low in mistletoe preparations from conifers and apple tree. The highest VT-A content is found in Iscucin® Crataegi (Fig. 2). However, the present analyses show that VT-A and purothionin in the tested concentration ranges, which correlate with the amount of VT in the Iscucin® preparations, had no effect on the cell viability of the investigated tumor cell lines (Fig. 4 A + B). Other cell lines, e. g. Yoshida sarcoma, Molt 4 and K562 HA1077 were sensitive to 0.7 – 15.2 μg/ ml VT (Urech et al., 1995). VT fractions isolated from fresh plant showed cytotoxic effects on KB cells and HeLa cells as well as a con-centration of 0.2 – 0.7 μg/ml (Konopa et al., 1980). 
4. Conclusion
The antiproliferative effect on human tumor cells is not exclusively dependent on ML-1 content. Rather, the complete mistletoe extracts are more potent to inhibit tumor cell proliferation as shown in Fig. 3. Therefore, there must be some ingredients inducing the inhibition of tumor cell growth. From the results of the present study, however, VT-A had no effect on human cancer cell growth in vitro. It is likely, that a concerted action of all MLs and VTs including ML II, ML III and VT B played a role in the tumor cell growth inhibition. Moreover, in all Is-cucin® preparations the phenolic compounds syringin and syringenin-4′-0-apiosylglucoside are present (Gärtner et al., 2015, 2016) and these components also possess anticancer effects (Lall et al., 2015; Xia, 2016; Zhang et al., 2007). This strengthens the hypothesis that a blend of
compounds has a stronger effect than isolated compounds, so that whole preparations from mistletoe are more effective than specific compounds. Further research is necessary to evaluate the role of VT and to find out which ingredients contribute to the antiproliferative effect of mistletoe extracts on human tumor cells.
Declaration of interest
All authors from WALA Heilmittel GmbH state that they are em-ployees of this company. The authors alone are responsible for the content and writing of this article. WALA Heilmittel GmbH financially supports the in vitro experiments.
Funding sources
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Submission declaration
This publication is approved by all authors. It will not be published elsewhere in the same form, in English or in any other language, in-cluding electronically without the written consert of the copyright-holder.
Acknowledgements
We thank ProQinase GmbH (Freiburg, Germany) for conducting the in vitro tests.
Glossary
ML Mistletoe lectin
ML-1 Mistletoe lectin 1
VT Viscotoxin
VT-A Viscotoxin A
RNA Ribonucleic acid
DNA Deoxyribonucleic acid
2D Two-dimensional
FCS Fetal calf serum
RPMI-1640 Roswell Park Memorial Institute (RPMI) 1640 Medium DMEM Dulbecco's Modified Eagle's Medium
DMSO Dimethyl sulfoxide
CE-UV Capillary Electrophoresis with Ultra Violet Detection
HPLC High Performance Liquid Chromatography
HPLC-UV High Performance Liquid Chromatography with Ultra Violet
Detection
ELISA Enzyme-linked Immunosorbent Assay
References
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