Functional and therapeutic effects of viscumins in the treatment of experimental glioblastoma

Abstract

Glioblastoma (GBM) is the most aggressive primary brain tumor, leading to death in less than 15 months despite tumor resection, radiation, and chemotherapy. The development of new and effective treatment options is therefore crucial. Mistletoe extracts (ME) have been used in complementary and alternative cancer therapy for decades. Mistletoe lectins I-III, also called viscumins (ML), and viscotoxins are the main active components, providing anti-cancer activity by different mechanisms including immuno-modulation, dose-dependent cytotoxicity, and mitigation of tumor cell motility. In clinical trials, viscumin treatment of cancer patients led to an improvement in the quality of life and to prolonged survival for some tumor entities. Here we show anticancer effects of the ME ISCADOR Qu, the recombinant ML-1 Aviscumine, and native ML-1. We found that each of the three viscumins reduced cell proliferation but even to a different extent. The reduction in proliferation was accompanied by changes in glioma cell cycle distribution and gene expression. In higher concentrations viscumins induced cell death in GBM cells in vitro. Nevertheless, at the same concentration we observed no toxic effects in organotypic mouse brain cultures in situ. The viscumin receptor CD75s was expressed in all tested GBM cell lines so far and its level of expression correlated well with the cell’s vulnerability towards viscumin induced cell death. We detected CD75s in 6 out of 7 human GBM tissues but not in human and murine healthy brain tissue. This makes viscumins suitable for a localized drug application in the brain. Viscumins significantly reduced glioma cell motility and the expression and activity of cancer motility-associated factors in GBM cell lines. Viscumins also provided beneficial immune-stimulating effects. They enhanced the vulnerability of GBM cells towards natural killer (NK) cell and T cell mediated killing. Besides, in GBM cells viscumins altered the expression of immune response related genes that are associated to pro-inflammatory processes. Furthermore, we found synergistic therapeutic effects in GBM cells if viscumins were used in combination with irradiation and chemotherapy. Additionally, adjuvant viscumin therapy prolonged the survival of glioma bearing mice in two different mouse models. In mice, serum levels of pro-inflammatory cytokines were increased after ISCADOR Qu injections. These findings provide the basis for clinical trials to investigate the therapeutic efficacy of adjuvant viscumin treatment in glioblastoma patients.