Interaction Between Cap- Derived Singlet Oxygen And Tumor Cell Protective Catalase: Update And Chances

Transformation of cells from various tissues requires NADPH oxidase-dependent generation of extracellular superoxide anions. These drive the proliferation, but also cause the elimination of malignant cells through the HOCl and the NO/peroxynitrite signaling pathways. These intercellular signaling pathways induce apoptosis selectively in malignant cells, due to site-specific concerted interaction of defined reactive oxygen and nitrogen species (ROS/RNS). Tumor progression requires the expression of membrane-associated catalase. This enzyme interferes with HOCl signaling through decomposition of H2O2, and with NO/peroxynitrite signaling through oxidation of NO and decomposition of peroxynitrite. Membrane-associated catalase has been found on all lines of bona fide tumor cells and represents a promising target for novel antitumor strategies. Inactivation of tumor cell-specific membrane-associated catalase reactivates intercellular ROS/RNS-dependent apoptosis-inducing signaling and leads to autocrine apoptotic selfdestruction of tumor cells.

Model experiments with defined ROS and RNS led to the conclusion that CAP-derived singlet oxygen might lead to site-specific inactivation of catalase, followed by tumor cell-specific generation of secondary singlet oxygen and further inactivation of catalase. This then allows for subsequent reactivation of intercellular ROS/RNS-dependent apoptosis-inducing signaling [1]. Only on the first sight, this model seemed a) to be in contradiction to the model on the dependence of CAP action from aquaporins [2] and b) to be independent of subsequent immunogenic cell death and activation of a cytotoxic T cell response [3, 4]. The analysis of existing experimental data from several groups and the alignment of site-specific mechanisms, defined by chemical biology and cell biology, allowed to establish an updated and comprehensive model [5] that includes the concepts from references [1-4] in a rational way. This model shows several biochemical amplification loops related to the generation of secondary singlet oxygen, a positive feed-back of HOCl signaling on immunogenic modulation, as well as a feedback loop from activated T cells to catalase inactivation and reactivation of intercellular ROS/RNS signaling. Thereby HOCl seems to play a role as mediator for apoptosis induction and enhancer of immunogenic stimulation.

The detailed biochemical analysis of generation of secondary singlet oxygen by tumor cells after initial interaction with exogenous singlet oxygen allowed to predict that an increase in the concentrations of tumor cell-derived extracellular superoxide anions and/or nitric oxide should cause a substantial synergistic effect with exogenous singlet oxygen. Therefore, a hybrid molecule, consisting of a plant-derived NOX stimulator and a plant-derived inhibitor of NO dioxygenase (NOD) (patent application PCT/EP2016/080169) has been synthesized in collaboration with Prof. R. Brückner and Dr. S. Braukmüller (Organic Chemistry, Freiburg). When applied in vitro at 0.1-0.5 nmoles/liter, this compound caused a remarkable synergistic effect with exogenous singlet oxygen, characterized by a reduction of the optimal dose of singlet oxygen by three log steps. We suggest to evaluate whether this principle can be translated into application of CAP for tumor treatment. If so, the reduction of necessary plasma doses might be useful for further development of microinvasive plasma-mediated tumor therapy