Reaction kinetics and targeting to cellular glutathione S-transferase of the glutathione peroxidase mimetic PhSeZnCl and its d,l-polylactide microparticle formulation

Abstract

Catalytic properties and cellular effects of the glutathione peroxidase (GPx)-mimetic compound PhSeZnCl or its d,l-lactide polymer microencapsulation form (M-PhSeZnCl) were investigated and compared with the prototypical Se-organic compounds ebselen and diselenide (PhSe)2. PhSeZnCl was confirmed to catalyze the ping-pong reaction of GPx with higher Vmax than ebselen and (PhSe)2, but the catalytic efficiency calculated for the cosubstrates glutathione (GSH) and H2O2, and particularly the high reactivity against thiols (lowest KM for GSH in the series of test molecules), suggested poor biological applicability of PhSeZnCl as a GPx mimetic. Cytotoxicity of PhSeZnCl was demonstrated in various cancer cell lines via increased reactive oxygen species (ROS) generation, depletion of intracellular thiols, and induction of apoptosis. Experiments carried out in GSH S-transferase P (GSTP)-overexpressing K562 human erythroleukemia cells and in GSTP1-1-knockout murine embryonic fibroblasts (MEFs) demonstrated that this cytosolic enzyme represents a preferential target of the redox disturbances produced by this Se-compound with a key role in controlling H2O2 generation and the perturbation of stress/survival kinase signaling. Microencapsulation was adopted as a strategy to control the thiol reactivity and oxidative stress effects of PhSeZnCl, then assessing applications alternative to anticancer. The uptake of this “depowered” GPx-mimetic formulation, which occurred through an endocytosis-like mechanism, resulted in a marked reduction of cytotoxicity. In MCF-7 cells transfected with different allelic variants of GSTP, M-PhSeZnCl lowered the burst of cellular ROS induced by the exposure to extracellular H2O2, and the extent of this effect changed between the GSTP variants. Microencapsulation is a straightforward strategy to mitigate the toxicity of thiol-reactive Se-organic drugs that enhanced the antioxidant and cellular protective effects of PhSeZnCl. A mechanistic linkage of these effects with the expression pattern and signaling properties of GSTP. This has overcome the GPx-mimetic paradigm proposed for Se-organic drugs with a more pragmatic concept of GSTP signaling modulators.

Kleine Organoselenverbindungen: mehr als nur Mimetika der Glutathion‐Peroxidase

“…Während Verbindungen mit aromatischen Substituenten R nicht spontan cyclisieren und als Diselenide stabil sind, können die hier vorgestellten Verbindungen 9 a – d nur als Isoselenazole isoliert werden. Ähnliche Diselenide12 und andere selenhaltige Verbindungen13 wurden ebenfalls als potentielle GPx‐Mimetika untersucht.…”

The Regulation of Pathways of Inflammation and Resolution in Immune Cells and Cancer Stem Cells by Selenium

“…Further, Se-organic compounds (diselenides) act as glutathione peroxidase (GPx) mimetic drugs in cells with high thiol content such as cells exhibiting a high rate of metabolism and proliferation, all characteristics of tumor (Bartolini et al, 2015b; Frenkel, Falvey & MacVicar, 1991). Furthermore, in malignant cells, toxicity to selenium is reached at considerably lower concentrations compared to non-malignant cells suggesting the cytotoxic effects are specific for tumor cells (Husbeck, Nonn, Peehl & Knox, 2006).…”

“…Furthermore, in malignant cells, toxicity to selenium is reached at considerably lower concentrations compared to non-malignant cells suggesting the cytotoxic effects are specific for tumor cells (Husbeck, Nonn, Peehl & Knox, 2006). While the exact mechanisms are unclear, it is plausible that redox modulation of signalling pathways through changes in cellular oxidant tone combined with changes in redox status of protein thiols, selenium could interfere with the pre-metastatic niche created by platelet mediated fibrin deposition on tumor cells (Bartolini et al, 2015a; Bartolini, et al, 2015b; Quail & Joyce, 2013). …”

<strong>Organoselenium Chemistry: after 200 years the "<em>Gold Rush</em>" is still open</strong>

“…[22] (Fig 6) Following these consideration, is now our opinion to change the paradigm that for a long time described organoselenium compounds simply as antioxidants and we need to answer to a different question: "is it possible to use prooxidant activity as specific bullet against new therapeutically relevant targets? Furthermore, we also demonstrated that mild electrophilic diselenides with their moderate prooxidant activity are able to stimulate a hormetic response Nrf2-mediated that reinforce the self-antioxidant defense of cells.…”