Mitogen-like Cerium-Based Nanoparticles Protect Schmidtea mediterranea against Severe Doses of X-rays

Abstract

Novel radioprotectors are strongly demanded due to their numerous applications in radiobiology and biomedicine, e.g., for facilitating the remedy after cancer radiotherapy. Currently, cerium-containing nanomaterials are regarded as promising inorganic radioprotectors due to their unrivaled antioxidant activity based on their ability to mimic the action of natural redox enzymes like catalase and superoxide dismutase and to neutralize reactive oxygen species (ROS), which are by far the main damaging factors of ionizing radiation. The freshwater planarian flatworms are considered a promising system for testing new radioprotectors, due to the high regenerative potential of these species and an excessive amount of proliferating stem cells (neoblasts) in their bodies. Using planarian Schmidtea mediterranea, we tested CeO₂ nanoparticles, well known for their antioxidant activity, along with much less studied CeF₃ nanoparticles, for their radioprotective potential. In addition, both CeO₂ and CeF₃ nanoparticles improve planarian head blastema regeneration after ionizing irradiation by enhancing blastema growth, increasing the number of mitoses and neoblasts' survival, and modulating the expression of genes responsible for the proliferation and differentiation of neoblasts. The CeO₂ nanoparticles' action stems directly from their redox activity as ROS scavengers, while the CeF₃ nanoparticles' action is mediated by overexpression of "wound-induced genes" and neoblast- and stem cell-regulating genes.

Keywords: X-ray; cerium fluoride nanoparticles; cerium oxide nanoparticles; planarians; radioprotection.

Figure 1

Transmission electron microscopy (a,b), dynamic light scattering (c,d) in water, and UV absorption spectra (e,f) of cerium oxide (CeO₂) and cerium fluoride (CeF₃) nanoparticles. Insets in (a,b) show selected-area electron diffraction data.

Figure 2

Concentration dependencies of hydrogen peroxide formation induced by X-ray irradiation (1 Gy) in a buffer solution containing CeF₃ and CeO₂ NPs. The mean values of three independent experiments and their standard errors are given.

Figure 3

Radioprotective effect of CeO₂ nanoparticles on regenerating planarian blastemas after X-ray irradiation (10 Gy and 15 Gy). Brown to light-brown bars represent the percentage of blastema regeneration rate; the regeneration rate of non-irradiated animals was taken as 100% (red line). * p < 0.05 (difference from the control group). M ± SD, n = 90. IR—X-ray irradiation.

Figure 4

Radioprotective effect of CeF₃ nanoparticles on regenerating planarian blastema after irradiation with 10 Gy and 15 Gy. Green to light-green bars represent the percentage of blastema regeneration rate; the regeneration rate of non-irradiated animals was taken as 100% (red line). * p < 0.05 (difference from the control group). M ± SD, n = 90. IR—X-ray irradiation.

Figure 5

Mitotic activity of regenerating planarians pretreated with CeO₂ and CeF₃ nanoparticles and X-ray irradiated. Photographs of immunostained planarians after X-rays irradiation with CeO₂ (a) and cerium fluoride (b) nanoparticles. Quantitative analysis of the level of mitotic cells (c). This concentration of nanoparticles (10⁻⁶ M) was chosen as it gave the most pronounced effect. Significance was estimated by one-way analysis of variance (one-way ANOVA). Data are presented as means. Significant statistical difference ** p < 0.001, *** p < 0.05 compared to the untreated control (indicated by the red line).

Figure 6

Expression of 3 classes of neoblast marker genes in regenerating planarians on days 3 and 10 after 10 Gy X-ray irradiation. The intensity scale of the standardized expression values ranges from −2 (green: low expression) to +2 (red: high expression), with a 1:1 intensity value (black) representing the control (non-treated). The data in the heat maps are from the non-irradiated control group. A non-irradiated control group without CeO₂ or CeF₃ nanoparticle pretreatment was taken as a control.