Suppressive effect of ascorbic acid on the mutagenesis induced by the bystander effect through mitochondrial function

Radiation-quality-dependent bystander cellular effects induced by heavy-ion microbeams through different pathways

We investigated the radiation-quality-dependent bystander cellular effects using heavy-ion microbeams with different ion species. The heavy-ion microbeams were produced in Takasaki Ion Accelerators for Advanced Radiation Application, National Institutes for Quantum Science and Technology. Carbon (12C5+, 220 MeV), neon (20Ne7+, 260 MeV) and argon (40Ar13+, 460 MeV) ions were used as the microbeams, collimating the beam size with a diameter of 20 μm. After 0.5 and 3 h of irradiation, the surviving fractions (SFs) are significantly lower in cells irradiated with carbon ions without a gap-junction inhibitor than those irradiated with the inhibitor. However, the same SFs with no cell killing were found with and without the inhibitor at 24 h. Conversely, no cell-killing effect was observed in argon-ion-irradiated cells at 0.5 and 3 h; however, significantly low SFs were found at 24 h with and without the inhibitor, and the effect was suppressed using vitamin C and not dimethyl sulfoxide. The mutation frequency (MF) in cells irradiated with carbon ions was 8- to 6-fold higher than that in the unirradiated control at 0.5 and 3 h; however, no mutation was observed in cells treated with the gap-junction inhibitor. At 24 h, the MFs induced by each ion source were 3- to 5-fold higher and the same with and without the inhibitor. These findings suggest that the bystander cellular effects depend on the biological endpoints, ion species and time after microbeam irradiations with different pathways.

External modulators and redox homeostasis: Scenario in radiation-induced bystander cells

Redox homeostasis is imperative to maintain normal physiologic and metabolic functions. Radiotherapy disturbs this balance and induces genomic instability in diseased cells. However, radiation-induced effects propagate beyond the targeted cells, affecting the adjacent non-targeted cells (bystander effects). The cellular impact of radiation, thus, encompasses both targeted and non-targeted effects. Use of external modulators along with radiation can increase radio-therapeutic efficiency. The modulators' classification as protectors or sensitizers depends on interactions with damaged DNA molecules. Thus, it is necessary to realize the functions of various radio-sensitizers or radio-protectors in both irradiated and bystander cells. This review focuses on some modulators of radiation-induced bystander effects (RIBE) and their action mechanisms. Knowledge about the underlying signaling cross-talk may promote selective sensitization of radiation-targeted cells and protection of bystander cells.

Evaluation of mitochondrial redox status and energy metabolism of X-irradiated HeLa cells by LC/UV, LC/MS/MS and ESR

To evaluate the metabolic responses in tumour cells exposed to ionizing radiation, oxygen consumption rate (OCR), cellular lipid peroxidation, cellular energy status (intracellular nucleotide pool and ATP production), and mitochondrial reactive oxygen species (ROS), semiquinone (SQ), and iron-sulphur (Fe-S) cluster levels were evaluated in human cervical carcinoma HeLa cells at 12 and 24 h after X-irradiation. LC/MS/MS analysis showed that levels of 8-iso PGF_2α and 5-iPF_2α-VI, lipid peroxidation products of membrane arachidonic acids, were not altered significantly in X-irradiated cells, although mitochondrial ROS levels and OCR significantly increased in the cells at 24 h after irradiation. LC/UV analysis revealed that intracellular AMP, ADP, and ATP levels increased significantly after X-irradiation, but adenylate energy charge (adenylate energy charge (AEC) = [ATP + 0.5 × ADP]/[ATP + ADP + AMP]) remained unchanged after X-irradiation. In low-temperature electron spin resonance (ESR) spectra of HeLa cells, the presence of mitochondrial SQ at g = 2.004 and Fe-S cluster at g = 1.941 was observed and X-irradiation enhanced the signal intensity of SQ but not of the Fe-S cluster. Furthermore, this radiation-induced increase in SQ signal intensity disappeared on treatment with rotenone, which inhibits electron transfer from Fe-S cluster to SQ in complex I. From these results, it was suggested that an increase in OCR and imbalance in SQ and Fe-S cluster levels, which play a critical role in the mitochondrial electron transport chain (ETC), occur after X-irradiation, resulting in an increase in ATP production and ROS leakage from the activated mitochondrial ETC.

The key role of miR-21-regulated SOD2 in the medium-mediated bystander responses in human fibroblasts induced by α-irradiated keratinocytes

Radiation-induced bystander effect (RIBE) is well accepted in the radiation research field by now, but the underlying molecular mechanisms for better understanding this phenomenon caused by intercellular communication and intracellular signal transduction are still incomplete. Although our previous study has demonstrated an important role of miR-21 of unirradiated bystander cells in RIBEs, the direct evidence for the hypothesis that RIBE is epigenetically regulated is still limited and how miR-21 mediates RIBEs is unknown. Reactive oxygen species (ROS) have been demonstrated to be involved in RIBEs, however, the roles of anti-oxidative stress system of cells in RIBEs are unclear. Using transwell insert co-culture system, we investigated medium-mediated bystander responses in WS1 human fibroblasts after co-culture with HaCaT keratinocytes traversed by α-particles. Results showed that the ROS levels in unirradiated bystander WS1 cells were significantly elevated after 30min of co-culture, and 53BP1 foci, a surrogate marker of DNA damage, were obviously induced after 3h of co-culture. This indicates the occurrence of oxidative stress and DNA damage in bystander WS1 cells after co-culture with irradiated keratinocytes. Furthermore, the expression of miR-21 was increased in bystander WS1 cells, downregulation of miR-21 eliminated the bystander responses, overexpression of miR-21 alone could induce bystander-like oxidative stress and DNA damage in WS1 cells. These data indicate an important mediating role of miR-21 in RIBEs. In addition, MnSOD or SOD2 in WS1 cells was involved in the bystander effects, overexpression of SOD2 abolished the bystander oxidative stress and DNA damage, indicating that SOD2 was critical to the induction of RIBEs. Moreover, we found that miR-21 regulated SOD2, suggesting that miR-21 might mediate bystander responses through its regulation on SOD2. In conclusion, this study revealed a profound role of miR-21-regulated SOD2 of unirradiated WS1 cells in bystander effects induced by α-irradiated HaCaT keratinocytes.

Role of ROS-mediated autophagy in radiation-induced bystander effect of hepatoma cells

PURPOSE

Autophagy plays a crucial role in cellular response to ionizing radiation, but it is unclear whether autophagy can modulate radiation-induced bystander effect (RIBE). Here, we investigated the relationship between bystander damage and autophagy in human hepatoma cells of HepG2.

MATERIALS AND METHODS

HepG2 cells were treated with conditioned medium (CM) collected from 3 Gy γ-rays irradiated hepatoma HepG2 cells for 4, 12, or 24 h, followed by the measurement of micronuclei (MN), intracellular reactive oxygen species (ROS), mitochondrial membrane potential (MMP), and protein expressions of microtubule-associated protein 1 light chain 3 (LC3) and Beclin-1 in the bystander HepG2 cells. In some experiments, the bystander HepG2 cells were respectively transfected with LC3 small interfering RNA (siRNA), Beclin-1 siRNA or treated with 1% dimethyl sulfoxide (DMSO).

RESULTS

Additional MN and mitochondrial dysfunction coupled with ROS were induced in the bystander cells. The expressions of protein markers of autophagy, LC3-II/LC3-I and Beclin-1, increased in the bystander cells. The inductions of bystander MN and overexpressions of LC3 and Beclin-1 were significantly diminished by DMSO. However, when the bystander cells were transfected with LC3 siRNA or Beclin-1 siRNA, the yield of bystander MN was significantly enhanced.

CONCLUSION

The elevated ROS have bi-functions in balancing the bystander effects. One is to cause MN and the other is to induce protective autophagy.