Clinically relevant radioresistant cells exhibit resistance to H2O2 by decreasing internal H2O2 and lipid peroxidation

The reversibility of cancer radioresistance: a novel potential way to identify factors contributing to tumor radioresistance

To understand the molecular mechanisms responsible for radioresistance in cancer cells, we previously established clinically relevant radioresistant (CRR) cell lines from several human cancer cell lines. These CRR cells proliferate even under exposure to 2 Gy/day of X-rays for more than 30 days, which is a standard protocol for tumor radiotherapy. CRR cells received 2 Gy/day of X-rays to maintain their radioresistance (maintenance irradiation; MI). Interestingly, CRR cells that did not receive MI for more than a year lost their radioresistance, indicating that radiation-induced radioresistance is reversible. We designated these CRR-NoIR cells. Karyotyping of the parental and CRR cells revealed that the chromosomal composition of CRR cells is quite different from that of the parental cells. However, CRR and CRR-NoIR cells were more similar compared with the parental cells because CRR cells repair X-ray-induced DNA damage with higher fidelity. To identify the factor(s) involved in tumor radioresistance, previously published studies including ours have compared radioresistant cells to parental cells. In this review, we conclude that a comparison between CRR and CRR-NoIR cells, rather than parental cells, is the best way to identify factors involved in tumor radioresistance.

Decreased mitochondrial membrane potential is an indicator of radioresistant cancer cells

AIMS

To overcome radioresistant cancer cells, clinically relevant radioresistant (CRR) cells were established. To maintain their radioresistance, CRR cells were exposed 2 Gy/day of X-rays daily (maintenance irradiation: MI). To understand whether the radioresistance induced by X-rays was reversible or irreversible, the difference between CRR cells and those without MI for a year (CRR-NoIR cells) was investigated by the mitochondrial function as an index.

MAIN METHODS

Radiation sensitivity was determined by modified high density survival assay. Mitochondrial membrane potential (Δψm) was determined by 5,5',6,6'-tetrachloro-1,1', tetraethylbenzimidazolocarbo-cyanine iodide (JC-1) staining. Rapid Glucose-Galactose assay was performed to determine the shift in their energy metabolism from aerobic glycolysis to oxidative phosphorylation in CRR cells. Involvement of prohibitin-1 (PHB1) in Δψm was evaluated by knockdown of PHB1 gene followed by real-time PCR.

KEY FINDINGS

CRR cells that exhibited resistant to 2 Gy/day X-ray lost their radioresistance after more than one year of culture without MI for a year. In addition, CRR cells lost their radioresistance when the mitochondria were activated by galactose. Furthermore, Δψm were increased and PHB1 expression was down-regulated, in the process of losing their radioresistance.

SIGNIFICANCE

Our finding reveled that tune regulation of mitochondrial function is implicated in radioresistance phenotype of cancer cells. Moreover, as our findings indicate, though further studies are required to clarify the precise mechanisms underlying cancer cell radioresistance, radioresistant cells induced by irradiation and cancer stem cells that are originally radioresistant should be considered separately, the radioresistance of CRR cells is reversible.

Mitochondrial Dysfunction in Diseases, Longevity, and Treatment Resistance: Tuning Mitochondria Function as a Therapeutic Strategy

Mitochondria are very important intracellular organelles because they have various functions. They produce ATP, are involved in cell signaling and cell death, and are a major source of reactive oxygen species (ROS). Mitochondria have their own DNA (mtDNA) and mutation of mtDNA or change the mtDNA copy numbers leads to disease, cancer chemo/radioresistance and aging including longevity. In this review, we discuss the mtDNA mutation, mitochondrial disease, longevity, and importance of mitochondrial dysfunction in cancer first. In the later part, we particularly focus on the role in cancer resistance and the mitochondrial condition such as mtDNA copy number, mitochondrial membrane potential, ROS levels, and ATP production. We suggest a therapeutic strategy employing mitochondrial transplantation (mtTP) for treatment-resistant cancer.

MiR-7-5p Is Involved in Ferroptosis Signaling and Radioresistance Thru the Generation of ROS in Radioresistant HeLa and SAS Cell Lines

In cancer therapy, radioresistance or chemoresistance cells are major problems. We established clinically relevant radioresistant (CRR) cells that can survive over 30 days after 2 Gy/day X-ray exposures. These cells also show resistance to anticancer agents and hydrogen peroxide (H₂O₂). We have previously demonstrated that all the CRR cells examined had up-regulated miR-7-5p and after miR-7-5p knockdown, they lost radioresistance. However, the mechanism of losing radioresistance remains to be elucidated. Therefore, we investigated the role of miR-7-5p in radioresistance by knockdown of miR-7-5p using CRR cells. As a result, knockdown of miR-7-5p increased reactive oxygen species (ROS), mitochondrial membrane potential, and intracellular Fe²⁺ amount. Furthermore, miR-7-5p knockdown results in the down-regulation of the iron storage gene expression such as ferritin, up-regulation of the ferroptosis marker ALOX12 gene expression, and increases of Liperfluo amount. H₂O₂ treatment after ALOX12 overexpression led to the enhancement of intracellular H₂O₂ amount and lipid peroxidation. By contrast, miR-7-5p knockdown seemed not to be involved in COX-2 and glycolysis signaling but affected the morphology of CRR cells. These results indicate that miR-7-5p control radioresistance via ROS generation that leads to ferroptosis.

VLX1570 induces apoptosis through the generation of ROS and induction of ER stress on leukemia cell lines

A novel proteasome deubiquitinase inhibitor, VLX1570, has been highlighted as a promising therapeutic agent mainly for lymphoid neoplasms and solid tumors. We examined in vitro effects of VLX1570 on eight myeloid and three lymphoid leukemia cell lines. From cell culture studies, 10 out of 11 cell lines except K562 were found to be susceptible to VLX1570 treatment and it inhibited cell growth mainly by apoptosis. Next, to identify the signaling pathways associated with apoptosis, we performed gene expression profiling using HL‐60 with or without 50 nmol/L of VLX1570 for 3 hours and demonstrated that VLX1570 induced the genetic pathway involved in “heat shock transcription factor 1 (HSF1) activation”, “HSF1 dependent transactivation”, and “Regulation of HSF1 mediated heat shock response”. VLX1570 increased the amount of high molecular weight polyubiquitinated proteins and the expression of HSP70 as the result of the suppression of ubiquitin proteasome system, the expression of heme oxygenase‐1, and the amount of phosphorylation in JNK and p38 associated with the generation of reactive oxygen species (ROS) induced apoptosis and the amount of phosphorylation in eIF2α, inducing the expression of ATF4 and endoplasmic reticulum (ER) stress dependent apoptosis protein, CHOP, and the amount of phosphorylation slightly in IRE1α, leading to increased expression of XBP‐1s in leukemia cell lines. In the present study, we demonstrate that VLX1570 induces apoptosis and exerts a potential anti‐leukemic effect through the generation of ROS and induction of ER stress in leukemia cell lines.

The Effects of Hydrogen Peroxide and/or Radiation on the Survival of Clinically Relevant Radioresistant Cells

BACKGROUND

Radiation therapy is a highly cost-effective treatment for cancer, but the existence of radio-resistant cells remains the most critical obstacle in radiotherapy. We have been established clinically relevant radioresistant (CRR) cell lines by exposure to a stepwise increase of fractionated X-rays. We are trying to overcome the radio-resistance by analyzing the properties of these cells. In this study, we tried to evaluate the effects of hydrogen peroxide (H₂O₂) on the CRR cells because this can evaluate the efficacy of Kochi Oxydol-Radiation Therapy for Unresectable Carcinomas (KORTUC) that treats H₂O₂ before irradiation. We also established H₂O₂-resistant cells to compare the radiation and H₂O₂ resistant phenotype.

MATERIALS AND METHODS

We used human cancer cell lines derived from hepatoblastoma (HepG2), oral squamous cell carcinoma (SAS), and cervical cancer (HeLa). We established HepG2, SAS, and HeLa CRR cells and HepG2, SAS, and HeLa H₂O₂-resistant cells. To evaluate their sensitivity to radiation or H₂O₂, high-density survival assay, or WST assay was performed. CellROX^TM was used to detect intracellular Reactive Oxygen Species (ROS).

RESULTS

CRR cells were resistant to H₂O₂-induced cell death but H₂O₂-resistant cells were not resistant to irradiation. This phenotype of CRR cells was irreversible. The intracellular ROS was increased in parental cells after H₂O₂ treatment for 3 h, but in CRR cells, no significant increase was observed.

CONCLUSION

Fractionated X-ray exposure induces H₂O₂ resistance in CRR cells. Therefore, it is necessary to carry out cancer therapy such as KORTUC with the presence of these resistant cells in mind, and as the next stage, it would be necessary to investigate the appearance rate of these cells immediately and take countermeasures.

Mitochondrial reactive oxygen species and heme, non-heme iron metabolism

Mitochondria are one of the most important organelles for eukaryotes, including humans, to produce energy. In the energy-producing process, mitochondria constantly generate reactive oxygen species as a by-product of electrons leaking out from the electron transport chain react with oxygen. The active oxygen, in turn, plays pivotal roles in mediating several signalings, including those that are implicated in the development of some diseases such as neurodegenerative disease, cardiovascular disease, and carcinogenesis. This signaling, derived from mitochondrial reactive oxygen species, also affects intracellular iron homeostasis by regulating the expression of transporters. Heme iron is incorporated into cells through HCP1, and non-heme iron is transported by DMT1 in absorptive cells. Intracellular iron is exported by ferroportin and bound with transferrin. In most types of cell including erythrocyte, transferrin-bound iron is incorporated through transferrin-transferrin receptor system. We previously reported that the expression of HCP1 and DMT1 was upregulated in cancer cells and that overexpression of manganese superoxide dismutase, which is a mitochondrial-specific superoxide dismutase, downregulated the expression. These findings indicate that mitochondrial reactive oxygen species is associated with iron-related oxidative reactions. Recently, a mitochondria-specific iron transporter, mitoferrin, was identified, and the relationships among mitochondria, iron transportation, and diseases have been increasingly clarified.

Disturbance in the regulation of miR 17-92 cluster on HIF-1-α expression contributes to clinically relevant radioresistant cells: an in vitro study

Cellular radioresistance is one of the major obstacles to the effectiveness of cancer radiotherapy. In an attempt to elucidate the implication of HIF-1α and miR-17-92 expressions in refractory radioresistant cells and also in order to study the potential applications of these molecules as novel therapeutic modalities to overcome radioresistant cancers, the current study was conducted. Clinically relevant radioresistant (CRR) cells from human cancer cell lines were established by exposing to long-term fractionated radiation of X-rays. Correspondingly, microarray analysis and real time RT-PCR were performed to find miRNA involved in the CRR phenotype. HIF-1α was down-regulated and miR17-92 cluster was overexpressed in CRR cells by transfection. The expression of miR 17-3p was inhibited by specific inhibitors and miR 19a was enforced by mimics, respectively in parental cells. Overexpression of HIF-1α in parental cells or down regulation of HIF-1α in CRR cells were not involved in radioresistance. However, when HIF-1α was genetically modified to constitutively express under normoxia condition, it was rendered for protection to cells. Exogenous overexpression of miR 17-92 cluster in CRR cells resulted in abolition of HIF-1α expression and restored sensitizations to ionizing radiation. Attenuated expression of miR-17-3p in parental cells protected them from irradiation. Overall, fine-tune deregulation of miR 17-92 cluster in CRR cells might account for the accumulation of HIF-1α in the CRR cells following exposure to irradiation.

MiR-7-5p is a key factor that controls radioresistance via intracellular Fe2+ content in clinically relevant radioresistant cells

MicroRNA (miRNA) is a non-coding RNA involved in regulating both cancer gene promotion and suppression. We investigated the role of miRNA in inducing radiation resistance in cancer cell lines using clinically relevant radioresistant (CRR) cells. Analysis using miRNA arrays and qPCR revealed that miR-7-5p is highly expressed in all examined CRR cells. Additionally, CRR cells lose their radioresistance when daily irradiation is interrupted for over 6 months. MiR-7-5p expression is reduced in these cells, and treating CRR cells with a miR-7-5p inhibitor leads to a loss of resistance to irradiation. Conversely, overexpression of miR-7-5p in CRR cells using a miR-7-5p mimic shows further resistance to radiation. Overexpression of miR-7-5p in parent cells also results in resistance to radiation. These results indicate that miR-7-5p may control radioresistance in various cancer cells at the clinically relevant dose of irradiation. Furthermore, miR-7-5p downregulates mitoferrin and reduces Fe²⁺, which influences ferroptosis. Our findings have great potential not only for examining radiation resistance prior to treatment but also for providing new therapeutic agents for treatment-resistant cancers.

Lipid peroxidation increases hydrogen peroxide permeability leading to cell death in cancer cell lines that lack mtDNA

4-Hydroxynonenal (HNE) is an important product of plasma membrane lipid peroxidation, which is a cause of cell and tissue injury. Mitochondrial DNA (mtDNA)-depleted ρ⁰ cells were established using human cervical cancer and oral squamous cell carcinoma cell lines. We investigated the effect of reactive oxygen species in ρ⁰ cells, especially the mechanism of hydrogen peroxide (H₂ O₂ )-mediated cell death. These cell were subjected to high oxidative stress and, compared with their parental cells, showed greater sensitivity to H₂ O₂ and high lipid peroxidation. Upregulation of HNE in the plasma membrane was observed prior to the increase in intracellular H₂ O₂ . The amount of oxidized lipid present changed H₂ O₂ permeability and administration of oxidized lipid led to further cell death after treatment with H₂ O₂ . Expression levels of lipoxygenase ALOX genes (ie ALOX5, ALOX12, and ALOX15) were upregulated in ρ⁰ cells, as were expression levels of ALOX12 and ALOX15 proteins. ALOX5 protein was mainly distributed in the nucleus, while ALOX12 and ALOX15 proteins were distributed in the nucleus and the cytoplasm. Although expression of COX2 gene was upregulated, its protein expression did not increase. ALOX (especially ALOX15) may be involved in the sensitivity of cancer cells to treatment. These data offer promise for the development of novel anticancer agents by altering the oxidation state of the plasma membrane. Our results showed that lipid peroxidation status is important for H₂ O₂ sensitivity and that ALOX15 is involved in lipid peroxidation status.

Challenges and Contradictions of Metal Nano-Particle Applications for Radio-Sensitivity Enhancement in Cancer Therapy

From the very beginnings of radiotherapy, a crucial question persists with how to target the radiation effectiveness into the tumor while preserving surrounding tissues as undamaged as possible. One promising approach is to selectively pre-sensitize tumor cells by metallic nanoparticles. However, though the "physics" behind nanoparticle-mediated radio-interaction has been well elaborated, practical applications in medicine remain challenging and often disappointing because of limited knowledge on biological mechanisms leading to cell damage enhancement and eventually cell death. In the present study, we analyzed the influence of different nanoparticle materials (platinum (Pt), and gold (Au)), cancer cell types (HeLa, U87, and SKBr3), and doses (up to 4 Gy) of low-Linear Energy Transfer (LET) ionizing radiation (γ- and X-rays) on the extent, complexity and reparability of radiation-induced γH2AX + 53BP1 foci, the markers of double stand breaks (DSBs). Firstly, we sensitively compared the focus presence in nuclei during a long period of time post-irradiation (24 h) in spatially (three-dimensionally, 3D) fixed cells incubated and non-incubated with Pt nanoparticles by means of high-resolution immunofluorescence confocal microscopy. The data were compared with our preliminary results obtained for Au nanoparticles and recently published results for gadolinium (Gd) nanoparticles of approximately the same size (2⁻3 nm). Next, we introduced a novel super-resolution approach-single molecule localization microscopy (SMLM)-to study the internal structure of the repair foci. In these experiments, 10 nm Au nanoparticles were used that could be also visualized by SMLM. Altogether, the data show that different nanoparticles may or may not enhance radiation damage to DNA, so multi-parameter effects have to be considered to better interpret the radiosensitization. Based on these findings, we discussed on conclusions and contradictions related to the effectiveness and presumptive mechanisms of the cell radiosensitization by nanoparticles. We also demonstrate that SMLM offers new perspectives to study internal structures of repair foci with the goal to better evaluate potential differences in DNA damage patterns.