Ionizing radiation accelerates Drp1-dependent mitochondrial fission, which involves delayed mitochondrial reactive oxygen species production in normal human fibroblast-like cells

Differential miRNA expression profiling reveals miR-205-3p to be a potential radiosensitizer for low- dose ionizing radiation in DLD-1 cells

Enhanced radiosensitivity at low doses of ionizing radiation (IR) (0.2 to 0.6 Gy) has been reported in several cell lines. This phenomenon, known as low doses hyper-radiosensitivity (LDHRS), appears as an opportunity to decrease toxicity of radiotherapy and to enhance the effects of chemotherapy. However, the effect of low single doses IR on cell death is subtle and the mechanism underlying LDHRS has not been clearly explained, limiting the utility of LDHRS for clinical applications. To understand the mechanisms responsible for cell death induced by low-dose IR, LDHRS was evaluated in DLD-1 human colorectal cancer cells and the expression of 80 microRNAs (miRNAs) was assessed by qPCR array. Our results show that DLD-1 cells display an early DNA damage response and apoptotic cell death when exposed to 0.6 Gy. miRNA expression profiling identified 3 over-expressed (miR-205-3p, miR-1 and miR-133b) and 2 down-regulated miRNAs (miR-122-5p, and miR-134-5p) upon exposure to 0.6 Gy. This miRNA profile differed from the one in cells exposed to high-dose IR (12 Gy), supporting a distinct low-dose radiation-induced cell death mechanism. Expression of a mimetic miR-205-3p, the most overexpressed miRNA in cells exposed to 0.6 Gy, induced apoptotic cell death and, more importantly, increased LDHRS in DLD-1 cells. Thus, we propose miR-205-3p as a potential radiosensitizer to low-dose IR.

Role of Ionizing Radiation in Neurodegenerative Diseases

Ionizing radiation (IR) from terrestrial sources is continually an unprotected peril to human beings. However, the medical radiation and global radiation background are main contributors to human exposure and causes of radiation sickness. At high-dose exposures acute radiation sickness occurs, whereas chronic effects may persist for a number of years. Radiation can increase many circulatory, age related and neurodegenerative diseases. Neurodegenerative diseases occur a long time after exposure to radiation, as demonstrated in atomic bomb survivors, and are still controversial. This review discuss the role of IR in neurodegenerative diseases and proposes an association between neurodegenerative diseases and exposure to IR.

Different mitochondrial fragmentation after irradiation with X-rays and carbon ions in HeLa cells and its influence on cellular apoptosis

Although mitochondria are known to play an important role in radiation-induced cellular damage, the mechanisms by which ionizing radiation modulates mitochondrial dynamics are largely unknown. In this study, human cervical carcinoma cell line HeLa was used to demonstrate the different modes of mitochondrial network in response to different quality radiations such as low linear energy transfer (LET) X-rays and high-LET carbon ions. Mitochondria fragmented into punctate and clustered ones upon carbon ion irradiation in a dose- and LET-dependent manner, which was associated with apoptotic cell death. In contrast, low-dose X-ray irradiation promoted mitochondrial fusion while mitochondrial fission was detected until the radiation dose was more than 1 Gy. This fission was driven by ERK1/2-mediated phosphorylation of Drp1 on Serine 616. Inhibition of mitochondrial fragmentation suppressed the radiation-induced apoptosis and thus enhanced the resistance of cells to carbon ions and high-dose X-rays, but not for cells irradiated with X-rays at the low dose. Our results suggest that radiations of different qualities cause diverse changes of mitochondrial dynamics in cancer cells, which play an important role in determining the cell fate.

Potential relationship between the biological effects of low-dose irradiation and mitochondrial ROS production

Exposure to ionizing radiation (IR) induces various types of DNA damage, of which DNA double-strand breaks are the most severe, leading to genomic instability, tumorigenesis, and cell death. Hence, cells have developed DNA damage responses and repair mechanisms. IR also causes the accumulation of endogenous reactive oxidative species (ROS) in the irradiated cells. Upon exposure to low-dose irradiation, the IR-induced biological effects mediated by ROS were relatively more significant than those mediated by DNA damage. Accumulating evidence suggests that such increase in endogenous ROS is related with mitochondria change in irradiated cells. Thus, in this review we focused on the mechanism of mitochondrial ROS production and its relationship to the biological effects of IR. Exposure of mammalian cells to IR stimulates an increase in the production of endogenous ROS by mitochondria, which potentially leads to mitochondrial dysfunction. Since the remains of damaged mitochondria could generate or leak more ROS inside the cell, the damaged mitochondria are removed by mitophagy. The disruption of this pathway, involved in maintaining mitochondrial integrity, could lead to several disorders (such as neurodegeneration) and aging. Thus, further investigation needs to be performed in order to understand the relationship between the biological effects of low-dose IR and mitochondrial integrity.

Diverse roles of mitochondria in ischemic stroke

Stroke is the leading cause of adult disability and mortality in most developing and developed countries. The current best practices for patients with acute ischemic stroke include intravenous tissue plasminogen activator and endovascular thrombectomy for large-vessel occlusion to improve clinical outcomes. However, only a limited portion of patients receive thrombolytic therapy or endovascular treatment because the therapeutic time window after ischemic stroke is narrow. To address the current shortage of stroke management approaches, it is critical to identify new potential therapeutic targets. The mitochondrion is an often overlooked target for the clinical treatment of stroke. Early studies of mitochondria focused on their bioenergetic role; however, these organelles are now known to be important in a wide range of cellular functions and signaling events. This review aims to summarize the current knowledge on the mitochondrial molecular mechanisms underlying cerebral ischemia and involved in reactive oxygen species generation and scavenging, electron transport chain dysfunction, apoptosis, mitochondrial dynamics and biogenesis, and inflammation. A better understanding of the roles of mitochondria in ischemia-related neuronal death and protection may provide a rationale for the development of innovative therapeutic regimens for ischemic stroke and other stroke syndromes.

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Review of current treatment of ischemic stroke indicates deficiency in the contemporary methods.

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Discuss the mitochondrial ROS-related signaling that affect neuronal fate after ischemic stroke.

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Mechanisms of mitochondrial dynamics and mitophagy could be pivotal for ischemic stroke.

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Inhibiting mitochondrion-induced inflammatory response is a potential treatment for ischemic stroke.

Calmodulin-dependent protein kinase II (CaMKII) mediates radiation-induced mitochondrial fission by regulating the phosphorylation of dynamin-related protein 1 (Drp1) at serine 616

Mitochondrial dynamics are suggested to be indispensable for the maintenance of cellular quality and function in response to various stresses. While ionizing radiation (IR) stimulates mitochondrial fission, which is mediated by the mitochondrial fission protein, dynamin-related protein 1 (Drp1), it remains unclear how IR promotes Drp1 activation and subsequent mitochondrial fission. Therefore, we conducted this study to investigate these concerns. First, we found that X-irradiation triggered Drp1 phosphorylation at serine 616 (S616) but not at serine 637 (S637). Reconstitution analysis revealed that introduction of wild-type (WT) Drp1 recovered radiation-induced mitochondrial fission, which was absent in Drp1-deficient cells. Compared with cells transfected with WT or S637A Drp1, the change in mitochondrial shape following irradiation was mitigated in S616A Drp1-transfected cells. Furthermore, inhibition of CaMKII significantly suppressed Drp1 S616 phosphorylation and mitochondrial fission induced by IR. These results suggest that Drp1 phosphorylation at S616, but not at S637, is prerequisite for radiation-induced mitochondrial fission and that CaMKII regulates Drp1 phosphorylation at S616 following irradiation.

N-Acetyl-L-cysteine protects thyroid cells against DNA damage induced by external and internal irradiation

We evaluated the effect of the antioxidant N-acetyl-L-cysteine (NAC) on the levels of reactive oxygen species (ROS), DNA double strand breaks (DSB) and micronuclei (MN) induced by internal and external irradiation using a rat thyroid cell line PCCL3. In internal irradiation experiments, ROS and DSB levels increased immediately after ¹³¹I addition and then gradually declined, resulting in very high levels of MN at 24 and 48 h. NAC administration both pre- and also post-¹³¹I addition suppressed ROS, DSB and MN. In external irradiation experiments with a low dose (0.5 Gy), ROS and DSB increased shortly and could be prevented by NAC administration pre-, but not post-irradiation. In contrast, external irradiation with a high dose (5 Gy) increased ROS and DSB in a bimodal way: ROS and DSB levels increased immediately after irradiation, quickly returned to the basal levels and gradually rose again after >24 h. The second phase was in parallel with an increase in 4-hydroxy-2-nonenal. The number of MN induced by the second wave of ROS/DSB elevations was much higher than that by the first peak. In this situation, NAC administered pre- and post-irradiation comparably suppressed MN induced by a delayed ROS elevation. In conclusion, a prolonged ROS increase during internal irradiation and a delayed ROS increase after external irradiation with a high dose caused serious DNA damage, which were efficiently prevented by NAC. Thus, NAC administration even both after internal or external irradiation prevents ROS increase and eventual DNA damage.

Mdivi-1 ameliorates early brain injury after subarachnoid hemorrhage via the suppression of inflammation-related blood-brain barrier disruption and endoplasmic reticulum stress-based apoptosis

Aberrant modulation of mitochondrial dynamic network, which shifts the balance of fusion and fission towards fission, is involved in brain damage of various neurodegenerative diseases including Parkinson's disease, Huntington's disease and Alzheimer's disease. A recent research has shown that the inhibition of mitochondrial fission alleviates early brain injury after experimental subarachnoid hemorrhage, however, the underlying molecular mechanisms have remained to be elucidated. This study was undertaken to characterize the effects of the inhibition of dynamin-related protein-1 (Drp1, a dominator of mitochondrial fission) on blood-brain barrier (BBB) disruption and neuronal apoptosis following SAH and the potential mechanisms. The endovascular perforation model of SAH was performed in adult male Sprague Dawley rats. The results indicated Mdivi-1(a selective Drp1 inhibitor) reversed the morphologic changes of mitochondria and Drp1 translocation, reduced ROS levels, ameliorated the BBB disruption and brain edema remarkably, decreased the expression of MMP-9 and prevented degradation of tight junction proteins-occludin, claudin-5 and ZO-1. Mdivi-1 administration also inhibited the nuclear translocation of nuclear factor-kappa B (NF-κB), leading to decreased expressions of TNF-ɑ, IL-6 and IL-1ß. Moreover, Mdivi-1 treatment attenuated neuronal cell death and improved neurological outcome. To investigate the underlying mechanisms further, we determined that Mdivi-1 reduced p-PERK, p-eIF2α, CHOP, cleaved caspase-3 and Bax expression as well as increased Bcl-2 expression. Rotenone (a selective inhibitor of mitochondrial complexes I) abolished both the anti-BBB disruption and anti-apoptosis effects of Mdivi-1. In conclusion, these data implied that excessive mitochondrial fission might inhibit mitochondrial complex I to become a cause of oxidative stress in SAH, and the inhibition of Drp1 by Mdivi-1 attenuated early brain injury after SAH probably via the suppression of inflammation-related blood-brain barrier disruption and endoplasmic reticulum stress-based apoptosis.

Involvement of reactive oxygen species in ionizing radiation-induced upregulation of cell surface Toll-like receptor 2 and 4 expression in human monocytic cells

Toll-like receptors (TLRs) are pattern recognition receptors that recognize pathogen-associated molecular patterns and are indispensable for antibacterial and antiviral immunity. Our previous report showed that ionizing radiation increases the cell surface expressions of TLR2 and TLR4 and enhances their responses to agonists in human monocytic THP1 cells. The present study investigated how ionizing radiation increases the cell surface expressions of TLR2 and TLR4 in THP1 cells. The THP1 cells treated or not treated with pharmaceutical agents such as cycloheximide and N-acetyl-L-cysteine (NAC) were exposed to X-ray irradiation, following which the expressions of TLRs and mitogen-activated protein kinase were analyzed. X-ray irradiation increased the mRNA expressions of TLR2 and TLR4, and treatment with a protein synthesis inhibitor cycloheximide abolished the radiation-induced upregulation of their cell surface expressions. These results indicate that radiation increased those receptors through de novo protein synthesis. Furthermore, treatment with an antioxidant NAC suppressed not only the radiation-induced upregulation of cell surface expressions of TLR2 and TLR4, but also the radiation-induced activation of the c-Jun N-terminal kinase (JNK) pathway. Since it has been shown that the inhibitor for JNK can suppress the radiation-induced upregulation of TLR expression, the present results suggest that ionizing radiation increased the cell surface expressions of TLR2 and TLR4 through reactive oxygen species-mediated JNK activation.

Analysis of the mechanism of radiation-induced upregulation of mitochondrial abundance in mouse fibroblasts

Mitochondria strongly contribute to the maintenance of cellular integrity through various mechanisms, including oxidative adenosine triphosphate production and calcium homeostasis regulation. Therefore, proper regulation of the abundance, distribution and activity of mitochondria is crucial for the maintenance of cellular homeostasis. Previous studies have shown that ionizing radiation (IR) alters mitochondrial functions, suggesting that mitochondria are likely to be an important target of IR. Though IR reportedly influences cellular mitochondrial abundance, the mechanism remains largely unknown. In this study, we examined how IR influences mitochondrial abundance in mouse fibroblasts. When mouse NIH/3T3 cells were exposed to X-rays, a time-dependent increase was observed in mitochondrial DNA (mtDNA) and mitochondrial mass, indicating radiation-induced upregulation of mitochondrial abundance. Meanwhile, not only did we not observe a significant change in autophagic activity after irradiation, but in addition, IR hardly influenced the expression of two mitochondrial proteins, cytochrome c oxidase subunit IV and cytochrome c, or the mRNA expression of Polg, a component of DNA polymerase γ. We also observed that the expression of transcription factors involved in mitochondrial biogenesis was only marginally affected by IR. These data imply that radiation-induced upregulation of mitochondrial abundance is an event independent of macroautophagy and mitochondrial biogenesis. Furthermore, we found evidence that IR induced long-term cell cycle arrest and cellular senescence, indicating that these events are involved in regulating mitochondrial abundance. Considering the growing significance of mitochondria in cellular radioresponses, we believe the present study provides novel insights into understanding the effects of IR on mitochondria.

Metabolic analysis of radioresistant medulloblastoma stem-like clones and potential therapeutic targets

Medulloblastoma is a fatal brain tumor in children, primarily due to the presence of treatment-resistant medulloblastoma stem cells. The energy metabolic pathway is a potential target of cancer therapy because it is often different between cancer cells and normal cells. However, the metabolic properties of medulloblastoma stem cells, and whether specific metabolic pathways are essential for sustaining their stem cell-like phenotype and radioresistance, remain unclear. We have established radioresistant medulloblastoma stem-like clones (rMSLCs) by irradiation of the human medulloblastoma cell line ONS-76. Here, we assessed reactive oxygen species (ROS) production, mitochondria function, oxygen consumption rate (OCR), energy state, and metabolites of glycolysis and tricarboxylic acid cycle in rMSLCs and parental cells. rMSLCs showed higher lactate production and lower oxygen consumption rate than parental cells. Additionally, rMSLCs had low mitochondria mass, low endogenous ROS production, and existed in a low-energy state. Treatment with the metabolic modifier dichloroacetate (DCA) resulted in mitochondria dysfunction, glycolysis inhibition, elongated mitochondria morphology, and increased ROS production. DCA also increased radiosensitivity by suppression of the DNA repair capacity through nuclear oxidization and accelerated the generation of acetyl CoA to compensate for the lack of ATP. Moreover, treatment with DCA decreased cancer stem cell-like characters (e.g., CD133 positivity and sphere-forming ability) in rMSLCs. Together, our findings provide insights into the specific metabolism of rMSLCs and illuminate potential metabolic targets that might be exploited for therapeutic benefit in medulloblastoma.

Grape seed proanthocyanidins prevent irradiation-induced differentiation of human lung fibroblasts by ameliorating mitochondrial dysfunction

Radiation-induced lung fibrosis (RILF) is a long-term adverse effect of curative radiotherapy. The accumulation of myofibroblasts in fibroblastic foci is a pivotal feature of RILF. In the study, we found the inhibitory effect of grape seed proanthocyanidins (GSPs) on irradiation-induced differentiation of human fetal lung fibroblasts (HFL1). To explore the mechanism by which GSPs inhibit fibroblast differentiation, we measured the reactive oxygen species (ROS) levels, mitochondrial function, mitochondrial dynamics, glycolysis and the signaling molecules involved in fibroblast transdifferentiation. GSPs significantly reduced the production of cellular and mitochondrial ROS after radiation. The increases in mitochondrial respiration, proton leak, mitochondrial ATP production, lactate release and glucose consumption that occurred in response to irradiation were ameliorated by GSPs. Furthermore, GSPs increased the activity of complex I and improved the mitochondrial dynamics, which were disturbed by irradiation. In addition, the elevation of phosphorylation of p38MAPK and Akt, and Nox4 expression induced by irradiation were attenuated by GSPs. Blocking Nox4 attenuated irradiation-mediated fibroblast differentiation. Taken together, these results indicate that GSPs have the ability to inhibit irradiation-induced fibroblast-to-myofibroblast differentiation by ameliorating mitochondrial dynamics and mitochondrial complex I activity, regulating mitochondrial ROS production, ATP production, lactate release, glucose consumption and thereby inhibiting p38MAPK-Akt-Nox4 pathway.

Monitoring Mitochondrial Complex-I Activity Using Novel PET Probe 18F-BCPP-EF Allows Early Detection of Radiotherapy Effect in Murine Squamous Cell Carcinoma

Objectives

Aerobic glycolysis, the main pathway of energy production in tumors (Warburg effect) allows detection of tumors by positron emission tomography (PET) using ¹⁸F-fluoro-2-deoxy-D-glucose (¹⁸F-FDG). Since ionizing radiation (IR) is reported to switch aerobic glycolysis to mitochondrial oxidative phosphorylation, radiotherapeutic efficacy was monitored by the activity of mitochondrial complex I (MC-I), using a new PET probe ¹⁸F-BCPP-EF, ¹⁸F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoro-ethoxy)-ethoxy] -pyridine-3-ylmethoxy}-2H-pyridazin-3-one, compared with ¹⁸F-FDG uptake and the apoptosis index.

Methods

Tumor uptake of ¹⁸F-BCPP-EF or ¹⁸F-FDG was examined in C3H/HeN mice inoculated with murine squamous cell carcinoma SCCVII at various time points after a single dose of x-ray irradiation at 0, 6, 15, or 30 Gy. Apoptosis incidence was determined by TUNEL staining in excised tumor tissue.

Results

Tumor growth suppression was dose-dependent; tumor grew 10-fold (0 Gy), 5-fold (6 Gy), 2-fold (15 Gy), and reduced to half in its volume (30 Gy) 14 days after treatment. ¹⁸F-BCPP-EF uptake was significantly increased as early as 3 days after 15 Gy or 30 Gy, when tumor size and apoptosis index showed no difference among radiation doses. In contrast, ¹⁸F-FDG uptake was initially increased dose-dependently, remained elevated up to 7 days, and eventually decreased 10 days after 30 Gy and also 14 days after 15 Gy when tumor size was already reduced. Apoptosis index was increased after irradiation but failed to correlate with tumor response.

Conclusion

Tumor uptake of ¹⁸F-BCPP-EF was increased dose-dependently early after effective doses of IR when ¹⁸F-FDG uptake as well as apoptosis incidence were not indicative of tumor response. The results suggest that ¹⁸F-BCPP-EF is a promising “positive” MC-I imaging PET probe for early detection of efficacy of tumor radiotherapy.

Ionizing Radiation Induces Innate Immune Responses in Macrophages by Generation of Mitochondrial Reactive Oxygen Species

During radiotherapy for tumors, the innate immune system also responds to ionizing radiation and induces immune modulation. However, little is known about the molecular mechanisms by which radiation modulates innate immune responses. In this study, we observed that radiation triggered the generation of mitochondrial reactive oxygen species (mROS), leading to innate immune responses in murine bone marrow-derived macrophages (BMDM). Radiation-induced mROS was essential for robust induction of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and IL-12p40 mRNA and protein in BMDM. Exposure to radiation also led to rapid activation of the mitogen-activated protein kinase (MAPK) and nuclear factor (NF)-κB pathways in BMDM. Notably, radiation-induced MAPK activation and NF-κB signaling were regulated by mROS in macrophages. Additionally, radiation-induced expression of TNF-α, IL-6 and IL-12p40 was dependent on JNK, p38 and NF-κB activation in BMDM. These data suggest a key role for radiation-induced pro-inflammatory responses and activation of the MAPK and NF-κB pathways through a triggering mechanism involving mROS generation.

Mdivi-1 Protects Epileptic Hippocampal Neurons from Apoptosis via Inhibiting Oxidative Stress and Endoplasmic Reticulum Stress in Vitro

Mitochondrial division inhibitor 1 (mdivi-1), a selective inhibitor of the mitochondrial fission protein dynamin-related protein 1, has been proposed to have a neuroprotective effect on hippocampal neurons in animal models of epilepsy. However, the effect of mdivi-1 on epileptic neuronal death in vitro remains unknown. Therefore, we investigated the effect of mdivi-1 and the underlying mechanisms in the hippocampal neuronal culture (HNC) model of acquired epilepsy (AE) in vitro. We found that mitochondrial fission was increased in the HNC model of AE and inhibition of mitochondrial fission by mdivi-1 significantly decreased neuronal apoptosis induced by AE. In addition, mdivi-1 pretreatment significantly attenuated oxidative stress induced by AE characterized by decrease of reactive oxygen species (ROS) production and malondialdehyde level and by increase of superoxide dismutase activity. Moreover, mdivi-1 pretreatment significantly decreased endoplasmic reticulum (ER) stress markers glucose-regulated protein 78, C/EBP homologous protein expression and caspase-3 activation. Altogether, our findings suggest that mdivi-1 protected against AE-induced hippocampal neuronal apoptosis in vitro via decreasing ROS-mediated oxidative stress and ER stress.

Peroxisome proliferator-activated receptor-gamma dependent pathway reduces the phosphorylation of dynamin-related protein 1 and ameliorates hippocampal injury induced by global ischemia in rats

BACKGROUND

Dynamin-related protein 1 (Drp1) is a mitochondrial fission protein that, upon phosphorylation at serine 616 (p-Drp1(Ser616)), plays a pivotal role in neuronal death after ischemia. In the present study, we hypothesized that peroxisome proliferator-activated receptor-gamma (PPARγ)-dependent pathway can reduce the expression of p-Drp1(Ser616) and ameliorate hippocampal injury induced by global ischemia in rats.

RESULTS

We found that pretreatment of the rats with Mdivi-1, a selective Drp1 inhibitor, decreased the level of transient global ischemia (TGI)-induced p-Drp1(Ser616) and reduced cellular contents of oxidized proteins, activated caspase-3 expression as well as the extent of DNA fragmentation. Delivery of siRNA against Drp1 attenuated the expression of p-Drp1(Ser616) that was accompanied by alleviation of the TGI-induced protein oxidation, activated caspase-3 expression and DNA fragmentation in hippocampal proteins. Exogenous application of pioglitazone, a PPARγ agonist, reduced the p-Drp1(Ser616) expression, decreased TGI-induced oxidative stress and activated caspase-3 expression, lessened the extents of DNA fragmentation, and diminished the numbers of TUNEL-positive neuronal cells; all of these effects were reversed by GW9662, a PPARγ antagonist.

CONCLUSIONS

Our findings thus indicated that inhibition of TGI-induced p-Drp1(Ser616) expression by Drp1 inhibitor and Drp1-siRNA can decrease protein oxidation, activated caspase-3 expression and neuronal damage in the hippocampal CA1 subfield. PPARγ agonist, through PPARγ-dependent mechanism and via decreasing p-Drp1(Ser616) expression, can exert anti-oxidative and anti-apoptotic effects against ischemic neuronal injury.

Ionizing radiation and heart risks

Cardiovascular disease and cancer are the two leading causes of morbidity and mortality worldwide. As advancements in radiation therapy (RT) have significantly increased the number of cancer survivors, the risk of radiation-induced cardiovascular disease (RICD) in this group is a growing concern. Recent epidemiological data suggest that accidental or occupational exposure to low dose radiation, in addition to therapeutic ionizing radiation, can result in cardiovascular complications. The progression of radiation-induced cardiotoxicity often takes years to manifest but is also multifaceted, as the heart may be affected by a variety of pathologies. The risk of cardiovascular disease development in RT cancer survivors has been known for 40 years and several risk factors have been identified in the last two decades. However, most of the early work focused on clinical symptoms and manifestations, rather than understanding cellular processes regulating homeostatic processes of the cardiovascular system in response to radiation. Recent studies have suggested that a different approach may be needed to refute the risk of cardiovascular disease following radiation exposure. In this review, we will focus on how different radiation types and doses may induce cardiovascular complications, highlighting clinical manifestations and the mechanisms involved in the pathophysiology of radiation-induced cardiotoxicity. We will finally discuss how current and future research on heart development and homeostasis can help reduce the incidence of RICD.

Inhibition of the mitochondrial fission protein dynamin-related protein 1 (Drp1) impairs mitochondrial fission and mitotic catastrophe after x-irradiation

The role of mitochondrial dynamics in cellular responses to ionizing radiation (IR) is still largely unknown. This study demonstrates that IR triggers Drp1-dependent mitochondrial fission and that Drp1 inhibition attenuates radiation-induced mitotic catastrophe, suggesting that Drp1 is involved in determining the fate of cells after irradiation.

Accumulating evidence suggests that mitochondrial dynamics is crucial for the maintenance of cellular quality control and function in response to various stresses. However, the role of mitochondrial dynamics in cellular responses to ionizing radiation (IR) is still largely unknown. In this study, we provide evidence that IR triggers mitochondrial fission mediated by the mitochondrial fission protein dynamin-related protein 1 (Drp1). We also show IR-induced mitotic catastrophe (MC), which is a type of cell death associated with defective mitosis, and aberrant centrosome amplification in mouse embryonic fibroblasts (MEFs). These are attenuated by genetic or pharmacological inhibition of Drp1. Whereas radiation-induced aberrant centrosome amplification and MC are suppressed by the inhibition of Plk1 and CDK2 in wild-type MEFs, the inhibition of these kinases is ineffective in Drp1-deficient MEFs. Furthermore, the cyclin B1 level after irradiation is significantly higher throughout the time course in Drp1-deficient MEFs than in wild-type MEFs, implying that Drp1 is involved in the regulation of cyclin B1 level. These findings strongly suggest that Drp1 plays an important role in determining the fate of cells after irradiation via the regulation of mitochondrial dynamics.

Targeting mitochondria in cancer cells using gold nanoparticle-enhanced radiotherapy: a Monte Carlo study

PURPOSE

Radiation damage to mitochondria has been shown to alter cellular processes and even lead to apoptosis. Gold nanoparticles (AuNPs) may be used to enhance these effects in scenarios where they collect on the outer membranes of mitochondria. A Monte Carlo (MC) approach is used to estimate mitochondrial dose enhancement under a variety of conditions.

METHODS

The penelope MC code was used to generate dose distributions resulting from photons striking a 13 nm diameter AuNP with various thicknesses of water-equivalent coatings. Similar dose distributions were generated with the AuNP replaced by water so as to estimate the gain in dose on a microscopic scale due to the presence of AuNPs within an irradiated volume. Models of mitochondria with AuNPs affixed to their outer membrane were then generated-considering variation in mitochondrial size and shape, number of affixed AuNPs, and AuNP coating thickness-and exposed (in a dose calculation sense) to source spectra ranging from 6 MV to 90 kVp. Subsequently dose enhancement ratios (DERs), or the dose with the AuNPs present to that for no AuNPs, for the entire mitochondrion and its components were tallied under these scenarios.

RESULTS

For a representative case of a 1000 nm diameter mitochondrion affixed with 565 AuNPs, each with a 13 nm thick coating, the mean DER over the whole organelle ranged from roughly 1.1 to 1.6 for the kilovoltage sources, but was generally less than 1.01 for the megavoltage sources. The outer membrane DERs remained less than 1.01 for the megavoltage sources, but rose to 2.3 for 90 kVp. The voxel maximum DER values were as high as 8.2 for the 90 kVp source and increased further when the particles clustered together. The DER exhibited dependence on the mitochondrion dimensions, number of AuNPs, and the AuNP coating thickness.

CONCLUSIONS

Substantial dose enhancement directly to the mitochondria can be achieved under the conditions modeled. If the mitochondrion dose can be directly enhanced, as these simulations show, this work suggests the potential for both a tool to study the role of mitochondria in cellular response to radiation and a novel avenue for radiation therapy in that the mitochondria may be targeted, rather than the nuclear DNA.

Live-cell imaging study of mitochondrial morphology in mammalian cells exposed to X-rays

Morphological changes in mitochondria induced by X-irradiation in normal murine mammary gland cells were studied with a live-cell microscopic imaging technique. Mitochondria were visualised by staining with a specific fluorescent probe in the cells, which express fluorescent ubiquitination-based cell-cycle indicator 2 (Fucci2) probes to visualise cell cycle. In unirradiated cells, the number of cells with fragmented mitochondria was about 20 % of the total cells through observation period (96 h). In irradiated cells, the population with fragmented mitochondria significantly increased depending on the absorbed dose. Particularly, for 8 Gy irradiation, the accumulation of fragmentation persists even in the cells whose cell cycle came to a stand (80 % in G1 (G0-like) phase). The fraction reached to a maximum at 96 h after irradiation. The kinetics of the fraction with fragmented mitochondria was similar to that for cells in S/G2/M phase (20 %) through the observation period (120 h). The evidences show that, in irradiated cells, some signals are continually released from a nucleus or cytoplasm even in the G0-like cells to operate some sort of protein machineries involved in mitochondrial fission. It is inferred that this delayed mitochondrial fragmentation is strongly related to their dysfunction, and hence might modulate radiobiological effects such as mutation or cell death.

Ionizing radiation-induced cell death is partly caused by increase of mitochondrial reactive oxygen species in normal human fibroblast cells

Radiation-induced cell death is thought to be caused by nuclear DNA damage that cannot be repaired. However, in this study we found that a delayed increase of mitochondrial reactive oxygen species (ROS) is responsible for some of the radiation-induced cell death in normal human fibroblast cells. We have previously reported that there is a delayed increase of mitochondrial (·)O2(-), measured using MitoSOX™ Red reagent, due to gamma irradiation. This is dependent on Drp1 localization to mitochondria. Here, we show that knockdown of Drp1 expression reduces the level of DNA double-strand breaks (DSBs) remaining 3 days after 6 Gy irradiation. Furthermore, cells with knockdown of Drp1 expression are more resistant to gamma radiation. We then tested whether the delayed increase of ROS causes DNA damage. The antioxidant, 2-glucopyranoside ascorbic acid (AA-2G), was applied before or after irradiation to inhibit ROS production during irradiation or to inhibit delayed ROS production from mitochondria. Interestingly, 1 h after exposure, the AA-2G treatment reduced the level of DSBs remaining 3 days after 6 Gy irradiation. In addition, irradiated AA-2G-treated cells were more resistant to radiation than the untreated cells. These results indicate that delayed mitochondrial ROS production may cause some of the cell death after irradiation.

Protracted Oxidative Alterations in the Mechanism of Hematopoietic Acute Radiation Syndrome

The biological effects of high-dose total body ionizing irradiation [(thereafter, irradiation (IR)] are attributed to primary oxidative breakage of biomolecule targets, mitotic, apoptotic and necrotic cell death in the dose-limiting tissues, clastogenic and epigenetic effects, and cascades of functional and reactive responses leading to radiation sickness defined as the acute radiation syndrome (ARS). The range of remaining and protracted injuries at any given radiation dose as well as the dynamics of post-IR alterations is tissue-specific. Therefore, functional integrity of the homeostatic tissue barriers may decline gradually within weeks in the post-IR period culminating with sepsis and failure of organs and systems. Multiple organ failure (MOF) leading to moribundity is a common sequela of the hemotapoietic form of ARS (hARS). Onset of MOF in hARS can be presented as “two-hit phenomenon” where the “first hit” is the underlying consequences of the IR-induced radiolysis in cells and biofluids, non-septic inflammation, metabolic up-regulation of pro-oxidative metabolic reactions, suppression of the radiosensitive hematopoietic and lymphoid tissues and the damage to gut mucosa and vascular endothelium. While the “second hit” derives from bacterial translocation and spread of the bacterial pathogens and inflammagens through the vascular system leading to septic inflammatory, metabolic responses and a cascade of redox pro-oxidative and adaptive reactions. This sequence of events can create a ground for development of prolonged metabolic, inflammatory, oxidative, nitrative, and carbonyl, electrophilic stress in crucial tissues and thus exacerbate the hARS outcomes. With this perspective, the redox mechanisms, which can mediate the IR-induced protracted oxidative post-translational modification of proteins, oxidation of lipids and carbohydrates and their countermeasures in hARS are subjects of the current review. Potential role of ubiquitous, radioresistant mesenchymal stromal cells in the protracted responses to IR and IR-related septicemia is also discussed.

Increased effectiveness of carbon ions in the production of reactive oxygen species in normal human fibroblasts

The production of reactive oxygen species (ROS), especially superoxide anions (O2 (·-)), is enhanced in many normal and tumor cell types in response to ionizing radiation. The influence of ionizing radiation on the regulation of ROS production is considered as an important factor in the long-term effects of irradiation (such as genomic instability) that might contribute to the development of secondary cancers. In view of the increasing application of carbon ions in radiation therapy, we aimed to study the potential impact of ionizing density on the intracellular production of ROS, comparing photons (X-rays) with carbon ions. For this purpose, we used normal human cells as a model for irradiated tissue surrounding a tumor. By quantifying the oxidization of Dihydroethidium (DHE), a fluorescent probe sensitive to superoxide anions, we assessed the intracellular ROS status after radiation exposure in normal human fibroblasts, which do not show radiation-induced chromosomal instability. After 3-5 days post exposure to X-rays and carbon ions, the level of ROS increased to a maximum that was dose dependent. The maximum ROS level reached after irradiation was specific for the fibroblast type. However, carbon ions induced this maximum level at a lower dose compared with X-rays. Within ∼1 week, ROS decreased to control levels. The time-course of decreasing ROS coincides with an increase in cell number and decreasing p21 protein levels, indicating a release from radiation-induced growth arrest. Interestingly, radiation did not act as a trigger for chronically enhanced levels of ROS months after radiation exposure.

Hematopoietic stem cell injury induced by ionizing radiation

SIGNIFICANCE

Exposure to ionizing radiation (IR) as the result of nuclear accidents or terrorist attacks is a significant threat and a major medical concern. Hematopoietic stem cell (HSC) injury is the primary cause of death after accidental or intentional exposure to a moderate or high dose of IR. Protecting HSCs from IR should be a primary goal in the development of novel medical countermeasures against radiation.

RECENT ADVANCES

Significant progress has been made in our understanding of the mechanisms by which IR causes HSC damage. The mechanisms include (i) induction of HSC apoptosis via the p53-Puma pathway; (ii) promotion of HSC differentiation via the activation of the G-CSF/Stat3/BATF-dependent differentiation checkpoint; (iii) induction of HSC senescence via the ROS-p38 pathway; and (iv) damage to the HSC niche.

CRITICAL ISSUES

Induction of apoptosis in HSCs and hematopoietic progenitor cells is primarily responsible for IR-induced acute bone marrow (BM) injury. Long-term BM suppression caused by IR is mainly attributable to the induction of HSC senescence. However, the promotion of HSC differentiation and damage to the HSC niche can contribute to both the acute and long-term effects of IR on the hematopoietic system.

FUTURE DIRECTIONS

In this review, we have summarized a number of recent findings that provide new insights into the mechanisms whereby IR damages HSCs. These findings will provide new opportunities for developing a mechanism-based strategy to prevent and/or mitigate IR-induced BM suppression. Antioxid.

Mitochondrial energy metabolism and redox signaling in brain aging and neurodegeneration

SIGNIFICANCE

The mitochondrial energy-transducing capacity is essential for the maintenance of neuronal function, and the impairment of energy metabolism and redox homeostasis is a hallmark of brain aging, which is particularly accentuated in the early stages of neurodegenerative diseases.

RECENT ADVANCES

The communications between mitochondria and the rest of the cell by energy- and redox-sensitive signaling establish a master regulatory device that controls cellular energy levels and the redox environment. Impairment of this regulatory devise is critical for aging and the early stages of neurodegenerative diseases.

CRITICAL ISSUES

This review focuses on a coordinated metabolic network-cytosolic signaling, transcriptional regulation, and mitochondrial function-that controls the cellular energy levels and redox status as well as factors which impair this metabolic network during brain aging and neurodegeneration.

FUTURE DIRECTIONS

Characterization of mitochondrial function and mitochondria-cytosol communications will provide pivotal opportunities for identifying targets and developing new strategies aimed at restoring the mitochondrial energy-redox axis that is compromised in brain aging and neurodegeneration.

Effects of ionizing radiation on mitochondria

The current concept of radiobiology posits that damage to the DNA in the cell nucleus is the primary cause for the detrimental effects of radiation. However, emerging experimental evidence suggests that this theoretical framework is insufficient for describing extranuclear radiation effects, particularly the response of the mitochondria, an important site of extranuclear, coding DNA. Here, we discuss experimental observations of the effects of ionizing radiation on the mitochondria at (1) the DNA and (2) functional levels. The roles of mitochondria in (3) oxidative stress and (4) late radiation effects are discussed. In this review, we summarize the current understanding of targets for ionizing radiation outside the cell nucleus. Available experimental data suggest that an increase in the tumoricidal efficacy of radiation therapy might be achievable by targeting mitochondria. Likewise, more specific protection of mitochondria and its coding DNA should reduce damage to healthy cells exposed to ionizing radiation.

Cytoplasmic irradiation results in mitochondrial dysfunction and DRP1-dependent mitochondrial fission

Direct DNA damage is often considered the primary cause of cancer in patients exposed to ionizing radiation or environmental carcinogens. Although mitochondria are known to play an important role in radiation-induced cellular response, the mechanisms by which cytoplasmic stimuli modulate mitochondrial dynamics and functions are largely unknown. In the present study, we examined changes in mitochondrial dynamics and functions triggered by α particle damage to the mitochondria in human small airway epithelial cells, using a precision microbeam irradiator with a beam width of 1 μm. Targeted cytoplasmic irradiation using this device resulted in mitochondrial fragmentation and a reduction of cytochrome c oxidase and succinate dehydrogenase activity, when compared with nonirradiated controls, suggesting a reduction in respiratory chain function. In addition, mitochondrial fragmentation or fission was associated with increased expression of the dynamin-like protein DRP1, which promotes mitochondrial fission. DRP1 inhibition by the drug mdivi-1 prevented radiation-induced mitochondrial fission, but respiratory chain function in mitochondria inhibited by radiation persisted for 12 hours. Irradiated cells also showed an increase in mitochondria-derived superoxide that could be quenched by dimethyl sulfoxide. Taken together, our results provide a mechanistic explanation for the extranuclear, nontargeted effects of ionizing radiation.

Long-term effects of ionising radiation on the brain: cause for concern?

There is no clear evidence proving or disproving that ionising radiation is causally linked with neurodegenerative diseases such as Parkinson's and Alzheimer's. However, it is known that high doses of ionising radiation to the head (20-50 Gy) lead to severe learning and memory impairment which is characteristical for Alzheimer's. The cumulative doses of ionising radiation to the Western population are accruing, mostly due to the explosive growth of medical imaging procedures. Children are in particular prone to ionising radiation as the molecular processes within the brain are not completely finished. Furthermore, they have a long lifespan under risk. We wish to open a debate if such low doses of radiation exposure may lead to delayed long-term cognitive and other defects, albeit at a lower frequency than those observed during application of high doses. Further, we want to sensitise the society towards the risks of ionising radiation. To achieve these aims, we will recapitulate the known symptoms of Parkinson's and Alzheimer's on the molecular level and incorporate data of mainly low- and moderate-ionising radiation (<5 Gy). Thus, we want to highlight in general the potential similarities of both the neurodegenerative and radiation-induced pathways. We will propose a mechanistic model for radiation-induced neurodegeneration pointing out mitochondria as a key element. This includes effects of oxidative stress and neuroinflammation-all fundamental players of neurodegenerative diseases.

Mitochondrial respiratory chain dysfunction: implications in neurodegeneration

For decades mitochondria have been considered static round-shaped organelles in charge of energy production. In contrast, they are highly dynamic cellular components that undergo continuous cycles of fusion and fission influenced, for instance, by oxidative stress, cellular energy requirements, or the cell cycle state. New important functions beyond energy production have been attributed to mitochondria, such as the regulation of cell survival, because of their role in the modulation of apoptosis, autophagy, and aging. Primary mitochondrial diseases due to mutations in genes involved in these new mitochondrial functions and the implication of mitochondrial dysfunction in multifactorial human pathologies such as cancer, Alzheimer and Parkinson diseases, or diabetes has been demonstrated. Therefore, mitochondria are set at a central point of the equilibrium between health and disease, and a better understanding of mitochondrial functions will open new fields for exploring the roles of these mitochondrial pathways in human pathologies. This review dissects the relationships between activity and assembly defects of the mitochondrial respiratory chain, oxidative damage, and alterations in mitochondrial dynamics, with special focus on their implications for neurodegeneration.

Ionizing radiation induces mitochondrial reactive oxygen species production accompanied by upregulation of mitochondrial electron transport chain function and mitochondrial content under control of the cell cycle checkpoint

Whereas ionizing radiation (Ir) instantaneously causes the formation of water radiolysis products that contain some reactive oxygen species (ROS), ROS are also suggested to be released from biological sources in irradiated cells. It is now becoming clear that these ROS generated secondarily after Ir have a variety of biological roles. Although mitochondria are assumed to be responsible for this Ir-induced ROS production, it remains to be elucidated how Ir triggers it. Therefore, we conducted this study to decipher the mechanism of Ir-induced mitochondrial ROS production. In human lung carcinoma A549 cells, Ir (10 Gy of X-rays) induced a time-dependent increase in the mitochondrial ROS level. Ir also increased mitochondrial membrane potential, mitochondrial respiration, and mitochondrial ATP production, suggesting upregulation of the mitochondrial electron transport chain (ETC) function after Ir. Although we found that Ir slightly enhanced mitochondrial ETC complex II activity, the complex II inhibitor 3-nitropropionic acid failed to reduce Ir-induced mitochondrial ROS production. Meanwhile, we observed that the mitochondrial mass and mitochondrial DNA level were upregulated after Ir, indicating that Ir increased the mitochondrial content of the cell. Because irradiated cells are known to undergo cell cycle arrest under control of the checkpoint mechanisms, we examined the relationships between cell cycle and mitochondrial content and cellular oxidative stress level. We found that the cells in the G2/M phase had a higher mitochondrial content and cellular oxidative stress level than cells in the G1 or S phase, regardless of whether the cells were irradiated. We also found that Ir-induced accumulation of the cells in the G2/M phase led to an increase in cells with a high mitochondrial content and cellular oxidative stress level. This suggested that Ir upregulated mitochondrial ETC function and mitochondrial content, resulting in mitochondrial ROS production, and that Ir-induced G2/M arrest contributed to the increase in the mitochondrial ROS level by accumulating cells in the G2/M phase.

Mitochondrial network in the heart

Mitochondria are subcellular organelles that provide energy for the cell. They form a dynamic tubular network and play an important role in maintaining the cell function and integrity. Heart is a powerful organ that supplies the motivation for circulation, thereby requiring large amounts of energy. Thus, the healthiness of cardiomyocytes and mitochondria is necessary for the normal cardiac function. Mitochondria not only lie in the center of the cell apoptotic pathway, but also are the major source of reactive oxygen species (ROS) generation. Mitochondrial morphological change includes fission and fusion that are regulated by a large number of proteins. In this review we discuss the regulators of mitochondrial fission/fusion and their association with cell apoptosis, autophagy and ROS production in the heart.

Mitochondrial dynamics in cancer and neurodegenerative and neuroinflammatory diseases

Mitochondria are key organelles in the cell, hosting essential functions, from biosynthetic and metabolic pathways, to oxidative phosphorylation and ATP production, from calcium buffering to red-ox homeostasis and apoptotic signalling pathways. Mitochondria are also dynamic organelles, continuously fusing and dividing, and their localization, size and trafficking are finely regulated. Moreover, in recent decades, alterations in mitochondrial function and dynamics have been implicated in an increasing number of diseases. In this review, we focus on the relationship clarified hitherto between mitochondrial dynamics and cancer, neurodegenerative and neuroinflammatory diseases.